Hybrid friction stir processing with active cooling approach to enhance superplastic behavior of AA7075 aluminum alloy

Friction stir processing (FSP) with active cooling approach can be considered as one of the variants of the FSP in order to enhance superplastic behavior. In this study, high strength AA7075 alloy was subjected to normal and hybrid FSP at different cooling medium such as compressed air, water, and CO₂ to obtain a variety of cooling rate during the process. Hybrid FSP samples were produced without any processing flaws at a lower processing temperature in comparison to normal FSP sample. Among the hybrid FSP samples, CO₂ cooling sample was produced at the lowest processing temperature. Optical and electron microscopy confirmed that microstructures of all the samples were characterized by fine equiaxed grain in the stir zone (SZ). Reduction of grain size in hybrid FSP samples was found due to higher cooling rate, which prevented the coarsening of grains in the SZ. Improvement in grain refinement was observed in the order of normal (4.12 μm), compressed air (3 μm), water (2.64 μm), CO₂ (1.96 μm) FSP samples. All FSP samples obtained the superplastic elongation. The highest elongation of 572% was achieved for CO₂ cooling sample due to the finest grained microstructure produced at the lowest heat input during FSP.