Tungsten Powder Metallurgy - Sintering
Tungsten Powder Metallurgy - Sintering 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.
Tungsten Powder Metallurgy - Sintering is commonly regarded as taking place in three stages：
*During the early stage, necks are formed between individual particles and grow by diffusion, increasing the interparticle contact area. The powder aggregate shrinks, involving center to center approach of the particles. In this stage, the degree of densification is still low and the pore structure is open and fully connected.
*With increasing neck formation (intermediate stage), the necks become and lose their identity. The pores are assumed to be cylindrical. Their radii vary along their lengths and, with increasing shrinkage, the pore channels break up into small, still partly interconnected segments. During this stage (channel closure stage), pronounced densification occurs and significant grain growth occurs concurrent with shrinkage.
*Finally, in the last stage (isolated pore stage), the pore segments further break up into chains of discrete, isolated pores of more or less spherical symmetry. This stage occurs when about 90% of the theoretical density is achieved. The sintering density then approaches asymptotically the practical limit of 92-98%.
Investigations have shown that the densification is controlled by grain boundary diffusion over most of the densification range, unless at very high densities it becomes controlled by lattice diffusion.
Since the motion of grain boundaries, necessary for grain growth, is impeded by the presence of pores, grain coarsening proceeds at a higher-rate above 97% density. Grain sizes of the as-sintered ingots are commonly in the range of 1 0 to 30µm.
Besides temperature and time, several other parameters influence densification, such as powder particle size, green density, sintering atmosphere, powder purity, compact size/weight, heating rate, thermal gradients, and the presence of insoluble phases such as oxides (Th02, La203, Ce02, 2r02) or metallic potassium (NS-tungsten).
The influence of temperature and time on densification can be estimated by using so-called density diagrams, which are based on approximate sintering models. Nevertheless, empirical rate equations are used for industrial purposes to calculate e necessary sintering times at different temperatures.
Tungsten sintering, in practice, is always performed in reducing atmosphere which removes the oxygen coating of the powder particle surfaces. High-purity dry hydrogen is commonly used. Under vacuum or in inert gas, sintering is retained by residual oxygen, and the desired density will not be achieved.
Since the ductility of tungsten is very sensitive to most of the impurities, purification is important. Therefore, special care must be taken so that, during sintering, evaporation can take place to the desired extent (i.e., as long as there is an open porosity). If the ingot densifies too quickly, impurities can be trapped. Due to the higher sintering temperature, direct sintering is more effective in cleaning than indirect sintering.
The sintering of doped tungsten is a peculiar case in sintering of tungsten. This includes dispersion-strengthened materials such as thoriated tungsten or tungsten with additions of CeO2, La2O3, and ZrO2 as well as NS (non-sag) tungsten used for lamp filaments.
NS-doped tungsten powder contains small inclusions of potassium aluminosilicates, which were incorporated during the reduction process. During sintering, the silicates dissociate thermally and submicron potassium bubbles form in the tungsten ingot. Similar to the oxides, these bubbles pin the-grain boundaries and inhibit grain coarsening during sintering. Since potassium is gaseous during sintering, the bubbles are under high pressure, which is balanced by the surface tension of' the pore. They can be seen as small pores in the fracture surfaces of NS-doped tungsten besides the significantly coarser residual sintering pores, as characteristic for undoped tungsten. They constitute the starting point for subsequent formation of rows of bubbles during thermo-mechanical processing.
Up to the sixties, the only sintering method used in practice was direct sintering. Although still in use for the production of doped tungsten, it has lost its importance. From then on, mainly because of the increasing demand for Iarger parts and the higher capacity of the aggregates, indirect sintering furnaces were developed. This technique is used nowadays as the main route for producing pure tungsten.