Effect of direct aging temperature on the microstructure and hardness of multi-pass friction stir processed additive manufactured AlSi10Mg aluminium alloy

Multi-pass friction stir processing was applied to homogenize laser-powder-bed fusioned AlSi10Mg alloy, followed by direct aging at 180 °C and 400 °C. The evolution of Silicon morphology, grain structure, and hardness was systematically evaluated in the base metal, stir zone, and thermomechanically affected zone. Friction stir processing fragmented the continuous eutectic Silicon network into fine, dispersed particles and refined the grain size, particularly in the stir zone, where the average grain size decreased to 2.9 μm compared with 7.9 μm in the base metal. Direct aging at 180 °C promoted the formation of fine Silicon precipitates while preserving this refined distribution. Transmission electron microscopy (TEM) further revealed very fine precipitates at 180 °C within the stir zone confirming a strong precipitation hardening effect. This condition improved hardness from 101 to 115 HV in the base metal and from 64 to 86 HV in the stir zone. In contrast, aging at 400 °C caused silicon coarsening and grain growth, with the base metal grain size increasing to 9.7 μm and hardness dropping to 66 HV. TEM confirmed that precipitates in this condition were coarser and more widely spaced, correlating with the reduced hardness. Similarly, the stir zone hardness decreased to 53 HV, although the profile became more uniform due to microstructural homogenization. Overall, the results demonstrate that aging at 180 °C, unlike 400 °C, effectively integrates recrystallization-driven refinement with precipitation hardening, providing a robust pathway for tuning microstructure and mechanical properties in AlSi10Mg alloy.