Understanding the processability, microstructure, and mechanical properties of molybdenum processed by electron beam powder bed fusion

This work aims to develop and characterize molybdenum (Mo) processed by Electron Beam Powder Bed Fusion (EB-PBF). Two Mo powders with different oxygen levels were used to investigate the processability, obtaining crack-free bulk samples. The process parameter optimization resulted in a residual porosity of around 0.15 %, showing that limiting the oxygen played a key role in reducing the formation of pores. The as-fabricated Mo samples displayed columnar grains with lengths of several millimeters along the building direction composed of numerous subgrain structures created by the thermal stresses of the EB-PBF process. Moreover, the high preheating temperature, as well as the remelting combined with high residual stresses, triggered the formation of a few recrystallized grains. The Mo samples were characterized by a strong <001> crystallographic fiber orientation along the building direction. The microstructure also revealed the formation of a limited quantity of molybdenum oxides along the grain boundaries. Finally, the bending strength, hardness, Young modulus, and coefficient of thermal expansion (CTE) were determined. The Mo processed by EB-PBF presented bending strength superior to the traditional recrystallized Mo, while the Young modulus and CTE were compatible with the traditional processed Mo.