Liquid metal embrittlement during resistance spot welding of Zn-coated high-strength steels

Abstract

Liquid Zn-assisted embrittlement during resistance spot welding of Zn-coated high strength steels induces risks of surface cracking. This paper presents a state-of-the-art review on the surface crack evolution due to liquid metal embrittlement during spot welding. Several aspects of cracking have been discussed such as macroscopic and microscopic features of liquid Zn-assisted cracks, prevalent crack locations in spot welds, sensitivity to welding conditions, and susceptibility of different types of steels and Zn-based coatings. Subsequently, the influence of governing factors like temperature, stress, microstructure, and nature of coating are presented by correlating thermo-mechanical studies with actual spot welding investigations on Zn-coated steels. Finally, the possible mechanisms of embrittlement are discussed and suitable methods to suppress surface crack formation during spot welding are suggested. This is the winning review of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining, run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.

Four Types of LME Cracks in RSW of Zn-Coated AHSS

“…Hence, various classifications of the cracks were proposed based on the crack location; however, these classifications differed in many ways (Refs. 11,18,23,25). Choi et al (Ref.…”

“…Furthermore, the geometry of the electrode also influences LME due to the variation in temperature, stress, and availability of zinc by manifesting different contact conditions (Refs. [21][22][23][24]. Con-sequently, it is important to consider the presence of the electrode and the electrode-to-sheet (E/S) interface during the study of LME.…”

Detection Method for Liquid Metal Embrittlement Cracks Inside the Intermediate Sheet Zone of Dissimilar Resistance Spot Welds

“…Providing an overview where research work on LME during resistance spot welding was conducted, Bhattacharya generates a reference as strength-elongation diagram for steel alloys. [13] Thereby, no available literature source for the field of austenitic stainless steels could be quoted. Even accumulation of liquid zinc was mentioned for the notch area of the intermediate sheet zone, the heat-affected steel surface inside the intermediate sheet zone was not classified as a frequent location of cracks.…”

Significance of cutting plane in liquid metal embrittlement severity quantification

“…Advanced high strength steel (AHSS) is being adopted to increase material strength for automotive parts. However, when some of these steels are resistance spot welded (RSW) during automotive assembly, the protective zinc coating on the steel surface melts and enters the grain boundaries of the steel substrate, leading to intergranular cracking [1][2][3]. This is known as liquid metal embrittlement (LME), and may decrease spot weld strength [4][5][6].…”

“…Due to concerns about the impact of LME cracks on RSW strength, automotive standards are strict on how large surface cracks may be before welds are no longer fitfor-service [7]. It should also be noted that no cracks in the weld shoulder are permissible, which is where LME cracks are often seen are seen [3]. These standards are in-line with research that shows that the depth of shoulder cracks can affect the degree of strength loss exhibited by welds with LME cracks when compared to uncracked welds [8,9].…”