Narrow Results

This paper exemplifies the feasibility of expanding a multi-criteria decision-making (MCDM) method to select optimum process parameters during the wire electrical discharge machining (WEDM) of nitinol. The application potential of combined desirability function analysis (DFA) and analytical hierarchy process (AHP) has been reported. Nitinol, a shape memory alloy (SMA), can memorize or retain its original shape when subjected to thermo-mechanical or magnetic loads. Four key input variables, like pulse on time ($T_{\mathrm{\text{ON}}})$, pulse off time ($T_{\mathrm{\text{OFF}}})$, wire tension (WT), and wire feed (WF) have been studied to optimize three correlated responses, like kerf width, material removal rate (MRR), and surface roughness ($R_{\text{a}})$. Process parameter permutations $T_{\mathrm{\text{ON}}}=120\mu$s, $T_{\mathrm{\text{OFF}}}=55\mu$s, $\mathrm{\text{WT}}=8$ kg-F and $\mathrm{\text{WF}}=3$m/min were found to yield the optimum results. For the desired kerf width, MRR and $R_{\text{a}}$, the optimum process parameters were also achieved expending Taguchi’s signal-to-noise ratio. Validation results affirmed that the MCDM approach, AHP–DFA is a proficient strategy to select optimal input parameters for a preferred output eminence for WEDM of nitinol.

In this paper, Nitinol, an equiatomic binary alloy of nickel and titanium, was surface modified for its potential biomedical applications by chemical and electrochemical etching. The main objective of the surface modification is to reduce the nickel content on the surface of Nitinol and simultaneously to a rough surface microstructure. As a result, better biocompatibility and better cell attachment would be achieved. The effect of the etching parameters was investigated, using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectrometry (XPS). The corrosion property of modified Nitinol surfaces was investigated by electrochemical work station. After etching, the Ni content in the surface layer has been reduced and the oxidation of Ti has been enhanced.

Shape memory alloy (SMA), a distinctive class of material, can possess its preceding form when subjected to definite thermo-mechanical energy. Nitinol, an SMA, having an admirable shape memory effect, super elastic, and biomechanical properties, has developed a vast application in the field of biomedical, automobile, robotics, aerospace, etc. Wire electrical discharge machining (WEDM) technique is employed for machining of electrically conductive materials like SMAs, high tech ceramics, smart materials, etc. This paper is focused on analyzing the effect of different significant input parameters on the vital machinability aspects of SMA nitinol during WEDM. Independent input variables like pulse-on time ($T_{\mathrm{\text{ON}}})$, discharge current ($I)$, wire-speed (WS), wire tension (WT) and flushing pressure (FP) were considered to find out their influence on the kerf width (KW), material removal rate (MRR), arithmetic mean roughness ($R_{\text{a}})$, and microhardness ($\mu$h). 3D optical profile, X-ray diffraction analysis, and scanning electron microscopy were also executed on the WEDMed surface to inspect the surface, microstructure, and phase changes in the machined surface. It was detected that $I$, $T_{\mathrm{\text{ON}}}$ and FP were more influential than WT and WS for most of the responses.

NiTinol Shape Memory Alloys (SMA) are becoming one of the ideal choices for biomedical industries due to their unique properties such as Shape Memory Effect (SME), Super Elasticity (SE) and Biocompatibility. In the process of making complicated biomedical implants, welding processes play a vital role. In this work, an attempt was made to study the effect of heat input and Post Weld Heat Treatment (PWHT) on the TIG-welded NiTinol SMA. TIG welding was carried out on 1-mm thick NiTinol sheets. With increase in heat input, there was a significant variation in Phase Transformation Temperature (PTT) of welded samples. The variation in PTT is attributed to the formation of intermetallic phases such as Ti₂Ni, Ni₃Ti and NiTiO₃ and coarse grain formation. Electron Back Scattered Diffraction (EBSD) analysis on the weld revealed that the average grain size of parent material was increased from 9.92851$\mu$m to 48.292345$\mu$m after the welding process. The PWHT was carried out on the best weld characteristic sample. PWHT did not produce significant effect on PTT. Austenite start and finish temperature slightly decreased after PWHT, whereas slight drift towards the positive side was noticed in martensite start and finish temperature.

Nickel ion release from NiTi shape memory alloy is an issue for biomedical applications. This study was planned to study the effect of C${}^{+}$ implantation on nickel ion release and affinity of calcium phosphate precipitation on NiTi alloy. Four annealed samples are chosen for the present study; three samples with oxidation layer and the fourth without oxidation layer. X-ray diffraction (XRD) spectra reveal amorphization with ion implantation. Proton-induced X-ray emission (PIXE) result shows insignificant increase in Ni release in simulated body fluid (SBF) and calcium phosphate precipitation up to $8\times 10^{13}$ions/cm². Then Nickel contents show a sharp increase for greater ion doses. Corrosion potential decreases by increasing the dose but all the samples passivate after the same interval of time and at the same level of $V_{\mathrm{\text{SCE}}}$ in ringer lactate solution. Hardness of samples initially increases at greater rate (up to $8\times 10^{13}$ions/cm${}^2)$ and then increases with lesser rate. It is found that $8\times 10^{13}$ions/cm² ($\approx 10^{14})$ is a safer limit of implantation on NiTi alloy, this limit gives us lesser ion release, better hardness and reasonable hydroxyapatite incubation affinity.

This paper compares some of the vigorous machinability characteristics of SMA-Nitinol during WEDM process using uncoated and zinc coated brass wire electrodes. A series of experiments were regulated based on Taguchi’s L${}_{27}$ orthogonal array with an objective of unveiling the benefits of using coated brass wire electrodes in comparison to uncoated counterparts. Five factors, namely pulse-on time ($T_{\mathrm{\text{ON}}}$), discharge current ($I$), wire tension (WT), wire speed (WS) and flushing pressure (FP), were considered, each at three different levels to scrutinize four responses, viz. surface roughness (Ra), kerf width (KW), machining time (MT) and micro-hardness ($\mu$h). It was perceived that zinc-coated brass wire was more preferable to get favorable responses like Ra, KW and $\mu$h when compared with brass wire counterparts. FESEM micrographs also revealed that micro and large cracks, wide craters, recast layer were more prominent on the WEDMed surface of brass wire compared to zinc-coated brass wire. Use of zinc-coated brass wire electrode significantly improves the machinability of the selected work material within the specified range of process variables.

Shape memory alloys (SMAs) are a class of materials that have unique properties, including Young's modulus-temperature relations, shape memory effects, superelastic effects, and high damping characteristics. These unique properties, which have led to numerous applications in the biomedical and aerospace industries, are currently being evaluated for applications in the area of seismic resistant design and retrofit. This paper provides a critical review of the state-of-the-art in the use of shape memory alloys for applications in seismic resistant design. The paper reviews the general characteristics of shape memory alloys and highlights the factors affecting their properties. A review of current studies show that the superelastic and high-damping characteristics of SMAs result in applications in bridges and buildings that show significant promise. The barriers to the expanded use of SMAs include the high cost, lack of clear understanding of thermo-mechanical processing, dependency of properties on temperature, and difficulty in machining.

The results of pilot in situ studies of the responses of Nitinol surfaces to deformation are presented. It is shown that the mechanical behavior of Nitinol surfaces differs, depending on oxide thickness and its chemical composition. The corrosion resistance of the surfaces evaluated in strain-free and strained states using potentiodynamic and potentiostatic cyclic polarization at the body potentials demonstrated quite stable behavior.