Material removal rate and electrode wear study on

the EDM of silicon carbide

From ： Journal of Materials Processing Technology 164–165 (2005) 889–896

Author ： C.J. Luis, I. Puertas, G. Villa

研究生：李政龍日期： 2010.06.11

Content

1. Introduction

2. Equipment used and conductive ceramic EDMed

3. Design of the experiments

4. Results and analysis of MRR

5. Results and analysis of EW

6. Conclusions

1. IntroductionIn this work, a material removal rate (MRR) and electrode wear (EW) study on the die-sinking electrical discharge machining (EDM) of siliconised or reaction-bonded silicon carbide (SiSiC) has been carried out.

This study was made only for the finish stages and has been carried out on the influence of five design factors: intensity supplied by the generator of the EDM machine (I), pulse time (ti ),duty cycle (η),open-circuit voltage (U) and dielectric flushing pressure(P ） .

2.Equipment used and conductive ceramic EDMed

The equipment used to perform the experiments was a die-sinking EDM machine of type ONA DATIC D-2030-S. Also,a jet flushing system in order to assure the adequate flushing of the EDM process debris from the gap zone was employed.

The dielectric fluid used for the EDM machine was a mineral oil (Oel-Held Dielektrikum IME 82) with a flash point of 82 . ℃The electrodes used weremade of electrolytic copper (with a cross-section of 12mm×8mm) and the polarity was negative.

2.1. Die-sinking EDM machine

The ceramic material,it has a high thermal conductivity(150–200Wm−1 K−1,20 ) and a low thermal expansion℃ coefficient (4.3–4.6×10−6 K−1, 20–1000 ◦C),which provides it with a good resistance to thermal shock. Reactionbonded silicon carbide performs better under chemical corrosion than other ceramicmaterials, such as tungsten carbide or alumina (Al2O3).

2.2. Reaction-bonded or siliconised silicon carbide

3.Design of the experiments3.1. Design factors selected

There are a large number of factors to consider within the EDM process, but in this work the level of the generator intensity (I), pulse time (ti), duty cycle (η), open-circuit voltage (U) and dielectric flushing pressure (P) have only been taken into account as design factors.

The reason why these five factors have been selected as design factors is that they are the most widespread and used amongst EDM researchers.

3.2. Response variables selected

The response variables selected for this study refer to the speed of the EDM process, i.e., material removal rate (MRR),and the efficiency of the copper electrode used, i.e., volumetric electrode wear (EW). These response variables are defined in Eqs. (1) and (2), respectively:

The low and high levels selected for intensity, pulse time,duty cycle, open-circuit voltage and dielectric flushing pressure were: 3 and 5, 30 and 70 μs,0.4 and 0.6, −120 and −200V and finally, 20 and 60 kPa, respectively Table 1. The levels of the intensity factor (3 and 5) are equivalent to 2 and 6 A, respectively.

3.3. Fractional factorial design employed

4.Results and analysis of MRRA first-order model was proposed for the response variable MRR, where this was rejected as a result of the values obtained for the curvature test that can be seen in Table 3.As in this case the P-value, which is equal to 0.0002, is lower than 0.05, the null hypothesis that there are no pure quadratic effects in the model is therefore rejected.in the first-order model, for a confidence level of 95%. Thus, that the proposed first-order model is suitable for a significance level α of 0.05 is rejected.

5.Results and analysis of EWIn the sameway as MRR, a first-order model was proposed for EW, where this is rejected as a result of the curvature test shown in Table 5. As in this case the P-value, which is equal to 0.0002, is lower than 0.05, it is accepted that there is statistical evidence of curvature in the first-order model, for a confidence level of 95% and then, the second-order model is selected.

6.ConclusionsA. In this work, a study on the influence of the most relevant EDM

factors over material removal rate (MRR) and electrode wear (EW) has been carried out. The study has been made for a conductive ceramic such as siliconised silicon carbide (SiSiC) and only for the finish stages.

B. The design factors selected in this case were: intensity (I), pulse time (ti), duty cycle (η), open-circuit voltage

(U) and flushing pressure (P), where all of them, except for the last one, are parameters widely used by the machinists to control the EDM machine generator.

C. In the case of MRR, the only influential design factors,for a confidence level of 95%, were: intensity and voltage.

The variation tendencies of the latter were those expected a priori

and according to these, in order to obtain a high value of MRR within the work interval of this study, both design factors ( I and U) should be fixed as high as possible.

D. With regard to EW and arranged in descending order of importance, intensity, pulse time and flushing pressure turned out to be the influential factors for a confidence level of 95%.

E. Moreover, in the case of flushing pressure, it was verified that an increase in the latter resulted in a decrease in the wear on the electrode.