Influence of ambient and cryogenic temperature on friction stir processing of severely deformed aluminum with SiC nanoparticles

The texture, microstructure, and mechanical properties of severely deformed aluminum subjected to friction stir processing (FSP) with SiC nanoparticles under ambient and cryogenic temperatures are comprehensively investigated. Even one pass of FSP in the ambient temperature results in the formation of quite large grain structure with random texture in the heat affected zone (HAZ). It suggests the occurrence of static recrystallization and subsequent grain growth because of the appreciable strain stored within the base metal. One FSP pass under condition of cryogenic temperature causes to the development of bimodal grain size distribution in HAZ, including the elongated grains with interior subgrains and very large grains as a result of texture-induced abnormal grain growth. Large grains with random texture are formed throughout HAZ during further FSP passes. In the stir zone where SiC nanoparticles are dispersed during FSP in the ambient temperature, fine grain structure with the shear texture components of C and A1∗ is obtained through the geometrically dynamic recrystallization mechanism. When FSP is performed in the cryogenic temperature, the grain size of stir zone is decreased because of the increased cooling rate. Although the shear texture components developed in the stir zone are the same at both mentioned processing conditions, the texture strength is less in the case of cryogenic temperature, suggesting the contribution of additional grain formation mechanism, namely discontinuous dynamic recrystallization. This mechanism gives rise to the formation of randomly oriented grains. The nano-indentation test results confirm that FSP under cryogenic temperature improves the mechanical properties not only in the stir zone, but also in HAZ.