Fabrication Of Shape Memory Alloy Research Articles

Design and fabrication of shape memory alloy based 4D-printed actuator for FWMAV: A performance study

Flapping Wing Micro Aerial Vehicles (FWMAVs) use flapping wings to create lift and thrust, akin to bird flight. In this work, we seek to design and fabricate a Shape Memory Alloy (SMA) based 4D-printed actuator that behaves as a wing structure to achieve the desired flapping action in an FWMAV. The integration of SMA in the static matrix facilitates the desired motion by bending SMA as a response to a stimulus. The properties of SMA such as the work density, stroke angle, etc., are greatly enhanced in contrast to more complex, conventional motor-driven mechanisms. The study includes the investigation of the actuator's performance, which is evaluated using image processing based on parameters such as the stroke angle, actuation speed, and recovery force, and helps in utilizing the actuator based on the needs and demands of specific targeted applications by referring to the baseline data recorded by conducting a series of experiments.

Shape Memory Alloys via Halide‐Activated Pack Equilibration

Fabrication of shape memory alloy (SMA) components based on NiTi is challenging due to the precision with which elemental composition and microstructure must be controlled during processing to achieve desired shape memory behavior. Herein, a method to control chemistry in an NiTi SMA via halide‐activated pack equilibration (SHAPE) against a constant chemical potential reservoir is described. To demonstrate the efficacy of the SHAPE process, an initially titanium‐deficient specimen (pure nickel foam) has been equilibrated against an excess of an intimately mixed two‐phase pack (NiTi + Ti2Ni) in the presence of a vapor phase transport agent (iodine). The two‐phase pack regulates chemical potentials in this two‐component system in accordance with Gibbs’ phase rule. Ti‐rich NiTi foams thus produced exhibit reproducible and well‐defined phase transformation behaviors. The SHAPE process is advantageous for the fabrication of shape memory components of varying areal dimension, shape, and/or complexity owing to independence of the equilibrium state of the system from either the initial state of the specimen or the details of the process kinetics. Current limitations and prospects for the application of this method to improve the quality of SMA components are briefly discussed.

Optimizing machining responses of homologous TiNiCu shape memory alloys using hybrid ANN-GA approach

Fabrication of shape memory alloys using wire electro discharge machining (WEDM) has gained popularity over the last few years. Most widely used machining parameters of WEDM process are pulse on time (Øon), pulse off time (Øoff), servo voltage (σ) and wire feed (ω). WEDM responses like material removal rate (MR), surface roughness (SR), kerf width (KW) and recast layer thickness (LT) have been evaluated by researchers to determine machining characteristics and are also considered for this study. These machining responses determine the quality of machining and are majorly influenced by thermal conductivity and melting temperature of the WEDM workpiece. Actuation behavior of shape memory alloys is a function of phase transformation characteristics which in turn depends on elemental composition of the selected alloys. Therefore, dissimilar machining responses of Ti50Ni40Cu10 and Ti50Ni25Cu25 have been observed even though similar machining input values were used. This study utilized artificial neural network (ANN) mapping to establish WEDM response function – which was used as fitness function to perform multi objective optimization using genetic algorithm (GA). It was found that ANN successfully predicted machining responses of selected homologous alloys and GA helped in identifying suitable input parameter values to optimize machining responses.

The electron beam freeform fabrication of NiTi shape memory alloys. Part I: Microstructure and physical–chemical behavior

Nickel–titanium alloys are the most widely used shape memory alloys due to their outstanding shape memory effect and superelasticity. Additive manufacturing has recently emerged in the fabrication of shape memory alloy but despite substantial advances in powder-based techniques, less attention has been focused on wire-based additive manufacturing. This work reports on the preliminary results for the process-related microstructural and phase transformation changes of Ni-rich nickel–titanium alloy additively manufactured by wire-based electron beam freeform fabrication. To study the feasibility of the process, a simple 10-layer stack structure was successfully built and characterized, exhibiting columnar grains and achieving one-step reversible martensitic–austenitic transformation, thus showing the potential of this additive manufacturing technique for processing shape memory alloys.

A numerically controlled shape memory alloy wire bending process using vat photopolymerization

Abstract The fabrication of shape memory alloy (SMA) based products requires the precise bending and high-temperature training of SMA wires to obtain desired two-dimensional (2D) and three-dimensional (3D) patterns. In the typical fabrication process, SMA wire is bent and fixed in a pre-built mold and is trained using a furnace. However, in the small batch production, the fabrication of a mold that can be put in a high-temperature oven is costly and requires a long time to fabricate. A novel SMA wire bending process using vat photopolymerization (VPP) is presented in this paper. A rotary wire extruding system was designed to control the feeding speed of the fed wire and the bending of the wire into the desired shape. The wire bending process was enabled by utilizing a VPP tool that was immersed in liquid resin to selectively cure the resin to firmly fix the extruded SMA wire on a substrate. An air bubble assisted separation process was developed to address the separation of the cured resin from the immersed VPP tool. After the bending of the SMA wire into the desired shape, the SMA wire was connected to an electric current to heat the wire to a high temperature required by the training process so the SMA wire can memorize the fabricated pattern. Finally, the photocured fixtures deposited by the VPP tool were dissolved by a chemical solution and the bent SMA wire was detached from the fixture that was ready to use. The developed numerically controlled wire bending process is flexible for designed patterns and can fabricate SMA wires into 2D/3D shapes without specific molds or tedious manual work. Several test cases were performed to demonstrate the effectiveness of the newly developed SMA wire bending process.

In-situ TEM analysis of the phase transformation mechanism of a Cu–Al–Ni shape memory alloy

Minimizing phase transformation (PT) hysteresis is of crucial importance for reliability of shape memory alloy (SMA)-based devices, where the lattice strain/stress caused by thermal hysteresis leads to functional degradation. As a result, understanding structural factors that control PT pathway is critical for development of materials with high reversibility. In this study, two distinct PT mechanisms (from γ1' martensite to β1 austenite phase) in Cu–Al–Ni SMAs were revealed by in-situ TEM observation. A growth-dominant conventional PT mechanism shows in the fast quench sample, whereas a nucleation-dominant PT mechanism that suppresses interface propagation and induces high thermal hysteresis displays in the slow quench sample. By characterizing the atomic scale composition and microstructure we discover that slow quenching induces nanoprecipitation that changes chemistry of the matrix alloy, which in turn causes the higher PT hysteresis and temperature, while fast quenching avoids the formation of these nanoprecipitates. Our finding provides valuable insights into the fabrication of SMAs with better reliability.

<i>In-Situ</i> TEM Analysis of the Phase Transformation Mechanism of a Cu-Al-Ni Shape Memory Alloy Fabricated with Different Quenching Speeds

A shape memory alloy (SMA) recovers its original shape after deformation through martensitic phase transformation (PT), which is diffusionless and involves shear deformation. Minimizing PT hysteresis is of crucial importance for reliability of SMA-based devices, where the stress caused by thermal hysteresis leads to functional degradation. As a result, understanding structural factors that control PT, including phase nucleation and growth, is critical for development of materials with high reversibility. In this study, the PT mechanism (from γ1' martensite to β1 austenite phase) in Cu-Al-Ni SMAs was investigated by preparing alloy with different quenching rates, resulting in very different transition temperatures. By characterizing the atomic scale composition and microstructure we show that slow quenching induces nanoprecipitation that change chemistry and strain field of the matrix alloy, while fast quenching avoids the formation of these nanoprecipitates. In-situ monitoring of the PT process in TEM demonstrate a growth-dominant conventional PT mechanism with low hysteresis in the fast quench sample. On the other hand, the alloy with precipitates shows a nucleation-dominant PT mechanism with suppression of phase growth that induces high thermal hysteresis. Our finding provides valuable insights into the fabrication of SMAs with better reliability.

Free-standing NiTi alloy nanowires fabricated by nanoskiving.

We report on free-standing NiTi alloy nanowires (120 nm × 75 nm) fabricated using a technique referred to as "nanoskiving", which complements conventional thin film sputter deposition with ultramicrotomy for thin sectioning. To date, the technique has been limited to pure metals without exploring metallic alloys. Leveraging the technique for the fabrication of shape memory alloy (SMA) nanostructures meets two critical requirements: compositional control (via film deposition) and controlled dimensions (via film deposition and programmable sectioning). Microstructure and composition analysis confirm continuity of the produced nanowires and Ni and Ti elemental uniformity. Free-standing NiTi nanowires are robust and remain intact throughout physical manipulation. The fabrication of NiTi alloy nanowires by nanoskiving will advance fundamental characterization of small scale SMA behavior.

524 傾斜機能特性を有する形状記憶合金の作製(OS6-2 先進材料とその機械的性質(2),OS6 先進材料とその機械的性質)

524 傾斜機能特性を有する形状記憶合金の作製(OS6-2 先進材料とその機械的性質(2),OS6 先進材料とその機械的性質)

Tailored processing of epoxy with embedded shape memory alloy wires

We present the development of a low-temperature liquid composite moulding cure schedulethat is compatible with the fabrication of shape memory alloy (SMA)–epoxy compositematerials. With this process, the SMA wires do not need to be maintained in place with anexternal frame, even though the peak post-cure temperature exceeds the activationtemperature of the SMA wires. The intrinsic interfacial shear strength of the final materialis experimentally determined from single wire pull-out tests, and is compared with theshear stress exerted at the interface by an activated SMA wire. These measurementsshow that the interface is strong enough to withstand the maximum activationstress. This is confirmed through tests involving the cyclic activation of SMA wiresembedded in epoxy samples. The paper successfully demonstrates that, by carefultailoring of the processing schedule, an SMA–epoxy composite that maintains astrong interfacial bond during both processing and subsequent activation of theembedded wires can be fabricated using standard composite processing methods.

Fabrication of shape memory alloys using the plasma skull push–pull process

In this work, the fabrication of shape memory alloys (SMAs) through the use of the plasma skull push–pull (PSPP) process was studied. In this process, small quantities of metallic elements can be quickly melted on a copper crucible using a rotating plasma torch and injected into a metallic mold. In order to validate the utilization of the PSPP process to produce SMA, several Ni–Ti-based and Cu–Al-based binary, ternary and quaternary alloys were tested. It was verified that five melts and re-melts of the SMA before injection are enough to obtain a homogeneous product. The obtained SMAs were characterized by optical microscopy and microhardness measurements. The thermoelastic martensitic transformation at the origin of the shape memory effect (SME) was studied via electrical resistance as a function of temperature and differential scanning calorimetry (DSC). All studied SMAs were shown to undergo a thermoelastic transformation, thus demonstrating the viability of the PSPP process to fabricate a great variety of SMAs.

製造技術(<小特集>先端機能部材として活躍する形状記憶合金)

製造技術(<小特集>先端機能部材として活躍する形状記憶合金)