W-Cu Thermal and Electrical Conductivity Comparison of Different Process

Except test of hardness, density and micro-structure, for tungsten copper composite materials, which is widely used in EDM electrode, high-voltage discharge tube and heat sink, test of thermal properties (includes thermal conductivity and the coefficient of thermal expansion) and electrical conductivity are also essential. Combined with micro-structure of them, the researchers conclude some main reasons:

1. Although tungsten copper composite material has uniform distribution of W and Cu phase in the molding process, it still remains some porosity, which has a great impact on thermal conductivity of tungsten copper composite material;

2. Under the optimum Cu infiltration, there is no pore inside the sample, but Cu phase can not connect and form net structure. W and Cu phases distribute unevenly, in the process of heat conduction, part of thermal conductivity convey by W phase so that it is a critical factor of the lower thermal conductivity;

3. Relatively, injection molding process can effectively avoid the two defects, not only improve the density of tungsten copper products, but also W and Cu two-phase evenly distributed, thus it has a higher thermal conductivity.

Theoretically, thermal expansion of solid materials is due to the thermal vibrations of atoms as a center from its equilibrium position, which called crystal vibration non-harmonic effect. When the sintering temperature is increasing, atomic vibrations also stronger, the greater the energy of atomic vibrations, so that the microscopic atomic lattice parameters increase, the macro is manifested in the thermal expansion of solid materials. For single-phase material, the thermal expansion will increase as the temperature rises.

While for tungsten copper (W-Cu) two phases heat sink material, it has lower coefficient of thermal expansion, which thermal expansion behavior is much more complex than a single-phase material. The experiment shows that t lower temperatures, tungsten-copper composite material showed a negative thermal expansion, but only when the temperature exceeds a certain value showed positive expansion. Tungsten copper sample coefficient of thermal expansion of injection molding and compression molding process under more stable than copper infiltration sample, the magnitude of change is smaller.

This is due to the phase change, as well as the internal organization of the reasons magnetic stretch, thermal expansion of the material will show some special law. By increasing the degree of constraint W phase at elevated temperatures in the expansion phase of Cu, thereby reducing the thermal expansion coefficient of tungsten copper composite material. In addition, since the difference of the coefficient of thermal expansion of the materials, tungsten copper composite material will produce complex stress inside, whose distribution will restrain the thermal expansion behavior.

As for the electrical conductivity, it was detected by eddy current method. When an alternating current is cut coil (also called probes) near the surface of a conductive material, since the coil alternating magnetic field, it has an effect on the material surface and near surface induced swirling current, which called the vortex. Materials and eddy currents generate their own magnetic field coil reacts, which is related to the size of the surface conductivity near the surface. Non-ferromagnetic conductive material can be directly detected by eddy current sensor. After testing found that the sample injection molded tungsten copper has the highest conductivity, reaches 37.43%IACS, which is higher than molding sample (29.85%IACS) and infiltrated sample (33.18%IACS).

Tungsten Copper Properties Comparison of Different Process

The commonly used molding technologies are molding process, extrusion molding and injection molding, wherein the molding process and injection molding process widely used in tungsten copper materials. Molding process is also known as compression molding or press molding, it is a kind of processes that put first powdered, granular or fibrous plastic into the mold cavity at a molding temperature, and then close the mold to shape and solidify. Molding process has many advantages, such as less raw material wastage, low cost of equipments, one-step forming, suitable for plate with large size. But it has long lead time, low efficiency, not suitable for products with complex structure or shape, the size is limited by the compressor.

The injection molding process here refers specifically to metal powder injection molding (MIM), it is a kind of new near net shape powder metallurgy technology that introduce from modern plastic injection molding technology into the field of powder metallurgy. The basic process is that Firstly, the solid powder with an organic binder uniformly kneaded, granulated after heating under plasticized state (~ 150 ℃) with an injection molding machine into the mold cavity solidifying-forming, and then by chemical or thermal decomposition method of forming the blank the binder removal, the final densification to give the final product. While the advantages of MIM are high density, high product consistency, a wide range of applications and can be formed smaller in size and shape of the structure of complex products. However, it has high requirements for the injection, high cost of mold design and only suitable for small batch production.

Take W-Cu tungsten copper product as a example to compare molding process with injection molding of tungsten copper material properties. W-Cu theoretical density is 17.28g / cm3, the trend of relative density and relative density. As we can see from this graph above, With increasing sintering temperature, density of the obtained two processes copper tungsten composites are showing trend of increased first and then decreased. The difference is the temperature of the maximum density, the highest density  of molded samples at 1350 ℃, reaches 17.16g / cm3, while the sample injected at 1400 ℃ highest density is 17.17 g / cm3. After the further increase in temperature, both the density began to decrease. This is due to the sample over-burnt, Cu phase inside of tungsten copper exudates to surface and the pores and defects left in side. Under the optimum sintering temperature, the density of injected sample reaches 99.31%, molded samples reaches 99.78%. In addition, the hardness of tungsten-copper composite material (HRB) was positively correlated to its density, the greater the density, the hardness of the respective materials is also greater.

Besides, we also compare the micro-structure of tungsten copper with different processes. The specific procedure is that milled, polished tungsten copper composite sample after etching with FeCl3 salt solution for 2 minutes, with water to clean the surface of the etching solution, then wipe the surface with alcohol cotton, and then drying, Finally placed under microscope observation of the metallurgical microstructure. By comparing the SEM photomicrographs and photographs can be found that tungsten copper molded sample of W, Cu two-phase distribution is more uniform, uniform grain size, but segregation occurs at a constant temperature inside, causing a regional copper-rich, there are some porosity. While tungsten copper sample by injection molding, W and Cu distribute uniform and does not exist W-rich phase and Cu-rich phase of the phenomenon and Cu phase forms net structure around tungsten grains.

Tungsten Copper Heat Sink Electrical Conductivity

Except Rockwell hardness (HRB) of detection, it is essential that electrical conductivity measuring in tungsten copper composite materials as heat sink materials. Electrical conductivity is an important indicator to measure ability to conduct electricity of tungsten copper heat sink, which directly affects the final properties of products. The stronger ability to conduct electricity, the smaller electrical resistance, on the contrary, electrical resistance is greater. For hardness testing, if tungsten copper material over-burning, the inside Cu phase will exude and lead to the emergence of the hole, making the overall decrease in hardness; while the precipitation of Cu phase of segregation occurred, and thus will make the hardness not dropped significantly. Based on the theory, the relevant researchers have concluded that the most likely two factors tungsten copper electrical conductivity of heat sink, one is copper content, the other is the porosity.

Theoretically, tungsten copper composite material is composed of tungsten (W) and copper (Cu). W phase has higher hardness and melting point, lower electrical conductivity; on the contrary, Cu has lower hardness and melting point, but it has excellent electrical conductivity. Therefore, it can be speculate that electrical conductivity of tungsten copper heat sink material depends on how much copper content, the higher the copper content, the corresponding tungsten-copper composite conductivity is higher. From chemical energy band, different metal has different equivalent orbital and atom distance, which band (or empty) overlap, constitutes a full conduction band, and easily to have a metallic conductivity. So as long as there is not full of the conduction band, whether it itself is not full band, or the empty band and full band formed by overlapping unfilled band, under the action of external electric field will form a directional flow of electrons, thereby such that the material has conductivity.

Under the intervention of external electric field, the outermost valence band electrons gain a little extra energy without violating the exclusion principle to reach many empty places within the band near. Compared with thermal excitation out-of-order, Excited electrons by the electric field and the field in the direction opposite to the momentum, it will produce a collective motion in the crystal, thereby forming current. For tungsten copper heat sink, due to different equivalent orbital and atom distance of W and Cu atoms, the band (or empty) overlap, constitutes a full conduction band, and has excellent electrical conductivity. For copper with divalent, its valence band is full band, which the valence band and a higher band overlaps, filled with electrons can occupy the vacated belt so that it has good conductivity; while tungsten with hexa-valence, its valence band is not full and the electrical conductivity is lower than copper phase. So this is a good proof of the conductivity of tungsten copper heat sink materials or conductivity depends on copper content.

The other influencing factor is porosity, which will hinder the movements of electrons. In other words, the higher porosity, the lower tungsten copper heat sink electrical conductivity. Due to tungsten copper is a kind of pseudo alloy, which composed of two kinds of metals with a big difference, there are a lot of interface and porosity between W and Cu grains. The experiment shows that tungsten copper composite material and its electrical conductivity were positively correlated with the density, the lower the density, the lower the conductivity. In addition, sintering temperature controlling is also important, electrical conductivity of tungsten copper has a sharp decline when it over-burnt. The main reasons comprise two, one is inside W phase has severe segregation and porosity, and as the main medium and Cu conductive phase may also undergone some degree of segregation, such that its penetration to the surface, the conductivity decreases with the decreasing content of Cu inside.

Tungsten Copper Electrode and Other Materials Comparison — Part II

Some researchers put graphite electrode into molten metal (such as Cu or Al) in order to improve the properties of graphite electrode in EDM. At the same time, pressing the liquid metal and make Cu or Al fill in the pore of graphite electrode for improving the strength and thermal conductivity of the electrode.

Except graphite, Cu, Cu-based and Cu composite material is also widely used as electrode in EDM, ECM or other machining.

2. Cu, Cu-based and Cu Composite Material

Copper (commonly known as red copper), which is a kind of electrode material, especially in machining non-ferrous metal. Cu has low melting point so that its wastage is high, the lifespan is short and the cost too much, which requires that other material with high melt point to improve its properties.

Tungsten copper (Cu-W) not only has high melt point, low coefficient of expansion and spark resistance, but also has good conductivity so that it is a new kind of high-performance electrode material. Tungsten copper electrode is widely used in die steel and WC work-pieces, the common ratio of Cu/W is 25:75. However, the higher price than graphite or copper, tungsten copper spreading has been blocked to some extent.

Some researchers use copper, tungsten copper, brass and aluminum as electrode for hardened steel machining. The result shows that Cu and Al have higher speed and better precision, Cu and tungsten copper (W-Cu) has the lowest electrode wastage and brass electrode wastage is the largest. Therefore, Cu is a kind of ideal electrode for hardened steel machining, which has low wastage of electrode and high MRR (Material Removal Rate).

In another experiment, researchers use copper (Cu), tungsten (W) and tungsten copper (Cu-W) as electrode for tungsten carbide machining. The result shows that tungsten copper (Cu-W) can remarkably improve the machining efficiency. What’s more, the wastage of electrode is lower under the low voltage so that Cu-W is the most ideal material for tungsten carbide machining. In addition, the researchers also use different content of Cu and ZrB2 or TiSi as electrode for EDM by PM (Powder Metallurgy). Compared with graphite, copper and tungsten copper, TiSi/Cu has higher wastage of electrode, the machining surface is rough and the efficiency is not so well so that it is not suitable used as electrode. While ZrB2/Cu can be used as electrode, but the combining power between Cu-base and ZrB2 is weak, and the content of ZrB2 will has an effect on EDM properties.

Besides graphite, copper, and Cu-based alloy, Cu-based composite material is also a kind of common materials of electrode.

3. Cu-based Composite Material

Cu-based composite electrode has good conductivity and thermo-plasticity and use air and water as medium, which usually for EDM or polishing. It is composed of 60%-65% solid carbon material (such as graphite powder, graphite slices or carbon nano-tubes and other mixture) distributes thermoplastic matrix and soften over and over again for molding. Compared with graphite and tungsten copper electrode, it has many advantages, like low cost, available for complicated shape and better efficiency, etc.

However, due to its density is much lower than tungsten copper, the electrical resistivity is higher and the wastage of electrode is increasing. This composite material is under development, which requires that low electrical resistivity, low coefficient of expansion, thermal cycling resistance, excellent conductivity and good dimensional stability in liquid.

Besides graphite, copper, and Cu-based alloy, Cu-based composite material, diamond is also a common kind of material of electrode. However, due to the cost of diamond is much higher than graphite and the size is limited, there are some scientists do researches about that.

4. Diamond

CVD (Chemical Vapor Deposition) diamond, which mixed by carbon-containing gas and oxygen was excitation-decomposition under the low standard atmospheric pressure, and come into active diamond carbon atoms. They deposit at the matrix and grow up to polycrystalline diamond (or monocrystalline diamond, mono like diamond).

In order to make it have the same properties of tungsten copper (such as conductivity, electrical resistance), researchers add boron (B) during CVD, which has excellent adsorption to oil medium in electrical discharge machining (EDM). CVD diamond has high MRR (Material Removal Rate) and low wastage of electrode, especially it can work in high current density under certain condition, which Cu, graphite and tungsten copper is unavailable. In addition, there is another kind of PCD (polycrystalline diamond), which has similarities with CVD diamond and its cost is lower than CVD diamond so that it is a kind of ideal material for electrode.

In Micro- EDM, the electrode will increase the energy of spark in unit area, which increase the wastage and decrease the precision of machining. If we change the parameters to reduce the discharge energy unit area, the processing will be longer and efficiency will decrease. Therefore, the choice of electrode materials is also important for Micro-EDM, the common electrode materials are tungsten (W), tungsten carbide (WC) and tungsten copper (W-Cu), etc. Drilling and milling in Micro-EDM often uses tungsten (W) or tungsten carbide (WC) rod or tube as the electrode of tools.

In conclusion, we should choose different materials of electrode according to different process parameters or different work-pieces. Therefore, learn some methods, materials, shape, customer requirements and cost performance before we choose electrode is significant to the quality of products. In addition, tungsten copper and other new materials will promote the development of EDM (Electrical discharge machining).

If you have any interest in our tungsten copper alloy products, please do not hesitated to contact us by Email:sales@chinatungsten.com or by telephone:86 592 5129696, we are at your service.

Tungsten Copper Electrode and Other Materials Comparison — Part I

The using of electrode material has a huge effect on electrical discharge machining (EDM), such as the wastage of tool, MRR (Material Removal Rate), the surface quality of work-piece and so on. Therefore, the choice of electrode material should not be ignored in EDM and it depends on the situation.

First of all, the electrode should have high melting point, good conductivity, low coefficient of expansion and some excellent mechanical properties so that it can be thermal resistance, not easy to deformation and has low wastage in EDM. Theoretically, the micro crystal structure of electrode is beneficial for decreasing the wastage of electrode and properly reduce the size of grain will be better. In addition, the material of electrode should pledge stability, good quality of surface, reasonable price and easily for machining to EDM.

With the development of EDM, the material of electrode and the related manufacture makes a great progress. At present, there are so much electrode materials, such as graphite, tungsten (W), copper (Cu) and some other metal or non-metal elementary substances. In addition, steel, cast steel, Cu-based and W-based composite materials or diamond can be also used as EDM electrode.

1. Graphite

As for the aspect of chemical, graphite is an allotrope of carbon element, which covalently bound covalent molecular bonds. Its has high boiling point and melting point, the melting point up to 3652 ℃, the boiling point of 4827 ℃, insoluble in water. And it also has excellent electrical and thermal conductivity, chemical stability and workability so that it is widely used as EDM electrodes. In addition, graphite also has different kinds, which can be specifically divided by graphite particle density, the size and mechanical and electrical properties.

Fine graphite has small grain, porosity and good mechanical strength, which has low wastage of electrode in EDM. But its MRR (Material Removal Rate) is undesirable. At present, the most of graphite grain size is below 10μm, which depends on the conditions electrode works (finishing, semi-finished or roughing) and its shape when we make a choice. In addition, the roughness of work-piece surface is related with the size of graphite grains, the average particle size below 1 μm is specialized for finishing.

Compared with other materials of electrode, graphite can be used in EDM with large discharge current, which remarkably increasing the efficiency. Although graphite has low proportion and price, it is too fragile to machining into the shape of thin. Therefore, it is limited in EDM and researchers find that high speed milling can be improved to some extent.

Some researchers put graphite electrode into molten metal (such as Cu or Al) in order to improve the properties of graphite electrode in EDM. At the same time, pressing the liquid metal and make Cu or Al fill in the pore of graphite electrode for improving the strength and thermal conductivity of the electrode.

If you have any interest in our tungsten copper alloy products, please do not hesitated to contact us by Email:sales@chinatungsten.com or by telephone:86 592 5129696, we are at your service.

Effect of TiC in Copper-tungsten Electrodes

Due to the high thermal conductivity of Cu, and the better spark erosion resistance, low thermal expansion coefficient and high melting temperature of W, copper-tungsten electrodes have been widely used for machining die steel and tungsten carbide work-piece. The materials normally used in EDM electrodes are various types of copper, graphite, tungsten, brass and silver. But copper-tungsten has better properties. The low melting point of Cu reduces the resistance to electrode wear.

At high tungsten content, there is porosity in the liquid phase sintered electrode due to the insolubility between the Cu and W. This greatly impedes densification during the solution-reprecipitation stage of liquid phase sintering. Hence, this makes it necessary to introduce another material with high melting point. Materials having good electrical and thermal conductivity with a high melting point are used preferably in copper-based electrodes to resist electrode shape-loss.

Titanium carbide (TiC) is an extremely hard refractory material with high melting temperature, and high thermal shock and abrasion resistance. It is used mainly for powder metallurgical parts including cutting tool tips, dies, wear parts and resistant coating. In industry, the manufacturing of Cu-W composites is usually done through infiltration of Cu into a porous, pre-sintered tungsten compact, or through the liquid phase sintering of compacts pressed from mixed powder.

In some researches, Cu-W/TiC was investigated and fabricated through liquid phase sintering. In is reported that the additive metals (iron, cobalt and nickel) can enhance densification in the liquid phase sintering of Cu-W. In order to increase the densification of the Cu-W matrix, nickel (Ni) was introduced. In addition, the densification of Cu-W/TiC sintered electrodes can be improved by the addition of Ni. However, due to the insolubility of Cu, W and Tic, and the amount of Cu apparently reduced by the cold welding in ball milling, porosity cannot be avoided. Nevertheless, with increasing TiC, the distribution of the particle size becomes narrow.

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LED Tungsten Copper Heat Base for Heat Dissipation

With the world's attention of environmental awareness, energy saving has been an irresistible trend of the moment. The LED industry is one of the fastest growing industries; LED products not only have a great advantage in terms of energy saving, but also have high efficiency, fast response time, long life cycle and do not contain toxic substances, which is outstanding in similar products. In general, the input power of LED high power products has about 15% of electrical energy into light energy, and another 85% of the electrical energy is dissipated into heat. So if the LED light emitting heat generated not been able to export, it will make the temperature of the LED screen is too high, thus affecting its luminous efficiency, stability and product life cycle. In order to improve the luminous efficiency, thermal dispersion management and design LED system become an important research topic.

The ways of heat dissipation mainly includes: air heat dissipation, heat the substrate to export, export gold wire cooling, through-hole heat dissipation and so on. Here we introduce the substrate cooling. In LED products typically requires multiple LED assembled on a circuit board. In addition to the circuit board is responsible for carrying LED module structure, on the other hand it also needs to play the role in heat dissipation. LED heat dissipation substrate mainly takes advantages of the excellent heat conductivity of its thermal substrate material to derived from the LED grain. Thus according to the ways of LED heat dissipation, can be divided into two types, LED grain board and the system circuit board. These two are multiplied by different heat dissipation board carrying the LED chip LED grain and the LED grain emits light generated by the circuit board to the system, and then absorbed by the atmosphere via the heat dissipating board LED die, to achieve the dissipation of the heat effect. Tungsten copper material has high strength, low coefficient of thermal expansion, excellent plasticity and thermal and electrical conductivity, which is the perfect choice for LED board. Compared with single metal, such as Al substance, it can avoid the thermal mismatch and have better stability and heat dissipation.

If you have any interest in our tungsten copper alloy products, please do not hesitated to contact us by Email:sales@chinatungsten.comor by telephone:86 592 5129696, we are at your service.