Powder mixed electric discharge machining (PMEDM) has offered itself as an effective variant of conventional EDM process. It involves adding certain conductive powders to the dielectric which influence spark attributes during machining and result into high surface quality and increased material removal rate [1–2]. The improvement in surface quality is attributed to increase in discharge gap which reduces the generated heat flux, that then helps form smaller craters (thus better surface quality) [2–5]. Material removal rate (MRR), on the other hand, gets improved because of the increased spark frequency, which is the result of reduced dielectric resistivity [2]. Ease of removal of debris which is made possible by wider discharge gap [5] and smaller craters further contributes to increase in MRR [2].
Review of the available PMEDM literature indicates that various powder relevant variables namely the ‘material’ itself, ‘concentration’ and ‘size’ influence the process outcomes differently [5–9]. Wong et al. [5] found specific combinations of workpiece and powder-mixed dielectric to be resulting into beneficial outcomes when they compared the effectiveness of Si, Al, SiC, Gr, MoS2 and crushed glass. Graphite and Si were found to be producing mirror like surfaces for SKH-54 tool steel workpiece material (even up-to a pulse current of 2 Amp) whereas, Al was reported to be more effective in producing mirror like surfaces for the workpiece material SKH-51 [5]. For other response measures as well, the favorable outcomes are not found to be specific to the use of any one powder material. Tzeng and Lee [8] reported Cr addition (in Kerosene) to be contributing most, to the increase in MRR than Cr, Cu, Al and SiC powders. They however found the effectiveness of powders towards tool wear rate to be different than pattern observed for MRR. Maximum tool deterioration was reported for the case of SiC, followed by Al and Cr. The Cu was not found to be much influential for either response measure in their work. The ineffectiveness of Cu as an additive was attributed to its high density that kept it from remaining suspended in dielectric [8–9]. Chow et al. [10] in their work on micro-slit EDM, reported SiC to be having higher material removal depth in comparison to Al at their respective optimum concentrations. In yet another work [11], addition of SiC powder in de-ionized water was seen to increase material removal depth. More recently, SiC powder mixed dielectric has been reported to have positive effect in reducing surface roughness for hardened steel [12]. In yet another work employing a different powder material i.e. Gr was not only reported to improve MRR and tool wear rate [2], but was also found to alloy with machined surface [11]. In a recent study Sahu and Mandal [13] reported Gr based PMEDM to be generating surface with maximum flatness for Nimonic 263 in the work that also employed alumina powder in EDM oil. Other than powder material itself, “powder concentration” has also been seen to play its part. Particles beyond an optimum level are mentioned to have similar effect to that of overcrowded debris in the gap thus causing unstable machining [2]. The ensuing bridging effect, short circuiting, arcing and settling issues have been attributed for adversely affecting surface characteristics [2, 4, 8]. Powder concentrations upto 20 g/l are seen to be mostly reported to be used with only limited work employing concentrations in excess [2]. Another important powder relevant variable seen to be investigated is “particle size. It is seen to relate directly with the discharge gap produced. In general, larger particles have been reported to result in wider discharge gaps whereas smaller particles produce smaller gaps [2, 8]. Correspondingly, improved material removal rate and tool wear ratio has been associated with smaller particles [8]. In this regard, work can be found in both micro and nano regimes [2, 14–15] wherein the micro-regime and nano-regime have been seen to be employed mostly in the size ranges of 1–55µm and 20-60nm respectively [2].
While the preceding section discussed how various powder relevant variables generally affect the process outcomes, another important context of discussion pertains to the way these powder relevant variables contribute in terms of their influence when compared with other process variables in a certain parametric combination. Kansal et al. [6] optimized the process parameters of powder mixed dielectric machining for MRR and SF using response surface methodology (RSM). Silicon powder in the range of 20–30 micron was added to kerosene oil (base dielectric) while machining EN-31 tool steel with Cu electrode. They found quadratic model to be statistically significant for explaining the relationship between the independent and the output variables. Powder addition not only increased surface finish but also its high concentration tended to increase MRR. They found maximum MRR at Si concentration of 2g/l and peak current of 12 Amp (the highest employed in their work). Later, Kumar et al. [16] optimized the process for another tool steel grade EN-24 while employing the same process parameters but with different electrode dia. They employed grey relation analysis for process optimization. Batish et al. [17], in another work, evaluated parametric combination that involved various powder specific and conventional EDM variables for various die steel grades. They found powder concentration to be more dominant than certain electrical parameters (such as pulse on time and current) in terms of significance for tool wear rate. For the case of overcut, they noted powder concentration to be second highest contributing factor behind pulse on time. Singh and Yeh [18] used L18 orthogonal array with gray relational analysis to optimize the process when SiC abrasive powder mixed dielectric was used to machine 6061Al/Al2O3p/20p aluminum matrix composite. Eight input variables were considered in their work with performance measures being material removal rate, tool wear rate and surface roughness. As per their confirmatory runs, an increase was noted for MRR (from 0.4267 to 0.530 g/min) whereas reduction was recorded for tool wear rate (from 0.0112 to 0.0096 g/min) and surface roughness (3.64 to 2.82 microns). They also concluded pulse on time to be relatively more important than other investigated parameters for the multiple performance characteristics studied in their work. Aspect ratio, tool electrode lift time, duty cycle, abrasive particle size, abrasive powder concentration, gap voltage, and pulse current were reported to follow the order of importance. In another work on composite machining via PMEDM, Singh et al. [19] used tungsten powder mixed dielectric to machine aluminum alloy 6061/10% SiC composite. They used RSM to model the effect of four electrical parameters (current, pulse on, pulse off and gap voltage) on MRR with and without powder presence. They reported that although all four electrical parameters had significant role in MRR for both i.e. powder mixed and conventional EDM, however change in MRR with respect to pulse off time was more with PMEDM than that obtained in simple EDM. Additionally, surface analysis of the machined workpiece indicated 42.85% reduction in thickness of the recast layer for the case of PMEDM in comparison to when only base dielectric was used. Long et al. [7] while machining different grades of steel in presence of titanium powder mixed dielectric concluded that interactions of titanium powder concentration with workpiece material and with electrode material were among the main factors that influenced MRR. In a more recent work, Kumar et al. [20] while employing different powder materials to machine Inconel-800 at different machining conditions, found powder particles along with current, pulse on time and tool material to be influential factors for tool wear rate.
As evident from above literature, among the parametric sets investigated in specific studies, the powder specific variables have played different (but important role) in differing scenarios. Interestingly, it is pertinent to note that all aforementioned studies were undertaken in “die sinking machining mode”. “Wire cut machining mode” which is an important process variant and otherwise enjoys a respectable place in research and industry due to its extended capabilities, has not been given much attention by researchers in PMEDM domain. Very limited research could be found for wirecut PMEDM and that too only considered special machining scenarios [21–22]. Jarin et al. [21] while employing wire cut mode, mixed carbon nanopowder (at three concentration levels of 0.2, 1 and 2 g/L) in commercially available dielectric to machine gold coated Si wafer at different voltage and capacitance values to compare the performance with unmixed dielectric. They noted higher MRR when machined with powder addition in general with maximum increase reported to be ~ 33% at certain conditions in comparison to un-mixed dielectric. It was also observed that specifically at lower discharge energies, MRR increased with increasing concentration of the powder. Incidentally the pattern of MRR increase was inconsistent at higher discharge energies which they attributed to more abrupt and uncontrolled erosion. They also noted higher spark gaps at higher discharge energies. In another study, Yeh et al. [22] mixed sodium pyrophosphate powder in pure water to machine polycrystalline silicon ingot in wire cut mode. The effects on cutting speed, kerf loss and surface roughness were evaluated. They reported 2.4 times higher cutting speed and an improvement of 16% in surface roughness with powder mixed dielectric in comparison to unmixed dielectric. An EDS analysis however, revealed infiltration of phosphorus into the machined surface for the case of sodium pyrophosphate mixed dielectric.
Ironically, other than the aforementioned special scenarios, no study could be located that thoroughly evaluates the role of powder mixed EDM in wirecut mode. It is important to realize that the dynamics of certain variables in wire cut mode are different than diesinker mode and therefore, the role of process specific variables in conjunction with powder addition is found to lack a thorough investigation. It has also been cited by others [9] that there exists a need to further explore powder material’s potential especially when so many variables get combined [9]. Looking at the effectiveness of powders for ‘die sinking machining mode’ and prevalence of significant research gap in ‘wire cut machining mode of EDM’, a thorough investigation in this regard is called for. It would be worthwhile to evaluate the role of variables and quantify the benefits for powder assisted wire cut machining mode.
This work quantifies the role of different process variables while employing SiC powder during machining of AISI D2 steel in EDM wire cut mode with re-circulating molybdenum wire which not only provides an economic proposition to be used as a tool but is also reported to have less tendency to chemically react and contaminate machined surface [23–24]. An average size of ~ 15 micron SiC powder was taken with three levels of concentration, stand-off distance, flushing pressure, pulse on time and current. Surface roughness, material removal rate, kerf width and wire wear ratio were the output criteria. Taguchi L27 array was used for experimentation followed by main effect plots and analysis of variance (ANOVA) for determination of significant parameters at 95% confidence level. SEM analysis was used to evaluate re-cast layer thickness of the optimized samples.