Effect of energy density on the superelastic property of Ni-rich NiTi alloy fabricated by laser powder bed fusion

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2022-09, Vol.854, p.143874, Article 143874
• Main Authors: Shen, F.L., Li, H.Q., Guo, H., Guo, N.N., Fang, X.Y.
• Format: Article
• Language: English
• Subjects: Alloying elements; Chemical composition; Chemical precipitation; Energy density; Intermetallic compounds; Laser powder bed fusion; Lasers; Martensite; Martensitic transformations; Mechanical analysis; Melt pools; Melting; Microstructure; Ni-rich NiTi alloy; Nickel base alloys; Nickel compounds; Nickel titanides; Parameter modification; Powder beds; Process parameters; Shape memory alloys; Superelasticity; Titanium compounds; Transformation temperature
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2022.143874

Laser powder bed fusion (LPBF) has proved to be a promising process for tuning the microstructure and mechanical response in NiTi-shaped memory alloys using a varied energy density input by modifying the process parameters such as laser power or scanning speed. The microstructure, precipitation, chemical composition, and their relationship with austenite–martensite transformation temperatures (TTs) and superelastic properties have been extensively investigated in Ni-rich NiTi alloys processed at varying volume energy densities (52–110 J/mm3) through LPBF. A specific amount of Ni3Ti precipitation was observed and primarily located along the boundary and at the center of the melt pool, and nanoscale spherical NiTi2/Ni2Ti4OX precipitation was homogeneously dispersed in the B2 matrix and B19’ martensite, but no Ni4Ti3 was observed as frequently reported in the literature. Decreasing the volume energy density (VED) led to lower TTs and a higher recovery strain ratio. However, there is an exception for the sample built with the highest laser power of 250 W that possesses the largest Ni/Ti atomic ratio and therefore the highest superelastic property despite the mediated VED (∼87 J/mm3) employed. We concluded that the variation in TTs and superelastic properties among the samples is caused by the varying amounts of Ni, which causes competition among alloying elements in evaporation, condensation, and precipitation during LPBF. •Lower energy density reduces TTs except for high laser power of 250 W.•Ni3Ti occurs along the boundary and center of the melt pool.•Nanoscale NiTi2/Ni2Ti4OX is dispersed in B2 and B19′ phases.•The high superelastic property corresponds to low TTs.•TTs depends on net effect of element evaporation, condensation and precipitation.