Thermomechanical Characterization of Superelastic Ni-Ti SMA Helical Extension Springs Manufactured by Investment Casting

José Joelson De Melo Santiago,Carlos José De Araújo,Jackson De Brito Simões

doi:10.1590/1980-5373-mr-2018-0852

Abstract

Shape memory alloy (SMA) helical springs are special mechanical parts that require a previous evaluation of its behavior for application. Therefore, in this paper thermal and mechanical behaviour of superelastic Ni-Ti SMA helical extension springs manufactured by investment casting (IC) are evaluated. Phase transformation temperatures were measured by Electrical Resistance as a function of Temperature (ERT) and Differential Scanning Calorimetry (DSC). Tensile tests were carried out within strain and temperatures ranges. The pitch angle and stiffness of each spring were determined. Results demonstrated that Ni-Ti SMA helical springs produced by IC presented phase transformation corresponding to the superelastic effect (SE). The reversible deformations under tensile test were of the order of 70%. The mechanical behavior as function of temperature revealed a linear relationship between maximum force and spring temperature.

Highlights

Since the discovery of the shape memory phenomenon in Ni-Ti alloys, shape memory alloys (SMA) have been applied in several areas such as aeronautics and biomedical
These two techniques yielded different transformation temperatures, which was mainly influenced by the sample state and sample dimensions
Differential Scanning Calorimetry (DSC) is a thermal analysis technique in which the heat flow of the material is measured as a function of temperature while ERT is based on changes in electric properties, usually applied to detect transformation temperatures between the martensite and austenite phases[24]

Summary

Introduction

Since the discovery of the shape memory phenomenon in Ni-Ti alloys, shape memory alloys (SMA) have been applied in several areas such as aeronautics and biomedical. The manufacture of Ni-Ti alloys is accomplished through two main techniques, casting processes and powder metallurgy, followed by other processes such as machining, heat treatments, forming and shape setting to obtain mechanical devices[3,4]. In order to establish an alternative route for the manufacture of Ni-Ti SMA parts, Simões and De Araújo[6] have recently used investment casting (IC) processes to produce various mechanical components such as Belleville washers, wire netting, honeycombs, helical springs and screws. Helical springs produced by investment casting techniques were obtained directly from the molten Ni-Ti and presented good mechanical properties[6,16]. This paper analysed the thermal and mechanical behaviour of Ni-Ti SMA helical extension springs obtained by an investment casting process corresponding to induction melting followed centrifugal casting (ICC)

Experimental Procedures

Thermal characterization

Dimensional and superficial analysis

Tensile tests

Test temperature influence

Conclusions