State of the art in processing of shape memory alloys with electrical discharge machining: A review

Abstract

Shape memory alloys (SMAs) have been well known for their superior and excellent properties which makes them an eligible candidate of paramount importance in real-life industrial applications such as; orthopedic implants, actuators, micro tools, stents, coupling and sealing elements, aerospace components, defense instruments, manufacturing elements, bio-medical appliances, etc. In spite of their exceptional properties, the effective processing of these alloys is always seen as a challenge by researchers around the globe. The present article has been therefore attempted to explore the numerous studies conducted to process these alloys by employing the principles of electrical discharge machining (EDM) and its allied approaches. The NiTi-based SMAs have been revealed to be explored majorly among the several types SMAs. The several investigations carried out in the domain of EDM, Wire-EDM, and some conventional processing of various types of SMAs have also been critically reviewed and reported. It also highlights the numerous experimental, theoretical, modeling, and optimization-based researches attempted in EDM of SMAs. It was also reported that the proper selection of process variables, tool electrode, and the dielectrics can substantially improve the overall process effectiveness. Among the various accessible EDM variants used for the processing of SMAs, attempted by the umpteen investigators, the wire-cut EDM process has been revealed as the most explored one for cutting SMAs than the other allied processes such as: die-sinking EDM and powder-mixed EDM. The micro-machining applications of EDM have also been deliberated briefly. The last section of the article reports about the opportunities and the challenges for future research.

Introduction

In today’s scenario of technological advancement, the demand for smart materials is continuously rising because of their inimitable and exceptional properties, and applications in various sectors ranging from bio-medical diligences to manufacturing industries. The influential properties of these materials push the technology toward a smart system with intelligent, unique, and adaptive features and functions, which necessitates the use of actuators, sensors, and micro-controllers. SMAs are one of the categories of shape memory materials (SMMs), which is going to memorize their shape or they have an ability to remember their previous form when exposed to the alterations in the temperature, and magnetic variations.^1–3 This is broadly called as—the shape memory effect (SME). Under certain conditions, often commonly observed is superelasticity (in alloys) or visco-elasticity (in polymers). ⁴ A broad range of SMAs in foam, solid, and film shapes have been developed presently. Among them, there are mainly three alloy systems reported, namely; NiTi-based, Copper-based (CuZnAl and CuAlNi), and Iron-based. ⁵ The demand for SMAs has been quite increasing in respect of engineering and technical applications in several commercial fields such as; in user goods and various applications in the industry,^6–8 composites and structures, ⁹ automobile,^10–12 robotics,^13–16 biomedical,^15,17–20 aerospace,^21–23 mini actuators and micro-electromechanical systems (MEMS),^{15,17,24–27} and even in fashion. ²⁸ However, for specific requirements or applications, each type of SMM has its advantage.

In spite of the excellent and superior properties of the SMAs, their effective processing with conventional processes is not easy and acquire high processing costs and low accuracy, which further limit its expansion in the market. In the era of innovation and development today, superior product quality is becoming a major issue. There are several advanced manufacturing processes that can be used to process different advanced engineering components and materials with greater quality, that is, thermal-based, mechanical energy, and electrochemical based. In advanced machining operations, EDM is one of the types of advanced manufacturing practices successfully employed for machining of materials that are difficult to cut.^29–32 This method has been applied in modern manufacturing to enable high precision machining, slice complicated form, and enhance surface conditions.^33–35 One of the strengths of EDM strategies is that a proper distance is conserved throughout amid the work material and tool (electrode); post-processing the EDM results in residual stress—free material. ³⁶

The present study aims to capsulize the immense researches conducted on the EDM and its allied machining of different SMAs. The paper is made to provide a comprehensive review on the machining of SMAs with the different EDM process variants. It starts with the basics of SMAs, problems conveyed with the conventional machining of SMAs, introduction, and applicability of EDM, its principle, machining of different SMAs with diverse EDM operations. Furthermore, it reports the critically reviewed several experimental, theoretical, modeling-based investigations undertaken in the EDM-based processing of SMAs. The final section discusses the different research problems found and the directions for SMAs future work in EDM.

An overview of shape memory alloys

Arne Ölander first found SMA or “intelligent alloy” in 1932, and Vernon first defined the word “shape memory” in 1941. ³ The function of shape memory components was not recognized until 1962. Wang and Buehler disclosed about the impact of shape memory in an alloy of nickel-titanium (Ni-Ti), often called nitinol (Drawn from the material structure and the place of discovery, i.e., a mixture of Ni-Ti and Naval ordnance lab). ¹ SMAs belongs to a group of metallic alloys, those when exposed to a process of memorization among two stages of transformation depending on temperature or magnetic field, can retain their actual form (size & shape). This phenomenon of transformation is referred to as the shape memory effect (SME). These kinds of materials have two basic phase systems, lower martensitic phase and higher austenitic phase of temperature. ³⁷ Evolution in such materials is widely referred to as shape memory transformation in phase schemes. The driving force behind this transition is the difference in Gibbs free energy for the altered phases and can be induced by stress or temperature. ³⁸

The SMAs exist with different structures of crystals and six transformations in two different phases in the two distinct phases with six different transformations and three structures of crystals, as shown in Figure 1. ³⁹ The three separate crystal forms are martensite twined, martensite detwined, and austenite. The shape change effect and pseudoelasticity are characterized into three categories, that is, Pseudoelasticity or Superelasticity, one-way shape memory effect (OWSME), and two-way shape memory effect (TWSME). Three main classes of SMA system are; Nickel-Titanium-based (Ni-Ti) SMAs, Copper-based (Cu) SMAs, and Iron-based (Fe) SMAs. Nickel-titanium (Ni-Ti) alloys have initiated the applicability of alloys that exhibit shape retention effects for saleable & industrial purposes.^40,41 NiTi SMA has high ductility, great corrosion resistance, excellent fatigue, and strain-controlling in diverse environments including fluids. ⁴² The other explores NiTi variants are Ni-Ti-Cu, Ni-Ti-Nb, Ni-Ti-Fe, Ni-Ti-Co, Ni-Fe-Ga, and Ni-Ti-Pd.

Figure 1.

SMA different phase changes and the crystal structure. ³⁹

Copper-based SMAs are considered as the economical alloys rather than NiTi SMAs because they are easy to manufacture using orthodox powder metallurgy and liquid metallurgy routes. Alloys based on Cu have extensive transforming temperature range, low hysteresis, Great superelastic effect, and high damping coefficient. All these anticipated properties have expanded Cu’s potential to shape memory applications. Cu–Zn & Cu–Al are the main Cu-based alloys, with a third alloy element which has been often added to alter the temperature or microstructure of the transformation. ⁴³

The third SMAs projecting group is Iron-based SMAs subsequently the Ni-Ti, and Copper-based SMAs. Detailed Classification of SMAs is depicting in Figure 2. This SMA class over the NiTi alloy system was economically viable due to its relatively low alloy component & easy manufacturing. ⁴⁴ Iron-based SMAs are also known as shape memory steel (SMS) and consist of various alloys; that is, Fe–Mn–Si, Fe–Pt, Fe–Pd, Fe–Ni–C, Fe–Mn–Al, and Fe–Ni–Co–Ti. ⁴⁴ For an economically viable process, SMS production rates must be as high as carbon steel (Table 1).

Figure 2.

Broad classification of SMAs.

Table 1.

Various characteristics of different SMAs.

Sr. No.	Process characteristics	Fe-based	Cu-based	NiTi
1	SME	Low 45	Low 46	High 38
2	Workability	Good 47	Low 43	Moderate 41
3	Fabrication	Moderate 44	Good 46	Low 39
4	Maximum recoverable strain	Less than 6% 25	5–6% 26	8–9% 38
5	Cost	Low 45	Low 43	High 23

SME: shape memory effect.

Electrical discharge machining and its allied processes

EDM is a thermo-erosive type of process in which spatially and temporally controlled separated pulsed discharges are utilized to machine electrically conductive materials regardless of their mechanical, chemical, and thermo-physical properties. ⁴ The EDM process was invented in the 1940s. ³⁵ EDM exists as one of the modern machining operations productively used for machining tough to cut materials. This machining method has been utilized in modern industrial sectors for the promotion of machining material with great accuracy, cut complex shapes with optimal surface parameters. One of EDM’s major advantages is that there has been no direct interaction amid the tool (electrode) & work material during the machining progression, this further leads to material free from residual stresses after the machining process. EDM is a production process using electric discharge (sparks) to obtain the desired shape. Furthermore, known as; die-sinking, machining method using spark, tool erosion machine.

Working principle of EDM

The overall concept of the EDM process is the use of thermos-electric energy to erode the material from a piece of work with the assistance of periodic electric sparks between the workpiece and the uncontacted electrode. The workpiece and the tool are submerged in a dielectric fluid, as explicated in Figure 3. The common types of liquid dielectrics used in the EDM are kerosene, de-ionized water, EDM oil, etc. In some other cases, dielectrics based on gaseous also used in the EDM process. ⁴⁸

Figure 3.

Basic working mechanism of EDM process.

The workpiece is made as anode and tool as a cathode. If the gap voltage is adequately high, then in the form of spark it discharges through the gap. The spark formation results in the formation of electrons and positive ions which results in the formation of a conductive channel. There is a collision between the electrons and ions ensuing in the creation of the plasma channel. Due to the creation of a channel of plasma, a very large thermal energy generated which results in very high temperature. Due to such high-temperature formation in the machining zone, removal of material takes because of instant melting and vaporization of the material. During the machining process, the flushing of dielectric fluid carries away the unwanted debris, residue material, and restores the sparking condition.

EDM input and response parameters

The different performance parameters guide the electrical discharge process and these are known as peak current, voltage, pulse-off time, discharge gap, pulse-on time, workpiece rotation, dielectric fluid flushing, pulse waveform, etc.^49,50 Spark voltage (V) is the mean voltage while performing machining in the interface amid the workpiece and the electrode. Spark gap size regulation and overcut are directly affected by the discharge voltage. ⁴⁸ The power expended during the discharge process is known as the peak current. The parameters which are directly prejudiced by the peak current are machining accuracy, rate of material removal, and electrode wear. ⁴⁹ The duration of discharge time is known as pulse-on-time. MRR is directly influenced by T_on, which upsurges with larger T_on. ⁵⁰ When there is no discharge applied during the machining process, then that particular time period is known as pulse off time (T_off). During the no discharge period, unwanted debris is flushed away with the dielectric from the machining area. ⁵¹ The positive or negative charge of the electrodes in the machining process is known as Polarity. The workpiece has different polarity as compared to the tool electrode. The tool and workpiece interface space is identified as the discharge gap (G). The value of the gap lies usually between 0.01 and 0.1 mm. The important non-electrical parameter in EDM is Flushing. It is defined as the flow rate of dielectric onto the machining area in the machining of the product in EDM. Flushing helps to keep the machining area clean with the removal of debris and control of temperature at the machining zone with the proper flow rate of dielectric fluid.

The overall process performance in EDM is measured by the various response parameters. These process characteristics can be articulated as; material removal rate (MRR), surface quality, electrode wear rate (EWR), dimensional deviation, overcut length, surface profile, etc. MRR is known as the amount of removed material from the workpiece in a unit time. The main goal of the industries is to increase the MRR by controlling the other parameters. Different techniques and methods are followed for obtaining the optimum response measures. Electrode wear rate (EWR) is defined same as the MRR but in this case, the tool is there instead of the workpiece, it is known as the amount of removed material from the tool during the process of machining in a unit time. Figure 4 is exploring various parameters that influence the EDM process. Surface quality is an imperative response factor which governs the surface characterization of the workpiece. It includes a heat-affected zone, surface roughness, microcrack density, crack cavity, etc. Based upon the various investigations carried out in the domain of EDM process, this machining method has been studied broadly by attempting into its numerous variants namely; Die-sinking EDM, dry EDM, micro-EDM, wire-EDM, and PM-EDM.^50–52 However, the die-sinking EDM (DS-EDM), and wire-EDM variants have been majorly attempted by the umpteen researchers around the globe.

Figure 4.

Fishbone representation of various process and machining characteristics in EDM operations.

EDM of various shape memory alloys: State-of-the-art

Machining with EDM-based processes is a new zone of experimental and theoretical research in the case of SMAs. There have been conducted numerous research studies with the purpose of investigating SMAs with EDM and its allied processes. The critically reviewed literature on SMAs machining with EDM-based methods has been thoroughly explored in the present section. These reviewed studies have further been categorized based on EDM variant investigated, machining appearances of interest, and other methods employed for processing of the SMAs.

Die-sinking EDM

Experimental studies on SMAs machining with EDM operation by various investigators have taken into consideration the impact of process inputs like; peak current, pulse-off-time, voltage, workpiece rotation, pulse on time, dielectric flushing, pulse waveform, and discharge gap on numerous machining features of interest. Several performance attributes in the interest of machining of SMAs with EDM are mainly as; MRR, EWR, and surface morphology.

Material removal rate (MRR)

The MRR has been investigated by various researchers in the electrical discharge machining of SMAs. MRR has been revealed as an important response parameter and the reported machining method claimed to be one of the suitable methods for offering superior material removal rates while processing SMAs. Alidoosti et al. ⁵³ investigated on NiTi SMA machining characteristics based on the full factorial design in EDM process. It was reported that with the rise in pulse current value it leads to an increase in MRR. However, with an upsurge in the value of pulse current, current density rises which further leads to an increase in MRR. MRR was found to be increased with a rise in pulse period due to large amassed electro discharge energy that speedily melt and evaporate the work. Abidi et al. ⁵⁴ analyzed and optimized the μEDM process with the help of MOGA-II approach. It had been reported that EDM has been used for micro-hole drilling. MRR was observed as greater for the electrode made of brass in the selected domain for the same discharge energy. Brass has less thermal conductivity than tungsten, so the workpiece absorbs extra heat and outcomes in elevated MRR than the tungsten electrode.

Gaikwad et al. ⁵⁵ conducted the optimization of the process parameter for the electrical discharge process to optimize the product removal rate through the fabrication of NiTi. The optimized rate of material abstraction achieved was 7.0806 mm³/min. Daneshmand et al. ⁵⁶ experimentally explored the impacts of EDM inputs namely; voltage, T_on, discharge current, and T_off on the MRR of NiTi alloy. The rise of I_p boosts both the energy and the positive ions that attack the workpiece surface and increases the temperature of the workpiece, thereby results in a rise in MRR due to melting and further evaporation of the material from the workpiece surface. Chen et al. ⁵⁷ inspected characteristics of EDM process while machining of Ni-Al-Fe SMA. Experimental results showed that in the EDM process, the MRR of Ti_35.5Ni_49.5Zr₁₅ and Ni₆₀Al_24.5Fe_15.5 exhibit a reverse relationship to alloy material’s thermal conductivity and melting temperature. Al-Ahmari et al. ⁵⁸ experimentally investigated the Ni-Ti SMA on the grounds of MRR in laser-EDM, and μ-EDM. For the case of laser-EDM, it was disclosed that the MRR was much greater than the standard μ-EDM process. Because of low short-circuiting and arcing, the higher MRR was attained in the case of laser-EDM and further, the dielectric helped the debris produced in the machined hole to flow away. They have employed Q-switched Nd: YAG laser operating at a wavelength of 1064 nm for machining purposes. Jatti ⁶² studied the MRR while conducting EDM of NiTi, NiCu, and BeCu Alloys. At the optimal parametric setting, the attained value of MRR was recorded as 9.157 mm³/min.

Tool wear rate/Wear rate of electrode

Wear of tool in EDM operations while machining of SMAs is an important parameter of concern in EDM processes. A lesser rate of tool wear is always approached for having optimized results in EDM of SMAs. In various studies carried out in the context of EDM of SMAs, the tool wear is frequently being reported as electrode wear because in EDM process the tool is considered as an electrode. Chen et al. ⁵⁹ explored Ti-Ni-X ternary SMAs using EDM and studied the influence of the machining characteristics on these SMAs in the EDM process.

Alidoosti et al. ⁵³ inspected on characteristics of electrode wear while machining of SMA of nickel-titanium in EDM process. The increased pulse current leads to enhanced relative electrode wear, which happened due to the production of more discharge energy caused into the tool wear. Abidi et al. ⁵⁴ examined the tool wear rate of Ni-Ti based SMA. The tool wear rate is one of the key parameters of concern of the life of the tool for the cost of machining and accuracy. An increased TWR was reported at greater concentrations of discharge energy and therefore it was suggested to employ shorter pulses to decrease the TWR.

Daneshmand et al. ⁵⁶ considered the impact of inputs of EDM on the tool wear and relative electrode wear while machining NiTi alloy. Tool wear was reported to be largely dependent on tool material and pulse energy, and the amount of tool wear increases with the increase in pulse energy for a given tool. The pulse duration versus the electrode wear rate against the different discharge current values in the case of Ni₆₀Al_24.5Fe_15.5 alloy has been shown in Figure 5. It shows the initial rise in EWR value, it touches the peak value and then fallen with an increase in pulse duration. ⁵⁷ Jatti ⁶² studied the TWR while conducting EDM of NiTi, NiCu, and BeCu Alloys. At the optimal parametric setting, the attained value of TWR was recorded as 0.128 mm³/min. Figures 6(a–c), reflects the numerous process outcomes, the different types of SMAs, and various process inputs investigated while performing EDM of several SMAs, respectively.

Figure 5.

EWR vs. Pulse duration in Ni₆₀Al_24.5Fe_15.5 alloy. ⁵⁷

Figure 6.

(a) Response parameters in EDM of SMAs, (b) summarized view of different SMAs processed with EDM, and (c) process parameters in EDM of SMAs.

Surface morphology

Surface morphology related studies include the investigation of the roughness of surface machined, microstructure analysis of the processed work surface, and the surface integrity-based aspects. This machining attribute has been seen as the utmost important process responses while attempting EDM of SMAs. Ahmari et al. ⁵⁸ analyzed the viability and characteristics of the micro and laser-EDM in combination for machining of 200 μm diameter micro-holes in Ni-Ti-based SMA. The evaluation of microstructure and morphology study of drilled micro-holes with the combination process of micro and laser-EDM has been obtained with scanning electron microscopy.

The surface quality of the holes has been shown in Figure 7. It had been revealed that in the hybrid machining method, imperative improvements in micro-hole quality relative to laser processes observed, and eminence of holes was as worthy as average micro-EDM. ⁵⁸ It was noted that the taperness had been considerably decreased using the hybrid method and the holes become more parallel as seen in conventional μ-EDM. ⁵⁸ Surface roughness has been described as irregularities on the surface that occur due to the impact of various performance factors on the machining surface. It had been witnessed that with the upsurge in voltage and capacitance, there was an increase in surface roughness. The Ra value was found to be constant irrespective of discharge voltage, in case of machining Ni-Ti SMA with a brass electrode at low capacitance. ⁵⁴ Daneshmand et al. ⁵⁶ explored the characteristics of SMA based on NiTi alloy and studied the impact on surface roughness of various process parameters. Taguchi’s method has been implemented to scheme the runs. The trials designate that the factors of T_on, voltage, and discharge current have put up great influence on machining responses in EDM.

Figure 7.

Surface quality produced with: (a) micro-EDM, (b) laser machining, and (c) hybrid machining. ⁵⁸

Dimensional accuracy

Dimensional accuracy is a vital factor from a machining point of view, it describes the closeness of the measurements of the machined product to the standard one’s. It covers various machining characteristics like dimensional shift, overcut, taperness, circularity. Overcut is regarded as the crucial factor for the micro-hole measurement of the machined samples after performing EDM operation. Overcut in the samples described as the distinction amid the micro-hole diameter generated and the tool electrode’s real diameter. ⁵⁸

Overcut rises with the upsurge in discharge energy, as at greater levels of energy, the spark conveys greater amounts of energy, decomposing big amounts of dielectric, releasing more oxygen, influencing machining effectiveness and resulting in subordinate sparks among the edges of the electrode (tool) and the internal surface of the hole due to incorrect debris flushing. The taper angle also observed as a significant feature in assessing the accuracy of micro-holes. In micro-EDM, wear of the corner of the tool revealed as a major factor accountable for the taperness of the processed micro-holes. ⁵⁴ With the rise in capacitance for both electrodes, the overcut rises in the case of brass electrode but with less variations, and greater values of overcut are achieved. The tapering is assessed as the distinction between the input diameter and the micro-hole output diameter and angle amid them is called as the angle of taper. ⁵⁴ With the greater capacitance value of the tungsten electrode, the smallest angle of taper was accomplished while with the small capacitance values of the brass electrode, the highest taper angle was obtained while performing machining operation of Ni-Ti based SMA on EDM. ⁵⁴ The circularity error has been explained as the radiated distance amid the least confining circle and the extreme inscription circle. It had been revealed that for the tungsten electrode, the circularity error was almost constant, and regardless of the capacitance, the lower values were obtained. On the other hand, a substantial variation was noted for brass tool, and circuitousness flaws get reduced with a rise in capacitance. For the tungsten electrode, the small circularity error was due to a low rate of tool wear rate and rate of material removal. ⁵⁴ Table 2 exemplifies a review of literature on EDM of SMAs. Figures 8 and 9 are reflecting the variation in the recast layer thickness by varying the pulse duration for the EDMed samples of Ti₅₀Ni₄₀Cu₁₀, and Ti_35.5Ni_48.5Zr₁₆ (Table 3).^57,60

Table 2.

Review of past studies on SMAs in EDM.

Sr. No.	Author(s)	Work material	Attempted processparameters	Studied process characteristics	Remarks/Conclusions
1	Alidoosti et al. 53	NiTi SMA	EM, Cp, Ton	MRR, SR, EWR	W-Cu electrode has more work stability, MRR enhances with an increment in pulse current.
2	Abidi et al. 54	NiTi SMA	CP, Dv, ET	MRR, TWR, SR,Oc, Tp, Cy	High MRR achieved with brass electrode. Capacitance and electrode materials are dominant parameters in machining process.
3	Gaikwad et al. 55	NiTi alloy	Cg, Toff, TC, WC, Ton	MRR	Significant impact on MRR. Based on optimum parameter configuration, the optimized MRR is 7.0806 mm3/min.
4	Daneshmandet al. 56	NiTi SMA	Dc, V, Toff, Ton	SR, MRR, TWR	Dc, voltage, Ton have direct impact on MRR. TWR diminishes with increase in Toff.
5	Ahmari et al. 58	Ni-Ti basedSMA	De, V	SR, TWR, MRR, DA	Hybrid machining results in a 50%–65% reduction in the machining time. MRR improved with less TWR, SR and better dimensional accuracy on optimized parameter settings.
6	Chen et al. 57	Ni-AlFe ternarySMA	Dc, Ton, Toff, Vg	MRR, SR, Hd	With growing Dc, MRR increases monotonically in EDM of Ni60Al24.5Fe15.5 ternary SMA. Ni60Al24.5Fe15.5 demonstrate increasing SR with increase in Dc and Ton. The hardness of the specimen near the outer surface of the EDMed alloy Ni60Al24.5Fe15.5 will exceed 527 Hv.
7	Chen et al. 59	TiNiX SMA	Dc, Ton, Toff, Vg	MRR, SR, Hd	MRR for Ti35.5Ni49.5Zr15 and Ti50Ni49.5Cr0.5 SMA significantly increases with De, having maximum value at τP =12µs at IP = 10A. At maximum MRR the the thickness of recast layer becomes minimal and it also varies with Ton. For EDMed Ti50Ni49.5Cr0.5 and Ti35.5Ni49.5Zr15 alloys, the hardness of the specimen near the outer surface can exceed 913 and 1087 Hv.
8	Hsieh et al. 60	Ti35.5Ni48.5Zr16 andNi60Al24.5Fe15.5 SMa	Dc, Ton, Toff, Vg	MRR, SR, Hd	With increasing Dc, MRR becomes monotonically increased. As the De increases, crater size increases. With increase in Ton, the thickness of recast layer get increased.
9	Abdul-Raniet al. 61	Zr-based BMG	Ip, DD, PC	Hd, SR	Surface hardness greatly improved from 49.9 to about 85.5 HRC. SR majorly impacted by peak current, Ton and their interaction. Lowest roughness of 1.976 µm achieved at current of 5 A and durations of 4 µs. Surface roughness reduces with the increase in HA powder concentration.
10	Jatti 62	NiTi alloy,NiCu alloyand BeCu alloy	WC, Cg, Vg, Ton, Toff	MRR, TWR	The optimal value of MRR is 9.157 mm3/min and TWR is 0.128 mm3/min obtained at optimal setting s of parameters.
11	Gaikwadet al. 63	NiTi 60	Dc, Vg, Ton, Toff	MRR, SR, WLT	MRR increases with increases in current. For maximum MRR the optimal process parameters are current 8A, Ton 20 μs, Toff at 7 µs and voltage 55 V. With increase in voltage and current, the SR get increased.
12	Ahmedet al. 64	Ti-6Al-4V	ET, Ip, PTR	MRR, SR	MRR improves with high discharge current. Work surface integrity gets to be coarser with high discharge current. Graphite has demonstrated to be the best electrode for material removal and less surface roughness value.
13	Hasçalik andÇaydaş 65	Ti-6Al-4V	ET, Cp, Ton	MRR, SR, SM	MRR increase with pulse on duration. Surface roughness and electrode wear increases with process parameters. Recast layer increases surface roughening. WLT increase with process parameters. Graphite proved to be best electrode.
14	Kou et al. 66	Ti-6Al-4V	Cp	MRR, SM, TWR, RSL	MRR enhances with increase in current. MRR obtained is five times than obtained with conventional EDM. SR and TWR is more at maximal value of MRR.
15	Huang et al. 67	TiNi-based SMA	Cp, Ton	MRR, EWR, SR	Increase in pulse current and duration leads to sustained surface melts, which leads to high MRR. With increase in Ton value the EWR get increased. SR value is less at low current value.
16	Daneshmand et al. 68	NiTi-60 SMA	Dc, Ton, Toff, Vg	MRR, TWR, SR	MRR increases with the rotation of tool, Al2O3 powder and upsurge in current strength and Ton. With tool rotation and Al2O3 powder, TWR declines with increased intensity of voltage and current. With increase in Ton and Toff, the TWR increases.
17	Daneshmand et al. 69	NiTi alloy	Vg, Dc, Ton, Toff	MRR, TWR, SM	In rotational spark, the material removal speed for NiTi alloy is lower than traditional spark. Due to the continual movement of electrode craters are less and surface roughness is less in rotational spark than the traditional ones.
18	Katiyar et al. 70	Fe-Cu alloy	Vg, Ton	SM	High pulse on time gives the molten metal adequate time to achieve spherical shape, while reduced pulse-on-time quenches the molten material to achieve arbitrary shape. TEM confirms the nano phase granular particles of Fe-Cu alloy. SEM revealed existence of spherical and non-spherical shape particles.
19	Lin et al. 71	TiNi SMA	Cp, Ton, Toff, Vg	MRR, SM, Hd	With the rise in Cp, MRR upsurges monotonically Recast layer consists of the oxides of TiNiO3, TiO2 and consumed Cu electrodes, deposition particles.
20	Theisen et al. 72	NiTi SMA	V, Ip, Fy	MRR, SR, SM	MRR increases with the increase in discharge current due to high thermal energy formation which leads to removal of more material. Formation of more craters at high discharge current value.
21	Lin et al. 73	TiNi SMA	Cp, Ton, Toff, Vg, ET	MRR, SR, SM, Hd	MRR directly relates with discharge energy mode. It increases with increase in pulse current and provide optimal results at τp = 6–12 µs at Ip = 12 A. The recast layer thickness firstly increases reaching a critical value and then decreases with increase in pulse duration.

C_p: pulse current; T_on: pulse duration/pulse on time; C_g: gap current; T_off: pulse off time/pause duration; D_c: discharge current; V_g: gap voltage; D_e: discharge energy; EM: electrode materials; CP: capacitance; ET: electrodes; TC: tool conductivity; WC: work conductivity; V: voltage; I_p: peak current/current; DD: discharge duration; PC: powder concentration; PTR: pulse time ratio; F_y: frequency; MRR: material removal rate; SR: surface roughness; EWR: electrode wear rate; TWR: tool wear rate; O_c: overcut; T_p: taperness; C_y: circularity; H_d: hardness; SM: surface morphology/surface characterization/surface topography; RSL: re-solidified layer.

Figure 8.

SEM micrographs near the EDMed surface layer for the Ti₅₀Ni_49.5Cr_0.5 SMA. ⁵⁷

Figure 9.

Micrographs of EDMed surface developed for the Ti_35.5Ni_48.5Zr₁₆. ⁶⁰

Table 3.

Summary of experimental and theoretical studies on SMAs with EDM.

Sr. No.	SMA types	Experimental studies	Theoretical studies
1	NiTi	Refs.53–57,62,63,68,69,72	Ref. 54
2	Ni-AlFe	Refs.57,60	Ref. 60
3	TiNiCr	Refs.59,67	Ref. 67
4	TiNiZr	Refs.59,60	Ref. 59
5	Ti-6Al-4V	Refs.64,65,66	Ref. 66
6	Fe-Cu	Ref. 70	Ref. 70
7	TiNi	Refs.67,71	Ref. 71
8	TiNiCo	Ref. 59	Ref. 59

In spite of the various quality aspects associated with the EDM method, there also exist some crucial and negative aspects too, as the EDM process is a spark erosion method. There is a high-temperature formation, that is, plasma in the machining area, which results in deterioration of the workpiece and tool. The operation of EDM process at large pulse on time and peak current results into more discharge formation that continuously results in large temperature which leads to more stress and wear of the SMAs. Long exposure to high-temperature results in HAZ formation, which changes the chemistry of the SMAs. ³⁹ Microstructural changes, grain orientation results in wear and tear of the workpiece. The shape of the SMA gets distorted due to stress induction with continuous exposure of large temperature. SMA has the property to memorize their previous form up to a particular temperature condition, which is known as shape memory effect (SME), ⁴ further increasing the temperature results into loss in shape memory effect (SME). The structural changes result in amorphization in SMA that ultimately destroys the SME, alloy loses its memorizable property. The surface properties particularly affect when performing machining operations at high-temperature conditions, as more surface roughness in SMAs hinders their practical use in industry.

Wire electrical discharge machining

WEDM is an imperative variant of advanced processing operations employed for cutting of advanced materials irrespective of their hardness. In this section different SMAs have been focused on the grounds of machining aspect with the utilization of WEDM has been discussed and reviewed. A study based on different response parameters has been presented. Figures 10(a–c) represent the various process inputs, the different types of SMAs, and the numerous process outcomes investigated while performing WEDM on several SMAs, correspondingly.

Figure 10.

(a) Studied process parameters in WEDM of SMAs, (b) machining of various SMAs with WEDM, and (c) response parameters studied in WEDM of numerous SMAs.

Material removal rate

The rate of material removal has been revealed as a vital machining performance characteristics in WEDM. It is explored as the amount of material removal takes place per unit time. Manjaiah et al. ⁷⁴ investigated the characteristics of Ti-Ni-based SMA with the usage of wire cut EDM process. It had been found that during the use of kerosene and additive fluids as dielectrics results in more oxides and carbides formation as compared to de-ionized water. Narendranath et al. ⁷⁵ investigated on experimentation of different performance characteristics in WEDM of Ti₅₀Ni_42.4Cu_7.6 SMA. The results based on parametric analysis reveal that low peak current with prolonged duration pulse leads to reduced roughness of the surface. The experiments had been planned according to Box-Behnken design (BBD). Manjaiah et al. ⁷⁹ explored the impacts of tool material on Ti₅₀Ni_50−xCu_x SMA in the wire electro discharge process. The study revealed that with the rise of pulse on time there was an upsurge in MRR for both the SMA alloys when they were machined with the use of both zinc-smeared and brass wires. Due to decreased surface roughness and improved MRR, the zinc-coated brass wire has been discovered to be appropriate for TiNiCu SMA machining compared to brass wire.

During the WEDM of Al 6063 alloy, Singh et al. ⁸⁰ examined the rate of material amputation in WEDM process and noted down the various influential process parameters while machining of SMA. It had been observed that MRR rises with a growing pulse-on moment, while the reaction parameter reduces with higher servo voltage & off-time pulse moment. Daneshmand et al. ⁸² explored on the processing of SMA based on Ni-Ti using Wire cut EDM. MRR was the parameter of concern in the machining of Ni-Ti SMA. The research showed that in the machining of NiTi₆₀ alloy’s the most substantial factor that affects the MRR was pulse current, so MRR gets improved when it rises.

Kumar et al. ⁸³ investigated the parameter of surface roughness using the RSM method in W-EDM. The lowest surface roughness value attained for different combinations of process factors. Kulkarni et al. ⁸⁵ investigated the optimization of nitinol superplastic alloy’s multi-performance characteristics in WEDM. Sharma et al. ⁸⁶ inspected on experimental study & parametric optimization of porous Ni₄₀Ti₆₀ alloy with the usage of wire cut EDM method. The machining of porous NiTi alloy in WEDM results in improved cutting efficacy in terms of the rate of material amputation as compare to orthodox machining processes. The cutting rate (CR) rises with a surge in current & T_on. T_on rises with an upsurge in the measure of pulse current, which leads to high erosion of work material. Bisaria and Shandilya ⁸⁷ investigated on crater depth of nickel-titanium-shaped memory alloy in WEDC on the grounds of material removal. The elevated energy of release creates more heat conduction in the work material that increases workpiece melting which ultimately leads to greater MRR. Manjaiah et al. ⁹⁰ investigated on Ti₅₀Ni_50xCu_x SMAs by assessment of wire cut EDM characteristics. The strenuous discharge energy of release per unit of spark leads to penetrate the material, further on the melting of work takes place owing to the high-temperature formation. Manjaiah et al. ⁹¹ investigated on the rate of material removal & its influential characteristics on the processing of Ti₅₀Ni_50-xCu_x SMA in wire-cut EDM. It has been witnessed that MRR was strongly influenced by T_on and SV. The titanium-based SMA, that is, Ti₅₀Ni₃₀Cu₂₀ shows high MRR and the factors that lead to large MRR were thermal conductivity & huge melting temperature.

Surface morphology

Surface morphology defined as the science of the surface of materials. It described as the study of surface roughness, geometry, integrity, crack formation, composition, grain structure, etc. of the various materials. This parameter is of utmost importance in the case of the study of the surface chemistry of SMAs. The SR of work material samples was evaluated at three distinct places and the process response was taken as the average. ⁷⁴ It was observed that cut off length chosen for surface roughness measurement was 4 mm at 0.25 mm/s stylus velocity. With enhanced T_on, SR reduces nonlinearly for low peak current value, while roughness rises at greater current. ⁷⁵

Hsieh et al. ⁷⁷ investigated the ability to recover the shape of ternary SMA TiNiX (X = Zr, Cr) and its characteristics of machining in wire-cut EDM. It has been observed that with the rise in duration of the pulse, the recast layer wideness decreases for the WEDMed TiNiX alloys. Lin et al. ⁷⁸ scrutinized on the features of iron-based SMAs in WEDM. It was observed that the hardness of the sample adjacent to the peripheral surface can achieve up to 550 Hv in the case of both SMAs. The wire-cut EDM of Fe-based SMAs revealed that they still show excellent profile retrieval, but a minor deprivation of recovery of shape happens owing to the depression of the recast layer. Owing to the decreased value of roughness and improved rate of removal of material, it was found that zinc-smeared brass wire is ideal for Ti50Ni50−xCux SMA processing compared to brass wire. ⁷⁹ Moreover, it was also verified from the SEM pictures that zinc-smeared wire generates fewer small cracks, molten droplets, and craters on the processed surface of SMA, as explored in Figure 11.

Figure 11.

SEM micrographs of Ti₅₀Ni₄₀Cu₁₀ SMA (using brass & Zinc wire). ⁷⁹

Sharma et al. ⁸¹ studied on the surface morphology of machined samples of Ni_55.8Ti SMA on wire spark erosion machining. The main causes of surface deterioration, defects in the samples, and white layer formation were the formation of concentrated heat at the large discharge setting of parameters and resolidification of the melted material caused by inappropriate flushing. Liu et al. ⁸⁴ investigated the SMAs crystallography, characteristics, and compositions of white layer formation by wire-EDM. The recast layer formation while machining of nitinol in the EDM process was a crystalline arrangement rather than a solid amorphous structure attributable to the lesser number of constituent composition in nitinol and elements like Cu, Zn and Ni have insignificant variations of atomic size in the formation of the white layer. Kulkarni et al. ⁸⁵ investigated the surface roughness characteristics of nitinol in WEDM. Taguchi’s utility and Quality loss function were used for optimal values of different parameters of process, that is, T_on, T_off, gap voltage, wire feed in WEDM simultaneously for the low value of surface roughness. Sharma et al. ⁸⁶ investigated on the surface characteristics of Ni₄₀Ti₆₀ SMA after machining with WEDM. It was revealed that SR improves with the rise in T_on. The discharge energy rises with the rise in Ton, which improves the melting of metal and reduces the size of the crater, thus leads to an increase in SR, as reflected in Figure 12.

Figure 12.

Reflection of un-machined surface while WEDM of NiTi-SMA: (a) small discharge energy, (b) average discharge energy, and (c) large discharge energy. ⁸⁷

Bisaria and Shandilya ⁸⁷ investigated Ni-Ti based SMA and it’s surface characteristics like depth of crater on the grounds of MRR in WEDM process. At greater discharge energy density, the 3D surface topography discloses the creation of profound and widespread craters with a large value of unevenness on the processed work as compared to the reduced discharge of energy density. Singh and Misra ⁸⁸ used RSM and ANN modeling to investigate the wire cut EDM samples based on the evaluation of the surface finish parameter. The research was concentrated on wire cut EDM of nimonic 263 (combustor material). With the rise in discharge energy & duration of the pulse, it has been revealed that there was an increase in RLT. Bisaria and Shandilya ⁸⁹ explored the features of NiTi SMA and its surface integrity features while machining with wire-cut EDM. In wire-EDMed Ti50Ni50-xCux SMA, the concentrated discharge energy per unit spark penetrates the material; leads to more melting and vaporizes material, and flushes away from the machined surface; leading to increased MRR. ⁹⁰

Dimensional accuracy

Dimensional accuracy defined as the measurement of the closeness of dimensions of the machined product as compared with the standard product range. From the machining point of view, dimensional accuracy plays an essential role in manufacturing, designing, and meeting the demands of the industry. Sharma et al. ⁸⁶ investigated on the parameter of dimensional deviation of porous Ni₄₀Ti₆₀ SMA after machining with WEDM. There was a substantial rise in d_sh by the growth in value of IP, T_off & T_on. It has been revealed that complete flushing of debris takes place with a rise in T_off at high energy of discharge and therefore an increase in d_sh value due to low re-deposition of melted material. Singh et al. ¹⁰⁰ investigated on the dimensional deviancy of M42 HSS samples, machined using treated brass wire in WEDM. The process parameters which had more impact on the dimensional deviation were on time and off-time pulse. Cryogenic treatment results in significant improvement in the dimensional shift value (Tables 4 and 5).

Table 4.

Review of past investigations on SMAs in WEDM.

Sr. No.	Author(s)	Work material	Process inputs attempted	Machiningcharacteristics	Results/Conclusions
1	Manjaiah et al. 74	TiNi SMA	Ton, Toff, SV, FP, WF	MRR, SR	The 3D surface morphology observed the peaks and valleys and SEM viewed the agglomerated debris globules.
2	Narendranath et al. 75	Ti50Ni42.4Cu7.6 SMA	Ton, Toff, SGV, WF, WT	MRR, SR	The results based on parametric analysis reveal that low peak current with prolonged duration pulse leads to reduced roughness of the surface.
3	Takale and Chougule 76	Ti49.4Ni50.6 SMA	Ton, Toff, SGV, WF, WT	MRR, SR	Smooth surface finishing was noted with less material deposition and micro-cracks, adequate hardness, low residual stress, and less heat-affected area formation (white layer).
4	Hsieh et al. 77	Ti-Ni-X (X = Zr, Cr)	Ton, Toff, Df, SGV, Ip, FP	MRR, SR	The maximum feeding rate of TiNiZr, and TiNiCr ternary SMAs without breakage of the wire electrode is significantly related to the electrical discharge energy.
5	Lin et al. 78	Fe–30Mn–6Si andFe–30Mn–6Si–5Cr SMA	Ton, Toff, Df, SGV, Ip, FP	SR, SM	Surface of FeMnSi and FeMnSiCr shape memory alloys observes electro-discharge craters and recast materials. FeMnSi’s recast layer is much thicker than WEDM’s FeMnSiCr SMAs.
6	Manjaiah et al. 79	Ti50Ni50−xCux SMA	Ton, Toff, SV, WF, SF	MRR, SR	For maximizing MRR and minimizing surface roughness, servo voltage is the most important variable. Due to decreased surface roughness and improved MRR, zinc-coated brass wire is discovered to be appropriate for TiNiCu SMA machining compared to brass wire.
7	Singh et al. 80	Al6063 alloy	Ton, Toff, Ip, SV	MRR	Pulse-on time, pulse-off time and servo voltage have a substantial impact on the reaction parameter while the peak current impact is discovered to be insignificant. MRR rises with growing pulse-on moment, while the reaction parameter reduces with higher pulse-off moment and servo voltage.
8	Sharma et al. 81	Ni55.8Ti SMA	Ton, Toff, SGV, WF	SR, SM	Maximum surface roughness improved with rise in pulse on time, servo voltage and pulse off time reduction. With optimum parameters, the WSEM of Ni55.8Ti resulted in a significant improvement in surface quality with optimum roughness value of 7.78 μm, recast layer thickness of about 4–6 μm.
9	Daneshmand et al. 82	Ni-Ti SMA	WF, Ip, Ton	MRR, SR	Increased power increases discharge energy and the number of spark that increases workpiece temperature and machining speed rapidly increases MRR and surface roughness. Most affective factor in MRR is pulse current, so MRR also improves when it rises.
10	Kumar et al. 83	Titanium Grade-2	Ton, Toff, Ip, SGV, WF, WT	SR	The roughness of the surface improved when Ton and Ip increased owing to the longer machining time that led to the enhanced likelihood of “double sparking” and localized sparking. The roughness of the surface ranged from 2.48 μm to 2.62 μm.
11	Liu et al. 84	Nitinol SMA	OCV, PI, Ton, SSI, SF, SGV, WT, LQ, WF	SM	The white layer formed during the WEDM of Nitinol is crystalline instead of amorphous solid, which shows miniature difference in the atomic size of Zn, Cu & Ni in the Nitinol white layer. WL consists of two different layers with different microstructure and chemistry.
12	Kulkarni et al. 85	Nitinol	Ton, Toff, WF, SGV	MRR, SR	Optimal combination of parameters obtained for maximum MRR and less surface roughness. Wire feed found to be most significant factor in machining followed by pulse on time, pulse off time and spark set voltage.
13	Sharma et al. 86	Ni40Ti60 alloy	Ton, Toff, SV, Ip	CR, Dsh, SR	Increase in Ton & Ip results into increase in cutting rate. Increase in Ip for longer duration results into increase in discharge energy which leads to more material removal from the workpiece. Increase in Ton and discharge energy leads to more melting of materials and formation of large craters which leads to more roughness.
14	Bisaria and Shandilya 87	Ni55.95Ti44.05 SMA	De, Sf, SGV, WF, WT	ACD, MRR, MC	MRR and crater depth rise monotonically with rise in discharge energy density because it leads to high thermal energy formation which leads to more MRR and large craters on workpiece surface. Analysis of XRD peaks of machined surface shows the existence of different compounds such as Ni4Ti3, NiTi, CuZn, TiO, TiO2, Cu2NiZn, NiTiO3, and NiZn.
15	Singh and Misra 88	Nimonic 263	Ton, Toff, Ip, SGV	SR, SM	Pulse on time is the most significant parameter followed by peak current for Ra and Rz. Longer pulse on time leads to more discharge energy, resulting in higher speed of material removal, surface roughness and thickness of recast layer. The thickness of the recast layer increases with an increasing duration of the pulse and energy discharge per spark. At low discharge energy, a minimum of 7.06 RLT was discovered.
16	Bisaria and Shandilya 89	NiTi	SGV, WT, Toff, WF, Ton	CE, SR	SR and CE are heavily affected by spark gap voltage, spark off time, and spark on moment, while wire velocity and wire tension are trifling.
17	Manjaiah et al. 90	Ti50Ni50-xCux	Ton, Toff, SV	MRR, SR	Increased pulse on time increases the amount of material removed from the workpiece. The concentrated discharge energy per unit spark penetrates into the material; leads to more melting and vaporizes material and flushes away from the machined surface; leading to increased MRR.
18	Manjaiah et al. 91	Ti50Ni30Cu20	Ton, Toff, SV	MRR, SR, RLT, H,and SM	The MRR and surface roughness are strongly influenced by pulse on time and SV. This can be due to the higher temperature of melting and thermal conductivity. An enhanced time pulse with a reduced SV aggravates the quality of the surface. As the longer pulse on time has a comparatively elevated accumulated discharge energy, resulting in more molten material and resolidification, the recast layer thickness rises with the rise in pulse on time.
19	Kulkarni et al. 92	NiTi super elstic alloy	Ton, Toff, SGV	MRR, SR	Pulse on and voltage are main contributing factors for good quality surface of NiTi alloy. Surface roughness increases with rise in Ton and decrease with increase in Toff. MRR increases with rise in Ton and decreases as Toff increases. Initial rise of SV (up to 40 V) improves MRR, but further rise of SV causes reduction of MRR.
20	Majumder and Maity 93	NiTi SMA	Ton, Ip, WT, WF, FP	AMR, MPVH, RMSR, Hd	Approximately ±5 percent mistake was viewed by the suggested GRNN model, which disclosed the fitness of the GRNN model to predict the WEDM outputs for Ni-Ti shape alloy. Vikor fuzzy hybrid multivariate process used for optimal combination of parameters in machining of NiTi. TON = 10 µs, I = 12 A, WT = 12 N, WS = 150 mm/s and FP = 6 bar optimized input parameter combination established for the best results. Pulse on time is most important parameter in machining.
21	Velmurugan et al. 94	NiTi SMA	SV, Ton, Toff, Ip, WF	MRR, SR	L18 orthogonal array of Taguchi employed for different combination of parameter to feed them in ANN modeling through neural power V 2.5. Genetic Algorithm (GA) used for optimization of input and output variables. With 20 neurons in the concealed layer, the Incremental Back Propagation and Batch Back Propagation learning algorithms suggested to estimate optimum output variables with fewer training data under ordinary multilayer feed and complete feed forward connections based on fewer RMSE values respectively.
22	Shandilya et al. 95	Ni-rich NiTi SMA	Ton, Toff, SGV, WF, WT	RLT, FEAC	SV, Ton, Toff are the influencing parameters for FEAC and RLT. At higher Ton, the recast layer with high FEAC observed on machined surface of NiTi.
23	Roy and Narendranath 96	Ti50Ni25Cu25	Ton, Toff, SGV, WF	VMRR, SM	At lower spark gap voltage smaller re-solidified debris present on machined surface and at higher voltage large debris accumulate on machined surface of work material. Higher spark gap voltage and greater wire electrode feed rate should be integrated to improve surface features at the cost of machining rate.
24	Reddy et al. 97	Ti50Ni48Co2 SMA	Ton, Toff, SGV	MRR, SR	GRA method is applied for maximizing the MRR and decreasing the SR respectively. Pulse on time = 125 µs, Pulse off time = 35 µs, Servo voltage = 40 V are the best combination of parameter setting to achieve high MRR and less surface roughness.
25	Sharma et al. 98	NiTi SMA	Ton, Toff, SGV	CR	More value of wire feed, spark voltage, pulse on time and less value of pulse off time favors the cutting rate. Toff & Ton significantly affects the CR as compared to the SV &WF. Cutting rate and productivity has been increased with the GA based optimization.
26	Arikatla et al. 99	Ti-6Al-4V	SV, IP, Toff, Ton	MRR, KW, SR, CR, Oc, SM	Sparking from both sides of the wire to the workpiece results into slightly increase in kerf width for the trim cut. During main cuts to trim cuts the SR value reduced from 3.75 µm to 1.25 µm. From rough to trim cut, the cutting speed rises and decreases slightly from the first trim cut to the last cut. And the over cut rises slightly during the trim cuts.

T_on: Pulse duration/Pulse on time/Pulse interval time/Spark on time; T_off: pulse off time/spark off time; SV: servo voltage; FP: flushing pressure; WF: wire speed/wire feed; SGV: spark gap set voltage/spark gap voltage; WT: wire tension; D_f: duty factor; I_p: peak current/current/discharge current; OCV: open circuit voltage; PT: power intensity; D_e: discharge energy density; S_f: spark frequency; SF: servo feed; SSI: second spark intensity; LQ: liquid quantity; IP: input power; MRR: material removal rate; SR: surface roughness; SM: surface morphology/surface characterization/surface topography; CR: cutting rate; D_sh: dimensional shift; ACD: average crater depth; H_d: micro-hardness/hardness; K_w: kerf width; O_c: overcut; RLT: recast layer thickness; CE: cutting efficiency; MC: metallographic changes; AMR: arithmetic mean roughness; MPVH: maximum peak to valley height; RMSR: root mean square roughness; FEAC: foreign element atomic content; VMRR: volumetric material removal rate.

Table 5.

Summary of experimental and theoretical studies on SMAs with WEDM.

Sr. No.	SMA types	Experimental studies	Theoretical studies
1	NiTi	Refs.25,81,82,86,87,89,92–95	Refs.87,93
2	Ti50Ni42.4Cu7.6	Ref. 75	Ref. 75
3	Ti35.5Ni49.5Zr15	Ref. 77	Ref. 77
4	Ti50Ni49.5Cr0.5	Ref. 77	Ref. 77
5	Fe–30Mn–6Si	Ref. 78	Ref. 78
6	Fe–30Mn–6Si–5Cr	Ref. 78	Ref. 78
7	Ti50Ni40Cu10	Refs.79,90,91	Ref. 79
8	Ti50Ni30Cu20	Refs.90,91	Ref. 90
9	TiNi	Refs.74,76	Ref. 74
10	Ti-6Al-4V	Refs.26,83	Ref. 83
11	Nitinol	Refs.84,85	Ref. 84
12	Ti50Ni25Cu25	Ref. 96	Ref. 96
13	Ti50Ni48Co2	Ref. 97	Ref. 97

Micro-EDM process: An emerging research perspective

Micro-EDM is an imperative and emerging EDM variant, designed and developed for machining on a micro-level scale. Miniaturization is the concept on which micro-EDM works, as industries, have a lot of demand for manufacturing small components with complex geometrical features, which are met with the use of this machine. Abidi et al. ⁵⁴ inspected the Ni-Ti based SMA characteristics with the micro-EDM process. A multi-objective genetic algorithm was used for the analysis and optimization of µEDM. Jahan et al. ¹¹⁶ investigated the Ni-Ti SMAs and their characteristics in the respect of their biocompatibility while performing a machining operation on micro-EDM. The migrated elements into the machining area after performing a machining operation on micro-EDM were carbon and oxygen. There was oxide formation on the machined surface of the Ni-Ti SMA and the formed important component was NiTiO₃. It has been revealed that the formation of NiTiO₃ worked as a protective layer while considering the application of Ni-Ti SMA as biomedical implants. Abidi et al. ¹¹⁷ investigated on Ni-Ti and its characteristics using grey relations coupled with principal component analysis while performing micro-EDM. The grey relational analysis based on Taguchi has been implemented to investigate the effects of selected process parameters. Various challenging areas in the domain of Micro-EDM is represented in Figure 13.

Figure 13.

Micro-EDM challenging areas.

Machining of SMAs with other methods

Apart from the non-traditional processing (i.e. EDM, Wire-EDM, PMEDM, Micro-EDM, Dry EDM, etc.) of the SMAs, the attempts have also been made to process these smart materials with some other advanced machining methods (i.e. Laser processing, additive manufacturing, ECM, etc.) and conventional manufacturing processes (namely; CNC turning, milling process, etc.). The experimental investigations related to these other advanced methods (apart from EDM-based methods) and the traditional processes have been reviewed and elaborated in this section.

Caputo et al. ¹⁰¹ investigated the path of additive manufacturing of functional net formed components from Ni-Mn-Ga powders of pre-alloyed magnetic form. Due to the comparatively simple control of part porosity and the chance of obtaining complicated formed components from Ni-Mn-Ga metals, additive production via powder bed binder jetting, also recognized as 3D printing (3DP) has been used in this studies. Binder jetting of Ni-Mn-Ga powders accompanied by curing and sintering demonstrated effective with excellent mechanical strength in generating net-shaped porous structures. Additive manufacturing is a viable technology to resolve the design problems of Ni-Mn-Ga MSMA functional parts. Pfeifer et al. ¹⁰² investigated the shape memory implants which are orthopedically adaptable. Nickel-titanium (NiTi) SMA derived implants were created to adjust the implant to the real healing scenario. Wang et al. ¹⁰³ investigated on the NiTi SMAs using pre-mixed powders in Additive Manufacturing. This research provides relative research on the in-situ alloy of NiTi SMAs using pre-mixed Ni-Ti powders using selective laser melting (SLM), selective melting of the electron beam, and direct energy deposition (DED). Xu et al. ¹⁰⁴ investigated burn resistant Ti40 alloy in Electrochemical machining. The study focused on the feasibility of making aero-engine components from Ti40 burn resistant alloy in Electrochemical Machining, thereby reducing the cost and expenses as compared to conventional machining techniques. A mixed electrolyte of NaCl and KBr is the best for ensuring the quality of machining of burn resistant Ti40 alloy in ECM. The surface quality obtained is good with surface roughness value as low as 0.371 µm and a higher material removal rate of 3.1 mm³/min. Oliveira et al. ¹⁰⁵ explored Cu-Al-Be based SMA & inspect their mechanical properties and microstructure with the help of laser welding. The tensile power of laser-welded joints has been revealed to be slightly smaller than the root material (686 v/s 631 MPa). Oliveira et al. ¹⁰⁸ investigated the CuAlMn and NiTi SMAs with dissimilar laser welding processes. Root material-namely beaches were created in the joint’s synthesis area. This is linked to the complicated chemical composition of the fusion area, the temperature of which was smaller than that of the foundation metals. It has been observed that the CuAlMn fusion border has an improved grain edifice than NiTi which was due to the greater thermic conduction of the prior root work material. The different types of lasers used in these laser-based machining and welding are Yb: YAG laser welding,^107,110 Nd Yag laser welding,^105,108 High-Power Diode Laser (HPDL), ¹¹⁴ Nd: YAG-based fiber laser. ¹⁰⁸ Sam et al. ²⁰⁵ have been studied the tensile shape memory actuation behavior of NiTi SMA fabricated using L-PBF additive manufacturing process. The ductility and actuation strain of the samples prepared using L-PBF found lower than that of the NiTi SMA samples conventionally manufactured. Shiva et al. ²⁰⁶ developed a laser additive manufacturing (LAM) technique for processing ternary SMA, that is, TiNiCu with different compositions. SEM analysis revealed the homogeneity in the deposition of material in all the samples. LAM manufactured properties of TiNiCu₁₀ found to be best among all other tested samples (Tables 6–9).

Table 6.

Review of past investigations on SMAs in other machining operations.

Sr. No.	Author(s)	Machining process	Work material	Processparametersconsidered	Machining characteristicsinvestigated	Results/Conclusions
1	Caputo et al. 101	3D printing	Ni-Mn-Ga	LT, BS, PR	SM, PT, ST	Using the binder jetting method, additive production of net-shaped components made of Ni-Mn-Ga magnetic shape memory alloys was conducted. Parts with complex geometries manufactured using Ni-Mn-Ga powders generated by spark erosion in liquid argon, nitrogen, and ball milling.
2	Pfeifer et al. 102	Laser cuttingand welding	NiTi	PD, PF	Kw, SR, TA	Nickel-titanium (NiTi) shape memory alloy (SMA) based implants created to adapt them to the real healing scenario. The SMA transition temperature reached due to contactless induction heating and causes the one-way shape memory effect (SME). This results in a slight alteration of the second moment of inertia and an adjustment of the implant’s bending rigidity.
3	Wang et al. 103	Directed energydeposition(DED), Selectivelaser melting(SLM) and Selectiveelectron beammelting (SEBM)	NiTi	LSS, LT, LP	MH, PM, TMP	Pre-mixed equiatomic Ni-Ti powder feedstocks used to manufacture metal components through DED, SLM and SEBM procedures to study the material’s printability in comparison. This material’s printability using various metal AM methods is listed in order as DED > SLM > SEBM.
4	Xu et al. 104	Electrochemicalmachining (ECM)	Ti40	PC, V, CFR, EC, EIP	MRR, SR	Mixed aqueous electrolyte of KBr and NaCl used for the electrochemical machining of Ti 40 burn resistant titanium alloy, ensures the stability and quality of ECM. Low surface roughness value 0.371 and high MRR more than 3.1 mm3/min obtained in ECM. High efficiency, good quality and no tool wear reported with ECM machining of Ti 40 alloy and this material has wide application in aerospace industry.
5	Oliveira et al. 105	Laser Welding	Cu-Al-Be	PD, PP	MS, TP	Joints with no defect obtained with laser welding of Cu-Al-Be SMA. The crystallographic fusion texture remain unchanged with laser welding with same orientation as base material. Tensile strain for the welds is higher as compare to tensile strength for the laser-welded joints. Superelastic behavior preserved with high transformation strain with laser welding. In the temperature range of −40°C to 100°C superelastic behavior shown by laser-welded joints.
6	Elahinia et al. 106	Selective lasermelting (SLM)	NiTiHf	LP, HS, LT, SV	MS, MTP	NiTiHf SMA additively manufactured using selective laser melting (SLM). Transformation temperature of SLM found to be more than 200°C. For stresses of up to 500 MPa, the compression in for shape memory response evaluated and conversion strains found very similar.
7	Bahador et al. 107	Laser Welding	Ti-Ni, Ti-Nb, Ti-Ta	WS, FD	MS, Hd	The effect of defocusing fiber laser welding distance was explored on Ti-Ni, Ti-Nb, and Ti-Ta shape memory alloys (SMAs). The Ti-Ni furnace-sintered welds have the highest porosity and the smallest welding performance. Although porosities were still observed, the microwave sintered alloys of Ti-Nb and Ti-Ta SMAs showed better weldability. Compared to the other welded joints, Ti-Ta SMA showed the highest welding performance.
8	Oliveira et al. 108	Laser Welding	NiTi and CuAlMn	PD, PP	MS	Different combinations of sophisticated alloys are essential for the growth of new applications. Laser welding was used to join NiTi to CuAlMn. Base material-like beaches were created in the joint’s fusion area. Due to the greater thermal conductivity of the former base material, the CuAlMn fusion border has a finer grain structure than NiTi.
9	Khalil et al. 109	CNC Turning machine	NiTi	DOC, CS, FR	CF, TW	In two cutting environments, dry and minimum quantities of nanolubricants, the influence of multiple machining parameters such as cutting velocity, cutting depth, feed rate on tool wear and cutting forces explored.
10	Sathiya and Ramesh 110	Laser Welding	Nitinol	LP, WS, SGBD, FP	BG, MS, Hd, CR	Present study focused on Yb:YAG laser welding of Nitinol SMA of 1 mm thickness. Weld with complete penetration, higher hardness and resistance to corrosion than the base metal obtained with the laser welding. Microstructure have dendritic structure, pores formed with the less gas blown distance. Micro-hardness values achieved are better, it influenced by the grain structure. Better bead profile, corrosion resistance, hardness achieved at gas blown distance = 16 mm, power = 900 W, focal position = −0.5, welding speed = 2100 mm/min.
11	Sadati and Javadi 111	Nd Yag Laser Welding	NiTi	PN, WF, FL, IPW, PE, LP	MS, SA, TT	Successful welding of NiTi samples done by laser. Junction created using optimized parameter settings. Tiny and outspread holes observed in microstructure. Elongation and tensile strength is higher when samples subjected to tension.
12	Yang et al. 112	Laser Welding	NiTi	LP, LSS	TP, MS, SMP	Increase in laser power and scanning speed leads to the increase in critical stress and decrease in Ms temperature. XRD and TEM shows the austenitic and martensitic phases of fabricated samples.
13	Oliveira et al. 113	Laser Welding	Cu-Al-Mn	PD, PP	MS, TP, Hd	Cu-Al-Mn has excellent weldability. Finer grained fusion zone more better than bamboo like grained base material. No degradation of properties of welded samples observed as compare to base material. In the coarse grained base material, the fracture of the welded specimens always occurred in ductile mode away from the fine grained fusion zone.
14	Mehrpouya et al. 114	Laser Welding	NiTi	LP, LSS, FD	MS	HPDL technique used for laser welding of Ni54.76Ti shape memory alloy. Weld bead of superior quality, uniformity, homogeneity and free from cracks have been obtained with laser power in the range of 350–450 W and scan speeds in the range of 3–5 mm/s.
15	Guo et al. 115	Milling	Nitinol	FR	SR	To assess mechanical behavior in cutting, quasi-static and dynamic stress-strain of Nitinol Ni50.8 Ti49.2 was achieved at big strain and elevated rates. Flank wear increases with increase in feed rate rate due to large mechanical load. It is possible to produce the lowest surface roughness in the milling at a certain feed rate.

LT: layer thickness; BS: binder saturation; PR: packing rate; PD: pulse duration; PF: pulse frequency; LSS: laser scanning speed/scanning speed; LT: laser thickness; LP: laser power; PC: pulse current; V: voltage; CFR: cathode feeding rate; EC: electrolytic concentration; EIP: electrolytic inlet pressure; PD: pulse duration; PP: peak power; LP: laser power; HS: hatch spacing; SV: scanning velocity; WS: welding speed; FD: focusing distance; DOC: depth of cut; CS: cutting speed; FR: feed rate; SGBD: shielding gas blown distance; FP: focal position; PN: pulse number; WF: welding frequency; FL: focal length; IPW: interval pulse width; PE: pulse energy; LP: laser power; SM: surface characterization; PT: porosity; ST: strain; K_w: kerf width; SR: surface roughness; TA: taper angle; MH: microstructure homogeneity; PM: phase formation; TMP: thermo-mechanical properties; MRR: material removal rate; SR: surface roughness; MS: microstructure/microstructural characterization; TP: tensile properties; MS: microstructure; MTP: mechanical & thermo-mechanical properties; H_d: micro-hardness; CF: cutting force; TW: tool wear; BG: bead geometry; CR: corrosion resistance; MS: microstructure; SA: surface analysis; TT: tensile testing; TP: tensile properties; SMP: shape memory properties.

Table 7.

Summary of studies on SMAs with other machining methods.

Sr. No.	SMA types	Experimental studies	Theoretical studies
1	Ni-Mn-Ga	Refs.101,116	Ref. 101
2	NiTi	Refs.102,103,107,108,109,111,112,114	Refs.103,112
3	Ti-Nb	Refs.107,117	Ref. 107
4	Ti40	Refs.104,118	Ref. 104
5	Cu-Al-Be	Refs.105,120	Ref. 105
6	NiTiHf	Refs.106,122	Ref. 106
7	Ti-Ni	Refs.107,108,119	Ref. 107
8	Cu-Al-Mn	Refs.105,108,113,121	Ref. 113
9	Ti-Ta	Refs.107,125	Ref. 107
10	Nitinol	Refs.110,115,123	Ref. 110
11	Cu-Al-Ni-Mn	Refs.108,124	Ref. 108

Table 8.

Summary of various optimization techniques on SMAs.

Sr. No.	Name of the author	Processattempted	Shape memory alloy	Single/Multi-response optimization	Optimization techniques
1	Abidi et al. 54	EDM	NiTi	Multi response	Multi-objective genetic algorithm(MOGA-II)
2	Gaikwad et al. 55	EDM	NiTi	Single response	Taguchi
3	Daneshmand et al. 56	EDM	NiTi	Single response	Taguchi
4	Jatti 62	EDM	NiTi, NiCu, BeCu	Multi response	Taguchi
5	Gaikwad et al. 63	EDM	NiTi-60	Single response	Taguchi
6	Daneshmand et al. 68	EDM	NiTi-60	Single response	Taguchi
7	Daneshmand et al. 69	EDM	NiTi	Single response	Taguchi
8	Manjaiah et al. 74	WEDM	TiNi	Multi response	Taguchi
9	Narendranath et al. 75	WEDM	Ti50Ni42.4Cu7.6	Single response	Response surface methodology (RSM)
10	Takale et al. 76	WEDM	Ti49.4Ni50.6	Single response	Taguchi
11	Sharma et al. 81	WEDM	Ni55.8Ti	Single response	Taguchi
12	Kulkarni et al. 85	WEDM	Nitinol	Multi response	Taguchi utility
13	Sharma et al. 86	WEDM	Ni40Ti60	Single response	RSM
14	Majumdar et al. 93	WEDM	NiTi	Multi response	General regression neural network(GRNN), Multi variate hybridapproach, Taguchi
15	Velmurugan et al. 94	WEDM	NiTi	Multi response	ANN, Genetic algorithm, Taguchi
16	Reddy et al. 97	WEDM	Ti50Ni48Co2	Multi response	Multi-objective Grey RelationalAnalysis (GRA), Taguchi
17	Sharma et al. 98	WEDM	NiTi	Multi response	GA, Taguchi

Table 9.

SMAs and their detailed applications.

Sr. No.	Name of alloy	Machine	Applications
1	Ti49.4Ni50.674,126–129	WEDM	Orthopedic implant
2	Ti50Ni48Co297,130–133	WEDM	Medical devices
4	Ni-rich NiTi SMA81,134–140	WEDM	Medical, aerospace, corrosion resistance
5	TiNiCo97,141–145,217	WEDM	Excellent biocompatibility, corrosionresistance
6	Ni-Ti SMA80,146–153	WEDM	Defense, aerospace, and medicine
7	Ti50Ni50 and Ti50Ni49.5Cr0.5 SMAs60,69,154–156	Dry EDM	Biomedical applications
8	Nitinol54,157–160,207	Laser Cutting, EDM	Biomedical and aerospace
9	Cryo-treated NiTi alloys57,161–164	EDM	Defense, aerospace, and medicine
10	Sintered porous NiTi SMA86,165–167	WEDM	Actuators, Corrosion resistance
11	NiTi SMA98,168–172	AWJM	Medical, aerospace, sensing
12	TiNi SMA76,129,140,173–175,213	WEDM	Orthopedic implant
13	Nitinol SMA85,176–179	Milling, EDM	Medical devices and actuators
14	Cryogenated NiTi SMA78,180–182,205	Dry & MQL (Minimum Quantity Lubrication)	Excellent biocompatibility, corrosionresistance
15	Ti50Ni50−xCu x SMA.79,183,184,206	WEDM	Actuators, micro tools and stents in biomedical components
16	TiNiCr and TiNiZr SMA59,185,186,208	EDM	Coupling and sealing
17	Ni-AlFe and TiNiZr SMA60,187,188,209	EDM	Limited to lower temperatures
18	NiTi SMA56,189,190,215,216	EDM	Medicine, aerospace
19	TiNi and TiNiCu SMA67,71,191,192	EDM	Coupling and sealing
20	Fe-30Mn-6Si and Fe-30M-6Si-5Cr SMA78,193,194	WEDM	Coupling elements
21	TiNiZr/Cr SMA77,195,196,210	ECDe, WEDM	Coupling and sealing
22	NiTi SMA56,151,160,197,198,211,212,219	EC polishing	Actuators and medical
23	NiTi SMA58,128,168,199–201,214,222,223,225–227	Micro-EDM, Water jet,Micro milling	Actuators and medical
24	NiTi SMA64,134,147,174,202–204,218,220,221,224,228	Wire-EDM, EDM, WJM	Actuators and medical

Figure 14(a) represents the various SMAs investigated with various processes (other than EDM/WEDM), whereas Figure 12(b) highlights the usage of different machining processes in other machining operations.

Figure 14.

(a) Machining of various SMAs with other machining operations and (b) utilization of different machining processes in other machining operations.

Various critical research areas and directions for future research

EDM has been acknowledged as a quite competent manufacturing technique in experimental-based research to effectively process various types of advanced engineering materials regardless of their various characteristics. The research on EDM is however conducted largely with a perspective to examining the method ability to process various products under the impact of system factors and model certain process results of interests too. Figure 15 illustrates the innumerable elements of futuristic studies in EDM. Based on past research work performed in the EDM field, there is an outstanding extent of forthcoming research in numerous crucial aspects of the EDM process. Figure 16 is explicating some other investigating aspects need to be further research in EDM and its variant processes. Quite a few of the research concerns/areas identified and are described as follows:

1. Very few experimental research studies in EDM of SMAs were reported in the existing literature, those employed the approaches to design the experiments, that is, Taguchi, RSM, etc. Maximum of the experimental works reported were found to be centered on the approach termed as—“one-factor-at-a-time”. However, for resolving any real-life manufacturing issue, it becomes very essential to explore the concurrent impact of several variables on machining efficiency concurrently.

2. In previous studies, some important material-based input factors, that is, homogeneity, distinct job thickness, substance’s anisotropic nature, and material density were hardly regarded. Inclusion of these influential factors as parameters becomes quite essential to investigate where the binder phase materials and their concentrations play a vital role in varying the work material properties.

3. Most of the experimental studies are found to be done with the use of only one type of electrode, that is, Copper (Cu) electrode. Other electrodes that can also be used are; tungsten (W), tungsten-copper (W-Cu), graphite, brass, aluminum, composite electrodes, etc.

4. The comparative experimental work by considering different electrodes as a process variable for achieving better machining performances has also rarely been attempted so far. Therefore, it is an area that further needed to be explored for optimizing the overall product quality, cost, and time, etc. in real-life industrial issues.

5. The dielectric based process parameters like flushing pressure, types of dielectric fluid, etc. are hardly been found to be investigated in performing EDM of SMAs. It is revealed from most of the studies conducted that the de-ionized water employed as a dielectric broadly. The variation of flushing pressure with the passage of time and the corresponding effect of these revulsions on the response parameters can be perceived as a promising area of interest in the advanced processing of SMAs. In addition, other dielectric mediums that are rarely employed are kerosene oil, hydrocarbons, EDM oils, powder-mixed dielectrics, and gaseous dielectrics like oxygen and argon can also be attempted to understand the process mechanism under the effect of these different dielectrics.

6. Study on the wear rate of tool (TWR) in EDM of SMAs is quite rarely explored in various research articles reported. This is one of the important response parameters from the machining point of view which further needed to be investigated in order to explore the deeper insights of the process mechanisms involved.

7. Parameters based on the geometrical accuracy namely as overcut, taperness, circularity of holes, etc. and dimensional tolerances are hardly been investigated in the past experimental research studies conducted while processing SMAs with EDM process. Studies based on these essential characteristics need to be addressed in future investigations in the domain of EDM for improving the process capability.

8. Limited studies reported on the use of different tool geometries for machining purposes in EDM. The shape and size of the tool is an important factor in optimizing the machining performance in EDM. The angle of projection of the tool for the different complex and intricate shapes on the workpiece, and different tool shapes at the lateral end (circular, rectangular, triangular, square, etc.) is an area of concern for the future studies in EDM of SMAs.

9. No credible investigations have been reported so far with the use of a micro variant of the EDM method for the effective processing of SMAs.

10. The relationship between the input parameters (i.e. gap voltage, T_on, gap current, and T_off) and EDM efficiency measurements through machining of SMAs has been attempted by very few scientists.

11. The effect of the electrical conductivity of the workpiece and the tool electrode on the machining responses has not been considered during the EDM of SMAs. The electrical and thermal conductivity of the workpiece and the tool plays an important role in enhancing the overall efficiency of the EDM process.

12. Limited attempts have been stated on the machining of ternary SMAs with EDM. These alloys are of quite an importance from the medical praxis point of view, as many nickel and titanium-based ternary SMAs possess significant applications in the biomedical industry.

13. Very few studies have been reported on machining of Fe and Copper-based SMAs in EDM processes. Copper and Fe-based SMA’s are a good alternative to the Ni-based SMA, as these are cost-effective and used as a substitute to Ni-based SMA in various industries.

14. Limited work reported on the study of white layer thickness (WLT) in machined samples of SMAs in the EDM process. The thickness varied according to the machining of different SMAs in EDM and this area is very less explored. Further studies can be conducted for understanding white layer thickness and optimizing surface morphological characteristics.

15. No substantial work has been observed on the cryogenic treatment of electrodes and workpieces used in the EDM process. Impact of cryogenic treatment of electrode on the workpiece and impact on different response parameters needed to be observed. Cryogenic treatment of workpiece and its effect on performance parameters, all these cases needed to be taken care of for future work in this area.

16. The modeling of cutting responses like SR, TWR, MRR, dimensional deviation, etc. are recorded in very few researches. Even a single study for modeling decisive responses, that is, sub-surface damage, recast layer formation, and power consumption, was not reported. Modeling these solemn machining responses becomes quite vital in order to predict the output at an early stage to make it safer to pick the process inputs accordingly.

17. Very less literature has been recorded regarding the movement of tool electrode and workpiece table for obtaining optimal performance parameters while processing SMAs with EDM. The effect of the movement of tool electrode on the MRR, TWR, SR, etc. is an area of quite an interest and future study in the broad domain of EDM.

18. Fuzzy logic and tool-based simulation modeling applications could also be used to validate the models developed. In addition, FEM based concepts would also offer a clearer knowledge of the system engaged in the process. There is a huge want to use such methods to develop models for the method outcomes of interest as an answer which would be more precise and applicable to the established models.

19. Most of the experimental research attempted in EDM of SMAs has been observed to be based on the notion of single-objective optimization. Determining the process parameters in such a way that multiple responses can be optimized simultaneously, particularly while deciphering some practical industrial issues, becomes quite essential. It is also possible to use different multi-response optimization methods, that is, traditional approaches (grey relational theory, utility theory, etc.), non-traditional methods (genetic algorithm, ant bee colony, particle swarm optimization, etc.), and multi-attribute decision-making methods (analytical hierarchy process, weighted product technique, etc.). To select a more competent optimization technique, a relative study of the augmented outcomes from these methods can be fairly supportive for future researchers and scientists.

20. There is also a need to explore the economic elements while attempting EDM of SMAs, specifically in terms of the costs engaged in the manufacture of tools, and the lifespan of tools. Furthermore, providing some cost-effective procedures for tool manufacturing purposes would further improve the economic-aspects of the EDM method.

Figure 15.

Potential areas for future research in EDM & its variants.

Figure 16.

Some other research issues need to be taken care in prospect of workpiece, process responses & environment based in EDM & its variant processes.

Conclusions

Electric discharge machining is a thermal-based contemporary processing method which has been widely employed to effectively process the vast range of technically-advanced engineering materials. It has also been used competently to commercially process a broad variety of advanced materials including SMAs. This work targets the working tenet of EDM, material removal mechanism involved, and study of different experimental and modeling techniques, its application, and influence of various process factors on the umpteen process characteristics. The gaps revealed from the previous investigations and opportunities for future attempts in EDM of SMAs have also been deliberated. The following conclusions are drawn as:

• EDM offers better concert in terms of rate of material amputation (MRR), rate of tool wear (TWR), surface roughness (SR), and healthier surface characterization as compared to the conventional machining methods.

• Miniature holes and complex geometries are important machining characteristics of EDM whereas, with the help of conventional machining processes, the complex geometries and intricate shapes cannot be possible due to direct tool interaction with the workpiece which further leads to tool failure and causes into more loss of power.

• SEM and XRD analysis of machined samples of SMAs in EDM reveal less number of cracks, uniform particle distribution as compare to work-samples machined and analyzed with conventional machining techniques.

• The use of different electrodes for having optimized results is a trending area of research in EDM. Machining with the use of composite electrodes results in better consequences in terms of MRR, SR as compare to single element electrodes used in the EDM process.

• SMAs are one of the types of advanced substances and they have numerous applications in the biomedical sector and commercial industries. Processing of these alloys with the EDM method results in enhanced outcomes in the rapport of MRR, SR, TWR, hardness, dimensional characteristics (dimensional deviation, overcut, circularity, taperness), etc. Machining of Cu and Fe-based SMAs is a new area of research in EDM. These SMAs have numerous applications in various industries and also be used in the medical sector.

• Material exclusion in EDM occurs with the help of electric discharge when the tool electrode and workpiece are kept close to the minimal distance. The intensity of the electric spark relies on the conductivity of the electrodes and workpiece. It further stoops upon the type of electrode and workpiece material and their properties.

• Cryogenic treatment of electrodes and workpiece is another area of interest in EDM. It results in better outcomes in terms of performance parameters while machined with the cryogenically treated electrodes as compare to the untreated ones. Microstructure analysis reveals quite an improvement while using the cryogenic treated electrode for machining of SMAs with EDM. Micrographs reflect very less creation of cavities, cracks, and craters on the surface of samples processed with cryogenically treated electrodes.

• Dielectrics play an important role in the EDM process. Powder-mixed dielectrics are a new area of concern and it would provide a better machining environment in performing EDM of SMAs. Flushing pressure is important for obtaining optimizing results in terms of different performance characteristics.