Influence of SiC interlayer on mechanical and electrical joint characteristics of Cu-Al-Cu sandwich spot welds obtained using a new friction processing technique

Abstract

Al busbars with Cu welded to Al on top and bottom, at lug connections, offer huge potential in carrying large currents at lower weight, cost and connection temperatures. Smaller weld areas increase joint electrical contact resistance and thinning of Cu, Al sheets during welding can cause premature failure in spot welds particularly in multilayer configuration. This work aims to overcome these drawbacks by obtaining large joint area Cu-Al-Cu sandwich spot welds using a new friction processing technique with and without SiC interlayer at joint interfaces. The technique eliminates direct stirring and reduces thinning of metal sheets to be joined. Localized melting and interdiffusion at Al-Cu interfaces at near eutectic temperatures is used to obtain the welds simultaneously at top and bottom joint interfaces. Thrust load and joint temperatures are used to investigate changes in weld microstructure and joint mechanical, electrical properties from presence of SiC particles at joint interfaces. Introduction of SiC particles, resulted in homogenous welds at top as well as bottom interfaces with smaller pores and thicker melt layer from enhanced interdiffusion. Improved wetting of eutectic melt with SiC particles and Al, Cu sheets, increased effective joining area, reduced electrical contact resistance, and increased joint strength. During fracture, cracks originated in brittle Al₂Cu intermetallic phase next to Cu. However, the intermetallic phase appeared as scattered with interspersed Al in between. Consequently, cracks took a tortuous route around the SiC particles embedded in eutectic melt region increasing fracture toughness.