The effect of impulses on precipitation behavior in 7075-T6 aluminum alloy joint by impulse friction stir welding

Joining procedure for aluminum alloy 7075-T6 (AlZnMgCu1.5) sheets was realized by impulse friction stir welding (IFSW) with different impulse frequencies. The additional mechanical impulses during IFSW enhance the forging action of the tool, resulting in the weld microstructure modification. The microstructural evolution in different zones of the weld (SZ and HAZ) was studied, focusing on the strengthening precipitation behavior of the welded joint as well as the overall mechanical properties. The results illustrate that the application of impulses during IFSW in SZ of the weld accelerates the reprecipitation and dynamic recrystallization processes, which lead to the formation of strengthening precipitates and the homogeneous grain microstructure, respectively. HAZ of the welds obtained by the FSW and IFSW represent dislocation-free grain interior, and the strengthening precipitation behavior was completely different: the HAZ of conventional FSW welds exhibits a high concentration of stable η phase, while in the HAZ of IFSW joints shows a high concentration of heat-sensitive η' precipitates, suggesting that even in the HAZ, only additional deformation generated by mechanical impulses even in HAZ is responsible for the induced formation of GP zones and after the transformation of GP zones to the metastable η' precipitates.