Ti modification mechanisms and effects of processing parameters on defect control and equiaxed grain formation in laser powder bed fused Al-Li-Cu alloy

Elements such as Sc, Zr and Ti have been employed for the modification of aluminum alloys to enhance their laser powder bed fusion (L-PBF) formability. However, critical knowledge gaps persist in elucidating the fundamental mechanisms and differential modification behaviors among these elements, while the parametric effects of laser processing on modification efficacy remain insufficiently characterized, necessitating comprehensive mechanistic investigations. In this study, Ti, with a lower density and cost, was incorporated into the Al-Li-Cu alloy to improve the formability. The results indicated that Ti incorporation notably refined the grain structure, eliminated the defects, and granted the alloy excellent mechanical properties with a tensile strength of 516.5 ± 2.7 MPa and an elongation of 6.78 ± 0.80 % obtained after heat treatment. Results indicated that the modification capability of Ti is predominantly constrained by forced undercooling in the melt pool. Al₃Ti particles are activated near the center of the molten pool with the development of undercooling during solidification, contributing to a microstructure with columnar grains at the boundary and equiaxed grains near the center of the molten pool. While the modification capabilities are restricted by the cooling rate for Sc and Zr, as the formation of Al₃Sc and Al₃Zr near the molten pool center is suppressed. The microstructure is controlled by the various sizes of Al₃Ti particles formed under different laser parameters. The size of Al₃Ti particles increased from 30 to 80 nm to about 200–300 nm with elevated laser energy density, which reduces the required activating undercooling for Al₃Ti particles, results in samples with complete fine equiaxed grains.