Improving the weldability of press hardened steel to aluminum alloy in resistance spot welding using interlayer and ultrasonic assistance

Vehicle light-weighting has driven growing interests in joining aluminum alloys with press hardened steel (PHS), a metal widely used for its high strength. However, the significant physical and chemical differences between aluminum and PHS lead to low welding compatibility in conventional resistance spot welding (RSW) process, and some typically hard and brittle Fe-Al intermetallic compounds (IMCs) forming during the process, which can significantly deteriorate joint performance. This work proposes an improved method for RSW process of AA6061 aluminum alloy and PHS by introducing a stainless steel interlayer (thickness: 0.25–0.5 mm) and ultrasonic longitudinal vibration. The effects of different interlayer thicknesses and process combinations: conventional RSW, with interlayer only, and with both of interlayer and ultrasonic assistance (UA) were investigated in terms of dynamic resistance, metallurgical characteristics, joint microstructure, and mechanical properties of the Al/PHS welded joint. Results show that the stainless steel interlayer could effectively prevent direct Al-PHS contact. Using 0.45 mm interlayer made the IMC thickness reduced from ∼50 μm without the interlayer to ∼2.3 μm. Ultrasonic vibration further reduced the IMC layer to ∼1.4 μm, and enhanced Fe diffusion toward the aluminum side. With a 0.45 mm thick interlayer, the joint obtained from UA-RSW process could achieve a peak load of 6.32kN, which was 65.6% higher than that of the process using the same thickness interlayer without UA. The fracture energy was increased from 4.393 J to 14.806 J, and the fracture mode of the joint changed from interfacial fracture to better button fracture. These findings demonstrate that the synergistic using of a stainless steel interlayer and ultrasonic vibration can enable effective joining of PHS and aluminum alloys, and offer a promising solution for joining dissimilar metals and ultra-high-strength steels in advanced lightweight structures.