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.
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