The volume shrinkage of tungsten skeleton infiltrated with increasing compacts after pressing force increases, after its infiltration volume shrinkage greater than shrinkage skimmed skeleton after sintering blanks. As the result of the main ingredient of compact is tungsten and a slight amount of copper-induced, when skimmed blank sinter at high temperature, volume shrinkage mainly by volume diffusion effects and some tungsten particles induced copper compacts pressing pressure, which is conducive mutual contact between the tungsten particles and the diffusion and sintering density improving

However, if the pressing pressure exceeds a certain limit, the tungsten particle volume shrinkage decreases. It occurs liquid phase sintering during copper-tungsten skeleton infiltration. By the action of capillary force, tungsten particles rearranges, and improves the density, the volume shrinkage caused much greater than the volume shrinkage by volume diffusion.

More information about pressing process effect on tungsten copper electrode properties, click here:

http://news.chinatungsten.com/en/tungsten-information/80252-ti-10309 
http://news.chinatungsten.com/en/tungsten-information/80294-ti-10312

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With the increasing pressure, the density of powder with same weight is increasing. This is due to the accumulation of powder loaded in loose, irregular surface friction generated between each other, the particles to each other to form a bridge arch effect. When pressure exerted, arch effect the destruction of the body powder, powder particles will fill the pores with each other, rearrangement occurs, increase the contact area, and displacement occurs inside powder body.

Therefore, in the process of powder formation, the increasing molding pressure makes the blank density increase. For the same particle size of infiltration sample, as the pressure increased, under the same conditions, which skimmed density, skeletal density have also been increased accordingly.

For the impact of tungsten copper alloy electrode volume change, the experimental data shows that the volume shrinkage skimmed blank with high-temperature sintering compacts after pressing pressure increases. When the pressing pressure is 410 MPa, the maximum volume contraction, and thereafter compacting pressure continues to increase, the volume shrinkage rate began to decline.

More information about pressing process effect on tungsten copper electrode properties, click here:

http://news.chinatungsten.com/en/tungsten-information/80252-ti-10309

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In last part we know that to obtain a prolonged tool’s life, tungsten carbide cutting tools are coated with layers of wear resistant materials via chemical vapor deposition in a vacuum furnace. We go on to explain PVC technology.

PVD-Physical Vapor Deposition

The inserts are mounted on racks so they are separated from each other. Each rack rotates and the whole assembly of racks revolves within the furnace, so every surface of the tungsten carbide cutting tools is exposed to the deposition process. The furnace is evacuated. Strong negative charge is applied to the tungsten carbide cutting tools. A piece of titanium, or titanium and aluminum is installed on the wall or floor of the furnace. The metal is vaporized by either an electric arc or an electron beam, liberating the positively charged metal ions. These ions are attracted to the negatively charged inserts. Nitrogen and methane are added as appropriate, to achieve the different types of coatings.

PVD coatings are typically about 2-4 microns thick. Different manufacturers use different numbers of layers. When the inserts are removed from the furnace they may be ground again, or directly packaged and shipped.

Tool manufacturers are meeting the pressures for ever increasing feeds and speeds, and the need for longer tool life and lower costs, by continually improving the designs of tungsten carbide cutting tools and developing better and better coating technologies.

 (The end -This article is divided into several parts and this is part 9, for part 8 please refer to http://news.chinatungsten.com/en/tungsten-information/80278-ti-10331)

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In last part we know that tungsten carbide cutting tools are obtained through a final machining process on carbide blanks, including grinding, cutting, drilling, milling and sawing, etc. But machining may not be the final step for some type of cutting tools, since thereafter they are usually coated with special layers of wear resistant materials for longer life and better cutting efficiency. We explain firstly CVD.

Coating

To prolong tool life under challenging cutting conditions, many types and combinations of coatings have been developed for tungsten carbide cutting tools. TiC layers were the first to be applied by CVD. Others followed like TiN, TiCN, Al₂O₃, AlON as well as combinations and permutations. They can be applied mainly in two ways: by chemical vapor deposition (CVD) or physical vapor deposition (PVD). Both types are applied in furnaces. Coated hardmetal is the dominant tool material for indexable inserts.

CVD-Chemical Vapor Deposition

For CVD, the coating is usually 5- 20 microns thick. Milling and drilling inserts usually receive 5–8 microns, as these operations require better surface finish, and they encounter more impact, so they require greater edge toughness. To apply a CVD coating, the parts are placed on trays and sealed in a furnace. The furnace is drawn down to a vacuum.

For each layer, the appropriate gases are introduced into the furnace, such as hydrogen, titanium tetrachloride, methane, nitrogen, aluminum chloride. A chemical reaction occurs, depositing the layer of coating on the inserts.

 (To be continued-This article is divided into several parts and this is part 8, for part 7 please refer to http://news.chinatungsten.com/en/tungsten-information/80277-ti-10330; for part 9 please refer to http://news.chinatungsten.com/en/tungsten-information/80279-ti-10332)

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In last part we know that the blank of a tungsten carbide insert comes into form with enough density and mechanical properties by sintering process. When we finally get a sintered tungsten carbide blank, we will go on to the machining process.

Machining

Before any tungsten carbide blank can be used as a tool, it has to be machined into a specific shape with a certain cutting function. Machining is any of various processes in which a piece of raw material is cut into a desired final shape and size by a controlled material-removal process. The three principal machining processes are classified as turning, drilling and milling. Other operations falling into miscellaneous categories include shaping, planning, boring, broaching and sawing.

For example, after the tungsten carbide inserts are removed from the furnace and cooled, they are dense and hard. After a quality control check, the inserts are usually ground or honed to achieve the correct dimensions and cutting edge. Honing to a radius of 0.001″ is typical, though some parts receive a cutting-edge radius of half a thousandth or as large as 0.002″, and some are left “dead sharp” as sintered.

But most of tungsten carbide cutting tools are much complex than a carbide insert, so they shall be machined in many different ways by different machining equipments with various machining tools. The modern equipment is the CNC machining center in high efficiency and precision, which can produce a final tungsten carbide drill in a complete sequence from grinding, cutting, boring , drilling, end milling to deburring, etc. The finished drill can be put into use directly, or go to the last process:coating.

 (To be continued-This article is divided into several parts and this is part 7, for part 6 please refer to http://news.chinatungsten.com/en/tungsten-information/80276-ti-10329; for part 8 please refer to http://news.chinatungsten.com/en/tungsten-information/80278-ti-10331)

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In last part we know that grade powder is compacted by ways such as uniaxial pressting or isostatic pressing to form a blank shape of tungsten carbide cutting tool and then it goes for sintering. Here we go forward to the process of sintering.

Sintering

In order to increase the strength of the green compacts, they are subjected to heat treatment, which is called sintering. The main aim of sintering is densification in order to provide the metal with the necessary physical and mechanical properties and a density which is suitable for subsequent thermomechanical processing. Sintering of tungsten carbide is commonly carried out in a temperature range of 1500 up to 1500℃ under flowing hydrogen, either by direct sintering (self resistance sintering) or indirect sintering (resistance element heating systems). The density thereby obtained should be a minimum of 90% of the theoretical density, but is commonly in the range of 92% to 98%.

Let’s take tungsten carbide inserts for instance. After pressing, the form looks like oversized inserts and is fairly delicate. They are removed from the molds and placed on graphite or molybdenum trays, and go into a sintering furnace where they are heated in a low-pressure hydrogen atmosphere. The cobalt melts, and the insert consolidates into a solid, smaller size. After the inserts are removed from the furnace and cooled, they are dense and hard. After a quality control check, the inserts are ready for further machining.

 (To be continued-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/80275-ti-10328; for part 7, please refer to http://news.chinatungsten.com/en/tungsten-information/80277-ti-10330)

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In last part we know how the grade powder is gained by mixing up the grade ingredients in a ball mill and drying them in a spray dryer, which also influences much the final quality of tungsten carbide cutting tools. Here we explain the procedure of compacting.

Compacting or Pressing

The grade powder is consolidated into a compact by two main routes: pressing in rigid dies (uniaxial pressing) and isostatic pressing in flexible molds (compaction under hydrostatic pressure). Other techniques, such as powder rolling, cold extrusion, explosive compaction, slip casting, vibratory compaction, or metal injection molding, have gained no industrial importance.

Tungsten powder is not easy to compact due to its relatively high hardness and difficult deformation. Nevertheless, in most cases compaction is performed without lubricant to avoid any contamination by the additive. The resulting compacts are generally sufficiently strong so that they can be handled without breaking. For machining the part, it must be pre-sintered beforehand.

Take tungsten carbide inserts as an example. The grade powder goes into insert shaped molds specially designed to allow for the shrinkage that will happen later on in the process. The powder is compressed into the molds, in a process similar to how pharmaceutical tablets are formed.

 (To be continued-This article is divided into several parts and this is part 5, for part 4 please refer to http://news.chinatungsten.com/en/tungsten-information/80274-ti-10325; for part 6 please refer to http://news.chinatungsten.com/en/tungsten-information/80276-ti-10329)

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In last part we know that tungsten carbide grain size is key to the properties of final tungsten carbide cutting tools, together with additives such as binder cobalt, carbides of titanium, tantalum, niobium, etc. Here we explain the process of mixing up.

Mixing Up

All these ingredients used to make tungsten carbide are blended with a liquid such as alcohol or hexane and placed in a mixing vessel, often a rotating drum called a ball mill. In addition to the grade ingredients, cemented balls 1/4″ to 5/8″ in diameter are added, to help the process of adhering the cobalt to the tungsten carbide grains. A ball mill may be as small as five inches in diameter by five inches long, or as large as a 55-gallon drum.

When the mixing is complete, the liquid must be removed. This typically happens in a spray dryer, which looks like a stainless steel silo. An inert drying gas, nitrogen or argon, is blown from the bottom up. When all the liquid is removed, the remaining dry material is “grade powder,” which looks like sand. The level of mixing up all ingredients affects the uniformity of grade powder and then finally has great effect on the final tungsten carbide cutting tools.

(To be continued-This article is divided into several parts and this is part 4, for part 3 please refer to http://news.chinatungsten.com/en/tungsten-information/80247-ti-10323; for part 5 please refer to http://news.chinatungsten.com/en/tungsten-information/80275-ti-10328)

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Two types of crystalline (NH) 4) 10[H 2W 12O 42]·10H 2O were precipitated in sequence when an ammonia solution of , peroxopolytungstate was neutralized with hydrochloric acid. It was found by X-ray diffraction that one of them is an unknown phase of ammonium paratungstate, and the other is the reported one. Heating the former, it turned directly to common triclinic WO 3. On the other hand, the latter yielded the hexagonal form of WO 3 (h-WO 3) in the temperature range 350-370 °C. This is a new synthetic method of h-WO 3, though the final precursor is a known compound. The color of h-WO 3 thus formed was pale yellow and contained a small amount of residual ammonia (N/W = 0.017 in molar ratio). Its crystallographic c-axis (7.689 Å) was shorter than than the reported value (7.798 Å) for the same WO 3 phase synthesized from 3WO 3·H 2O. 

 

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Tungsten is almost three times as dense as iron, almost twice that of lead and virtually the same as gold, its high density made itself that earned it is name which is comes from Swedish: tungsten heavy stone.

All sorts of weird applications have evolved to exploit tungsten's unique qualities. As we know, its density could rage from 17.0~18.5g/cm3, and the hardness usually from 24~28HRC, besides, when it comes to be used as the bullet, then the hardness should be controlled at least 40HRC, that is the most important reason why the military has called tungsten into service in another kind of evolutionary arms race, and also it is a prefer material called green bullet for tungsten missiles for it is non-toxic.

Tungsten makes very good bullets, which is the kind of thing that if you fire it at someone else's armour, and it will go right through it and kill it. Just like the creatures of the Cambrian period, once someone starts using teeth (or tungsten bullets and missiles) you need to do something about it. If you introduce tungsten into your bullets you've got to introduce tungsten into your armour. The fascinating balance military engineers have to negotiate between the strength of tungsten and the costs in fuel and manoeuvrability all that extra weight brings.

Now they could put the tungsten on the side of the tank but not on the top. Then people are developing warheads that will fly towards the tank and then at the last minute go up and then drop on it, so then you have to start arming the top of the tank. So tungsten metal is a perfect for bullets and missiles.

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