The machining characteristics and surface integrity of Ni-rich NiTi shape memory alloy using wire electric discharge machining

Abstract

Owing to the increasing demand for Ni-rich shape memory alloys in various sectors such as biomedical, aerospace, and robotics, the efficient machining of shape memory alloys is vital for their productive exploitation. The aim of this experimental investigation is to explore the influence of wire electric discharge machining process parameters such as spark gap voltage, wire tension, spark off time, wire speed, and spark on time, on the cutting efficiency and surface roughness of Ni50.89Ti49.11 SMA using one factor at a time approach. The results reveal that cutting efficiency and surface roughness are strongly influenced by spark off time, spark on time, and spark gap voltage, whereas wire speed and wire tension have the inconsequential effect. The presence of many microcracks, craters, voids, bulges of debris, and the re-solidified layer of molten material on the machined surface have been detected in scanning electron micrographs. The results of phase analysis using energy-dispersive X-ray spectroscopy and X-ray diffraction divulge the migration of foreign elements from the brass wire and dielectric to the machined surface. Due to the formation of recast layer and various oxides, the hardening effect near the machined surface was also observed. The hardness near the machined surface has been increased several times in comparison to bulk hardness.

Study on crater depth during material removal in WEDC of Ni-rich nickel–titanium shape memory alloy

“…Spark frequency (number of sparks per second) is given according to Eq. (3) [6] which increases with the decrease in t ON . Here t ON is pulse on time, the five levels of t ON , 105, 100, 115, 120, and 125 µs, and pulse off time (t OFF ), 52, 55, 57, 60, and 63 µs, were considered during experiments.…”

“…Currently, SMAs are being widely used in various sectors, such as robotics, biomedical, aerospace, and other important industries due to their outstanding properties [2,3]. Ni-rich NiTi SMAs show better superelasticity and shape memory effect compared to equiatomic NiTi SMAs because of higher intrinsic strength, and they can be precipitation-hardened [4][5][6]. Conventional machining of NiTi SMAs is the most challenging task because of the high reactivity of titanium which causes poor surface finish, high tool wear, and high machining time [7,8].…”

“…It was found that material removal rate (MRR) shows increasing trend with pulse on time, whereas pulse off time and spark gap voltage show reverse trend. Bisaria et al [6] conducted the experimental study on machinability of Ni-rich NiTi SMA in WEDM and suggested that a thin recast layer with less transfer of foreign elements and small size craters was deposited on the machined surface at the lower level of discharge energy.…”

Surface Analysis of Wire-Electrical-Discharge-Machining-Processed Shape-Memory Alloys

“…High pulse-on time and high discharge current in WEDM meant high discharge energy and as a result of that a large chance of cracks, globules and other surface defects [28,29]. However, with the increase in pulse-off time, discharge energy and spark intensity decreased, which resulted in a reduction in cracks and surface defects [30]. Integrity is key in many applications in high-performance alloys [31].…”

“…Elemental analysis was carried out by using energy dispersive X-ray (EDX)analysis. The researchers used a brass wire as a tool electrode, and reported that the element of the wire material was deposited on the surface of the machined surface [28,30]. Figure 9 shows the result of EDX analysis for the machined sample obtained at the optimized parameter settings.…”