Friction stir welding of steel and aluminium: A review

Friction stir welding (FSW) joins two metals in a solid state, requiring moderate heat input and tool pressure. Steel is heavier than aluminium, but because of its low cost and high strength, aluminium can’t entirely replace steel. Unlike fusion welding, FSW is a viable technique for joining aluminium and steel, since it reduces solidification-related defects and produces thin intermetallic compounds (IMCs) and low residual stress. There is a lack of research on the optimisation of FSW parameters for joining aluminium and steel sheets, as well as a comparative discussion on the effects of multipass and filler material on friction stir-welded joints of aluminium and steel. This literature aims to compile research articles for comparative discussion on welding parameters optimisation, filler material, and multipass for friction stir welding of aluminium and steel. The influences of welding parameters and current strategies, such as hybrid FSW and ultrasonic vibration FSW, have been discussed. Key findings from the literature suggest that, under ideal conditions, the mean IMC thickness was found to be 4 to 5 µm, and the mean ultimate tensile strength was 85% of that of the Al-base metal for the butt joint configuration. The IMC thickness for the lap joint configuration was less than 1.2 µm at a pin plunge depth of 0.2 mm and at a pitch of 0.4 to 1.2 (mm/rev). After applying proper filler material with suitable plunge depth, no IMC of the base material was found, hook flaws were minimised, and better tensile strength was attained. Joining of sheets less than 3 mm with filler material is better than a second pass for the butt joint. Filler material, which is a solid soluble in aluminium, produces a stronger weld compared to that which is a solid soluble in iron.