Abstract
Metal additive manufacturing processes often use gas-atomized powder as feedstock, but these processes use different methods for consolidation. Depending on the consolidation temperature, secondary phases may be retained during processing, making it important to understand powder microstructure prior to consolidation. Commercial alloy compositions are typically used for these powders because they have been widely studied and qualified; however, the microstructure of the powder form of these compositions has not been studied. This paper aims to understand the commercial Al 6061 powder: how the internal microstructure of the powder differs from wrought both in the as-manufactured and thermally-treated conditions. A specific focus is put on the Mg-rich phases and their morphologies. This was accomplished through transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray spectroscopy. Both the size and morphology of the phases in the powder differ greatly from those in the wrought form.
Highlights
Commercial alloy compositions are often used to create powders that are used for feedstock for many metal additive manufacturing (AM) processes
It is expected that the phases present in the as-manufactured powder microstructure would more closely match that predicted by the Scheil solidification diagram (Fig. 2b)
This demonstrates the predicted phases in the as-manufactured powder for rapid solidification (Scheil results), the phases expected if the initial powder were at equilibrium, and the phases expected after thermal treatment of the powder
Summary
Commercial alloy compositions are often used to create powders that are used for feedstock for many metal additive manufacturing (AM) processes. Additional material qualification for a new alloy can be avoided, making qualification for use more feasible. While these compositions have been widely studied in the wrought or cast form, limited work has been carried out to analyze the microstructure of the powder form of these alloys. During the gas-atomization process, liquid droplets undergo rapid solidification, experiencing cooling rates on the order of 104–105°C/s.1 This cooling rate is radically different that those experienced by similar alloys in a casting process, which are on the order of 10À1–102°C/s, leading to different microstructures and resultant properties in powders as compared to their cast or wrought counterparts.[1] During the gas-atomization process, liquid droplets undergo rapid solidification, experiencing cooling rates on the order of 104–105°C/s.1 This cooling rate is radically different that those experienced by similar alloys in a casting process, which are on the order of 10À1–102°C/s, leading to different microstructures and resultant properties in powders as compared to their cast or wrought counterparts.[1]
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