Study of microstructure evolution and mechanical properties in friction stir based additive multi-layer manufacturing of Al 6061 alloy: Effect of feedstock material form and heat treatment

Abstract

This paper presents study on the effects of heat treatment and feedstock material form (i.e. powder and rod form) on evolution of microstructure, phase, precipitates and porosity and their correlation with mechanical properties (in terms of microhardness, tensile properties, and fracture behavior) for multi-layer deposition of Al 6061 alloy build by friction stir based additive manufacturing process. Influence of feedstock material form on surface topography, geometry, and efficiency of multi-layer deposition of Al 6061 alloy is also presented. The manufactured multi-layer depositions were subjected to T6 heat treatment involving solution treatment and artificial aging. Untreated multi-layer deposition obtained using powder form of Al 6061 alloy has smaller and refined grains, less porosity and hardening precipitates, and better mechanical properties than multi-layer depositions obtained using it in rod form due to dominance of refined grains and strain hardening. Heat-treated multi-layer deposition obtained from rod form of Al 6061 alloy has larger grains, more amount of porosity but better mechanical properties than those obtained using it in the powder form due to dominance of precipitates and localized improvement in metallurgical bonding. Untreated depositions showed ductile mode of tensile fracture whereas heat-treated depositions showed brittle-ductile mixed mode of fracture behavior. Using feedstock of Al 6061 alloy in powder form yielded 40% more deposition efficiency, higher temperature (413 °C) and effective width of deposition, and smaller maximum and average surface roughness (35.3

3.8 µm) than using it in the rod form whereas its use in rod form resulted in higher deposition height in a smaller number of deposition layers. Findings of this study will be helpful in selection of appropriate form of feedstock material and heat treatment of friction stir based additively manufactured multi-layer component to get its better microstructure and preferred properties. © 2022 Elsevier Ltd