Effect of laser microcutting on thermo-mechanical properties of NiTiCu shape memory alloy

The machining of shape memory alloys (SMAs), such as NiTi based alloys, is a very interesting and relevant topic for several industrial applications in the biomedical, sensor and actuator fields. Laser technology is one of the most suitable methods for the manufacturing of products in the aforementioned fields, mainly when small and precise features have to be included. Due to the thermal nature of this process, study of its effect on the functional properties of these materials is needed. Except for binary NiTi, few results on the laser machining of NiTi based alloys are available in the literature. In this work, thin sheets of Ni40Ti50Cu10 (at.%) were processed by a fibre laser and the effect of process speed on the material properties was analysed. Scanning electronic microscopy was adopted for observation of the laser cut edges’ morphology. Chemical composition of the processed material was evaluated by energy dispersion spectroscopy and nanohardness measurements were used to estimate the heat affected zone. SMA functional properties were studied on both base and laser machined material. These characteristics are affected by laser machining for the presence of melted material; this effect can be minimised by increasing the laser process speed.     

Characterisation of surface oxidation of nickel-titanium alloy by ion-beam and electrochemical techniques

The passivating effects produced on Ni₄₉Ti₅₁ (at.%) shape memory alloy by electropolishing in methanol-sulphuric acid electrolyte or thermal treatment in air are investigated with ion-beam techniques (secondary ion mass spectrometry (SIMS) and mass resolved ion scattering spectrometry (MARISS)) and electrochemical techniques (polarisation curves, photocurrent spectroscopy and impedance). Electropolishing produces a limited passivation due to formation of a thin layer of amorphous TiO₂, which causes only a slight hindrance to the oxygen evolution reaction. Thermal treatment in air at 450 °C causes a more substantial passivation due to formation of a thick oxide layer, estimated at about 80 nm for a 16 h treatment on the basis of weight increase, disregarding roughness. Data indicate that the film produced by thermal oxidation has probably a duplex structure, with a porous outer layer with inclusions of Ni species and a compact inner layer, in contact with the substrate, consisting of (essentially) nickel free TiO₂ rutile.     

Application of SMA Composites in the Collectors of the Railway Pantograph for the Italian High-Speed Train

In high-speed train operation the flexural modes of the collector play a significant role in the vibrations for the frequencies over 40-50 Hz. In a previous work it has been established that a possible way to increase the modal damping of these flexural modes, without deep modification of the collector structure, is to increase the specific damping of the lateral horns, usually made of glass fiber polymer. Ni-Ti alloy yarns can be used as “smart fibers” embedded in this conventional material in order to make new horns with increased damping capacity, with a configuration of laminated composite material. The first step of the work herein presented consists in setting, through a proper thermal treatment, martensitic structure within the pantograph working temperature range, in order to obtain damping capabilities at low amplitude strain in the range 10^?4-10^?3. Afterwards a series of dynamic tests aimed at identifying the damping capacity of the NiTi wires has been undertaken. A finite element (FE) model of the SMA composites horn has been finally validated, comparing the results of dynamic numerical analysis with the results of measurements.     

Nonequiatomic NiTi Alloy Produced by Self Propagating High Temperature Synthesis

Shape memory alloy NiTi in porous form is of high interest as implantable material, as low apparent elastic modulus, comparable to that of bone, can be achieved. This condition, combined with proper pore size, allows good osteointegration. Porous NiTi can be produced by self propagating high temperature synthesis (SHS), starting from mixed powders of pure Ni and Ti. Process parameters, among which powder compaction degree and preheating temperature, strongly influence the reaction temperature and the resulting product: at low reaction temperatures, high quantity of secondary phases are formed, which are generally considered detrimental for biocompatibility. On the contrary, at higher reaction temperatures, the powders melt and crystallize in ingots. The porous structure is lost and huge pores are formed. Mechanical activation of powders through ball milling and addition of TiH _x are investigated as means to reduce reaction temperature and overheating, in order to preserve high porosity and limit secondary phases content. Both processes affect SHS reaction, and require adjustment of parameters such as heating rate. Changes in porous shape and size were observed especially for TiH _x additions: the latter could be a promising route to obtain shaped porous products of improved quality.     

Electrical Pulse Shape Setting of Thin Ni49Ti51 Wires for Shape Memory Actuators

The effect of non-conventional electrical pulse treatment on microstructure and shape memory response of Ni₄₉Ti₅₁ wires was studied. High-power rapid current square pulses were applied to cold-drawn wires of 25 µm diameter. The TEM microstructures showed equiaxial recrystallization, with average grain size in the 100 nm range, after a double current pulse lasting 10 ms. Rapid current pulse treatment in Ti-rich NiTi induces shape memory effect with reduced thermal hysteresis and high recovery strain values. This novel treatment might replace conventional pre-device shape setting/training of the wire with a post-device shape setting with promising industrial advantages.     

Effect of Current Pulses on Fatigue of Thin NiTi Wires for Shape Memory Actuators

Shape memory alloys (SMA) are used in many technological applications, thanks to their unique properties: superelasticity and shape memory (SM) effect. Many efforts have been made to improve performance of SMA wires to utilize them as thermal actuators also for many thousands of cycles. Near-equiatomic nickel-titanium compound is the most used materials for SM actuators because of its high recoverable values of strain and good cycling stability, if compared to the other known SMA. In this study, the functional properties of NiTi thin wires (80 μm) thermally cycled under a constant load (200 MPa) were investigated. In particular, the effect of two heating conditions, carried out by a step and a ramp current pulses, on functional fatigue of SMA has been studied. By means of an experimental apparatus, thermomechanical cycling, thermal loop under constant load and actuation time (AT) tests were carried out to investigate the alteration and the trend of recovered strain, irreversible strain, characteristic temperatures, and ATs of the thin SM wires.     

Functional Response of NiTi Elements for Smart Micro-actuation Applications

Shape memory alloys (SMAs) can be considered a good candidate for actuation applications in the current micro-technology field. In the micro-scale, the temporal response of the SMA actuators can be improved, because of faster cooling during the austenite-martensite transformation. One of the most investigated geometries for this purpose has been the snake-like arrangement, which allows high strokes with considerable forces to be obtained. In this work, SMA elements for micro-actuators were patterned by laser machining in a snake-like shape. Subsequent surface chemical etching was adopted to improve the functional properties of the micro-elements. Calorimetric analysis and thermo-mechanical response of 90 μm thick SMA elements were reported for the evaluation of their functional performances. Moreover, the effect of post-thermal treatment and grain orientation were also evaluated on the final performances.     

Laser Micro-processing of Zr50Cu28Ni7Co15 High-Temperature Shape Memory Alloys

In this paper, an experimental study of laser micro-processing on a Cu-Zr-based shape memory alloy (SMA), which is suitable for high-temperature (HT) applications, is discussed. A first evaluation of the interaction between a laser beam and Zr₅₀Cu₂₈Ni₇Co₁₅ HT SMA is highlighted. Single laser pulses at various levels of power and pulse duration were applied to evaluate their effect on the sample surfaces. Blind and through microholes were produced with sizes on the order of a few hundreds of microns; the results were characterized from the morphological viewpoint using a scanning electron microscope. The high beam quality allows the holes to be created with good circularity and little melted material around the hole periphery. An analysis of the chemical composition was performed using energy dispersive spectroscopy, revealing that compositional changes were limited, while important oxidation occurred on the hole surfaces. Additionally, laser micro-cutting tests were also proposed to evaluate the cut edge morphology and dimensions. The main result of this paper concerned the good behavior of the material upon interaction with the laser beam, which suggests that microfeatures can be successfully produced in this alloy.