Voxel-Level Design of Functionally Graded NiTi and NiTiHf Shape Memory Alloys via Additive Manufacturing

This study presents a comprehensive framework for the successful additive manufacturing of functionally graded shape memory alloys (FG-SMAs) via Laser Powder Bed Fusion (LPBF), encompassing both NiTi and high-temperature NiTiHf systems. The framework allows precise voxel-level control of local chemistry and microstructure starting from a single feedstock, enabling location-specific phase transformation characteristics. By systematically analyzing the relationships between volumetric energy density, Ni evaporation, and transformation temperatures, the study achieved accurate control of Ni content across different segments of the functionally graded materials (FGMs). Thermal modeling elucidated the influence of thermal history change, due to changes in the process parameters and sample size, on Ni evaporation and oxide formation, findings that were experimentally validated by transmission electron microscopy. Wavelength dispersive spectroscopy confirmed the localized control of Ni content, while differential scanning calorimetry identified multiple transformation peaks, indicating multi-stage transformation behavior throughout the FG segments. The transformation range spanned over 150°C in the FG NiTiHf SMAs, demonstrating the capability to engineer tailored thermomechanical responses across different regions within the printed components. Simultaneously, the FG NiTi parts exhibit combined superelastic and shape memory responses under variable loading, with transformation strains up to 6%. A novel thermomechanical modeling approach effectively predicted the global actuation response of the FG parts, and mechanical testing demonstrated broader transformation ranges and high strain recovery in the samples. These results highlight the potential of FG SMAs for applications requiring tailored actuation and reliable performance across varying temperature ranges, offering a viable path forward for enhancing their functionality.