The electric current is used to control the sintering progress and as an indicator of the prevailing sintering temperature. The current is kept low-at the beginning due to the higher resistance of the bar (less contact between the grains). It is then gradually raised up to 2700-3000℃. Proper hold points are important to allow the outgassing of impurities. This is particularly important for the sintering of NS-tungsten, because both Al and Si evaporate rapidly above 2150℃. If densification occurs too rapidly, excess dopants will be trapped and may cause difficulties during subsequent working or even bursting of the bar during sintering. Commonly, less than 10ppm Si and Al but 60 to 100ppm K remain in the ingot, the latter in the form of desire potassium bubbles.
Depending on the bar size, the heating up time is between 20 and 60 minutes and the holding time between 30 and 60 minutes. Typical bar dimensions are 15-25 mm *15-25mm*600-920mm. Typical weights are between 1.5 and 6kg. The density of sintered bars is between 88 and 96% of the theoretical value. It is always lower for NS-tungsten (88 to 92%) than for pure tungsten, due to the presence of trapped potassium.
Advantages compared to indirect sintering are short sintering times; temperature inside the bar is higher than on the surface, which favors the diffusion and evaporation of impurities; higher temperature results in a better cleaning effect; a simpler aggregate with comparatively low maintenance costs.
Disadvantages are the dimensional restrictions (only simple shapes can be sintered); low capacity; the loss of the “head” (clipped parts) which did not obtain the temperature due to the heat sink effect of the contacts.
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