Evolution of Fe-Rich Phases in Thermally Processed Aluminum 6061 Powders for AM Applications

Gas-atomized powders are frequently used in metal additive manufacturing (MAM) processes. During consolidation, certain properties and microstructural features of the feedstock can be retained. Such features include porosity, secondary phases, and oxides. Of particular importance to alloys such as Al 6061, secondary phases found in the feedstock powder can be directly related to those of the final consolidated form, especially for solid-state additive manufacturing. Al 6061 is a heat-treatable alloy that is commonly available in powder form. While heat treatments of 6061 have been widely studied in wrought form, little work has been performed to study the process in powders. This work investigates the evolution of the Fe-containing precipitates in gas-atomized Al 6061 powder through the use of scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDS). The use of coupled EDS and thermodynamic modeling suggests that the as-atomized powders contain Al₁₃Fe₄ at the microstructure boundaries in addition to Mg₂Si. After one hour of thermal treatment at 530 °C, it appears that the dissolution of Mg₂Si and Al₁₃Fe₄ occurs concurrently with the formation of Al₁₅Si₂M₄, as suggested by thermodynamic models.