Numerical simulation of ultrasonic spot welding of superelastic NiTi alloys: Temperature distribution and deformation behavior

Ultrasonic spot welding (USW) has attracted increasing attention due to its high- throughput solid-state bonding mechanism, which shows great potential in the semiconductor and automotive industry for the joining of metal sheets. However, the short welding cycle makes it challenging to effectively monitor the temperature history and deformation of the workpieces during the USW process, especially for the materials with some special properties. In this study, a three-dimensional (3D) finite element analysis model for USW of superelastic NiTi shape memory alloy (SMA) with Cu interlayer was developed using ANSYS Workbench. The thermal-stress coupled phenomena including the heat generation and stress distribution during the welding process was simulated and analyzed. Firstly, the superelastic constitutive model of NiTi SMAs was constructed. The distribution of temperature and stress field was then obtained by thermal-stress analysis using the direct coupling method, and the superelasticity of SMAs was observed. The simulation results showed that the highest temperature occurred in the center of the welding area during USW, which is proportional to the welding time and inversely proportional to the clamping pressure. In addition, the maximum stress occurred at the center of the contact surface between upper NiTi and Cu interlayer. After that, the validity of the simulation results was verified by setting up a thermocouple temperature measurement platform to collect the temperature data, which exhibited a good agreement with the simulated results. The simulation procedure demonstrates its potential to predict temperature and stress distribution during USW process.

Evolution of Transformation Strain During Partial Transformation Cycling of an NiTi Shape Memory Alloy

This paper deals with how the magnitude of transformation strain changes on partial transformation cycling of an NiTi shape memory alloy. A near-equiatomic NiTi shape memory alloy was allowed to undergo partial thermal cycling keeping the stress constant at 100 MPa for various upper cycle temperatures (between austenite start and austenite finish), using a custom-built thermomechanical cycling test setup. The displacement and the temperature of the sample during cycling were measured using a LASER extensometer and an optical pyrometer, respectively. The test results show that the recovery strain and thermal hysteresis width decrease with increasing number of cycles during partial cycling. In addition, martensite start and martensite finish temperatures increase during the initial cycles, whereas austenite start and austenite finish temperatures decrease during the initial cycles, followed by their saturation.

XGBoost automates the characterisation of reversibly actuating planar-flow-casted NiTi shape memory alloy foil

Nickel-Titanium (NiTi) shape memory alloys (SMAs) are smart materials able to recover their original shape under thermal stimulus. Near-net-shape NiTi SMA foils of 2 meters in length and width of 30 mm have been successfully produced by a planar flow casting facility at CSIRO, opening possibilities of wider applications of SMA foils. The study also focuses on establishing a fully automated experimental system for the characterisation of their reversible actuation, significantly improving SMA foils adaptation into real applications. Artificial Intelligence involving Computer Vision and Machine Learning based methods were successfully employed in the development of the automation SMA characterisation process. The study finds that an Extreme Gradient Boosting (XGBoost) Regression model based predictive system experimented with over 175,000 video samples could achieve 99% overall prediction accuracy. Generalisation capability of the proposed system makes a significant contribution towards the efficient optimisation of the material design to produce high quality 30 mm SMA foils.

Influence of Batch Mass on Formation of NiTi Shape Memory Alloy Produced by High-Energy Ball Milling

A high-energy ball milling technique was used for production of the equiatomic NiTi alloy. The grinding batch was prepared in two quantities of 10 and 20 g. The alloy was produced using various grinding times. Scanning electron microscopy, X-ray diffraction, hardness measurement and differential scanning calorimetry were used for materials characterization at various milling stages. The produced alloy was studied by means of microstructure, chemical and phase composition, average grain and crystallite size, crystal lattice parameters and microstrains. Increasing the batch mass to 20 g and extending the grinding time to 140 h caused the increase in the average size of the agglomerates to 700 µm while the average crystallites size was reduced to a few nanometers. Microstrains were also reduced following elongation of milling time. Moreover, when the grinding time is extended, the amount of the monoclinic phase increases at the expense of the body-centered cubic one—precursors of crystalline, the B2 parent phase and the B19′ martensite. Crystallization takes place as a multistage process, however, at temperatures below 600 °C. After crystallization, the reversible martensitic transformation occurred with the highest enthalpy value—4 or 5 J/g after 120 and 140 h milling, respectively.

Study on Interface and mechanical property of laser welding of NiTi shape memory alloy and 2A12 aluminum alloy joint with a TC4 wire

2A12 aluminum alloy had the advantages of light weight and high strength. It could be used to manufacture the skin of the hypersonic aircraft. Due to the thermal deformation of fuselage and wing under long-term thermal and mechanical load, the accuracy of flight control was reduced. The shape memory effect of NiTi shape memory alloy (SMA) could be used to reduce the thermal deformation by realizing the laser welding of NiTi SMA and 2A12 aluminum alloy. According to previous studies on laser welding of NiTi SMA and TC4, the tendency to crack for the welded joints could be reduced by placing the laser beam on the side of TC4. Therefore, TC4 wire was used as the filling material. As the TC4 wire was constantly sent into the molten pool to absorb laser energy, the melting amount of NiTi SMA and 2A12 aluminum alloy were reduced. It was beneficial to reduce the formation of brittle intermetallic compounds. There were mainly the fusion zone (FZ), NiTi SMA/FZ interface, and the 2A12 aluminum alloy/FZ interface in the welded joints. With the increase of laser power, the growing distance of Ti2Ni phase also increased. In addition, the Ti-Al intermetallic compounds and the fracture load of joints firstly increased and then decreased. When the laser power was high, Ni-Al intermetallic compounds increased. This caused the decrease of fracture load of welded joints. Besides, pores caused by the burning of elements in the FZ would also weaken the fracture load of welded joints. When the laser power was 2.4 kW, more Ti-Al intermetallic compounds appeared at the interface and the maximum fracture load of welded joint was 211 N/mm. The fracture mode was intergranular brittle fracture. The heat affected zone (HAZ) with optimal mechanical properties basically retained the shape memory effect of NiTi SMA.