Heat treatment of NiTi alloys: Influence of volumetric energy density on ageing parameters and the resulting physical properties

This study comprehensively examines the effects of ageing temperatures (400, 500, and 600 ᵒC); age-dwelling times (30, 60, and 90 min) and the role of volumetric energy densities (VEDH) of initial processing, on the physical properties of additively manufactured (AM) NiTi samples. Response Surface Modelling (RSM) and curve fitting analysis were employed to establish correlations between the parameters. The samples were processed at different VEDH of 40, 80, and 120 J/mm3 using NiTi (50 at. %) powders via the AM technique of powder bed fusion with a laser beam source (PBF-LB). The melt pool thermal radiation was in-situ monitored using infrared (IR) pyrometers and the collated data post-analysed. Physical properties investigated include microstructure, relative density, composition and phase characteristics, thermal gradient, melt layer temperatures, transformation temperatures, nanohardness and Young's modulus. Mean melt layer temperatures (normalised) increased with VEDH resulting; in all as-built samples being Ti-rich NiTi (>50 at. % Ti), the co-existence of NiTi2, martensite and residual austenite phases, while density was highest in samples processed at VEDH of 80 J/mm3. Ageing at higher temperatures >400 ᵒC for longer dwell times led to recrystallisation and crystallographic re-orientation, but at different degrees depending on the initial thermal history. This recrystallisation effect and initial densities had compounding effects on the observed trends of other examined properties in the aged samples. These findings reveal for the first time, the significant importance of the VEDH of initial processing compared to the ageing temperatures and dwell times.