Process-structure-property relationships governing functional response in additively manufactured NiTi shape memory alloys: A review

Additive manufacturing (AM) is redefining the design space of NiTi shape memory alloys (SMAs), enabling architected geometries and spatially tailored functionality not accessible through conventional processing. However, the extreme thermal gradients and rapid solidification inherent to AM fundamentally reshape transformation thermodynamics, microstructure, and functional reliability. This review synthesizes current understanding of process-structure-property relationships across laser powder bed fusion, electron-beam powder bed fusion, directed energy deposition, and solid-state AM routes, framing them within a unified thermal-compositional landscape. NiTi performance in AM cannot be evaluated solely based on densification metrics. Instead, subtle variations in local energy distribution, evaporation-driven nickel redistribution, oxygen uptake, and crystallographic texture govern transformation temperatures, anisotropy, and superelastic stability. A curated database compiled from over 300 studies is used to perform cross-study statistical mapping, revealing that energy-density metrics are non-unique descriptors, where identical nominal values can produce divergent phase states depending on melt-pool mode and time-temperature history. Texture control via build orientation and scan strategy has emerged as a powerful lever for engineering anisotropic functional response, while post-build heat treatments enable secondary-phase tuning but remain highly sensitive to as-built chemistry. Looking forward, the field is transitioning from empirical parameter optimization toward physics-informed and data-driven design frameworks. Standardized reporting of composition shifts, thermal history, and transformation metrics is needed to enable cross-platform comparability. Integration of in situ monitoring, predictive modeling, and curated databases offers a pathway toward closed-loop control of transformation behavior. Establishing thermodynamically grounded design principles will be essential for translating AM NiTi into reliable biomedical, aerospace, and adaptive structural applications. This framework provides a basis for rational process design and standardized reporting in AM NiTi systems.