Now showing 1 - 10 of 28
  • 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
    Numerical calculation of residual stress development of multi-pass gas metal arc welding
    ( 2012)
    Heinze, C.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    In various applications, welding-induced residual stresses have a substantial impact on the integrity of welded constructions. Tensile residual stress can promote stress-corrosion cracking, brittle fracture, and reduces the fatigue life in service, as well as influences component design due to critical stress concentrations within the component. In the present paper, a six bead multi-pass gas metal arc weld of 20 mm thick structural steel S355J2+N is experimentally and numerically investigated. The studies include transient 2D and 3D numerical calculations which consider temperature-dependent material properties, phase transformations, "thermal" tempering, transformation plasticity, volume change due to phase transformation, an elastic-plastic material model, and isotropic strain hardening. The experimentally determined and calculated residual stresses are in a good agreement. Furthermore, the influence of the preheat and interpass temperature on welding-induced residua l stresses is shown in the present investigation.
  • 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
    The effect of tack welding on numerically calculated welding-induced distortion
    ( 2012)
    Heinze, C.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    A single-layer pulsed gas metal arc weld of structural steel S355J2+N with a thickness of 5 mm is experimentally and numerically investigated. Two tack welds are considered in the numerical simulation into two different ways. First, the tack welds are represented by elements belonging to the initial material. This implies that the "tack weld material" was not exposed to any thermal load or phase transformation before actual welding was performed. The weld seam is shortened and there is an influence on the stiffness of the whole structure affecting the calculation result. Secondly, the tack welds were simulated as conducted in the experimental welding procedure. The cases considering tack welding are compared to a simulation neglecting tack welding and to the experimental results. The influence of tack welds on the calculated welding-induced distortion is clarified and a contribution to an improved simulation-based prediction of welding-induced distortion is possible by modeling tack welding according to the realistic fabrication procedure.
  • Publication
    Numerical calculation of residual stress development of multi-pass gas metal arc welding under high restraint conditions
    ( 2012)
    Heinze, C.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    During welding, residual stresses build-up created by the steep thermal gradient that occurs in the weld zone from localized heating and cooling, and phase transformations appearing in low-alloyed structural steel is inevitable. Welding of rather simple test plates do not cover the actual structural effects, which have to be considered during real component welding. However, the resulting welding-induced residual stress state is highly influenced by the structural characteristics, i.e. restraint conditions, of the welded construction. Therefore, a unique large-scale testing facility providing a specific shrinkage restraint while welding and subsequent cooling was used for the present investigations. Hereby, a six bead multi-pass gas metal arc weld of 20. mm thick structural steel S355J2 + N was welded under shrinkage restraint. The residual stresses were experimentally and numerically investigated, and compared to an analysis of plates welded under force-free support an d free shrinkage conditions.The experimentally determined and calculated residual stresses using both 2D and 3D numerical models are in a good agreement. Furthermore, the influence of a shrinkage restraint on the residual stress distribution is both experimentally and numerically shown for the present test set-up.
  • Publication
    Effect of heat source configuration on the result quality of numerical calculation of welding-induced distortion
    ( 2012)
    Heinze, C.
    ;
    Schwenk, C.
    ;
    Rethmeier, M.
    The results of numerical welding simulations strongly depend on its temperature field. In the present paper, the temperature field of a pulsed gas metal arc weld of structural steel S355J2+N (ASTM A572 Gr. 50) with a thickness of 5 mm is experimentally and numerically investigated. In the case of temperature field validation, volumetric Gauss and double-ellipsoid Goldak heat sources are applied. Additionally, different heat source configurations, including adaptations of thermal conductivity, are analyzed regarding their influence on the calculation of welding-induced distortion. The investigations clarify the influence of heat source configurations on the calculated results, thus, contribute to an improved prediction of welding-induced distortion.
  • 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
    Case study for welding simulation in the automotive industry
    ( 2012)
    Rethmeier, M.
    ;
    Perret, W.
    ;
    Thater, R.
    ;
    Alber, U.
    ;
    Schwenk, C.
  • Publication
  • Publication
    Different modeling approaches for efficient distortion calculation of an automotive assembly
    ( 2011)
    Thater, R.
    ;
    Perret, W.
    ;
    Schwenk, C.
    ;
    Alber, U.
    ;
    Rethmeier, M.
    In contrast to other simulation fields like forming simulation, welding simulation is still not widely used in industrial environments. A high user expertise, a high time-to-solution and the result accuracy are the most important problems that hinder its extensively application. Different modeling approaches influence these aspects and an analysis of their implementation is of interest especially for the automotive industry as a key user for production simulation. In this study a MIG welded aluminum assembly from the automotive industry is investigated. Experimental and numerical methods are performed. On the experimental side, temperature cycles are obtained with thermocouples and transient distortion fields are measured using a 3D optical analysis system. On the numerical side, threedimensional nonlinear welding simulations are performed using the welding simulation software simufact.welding to obtain the transient distortion field of the investigated industrial assembly. Several modeling approaches are considered and are discussed with respect to the time-to-solution (modeling effort and the calculation time) and the result accuracy. Finally, a FE model with a good result accuracy and a reasonable calculation time for an industrial application is presented.