Liquid metal embrittlement during the resistance spot welding of galvannealed steels: synergy of liquid Zn, α-Fe(Zn) and tensile stress

Abstract

In this study, the influence of the microstructural evolution of galvannealed (GA) coating and the thermomechanical characteristics of liquid metal embrittlement (LME) cracking during resistance spot welding (RSW) of steels were investigated. The analysis of LME cracks, coating microstructure and thermomechanical characteristics at various weld regions revealed a synergistic effect of liquid Zn, α-Fe(Zn), and tensile stress on LME cracking during RSW. A continuous α-Fe(Zn) layer inhibits liquid Zn from contacting the steel substrate; hence, LME was suppressed. However, a long-duration, high-magnitude tensile stress can fragment the α-Fe(Zn) layer and enable LME cracking. Conversely, α-Fe(Zn) particles allow the easy infiltration of liquid into the steel and make the crack on tensile stress development.

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

“…Low strain rates combined with high temperatures are the reason for the formation of these intermetallic phases [ 14 ]. The intermetallic phases form at the interface of Fe-Zn and suppress the contact between steel and liquid zinc while acting as a protection layer of the base steel [ 33 ]. High-magnitude tensile stresses can cause fracture of the α-Fe(Zn) layer and continue in the steel sheet, while liquid zinc can penetrate the cracks and initiate LME.…”

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

“…7 The main drawback of analysing LME cracking using the HTTT test is the inherent differences in RSW thermal cycles and the simulated thermal cycles during HTTT tests. 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.…”

“…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…”

“…Researchers have attempted to understand the LME crack behaviour using various approaches such as thermomechanical simulations, finite-element simulations, and sectioning spot-welded joints. [4][5][6][7] Thermomechanical simulation methods such as high-temperature tensile test (HTTT) can simplify the complexities associated with the RSW process. 5,7 Significant loss of ductility shown in the stress-strain curves of Zn-coated 3G-AHSS at temperatures above 419°C (Zn melting temperature) was primarily attributed to the LME phenomenon.…”

“…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. It was reported that high thermal stress promotes the formation of LME cracks.…”