Tungsten Copper Electrode Sintering Mechanism and Densification II

Viewed from the diagram, the particle with deep color, cladded internal is tungsten (W), and the external particles in white is copper (Cu), the particles contacts by point. In the solid-phase state, the shrinkage of powder takes up 1/3 shrinkage of total volume. Generally, tungsten copper W-Cu powder does not shrink in the solid-phase state. However, if the temperature rises up to or exceeds copper recrystallization temperature during the copper powder sintering, it will remarkably sintering in solid-phase.

In addition, tungsten grain of tungsten copper nano composite powder is disperse at the beginning, basically Cu phase contacts with each other. Therefore, some related researchers propose that Cu solid-phase sintering at the temperature lower than copper liquid is the main mechanism of sintering densification, and the internal tungsten grains cladded in Cu blocks out the sintering process to some extent.

Picture a to picture c shows that due to the diffusing and flowing effect of copper liquid, the contact area of particles is expanding, the internal gas is extruded so that the compact densification. Tungsten particles close to each other and in contact gradually under the action of the flow of fluid passing copper will eventually gather and grow up under the influence of interfacial tension.

Picture a is the initial status of compact, which powder is disperse and the contact of particles is point with small contact area, the small tungsten grain inside of Cu phase is separated by the larger Cu grain; Picture b shows that with the increasing sintering temperature gradually, the tension of diffusion of surface atoms and surface tension force it to flow to the contact, the contact area expands, the pores shrinks and the internal tungsten grain cladded by Cu phase get close, gathers and grows up by the flowing copper liquid and interfacial energy; Picture c, with the temperature increasing further, the contact area is becoming larger, the pore is shrinking to ball-shaped, tungsten grain is gathering and composed of the larger grain, which blocks out copper solid-phase sintering further.

Nano Copper powder of Tungsten Copper Electrode Preparation

Compared with traditional powder metallurgy (PM), tungsten copper electrode nano composite material including powder milling, compacting and sintering process, but due to the characteristics of nano particles, there are some differences between traditional powder metallurgy and nano copper powder preparation.

Gas Evaporating
Copper evaporates in inert gas atmosphere, then a conflict with an inert gas and the particles, cooling, condensation forming nano copper particles. The forming powder process including 3 stages: one is metal evaporates and produces vapor, another one is metal vapor diffuses in inert gas and nucleation and crystal nucleus growth. The powder particle size is mainly determined by the growth process from the evaporation surface to this temperature region, and this process is affected by evaporation temperature and the pressure and the type of inert gas, the influence of temperature gradients within the device and convection conditions. Metal ultra micro powder with 10nm-1μm can be fabricated by the process parameters controlling. This method is the most direct and effective in metal ultra micro powder fabricating, a company from France uses induction heating to improve the gas evaporating and succeed in fabricating copper ultra micro powder.

Plasma
Plasma has high temperature and high reaction speed, you can get a uniform, nano-powder of small particles, easily to achieve mass production, can be prepared almost any nanomaterials. It can be specifically divided into direct current arc (DC), high frequency (RF) and mixture (HP). DC has many advantages, such as simple equipment and operation, high efficiency and wide application, but it will easily melt and evaporate at high temperature, which will pollute the product; RF has no pollution of electrode, high speed of reaction, but its energy utilization rate is lower and instability. HP is combined with DC and RF, which has large space of plasma, high purity, high efficiency and good stability.

Tungsten Copper Electrode Infiltration Process

Tungsten copper electrode production process as follow:
1. Preparation: Firstly, put tungsten powder, induced copper powder in ball grinding mill, and add 1% of stearate and dried and sieved to thereby obtain mixed powder after a while.

2. Compacting: Hydraulic machine pressing, we should concern the weighing in case of the loss of powder loss during the compacting process,

3. Degreasing: Using H₂ as protective atmosphere when thermal degreasing, adhesive must be removed in order to avoid contamination and clean sintering furnace.

4. Sintering: Sintering in molybdenum wire furnace, H₂ as protective atmosphere.

5. Infiltration: Using H₂ as protective atmosphere and infiltrated copper at 1200-1300℃.

6. After-processing: The infiltrated tungsten copper alloy electrode after annealing treatment to improve the performance of the alloy, and then subjected to a surface treatment to remove excess copper.

In the process of tungsten copper, silver-tungsten, ferro tungsten mechanical parts and metal-ceramic materials, the infiltration method is one of the most major processes. Take tungsten copper electrodes for example, the infiltration process is immerse tungsten skeleton in liquid copper after high temperature sintered, so that the pores are filled with liquid copper, then by annealing and cooling to obtain a dense tungsten copper materials.

Thus, infiltration process is the preparation of tungsten-copper electrode in the most critical a process, which directly affects the overall performance of tungsten copper electrodes. However, the most of present study concentrated on the parameters of infiltration, for infiltration mechanism is quite few, and most of these studies are directed to coarse particles of tungsten powder, and rarely comes to fine particles of tungsten powder. Infiltration is the process relies on an external metal powder wetted porous body (tungsten skeleton), under the action of capillary force, the liquid metal (Cu) along the inter-particle porosity within the particle porosity flow or until completely filled up the pores.

Effect of TiC in Tungsten Copper Electrode

Due to the high thermal conductivity of Cu, and the better spark erosion resistance, low thermal expansion coefficient and high melting temperature of W, tungsten copper 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.

Sintering Process Effect on Tungsten Copper Electrode Properties

For a certain density of compacts, sintering of tungsten skeleton is undoubtedly key processes of manufacturing tungsten copper electrode. High-temperature sintering can remove the impurities easily and ensure the electric conductivity. While the choice of the sintering temperature is mainly consider that the impact of tungsten powder granularity, particle size distribution and pressing density and other factors.

If the temperature is too low, the porosity, the hardness and density of tungsten copper electrode will decrease; If the temperature is too high, the porosity is low, the copper content is relatively reduced, high product density, electrical conductivity, and formability is deteriorated. Therefore, under normal circumstances would control the sintering temperature range between 1600 ℃ -2200 ℃. The sintering time and the sintering temperature can be achieved relatively matched state by some adjustments to improve the sintering temperature sintering time can be shortened, thereby improving the processing efficiency, and sintering time may be extended to some extent reduce the sintering temperature.

But overall, tungsten copper electrode sintering temperature sintering property performance is more sensitive than the sintering time. The properties of tungsten copper electrode at different sintering temperature for 1h as follow:

As the sintering temperature increases, the density of tungsten skeleton also increases. While to prepare infiltrated tungsten skeleton well, on the one hand requires that tungsten skeleton has a certain hardness and strength, on the other hand requires tungsten skeleton excellent connectivity inside, which is conducive to metallic copper infiltration.

Annealing Process Effect on Tungsten Copper Electrode Properties

Annealing process controlling is crucial in the process, its time and temperature has an effect on the properties of tungsten copper electrode directly. After infiltration cooling rate has a great impact on the copper layer quality, too fast cooling will be easy to produce pores and tiny cracks. Meanwhile, due to the furnace temperature unevenness, the copper layer thickness of products are not easy to be uniform, so it requires the use of a copper infiltrated after annealing treatment, which further improving the performance of a tungsten-copper electrode. Some experiments shows that annealing process has a great influence on the electrical conductivity of tungsten copper electrode, there is a table of the electrical conductivity of W-25Cu tungsten copper electrode at different temperature.

Some experiments shows that annealing process has a great influence on the electrical conductivity of tungsten copper electrode, there is a table of the electrical conductivity of W-25Cu tungsten copper electrode at different temperature in the part one. The table shows that when the annealing temperature reaches 800 ℃, the electrical conductivity of tungsten copper electrode is the highest, after which the temperature continues to rise, the conductivity decreased. Theoretically, this is due to the elimination of internal stress and the Cu phase recrystallization. On the one hand, when tungsten skeleton is starting cooling process after infiltrated at high temperature, it produce large internal stress because of the a great difference between the coefficient of thermal expansion of tungsten (W) and copper (Cu), which affects the electrical conductivity of tungsten copper electrodes. On the other hand, copper phase recrystallized at 400 ℃, and when the temperature rises to 800 ℃, copper atom lattice distortion reduced, the crystal defects supplemented and improved conduction mechanism of Cu is further reflected. However, with further increase of annealing temperature, changes in Cu phase grain boundaries will gradually slow, the impact on the conductivity becomes negligible.