Advanced high strength steels are usually coated by a zinc layer for an increased resistance against corrosion. During the resistance spot welding of zinc coated steel grades, liquid metal embrittlement (LME) may occur. As a result, cracking inside and around the spot weld indentation is observable. The extent of LME cracks is influenced by a variety of different factors. In this study, the impact of the used electrode geometry is investigated over a stepwise varied weld time. A spot welding finite element simulation is used to analyse and explain the observed effects. Results show significant differences especially for highly increased weld times. Based on identical overall dimensions, electrode geometries with a larger working plane allow for longer weld times, while still preventing LME within the investigated material and maintaining accessibility.

Christoph Böhne (M.Sc.) studied mechanical engineering at the University of Paderborn and graduated in 2017. Currently he is working as a research assistant in the work group thermal joining with focus on resistance welding processes. Prof. Dr.-Ing. Gerson Meschut, graduated in 1998 at the University of Paderborn in Mechanical Engineering. After continuing research as chief-engineer, he joined the research and development department of Volkswagen AG in Wolfsburg in 2000. From 2005 to 2011 he was the technical managing director of Wilhelm Böllhoff GmbH & Co. KG in Bielefeld. Currently, he is the head of the Laboratory for Materials and Joining Technology (LWF) of the University of Paderborn. Max Biegler (M.Sc.) finished his studies in mechanical engineering at Technical University of Munich in 2015. He is currently working as a research associate at Fraunhofer IPK with focus on numerical modelling of welding processes. Univ.-Prof. Dr.-Ing. Michael Rethmeier graduated in 2003 at the Technical University Carolo Wilhelmina Braunschweig in Mechanical Engineering. Before joining the Technical University of Berlin (TUB) and the Federal Institute for Materials Research and Testing (BAM) in 2007, he worked at the Volkswagen AG. Currently, he is head of department 'Welding Technologies' (BAM), the field of 'Joining Technologies' at the Institute for Machine Tools and Factory Management (TUB) and the department of 'Joining and Coating Technology' (Fraunhofer IPK). Avoidance of liquid metal embrittlement during resistance spot welding by heat input dependent hold time adaptionLiquid 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.

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.

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