Improved formability and microstructure of laser powder bed fusion Ti-43Al-4Nb-1Mo-0.1B alloy with circular beam oscillation

To improve the formability and microstructure of laser powder bed fusion (LPBF) Ti-43Al-4Nb-1Mo-0.1B (TNM) alloy, a circular beam oscillation strategy was employed in this study. The metallurgical defects, microstructure, and mechanical properties of the circularly oscillating and non-oscillating (linear) scanning are first compared. Based on the comparative analysis of solidification and cooling processes during linear and oscillating scanning, the formation and inhibition mechanisms of metallurgical defects were revealed. The lack-of-fusion defects and cracks of LPBFed TNM alloy can be eliminated by circularly oscillating scanning, and the relative density of the crack-free specimen is 99.9 % at an oscillating velocity of 100 mm/s. Compared to linear scanning, circularly oscillating scanning promotes an increase in the alpha(2) and gamma phases, a decrease in the B-2 phase, a uniform distribution of elements, and a relatively random texture. The average microhardness under oscillating scanning (479-507 Hv) is lower than that under linear scanning (584-614 Hv). The yield strength, ultimate strength, and ultimate compression strain of the oscillating LPBFed TNM specimens are similar to 1165.65 MPa, similar to 1738.03 MPa, and similar to 13.21 %, respectively. The inhibition of lack-of-fusion defects is attributed to the more sufficient liquid convection and enhanced liquid diffusion under the prolonged laser action time and solidification time as well as the generation of turbulence. The inhibition of cracks is due to the generation of finer equiaxed grains and a decline in the fraction of brittle phases under the enhanced stirring effect, reduced cooling rate, and more uniform temperature distribution.