Nickel–titanium (NiTi) shape memory alloys are widely studied due to their excellent shape memory effect, superelasticity, and mechanical properties, and show great potential in automotive, biomedical, and functional device fields. However, the mechanical properties and microstructural uniformity of laser additively manufactured NiTi alloys are highly sensitive to process parameters, and defects such as pores, cracks, and inhomogeneous microstructure remain difficult to eliminate. This study aims to systematically investigate the effects of laser power and scanning speed on the microstructure, crystallographic characteristics, and mechanical properties of laser additively manufactured NiTi alloys, and to provide an experimental basis for process optimization.

NiTi alloys were fabricated by laser additive manufacturing under an argon protective atmosphere. Three scanning speeds (500 mm/s, 600 mm/s, 700 mm/s) and three laser powers (70 W, 80 W, 90 W) were set, with scanning spacing and layer thickness kept constant, and nine parameter combinations were designed to screen the optimal process (Table 1). Based on preliminary evaluation of density and tensile strength, three groups of representative samples were selected for in-depth characterization. Density was measured by the Archimedes method. Metallographic microstructures were observed using optical microscopy (Fig.5). Tensile tests were carried out at room temperature, and fracture morphologies were analyzed using scanning electron microscopy (Fig.3). Microhardness distribution on different cross-sections was measured using a Vickers hardness tester (Fig.4). Phase composition was analyzed using X-ray diffraction (Fig.6). Electron backscatter diffraction (EBSD) was used to characterize grain morphology, grain boundary characteristics, crystallographic orientation, and dislocation density distribution in the x–y and x–z planes (Fig.7, Fig.8).

Laser energy density played a decisive role in the densification behavior and mechanical properties of NiTi alloys. As energy density increased, the density and tensile strength of the samples first increased and then decreased, indicating the existence of an optimal processing window (Fig.2). Low energy density resulted in insufficient melting, poor interlayer bonding, increased porosity, and decreased strength. Excessively high energy density induced metal evaporation, oxidation, and defect formation, reducing densification and mechanical properties.

Metallographic observation showed that the microstructure of the samples mainly consisted of needle-like grains formed by rapid solidification, accompanied by unevenly distributed black regions (Fig.5), which were related to oxide inclusions, micropores, or segregation. The sample prepared at 600 mm/s and 90 W exhibited a more uniform microstructure with fewer defects, corresponding to better mechanical properties.

EBSD analysis indicated that the highest-strength sample (600 mm/s, 90 W) exhibited typical microstructural features within the melt pool (Fig.7). Equiaxed grains were mainly distributed at the melt pool boundaries, while the center presented a mixed structure of equiaxed and columnar grains. This distribution was closely related to the temperature gradient and solidification conditions during laser processing. The sample contained a high proportion of high-angle grain boundaries and a relatively high density of geometrically necessary dislocations. Crystallographic orientation analysis revealed that the x–y plane had a 101 preferred orientation, and the x–z plane was dominated by 001 (Fig.7, Fig.8). The overall texture intensity was weak, indicating low anisotropy.

Tensile tests showed that the sample fabricated at 600 mm/s and 90 W had the highest tensile strength, approximately 405 MPa (Fig.3). The fracture surface presented a mixed ductile–brittle fracture feature, with fatigue striations, microcracks, and unmelted particles being observed, showing that cracks originated from micro-defects, while the uniform grain structure effectively inhibited crack propagation. The sample with optimized parameters showed a more uniform hardness distribution, while the 700 mm/s sample exhibited the most significant hardness fluctuation (Fig.4).

This study systematically investigates the effects of laser additive manufacturing process parameters on the microstructure and mechanical properties of NiTi shape memory alloys. The results show that a scanning speed of 600 mm/s and a laser power of 90 W achieve an optimal match between energy input and cooling rate, resulting in increased densification, uniform microstructure, suitable crystallographic characteristics, and improved tensile strength and hardness uniformity. The equiaxed grains at melt pool boundaries, the mixed grain structure in the center, and the high dislocation density are key factors for the strengthening of the alloy. This study provides experimental and theoretical support for the process optimization of NiTi alloys by laser additive manufacturing, and helps promote their application in high-performance functional components.