Additive manufacturing of pure niobium by laser powder bed fusion: Microstructure, mechanical behavior and oxygen assisted embrittlement

In this study, pure niobium (Nb) was additive manufactured by laser powder bed fusion (LPBF). The effect of energy density and oxygen content on the densification behavior, microstructure and mechanical properties of the LPBF-built Nb were investigated. It was demonstrated that the density steadily increased and the pores were gradually eliminated with the increase of energy density. A maximum density of 99.7% was achieved at the energy density of 146 J/mm3. Further increase of energy density led to an intense Marangoni convection and a slightly decrease of density. The average grain size of the LPBF-built Nb was 70 mu m approximately. A large fraction of low-angle grain boundaries and a high density of dislocations were observed in the LPBF-built Nb. The ultimate tensile strength (UTS) of the LPBF-built Nb increased steadily with the increase of energy density due to the increment of density. The fine grains and high-density dislocations resulted in high strength of the LPBF-built Nb. The maximum yield strength and UTS were as high as 550 MPa and 630 MPa respectively with a consid-erable elongation of 26%. Oxygen deteriorated the mechanical properties of the LPBF-built Nb significantly. The increase of oxygen enhanced the tensile strength of Nb and reduced its plasticity significantly. The LPBF-built Nb became brittle when the oxygen content increased from 0.05 wt % to 0.23 wt %. The elongation to fracture of the LPBF-built Nb decreased to less than 2% regardless of the input energy density. It was proposed that the interstitial oxygen atoms exert a repulsive force on the dislocation and block its movement, leading to a sharp decrease in the plasticity of LPBF-built Nb.