Microstructure and property assessment of dissimilar joints of 6061-T6 Al/dual-phase steel fabricated by friction stir spot welding
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Sound dissimilar joints of 6061-T6 Al (top member)/DP590 steel sheets (1.6 and 1.2 μm 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 ~ 2950 N was achieved for the joint with an IMC layer of ∽ 2 μm 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.
KeywordsFriction stir spot welding Microstructure Aluminum alloys Iron alloys Fractures
Partial financial support by the Center of Excellence for Surface Engineering and Corrosion Protection of Industries and University of Tehran is gratefully acknowledged.
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