Tungsten Copper Sintering Technologies Comparison

There are many kinds of tungsten copper sintering technologies, such as conventional infiltration, mixed-pressing, pressure sintering and new SPS (Spark Plasma Sintering), etc, which have their own advantages and disadvantages. Next, we analyze these different sintering technologies by the principle and the applications. First of all, infiltration, which is called melt immersion, is the most widely used in tungsten copper products currently. It is obtained by adding a small amount of tungsten powder or copper powder compacts made of tungsten powder and tungsten and copper infiltrated compacts stacked together, and thereafter sintered at a temperature above the melting point of copper and under reducing atmosphere or vacuum.

In the process of sintering, copper in a molten state to rely on capillary action infiltrated tungsten skeleton, two sintering and infiltration step can also be performed separately. Infiltration has many advantages, such as high density, excellent sintering properties and electrical and thermal conductivity, good arc ablation resistance. But liquid copper barely rely on capillary forces infiltrated tungsten skeleton, this will lead to coarse and uneven distribution of copper particles sintered tungsten particles at high temperatures easily grew up together. In addition, after infiltration machined to remove excess copper and will increase costs and reduce the yield.

Mixed - pressing sintering process is one of the most conventional powder metallurgy, which basic process is preparation →ball milling → compacting →sintering. Sintering can be also divided into two parts, one is solid phase sintering (temperature lower than the melting point of Cu), the other is liquid phase sintering (temperature higher than the melting point of Cu). Since the solid solubility in the liquid phase W Cu is small, the material can not be conveyed by dissolution and precipitation particles rounded manner, and tungsten and copper infiltration is poor, it is difficult to achieve densification.

In order to improve mixed-pressing, relevant scholars uses activated agent (Ni, Pd and other activated elements) to increase the solubility of W in liquid phase, which is beneficial for the sintering process. However the additives have a bad influence on the electrical and thermal conductivity of tungsten copper products. Pressure sintering is a kind of secondary processing after sintering, especially for the conditions that one-step forming can not meet the requirements. Although hot pressing, repressing and re-sintering and HIP (Hot Isostatic Pressing) can remarkably improve the properties of tungsten copper composite material, they are limited by the costs and the efficiency. In addition, using hammering or cold rolling of tungsten-copper composite material for secondary processing can greatly increase the density and mechanical properties of the material. But when the content of W is higher, the ductility of tungsten copper will decrease and the difficulty of machining will increase.

Spark Plasma Sintering (SPS), which is also known as Plasma Activated Sintering (PAS) or Field Assisted Sintering Technology (FAST). According to different powder media, SPS can be divided into conductive and non-conductive, there is a big difference in mechanism between them. Generally, conductive SPS will produce a large amount of Joule heat generated by DC pulse current graphite mold; and the current through the powder will induce electric discharge between powder particles and arouse plasma. With the increasing density of plasma, High-speed reverse movement of the particle surface of the particles have a greater impact, so that the gas adsorption of escape or break an oxide film, so that the surface to be purified and activated, will be conducive to sintering. However, instantaneous high temperature generated by the discharge causes evaporation and melting of the grain surface, the grain at the point of contact to form sintered neck.

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.