Elucidation of intermetallic compounds and mechanical properties of dissimilar friction stir lap welded 5052 Al alloy and DP590 steel

The friction stir lap welding (FSLW) was employed to join the dissimilar 5052 Al alloy and DP590 steel. The intermetallic compound (IMC) formation area and thickness increased linearly with increasing rotation speeds at 300 mm/min welding speed. Microvoid and gap defects existed on low-rotational-speed-produced lap interfaces, disappearing with increasing rotation speeds. However, cavities defects neighbouring the large-sized steel fragment appeared in the stir zone of the Al matrix with rotation speeds higher than 750 rpm. The negative influences from the Al-rich IMCs layer exceeding 1.5 µm thick and the aggravating internal cavities defects significantly reduced the joint strength. Correspondingly, the fracture mode of the friction stir lap welded 5052 alloy and DP590 steel was converted from the semi-brittle fracture into the full-brittle fracture. The maximum tensile shear force of 4.52 kN reached 75.2% of the Al 5052 base metal, obtained at the rotation speed of 750 rpm. However, from the combined perspective of integral bearing load capacity and local shear strength, the rotation speed of 500 rpm was recommended to achieve sound FSLW joints of Al5052/DP590. • Well-formed dissimilar Al5052/DP590 joints were obtained by friction stir lap welding (FSLW). • IMCs layer distribution was correlated to interfacial strength and fractured behaviour. • Material flow characterization and IMCs formation sequence were discussed. • IMCs layer with thickness exceeding 1.5 µm was found to deteriorate the FLSWed Al 5052/DP590 joint significantly.