Effect of polygonal pin profiles on friction stir processed superplasticity of AA7075 alloy

Recently friction stir processing (FSP) has shown keen interest to achieve superplasticity in different aluminum alloys. The pin profile of FSP tool is one of the important process parameter which controls the mechanical and metallurgical properties of stir zone (SZ), like other variables of tool rotational speed, travel speed, and tool tilt. The high strength 7075 aluminum (Al-Zn-Mg-Cu) alloy was subjected to FSP to investigate effects of pin profiles on the superplastic behavior. Three different polygonal pin profiles of square, pentagon and hexagon were used. Microstructure, microhardness and grain size measurements were performed for all FSP samples. Fine grain uniform microstructure without cavitation in the SZ was observed in sample produced by square pin only. All polygonal pin profiles indicated sticking of workpiece material around tool pin that resulted in non-uniform grain microstructure in the SZ. Hot tensile testing was carried out for square pin under the superplastic condition of 3 × 10−4 s−1 and 400 °C to study the superplastic behavior. Uniform superplastic elongation of 227% was obtained in the gage region of the square pin sample.