Tungsten Powder Metallurgy
Although the Wollaston process of producing platinum metal from platinum powder is regarded as the birth of modern powder metallurgy (1808-1815), it was the pioneering work of W.D.Coolidge on the production of ductile tungsten wires in 1909-1913 which led to its first commercial application. Over the years, only few changes were made in the industrial production of tungsten metal. Powder metallurgy is still today the main route in tungsten and tungsten alloy manufacture. Unlike other refractory metals, such as Zr, Hf, Nb, or Ta, melting technology has assumed no industrial importance for metal production, since the very high temperature, necessary for melting, and the resulting coarser microstructure of the "as cast" material make further processing both difficult and costly.
Tungsten powder metallurgy comprises two steps: compaction and sintering.
Tungsten powder metallurgy comprises step 1 - compaction
Tungsten powder is consolidated into a compact by two main routes: pressing in rigid dies (uniaxial pressing) an 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 generaIIy sufliciently strong so that they can be handled without breaking. For machining the part, it “must be pre-sintered beforehand.
Tungsten powder metallurgy comprises step 1 - sintering
ln 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 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 is commonly carried out in a temperature range of 2000 up to 3050℃ under flowing hydrogen either by direct sintering (self- resistance heating) 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 between 92 to 98%.
The main driving force for sintering is the lowering of free energy, which takes place when individual particles grow together, pores shrink, and the high surface area of the compact (i.e., its high excess surface energy) decreases. The decrease in surface area is accomplished by diffusional flow of matter into the pore volume under the action of capillary forces (surface tension force). Besides shrinkage, recovery (change of subgrain structures and strain relief), recrystallization (formation of strain free crystals low in dislocation density), and grain growth occur during sintering, also contributing to the minimization of free energy.
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