For tantalum and niobium, electron beam smelting is often used in industrial production. There are two ways of electron beam melting: drop melting and cold flow tank melting. The dropping electron beam smelting is to focus the electron beam from the electron gun on the tantalum and niobium billet. Under the high speed impact of the electron beam, the temperature there rises rapidly and is melted, and then flows into the water-cooled copper casting mold or mold below (dropping), so that the focus sweeps on the billet continuously and the billet is continuously melted. Molten tantalum and niobium dripped down and solidified into ingots. An electron beam focuses on the surface of the molten pool and sweeps it away. In addition to maintaining and controlling the temperature of the molten pool, it also melts the solid particles that may fall into the pool, giving the ingot a good appearance and internal quality. During cold flow tank smelting, tantalum and niobium billets are melted and then dripped into the water-cooled copper flow tank, where they continue to be heated by the electron beam for refining, and then flow to the crystallizers for solidification, that is, their melting, refining and solidification are carried out in different parts of the furnace, which is a continuous operation.
Electron beam smelting is carried out in vacuum, high vacuum degree can make the electron beam stable, therefore, it should be equipped with a strong vacuum system, the gas content of the blank block should also be limited, generally carbon < 800×10-6, oxygen < 1500×10-6, nitrogen < 300×10-6, hydrogen < 50×10-6. For this reason, the billets used for smelting are usually sintered or pre-sintered.
Tantalum and niobium have high melting points of 2996℃ and 2470℃, respectively. Smelting is carried out in vacuum, so impurities with high vapor pressure are evaporated. Therefore, electron beam smelting has a good purification effect. The power of electron beam can be adjusted to control the melting rate and melt temperature to improve the purification effect. According to the purity of the blank block and the purity of the finished product, it can be smelted twice or even three times to further improve the purity.
Electron beam smelting is carried out under the conditions of high temperature, vacuum and open, which has the advantages of good purification effect, but also has the disadvantages of high volatilization loss. Therefore, the amount of alloying elements should be precisely controlled.
