Graphene is currently known as the most hard, and has the highest conductivity and thermal conductivity of the material.

　　Therefore, graphene and graphene sheets in the defense industry, aerospace industry, automotive industry, communications industry, energy industry and other fields, will have a wide range of applications. In recent years, many domestic and foreign organizations have focused on the application of the possibility of the role of graphene.

　　Graphene has been in high performance composite materials, display and sensing materials, such as precision electronic functional materials, super capacitors, fuel cells, energy storage materials and other new energy materials application development aspects, have very outstanding performance. Especially in the high performance composite materials, the performance is excellent.

　　It should be emphasized that the graphite has a very large shape ratio (diameter / thickness ratio), and has the basic electrical conductivity and good lubrication, corrosion resistance and high temperature resistance. It is found that graphene is the material which is found to have the highest strength, and the strength of graphene is even more than that of diamond. At the same time, graphene sheets with nano thickness, easy to other materials such as polymer materials, composite, and form a good composite interface. Therefore, graphene can be prepared by high strength, high temperature resistance, high temperature resistance and electrical conductivity. Graphene nanoplatelets can make the original plastic strength, high hardness, can make the coating improved in corrosion resistance, improve the wear resistance; in addition, graphene nanoplatelets would also give composite materials (plastics, rubber, fiber, ink) excellent conductive, antistatic performance.

　　In a word, graphene microchip is excellent modifier, is expected in the polyimide (PI), nylon (PA), polycarbonate (PC), polyphenylene sulfide (PPS), poly (POM) and polyurethane (PU), poly (ethylene terephthalate) (PET), poly (butylene terephthalate) (PBT), polypropylene (PP), polyethylene (PE), polystyrene (PS), acrylonitrile - butadiene - styrene (ABS) etc. all kinds of plastic changes in play an important role in all kinds of plastics, rubber, fiber, ink and other products.

In EDM process, due to EDM parameter selection of inappropriate, or processing chip discharge is not so good reasons to produce concentrated discharge arcing, such as electric sparks are scarce and red sparks, the electric discharge sound has become irregular, then produce the so-called unstable discharge. Processing is not stable, the processing current is not in accordance with the set of the current work, the working current is small, and in the process of processing will appear a variety of processing defects, resulting in waste. Often the following abnormal discharge phenomenon.

　　(1) the discharge is not stable at the start of the process

　　At the beginning of the processing of graphite electrode, due to the workpiece being chip cutting, burr, resulting in concentrated discharge; and due to the discharge energy (high peak current, pulse width), and pulse interval is too narrow, jet pressure is too large, resulting in at the beginning of processing discharge instability. Therefore, the machining process of the first complete removal of the workpiece on the workpiece, burr, and the removal of heat treatment due to the oxide film, paint, rust, etc.. At the start, the current is set at a small value, and then gradually increased to the peak current, and the jet pressure is set to a small number of.

　　(2) a granular projection

　　In the process of electric spark machining of graphite electrode, the machining depth is too deep to produce electric arc, and the granular material is formed on the workpiece, which causes the workpiece to be scrapped (as shown in Figure 1).

　　Cause of:

　　Pulse width is set too large, the angle of the edge of the electrode to generate granular protuberance, thus causing short circuit, causing arcing; electric corrosion product processing chip too much, will not be drained; processing liquid nozzle angle setting is wrong, the machining liquid can not be fully spray into the gap, electric corrosion product processing chip can not be fully discharged; processing depth, processing chips can not be fully discharged, stranded at the bottom.

　　Solution scheme:

　　Shorten the pulse width ton, longer pulse interval toff, inhibit the production of granular protrusion on the generation and electric corrosion product processing chip, and as far as possible to the side of the nozzle is arranged on the electrode; if the processing depth and improve the number of vibrations of the electrode and to accelerate the speed of vibration.

　　(3) machining of the ground

　　In EDM process and pulse interval is too small, the electrode vibration speed slow, jet pressure is weak, resulting in electric corrosion product processing chips can not be fully discharged and many electric corrosion product bonded on the electrode surface, forming carbon block, in the electrode motion of easy detachment, in ground processing produce depression (Figure 2).

　　Solution:

　　Extend the pulse interval, improve the electrode vibration velocity, jet pressure increases, also can in the process to suspend processing, with a brush cleaning electrode surface and processing the bottom surface of the processing chip.

　　(4) the entrance space is large

　　Because of the graphite electrode lifting speed is too slow, at the lower end of the processing time is too long, the absolute loss, and at the entrance to the electrode of the discharge gap exist electric corrosion product, resulting in "secondary discharge" (machined surface due to the involvement of electrical corrosion products such as again for unnecessary discharge), the discharge gap to expand, in the depth direction of processing produce processing angle, entrance space (as shown in Figure 3). Therefore, in the process of processing, the graphite electrode can not be too slow.

　　(5) the process of machining the bottom surface or the bottom surface roughness is not uniform

　　Due to the pulse interval is too small, especially jet pressure uneven, inter electrode gap is too small, electrical corrosion products can not fully discharged, and at the bottom of the machining surface is not uniformly distributed, so along with the processing continuously, on the bottom surface bending (as shown in Figure 4 (a)) or cause at the end of processing rough surface is not uniform (as shown in Figure 4 (b) below).

　　Solution:

　　Increasing the pulse interval, setting the constant jet pressure, increasing the gap between the two electrodes, often check the chip removal.

　Characteristics of graphite

　　1 light weight: the proportion of 1/5, copper, suitable for the production of large electrodes

　　2 machine processing performance is good: cutting resistance for copper 1/3

　　3 surface easy to deal with: no burr after machining, easy manual processing

　　4 high temperature: when the temperature reached 3650 degrees Celsius, it can withstand higher current.

　　5 the thermal expansion coefficient is low: 1/4 for copper, and the heat is not deformed

　　6 discharge machining speed: discharge machining speed than copper block 3~5 times, large area or rough machining more obvious.

　　7 electrode loss is small: the ratio of the copper and copper is 2:4, it can be processed without damage.

　　Machining method of graphite electrode:

　　1 tool requirements: sharp, there is a large front and rear corner and the capacity of the chip

　　2 tool material: carbide knife coating knife

　　3 feed speed: 5500~8000 roughing finishing 4000~6000

　　4 machine speed: rough machining 6500~10000 turn; finishing 6000~20000 turn

　　5 processing margin: rough machining 0.6mm; finishing 0.3mm

　　6 cutting capacity: the amount of lateral cutting is 1/2~2/3, and the depth of cutting is 1~1.5 times of the diameter of the cutter.

　　7 cutting direction: the direction of the cutting is better than that of the cutting, and the milling is better than the reverse.

　　CNC machining speed, cutting, easy dressing: high speed milling rough machining a copper block 3 times, high speed milling finishing a copper block 5 times, and its strength is very high, for high (50-90mm) ultrathin (0.2 0.5 mm) of the electrode, processing is not easy to deform. Many times the product needs to have good ruled surface effect, which requires to do the whole of the electrode and in the production of the whole electrode exists all sorts of hidden corner cleaning, due to the characteristics of graphite is easy to repair, making this problem is easy to solve, and greatly reduce the electrode number, can realize the complex geometry, and copper is unable to do so

　　Rapid EDM molding, thermal expansion is small, low loss: because the conductivity of the graphite is better than copper, so it is faster than the rate of copper, copper, 3-5 times. And the discharge can withstand the larger current, electric spark rough machining more favorable. At the same time, under the same volume, the graphite weight is 1/5 times of copper, which greatly reduces the load of EDM. For the production of large electrodes, the overall electrode has a great advantage. The sublimation temperature of graphite is 4200 for copper 3-4 times (the sublimation temperature of copper is 1100). At high temperature, the deformation is very small (under the same electrical conditions for copper 1/3-1/5) is not softened. Can be efficient, low consumption of the discharge energy transfer to the workpiece. Because of the high temperature strength of graphite, the discharge loss (1/4) can be reduced effectively, and the quality of the process can be ensured.

　　Light weight, low cost, production costs for a set of mold, electrode CNC machining time, EDM, electrode wear accounted for overall body most, and these are determined by the electrode material itself. Graphite and copper are compared, the machining speed and CDM speed of graphite are 3-5 times of copper. At the same time, the characteristics of wear minimum and the overall graphite production, can reduce the number of electrodes, but also reduces the electrode material and machining time. All of these can reduce the production cost of the mold.

　　How to use graphite materials and other relevant factors to consider is, of course, die by the copper electrode conversion of graphite electrode. First of all, should be clear. Today part of the spark machine customers using graphite electrode discharge machining, which eliminates the mold cavity polishing and chemical polishing process can still achieve the desired surface finish. Without increasing the time and polishing process, the copper electrode can not produce such a workpiece. In addition, graphite is divided into different levels, in a specific application using the appropriate levels of graphite and electric spark discharge parameters to achieve the ideal processing effect, if the graphite electrode spark machine uses the operating personnel and the copper electrode of the same parameters, then the result would is disappointing. If you want to strictly control the electrode material, the graphite electrode can be used in the rough machining non loss state (loss of less than 1%), but the copper electrode is not used.

　　1. The increasing complexity of the mold geometry and the diversification of product applications require more and more high discharge accuracy of the EDM machine. The graphite electrode has the advantages of easy processing, high removal rate and low loss of graphite. Therefore, some of the spark machine customers have abandoned the copper electrode and switched to the graphite electrode. Also some special shape electrode cannot be made of copper, but graphite is easy molding and copper electrode is heavy, not suitable for the processing of large electrode, these factors caused part of the spark client application of graphite electrode.

　　2.Graphite electrode is easy to process, and the processing speed is significantly faster than the copper electrode. For example, the use of the milling process of graphite, the processing speed is 2 ~ 3 times faster than other metal processing, and no additional manual processing, and copper electrodes need to be put on the ground. Similarly, if the use of high speed graphite machining center manufacturing electrode, the speed will be faster, more efficient, it will not produce dust. In these processes, the selection of suitable tools and graphite can reduce the wear and tear of the cutting tool. If more specific milling time of the graphite electrode and the copper electrode and graphite electrode, graphite compared to copper electrode fast 67%, in general electrical discharge machining by graphite electrode machining than the copper electrode fast 58%. As a result, the processing time is greatly reduced, but also reduces manufacturing costs.

　　3.The graphite electrode and the traditional copper electrode design. Many die companies usually in copper electrode of rough machining and finish machining have different amount of reserve, and graphite electrode is used almost the same amount of reserved. This reduces the number of CAD / CAM and machine processing, for this reason alone is sufficient to improve the accuracy of the mold cavity in a large extent.

　Compared with metal materials,the carbon carbon composite material has outstanding features.

　　1、Quality light

　　The density of carbon carbon composite material is only 1/4 of the steel,after using the carbon carbon composite material,it can can greatly reduce the quality of the aircraft. For example,it can reduce 816.5kg of Boeing 747-400,and can reduce 998kg of A340.

　　2、Energy load

　　Energy load is the index of absorbing the kinetic energy of the brake device,it’s usually indicated by the heat load ratio of the friction surface,the unit is J/cm2.As the heat capacity of C/C composite braking material is much higher than the powder metallurgy metal base material,so the energy load of carbon carbon composite material is 3 times higher than the powder metallurgy metal base material.

　　3、High temperature resistance, no deformation and no bonding phenomenon.

　　Because of the high strength under the high temperature,high thermal conductivity and low coefficient of thermal expansion,so the seismic performance is much better than the metal base braking material,and in the course of using, the deformation will not occur.

　　4、Good friction and wear,long service life.

　　Some researchs show that:during the brake processing,the friction coefficient of the carbon carbon composite material is stable,the curve of moment and the time is smooth,under the different conditions of speed, specific pressure and energy load,the property of friction is very stable;if ander the same conditions,it will show a good repeat performance,although under a high energy load,the moment will reduce little.

　　The carbon carbon composite material will have a good self-lubricating property after appropriate graphite processing,the friction rate will be 1/5 of the metal base material,so the service life will be longer,the surface of the friction will be smooth and bright. In addition,the C/C composite material can be used in overload condition.

　　C/C composite material was used for braking in the airplane for the first time in 1973,and now,over 60% of the carbon carbon composite material still be used for the airplane braking material,meanwhile, C/C composite material is also used for rocket nozzle, missile nose cone,parts on for re entering the atmosphere, racing car,high speed rail,tank etc.

　　When the plane take off and land,the kinetic energy is very large,the kinetic energy of Boeing 767 aircraft is about 670MJ.the function of the brake disc is to make the airplane,with high kinetic energy and high momentum,slow and stop. The dynamic and stationary disc of aircraft will rub against each other to absorb the huge kinetic energy.the C/C composite material belong to the brittle materials,the energy generated during the friction can not transform into other form through plastic deformation,so it can only transform into thermal energy,so the temperature of the surface between dynamic and stationary disc is very high,it can reach 2000℃ at the moment. So that could have massive significance to do the research of C/C composite material.

　　The influence of thermal conductivity of the C/C composite material is very complex. Different kinds of fibers and volume fraction,different kinds of matrix carbon and weave pattern,all these factors can lead to the thermal conductivity with a big difference. For example,there are different fiber type,such as: Nylon base,PAN base, isotropic pitch base and mesophase pitch base; The matrix carbon has thermosetting resin carbon, thermoplasticity pitch carbon and pyrolytic carbon vapor deposition;the weave pattern has fiber felt, non weft fabric,3D weave and 2.5d weave and 2d lamination etc. One of so many factors changed can have a big influence on the carbon carbon composite material.

　　Graphite is a super material of chemical industry, the chemical stability of the very prominent, acid and alkali resistance, corrosion resistance, in the chemical industry has a special position.

　　Natural graphite with bulk graphite and flake graphite is the best, the most widely used. I haven't found massive blocky Graphite Deposit, flake graphite and aphanitic graphite has a large deposit, and formed a large-scale mining base.

　　Natural graphite has many excellent properties, so it is widely used in various sectors of the national economy, especially in the metallurgical, mechanical, electrical, chemical industry has been widely used.

　　Graphite has outstanding resistance to high temperature and low temperature resistance properties, a small expansion coefficient, volume stability, so it can anti quench heat changes, the graphite resistance high temperature properties and general metal different when the temperature rises, it is not tend to soften, and its strength instead of increased.

　　Graphite is a good conductor of heat and electricity, similar to how the heat transfer performance and conductive performance and most metals, but also its characteristics, general metal when the temperature increases, thermal conductivity increased, and the thermal conductivity of graphite on the contrary, at elevated temperature, the thermal conductivity instead. At extremely high temperatures, the graphite tends to be insulated, and thus, in the case of high temperature, graphite Heat insulation performance is very reliable.

　　Graphite plasticity and lubrication has been obtained in industry, larger scales, friction coefficient is small, so graphite flake is the most slippery, lubrication best, dense crystalline graphite times, earthy graphite lubricant is poor.

　　Graphite or atomic reactor excellent moderator, the use of graphite has entered a new historical stage, in order to manufacture excellent retarding agent, graphite purification process was further developed, so graphite has become important materials in atomic energy industry.

　　Because of its special structure, graphite has special properties

　　1) high temperature resistant: Graphite melting point of 3850 + 50 DEG C, the boiling point of 42.5 DEG C, even the ultra high temperature arc ignition, weight loss is very small, coefficient of thermal expansion is also very small. Graphite strength increases with temperature, and the intensity of graphite is increased by one time at 2000.

　　2) the conductivity of graphite is one hundred times higher than that of general non metallic mineral. Thermal conductivity over steel, iron, lead and other metal materials. The thermal conductivity decreases with the increase of temperature, even in high temperature, graphite as insulator.

　　3) the lubrication performance of graphite is determined by the size of graphite flake, the bigger the scale, the smaller the friction coefficient, the better the lubrication performance.

　　4) chemical stability: graphite has good chemical stability at room temperature, acid, alkali and corrosion resistance to organic solvents.

　　5) plasticity: the toughness of the graphite is good, can be a very thin slice.

　　6) thermal shock resistance: graphite can withstand the temperature of the severe changes in the temperature of the temperature change and not cause damage, when the temperature is changed, the volume of graphite is not big, will not produce cracks.

　　The main factors of the influence of the carbon carbon composite material include the type of carbon fiber, volume fraction and the type to reinforce,the type and structure of the matrix carbon,material density and the heating temperature(graphitization) etc.the mechanical property of the carbon carbon composite material will increase with the increasing of the density,but the heating will reduce the strength of the C/C composite material.

　　As to the densification process of embryo body is different,so the mechanical property of carbon composite material is usually different,different scolars will get different results.In general, we think that CVI carbon and the carbon fiber can form a good combination,so the CVI can make the carbon carbon composite with a better mechanical property than the liquid phase impregnation.

　　Actually ,no matter CVI or liquid phase impregnation to treat the carbon carbon composite material,the structure of the matrix carbon has a great influence on the mechanical property,so to the C/C composite material used aviation brake,the design of matrix carbon is very important to the mechanical property.And the processing of the matrix carbon also need to consider the cost of the raw material and other properties,like heating conductivity and the friction property.

　　The volume fraction of the the carbon fiber is also a very important parameter.if the content of the carbon fiber too short,the function of the carbon fiber is hardly to show up.the mechanical property of the C/C composite material will increase with the increasing of the carbon fiber content,when it reach to a certain limit,the property will reduce will the increasing of the carbon fiber.

　　To the carbon composite material with CVI processing,another influence is the structure of the CVI carbon.but the research on this part is short,especially for the 2D and 3D C/C composite material.the influence of the CVI carbon to the carbon carbon composite material need the further research.

　　The physical properties of carbon/carbon composite material are mainly included:density, degree of graphitization,hardness, thermal conductivity, electric conductivity, specific heat, coefficient of thermal expansion etc.Among these properties,the density,thermal conductivity and the graphitization are very important to the carbon carbon composite material which is used for aviation brake.

　　The density can be the direct response of the C/C material,and it’s the basis of other properties,and the density depends on the density of the carbon fiber and the matrix carbon and the porosity.

　　The degree of the graphitization is another important parameter of the C/C composite material,it has a close contact with the strength and friction property.now,there is no uniform method to test the graphitization degree,the ways to test it are mainly included XRD, laser Raman spectrometry, magnetic resistance method, electrical resistivity method etc.

　　The thermal conductivity is also important to the carbon carbon composite material,it’s a symptom during the heat absorption and heat release. In the brake process,the C/C material must have the ability to absorb and release much heat to make the brake device work well.the thermal conductivity of C/C material depends on the carbon fiber and matrix carbon,and the structure, density ,and the graphitization also have a contribution to the thermal conductivity. The volume fraction and the direction of the carbon fiber have a big effect on the thermal conductivity of the carbon carbon composite material.Some researches show that the larger density can lead to a higher thermal conductivity.