Phase transformations in Ni-rich additively manufactured NiTi alloys

The objective of the research outlined in the article was to evaluate the conventional parameter known as volumetric energy density (VED), which serves as the primary indicator determining the quality of products manufactured using the additive technique with the Selective Laser Sintering (SLS) method. An independent goal was to examine the impact of secondary phase precipitates (Ni4Ti3) on the temperature ranges of phase transformations in nickel-rich NiTi alloys. Design/methodology/approach The assessment of Volumetric Energy Density (VED), was conducted based on the measurement of sample density, analysis of phase transformation sequences determined by scanning differential calorimetry (DSC), and phase composition determined by X-ray diffraction (XRD). VED was calculated based on laser power, scanning speed, hatch spacing, and single-layer thickness in the study on the influence of secondary phase precipitates (Ni4Ti3), formed during the ageing of the samples at 500°C for 20 and 100 hours, on the sequence of phase transformations, as well as the initiation and completion temperatures of martensitic (Ms, Mf) and austenitic (As, Af) transformations, scanning differential calorimetry (DSC) and X-ray diffraction (XRD) were employed. Findings It has been demonstrated that the value of Volumetric Energy Density (VED) is not a decisive parameter for the quality of the result. The primary influences on the quality of samples after the SLS process were found to be the laser energy and hatching space values. By varying the ageing time at a temperature of 500°C for NiTi alloy samples with excess nickel content (51.7 at.%), it is possible to influence the temperature of thermoelastic martensitic transformation effectively. This allows for utilising its unique functional properties for various advanced applications. Research limitations/implications The absolute condition for determining the temperature and post-processing time, which dictates specific phase transformation temperatures in NiTi alloys produced by additive techniques (SLS), is the selection of process parameters. The parameters include laser power and hatch spacing, ensuring an appropriate scanning speed and layer thickness. The factors guarantee certain product properties characterised by specific density and direct austenite-to-martensite transformation occurring during the cooling of samples. Practical implications Influencing the Ni/Ti ratio and microstructure of NiTi alloy through appropriate post-processing allows for the utilisation of its functional features, such as the shape memory effect combined with superelasticity, for advanced applications. Originality/value Based on the conducted research, it has been demonstrated that the quality of the product after the SLS process primarily depends on laser power and hatch spacing. It has also been shown that, for precise tuning of phase transformation temperatures in additively manufactured NiTi alloys, the selection of post-processing parameters is essential and is influenced by the initial composition of the powder used.