In Situ Synthesis and Characterization of Shape Memory Alloy Nitinol by Laser Direct Deposition

Abstract

Nitinol is well known for its unique shape-memory and super-elastic properties along with its excellent biomechanical compatibility and corrosion resistance. In this study, a laser direct deposition technique was explored to synthesize high-quality, near-net-shape nitinol components directly from elemental nickel and titanium powders as opposed to using expensive prealloyed nitinol powder. The systematic characterization of samples was done using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Transformation temperatures were obtained using differential scanning calorimetry (DSC). With an optimum ratio of nickel and titanium powder mixture, optimal laser parameters, and post-heat treatment, samples with homogeneous and nearly fully dense NiTi phase were synthesized with less unwanted secondary phases occupying less than 3.2 pct volume fraction. Furthermore, these results were compared with those obtained for samples deposited using prealloyed nitinol powder. This technique offers maximum flexibility and cost benefit in the manufacturability of near-net-shape nitinol components.

Phase transformation characteristics and mechanical characterization of nitinol synthesized by laser direct deposition

“…According to Halani and Shin's earlier investigation [26], Ni content between 55 and 60 atomic pct. (Ni/Ti ratio between 1.22 and 1.5) in the initial elemental Ni and Ti powder mixture gave a minimum volume fraction of unwanted secondary phases (NiTi 2 and Ni 3 Ti).…”

“…Halani and Shin [26] have recently used laser direct deposition, a laser-based additive manufacturing technique to synthesize nitinol directly from the less expensive elemental nickel and titanium powders as opposed to using expensive pre-alloyed nitinol powder. With optimized laser parameters, a nickel to titanium atomic ratio between 1.22 and 1.5 in the initial powder mixture and a post-heat treatment, they were able to achieve nearly fully dense samples with a homogeneous NiTi phase and a minimum volume fraction of unwanted secondary phases which are NiTi 2 and Ni 3 Ti.…”

Metals for bone implants. Part 1. Powder metallurgy and implant rendering

“…For processing NiTi, the entire process is usually done in an enclosed chamber filled with Argon to minimize oxidation. While in principle it is possible to use elemental powder mixtures [137,138,[152][153][154], the majority of published results are from pre-alloyed NiTi powder [137-146, 148, 150, 151].…”

Microstructure and corrosion behavior of laser processed NiTi alloy

“…Conventional processing methods such as metal working and powder metallurgical routes are highly susceptible to oxygen and carbon contamination, and segregation of secondary phases [13,14]. Several alternative methods such as photolithographic techniques [15], thermal spray method for foils [13,16,17] have been used to produce NiTi alloys in different forms and shapes depending on the purpose and application. Some of these methods also involve certain handling difficulties.…”