Effect of heat treatments on microstructure and strengthening mechanisms of friction stir powder additive-manufactured Al-Zn-Mg-Cu aluminium alloy

Abstract

Solid-state additive manufacturing is gaining attention for fabricating lightweight alloys with enhanced mechanical properties due to its low thermal input. In this work, a multilayer AA7075 (Al-Zn-Mg-Cu) alloy structure was developed using Friction Stir Powder Additive Manufacturing (FSPAM). A detailed study was then conducted to evaluate the influence of post-deposition heat treatments (T6 and T73) on the resulting microstructure, microhardness, and tensile behaviour of the as-deposited (AD) samples. The AD condition exhibited fine, equiaxed grains resulting from continuous dynamic recrystallization (CDRX). While the heat treatments had little effect on grain size, they significantly altered the nature and distribution of precipitates. After T6 treatment, fine η′ precipitates were observed within grains, along with coarser η precipitates at grain boundaries. The T6-treated sample achieved the highest ultimate tensile strength (UTS) of 560 MPa, with reduced ductility compared to both the AD and T73-treated specimens. Following the T73 heat treatment, the η′ precipitates underwent coarsening into the more stable η phase, which resulted in a reduction in UTS while enhancing ductility, as evidenced by the highest observed elongation among the tested conditions. Fractographic analysis of the as-deposited (AD) tensile specimens revealed features indicative of partial ductile fracture. In contrast, the T6-treated samples exhibited a reduced number of dimples containing fine precipitate particles, suggesting a more brittle fracture behaviour. The T73 specimens, however, displayed a higher density of larger dimples with coarser precipitates, consistent with the improved ductility resulting from the overaged microstructure. These findings demonstrate that the mechanical performance of FSPAM-processed aluminium alloys can be effectively tailored through the selection of appropriate post-heat treatment (T6 or T73), depending on the targeted application requirements. © 2026