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Der Einsatz von hochfesten Stählen im Karosseriebereich des Automobilbaus hat während der letzten Jahre stark zugenommen. Hierzu zählen Dual- und Komplexphasenstähle, welche durch Kombination unterschiedlicher Gefügebestandteile auch deren Vorteile kombinieren, sowie TRIP (TRansformation Induced Plasticity) und Mangan-Bor Stähle, welche sehr gute Umformeigenschaften mit hohen Festigkeiten durch Martensitbildung bei der Umformung kombinieren. TWIP (Twinning Induced Plasticity) Stähle erreichen ähnliche Effekte durch forcierte Zwillingsbildung.
Die Ursachen für den Einsatz dieser Stähle liegen in dem Potential dieser Materialien zur Gewichts- und Kostenreduzierung, bei gleichzeitiger Erhöhung der Fahrgastsicherheit. Auf Grund der prinzipiell gegebenen Schweißeignung dieser Stähle, werden die klassischen Fügeverfahren im Karosseriebau wie das kostengünstige und effektive Widerstandspunktschweißen, das Metall-Schutzgas (MSG)-Schweißen oder das Laserschweißen angewendet. Allerdings treten teilweise Herausforderungen, beispielsweise durch Gefügeveränderungen in den Fügestellen auf, die zu ungewollten Aufhärtungen oder Erweichungen führen.
In diesem Projekt wird ein Verfahren entwickelt, mit welchem die lokalen Werkstoffeigenschaften von im Automobilbau typischen Werkstoffen und deren Fügestellen bestimmt werden können. Relevante Kennwerte sind in erster Linie das SpannungsDehnungs-Verhalten der verschiedenen Zonen einer Schweißverbindung; relevante Zonen wiederum sind neben dem Grundwerkstoff die Wärmeeinflusszone und das Schweißgut. Zu diesem Zweck wird das Verfahren der instrumentierten Eindringprüfung für den Einsatz bei hochfesten Stählen weiterentwickelt. Zunächst werden hierzu Zugversuche an einfachen Grundwerkstoffgeometrien durchgeführt. Im Anschluss wird die optische Dehnungsfeldmessung an stark taillierten, geschweißten Zugversuchsproben durchgeführt.
Die Taillierung dient dem Zweck, die WEZ auch mittels WPS über den gesamten Querschnitt der Probe erzeugen zu können, bzw. im Versuch auch Dehnungen in den relevanten Bereichen herbeizuführen.
Das im Projekt angewendete Auswerteverfahren, welches auf nichtlinearen Regressionsmodellen in Form von künstlichen, neuronalen Netzwerken beruht, ermöglicht die Vorhersage des Festigkeitsverhaltens des Werkstoffes anhand der gemessenen Krafteindringwegdaten.
Motivated by the tensile strength loss of 9%Ni steel arc welded joints made using Ni-based austenitic filler metals, the feasibility of maintaining the tensile strength using matching ferritic filler metal has been demonstrated. In comparison with shielded metal arc welded joint made using Ni-based austenitic electrode ENiCrMo-6, higher tensile strength comparable to that of the base metal was obtained using matching ferritic electrode. Besides, sufficient impact toughness energies with much lower mismatch were obtained for weld metal and heat-affected zone.
Welded joint with a lower mechanical mismatching is of considerable importance for achieving acceptable combination of tensile strength and impact toughness. A better combination of These mechanical properties is ensured by applying a post weld heat treatment.
Ni-based superalloys are well established in various industrial applications, because of their excellentmechanical properties and corrosion resistance at high temperatures. Despite the high development stage anda common industrial use of these alloys, hot cracking remains a major challenge limiting the weldability ofthe materials. As commonly known, the hot cracking susceptibility during welding increases with the amountof precipitation phases. Hence, a large amount of highstrength Ni-Alloys is rated as non-weldable. A newapproach based on electron beam welding at low feed rates shows great potential for reducing the hotcracking tendency of precipitation-hardened alloys. However, geometry and properties of the weld seamdiffer significantly in comparison to the common process range for practical uses. The aim of this study is toinvestigate the influence of welding parameters on the seam geometry at low feed rates between 1 mm/s and10 mm/s. For this purpose, 25 bead on plate welds on a 12 mm thick sheet made of Inconel 718 are carriedout. First, the relevant parameters are identified by performing a screening. Then the effects discovered arefurther studied by using a central composite design. The results show a significant difference between theanalyzed weld seam geometry in comparison to the well-known appearance of electron beam welded seams.
Motivated by the tensile strength loss of 9%Ni steel arc welded joints made using Ni-based austenitic filler metals, the feasibility of maintaining the tensile strength using matching ferritic filler metal has been demonstrated. In comparison with shielded metal arc welded joint made using Ni-based austenitic electrode ENiCrMo-6, higher tensile strength comparable to that of the base metal was obtained using matching ferritic electrode. Besides, sufficient impact toughness energies with much lower mismatch were obtained for weld metal and heat-affected zone.
Welded joint with a lower mechanical mismatching is of considerable importance for achieving acceptable combination of tensile strength and impact toughness.Abetter combination of These mechanical properties is ensured by applying a post weld heat treatment.
Notch impact toughness of laser beam welded thick sheets of cryogenic nickel alloyed steel X8Ni9
(2020)
This research work deals with the investigations of the impact toughness of laser beam welded 14.5 mm thick sheets made of cryogenic steel X8Ni9 as a function of preheating. This 9% nickel alloyed steel is widely used in the liquefied natural gas (LNG) industry. An application of highly efficient welding processes such as high-power laser beam welding (LBW) in LNG sector requires an understanding of the interactions between the LBW process parameters and weld properties, in particular the impact toughness. The results show that the original fine-grained martensitic microstructure of the base metal (BM) is significantly changed by melting and crystallization during the LBW, what can lead to a decrease in the impact toughness of the weld metal (WM) below the required level. An optimal preheating temperature range leads to the favorable thermal welding cycle and is of remarkable importance for maintaining the notch impact toughness of laser beam welded joints of these thick steel sheets. A parameter window was identified in which V-notch impact toughness comparable to that of the BM at -196 °C was achieved.
In diesem Beitrag werden Ergebnisse der Untersuchungen eines Verfahrens zum fehlerfreien Laserstrahl-Hybridschweißen von geschlossenen Rundnähten vorgestellt. Das Verfahren zielt auf die Vermeidung von Schweißimperfektionen im Überlappbereich einer laserstrahlhybridgeschweißten Rundnaht. Eine Strategie der Prozessführung beim Schließen der Rundnaht wurde entwickelt, mit der ein fehlerfreier Überlappbereich durch die Kontrolle der Erstarrungsbedingungen am Schweißnahtende erreicht wird. Die kontrollierte Wärmeführung wird durch eine Anpassung der Parameter von beiden beteiligten Schweißprozessen, dem Laserstrahl- sowie MSG-Schweißprozess realisiert. Experimentelle Untersuchungen wurden an 12 mm bis 15 mm dicken Rohrabschnitten durchgeführt. Der Einfluss von Prozessparametern wie der Laserleistungsrampe und Rampenzeit, der Veränderung des Abbildungsmaßstabes und der Defokussierung des Laserstrahls auf die Erstarrungsbedingungen am Ende der Rundnaht wurde untersucht, um eine optimale Strategie zum Herausführen der Prozessenergie zu finden. Im Rahmen der experimentellen Untersuchungen konnte gezeigt werden, dass eine Defokussierung des Laserstrahls im Bereich zwischen 60 mm und 100 mm über einen kurzen Auslaufbereich der Naht von ca. 15 mm zu einer deutlich besseren Nahtausbildung im Überlappbereich führte. Es konnte eine günstige kelchförmige Schweißnahtform ohne eine Tendenz zur Rissbildung erzielt werden. Die Laseroptik mit motorisch angesteuertem Linsensystem ermöglichte dabei eine Vergrößerung des Laserstrahldurchmessers ohne eine Veränderung der Position des MSG-Lichtbogens relativ zur Bauteiloberfläche.
Bislang kommt das Laserstrahlschweißen bzw. Laserhybridschweißen hauptsächlich bei Dickblechen mit einer Wandstärke von bis zu 15 mm zum Einsatz. Für Anwendungen über 20 mm war dieses Verfahren aufgrund einiger Herausforderungen bisher nur bedingt realisierbar. Eine von der Bundesanstalt für Materialforschung und -prüfung (BAM) entwickelte elektromagnetische Schmelzbadunterstützung ermöglicht nun ein einlagiges Schweißen von bis zu 30 mm Wandstärke.
The transport phenomena in the wire feed laser beam welding are investigated numerically. A three-dimensional transient heat transfer and fluid flow model coupled with free surface tracing and element transport is developed. A ray-tracing method with local grid refinement algorithm is used to calculate the multiple reflections and Fresnel absorption on the keyhole wall. The filler material flows backward along the lateral side of the weld pool, and subsequently flows forward along the longitudinal plane. The occurrence of the bulging phenomenon may further prevent the downward transfer of the additional elements to the root of the weld pool.
The study deals with the application of the high-power hybrid-laser arc welding process on up to 15 mm thick pipe segments with the intention to avoid end crater imperfections during closing of the circumferential welds, where the pipes were turned during welding in 1G- and 2Gpositions.
Different techniques such as laser power ramp-down, abrupt switch-off of the laser power and change of the magnification of the laser spot diameter and defocusing of the laser beam relative to the workpiece were tested to remove the laser energy from the process. It could be shown that a high defocusing of the optic system above 40 mm with a resulting beam diameter > 2.9 mm in a short overlap length of approx.
20 mm leads to the formation of a cup-shaped weld seam, which is preferred for avoidance of cracks and pores in the end crater. A laser optics with motor-driven lens system was used for the welding experiments to defocuse the laser beam without changing the position of the arc.
Notch impact toughness of laser beam welded thick sheets of cryogenic nickel alloyed steel X8Ni9
(2020)
The paper deals with the investigations of the impact toughness of laser beam welded 14.5 mm thick sheets made of cryogenic steel X8Ni9 as a function of preheating. This 9% nickel alloyed steel is widely used in the liquefied natural gas (LNG) industry. An application of highly efficient welding processes such as high-power laser beam welding (LBW) in LNG sector requires an understanding of the interactions between the LBW process parameters and weld properties, in particular the impact toughness. The results show that the original fine-grained martensitic microstructure of the base metal (BM) is significantly changed by melting and crystallization during the LBW, what can lead to a decrease in the impact toughness of the weld metal (WM) below the required level. An optimal preheating temperature range leads to the favorable thermal welding cycle and is of remarkable importance for maintaining the notch impact toughness of laser beam welded joints of these thick steel sheets. A parameter window was identified in which V-notch impact toughness comparable to that of the BM at -196 °C was achieved.
In diesem Beitrag werden Ergebnisse der Untersuchungen eines Verfahrens zum fehlerfreien Laserstrahl Hybridschweißen von geschlossenen Rundnähten vorgestellt. Das Verfahren zielt auf die Vermeidung von Schweißimperfektionen im Überlappbereich einer laserstrahlhybridgeschweißten Rundnaht. Eine Strategie der Prozessführung beim Schließen der Rundnaht wurde entwickelt, mit der ein fehlerfreier Überlappbereich durch die Kontrolle der Erstarrungsbedingungen am Schweißnahtende erreicht wird. Die kontrollierte Wärmeführung wird durch eine Anpassung der Parameter von beiden beteiligten Schweißprozessen, dem Laserstrahl- sowie MSG Schweißprozess realisiert. Experimentelle Untersuchungen wurden an 12 mm bis 15 mm dicken Rohrabschnitten durchgeführt. Der Einfluss von Prozessparametern wie der Laserleistungsrampe und Rampenzeit, der Veränderung des Abbildungsmaßstabes und der Defokussierung des Laserstrahls auf die Erstarrungsbedingungen am Ende der Rundnaht wurde untersucht, um eine optimale Strategie zum Herausführen der Prozessenergie zu finden. Im Rahmen der experimentellen Untersuchungen konnte gezeigt werden, dass eine Defokussierung des Laserstrahls im Bereich zwischen 60 mm und 100 mm über einen kurzen Auslaufbereich der Naht von ca. 15 mm zu einer deutlich besseren Nahtausbildung im Überlappbereich führte. Es konnte eine günstige kelchförmige Schweißnahtform ohne eine Tendenz zur Rissbildung erzielt werden. Die Laseroptik mit motorisch angesteuertem Linsensystem ermöglichte dabei eine Vergrößerung des Laserstrahldurchmessers ohne eine Veränderung der Position des MSG-Lichtbogens relativ zur Bauteiloberfläche.
Dieser Beitrag stellt Ergebnisse der Untersuchungen zum Auftragschweißen als Plasma-Laserstrahl-Hybrid-Prozess dar. Es hat sich gezeigt, dass ein Laserstrahl, der in einer gemeinsamen Prozesszone mit einem Plasma-Pulver-Auftragschweißprozess vorlaufend angeordnet ist, diesem Prozess eine erhebliche Geschwindigkeitssteigerung sowie eine Verbesserung der Stabilität ermöglicht. Der Hybrid-Prozess konnte mit Verschleißschutzwerkstoffen
sowie dem Korrosionsschutzwerkstoff Inconel 625 bei Vorschubgeschwindigkeiten von bis zu 10 m/min erfolgreich validiert werden.
Im Hinblick auf die aktuellen Entwicklungen zu Hochgeschwindiglceits-Laserstrahlauftragschweißen kann der Plasma-Laserstrahl-Hybrid-Prozess
zwischen diesen und den konventionellen Verfahren eingeordnet werden.
Nickelbasis-Superlegierungen sind seit vielen Jahren in unterschiedlichen Industrieanwendungen im Einsatz. Aufgrund der großen Heißrissneigung ist das Schweißen dieser Werkstoffe jedoch bei einer Vielzahl von Legierungen problematisch. Neue Arbeiten auf dem Gebiet zeigen, dass entgegen den gängigen Theorien auch reduzierte Schweißgeschwindigkeiten eine Tendenz zur Verringerung der Rissneigung aufweisen. Bisher existieren jedoch kaum Erkenntnisse zum Prozessverhalten in diesem Parameterbereich. In dieser Arbeit wird daher der Einfluss der relevanten Prozessparameter beim Elektronenstrahlschweißen (EBW) auf die Nahtgestalt im Bereich geringer Vorschubgeschwindigkeiten untersucht. Auf Grundlage der gewonnenen Erkenntnisse soll ein Ansatz zum rissfreien Fügen von komplexen Nickelbasis-Superlegierung gebildet werden. Die praktische Umsetzbarkeit wird abschließend anhand einiger Probeschweißungen an einem besonders heißrissgefährdeten Werkstoff demonstriert. Um fehlerfreie Verbindungen zu ermöglichen, wurden zunächst die relevanten Parameter für die Einstellung von Nahtbreite, Einschweißtiefe, Aspektverhältnis und Nahtfläche anhand einer Versuchsreihe mit 17 Blindschweißungen auf einer 12 mm dicken Platte aus Inconel 718 bestimmt. Die genaue Beschreibung des Einflusses der als signifikant identifizierten Faktoren erfolgte über die Anwendung einer Regressions- und Varianzanalyse. Die Ergebnisse zeigen, dass die Einschweißtiefe, die Nahtbreite, das Aspektverhältnis sowie die Nahtfläche vorrangig über den Strahlstrom, die Fokuslage sowie den Vorschub beeinflusst werden können. Auf Basis der gebildeten statistischen Modelle erfolgte die Vorhersage geeigneter Parameter für eine finale Versuchsreihe. Die abschließenden Demonstratorschweißungen wurden exemplarisch an einer Nickelbasis-Gusslegierung mit besonders hohem Ausscheidungsphasenanteil durchgeführt. Hierfür wurden Schweißungen im I- Stoß an 6,5 mm und 10 mm dicken Blechen ausgeführt. Trotz der mangelnden Schweißeignung und dem hohen Anteil an Ausscheidungsphase des Werkstoffes, zeigten sich nach Optimierung der Prozessparameter keine Heißrisse mehr.
Directed energy deposition is a metal additive manufacturing process that builds parts by joining material in a layer-by-layer fashion on a substrate. Those parts are exposed to rapid thermo-cycles which cause steep stress gradients and the layer-upon-layer manufacturing fosters an anisotropic microstructure, therefore stress relief heat treatment is necessary. The numerical simulation can be used to find suitable parameters for the heat treatment and to reduce the necessary efforts to perform an effective stress relieving. Suitable validation Experiments are necessary to verify the results of the numerical simulation.
In this paper, a 3D coupled thermo-mechanical model is used to simulate the heat treatment of an additive manufactured component to investigate the application of a C-ring geometry for the distortion-based validation of the numerical simulation.
Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to quantify the distortion after each process step.
Directed energy deposition additive manufactured parts have steep stress gradients and an anisotropic microstructure caused by the rapid thermo-cycles and the layer-upon-layer manufacturing, hence heat treatment can be used to reduce the residual stresses and to restore the microstructure. The numerical simulation is a suitable tool to determine the parameters of the heat treatment process and to reduce the necessary application efforts. The heat treatment simulation calculates the distortion and residual stresses during the process. Validation experiments are necessary to verify the simulation results. This paper presents a 3D coupled thermo-mechanical model of the heat treatment of additive components. A distortion-based validation is conducted to verify the simulation results, using a C-ring shaped specimen geometry. Therefore, the C-ring samples were 3D scanned using a structured light 3D scanner to compare the distortion of the samples with different post-processing histories.
A self-restraint hot cracking test (free edge test) was used in combination with a novel optical measurement technique to determine the critical straining conditions for solidification cracking for the stainless steel grade 1.4828 (AISI 309). The Lucas-Kanade algorithm for the optical flow (OF) calculation was implemented to obtain the full-field displacement and then the full-field strain.
The use of external laser illumination with appropriate filters allows to obtain good image quality with good contrast. The critical straining conditions required for solidification cracking can be obtained by means the proposed technique in the immediate vicinity of the solidification front. A very good repeatability was demonstrated for the used measurement technique. The critical straining conditions for solidification cracking for the tested steel und under this welding conditions has been detected
The development of high energy laser sources enables single-pass welds of thick plates up to 30 mm, but often additional materials are needed to influence the properties of the weld seams. However, the homogenous distribution of filler materials in form of e.g. electrodes is only possible up to 7 mm while the elements are only traceable up to a depth of 14 mm. To overcome this problem a two-step process is used where first the edges of the weld partners are coated with the filler material by laser metal deposition (LMD) and afterwards are welded by laser beam. Single-pass welds with electromagnetic weld pool support of 30 mm thick S355 J2+N-plates with austenitic AISI 316L-coatings were investigated as well as the influence of the coatings to the penetration depth of the laser beam without electromagnetic weld pool support in double-sided joints. The weld seams were tested by X-ray inspection and cross sections.
Recent studies have confirmed the widening of the weld pool interface, known as a bulge effect, during deep penetration high power laser beam welding. The link between such geometric particularities of the weld pool shape and the hot cracking phenomena is significant. The present work seeks to extend the level of understanding by investigating their relationship. A coupled multiphysics, multiscale numerical framework is developed, comprising a series of subsequent analyses. The study examines the influences of the bulge on the three most dominant effects causing hot cracking, namely the thermal cycles, the mechanical loading, and the local microstructure. The bulge in the weld pool shape forms approximately in the middle of the plate, thus correlating with the location of hot cracking. It increases the hot cracking susceptibility by enhancing the three dominant effects. The numerical results are backed up by experimental data.
Die numerische Simulation hilft, Probleme bei additiven Bauprozessen früh zu erkennen und Optimierungspotentiale auszuschöpfen. Ziel ist, im additiven Auftragschweißen (DED) die Zahl der nötigen Versuche durch Vorhersagen zu verringern und Prozessgrößen zu visualisieren. Eine besondere Anwendung der Simulation ist die Generierung verzugskompensierter Geometrien: Durch die Berechnung des Bauteilverzugs kann die Geometrie vor dem Bauen so verändert werden, dass sie mit Verzug die gewünschte Toleranz erreicht. So kann Zerspanvolumen und Aufmaß reduziert werden.
Liquid metal embrittlement (LME) cracking can occur during resistance spot welding (RSW) in zinc-coated advanced high-strength steels (AHSS) for automotive production. In this study, a methodological variation of hold time is performed to investigate the process-related crack influence factors. A combination of numerical and experimental investigations confirms, that the extent of heat dissipation and re-heating of the sheet surface can be influenced and thus the degree of crack formation can be controlled in a targeted manner by the parameterisation of the hold time. The temperature and stress history of crack-free and crack-afflicted spot welds are analysed and a conclusion on the borders defining the LME active region is derived.
Duplexstähle finden in vielen industriellen Bereichen Anwendung, dies ist nicht zuletzt ihren hervorragenden Eigenschaften, wie einer guten Korrosionsbeständigkeit, einer guten Duktilität bei trotzdem hoher Festigkeit, zuzuschreiben. Diese Eigenschaften werden jedoch durch das Schweißen, vor allem das Laserstrahlschweißen, beeinträchtigt, da die hohen Abkühlraten zu erhöhten Ferritanteilen im Schweißgut führen. Mit Hilfe eines zweistufigen Prozesses, bei dem die Kanten der Fügepartner vor dem Schweißen mit nickelhaltigem Pulver beschichtet werden, soll dieses Problem für dickwandige Bleche gelöst werden.
In diesem Zusammenhang wurden verschiedene Prozessparameter für den Laser-Pulver-Auftragschweiß-Prozess untersucht sowie die defektfreie Schweißung dieser beschichteten Kanten mit unterschiedlichen Prozessgasen.
Duplexstähle finden aufgrund ihrer Eigenschaften, wie der guten Korrosionsbeständigkeit, einer hohen Festigkeit bei gleichzeitig guter Duktilität häufig industrielle Anwendung. Durch die hohen Abkühlraten beim Laserstrahlschweißen weisen Schweißnähte jedoch anstelle eines ausgeglichenen Duplexgefüges einen deutlich erhöhten Ferritanteil, im Vergleich zum Basiswerkstoff, auf. Dies führt zu einer verringerten Duktilität sowie Korrosionsbeständigkeit. Um dieses Problem zu lösen, wurde ein Prozess entwickelt, der auf einer Kantenbeschichtung mit nickelhaltigem Zusatzmaterial der zu fügenden Bleche mittels Laser-Pulver-Auftragschweißen (LPA) basiert.
Die resultierenden Schweißnähte wurden zerstörend anhand von Schliffbildern, EDX-Aufnahmen, Härtemessungen und Kerbschlagbiegeversuchen geprüft.
In additive manufacturing (AM) directed energy deposition (DED), parts are built by welding layers of powder or wire feedstock onto a substrate with applications for steel powders in the fields of forging tools, spare parts, and structural components for various industries. For large and bulky parts, the choice of toolpaths influences the build rate, the mechanical performance, and the distortions in a highly geometry-dependent manner. With weld-path lengths in the range of hundreds of meters, a reliable, automated tool-path generation is essential for the usability of DED processes. This contribution presents automated tool-path generation approaches and discusses the results for arbitrary geometries. Socalled “zig-zag” and “contour-parallel” processing strategies are investigated and the tool-paths are automatically formatted into machine-readable g-code for experimental validation to build sample geometries. The results are discussed in regard to volume-fill, microstructure, and porosity in dependence of the path planning according to photographs and metallographic cross-sections.
Components distort during directed energy deposition (DED) additive manufacturing (AM) due to the repeated localised heating. Changing the geometry in such a way that distortion causes it to assume the desired shape – a technique called distortion-compensation – is a promising method to reach geometrically accurate parts. Transient numerical simulation can be used to generate the compensated geometries and severely reduce the amount of necessary experimental trials. This publication demonstrates the simulation-based generation of a distortioncompensated DED build for an industrial-scale component. A transient thermo-mechanical approach is extended for large parts and the accuracy is demonstrated against 3d-scans. The calculated distortions are inverted to derive the compensated geometry and the distortions after a single compensation iteration are reduced by over 65%.
The additional element from the filler wire in the laser beam welding is usually distributed inhomogeneously in the final weld due to the high solidification rate of weld pool. It has been found that the electromagnetic stirring produced by an external oscillating magnetic field can enhance the material mixing in the weld pool to achieve a more uniform element distribution. However, the magnetic field has a highly nonlinear and multicoupled interaction with the weld pool behavior, which makes the quantitative explanation of the physical mechanism difficult. In this study, the effect of electromagnetic stirring on the transport phenomena in the wire feed laser beam welding is investigated by a numerical modeling. A 3D transient multiphysical model considering the magnetohydrodynamics, heat transfer, fluid flow, keyhole dynamics, and element transport is developed. The multiple reflections and the Fresnel absorption of the laser on the keyhole wall are calculated using the ray tracing method. The numerical results show that a Lorentz force produced by the oscillating magnetic field and its induced eddy current gives significant influence on the transport phenomena in the molten pool. The forward and downward flow is enhanced by the electromagnetic stirring, which homogenizes the distribution of the additional elements from a nickel-based filler wire in a steel weld pool. The numerical results show a good agreement with the high-speed images of the molten pool, the fusion line from the optical micrograph, and the element distribution from the energy dispersive x-ray spectroscopy. This work provides a physical base for the electromagnetic-controlled laser beam welding and some guidance for the selection of electromagnetic parameters.
A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby, the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection, and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes’ temperature by the melting temperature. The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g., the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.
The geometrical characteristics of the weld end crater are commonly used as a means of validating numerical results in welding simulations.
In this paper, an analytical model is developed for calculating the cooling stage of the welding process after the moving energy source is turned off. Solutions for various combinations of heat sources and heated bodies are found. It is shown that after turning off the Energy source, additional melting of the base material in the longitudinal direction may occur due to the overheated liquid metal. The developed technique is applied to complete-penetration keyhole laser beam welding of 2 mm thick austenitic stainless-steel plate 316L at a Welding speed of 20 mm/s and a laser power of 2.3 kW. The results show a theoretical increase in the weld end crater length of up to 19% compared to the length of the steady-state weld pool. It is found that at the moment of switch off, the weld end crater center, where solidification of the liquid metal ends, is shifted from the heat source axis toward the weld pool tail. The solidification rate and the direction of crystallization of the molten material during the welding process and those in the weld end crater differ significantly. A good agreement between the computational results and the welding experiments is achieved.
Laser beam welding of aluminum die casting is challenging. A large quantity of gases (in particular, hydrogen) is absorbed by aluminum during the die-cast manufacturing process and is contained in the base material in solved or bound form. After remelting by the laser, the gases are released and are present in the melt as pores. Many of these metallurgic pores remain in the weld seam as a result of the high solidification velocities. The natural (Archimedean) buoyancy is not sufficient to remove the pores from the weld pool, leading to process instabilities
and poor mechanical properties of the weld. Therefore, an electromagnetic (EM) system is used to apply an additional buoyancy
component to the pores. The physical mechanism is based on the generation of Lorentz forces, whereby an electromagnetic pressure is introduced into the weld pool. The EM system exploits the difference in electrical conductivity between poorly conducting pores (inclusions) and the comparatively better conducting aluminum melt to increase the resulting buoyancy velocity of the pores. Within the present study, the electromagnetic supported degassing is investigated in dependence on the laser beam power, welding velocity, and electromagnetic flux density. By means of a design of experiments, a systematic variation of these parameters is carried out for partial penetration laser beam welding of 6mm thick sheets of wrought aluminum alloy AlMg3 and die-cast aluminum alloy AlSi12(Fe), where the wrought alloy serves as a reference. The proportion of pores in the weld seams is determined using x-ray images, computed tomography images, and cross-sectional images. The results prove a significant reduction of the porosity up to 70% for both materials as a function of the magnetic flux density.
In this study, the influence of the welding speed, the arc power, and the laser focal position on the solidification crack formation for partial penetration laser hybrid–welded thick-walled plates was investigated.
The solidification cracking in the weld root is a result of interaction between metallurgical and geometrical and thermomechanical factors.
Experimentally, a direct correlation between the welding speed and the crack number was observed. That is by reducing the welding velocity, the crack number was decreased. The focal position shows also a significant influence on the crack number. By focusing the laser on the specimen surface, the crack number has been significantly diminished.
The wire feed speed showed a very slight influence on the crack formation. That is due to the large distance between the critical region for cracking and the arc region.
The numerical model shows a high stress concentration in the weld root for both components (vertical and transversal).
Numerically, the reduced welding speed showed a strong impact on stress, as the model demonstrated a lower stress amount by decreasing the welding speed.
The metallurgical factors, such as the assumed accumulation of the low-melting eutectics in the weld root, should be a contribution for solidification cracking, where the tensile stress is acting.
This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone.
Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials’ plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve; (2) profile of the indented surface; (3) analyzing of the 3D-measurement image; and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good Agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface.
МОДЕЛИРОВАНИЕ ГИДРОДИНАМИЧЕСКИХ И ТЕПЛОВЫХ ПРОЦЕССОВ ПРИ ЛАЗЕРНОЙ СВАРКЕ СО СКВОЗНЫМ ПРОПЛАВЛЕНИЕМ
(2020)
Разработана модель физических процессов при сварке плавлением на основе концепции эквивалентных источников теплоты. Модель включает в себя две части: термогидродинамику сварочной ванны и теплопроводность свариваемого тела вне ванны. В задаче термогидродинамики учитываются температурные зависимости свойств материала, форма парогазового канала, термокапиллярная и естественная конвекция, фазовые превращения и другие физические явления.Приведено решение задачи термогидродинамики методом конечных элементов на примере сварки стальной пластины толщиной 15 мм со сквозным проплавлением лазерным лучом (по технологии "замочная скважина"). Показано, что термокапиллярная конвекция жидкого металла является основной причиной сложной выпукло-вогнутой формы границы ванны с увеличенными размерами в приповерхностных областях. Получено удовлетворительное совпадение расчетных и экспериментальных размеров сварочной ванны.
Because of its excellent corrosion resistance, high tensile strength and high ductility, duplex stainless steel 2205 offers many areas of application. Though laser beam welding accompanied by high cooling rates, duplex steels tend to perform higher ferrite contents in weld metal as the base metal, which leads to a reduction of ductility and corrosion resistance of the weld joint. To overcome this problem, a solution, based on buttering the plate edges by laser metal deposition (LMD) with material containing higher Ni concentrations prior to laser welding was suggested.
In this context different process parameters for LMD process were investigated. In a second step the possibility of welding those edges defect free while achieving balanced austenite-ferrite ratio was verified with metallographic analysis, Electron Backscatter Diffraction (EBSD) and impact testing according to Charpy.
The influence of heat input and welding speed on the microstructure and mechanical properties of single-pass hybrid laser arc welded 20mm thick plates of high-strength pipeline steel X120 were presented. The heat Input was varied in the range of 1.4 kJ mm−1 to 2.9 kJ mm−1, while the welding speed was changed between 0.5m min−1 and 1.5m min−1. A novel technique of bath support based on external oscillating electromagnetic field was used to compensate the hydrostatic pressure at low welding velocities. A major advantage of this technology is, that the welding speed and thus the cooling time t8/5 can be variated in a wide parameter window without issues regarding the weld root quality. The recommended welding thermal cycles for the pipeline steel X120 can be met by that way. All tested Charpy-V specimens meet the requirements of API 5 L regarding the impact energy. For higher heat inputs the average impact energy was 144 ± 37 J at a testing temperature of −40 °C. High heat Input above 1.6 kJ mm−1 leads to softening in the weld metal and heat-affected-zone resulting in loss of strength. The minimum tensile strength of 915 MPa could be achieved at heat inputs between 1.4 kJ mm−1 and 1.6 kJ mm−1.