Processing and property assessment of NiTi and NiTiCu shape memory actuator springs

Abstract

Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one-way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni 50 Ti 50 and ternary Ni 40 Ti 50 Cu 10 SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape-constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring-making are documented. The mechanical and functional characteristics of the wires are investigated in the stress-strain-temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported.

Twinning-Induced Elasticity in NiTi Shape Memory Alloys

“…After the final deformation step, the wires were straightened and again subjected to RA. Our procedure establishes a fully recrystallized microstructure with a medium grain size close to 30 lm [45,46]. The resulting NiTi wires were used for all subsequent mechanical tests, where the processing stress/strain history was no longer considered and the deformation of the asprocessed material was reset to u = 0.…”

“…Wires with diameters of 1.66, 1.6, 1.54, 1.44, 1.34, and 1.25 mm (corresponding to pre-deformation levels of u = 0.07, 0.14, 0.22, 0.35, 0.49, and 0.63) were thus obtained. All further details on thermomechanical treatments are given elsewhere [21,[45][46][47].…”

Effect of Cu Content on Atomic Positions of Ti50Ni50−xCux Shape Memory Alloys Based on Density Functional Theory Calculations

“…In order to design new SMAs possessing very small transformation temperature hysteresis, understanding the effect of alloying on martensite structures is of vital importance. Previously reported works have studied the effects of Cu content on transformation temperature [10], microstructure [7,8,11,12], stress-strain characteristic and fatigue life [13][14][15], as well as applications as actuators [5,16]. Earlier research has found that the Cu content in TiNiCu alloys has little influence on the MT temperature but obvious effect on the MT pathway [11,12,17].…”

Mechanical Properties of NiTi-Based Foam with High Porosity for Implant Applications

“…Gil et al [14,15] showed that NiTiCu SMAs exhibit a much more narrow stress hysteresis, based on transformation stresses, as compared to binary NiTi SMAs. Fatigue testing of NiTiCu SMA actuator springs were conducted by Grossmann et al [16,17]. They reported that the functional fatigue resistance of NiTiCu SMAs is strongly improved by the addition of Cu.…”

“…Sehitoglu et al [18,19] and Biscarini et al [20] investigated the mechanical properties of NiTiCu single crystal SMAs under compression loading, and reported that the slip resistance increases when Ni-rich precipitates are present. To date, the mechanical behavior of bulk NiTiCu SMA is well established [7,[14][15][16][17][18][19][21][22][23][24]; however, few studies have focused on the effect of porosity on NiTiCu SMAs [25][26][27][28]. Goryczka et al [26,27] examined powder processing methods for producing homogenous NiTiCu SMAs.…”