Robust bonding and microstructure behavior of aluminum/high-strength steel lap joints using resistance element welding process for lightweight vehicles: Experimental and numerical investigation

Here we present the resistance element welding (REW) process as a novel alternative to aluminum (Al)/steel (Fe) joints for the automotive structural assembly process. Before the REW process on the SPFC980 steels and AA5052 alloys, the low-carbon steel, S20C elements were riveted to AA5052 sheets to resolve the inconsistency of properties between Al and Fe. The REW processes were conducted with the 6 input parameters from 3.5 kA to 12 kA with a welding current under 200 ms of an energization time. As a result, robust bonding was successfully achieved exceeding 9 kN of tensile-shear strengths with excellent failure energies from the welding currents in 6.5 kA–10.5 kA without any common weld defects, e.g., crack, lack of fusion, lack of penetration, undercut, etc. These results present exceptional characteristics compared with the strength of previous reported Al/Fe joining technologies. The crystallographic characteristics of the welded metals in this process were quantitatively investigated using electron backscatter diffraction (EBSD) analysis. Due to the cooling effect the phase transformation was verified by theoretical values and finite element method (FEM) computational calculation. Furthermore, the fracture behavior analysis was thoroughly performed using the in-situ digital image correlation (DIC) method. This study provides a substantial alternative to achieve robust bonding of the automotive structural Al alloy/high tensile steel through the REW process and can be extended to mass production of lightweight structural assembly applications as well.