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

Abstract

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.

Half-sectioned spot weld parameter optimisation for liquid metal embrittlement analysis in advanced high-strength steels

“…6 Differences in HTTT parameters such as set temperature, hold time, and strain rate can result in discrepancies in the thermal cycles and LME cracking behaviour. Moreover, it has been shown that these parameters can influence the morphology and microstructure of the Fe-Zn intermetallic in the coating layer, which is critical to LME cracking behaviour assessment 7,8 Discrepancies in the Fe-Zn intermetallic compound morphology between HTTT and RSW have been reported and attributed to different thermal cycles of each process. 6 In the case of finite-element modelling, DiGiovanni et al 9 demonstrated that a correlation exists between electrode collapse and thermal stresses, especially in the shoulder area of the weldment.…”

Multiscale Investigation of Microcracks and Grain Boundary Wetting in Press-Hardened Galvanized 20MnB8 Steel

“…Zn can embrittle the steel, and macroscopic cracks can occur during the further processing steps. Grain boundary (GB) weakening by Zn is supposed to be the underlying mechanism of this metal-induced embrittlement (MIE), which is therefore highly influenced by different alloy compositions and their resulting microstructures [3][4][5][6]. Already a monolayer or even a sub-monolayer of Zn at a GB has a strong impact on its strength [7].…”