Multi-Level Process Optimization of Dissimilar AA 6061-T6/ Cold-Rolled Low Carbon Steel Resistance Spot Welding via Taguchi L25
Abstract
Dissimilar aluminum to steel joints in automotive body-in-white structures fail at the weld interface because brittle Fe-Al intermetallic compounds (IMCs) form rapidly during welding and reduce joint strength. Resistance Spot Welding (RSW) can produce these joints at an industrial scale. Still, the individual and combined effects of its process parameters on joint strength, fracture mode, and IMC growth have not been systematically quantified. RSW of dissimilar aluminum to steel assemblies is a key technology for the automotive lightweight body-in-white structures. However, the combined effects of RSW process parameters on strength, failure mode, and microstructural evolution in aluminum-to-steel assemblies are not sufficiently quantified in the literature. In this research, a statistically rigorous, mechanistically integrated approach is used to analyse RSW of lap joints between aluminum 6061-T6 (upper sheet) and cold-rolled low-carbon steel (lower sheet), both 1.0 mm thick. Taguchi's L25 Orthogonal Array (OA) is used to evaluate the RSW process parameters of current (5.8 to 8.8 kA), time (0.35 to 0.75 s), and squeeze time (0.90 to 1.35 s) on Shear Failure Force (SFF) of the lap weldments; all tests are run in triplicate. Analysis of variance (ANOVA) is performed on RSW process parameters for SFF. Welding current is found to be the predominant process parameter, accounting for 64.6% of the total variance in SFF. The results also indicate that squeeze time accounted for 11.1 % and welding time accounted for 6.2%. The optimal parameter set of 6.5 kA for welding current, 0.55 s for welding time, and 1.35 s for squeeze time is validated through a confirmation experiment that produced an SFF of 1,052 N. A linear regression model is constructed for interpolative prediction strictly within the parameter space (R2 = 0.817). Vickers micro-hardness mapping is carried out. The peak microhardness values of the steel nugget center and aluminum nugget center are 193.4 HV and 106.7 HV, respectively. Heat Affected Zone (HAZ) softening on the aluminum side is caused by Mg2Si overaging.