Tunable phase transformation temperature and excellent superelasticity in as-printed NiTiCu shape memory alloys fabricated by the laser powder bed fusion

Abstract

Shape memory alloys (SMAs) fabricated via additive manufacturing (AM) not only provide a feasible way to build components with complex shapes but also pioneer 4D printing applications due to their unique functional properties. However, the evolution of martensitic transition behavior and superelasticity in NiTiCu SMAs, especially in relation to process parameters and their underlying mechanisms, remains unclear. In this work, a novel Ni46Ti49Cu5 SMA fabricated via laser powder bed fusion (LPBF) exhibit outstanding functionality with a tunable phase transformation temperature without post-heat treatment. By systematically varying laser power (75–250 W), scanning speed (500–1500 mm/s), and hatch spacing (40–110 μm), we identified a distinct input energy density window (62.5–78.13 J/mm³) that achieves high relative density while eliminating cracks and minimizing porosity. Microstructural analysis reveals columnar grains formed by epitaxial growth along the maximum thermal gradient, resulting in a pronounced < 001 > crystallographic texture on the plane perpendicular to the build direction. The phase transformation temperatures change nearly monotonically with input energy density, primarily due to evaporation of Ni and Cu during processing, which alters the equivalent (Ni+Cu) content of the matrix. The input energy density also strongly influences the superelastic response by controlling porosity. The alloy fabricated with an energy density of 69.44 J/mm3 yields exceptional superelasticity, with a recovery ratio of 95.33 % and a recoverable strain of 5.72 %. Notably, the phase transformation behavior of the Ni46Ti49Cu5 SMA can be precisely tailored by adjusting processing parameter combinations while maintaining a constant energy density. These findings establish a framework for fabricating NiTi-based SMAs with superior superelasticity and tunable phase transformation temperatures via LPBF, enabling their potential application in advanced smart materials.