Performance of Difference Electrode Materials in Electrical Discharge Machining of Tungsten Carbide

Problem statement: Electrical Discharge Machining (EDM) is now a well stablished machining option in many industries. Tungsten carbi de (WC-Co) is an important tool and die material mainly because of its high hardness, strength, wear resistance and high melting point. Normally, EDM is capable of machining geometrically complex or ha d material component, that are precise and difficult to machine. The objective of this research is to study the perf ormance of different electrode materials on tungsten carbide workpiece with EDM pr ocess. Approach: The electrode materials were graphite (Poco EDM-3), copper-graphite (Poco EDM-C3 ) and copper-tungsten (solid).The important parameters were discharge current, on time, off tim e, open-circuit voltage and electrode polarity. A workpiece material was a tungsten carbide (W 90-Co1 0). Results: The results show that the electrode negative polarity performs very well, Poco EDM-3 gi ves higher Material Removal Rate (MRR). Both powder electrode (EDM-3 and EDM-C3) give the better MRR and EWR more than solid electrode. Conclusion: The suitable duty factor is 11%. The Surface Rough ness (SR) of copper-tungsten give the best when current peak intensity not over 20 ampere s.


INTRODUCTION
Electrical Discharge Machining (EDM) is a machine that is used as non-traditional manufacturing and this machine is continually developing further technology that would be impossible to produce with faster and conventional (Mahdavinejad and Mahdavinejad, 2005). This machine produces tools with complex-shapes and being used extensively in industries. Furthermore EDM can operate as surface finish in last stage of tool production (Puertas et al., 2004).
Tungsten carbide (WC-Co) is an important tool and die material mainly because of its high hardness, strength and wear resistance. The melting point of tungsten carbide is 2,800°C. Due to its properties, it cannot be processed easily by conventional machining technique (Singh et al., 2004). So EDM process will open up an opportunity for the machining of tungsten carbide. Tungsten carbide is some kind of cemented carbide; the particle of carbide is bind with the process of powder metallurgy (George et al., 2004), such as Tungsten carbide (WC-Co), Titanium Carbide (TaC).
In an EDM operation, it is need to select right parameters for sparking performance (Lee and Li, 2003). However, the right and desired parameters that base on the experience, instruction manual or a large number of test of experiment that require a lot of time and materials. The objective of this research is to study the performance of different electrode materials on tungsten carbide workpiece with EDM process.

Raw material:
The workpiece material was a tungsten carbide with 90% of W and 10% of Co. The electrode materials were graphite, copper graphite and copper tungsten. The workpiece material from United Tungsten Co, Ltd. which is show in Table 1.
The electrode materials were graphite (EDM-3), copper-graphite (EDM-C3) and copper-tungsten (solid). The graphite and copper graphite electrode were bought from Poco Graphite Co, Ltd. The electrodes were made of powder metal. The copper tungsten is made of solid copper tungsten electrode. The physical properties of materials electrode which is show in Table 2.

Sample preparation:
The experiment of materials electrode diameter is 3 mm and depth of EDM spark is 3 mm. The machine is used as a numerical control model FORM-2-LC Charmilles Technologies (Fig. 1). Dielectric oil of spark is Shell EDM Fluid 2A from Shell Co, Ltd. (Thailand). For this experiment of research, to start at determine try-out of suitable parameter relative in EDM process, such as polarity, duty factor, on time, off time, open circuit voltage, discharge current and electrode materials type. The detailed machining conditions used in this investigation were given in Table 3. Finally, the experiment result comparative the optimal parameters of material removal rate, electrode wear ratio, surface roughness. However, the optimal EDM parameter are considered.    Fig. 3 represent that, lower duty factor have Material Removal Rate (MRR) higher than upper duty factor and negative polarity graphite electrode show the most MRR is 7%.
As Fig. 3, represents positive polarity cuppertungsten electrode has duty factor 89% at 0 mm 3 min −1 and the same this condition in Fig. 2 has not results because of an inappropriate sparking condition that is not able to produce a material removal rate. Effect of open circuit voltage: Previously experiment of a variation of on-time and off-time condition show that negative polarity and graphite electrode are appropriate parameters to sparking carbide tungsten material. Therefore, negative polarity is used only in this experimental procedure. Experimental parameter; current 12 A, duty factor 11%.   Figure 7 shows the relation between current and MRR of workpiece were found that an increased current have influence to increasing of MRR, graphite electrode has the most MRR at 55 A. Figure 8 shows the relation between current and electrode wear rate of workpiece were found that an increased current have influence to deceasing of electrode wear rate, unless cupper-tungsten is performed, In case of cupper-tungsten electrode has the least electrode wear rate at 6 A. Figure 9 shows the relation between current and rough surface (Lee and Li, 2001) were found that an increasing rough of surface have an effect from increased current, cupper-tungsten electrode has the least rough surface at 6 A.

CONCLUSION
From this research it can be concluded that following below: • The duty factor value decreased give the less material removal rate and the effectiveness of the process is evaluated of MRR increases with the discharge current intensity • The graphite electrode gives the most material removal rate and gives the better than surface roughness but it gives high electrode wear ratio. • The results show that the electrode negative polarity performs very well, Poco EDM-3 gives significantly higher Material Removal Rate (MRR) and lower surface micro-crack density than the Poco EDM-C3 and copper-tungsten • The material powder electrode (EDM-3 and EDM-C3) give the better MRR and less micro-cracks than solid electrode