• Chmielewska, Agnieszka
• Wysocki, Bartlomiej
• Buhagiar, Joseph
• Gloc, Michał
• Michalski, Bartosz
• Swieszkowski, Wojciech
• Adamczyk-Cieślak, Bogusława

Abstract

NiTi alloys are widely used in different industrial and medical applications. Due to the inherent difficulty in themachining of these alloys, the use of Additive Manufacturing (AM) methods has become a popular method fortheir production. When working with NiTi alloys, there is a requirement on the precise control of their chemicalcomposition, as this determines the phase transition temperatures which are responsible for their shape memoryor superelastic behaviour. The high energies used in AM to melt the NiTi alloy leads to nickel evaporation,resulting in a chemical change between the batch powder and the additively manufactured part. Therefore, inAM techniques applied to different NiTi alloys, understanding the relationship between the melting strategy andnickel evaporation is crucial during the developing the desired chemical composition of the final-fabricatedmaterial. In this study, three NiTi alloys were fabricated using laser powder bed fusion (LPBF) starting fromelementally blended Ni and Ti powders. Different melting strategies, including single and multiple melting, werestudied in this work. Remelting improved the density and reduced cracking of the AM part. Microscopic observations,using a Scanning Electron Microscope (SEM) with a Backscattered Electron (BSE) detector, showedthat the chemical homogeneity of the materials was enhanced by multiple remelting. Pure Ni and Ti were notfound in any of the samples, proving that the applied melting strategies ensured good alloying of both powders.Regardless of the number of melting runs, X-ray diffraction (XRD) analysis showed the presence of NiTi (B2) and(B19′ ) phases, as well as NiTi2, Ni4Ti3 and Ni3Ti precipitates in all samples. The research demonstrated that,during the AM process, and depending on the melting strategy, 1.6–3.0 wt% of nickel evaporates from thematerial. It was demonstrated that the amount of evaporated nickel increased with the increasing number of meltcycles.

Topics

• density
• impedance spectroscopy
• nickel
• scanning electron microscopy
• x-ray diffraction
• melt
• selective laser melting
• phase transition
• chemical composition
• precipitate
• evaporation