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 trans-formation 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.