Qualified billet is one of the keys to tungsten material production. To make a good billet, you must first select qualified tungsten powder. The characteristics of the powder (average particle size, particle size distribution, chemical composition), mixing, forming and sintering processes have a direct impact on the composition, density and microstructure of the blank, and strongly affect the processing and use performance of the product.
The silicon, aluminum, and potassium added to the non-sagging tungsten wire are added in the form of oxides in tungsten trioxide or "blue tungsten" (a mixture of a variety of low-order tungsten oxides). The mixture is usually containing hydrofluoric acid The solution is washed to remove impurities in the powder. The blanks for the production of filaments and small sheets are mostly formed on a press, and can also be formed by isostatic pressing.
The size of the powder blank is generally 12×12×400mm, and there are also larger round rods, square rods or rectangular rods. The powder body is pre-fired in a hydrogen atmosphere at 1200°C for 1 hour to make it have a certain strength and conductivity, and then conduct energization and self-resistance sintering.
Electrified self-resistance sintering, commonly known as "vertical melting", is a method developed in tungsten processing. The principle is to pass the current directly through the sintered billet, which generates Joule heat due to the electrical resistance of the billet itself. This heat is used to sinter the billet. The sintering current is usually 90% of the fusing current. The resulting blank is a self-resisting sintered bar (also called a vertical melting bar). The general standard for vertical frit bars that can be processed into wires is to control the number of cross-sectional crystal grains to be about 10,000 to 20,000 per square millimeter, and to have a density of 17.8 to 18.6 g/cm3. For pipes, sheets or other large-size products, isostatic pressing (pressure above 2500kg·N/mm2) is often used for forming, and sintered under vacuum or hydrogen protection at a high temperature of 2300-2700℃.