Fabrication of Tungsten
Tungsten Powder Metallurgy - Fabrication of wrought P/M Tungsten.
With only some exceptions, tungsten is used in the form of pore-free p. reforms ("wrought" P/M tungsten). To obtain a completely dense material, as well as the desired shape and mechanical properties, a complex, multistage, hot and cold forming process is required. The most important forming techniques for tungsten are rolling (for rods and sheet products), round forging (for large diameter parts), swaging (for rods), forging (for large parts), drawing (for wires and tubes). Secondary forming processes include flat rolling of wires, flow turning, spinning, deep drawing and wire coiling. For a detailed description of the most important forming processes, we refer the reader to the book, Tungsten, by Yih and Wang. In general, plastic forming of tungsten is difficult and needs experience. In the as sintered condition, tungsten is brittle except at quite high temperatures, because it is recrystallized (coarse grained) and not fully-dense. Unlike most metals, the low-temperature ductility of tungsten increases with progressive deformation, because embrittlement is due to grain boundary segregations of interstitial soluble elements, such as oxygen, carbon, and nitrogen. With the breakdown of the coarse microstructure during deformation, these impurities are distributed over a larger intergranular area, which makes the material more ductile and less sensitive to cracking during forming at lower temperatures.
The first forming step is usually carried out at 1500-1700℃. As a result of the low specific heat and high thermal conductivity of tungsten, several reheating stages are necessary in the first stages of shaping, because the heat is lost rapidly at these temperatures and the ingot cools down rap idly. If the metal is worked at too low a temperature, cracks and splits will easily develop. As the forming process continues, the forming temperature is reduced progressively since the recrystallization temperature decreases as deformation proceeds. Tungsten is generally worked below its recrystallization temperature, because recrystallization is combined with grain boundary embrittlement. With increasing work hardening during deformation, both the hardness and strength of the worked part increase significantly, and intermediate stress relief annealings are necessary to minimize the hazards of cracking (laminating) and to avoid overstraining the working tool.
Forming of tungsten is commonly carried out without protecting atmosphere. In air, tungsten is readily oxidized. Tungsten trioxide forms on the surfaces of the worked piece and, above 800℃, it volatilizes. The oxide layer acts as a protective layer against contamination from the working tools and is removed at certain stages of deformation by pickling and/or machining. Intermediate annealing and stress-relieving annealing is performed under hydrogen to avoid enhanced oxidation of the metal and sublimation of WO3.
Tungsten Powder Metallurgy - Fabrication of Tungsten - Shaping.
Compared to ductile metals and alloys, the fabricability of tungsten is rather poor: Tungsten should always be heated before shaping. The temperature range for forming has a lower limit, set by the brittle-to-ductile transformation temperature, and an upper limit, set by the recrystallization temperature. This temperature is mainly dependent on the purity, the history of deformation, and heat treatment of the material. Highly deformed products, such as thin tungsten wires, ribbons, or foils, are ductile at room temperature.
Thin, strongly deformed sheet and foil have a pronounced structure in the longitudinal direction due to elongation of the grains during rolling. The bending properties long the direction of rolling are therefore different from those across it. Therefore, tungsten sheet should always be bent in a way such that the bending edge is perpendicular to the rolling direction. If bending in the longitudinal direction cannot be avoided, owing to the design, much higher bending temperatures are required. At high temperatures, tungsten sheet can be stamped, punched, and sheared. Sharp tools are essential to clean cutting action without sheet cracking or delamination. Tungsten cylinders and cones can be formed by spinning, flow turning, or forging. The use of stress-relieved tungsten is suggested for optimum fabrication results.
Tungsten Powder Metallurgy - Fabrication of Tungsten - Bonding of Tungsten
Mechanical Joining. Mechanical joining, such as riveting, tacking, or lacing, comprises the simplest methods of joining tungsten, provided the joint need not be impermeable to liquids and/or gases. Tungsten and molybdenum parts can be used. Mechanical fastening is used for constructional parts, such as heating elements, containers, large shields, etc.
Brazing. The parts to be joined must be free from grease, oils, oxides, or other impurities. As tungsten is very sensitive to oxidation, brazing must be carried out under a protective gas, hydrogen, or in vacuum. If maximum strength of the joint is required, tungsten has to be brazed below its recrystallization temperature. Typical brazing metals and temperatures are; Rh (1970℃), Ni (1430℃), or AgCu20Pd15 (700-900℃). More derailed information is available. Brazing of tungsten to ceramics, graphite, and silicon has gained importance for the fabrication of refractory-metal composites.
Welding. Tungsten possesses only moderate welding properties. Welding must be carried out under controlled weld atmospheres, preferably in a dry box, since any contamination by oxygen will reduce the ductility of the joint. Before welding, the metal has to be degreased and pickled, commonly with a mixture of nitric acid and hydrofluoric acid (90/10 vol%). The weld seams have a coarse-grained structure in the hot-fusion zone, owing to recrystallization, and can therefore withstand only low mechanical stresses. Tungsten can be welded by tungsten inert gas (TIG) welding, laser beam welding (for thin parts), plasma welding, friction welding and electron beam welding, the latter being the preferred fusion-welding method. Tungsten can be welded to W-Re and Mo-Re alloys. W-Re alloys (in particular W-26Re) are recommended as filler metal for welding tungsten. Tungsten can be diffusion welded to tungsten and other metals. Despite the high melting point of tungsten, temperatures of 1300-2000℃ and pressures of 2—20N•mm-2 give satisfactory joints. Intermediate layers of Ni, Pt, Rh, Ru, and Pd accelerate the diffusion process. Tungsten wires can be spot-welded to other metals under a protective gas blanket.
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