Shape Setting and Thermomechanical Characterization of Shape Memory Alloy Springs

Shape memory alloys (SMA's) are a class of smart materials with the remarkable properties of shape memory effect (SME) and pseudoelasticity (PE) due to their martensitic phase transformations under various thermomechanical loadings. To fabricate SMA elements with a desirable shape (e.g. springs, hooks, and Belleville washers) from elementary products (e.g. bars, wires, ribbons, and sheets), specific treatments called shape setting are required. Shape setting is accomplished by deforming an SMA part to a specified shape, constraining the configuration, and appropriate heat treatments. In this paper, SMA helical springs were produced by using two sets of NiTi (Ti-55.87 at. % Ni) wires, one of which showing SME and another showing PE at the ambient temperature. Annealing was performed with the use of a tube furnace under inert atmospheric (Ar). Different pitches as well as annealing temperatures were tried to investigate the effect of such parameters on the thermomechanical characteristics of the fabricated springs. Phase transformation temperatures of the products were measured by differential scanning calorimetry (DSC) and compared with those of the original wires. Compression tests were also carried out, and stiffness of each spring was determined. The different desired pitches were so that some springs experienced phase transition during loading while the others did not. The former showed a varying stiffness with the applied load, but the latter act as passive springs with a predetermined stiffness. Stiffness and phase transformation temperatures of the made springs were concluded to be influenced by the annealing temperature as well as the adjusted pitches.

کلیدواژه ها:

shape memory alloy (SMA) ، spring ، shape setting ، thermomechanical ، heat treatment ، transformation temperatures