Laser Powder Bed Fusion of Superelastic Ti-Ni Lattice Structures: Process Design and Testing

Abstract

Laser powder bed fusion allows the production of complex geometries and eases the shaping of difficult-to-transform materials, such as near-equiatomic Ti-Ni shape memory alloys. In this study, a numerical model was used to select 11 sets of printing parameters with different volumetric energy densities (VEDs) and build rates (BRs) to produce bulk Ti-50.26at%Ni alloy specimens. The manufactured specimens were studied in terms of their structural integrity, printed density, chemical composition, transformation temperatures, and crystalline phases. At high VEDs and low BRs, a significant decrease in the nickel content was observed. VED = 90 J/mm3 and BR = 10 cm3/h yielded a printed density of 99.94% and an austenite finish temperature of Af = 26.3 °C. The same printing conditions were used to produce 60% porous diamond and gyroid lattice structures. After heat treatment at 500 °C for 30 min, the diamond lattices manifested larger apparent recovery strains (7 vs. 6%), higher compliance (2.9 vs. 3.4 GPa), and similar yield stresses (~48 MPa) compared to their gyroid equivalents. The numerical model predicted that at an equivalent apparent compression strain of 6%, only a ~2% volume fraction of the diamond lattice material underwent plastic deformation as compared to ~20% for its gyroid equivalent.