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Im Rahmen eines Kooperationsprojekts mit der Schottel GmbH wurde eine bauteilnahe Probengeometrie entwickelt, mit der sich unter Verwendung einer einachsigen Prüfmaschine das Spannungstiefenprofil in dem unter überlagerter Biege- und Zugspannung beanspruchtem Querschnitt im Schweißnahtübergang des Bauteils reproduzieren ließ.
Proben dieser Geometrie wurden vom Kooperationspartner im geschweißten bzw. geschweißten und spannungsarmgeglühten Zustand bereitgestellt. An der BAM wurden je Wärmebehandlungszustand mittels Senkerodieren zwei verschiedene künstliche Defektgeometrien im Schweißnahtübergang eingebracht.
Anhand der Proben wurden experimentell für alle vier Probenzustände Wöhlerlinien für ein Spannungsverhältnis von R = 0,1 ermittelt.
Zusätzlich wurde das u.a. an der BAM entwickelte IBESS-Verfahren angewandt, um die Wöhlerlinien analytisch zu bestimmen. Hierbei wurde eine sehr gute Übereinstimmung von Berechnung und Experiment erreicht, wobei die Berechnung zu einer leicht konservativen Lebensdauervorhersage führte.
In situ chemical analysis of duplex stainless steel weld by laser induced breakdown spectroscopy
(2024)
The high corrosion resistance and good mechanical properties of duplex stainless steel (DSS) are due to its special chemical composition, which is a balanced phase ratio of ferrite (α) and austenite (γ). Many industrial applications require the integration of DSS components. For this, Gas tungsten arc welding (GTAW) is an excellent choice, as it allows an automated operation with high reproducibility. However, when the weld pool solidifies, critical ratios of α- and γ- phases can occur, which lead to solidification cracking, increased susceptibility to corrosion, and a decrease in ductility and critical strength. Previous studies have shown that these defects can be caused by the accumulation of manganese and chromium in the heat affected zone (HAZ), requiring ongoing monitoring of this accumulation. A suitable method for such monitoring is laser-induced breakdown spectroscopy (LIBS), which can be used in two operating modes: calibration using standard reference samples and calibration-free. Unlike conventional quantitative LIBS measurements, which require reference samples to generate a calibration curve, calibration-free LIBS (CF-LIBS) allows chemical compositions to be determined solely from the emission spectrum of the plasma. Numerous publications show that CF-LIBS is a fast and efficient analytical method for the quantitative analysis of metal samples. In this work, CF-LIBS is applied to spectra obtained during GTAW DSS welding and the result is compared with those obtained by PLS analysis. A good correlation was found between both types of analysis, demonstrating the suitability of the CF-LIBS method for this application. The CF-LIBS method has a significant advantage over conventional LIBS due to the rapid in situ measurement of concentrations of major alloying elements without calibration procedure. This, combined with fast feedback and appropriate adjustment of welding parameters, helps prevent welding defects.
The weldability of stainless steels is largely controlled by the chemical composition, and alloys with ferritic or ferritic-austenitic solidification show the highest resistance to hot cracking. As the resulting phase balance also
affects the final properties, it may be beneficial to both foresee and measure the weld metal ferrite content. The WRC ’92 constitution diagram is currently the most accurate prediction tool available, but it does not take the cooling rate into consideration and the precision may be less accurate for stainless steels with high ferrite numbers (FN). This study aims to validate the reliability of the WRC ’92 diagram for weld metals with FN > 50. The chemical composition was altered through gas tungsten arc welding (GTAW) of UNS S32205 with ER347 filler wire that had been PVD-coated with either niobium (Nb), copper (Cu), nickel (Ni), manganese (Mn), carbon (C), or silicon (Si). The actual ferrite content was assessed using image analysis, FERITSCOPE® and X-ray diffraction (XRD). While the WRC ’92 diagram predictions were deemed accurate to acceptable for Ni, Si, and Mn, notable deviations were observed for Nb, Cu, and C. The FeriteScope exhibited a consistent trend with image analysis, though the values were higher, and scatter was wider, and the conversion factor is open for discussion. The lowest accuracy and largest spread were obtained using non-contact XRD, rendering it unsuitable for ferrite measurements of welds.
The development of hydrogen technologies entails high safety requirements in distribution and dispensing infrastructure. Therefore, it is necessary to pursue research on material compatibility in hydrogen, especially for critical parts with tribological issues.
The focus of this study is to evaluate the influence of hydrogen on a wider range of commercially available polymer materials. Thereby, the friction and wear behavior of different grades of TPE, POM, PA66, PA12, PPA, PEEK, PPS, PTFE, PAI, PI and PBI were investigated against a rotating steel disk (AISI 304). Filled and unfilled polymers from different suppliers were evaluated at room temperature in air, vacuum and hydrogen gas (H2) as well as in liquid hydrogen at - 253°C (LH2).
The sliding behavior of the polymer materials is discussed by means of surface analyses, whereby special attention is paid to the formation of a transfer film.
According to the results at ambient temperature, the effect of hydrogen environment on the tribological behavior of neat polymers may be related to lack of moisture, but also to saturated hydrocarbons in gaseous hydrogen. In liquid hydrogen, the best tribological performances were achieved with neat PA polymers as well as PPS and PI composites.
Effect of high-pressure hydrogen environment on the physical and mechanical properties of elastomers
(2024)
This study presents the influence of high-pressure hydrogen environment on the physical and mechanical properties of two types of cross-linked hydrogenated acrylonitrile butadiene rubbers. Based on the CSA/ANSI standard, static exposures in hydrogen experiments were performed up to 100 MPa at 120 °C. Characterization before and after exposure was conducted by means of density and hardness measurements, dynamic mechanical analysis (DMA), tensile tests, compression set, FT-IR and AFM analyses to assess effects after decompression. While the effect of high-pressure exposure is significant immediately after exposure, most of the physical and mechanical properties recover after 48 hours. FT-IR, AFM, SEM and compression set results indicate, however, permanent effects.
A fundamental study of physical mechanisms of wineglass-shaped fusion zone profile in laser melting
(2024)
The fusion zone geometry in laser melting processes e.g., laser welding and laser-based additive manufacturing, of metallic materials has commonly a wineglass-shaped profile which is critical to the grain orientation and stress distribution. Hereby, we adopt for the first time a decoupling work through a combination of multi-physics modelling and experiments to reveal the fundamental mechanisms of this special morphology. Two physicsbased easy-to-use metal vapour models are proposed to consider the vapour’s momentum and thermal effects separately. It is found that the direct laser energy absorption and Marangoni shear stress which are widely hypothesised to dominate the wineglass-shape formation show only a minor influence. The additional heating from the metallic vapour plume rather than its momentum impact contributes predominantly to the enlarging of the molten pool top region, resulting directly in the formation of the wineglass-shaped fusion zone. The generality of the plume heating effect is also validated in two types of materials (steel and Al) in a wide range of parameters.
Despite the advances in hardware and software techniques, standard numerical methods fail in providing real-time simulations, especially for complex processes such as additive manufacturing applications. A real-time simulation enables process control through the combination of process monitoring and automated feedback, which increases the flexibility and quality of a process. Typically, before producing a whole additive manufacturing structure, a simplified experiment in the form of a beadon-plate experiment is performed to get a first insight into the process and to set parameters suitably. In this work, a reduced order model for the transient thermal problem of the bead-on-plate weld simulation is developed, allowing an efficient model calibration and control of the process. The proposed approach applies the proper generalized decomposition (PGD) method, a popular model order reduction technique, to decrease the computational effort of each model evaluation required multiple times in parameter estimation, control, and optimization. The welding torch is modeled by a moving heat source, which leads to difficulties separating space and time, a key ingredient in PGD simulations. A novel approach for separating space and time is applied and extended to 3D problems allowing the derivation of an efficient separated representation of the temperature.
The results are verified against a standard finite element model showing excellent agreement. The reduced order model is also leveraged in a Bayesian model parameter estimation setup, speeding up calibrations and ultimately leading to an optimized real-time simulation approach for welding experiment using synthetic as well as real measurement data.
It is proposed to model the experimentally observed weld pool boundary with superellipses (Lamé curves) and to find the unknown parameters of the curves using optimization methods. It has been shown experimentally that during laser welding of austenitic stainless steel with a thickness of 2 mm at a speed of 20 mm/s, the rear weld pool part has a shape close to triangular which can be accurately approximated by a superellipse. Analytical dependences of the trajectory and growth rate of the crystal and its cross-sectional area on the geometry of the rear weld pool part are obtained.
A method of solving the thermo-fluid dynamics problem is presented, enabling the prediction of the shape and dimensions of the weld pool during keyhole laser welding of thick plates. It is shown that the rear weld pool boundary can be satisfactorily approximated by a set of superellipses (Lamé curves). The presence of a convex rear weld pool boundary in the mid-plane has been observed experimentally and reproduced numerically. It was shown that in this zone the concentration of liquating impurities increases and the local solidification temperature decreases, contributing to the susceptibility to hot cracking.
Multiple principal element alloys encompass the well-known high entropy alloys (HEA). The alloy system represents a new class of materials consisting of at least three alloying elements, each containing 5 to 35 at.%. Thus, this alloying concept differs fundamentally from conventional materials such as steel or nickel alloys. For this purpose, the alloying elements are specifically selected, the microstructures are adjusted in a single-phase and, in some cases, multi-phase manner. In particular, conflicting goals, such as the trade-off between strength and ductility in conventional steels, are overcome. However, in the last 20 years, the focus has been on material synthesis. With the increase in available material quantities, the focus is now on processing issues such as joining and welding processes. The weldability of HEAs has received very little attention so far. The experience with dissimilar metal welds is completely lacking but is essential for the application of these materials in combination with conventional materials. The present study presents comprehensive experimental results on the weldability of an equimolar CoCrFeMnNi-HEA in cold-rolled and heat-treated condition, which was joined by tungsten inert gas welding to an austenitic steel AISI 304. The mechanical properties of the dissimilar metal welds were characterized by cross-weld tensile samples, whereas the local deformation in the weld of the different welding zones was measured by digital image correlation. In accordance with the respective initial HEA condition (cold-rolled vs. heat-treated), the local strain behavior was divergent and influenced the global mechanical properties of both DMW types. Nonetheless, the experiments provided proof in principle of the weldability for dissimilar joints of the CoCrFeMnNi-HEA welded to conventional materials like austenitic stainless steels ensuring a corresponding capability for mechanical loading. This allows further considerations on the application of these innovative materials.
Die Erzeugung rissschließfreier Anrisse in gekerbten bruchmechanischen Proben für anschließende Ermüdungsversuche kann durch eine zyklische Druckbelastung an der Kerbe erfolgen: Bei der Erstbelastung wird eine plastische Verformung an der Kerbspitze erzeugt, welche Zugeigenspannungen und damit eine risstreibende Kraft bei der folgenden zyklischen Druckbelastung hervorruft. Durch das Risswachstum kommt es zu einer Entspannung, bis der Riss schließlich arretiert, weil die effektive risstreibende Kraft auf den Schwellenwert abgesunken ist. Um Kurzrisswachstum in additiv, mittels pulverbettbasiertem Laserstrahlschmelzen (Laser Powder Bed Fusion - LPBF) gefertigtem AISI 316L Stahl und seinem konventionell erzeugten, warmgewalzten Pendant zu untersuchen, wurde dieses Verfahren im Rahmen der vorliegenden Arbeit genutzt. Im Falle des additiv gefertigten Materials kam es zu unerwartet langem Ermüdungsrisswachstum, bevor der Riss arretierte. Zur Ermittlung der Ursache erfolgte eine Messung der Eigenspannungsverteilungen mittels Neutronendiffraktion in der Kerbebene der additiv gefertigten Proben. Anschließend wurde ein Finite-Elemente-Modell zur Simulation des Ermüdungsrisswachstums in konventionellem und additiv gefertigtem Material implementiert. Im Rahmen dieses Vortrages wird insbesondere auf das Vorgehen bei der Simulation und auf die getroffenen Annahmen und Vereinfachungen eingegangen. Die Resultate werden mit den experimentellen Ergebnissen verglichen und das für die Simulation von Risswachstum genutzte Knoten-Freigabe-Verfahren (Node-Release), das Einbringen von Eigenspannungen als initiale Spannungen in der Simulation, und die Rechengrößen zur Quantifizierung der risstreibenden Kraft kritisch diskutiert.
Direct energy deposition additive manufacturing technologies utilizing an electric arc offer a great potential in generating large volume metal components. However, the selection of process parameters that yield the desired near net shape design as well as the requested mechanical component behavior is not a trivial task due to the complex relationship. Exemplarily for additive manufacturing of high-strength precipitation hardening AlMgSi-aluminum alloy this paper shows the application of a newly developed matching solid welding wire doped with TiB as grain refiner. The correlation between process parameters and component quality is examined analyzing the size and distribution of pores as well as the grain morphology. Furthermore, the influences of different post-weld heat treatments are evaluated to meet the reference mechanical properties of the corresponding wrought material. Finally, the digital integration of the entire additive manufacturing chain enables an overall traceability of the relevant process steps which is the basis for a reliable subsequent quality assessment.
AbstractThe sustainable and resource-efficient production of wind energy plants requires the use of modern high-strength fine-grain structural steels. This applies to both foundation and erection structures, like mobile or ship cranes. During the assembly of steel structures, unacceptable defects can occasionally be found in the weld area. In most cases, the economical solution would be local thermal gouging of the affected areas and re-welding. Due to the high shrinkage restraint of the joint groove in the overall structure, the superposition of global and local welding-induced stresses may lead to crack formation and component failure, particularly in interaction with the degradation of the microstructure and mechanical properties of high-strength steels during the repair process. However, manufacturers hardly have any information about these issues and there is a lack of recommendations and guidelines to take these safety-relevant aspects into account in adequate repair concepts. The aim of this research is to derive recommendations for repair concepts appropriate to the stresses and materials involved providing a basis for standards and guidelines to avoid cold cracking, damage and expensive reworking especially for high-strength steels. Part 1 of this study involves systematic investigations of influences of shrinkage restraint during repair welding of two high-strength steels S500MLO for offshore application and S960QL for mobile crane structures. The quantification of the shrinkage restraint of repair weld joints was achieved by means of experimental and numerical restraint intensity analysis. In welding experiments with self-restrained slot specimens, restraint intensity and introduction of hydrogen via the welding arc using anti spatter spray were varied systematically to analyse the effect on welding result, residual stresses and cold cracking. It could be shown that increasing restraint intensities result in significantly higher transverse residual stress levels. In the case of hydrogen introduction S500MLO showed no cold cracking independent of the restraint conditions. However, S960QL was found to be considerably cold cracking sensitive if hydrogen is introduced. With increasing restraint intensity length and number of cold cracks increases significantly. Part 2 [1] of this study is focussed on microstructure and residual stresses due to gouging and stress optimization via adequate heat control parameters in repair welding.
In manufacturing, fusion welding processes use a lot of resources, which presents an opportunity to reduce environmental impact. While there is a general understanding of the environmental impact of these processes, it is difficult to quantitatively assess key parameters. This study introduces a welding-specific methodology that uses life cycle assessment (LCA) to evaluate the environmental impact of fusion welding technologies. Our approach analyses the main parameters that affect the environmental performance of different welding techniques, including traditional methods and additive manufacturing through the Direct Energy Deposition-Arc (DED-Arc) process. We integrate real-time resource usage data to offer an innovative framework for directly deriving environmental impacts. This research contributes to optimising welding processes by providing a precise and quantifiable measure of their ecological impact, facilitating the advancement of sustainable manufacturing practices.
Life cycle assessment of fusion welding processes considering upstream and downstream process steps
(2023)
In manufacturing, fusion welding processes consume significant resources, presenting a significant opportunity for reducing environmental impact. Although there is a qualitative understanding of the environmental implications of these processes, a quantitative assessment of key parameters remains complex. This study introduces a welding-specific methodology that employs life cycle assessment (LCA) to quantitatively evaluate the environmental footprint of fusion welding technologies. Our approach identifies and analyses the principal parameters affecting the environmental performance of various welding techniques, including traditional joint welding and additive manufacturing via the Direct Energy Deposition-Arc (DED-Arc) process. Real-time resource usage data is integrated to offer an innovative framework for directly deriving environmental impacts. This research contributes to optimising welding processes by providing a precise and quantifiable measure of their ecological impact. This facilitates the advancement of sustainable manufacturing practices.
Numerical and experimental assessment of liquid metal embrittlement in externally loaded spot welds
(2024)
Zinc-based surface coatings are widely applied with high-strength steels in automotive industry. Some of these base materials show an increased brittle cracking risk during loading. It is necessary to examine electrogalvanized and uncoated samples of a high strength steel susceptible to liquid metal embrittlement during spot welding with applied external load. Therefore, a newly developed tensile test method with a simultaneously applied spot weld is conducted. A fully coupled 3D electrical, thermal, metallurgical and mechanical finite element model depicting the resistant spot welding process combined with the tensile test conducted is mandatory to correct geometric influences of the sample geometry and provides insights into the sample’s time dependent local loading. With increasing external loads, the morphology of the brittle cracks formed is affected more than the crack depth. The validated finite element model applies newly developed damage indicators to predict and explain the liquid metal embrittlement cracking onset and development as well as even ductile failure.
Beim Unterpulverschweißen sind die Prozessgeräusche ein Indikator für eine gute Fügequalität. Diese Beurteilung kann i.d.R. nur von einer erfahrenen Fachkraft durchgeführt werden. Eine kürzlich entwickelte künstliche Intelligenz kann automatisch das akustische Prozesssignal anhand vortrainierter Merkmale klassifizieren und die Fügequalität anhand des Geräuschs beurteilen. Der Algorithmus, einmal richtig trainiert, kann den Prüfaufwand beim Unterpulverschweißen deutlich reduzieren.
Die additive Fertigung mittels Schweißverfahren bietet große ökonomische Vorteile für eine ressourceneffiziente Bauteilherstellung. Offene Fragen bezüglich Homogenität, Anisotropie der Schweißgefüge und den damit verbundenen Bauteileigenschaften stehen einer wirtschaftlichen Verarbeitung oftmals im Wege. Finale Bauteilgeometrie und Oberflächengüte erfordern meist komplementäre subtraktive Fertigungsschritte. Werkstoffe für hochbelastbare Komponenten sind oftmals schwer spanbar. In einem Vorhaben der BAM und des ISAF wurde untersucht, wie die Modifikation der AM-Schweißzusätze und das ultraschallunterstützte Fräsen (US) die Zerspanungssituation verbessern. Der vorliegende Artikel stellt wesentliche Zusammenhänge zwischen Legierung, Gefüge und Zerspanung zweier schwer spanbarer Hochleistungslegierungen (FeNi und CoCr) dar. Großes Potenzial zeigte neben dem US die Modifikation mit Zr und Hf bei Zulegierung in das Schweißgut mittels Beschichtung von Massivdrähten bzw. Herstellung von Fülldrähten.
Zur Umsetzung der wasserstoffbasierten Energiewende ist eine zuverlässige Transportinfrastruktur unverzichtbar. Hierfür ist die Nutzung großer Teile des vorhandenen Erdgasnetzes vorgesehen. Im Rahmen von Reparatur- und Wartungsarbeiten ist das Schweißen im Betrieb an Erdgaspipelines Stand der Technik. In Verbindung mit Wasserstoff wirft diese Technik jedoch Fragen der Materialverträglichkeit auf. Das im Vortrag vorgestellte Projekt H2SuD leistet Beiträge zur Aufklärung des Einflusses von Reparaturschweißprozessen auf die Wasserstoffaufnahme und Degradation im Betrieb befindlicher H2 Gasleitungen. Ein Schwerpunkt des Vortrages liegt auf der im Projekt erarbeiteten Prüfmethode für Modellversuche, mit denen der Einfluss von Schweißen im Betrieb an Pipelines auf die Wasserstoffaufnahme getestet wird. Die Durchführung erster Versuche und dabei erzielte Ergebnisse werden präsentiert.
Abstract
In the post-processing of large maritime components, a considerable amount of waste in the form of milling and grinding chips is produced. At the same time, additive manufacturing technologies have shown great potential in producing high-volume parts for maritime applications, allowing novel design approaches and short lead times. In this context, this study presents a sustainable approach to recycle and use aluminium bronze waste material, generated during post-processing of large cast ship propellers, as feedstock for laser-powder directed energy deposition. The recycling technology used to produce powder batches is inductive re-melting in combination with ultrasonic atomization. The derived metal powders are characterized using digital image analysis, powder flowability tests, scanning electron microscopy as well as energy dispersive X-ray spectroscopy. Compared to conventional metal powders produced by gas atomization, the recycled material shows excellent sphericity and a powder size distribution with a higher content of finer and coarser particles. Metallographic sections of deposited additively produced specimens show an increased hardness and reduced ductility, but also competitive densities and higher yield and ultimate tensile strength compared to cast material. The process chain shows high potential for the maritime sector to enable circular and sustainable manufacturing.
Die neue Generation von bainitischen Schmiedestählen bietet auf der einen Seite ein erhebliches Festigkeitspotenzial, das es im Sinne des Leichtbaus auch für zyklisch belastete Sicherheitsbauteile zu heben gilt. Auf der anderen Seite geht mit dieser Festigkeitssteigerung im Vergleich zu Vergütungsstählen ein Verlust an Duktilität und Zähigkeit einher. Am Beispiel des Demonstratorbauteils Achsschenkel ging das Forschungsvorhaben der Frage nach, wie ein Freigabeprozess zu gestalten ist, damit das Festigkeitspotenzial der neuen Stähle zuverlässig ausgenutzt werden kann. Als Ergebnis wird eine Vorgehensweise dargestellt, die den Eigenschaften der neuen Generation von Schmiedestählen Rechnung trägt, um den sicheren Betrieb der Komponenten zu gewährleisten.
In der Abschlusspräsentation des ZIM-Kooperationsvorhabens "TeWuFeAl" werden die wichtigsten Ergebnisse zusammengefasst dargestellt. Ziel des Projektes war die Entwicklung einer Prozess- und Werkzeugtechnologie für das ultraschallunterstützte Fräsen von im Kokillenguss hergestellter Eisen- und Titan-Aluminid-Bauteilen. Auf Basis einer eigens entwickelten Werkzeugtechnologie und Prozessführung konnten die Belastungen auf das Werkzeug und die Oberfläche von Bauteilen aus dem schwer zerspanbaren Eisenaluminid bei der frästechnischen Bearbeitung signifikant verringert werden. Die Messung der Prozesskräfte und der thermischen Belastung der Werkzeugschneide ermöglicht die Ermittlung geeigneter Zerspanungsparameter für eine maximale Werkzeuglebensdauer. Die Anwendung des ultraschallunterstützten Fräsens (USAM) wirkt sich vorteilhaft auf den Zerspanprozess aus. Neben einer deutlichen verlängerung der Werkzeuglebensdauer, zeigen sich Vorteile in Bezug auf die topographischen, metallurgischen sowie mechanischen Eigenschaften der erzeugten Oberflächenintegrität. Die durch USAM erzeugte homogene Bauteiloberfläche weist eine reduzierte Defektdichte und verringerte Rauheitskennwerte auf. Besonders vielversprechend sind die Ergebnisse der Messungen der Eigenspannungen im Tiefenprofil der Oberfläche. Die Werkzeugoszillation bewirkt eine, vergleichbar zum Strahlprozess, Verdichtung der Oberfläche was in Druckeigenspannungen resultiert. Diese potenziell vorteilhaften Druckeigenspannungen können maßgeblich zur Risssicherheit hochbelasteter Komponenten und somit zum Leichtbau und Ressourceneffizienz beitragen.
High-strength steels offer potential for weight optimization due to reduced wall thicknesses in modern constructions. Additive manufacturing processes such as Wire Arc Additive Manufacturing (WAAM) enable the resource-efficient production of structures. In the case of defects occurring in weld seams orWAAM components due to unstable process conditions, the economical solution is local gouging or machining and repair welding. It is important to understand the effects of machining steps on the multiaxial stress state in conjunction with the design-related shrinkage restraints. Research into how welding and slot milling of welds andWAAM structures affects residual stresses is still lacking. For this reason, component-related investigations with high-strength steels with yield strengths ≥790 MPa are carried out in our research. In-situ digital image correlation (DIC) and ex-situ X-ray diffraction (XRD) were used to analyze the stresses and strains induced on specimens during and after milling. The systematic analyses revealed a significant interaction of the stiffness and microstructure of the specimens with the initial residual stresses induced by welding. Subsequent repair welds can result in significantly higher residual stresses.
Additive manufacturing (AM) processes enable the efficient production of advanced constructions. New developments in topology optimization are leading to weight-optimized designs of increasing complexity. Direct energy deposition processes (DED) such as wire and arc-based additive manufacturing are an important method of additive manufacturing. The wire filler metals enable a wide range of materials, while the arc process provides a high deposition rate compared to laser and powder-based processes. Combined with the use of high-strength steels, the thickness of walls or components can be significantly reduced in the context of lightweight construction, which results in significant savings in energy, costs, time and resources. Suitable high-strength steel filler metals are commercially available for DED-arc AM processes. However, guidelines and quantitative knowledge about welding stresses and cold cracking issues during component production and service are lacking. This limits the industrial application considerably. In a joint project of BAM and Chemnitz University of Technology, the main influences and complex interactions of material, production process, design and processing steps on the residual stress level are investigated. The aim is to develop processing recommendations and a cold cracking test for economical processing and stress-related design of high-strength steels with DED-arc. This study focuses on residual stress analysis by neutron diffraction (ND) and X-ray diffraction (XRD) on defined test specimens. The ND analysis were performed at the Paul Scherrer Institute- Villigen, Switzerland (PSI) and the XRD analysis at BAM. The study shows a quantitative and qualitative comparison of the residual stress magnitudes and distribution between the component bulk (ND) and surface (XRD) analyses. The ND analysis reveals that in DED-arc AM walls the residual stresses dominate in the direction of welding and are negligibly small in each case transverse to the direction of welding. The topology of the analyzed residual stresses shows almost identical residual stress maps compared to XRD. In addition, the residual stresses are significantly influenced by the solid phase transformation of the material due to low cooling times and less post heat treatment cycles of following AM layers in the area of the top layer.
Additive manufacturing (AM) processes enable the efficient production of advanced constructions. New developments in topology optimization are leading to weight-optimized designs of increasing complexity. Direct energy deposition processes (DED) such as wire and arc-based additive manufacturing are an important method of additive manufacturing. The wire filler metals enable a wide range of materials, while the arc process provides a high deposition rate compared to laser and powder-based processes. Combined with the use of high-strength steels, the thickness of walls or components can be significantly reduced in the context of lightweight construction, which results in significant savings in energy, costs, time and resources. Suitable high-strength steel filler metals are commercially available for DED-arc AM processes. However, guidelines and quantitative knowledge about welding stresses and cold cracking issues during component production and service are lacking. This limits the industrial application considerably. In a joint project of BAM and Chemnitz University of Technology, the main influences and complex interactions of material, production process, design and processing steps on the residual stress level are investigated. The aim is to develop processing recommendations and a cold cracking test for economical processing and stress-related design of high-strength steels with DED-arc. This study focuses on residual stress analysis by neutron diffraction (ND) and X-ray diffraction (XRD) on defined test specimens. The ND analysis were performed at the Paul Scherrer Institute- Villigen, Switzerland (PSI) and the XRD analysis at BAM. The study shows a quantitative and qualitative comparison of the residual stress magnitudes and distribution between the component bulk (ND) and surface (XRD) analyses. The ND analysis reveals that in DED-arc AM walls the residual stresses dominate in the direction of welding and are negligibly small in each case transverse to the direction of welding. The topology of the analyzed residual stresses shows almost identical residual stress maps compared to XRD. In addition, the residual stresses are significantly influenced by the solid phase transformation of the material due to low cooling times and less post heat treatment cycles of following AM layers in the area of the top layer.
Bei der Montage von Stahlkonstruktionen kommt es trotz anforderungsgerechter schweißtechnischer Fertigung vereinzelt zur Detektion von unzulässigen Unregelmäßigkeiten im Schweißbereich. Die Verarbeitungsregelwerke empfehlen das lokale thermische Ausfugen betroffener Bereiche und erneutes Schweißen, geben aber aufgrund fehlender wissenschaftlich fundierter Untersuchungen kaum Informationen zu adäquaten Reparaturkonzepten. Dies betrifft insbesondere die Berücksichtigung und Optimierung resultierender schweißbedingter Beanspruchungen durch hohe Schrumpfbehinderungen der Ausfugenuten sowie der Gefügedegradation angrenzender Bereiche durch das Ausfugen und erneute Schweißen. Gerade bei hochfesten Stahlgüten ergeben sich dadurch häufig reduzierte mechanische Eigenschaften und zusätzliche schweißbedingte Beanspruchungen sowie erneut auftretende Nahtdefekte.
Deshalb wurden für das Forschungsvorhaben systematische bauteilrelevante Untersuchungen der schweißbedingten Beanspruchungen und Gefügeveränderungen reparierter Schweißnähte in Abhängigkeit von der Schrumpfbehinderung und Wärmeführung beim Schweißen und Ausfugen sowie von der Reparaturzyklenanzahl durchgeführt. Die Untersuchungsergebnisse zeigen auf, welche Faktoren sich für eine Beanspruchungsreduzierung auch bei hohen Einspannbedingungen eignen und wie eine Degradation des Gefüges und der Eigenschaften der Schweißnaht sowie wiederholte Schweißnahtdefekte in der Reparaturnaht vermieden werden können. Insbesondere können mittels adaptiver Wärmeführung geringere schweißbedingte Beanspruchungen in den Reparaturschweißnähten bewirkt werden. Bauteilversuche sichern zudem die Übertragbarkeit der Schweißexperimente in die Praxis ab.
Aufgrund der Erkenntnisse konnten Empfehlungen für beanspruchungs- und werkstoffgerechte Reparaturkonzepte abgeleitet und ausgesprochen werden. Die Analysen wurden an den hochfesten Stahlgüten S500MLO für den Offshore-Bereich und S960QL für den Mobilkranbau realisiert. Damit wird insbesondere der wirtschaftlichen Fertigung hocheffizienter Konstruktionen für Windenergieanlagen und hochfester Strukturen, die für deren Errichtung notwendig sind, Rechnung getragen.
So bieten die Forschungsergebnisse eine wesentliche Grundlage für die Weiterentwicklung entsprechender Normen und Regelwerke. Damit können letztlich Schäden und zumeist teure Nacharbeiten verhindert und eine verbesserte Ausnutzung des hohen Festigkeitspotentials hochfester Stähle erreicht werden. Gerade auch KMU können mit Blick auf die Kosten für Fertigung, Schweißarbeit und Material von den Erkenntnissen beim Einsatz hochfester Stähle, die für eine effiziente Realisierung der Energiewende in Deutschland notwendig sind, profitieren.
Additive manufacturing (AM) processes enable the efficient production of advanced constructions. New developments in topology optimization are leading to weight-optimized designs of increasing complexity. Direct energy deposition processes (DED) such as wire and arc-based additive manufacturing are an important method of additive manufacturing. The wire filler metals enable a wide range of materials, while the arc process provides a high deposition rate compared to laser and powder-based processes. Combined with the use of high-strength steels, the thickness of walls or components can be significantly reduced in the context of lightweight construction, which results in significant savings in energy, costs, time and resources. Suitable high-strength steel filler metals are commercially available for DED-arc AM processes. However, guidelines and quantitative knowledge about welding stresses and cold cracking issues during component production and service are lacking. This limits the industrial application considerably. In a joint project of BAM and Chemnitz University of Technology, the main influences and complex interactions of material, production process, design and processing steps on the residual stress level are investigated. The aim is to develop processing recommendations and a cold cracking test for economical processing and stress-related design of high-strength steels with DED-arc. This study focuses on residual stress analysis by neutron diffraction (ND) and X-ray diffraction (XRD) on defined test specimens. The ND analysis were performed at the Paul Scherrer Institute- Villigen, Switzerland (PSI) and the XRD analysis at BAM. The study shows a quantitative and qualitative comparison of the residual stress magnitudes and distribution between the component bulk (ND) and surface (XRD) analyses. The ND analysis reveals that in DED-arc AM walls the residual stresses dominate in the direction of welding and are negligibly small in each case transverse to the direction of welding. The topology of the analyzed residual stresses shows almost identical residual stress maps compared to XRD. In addition, the residual stresses are significantly influenced by the solid phase transformation of the material due to low cooling times and less post heat treatment cycles of following AM layers in the area of the top layer.
Wear behavior of innovative niobium carbide cutting tools in ultrasonic-assisted finishing milling
(2023)
The resources of niobium exceed the ones of tungsten by an order of magnitude. With 92%, Brazil is today the main global producer of niobium. Hence, niobium carbides (NbC) are a sustainable and economic alternative to conventionally used cutting materials, especially tungsten carbides (WC). Moreover, NbC can be used in Ni alloy matrix and thus offer significant advantages by substituting WC in Co matrix as cutting materials in terms of health risks and raw material price and supply risk. Based on recent studies which found an increased performance of NbC compared to WC cutting tools in machining higher strength steels, the composition NbC12Ni4Mo4VC was chosen for finish machining of a high-strength steel S960QL in this study. The experiments were carried out on an ultrasonic-assisted 5-axis milling machine using NbC tools specially made to benchmark them with commercially available coated WC cutting inserts. In addition, the influence of a coating system for the NbC inserts is tested and evaluated for its performance in the cutting process. Tool wear and cutting force analyses are implied to identify optimal parameter combinations as well as tool properties for the novel NbC tool. Together with the oscillation of ultrasonic-assisted milling, the loads on the component surface and the tool can be reduced and the wear behavior of the novel NbC tool can be refined. These milling tests are accompanied by standardized wear tests, i.e., pin-on-disc, between the aforementioned material combinations, and the results are correlated with each other. Finally, the behavior when using hard-to-cut materials such as Ni alloys, or innovative materials such as iron aluminide is also being tested, as these are constantly in the focus of machining optimization. With this strategy, comprehensive knowledge is achievable for future efficient application of NbC for milling tools, which have already been researched for decades using WC.
The new multi-element alloying concept of systems with defined entropy (HEA — high-entropy alloy or MEA — medium-entropy alloy) is increasing in material research interest. Improved properties or combinations of properties are shown by several systems. Thus, the resulting microstructures and production of HEA/MEA as well as properties have been primarily investigated so far. Furthermore, processing is a key issue to transfer HEA/MEA systems to real components. Since welding is the most important joining process for metals, it is crucial to investigate the influence of welding to guarantee component integrity. Since most HEA are made of expensive alloying elements such as Co or Ni, they will not be used entirely as structural materials. Thus, it can be advantageous to weld conventional alloys such as austenitic stainless steels with the HEA and MEA to produce components that are both application-oriented and economically viable. Therefore, in this paper, first results of dissimilar metal welding, by tungsten inert gas (TIG) and friction stir welding (FSW), of a CoCrFeMnNi HEA as well as a CoCrNi MEA with a conventional AISI 304 austenitic stainless steel are presented. The focus is on the microstructure formation due to the two welding processes. The results of TIG welding show a dendritic microstructure, whereas in FSW both materials are stirred but still coexist.
Stresses in repair welding of high-strength steels Part 2: Heat Control and Stress Optimization
(2023)
In welding of high-strength steels, e.g., for foundations and erection structures of wind energy plants, unacceptable defects can occasionally be found in the weld area, which should be removed by thermal gouging and subsequent re-welding. High shrinkage restraint of repair welds may lead to crack formation and component failure, particularly in interaction with degraded microstructures and mechanical properties due to repair cycles. This study aims for elaboration of recommendations for repair concepts appropriate to the stresses and materials involved to avoid cold cracking, damage and expensive reworking. In part 1 [1] of this study systematic investigations of influences of shrinkage restraint on residual stresses and cold cracking risk during repair welding of two high-strength steels S500MLO for offshore application and S960QL for mobile crane structures were focussed. In this part 2, the microstructure and residual stresses due to gouging and influences of heat control parameters in repair welding are analysed. A clear reduction in residual stress after gouging can be observed, especially for the specimens with restrained transverse shrinkage. Gouging to a depth of approx. 2/3 of the seam height does not lead to a complete relaxation of the observed reaction forces. Particularly for the higher strength steel S960QL, there are pronounced areas influenced by the gouging process in which a degradation of the microstructure and properties should be assumed. Overall, the repair welds show a significant increase in the width of the weld and HAZ compared to the original weld, especially in the case of S960QL/G89. The repair welds show higher welding-induced stresses than the original welds, especially in the areas of the HAZ and the base metal close to the weld seam. This behaviour can be attributed overall to increased restraint conditions due to the remaining root weld or shorter gouge grooves. In good agreement with earlier investigations, the residual stresses transverse to the weld can be significantly reduced by upwardly limited working or interpass temperatures and the reaction stresses resulting from high restraint conditions can be effectively counteracted. The influence of the heat input on the stress formation is low compared to the interpass temperature for both test materials.
The sustainable and resource-efficient production of wind energy plants requires the use of modern high-strength fine-grain structural steels. This applies to both foundation and erection structures, like mobile or ship cranes. During the assembly of steel structures, unacceptable defects can occasionally be found in the weld area. In most cases, the economical solution would be local thermal gouging of the affected areas and re-welding. Due to the high shrinkage restraint of the joint groove in the overall structure, the superposition of global and local welding-induced stresses may lead to crack formation and component failure, particularly in interaction with the degradation of the microstructure and mechanical properties of high-strength steels during the repair process. However, manufacturers hardly have any information about these issues and there is a lack of recommendations and guidelines to take these safety-relevant aspects into account in adequate repair concepts. The aim of this research is to derive recommendations for repair concepts appropriate to the stresses and materials involved providing a basis for standards and guidelines to avoid cold cracking, damage and expensive reworking especially for high-strength steels. Part 1 of this study involves systematic investigations of influences of shrinkage restraint during repair welding of two high-strength steels S500MLO for offshore application and S960QL for mobile crane structures. The quantification of the shrinkage restraint of repair weld joints was achieved by means of experimental and numerical restraint intensity analysis. In welding experiments with self-restrained slot specimens, restraint intensity and introduction of hydrogen via the welding arc using anti spatter spray were varied systematically to analyse the effect on welding result, residual stresses, and cold cracking. It could be shown that increasing restraint intensities result in significantly higher transverse residual stress levels. In the case of hydrogen introduction S500MLO showed no cold cracking independent of the restraint conditions. However, S960QL was found to be considerably cold cracking sensitive if hydrogen is introduced. With increasing restraint intensity length and number of cold cracks increases significantly. Part 2 [1] of this study is focussed on microstructure and residual stresses due to gouging and stress optimization via adequate heat control parameters in repair welding.
Der Vortrag gibt einen Überblick über den Einfluss der Prozessparameter auf die Eigenspannungen sowie die Härte in additiv gefertigten Bauteilen aus hochfestem stahl. Des Weiteren wird dargestellt, wie sich das Bauteildesign und trennende Fertigungsschritte auf die Eigenspannungen der Bauteile auswirken.
Additive manufacturing (AM) processes enable the efficient production of advanced constructions. New developments in topology optimization are leading to weight-optimized designs of increasing complexity. Direct energy deposition processes (DED) such as wire and arc-based additive manufacturing are an important method of additive manufacturing. The wire filler metals enable a wide range of materials, while the arc process provides a high deposition rate compared to laser and powder-based processes. Combined with the use of high-strength steels, the thickness of walls or components can be significantly reduced in the context of lightweight construction, which results in significant savings in energy, costs, time and resources. Suitable high-strength steel filler metals are commercially available for DED-arc AM processes. However, guidelines and quantitative knowledge about welding stresses and cold cracking issues during component production and service are lacking. This limits the industrial application considerably. In a joint project of BAM and Chemnitz University of Technology, the main influences and complex interactions of material, production process, design and processing steps on the residual stress level are investigated. The aim is to develop processing recommendations and a cold cracking test for economical processing and stress-related design of high-strength steels with DED-arc. This study focuses on residual stress analysis by neutron diffraction (ND) and X-ray diffraction (XRD) on defined test specimens. The ND analysis were performed at the Paul Scherrer Institute- Villigen, Switzerland (PSI) and the XRD analysis at BAM. The study shows a quantitative and qualitative comparison of the residual stress magnitudes and distribution between the component bulk (ND) and surface (XRD) analyses. The ND analysis reveals that in DED-arc AM walls the residual stresses dominate in the direction of welding and are negligibly small in each case transverse to the direction of welding. The topology of the analyzed residual stresses shows almost identical residual stress maps compared to XRD. In addition, the residual stresses are significantly influenced by the solid phase transformation of the material due to low cooling times and less post heat treatment cycles of following AM layers in the area of the top layer.
Additive Fertigungsverfahren wie das Wire Arc Additive Manufacturing (WAAM) ermöglichen die effiziente Fertigung von gewichtsoptimierten endkonturnahen Strukturen in modernen Stahlkonstruktionen. Ihre Effizienz kann weiter durch die Verwendung von hochfesten Stählen gesteigert werden. Dies erlaubt eine signifikante Einsparung von Kosten, Zeit und Ressourcen. Entsprechende Schweißzusatzwerkstoffe für Lichtbogenschweißverfahren sind kommerziell verfügbar. Fehlende Richtlinien und fehlende quantitative Kenntnisse über die schweißtechnische Beanspruchung während der Fertigung und im Betrieb limitieren den industriellen Einsatz stark. Daher werden in einem aktuellen Vorhaben (IGF-Nr. 21162 BG) der BAM und TU Chemnitz die wesentlichen Einflüsse und komplexen Interaktionen durch Werkstoff, Fertigungsprozess, Konstruktion und trennende Fertigungsschritte auf den Beanspruchungszustand untersucht. Der vorliegende Beitrag fokussiert hierfür die Auswirkungen trennender Fertigungsschritte auf Verzug und Eigenspannungen definierter WAAM-Prüfkörper. Die Eigenspannungsanalyse erfolgt mittels Röntgenbeugung. Die große Anzahl an Ergebnissen von zuvor untersuchten Referenzproben, die mit einem speziellen WAAM-Massivdraht (Streckgrenze >820 MPa) bei unterschiedlicher Wärmeführung und Geometrie vollautomatisiert geschweißt wurden, lässt eine Korrelation der Messdaten zu. Hierzu erfolgt die Analyse des Ausgangszustandes und abschließend des Zustandes nach trennender Bearbeitung, welche begleitend mittels digitaler Bildkorrelation hinsichtlich des Verzuges untersucht wurden. So ist zu beobachten, dass die Geometrie deutlichen Einfluss auf Relaxation und Umlagerung der Eigenspannungen hat und damit die Risssicherheit positiv beeinflussen kann.
Auf der Grundlage einer speziell entwickelten Werkzeugtechnologie sowie Prozessführung können die Belastungen auf das Werkzeug und die Bauteiloberfläche bei der frästechnischen Bearbeitung von Bauteilen aus dem innovativen jedoch schwer zerspanbaren Eisenaluminid deutlich reduziert werden. Die Messung der Prozesskräfte sowie der thermischen Belastung der Werkzeugschneide ermöglicht die Identifikation geeigneter Zerspanungsparameter hinsichtlich einer maximalen Werkzeuglebensdauer. Die Verwendung des ultraschallunterstützten Fräsens (USAM) besitzt vorteilhafte Einflüsse auf den Zerspanprozess, insbesondere hinsichtlich einer homogenen Bauteiloberfläche. Im Rahmen von Standzeitversuchen erfolgt durch USAM ein gleichmäßiger und im Vergleich zum konventionellen Fräsen (CM) deutlich reduzierter Werkzeugverschleiß. Mit wachsendem Zerspanvolumen verstärkt sich der positive Effekt von USAM und äußert sich in einer deutlichen Reduzierung der Rauheit und Defektdichte auf der Bauteiloberfläche.
Intermetallische Werkstoffe werden für Hochtemperaturanwendungen immer wichtiger. Insbesondere Aluminide mit hohen spezifischen Festigkeiten haben ein großes Potenzial für Leichtbauanwendungen, z. B. werden Titanaluminide bereits in Flugzeugturbinen eingesetzt. Ökonomische und ökologische Aspekte sind die treibende Kraft zur Substitution konventioneller Legierungen, z. B. Nickelbasislegierungen, die eine wesentlich höhere Kostenintensität und ein höheres Gewichts-Festigkeits-Verhältnis aufweisen. Insbesondere Eisenaluminide (FeAl) sind aufgrund guter mechanischer Eigenschaften und der guten Verfügbarkeit der wichtigsten Hauptlegierungselemente attraktiv. Speziell entwickelte FeAl-Legierungskonzepte ermöglichen eine wirtschaftliche Produktion im Kokillenguss, bei der die Erstarrung der Schmelze spezielle komplexe Gefüge ermöglicht, um die hohen thermischen und mechanischen Eigenschaften durch eine hohe Anzahl von harten Ausscheidungen entlang der Korngrenzen zu erreichen. Zu diesen Eigenschaften gehört aber auch, dass notwendige Bearbeitungsschritte, wie das Schlichtfräsen, zur Erzielung komplexer Endkonturen, zu vorzeitigem Werkzeugverschleiß und geringen Oberflächengüten führen. Für das finale FeAl-Bauteil ist eine hohe Oberflächenintegrität insbesondere in sicherheitsrelevanten Anwendungen in der Luftfahrt oder im Energiebereich erforderlich. Aus diesem Grund konzentriert sich die vorliegende Untersuchung auf die Mechanismen und die Minimierung des Werkzeugverschleißes und die Auswirkungen auf die Oberfläche eines in Kokille vergossenen FeAl-Bauteils. Neben den Optimierungen des Fräsprozesses und der Werkzeugparameter (Geometrie, Material) wird ein modernes hybrides Bearbeitungsverfahren, das ultra-schallunterstützte Fräsen (USAM), eingesetzt, bei dem eine Überlagerung mit einer hochfrequenten Schwingung des rotierenden Werkzeugs erzeugt wird. Analysen der Kräfte und Temperaturen im Fräsprozess, der Verschleißerscheinungen am Werkzeug und der Oberflächenintegrität zeigen positive Auswirkungen durch USAM. Darüber hinaus wird eine valide Grundlage für die Optimierung des Werkzeugs und des Fräsprozesses geschaffen, um eine hohe Oberflächenintegrität (z. B. Verringerung von Zugeigenspannungen und Defekten) und eine längere Werkzeuglebensdauer zu gewährleisten, die zukünftig eine wirtschaftliche Herstellung von FeAl-Bauteilen ermöglicht.
Hochfeste Feinkornbaustähle verfügen über großes Potenzial für gewichtsoptimierte, effiziente Strukturen in vielen modernen Stahlkonstruktion. Weitere Effizienzsteigerungen können durch additives Fertigen sowie bionische Bauweisen erreicht werden. Hochfeste Zusatzwerkstoffe für formgebendes MSG-Schweißen (engl.: Wire Arc Additive Manufacturing, WAAM) sind bereits von den Schweißzusatzherstellern lieferbar. Eine breite industrielle Anwendung insbesondere für KMU ist aufgrund fehlender quantitativer Erkenntnisse und Richtlinien bezüglich schweißbedingter Beanspruchungen und Bauteilsicherheit während der Herstellung und des Betriebs derzeit stark limitiert. In einem gemeinsamen Forschungsprojekt (FOSTA-P1380/IGF21162BG) der BAM und der TU Chemnitz werden daher die mit Eigenspannungsausbildung und der Gefahr einer Kaltrissbildung verbundenen prozess- und werkstoffbedingten sowie konstruktiven Einflüsse beim WAAM hochfester Stähle untersucht. Der vorliegende Beitrag fokussiert die Analyse der Auswirkungen von Schweißwärmeführung und Design der WAAM-Bauteile auf Nahtgeometrie, Materialausnutzung und Eigenspannungen. Hierfür werden geometrisch definierte Probenkörper (Hohlquader) vollautomatisiert mit einem speziellen, hochfesten WAAM-Massivdraht (Streckgrenze >790 MPa) geschweißt. Die Wärmeführung wird innerhalb eines statistischen Versuchsplans so variiert, dass die Dt8/5-Abkühlzeiten im empfohlenen Verarbeitungsbereich (ca. 5 s bis 20 s) liegen. Das WAAM-Schweißen komplexer Geometrien mit variierenden Schweißwärmeführungen und geometrischen Verhältnissen bzw. Wanddicken bedingt neben den Auswirkungen auf die Abkühlbedingungen, Abkühlzeiten und Gefüge deutliche Einflüsse auf die konstruktiven Steifigkeitsverhältnisse beim Schweißen. Es zeigen sich anhand der Schweißexperimente signifikante Auswirkungen durch Probenskalierung und Wärmeeinbringung auf die resultierenden lokalen Eigenspannungen. Die Folge können ungünstige Bauteileigenschaften und risskritische Zugeigenspannungen sein.
Die Verringerung der CO2-Emissionen ist eng mit der Entwicklung hocheffizienter und wirtschaftlicher Komponenten aus Stahl in der Anlagen- und Verfahrenstechnik verknüpft. Um den hohen kombinier-ten korrosiven, tribologischen, thermischen und mechanischen Beanspruchungen stand zu halten, werden auf Anwendungsfall und Stahlwerkstoff abgestimmte Verschleißschutzschichten eingesetzt. Neben der zunehmenden Forderung konventionelle Kobalt- durch Nickellegierungen zu ersetzen, wächst auch der Bedarf nach definierten Ober- bzw. Funktionsflächen hoher Güte. Aufgrund hohen Werkzeugverschleißes ist die erforderliche Fräsbearbeitung zu Herstellung der komplexen Geomet-rien der Bauteile für KMU oftmals wirtschaftlich nicht möglich. Mittels Legierungsmodifikation der Schweißzusätze für nickelbasierte plasmaauftraggeschweißte Verschleißschutzschichten und durch den Einsatz innovativer ultraschallunterstützter Fräsprozesse soll eine günstigere Zerspanbarkeit er-reicht werden, ohne das Verschleißschutzpotential zu mindern. Im vorliegenden Beitrag wird der Ein-fluss der mittels Legierungsmodifikation eingestellten Gefüge- und Ausscheidungsmorphologie auf die Zerspanbarkeit untersucht. Dies erfolgt anhand einer typischerweise für Schneckenmaschinen einge-setzten Verschleißschutzlegierung NiCrMoSiFeB-Legierung („Colmonoy 56 PTA“), die konventionelle CoCr-Legierungen (Stellite) substituiert. Metallurgische Untersuchungen und In-situ-Messungen auf-tretender Prozesskräfte an der Werkzeugschneide beim Fräsen sowie anschließende Untersuchungen von Werkzeugverschleiß und Oberflächenintegrität ermöglichen eine detaillierte Analyse und Korrela-tion zwischen mikrostrukturellen Eigenschaften und Zerspanbarkeit.
Die Ziele zur Verringerung der CO2-Emissionen sind eng verknüpft mit der Entwicklung hocheffizienter und wirtschaftlicher Komponenten aus Stahl in der Anlagen-, Verfahrens- und Kraftwerktechnik, die für hohe kombinierte korrosive, tribologische, thermische und mechanische Beanspruchungen auf Anwendungsfall und Stahlwerkstoff abgestimmte Verschleißschutzschichten erfordern. Neben zunehmenden Forderungen infolge des Preis- und Lieferrisikos konventionelle Kobalt- durch Nickellegierungen zu ersetzen, wächst in der Industrie der Bedarf nach definierten Oberflächen hoher Güte bzw. Funktionsflächen für die Schutzschichten. Eine für Bauteile mit komplexer Geometrie erforderliche Fräsbearbeitung ist insbesondere für KMU aufgrund hohen Werkzeugverschleißes oftmals nicht wirtschaftlich realisierbar, jedoch für viele Einsatzfälle dringend notwendig.
In einem Gemeinschaftsvorhaben der BAM und des ISAF der TU Clausthal (Fosta P1550/IGF 21959 N) wird daher untersucht, wie mittels Legierungsmodifikationen der Schweißzusätze für nickelbasierte plasmaauftraggeschweißte Verschleißschutzschichten und durch Einsatz innovativer ultraschallunterstützter Fräsprozesse eine günstigere Zerspanbarkeit erreicht werden kann, ohne das Verschleißschutzpotential zu mindern.
Im vorliegenden Beitrag wird der Einfluss der mittels Legierungsmodifikation eingestellten Gefüge- und Ausscheidungsmorphologie auf die Zerspanung untersucht. Dies erfolgt anhand einer typischerweise für Schneckenmaschinen eingesetzte Verschließschutzlegierung zur Substitution entsprechender CoCr-Legierungen (Stellite), einer NiCrMoSiFeB-Legierung (Handelsname: Colmonoy 56 PTA). Durch metallurgische Untersuchungen und In-situ-Messung auftretender Prozesskräfte und Temperaturen an der Werkzeugschneide beim Fräsprozesses sowie der anschließenden Untersuchung von Werkzeugverschleiß und Oberflächenintegrität ist eine detaillierte Analyse und Korrelation zwischen den mikrostrukturellen Eigenschaften und der Zerspanbarkeit möglich. Die Vorgehensweise erlaubt einer Beurteilung des Einflusses der ultraschallunterstützten Fräsbearbeitung auf den Prozess sowie die resultierenden Oberflächenintegrität. Unter systematischer Anwendung dieser Methodik sowie der Berücksichtigung der Anbindung zum Stahlsubstratwerkstoff und der Wirksamkeit des Verschleißschutzes lässt sich letztlich eine gezielte Optimierung der Zerspanungssituation und des Verschleißschutzes erreichen. Die Erkenntnisse erlauben Handlungsanweisungen und Empfehlungen für Normen und Verarbeitungsrichtlinien, die besonders KMU eine sichere und wirtschaftliche Fertigung hochbelasteter Stahlkomponenten mit unkritischen, kostenreduzierten Werkstoffen ermöglichen sollen.
Offshore wind turbines continuously increase in size and weight and demand adequate offshore foundations concepts like monopiles, tripods, or jackets. These components are typically constructed using submerged arc welding (SAW) with high-strength thick steel plates like the S420ML. During welding, the occurrence of delayed hydrogen-assisted cracking (HAC) must be anticipated. HAC is a critical combination of the local hydrogen concentration within a susceptible microstructure under certain mechanical load, i.e., the occurring (welding) residual stresses. The welding sequence of the thick-walled plates complicates the residual stress distribution due to the necessary repeated thermal cycling, i.e., welding seam/layer deposition to fill the joint. For that purpose, SAW with two-wire-technique was used to weld a specially designed and prototype-like mock-up of a real component with a thickness of 50 mm, filled with over 20 passes and a seam length of 1000 mm. Additional welded stiffeners simulated the effect of a high restraint, to achieve critical HAC conditions. The necessity of a minimum waiting time (MWT) before the NDT can be conducted (to exclude HAC) was critically verified by the application of ultrasonic testing of the welded joint at different time-steps of the NDT of up to 48 h after the completion welding. The residual stresses were determined by a robot XRD goniometer. Tensile residual stresses up to the yield limit are found both in the weld metal and in the heat-affected zone. Numerical modeling allowed the qualitative estimation of the hydrogen diffusion in the weld. No noticeable HAC occurrence was identified and confirms the high cracking resistance of the investigated material. Finally, the applicability of the MWT concept should be critically discussed.
Additive manufacturing (AM) technologies are becoming increasingly important, not only for the manufacture of parts, but also as repair technology that complement existing production technologies. Powder bed fusion of metals by laser beam (PBF-LB/M) combines the freedom in design with high achievable accuracy, making it ideal as a repair approach. However, there are still challenges in adapting process for repair applications. When mounting parts inside PBF-LB/M machines, their real position within the build volume is unknown. One goal of a repair process is to minimize the offset between the base component and the additively manufactured structure to reduce additional rework. For a minimum offset between component and additively manufactured structure, the actual position of the component has to be identified with high precision within the machine coordinate system (MCS). In this work a process setup is presented that allows the actual position of a gas turbine blade to be detected inside a PBF-LB/M machine. A high resolution camera with 65 megapixel is used for this purpose. The presented setup is implemented on a SLM 280 HL PBF-LB/M machine. In addition to the setup, a novel repair workflow using PBF-LB/M is presented. The developed setup and workflow consider inaccuracies in the component and camera mounting, as well as process inaccuracies. This includes keystone distortion correction by homography. The machine setup and workflow are used to repair a real gas turbine blade. Subsequently the offset between the turbine blade and the additivley manufactured structure is validated by 3D scanning the repaired part. The maximum offset is 160 µm. The presented approach can be extended to other geometries and PBF-LB/M machine manufacturers. The high-resolution camera approach is platform independent, which facilates the market penetration of PBF-LB/M repair processes.
Residual stresses Analysis in Additively Manufactured alloys using neutron diffraction (L-PBF)
(2023)
An overview of recent progress at BAM of residual stress analysis in additively manufactured, in particular Laser Powder Bed Fusion of metallics materials, using neutron diffraction will be presented. This will cover important topics of the stress-free reference, the diffraction elastic moduli and principal stress determination.
The main solidification parameters, namely the crystal axis, the growth rate, and the cross-sectional area of the columnar crystal control the primary microstructure and thus the final weld seam properties.
Limited understanding of solidification parameters due to complexities in experimental and theoretical weld pool boundary determination and
its mathematical description.
Application of local Lamé curves for efficient weld pool boundary reconstruction Derivation of general analytical solutions for the main solidification parameters of a two-dimensional columnar crystal growth during laser beam welding of thin steel sheets.
The Lamé curves approximation technique was successfully applied for the reconstruction of the rear part of the two-dimensional steady-state weld pool boundary. General analytical expressions for the main solidification parameters, namely the crystal axis, the growth rate and the cross-sectional area of the crystal were derived.
The derived expressions and herewith obtained results were verified and validated by comparing them to known theoretical solutions and experimental measurements. Dimensionless analysis of the influence of the size and shape of the rear weld pool boundary on the solidification parameters was provided.
The derived general solutions and the analysis provided allow for critical welding parameters to be estimated and adapted accordingly to improve the welding process.
Die Wärmeverteilung des Lasers beim Laserstrahltiefschweißen ist für die Formgebung der Dampfkapillare und für die Schweißbaddynamik entscheidend. In dieser Arbeit werden die Laserwärmeverteilung und deren Einflüsse auf die Schweißbadtiefe sowie -breite numerisch anhand des Ray-Tracing-Verfahrens analysiert. Hierbei wird der La-serstrahl in mehreren Strahlenbündeln bzw. Subrays unterteilt. Diesbezüglich soll der Pfad der Subrays präzis be-rechnet werden, um die Dynamik der Dampfkapillare und des Schweißbades eines realen Schweißprozesses rich-tig abzubilden. Zu diesem Zweck beschäftigt sich die vorliegende Arbeit mit der Genauigkeitsverbesserung der Kontaktposition und der Reflexionsrichtung der Subrays auf der freien Oberfläche anhand der Level-Set-Methode. Um die Güte dieses Simulationsansatzes zu gewährleisten, wurde eine Gegenüberstellung mit den zwei klassischen Ray-Tracing-Verfahren mittels drei verschiedenen Benchmark-Testreihen durchgeführt. Anschließend wurden die Versuchsergebnisse zur Validierung der implementierten numerischen Ansätze verwendet. Im Rahmen dieser Arbeit kann es gezeigt werden, dass unterschiedliche Wärmeverteilung aufgrund der verschiedenen Ray-Tracing-Verfahren deutlich zu erkennen ist, welche wiederum die Schweißbaddynamik sowie die lokalisierte Dampfkapil-lardynamik stark beeinflussen. Ferner wurde es bestätigt, dass die implementierte Level-Set-Methode zu einer ge-naueren Ermittlung der Kontaktposition und der Reflexionsrichtung der Subrays und somit zu einer Verbesserung der simulierten Schmelzkontur führt.
Die Additive Fertigung gewinnt zunehmend an Bedeutung für die Verarbeitung von Kupferwerkstoffen im industriellen Umfeld. Hierbei wird verstärkt auf drahtförmige Ausgangswerkstoffe gesetzt, da diese Vorteile im Handling bieten, bereits aus der Schweißtechnik bekannt sind und sich zumeist durch geringere Beschaffungskosten auszeichnen. In den letzten Jahren entwickelte sich unter den drahtbasierten Verfahren der Directed-Energy-Deposition (DED) eine Prozessvariante unter Nutzung des Elektronenstrahls zur industriellen Marktreife. Dabei zeigt die Technologie Wire Electron Beam Additive Manufacturing (DED-EB) besondere Vorteile gegenüber anderen DED-Prozessen für die Anwendung an Kupfer. Um das Verfahren einem breiten Anwenderkreis in der Industrie zugänglich zu machen, fehlen jedoch Daten zu Leistungsfähigkeit, Prozessgrenzen und Anwendungsmöglichkeit- en. Die vorliegende Untersuchung beschäftigt sich mit dieser Problemstellung am Beispiel der Legierung CuSn1MnSi. Über mehrstufige Testschweißungen werden die physikalisch möglichen Prozessgrenzen ermittelt und Rückschlüsse über die Eignung der Parameter zum additiven Aufbau gezogen. An verschiedenen additiv gefertigten Probekörpern werden anschließend Kennwerte für Aufbaurate, Härte, Mikrostruktur, Oberflächenqualität sowie mechanische Festigkeitswerte ermittelt. Es zeigt sich, dass das die durch DED-EB hergestellten Proben, trotz des groben Gefüges sowie der thermischen Belastung im Aufbauprozess, in ihren Eigenschaften gut mit den Spezifikationen des Ausgangsmaterials übereinstimmen.
Um Bauteile vor Verschleiß und Korrosion zu schützen werden Beschichtungen aus resistenteren Materialien aufgetragen. Hierzu zählen unter anderen die Legierungen auf Cobalt-Chrom Basis. Der diskrete Materialsprung ist jedoch unter thermischen und mechanischen Belastungen häufig Ursache für das Versagen der Beschichtung. In dieser Arbeit werden daher Materialgradierungen von verschiedenen Stahllegierungen zu einer Cobalt-Chrom Basislegierung untersucht. Die Ergebnissen werden dafür auch mit Resultaten zu vorangegangenen Untersuchungen verglichen. Kern der Arbeit bilden geätzte Schliffbilder der Materialpaarungen und Auswertungen mittels Farbeindringprüfung sowie die metallografische Bestimmung der Porosität. Ziel der Arbeit ist ein defektfreier Aufbau der funktional gradierten Materialpaarungen.
Reducing CO2 emissions to 62 percent up to 2030 and achieving greenhouse neutrality up to 2050 are the key goals of the EU's climate protection legislation. The expansion of technologies for climate-neutral energy generation is not the only important contribution to reducing greenhouse gases, the efficient use of material systems is also a key factor. In the area of plant engineering, steel components are provided with a wear-protection coating for efficient use to protect them against corrosive, tribological, thermal and mechanical stresses. The wear-protection coatings are primarily made of Co alloys, which are to be replaced by alternative materials, such as nickel alloys, because of the poor working conditions regarding the extraction of the cobalt and because of the health risks. In terms of corrosion behavior, many applications require a crack-free wear protection layer and additional machining. Machining by milling is not economical for wear materials, especially for SME because of the high tool wear, but it is essential to produce defined surfaces with high quality.
Alloy modifications of the filler metals for nickel-based plasma deposition welded wear protection coatings and the use of innovative ultrasonic-assisted milling processes are examined to determine how more favorable machinability can be achieved without reducing the wear protection potential. The focus is on the NiCrSiFeB alloy, which is intended to replace CoCr alloys in the area of screw machines. The modification of hard facing layers in terms of microstructure and precipitation morphology as well as suitability for machining is investigated and compared with the CoCr alloy. In this aspect, the alloy modifications are generated by a PTA process by systematically adjusting the preheating and interpass temperatures, a crack-free wear-resistant layer can be generated, which is subsequently machined by a milling process. In addition to the crack-free properties,
the microstructure, the bonding as well as the mixing between the NiCrFeB alloy and a 1.8550 as well as between the CoCr alloy and a 1.4828 are analyzed and compared in the joining areas. In addition, heating and cooling rates are determined and a chemical analysis of the weld metals is performed. This study provides important knowledge about the differences between NiCrFeB alloy and CoCr alloys in terms of microstructure morphology, bonding and mixing behavior in the joining zones to the basic material.
Alloy Modification and ultrasonic assisted milling of wear resistant alloys with defined surfaces
(2023)
The targets for reducing CO2 emissions are closely linked to the development of highly efficient and economical steel components in plant, process and power plant technology, which require wear protection coatings tailored to the application and steel material for high combined corrosive, tribological, thermal and mechanical stresses. There is a growing demand in industry for defined functional surfaces of high quality for these coatings. Milling is a standard process for finish machining. The desired properties of wear resistant alloys imply significant challenges for the milling process due to high tool wear and surface defects. Besides the hardness of the coating materials, especially due to the precipitations, inhomogeneous, anisotropic weld structures of the claddings lead to further deteriorations of milling processes due to unstable milling conditions and process forces.
A joint project of BAM and ISAF of TU Clausthal (Fosta P1550/IGF 21959 N) investigates the optimization of these challenging machining conditions by means of alloy modifications of the welding powder for plasma transferred arc cladding, without reducing the wear protection potential and using ultrasonic assisted milling process.
In this paper, the influence of the microstructure and precipitation morphology adjusted by means of alloy modification on machining for a NiCrMoSiFeB alloy (trade name: Colmonoy 56 PTA) is investigated. Through metallurgical investigations and in-situ measurement of cutting forces and temperatures at the cutting edge during the milling process as well as the subsequent investigation of tool wear and surface integrity, a detailed analysis and correlation between microstructural properties and machinability is feasible. The findings allow recommendations for standards and processing guidelines, enabling safe and economical production of highly stressed steel components with non-critical, cost-reduced materials.
Alloy Modification and ultrasonic assisted milling of wear resistant alloys with defined surfaces
(2023)
The targets for reducing CO2 emissions are closely linked to the development of highly efficient and economical steel components in plant, process and power plant technology, which require wear protection coatings tailored to the application and steel material for high combined corrosive, tribological, thermal and mechanical stresses. There is a growing demand in industry for defined functional surfaces of high quality for these coatings. Milling is a standard process for finish machining. The desired properties of wear resistant alloys imply significant challenges for the milling process due to high tool wear and surface defects. Besides the hardness of the coating materials, especially due to the precipitations, inhomogeneous, anisotropic weld structures of the claddings lead to further deteriorations of milling processes due to unstable milling conditions and process forces.
A joint project of BAM and ISAF of TU Clausthal (Fosta P1550/IGF 21959 N) investigates the optimization of these challenging machining conditions by means of alloy modifications of the welding powder for plasma transferred arc cladding, without reducing the wear protection potential and using ultrasonic assisted milling process.
In this paper, the influence of the microstructure and precipitation morphology adjusted by means of alloy modification on machining for a NiCrMoSiFeB alloy (trade name: Colmonoy 56 PTA) is investigated. Through metallurgical investigations and in-situ measurement of cutting forces and temperatures at the cutting edge during the milling process as well as the subsequent investigation of tool wear and surface integrity, a detailed analysis and correlation between microstructural properties and machinability is feasible. The findings allow recommendations for standards and processing guidelines, enabling safe and economical production of highly stressed steel components with non-critical, cost-reduced materials.
Steelmaking slag is a by-product of steel production, of which 4.5 Mt were produced in 2020 in Germany alone. It is mainly used in road construction, earthwork and hydraulic engineering. A smaller part is returned to the metallurgical cycle, used as fertiliser or landfilled.
With this use, iron oxides still contained in steelmaking slag are lost. In addition, the possibility of producing higher-grade products from steelmaking slag is foregone. In recent decades, many researchers have investigated the production of Portland cement clinker and crude iron from basic oxygen furnace slags (BOFS) via a reductive treatment. Carbothermal treatment of liquid BOFS causes a reduction of iron oxides to metallic iron, which separates from the mineral phase due to its higher density. Simultaneously, the chemical composition of the reduced slag is adapted to that of Portland cement clinker.
In this study, German BOFS was reduced in a small-scale electric arc furnace using petrol coke as a reducing agent. The resulting low-iron mineral product has a similar chemical composition to Portland cement clinker and was rich in the tricalcium silicate solid solution alite (Ca3SiO5). Based on its chemical and mineralogical composition, similar to that of Portland cement clinker, the reduced BOFS has the potential to react comparably. In our study, the reduced BOFS produced less hydration heat than OPC, and its hydraulic reaction was delayed. However, adding gypsum has shown to accelerate the hydration rate of the reduced BOFS compared to that known from the calcium silicates of Portland cement clinker.
Further research to improve the hydraulic properties of the reduced slag is essential. If successful, producing a hydraulic binder and crude iron from BOFS has economic and ecological benefits for both the cement and steel industries.
Die Wasserstofftechnologien der näheren Zukunft erfordern sichere Komponenten. Die Füge- und Schweißtechnik ist hier von zentraler Bedeutung, insbesondere für die Transportinfrastruktur (wie Pipelines). Der vorliegende Vortrag gibt hierzu einen umfassenden Überblick, beginnend bei der Erzeugung, Speicherung über Transport und Nutzung. Zusätzlich wird die zunehmende Bedeutung der additiven Fertigung beleuchtet und ein kurzer Ausblick auf die aktuelle Normungsroadmap der Wasserstofftechnologien gegeben.
The process-structure-property-performance relationship is a fundamental paradigm in metal additive manufacturing (AM). The knowledge of how process parameters and post-process treatments influence the material properties and therefore the structural integrity of additively manufactured metallic components is of paramount importance. With respect to this, one of the unsolved problems related to the unique AM process is the reliability of witness samples for part qualification. Focusing on the Laser Powder Bed Fusion (PBF-LB/M) process, the microstructure and therefore the mechanical performance of test coupons are not representative of parts of the same material in every case. This work summarizes part of the results of an ongoing cooperation between BAM and BASF, which aims at addressing this issue by designing dedicated specimens, representative of the fatigue properties of the real component. The considered case study consisted in the fabrication and characterization of the fatigue properties of a pressure vessel made of AISI 316L. The first phase of the project was devoted to the process monitoring to understand the relationship between process parameters and temperature distribution during the fabrication of the component. Finite element models were tuned on the experimental data from the process monitoring and simulations of the temperature field in the component were conducted. The numerical results were in good agreement with the temperature profiles measured during the fabrication of the component. The second phase dealt with the evaluation of the fatigue and fatigue crack propagation properties on specimens extracted from different regions of the pressure vessel. The experimental investigations showed significant differences in hoop and longitudinal direction. A clear dependence of the fatigue properties on heat accumulation (temperature distribution) and defect size was pointed out. The third phase encompassed the fabrication and testing of the representative specimens. Numerical simulations were used to design specimens displaying the same temperature profile as in the most critical region of the component. The fatigue testing of the representative specimens is subject of ongoing work.
Alloy modification and ultrasonic assisted milling of wear resistant alloys with defined surfaces
(2023)
The targets for reducing CO2 emissions are closely linked to the development of highly efficient and economical steel components in plant, process and power plant technology, which require wear protection coatings tailored to the application and steel material for high combined corrosive, tribological, thermal and mechanical stresses. In addition to increasing demands to replace conventional cobalt alloys with nickel alloys as a result of price and supply risks, there is a growing demand in industry for defined functional surfaces of high quality for these coatings. Milling is a standard process for finish machining. The desired properties of wear resistant alloys imply significant challenges for the milling process due to high tool wear and surface defects. Besides the hardness of the coating materials, especially due to the precipitations, inhomogeneous, anisotropic weld structures of the claddings lead to further deteriorations of milling processes due to unstable milling conditions and process forces.
A joint project of BAM and ISAF of TU Clausthal (Fosta P1550/IGF 21959 N) investigates the optimization of these challenging machining conditions by means of alloy modifications of the welding powder for plasma transferred arc cladding, without reducing the wear protection potential and using ultrasonic assisted milling process.
In this paper, the influence of the microstructure and precipitation morphology adjusted by means of alloy modification on machining is investigated. The alloy used is a NiCrMoSiFeB alloy (trade name: Colmonoy 56 PTA). Through metallurgical investigations and in-situ measurement of cutting forces and temperatures at the cutting edge during the milling process as well as the subsequent investigation of tool wear and surface integrity, a detailed analysis and correlation between microstructural properties and machinability is feasible. The findings allow recommendations for standards and processing guidelines, enabling safe and economical production of highly stressed steel components with non-critical, cost-reduced materials.
The long-term global goals of achieving almost net zero carbon emissions in the next decades are closely linked to the development of highly efficient components in plant, process and energy engineering, and their sustainable and resource-saving production. Plant components must increasingly withstand tribological loads in addition to high thermal, mechanical, and corrosive stresses. Such combined stresses demand high-performance alloys economically tailored to the application for wear protection and components produced as semi-finished products, via additive manufacturing (AM) or claddings via deposition welding. For instance, the protection of special components made of less cost-intensive materials, e.g., steel in process engineering for screw machines or exhaust gas separation, is feasible applying cost-intensive Ni- or Co-based hard-phase claddings.
Today an increasing number of above-mentioned applications demand precise finishing machining of components to ensure defined compact surfaces with a high integrity and complex contours. Contour milling is standard process for finishing machining of metals. Especially, the desired properties of wear resistant materials (e.g., high strength, hard precipitations) imply significant challenges for milling processes and tools, leading frequently to uneconomic milling conditions due to intolerable high tool wear and surface defects. Inhomogeneous, anisotropic weld structures due to cladding or AM of wear resistance alloys lead to further deteriorations of milling processes due to unstable milling conditions and process forces during chip removal.
To tackle these challenges, already several approaches exist, (1) to enhance machinability of the claddings by alloy modifications to specifically influence solidification and hard phases morphology (precipitation shape, size, distribution) and (2) to achieve significant improvements of the machining situation (e.g., increase of tool life and surfaces integrity) by means of modern hybrid machining processes such as ultrasonic-assisted milling. This contribution shows a comprehensive overview of recent results with these promising approaches for additively welded Ni- CoCr-alloys.
The laser hybrid welding process offers many advantages such as the high penetration depth and high welding speed, and it is characterized by its low heat input compared to the arc-based welding processes, which makes the laser hybrid welding process as a suitable alternative process when welding thick-walled steels. However, there are some challenges when using laser hybrid welding process for thick steels. Due to the uneven cooling conditions and the inhomogeneous filler wire mixing, a typical laser hybrid weld can be divided into two different zones over the depth: the arc-dominated zone on the upper part and the laser-dominated zone in the root part. This leads to different mechanical properties in a laser hybrid welded joint. Due to the high cooling rates and the lack of filler wire in the laser-dominated zone, this area is more critical regarding the mechanical properties, especially the Charpy impact toughness. A low heat input can lead to undesired microstructure consisting of martensite, hardening and deteriorated impact toughness due to the high cooling rate, where higher heat inputs can lead to grain-coarsening and even loss of impact strength. This study deals with the influence of the welding speed and resulting heat input on the cooling rates, the microstructure and the mechanical properties of single-pass laser hybrid welded steels of S355J2 with thickness up to 30 mm. The experiments were performed with a 20-kW fibre laser system and a contactless electromagnetic weld backing on up to 30 mm thick steels in butt-joint configuration in 1G welding position. The cooling time was measured in three different locations near to fusion lines corresponding to different heights of the seam using a special configuration with pyrometers, collimators, and optical fibres. The test specimens for the Charpy impact testing and tensile testing were extracted in up to three different depths. Based on the experiments, a heat input of 1.3 kJ/mm - 1.6 kJ/mm, 2 kJ/mm - 2.4 kJ/mm and 3.7 kJ/mm were recommended when single-pass laser hybrid welding of 20 mm, 25 mm and 30 mm thick structural steels in regard to the minimum requirements of the mechanical properties, respectively. The optical measurement of the cooling times in different depths could be carried out reproducibly.
Offshore-Windenergieanlagen erfordern Gründungskonzepte aus unterpulver-(UP-)geschweißten Dickblechen (bspw. der Güte S420ML). Während der Schweißfertigung kann eine zeitverzögerte wasserstoffunterstützte Kaltrissbildung auftreten, deren Bewertung aufgrund der Bauteilgröße von Offshore-Strukturen sehr komplex ist. Deswegen wurde eine bauteilähnliche Geometrie (Mock-Up) entwickelt, um reale Steifigkeitsverhältnisse auf den Labormaßstab zu übertragen. Zusätzliche Versteifungen simulieren die Wirkung einer Einspannung bzw. Schrumpfbehinderung der Schweißnaht. Über die Verwendung von Schweißpulvern mit definierter Feuchte wurden zudem ein Extremszenario der Wasserstoffaufnahme simuliert. Entsprechend der vorgegebenen Mindestwartezeit für die ZfP von bis zu 48 h wurde die Schweißnaht zerstörungsfrei mit Phased-Array-Ultraschall-Prüfung (PAUT) geprüft und die Eigenspannungen über Röntgendiffraktometrie (XRD) bestimmt. Zusätzlich wurde die Wasserstoffverteilung in der Schweißverbindung numerisch simuliert. Außer zulässigen Defekten (wie Poren), wurde keine verzögerte Kaltrissbildung in den Mock-Ups festgestellt, was auf hohe Rissbeständigkeit hindeutet.
The Kitagawa-Takahashi (KT) diagram is a well-established tool in the fatigue community for the prediction of the fatigue limit of metallic materials in presence of defects. Nevertheless, its determination is associated with a large number of fatigue tests carried out on specimens with artificial defects (notches) to describe the fatigue limit-defect size relationship in the short crack regime. The preparation of the specimens and the execution of the tests is time expensive. Therefore, few phenomenological models have been introduced in the past to provide a first approximation of the KT diagram based solely on the fatigue limit for smooth specimen and the fatigue crack propagation threshold for long cracks. Despite the use of such models is widely spread, these suffer from problems related to the uncertainties of the material parameters. Furthermore, the mechanics of short cracks, which is the physical mechanism behind the concept of the fatigue limit, is not considered. This work discusses the advantages and drawbacks of using short fatigue crack propagation models for predicting the fatigue limit of flawed metallic materials. The KT diagrams for a EA4T railway axle steel in as-received and full-hardened condition are approximated by means of the cyclic R-curve method.
Structural alloys are largely employed in key industrial sectors and their demand is predicted to rise rapidly for the next decades. Most of these materials require a large amount of energy for extraction and manufacturing, which causes the emission of greenhouse gases and other pollutants. Therefore, strategies for improving the sustainability of structural metallic alloys are urgently needed.
Additive Manufacturing (AM), in particular Laser Powder Bed Fusion (PBF-LB/M), aims to be a sustainable manufacturing process, as it allows the build-up of complex geometry in near net-shape from 3D models, while minimizing material waste and the energy required for the process and post-process treatments.
Nevertheless, the application of additively manufactured parts in structural safety-relevant applications is still hindered by the poor fatigue performance. The cause of this has been mainly attributed to the presence of manufacturing defects and surface roughness. Therefore, a huge effort has been made to optimize the process parameters and to introduce post-process treatments to minimize the defect content. However, material flaws cannot be fully eliminated, but these can be considered in a damage tolerance framework for the prediction of the fatigue performance of additively manufactured metallic materials, which is essential for part design and qualification.
This work aims at presenting different modelling strategies for the prediction of the fatigue limit of AM metals. Simple empirical models and more complex models based on fatigue short crack propagation are proposed. The investigated material is an AlSi10Mg alloy fabricated by PBF-LB/M and subjected to two different low-temperature heat-treatments (265°C for 1 h and 300°C for 2h). The results show that the models can provide good approximation of the fatigue limits and help in the interpretation of the scatter of fatigue data.
In vielen Anwendungen werden aufgrund zunehmender Anforderungen an den Widerstand gegenüber hohen, kombinierten mechanischen, tribologischen, thermischen und korrosiven Beanspruchungen Hochleistungsle¬gierungen eingesetzt. Den vielfach komplexen Bauteilstrukturen, herausfordernden Fertigungseigenschaften und der Erhöhung der Ressourceneffizienz lässt sich heute mit der additiven Fertigung (AM) begegnen, dem oftmals noch unzureichende Kenntnisse über die resultierende Homogenität und Anisotropie der speziellen Gefüge und den damit verbundenen Eigenschaften entgegensteht. Viele Anwendungen benötigen zudem definierte Oberflächen mit hohen Anforderungen an die Oberflächenintegrität, dem durch komplementären Einsatz von additiven und abtragenden Fertigungsschritten Rechnung getragen wird. Hochleistungslegierungen auf der Basis von Nickel oder Kobalt sind infolge der niedrigen Wärmeleitfähigkeit verbunden mit hoher Festigkeit und Zähigkeit äußerst herausfordernd mit Blick auf eine wirtschaftliche Zerspanung mit geometrisch bestimmter Schneide. Ein Vorhaben (IGF-Nr. 20.979 N/DVS-Nr. 1.3211) der BAM und TU Clausthal/ISAF befasst sich mit der Lösung dieser Problemstellung durch gezielte Beeinflussung des Gefüges mittels Modifikation der Schweißzusatzwerkstoffe und den Einsatz des ultraschallunterstützten Fräsens (US), bei gleichzeitiger Sicherstellung der geforderten Eigenschaften der untersuchten Werkstoffe. Es werden die Ergebnisse der Mikrostruktur und der Zerspanbarkeit der Modifikationen der MSG-Schweißung der FeNi36-Referenzlegierung mit 0,4 Gew.-% Zr und der Modifikationen der MSG-Schweißung der CoCr-Referenzlegierung mit 1 Gew.-% Zr sowie Hf vorgestellt. Abschließend werden aus den Ergebnissen Handlungsempfehlungen abgeleitet.
The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by the experimental and numerical method. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer-Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that the additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the molten pool shape. The probability of keyhole collapse at the bottom increases significantly to 72% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and the keyhole collapse are obtained. The developed model has been validated by the experiments, which shows a good agreement.
Fracture mechanics-based methods can be applied to the fatigue assessment of mechanical components containing defects or cracks subjected to cyclic loading. The knowledge of the crack growth behavior for short and long cracks enables the determination of both finite and infinite life regimes. Nevertheless, a first problem arises from the empirical determination of the fatigue data, which vary according to the test conditions, e.g., the test method, frequency and the stress ratio R. This is due to crack closure and further environmental phenomena affecting crack propagation behavior. A second issue regards the lack of standard experimental procedures for the determination of the fatigue crack propagation threshold ΔKth dependency on the crack size. This is problematic, since ΔKth increases with the build-up of the closure effects in the short crack regime and its inaccurate experimental determination leads to large deviations in the assessment and, even more critical, to non-conservative predictions. Aiming at providing more reliable and safer assessment procedures, the present work proposes a method based on effective crack growth data obtained at high stress ratios (R ≈ 0.8). A short-crack model based on the cyclic R-curve and CPLR data is employed for describing the development of the closure effects. The predictions are compared to established methods and validated by experimental tests.
Die Wärmeverteilung des Lasers beim Laserstrahltiefschweißen ist für die Formgebung der Dampfkapillare und für die Schweißbaddynamik entscheidend. In dieser Arbeit werden die Laserwärmeverteilung und deren Einflüsse auf die Schweißbadtiefe sowie -breite numerisch anhand des Ray-Tracing-Verfahrens analysiert. Hierbei wird der La-serstrahl in mehreren Strahlenbündeln bzw. Subrays unterteilt. Diesbezüglich soll der Pfad der Subrays präzis berechnet werden, um die Dynamik der Dampfkapillare und des Schweißbades eines realen Schweißprozesses richtig abzubilden. Zu diesem Zweck beschäftigt sich die vorliegende Arbeit mit der Genauigkeitsverbesserung der Kontaktposition und der Reflexionsrichtung der Subrays auf der freien Oberfläche anhand der Level-Set-Methode. Um die Güte dieses Simulationsansatzes zu gewährleisten, wurde eine Gegenüberstellung mit den zwei klassi-schen Ray-Tracing-Verfahren mittels drei verschiedenen Benchmark-Testreihen durchgeführt. Anschließend wur-den die Versuchsergebnisse zur Validierung der implementierten numerischen Ansätze verwendet. Im Rahmen dieser Arbeit kann es gezeigt werden, dass unterschiedliche Wärmeverteilung aufgrund der verschiedenen Ray-Tracing-Verfahren deutlich zu erkennen ist, welche wiederum die Schweißbaddynamik sowie die lokalisierte Dampfkapillardynamik stark beeinflussen. Ferner wurde es bestätigt, dass die implementierte Level-Set-Methode zu einer genaueren Ermittlung der Kontaktposition und der Reflexionsrichtung der Subrays und somit zu einer Ver-besserung der simulierten Schmelzkontur führt.
State-of-the-art laser powder bed fusion (PBF-LB/M) machines allow pre-heating of the substrate plate to reduce stress and improve part quality. However, two major issues have been shown in the past: First, with increasing build height the apparent pre-heat temperature at the surface can deviate drastically from the nominal pre-heat temperature in the substrate plate. Second, even within a single layer the local surface pre-heat temperature can show large gradients due to thermal bottlenecks in the part geometry underneath the top surface. Both lead to unwanted changes in microstructure or defects in the final parts. In this study, a first attempt is taken to show the feasibility of pre-heating the top surface with the onboard laser beam to overcome the mentioned issues. A single layer of a group of three parts built from IN718 to a height of 33.5 mm is pre-heated in a commercially available PBF-LB/M machine to an average steady state surface temperature of 200 °C using the onboard laser beam. The parts are continuously heated, omitting powder deposition and melting step. Temperatures are measured by thermocouples underneath the surface. The experiments are supported by a thermal finite element (FE) model that predicts the temperature field in the parts. When heating the parts uniformly with the laser beam, differences in surface temperatures as large as 170 K are observed. To overcome this inhomogeneity, the heat flux supplied by the laser beam is modulated. An optimized, spatial heat flow distribution is provided by the thermal FE model and translated into a scan pattern that reproduces the optimized heat distribution on the PBF-LB/M machine by locally modulating hatch distance and scan velocity. This successfully reduces the differences in surface temperature to 20 K. Thermographic imaging shows that a homogeneous surface temperature can be achieved despite the localized heat input by the beam. The potential for industrial application of the optimized laser-heating technique is discussed.
Through experimental observation and auxiliary numerical simulation, this investigation studies the different types of grain refinement of 5754 aluminum alloy laser beam welding by applying a transverse oscillating magnetic field. Scanning electron microscope results have proved that the application of a magnetic field can reduce the average crystal branch width and increase its number. The interaction between the induced eddy current generated by the Seebeck effect and the applied external magnetic field produces a Lorentz force, which is important for the increase of the number of crystal branches. Based on the theory of dendrite fragmentation and the magnetic field-induced branches increment, the grain size reduction caused by the magnetic field is studied. Furthermore, the effects of the magnetic field are analyzed by combining a phase field method model and simulations of nucleation and grain growth. The grain distribution and average grain size after welding verifies the reliability of the model. In addition, the introduction of a magnetic field can increase the number of periodic three-dimensional solidification patterns. In the intersection of two periods of solidification patterns, the metal can be re-melted and then re-solidified, which prevents the grains that have been solidified and formed previously from further growth and generates some small cellular grains in the new fusion line. The magnetic field increases the building frequency of these solidification structures and thus promotes this kind of grain refinement.
The weld quality and the possible defect formation are directly determined by the weld pool shape and the thermo-fluid dynamics therein. In this paper, an untypical weld pool profile, i.e., elongated at its top and bottom but narrowed at the middle, is found experimentally and numerically in the wire feed laser beam welding. The detrimental influence of the weld pool narrowing on the element transport is analyzed and discussed. A magnetohydrodynamic technique is utilized to suppress the narrowing, aiming at a more homogenous element distribution. It is found that a low-temperature region is formed in the middle of the weld pool due to the interaction of the two dominant circulations from the top and bottom regions. The weld pool is significantly narrowed due to the untypical growth of the mushy zone in the low-temperature region, which results in a direct blocking effect on the downward flow and the premature solidification in the middle region. The Lorentz force produced by a transverse oscillating magnetic field shows the potential to change the flow pattern into a single-circulation type and the low-temperature-gradient region is mitigated.
Therefore, the downward transfer channel is widened, and its premature solidification is prevented. The numerical results are well validated by experimental measurements of metal/glass observation and X-ray fluorescence element mapping.
Numerical analysis of ultrasonic vibration enhanced friction stir welding of dissimilar Al/Mg alloys
(2023)
The ultrasonic vibration enhanced friction stir welding (UVeFSW) process has unique advantages in joining dissimilar Al/Mg alloys. While there are complex coupling mechanisms of multi-fields in the process, it is of great significance to model this process, to reveal the influence mechanism of ultrasonic vibration on the formation of Al/Mg joints.
In this study, the acoustic-plastic constitutive equation was established by considering the influence of both ultrasonic softening and residual hardening on the flow stress at different temperatures and strain rates. And the ultrasonic induced friction reduction (UiFR) effect on friction coefficient in different relative directions at the FSW tool-workpiece interface was quantitatively calculated and analyzed.
The Al/Mg UVeFSW process model was developed through introducing the above acoustic effects into the model of Al/Mg friction stir welding (FSW). The ultrasonic energy is stronger on the aluminum alloy side. In the stirred zone, there is the pattern distribution of ultrasonic sound pressure and energy. The heat generation at the tool-workpiece contact interface and viscous dissipation were reduced after applying ultrasonic vibra-tion. Due to the UiFR effect, the projection of friction coefficient and heat flux distributions at the tool-workpiece interface present a "deformed" butterfly shape. The calculated results show that ultrasonic vibra-tion enhanced the material flow and promoted the mixing of dissimilar materials.
The contactless magnetohydrodynamic technology has been considered as a potential and promising method to improve the weld qualities of deep penetration laser beam welding. In this paper, numerical investigations are conducted to study the influence of the auxiliary oscillating magnetic field on the porosity suppression in laser beam welding of 5754 aluminum alloy. To obtain a deeper insight into the suppression mechanism, a three-dimensional transient multi-physical model is developed to calculate the heat transfer, fluid flow, keyhole dynamic, and magnetohydrodynamics. A ray tracing algorithm is employed to calculate the laser energy distribution on the keyhole wall. A time-averaged downward Lorentz force is produced by an oscillating magnetic field. This force acts in the molten pool, leading to a dominant downward flow motion in the longitudinal section, which blocks the bubble migration from the keyhole tip to the rear part of the molten pool. Therefore, the possibility for the bubbles to be captured by the solidification front is reduced. The electromagnetic expulsive force provides an additional upward escaping speed for the bubbles of 1 m/s ~ 5 m/s in the lower and middle region of the molten pool. The simulation results are in a good agreement with experimental measurements. Based on the results obtained in this study, a better understanding of the underlying physics in laser beam welding enhanced by an auxiliary oscillating magnetic field can be provided and thus the welding process can be further optimized reducing the porosity formation.
A three-dimensional numerical model was developed to accurately predict the steady-state weld pool shape in full penetration laser beam welding. The model accounts for the coupling between the heat transfer and the fluid dynamics by considering the effects of solid/liquid phase transition, thermo-capillary convection, natural convection, and phase-specific and temperature-dependent material properties up to the evaporation temperature. A fixed right circular cone was utilized as a keyhole geometry to consider the heat absorbed from the laser beam. The model was used to analyze the influence of the thermodynamic activity of solutes and turbulence on the weld pool shape. A mesh sensitivity analysis was performed on a hybrid mesh combining hexahedral and tetrahedral elements. For the case of full penetration laser beam welding of 8 mm thick unalloyed steel sheets, the dependence of the weld pool shape on the surface-active element sulfur was found to be negligible. The analysis of the results showed that a laminar formulation is sufficient for accurately predicting the weld pool shape since the turbulence has a minor impact on the flow dynamics in the weld pool. The validity of the numerical results was backed up by experimental measurements and observations, including weld pool length, local temperature history, and a range of metallographic crosssections.
Numerical study on the formation of a bulging region in partial penetration laser beam welding
(2023)
A transient three-dimensional thermo-fluid dynamics numerical model was developed to study the formation of a bulging region in partial penetration laser beam welding. The model accounts for the coupling between the fluid flow, the heat transfer, and the keyhole dynamics by considering the effects of multiple reflections and Fresnel absorption of the laser beam in the keyhole, the phase transitions during melting and evaporating, the thermo-capillary convection, the natural convection, and the phase-specific and temperature-dependent material properties up to the evaporation temperature. The validity of the model was backed up by experimentally obtained data, including the drilling time, the weld pool length, the local temperature history outside the weld pool, the process efficiency, and a range of metallographic crosssections.
The model was applied for the cases of partial penetration laser beam welding of 8 mm and 12 mm thick unalloyed steel sheets. The obtained experimental and numerical results reveal that the bulging region forms transiently depending on the penetration depth of the weld, showing a tendency to transition from a slight bulging to a fully developed bulging region between penetration depths of 6 mm and 9 mm, respectively.
Der Absorptionsgrad metallischer Werkstoffe spielt bei Fügeprozessen mit einer Strahlungsquelle wie dem Hochleistungslaserstrahltiefschweißen eine bedeutende Rolle. Dieser beeinflusst die Menge der absorbierten Laserenergie, welche zum Aufschmelzen sowie zur lokalen Verdampfung des zu verbindenden Materials führt. Eine der wichtigsten Eigenschaften der Laserstrahlabsorption ist die Temperaturabhängigkeit. Dennoch wird sie in vielen Simulationsarbeiten häufig ignoriert und stattdessen ein angepasster Parameter zur Kalibrierung der Ergebnisse implementiert. Diese Vorgehensweise wirkt sich negativ auf die Zuverlässigkeit sowie die Genauigkeit des numerischen Modells sowie auf die Vorhersagbarkeit der Simulationsergebnisse aus. In der vorliegenden Arbeit wird die Temperaturabhängigkeit der Laserabsorption in ein selbstkonsistentes zweiphasiges Modell unter Berücksichtigung der gekoppelten 3D-Fluidströmung und Wärmeübertragung einbezogen. Die berechnete Laserstrahlabsorption wird durch die temperaturabhängigen Materialeigenschaften, die Lasercharakteristik und den Einfallswinkel des Laserstrahls bestimmt. Die freie Oberfläche jeder Phase wird mithilfe der volume-of-fluid Methode, kurz VOF, berechnet und die Laser-Material-Wechselwirkung wird durch ein neulich implementiertes Ray-Tracing-Verfahren modelliert, die auf einem lokalisierten Level-Set-Algorithmus basiert. Anschließend wird die transiente Wärmeeinbringung und die Geometrie der Schweißnaht analysiert und mit dem temperaturunabhängigen Absorptionsmodell verglichen. Es wurde festgestellt, dass die Temperaturabhängigkeit der Laserabsorption entscheidend für die genaue Bestimmung der Wärmeeinbringung und der Schmelzbadgeometrie ist. Das entwickelte Modell wurde anhand der experimentellen Untersuchungen validiert und die Bedeutung der Temperaturabhängigkeit bei der Laserstrahlabsorption wurde für die Simulation des Laserstrahltiefschweißens aufgeklärt.
Magnetohydrodynamic technology is increasingly recognized as a promising approach for enhancing the quality of deep penetration laser beam welding. This study employs experimental and numerical methods to investigate the effects of an auxiliary oscillating magnetic field on reducing porosity in the laser beam welding of 5754 aluminum alloy. The experimental results clearly demonstrate a significant reduction in the porosity ratio, thereby validating the efficacy of applying MHD technology in mitigating porosity during the laser beam welding process. In addition, a transient 3D multi-physical model has been developed, integrating the magnetohydrodynamic and metal vapor plume, to gain a more comprehensive understanding of the porosity suppression mechanism. The introduction of an oscillating magnetic field generates a time-averaged downward Lorentz force. This Lorentz force, in turn, induces an electromagnetic expulsive force, which effectively increases the upward escape velocity of bubbles in the molten pool. Furthermore, the molten pool shape is significantly enlarged, which further facilitate the escape of bubbles. The simulation results agree well with the experimental results.
The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.
The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.
Die Kombination aus hoher Korrosionsbeständigkeit und guten mechanischen Eigenschaften von Duplexstählen (DSS) ist auf ihre chemische Zusammensetzung und das ausgewogene Phasenverhältnis von Ferrit (α) und Austenit (γ) zurückzuführen.
Viele industrielle Anwendungen erfordern eine stoffschlüssige Verbindung von DSS. Das Wolfram-Inertgas-Schweißen (WIG) ist relativ einfach zu handhaben, benötigt nur wenig Platz und ermöglicht ein automatisiertes Schweißen, mit sehr hoher Reproduzierbarkeit und ist daher hervorragend zum Schweißen von DSS.
Während der Erstarrung dieser Dualphasenstähle kann es zu kritischen Phasenverhältnissen von α und γ kommen, was zu Erstarrungsrissen, Korrosionsanfälligkeit, geringerer Duktilität und kritischen Festigkeitswerten führt. Um die gewünschten Werkstoffeigenschaften zu erhalten, muss daher die α/γ-Verteilung zuverlässig vorhergesagt werden. Dies geschieht in der Regel mit Hilfe des WRC1992-Diagramms. Die Vorhersagegenauigkeit des Ferritgehalts in diesem Diagramm ist jedoch meist nicht genau genug und muss daher optimiert werden. Daher ist es notwendig, selbst kleinste Veränderungen in der chemischen Zusammensetzung des Schweißguts idealerweise während des Schweißens zu überwachen. Dies wird in diesen Experimenten mit Hilfe der laser-induzierten Plasmaspektroskopie (LIBS) durchgeführt. Ein großer Vorteil dieser Technik ist die hochgenaue zeit- und ortsaufgelöste Messung der chemischen Zusammensetzung während des Schweißens. In vorherigen Arbeiten wurde bereits die chemische Zusammensetzung im Schweißgut und der WEZ quantifiziert. In der präsentierten Untersuchung wird der Einfluss einzelner Elemente, wie Nb und Cu, auf das resultierende Schweißmikrogefüge untersucht.
The amount of absorbed energy in the keyhole as well as its spatial and temporal distribution is essential to model the laser beam welding process. The recoil pressure, which develops because of the evaporation process induced by the absorbed laser energy at the keyhole wall, is a key determining factor for the macroscopic flow of the molten metal in the weld pool during high-power laser beam welding. Consequently, a realistic implementation of the effect of laser radiation on the weld metal is crucial to obtain reliable and accurate simulation results. In this paper, we discuss manyfold different improvements on the laser-material interaction, namely, the ray tracing method, in the numerical simulation of the laser beam welding process. The first improvement relates to locating the exact reflection points in the ray tracing method using a so-called cosine condition in the determination algorithm for the intersection of reflected rays and the keyhole surface. A second correction refers to the numerical treatment of the Gaussian distribution of the laser beam, whose beam width is defined by a decay of the laser intensity by a factor of 1/e2, thus ignoring around 14% of the total laser beam energy. In the third step, the changes in the laser radiation distribution in the vertical direction were adapted by using different approximations for the converging and the diverging regions of the laser beam, thus mimicking the beam caustic. Finally, a virtual mesh refinement was adopted in the ray tracing routine. The obtained numerical results were validated with experimental measurements.
The spatial laser energy absorption inside the keyhole is decisive for the dynamic molten pool behaviors and the resultant weld properties in high-power laser beam welding (LBW). In this paper, a numerical simulation of the LBW process, considering the 3D transient heat transfer, fluid flow, and keyhole dynamics, is implemented, in which the free surface is tracked by the volume-of-fluid algorithm. The underlying laser-material interactions i.e., the multiple reflections and Fresnel absorption, are considered by an advanced ray-tracing method based on a localized Level-Set strategy and a temperature-dependent absorption coefficient. The laser energy absorption is analyzed from a time-averaged point of view for a better statistical representation. It is found for the first time that a noticeable drop of the time-averaged laser energy absorption occurs at the focus position of the laser beam, and the rest region of the keyhole has relatively homogenous absorbed energy. This unique absorption pattern may lead to a certain keyhole instability and have a strong correlation with the detrimental bulging and narrowing phenomena in the molten pool. The influence of the different focus positions of the laser beam on the keyhole dynamics and molten pool profile is also analyzed and compared. The obtained numerical results are compared with experimental measurements to assure the validity of the proposed model.
The spatial laser energy absorption inside the keyhole is decisive for the dynamic molten pool behaviors and the resultant weld properties in high-power laser beam welding (LBW). In this paper, a numerical simulation of the LBW process, considering the 3D transient heat transfer, fluid flow, and keyhole dynamics, is implemented, in which the free surface is tracked by the volume-of-fluid algorithm. The underlying laser-material interactions i.e., the multiple reflections and Fresnel absorption, are considered by an advanced ray-tracing method based on a localized Level-Set strategy and a temperature-dependent absorption coefficient. The laser energy absorption is analyzed from a time-averaged point of view for a better statistical representation. It is found for the first time that a noticeable drop of the time-averaged laser energy absorption occurs at the focus position of the laser beam, and the rest region of the keyhole has relatively homogenous absorbed energy. This unique absorption pattern may lead to a certain keyhole instability and have a strong correlation with the detrimental bulging and narrowing phenomena in the molten pool. The influence of the different focus positions of the laser beam on the keyhole dynamics and molten pool profile is also analyzed and compared. The obtained numerical results are compared with experimental measurements to assure the validity of the proposed model.
The spatial laser energy absorption inside the keyhole is decisive for the dynamic molten pool behaviors and the resultant weld properties in high-power laser beam welding (LBW). In this paper, a numerical simulation of the LBW process, considering the 3D transient heat transfer, fluid flow, and keyhole dynamics, is implemented, in which the free surface is tracked by the volume-of-fluid algorithm. The underlying laser-material interactions, i.e., the multiple reflections and Fresnel absorption, are considered by an advanced ray-tracing method based on a localized level-set strategy and a temperature-dependent absorption coefficient. The laser energy absorption is analyzed from a time-averaged point of view for a better statistical representation. It is found for the first time that a significant drop in the time-averaged laser energy absorption occurs at the focus position of the laser beam and that the rest of the keyhole region has relatively homogeneous absorbed energy. This unique absorption pattern may lead to a certain keyhole instability and have a strong correlation with the detrimental bulging and narrowing phenomena in the molten pool. The influence of different focus positions of the laser beam on the keyhole dynamics and molten pool profile is also analyzed. The obtained numerical results are compared with experimental measurements to ensure the validity of the proposed model.
The present work deals with the recently confirmed widening of the weld pool interface, known as a bulging effect, and its relevance in high power laser beam welding. A combined experimental and numerical approach is utilized to study the influence of the bulge on the hot cracking formation and the transport of alloying elements in the molten pool. A technique using a quartz glass, a direct-diode laser illumination, a high-speed camera, and two thermal imaging cameras is applied to visualize the weld pool geometry in the longitudinal section. The study examines the relevance of the bulging effect on both, partial and complete penetration, as well as for different sheet thicknesses ranging from 8 mm to 25 mm. The numerical analysis shows that the formation of a bulge region is highly dependent on the penetration depth and occurs above 10 mm penetration depth. The location of the bulge correlates strongly with the cracking location. The obtained experimental and numerical results reveal that the bulging effect increases the hot cracking susceptibility and limits the transfer of alloying elements from the top of the weld pool to the weld root.
Liquid metal embrittlement (LME) cracking is a phenomenon observed during resistance spot welding (RSW) of zinccoated advanced highstrength steels (AHSS) in automotive manufacturing. In this study, severe cracks are observed at the edge of the sheet under reduced flange widths. These cracks, traversing the AHSS sheet, culminate at the edge with a width of approximately 1.2 mm.
Through combined numerical and experimental investigations, and material testing, these cracks are identified and validated as a new type of LME crack. The mechanism behind this crack formation is attributed to unique geometric conditions that, when compared to center welding, amplify radial material flow by ninefold to 0.87 mm. The resultant tangential tensile stresses approximate 760 MPa, which exceed the yield strength of the examined advanced highstrength steel (AHSS) under heightened temperature conditions, and when combined with liquid zinc, promote the formation of this new type of LME crack.
Eine typische Automobilkarosserie kann bis zu 5000 Widerstandspunktschweißverbindungen aufweisen, welche hohen Qualitätsanforderungen genügen müssen. Daher ist eine durchgehende Prozessüberwachung unerlässlich. Die Transformation zur E-Mobilität in der Automobilindustrie und die damit einhergehende Reichweitenproblematik treiben die Entwicklung und Einführung neuer hochfester Stähle an. Dies resultiert in einem gesteigerten Fertigungsaufwand hinsichtlich einer stabilen Prozess-führung in der Fügetechnik. Um diesen Anstieg an Komplexität zu bewältigen, sind die Methoden der künstlichen Intelligenz ein geeignetes Mittel. Mit ihnen kann, durch Auswertung der Prozessparameter und -signale, die individuelle Schweißpunktqualität sichergesellt werden. Die Vorhersagegenauigkeit von neuen Daten, also das extrapolieren, stellt für die meisten Algorithmen eine große Herausforderung dar.
In dieser Arbeit wird ein künstliches neuronales Netz zur Vorhersage des Punktdurchmessers von Widerstandspunktschweißungen anhand von Prozessparametern implementiert. Die Vorhersagegenauigkeit und Extrapolationsfähigkeit des Modells wird durch die Auswertung des dynamischen Widerstandssignals verbessert.
Um die Extrapolationsfähigkeit zu untersuchen, wird die Vorhersagegenauigkeit des Modells mit Daten getestet, die sich in Bezug auf den Werkstoff und der Beschichtungszusammensetzung deutlich von den Trainingsdaten unterscheiden. Dazu wurden mehrere Schweißexperimente mit Werkstoffen verschiedener Hersteller durchgeführt und nur ein Teil der Daten in das Training einbezogen. Die Ergebnisse dieser Arbeit verdeutlichen den positiven Einfluss der Prozesssignale auf die Robustheit des Modells und die Skalierbarkeit der Algorithmen
künstlicher neuronaler Netze auf Daten außerhalb des Trainingsraums.
Durch pulverbasiertes Directed-Energy Deposition lassen sich Gradierungen fertigen, um diskrete Materialübergänge zu vermeiden und die Lebensdauer von Hartschichten zu erhöhen. Die Kombination aus Stahl als Basiswerkstoff und einer verschleiß- und korrosionsbeständigen Co-Cr Legierung verspricht durch Vermeiden von Spannungskonzentrationen das Verhindern von Abplatzungen und Rissen in der Schutzschicht. Um die Qualität des gefertigten Bauteils zu beurteilen, liegen für solche Funktional Gradierten Materialien (FGM) wenig Erkenntnisse vor. Daher wird im Rahmen dieser Studie eine Methodik erarbeitet, um die relative Dichte eines Funktional Gradierten Materials auf Stahl und Co-Cr Basis mittels Maschinendaten zu bestimmen. Anschließend wird unter Einsatz eines künstlichen neuronalen Netzes anhand von Sensordaten die relative Dichte vorhergesagt. Das trainierte Netz erreicht eine Vorhersagegenauigkeiten von 99,83%. Abschließend wird eine Anwendung anhand von einem Demonstrator gezeigt.
Effect of Tensile Loading and Temperature on the Hydrogen Solubility of Steels at High Gas Pressure
(2023)
The hydrogen solubility in ferritic and martensitic steels is affected by hydrostatic stress, pressure, and temperature. In general, compressive stresses decrease but tensile stresses increase the hydrogen solubility. This important aspect must be considered when qualifying materials for high‐pressure hydrogen applications (e.g., for pipelines or tanks) by using autoclave systems. In this work, a pressure equivalent for compensating the effect of compressive stresses on the hydrogen solubility inside of closed autoclaves is proposed to achieve solubilities that are equivalent to those in pipelines and tanks subjected to tensile stresses. Moreover, it is shown that the temperature effect becomes critical at low temperatures (e.g., under cryogenic conditions for storing liquid hydrogen). Trapping of hydrogen in the microstructure can increase the hydrogen solubility with decreasing temperature, having a solubility minimum at about room temperature. To demonstrate this effect, the generalized law of the hydrogen solubility is parameterized for different steels using measured contents of gaseous hydrogen. The constant parameter sets are verified and critically discussed with respect to the high‐pressure hydrogen experiments.
In the course of tomorrow's hydrogen-based energy transition, the construction of the corresponding infrastructure will play a central role. The majority of materials used to date are typically welded for component fabrication. In that context, steels are widely applied and can be prone to hydrogen embrittlement. This includes the classical delayed cold cracking during welding processing as well as embrittlement phenomena during operation. For the evaluation of any hydrogen effect on, for example, the mechanical properties of a welded metallic material, the hydrogen content must be precisely determined. In the case of welds, for example, according to ISO 3690, this is the isothermal carrier gas hot extraction (CGHE). CGHE is based on accelerated hydrogen degassing due to thermal activation of hydrogen at elevated temperatures. In addition to the pure quantification of hydrogen, thermal desorption analysis (TDA) with varied heating rates can be used to determine and evaluate the bonding state at microstructural defects in the material. For both techniques, experimental and measurement influences have to be considered, which have a great effect on the result. For CGHE, for example, ISO 3690 suggests different sample geometries as well as minimum extraction times. The present study summarizes results and experiences of numerous investigations with different sample temperatures and geometries (ISO 3690 type B and cylindrical TDA samples) regarding: the influence of the sample surface (polished/welded), measurement accuracies depending on the sample volume as well as the insufficient monitoring of the effect of the PI controller on the extraction temperature. In particular, a deviating extraction temperature to the set temperature, can significantly falsify the measurement results. Based on the results, methods are shown to quickly reach the desired extraction temperature without having to physically interfere with the measurement equipment. This serves to substantially improve the reliability of hydrogen measurement through increased signal stability and accelerated hydrogen desorption. In general, an independent temperature measurement with dummy samples for the selected heating procedure is advisable to exclude possible unwanted temperature influences already before the measurement. In addition (and way more important), the methods described can be transferred directly to industrial applications.
Multi-principal element alloys (MPEAs) are innovative materials that have attracted extensive research attention within the last decade. MPEAs are characterized by a solid solution of equiatomic metallic elements. Depending on the number of elements, they are also referred as high entropy alloys (HEAs with n ≥ 4 elements like CoCrFeMnNi) and medium-entropy alloys (MEA with n = 3 elements CoCrNi). Depending on the alloy concept, MPEAs show exceptional properties in terms of mechanical performance or corrosion resistance at extreme environments. In that connection, hydrogen and its challenges for the most metallic materials gets more and more important. MPEAs are candidate materials for the substitution of conventional materials like austenitic stainless steels e.g., at very high-pressure up to 1000 bar. Those pressures are typically reached in valves or compressors for refueling of tanks with operational pressure of 700 bar. So far, the susceptibility of HEA/MEAs to hydrogen assisted cracking (if any) and the especially the underlying hydrogen uptake and diffusion was not within the scientific scope and not investigated in detail yet. For that reason, we focused on the hydrogen absorption the characterization of the hydrogen diffusion and trapping at elevated temperatures in a CoCrFeMnNi-HEA (each element with 20 at.-%) and CoCrNi-MEA, each element with 33.3 at.-%). As reference grade, the commercially available austenitic stainless steel AISI 316L was investigated. High-pressure hydrogen charging was conducted at different pressures in autoclave environment with maximum value of 1,000 bar. Thermal desorption analysis (TDA) via carrier gas hot extraction with coupled mass spectrometry was used with a max. heating rate of 0.5 K/s up to 650 °C. The measured desorption spectra of the different samples were deconvoluted into a defined number of individual peaks. The individually calculated peak temperatures allowed the definition of activation energies for predominant trap sites in the respective materials as well as the percentage share of the totally absorbed hydrogen concentration. The results present for the first time the complex interaction of both MPEAs and high-pressure hydrogen charging. A deconvolution of four peaks was selected and a main desorption peak was identified the dominant hydrogen trap containing the biggest share of the absorbed hydrogen concentration. The chemical composition an austenitic phase of both MPEAs is responsible for delayed hydrogen diffusion and strong, but mostly reversible, trapping. The comparison with the 316L samples showed significantly higher activation energies in the MPEAs, whereas hydrogen was also trapped at very high extraction temperatures. The absorbed maximum hydrogen concentration at 1,000 bar was 130 ppm for the CoCrFeMnNi-HEA, 50 ppm for the CoCrNi-MEA and 80 ppm for the 316L. It is interesting that the CoCrFeMnNi-HEA has obviously a way higher trapping capability compared to the conventional austenitic 316L, which could be a major advantage in terms of resistance to hydrogen assisted cracking.
The study provides an overview of the aspects of joining and its importance in manufacturing of components for the more and more important field of hydrogen as key factor for the energy transition to a decarburized future. To this end, the fundamentals of the technology fields of hydrogen production, storage, transport, and application are presented and the state of the art of manufacturing of components for hydrogen technologies by joining is summarized. Based on representative examples from practice, research and development, the importance of joining technology in hydrogen technologies is clearly highlighted and perspectives for the future are derived. From a macroeconomic perspective, the focal points, or trends of joining technologies here include: the erection of new infrastructure for hydrogen storage and transport, and the safe conversion of existing natural gas infrastructure and its challenges for welded materials. In addition, we show the problems that are anticipated with in-service repair welding of hydrogen pipelines. In hydrogen applications, the efficient mass production of fuel cells and electrolysers is becoming increasingly important. For that reason, the importance of additive manufacturing is highlighted. Finally, the challenges for technical regulations and standardization by using hydrogen are shown.
Offshore wind turbines (OWT) are a key factor of the sustainable energy generation of tomorrow. The continuously increasing installation depths and weight of the OWTs require suitable foundation concepts like monopiles or tripods. Typically, mild steels like the S420ML are used with plate thicknesses up to several hundreds of mm causing high restraints in the weld joints. Due to the large plate thickness, submerged arc welding (SAW) with multiple wires is the state-of-the-art welding procedure. As a result of the very high stiffness of the construction, a certain susceptibility for time-delayed hydrogen-assisted cracking (HAC) may occur. The evaluation of crack susceptibility is very complex due to the component size and stiffness of real offshore structures. For this purpose, a near-component test geometry was developed to transfer the real stiffness conditions to laboratory (i.e., workshop) scale. The investigated mock-up, weighing 350 kg, comprised heavy plates (thickness 50 mm, seam length 1,000 m) joined by a 22-pass submerged arc weld. Additional stiffeners simulated the effect of high restraint or shrinkage restraint of the weld. Extreme scenarios of hydrogen absorption during welding were simulated via the use of welding fluxes in dry (HD < 5 ml/100g Fe) and moisture condition (HD > 15 ml/100g Fe). The residual stresses were determined by a robot X-ray diffractometer. Areas of critical tensile residual stress (at the level of the yield strength) were found in the weld metal and heat affected zone. To identify possible delayed cracking, the welds were tested by phased array ultrasonic testing (PAUT) after 48 h. Summarized, no significant occurrence of HAC was detected, indicating the high crack resistance of the welded joint, i.e., a suitable combination of base material, welding consumable and welding parameters.
Multiple principal element alloys (MPEA) represent a class of materials consisting of at least three alloying elements, each with 5 to 35 atomic %. The MPEAs encompass the so-called high-entropy (HEA) and medium-entropy alloys (MEAs) and are fundamentally different from conventional materials like the Fe-based steel. Within the last 20 years, highly innovative MPEA with individually adjustable properties for industrial applications were identified. However, the focus has been on pure material synthesis. With the increase in available material quantities, the focus is now on processing issues such as joining and welding. In that connection, the weldability of MPEAs has received very little attention so far and experience in dissimilar metal welds (DMWs) is lacking so far but. The present study summarizes comprehensive experimental results on the weldability of MPEA-DMWs and their resulting microstructure. For this purpose, two equiatomic MPEAs, CoCrFeMnNi (HEA) and CoCrNi (MEA) in cold-rolled and annealed conditions were joined by solid-state friction stir welding (FSW) to an austenitic stainless steel 316L. The DMWs showed very interesting microstructure features. In addition, the mechanical-technological properties were obtained by instrumented tensile tests, and the local straining was determined in-situ by digital image correlation (DIC). A significant influence of the FSW processing on the mechanical performance was identified,in terms of the formation of FSW-specific defects like whitebands or tunnel defects. The experiments proofed the general (but currently limited) FS weldability of the MPEAs to conventional austenitic steel grade AISI 304. This enables targeted further considerations of these highly innovative MPEAs.
Der Vortrag gibt einen tiefen Überblick über die Bedeutung der Füge- und Schweißtechnik in Wasserstofftechnologien. Dazu gliedert sich der Vortrag in die Komplexe H2-Erzeugung, Speicherung, Transport, Anwendung auf und gibt jeweils repräsentative Industriebeispiele für den heutigen Anwendungsstand. Insbesondere werden hier Fertigungstechnologien für Brennstoffzellen vorgestellt, sowie Herausforderungen beim Reparaturschweißen von Wasserstoffpipelines. Zeitgleich wird in einem eigenen Kapitel die Bedeutung der additiven Fertigung ebenso erläutert, wie die aktuellen und umfassenden Tätigkeiten auf dem Gebiet der Normung zu H2-Technologien.
Various industrial applications require the joining of DSS components. Tungsten Inert Gas (TIG) welding is particularly well-suited for this purpose due to the ability to achieve highly reproducible automated welds. However, during solidification of the weld pool, critical phase ratios of ferrite (α) and austenite (γ) may occur, leading to solidification cracking, increased corrosion susceptibility, lower ductility and critical strength values. Hence, in order to achieve the desired material characteristics, it is crucial to accurately predict the α/γ phase ratio within the weld. Conventionally, the WRC-1992 diagram is employed for this purpose. In our study, we used Laser-Induced Breakdown Spectroscopy (LIBS) to track alterations in the chemical composition of the weld metal on the surface throughout the welding process. One significant benefit of this method is its capability to provide precise and real-time measurements of chemical compositions during welding, both temporally and spatially. In previous investigations, we could develop routines to quantify the measurement of chemical compositions within the weld metal and the Heat-Affected Zone (HAZ). The conducted research focuses on examining the alterations in chemical concentrations of specific alloying elements, namely Cu and Mn as γ-forming elements, and Nb and Cr as α-forming elements, during welding. These changes are measured in real-time using Laser-Induced Breakdown Spectroscopy (LIBS). To achieve this, both the ferrite number, determined through magnetic-inductive techniques, and the weld microstructure are analyzed. Through image-analytical methods, a correlation is established between the microstructure and the LIBS data obtained.
The capability to produce complexly and individually shaped metallic parts is one of the main advantages of the laser powder bed fusion (PBF LB/M) process. Development of material and machine specific process parameters is commonly based on results acquired from small cubic test coupons of about 10 mm edge length. Such cubes are usually used to conduct an optimization of process parameters to produce dense material. The parameters are then taken as the basis for the manufacturing of real part geometries. However, complex geometries go along with complex thermal histories during the manufacturing process, which can significantly differ from thermal conditions prevalent during the production of simply shaped test coupons. This may lead to unexpected and unpredicted local inhomogeneities of the microstructure and defect distribution in the final part and it is a root cause of reservations against the use of additive manufacturing for the production of safety relevant parts. In this study, the influence of changing thermal conditions on the resulting melt pool depth of 316L stainless steel specimens is demonstrated. A variation of thermo-graphically measured intrinsic preheating temperatures was triggered by an alteration of inter layer times and a variation of cross section areas of specimens for three distinct sets of process parameters. Correlations between the preheating temperature, the melt pool depth, and occurring defects were analyzed. The limited expressiveness of the results of small density cubes is revealed throughout the systematic investigation. Finally, a clear recommendation to consider thermal conditions in future process parameter optimizations is given.
The capability to produce complexly and individually shaped metallic parts is one of the main advantages of the laser powder bed fusion (PBF LB/M) process. Development of material and machine specific process parameters is commonly based on results acquired from small cubic test coupons of about 10 mm edge length. Such cubes are usually used to conduct an optimization of process parameters to produce dense material. The parameters are then taken as the basis for the manufacturing of real part geometries. However, complex geometries go along with complex thermal histories during the manufacturing process, which can significantly differ from thermal conditions prevalent during the production of simply shaped test coupons. This may lead to unexpected and unpredicted local inhomogeneities of the microstructure and defect distribution in the final part and it is a root cause of reservations against the use of additive manufacturing for the production of safety relevant parts. In this study, the influence of changing thermal conditions on the resulting melt pool depth of 316L stainless steel specimens is demonstrated. A variation of thermo-graphically measured intrinsic preheating temperatures was triggered by an alteration of inter layer times and a variation of cross section areas of specimens for three distinct sets of process parameters. Correlations between the preheating temperature, the melt pool depth, and occurring defects were analyzed. The limited expressiveness of the results of small density cubes is revealed throughout the systematic investigation. Finally, a clear recommendation to consider thermal conditions in future process parameter optimizations is given.
The performance of titanium alloy (Ti6Al4V) surfaces was investigated in lubricated reciprocating sliding tribological tests (RSTT). Special emphasis was laid on the effect of surface nanostructures in area of contact on the respective friction and wear behaviour. These so-called laser-induced periodic surface structures (LIPSS, ripples) were produced on the titanium alloy surface upon scan processing in air by an ultrashort pulsed femtosecond (fs) laser. As lubricant served two types of base oils, a pure polyalcylene-glycol, and an SAE 0W30 oil containing only antioxidants and temperature stabilizers. Tribological tests were carried out on polished as well as LIPSS covered areas using both types of base oil. A test metrics was established, combining the additive 2-ethylhexyl-zincdithiophosphate (ZDDP) or the ionic liquid [P6,6,6,14] [DEHP] (98% purity) with the respective base oils. The test metrics also considered the orientation of motion with respect to the orientation of the structures formed on the surface. Results are presented which show that the interplay between LIPSS and the local chemistry formed by the respective additives is beneficial for the tribological behaviour of the titanium alloy. Certain combinations of base oil, additive and LIPSS reduced friction and wear significantly in the tribological contact.