EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Zn-coated advanced high strength steels (AHSS) of the 3rd generation are an important part of modern car body lightweight constructions with excellent corrosion properties. However, liquid metal embrittlement (LME) during manufacturing processes such as resistance spot welding of AHSS is still a major concern that has attracted a great attention of industries and academic researchers. Despite numerous studies, a thorough understanding and a conclusive mechanism of how different alloy elements, for instance, boron and Si influence LME behavior is still missing.
The present study was therefore undertaken to demonstrate how the variation of Boron and Silicon in a high ductility dual phase (DP-HD) steel could affect LME. Next to the crystallographic investigation of the initial material, hot tensile tests were conducted to interpret LME susceptibility of different electrogalvanized DP steels. Results indicated that the presence of boron mitigates LME sensitivity at elevated temperature. Additionally, it was proved that decreasing Si-content from 1.5% to 0.9% led to lower LME susceptibility. Furthermore, detailed electron backscatter diffraction in conjunction with transmission electron microscopy results verified the intergranular penetration of Zn along prior austenite grain boundaries, where Zn as a ferrite stabilizer can promote the formation of α-Fe(Zn). Thus those Zn-wetted and α-Fe(Zn) decorated grain boundaries could be a promising path for LME crack propagation. Eventually, LME cracks in resistance spot welded DP-HD steels were investigated and the grain boundaries that are prone to LME crack propagation were analyzed in detail. The gained results help to develop a simple model system based on an interstitial free steel to better understand the basics of Zn diffusion along different steel grain boundaries, which consequently will also lead to insight into the embrittlement mechanism in modern AHSS.