Nitinol (NiTi) shape memory alloys fabricated by Laser Powder Bed Fusion (L-PBF) Additive Manufacturing (AM) have attracted much attention in recent years, as compared with conventional manufacturing processes it allows to produce Nitinol parts with high design complexity. Avoidance of defects during L-PBF is crucial for the production of high quality Nitinol parts. In this study, analytical models predicting melt pool dimensions and defect formation criteria were synergistically used to develop processing maps demonstrating boundary conditions for the formation of such defects, as balling, keyhole-induced pores, and lack of fusion. Experimental validation has demonstrated that this method can provide an accurate estimation and guide manufacturability of defect-free Nitinol alloys. Moreover, the crack formation phenomena were experimentally analysed, which showed that a low linear energy density (E_l) should be chosen to avoid cracks in the optimized process windows. Based on model predictions and experimental calibrations, Nitinol samples with a relative density of more than 99% were successfully fabricated.

Nitinol (nickel-titanium or Ni-Ti) is one of the most utilized alloys exhibiting the Shape Memory Effect, which makes it possible to use it in many applications, such as aerospace, automotive, biomedical and others. The development of Additive Manufacturing technologies allows creating the latest time-dependent 4D products from nitinol that can realize the Shape Memory Effect after 3D-printing, which in the future will further increase the use of nitinol. However, processing nitinol is highly sensitive to compositional and thermal changes, affecting the final phase structure and thus temperature of the martensitic transformation. This paper presents a review of various effects of composition and 3D-printing process parameters on changes in transformation characteristics, as well as possible methods for their prevention and post-process heat treatments. It is reported that only by precise process and temperature control it is possible to create 4D products with the ability to realize the multi-stage shape memory effect. Finally, the paper discusses the various application of 3D-printed Nitinol and its advantages as compared to conventional processing routes.