Resultant physical properties of as-built nitinol processed at specific volumetric energy densities and correlation with in-situ melt pool temperatures

Abstract

In this study, direct comparisons between the properties of three groups of as-built nitinol (NiTi) samples fabricated using three pre-determined volumetric energy densities (VED) as calculated based on hatch spacings (VEDH = 40, 80, and 120 J/mm3), were examined. The additive manufacturing technique adopted is the powder bed fusion using a laser beam (PBF-LB), and the powder feedstock is Ni50Ti50. Although the three VEDH's increased by 100% for each sample group, each VEDH value was within the range of those recommended in the literature, as well as equivalent to 100 J/mm3 (VEDf), VED as calculated based on laser spot size. Significant differences were found between the thermal profiles and normalised thermal gradients, resulting in compositional differences, defects, phase, and phase transformation behaviour, Young's modulus, hardness, and microstructure. Thermal gradients and full-wave amplitude of the in-situ melt pool thermal radiation waveforms increased as VEDH increased. Higher VEDH's resulted in more martensitic NiTi samples with larger melt pool volume and >98% sample relative density. Austenite transformation temperatures increased with an increase in at.% Ti for each increase in VEDH, consequent on the higher normalised average melt pool temperatures. The calculated thermal gradient (normalised) was correlated to higher thermal stresses as VEDH increased, resulting in residual stress-induced weld-metal liquation microcracks. This work has demonstrated the potential of post-processed in-situ melt pool thermal radiation data, in providing insights into the physical property variations of as-built nitinol samples fabricated via PBF-LB.

Keywords

• Volumetric energy density
• NiTi alloys
• In-situ monitoring
• Residual stresses
• Laser powder bed fusion
• Liquation cracking