Effect of Nb content on microstructural evolution, mechanical and tribological properties of in situ alloyed copper-modified titanium produced using laser powder bed fusion

Control of the columnar to equiaxed transition (CET) is a major challenge in additively manufactured β titanium alloys. In this work, the promotion of CET was successfully achieved through in-situ fabrication of Ti-5Cu (wt.%) alloys with additions of 5, 15, and 25 wt.% Nb using elemental Ti, Cu, and Nb powders by employing laser powder bed fusion (LPBF). The alloy containing 5 wt.% Nb consisted of α lamellae, Ti₂Cu precipitates, and unmelted β-Nb inclusions, whereas the 25 wt.% Nb alloy consisted of equiaxed β grains, ω precipitates, and Ti₂Cu precipitates at the grain boundaries. In terms of mechanical properties, despite the presence of Nb inclusions and liquation cracks in the 5 wt.% Nb alloy, it showed a yield strength of 1051 ± 40 MPa and an elongation of 5.2% ± 1.3%. Both the strength and ductility decreased with increasing Nb content, e.g., the 25 wt.% Nb alloy exhibited a yield strength of 808 ± 53 MPa and an elongation of 1.6% ± 0.2%. As the Nb content increased from 5 to 25 wt.%, the Young's modulus decreased from 110 to 65 GPa. The 25 wt.% Nb alloy showed a high ratio of hardness to Young's modulus (H/E) and yield pressure (H³/E²). However, due to its brittle nature, the material manifested high wear rates. These findings provide a basis for the future development of novel low-modulus isotropic β-titanium alloys using LPBF.