Effect of Heat Treatment on the Microstructure and Vickers Hardness of Aluminum Alloy Gradient Plate Prepared Through Friction Stir Processing

Aluminum alloy plates with varying properties were prepared through the superposition friction stir processing of three different aluminum alloys with friction stir joining technology and subjected to solid solution aging. Experimental results showed that friction stir joining yielded a relatively smooth aluminum alloy surface gradient change plate. The plate surface presented a circular superposition trajectory with a clear outline and a rough surface with a flying edge during preparation. The three superimposed aluminum alloy plates achieved good fusion. In particular, fusion was most evident between the uppermost 5052 aluminum alloy and the middle layer of the 6061 aluminum alloy. The fusion of the middle layer of the 6061 aluminum alloy and the lower layer of the 7075 aluminum alloy weakened, and an obvious dividing line was observed. The average Vickers hardness values of the uppermost and undersurfaces of the samples heat-treated at 540°C were the highest, which were 1.34 and 119.06% higher than those of the samples without heat treatment, respectively. The most notable cross-section Vickers hardness gradient was obtained at the solution temperature of 500°C.