Comprehensive study of microstructure, phase transformations, and mechanical properties of nitinol alloys made of shape memory and superelastic wires and a novel approach to manufacture Belleville spring using wire arc additive manufacturing

Abstract

In the present work, using wire arc additive manufacturing (WAAM), a novel approach is implemented to manufacture the Belleville spring/washer made from superelastic (SE) and shape memory (SM) Nitinol (NiTi) wires. A detailed correlation between the chemical composition, microstructure, and martensite transformation of deposited layers and their mechanical properties has been established. The energy dispersive spectroscopy (EDS) results revealed that the first track deposited using both SE and SM effect wire consisted of significant Fe constituents, which was further reduced while increasing the number of tracks. Moreover, as compared to the sample manufactured with SE effect NiTi wire (S1), the X-ray diffraction (XRD) of the sample manufactured with SM effect NiTi wire (S2) showed a secondary phase like NiTi2 along with the major NiTi (B19′ martensitic). It has been found that the presence of Ni4Ti3 precipitate in the microstructure of SE causes an enhanced superelasticity desired for the Belleville spring applications. Furthermore, the mechanical properties, microstructure, and phase transformation temperature of both S1 and S2 were investigated and compared. It was found that S2 has a finer microstructure than S1. Also, the compressive strength and hardness of S2 were higher than that of S1. In contrast, the tensile strength of the S2 was found to be lower than the S1. Regarding manufacturing the Belleville spring using WAAM, S1 showed promising mechanical and martensitic transformation results.