Effect of Heat Treatments on the Thermomechanical Behaviour of Ni-Ti Superelastic Mini Coil Springs

Abstract

Abstract. Shape Memory Alloys are susceptible to annealing heat treatments, which are capable of partially or fully recover atomic mobility and, therefore, affect the overall thermomechanical response of the material. In this work, NiTi SMA orthodontic minicoil springs in superelastic state, widely commercialized, were submitted to annealing treatments as a way to modify their thermomechanical response and adapt it to the use in mechanical systems in other fields besides orthodontics. The main objective is to study the influence of temperature and time of annealing on the thermomechanical behaviour of the coil springs, originally superelastic at room temperature. Using a factorial design, three mechanical properties of interest were studied: spring constant, shear modulus and energy dissipation capacity. It was demonstrated that annealing in the range of 500°C-600°C is capable of converting superelastic springs to an apparently actuator state, as residual strain after loading/unloading at room temperature was observed, when a maximum 7% shear strain was attained in the cross section of the spring's wire.

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

“…However, in the initial linear region of the force (F) vs displacement (y) curve, with small variations in the pitch angle (α), the mechanical behaviour is similar to a conventional linear spring. Thus, it is possible to use a classic mathematical approach 19,20 to calculate the corresponding shear modulus (G) from stiffness (k) for SMA helical springs [18][19][20][21] in the austenitic phase using Eq. (1).…”

“…Therefore, from force-displacement curves of superelastic SMA springs, as illustrated in Figure 2, stiffness (k) of the austenite phase is determined in the initial linear region from 10% to 20% of maximum load, before the transformation had taken place 21 .…”

An experimental investigation of the superelastic fatigue of NiTi SMA wires

“…Currently, the main developed SMA system is that of NiTi-based. These NiTi SMA, and some other copperbased alloys (CuAlNi, CuAlBe, CuAlMn, etc) [1], have been used for the development of mechanical components such as superelastic bolts [3], Belleville washers [4], and orthodontic devices like micro-coil springs [5] and arch wires. In this context, but in a dynamic way, some mechanical devices using superelastic SMA as vibration and impact absorbers have been developed [6,7], due to the aforementioned energy dissipation capacity.…”

Pull-out resistance of shape memory alloy nickel-titanium ribbons embedded in silicone matrix for development of flexible composites

“…After cold rolling, the ribbons with 0.15 mm in thickness were submitted to heat treatments of 450°C for 20 min and 550°C for 180 min to obtain ribbons with characteristics of superelasticity (SE) and Shape Memory Effect (SME), respectively (Grassi et al, 2015). After heat treatments, the NiTi ribbons were 0.15 mm thick and 0.8 mm wide.…”