Surface states of gas-atomized Al 6061 powders – Effects of heat treatment

Surface oxides formed on powder feedstocks used for cold spray deposition can play an important role in the bonding of the particles and in the development of defects in the deposit. A combination of scanning transmission electron microscopy and x-ray photoelectron spectroscopy was used to investigate the oxides formed on gas-atomized Al 6061 alloy feedstock powders. The powders were studied in the as-atomized condition and after two different thermal exposures that correspond to typical feedstock pre-treatment conditions. The surface features and internal microstructures are consistent with those reported previously for these powders. The as-atomized powders have 5.2 nm thick amorphous oxide layers, with an outer Mg-rich sub-layer and an inner Mg-lean sub-layer. Powders heat-treated at 230 °C in air exhibit slightly thicker oxide layers with a crystalline MgAl2O4 spinel outer sub-layer and an amorphous aluminum oxide inner sub-layer. Powders homogenized under Ar at 400 and 530 °C have significantly thicker (8.9 nm) oxide layers with evidence for a defect inverse spinel Al(Mg,Al)2O4 inner sub-layer between the MgAl2O4 spinel outer sub-layer and the alloy. Differences between these observations and those reported previously for oxidation of bulk alloys are explained on the basis of Mg surface segregation during the gas atomization process.