Effect of Electrode Material on Electrical Discharge Machining of DIN 1.2379 and DIN 1.2080 Surface

Electrical Discharge Machining (EDM) is used in cutting special steel accurately; in this paper the effect of electrode material will be studied to avoid the surface cracks. Two grades of tool steels are chosen as test materials, DIN 1.2080 and DIN 1.2379 they are a high carbon, high chromium alloy tool steels with excellent resistance to wear and abrasion four types of EDM electrode material has been chosen, Dura graph 11, Dura graph 15, Poco graphite “EDM-C 3”, and copper electrodes.


Introduction
Electrical Discharge Machining (EDM) is a cost-effective way of shaping complex geometry materials with high accuracy, and is widely used in the hardmetal industry [1].Melting is observed to be the main material removal mechanism in EDM machining of hardmetals.Graphite is widely used as electrode material in EDM; this is due to its good electrical and thermal properties, along with its machinability [2,3].For this study, it is expected that carbon from graphite electrode implant and generate a conductive layer.In this paper, copper, Dura graph 11, Dura graph 13, and copper-infiltrated-graphite (Poco EDM-C3) electrodes were used to compare their effects on the surface of DIN 1.2080 and DIN 1.2379 corresponding to EDM properties.Spark or Cavity Type EDM consists of an electrode and workpiece that are submerged in an insulating liquid.The electrode and workpiece are connected to a suitable power supply which generates an electrical potential between the two parts.As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid and a small spark jumps.The resulting heat and capitations vaporize the base material, and to some extent, the electrode [4].

Factors Affecting EDM Applications
A common belief in the mold making industry is that a lower quality electrode material can be used to produce superior surface finishes if a high quality EDM machine is used.while the technologies of the newer generation EDM sinkers have allowed the EDM user to become more proficient in the application, this technology only goes so far when it comes to producing fine surface finishes economically with low quality electrode materials.True, improved EDM sinker technologies have the ability to monitor the condition of the EDM process and make adjustments to run more efficiently.However, these changes are generally limited to optimization in the roughing stage or to eliminate arcing in the EDM cavity.Often, the production of fine surface finishes in the cavity does not come with the technological improvements of the EDM sinker unless an electrode material of higher quality is used [5].

Electrode material
In general, the material characteristics of the electrode play a much larger role in achieving a fine surface finish than one might think.When we think of fine finishes, we may envision something similar to a glossy finish that reflects light very easily.Conversely, in today's mold design a fine finish means more than a surface that has been polished.Many of the molds that are currently being produced require a textured finish that must be consistent throughout the cavity.
Electrode materials can be silver tungsten, copper tungsten, copper graphite "combination between metallic and non-metallic material", chromium copper, graphite "non-metallic material", and brass.Of all these, electrolytic copper is the most popular.Silver tungsten and copper tungsten are quite expensive, so their uses are limited to very small high precision parts.Other factors in electrode selection are low wear and ease of manufacturing.
Graphite features low electrode wear and high removal rate but due to low density, it breaks easily.Graphite is the beat employed when fine finish is not required [5].

Experimental Work Test materials
Two grades of tool steels has been chosen and analyzed using X-Ray diffraction as follows: DIN 1.2080: is a high carbon, high chromium alloy tool steel with excellent resistance to wear and abrasion high-performance blanking and punching dies for sheet thickness up to 1.5 mm; thread rolling dies, plastic molds.As follows in Table 1.DIN 1.2379: is a high carbon, high chromium, molybdenum, vanadium tool steel which offers good wear resistance, highperformance blanking and punching dies for sheet thickness up to 3 mm; thread rolling dies, plastic molds.As follows in Table 2.
The EDM specimens were further machined by EDM, and then examined by Scanning Electron Microscope "SEM".

Samples under investigation
The experimental samples of dimensions 40×25×10 mm were made from DIN 1.2080 and 45×25×15 mm were made from DIN 1.2379, as shown in Figure 2.

SEM Observations Surface observations
The distinctive morphology of a surface which has undergone EDM machining, is due to the enormous amount of heat generated by the discharges, which causes melting and vaporization of the material, followed by rapid cooling.The surface topography presented in Figures 3-10 reveals that the surface roughness is caused by an uneven fusing structure, pockmarks, voids and cracks, also on DIN1.2379 machined by EDM-C 3 electrode reveals more dissolved copper on the surface.

White layer observations
The white layer is formed by molten metal which is not flushed away by the dielectric, but solidifies on the sample's machined surface during cooling (Figure 4).The white layer formed on the test samples, revealed that EDM machining exhibits less thick white layer and less micro-crack while using EDM-C 3 electrode as presented in Figures 11-14.

Cracks observations
Observation of the machined surface, Figures 15-22, and the sample sections, reveals that the surface cracks are often micro-cracks.
The high magnification microscope shows that cracks exist in the white layer; initiating at its surface, and travelling down perpendicularly towards the parent material.In the vast majority of cases the cracks C.

Conclusions
The EDM process induces surface damage such as pinholes, and craters in the surface layer.Also the surface morphology of the EDM specimen has revealed a poor surface structure.The surface cracks are often micro-cracks and exists in the white layer; initiating at its surface, and travelling down perpendicularly towards the parent material.Using copper electrodes upon DIN 1.2379 less microcracks appeared on the white layer surface than on DIN 1.2080.Spark erosion EDM machining exhibits less thick white layer and less micro-crack while using EDM-C 3 electrode.