Additive manufacturing of thin structures in NiTi shape memory alloy by laser metal wire deposition and laser powder bed fusion: martensitic transformation and microstructures

Additive manufacturing (AM) of NiTi shape memory alloys has been a challenging topic for developing advanced smart elements for sensors and actuator as well as biomedical devices, such as stents. The laser melting of different feed stocks, such as powder and wire, has an important rule on inducing tailored microstructure and functional performances, due to different cooling rates. In this work Nitinol characteristics induced by using two laser based AM techniques, namely laser metal wire deposition and selective laser melting, were investigated. The evolution of the martensitic transformation was studied from the initial feed stock, wire and powder, to the AMed samples, in as built and heat treated condition, via differential scanning calorimetry and X-Rays diffraction in temperature. The microstructure and chemical composition were investigated through optical and scanning electron microscopy coupled with energy dispersive spectroscopy. Aspect ratio and size of the solidified liquid pools are strongly associated to the different AM techniques and feed stock materials, while the rule of post- heat treatments is fundamental for fixing the functional properties of the NiTi built parts.