**1. Introduction**

Nickel-titanium (Ni-Ti) stent wire is a biomaterial, which have been widely used in various applications including medical devices [1–3]. For example, it is used for endovascular stents, which are useful in treating various heart diseases. Blood flow can be improved by inserting a collapsed nickel Ni-Ti stent into a vein and heating the wire, and it can serve as a substitute for sutures. As a result, high-quality surface finishes and mechanical functionality have become desirable characteristics of such biomaterials [4,5]. Conventional polishing or grinding can produce high-quality surfaces [6,7]. In previous works, many researchers have adopted some surface finishing methods for improving the surface accuracy of their products. Chang et al. [8] used the magnetic abrasive finishing process for improving a surface roughness (Ra) of cylindrical SKD11 materials using unbonded magnetic abrasive

tools. According to his results, the Ra of cylindrical SKD11 materials was enhanced to 0.042 μm by the unbonded magnetic abrasive tools. Heng et al. [9] proposed a new manufacturing precision microdiameter ZrO2 bar 800 μm in diameter using new magnetic pole designs via ultraprecision magnetic abrasive finishing. According to his results, a surface roughness Ra of ZrO2 ceramic bar was enhanced to 0.02 μm within 40 s under the optimal conditions. Singh et al. [10] applied the magnetic abrasive finishing process for enhancing the accuracy of a plane workpiece with various important parameters (i.e., voltage (DC), finishing gap, rotating speed, and abrasive grain size). According to his study, the voltage and working gap are obtained to be the best parameters for a change in surface roughness ( ΔRa). However, such techniques typically use industrial processing oils (e.g., light oil, oil mist, SEA oil), which contain toxic substances that are likely to exist on the finished surface [11], reducing the appeal of such products [12]. Park et al. [13] have explained that, in the machining process, when industrial oils such as petroleum-based oils are used, they possibly are more harmful to human health than ecological oils. Benedicto and Carou et al. [14] have reviewed the application of various machining fluids on the machining processes. The machining fluid have been widely used for reducing the machining temperature, and for removing microchips of metal workpieces. Despite these critical works, some disadvantages still remain, such as the environmental impact and health risks to workers. Li and Aghazadeh et al. [15] have studied the health e ffects associated with the toxicity from metalworking fluids (MWFs) in grinding processes. They have reported that a waste oil, coolant, or another lubricrant can generate a toxicity characteristic leachate procedure, which could be e ffective in human health.

In recent years, a surface accuracy and dimensional accuracy of wire products have been improved by some advanced surface treatment methods (i.e., ion implantation and plasma coating) [16]. The ion implantation and plasma-coating technique have successfully improved the surface quality of some biomedical wire materials (i.e., Ni-Ti wire, TMA wire, beta-titanium wire, etc.) [17]. However, despite the potential advantages of these surface treatment methods, various limitations still exist. In the ion implantation process, some highly toxic gases (i.e., arsine (AsH3), and phosphine (PH3)) have been used [18]. Therefore, these toxic gases probably remain on the wire workpiece surface after processing by ion implantation. Rahman et al. [19] demonstrated that in the plasma coating process, the high voltage electrical shock has been supplied to the gap of electrodes for producing the plasma. Therefore, in order to overcome these problems, the environmentally friendly oils have been applied to the ultraprecision magnetic abrasive finishing (UPMAF) process for ultraprecision finishing of Ni-Ti stent wire. In this study, we evaluated the e ffectiveness of light oil, olive oil, and castor oil for the UPMAF process of Ni-Ti stent wire in terms of Ra, and removed diameter (RD). This research aims to elucidate the characteristics of Ni-Ti stent wire produced with an UPMAF process and a rotating magnetic field according to di fferent processing oils. In addition, the e ffects of important input parameters (i.e., the rotating speed of the magnetic field, and di fferent processing times) on Ra and removed diameter were studied in this research.

#### **2. Experimental Method and Setup**

A photograph and a schematic diagram of an UPMAF process using a rotating magnetic field for processing Ni-Ti stent wire are shown in Figures 1 and 2, respectively. The equipment comprised a magnetic abrasive finishing part, a spool-driving stepping motor, two driven spools, two fixing rollers, two proximity sensors, a sensor controller, an electric slider, a power supply, a programmable controller, an electrical slider controller, a stepping motor controller, and a stepping motor (speed range: 350–4000 rpm). To achieve a high e fficiency of finishing accuracy of a Ni-Ti stent wire, two sets of Nd-Fe-B permanent magnets were used. The permanent magnets were composed of a south pole and north pole, which generated lines of magnetic force between the poles. The mixture for the magnetic abrasive particles consists of electrolytic iron particles, diamond abrasive particles, and processing oil. The abrasive particles were controlled by the magnetic force at room temperature (25 ◦C). To perform ultraprecision magnetic abrasive finishing, a Ni-Ti stent wire 120 mm in length was inserted inside the

particulate brush of the magnetic abrasive tool and vibrated at 10 Hz. The finishing part rotated at up to 2000 rpm. The schematic view of magnetic force acting on magnetic abrasive particle during the UPMAF process is shown in Figure 3. As the Nd-Fe-B permanent magnets were used, 520-mT of magnetic flux density was obtained in the finishing zone. For the finishing mechanisms of an UPMAF process, "M" is a position, where a magnetic force, *Fm*, strongly pushes on the Fe particle. A magnetic force, *Fm*, is generated by the two forces, *Fx* on the x-component and *Fy* on the y-component. The force *Fx* acts on the magnetic abrasive particle along the direction of the line of the magnetic field. The force *Fy* is produced by the line of the magnetic field when the Ni-Ti wire material pushes out the bridges formed in the direction of magnetic equipotential lines. A magnetic force, *Fm*, acting on a magnetic abrasive particle can be expressed by Equations (1) and (2) [20].

**Figure 1.** Photograph of ultraprecision magnetic abrasive finishing equipment for an ultraprecision Ni-Ti wire stent.

**Figure 2.** Diagram of ultraprecision magnetic abrasive finishing equipment for an ultraprecision of Ni-Ti wire.

**Figure 3.** Schematic view of magnetic force acting on magnetic abrasive particle.

Where, x is direction of the line of magnetic field, *y* is direction of the magnetic equipotential line, χ*FP* is the material magnetic susceptibility, μ is the permeability of free space, *V* is the volume of the magnetic abrasive particles, *H* is a magnetic field strength at point "M", *dHdX* and, *dHdY* are the variation rates of magnetic field strength in *x* and *y* components, respectively.

$$F\mathbf{m} = F\mathbf{x} + F\mathbf{y} \tag{1}$$

$$F\mathbf{x} = \chi\_{\rm FP} \mu V H \left(\frac{dH}{dX}\right) \text{ and } F\mathbf{y} = \chi\_{\rm FP} \mu V H \left(\frac{dH}{dY}\right) \tag{2}$$

Figure 4 is a photograph of the finishing part of an UPMAF process using a rotating magnetic field for the Ni-Ti stent wire. As shown in Figure 4, the Ni-Ti stent wire workpiece was put the gap between both magnetic poles, and surrounded by a mixture of unbonded magnetic abrasive particles. These magnetic abrasive particles were governed by magnetic forces. To perform an UPMAF process, a workpiece was moved inside the particulate brush of magnetic abrasive particles while the finishing part rotated. With this working procedure, the ultraprecision finishing of Ni-Ti stent wire was achieved.

**Figure 4.** Photograph of the finishing part for ultraprecision finishing of a Ni-Ti wire stent.
