Sound dissimilar linear friction welding of A7075-T6 Al and mild steel by simultaneous interfacial deformation using higher forging speed

Abstract

Sound dissimilar welding of heat-treatable A7075-T6 aluminum and mild steel is extremely difficult due to the formation of interfacial cracks. The present study aims to attempt linear friction welding (LFW) of high-strength A7075-T6 Al alloy and mild steel to obtain sound joints with the highest strength ever by controlling the quality deteriorating phenomena that previous studies have faced. In this study, LFW joints were fabricated under two different applied pressures of 100 MPa and 300 MPa at a forging speed of 5 mm/s, along with various forging speeds ranging from 5 mm/s, 10 mm/s, to 20 mm/s under an applied pressure of 300 MPa, and the joint microstructure, mechanical properties and interface macrostructure were investigated. Furthermore, thermal dependence behaviors of both alloys were systematically investigated to optimize the processing parameters, especially applied pressure during LFW. At lower forging speeds of 5 mm/s and 10 mm/s, the weld defects and un-bonded regions were observed near the center of the weld accompanied by unevenness of butt surface due to the relatively thick intermetallic compound (IMC) formed in the interface which greatly influenced the joint strength. In contrast, at the higher forging speed of 20 mm/s, weld defects at the joint interface were effectively suppressed by promoting the simultaneous and uniform interfacial deformation of both alloys thanks to the extremely thin IMC during LFW. Consequently, a highly efficient joint with 100 % efficiency was successfully obtained at higher forging speed of 20 mm/s and despite the occurrence of softening towards A7075, the fabricated joint displayed a base metal fracture towards the steel side.