EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Shape memory alloys (SMA) are used in a wide range of applications where complex or customized geometries are required, as actors and sensors in aviation and aerospace or personalized medical devices. However, conventional shaping technologies are difficult to apply, mainly because of the high work hardening, strength, toughness and ductility of NiTi shape memory alloys. Hence, there is an increasing demand for the fabrication of complex NiTi structures by alternative shaping technologies like Additive Manufacturing (AM). However, as the functional properties of NiTi SMAs are very sensitive to compositional and microstructural changes, it is essential to understand the interrelation of the shaping process, the microstructure and the material properties. It was found in previous work that Additive Manufacturing by Laser Powder Bed Fusion (LPBF) is capable of producing complex NiTi parts, but the unique microstructure created by laser melting and rapid cooling is causing variations of the composition, formation of precipitations and dislocations, and strong texture and residual stresses, making it very challenging to achieve controlled and homogeneous functional properties.

Hence, in this work the sinter-based AM technology Metal Binder Jetting (MBJ) is investigated as an alternative processing route. MBJ is expected to provide a more homogeneous microstructure and less internal stresses compared to LPBF, but the printing process and sintering parameters have to be carefully controlled to achieve high density and avoid the formation of precipitations. Furthermore, the organic binder components added during printing can lead to severe uptake of oxygen and carbon to the material, so optimizing the binder composition and debinding conditions is another key aspect in successfully applying MBJ to NiTi SMAs. Results on the influence of different starting powders, printing parameters, debinding and sintering conditions on the density, microstructure, impurity content and functional properties will be presented.