Comparative Study on the Crack Formation Tendency of the Laser Lap-Welded Steels with GA, GI and ZAM Coatings

Steels coated with Zn-based coatings have been extensively employed in the manufacturing industry due to their excellent corrosion resistance. Unfortunately, crack defects can occur during the laser welding process. Three kinds of coatings were involved in the present study, namely galvannealing (GA), galvanizing (GI) and zinc–aluminum–magnesium (ZAM) coatings. To investigate the influence of different coatings (namely GA, GI and ZAM) on crack formation tendency, we carried out a comparative study on laser lap-welded joints with similar coating combinations (including Steel A (GA)/Steel A (GA), Steel B (GI)/Steel B (GI) and Steel C (ZAM)/Steel C (ZAM)) and dissimilar coating combinations (including Steel A (GA)/Steel C (ZAM) and Steel B (GI)/Steel C (ZAM)). The results showed that the joints with different zinc-based coatings exhibited distinct crack behaviors. Only the similar ZAM/ZAM joints revealed both lateral and central cracks, while the dissimilar GA/ZAM and GI/ZAM joints produced lateral cracks in the zones adjacent to the heat affected zone (HAZ). In particular, all the lateral cracks existed in the dissimilar joints, initiated and distributed on the ZAM-coated side. The microstructure and coating melting characteristics were analyzed in the process of crack initiation and propagation. It was considered that the lower melting temperature of ZAM (about 349 °C) compared with GA (about 423 °C) and GI (about 425 °C) could prolong the time to hold the melt status of the coating, accelerating the Fe dissolution, Al-Zn eutectic reaction (inducing lateral cracks) and liquid Zn penetration along the grain boundaries (causing central cracks), thereby increasing the crack susceptibility of the laser lap welding ZAM-coated steel plates.