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Investigation into some surface characteristics of electrical discharge machined SKD-11 using powder-suspension dielectric oil
Journal of Materials Processing Technology 170 (2005) 385–391
Investigation into some surface characteristics of electrical discharge machined SKD-11 using powder-suspension dielectric oil Tzeng Yih-fong a,∗ , Chen Fu-chen b a
Department of Mechanical and Automation Engineering, National Kaohsiung First University of Science and Technology, 1 University Road, Yenchao, Kaohsiung 824, Taiwan b Department of Mechanical Engineering, Kun Shan University of Technology, Taiwan Received 24 August 2002; received in revised form 3 December 2004; accepted 1 June 2005
Abstract This project investigates the property effect of additives on surface quality of EDMed SKD-11. The additives with significantly different thermophysical properties, including aluminum (Al), chromium (Cr), copper (Cu), and silicon carbide (SiC) powders are studied. Experimental results show that the particle size of additives in the dielectric oil affects the surface quality of EDMed work. While the smallest particles (70–80 nm) generates the best surface finish of the machined work, the greater the particle size the less the improvement in the surface roughness. However, particle size has opposite effect on the recast layer, as the smallest particles generated the thickest recast layer of the EDMed surface, and the greater the particle size the thinner the recast layer. Among the additives, Al powder produces the best surface finish and the thinnest recast layer in the machined work, whereas the process without foreign particles and with copper powder, generates the worst surface characteristics. © 2005 Elsevier B.V. All rights reserved. Keywords: Electro-discharge machining (EDM); Powder-suspensions; Surface roughness; Recast layer; Thermo-physical property
1. Introduction Electro-discharge machining (EDM) is a process that is used to remove metal through the action of an electrical discharge of short duration and high current density between the tool and the workpiece [1]. EDM on ferrous metals results in surface changes, with the formation of a re-solidified layer, usually known as the recast layer, which varies in thickness. The recast layer undergoes complex structural changes associated with extremely high cooling rate. It was reported that the hard recast layer was often found to contain microcracks, caused by high tensile residual stresses exceeding the ultimate strength of the material [2]. This surface imperfection further reduces the fatigue, wear and corrosion resistance of EDMed components [3,4]. To remove the damaged surface layer and to restore the surface properties, post-machining processes are thus needed. However, post-processing means ∗
Corresponding author. Tel.: +886 7 6011000x2226; fax: +886 7 6011066. E-mail address: [email protected] (T. Yih-fong).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.06.006
an additional cost. To reduce the unit production cost for industry, it is essential to minimise the damaged surface layer of machined work in the EDM process. In 1989, Narumiya et al. began to notice and examined the effects of silicon powder in the working fluid on EDM [5,6]. They found that foreign particles were capable of reducing the recast layer, preventing cracks, and producing a mirrorlike surface finish in the EDMed components. In 1993, Yan et al. continued to investigate the effects of dielectrics with suspended aluminum, alumina, and silicon carbide powders, respectively on EDM [7–9]. They observed the same results as those produced by silicon powders. In 1995 Quan et al. further studied the effects of unspecified conductive particles and inorganic-oxide particles on EDM, and agreed that the introduced particles produced better surface quality [10]. Although the advantages of properly introducing foreign particles including metals and non-metals into EDM have since been widely recognized, very few attention was paid to the possible effects of the characteristics of the particles
386
T. Yih-fong, C. Fu-chen / Journal of Materials Processing Technology 170 (2005) 385–391
on the surface quality of the machined components. Without such information, it would, nevertheless, be very difficult for industry to determine the optimal selection or combination of particles. A systematic study of the factors of the particles with significant difference in properties was undoubtedly necessary. This paper thus attempts to fill into the gap of literature, in the hope of providing a better understanding of the correlation between the characteristics of chemical additives and the surface quality.
2. Material removal mechanism using powder-suspension dielectric oil As debris in a spark gap usually consists of metal and carbon particles, which will drastically lower the breakdown strength of dielectric, gap debris evidently would facilitate ignition process and increases gap size. Absence of the debris can result in arcing due to a lack of precise feeding mechanism with extremely high position resolution, which occurs frequently in the early stages of the ignition process particularly. Moreover, the amount of debris matters. While the absence of debris does not help improve sparking frequency, too much debris is generally believed to be the dominant cause of spark concentration i.e. arcing that leads to an unstable and inefficient process. In fact, gap debris is of somewhat help. Gap debris reportedly is comparatively a most crucial factor to the stability of machining process, which demands evenly disperse discharge locations that mainly depend upon debris concentration and distribution, bubbles, de-ionisation, and surface irregularities. Nevertheless their concurrent presence poses great difficulty to segregate the effects attributed to each factor [11]. Most present control systems thus cannot directly
regulate discharge location. Gap debris in this regard can significantly control discharge transitivity, gap size, breakdown strength, and de-ionisation [11]. The remaining core issue is how to decide the function of gap debris. Theoretically, the function of debris will be to a great extent controlled by the characteristics of the additives if they are added in suitable particle size, particle concentration, particle density, thermal conductivity, electrical resistivity, melting point, evaporation point, specific and latent heat, etc. Whether and how different character combinations affecting EDM process, however, has been rarely discussed and needs a further investigation, so as to provide the best formula for wise selection and control of these additives in a precise EDM. The addition of particles alters the material removal mechanism in the EDM process. Fig. 1 shows a schematic diagram of a normal single electrical discharge in a spark gap without and with suspended particles. It is noted that the addition of powders lead to an increase in gap size that subsequently resulted in a reduction in electrical discharge power density and in gas explosive pressure for a single power pulse. As seen in Fig. 2, the modified material removal mechanism for addition of powders during normal single electrical discharge time is the combined effect of mechanical thrust driven by the gas explosion mainly from the working fluid evaporation with the striking impact by the suspended particle. The materials removed by the grinding effect of suspended particles within the interspace are negligible. It is worth noting that the weaker gas explosion in the interspace after powder addition might lead to a reduction in the material removal rate (MRR) during normal single electrical discharge process. To enhance the machining efficiency of the whole EDM process, the particles striking effect and the discharge transitivity, therefore, play a decisive role. Especially, the latter decides the spark-
Fig. 1. Schematic diagrams of the electric discharge for single power pulse (a) without powder and (b) with powders [10].
T. Yih-fong, C. Fu-chen / Journal of Materials Processing Technology 170 (2005) 385–391
387
Fig. 2. Schematic diagrams of material removal mechanism during the normal discharge for the working fluid (a) without powder and (b) with powders [10].
ing frequency that governs the entire MRR, while the first has minor cutting effect contributing mainly to the improvement of the surface finish.
Table 2 EDM process parameters set-up Workpiece (−) Electrode (+)
3. Experiments
Machining time (min) Dielectric fluid
A locally designed electrical discharge machine Castek03 is used throughout the experiments. A range of different powders is added to HERCULES ED 320H dielectric oil as the working fluid in EDM process. Some of the important thermophysical properties of used particles are displayed in Table 1. Mould steel SKD-11 workpieces (16 mm in diameter) which are quenched and tempered to raise microhardness from 20 to 60 Hrc are spark eroded using copper tools with 8 mm in diameter using the EDM process parameters set-up as shown in Table 2. The applied discharge currents and power-on times are intentionally kept low to simulate the conditions of precision machining. The filter system for re-circulation of dielectric fluid is designed in such a way in order not to filter out the powders that are added into the dielectric fluid. It has to be noted that the powders investigated is non-magnetic. The new filter system shown in Fig. 3 comprises two subsystems: (a) Filtering: uses the magnetic force of hundreds of magnets closely arranged in a series in the filter to sufficiently separate the work debris from the dielectric oil, while the powders pass through for constant reuse. (b) Re-circulation: the pump sends the dielectric fluid back into the tank.
SKD11 (16 mm in diameter) Cu (8 mm in diameter) 20 Pure dielectric oil
SKD11 (16 mm in diameter) Cu (8 mm in diameter) 20 Dielectric oil + powders 3.5
Dielectric flowing rate (l/min) Powders Powder size (m)
3.5
Particle concentration (cm3 /l) Breaking voltage (V) Discharge peak current IP (A) Pulse-on time Ton (s) Duty cycle (CD )
No
Al, Cr, Cu, SiC 0.07–0.08, 10–15, 100 0.25, 0.5, 1.0
120 1.5, 4.0
120 1.5, 4.0
6, 25, 75 1/2, 2/3
6, 25, 75 1/2, 2/3
No No
Two of the dominant features of the surface integrity of EDMed components are the surface roughness and the recast layer. In order to obtain quantitative data on the extent of these phenomena, analyses are carried out using a metrology instrument and an optical microprobe. The roughness value is the average of the readings scanned along the centreline of the top surface of the EDMed components in three directions using a Taylor–Hobson metrology instrument. In order to measure the recast layer, specimens are first carefully prepared by properly cutting the EDMed components along the centreline in the longitudinal direction. The surface cross-
Table 1 Thermophysical properties of additives Powders
Density (g/cm3 )
Thermal conductivity (W/cm K)
Electrical resistivity ( cm)
Melting point (◦ C)
Specific heat (cal/g ◦ C)
Al Cr Cu SiC
2.70 7.16 8.96 3.21
2.38 0.67 4.16 1.0–5.0
2.45 2.60 1.59 1 × 109
660 1875 1083 2987
0.215 0.11 0.092 0.18
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Fig. 3. Schematic diagram of a self-designed filter system for the electrodischarge machine used for the experiments.
sections are then mounted in a resin, ground and polished for metallurgical examination. These sections are observed under an optical microscope equipped with a micrometer to calculate the average thickness of the recast layer of these EDMed components.
Fig. 4. The dependence of the surface roughness of the EDMed components on particle size and particle concentration of Al powder, and on discharge current.
4. Experimental results and discussions 4.1. General discussions It is found during the experiments that the particle size, particle density, particle concentration, electrical resistivity, and thermal conductivity of the powders are the relatively important powder characteristics affecting the surface quality in the EDM process. The effect of each factor will be discussed in the following sections. Other typical properties, such as melting point, evaporation point, and the latent heat of the particles, are observed to have no appreciable effects on surface characteristics of the EDMed components. 4.2. Effects of particle size on surface roughness Fig. 4 describes the dependence of surface roughness of the EDMed components on the particle size and on the concentration of aluminum powder within the dielectric oil, and also on the discharge current. It is observed that foreign particles are capable of reducing the surface roughness of the machined parts, and the improvement is more associated with the particle size rather than the particle concentration. This is due to a relatively low particle concentration being used in the experiments. Experimental results further reveal that the smallest particle (70–80 nm) generates the best surface finish and the greater the particle size, the worse the surface roughness. It is due to small particles producing fine cutting effects in a complementary way. This can be seen in the optical microphotographs in Figs. 5–8, referring to the surface finish of the specimens, which are electro-discharge machined by three levels of particle sizes. Slight differences
Fig. 5. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and without additives.
are observed in the comparison of the surface finishes from Figs. 5–8. Fig. 5 displays more deep craters than Figs. 6–8. This indicates higher surface roughness produced by the process without powder additions. It is well worth noting in Fig. 4 that the effects of foreign particles on surface finish of EDMed components are less influential than the pulsed discharge current set-up. It is due to the fact that the discharge current governs the heating effects during EDM process, whereas particle striking plays a secondary role.
Fig. 6. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 70–80 nm Al powders.
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Fig. 7. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 10–15 m Al powders.
4.3. Effects of particle concentration on surface roughness The effects of a range of particle concentrations of aluminum powder on the surface roughness can also be seen in Fig. 4. Experimental results show that the particle concentration of 0.25 cm3 /l leads to a best improvement in the surface finish of ED machined SKD-11. A small difference in surface finish is made when varying the particle concentration from 0.25 to 1.0 cm3 /l. This is primarily due to the fact that only a very small fraction of particle content existing between the spark gap is used to strike the melted zone of the machined components in a complementary manner. However, it can also be observed from Fig. 4 that the effects of particle concentrations on surface roughness are less influential than the discharge current. This reason is same as that explained in Section 4.2. 4.4. Effects of particle properties on surface roughness Among the additives investigated, the addition of Al powder, as shown in Fig. 9, produces the best surface finish in the machined work than those of Cr and SiC. It is noted that the effects of Cu are not included in Fig. 9 because the process with Cu powder does not work at all in improving the surface quality, due to its large density effects. The very high density of Cu powder overwhelms the suspension capability of the dielectric fluid. Almost all of the Cu powders are deposited
Fig. 9. The dependence of the surface roughness of the EDMed components on particle concentration of Al, Cr, and SiC powders, power-on time, and on discharge current.
at the bottom of the dielectric fluid contributing nothing to reduce the surface roughness. Al powder, on the other hand, proves to be the best addition in terms of particle concentration, mainly due to their combined effects of low electrical resistivity, high thermal conductivity and low density. Low electrical resistivity creates a high spark gap and high thermal conductivity takes more heat away. Both effects together lead to low electrical discharge power density resulting in low gas explosion, thus only shallow craters are produced in the machined surface. Furthermore, the low density of the Al powder corresponds with low explosive impact upon the melted zone, generating fine grinding effects. The related theories have been discussed in Section 2. Comparatively, Cr powder generates the second best results due to its higher density than Al. Its higher density produces heavier impact effects on the melted zone causing a slightly higher surface roughness value. SiC basically produces a larger explosion force for a normal pulse discharge and this leads to deeper craters, because of its higher electrical resistivity and lower thermal conductivity than Cr and Al. As a result, SiC powder produces a worse surface roughness than Al and Cr. 4.5. Effects of particle size on recast layer
Fig. 8. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 100 m Al powders.
Fig. 10 shows the effects of the particle size of aluminum powder on the recast layer of the machined components. It can be seen from Fig. 10 that the introduction of foreign particles is capable of reducing the recast layer. The particle size of 100 m produces the thinnest recast layer in the EDMed components, whereas 70–80 nm generates the thickest (Fig. 10). It is observed that the thickness of the recast layer decreases, while the particle size of the additives increases. The difference made by particle size can be seen in the microphotographs in Figs. 11 and 12 where can be observed the thickness of the white layer obtained after
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Fig. 10. The dependence of the recast layer of the EDMed components on discharge current, particle concentration and on particle size.
Fig. 13. The dependence of the recast layer of the EDMed components on particle concentration of Al, Cr, and SiC powders, and on discharge current.
4.6. Effects of particle concentration on recast layer
Fig. 11. Optical microphotograph of the recast layer in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and without additives.
metallurgical etching on specimens. It is interesting to find (see Fig. 10) that the thickness produces at a high discharge current of 4.0 A with the aid of the large particle size 100 m can be even less than that produced at a low discharge current of 1.5 A with the aid of smaller particle size 10–15 m and 70–80 nm. This is due to a reduction in electrical power density resulting from an increase in spark size, plus poor discharge transitivity and hard striking effects of large particles.
The effects of particle concentrations on the recast layer of the EDMed components can also be checked out in Fig. 10. It is revealed that there is a decrease in the recast layers when adding the particles with concentrations ranging from 0.25 to 1.0 cm3 /l to the dielectric oil. It is noticed that the particle concentration 0.5 cm3 /l is less effective in decreasing the recast layer. It thus produces a thicker recast layer compared to both 0.25 and 1.0 cm3 /l that generally generate thinner recast layers. However, from the machining point of view, particle concentration 0.5 cm3 /l is found to be the optimal choice in improving the machining efficiency and reducing the tool wear rate, by enhancing the process stability most [10]. The experimental results here may further indicate that the thickest recast layers produced by introducing particle concentration 0.5 cm3 /l is the result of strongest accumulated heating effects due to its greater enhancement of discharge transitivity during EDM process. In other words, the thinner recast layer produced by the particle concentration of 0.25 and 1.0 cm3 /l is due to their weaker accumulated heating effects because of their having higher possibility of abnormal discharging. 4.7. Effects of particle properties on recast layer
Fig. 12. Optical microphotograph of the recast layer in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 100 m Al powders.
Fig. 13 demonstrates the dependence of the recast layer of the EDMed components on particle concentration of Al, Cr, and SiC powders, and on discharge current. The effects of the addition of Cu powders on the recast layer of EDMed components are not included because the addition of Cu particles makes no difference to the recast layer. This is due to its very high-density effects resulting in a sufficient deposition at the bottom of the tank. Therefore, the process with no foreign particles and with copper powder produces the thickest recast layer. Again it can be clearly identified that that addition of Al powders generates the thinnest recast layer in
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the machined work, compared to Cr and Sic powders, due to its better combined effects of low electrical resistivity, and high thermal conductivity as previously mentioned. The first produces a high sparking gap size, which leads to low electric discharge power density and hence low explosion force. The latter takes more heat away resulting in less thermal input to the workpiece. A shallower melted zone in the machined surface is thus produced with Al powders. On the other hand, Cr powder generates the second best results on the reduction in the recast layer thickness. Its lower thermal conductivity than Al is responsible for a deeper thickness of the recast layer. As for SiC, because of its higher electrical resistivity and lower thermal conductivity than both Cr and Al, it produces deeper recast layers than those by Al and Cr powders for a normal electric pulse discharge. 5. Conclusions This project concentrates on the qualitative and quantitative analysis of surface quality of EDMed components. The work completed is summarised as: I. The most critical factors of the powder characteristics governing surface roughness and recast layer are identified as particle size, particle concentration, particle density, electrical resistivity, and thermal conductivity. II. The effect of particle size of additives on surface roughness is found as 70–80 nm giving the best surface finish, followed by 10–15, and 100 m. III. The difference in the surface roughness caused by the variation in the selected particle concentration of additives is found to be negligible. IV. Aluminum powder produces the best surface finish, followed by chromium, silicon carbide, and copper powder the worst. V. The introduction of foreign particles is proved to reduce the recast layer of EDMed components.
391
VI. The particle concentration of 0.5 cm3 /l is found to be the least effective in decreasing the recast layer. VII. The effect of particle size of additives on the recast layer is found as 100 m producing the thinnest layer, followed by 10–15 and 70–80 nm the thickest. VIII. Aluminum powder is found to be able to reduce the recast layer the most, followed by chromium, silicon carbide, and copper powder the worst.
References [1] P.F. Thomson, Surface in electro-discharge machining, Mater. Sci. Technol. 5 (1989) 1153–1157. [2] J. Wallbank, V.B. Phadke, Some metallurgical aspects of die failure, Met. Technol. 9 (1982) 405–412. [3] L.C. Lee, L.C. Lim, Y.S. Wong, H.H. Lu, Towards a better understanding of the surface features of electro-discharge machined tool steels, J. Mater. Process. Technol. 24 (1990) 513–523. [4] H. Narumiya, N. Mohri, N. Saito, H. Otake, Y. Tsnekawa, T. Takawashi, K. Kobayashi, EDM by powder suspended working fluid, in: Proceedings of the ninth International Symposium for Electromachining, 1989, pp. 5–8. [5] N. Mohri, N. Saito, M. Suzuki, T. Takawashi, K. Kobayashi, A new process of finish machining on free surface by EDM methods, Ann. CIRP 40 (1) (1991) 207–210. [6] B.H. Yan, S.L. Chen, Effects of dielectric with suspended aluminum powder on EDM, J. Chin. Soc. Mech. Eng. 14 (3) (1993) 307–312. [7] B.H. Yan, S.L. Chen, Characteristics of SKD11 by complex process of electrical discharge machining using liquid suspended with alumina powder, J. Jpn. Inst. Met. 58 (9) (1994) 1067–1072. [8] B.H. Yan, M.D. Chen, Effect of ultrasonic vibration on electrical discharge machining characteristics of Ti–6Al–4V alloy, J. Jpn. Inst. Light Met. 44 (5) (1994) 281–285. [9] Y.M. Quan, Y.H. Liu, Powder-suspension dielectric fluid for EDM, J. Mater. Process. Technol. 52 (1995) 44–54. [10] C.Y. Lee, Effects of powder characteristics on electro-discharge machining process, Master Thesis, Taiwan Chang Gung University, 1999. [11] Y.F. Luo, The Dependence of interspace discharge transitivity upon the gap debris in precision electro-discharge machining, J. Mater. Process. Technol. 68 (1997) 127–131.
Investigation into some surface characteristics of electrical discharge machined SKD-11 using powder-suspension dielectric oil Tzeng Yih-fong a,∗ , Chen Fu-chen b a
Department of Mechanical and Automation Engineering, National Kaohsiung First University of Science and Technology, 1 University Road, Yenchao, Kaohsiung 824, Taiwan b Department of Mechanical Engineering, Kun Shan University of Technology, Taiwan Received 24 August 2002; received in revised form 3 December 2004; accepted 1 June 2005
Abstract This project investigates the property effect of additives on surface quality of EDMed SKD-11. The additives with significantly different thermophysical properties, including aluminum (Al), chromium (Cr), copper (Cu), and silicon carbide (SiC) powders are studied. Experimental results show that the particle size of additives in the dielectric oil affects the surface quality of EDMed work. While the smallest particles (70–80 nm) generates the best surface finish of the machined work, the greater the particle size the less the improvement in the surface roughness. However, particle size has opposite effect on the recast layer, as the smallest particles generated the thickest recast layer of the EDMed surface, and the greater the particle size the thinner the recast layer. Among the additives, Al powder produces the best surface finish and the thinnest recast layer in the machined work, whereas the process without foreign particles and with copper powder, generates the worst surface characteristics. © 2005 Elsevier B.V. All rights reserved. Keywords: Electro-discharge machining (EDM); Powder-suspensions; Surface roughness; Recast layer; Thermo-physical property
1. Introduction Electro-discharge machining (EDM) is a process that is used to remove metal through the action of an electrical discharge of short duration and high current density between the tool and the workpiece [1]. EDM on ferrous metals results in surface changes, with the formation of a re-solidified layer, usually known as the recast layer, which varies in thickness. The recast layer undergoes complex structural changes associated with extremely high cooling rate. It was reported that the hard recast layer was often found to contain microcracks, caused by high tensile residual stresses exceeding the ultimate strength of the material [2]. This surface imperfection further reduces the fatigue, wear and corrosion resistance of EDMed components [3,4]. To remove the damaged surface layer and to restore the surface properties, post-machining processes are thus needed. However, post-processing means ∗
Corresponding author. Tel.: +886 7 6011000x2226; fax: +886 7 6011066. E-mail address: [email protected] (T. Yih-fong).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.06.006
an additional cost. To reduce the unit production cost for industry, it is essential to minimise the damaged surface layer of machined work in the EDM process. In 1989, Narumiya et al. began to notice and examined the effects of silicon powder in the working fluid on EDM [5,6]. They found that foreign particles were capable of reducing the recast layer, preventing cracks, and producing a mirrorlike surface finish in the EDMed components. In 1993, Yan et al. continued to investigate the effects of dielectrics with suspended aluminum, alumina, and silicon carbide powders, respectively on EDM [7–9]. They observed the same results as those produced by silicon powders. In 1995 Quan et al. further studied the effects of unspecified conductive particles and inorganic-oxide particles on EDM, and agreed that the introduced particles produced better surface quality [10]. Although the advantages of properly introducing foreign particles including metals and non-metals into EDM have since been widely recognized, very few attention was paid to the possible effects of the characteristics of the particles
386
T. Yih-fong, C. Fu-chen / Journal of Materials Processing Technology 170 (2005) 385–391
on the surface quality of the machined components. Without such information, it would, nevertheless, be very difficult for industry to determine the optimal selection or combination of particles. A systematic study of the factors of the particles with significant difference in properties was undoubtedly necessary. This paper thus attempts to fill into the gap of literature, in the hope of providing a better understanding of the correlation between the characteristics of chemical additives and the surface quality.
2. Material removal mechanism using powder-suspension dielectric oil As debris in a spark gap usually consists of metal and carbon particles, which will drastically lower the breakdown strength of dielectric, gap debris evidently would facilitate ignition process and increases gap size. Absence of the debris can result in arcing due to a lack of precise feeding mechanism with extremely high position resolution, which occurs frequently in the early stages of the ignition process particularly. Moreover, the amount of debris matters. While the absence of debris does not help improve sparking frequency, too much debris is generally believed to be the dominant cause of spark concentration i.e. arcing that leads to an unstable and inefficient process. In fact, gap debris is of somewhat help. Gap debris reportedly is comparatively a most crucial factor to the stability of machining process, which demands evenly disperse discharge locations that mainly depend upon debris concentration and distribution, bubbles, de-ionisation, and surface irregularities. Nevertheless their concurrent presence poses great difficulty to segregate the effects attributed to each factor [11]. Most present control systems thus cannot directly
regulate discharge location. Gap debris in this regard can significantly control discharge transitivity, gap size, breakdown strength, and de-ionisation [11]. The remaining core issue is how to decide the function of gap debris. Theoretically, the function of debris will be to a great extent controlled by the characteristics of the additives if they are added in suitable particle size, particle concentration, particle density, thermal conductivity, electrical resistivity, melting point, evaporation point, specific and latent heat, etc. Whether and how different character combinations affecting EDM process, however, has been rarely discussed and needs a further investigation, so as to provide the best formula for wise selection and control of these additives in a precise EDM. The addition of particles alters the material removal mechanism in the EDM process. Fig. 1 shows a schematic diagram of a normal single electrical discharge in a spark gap without and with suspended particles. It is noted that the addition of powders lead to an increase in gap size that subsequently resulted in a reduction in electrical discharge power density and in gas explosive pressure for a single power pulse. As seen in Fig. 2, the modified material removal mechanism for addition of powders during normal single electrical discharge time is the combined effect of mechanical thrust driven by the gas explosion mainly from the working fluid evaporation with the striking impact by the suspended particle. The materials removed by the grinding effect of suspended particles within the interspace are negligible. It is worth noting that the weaker gas explosion in the interspace after powder addition might lead to a reduction in the material removal rate (MRR) during normal single electrical discharge process. To enhance the machining efficiency of the whole EDM process, the particles striking effect and the discharge transitivity, therefore, play a decisive role. Especially, the latter decides the spark-
Fig. 1. Schematic diagrams of the electric discharge for single power pulse (a) without powder and (b) with powders [10].
T. Yih-fong, C. Fu-chen / Journal of Materials Processing Technology 170 (2005) 385–391
387
Fig. 2. Schematic diagrams of material removal mechanism during the normal discharge for the working fluid (a) without powder and (b) with powders [10].
ing frequency that governs the entire MRR, while the first has minor cutting effect contributing mainly to the improvement of the surface finish.
Table 2 EDM process parameters set-up Workpiece (−) Electrode (+)
3. Experiments
Machining time (min) Dielectric fluid
A locally designed electrical discharge machine Castek03 is used throughout the experiments. A range of different powders is added to HERCULES ED 320H dielectric oil as the working fluid in EDM process. Some of the important thermophysical properties of used particles are displayed in Table 1. Mould steel SKD-11 workpieces (16 mm in diameter) which are quenched and tempered to raise microhardness from 20 to 60 Hrc are spark eroded using copper tools with 8 mm in diameter using the EDM process parameters set-up as shown in Table 2. The applied discharge currents and power-on times are intentionally kept low to simulate the conditions of precision machining. The filter system for re-circulation of dielectric fluid is designed in such a way in order not to filter out the powders that are added into the dielectric fluid. It has to be noted that the powders investigated is non-magnetic. The new filter system shown in Fig. 3 comprises two subsystems: (a) Filtering: uses the magnetic force of hundreds of magnets closely arranged in a series in the filter to sufficiently separate the work debris from the dielectric oil, while the powders pass through for constant reuse. (b) Re-circulation: the pump sends the dielectric fluid back into the tank.
SKD11 (16 mm in diameter) Cu (8 mm in diameter) 20 Pure dielectric oil
SKD11 (16 mm in diameter) Cu (8 mm in diameter) 20 Dielectric oil + powders 3.5
Dielectric flowing rate (l/min) Powders Powder size (m)
3.5
Particle concentration (cm3 /l) Breaking voltage (V) Discharge peak current IP (A) Pulse-on time Ton (s) Duty cycle (CD )
No
Al, Cr, Cu, SiC 0.07–0.08, 10–15, 100 0.25, 0.5, 1.0
120 1.5, 4.0
120 1.5, 4.0
6, 25, 75 1/2, 2/3
6, 25, 75 1/2, 2/3
No No
Two of the dominant features of the surface integrity of EDMed components are the surface roughness and the recast layer. In order to obtain quantitative data on the extent of these phenomena, analyses are carried out using a metrology instrument and an optical microprobe. The roughness value is the average of the readings scanned along the centreline of the top surface of the EDMed components in three directions using a Taylor–Hobson metrology instrument. In order to measure the recast layer, specimens are first carefully prepared by properly cutting the EDMed components along the centreline in the longitudinal direction. The surface cross-
Table 1 Thermophysical properties of additives Powders
Density (g/cm3 )
Thermal conductivity (W/cm K)
Electrical resistivity ( cm)
Melting point (◦ C)
Specific heat (cal/g ◦ C)
Al Cr Cu SiC
2.70 7.16 8.96 3.21
2.38 0.67 4.16 1.0–5.0
2.45 2.60 1.59 1 × 109
660 1875 1083 2987
0.215 0.11 0.092 0.18
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Fig. 3. Schematic diagram of a self-designed filter system for the electrodischarge machine used for the experiments.
sections are then mounted in a resin, ground and polished for metallurgical examination. These sections are observed under an optical microscope equipped with a micrometer to calculate the average thickness of the recast layer of these EDMed components.
Fig. 4. The dependence of the surface roughness of the EDMed components on particle size and particle concentration of Al powder, and on discharge current.
4. Experimental results and discussions 4.1. General discussions It is found during the experiments that the particle size, particle density, particle concentration, electrical resistivity, and thermal conductivity of the powders are the relatively important powder characteristics affecting the surface quality in the EDM process. The effect of each factor will be discussed in the following sections. Other typical properties, such as melting point, evaporation point, and the latent heat of the particles, are observed to have no appreciable effects on surface characteristics of the EDMed components. 4.2. Effects of particle size on surface roughness Fig. 4 describes the dependence of surface roughness of the EDMed components on the particle size and on the concentration of aluminum powder within the dielectric oil, and also on the discharge current. It is observed that foreign particles are capable of reducing the surface roughness of the machined parts, and the improvement is more associated with the particle size rather than the particle concentration. This is due to a relatively low particle concentration being used in the experiments. Experimental results further reveal that the smallest particle (70–80 nm) generates the best surface finish and the greater the particle size, the worse the surface roughness. It is due to small particles producing fine cutting effects in a complementary way. This can be seen in the optical microphotographs in Figs. 5–8, referring to the surface finish of the specimens, which are electro-discharge machined by three levels of particle sizes. Slight differences
Fig. 5. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and without additives.
are observed in the comparison of the surface finishes from Figs. 5–8. Fig. 5 displays more deep craters than Figs. 6–8. This indicates higher surface roughness produced by the process without powder additions. It is well worth noting in Fig. 4 that the effects of foreign particles on surface finish of EDMed components are less influential than the pulsed discharge current set-up. It is due to the fact that the discharge current governs the heating effects during EDM process, whereas particle striking plays a secondary role.
Fig. 6. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 70–80 nm Al powders.
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Fig. 7. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 10–15 m Al powders.
4.3. Effects of particle concentration on surface roughness The effects of a range of particle concentrations of aluminum powder on the surface roughness can also be seen in Fig. 4. Experimental results show that the particle concentration of 0.25 cm3 /l leads to a best improvement in the surface finish of ED machined SKD-11. A small difference in surface finish is made when varying the particle concentration from 0.25 to 1.0 cm3 /l. This is primarily due to the fact that only a very small fraction of particle content existing between the spark gap is used to strike the melted zone of the machined components in a complementary manner. However, it can also be observed from Fig. 4 that the effects of particle concentrations on surface roughness are less influential than the discharge current. This reason is same as that explained in Section 4.2. 4.4. Effects of particle properties on surface roughness Among the additives investigated, the addition of Al powder, as shown in Fig. 9, produces the best surface finish in the machined work than those of Cr and SiC. It is noted that the effects of Cu are not included in Fig. 9 because the process with Cu powder does not work at all in improving the surface quality, due to its large density effects. The very high density of Cu powder overwhelms the suspension capability of the dielectric fluid. Almost all of the Cu powders are deposited
Fig. 9. The dependence of the surface roughness of the EDMed components on particle concentration of Al, Cr, and SiC powders, power-on time, and on discharge current.
at the bottom of the dielectric fluid contributing nothing to reduce the surface roughness. Al powder, on the other hand, proves to be the best addition in terms of particle concentration, mainly due to their combined effects of low electrical resistivity, high thermal conductivity and low density. Low electrical resistivity creates a high spark gap and high thermal conductivity takes more heat away. Both effects together lead to low electrical discharge power density resulting in low gas explosion, thus only shallow craters are produced in the machined surface. Furthermore, the low density of the Al powder corresponds with low explosive impact upon the melted zone, generating fine grinding effects. The related theories have been discussed in Section 2. Comparatively, Cr powder generates the second best results due to its higher density than Al. Its higher density produces heavier impact effects on the melted zone causing a slightly higher surface roughness value. SiC basically produces a larger explosion force for a normal pulse discharge and this leads to deeper craters, because of its higher electrical resistivity and lower thermal conductivity than Cr and Al. As a result, SiC powder produces a worse surface roughness than Al and Cr. 4.5. Effects of particle size on recast layer
Fig. 8. Optical microphotograph of the surface roughness in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 100 m Al powders.
Fig. 10 shows the effects of the particle size of aluminum powder on the recast layer of the machined components. It can be seen from Fig. 10 that the introduction of foreign particles is capable of reducing the recast layer. The particle size of 100 m produces the thinnest recast layer in the EDMed components, whereas 70–80 nm generates the thickest (Fig. 10). It is observed that the thickness of the recast layer decreases, while the particle size of the additives increases. The difference made by particle size can be seen in the microphotographs in Figs. 11 and 12 where can be observed the thickness of the white layer obtained after
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Fig. 10. The dependence of the recast layer of the EDMed components on discharge current, particle concentration and on particle size.
Fig. 13. The dependence of the recast layer of the EDMed components on particle concentration of Al, Cr, and SiC powders, and on discharge current.
4.6. Effects of particle concentration on recast layer
Fig. 11. Optical microphotograph of the recast layer in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and without additives.
metallurgical etching on specimens. It is interesting to find (see Fig. 10) that the thickness produces at a high discharge current of 4.0 A with the aid of the large particle size 100 m can be even less than that produced at a low discharge current of 1.5 A with the aid of smaller particle size 10–15 m and 70–80 nm. This is due to a reduction in electrical power density resulting from an increase in spark size, plus poor discharge transitivity and hard striking effects of large particles.
The effects of particle concentrations on the recast layer of the EDMed components can also be checked out in Fig. 10. It is revealed that there is a decrease in the recast layers when adding the particles with concentrations ranging from 0.25 to 1.0 cm3 /l to the dielectric oil. It is noticed that the particle concentration 0.5 cm3 /l is less effective in decreasing the recast layer. It thus produces a thicker recast layer compared to both 0.25 and 1.0 cm3 /l that generally generate thinner recast layers. However, from the machining point of view, particle concentration 0.5 cm3 /l is found to be the optimal choice in improving the machining efficiency and reducing the tool wear rate, by enhancing the process stability most [10]. The experimental results here may further indicate that the thickest recast layers produced by introducing particle concentration 0.5 cm3 /l is the result of strongest accumulated heating effects due to its greater enhancement of discharge transitivity during EDM process. In other words, the thinner recast layer produced by the particle concentration of 0.25 and 1.0 cm3 /l is due to their weaker accumulated heating effects because of their having higher possibility of abnormal discharging. 4.7. Effects of particle properties on recast layer
Fig. 12. Optical microphotograph of the recast layer in the specimen machined at CD = 2/3, IP = 4.0 A, Ton = 25 s, concentration 0.5 cm3 /l, and with 100 m Al powders.
Fig. 13 demonstrates the dependence of the recast layer of the EDMed components on particle concentration of Al, Cr, and SiC powders, and on discharge current. The effects of the addition of Cu powders on the recast layer of EDMed components are not included because the addition of Cu particles makes no difference to the recast layer. This is due to its very high-density effects resulting in a sufficient deposition at the bottom of the tank. Therefore, the process with no foreign particles and with copper powder produces the thickest recast layer. Again it can be clearly identified that that addition of Al powders generates the thinnest recast layer in
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the machined work, compared to Cr and Sic powders, due to its better combined effects of low electrical resistivity, and high thermal conductivity as previously mentioned. The first produces a high sparking gap size, which leads to low electric discharge power density and hence low explosion force. The latter takes more heat away resulting in less thermal input to the workpiece. A shallower melted zone in the machined surface is thus produced with Al powders. On the other hand, Cr powder generates the second best results on the reduction in the recast layer thickness. Its lower thermal conductivity than Al is responsible for a deeper thickness of the recast layer. As for SiC, because of its higher electrical resistivity and lower thermal conductivity than both Cr and Al, it produces deeper recast layers than those by Al and Cr powders for a normal electric pulse discharge. 5. Conclusions This project concentrates on the qualitative and quantitative analysis of surface quality of EDMed components. The work completed is summarised as: I. The most critical factors of the powder characteristics governing surface roughness and recast layer are identified as particle size, particle concentration, particle density, electrical resistivity, and thermal conductivity. II. The effect of particle size of additives on surface roughness is found as 70–80 nm giving the best surface finish, followed by 10–15, and 100 m. III. The difference in the surface roughness caused by the variation in the selected particle concentration of additives is found to be negligible. IV. Aluminum powder produces the best surface finish, followed by chromium, silicon carbide, and copper powder the worst. V. The introduction of foreign particles is proved to reduce the recast layer of EDMed components.
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VI. The particle concentration of 0.5 cm3 /l is found to be the least effective in decreasing the recast layer. VII. The effect of particle size of additives on the recast layer is found as 100 m producing the thinnest layer, followed by 10–15 and 70–80 nm the thickest. VIII. Aluminum powder is found to be able to reduce the recast layer the most, followed by chromium, silicon carbide, and copper powder the worst.
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