Resistance spot welding of high-strength low-alloyed (HSLA) 420 steel and bake-hardening (BH) 220 steel

In this study, a number of models for estimating the mechanical attributes of dissimilar joints produced by resistance spot welding (RSW) of bake-hardening (BH) 220 steel and high-strength low-alloy (HSLA) 420 steel were developed based on regression analysis. Welding conditions were achieved via design of experiment (DOE), and the Box-Behnken design (BBD) method was selected. Welding time, welding current and electrode pressure were recorded as input parameters, while maximum displacement, peak load and failure energy of the welded samples accomplished from tensile shear tests were outputs. The influence of the welding current is superior to the influence of the welding time on the increase of the weld strength as stated by the analysis of variations (ANOVA). Relatively high welding heat input conditions enhance the mechanical attributes of the welds by increasing the nugget size. The largest mechanical properties obtained under the optimum welding conditions are welding time of 14 cycles, welding current of 10.8 kA and electrode force of 0.4 MPa. By verifying the regression models of the optimal welding conditions, the experimental results of the welded specimens with the maximum mechanical properties are about maximum displacement of 2.69 mm, peak load of 11.10 kN and failure energy of 23.90 J. The results show that the proposed spot weld strength models have an estimated accuracy of 5.32%. Due to the complete growth of austenite grains, the martensite in the coarse grain heat-affected zone (CGHAZ) is much larger than that of other zones, while in the HAZ at the side of BH steel, other phases besides martensite are also spotted, such as bainite, Widmanstatten ferrite and grain boundary ferrite. This is because BH steel has a lower carbon content. All welded specimens display hardening in the fine grain heat-affected zone (FGHAZ).