Laser direct deposition of shape memory alloy- nitinol and its characterization
Abstract
Nitinol is well known for its unique shape-memory property and super-elastic effect along with its excellent bio-mechanical compatibility and corrosion resistance. These properties have enabled its applications especially in the bio-medical and aerospace industry. In spite of these exceptional properties, processing of nitinol by conventional techniques is very difficult and expensive and hence needs to be explored. In this study, the feasibility of laser direct deposition technique to synthesize high quality near-net-shape nitinol components directly from elemental nickel and titanium powders as opposed to using expensive pre-alloyed nitinol powder was evaluated. Systematic characterization of samples was done using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) and differential scanning calorimeter (DSC). With optimized ratio of nickel and titanium powder mixture, laser direct deposition parameters and post-heat treatment, nearly fully dense sample with homogeneous NiTi phase and minimum volume fraction of unwanted secondary phases was synthesized. Further, these results, when compared with those obtained for samples deposited using pre-alloyed nitinol powder were similar in micro-structure and phase transformation characteristics. After being able to successfully synthesize nitinol in-situ from elemental nickel and titanium powders, experiments were carried out to understand nitinol's phase transformation characteristics using DSC. Different transformation temperatures were achieved by changing the initial chemistry in-situ during laser direct deposition and by subjecting the samples to aging heat treatment. Further uni-axial compressive testing was done at different temperatures to understand the mechanical behavior of laser synthesized nitinol. The results obtained from these tests indicated that nitinol samples showed effective shape memory and super-elasticity effect with complete strain recovery.
Degree
M.S.M.E.
Advisors
Shin, Purdue University.
Subject Area
Mechanical engineering
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