Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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  • Publication
    Influence of welding-induced cracks on the fatigue strength of resistance-spot-welded joints made of high-strength austenitic steel
    ( 2012)
    Rethmeier, M.
    ;
    Brauser, S.
    ;
    Schwenk, C.
    ;
    Noack, T.
    ;
    Jüttner, S.
    In the rough conditions in the fabrication of automobile bodies, it is not always possible to avoid welding-induced imperfections such as cracks during the resistance spot welding of high-strength steels. In this respect, the influence of such cracks on the fatigue strength particularly of modern high-strength austenitic steels is not sufficiently well-known at present. The influence of welding cracks with various positions and formations was therefore investigated within the framework of this paper. In this case, the analysis of the standardised stiffness courses of specimens and the comparison of the numbers of failure stress cycles served to prove that the surface cracks produced without any spatter in the centre, interfacial region and peripheral region of the weld nugget do not have any negative influence on the fatigue strength of the high-strength austenitic material investigated here. Specimens which were manufactured with welding spatter and exhibit cracks in the peripheral region show considerably higher numbers of failure stress cycles than crack-free reference specimens.
  • Publication
    Einfluss von schweißbedingten Rissen auf die Schwingfestigkeit von Widerstandspunktschweißverbindungen aus hochfestem austenitischen Stahl
    ( 2012)
    Brauser, S.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    ;
    Noack, T.
    ;
    Jüttner, S.
    Unter den rauen Bedingungen in der Automobilkarosseriefertigung lassen sich schweißbedingte Imperfektionen wie Risse beim Widerstandspunktschweißen von hochfesten Stählen nicht immer vermeiden. Dabei ist der Einfluss solcher Risse auf die Schwingfestigkeit insbesondere von modernen hochfesten austenitischen Stählen derzeit nicht hinreichend bekannt. Im Rahmen dieser Arbeit wurde daher der Einfluss von Schweißrissen verschiedener Lage und Ausbildung untersucht. Die erzielten Ergebnisse weisen nach, dass Oberflächenrisse bis zu einer Tiefe von 0,8 mm (Einzelblechdicke: 1,7 mm) bei den hier geprüften Bedingungen keinen negativen Einfluss auf die Schwingfestigkeitvon widerstandspunktgeschweißtem hochfesten austenitischen Stahl haben. Weiterhin ist nach derzeitigem Kenntnisstand davon auszugehen, dass diese Ergebnisse auf andere hochfeste austenitische Stähle übertragbar sind. Demzufolge können für Bauteile und Baugruppen aus hochfestem austenitischen Stahl im Hinblick auf eine wechselnde Belastung Oberflächenrisse bis zu 0,8 mm zunächst als unkritisch bewertet werden. Inwiefern eine Übertragbarkeit der Ergebnisse auf andere Belastungszustände (Zug-Druck-Wechsel, Scherzug usw.) besteht, ist in weiteren Untersuchungen zu klären. Darüber hinaus weisen die Ergebnisse darauf hin, dass auch eine Spritzerbildung beim Punktschweißen in Kombination mit den resultierenden Schweißrissen nicht zu einer Verringerung der Schwingfestigkeit führt. Vielmehr zeigt sich, dass die Spritzerbildung zu einer Erhöhung der Versagensschwingspielzahlen führen kann, sodass bezüglich der Schwingfestigkeit eine Spritzerbildung ebenfalls nicht als kritisch anzusehen ist. Da sich die hier dargestellten Ergebnisse auf gleichartige Verbindungen beziehen, im Rohkarosseriebau jedoch überwiegend Mischverbindungen, das heißt Schweißverbindungen aus unterschiedlichen Werkstoffen auftreten, ist eine Übertragbarkeit der Ergebnisse beispielsweise auf ferritisch-austenitsche Mischverbindungen in kommenden Untersuchungen zu prüfen.
  • Publication
    Case study for welding simulation in the automotive industry
    ( 2012)
    Rethmeier, M.
    ;
    Perret, W.
    ;
    Thater, R.
    ;
    Alber, U.
    ;
    Schwenk, C.
  • Publication
    Influence of Ti and B additions on grain size and weldability of aluminium alloy 6082
    ( 2012)
    Schempp, P.
    ;
    Cross, C.E
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    Grain refinement is an important possibility to enhance the weldability of aluminium weld metal that is usually defined by its susceptibility to solidification cracking. In this study, grain refinement was achieved through the addition of commercial grain refiner containing titanium and boron to the GTA weld metal of aluminium alloy 6082. The weld metal mean grain size could be reduced significantly from about 70 µm to a saturated size of 21 µm with a change in grain shape from columnar to equiaxed. The grain refinement prevented the formation of centreline solidification cracking that was present only in welds with unrefined grain structure. A variation of torch speed led to a strong change of solidification parameters such as cooling rate that was measured in the weld metal and the corresponding solidification rate and thermal gradient. The ratio thermal gradient/growth rate (G/R) decreased from 50 K s/mm2 (high torch speed) to 10 K s/mm2 (low torch speed). However, the variation of torch speed did not change the tendency for solidification cracking. The microstructure of unrefined and completely refined weld metal was compared. The observed change in size and distribution of the interdendritic phases was related to the change in susceptibility to solidification cracking.
  • Publication
    Weld metal grain refinement of aluminium alloy 5083 through controlled additions of Ti and B
    ( 2011)
    Schempp, P.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    ;
    Cross, C.E.
    The refinement of the weld metal gram structure may lead to a significant change in its mechanical properties and in the weldability of the base metal. One possibility to achieve weld metal grain refinement is the inoculation of the weld pool. In this study, it is shown how additions of titanium and boron influence the weld metal grain structure of GTA welds of the aluminium alloy 5083 (Al Mg4.5Mn0.7). For this purpose, inserts consisting of base metal and additions of the master alloy Al Ti5Bl have been cast, deposited in the base metal and fused in a GTA welding process. The increase of the Ti and B content led to a significant decrease of the weld metal mean grain size and to a change in grain shape. The results provide a basis for a more precise definition of the chemical composition of commercial filler wires and rods for aluminium arc welding.
  • Publication
    Numerical sensitivity analysis of welding-induced residual stress depending on variations in continuous cooling transformation behavior
    ( 2011)
    Heinze, C.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    ;
    Caron, J.
    The usage of continuous cooling transformation (CCT) diagrams in numerical welding simulations is state of the art. Nevertheless, specifications provide limits in chemical composition of materials which result in different CCT behavior and CCT diagrams, respectively. Therefore, it is necessary to analyze the influence of variations in CCT diagrams on the developing residual stresses. In the present paper, four CCT diagrams and their effect on numerical calculation of residual stresses are investigated for the widely used structural steel S355J2 + N welded by the gas metal arc welding (GMAW) process. Rather than performing an arbitrary adjustment of CCT behavior, four justifiable data sets were used as input to the numerical calculation: data available in the Sysweld database, experimental data acquired through Gleeble dilatometry tests, and TTT/CCT predictions calculated from the JMatPro and Edison Welding Institute (EWI) Virtual Joining Portal software. The performed numerical analyses resulted in noticeable deviations in residual stresses considering the different CCT diagrams. Furthermore, possibilities to improve the prediction of distortions and residual stress based on CCT behavior are discussed.
  • Publication
    Approach to assess a fast welding simulation in an industrial environment - application for an automotive welded part
    ( 2011)
    Perret, W.
    ;
    Thater, R.
    ;
    Alber, U.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    Fusion welding processes are widely used for joining metal structures, such as pipes, ships, and cars. In general, these joining processes offer a very good compromise between reliability, safety, cost and maintenance which are important issues in the current economical context. The negative heat effects of welding, i.e. distortions and residual stresses of the welded parts, are well known and many researches in this field have already been done in the last decades in order to minimize them. On the experimental side, many sophisticated procedures have become state of the art to deal with this problem. On the computational side, the improvement of the simulation algorithms and the computing power enables the simulations of many physical phenomena occurring during the welding process. The implementation of welding simulation techniques is nevertheless not an easy task and often associated with expert knowledge which hinders their global application in an industrial environment. This paper is focused on the industrial requirements of a welding simulation software with special respect to the needs of the automotive industry. The necessary information to run a welding simulation and the expectations of a weld specialist without deep knowledge in numerical methods are investigated. These expectations are tested on an automotive welded assembly with a commercially available welding simulation software designed especially for the needs of the automotive industry. A welding experiment is done and the measured temperature distributions and distortions serve as reference to validate the simulation results. The result quality of the simulations of temperature fields and distortions is in best agreement with experimental data. The workflow is well adapted for the considered industrial requirements and the time-tosolution as well as the computational costs are acceptable, whereas the efficient calibration of the heat input model is still a point which will be further investigated in current and future research works.
  • Publication
    General standard for welding simulation
    ( 2011)
    Schwenk, C.
    ;
    Tikhomirov, D.
    ;
    Eßer, G.
    ;
    Rethmeier, M.
    Für die Abgrenzung der Anwendbarkeit verschiedener Methoden der numerischen Schweißsimulation sowie für die Vereinheitlichung der Voraussetzungen und der durchzuführenden Schritte bei der Simulation sind normative Regelwerke für den Anwender erforderlich. Da es derzeit noch keine normähnlichen Dokumente auf diesem Gebiet gibt, wurde vom Deutschen Institut für Normung e.V. DIN in Zusammenarbeit mit der Forschungsvereinigung des DVS Deutscher Verband für Schweißen und verwandte Verfahren e.V. ein Arbeitsausschuss gegründet, welcher sich mit der Erarbeitung der entsprechenden Dokumente befasst. Der vorliegende Beitrag konzentriert sich auf die Vorstellung der neuen DIN Spec 32534-1, welche die grundlegenden Simulations-Schritte erläutert und ihre Anwendungsfelder sowie die Schlüsselbegriffe s pezifiziert. Des Weiteren wurde eine allgemein gültige Simulationsstruktur erarbeitet, welche als Empfehlung für den Auftraggeber und den Auftragnehmer bei der Formulierung und Abwicklung eines Dienstleistungsauftrages sowie für den Neueinstieg in die Schweißsimulation dienen soll. Schließlich wird ein Ausblick auf die weiteren Themenfelder des Arbeitsausschusses sowie auf die internationalen Aktivitäten auf diesem Gebiet gegeben.
  • Publication
    Case study for welding simulation in the automotive industry
    ( 2011)
    Perret, W.
    ;
    Thater, R.
    ;
    Alber, U.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    Welding is one of the most widely used joining processes in structural applications, like in car body production in the automotive industry. It is well-known that distortions and residual stresses occur during and after the welding process. Many procedures exist to decrease these negative heat effects of welding, but are often coupled with highly cost intensive experiments. For several decades, simulation models have been developed to understand and predict the heat effects of welding and to reduce experimental effort. In the production planning of various Original Equipment Manufacturers (OEM), some simulation tools are already well established, e.g. for crash test, forming or casting simulations. For welding, the demand is high but the implementation of welding simulation software is sti ll not established yet. Welding is a complex process and the development of a flexible simulation tool, which produces good simulation results without expert knowledge in simulation, is not an easy task. In this paper, a welded assembly from the automotive industry has been simulated and compared to experimental data. Temperature fields and transient distortion distributions have been measured with thermocouples and with an optical 3D deformations analysis tool, respectively. The simulation has been run with a commercially available welding simulation software. The simulated temperature fields match the numerical ones perfectly. The simulated distortions are also qualitatively in best agreement with the experimental ones. Quantitatively, a difference of approximately 20 % between the simul ated and the measured distortions is visible; this is acceptable considering the simplifications and assumptions of the simulation model. The global time to solution to get these results without expert knowledge in welding simulation was between 4 and 6 weeks, which is a reasonable time frame for an industrial application of welding simulation.
  • Publication