Optimizing Electrode Combinations to Improve Welding Current Density Distribution and Intermetallic Layer Morphology Growth for Resistance Spot Welding of Aluminum Alloys to Steels

In this study, a new resistance spot welding (RSW) process has been developed by optimizing electrode combinations to homogenize the current density, aiming to hinder the fast heat conduction and produce a thin and uniform intermetallic layer for RSW of 0.7 mm DC06 steel and 1.0 mm 6016-T4 aluminum alloy. The distribution of welding current density and the growth of intermetallic compounds (IMC) layer at Al/steel interface were studied using three electrode combinations of 16-6/16-6, 16-10/16-10 and 16-5/16-16. The desired button pull out fracture mode, characterized by dimple features and plastic fracture, can be achieved with the optimized asymmetric 16-5/16-16 electrode combination. Compared with the 16-6/16-6 and 16-10/16-10 electrode combinations, the peak load of the weld obtained is increased by 42.9% and 51.8%. The optimized electrode combination led to a larger and more uniform current density distribution on aluminum side and produced a larger adequate nugget size without expulsion and finer columnar dendritic grain structure. Additionally, a comprehensive and uniform IMC layer platform area was obtained, with the maximum IMC thickness being only 2.83 μm, which prevents the crack propagation along the interface and produces higher joint performance. Furthermore, it was first found that Fe and Cu atoms can enter the aluminum nugget by diffusion, and segregate at the grain boundary to strengthen the grain boundary, which increased the hardness of the nugget.