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Recent Developments for Thermo-Mechanically Coupled Simulations in LS-DYNA with Focus on Welding Processes

With increased mechanical and functional requirements put on many parts produced by the manufacturing industry, the numerical simulation of the process has gained importance within the last years. The main objective when applying numerical tools is an accurate prediction of the finished geometry. In order to allow for an efficient optimization procedure in the design phase of the process the complete manufacturing process chain has to be included in the simulation. For many processes in sheet metal forming this is state of the art. On the other hand, welding stages are often neglected in the virtual process chain, although the deformations that are induced have to be compensated for. Furthermore, high temperatures evolving in the structure and thus induce significant internal stresses and local changes in the microstructure of the metal alloys. The combination of these effects poses many challenges on the numerical simulation tool. In this paper novel developments in LS-DYNA will be presented that allow for an accurate and efficient modelling of different welding processes. One challenge in welding simulations is a realistic definition of the heat source including not only the correct amount of energy that is input into system but also the modelling of the power density distribution and the possibly complicated motion of the weld torch. The shape of this distribution can vary significantly between certain welding process and different choices of process parameters. The second challenge addressed in this paper is the modelling of weld seams. In many of the line welding processes a filler material is added to connect the parts. Other approaches as for example laser welding and spot welds, on the other hand do not require an additional material. Independent of the existence of a filler material, the connection between the processed parts is only established if the temperature has reached a certain value. Only in this case shear and tensile stresses can be transferred between the processed parts. Finally the effects of the process on the microstructure evolution in metal alloys have to be addressed. So far this issue had only been of interest for steel alloys in the hot stamping (press hardening) processes, in which the microstructure of the material (mostly boron-alloyed steel 22MnB5) is systematically manipulated in order to produce parts with ultra-high strength properties, but also to obtain areas within the same parts that have lower strength and increased ductility. In general, this is realized by locally varying cooling rates. In contrast to hot stamping, the heating of the material is of great importance for the application in welding simulation resulting in a still more complex phase kinetic description. As temperatures above the melting points are locally obtained, annealing is also to be considered.