Third generation (GEN3) automotive steels are advanced high-strength steels (AHSS) known for their high strength-to-weight ratio, which makes them ideal for use in lightweight structures. They also have excellent formability, which allows them to be easily shaped into complex geometries. These steels are increasingly utilized by the automotive industry to replace heavier gauge grades with the goal of achieving fuel efficiency by lowering the total car body weight. For example, manufacturing components such as hinge pillars, door beams, rear rails and roof headers in GEN3 steels can all benefit by more than 15% weight saving.
Similar to other AHSS, GEN3 steels are typically coated with Zn for corrosion protection. However, unlike other generations of AHSS, GEN3 steels have been found highly susceptible to liquid metal embrittlement (LME) where the steel experiences a sudden loss of ductility at high temperatures when Zn in the coating becomes molten. The mechanisms of LME depends on the substrate-liquid metal pair, and those for GEN3 steels are being actively researched. Engineeringly, the methodology used to evaluate and mitigate LME susceptibility of GEN3 steels is largely based on post-mortem visual identification, counting and measurement of the cracks in welds made with different parameters.
In this work, a novel LME calculation method is developed based on electro-thermo-mechanical finite element model that computes the evolution of stress and temperature fields during spot welding. The model employs a special glue contact for the weld nugget that allows for simulating the welding process in its entirety especially for the final cooling stage after electrode retraction. An LME ratio for crack initiation is established based on experimental data of peak failure stress from high-temperature tensile testing. An algorithm encompassing the LME ratio is embedded into the finite element model to track LME crack initiation throughout time as well as to differentiate different types of cracking based on the LME location. The model is applied to study how LME cracking severity is affected by welding schedule as well as electrode misalignment, demonstrating its utility in engineering solution to LME in GEN3 steel welding.