Investigation of Liquid Metal Embrittlement during Resistance Spot Welding of Martensitic Steel with Zn Jet Vapor-Deposited Coating

Advanced high-strength steels protected by zinc coatings have contributed to a reduction in CO2 emissions in the automotive industry. However, the liquid metal embrittlement (LME) of the Fe/Zn couple induced by simultaneously acting stresses and high temperatures during resistance spot welding could be the cause of unexpected failure. We investigated the possible risk of LME in spot-welded martensitic steel with Zn jet vapor-deposited coating and its influence on weld strength. The weld nugget cross-sections were analyzed (optical microscopy, SEM-EDS), and their tensile shear strengths were compared with their uncoated counterparts. LME cracks were observed in all samples meeting the process window (6, 6.5, 7 kA) located at the edge of the sheet/electrode indentation area. The frequency and length of cracks increased with current, and the occurrence of Zn within cracks indicated the LME mechanism. The shear tests showed the Zn-coated sample underwent a decrease in tensile shear strength that was most evident at a welding current of 7 kA (13.2%). However, LME was excluded as a cause of lower strength. The decrease was attributed to the smaller nugget diameter and the thin slit of Zn coating remaining in the weld notch.

Liquid Metal Embrittlement of Advanced High Strength Steel: Experiments and Damage Modeling

In the automotive industry, corrosion protected galvanized advanced high strength steels with high ductility (AHSS-HD) gain importance due to their good formability and their lightweight potential. Unfortunately, under specific thermomechanical loading conditions such as during resistance spot welding galvanized, AHSS-HD sheets tend to show liquid metal embrittlement (LME). LME is an intergranular decohesion phenomenon leading to a drastic loss of ductility of up to 95%. The occurrence of LME for a given galvanized material mainly depends on thermal and mechanical loading. These influences are investigated for a dual phase steel with an ultimate tensile strength of 1200 MPa, a fracture strain of 14% and high ductility (DP1200HD) by means of systematic isothermal hot tensile testing on a Gleeble® 3800 thermomechanical simulator. Based on the experimental findings, a machine learning procedure using symbolic regression is applied to calibrate an LME damage model that accounts for the governing quantities of temperature, plastic strain and strain rate. The finite element (FE) implementation of the damage model is validated based on the local damage distribution in the hot tensile tested samples and in an exemplary 2-sheet resistance spot weld. The developed LME damage model predicts the local position and the local intensity of liquid metal induced cracking in both cases very well.