EDM Technical Manual
CHAPTER 4: ELECTRODE MATERIALS
The purpose of any material used as an electrode in the EDM process is to transmit the electrical machining impulses, allowing workpiece erosion to take place with little or no self erosion. In doing this, the material should possess the essential qualities of good metal removal, low wear, and the ability to be accurately machined and fabricated at low cost. It is a known fact that electrode materials differ in performance, depending on the application and workpiece material. Some will remove metal very efficiently, with poor wear resistance, while others in the same application will give good wear resistance and relatively low metal removal. This is due to the physical characteristics and properties of the electrode materials and the conditions that occur in the workgap.
The five commonly used electrode materials are copper, brass, zinc, tungsten, and graphite. These materials can fall into two main categories: Metallics and Graphite. The Metallics are further divided into three groups: Common Metals, Tungsten and Tungsten Composites, and Exotics.
Graphites are divided into the following six classes according to their particle size:
Angstrofine <1 micron
Ultrafine 1-5 microns
Superfine 6-10 microns
Fine 11-20 microns
Medium 21-100 microns
Coarse >100 microns
Graphites in the Coarse classification are not suitable for EDM purposes.
Common Metals—This is the original group of metals used as electrode materials in those early EDM days between World Wars I and II. Copper is the best known; brass and zinc complete the group.
These materials have some very attractive qualities as electrode materials. They have high electrical and thermal conductivities. Copper, brass, and zinc are easily obtainable, consistent in quality, and low in cost, but they have their drawbacks as well. Copper has the highest melting point of this group, but it melts at about 1100°C. Temperatures in the gap surpass this by several thousand degrees, so rapid electrode wear is a problem. Other problems with common metals are slow metal removal rates and electrode fabrication limitations.
Tungsten and Tungsten Composites—Theoretically, tungsten is the best of the metallics for use as an electrode. With its very high strength, density, hardness, and a melting point near 3400°C, tungsten resists the damaging effects of the EDM process very well indeed. There are two main problems associated with using pure tungsten as an electrode material. It is very difficult to machine and extremely expensive, which limits its usefulness as an electrode material.
However, there are ways to make tungsten more attractive for certain applications. One of the more common is to combine it with a more ductile material such as copper. The resulting material is both easier to machine and more conductive, as well as being extremely strong and wear resistant.
Other tungsten composites that have been used with varying degrees of success include silver tungsten and tungsten carbide. Of course, since tungsten is an expensive metal to begin with, the addition of other metals makes its use cost effective only in limited applications. However, in those applications, as noted in Chapter 5, tungsten composites can be cost effective performers.
Exotics—This group contains just about every other metal that is conductive, but rarely used as electrode materials. Exotic materials that we know have been used for specific applications include tantalum, nickel, and molybdenum.
Graphite is a nonmetallic material usually classified as a metalloid, because it exhibits characteristics representative of both metals and nonmetals.
Graphite possesses a very high sublimation temperature (figure 4-1), transforming from a solid to a gaseous state without becoming a liquid. Good electrical and thermal properties, along with its machinability, make graphite an excellent electrode material.
There are many brands and grades of graphite available for selection by the EDMer. The performance of each grade is dependent on its particle size, microstructural consistency, and inherent physical properties. To help separate the different graphites they are classified by their particle size. Within each classification there can be a number of grades available for selection (figure 4-2). The reason that there are so many EDM grades of graphite is that each is tailored for specific applications with an expected performance level.
Figure 4-1. Only graphite and tungsten composites remain solid at temperatures remotely near gap conditions. *Graphite does not change to a liquid when heated, but sublimes.
Figure 4-2. Examples of each class of graphite (100x photomicrographs). Each class is as different in performance as in appearance.