Development of NiTiCu shape memory alloy by directed energy deposition processes

Abstract

Alloys possessing super-elasticity and shape memory effect are referred to as shape memory alloys (SMAs). They are used for a wide range of applications including actuators, micro-electro-mechanical systems (MEMS), and biomedical implants. NiTi (commonly known as Nitinol) is the most commonly used SMA. But it suffers from the limitations of relatively narrow working temperature range and high hysteresis loss. Adding copper to NiTi alloy to form NiTixCu SMAs addresses these limitations by reducing the hysteresis temperature thus enhancing functional lifespan and thermal stability of the resultant SMA. This improvement is crucial for those applications that require precise thermal control and consistent performance over time. Therefore, objectives of present research are: (i) Development of multi-layer deposition of NiTi20Cu SMA from the wires of NiTi and Cu by the twin wire arc additive manufacturing (T-WAAM) process using the identified optimum parameters from its single-layer single-track depositions manufactured during the main experiments, (ii) Development of multi-layer depositions of NiTi, NiTi5Cu and NiTi20Cu using powders of Ni, Ti and Cu by the micro-plasma arc metal additive manufacturing (μ-PAMAM) process using the identified optimum parameters from their single-layer single-track depositions manufactured during the main experiments. Objective is to study the effect of varying amount of Cu in NiTi on mechanical properties and shape memory effect, (iii) Characterization of multi-layer depositions of NiTi, NiTi5Cu, and NiTi20Cu using microstructure, chemical composition, formation of phases, tensile properties, microhardness, phase transformations to illustrate shape memory effect, and (iv) Comparative study of NiTi, NiTi5Cu, and NiTi20Cu using their characterizations. The experimental investigations were performed in the following three stages: (i) pilot experiments to bracket the ranges of variable process parameters and to identify values of the fixed parameters for both T-WAAM and μ-PAMAM processes, (ii) main experiments manufacturing single-track single-layer depositions of NiTixCu SMAs by T-WAAM and μ-PAMAM processes using one factor at a time approach for both the processes. The objective was to identify their optimum parameters for multi-layer depositions, and (iii) multi-layer depositions of NiTixCu SMAs by T-WAAM and μ-PAMAM processes using their identified optimum parameters.