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Eingeladener Vortrag
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A 3D electro-thermomechanical model is established in order to investigate liquid metal embrittlement.
After calibration to a dual phase steel of the 1000 MPa tensile strength class, it is used to analyse the thermo-mechanical system of an experimental procedure to enforce liquid metal embrittlement during resistance spot welding. In this procedure, a tensile stress level is applied to zinc coated advanced high strength steel samples during welding. Thereby, liquid metal embrittlement formation is enforced, depending on the applied stress level and the selected material.
The model is suitable to determine and visualise the corresponding underlying stresses and strains responsible for the occurrence of liquid metal embrittlement. Simulated local stresses and strains show good conformity with experimentally observed surface crack locations.
Laser welding is a widely established manufacturing process in many industry sectors. Solidification cracking represents one of the most inadequately solved problems in welding and has major economic implications. The avoidance of hot crack is for most fusion welding processes a key challenge for an important range of metallic construction materials and affects not only the manufacturers of welding equipment and material manufacturers, but also a large number of customers using welding technologies, as well as welding standardization and research. In this study a new investigation programme has been developed to qualify the hot cracking susceptibility of a variety of austenitic stainless steels. The results show the possibility of using this technique to determinate the critical values that occur with initiation of solidification cracking during laser beam welding
Laser beam welding is a widely used joining technique in many industrial applications. This is mainly due to its many unique advantages, especially compared to conventional arc welding processes. These advantages include, among others, highly concentrated energy deposition, low total heat input and a capacity to penetrate deep into the material while causing only small welding distortions.
However, at the same time, the small dimension of the laser spot, high solidification rates, and small dimensions of the weld pool itself can provoke issues regarding the assembly tolerances of the workpiece, the hot-cracking phenomena, as well as keyhole-induced bubbles escaping from the melt.
Weld pool shapes in laser beam welding are elongated at the external, free surfaces under the action of the main driving forces in the melt – such as recoil pressure and surface tension forces – while being shorter in the internal areas of the weld pool. This leads to a regular solidification sequence from the internal zones toward the free surfaces, e.g. from the bottom to the top in partial penetration welding. However, in recent studies reported in the literature and seen in the experimental and numerical investigations of BAM Bundesanstalt für Materialforschung und -prüfung in Berlin, it was found that an internal narrowing phenomenon can occur that is often accompanied by a distinct bulging of the weld bead in deeper zones.
As the internal behaviour of the melt during the process is hardly optically accessible, several numerical models and experimental techniques were established to visualise the mechanisms of the formation of the bulging and the narrowing phenomenon and to reveal the consequences on the solidification sequence, pore formation, and filler metal dilution.
size limitations and high production costs of L-PBF make it competitive for smaller, highly complex components, while the less complex elements of an assembly are manufactured conventionally. This leads to scenarios that use L-PBF only where it’s beneficial, and it require an integration and joining to form the final product. For example, L-PBF combustion swirlers are welded onto cast parts to produce combustion systems for stationary gas turbines. Today, the welding process requires complex welding fixtures and tack welds to ensure the correct alignment and positioning of the parts for repeatable weld results. In this paper, L-PBF and milled weld preparations are presented as a way to simplify the Tungsten inert gas (TIG) welding of rotationally symmetrical geometries using integrated features for alignment and fixation. Pipe specimens with the proposed designs are manufactured in Inconel 625 using L-PBF and milling. The pipe assembly is tested and TIG welding is performed for validation. 3D scans of the pipes before and after welding are evaluated, and the weld quality is examined via metallography and computed tomography (CT) scans. All welds produced in this study passed the highest evaluation group B according to DIN 5817. Thanks to good component alignment, safe handling, and a stable welding process, the developed designs eliminate the need for part-specific fixtures, simplify the process chain, and increase the process reliability. The results are applicable to a wide range of components with similar requirements.
The determination of mechanical properties of welded Steel structures such as strength or ductility is a subject of high interest for the majority of Companies in the area of metal Processing. The material Parameters can be obtained by performing the tensile test on the samples made from a part of a component. In some cases, it is highly expensive to produce the tensile specimens especially from the weld metal, which contains different type of microstructure such as weld seam or heat affected zone in an extremely small area. Therefore, a method is described in this paper to determine the material Parameters of high strength Steel structures and welded joints locally and without any additional effort to perform the tensile test. In this method, instrumented indentation technique (IIT), an indenter is pushed on the flat surface of a specimen in a certain period of time and simultaneously the applied force and the corresponding indentation path are measured. The data related to the force-indentation diagram is given as input to an artificial neural network (ANN) to obtain the material Parameters. The ANN can be trained by generating the large qualitative data sets with numerical Simulation of the IIT procedure. The Simulation must be run several times with the different material model parameter sets to generate the numerous
force-indentation diagrams as the inputs of ANN. Then, the trained ANN is validated by performing the IIT on the welded joints and comparing the obtained material Parameters from ANN with the tensile test.
Consequently, the mechanical properties of welded joints can be determined by performing the IIT and evaluating the resulting data by the ANN.
Innovative Design- und Fertigungsstrategien zur Steigerung der Leichtbaupotenziale im Stahlbau
(2022)
Ziel des Verbundvorhabens „SmartWeld“ ist es, für die schweißtechnische Fertigung von Stahlbaustrukturen exemplarisch am Beispiel von Hohlprofilknoten, welche als designrelevante Elemente von Gründungsstrukturen für Offshore-Windenergieanlagen (OWEA) einsetzbar sind, die Möglichkeiten des Leichtbaus unter konsequenter Digitalisierung der Fertigungskette aufzuzeigen, um einerseits die Ressourceneffizienz zu forcieren und gleichzeitig wettbewerbsfähige Fertigungsstrategien aufzuzeigen.
Innerhalb von „SmartWeld“ werden daher die Möglichkeiten einer wirtschaftlichen Fertigung von Leichtbau-Gründungsstrukturen am Beispiel aufgelöster Tragstrukturen (Jackets) aufgezeigt. Eine lastgerechte Auslegung der Schweißverbindungen nach bionischen Prinzipien ermöglicht, weitere Einsparpotenziale an Ressourcen bei gleichzeitiger Erhöhung der Lebensdauer zu erschließen.
Die Ausnutzung konstruktiver Prinzipien des Leichtbaus zur Erzeugung innovativer und ressourcenschonender Produkte erfordert den Einsatz digitaler Werkzeuge in der Designphase, der virtuellen Absicherung, der Fertigung und Qualitätsüberwachung sowie nachgelagerten Bauteilprüfung zur Abbildung der Nutzungsphase. Im Rahmen der Arbeiten wird daher der gesamte Produktentstehungs- und Prüfprozess betrachtet. Dies beinhaltet das Design im Kontext des konstruktiven Leichtbaus, den schweißtechnischen Fertigungsprozess, die Bauteilprüfung sowie den Transfer der Ergebnisse in die Industrie durch serientaugliche Demonstratoren. Der Fokus der Arbeiten ist, die Fertigung auch im Umfeld eines für großskalige Stahlstrukturen üblichen hohen Individualisierungsgrades schweißtechnisch zu automatisieren und das Leichtbaupotenzial gezielt umzusetzen. Die Vorgaben aus der Designphase hinsichtlich einer lastgerechten Gestaltung bzw. Implementierung bionischer Ansätze können so gezielt durch einen adaptiven, automatisierten Schweißprozess eingestellt werden. Dies führt zu einer Erhöhung der Ermüdungs¬festigkeit und damit Produktlebensdauer, wodurch die Strukturen dünnwandiger ausgeführt und Ressourcen sowie CO2 eingespart werden können.
Innovative Design- und Fertigungsstrategien zur Steigerung der Leichtbaupotenziale im Stahlbau
(2022)
Offshore wind energy plays an important role for the desired transition towards a carbon dioxide free industry within the next decades. However, the grounding of the offshore wind plants governs the overall installation process besides bureaucratic bottlenecks.
The application of lightweight principles in steel construction, e.g. the usage dissolved load-bearing structures, so called Jackets foundations, offer great potential in reducing the resource consumption, especially with respect to the needed amount of steel.
In this context this presentation focuses on a fully digitalization of the welding manufacturing and as well as testing chain to enable a fully automated manufacturing as well as quality assessment of tubular nodes as key element of Jackets foundation structures. Furthermore, the relationship between the seam shape geometry and resulting fatigue strength is evaluated by numerical methods incorporating bionic principles. It is shown that tubular nodes can be welded fully automatically taking geometry tolerances into account. Moreover, the seam shape could be manufactured as requested by the numerical models which offers great potential for an extended lifetime. Subsequently, the improvements in resource efficiency and reduction of carbon dioxide emissions are evaluated by a life-cycle-assessment.
Innovative Design- und Fertigungsstrategien zur Steigerung der Leichtbaupotenziale im Stahlbau
(2022)
Ziel des Verbundvorhabens „SmartWeld“ ist es, für die schweißtechnische Fertigung von Stahlbaustrukturen exemplarisch am Beispiel von Hohlprofilknoten, welche als designrelevante Elemente von Gründungsstrukturen für Offshore-Windenergieanlagen (OWEA) einsetzbar sind, die Möglichkeiten des Leichtbaus unter konsequenter Digitalisierung der Fertigungskette aufzuzeigen, um einerseits die Ressourceneffizienz zu forcieren und gleichzeitig wettbewerbsfähige Fertigungsstrategien aufzuzeigen.
Innerhalb von „SmartWeld“ werden daher die Möglichkeiten einer wirtschaftlichen Fertigung von Leichtbau-Gründungsstrukturen am Beispiel aufgelöster Tragstrukturen (Jackets) aufgezeigt. Eine lastgerechte Auslegung der Schweißverbindungen nach bionischen Prinzipien ermöglicht, weitere Einsparpotenziale an Ressourcen bei gleichzeitiger Erhöhung der Lebensdauer zu erschließen.
Die Ausnutzung konstruktiver Prinzipien des Leichtbaus zur Erzeugung innovativer und ressourcenschonender Produkte erfordert den Einsatz digitaler Werkzeuge in der Designphase, der virtuellen Absicherung, der Fertigung und Qualitätsüberwachung sowie nachgelagerten Bauteilprüfung zur Abbildung der Nutzungsphase. Im Rahmen der Arbeiten wird daher der gesamte Produktentstehungs- und Prüfprozess betrachtet. Dies beinhaltet das Design im Kontext des konstruktiven Leichtbaus, den schweißtechnischen Fertigungsprozess, die Bauteilprüfung sowie den Transfer der Ergebnisse in die Industrie durch serientaugliche Demonstratoren. Der Fokus der Arbeiten ist, die Fertigung auch im Umfeld eines für großskalige Stahlstrukturen üblichen hohen Individualisierungsgrades schweißtechnisch zu automatisieren und das Leichtbaupotenzial gezielt umzusetzen. Die Vorgaben aus der Designphase hinsichtlich einer lastgerechten Gestaltung bzw. Implementierung bionischer Ansätze können so gezielt durch einen adaptiven, automatisierten Schweißprozess eingestellt werden. Dies führt zu einer Erhöhung der Ermüdungs¬festigkeit und damit Produktlebensdauer, wodurch die Strukturen dünnwandiger ausgeführt und Ressourcen sowie CO2 eingespart werden können.
The paper describes a systematic investigation of the EM influenced laser beam welding of the aluminum die casting alloy AlSi12(Fe) in comparison to a reference material, a wrought aluminum alloy AlMg3. By using of a face centred CCD test plan, the influencing variables laser power, welding velocity and magnetic flux density are varied with regard to their influence on the remaining porosity. The global pore fraction of the weld seams was analyzed by X-ray images with ImageJ. This enabled a qualitatively very good regression model to be derived for the respective material, which identifies the dominant influencing variables. The results prove, statistically verified, for the investigated parameter range, that
- the magnetic flux density is the main cause for the porosity reduction,
- the porosity rises with increasing laser power the porosity in the weld seams rises,
- the influence of the welding velocity is negligible,
- the pore quantity in wrought alloy is more strongly minimized by the magnetic flux density than in die casting,
- the porosity decreases due to the EM influence by approx. 70 % compared to the unaffected welds. This effect is emphasized by the contour line charts, which illustrate the relationship between laser power and magnetic flux density.
With the exception of the quadratic influence of B at the wrought alloy, the statistical correlation shows a linear development of the respective influence variables for both aluminum alloys. In order to investigate these deviations, further simulations with a focus on weld pool geometry and weld pool flow are to be performed. In addition, the welding results can be classified in accordance with DIN EN ISO 13919-2 in the highest evaluation group B for AlMg3 and in evaluation group C for AlSi12(Fe) by applying a magnetic flux density of 350 mT. The analysis of the CT images at constant laser power and welding velocity allows a direct comparison both between the two alloys and also as a function of the magnetic flux density with regard to the number and size of pores. An increase in the magnetic flux density leads to a significant decrease in the number and volume of pores, which can be seen more clearly in wrought alloy than in die casting. Very acceptable results can be achieved for both materials and different welding parameters. This successfully demonstrates the desired process robustness and functionality of the EM system for practical applications. For subsequent investigations of overlap joints, the lowest possible laser power and a high magnetic flux density are recommended.
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.
The influence of two different weld imperfections (gap/crack) on the fatigue behaviour of spot welded joints was tested
Surface cracks havenoinfluenceon thefatiguelife
Samples with cracks in the electrode indentation /HAZ where tested
Stiffness curves are not significantly influenced by surface cracks
3D FE-Simulation demonstrate no changes of stress due to surface imperfections
Solidification cracking is still a serious problem in laser beam welding, especially in the welding of thick-walled plates. The influence of weld pool geometry on solidification cracking in partial penetration welding of thick plates is investigated within scope of this study. Therefore, a specific experimental setup of steel and quartz glass in butt configuration and on the side with high-speed camera were used to record the weld pool shape. In addition, the influence of laser inclination angle on the weld pool geometry and on solidification crack formation was investigated. The results show a bulge in the weld pool root, which is separated from an upper region by a necking region. This leads to a case where there are three different longitudinal region lengths with different solidification zones. This temporal sequence of solidification strongly promotes the formation of solidification cracks in the weld root.
An accurate and efficient description of the spatial distribution of laser energy is a crucial factor for the modeling of laser material processing, e.g., laser welding, laser cutting, or laser-based additive manufacturing. In this study, a 3D heat transfer and fluid flow model coupled with the volume-of-fluid algorithm for free surface tracking is developed for the simulation of molten pool dynamics in high-power laser beam welding. The underlying laser-material interactions, i.e., the multiple reflections and Fresnel absorption, are considered by a raytracing method. Two strategies of free surface reconstruction used in the ray-tracing method are investigated: a typical piecewise linear interface calculation (PLIC)-based method and a novel localized level-set method. The PLIC-based method is discrete, resulting in noncontinuous free surface reconstruction. In the localized level-set method, a continuous free surface is reconstructed, and, thus, the exact reflection points can be determined. The calculated spatial laser energy distribution and the corresponding molten pool dynamics from the two methods are analyzed and compared. The obtained numerical results are evaluated with experimental measurements to assure the validity of the proposed model. It is found that distinct patterns of the beam multiple reflections are obtained with the different free surface reconstructions, which shows significant influence not only on the molten pool behaviors but also on the localized keyhole dynamics.
In this study, the influence of the welding speed and the arc power on the solidification crack formation for partial penetration laser hybrid welded Thick-Walled plates were investigated. Experimentally, a linear correlation between the welding velocity and the crack number was observed. That is by reducing the welding velocity the crack number was reduced.
The reduced welding velocity showed a strong impact on stress, as the model demonstrated a very lower stress amount in comparison to the reference case. The reduction of the welding speed could be a helpful technique to reduce the hot cracking. The wire feed speed showed a very slight influence on the crack formation. That can be returned to the large distance between the critical region for cracking and the arc region.
The stability of the keyhole decreases for deep penetrated high-power laser beam welding. The keyhole tends to collapse with increasing laser power and e.g. keyhole induced porosity can occur. This study deals with the observation of the keyhole during high-power laser beam welding in partial penetration mode by means of a high-speed camera. A butt configuration of 25 mm thick structural steel and transparent quartz glass was used for the experiments. An oscillating magnetic field was applied perpendicular to the welding direction on the root side of the steel plate. The keyhole was highlighted with a coaxial diode laser. It was ascertained that the stability of the keyhole and the weld penetration depth were increased by applying an oscillating magnetic field with an oscillating frequency of 1.2 kHz and a magnetic flux density of 50 mT.
A combined experimental and numerical approach is applied for a numerical analysis of the non-uniform martensitic transformation kinetic on welding residual stresses and distortion of a single pass weld. The (γ → α)-transformation kinetic within the weld pool region is governed by a non-uniform distribution of the elements chromium and nickel. The single-pass weld was performed by use of the low-alloyed high-strength steel S960QL with the high-alloyed high-strength filler wire CN 13/4-IG®. A thermo-mechanical FE model of the welding process was experimentally validated against temperature field, solid phase distribution, transformation behaviour, X-Ray stress measurements and transient optical distortion measurements.
The experimentally determined and calculated weld residual stresses and transient distortion are in good agreement. It can be shown that the change on the (γ → α)-transformation kinetic driven by the inhomogeneous distribution of the chemical contents causes a strong influence on the weld residual stresses within the volume of the weld pool, which could promote crack propagation within the solidified weld pool by use of high-alloyed filler materials. Furthermore, a significant influence on the development of the transient welding distortion is visible. This influence should be respected during numerically calculation of welding distortion in case of multi-pass welding using interpass temperatures and high-alloyed filler materials.
The complex microstructure and the advanced mechanical properties of AHSS steel types lead to an increased susceptibility to weld cracking, especially during resistance spot welding under ‘rough’ manufacturing conditions.
Various activities have been conducted in recent years at the Federal Institute for Materials Research and Testing (BAM) and the Fraunhofer IPK on this matter. Understandings about the safe spot weldability and the process of crack-forming itself were gained, and solutions for handling this issue given.
The performed work includes investigations on the influence of initial gaps as well as restraint intensity. An experimental method for the determination of the general cracking susceptibility, i.e. a material ranking of AHSS in respective to current restraints was developed. Coupled mechanical and thermal simulations of the welding process show the present internal stress-strain-conditions and values responsible for the formation of cracks, deepening the understanding of the process.
By using this easy to perform and material-independent method for the determination of the cracking susceptibility in resistance spot welded joints, small and medium sized companies can profit by rapidly evaluating the joinability of their handled material combinations as well as OEMs.
The influence of two different weld imperfections (gap/crack) on the fatigue behaviour of spot welded joints was tested
Surface cracks have no influence on the fatigue life
Samples with cracks in the electrode indentation /HAZ where tested
Stiffness curves are not significantly influenced by surface cracks
3D FE-Simulation demonstrate no changes of stress due to surface imperfections
•
Welding under external forces was found suitable for characterizing AHSS materials
New quick and easy-to-perform procedure for establishing a material ranking regarding cracking susceptibility
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.
AISI 2205 duplex stainless steel is used in a variety of industries, including the chemical and petrochemical industries. This is due to its high tensile strength combined with good ductility and corrosion resistance. However, in laser beam welding, these properties are negatively afected by the high cooling rates typical of the welding process. The resulting higher ferrrite content in the weld metal than in the base material leads to a reduction in the ductility and corrosion resistance of the welded joint. To overcome this problem, in this study, thick plates were coated by direct energy deposition (DED) prior to laser beam welding, whereas a duplex powder mixture containing a higher nickel concentration was used as a coating material. To improve the weld quality for the proposed two-step process, a method of additional material deposition instead of conventional tack weld was investigated. The resulting welded joints showed a well-balanced austenite to ferrite ratio and their properties and microstructure were verifed by metallographic analysis, electron backscatter difraction and Charpy impact testing. Using the standard ASTM G48 test method, it was found that the corrosion resistance of the welds was improved by a factor of four in average compared to the conventionally welded joints. The resulting properties, such as good ductility and corrosion resistance, of the welds with pre-coated edges showed good agreement with those of the base metal and confrmed the proposed two-step process as a promising alternative to the conventional approaches for welding thick duplex stainless steel plates.