12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Corrosion protected galvanized advanced high strength steels with high ductility (AHSS HD) like the investigated dual phase steel (DP1200HD) tend to show an elevated risk of liquid metal cracking (LMC) during resistance spot welding (RSW). LMC is an intergranular cracking mechanism driven by temperature, tensile stresses, plastic deformation and the presence of liquid zinc. Crack initiation and growth is caused by liquid metal embrittlement (LME) where the material experiences a drastic loss of ductility of up to 95 percent depending on the loading conditions. The aim of this work is to experimentally investigate LME and to develop a model to predict the local LMC during RSW.
The prediction of LMC is addressed with laboratory scale hot tensile tests with uncovered and galvanized steel sheets at a Gleeble 3800 thermomechanical simulator and extensive laboratory scale resistance spot-welding tests. All gained material and process data are subsequently used for creating a physically based and validated LMC indicator, which is dependent on temperature, plastic strain and strain rate. Newly developed and validated finite element models of the welding process accounting for all relevant multi-physical phenomena provide deep insight in the RSW process and help to understand the influence of different conditions leading to LMC. The LMC indicator enhances the capabilities of these models and provides predictions for the onset of LMC. Taking advantage of the LMC indicator allows modifying the welding process such that a significant reduction of LMC during RSW can be achieved.