Progress from 3D to 4D printing of Nickel-titanium (NiTi) alloys for engineering applications: a review

Abstract Additive manufacturing (AM) has progressed from three-dimensional (3D) printing to four-dimensional (4D) printing, which incorporates the time dimension to enable adaptive responses to external stimuli. Nickel–titanium (NiTi) alloys, a shape memory alloy (SMA) distinguished by their shape memory effect (SME) and superelasticity, have emerged as key materials for 4D printing. This review examines AM techniques for NiTi alloys, focusing on Laser Directed Energy Deposition (LDED), Selective Laser Melting (SLM), and Electron Beam Melting (EBM). Each method is analyzed in terms of process parameters, manufacturing challenges, and medical and aerospace engineering applications. The discussion highlights that the thermomechanical and microstructural characteristics of NiTi require precise process control to ensure optimal functional performance. Demonstrated applications in medical implants and lightweight aerospace components underscore the potential of NiTi-based 4D printing for intelligent, adaptive devices. The remaining challenges include accurate prediction of final shapes, ensuring phase homogeneity, and the need for specialized design software. Overall, this study provides insights into the integration of NiTi alloys with 4D printing and identifies key directions for advancing next-generation AM systems. This review presents a unique process–structure–property–application relationship for NiTi-based 4D printing, demonstrating that control of energy input (150–800 W) and scan rate (>1000 mm s −1 ) directly influences transformation temperature (Af 10 °C–30 °C) and SME strain recovery (3%–6%).