Heterogeneous microstructure development in additive friction-stir deposited Al-Mg-Si alloy

Aluminum alloy 6061-T6511 was printed by additive friction stir deposition onto a cast aluminum A206-T4 plate. Metallography, X-ray powder diffraction, X-ray pole figure analysis, and electron backscatter diffraction of the center, advancing side, and retreating side of the 6061 deposit reveals different texture and grain structure characteristics across the transverse direction and through each layer. These distinct microstructural domains are indicative of spatially varied temperature, strain rate, and strain accumulation during deposition, and consequently of regions differing in the fraction of dynamic recovery and dynamic recrystallization during deposition. Hardness mapping over the complete cross section at 0.75 mm resolution shows the alloy continues to age after active deposition, and hardness increases over the 5 mm nearest the final printed layer. Additional mapping at 0.10 mm resolution shows intralayer hardness variations of the deposit on the retreating side. These observations demonstrate that complex and non-uniform thermal-mechanical transients occur during additive friction stir deposition, resulting in spatially non-uniform microstructure and properties.