2. Sintering
2.1 General. 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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