abstract

Additive manufacturing (AM) of NiTi has been mainly studied with the laser powder bed fusion (LPBF) technique. At the early stage of these studies, the printability and processing of NiTi were investigated to produce dense parts. Building on the results of the printability map, the involved process parameters (PPs) have been optimized to improve the properties. While low volumetric energy density 𝐸!, –the combined effect of laser power, scanning speed, hatch distance, and layer thickness– results in parts with a lack of fusion and formation of cracks, high 𝐸! leads to element evaporation and unintended porosity. In a slightly Ni-rich NiTi, low laser power results in higher strain recovery and lower mechanical hysteresis. At 100 W and a scanning speed of 1250 mm/s it is possible to arrive at perfect superelasticity without heat treatments.

Other PPs, including scanning strategy and build direction, have shown an impact on the microstructure, texture, and thermomechanical response of the part. Build orientation can affect the texture. With a 90° orientation, the preferred texture is <001>. By altering the orientation to 45°, the texture changes to <101>. As a result, the recoverable strain in torsion varies from 3.7% to 4.9% for angles of 0, 35, 45, 64, and 90 degrees. A concentric scan strategy strategy increases residual stress and warping, whereas bidirectional strategies significantly lower residual stress. With the goal of defect-free parts with desired microstructure, post-processing such as HIP and heat treatment are used to decrease pores and cracks and precipitate harden, respectively.

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Recently, AM of NiTi has also been conducted in Direct Energy Deposition (DED) and Binder Jetting (BJ). In laser wire DED, laser power and scanning speed in the range of 400-1000 W and 300-900 mm/min, respectively, were evaluated to obtain optimized PP to print defect-free parts. The quality was assessed by lack of fusion, wire stubbing and dripping, melt pool characteristics, and hardness measurements.

BJ, a more cost-effective method, holds the advantage of eliminating direct exposure to high temperatures and consequently reducing cracks, residual stress, and warping. Optimization of sintering of the green part has been studied by investigating the effect of chamber atmosphere (argon/vacuum) and sintering temperature (1250 – 1350 °C) for different binder saturation levels on density and dimensional accuracy. Vacuum sintering at higher temperatures increases the density without significant change in dimension. Sintering in the argon atmosphere resulted in lower carbon and oxygen levels in the parts.

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