Defect elimination and microstructure improvement of laser powder bed fusion β-solidifying γ-TiAl alloys via circular beam oscillation technology

To eliminate metallurgical defects and improve microstructures in laser powder bed fusion (LPBF) beta-solidifying gamma-TiAl alloys, circular beam oscillation technology was used to fabricate a Ti-43Al-9V-0.5Y alloy in this work. The metallurgical defects and microstructures under conventional linear (non-oscillating) scanning and circular oscillating scanning are first studied in comparison. Then, the microstructural evolution and defect suppression mechanisms under circular oscillating scanning are revealed based on the melt flow behavior. In contrast to the non-oscillating scanning, circular oscillating scanning can better eliminate the pores and cracks of LPBFed Ti-43Al-9V-0.5Y alloy. As the oscillating velocity is decreased, the crack density decreases and the micro-structure changes from columnar to equiaxed grains. The crack-free samples with a relative density of 99.98% and fine equiaxed grains are obtained as the oscillating velocity is less than or equal to 100 mm/s. The micro-hardness (473-581 Hv) of the non-oscillating samples is higher than that (440-487 Hv) of the oscillating samples. The room-temperature compressive strengths (similar to 1222 MPa and similar to 1931 MPa) and tensile strength (similar to 253 MPa) of the crack-free oscillating LPBFed samples are superior to those of the as-cast counterparts. The suppression of pores is ascribed to the improved keyhole stability, enhanced melt convection as well as escape of bubbles under circular beam oscillation. When the oscillating velocity is less than or equal to 100 mm/s, the sufficient stirring effect of the laser beam promotes the formation of fine equiaxed grains and the reduction of brittle B-2 phase content, which effectively inhibits cracking.