POWDER METALLURGY-3. Fabrication of Tungsten

3. Fabrication of Tungsten

3.1 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.

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