Tungsten Powder Granularity Effect on Tungsten copper electrode

Since the high strength, excellent thermal and electrical conductivity and arc erosion resistance, tungsten copper has been becoming the one of the important materials for electrode application. There are many influencing factors of tungsten copper electrode materials, but the vast majority of literature can consult to affect mostly concentrated in the preparation process (tungsten skeleton, density, hardness, uniformity) of tungsten copper electrode material. We introduce the effect of tungsten powder granularity on tungsten copper electrode. Viewed from the distribution and structure, tungsten copper with the same composition, the finer particle size of tungsten will distribute more uniformly. But the possibility of closed-pore and defects appearing is also increasing, Cu enrichment increasing and the overall uniformity decreasing.

For the aspect of the hardness and the density, with the increasing of powder granularity, the pressing density and degreasing density of the tungsten skeleton fabricated by different granularity of tungsten powder is basically the same, and the overall density of tungsten copper also decreased after the infiltration. Take tungsten copper W-30Cu as example, When the tungsten powder particle size of 2.65μm, the highest hardness, Brinell hardness up to 222HB. With the powder size increases, the hardness decreases when the tungsten powder particle size of 8μm minimum hardness, Brinell hardness is 190HB. This is due to the higher density brings the greater brittleness. The powder granularity decreases, the specific surface area increases, the higher surface energy, the greater sintering activity and the more obvious volume shrinkage after sintering, which increases the content of W, improves the overall density and hardness. In the electrical conductivity, tungsten powder particle size of the smaller and more easily when pressed cause tissue inhomogeneity of skeleton sintered in the passage is closed and prone to clogging, thus resulting in voids or defects in copper-rich material and cause copper deficiency or liquid infiltration feeding can not effectively reduce the integrity of the copper network grid after infiltration, so that the electrical conductivity decreased.

Tungsten Copper Electrode and Graphite Electrode Comparison

EDM (Electrical Discharge Machining), which the properties of the using electrode material have a direct effect on the overall performance (such as the instrument consumption, the removal rate of material, the surface quality and so on). Therefore, in the process of EDM, the choice of the electrode materials is the key factor in the manufacturing process. Electrode materials not only need the good performance in thermal and electrical conductivity, but also need the low coefficient of thermal expansion and other mechanical properties requirements so that the excellent heat resistance, deformation resistance and low consumption can be ensured. With the popularization and developments of EDM technology, the related electrode materials have been reforming gradually. At present, already in production and are being developed in the electrode materials include graphite (C), tungsten (W), copper (Cu) and other elemental metal or nonmetallic substance, steel, cast iron, copper-based or tungsten-based alloy composite material, polymer composites, diamond, etc.

The graphite can be used in EDM with large discharge current, which has low consumption in roughing process and has high consumption in finishing process and it is difficult to meet the surface roughness required. In addition, the proportion of graphite electrodes is relatively light, brittle, difficult to use in general machining thin or fine hair processed into a shape, which in some fine or complex shapes EDM application has been subject to certain restrictions. By comparison, tungsten copper electrode material is a kind of new high-performance electrode material that has high strength, low coefficient of thermal expansion and excellent spark erosion resistance, which has been widely used in machining tool steel and tungsten carbide workpiece. However, as the price of tungsten copper electrode is relatively higher than ordinary graphite electrodes or copper electrode material prices, which is also the one of barrier to popularize tungsten copper electrode material.

Tungsten Copper Electrode New Process

With the rapid development of EDM (Electrical Discharge Machining) industry, it has higher and higher requirements for the used electrode. Both tungsten copper with high content of W (mass fraction of W reaches 50%-90%) and tungsten copper with high content of Cu (mass fraction of W reaches 50%-90%), the most widely used are mixing method and infiltration method. Theoretically, the mixing method has simple operations and can fabricate tungsten copper products with any ratio of W and Cu. But the powder grain of raw material by this kind of conventional process is too large and the density distribution is uneven, which is difficult to fabricate tungsten copper electrode with compact structure after pressing and sintering process.

While the infiltration process is suitable to fabricate tungsten copper with 8%-40% content of Cu, tungsten skeleton infiltrated by copper in molten state to obtain the densification. Although the infiltration process is the most widely used in tungsten copper electrode now and it has stable working properties, it also has some problems, such as many controlling parameters, difficulties in porosity controlling, difficulties in porosity distribution and so on. So the relevant scholars and researches have further explosion surrounding the raw material preparation and molding process in recent years. Some new technology has gradually developed, which is more practical and operational effects are:

1. Pre-alloying: Its powder refinement and even micro-alloying, so as to achieve increased activity and split the purpose of promoting densification by mechanical and high energy ball milling;

2. Ultrafine particle / nano powder: By the addition of grain growth inhibitors, high-energy ball milling, adding elements and changing the material of the powder form and other methods to refine, even to the nanometer level;

3. Thermoforming: it is pre-loaded with a good blend in the mold cavity, rapid molding under heat and pressure to achieve integration and briquettes pressed sintered compact so as to achieve the densification effect;

4. Injection molding: the process is suitable for the manufacturing of parts with complex structure and a smaller volume, which prepared a uniform structure, precise size, but the control of process parameters is more complex, and higher production cost of the mold;

5. Coated powder process: based on Cu cladded by W powder, which will be an oxide, a compound or compound solution both doped together, dried calcined powder coated after the reduction.

In addition, vacuum sintering, microwave sintering, activated sintering, plasma arc welding sintering process all have own advantages and application scope. Here we mainly introduce the coated powder process by thermal chemical method.

New Technology of Tungsten Copper Electrode—Microwave Infiltration

Actually, microwave infiltration is one of new methods of tungsten copper electrode machining, which combines microwave sintering with traditional infiltration. It has many advantages, such as selective heating (the coupling ability of material and microwave, the ability of microwave absorption is related to the properties of itself, such as electrical conductivity, magnetic conductivity and dielectric constant, which will get diverse heating properties when the material is different in a given microwave field. ), volume heating (Each portion of the object in the portion of the material is heated after the heat acquired, so that the part of the material temperature rises.), non-thermal effect (It general refers to compared with traditional sintering under the same thermal dynamic conditions has remarkable superiority in physical, such as the active sintering of powder material and atomic diffusion rate.) , and uniform structure of infiltration processing. Therefore, microwave infiltration has higher speed of increasing temperature, shorter sintering time, higher energy utilization ratio and uniform structure or excellent properties of the products. Microwave infiltration of microwave sintering equipment is produced by DC magnetron microwave waveguide by introducing the heating chamber, the sample is placed in the cavity is heated sintering. The sketch of structure of the equipment as follow:

The major technical parameters of microwave sintering furnace includes work area, the highest working temperature, thermometric, methods, the range of infrared detection, the precision of temperature control, the out power of microwave, vacuum limit of furnace, the leaking of microwave. At present, the use of microwave sintering heating chamber includes a resonant and non-resonant two, while the cavity can be subdivided into single-mode and multi-mode resonant cavity resonator. Under the same electromagnetic power, single-mode resonant cavity having a stronger magnetic field strength is more suitable for low heating medium. The multimode resonator having multiple resonant modes, the electromagnetic field distribution is more uniform, the structure is relatively simple, suitable for a variety of heating load, the more it is difficult to accurately analyze the data.

Infiltration Time Effect on Tungsten Copper Electrode Properties

Take W-Cu tungsten copper electrode as an example, with increasing infiltration time, the density of tungsten copper electrode increases. And after that the infiltration density reaches the maximum, the density of tungsten copper electrode decreases by the time. The density and hardness of tungsten copper W-25Cu in different infiltration time as follow:

In the process of infiltration, copper solution completely infiltrated tungsten skeleton will take some time, with the increasing infiltration time, tungsten skeleton has infiltrated more fully, the corresponding density of tungsten copper electrode was improved. However, when tungsten skeleton was completely infiltrated by copper solution, if the temperature continues to increase the infiltration density changed a little longer this time but likely to cause the liquid copper tungsten particle erosion, some places inside may appear copper pool, which decrease the density of tungsten copper electrode.In addition, due to the diffusion of crystal grains, the odds of the liquid copper into the tungsten particles increases, the lattice stress decreases, and the tungsten particles are rearranged so that the partial so that the construction is more uniform. Viewed from the cost, it is useless to increase infiltration time.

In the early of infiltration, tungsten skeleton was not completely infiltrated by copper solution, which lack of density and hardness. However, the infiltration has become more and more fully as time goes by so that the density and hardness of tungsten copper electrode will rise. But when infiltration is completely and time continue to increase, it may lead tungsten particle to grow up and evaporated tungsten particles in liquid copper, which makes tissue distribution is uneven, the hardness decreased accordingly. Except that the density and hardness of tungsten copper electrode, the electrical conductivity is also an crucial property when it used as electrode in EDM or ECM.

Tungsten Copper Liner Warm Flow Compaction

Tungsten copper liner is a kind of psudoalloy that composed of W and Cu, which is immiscible and forming compound with each other. So it is difficult to fabricate by common pressing-sintering process, the most widely used in tungsten copper products is powder metallurgy (PM), which includes infiltration and activated liquid-phase sintering. Infiltration presses tungsten powder into a compact, at a certain temperature pre-sinter the porous tungsten skeleton with density and strength. And then the low melting point of the molten liquid copper is infiltrated tungsten skeleton in order to obtain tungsten copper alloy with fully densified. The main mechanism is the liquid metal wetting porous matrix metal, under the effect of capillary forces along the inter-particle pore liquid copper flows and fills pores of the porous tungsten skeleton.

According to the sintering temperature, infiltration process can be specifically divided into sinter tungsten skeleton at high temperature + Cu infiltrated and sinter composite powder at low temperature + Cu infiltrated. Sinter tungsten skeleton at high temperature + Cu infiltrated presses tungsten powder into a compact before sinter and infiltrated Cu at 1800-2200℃. Since the tungsten powder at a high temperature to fully restore the remaining low-melting impurities and sub-oxides can be restored by evaporation gasification and thermal decomposition was removed, thus prepared tungsten copper shaped relative density of 99.2% comprehensive and excellent performance.

However, it also has some disadvantages, such as long production cycle, the control parameters are many and complex and high production cost. While sinter composite powder at low temperature + Cu infiltrated is mixed with a small amount of induced copper before sintering and infiltration. Generally, induced copper content of about 2.5% -3.5% and the granularity is similar to tungsten powder, which can effectively improve the compacting performance and the flowability of liquid Cu. But too much of the added amount of copper will result in enrichment-induced infiltration of copper, so that the overall the density of tungsten copper liner decreases.

Overall, compared with the injection molding suitable for the smaller components, warm flow compaction is more suitable for the larger components with complex structure and shape. It has higher requirements on the granularity of raw material and is sensitive to the temperature changes and the amounts of binder. Low temperature may cause mixing powder flow properties deteriorated, so that the tungsten copper shaped charge density distribution can not be molded or uneven. High temperature may obviously separate the two phase (the mixed powder and the binder). The excessive amount of adhesive may cause infiltrated tungsten skeleton pore through the large, so that the density decreases and easily deformed. In addition, warm flow compaction combines the advantages of injection molding and warm compacting process, and it can effectively improve the density, the penetration depth and armor capability of tungsten copper liner, which is the ideal choice for tungsten copper liner manufacturing in the future.