Microstructure and mechanical properties of dissimilar friction stir welding of 6061-to-7050 aluminum alloys

• Dissimilar 6061 and 7050 aluminum alloys were joined via friction stir welding.
• Variations in the degree of intermixing were correlated with tool rotational speed.
• An increase in strength was observed with an increase in the tool rotational speed.
• Fracture under tensile loading occurred on the AA6061 side of the joint.

In this work, the microstructure and mechanical properties of friction stir welded dissimilar butt joints of 6061-to-7050 aluminum alloys were evaluated. Microstructure analysis of the cross-section of the joints revealed distinct lamellar bands and various degrees of intermixing that were correlated with tool rotational speed. Due to the distinct mechanical properties of the two alloys, microhardness measurements showed a consistent asymmetric hardness distribution profile across the weld nugget, regardless of tool rotational speed. Under monotonic tensile loading, an increase in the joint strength was observed with the increase in the tool rotational speed. Regarding fracture, the joints consistently failed on the 6061 aluminum alloy side. Furthermore, two modes of failure were observed, one through the stir zone and the other through the heat-affected zone. Inspection of the fracture surfaces suggested that inadequate material intermixing produced at low tool rotational speeds was the cause for the low mechanical strength and failure through the stir zone. On the other hand, the failure observed through the heat-affected zone at high rotational speeds was produced due to the material softening as confirmed by the microhardness measurements.

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