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EDM technology and strategy development for the manufacturing of complex parts in SiSiC
Journal of Materials Processing Technology 210 (2010) 631–641
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
EDM technology and strategy development for the manufacturing of complex parts in SiSiC S. Clijsters, K. Liu ∗ , D. Reynaerts, B. Lauwers Department of Mechanical Engineering, Division PMA, Catholic University Leuven (K.U.Leuven), Celestijnenlaan 300B, Bus 2420, B-3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 17 November 2009 Accepted 19 November 2009
Keywords: Electrical discharge machining Die-sinking EDM Ceramic composite SiSiC
a b s t r a c t Silicon carbide (SiC) is an extremely hard and difficult-to-shape engineering ceramic material used extensively in industry because of its superior mechanical properties, wear and corrosion resistance even at elevated temperature. Conventional ceramic processing and structuring techniques such as injection molding and grinding are costly and difficult to obtain flawless complex shaped components. By infiltrating free Si into the SiC, the electrical conductivity of the matrix is largely improved. Thus it can be machined by electrical discharge machining (EDM). In this paper, a die-sinking EDM technology for manufacturing components in a commercial available silicon infiltrated silicon carbide (SiSiC) is developed. The influences of the major operating EDM parameters (discharge current ie , open gap voltage ui , discharge duration te and pulse interval to ) of the iso energetic generator on the machining performances like Material Removal Rate (MRR), Tool Wear Ratio (TWR) and surface roughness (Ra) are examined. Relaxation pulses which have lower energy input are also considered to improve the surface quality. Furthermore, the topography and surface integrity of the material after the EDM process are analysed to determine the corresponding material removal mechanism. With the developed machining strategy, a sample piece with designed features such as ribs, a deep cavity and chamfers are manufactured to examine the machining performances. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide (SiC) is regarded as one of the most promising engineering ceramic materials because of its combination of excellent physical and mechanical properties such as low density, high toughness, high thermal conductivity, low thermal expansion coefficient and wear and corrosion resistance even at elevated temperature. Thus it has been extensively used in high temperature applications such as heat exchangers, engine components, high temperature bearings, fixtures and nozzles (Wilhelm et al., 1999). However, because of its hardness, it is difficult to be manufactured into complex shapes with desired accuracy cost-effectively using traditional ceramic processing methods or conventional manufacturing techniques like grinding. Electrical discharge machining is a non-conventional manufacturing process and proves to be an effective alternative comparing to traditional manufacturing processes. In this electro-thermal process, material is mainly removed by a series of carefully controlled discrete sparks (electrical discharges) generated between the tool electrode and workpiece both of which are submerged in a dielectric. The high temperature of the discharges normally melt and
∗ Corresponding author. Tel.: +32 0 16 322480; fax: +32 0 16 322987. E-mail address: [email protected] (K. Liu). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.11.012
evaporate the workpiece material locally and leave overlapped craters on the surface. Since there is no mechanical contact between the two electrodes, the process is able to machine any conductive component into accurate and complex shapes regardless of its hardness (Kruth, 2008). The major drawback of EDM ceramic material, however, is that most ceramics are not conductive enough. Research implicated that electrical resistivity of 100 cm seems the upper limit for a workpiece to be machined by die-sinking EDM (König et al., 1988). As most sintered ceramics, SiC has a very high electrical resistivity of 103 to 105 cm, it is practically impossible to be directly machined using EDM process. However, by infiltrating free Si into the SiC bulk material, the electrical conductivity can be increased dramatically without effecting the major mechanical properties of the components (10 cm) (Wilhelm et al., 1999). In last years, research has already been performed on the EDM milling of SiC, which seems to have a better machinability than die-sinking EDM (Lauwers et al., 2007; Liu et al., 2008). Researches on the machining of SiSiC also can be found, but these developments mostly aim at the finishing stage of the EDM process using a narrow process window of the input parameters (Mahdavinejad et al., 2006; Luis et al., 2005; Puertas et al., 2005). In these experiments, a variant SiSiC composite is investigated. The discharge current and discharge duration are in the range of 3–18 A and 30–500 s, respectively, with an open gap voltage from 120 to 200 V. However, the type of discharge pulses
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applied in the experiments is not specified explicitly. With the peak current of 6–8 A, a maximum material removal rate is achieved up to 0.5 mm3 /min and an average tool wear ratio around 10%. The EDMed surface has a minimum roughness Ra around 0.8 m with a small discharge energy, and it is noticed that the pulse duration merely has effects on it. Apparently, the influences of using high current and correspondingly high energy input on machining performances have not yet been investigated. Furthermore, limited information on the material removal mechanisms of EDMing this material is presented. As a consequence, in this paper, research is conducted on a commercially available SiSiC to determine the influences of the EDM electrical parameters using iso energetic pulses: discharge current ie , open gap voltage ui , discharge duration te and pulse interval time to , on the machining performances, which are the Material Removal Rate (MRR), Tool Wear Ratio (TWR), surface roughness (Ra). RCbased relaxation pulses are also examined to further improve the surface quality. Surface integrity after EDM process is also examined and the Material Removal Mechanism (MRM) is analysed. With the obtained results, a EDM machining strategy is developed and the performances are examined in a case study.
Table 1 The physical and mechanical properties of Silit® SKD SiSiC (Manufacturer’s data). Silicon carbide content Maximum service temperature Density Flexural strength Young’s modulus Thermal conductivity Thermal expansion coefficient Electrical resistivity
20 ◦ C 1000 ◦ C 1000 ◦ C 1000 ◦ C ˛ (RT. . .1300 ◦ C)
88% 1380 ◦ C 3.05 kg/dm3 250 MPa 250 MPa 360 GPa 36 W/(m K) 4.1 K−1 ×10−6 10 cm
RT: room temperature
2. Experimental setup The die-sinking EDM experiments are performed on a commercial machine, Charmilles Roboform 350, which has the ability to generate iso energetic pulses and RC-based relaxation pulses. The iso energetic pulse, which is controlled by the opening and closing of a transistor, normally has a longer pulse duration and larger discharge input energy. On the contrary, the released energy in the RC-based relaxation pulses mainly depends on the energy restored in the RC circuit and the local gap condition. In this research, the effects of both iso energetic and relaxation pulses on machining performances are investigated. Negative polarity is applied through the entire research. The dielectric fluid is hydrocarbon oil (TOTAL: DIEL MS7000). No side flushing but the electrode pulsation is used. POCO copper infiltrated graphite EDM-C3 is used as tool electrode material. The surface/subsurface integrity and cross-section has been examined on a Scanning Electron Microscope (SEM, Philips XL30FEG). All the specimens are cleaned ultrasonically. The SiSiC employed in the experiments is Silit® SKD commercially available from Saint-Gobain (Ceramics, 2009). It is produced by infiltrating a porous SiC–C green product with liquid Silicon. At a specific temperature (1600 ◦ C), the silicon melts and flows into the green product. There the silicon reacts with the carbon and generates a new SiC which grows on the original SiC particles (Wilhelm et al., 1999; US-Patent, 1990). The pores of the green product are filled with liquid silicon during the infiltration process that a practically dense product can be obtained. The SiSiC preserves the high hardness, good resistance against oxidation and corrosion, and an excellent wear resistance similar like SiC. Furthermore, despite the free silicon contributes to the improvement the electrical conductivity of the ceramic composite, the amount of them limits the bending strength to around 250 MPa. Details of the physical and mechanical properties of the ceramic can be found in Table 1. A microstructure of the SiSiC ceramic matrix is illustrated in Fig. 1. In this image the gray grains are SiC, which have an average particle size about 30 m. The white dots spread around the SiC grains are free Si. Among the silicon, the smaller gray dots are also SiC generated from the reaction of Si and C during the infiltration process. The die-sinking EDM experiments consist of machining a rectangular cavity with dimensions around 10 mm × 10 mm. The geometry of the EDMed cavities are measured by an optical coordinate measurement machine (Quickvision Pro, Mitutoyo). The depth
Fig. 1. Microstructure of the Silit® SKD SiSiC matrix: gray = SiC and white = Si.
and surface roughness Ra of the cavities are examined by a surface profilemeter (Form Talysurf 120, Taylor Hobson). With these results the MRR and TWR can simply be calculated: Volume removed from workpiece Time of machining Volume removed from electrode TWR = Volume removed from workpiece MRR =
3. Experiments on process investigation This research differs from the previous researches by the use of a machine which is able to generate both iso energetic and relaxation pulses. As a result, the methodologies to determine the influences of the input pulse parameters on the EDM performances are different for the iso energetic and relaxation pulses, since the machine provides more possibilities to modify the input parameters for iso energetic pulse. To explore the effects of major electrical parameters for iso energetic pulses, Design of Experiments (DoE) is used . It allows to analyse the influences of the input parameters on the responses with a minimum amount of experiments (Montgomery, 2001). A screening DoE is firstly executed on the chosen four parameters for iso energetic pulses, in order to determine a stable machining region and to check which parameter has a significant effect on the machining performances. The most significant parameters are then further investigated using two more DoEs with extended variant levels. Considering the small discharge energy input for relaxation pulses, a surface roughness optimization is focused using relaxation pulses at the last. In the following sections, the detailed investigation and experimental results are presented and discussed. 3.1. Screening DoE on iso energetic pulses The screening DoE is using a full factorial design of four major electrical parameters on two levels in order to determine a stable
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Table 4 Levels of the input parameters for the further design of experiments.
Table 2 Levels of the input parameters for the screening design of experiment.
DoE ie , to
DoE ie , ui
Parameters
Levels −
+
ie (A)
to (V)
ie (A)
ui (V)
ie te to ui
12 A 3.2 s 25 s −80 V
64 A 12.8 s 100 s −120 V
6 12 24 48
12.8 25 50 100
6 8 32 64
−80 −120 −160 −200
Table 3 Significant effects analysis on machining performances of screening DoE for SiSiC. Machining performances
MRR
TWR
Ra
Significance level (˛ = 0.05) 1 2 3 4
ui – – –
ie ie · to ie · ui –
ie ui te to
R2
0.80
0.97
0.98
machining region and the effect of each parameter on the machining performances. The selected low and high levels of the input parameters are listed in Table 2. These nominal values are equal to the effective values for iso energetic pulses, as can be seen in Fig. 2. A slightly increased discharge voltage is noticed (46 V) comparing to the machining of the steel (32 V), which is probably due to the lower electrical conductivity of the SiSiC. Since this variation is small, the effect of electrical resistivity for this ceramic composite on the machining process is negligible. The choice of these two levels aims to cover the entire available range of each parameter on the machine. In Fig. 3, the main effects of these parameters on the machining performances of MRR, TWR and Ra are illustrated. As can be seen in Fig. 3, the largest variation on MRR is given by ui in the investigated regions. The increased open gap voltage leads to a reduced MRR. Though the deviation of MRR between two levels of ie is considerably large, the effect is smaller comparing to parameter ui . However, unlike normal diesinking EDM of metal or alloy, higher discharge current prohibits the increase of machining speed, although a significantly lowered tool wear ratio is noticed. Furthermore, elevated discharge energy input by raising ie and te gives a worse surface quality than has been predicted. Increased gap voltage and pulse interval time also shows noticeable difference on improving the surface quality. The ANOVA (Analysis of Variance) analysis results of the screening DoE for all machining performances to characterise the significant factors are presented in Table 3. Confidence level of 95% (˛ = 0.05) is chosen. All the machining performances are fit in a second order model. The percentage between the variance of the
model’s predictions with the total variance of the data, defined as R2 , is used to evaluate the validity of the model. If R2 is very close to 1, the selected significant factors (main effects or interactions) are sufficient to explain most of the measured deviations in the output values and it is confirmed that the omitted effects are not important. As shown in this table, ui plays a significant role on affecting the MRR and Ra, so does the ie on TWR and Ra. Interaction of ie with ui and to also have significant influences on the tool wear. During the experiments, it is noticed that high energetic pulses with low pulse interval time result in arcs while pulses with low open gap voltage result in short-circuits. These phenomena should be avoided for stable and efficient machining. Therefore according to the results of DoE, the stable region can be determined. 3.2. Further investigation on the iso energetic pulses After determining the stable region in the previous section, the two most significant parameters ui and ie for the machining performances MRR and Ra, ie and to for the TWR from the screening DoE (Table 3) are further examined in two sets of experiments (Fig. 4). Since the input parameters are reduced to two, extended levels can be employed in order to study the non-linearity influences of the factors. Full factorial design at four levels are executed three times in random sequences to reduce the effects of noise and to check the repeatability of the tests. The different levels of each investigated input parameters are listed in Table 4. Rest of the parameters are kept constant and can be found in Fig. 4. The ANOVA analyses results of these two DoEs are listed in Table 5. The effects of input parameters on the machining performances are illustrated in surface plots in Fig. 5. As can be seen in Fig. 5(a), the non-linear effects of input parameter ie are obvious. In contrary with the screening DoE results (Fig. 3(a)), raising the discharge current shows an increase of MRR to a certain extent. An optimal discharge current 32 A is noticed to provide a maximum material removal of 6.1 mm3 /min when the biggest open gap voltage 200 V is applied. According to the experiments, the process will get unstable if the current keeps raising. Above this optimum current (32 A), arcs are observed during machining and leads to unsteady machining conditions, thus the
Fig. 2. The measured discharge voltage and current waveforms of iso energetic pulses.
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Fig. 4. The chosen parameters for the further design of experiments.
ment on the surface quality. As a consequence, in order to increase the machining speed, the discharge current should be chosen in a moderate value, and maximise the open gap voltage and prolong the discharge interval. As can be seen in Fig. 5(a) and (d), a decrease in TWR is noticed by raising the current, especially lower values lie in the high currents region (32 A). According to theoretical models (Dauw, 1985), it is found that the electrode wear depends mainly upon the electron emission from the cathode. Increasing the discharge current leads to a larger current density, and consequently, a large anode material removal. When applying a negative polarity together with a pulse duration which is not too long (6.4 s), it explains the increase in an absolute workpiece (anode) material removal with the raised ie . As a result, a smaller ratio of eroded masses of the tool electrode (cathode) to the workpiece is obtained, or a lower TWR. However, it also could be due to the interaction effects with other parameters which are not examined in this research. The interaction of ie and ui on the effect of TWR is also shown in these experiments: bigger discharge current in combination with higher open gap voltage clearly reduces the consumption of the tool, and average value of 22% is examined when increasing both of them. Meanwhile sufficient interval time between discharges is necessary for keeping the shape intactness of the tool especially when lower discharge current is used. Still, the mean of the TWR over the entire investigation Fig. 3. Main effects of the input electrical parameters on the machining performances.
MRR is correspondingly reduced. Thermo physical models for single spark discharges reveal the existence of an optimal discharge duration te for a particular discharge current ie , yielding an optimal removal efficiency for workpiece material. These optimal values are believed relating to the erosion resistivity of a material (Dauw, 1985). This could explain a decrease of MRR when the ie is further enlarged (Fig. 5(c)). Meanwhile, it is also noticed that an increase of pulse interval time to leads to a higher MRR. This is because the longer pulse interval provides sufficient time for the dielectric to regain its strength in the sparking gap and the generation of unstable discharges can be avoided. Though with to at 100 s it is possible to work at even high currents of 48 A, no increase of MRR is observed. However, the combination of long to and higher ie did provide a slightly improve-
Table 5 ANOVA analyses results of the further design of experiments. Significance
ie to Interaction R2 Significance
ie ui Interaction R2
DoE with ie and to MRR √ √ √
TWR √ √ √
Ra √
0.96
0.87
0.67
MRR √ √ √
TWR √ √
Ra √ √ √
0.93
0.69
0.92
DoE with ie and ui
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Fig. 5. Surface plots of the machining performances in relation with the input design of experiments parameters.
region is rather high around 30%, which is in consistency with the results from the screening DoE. Therefore, in order to reduce the TWR and maintain the machining accuracy, the effective way to lower the tool consumption is by increasing the current, open gap voltage and pulse interval time, which the latter two are in favour of the material removal rate. Thus, concerning effects of ie on the MRR, a compromised value has to be considered. The influences of the investigated parameters on the Ra are consistent with the results from screening DoE. As shown, lowering the input of the discharge energy directly improves the surface quality. Longer pulse interval time and higher absolute voltage are also essential to result in a smoother surface. 3.3. Surface quality optimization: relaxation pulses Generally, to improve the surface quality, it is recommended to lower the energy input of the discharge pulses, which will make the impact of the discharges smaller and result in a smaller and flatter crater. Thus the iso energetic pulses sometimes may not give the best results because of the limitations in further reducing the pulse duration and discharge current. The RC-based relaxation pulses, on the contrary, have the possibility to provide shorter pulse duration and even lower peak current by reducing the capacitance of the discharge circuit. Thus, these pulses are generally called for surface finishing. However, there are few modification possibilities on the machine to change the input of the electrical parameters for the relaxation pulses, the influences on machining performances are only examined by gradually reducing the input energy, which mainly lowers the capacitance of the discharge circuit. During the experiments, a group of pre-defined settings available on the machine, are used for the experiments. In Table 6, the actual measured machining parameters from the oscilloscope of
each test are listed. In Fig. 6, examples of the waveforms of relaxation discharge pulses are illustrated. As can be seen in the table, the input discharge energy, indicated as the product of discharge duration and peak current, is gradually reduced in corresponding with the descending energy level numbers; the effects of the pulse interval and open gap voltage at the different power levels on the machining behaviour are also demonstrated. The results of the machining performances of each experiment are presented in Fig. 7. Clear trends of decreased material removal rate and surface roughness are observed when the indicative energy level is lowered. At the same energy level, a slightly improved surface quality is also observed when prolongs the pulse interval time to . Meanwhile, it is also noticed that even using relaxation pulses, raising the absolute open gap voltage results in a more stable EDM process: faster machining accompanies with smoother surface. Even at the limits of the machine, a surface roughness of only 1.0 m is Table 6 Means of the measured actual discharge parameters using the relaxation pulses. Test number 1 2 3 4 5 6 7 8 9 10 11 12 13
Energy level (indicative)
ie (A)
te (s)
to (s)
ui (V)
230 230 230 230 170 170 151 151 131 131 90 111 70
7 7 7 7 1 1 0.86 0.86 0.4 0.4 0.6 0.4 0.2
6.5 6.5 6.5 6.5 4 4 9 7 15 15 4 4 4
25 100 25 100 25 25 25 45 45 25 25 25 25
−200 −200 −120 −120 −200 −120 −200 −200 −160 −160 −200 −200 −200
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Fig. 6. The measured waveforms of relaxation pulses.
achieved. Yes, this value is much higher than the result obtained on steel using the same setting (0.2 m). However, an increase of the tool wear ratio is noticed though the distribution of the points are rather scattered.
3.4. Optimization and strategy development
Fig. 7. Results of the machining performances using relaxation pulses. (a) Material removal rate and surface roughness of each experiment. (b) Tool wear ratio of each experiment.
By summarizing the conclusions from previous experiments, the final strategy for die-sinking EDM of SiSiC can be developed. Roughing and semi-roughing regimes use high energy iso energetic pulses in order to remove the bulk of material at a minimum amount of time. Semi-finishing and finishing regimes are preferably using low energy relaxation pulses to achieve a good surface finish and geometry accuracy. The proposed strategy with all input electrical parameters are listed in Table 7 and confirmation experiments on each setting are also conducted. This strategy consists of the following settings: high energetic rough machining in a stable regime can be achieved using high current (32 A), high pulse interval time (100 s) and maximum open gap voltage (−200 V). The discharge duration te is guided by the screening DoE and an average value 3.2 s is chosen. For semi-roughing regime, the energy input is slightly reduced (ie = 6 A and te = 6.4 s), which is necessary to smoothen the surface and keep an acceptable MRR (1.01 mm3 /min). Semi-finishing further improves the surface quality and the reasonable wear assures the shape accuracy. The last and finishing step is executed with one of the lowest energetic relaxation pulses which allows to obtain a surface roughness of 1 m Ra. These settings are tested with at least three repetitions, not just to check the machining performances, but also to estimate the sparking gap. As a result, a proper undersize can be used for electrodes fabrication. Furthermore, the SiSiC flexural strength after each die-sinking EDM regime is also examined using a three-point bending test. Results from average of at least five experiments are shown in Table 7 as well. As can be seen, higher input discharge pulses lead to a rougher surfaces and also to a degraded flexu-
Table 7 Proposed machining strategy for die-sinking EDM of SiSiC and corresponding machining performances, sparking gap sizes and mechanical properties. Strategy step
ie (A)
ui (V)
te (s)
to (s)
MRR (mm3 /min)
TWR (%)
Ra (m)
Sparking gap (m)
Flexural strength (MPa)
Roughing Semi-roughing Semi-finishing Finishing
32 6 7 0.5
−200 −200 −200 −200
3.2 6.4 6.4 6
100 100 25 25
3.60 1.01 0.94 0.01
28 46 35 54
2.93 2.62 1.49 1.05
94 45 30 21
241 ± 15 267 ± 13 264 ± 6 294 ± 21
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Fig. 8. Comparison of microstructures on the EDMed surfaces with variable settings using iso energetic pulses. EDM parameters: (a) ie = 64 A, te = 12.8 s, ui = −80 V and to = 25 s; (b) ie = 64 A, te = 12.8 s, ui = −80 V and to = 100 s; (c) ie = 12 A, te = 12.8 s, ui = −80 V and to = 25 s; (d) ie = 12 A, te = 3.2 s, ui = −120 V and to = 100 s.
ral strength. Fine EDM finished samples have a strength around 300 MPa which is slightly better than the value provided by the manufacturer; no dramatic degradation due to EDM process is noticed.
Since the unstable machining is only noticed when applying high energy discharges especially with high current, it is much likely that they are caused by arcs. These long, continuous arc plasmas with an extreme high temperature, results in a high carbon release
4. Discussion of material removal mechanism In Fig. 8, the topography of the EDMed surfaces with variable settings are presented. The material removal mechanisms are mainly recognised as melting and evaporation similar like EDM of steel, no spalling or any mechanical erosion is observed over the EDMed surfaces. As can be seen in these figures, the size and depth of the formed craters are explicitly reduced when lowering the discharge energy input; consequently, improved surface quality is obtained. Furthermore, on Fig. 8(a), microcracks are also visible on the rougher surfaces, which also explains the flexural strength degradation. More material removal information is revealed on the crosssection SEM images (Fig. 9). During the discharge, the center of the plasma which has a high temperature of 8000–10,000 ◦ C (Schumacher, 2004) leads to a melting/decomposition of SiC grains (∼2700 ◦ C) at the top layer according to the Si–C phase diagram (Fig. 10). This heat also evaporates the free silicon (∼3200 ◦ C) among the boundaries of SiC particles and the grown SiC grains which are generated during the infiltration process. The escaped Si leaves voids in the matrix. This phenomenon, nevertheless, is found very localised and shallow. Free Silicon into the bulk material is still kept intact. The topography of machined surfaces at unstable region and the cross-sections are presented in Fig. 11(a)–(c), respectively. The microstructure shows a distinctive difference comparing to the stable machining regions: a thermal inducted layer of around 150 m is visible. It consists of an extra layer identified as SiC with a thickness of about ±23 m formed or resolidified over the top of original EDMed surfaces. Beneath of that a porous layer around ±100 m with plenty of voids can be seen. Furthermore, microcracks are visible and penetrate into the bulk material till a depth around 175 m.
Fig. 9. SEM images of cross-sections of die-sinking EDMed SiSiC surfaces at stable machining using iso energetic pulses. EDM parameters: (a) ie = 12 A, te = 12.8 s, ui = −120 V and to = 25 s; (b) ie = 12 A, te = 3.2 s, ui = −120 V and to = 100 s.
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Table 8 Quality control results of the case study workpiece after EDM process and the comparisons with the specifications. Feature
Tolerance specification (mm)
Tolerance measurement (mm)
Dimension cavity Flatness cavity bottom Flatness cavity top Depth cavity
±0.10 0.05 0.05 ±0.10
−0.028 0.024 0.005 −0.007
OK? √ √ √ √
Surface roughness (m) Ra: 1.03 Rz: 7.15 Rt: 10.81
5. Case study
Fig. 10. Si–C phase diagram (Scace and Slack, 1959).
(hydrocarbon dielectric and graphite electrode) and heavily contaminated environment. The evaporated and decomposed SiC thus could easily react in this carbon rich condition at rather low temperature (∼1400 ◦ C) and quickly resolidify as a new layer of SiC over the EDMed surface under a sudden cooling impact of the electric. Apparently, the integrity of the surface and subsurface after unstable EDM machining is severely damaged and it must be avoided in the process.
As a case study, a workpiece with complex features is executed to verify the developed EDM machining strategy. The designed features include ribs, a complex cavity and chamfers (Fig. 12). Dimensions and tolerances of the workpiece are shown in Fig. 12(c). As can be seen, a flatness of 0.05 m is required on both the top and the bottom of the cavity; meanwhile dimensions of the features generally have tolerances of ±0.1 mm. The negative shape of the features are machined on the graphite electrodes using a micromilling machine (KERN MMP) directly on a clamp which also can be re-clamped on the die-sinking EDM machine in order to reduce the systematic error. Since the ribs, the cavity and the chamfers are located on the two sides of the workpiece, two sets of electrodes are used. The chamfers and the cavity thus are machined at the same time. EDMing of these features is done by the strategy developed from previous results as mentioned in Table 7. At roughing and semiroughing regimes, two electrodes are used for each because of the high tool wear ratio, especially when machining deep cavity and ribs. In Fig. 13, the graphite electrodes used to machine the ribs after first roughing, first semi-roughing and finishing regimes are shown. The roughing electrodes are severely worn and in order to assure the shape tolerance, extra electrodes are applied. The second electrode after semi-roughing exhibits much less wear; thus, in order to improve the machining efficiency, the semi-finishing regime is omitted. Still, the production of the entire workpiece takes around 72 h. Most of the machining time is consumed on the first roughing (∼40%) and finishing (∼45%) regimes. Images of the finished sample and the detailed features can be found in Fig. 14. The geo-
Fig. 11. SEM images of die-sinking EDMed SiSiC surfaces at unstable machining region. EDM parameters: ie = 64 A, te = 12.8 s, ui = −120 V and to = 25 s.
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Fig. 12. The CAD drawings of the case study: (a) front view; (b) back view; (c) dimensions and tolerances of the features.
Fig. 13. Images of the graphite electrodes used for machining the ribs after each EDM regimes with same magnification: (a) roughing; (b) semi-roughing; (c) finishing.
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Fig. 14. The machined sample of the case study.
metrical and dimensional quality control is performed on a CMM (Mitutoyo FN905). In Table 8, the measured results comparing to the specifications are listed, together with the surface roughness measured on the bottom surface of the cavity. As can be seen, all the features are within the tolerance and meet the required shape and geometry accuracy. The cavity bottom corners are sharp and the chamfers preserve a good geometry. However, the corners at the ribs is comparably large and possibly still can be improved. 6. Conclusion In this research, a strategy for die-sinking EDM an electrical conductive Silicon infiltrate SiC is developed. To machine this relatively low conductive ceramic composite, it is necessary to determine a stable EDM region at the start of strategy development. The experiments indicate that high energetic settings, i.e. high peak current ie and long discharge duration te , especially with low to , result in unstable machining. This region needs to be avoided for an optimal die-sinking machining, as well as the surface and subsurface damage. Following conclusions can be drawn from the experiments. For rough machining it is recommended to use a high peak current, together with long pulse interval time and high open voltage. These last two parameters are necessary to achieve a stable machining process. This can boost the material removal rate as high as up to 6.10 mm3 /min. As for Semi-roughing, a reduced energy input (lowering ie or/and te ) is able to acquire a lowered surface roughness, meanwhile a moderate material removal rate (1.01 mm3 /min) and TWR of 46% are obtained. By using relaxation pulses, better surface finish can be achieved. A roughness of 1 m Ra is obtained but with a extremely low machining speed. A further improvement of the surface quality is not possible. The average tool wear ratio for all the tests is considerably high (around 30%) even at roughing and semi-roughing regimes. To obtain the machining accuracy, several electrodes for each machining regime are required.
The case study shows that the developed strategy sequences for roughing, semi-roughing and finishing are sufficient to manufacture some complex features very accurately in dimensions and shapes. Yet small variations in MRR or surface quality can occur by the alternation of machining conditions such as features geometry, flushing conditions and complexity. These differences demand small changes in the strategy. However the machining efficiency is comparably low, especially for the finishing regime. The main material removal mechanism for EDM of SiSiC is melting and evaporation. Voids are generated at the subsurface due to the melting of the silicon. Microcracks are formed when the discharge energy input is high, which also leads to a flexural strength degradation to around 240 MPa. No visible microcracks are observed on fine finished surfaces and the maximum bending strength is up to 290 MPa. Furthermore, unstable machining leads to a dramatic damage of surface integrity and should be avoided during the process. This research only investigates the main generator parameters. Other factors such as servo related parameters, flushing condition, polarity and electrode materials still need to be studied to examine their influences on the machining performances. References Ceramics, 2009. Saint-Gobain Ceramics. http://www.refractories.saintgobain.com. Dauw, D., 1985. On-line identification and optimization of Electro-Discharge Machining. PhD Thesis. Katholieke Universiteit Leuven, Belgium. König, W., Dauw, D., Levy, G., Panten, U., 1988. EDM-future steps towards the machining of ceramics. Annals of the CIRP 37 (2), 623–631. Kruth, J.-P., 2008. Non-conventional Manufacturing Methods. ACCO, Belgium. Lauwers, B., Kruth, J.-P., Brans, K., 2007. Development of technology and strategies for the machining of ceramic components by sinking and milling EDM. Annals of the CIRP 56 (1), 225–228. Liu, R., Ji, Y., Li, Q., Yu, L., Li, X., 2008. Electric discharge milling of silicon carbide ceramic with high electrical resistivity. International Journal of Machine Tools and Manufacture. 48, 1504–1508. Luis, C., Puertas, I., Villa, G., 2005. Material removal rate and electrode wear study on the EDM of silicon carbide. Journal of Materials Processing Technology 164 (165), 889–896.
S. Clijsters et al. / Journal of Materials Processing Technology 210 (2010) 631–641 Mahdavinejad, R., Mehraban, M., Mahdavinejad, D., 2006. The behaviour of REFEL SiC under electrodischarge machining. Journal of Engineering Manufacture 220, 1635–1646. Montgomery, D.C., 2001. Design and Analysis of Experiments, 5th ed. Wiley Interscience, New York, USA. Puertas, I., Luis, C., Villa, G., 2005. Spacing roughness parameters study on the EDM of silicon carbide. Journal of Materials Processing Technology 164 (165), 1590–1596.
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Scace, R., Slack, G., 1959. Solubility of carbon in silicon and germanium. The Journal of Chemical Physics 30, 1551–1555. Schumacher, B., 2004. After 60 years of EDM the discharge process remains still disputed. Journal of Materials Processing Technology 149, 376–381. US-Patent US-Patent:4957811, 1990. Wilhelm, M., Kornfeld, M., Wruss, W., 1999. Development of SiC–Si composites with fine-grained SiC microstructures. Journal of the European Ceramic Society 19, 2155–2163.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
EDM technology and strategy development for the manufacturing of complex parts in SiSiC S. Clijsters, K. Liu ∗ , D. Reynaerts, B. Lauwers Department of Mechanical Engineering, Division PMA, Catholic University Leuven (K.U.Leuven), Celestijnenlaan 300B, Bus 2420, B-3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 17 November 2009 Accepted 19 November 2009
Keywords: Electrical discharge machining Die-sinking EDM Ceramic composite SiSiC
a b s t r a c t Silicon carbide (SiC) is an extremely hard and difficult-to-shape engineering ceramic material used extensively in industry because of its superior mechanical properties, wear and corrosion resistance even at elevated temperature. Conventional ceramic processing and structuring techniques such as injection molding and grinding are costly and difficult to obtain flawless complex shaped components. By infiltrating free Si into the SiC, the electrical conductivity of the matrix is largely improved. Thus it can be machined by electrical discharge machining (EDM). In this paper, a die-sinking EDM technology for manufacturing components in a commercial available silicon infiltrated silicon carbide (SiSiC) is developed. The influences of the major operating EDM parameters (discharge current ie , open gap voltage ui , discharge duration te and pulse interval to ) of the iso energetic generator on the machining performances like Material Removal Rate (MRR), Tool Wear Ratio (TWR) and surface roughness (Ra) are examined. Relaxation pulses which have lower energy input are also considered to improve the surface quality. Furthermore, the topography and surface integrity of the material after the EDM process are analysed to determine the corresponding material removal mechanism. With the developed machining strategy, a sample piece with designed features such as ribs, a deep cavity and chamfers are manufactured to examine the machining performances. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide (SiC) is regarded as one of the most promising engineering ceramic materials because of its combination of excellent physical and mechanical properties such as low density, high toughness, high thermal conductivity, low thermal expansion coefficient and wear and corrosion resistance even at elevated temperature. Thus it has been extensively used in high temperature applications such as heat exchangers, engine components, high temperature bearings, fixtures and nozzles (Wilhelm et al., 1999). However, because of its hardness, it is difficult to be manufactured into complex shapes with desired accuracy cost-effectively using traditional ceramic processing methods or conventional manufacturing techniques like grinding. Electrical discharge machining is a non-conventional manufacturing process and proves to be an effective alternative comparing to traditional manufacturing processes. In this electro-thermal process, material is mainly removed by a series of carefully controlled discrete sparks (electrical discharges) generated between the tool electrode and workpiece both of which are submerged in a dielectric. The high temperature of the discharges normally melt and
∗ Corresponding author. Tel.: +32 0 16 322480; fax: +32 0 16 322987. E-mail address: [email protected] (K. Liu). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.11.012
evaporate the workpiece material locally and leave overlapped craters on the surface. Since there is no mechanical contact between the two electrodes, the process is able to machine any conductive component into accurate and complex shapes regardless of its hardness (Kruth, 2008). The major drawback of EDM ceramic material, however, is that most ceramics are not conductive enough. Research implicated that electrical resistivity of 100 cm seems the upper limit for a workpiece to be machined by die-sinking EDM (König et al., 1988). As most sintered ceramics, SiC has a very high electrical resistivity of 103 to 105 cm, it is practically impossible to be directly machined using EDM process. However, by infiltrating free Si into the SiC bulk material, the electrical conductivity can be increased dramatically without effecting the major mechanical properties of the components (10 cm) (Wilhelm et al., 1999). In last years, research has already been performed on the EDM milling of SiC, which seems to have a better machinability than die-sinking EDM (Lauwers et al., 2007; Liu et al., 2008). Researches on the machining of SiSiC also can be found, but these developments mostly aim at the finishing stage of the EDM process using a narrow process window of the input parameters (Mahdavinejad et al., 2006; Luis et al., 2005; Puertas et al., 2005). In these experiments, a variant SiSiC composite is investigated. The discharge current and discharge duration are in the range of 3–18 A and 30–500 s, respectively, with an open gap voltage from 120 to 200 V. However, the type of discharge pulses
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applied in the experiments is not specified explicitly. With the peak current of 6–8 A, a maximum material removal rate is achieved up to 0.5 mm3 /min and an average tool wear ratio around 10%. The EDMed surface has a minimum roughness Ra around 0.8 m with a small discharge energy, and it is noticed that the pulse duration merely has effects on it. Apparently, the influences of using high current and correspondingly high energy input on machining performances have not yet been investigated. Furthermore, limited information on the material removal mechanisms of EDMing this material is presented. As a consequence, in this paper, research is conducted on a commercially available SiSiC to determine the influences of the EDM electrical parameters using iso energetic pulses: discharge current ie , open gap voltage ui , discharge duration te and pulse interval time to , on the machining performances, which are the Material Removal Rate (MRR), Tool Wear Ratio (TWR), surface roughness (Ra). RCbased relaxation pulses are also examined to further improve the surface quality. Surface integrity after EDM process is also examined and the Material Removal Mechanism (MRM) is analysed. With the obtained results, a EDM machining strategy is developed and the performances are examined in a case study.
Table 1 The physical and mechanical properties of Silit® SKD SiSiC (Manufacturer’s data). Silicon carbide content Maximum service temperature Density Flexural strength Young’s modulus Thermal conductivity Thermal expansion coefficient Electrical resistivity
20 ◦ C 1000 ◦ C 1000 ◦ C 1000 ◦ C ˛ (RT. . .1300 ◦ C)
88% 1380 ◦ C 3.05 kg/dm3 250 MPa 250 MPa 360 GPa 36 W/(m K) 4.1 K−1 ×10−6 10 cm
RT: room temperature
2. Experimental setup The die-sinking EDM experiments are performed on a commercial machine, Charmilles Roboform 350, which has the ability to generate iso energetic pulses and RC-based relaxation pulses. The iso energetic pulse, which is controlled by the opening and closing of a transistor, normally has a longer pulse duration and larger discharge input energy. On the contrary, the released energy in the RC-based relaxation pulses mainly depends on the energy restored in the RC circuit and the local gap condition. In this research, the effects of both iso energetic and relaxation pulses on machining performances are investigated. Negative polarity is applied through the entire research. The dielectric fluid is hydrocarbon oil (TOTAL: DIEL MS7000). No side flushing but the electrode pulsation is used. POCO copper infiltrated graphite EDM-C3 is used as tool electrode material. The surface/subsurface integrity and cross-section has been examined on a Scanning Electron Microscope (SEM, Philips XL30FEG). All the specimens are cleaned ultrasonically. The SiSiC employed in the experiments is Silit® SKD commercially available from Saint-Gobain (Ceramics, 2009). It is produced by infiltrating a porous SiC–C green product with liquid Silicon. At a specific temperature (1600 ◦ C), the silicon melts and flows into the green product. There the silicon reacts with the carbon and generates a new SiC which grows on the original SiC particles (Wilhelm et al., 1999; US-Patent, 1990). The pores of the green product are filled with liquid silicon during the infiltration process that a practically dense product can be obtained. The SiSiC preserves the high hardness, good resistance against oxidation and corrosion, and an excellent wear resistance similar like SiC. Furthermore, despite the free silicon contributes to the improvement the electrical conductivity of the ceramic composite, the amount of them limits the bending strength to around 250 MPa. Details of the physical and mechanical properties of the ceramic can be found in Table 1. A microstructure of the SiSiC ceramic matrix is illustrated in Fig. 1. In this image the gray grains are SiC, which have an average particle size about 30 m. The white dots spread around the SiC grains are free Si. Among the silicon, the smaller gray dots are also SiC generated from the reaction of Si and C during the infiltration process. The die-sinking EDM experiments consist of machining a rectangular cavity with dimensions around 10 mm × 10 mm. The geometry of the EDMed cavities are measured by an optical coordinate measurement machine (Quickvision Pro, Mitutoyo). The depth
Fig. 1. Microstructure of the Silit® SKD SiSiC matrix: gray = SiC and white = Si.
and surface roughness Ra of the cavities are examined by a surface profilemeter (Form Talysurf 120, Taylor Hobson). With these results the MRR and TWR can simply be calculated: Volume removed from workpiece Time of machining Volume removed from electrode TWR = Volume removed from workpiece MRR =
3. Experiments on process investigation This research differs from the previous researches by the use of a machine which is able to generate both iso energetic and relaxation pulses. As a result, the methodologies to determine the influences of the input pulse parameters on the EDM performances are different for the iso energetic and relaxation pulses, since the machine provides more possibilities to modify the input parameters for iso energetic pulse. To explore the effects of major electrical parameters for iso energetic pulses, Design of Experiments (DoE) is used . It allows to analyse the influences of the input parameters on the responses with a minimum amount of experiments (Montgomery, 2001). A screening DoE is firstly executed on the chosen four parameters for iso energetic pulses, in order to determine a stable machining region and to check which parameter has a significant effect on the machining performances. The most significant parameters are then further investigated using two more DoEs with extended variant levels. Considering the small discharge energy input for relaxation pulses, a surface roughness optimization is focused using relaxation pulses at the last. In the following sections, the detailed investigation and experimental results are presented and discussed. 3.1. Screening DoE on iso energetic pulses The screening DoE is using a full factorial design of four major electrical parameters on two levels in order to determine a stable
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Table 4 Levels of the input parameters for the further design of experiments.
Table 2 Levels of the input parameters for the screening design of experiment.
DoE ie , to
DoE ie , ui
Parameters
Levels −
+
ie (A)
to (V)
ie (A)
ui (V)
ie te to ui
12 A 3.2 s 25 s −80 V
64 A 12.8 s 100 s −120 V
6 12 24 48
12.8 25 50 100
6 8 32 64
−80 −120 −160 −200
Table 3 Significant effects analysis on machining performances of screening DoE for SiSiC. Machining performances
MRR
TWR
Ra
Significance level (˛ = 0.05) 1 2 3 4
ui – – –
ie ie · to ie · ui –
ie ui te to
R2
0.80
0.97
0.98
machining region and the effect of each parameter on the machining performances. The selected low and high levels of the input parameters are listed in Table 2. These nominal values are equal to the effective values for iso energetic pulses, as can be seen in Fig. 2. A slightly increased discharge voltage is noticed (46 V) comparing to the machining of the steel (32 V), which is probably due to the lower electrical conductivity of the SiSiC. Since this variation is small, the effect of electrical resistivity for this ceramic composite on the machining process is negligible. The choice of these two levels aims to cover the entire available range of each parameter on the machine. In Fig. 3, the main effects of these parameters on the machining performances of MRR, TWR and Ra are illustrated. As can be seen in Fig. 3, the largest variation on MRR is given by ui in the investigated regions. The increased open gap voltage leads to a reduced MRR. Though the deviation of MRR between two levels of ie is considerably large, the effect is smaller comparing to parameter ui . However, unlike normal diesinking EDM of metal or alloy, higher discharge current prohibits the increase of machining speed, although a significantly lowered tool wear ratio is noticed. Furthermore, elevated discharge energy input by raising ie and te gives a worse surface quality than has been predicted. Increased gap voltage and pulse interval time also shows noticeable difference on improving the surface quality. The ANOVA (Analysis of Variance) analysis results of the screening DoE for all machining performances to characterise the significant factors are presented in Table 3. Confidence level of 95% (˛ = 0.05) is chosen. All the machining performances are fit in a second order model. The percentage between the variance of the
model’s predictions with the total variance of the data, defined as R2 , is used to evaluate the validity of the model. If R2 is very close to 1, the selected significant factors (main effects or interactions) are sufficient to explain most of the measured deviations in the output values and it is confirmed that the omitted effects are not important. As shown in this table, ui plays a significant role on affecting the MRR and Ra, so does the ie on TWR and Ra. Interaction of ie with ui and to also have significant influences on the tool wear. During the experiments, it is noticed that high energetic pulses with low pulse interval time result in arcs while pulses with low open gap voltage result in short-circuits. These phenomena should be avoided for stable and efficient machining. Therefore according to the results of DoE, the stable region can be determined. 3.2. Further investigation on the iso energetic pulses After determining the stable region in the previous section, the two most significant parameters ui and ie for the machining performances MRR and Ra, ie and to for the TWR from the screening DoE (Table 3) are further examined in two sets of experiments (Fig. 4). Since the input parameters are reduced to two, extended levels can be employed in order to study the non-linearity influences of the factors. Full factorial design at four levels are executed three times in random sequences to reduce the effects of noise and to check the repeatability of the tests. The different levels of each investigated input parameters are listed in Table 4. Rest of the parameters are kept constant and can be found in Fig. 4. The ANOVA analyses results of these two DoEs are listed in Table 5. The effects of input parameters on the machining performances are illustrated in surface plots in Fig. 5. As can be seen in Fig. 5(a), the non-linear effects of input parameter ie are obvious. In contrary with the screening DoE results (Fig. 3(a)), raising the discharge current shows an increase of MRR to a certain extent. An optimal discharge current 32 A is noticed to provide a maximum material removal of 6.1 mm3 /min when the biggest open gap voltage 200 V is applied. According to the experiments, the process will get unstable if the current keeps raising. Above this optimum current (32 A), arcs are observed during machining and leads to unsteady machining conditions, thus the
Fig. 2. The measured discharge voltage and current waveforms of iso energetic pulses.
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Fig. 4. The chosen parameters for the further design of experiments.
ment on the surface quality. As a consequence, in order to increase the machining speed, the discharge current should be chosen in a moderate value, and maximise the open gap voltage and prolong the discharge interval. As can be seen in Fig. 5(a) and (d), a decrease in TWR is noticed by raising the current, especially lower values lie in the high currents region (32 A). According to theoretical models (Dauw, 1985), it is found that the electrode wear depends mainly upon the electron emission from the cathode. Increasing the discharge current leads to a larger current density, and consequently, a large anode material removal. When applying a negative polarity together with a pulse duration which is not too long (6.4 s), it explains the increase in an absolute workpiece (anode) material removal with the raised ie . As a result, a smaller ratio of eroded masses of the tool electrode (cathode) to the workpiece is obtained, or a lower TWR. However, it also could be due to the interaction effects with other parameters which are not examined in this research. The interaction of ie and ui on the effect of TWR is also shown in these experiments: bigger discharge current in combination with higher open gap voltage clearly reduces the consumption of the tool, and average value of 22% is examined when increasing both of them. Meanwhile sufficient interval time between discharges is necessary for keeping the shape intactness of the tool especially when lower discharge current is used. Still, the mean of the TWR over the entire investigation Fig. 3. Main effects of the input electrical parameters on the machining performances.
MRR is correspondingly reduced. Thermo physical models for single spark discharges reveal the existence of an optimal discharge duration te for a particular discharge current ie , yielding an optimal removal efficiency for workpiece material. These optimal values are believed relating to the erosion resistivity of a material (Dauw, 1985). This could explain a decrease of MRR when the ie is further enlarged (Fig. 5(c)). Meanwhile, it is also noticed that an increase of pulse interval time to leads to a higher MRR. This is because the longer pulse interval provides sufficient time for the dielectric to regain its strength in the sparking gap and the generation of unstable discharges can be avoided. Though with to at 100 s it is possible to work at even high currents of 48 A, no increase of MRR is observed. However, the combination of long to and higher ie did provide a slightly improve-
Table 5 ANOVA analyses results of the further design of experiments. Significance
ie to Interaction R2 Significance
ie ui Interaction R2
DoE with ie and to MRR √ √ √
TWR √ √ √
Ra √
0.96
0.87
0.67
MRR √ √ √
TWR √ √
Ra √ √ √
0.93
0.69
0.92
DoE with ie and ui
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Fig. 5. Surface plots of the machining performances in relation with the input design of experiments parameters.
region is rather high around 30%, which is in consistency with the results from the screening DoE. Therefore, in order to reduce the TWR and maintain the machining accuracy, the effective way to lower the tool consumption is by increasing the current, open gap voltage and pulse interval time, which the latter two are in favour of the material removal rate. Thus, concerning effects of ie on the MRR, a compromised value has to be considered. The influences of the investigated parameters on the Ra are consistent with the results from screening DoE. As shown, lowering the input of the discharge energy directly improves the surface quality. Longer pulse interval time and higher absolute voltage are also essential to result in a smoother surface. 3.3. Surface quality optimization: relaxation pulses Generally, to improve the surface quality, it is recommended to lower the energy input of the discharge pulses, which will make the impact of the discharges smaller and result in a smaller and flatter crater. Thus the iso energetic pulses sometimes may not give the best results because of the limitations in further reducing the pulse duration and discharge current. The RC-based relaxation pulses, on the contrary, have the possibility to provide shorter pulse duration and even lower peak current by reducing the capacitance of the discharge circuit. Thus, these pulses are generally called for surface finishing. However, there are few modification possibilities on the machine to change the input of the electrical parameters for the relaxation pulses, the influences on machining performances are only examined by gradually reducing the input energy, which mainly lowers the capacitance of the discharge circuit. During the experiments, a group of pre-defined settings available on the machine, are used for the experiments. In Table 6, the actual measured machining parameters from the oscilloscope of
each test are listed. In Fig. 6, examples of the waveforms of relaxation discharge pulses are illustrated. As can be seen in the table, the input discharge energy, indicated as the product of discharge duration and peak current, is gradually reduced in corresponding with the descending energy level numbers; the effects of the pulse interval and open gap voltage at the different power levels on the machining behaviour are also demonstrated. The results of the machining performances of each experiment are presented in Fig. 7. Clear trends of decreased material removal rate and surface roughness are observed when the indicative energy level is lowered. At the same energy level, a slightly improved surface quality is also observed when prolongs the pulse interval time to . Meanwhile, it is also noticed that even using relaxation pulses, raising the absolute open gap voltage results in a more stable EDM process: faster machining accompanies with smoother surface. Even at the limits of the machine, a surface roughness of only 1.0 m is Table 6 Means of the measured actual discharge parameters using the relaxation pulses. Test number 1 2 3 4 5 6 7 8 9 10 11 12 13
Energy level (indicative)
ie (A)
te (s)
to (s)
ui (V)
230 230 230 230 170 170 151 151 131 131 90 111 70
7 7 7 7 1 1 0.86 0.86 0.4 0.4 0.6 0.4 0.2
6.5 6.5 6.5 6.5 4 4 9 7 15 15 4 4 4
25 100 25 100 25 25 25 45 45 25 25 25 25
−200 −200 −120 −120 −200 −120 −200 −200 −160 −160 −200 −200 −200
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Fig. 6. The measured waveforms of relaxation pulses.
achieved. Yes, this value is much higher than the result obtained on steel using the same setting (0.2 m). However, an increase of the tool wear ratio is noticed though the distribution of the points are rather scattered.
3.4. Optimization and strategy development
Fig. 7. Results of the machining performances using relaxation pulses. (a) Material removal rate and surface roughness of each experiment. (b) Tool wear ratio of each experiment.
By summarizing the conclusions from previous experiments, the final strategy for die-sinking EDM of SiSiC can be developed. Roughing and semi-roughing regimes use high energy iso energetic pulses in order to remove the bulk of material at a minimum amount of time. Semi-finishing and finishing regimes are preferably using low energy relaxation pulses to achieve a good surface finish and geometry accuracy. The proposed strategy with all input electrical parameters are listed in Table 7 and confirmation experiments on each setting are also conducted. This strategy consists of the following settings: high energetic rough machining in a stable regime can be achieved using high current (32 A), high pulse interval time (100 s) and maximum open gap voltage (−200 V). The discharge duration te is guided by the screening DoE and an average value 3.2 s is chosen. For semi-roughing regime, the energy input is slightly reduced (ie = 6 A and te = 6.4 s), which is necessary to smoothen the surface and keep an acceptable MRR (1.01 mm3 /min). Semi-finishing further improves the surface quality and the reasonable wear assures the shape accuracy. The last and finishing step is executed with one of the lowest energetic relaxation pulses which allows to obtain a surface roughness of 1 m Ra. These settings are tested with at least three repetitions, not just to check the machining performances, but also to estimate the sparking gap. As a result, a proper undersize can be used for electrodes fabrication. Furthermore, the SiSiC flexural strength after each die-sinking EDM regime is also examined using a three-point bending test. Results from average of at least five experiments are shown in Table 7 as well. As can be seen, higher input discharge pulses lead to a rougher surfaces and also to a degraded flexu-
Table 7 Proposed machining strategy for die-sinking EDM of SiSiC and corresponding machining performances, sparking gap sizes and mechanical properties. Strategy step
ie (A)
ui (V)
te (s)
to (s)
MRR (mm3 /min)
TWR (%)
Ra (m)
Sparking gap (m)
Flexural strength (MPa)
Roughing Semi-roughing Semi-finishing Finishing
32 6 7 0.5
−200 −200 −200 −200
3.2 6.4 6.4 6
100 100 25 25
3.60 1.01 0.94 0.01
28 46 35 54
2.93 2.62 1.49 1.05
94 45 30 21
241 ± 15 267 ± 13 264 ± 6 294 ± 21
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Fig. 8. Comparison of microstructures on the EDMed surfaces with variable settings using iso energetic pulses. EDM parameters: (a) ie = 64 A, te = 12.8 s, ui = −80 V and to = 25 s; (b) ie = 64 A, te = 12.8 s, ui = −80 V and to = 100 s; (c) ie = 12 A, te = 12.8 s, ui = −80 V and to = 25 s; (d) ie = 12 A, te = 3.2 s, ui = −120 V and to = 100 s.
ral strength. Fine EDM finished samples have a strength around 300 MPa which is slightly better than the value provided by the manufacturer; no dramatic degradation due to EDM process is noticed.
Since the unstable machining is only noticed when applying high energy discharges especially with high current, it is much likely that they are caused by arcs. These long, continuous arc plasmas with an extreme high temperature, results in a high carbon release
4. Discussion of material removal mechanism In Fig. 8, the topography of the EDMed surfaces with variable settings are presented. The material removal mechanisms are mainly recognised as melting and evaporation similar like EDM of steel, no spalling or any mechanical erosion is observed over the EDMed surfaces. As can be seen in these figures, the size and depth of the formed craters are explicitly reduced when lowering the discharge energy input; consequently, improved surface quality is obtained. Furthermore, on Fig. 8(a), microcracks are also visible on the rougher surfaces, which also explains the flexural strength degradation. More material removal information is revealed on the crosssection SEM images (Fig. 9). During the discharge, the center of the plasma which has a high temperature of 8000–10,000 ◦ C (Schumacher, 2004) leads to a melting/decomposition of SiC grains (∼2700 ◦ C) at the top layer according to the Si–C phase diagram (Fig. 10). This heat also evaporates the free silicon (∼3200 ◦ C) among the boundaries of SiC particles and the grown SiC grains which are generated during the infiltration process. The escaped Si leaves voids in the matrix. This phenomenon, nevertheless, is found very localised and shallow. Free Silicon into the bulk material is still kept intact. The topography of machined surfaces at unstable region and the cross-sections are presented in Fig. 11(a)–(c), respectively. The microstructure shows a distinctive difference comparing to the stable machining regions: a thermal inducted layer of around 150 m is visible. It consists of an extra layer identified as SiC with a thickness of about ±23 m formed or resolidified over the top of original EDMed surfaces. Beneath of that a porous layer around ±100 m with plenty of voids can be seen. Furthermore, microcracks are visible and penetrate into the bulk material till a depth around 175 m.
Fig. 9. SEM images of cross-sections of die-sinking EDMed SiSiC surfaces at stable machining using iso energetic pulses. EDM parameters: (a) ie = 12 A, te = 12.8 s, ui = −120 V and to = 25 s; (b) ie = 12 A, te = 3.2 s, ui = −120 V and to = 100 s.
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Table 8 Quality control results of the case study workpiece after EDM process and the comparisons with the specifications. Feature
Tolerance specification (mm)
Tolerance measurement (mm)
Dimension cavity Flatness cavity bottom Flatness cavity top Depth cavity
±0.10 0.05 0.05 ±0.10
−0.028 0.024 0.005 −0.007
OK? √ √ √ √
Surface roughness (m) Ra: 1.03 Rz: 7.15 Rt: 10.81
5. Case study
Fig. 10. Si–C phase diagram (Scace and Slack, 1959).
(hydrocarbon dielectric and graphite electrode) and heavily contaminated environment. The evaporated and decomposed SiC thus could easily react in this carbon rich condition at rather low temperature (∼1400 ◦ C) and quickly resolidify as a new layer of SiC over the EDMed surface under a sudden cooling impact of the electric. Apparently, the integrity of the surface and subsurface after unstable EDM machining is severely damaged and it must be avoided in the process.
As a case study, a workpiece with complex features is executed to verify the developed EDM machining strategy. The designed features include ribs, a complex cavity and chamfers (Fig. 12). Dimensions and tolerances of the workpiece are shown in Fig. 12(c). As can be seen, a flatness of 0.05 m is required on both the top and the bottom of the cavity; meanwhile dimensions of the features generally have tolerances of ±0.1 mm. The negative shape of the features are machined on the graphite electrodes using a micromilling machine (KERN MMP) directly on a clamp which also can be re-clamped on the die-sinking EDM machine in order to reduce the systematic error. Since the ribs, the cavity and the chamfers are located on the two sides of the workpiece, two sets of electrodes are used. The chamfers and the cavity thus are machined at the same time. EDMing of these features is done by the strategy developed from previous results as mentioned in Table 7. At roughing and semiroughing regimes, two electrodes are used for each because of the high tool wear ratio, especially when machining deep cavity and ribs. In Fig. 13, the graphite electrodes used to machine the ribs after first roughing, first semi-roughing and finishing regimes are shown. The roughing electrodes are severely worn and in order to assure the shape tolerance, extra electrodes are applied. The second electrode after semi-roughing exhibits much less wear; thus, in order to improve the machining efficiency, the semi-finishing regime is omitted. Still, the production of the entire workpiece takes around 72 h. Most of the machining time is consumed on the first roughing (∼40%) and finishing (∼45%) regimes. Images of the finished sample and the detailed features can be found in Fig. 14. The geo-
Fig. 11. SEM images of die-sinking EDMed SiSiC surfaces at unstable machining region. EDM parameters: ie = 64 A, te = 12.8 s, ui = −120 V and to = 25 s.
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Fig. 12. The CAD drawings of the case study: (a) front view; (b) back view; (c) dimensions and tolerances of the features.
Fig. 13. Images of the graphite electrodes used for machining the ribs after each EDM regimes with same magnification: (a) roughing; (b) semi-roughing; (c) finishing.
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Fig. 14. The machined sample of the case study.
metrical and dimensional quality control is performed on a CMM (Mitutoyo FN905). In Table 8, the measured results comparing to the specifications are listed, together with the surface roughness measured on the bottom surface of the cavity. As can be seen, all the features are within the tolerance and meet the required shape and geometry accuracy. The cavity bottom corners are sharp and the chamfers preserve a good geometry. However, the corners at the ribs is comparably large and possibly still can be improved. 6. Conclusion In this research, a strategy for die-sinking EDM an electrical conductive Silicon infiltrate SiC is developed. To machine this relatively low conductive ceramic composite, it is necessary to determine a stable EDM region at the start of strategy development. The experiments indicate that high energetic settings, i.e. high peak current ie and long discharge duration te , especially with low to , result in unstable machining. This region needs to be avoided for an optimal die-sinking machining, as well as the surface and subsurface damage. Following conclusions can be drawn from the experiments. For rough machining it is recommended to use a high peak current, together with long pulse interval time and high open voltage. These last two parameters are necessary to achieve a stable machining process. This can boost the material removal rate as high as up to 6.10 mm3 /min. As for Semi-roughing, a reduced energy input (lowering ie or/and te ) is able to acquire a lowered surface roughness, meanwhile a moderate material removal rate (1.01 mm3 /min) and TWR of 46% are obtained. By using relaxation pulses, better surface finish can be achieved. A roughness of 1 m Ra is obtained but with a extremely low machining speed. A further improvement of the surface quality is not possible. The average tool wear ratio for all the tests is considerably high (around 30%) even at roughing and semi-roughing regimes. To obtain the machining accuracy, several electrodes for each machining regime are required.
The case study shows that the developed strategy sequences for roughing, semi-roughing and finishing are sufficient to manufacture some complex features very accurately in dimensions and shapes. Yet small variations in MRR or surface quality can occur by the alternation of machining conditions such as features geometry, flushing conditions and complexity. These differences demand small changes in the strategy. However the machining efficiency is comparably low, especially for the finishing regime. The main material removal mechanism for EDM of SiSiC is melting and evaporation. Voids are generated at the subsurface due to the melting of the silicon. Microcracks are formed when the discharge energy input is high, which also leads to a flexural strength degradation to around 240 MPa. No visible microcracks are observed on fine finished surfaces and the maximum bending strength is up to 290 MPa. Furthermore, unstable machining leads to a dramatic damage of surface integrity and should be avoided during the process. This research only investigates the main generator parameters. Other factors such as servo related parameters, flushing condition, polarity and electrode materials still need to be studied to examine their influences on the machining performances. References Ceramics, 2009. Saint-Gobain Ceramics. http://www.refractories.saintgobain.com. Dauw, D., 1985. On-line identification and optimization of Electro-Discharge Machining. PhD Thesis. Katholieke Universiteit Leuven, Belgium. König, W., Dauw, D., Levy, G., Panten, U., 1988. EDM-future steps towards the machining of ceramics. Annals of the CIRP 37 (2), 623–631. Kruth, J.-P., 2008. Non-conventional Manufacturing Methods. ACCO, Belgium. Lauwers, B., Kruth, J.-P., Brans, K., 2007. Development of technology and strategies for the machining of ceramic components by sinking and milling EDM. Annals of the CIRP 56 (1), 225–228. Liu, R., Ji, Y., Li, Q., Yu, L., Li, X., 2008. Electric discharge milling of silicon carbide ceramic with high electrical resistivity. International Journal of Machine Tools and Manufacture. 48, 1504–1508. Luis, C., Puertas, I., Villa, G., 2005. Material removal rate and electrode wear study on the EDM of silicon carbide. Journal of Materials Processing Technology 164 (165), 889–896.
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