Microstructure and mechanical properties of unalloyed molybdenum fabricated via wire arc additive manufacturing

Molybdenum is an important high-temperature structural material but has poor processability. Additive manufactured unalloyed Mo is generally very small and the mechanical properties is seldomly studied. In this work, wire arc additive manufacturing was adopted, and crack-free molybdenum parts with high-density (99.0%) and characteristic size of 20 mm×20 mm×120 mm were successfully fabricated by short-track scanning. The microstructure and mechanical properties of samples in both the as-deposited and heat-treated states were studied and compared. Large columnar grains were observed, which were basically along the 〈001〉 direction. Heat treatment leads to grain coarsening, and the elimination of some sub-grain boundaries. Due to the weakened effect of grain and sub-grain boundary hardening, the mechanical properties of heat-treated specimens were worse than that of as-deposited specimens at room temperature. Both of them exhibit brittle fracture features. Under high temperature, the ductile fracture is observed, and the as-deposited specimen has similar strength and ductility, compared with the heat-treated specimens, suggesting a weak role of grain and sub-grain boundary at high temperature. A large number of fragments were observed at the fracture surface after high-temperature tests, which was MoO₃ by energy dispersive spectroscopy test.