In recent years, people have pay more and more attention to metal injection molding of refractory metal, which is mainly used to produce parts of complex shape of small size, we call it MIM for short, and its full name is Metal Injection Molding. Under normal circumstances, this type of parts is functional unit under high temperature, and the requirement of high temperature strength and fatigue performance are relatively not so harsh, although this reduces the density requirements, but it achieves a nearly net shape, reducing the machine processing costs. Tungsten, owns the highest melting point at the refractory metals, which is the most widely used lighting electrode material.

The traditional preparation process of tungsten electrodes is showed as bellow: machining the tungsten rod which has been sintered and deformed to give pure tungsten / tungsten alloy electrodes. Because tungsten belongs to rare metal and its machining is difficult, therefore, net forming of tungsten electrodes are more and more paid attention in recent industry. Philips Advanced Metrology Systems Corporation is one of the earlier units to carry out a refractory metal injection molding research and development, the use of MIM to manufacture high-pressure gas discharge lamp with tungsten electrodes, in comparison with conventional tungsten electrode wounded with tungsten wire, MIM can achieve tungsten electrodes with different and complex shapes, which facilitate designers to design its shape according to the function of the electrode.



 

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Tungsten has increasingly been applied in aerospace, chemical and nuclear and other fields, it is suitable for welding almost all of metal, especially active metals of refractory metals, nonferrous metals and alloys, titanium and its alloys, stainless steel and high temperature alloys.

Pure tungsten electrodes under high temperature will recrystallize to form equiaxial and coarse grain structure, what will result in the decrease of its mechanical properties. What is more, because the current capacity of pure tungsten is small, the work function (4.52eV) is high, so that the burning would be more serious under large currents. Therefore, the scientists discovered thorium tungsten electrodes material in 1913. Thorium tungsten electrode is the first used electrode in rare earth tungsten electrodes, its good performance and higher market share, what makes it one of the main hot electron emission materials currently, however, thorium element have a radioactive, its half-life grew 1.39 * 10¹⁰, In the production and use processes and its waste will have a great harm to the environment.

In view of this, in the 1970s, the former Soviet Union has started to develop a new type of electrode material, while China factories are almost successful trial-manufacture the cerium tungsten electrodes and make it popularization and application at this time. Thanks to cerium tungsten electrodes themselves own advantages, such as, no radiation pollution, good low-arc arcing performance, so it has become the preferred alternative between the small-diameter welding and low-current welding. However, there are still some limitations in the use of cerium tungsten electrodes, and therefore it can not fully replace the thorium tungsten electrodes.

 

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This article describes that the RE (rare earth) tungsten electrode, as green tungsten electrode, has been gradually developed to replace the traditional radioactive thorium tungsten electrode material.

Chinese tungsten and rare earth resources reserve and production occupied an absolute advantage. Tungsten is known as the "war metal", "Industrial salt", and the rare earth is called "industrial MSG", as you can know from the name, tungsten and rare earth is playing an irreplaceable role in today's high-tech areas. Because they all have higher temperature performance, so they have a wide application in metallurgy, energy, defense industry, electronic information, the nuclear industry and aerospace and many other fields, and they have become indispensable material in the national defense and civilian facilities which is related to human lives.

Under normal circumstances, rare earth tungsten electrode material can be successfully prepared when tungsten acts as a host material and doped with rare earth oxide (with relatively low electron work function). Because the rare earth metal oxides own excellent thermal electron emission capability, rare earth tungsten electrode material may be, on the one hand, to improve the recrystallization temperature of the material, on the other hand, it can start the electron emission. Therefore, rare earth tungsten electrode material has been to a development direction on replacing the traditional radioactive thorium tungsten electrode material.



 

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Because it is the coordination that required by the welder, manual gas tungsten arc welding is a relatively difficult welding method. What is similar to torch welding is that GTAW normally requires two hands. Since that most applications require the welder manually feed a filler metal into the weld area with one hand while manipulate the welding torch with the other. Maintaining a short arc length is also important while preventing contact between the electrode and the workpiece.

To strike the welding arc, a high frequency generator, which is similar to a Tesla coil, provides an electric spark. This spark is a conductive path for the welding current through the shielding gas and allows the arc to be initiated while the electrode and the workpiece are separated, typically about 1.5–3 mm (0.06–0.12 in) apart.

Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, the size of which depends on the size of the electrode and the amount of current. While maintaining a constant separation between the electrode and the workpiece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed.

Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is always kept inside the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with a low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld. 

 

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Carbide drill cutting surfaces are often useful when machining through materials such as carbon steel or stainless steel, as well as in situations where other tools would wear away, such as high-quantity production runs. Most of the time, carbide will leave a better finish on the part, and allow faster machining. Carbide tools can also withstand higher temperatures than standard high speed steel tools. The material is usually tungsten-carbide cobalt, also called "cemented carbide", a metal matrix composite where tungsten carbide particles are the aggregate and metallic cobalt serves as the matrix. The process of combining tungsten carbide with cobalt is referred to as sintering or Hot Isocratic Pressing (HIP). During this process cobalt eventually will be entering the liquid stage and WC grains (>> higher melting point) remain in the solid stage. As a result of this process cobalt is embedding/cementing the WC grains and thereby creates the metal matrix composite with its distinct material properties. The naturally ductile cobalt metal serves to offset the characteristic brittle behavior of the tungsten carbide ceramic, thus raising its toughness and durability. Such parameters of tungsten carbide can be changed significantly within the carbide manufacturers sphere of influence, primarily determined by grain size, cobalt content, donation (e.g. alloy carbides) and carbon content.

Machining with carbide drills can be difficult, as carbide is more brittle than other tool materials, making it susceptible to chipping and breaking. To offset this, many manufacturers sell carbide inserts and matching insert holders.

With this setup, the small carbide drill insert is held in place by a larger tool made of a less brittle material (usually steel). This gives the benefit of using carbide without the high cost of making the entire tool out of carbide. Most modern face mills use carbide drill inserts, as well as some lathe tools and end mills.

To increase the life of carbide drill tools, they are sometimes coated. Four such coatings are TiN titanium nitride TiC titanium carbide, TIALN (titanium carbide-nitride), and TIALN Titanium Aluminum Nitride. (Newer coatings, known as DLC (Diamond Like Coating) are beginning to surface, enabling the cutting power of diamond without the unwanted chemical reaction between real diamond and iron.) Most coatings generally increase a tool's hardness and/or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material gall (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool. The coating is usually deposited via thermal Chemical vapor deposition CVD and, for certain applications, with the mechanical Physical vapor deposition" PVD method. However if the deposition is performed at too high temperature, an eta phase of a Co6 W6 C tertiary carbide forms at the interface between the carbide and the cobalt phase, facilitating adhesion failure of the coating.

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Analysis of electron-optical studies of experimental alloys of Fe-W and ferrotungsten

In this paper the experimental alloys of the Fe-W, which melted with refractory W - Ni - Fe scrap, compared with the standardized grade ferrotungsten ΦB 70. The comparative analysis of the microstructure, phase and chemical composition of the experimental alloys. Investigated the number of non-metallic impurities, the number of which is within the nominated GOST 17293-93. It is proposed to re-use as a substitute for ferrotungsten alloys for the manufacture high tungsten special materials based on iron.





 

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The main use of tungsten (in the form of tungsten carbide) is now in the manufacture of cemented carbide. Cemented carbide, or hard metal as it is often called, is a material made by "cementing" very hard tungsten mono-carbide (WC) grains in a binder matrix of tough cobalt metal by liquid phase sintering. This material is classified technically as a “brittle” material since it exhibits little or no plastic deformation preceding the initiation of a crack. All materials contain some amount of defects in the form of voids, pores or micro-cracks. These defects lead to reduced material strength. In the case of ductile materials such as aluminum, mild steels or copper, the frequency of defects is less critical than in brittle materials.

Sintered tungsten carbide exhibits a broader range of scatter-of-fracture stresses, due primarily to the existence of micro-voids, when compared to ductile materials. The value of the stress at fracture can also vary widely with size, stress state (tensile, bending, torsion), shape, and type of loading. Despite the significant variability of the stress at failure, cemented carbide has considerably high strength for what some consider to be a “ceramic” or cermet material.

The definition of a ceramic material is the marriage of a metal to a nonmetal, for example silicon (metal) carbide (carbon, non-metal), aluminum oxide, silicon nitride or tungsten carbide. A cermet is a composite material composed of ceramic (cer) and metallic (met) materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder with oxide, boride, carbide, or alumina. Generally, the metallic elements used are cobalt, nickel, and molybdenum. It is the addition of the metallic binder, i.e. cobalt or nickel that makes the cemented carbide (WC - Co) a cermet and differentiates it from truly brittle materials, that is, the ceramic family of materials.

The combination of WC and metallic cobalt as a binder is a well-adjusted system not only with regard to its properties, but also to its sintering behavior. The high solubility of WC in cobalt at high temperatures and a very good wetting of WC by the liquid cobalt binder result in an excellent densification during liquid phase sintering and in a pore-free structure. As a result of this, a material is obtained which combines high strength, toughness and high hardness.

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Tungsten is very valuable as a rare metal resources. Tungsten industry is significant to be one of the important material industries in modern time.
 
Tungsten has high hardness, high melting point, high strength, and it’s widely used in machinery, special steel, defense industry and other fields. Its main application is the carbide industry, which is often said as tungsten steel. Carbide is called the teeth of modern industry. As an important strategic resource, tungsten resource is emphasis protected by world powers. Our country has also set up Tungsten Industry Association since 1990s, introduced a number of laws and regulations, and keeps strict supervision on tungsten resources.
 
Global reserves of tungsten resources are scarce and centralizedly distributed. 65.5% of the world’s reserves are in China, which has obvious advantages. Since 2001, China began to imply tungsten resource extraction quotas and export quotas, effectively inhibited the global tungsten supply. Coupled with decreasing amount of foreign supply and limited availability of recycling waste tungsten, tungsten supply growth was slow.
 
Tungsten is mainly used in carbide, steel and alloys, chemical industry etc. About 50% of tungsten materials are used for carbide industry. In China production of carbide cutting tools only accounts about 25% of total production, while 65% of consumption in developed countries accounts for carbide cutting tool consumption. China hass broad space for development of carbide in the future. "Twelfth Five-Year Plan" clearly encourages new materials containing tungsten may become a new impetus to the demand of tungsten.
 
Tungsten resources are rare metal which has absolute advantage in China. As the country gradually strengthen the industry consolidation, supply growth will gradually slow down, while demand in areas such as carbide rapid growth in the long term, the future supply of tungsten would be probably in the tight state. Our carbide industry will get more rapid developement relied to the absolute advantage of tungsten resouces. 

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The paper introduces the use of W and Mo in alloy steel, describes the mechanism of ferro-tungsten-molybdenum smelting in three-phase AC working frequency plasmafurnace directly with mixed charge of W and Mo concentrates and gives the pilot experiment results. The ferro-tungsten-molybdenum, obtained with Mo/W of 0. 05~0.5 at recovery of Mo and W 90% respectively, is applicable to smelting ahoy steel. This new technologicalroute is open to utilization of the W-Mo intergrowth mineral resource.





 

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