Shape Memory Alloy Tubes Research Articles

The influence of pre-strain on the microstructure and recovery properties of cold-rolled TiNiFe alloy tube

Abstract In the present work, the influence of pre-strain on the microstructures and recovery properties of cold-rolled TiNiFe shape memory alloy tube are studied by pre-strain system, phase transformation testing system, electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). It is demonstrated that the pre-strain has significant effects on the recovery properties, transformation temperatures, and microstructures of TiNiFe alloy. With the increase of pre-strain, the kernel average misorientation (KAM) value and the dislocation pill-up increase, causing the recovery rate of the TiNiFe alloy decreases linearly, and maximum of recovery strain is 8%. And the reverse martensite transformation start temperature (TRs) increases from -133°C to -105°C and TRf increases from -124°C to -97°C, as the increase of pre-strain.

Effect of axial preloads on torsional behavior of superelastic shape memory alloy tubes – experimental investigation and simulation/predictions of intricate inner loops

Shape memory alloy (SMA) devices (utilizing wires/rods, springs or tubes) engage unique superelastic/pseudoelastic (SE) phenomenon for large stroke recovery, energy dissipation and damping applications. In particular, SMA components in torsion are also generally subjected to some pre-tension loads and hence understanding their combined effects on the torsional response is vital. In this work, some intricate internal loop and outer loop responses (large twists > 1500°) of SMA tubes under different twisting-untwisting scenarios are investigated to understand the effects of static pre-load on torsional responses. Key SMA hysteretic features like sink point memory (SPM) and return point memory (RPM) have not been well understood under torsion (with and without axial loads) have been investigated here. Further, a modified two variant thermodynamic Preisach modeling approach is proposed to simulate responses of twisted tubes with varying extents of twist and different axial loads. The central principle here is the use of a Gibbs Potential framework to separate the dissipative and thermoelastic parts of the SE responses using well established thermodynamic principles and then utilizing a non-ideal switching type disjunctive Preisach model to fit the nonlinear hysteretic dissipative section of the SE response. By using the outer loop response for model calibration, other complex loading unloading scenarios leading to intricate internal loops and effects of axial loads on these can be predicted. It is also shown that addition of a single interior loop detail for model calibration significantly enhances prediction of other more intricate internal loops. This approach dramatically simplifies the experimental data needed for the simulation of SMA components even under complex loading.

Multiphase reactive flow and heat transfer simulations in the unlocking components of an ultra-low shock pyrotechnic separation nut

This study presents a multiphase reactive flow and conjugate heat transfer model for the unlocking components in an ultra-low shock pyrotechnic separation nut. The process of the shape memory alloy (SMA) tube heated by the heat released from the combustion of the micro gas pyrotechnic charges was simulated. The established model was validated by comparing the simulation with experimental results for the temperature of the outer wall of the SMA tube. Then, the detailed ignition and combustion of micro gas pyrotechnic charges, transient flow of combustion products, and heat transfer in the SMA tube were analyzed. The effects of the amount and burning rate of micro gas pyrotechnic charges on the unlocking time were investigated. It is found that the unlocking time decreases with an increase in the amount of micro gas pyrotechnic charges. Moreover, the reduction range of the unlocking time decreases with the increase in the amount of micro gas pyrotechnic charges. Reducing the differences in the burning rate contributes to enhancing the consistency of unlocking time.

The Effect of Annealing Treatment on the Microstructure and Texture of a Cold-Rolled TiNiFe Shape Memory Alloy Tube

The effect of annealing treatment on the microstructure and texture of a Ti50Ni47Fe3 shape memory alloy tube was studied. The results show that the recrystallization process of a cold-rolled Ti50Ni47Fe3 alloy tube occurs at 600 °C. The microstructure changes from long striped grains to equiaxed recrystallized grains. The main texture of the alloy tube is the fiber texture <111> parallel to RD. With the increase in the annealing temperature, the crystal orientation of the alloy gradually turned to (111)<112>, and a series of secondary textures were distributed along the γ orientation line when the alloy was annealed at a lower temperature (450~600 °C). When the alloy was annealed at 650 °C, the growth of recrystallized grains made the grain orientation change, which led to the weakening of the γ-fiber texture and the formation of recrystallization textures.

Contact Pressure Evolution in Heat-Treated Iron-Based Shape Memory Joints

This paper presents an experimental investigation into the behavior of open-ended Fe-based Shape Memory Alloy (Fe-SMA) tubes under biaxial pre-straining conditions, focusing on development of interface contact pressure as a result of the shape memory effect (SME). During the pre-straining process of Fe-SMA tubes, the cross-section undergoes complete inelastic deformation with a non-uniform distribution. Consequently, non-uniform biaxial pre-strain emerges across the cross-section, leading to the formation of stress-induced martensite in both the radial and circumferential directions. This results in the alteration of the SME due to the direction and interaction of biaxial martensite. Consequently, the material experiences a complex pre-strain state, posing challenges in assessing the SME in Fe-SMA tubes. Nevertheless, it is possible to interpret the complex overall SME performance on the basis of measurements of the resultant pressure exerted by the Fe-SMA tube on a substance, thereby limiting its free recovery. This study explores the development of the interface contact pressure highlighting the impact of heat-treatment on the gripping capacity in Fe-SMA tubes. Via coupling of analytical models with results extracted from an experimental campaign, the resultant pressure at the interface throughout the course of activation is quantified aiding to assess the SME performance. The experimental setup involves Fe-SMA and steel tubes enabling the quantification of interface contact pressure through principal strain measurements at the inner diameter of the steel tube, allowing for differentiation of SME performance under as-received and heat-treated conditions.

Development of a crankshaft driven single long NiTi tube compressive elastocaloric cooler

Elastocaloric cooling has no environmental effects during operation, and achieving a compact structure especially the driver is significant for its commercialization. In this article, a compact, standalone crankshaft driven single long NiTi tube compressive elastocaloric cooler is developed. A crankshaft driver was designed and fabricated to drive a compressive elastocaloric regenerator utilizing a single long polycrystalline superelastic NiTi shape memory alloy tube (outer diameter 5 mm, wall thickness 1 mm, and initial length 305 mm). A novel design of ceramic heat insulation plate was applied to the cooler to reduce the conduction heat loss from the NiTi tube to the stainless-steel loading heads. The cooling performance of the cooler was characterized using synchronized thermocouples and infrared thermography, and the specific cooling(heating) power, temperature span, and coefficient of performance of up to 65(125) W·kg−1, 9.1 K, and 5.0, respectively were measured. The progressions of the temperature span, specific cooling(heating) power, and coefficient of performance with the operation cycle and temperature lift were analyzed. An energy analysis revealed that the heat transfer fluid carried out only 14% of the latent heat generated by the NiTi tube, which demonstrated a potential to enhance the cooling performance by the improvement in the regenerator structure.

Tuning the temperature range of superelastic Ni-Ti alloys for elastocaloric cooling via thermal processing

Caloric cooling enlisting solid-state refrigerants is potentially a promising eco-friendly alternative to conventional cooling based on vapor compression. The most common refrigerant materials for elastocaloric cooling to date are Ni-Ti based superelastic shape memory alloys. Here, we have explored tuning the operation temperature range of Ni50.8Ti49.2 for elastocaloric cooling. In particular, we have studied the effect of thermal treatments (a.k.a. aging) on the transformation temperature, superelasticity, and elastocaloric effects of Ni50.8Ti49.2 shape memory alloy tubes. The isothermal compressive test revealed that the residual strain of thermally-treated Ni-Ti tubes at room temperature approaches zero as aging time is increased. Short-time aging treatment at 400 °C resulted in good superelasticity and elastocaloric cooling performance with a large tunable austenite finish (A f) temperature range of 24.7 °C, as determined from the A f temperature of the samples that were aged 5–120 min. The main reason of the property change is the formation of a different amount of Ni4Ti3 precipitates in the NiTi matrix. Our findings show that it is possible to tailor the A f temperature range for development of cascade elastocaloric cooling systems by thermally treating a starting single composition Ni-Ti alloy.

Highly efficient grooved NiTi tube refrigerants for compressive elastocaloric cooling

Elastocaloric (eC) cooling without environmental issues has been a promising substitute for the conventional vapor-compression refrigeration. Commercial superelastic NiTi shape memory alloy tubes show large elastocaloric cooling capacity and ultrahigh fatigue life under cyclic compression but suffer from the limited specific heat transfer area (Aht) and insufficient cooling performance. Up to now, compressive NiTi tube eC refrigerant with Aht larger than 1 m2 Kg−1 has not been available due to the easy buckling problem. In this work, the buckling-free vertically and spirally grooved NiTi tubes (VGT and SGT) with Aht of 1.04 m2 Kg−1 and 1.25 m2 Kg−1 respectively are developed as the compressive eC refrigerants. The VGT and SGT are fabricated by the slow-feeding wire electric discharge machining based on the bared NiTi tubes. Then a compressive eC refrigeration prototype (ERP) is built to experimentally characterize the specific cooling power (SCP) and system temperature span (ΔTspan) of the grooved tubes. Under the regenerative flow configuration, the VGT and SGT achieve large zero-ΔTspanSCP of 2.18 W g−1 and 2.61 W g−1 at the operation frequency (fERP) = 0.5 Hz. Besides, the VGT and SGT produce maximum no-cooling-load ΔTspan of 25.2 K and 27.7 K respectively at fERP = 0.417 Hz. The experimental SCP and ΔTspan exceed most of the reported eC regenerators and there are still rooms for enhancing the cooling results by improving the ERP in future. Overall, the developed VGT and SGT with large Aht records and excellent cooling performance show big potentials of heat transfer structure design in developing highly efficient eC devices.

Elastocaloric effect characterization of a NiTi tube to be applied in a compressive cooler

In this article, the elastocaloric effect of a commercial superelastic NiTi shape memory alloy (SMA) tube (with an outer diameter of 5 mm and wall thickness of 1 mm) to be applied in a compressive cooler was measured and analyzed. The elastocaloric effect of the tube was measured vs the applied strain and strain rate. The largest temperature changes of 21 K during loading and 16 K during unloading were measured at an applied strain of 3.30% and strain rate of 0.33 s−1. In the fatigue testing of the sample, only 0.20% of the residual strain accumulated after a runout of 1 × 106 sinusoidal force-controlled loading–unloading cycles at a maximum compressive stress of 1100 MPa and frequency of 20 Hz. Numerical results of the cooling characteristics of a compressive device using a single NiTi tube with the above-mentioned cross section and an aspect ratio of 60:1 as the refrigerant showed that the device could produce a total cooling power of up to 20 W and a coefficient of performance of up to 6.5. The results of this article demonstrate that superelastic NiTi SMA tubes of suitable wall thickness and aspect ratios are good candidates to be applied in a compressive elastocaloric cooler.

Numerical simulation of extrusion of FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube based on finite element method and cellular automaton

FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube is firstly put forward and it can be fabricated by means of isothermal extrusion. Based on Arrhenius constitutive model of FeMnSiCrNi and NiTiNb shape memory alloys, isothermal extrusion of FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube is simulated by rigid viscoplastic finite element method. It can be found that the deformation zone of the dissimilar shape memory alloy composite tube is always in a three-dimensional compressive stress state during extrusion, and the deformation of the inner tube is obviously higher than that of the outer tube. It is necessary to guarantee the interface compatibility between the inner tube and the outer tube during isothermal extrusion of FeMnSiCrNi/NiTiNb dissimilar shape memory alloy composite tube. The relationship between macroscopic process variables and microscopic variables during plastic deformation of FeMnSiCrNi shape memory alloy tube at high temperature is established by coupling finite element simulation and cellular automaton simulation. The microstructural evolution of FeMnSiCrNi shape memory alloy in different deformation zones during isothermal extrusion of dissimilar shape memory alloy composite tube is simulated. It can be concluded that the grain size of dynamic recrystallization is reduced with the increase of plastic strain in the deformation zone.

Actuation performance of machined helical springs from NiTi shape memory alloy

Machined helical springs were designed and fabricated from NiTi shape memory alloy tube stock by cutting helical slots to make the coils, which provided them with high design flexibility and actuation potential. The springs were then subjected to isothermal tensile testing at temperatures of 20°C and 80°C, the experimental data showed that they could generate more than 50% actuation stroke at the applied force of 300 N. A material model was introduced to describe the thermally-induced actuation behavior of the machined NiTi spring. Finite element simulation of the springs was carried out to investigate the effects of geometries and applied loads on the actuation performance. With increasing applied load, the actuation stroke and transformation temperature increased as well, indicating more actuation potential at high loads. The actuation stroke was positively correlated to the outer diameter and the cross-sectional aspect ratio, negatively correlated to the cross-sectional area, and independent of the coil pitch. A variable-sweep wing actuated by two antagonistic machined springs was designed and numerically analyzed, the actuator could generate a steady cyclic motion up to 40 mm and allowed the swept-back angle of the wings to be continuously varied from zero to 90 degrees.

Enhancing cooling performance of NiTi elastocaloric tube refrigerant via internal grooving

• A grooved NiTi tube is designed and fabricated as the elastocaloric refrigerant. • A buckling-prevention frame is developed to ensure uniform compression of the tube. • Compared with ungrooved one, the grooved tube has 53% larger SCP (1.30 W g −1 ). • With the buckling-prevention frame, the grooved tube endures 2 million cycles. Developing novel heat transfer surface structure on NiTi shape memory alloy tubes has big potential to enhance the elastocaloric cooling performance. In this work, an internally grooved superelastic NiTi tube (outer diameter 5 mm, wall thickness 1 mm and length 50 mm) is designed and fabricated as the refrigerant in a compressive regenerative elastocaloric cooling prototype. The geometric parameters of the longitudinal grooves (groove width, groove depth, and groove spacing) are optimized by a numerical heat transfer simulation. A buckling-prevention frame is developed to ensure uniform compression of the NiTi tube in the cooling prototype. Compared with the ungrooved tube of the same size, the specific internal heat transfer area of the grooved NiTi tube (3.44 mm −1 ) is increased by 360%, producing 53% larger specific cooling power (1.30 W g −1 ) and 33% larger maximum system temperature lift (8 K). With the buckling-prevention frame, the grooved NiTi tube endures 2 million reversible phase transition cycles under the compressive stress of 800 MPa in the prototype. Such significantly enhanced cooling power of the grooved NiTi tube with high fatigue life demonstrates the critical roles of heat transfer surface structure and buckling prevention in enhancing the elastocaloric cooling performance under compression.

Morphology of Fatigue Fracture Surface for TiNi Shape Memory Alloy Stent

The fatigue life of a high-dilatation stent fabricated from TiNi shape memory alloy tube with a high reduction of area was investigated in the radial and uniaxial compression mode. In this study, fatigue properties of the stent were discussed focusing on morphology of the fatigue fracture surface. In addition, the local strain during compressive deformation was examined by observation of the stent subjected to the compression. The main points obtained in the study are as follows (1) Fatigue fracture occurs at base of a strut of the stent specimen. (2) Fracture surface appears at an angle to the strut axis. This means torsional deformation occurs at base of the strut of the stent specimen. (3) The fatigue crack initiation point appears on the outside of the strut part because of the maximum positive bending strain.

Experiments on active stiffness control of a thin-walled cylindrical shell with novel SMA driven screw-type actuators

Thin-walled cylindrical shells have been widely used as key components in various engineering applications. The stiffness of the thin-walled cylindrical shell is an important factor affecting its working performance. This study aims to investigate the active stiffness control of thin-walled cylindrical shells with a new SMA (shape memory alloy) driven actuator. The configuration of the actuator is designed first according to the circumferential and axial modal characteristics of the cylindrical shell. Geometric parameters of the actuator are optimized based on the statics and modal analysis of the system. SMA tubes are used as the driving sources of the actuator and their mechanical performance, including compressive mechanical property and recovery mechanical property are tested. The active stiffness control experiments are carried out on a simply supported thin-walled cylindrical shell. The control performance of the actuator is indicated by the enhancement ratio of natural frequencies of different modes. The influences of actuator number and actuation time on the control performance are also investigated. The results demonstrate that the proposed actuator can significantly improve the stiffness of the thin-walled cylindrical shell by nearly 30%.

Torsional behavior and microstructure characterization of additively manufactured NiTi shape memory alloy tubes

The large reversible strain upon heating (shape memory effect) or unloading (superelasticity), high power-to-weight ratio, good functional stability, compact size, and lightweight make the rotary NiTi shape memory alloys actuators an interesting candidate for various engineering applications. Additive manufacturing (AM) provides a single step freeform manufacturing process that not only fabricates the complex geometries but also tailors the properties of the printed parts profoundly. In the selective laser melting (SLM) technique, the process parameters (PPs) and scanning strategy show the remarkable effect on the microstructure, the properties, and the size accuracy of as-built parts. Impurity pick-up during the AM process is an unintended incident altering the microstructure and thermomechanical properties of fabricated parts significantly. In this paper, slightly Ni-rich NiTi powder is utilized to fabricate the NiTi tubes with three different thicknesses via the SLM method. It is shown that the bidirectional scanning strategy results in the size deviation of thin-wall tubes. Transformation temperatures (TTs) of the as-fabricated samples are assessed and compared with those of the starting powder. A large shift in TTs is found between the powder and the SLM tubes. The x-ray diffraction pattern shows the martensite phase at room temperature for the starting powder, while the as-built tubes are in the austenite phase coexisting with a secondary phase of Ti-rich oxide. Scanning electron microscopy (SEM) confirms Ti-rich Ti4Ni2Ox precipitates form along the grain boundaries. The characterization of tubes under pure torsional loading shows the localized shear strain on the tube surface. The thermomechanical behavior of the as-fabricated tubes is investigated and shown to exhibit superelastic response with a stable transformation strain of 2.3% after 10 cycles.

Shape memory alloy tube actuators inherently enable internal fluidic cooling for a robotic finger under force control

This paper presents the design, control and evaluation of a novel robotic finger actuated by shape memory alloy (SMA) tubes which intrinsically afford an internal conduit for fluidic cooling. The SMA tubes are thermomechanically programmed to flex the robotic finger when Joule heated. A superelastic SMA plate provides a spring return motion to extend the finger when cooling liquid is pumped through the internal channel of the SMA tube actuators. The mechanical design and nonlinear force controller are presented for this unique robotic finger. Sinusoidal and step response experiments demonstrate excellent error minimization when operated below the bandwidth which was empirically determined to be 6 rad s−1. Disturbance rejection experiments are also performed to demonstrate the potential to minimize externally applied forces. This method of internal liquid cooling of Joule heated SMA tubes simultaneously increases the system bandwidth and expands the potential uses of SMA actuators for robotic applications. The results show that this novel robotic finger is capable of precise force control and has a high strength to weight ratio. The finger can apply a force of 4.35 N and has a mass of 30 g. Implementing this design into wearable prosthetic devices could enable lightweight, high strength applications previously not achievable.

Design, fabrication and modeling analysis of a spiral support structure with superelastic Ni-Ti shape memory alloy for continuum robot

In order to improve the working ability of continuum robots in confined space, a novel central support structure with the spiral cutting pattern is proposed in this paper. The spiral hollow scheme was selected through analysing different cutting patterns of the thin-walled superelastic Ni-Ti shape memory alloy (SMA) tube. Then the laser cutting process and heat treatment for fabricating the structure were analysed. The analysis results of their impact on the material characteristics proves that the laser cutting and heat treatment ensure the performance of the material. The single-segment and multi-segment prototypes both showed prospective motion ability in the flexural rigidity and continuous deformation tests respectively. Then the flexural rigidity model of the spiral structure was proposed, which was proved effective by comparing the model simulation and finite element analysis results. Furthermore, the central support structures with different flexural rigidities can be customized, which has broad application prospects in continuum robots.

Proposal of a Feedback Control System for a Thermally Driven High-Power-Density Actuator

Shape memory alloy (SMA) actuators can generate force via the phase transition of the SMA, related to its temperature variations. The amount of force and the response duration length are approximately proportional to the SMA’s volume and mass. The higher-power actuator, which consists of a single SMA tube, has mechanical simplicity and high reliability; however, it has a relatively long driving duration, and actuation timing uncertainties caused by the environmental temperature variations. Because of its huge heat capacity, the environment temperature variations affect the actuator’s temperature, the phase transformation uncertainties, and force generation profiles, resulting in differences in the actuation timing that are tens of seconds. This paper proposes a suitable feedback control system consisting of two methods. One is the construction of a physical model for SMA tube thermomechanical coupling response that has suitable complexity for the real-time control system design by the limited application condition. The other is a reference trajectory generation and tracking method by using the model predictive control method with a constructed physical model, resulting in a possible drive timing uncertainty reduction. The effectiveness of the proposed methods was verified by specific test piece system identification and control simulations performed both numerically and experimentally. In addition, the effectiveness of the online control system itself was confirmed by comparing the results of a traditional control system using the proportional integral derivative method. The result indicates the potential of the possible application system for SMA actuators.

Scale-up of NiTiHf shape memory alloy tubes with high torque capability

High-torque, NiTiHf high-temperature shape memory alloy tubes were examined by means of constant-torque thermal cycling experiments. The alloy, with a Ni-rich composition of Ni50.3Ti29.7Hf20 (at%), was hot worked via extrusion from large ingots to produce final rods with an outer diameter of 25.4 mm. After aging at 550 °C for 3 h and air-cooling, the manufactured torque tubes were subjected to shear stresses from 0 to 500 MPa (torques of 0–1400 N m) and thermally cycled 20 times under stress, and twice after stress removal to evaluate the two-way shape memory effect. A transformation shear strain of 6% was obtained, and a maximum work output of 27.5 J cm−3. Two-way shear strains approached 3% strain without any extended training or cycling. The strain values obtained for the first time in this large-diameter, high-torque tube consistently matched smaller diameter tubes previously reported. This highlights the successful production and scale-up of both the material and the component level in torque tubes without degrading the actuation response, while providing much higher torque capability.

Two way shape memory effect in NiTiHf high temperature shape memory alloy tubes

Two-way shape memory effect (TWSME) in nano-precipitation hardened, Ni50.3Ti29.7Hf20 high temperature shape memory alloy (HTSMA) thin walled tubes and its thermal stability were investigated. Torsional TWSME was induced by repeated thermal cycling across their martensitic transformation under applied shear stress. The effects of training parameters and geometric factors, such as the number of training cycles, shear stress levels, and thickness of the tube walls, on the resulting TWSME were evaluated. Thermal stability of TWSME was characterized as a function of annealing treatments at elevated temperatures. It was found that under 200 MPa, 600 thermal cycles were sufficient to reach a two-way shape memory strain (TWSMS) as high as 2.95%, which was shown to be stable upon annealing up to 400 °C for 30 min. This TWSMS was 85% of the maximum measured actuation strain under 200 MPa. The microstructure after thermo-mechanical training was investigated using transmission electron microscopy (TEM), which did not indicate a significant change in precipitate structure and size after the training. However, small amount of remnant austenite was revealed at 100 °C below the martensite finish temperature, with notable amount of dislocations. Overall, it was found that nano-precipitation hardened Ni50.3Ti29.7Hf20 shows relatively high TWSMS and stable actuation response after much less number of training cycles as compared to binary NiTi and nickel lean NiTiHf compositions. Tube wall thickness and training stress levels have been found to have negligible effect on shape memory strains and number of cycles to reach the desired training level, for the ranges studied.