Microstructure formation during gas flow-assisted additive manufacturing of a metallic glass powder on ground and in microgravity

We studied bulk metallic glasses produced from gas flow-assisted laser-based powder bed fusion process, which is capable of additive manufacturing metallic parts in microgravity. A Zr-based bulk metallic glass composition ZrCuAlNb has been processed on ground and in microgravity in a compact sounding rocket payload MARS-M. Microstructure characterization was performed using electron microscopy and X-ray diffraction computed tomography, which cope with small amounts of sample materials, especially for those fabricated under microgravity conditions. Very similar microstructures and crystalline fractions are observed in sample manufactured on ground and in microgravity, which shows that process parameters of conventional laser powder bed fusion for manufacturing metallic glasses can be transferred to the processes in microgravity. Two different origins of crystallization have been identified in the ZrCuAlNb sample. The preferred occurrence of CuZr at the interlayer boundaries is likely a result of recrystallization from the undercooled melt and hence associated with laser scanning strategy. In contrast, the more uniformly distributed AlZr phase is considered to be triggered by the formation of CuZrO. Thus, for the fabrication of fully amorphous builds both on ground and in space, our findings point to higher scanning speeds and lower oxygen contents, while the latter can also be used to tune the crystalline fractions in the sample.