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Correlation between precipitate microstructure and mechanical properties in AA7075‐T6 aluminum alloy friction stir welded joints

Microstructural evolution and mechanical properties of friction stir welded AA7075‐T6 aluminum alloy were examined. Grain structure and precipitate evolution in the stir zone and heat‐affected zone were evaluated using optical microscope and differential scanning calorimetry. A significant grain refinement and dissolution of η′ precipitates in the stir zone were found, but chromium‐bearing dispersoids remained nearly unchanged. The main particles in the stir zone and heat‐affected zone were η precipitates as well as Guinier‐Preston zones formed during post‐weld natural aging. The small recrystallized grains were observed in the thermo‐mechanically affected zone next to the stir zone. A W‐shaped hardness distribution where soft region was produced in the heat‐affected zone at a short distance from the stir zone were obtained. Hardness profiles of the welds were explained by precipitate distributions. Friction stir welding resulted in the reversion and coarsening of η′ precipitates. The formation of Guinier‐Preston zones in the stir zone and some parts of the heat‐affected zone during post‐weld natural aging increased the hardness. In transverse tensile specimens, fracture occurred in a location with the minimum hardness at either advancing or retreating side randomly. Further, influences of welding parameters on mechanical properties were investigated.