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Applications of Tungsten |
| There is an increasing demand from the electronics, nuclear, and aerospace industries for materials that maintain reliability under ever-increasing temperature conditions. Because its properties meet these requirements, tungsten also is experiencing an increasing demand. Characteristics that support the demand for tungsten in many electronics applications are its: strength and stiffness at high temperatures good thermal conductivity low thermal expansion low emissivity Tungsten is also relied upon in high temperature furnace applications because of its properties that provide: strength and stiffness thermal expansion low vapor pressure electrical resistivity emissivity productivity and fabricability Glass-to-Metal Seals Tungsten has a coefficient of expansion approximating that of hard glass. For this reason, it is used extensively in glass-to-metal seals in hard glass lamp and electronic applications. Under special conditions, it may also be used with quartz. All rod intended for sealing purposes is processed and inspected to produce split-free material with no surface imperfections. Supports Since tungsten rod has a high degree of strength at elevated temperatures, it is utilized structurally to hold or support high temperature sources such as filaments and heaters for lamp and electronic uses. Electrodes Tungsten rod that is specially processed and manufactured for welding rod applications is used extensively in such processes as inert-gas-shielded arc welding and atomic hydrogen arc welding. Other types of tungsten rod are used for electrodes. These types, both regular and thoriated, are used for electrodes in vacuum melting processes, resistance welding, and electro-discharge machining. The resistance welding of most refractory metals and their alloys is not normally done for several reasons : The resulting cast, coarse grained microstructure has essentially no ductility or toughness. The coefficients of thermal expansion are much lower than most common metals, that results in cracking upon cooling if dissimilar metals are joined. The Refractory Metals are also chemically reactive and will oxidize unless heated in inert gas or vacuum. Some Electron Beam Welding is perform to join these metals for low stress applications. Cathodes/Anodes For tube applications, especially flash and xenon tubes, tungsten is used either as pure or thoriated at 1% and 2% for greater emissivity. Working Characteristics Because of Tungsten's high hardness and low ductility, it is a difficult material to fabricate. The best method for machining that involves metal removal is E.D.M. For parts such as tubes, crucibles and other small, thin-walled items, C.VD. is an effective, but costly, method of achieving them. There are times when neither of these methods will suffice and then conventional methods must be used. Cleaning of Tungsten A cleaning process is designed to deal with one or more of the following: surface scale general contamination removal of a basis metal Of all the potential contaminants in wrought products, iron is of primary concern. Others, such as aluminum, carbon, calcium, copper, nickel, etc., may also be present as elements, but they are more frequently present in the form of oxides. Removal of a controlled amount of basis metal may be desired to insure complete removal of contaminants. There are four main processes used to clean tungsten: Molten Salt This is one of the most common cleaning processes, requiring simple immersion in a molten bath containing oxidizing agents. This process will not attack the basis metal. Aqueous Alkaline Solutions This process works well on oxidized (yellow tungsten) surfaces. Reduced or intermediate oxides (brown, purple, etc.) will react more slowly to this process, if at all. A tightly adherent black scale, with or without carbon, is commonly found on tungsten that has been worked at high temperature. Despite the fact that it is predominantly W03, normally yellow in bulk, it is only slowly attacked even by a hot, concentrated solution. This is probably due more to its dense, fused, physical state than because of its chemical nature. This process is similar to the use of molten salts in that it will not attack the basis metal and it requires an oxidizing agent to work. Add Solutions Tungsten is much less reactive to individual acids than most common metals. HCI, HF, and H2SO4 have essentially no effect. When tungsten is treated with acid solutions, it frequently is stained by residual oxides even if rapid and thorough rinsing is used. Electrolytic Methods Electrolytic etching is the removal of basis metal by an applied voltage in a medium capable of dissolving the products of the electrolytic reaction. This may be done in molten salts or aqueous solutions. Electrical current and time determine the amount of metal removal. There are five primary cleaning methods for cleaning tungsten: Immerse the material in a 20% solution of potassium hydroxide that has been brought to a boil. Etch the material in a 20% solution of potassium hydroxide. Etch the material in a 50 vol. % HN03 - 50 vol. % HF solution. Immerse in molten sodium hydroxide. Immerse in molten sodium hydride. For rapid attack of heavy scale, molten salt is far superior to the other methods. In addition, if no oxidizer is present, it can be performed with no fear of basis metal loss. If appreciable sizes or volumes of material are to be processed, particularly with significant basis metal removal, acid solutions present a disposal, as well as an operational, problem. The utility of electro-etching is more dependent on geometry than the other methods. It will work well for treating continuous lengths of wire, however there is a contact problem if the cleaning is to be performed on many small parts. For all systems involving basis metal attack, rate control must be achieved by definition of concentration, temperature, and time of exposure. |
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