During sintering, the pressed porous powder will undergo many complex chemical and physical changes, along with the interaction between impurities and the elimination of pores, etc., which ultimately lead to the densification of the billet and the removal of impurities. The sintering of Ta and W is carried out at a temperature lower than the melting point, which belongs to the solid-phase binary system sintering. The formation of the alloy is mainly achieved by diffusion. After sintering at 4 temperatures, the appearance of Ta-W alloy strip is different. Among them, the alloy strip sintered at 1 800 ℃ and 2 100 ℃ has a good straightness. After measurement, the bending degree is less than 3 mm/500 mm (the ratio of the total chord height of the total length bending to the total length). The Ta-W alloy strip is slightly bent after sintering at 2 300 ℃, and the bending degree is less than 5 mm/500 mm. The alloy strip sintered at 2 500 ℃ is seriously bent, the bending degree is greater than 10 mm/500 mm, and the bonding is serious, affecting the normal operation of the oven. Density is an important index to measure the quality of blank strip and whether it can be processed. The relative density of the Ta-W alloy strip was measured by drainage method. The relative density (the ratio of actual density to theoretical density) of the Ta-W alloy strip at 2 300 ℃ and 2 500 ℃ reached more than 90%, but the relative density of the Ta-W alloy strip at 1 800 ℃ was less than 70%.
In electron beam melting, the alloy strip must be bundled and welded into a certain length of electrode, and the whole step in the melting process, so there are certain requirements for the strength and impact resistance of the alloy bar. The Ta-W alloy strips at 4 sintering temperatures were taken for 2 m high free drop test. It was found that the Ta-W alloy strips at 1 800 ℃ and 2 100 ℃ were easy to break brittle, while the Ta-W alloy strips at the other two sintering temperatures had higher strength and toughness, and passed the test successfully.
Generally speaking, impurities exist in the raw material powder in two forms: (1) in the form of compounds included in the metal pores or grains; (2) Dissolved in metal in solid solution form. At 400 ~ 800 ℃, most of the H2 in the raw material exits, and the rest H exists in the form of solid solution in Ta, which is very stable at room temperature. The solubility in Ta decreases with the increase of temperature, and decomposing and escaping at 1 000 ~ 1 200 ℃. Nitride is also very stable, at 2 000 ℃ above diffusion to the metal surface, volatilization removal. During high temperature sintering, O and low melting point impurities Fe, Ni, Cr and Mo form low cost oxides. Due to their high vapor pressure, they volatilize at 1 500 ~ 1 900 ℃. Si is volatilized at 1 600 ~ 1 900 ℃ in the form of low-priced oxides. Ti has a high melting point and begins to volatilize in the form of low-priced oxides at 1 800 ~ 2 000 ℃.
