Resistance Spot Welding Metallurgy of Thin Sheets of Zinc-Coated Interstitial-Free Steel

The extensive use of galvanized interstitial-free (IF) steels in the automotive industry makes their resistance spot welding (RSW) metallurgy important. In this study, the relationships between microstructure, macrostructure, mechanical performance, and failure mode of resistance spot welds of galvanized IF steels were investigated. In order to characterize the macro- or microstructure, geometry, mechanical performance, and failure mode of the welds, stereographic microscopy, optical microscopy, scanning electron microscopy (SEM), and microhardness techniques were used. The results showed that the heat-affected zone (HAZ) includes ferrite grains that were elongated in the direction of heat transfer from the weld pool boundary to the base metal (BM). In addition, it was found that the nugget microstructure contains lath martensite, bainite, and different ferrite morphologies. Increasing the amount of heat input led to a decrease in martensite phase content in the weld nugget (WN) microstructure. Microhardness test results showed that the hardness of the WN is higher than the HAZ and BM. In the tensile shear tests, interfacial fracture and pullout fracture followed by BM sheet tearing were observed. It was seen that a WN with size 4 x root t (t = sheet thickness) does not lead to pullout fracture. Finally, it was found that due to lower electrical resistivity of the steel in contrast to advanced high-strength steels, higher welding currents and longer welding times should be used in order to ensure the formation of large enough WNs and, thus, the satisfactory mechanical performance of the resistance spot welds. (C) The Minerals, Metals & Materials Society and ASM International 2019