Graphite Advantages

Graphite Advantages

Graphite has many advantages that have made it the material most widely used for EDM electrodes. According to Google, "Graphite has outstanding electrical conductivity, chemical resistivity, and mechanical properties, which make it a preferred choice in a variety of industrial applications."

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Key Advantages of Graphite

Graphite is highly prized in various industrial processes due to its multifaceted benefits. Here is an overview:

• It is easy to machine.
• It is very resistant to thermal shock.
• It has a low coefficient of thermal expansion (3 times lower than copper) which guarantees the stability of electrode geometry during electro discharge machining.
• It is available in large blocks.
• It does not melt, but goes directly from the solid state to the gaseous at 3,400°C,
• which reduces wear.
• Its density is 5 times lower than that of copper, which results in lighter electrodes.
• It provides a higher metal removal rate than copper with less wear.
• It has the unique characteristic that the wear ratio tends to decrease as the peak current increases.

More About Graphite

Graphite used for EDM machining is an isotropic material with a grain size ranging from a few microns to about 20 microns. In the 1970s, improvements made by graphite manufacturers (isotropic properties, consistent quality, large size billets) combined with the emergence of EDM machines equipped with iso-plus generators, allowed graphite to become the most commonly used material for EDM machining electrodes.

Three separate groups of graphite can be defined:

• Large grain graphite (about 20 µm) with low densities (1.76 g/cm3)
• Fine grain graphite (~10 µm) of high density (1.82 g/cm3)
• Very fine grain graphite (~4 µm) with densities greater than 1.86 g/dm3

Larger-grained graphite is used for machining in roughing modes, while fine-grained graphites produce the best surface finishes for finishing operations. As graphite has become more affordable, EDM machining shops will often inventory two or even three types or grades of graphite: a less expensive large-grained graphite for the roughing operation, followed by a finer-grained graphite for finishing or a combination of both roughing and finishing performance; and possibly an expensive very fine-grained graphite for fine finishing and precision operations.

Why Graphite?

Graphite has several advantages over other materials. It is resistant to thermal shock. It is the only material in which mechanical properties increase with temperature. It has a low CTE for geometrical stability. It is easily machined. It does not melt but sublimes at very high temperature (3,400ºC), and finally, its density is lower (five times less than copper) which means lighter electrodes. Graphite removes material better than copper or copper-tungsten while wearing slower. The wear rate tends to diminish as the discharge increases, unlike copper, whose wear increases at higher currents. Therefore, graphite is suited for the machining of large electrodes since working with a high current intensity provides decreased roughing time.

Although graphite is prone to abnormal discharge, this can be eliminated through quality flushing and lowering the intensity of discharge during negative polarity machining. However, as a result of this tradeoff, machining tungsten carbides is more difficult than with copper-tungsten electrodes. Also, since graphite is a ceramic, it is sensitive to mechanical shock and consequently must be handled and machined with care.

Comparing Graphite Grades

It is not advisable to compare a grade of graphite to another just by looking at physical properties without also performance testing the graphite in actual EDM operations. However, the following is a list of physical properties of graphite that exhibit some effect on performance in EDM operations.

• apparent density
• average grain size
• electrical resistivity
• flexural strength
• hardness

Advantages and Disadvantages of Graphite As a Reactor Material

The advantages and disadvantages of graphite as a reactor material are as follows:

Thermal Properties

The graphite has a high scattering cross-section and a very low thermal neutron absorption cross-section, the higher scattering cross-section is used to slow the neutron, and the lower absorption section prevents the neutron from being absorbed, allowing the nuclear reactor to use a small amount of fuel to achieve critical or normal operation.

High Temperature Resistance

Graphite is a high-temperature resistant material, its three-phase point, 15MPa is 4,024 ℃, so can not be used in melting, casting, forging and other thermal processing methods can only be used similar to powder metallurgy method. It is not as strong as the metal as the temperature decreases, but slightly increases, in 2000 ℃ the following applications, there will be no problem.

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Thermal Conductivity

Graphite has good thermal conductivity, in the reactor can effectively reduce the temperature gradient, do not produce too much thermal stress.

Stability

Graphite property is very stable. In addition to high temperature oxidation, water vapor, can be acid, alkali, salt corrosion, and thus can be used as molten salt nuclear reactor and uranium-bismuth nuclear reactor core components.

Radiation Resistance

The radiation resistance of graphite is excellent and can serve for 30-40 years in a long period.

Processability

Graphite can be processed into various shapes and components.

Availability

Graphite is rich in raw materials, cheap, easy to produce high-purity, strong, different density requirements of a variety of nuclear graphite, but graphite also has shortcomings, it is anisotropic crystal structure, in a layered distribution, atoms are concentrated in a, B-plane, the same layer of the nearest distance of 0.141nm, each other for the covalent bonding, with strong adhesion, and the layer distance of 0.335nm, the cohesion of the interlayer is Vandeval force and the binding force is weaker. The anisotropy is strongly manifested in the physics, intensity and irradiation of graphite.

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