Sinter-Based Additive Manufacturing of Ni-Ti Shape Memory Alloy

Additive manufacturing (AM) of Shape Memory Alloys (SMA) is an emerging technology that can open the route for numerous new applications in the fields of actuation, sensing, energy harvesting, and heat management. Currently, most AM processes of SMA rely on melting-based methods that locally melt the metallic feedstock. However, the repeated melting impairs the resulting microstructure, thus limiting the ability of the printed material to undergo a reversible thermo-elastic martensitic transformation. Recent advances in sinter-based AM have the potential to facilitate the control over the final microstructure and properties of the printed SMA. Here, we present the production and in-depth characterization of Ni-Ti SMA produced using two sinter-based AM methods: 1. Lithography-based Metal Manufacturing (LMM), and 2. Moldjet, a unique modification of conventional metal injection molding. Ni-Ti produced via LMM exhibits a recoverable strain of up to 2.3% under compression, while the amount of irrecoverable plastic strain is smaller than 0.05% up to a stress of 800 MPa. The high strength with moderate recoverable strain is attributed to the carbon content that promotes the formation of Ti-carbides during high temperature sintering. Higher recoverable strains of up to 5% in compression are obtained for Ni-Ti produced via the Moldjet process, with irrecoverable strains smaller than 0.07%. These high recoverable strains are attributed mainly to the lower carbon content that is inherent to the Moldjet process. Our results indicate that both methods are feasible and promising to produce net-shaped Ni-Ti SMA and encourage future studies of other sinter-based AM processes.