Effect of Elementally Blended Ni-Ti and Pre-alloyed Nitinol Powders on the Formation of Solidification Tracks by Laser Bed Powder Fusion (L-PBF)

Powder characteristics such as flow energy, bulk density, compressibility, morphology, particle size distribution (PSD) and laser energy absorbtivity have a significant impact on the Laser Bed Powder Fusion (L-PBF) process. The impact of powder characteristics on in-situ nickel-titanium alloy formation within the L-PBF process is not well understood. In this work, the characteristics of nickel, titanium and various elemental blends of nickel-titanium powder are compared with those of a pre-alloyed nitinol powder. L-PBF solidification tracks are generated from selected powders and characterised dimensionally and for chemical homogeneity. This study demonstrates that the melt homogenisation time and the size of titanium particles relative to nickel particles are critical factors for successful in-situ alloying of nitinol within the L-PBF process. The variation in elemental composition within solidification track cross sections, as measured by EDX point spectra, increases significantly for laser scanning speeds of 400mm/s and above. Furthermore, the rate of increase of variation in elemental composition with increasing laser scan speed is significantly more pronounced for a nickel-titanium powder blend of spherical particles with average titanium particle size of 48 µm compared to an otherwise similar blend with an average titanium particle size of 32 µm. There is no significant difference between solidification track widths of in-situ alloyed blends compared to pre-alloyed blends, indicating that optimal hatch spacing for in-situ alloying is likely to be similar to that of pre-alloyed powders.