Microstructure and property assessment of dissimilar joints of 6061-T6 Al/dual-phase steel fabricated by friction stir spot welding

Sound dissimilar joints of 6061-T6 Al (top member)/DP590 steel sheets (1.6 and 1.2 mu m thickness, respectively) were achieved using friction stir spot welding. A joint microstructure showed fine aluminum grains, dominant ultrafine ferrite grains, and a minor martensite phase at ferrite grain boundaries which showed no marked effect on tensile-shear load. Instead, the thickness of the intermetallic compound layer (IMC) and the hook shape at the joint interface were found to be the dominant and minor factors on tensile-shear load, respectively. The IMC layer showed linear kinetics growth, which indicates a reaction-controlled mechanism. A well-shaped hook was formed by the strategy of selecting a relatively high pin length, low plunge rate, and high dwell time. A maximum tensile-shear load of similar to 2950 N was achieved for the joint with an IMC layer of similar to 2 mu m thickness and a well-shaped hook; this is higher than the ultimate shear load of the weaker member (6061-T6 Al). Joints with thinner IMC layers showed weak Al/IMC interfaces, but those with thicker ones exhibited brittle IMC layers since a second IMC layer with lower Al content is formed; their lower joint strengths were attributed to the formation of Kirkendall voids at the interfaces of the Al/IMC layer and initial/second IMC layers, respectively.