Progress in laser directed energy deposition of NiTi alloys: Microstructure, mechanical and functional properties, and future perspectives

Nickel‑titanium (NiTi) shape memory alloys (SMAs) combine the shape memory effect (SME), pseudoelasticity (PE), high damping capacity, and biocompatibility, yet their conventional processing is constrained by extreme compositional sensitivity, interstitial contamination, and poor machinability. Laser directed energy deposition (L-DED) enables near-net-shape fabrication, repair, and in-situ alloying of NiTi, but its steep thermal gradients and cyclic reheating couple the processing history directly to the functional response. The present review consolidates L-DED-specific evidence within a process-structure-property framework and identifies the mechanisms behind the reported trends. Preferential Ni evaporation, governed by energy input and molten-pool residence time, is the dominant source of transformation-temperature drift, because a deviation of about 0.1 at.% Ni shifts the martensite start temperature by roughly 10 °C to 15 °C. Feedstock selection sets the secondary-phase landscape: pre-alloyed powders yield more homogeneous B2/B19′ matrices, whereas pre-mixed elemental powders enable low-cost compositional tuning but promote Ti 2 Ni, Ni 3 Ti, and Ni 4 Ti 3 through incomplete mixing and local Ni depletion or enrichment. Ultrasonic vibration assistance, in-situ laser reheating, and solution-aging treatments homogenize the chemistry and control Ni 4 Ti 3 precipitation, which governs the transformation sequence, hardness, recoverable strain, and cyclic stability. With appropriate precipitate control, L-DED NiTi achieves full compressive pseudoelastic recovery, stable SME after short training, high damping, and fatigue-resistant elastocaloric performance. Closed-loop composition control, quantitative residual-stress data, tensile functionality, and standardized qualification protocols remain the principal barriers, and research directions for multi-material, porous, and sustainable L-DED NiTi components are outlined.