Shahadat Hussain, PhD

Laser powder bed fusion (LPBF) is an appealing additive manufacturing technique suitable for producing intricate products with complex shapes, where traditional subtractive manufacturing methods may be impractical. When dealing with NiTi shape memory alloys, the fabrication process encounters additional challenges due to the material’s high ductility and work hardening characteristics. These properties lead to elevated tool wear and poor workability. However, LPBF overcomes these challenges by eliminating the need for tooling and enabling direct fabrication of complex shapes based on predefined computer-aided designs. Although there has been a growing interest in the LPBF of NiTi in recent years, the fabrication of NiTi structures with architected designs has received limited attention. This study aims to bridge this research gap by investigating the phase transformation behavior of two specific types of triply periodic minimal surface (TPMS) architected NiTi lattices, namely primitive and gyroid, manufactured using LPBF. The study utilized a methodology involving the design of TPMS structures, additive manufacturing, and the characterization of printed samples through electron microscopy, x-ray diffraction, and thermal analysis to measure phase transformation temperatures. The research not only examines the impact of process parameters on the behavior of the fabricated samples but also establishes the influence of cellular geometry on the functional response of TPMS NiTi structures. It was noteworthy to observe that gyroid samples displayed a higher thermal hysteresis and lower reverse transformation enthalpy in comparison with primitive samples.

Due to their high ductility and superior strength, the machining of NiTi shape memory alloys using conventional subtractive manufacturing technologies poses significant challenges. However, additive manufacturing (AM) offers a viable solution by circumventing the limitations of machinability through the elimination of tooling, thereby enabling the production of NiTi structures with previously unachievable intricacy. This study focuses on the fabrication of base layers of architected triply periodic minimal surface (TPMS) lattices using laser powder bed fusion (LPBF) and explores primitive and gyroid topologies. The investigation involves a comprehensive analysis and discussion of the influence of geometric properties and process parameters on the microstructural characteristics and the distribution of solid phases within the samples. Notably, the study reveals a substantial impact of process parameters and structural topology on the microstructural features. Additionally, notable observations are made concerning nickel evaporation, as well as the formation of oxide- and titanium-rich phases in relation to their distance from the base plate. Investigating intricate TPMS geometries in NiTi alloys in conjunction with varying laser process parameters represents a relatively new and unexplored area of research. These geometries may offer unique structural and functional properties that can potentially lead to innovative applications and advancements in various fields.