Journal of Manufacturing ProcessesVolume 15, Issue 4, October 2013, Pages 474–482

Technical PaperMachining efficiency of powder mixed near dry electrical discharge machining based on different material combinations of tool electrode and workpiece electrode

Xue Baia, b,  ,  Qinhe Zhanga, b,  ,  ,  Jianhua Zhanga, b,  ,  Dezheng Konga, b,  ,  Tingyi Yanga, b, 

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AbstractPowder mixed near dry electrical discharge machining (PMND-EDM) is a novel electrical discharge machining (EDM) process. It is proposed to further improve the machining efficiency of dry EDM. The principle of material removal in PMND-EDM is illustrated and its deionization principle is proposed. The influence of residual heat on MRR is analyzed. The concept of superfluous residual heat is proposed. The material removal rate (MRR), the main index of machining efficiency for PMND-EDM process, is researched. Single factor experiments are performed to get effect of peak current, pulse on time, pulse off time, flow rate, tool rotational speed, air pressure and powder concentration on MRR under different material combinations of tool electrode and workpiece electrode. Thermal phenomena in PMND-EDM are illustrated. Effect of each process parameter on MRR of PMND-EDM is gotten and analyzed based on the deionization principle of PMND-EDM. Differences in MRR under different material combinations are found out. Brass tool electrode and W18Cr4V workpiece gain higher MRR under most of discharge conditions, while the superiority of copper tool electrode and 45 carbon steel workpiece in MRR arise when there is improper heat dissipation. The difference is analyzed based on the deionization principle of PMND-EDM.

Keywords Powder mixed near dry EDM; 

Material removal rate;  Single factor experiment;  Deionization principle

1. IntroductionElectrical discharge machining (EDM) is one of the most commonly used non-traditional material removal process. It faces challenges coming from the need of environmental protection. Many researches are done to meet the need. Kunieda et al. [1] firstly reported that EDM can be successfully performed in the gas dielectric medium in the year 1997. Hereafter, to improve the machining efficiency of dry EDM, Tao et al. [2] and [3]studied the near dry EDM and Gu et al. [4], [5] and [6] studied the mist-jetting EDM. Material removal rate (MRR) of dry EDM is improved due to the addition of liquid phase to the gas continuous phase. Zhang et al.[7] and [8] applied ultrasonic actuation to the workpiece in dry EDM and MRR was found to be improved. Gao et al. [9] and [10] proposed powder mixed near dry electrical discharge machining (PMND-EDM) to further improve the machining efficiency and the surface quality. Bai et al. [11] did research on the breakdown mechanism of the three phase dielectric medium. And it was found out that peak current, pulse on time and flow rate of the powder–liquid mixtures influence the MRR of PMND-EDM significantly [12].The identification of the basic physical process involved in EDM processes makes for the better control of processing properties. Simulation is used to reveal the physical process from the micro perspective. Joshi and Pande [13] developed a non-linear transient thermo-physical model for die-sinking EDM process, and forecast the effect of EDM process parameters such as discharge current, discharge duration, discharge voltage and duty cycle on the machining efficiency by finite element method. In their study, the influence of energy distribution factor on the prediction accuracy was also researched and it was found that the selection of the energy distribution factor affected the prediction accuracy markedly. Pradhan [14] predicted the effects of pulse current and pulse duration on temperature distribution of a single spark based on an axial symmetry model for AISI D2 steel by finite element simulation, and showed that energy distribution to the electrode was vital as it governed the fraction of energy transmitted to the electrode and then determined the precision of the developed model. Salonities et al. [15] developed a theoretical thermal model for the determination of MRR achieved as a function of the process parameters, and pointed out that the fraction of the generated heat entering the workpiece depended on thermal properties of workpiece material and tool electrode material. Experimental research could be used to study EDM process from the macroscopic point of view. Saha and Choudhury [16] studied the effect of gap voltage, discharge current, pulse on time, duty factor, air pressure and spindle speed on MRR, and found out that MRR increased with the increase of discharge current, duty factor, air pressure and spindle speed. Kung et al. [17] studied the effect of discharge current, pulse on time, grain

size and concentration of aluminum powder particle on MRR in powder mixed EDM and developed a mathematical model based on experimental results for investigating the influence of processing parameters on performance characteristics. Kansal et al. [18] chose pulse on time, duty cycle, peak current and concentration of the silicon powder that is added into the EDM dielectric fluid to study the process performance in terms of MRR.Research results of simulation reveal that materials of tool electrode and workpiece electrode determine the energy distribution factor, and then affect the prediction accuracy of the thermo-physical model markedly. However, the physical properties of electrode materials are usually considered to be fixed in research on MRR. There are some researches on the influence of electrode materials.

Singh et al. [19] used different kinds of tool electrode materials to machine the workpiece, and it was found out that the best machining rates were achieved with copper and aluminum electrodes. Aas [20] did research on the performance of two graphite electrode qualities in EDM of seal slots in a jet engine turbine vane on the purpose of correct selection of electrode material. These two kinds of graphite are Poco EDM3 and Poco AF5 (Poco EDM3 is an isotropic ultrafine grain graphite which offers high strength with outstanding wear and fine surface finish characteristics. The AF5 has even finer grains than EDM3 and the AF5 is expected to have lower MRR but higher resistance to wear). Experimental results showed that EDM3 graphite performed very well giving significantly higher MRR than AF5, but still with acceptable relative electrode wear. The AF5 gave significantly lower wear, but also lower MRR. The main purpose of these studies is to find out the suitable electrode materials rather than study the influence of materials of the workpiece electrode and the tool electrode. Younis [21] studied the effect of electrode materials on the EDM machined surface through experiments. Its focuses are the white layer and the micro-crack on the machined surface, which may induce surface damage.In this paper, researches are done on the machining efficiency of PMND-EDM based on different material combinations of tool electrode and workpiece electrode. The principle of material removal in PMND-EDM is illustrated. The influence of residual heat on MRR is analyzed. The deionization principle of PMND-EDM is proposed. Single factor experiments are performed. Thermal phenomena in PMND-EDM are illustrated. Influences of process parameters on MRR are obtained. Differences in MRR under different material combinations are found out. Analysis is done based on the proposed deionization principle of PMND-EDM.

2. Principle of PMND-EDM

2.1. Material removal principle of PMND-EDMThe principle of PMND-EDM is shown in Fig. 1 [12] . In this new process, dielectric medium in liquid state in normal EDM process is replaced by three phase dielectric medium. These three phases are gas continuous phase, liquid dispersed phase and

solid dispersed phase. The breakdown mechanism of dielectric medium is changed due to the addition of dispersed phases and discharge gap is enlarged [11]. As a result, the deionization effect of dielectric medium is improved. Spark initiation becomes easy and MRR is improved.

Fig. 1. Principle of PMND-EDM [12].

Figure optionsThe material removal principle of PMND-EDM is similar to that of general EDM. It is mainly based on heat erosion. During sparking, the dielectric medium between tool electrode and workpiece electrode is breakdown in the place where the distance between two electrodes is the shortest. Then, a plasma channel is formed between two electrodes. And heat is transferred into the tool and the workpiece mainly through discharge channel. Plane heat source in a circular form is formed on the surface of workpiece. Material is heated. Fusion and gasification is induced. The heated material is ejected and a crater is formed on the surface of the workpiece after one spark. The next spark could be induced after deionization. After a succession of sparks, a certain amount of materials are removed from the workpiece and the required shape and size can be obtained.

During sparking, there are three zones from areas contacting with the discharge channel to matrix materials of the workpiece, namely gasification zone, fusion zone and heat affected zone (see Fig. 2) [22]. The heat absorption capacities of materials per unit volume in these three zones are decreasing in sequence. Regardless of the deionization effect of the dielectric medium between the tool electrode and the workpiece electrode, the machining efficiency of PMND-EDM increases with the increase of the volume of the fusion zone and decreases with the increase of the volume of the gasification zone and the heat affected zone. However, PMND-EDM is a complicated material removal process. The deionization effect of the dielectric medium between two electrodes also influences the machining efficiency.

Fig. 2. Section of the crater in PMND-EDM process.

Figure options

2.2. Deionization principle of PMND-EDMGenerally speaking, the deionization course of PMND-EDM process includes two parts. One is the neutralization of positive particles and negative particles; the other is the dissipation of the residual heat and the debris in the discharge gap. In a proper temperature which is higher enough, discharge is easy to be induced in the dielectric medium according to the theory of thermal breakdown [22]. Based on the theory of thermal breakdown and the thermal phenomena in PMND-EDM, it can be inferred that the existence of a certain amount of residual heat can facilitate the improvement of MRR; on the contrary, the MRR will be reduced due to insufficient residual heat during continuous discharges. However, poor deionization effect will be brought about when there is too much residual heat. This goes against the increase of the machining efficiency.Residual heat, which is too much or too little, does not facilitate the improvement of MRR and they correspond to improper heat dissipation conditions. Under a certain combination of process parameters, the heat dissipation ability of the dielectric medium is finite. There is a threshold under which the residual heat can be dissipated properly and the largest MRR can be obtained. The residual heat, which goes beyond the threshold, is defined as the superfluous residual heat.

It is usually thought that the dissipation of the residual heat occurs at the end of the spark. If the superfluous residual heat can be transferred out of the dielectric medium during sparking, there will no superfluous residual heat be left in the dielectric medium and a perfect processing condition with the highest MRR can be obtained. In fact, the residual heat can be dissipated during sparking. The variation of process parameters could influence the dissipation of the residual heat and result in different variation tendencies of the MRR. The change of the heat dispersion property of the electrodes could also influence the course of heat dissipation, and lead to differences in MRR.

3. Experimental setup and procedures

3.1. Experimental setupThe PMND-EDM experiments are conducted on SF201 computer numerical control (CNC) die-sinking EDM machine that is manufactured by AgieCharmilles (see Fig. 3(a)). A rotary spindle with center flushing capability, which is manufactured by System3R International AB, is used to hold the tubular tool electrode. It has eleven gears to transform rotational speed of tool electrode from 0 rpm to 2000 rpm. The minimal quantity lubrication (MQL) device (see Fig. 3(b)), produced by EUROLINK, is used to inject three phase dielectric medium into the discharge gap between tool electrode and workpiece electrode. The driving pressure of it ranges from 0.4 MPa to 0.8 MPa. And it could provide flow rate ranging from 0 to 2.68 cm3/min. An air compressor, manufactured by Beijing Zhengli Precision Machinery Limited Corporation, is used to supply compressed air. The maximum air pressure, which it can provide, is 0.65 MPa. A cooling and drying machine, manufactured by Guangzhou HAN FILTER Equipment Co. Ltd., is used to improve the quality of compressed air used in the experiments. Discharge status can be seen in Fig. 3(c).

Fig. 3. Experimental setup and discharge status of PMND-EDM.

Figure optionsTubes with 4 mm outer diameter and 2 mm inner diameter are used as tool electrode. Compressed air is used as gas continuous phase. EDM fluid is used as fluid dispersed phase. Silicon powders are used as solid dispersed phases. Silicon powders are mixed into the EDM fluid in the oil box of the MQL device. Air–liquid–powder three phase mixtures are formed by the MQL device under the driving of the air supply system and delivered into discharge gap between tool electrode and workpiece electrode.

3.2. Experimental proceduresTo study the MRR of PMND-EDM based on influences of electrode materials, single factor experiments are performed. Peak current, pulse on time, pulse off time, flow rate, powder concentration, air pressure and tool rotational speed are selected as processing parameters in the single factor experiment. In EDM processes, electrodes

include tool electrode and workpiece electrode. In order to obtain the influence clearly, the material of one electrode is fixed and two kinds of materials for the other are used during experiments.

Two sets of experiments are carried out. In the first set, differences caused by brass tool electrode and copper tool electrode in single factor experiments, when they are used to machine W18Cr4V high speed steel workpiece under different process parameters, are studied. In the second set, differences caused by 45 carbon steel and W18Cr4V high speed steel in single factor experiments, when they are machined by copper tubular tool electrode under different process parameters, are studied. The physical properties of brass, copper, 45 carbon steel and W18Cr4V high speed steel can be seen in Table 1.

Table 1.Physical properties of brass, copper, 45 carbon steel and W18Cr4V high speed steel [23] and [24].Material Density

(g/cm3)Melting point (°C)

Electrical resistivity (Ω mm2/m)

Specific heat (J/kg K)

Thermal conductivity (W/m K)

Brass 8.47 930 0.064 380 116Copper 8.94 1083 0.017 385 39145 carbon steel

7.85 1460 – 460 51.9

W18Cr4V 8.67 1700 – 473 27.21

Table optionsThermal phenomena in PMND-EDM are illustrated. The variation tendency of MRR caused by the variation of each process parameter is analyzed. MRR curves of two kinds of electrode materials, which are influenced by the same process parameter, are drawn in the same figure in each set of experiment. Their differences are found out. Physical properties of tool electrode materials and workpiece electrode materials are compared, respectively. Differences in MRR caused by the difference of electrode materials are analyzed.

Throughout experiments, the polarity for tool electrode is negative, the reference voltage is 40 V and the open circuit voltage is 300 V. In single factor experiments, the effect of each process parameter on MRR is obtained by changing one parameter at different levels while fixing others at the basic level. The basic levels and levels for each factor are listed in Table 2. The levels of powder concentration are selected based on our experimental experience. The levels of other factors are selected in the capable parameter range of the equipment. During selection, the maximum and the minimum are selected first, and then several points are selected in the interval based on the uniform principle.

Table 2.Experimental factors, basic levels and levels for each factor in single factor experiments.Factors Basic levels Levels for each factorPeak current (Ip/A) 9.4 3.2, 9.4, 25.6, 37, 50, 64Pulse on time (τon/μs) 32 1.8, 32, 130, 240, 420, 560

Factors Basic levels Levels for each factorPulse off time (τoff/μs) 32 2.4, 32, 75, 180, 320, 560, 750Flow rate (F/(ml/min)) 0.722 1.502, 1.110, 0.722, 0.398, 0.066Powder concentration (Con/(g/l)) 3 0, 3, 6, 9, 12, 15Tool rotational speed (N/rpm) 200 2000, 1600, 1000, 600, 200Air pressure (P/MPa) 0.5 0.4, 0.5, 0.6

Table optionsHoles are drilled. Machining times are recorded by the EDM machine. The MRR is equal to the machining depth times the cross section of the tool electrode divided by the machining time. The machining depth is set at 1 mm throughout single factor experiments. Experiments are repeated three times under each machining condition. And the mean value is used as the experimental result for each condition. Error bars are given to study the experimental results including the standard deviation.

4. MRR variations of PMND-EDM based on different tool electrode materials

4.1. Influence of process parameters on MRR based on different tool electrode materialsEffects of peak current, pulse on time, pulse off time, flow rate, powder concentration, air pressure and tool rotational speed on MRR under different tool electrode materials are shown in Fig. 4.

Fig. 4. Effect of each process parameter on MRR based on different tool materials.

Figure options

4.1.1. Thermal phenomena in PMND-EDMIt is known that the MRR of EDM is determined by the pulse discharge energy and the pulse discharge energy can be calculated by Eq. (1) [22] :

equation(1)

Turn  MathJax on

where W is the pulse discharge energy; U is the discharge voltage; I is the discharge current; τon is the pulse on time. Obviously, pulse on time and discharge current are two factors that influence the pulse discharge energy. Actually, the pulse off time could also influence the obtained heat of the machined workpiece during the PMND-EDM process. Fig. 5 is the scanning electron microscope (SEM) photographs of the workpiece surfaces machined by the copper tool electrode, which are obtained based on the single factor experiment of pulse off time. The pulse off time is 2.4 μs, 32 μs and 75 μs, respectively. The depth of crater gets obviously larger when the pulse off time is small enough, and discharge concentration is found in some area on the machined surface. The increase of MRR can be caused due to the increase of the volume of a single crater.

Fig. 5. The SEM photograph of the machined surface influenced by pulse off time.

Figure optionsAccording to Eq. (1), the increase of pulse on time and discharge current could result in the increase of heat. The effect of pulse on time on MRR with copper tool electrode in single factor experiments can be seen in Fig. 4(b). The effect of peak current on MRR with copper tool electrode in single factor experiments can be seen inFig. 4(a). The excessive increase of pulse on time could result in the decrease of MRR. For the effect of peak current on MRR, there is no decrease of MRR. To verify that the increase of peak current could result in the increase of residual heat, the SEM photographs of the machined workpiece surface are gotten (see Fig. 6). It can be seen that the reattachment on the machined surface increases when the peak current grows larger. These reattachments are formed mainly due to the increase of the residual heat. There are molten materials that could not be cooled sufficiently and then expelled out of the discharge gap. As a result, the increase of the reattachment is caused. The increase of peak current results in the increase of discharge energy. More material melts during sparking. If these molten materials are completely thrown out from the surface of the machined workpiece, the maximum

increment of the MRR could be reached. However, reattachments increase with the increase of peak current. This means that part of the molten materials would not be thrown out from the surface of the machined workpiece but re-solidify on the surface of the machined workpiece. The increase of reattachment goes against the acquirement of the maximum increment of the MRR. And the increase of the MRR is slowed down.

Fig. 6. The SEM photograph of the machined surface influenced by peak current.

Figure options

4.1.2. Influences of process parameters on MRR and difference caused by tool electrode materialsMRR increases rapidly with the increase of peak current according to Fig. 4(a). There are two reasons for this. Firstly, peak current determines the discharge energy. With the increase of peak current, the energy increases rapidly. Secondly, peak current influences the ejection of molten materials. Woo et al. [25]observed the effects of discharge energy on the specific impulsive force. Experimental results showed that the specific impulsive force increased with the increase of discharge energy. Meanwhile, it was observed that discharge energy was the most significant factor that influenced the specific impulsive force. According to Eq.(1), the increase of peak current could result in the increase of discharge energy. Therefore, force, which is used to eject molten materials, increases rapidly with the increase of peak current. Meanwhile, according to reference [22], the discharge channel is a kind of high-temperature ionized gas actually. The inner pressure of the discharge channel can be calculated by Eq. (2):equation(2)p 0 = ( n e + n i ) k T

Turn  MathJax on

where p0 is the pressure of the discharge channel; ne is the electron number per unit volume; ni is the ion number per unit volume; k is the Boltzmann constant; T is the absolute temperature. The increase of peak current can result in the increase of the electron number per unit volume and the ion number per unit volume due to the

enhancement of ionization. Thus the pressure of the discharge channel increases due to the increase of peak current. As a result, the ejection force increases with the increase of peak current. There will be more materials that are molten and ejected to discharge channel from the crater on the surface of workpiece and expelled from the discharging area with the increase of peak current. As a result, MRR improves with the increase of peak current. There are differences between MRR with brass and copper. MRR with copper and brass are nearly the same when peak current is low. However, copper obtains larger MRR when peak current gets large.In Fig. 4(b), MRR increases with the increase of pulse on time due to the increase of input energy. However, it decreases when pulse on time is beyond a certain value. The reason for this is that too big pulse on time may induce superfluous residual heat. The problem of heat dissipation is caused and MRR under such conditions is relatively low. It is also seen that MRR with brass tool electrode is higher than that with copper tool electrode, though the former increases quickly than the latter with the increase of MRR to its largest value and decreases quickly than the latter when the deionization effect goes improper.Discharge frequency increases with the decrease of pulse off time when the pulse on time is a constant. On the other hand, the residual heat increases with the decrease of pulse off time. Therefore, MRR increases with the decrease of pulse off time. See in Fig. 4(c). It also can be seen that when pulse off time is too small or too big, MRR with copper is similar to MRR with brass. Nevertheless, brass achieves higher MRR relative to copper under other conditions. The advantage of copper grows gradually with the decrease of pulse off time due to the decrease of deionization time. In the middle of the curves, proper heat dissipation can be gained. The advantage of brass becomes obvious. However, the superiority of brass decreases when the pulse off time is too big due to excessive dissipation of the residual heat which leads to vanish of the accumulation effect of the residual heat.Flow rate means the volume of solid–liquid mixtures that is delivered into discharge gap by compressed air per minute. The functions of solid–liquid mixtures are given as follows: facilitate the discharge initiation by changing electric field strength; make for the sufficient heat dissipation by enlarging discharge gap [11]; eject more molten materials by the explosive force of the gasification of solid and liquid phases; and restrain the excessive expansion of discharge channel in its radius direction. The increase of flow rate may facilitate the increase of MRR according to reasons mentioned above. The results shown in Fig. 4(d) are in accordance with the above analysis. It is also found that MRR with copper is larger than that with brass when flow rate is too small; however, brass gains higher MRR when flow rate is larger than a certain value. The superiority of brass to copper in MRR grows with the increase of flow rate.MRR increases with the increase of powder concentration and then decreases when powder concentration goes beyond a certain critical value according to Fig. 4(e). Powders that are delivered into discharge gap with an appropriate volume ratio can enlarge the discharge gap and then facilitate the dissipation of the residual heat.

However, too much powder in the discharge gap will destroy the dielectric property of the medium and the induction of spark will become difficult. So the effective discharge frequency is reduced and the MRR is decreased. From Fig. 4(e), MRR with brass is higher than that with copper, especially when powder concentration goes close to the point that stands for the largest MRR. The difference between brass and copper in MRR reduces with the deterioration of the deionization effect and grows with the improvement of the deionization effect.Air pressure has close relationship with the ejection force for molten materials and the dissipation of the residual heat. The dielectric medium grows to sufficient deionization conditions with the increase of air pressure. So the increase of MRR is induced (see Fig. 4(f)). It also can be seen that brass results in larger MRR than copper. The growth of MRR with brass is quicker than that with copper when air pressure grows from 0.5 MPa to 0.6 MPa due to the reason that sufficient deionization effect has been gotten based on copper tool electrode at about 0.5 MPa and excessive deionization is induced along with the further increase of air pressure.The decrease of MRR is found with the increase of tool rotational speed (see Fig. 4(g)). This can be explained by the interruption effect of tool rotation to discharge. In PMND-EDM, compressed air is continuous phase, liquid and solid dispersed phases are fed into the continuous phase to stimulate sparking. The discharge channel is easily formed in places where the dispersed liquid or solid phase stays, and it is influenced by the movement of the dispersed phase that induces the sparking. The rotation of the tool electrode leads to the change of relative positions among the spark point on the surface of tool electrode, the spark point on the surface of workpiece electrode and the dispersed liquid or solid phase in discharge channel. Furthermore, the tool rotation leads to stirring of dielectric medium and the dispersed liquid or solid phase may be ejected out of discharge channel easily. Because of reasons mentioned above, the interruption of sparks grows easily with the increase of tool rotational speed. As a result, MRR decreases with the increase of tool rotational speed. Moreover, for this process parameter, brass tool electrode gets higher MRR compared with copper tool electrode and their difference nearly maintains the same with the variation of tool rotational speed.

4.2. Influence of tool materials on MRR based on influences of process parametersTo find reasons for the difference in MRR, physical properties of brass and copper (listed in Table 1) are compared. Density and specific heat of brass are slightly less than that of copper. It is not thought to be the main reason. However, electrical resistivity of brass is about 4 times that of copper. This means that brass may produce more heat during sparking under the same discharge conditions. In addition, thermal conductivity of brass is about a quarter of that of copper. This means that heat transport capability of brass is not as good as copper. So more time is needed by brass to dissipate heat produced by itself before it can transfer heat that comes

from discharge channel to atmosphere. Melting point of brass is less than that of copper; therefore, more heat is needed for copper to reach its melting point. This also means that copper has the ability to absorb more heat under the same heat source on the surface of tool electrode.Under discharge conditions with superfluous residual heat, these physical properties of brass will go against the heat dissipation. The superfluous residual heat will increase and the MRR decreases because of short circuit and arcing. Nevertheless, higher thermal conductivity will facilitate heat dissipation and lower electrical resistivity will reduce the heat produced by tool electrode itself. So that copper tool electrode may have higher MRR than that of brass. However, when heat dissipation is excessive, more heat in discharge gap means that spark initiation will be easier and more heat will be transferred into workpiece. As a result, MRR will be improved. Under such conditions, larger electrical resistivity with lower thermal conductivity will facilitate the increase of MRR.

Ejective force grows with the increase of peak current and more molten materials will be thrown into discharge gap. Their cooling and solidification makes more heat transferred into discharge area and heat dissipation problem can be induced. Arcing increases with the increase of pulse on time. Much more heat is produced and its dissipation becomes difficult. Pulse off time that is too low and powder concentration that is too high or too low also lead to the heat dissipation problem. Moreover, low flow rate goes against enlargement of discharge gap and heat dissipation becomes difficult. Under discharge conditions mentioned above, MRR with copper tool electrode is higher or grows approximately equal to that with brass.

5. MRR variations of PMND-EDM based on different workpiece electrode materials

5.1. Influence of process parameters on MRR based on different workpiece electrode materialsEffects of peak current, pulse on time, pulse off time, flow rate, powder concentration, air pressure and tool rotational speed on MRR under different workpiece electrode materials are shown in Fig. 7.

Fig. 7. Effect of each process parameter on MRR based on different workpiece materials.

Figure optionsThe influence of process parameter on MRR in Fig. 7 is similar to that of the corresponding parameter in Fig. 4. Reasons for these variations have been given in Section 4.1.2. Therefore, research emphasis is shifted to the difference of MRR caused by materials of workpiece electrodes, which are made of 45 carbon steel and W18Cr4V high speed steel respectively (see Section 5.2).

5.2. Influence of workpiece materials on MRR based on influences of process parameters

5.2.1. Physical properties of workpiece materials and their influences on MRRDensity and specific heat are nearly the same for 45 carbon steel and W18Cr4V high speed steel (see Table 1). So they are not thought to be main reasons which cause difference in MRR. Melting point of 45 carbon steel is much lower and its thermal conductivity is much higher. Melting point decides the volume of steel in the molten state when the same heat is transferred into the steel, while thermal conductivity will determine the volume that is heated. The difference in MRR can be gotten due to the difference in melting point and thermal conductivity.Densities and specific heat of 45 carbon steel and W18Cr4V high speed steel are similar, respectively. So that higher melting point of materials demands more heat to remove the same volume of materials. Higher thermal conductivity makes more heat transferred into heat affected zone and the volume of this zone will grow. Under discharge conditions with different deionization effect, influences of the melting point and the thermal conductivity on MRR are different. Firstly, their influences under discharge conditions without superfluous residual heat are analyzed as follow. For 45 carbon steel, lower melting point makes it easier to be reached; however, higher thermal conductivity lets more heat be transferred into the heat affected zone. The former facilitates the MRR increase, while the latter goes against the increase. For W18Cr4V high speed steel, higher melting point makes it hard to be reached. However, lower thermal conductivity lets less heat be transferred into the heat affected zone. The former goes against the MRR increase, while the latter facilitates the increase. Nevertheless, under discharge conditions with superfluous residual heat, both of higher melting point and higher thermal conductivity contribute to the heat absorption from discharge channel. The residual heat in discharge gap may be dissipated due to higher melting point or higher thermal conductivity. And then the deionization effect is improved. As a result, the MRR can be improved.

5.2.2. Difference in MRR caused by workpiece material variationThe deionization effect is mainly influenced by duty cycle, which equals to the ratio of pulse on time to pulse off time. Higher duty cycle does not facilitate deionization, namely, too small pulse off time or too big pulse on time will go against the deionization when the discharge cycle is a constant. Higher MRR of 45 carbon steel is found with small pulse off time (see Fig. 7(c)). And lower MRR of W18Cr4V is found with large pulse on time (see Fig. 7(b)). Flow rate and powder concentration also influence the deionization effect. The difference for MRR of 45 carbon steel and MRR of W18Cr4V can be found from Fig. 7(d) and (e), respectively. Too small or too big powder concentration and too small flow rate do not facilitate deionization. Under these conditions, MRR of 45 carbon steel is found to go close to MRR of W18Cr4V and the reduction of their difference is caused. Nevertheless, MRR of W18Cr4V grows relatively quick when the powder concentration goes to some value, under which the largest MRR is gotten, and along with the increase of flow rate, and then the difference in MRR of 45 carbon steel and W18Cr4V high speed steel increases.

Low peak current or pulse on time could lead to insufficient energy input and inadequate residual heat. Under conditions with low peak current in Fig. 7(a) and conditions with low pulse on time in Fig. 7(b), difference in MRR of 45 carbon steel and W18Cr4V is found to be reduced because less heat is needed for 45 carbon steel to get molten. The usage of copper as tool electrode facilitates the effective dissipation of residual heat and the formation of superfluous residual heat is easier to be avoided. Residual heat increases with the increase of peak current. The superiority of 45 carbon steel decreases and a higher MRR is gotten by W18Cr4V under discharge conditions with higher peak current.Higher air pressure facilitates the heat dissipation. It can be seen in Fig. 7(f), MRR of W18Cr4V is a little higher than that of 45 carbon steel at 0.4 MPa. However, MRR of W18Cr4V increases more rapidly than that of 45 carbon steel with the increase of air pressure, though, nearly no increase of 45 carbon steel is found when air pressure grows from 0.5 MPa to 0.6 MPa. The deionization effect with the action of tool rotation is good and the variation of tool rotational speed has nearly no influence on the deionization effect. So that MRR variation tendencies are nearly the same for 45 carbon steel and W18Cr4V, though, the MRR of the latter is always higher than that of 45 carbon steel.

6. ConclusionsThe material removal of PMND-EDM depends on heat erosion. However, the deionization effect of the dielectric medium between two electrodes will influence its machining efficiency. Thermal phenomena in PMND-EDM are illustrated. The effect of residual heat on MRR of PMND-EDM is analyzed. It is indicated that there is superfluous residual heat during discharging of PMND-EDM. The concept of superfluous residual heat is defined. And the deionization principle of PMND-EDM is proposed. MRR is studied based on different material combinations of tool electrode and workpiece electrode through single factor experiments of peak current, pulse on time, pulse off time, flow rate, powder concentration, air pressure and tool rotational speed. The effect of each process parameter on MRR is analyzed based on the deionization principle of PMND-EDM. Similar effect of each process parameter is found out under different material combinations of tool electrode and workpiece electrode. However, there are differences caused by material changes of the tool and the workpiece, respectively. The brass tool electrode gains higher MRR under most of discharge conditions. Therefore, from the perspective of improving the MRR of W18Cr4V, brass should be used as the tool electrode material. Adequate heat dissipation is easier to be gotten by copper tubular tool electrode due to its relatively higher thermal conductivity, relatively higher melting point and relatively lower electrical resistivity when there is superfluous residual heat and then its superiority in MRR emerges. When machined by copper tubular electrode, W18Cr4V gains higher MRR under most of discharge conditions. However, the superiority of 45 carbon steel in MRR arises, when there is improper heat dissipation, due to its higher thermal

conductivity which facilitates the dissipation of the superfluous residual heat and the lower melting point which is easier to be gotten. Differences in physical properties of electrode materials result in their different influences on the residual heat, and then generate differences in MRR.

AcknowledgementsThe work was supported by a grant from the National Natural Science Foundation of China (Grant No.51075239), Graduate Independent Innovation Foundation of Shandong University (GIIFSDU, no. yzc12126) and Scientific Starting Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education, China (2010).

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