Influence of Laser Spot Size on the Microstructure, Transformation Temperatures, and Ni Content of a Nickel-Rich LPBF Ti-Ni Alloy

This study investigates the influence of laser spot size (Ø100 µm vs. Ø50 µm) on the microstructure, transformation behavior, and hardness of Ti-51.17 at.%Ni shape memory alloy samples fabricated by laser powder bed fusion. Samples were produced using identical processing parameters covering laser powers of 50–150 W, scanning speeds of 500–750 mm/s, and matched volumetric energy densities (28–83 J/mm³), with spot size as the only variable. Although both laser spot sizes produced comparable melt pool geometries and predominantly B2 austenite matrices with minor fractions of B19′ martensite, the Ø50 µm samples exhibited finer microstructures, higher residual stresses, and lower martensitic transformation temperatures in the as-built state. Heat treatment at 800 °C reduced residual stresses and partially homogenized the microstructure; however, differences in their transformation behavior remained. These differences were attributed to the spot-size-dependent Ni evaporation. Although transformation temperatures increased with increasing energy density for both conditions, the Ø50 µm samples consistently showed lower values. In the 55–83 J/mm³ volumetric energy density range, the estimated Ni content ranged from 51.15 to 50.42 at.%, and the Ø50 µm samples showed approximately 0.1–0.2 at.% greater Ni loss than their Ø100 µm counterparts. These findings demonstrate that Ni evaporation is a laser-spot-size-dependent phenomenon that must be considered when processing near-equiatomic Ti-Ni alloys.