Comparative review of additive manufacturing techniques for shape memory alloys: Process optimization, property-functionality relationships, and future 4D printing opportunities

Abstract

Shape Memory Alloys are advanced metals with unique features, including the shape memory effect and superelasticity, resulting from their reversible phase transformation. These qualities have made them highly appealing for applications in aerospace, biomedical fields, robotics, and adaptive engineering. However, traditional manufacturing methods face limitations in geometry, compositional control, and complexity. Additive Manufacturing offers transformative potential through its layer-by-layer fabrication, enabling the production of complex shape memory alloy structures with customizable microstructures and enhanced functional performance. This review highlights recent developments in the Additive Manufacturing of NiTi-, Cu-based, and Fe-based shape memory alloys. Unlike previous reviews, it compares Laser Powder Bed Fusion, Electron Beam Powder Bed Fusion, Directed Energy Deposition, and Binder Jetting using a unified process-microstructure-property-functionality framework. The discussion includes quantitative aspects such as energy density, cooling rate, porosity, shifts in transformation temperature, recoverable strain, and fatigue behavior. It also addresses unresolved controversies like rapid cooling, the trade-offs between densification and compositional stability, and cyclic functional reliability. Additionally, future trends in smart structures are presented, including emerging ideas for 4D printing, functionally graded shape memory alloy systems, and intelligent process optimization.