In part 2 we know that the first character of tungsten carbide spade-type micro drills during micro drilling is that it has no point at the tip and is easy to make an eccentric motion. Here we talk about the cutting energy distribution when micro drilling and the point angle characters.

A second consequence of the chisel edge of a tungsten carbide spade-type micro drill is its relatively long length compared to the drill diameter. This results in a relatively high thrust force along the drill axis. While the sloped cutting edges are increasing the diameter of the hole machined by the chisel edge, the specific cutting energy along the cutting edge is normally lower than at the chisel edge. The result is a large thrust force compared to the diameter of the drill. Again, this size effect works against the micro drilling process similar to the size effect in micro milling.

Micro drills are typically made of either cobalt steel or micro grain tungsten carbide. The steel drills are less expensive and easier to grind but are not as hard or strong as the tungsten carbide drills. The drill point angle is based on the material to be drilled. The normal point angle is 118 degrees and 135 degrees is used for hard materials. The larger included point angle provides more strength at the drill point.

 (To be continued. This article is divided into several parts and this is part 3, for part 2 please refer to http://news.chinatungsten.com/en/tungsten-information/79982-ti-10251; for part 4 please refer to  http://news.chinatungsten.com/en/tungsten-information/80043-ti-10264)

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In the process of mechanical alloying (MA), the powder will be refined by the impact of metal ball and get nano-crystalline pre-alloyed powder. The preparation of nano composite powder (＜20nm) by MA can direct sinter tungsten copper material, which has high density and uniform tissue. However, it has low efficiency and will bring in impurities during the long-time ball milling.

Compared with MA, co-reduction is a kind of process that refining tungsten and copper oxide ball or salts through ball milling, or Using a mixed solution of salts obtained ultrafine or nano-crystalline tungsten-copper compound mixed powder, and then get ultrafine or nano-crystalline tungsten copper mixed powder by reductive hydrogen H₂. After pressing and sintering, tungsten copper material properties can be remarkably improved. Co-reduction can be suitable for different raw material and different process, will not bring in any impurity so that it has bright prospects.

More information about tungsten copper electrode powder direct sintering, click here:

http://news.chinatungsten.com/en/tungsten-information/79974-ti-10240

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Mechanical alloying powder is different from casting metal or alloy material, its atoms coupling tightly and come into being solid solution or compounds. In general, Milling in a limited time only make contact at those points, lines and surfaces at or approaching the atomic distance of each component, and end up with a very uniform distribution of each component of a mixture or compound.

The principle is that extruding, crushing, welding, re-extruding and forming composite grains in the beginning of milling. And composite grains will emerge new atom surface, layered structure refine under the mechanical force. In the process of mechanical alloying, layered structure forming is a symbol of the beginning of elements alloying, which shortens the diffusion path of solid state atoms and accelerates tungsten copper alloying process. In addition, the larger lattice dislocation density, which in turn provides a fast diffusion path for the alloying proceeds, the alloy further accelerates the process.

More information about tungsten copper electrode powder direct sintering, click here:

http://news.chinatungsten.com/en/tungsten-information/79974-ti-10240

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In part 1 we know that micro drilling means a special drilling cycle and precise rotation of micro drills which is made to spade shape of diameter less than 50μm. Here we talk about the first important geometric characteristic of tungsten carbide spade-type micro drills.

There are several important geometric characteristics of tungsten carbide spade-type micro drills. First, the point of the micro drill is not a point at all. Even on conventional twist drills, the end is not truly pointed. Instead, the end of the micro drill consists of a cutting edge (called the chisel edge) made by two intersecting planes which also define the two primary cutting edges of the drill. The chisel edge removes material primarily by extrusion and cutting at high negative rake angle. The specific cutting energy along the chisel edge is relatively large compared to the drill's primary cutting edges. The chisel edge also adds to the drilling complexity because of the lack of a point. As the rotating micro drill first contacts the work piece (remember the micro drill has a very small structural rigidity) anything on the surface, including micro roughness and material slope, will cause the drill to walk on the surface as it is trying to begin removing material. Walking is characterized by an eccentric motion of the drill as it turns perhaps coupled with a non-time- varying bending of the drill about its longitudinal axis. Depending on the feed per revolution of the drill during hole start-up, the drill may begin drilling at a slant with the drill deflected like an end-loaded cantilever beam (which it is with superimposed column loading). If permitted to continue, the drill will quickly break. If the drill is strong enough to survive the large stress imposed in it due to micro drilling at a slant, the resulting hole will be slanted rather than normal to the work surface.

(To be continued. This article is divided into several parts and this is part 2, for part 1 please refer to http://news.chinatungsten.com/en/tungsten-information/79981-ti-10250)

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Micro drilling is characterized not just by small drills but also a method for precise rotation of the micro drills and a special drilling cycle. In addition, the walls of a micro drilled hole are among the smoothest surfaces produced by conventional processes. This is largely due to the special drilling cycle called a peck cycle.

The smallest tungsten carbide micro drills are of the spade type. The drills do not have helical flutes as do conventional tungsten carbide drills and this makes chip removal from the hole more difficult. Drills with a diameter of 50 micrometers and larger can be made as twist drills. Drills smaller than this are exclusively of the spade type because of the difficulty in fabricating a twist drill of this size.

 (To be continued. This article is divided into several parts and this is part 1, for part 2 please refer to http://news.chinatungsten.com/en/tungsten-information/79982-ti-10251)

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With the rapid development of nanomaterials, powder granularity becomes smaller and sintering temperature is decreasing, which is beneficial for products densification. Using ultra-fine or nano tungsten copper powder can directly sinter at the lower temperature so that it is the new research direction of tungsten copper processing. The general process of powder direct sintering is that: W powder, Cu powder mixed + forming additives → forming →degreasing → sintering → after-processing. Therefore, researchers start their study with ultra-fine powder manufacturing, molding process and sintering process.

1. Powder preparation

There are many methods of ultra-fine tungsten copper powder, mechanical alloying and co-reduction is the most popular. Mechanical alloying (MA), the powder technology invented by American in 1970s, which milling with W and Cu powder in ball grinder. During the milling, it uses inert gas for protection in case of powder oxidation.

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For instance, the density of W60-Cu tungsten copper electrode increases 12.7-12.8g/cm³ to 13.2-13.3g/cm³ after hot isostatic pressing (HIP), the hardness increases 50-70HRB to 80HRB, the bending strength increases 550-750MPa to 1000MPa and the electric conductivity can reaches 60%IACS. Foreign scholars adopt cold-rolling to tungsten copper machining and study the effects of distortion on properties. The result shows that with the increasing distortion, hardness and strength will be remarkably improved and annealing in time will promote the mechanical properties.

In conclusion, tungsten copper electrode infiltration has many advantages, such as high density, good thermal and electric conductivity. However, there are some problems that difficult for complex-shape machining, copper solidifying phase uneven distribution and the higher cost of surplus copper removal.

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The most of study of infiltration focus on qualitative analysis in the parameters, such as temperature, holding time, atmosphere and so on. For tungsten copper, infiltration mechanism refers to copper solution moisturizes porous tungsten skeleton, which flows along the gap of the grain and fills in the pore of the matrix by capillary force. It is important that proper temperature will ensure the small viscosity and large surface tension.

Generally, infiltration often uses H₂ (reducibility) for reducing the oxide of tungsten skeleton, which improves the moisture of the skeleton and the density of tungsten copper electrode. On the contrary, if we use oxidizing gas or carburization atmosphere, the porosity and density or other properties will be remarkably decreased. The study of after-processing includes repressing, hot forging, heat pressing, high temperature insostatic pressing (HIP) and so on. HIP can effectively eliminate the porosity and defects of infiltration.

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Compared with extrusion molding, Injection molding has been widely used, it makes porous tungsten blank by injection molding. The researchers adopt injection molding to make tungsten skeleton and observe 10%, 15%, 20% tungsten blank. Tungsten skeleton pre-sintering at 900℃, infiltrating 90-120min at 1500℃ can get tungsten copper composite material with 99.2% density, 1500MPa breaking strength.

The sintering process of porous tungsten skeleton plays an important role in tungsten copper electrode infiltration, including sintering temperature, holding temperature, sintering atmosphere, gas aggregate and so on. Sintering temperature choose by tungsten powder granularity, distribution of granularity, suppression density and other factors.

The experiment result shows that 5-9μm (tungsten powder granularity), 1000℃ (pre-sintering temperature), 1350℃ (infiltration temperature) is the ideal condition for tungsten copper electrode. In another experiment, researchers study tungsten copper contact of high-voltage switch sintering in the N₂ atmosphere, which find that there is non-oxidation, non carbonization in joint surface and tungsten powder grains distribute uniformly with Cu covered.

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In part 5 we know that there are about 5 distinct solid tungsten carbide end mill types, among HSM type featuring short cutting edge, larger corner radii, and HPM featuring long cutting edge length, chip dividers, helix angles, etc., are just popular milling cutter types. Here we talk about replaceable solid carbide head end milling system.

 (The end-This article is divided into several parts and this is part 6, for part 5 please refer to http://news.chinatungsten.com/en/tungsten-information/79930-ti-10235)

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