Surface Integrity of Ni-Rich NiTi Shape Memory Alloy at Optimized Level of Wire Electric Discharge Machining Parameters
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JMEPEG https://doi.org/10.1007/s11665-019-04477-2
Surface Integrity of Ni-Rich NiTi Shape Memory Alloy at Optimized Level of Wire Electric Discharge Machining Parameters Himanshu Bisaria
and Pragya Shandilya
(Submitted September 19, 2018; in revised form October 13, 2019) The concern of this experimental work is to study the surface integrity aspects such as surface morphology, three-dimensional surface topography, recast layer, phase analysis, and micro-hardness for Ni55.95Ti44.05 shape memory alloy at the optimized level of wire electric discharge machining parameters. A mathematical model was developed for surface roughness and material removal rate considering servo voltage, pulse on time, wire tension, wire feed rate, and pulse off time using response surface methodology technique. In order to obtain the optimized parameters, multi-objective optimization technique grey relation analysis was utilized. The adequacy of the developed model was also checked by analysis of variance. At optimal parameters setting, i.e., pulse on time 123 ls, pulse off time 58 ls, servo voltage 50 V, wire tension 3 N, and wire feed rate 5 m/min, maximum material removal rate (8.223 mm3/min) and minimum surface roughness (1.93 lm) were achieved. Surface characteristics of machined surface divulge the presence of discharge craters, debris, molten droplets, micro-voids, spherical nodules, and cracks. A recast layer of thickness 19 lm with approximately 21% of foreign elements was deposited on the machined surface at optimized parameters, whereas the micro-hardness of the outer machined surface was found to be increased approximately 1.98 times as compared to micro-hardness of bulk material. X-ray diffraction analysis shows the presence of the following compounds on the machined surface NiTi, Ni4Ti3, Ti4O3, Cu5Zn8, Ni(TiO3), and NiZn. Keywords
optimization, RSM, shape memory alloy, surface integrity aspects, WEDM
List of Symbols
TON TOFF Wg d
Pulse on time (ls) Pulse off time (ls) Spark gap Wire diameter
1. Introduction Abbreviations
A wide spectrum of NiTi SMAsÕ unique properties stimulates the interest of its usage in many sectors such as aerospace, biomedical, robotics, and other important domains because of their exceptional properties (Ref 1-5). In the past few decades, NiTi SMAs undergo considerable modifications in their phenomenal properties like shape memory effect, high wear and corrosion resistant, superelasticity and high specific strength. The same properties that impart uniqueness are also responsible for poor machinability in NiTi SMAs (Ref 6-8). A great proportion of SMA product market is occupied by Ni-rich SMAs (Ref 9). Owing to the poor machinability with conventional methods, it is difficult to maintain simultaneously low surface roughness, high MRR, and high dimensional accuracy (Ref 10, 11). Therefore, these issues must be dealt with extra care; to control these responses, the advanced machining processes must be exploited for effective and profitable machining of NiTi SMAs. However, unconventional machining pro
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