Material properties: Niobium has the characteristics of low elastic modulus, poor thermal conductivity, low mechanical strength, high expansion coefficient and high plasticity in refractory alloys. Tantalum has a high melting point, a melting point as high as 2996 °C, high temperature strength second only to molybdenum and tungsten, good corrosion resistance, and can heat most acidic corrosive media other than sulfuric acid.
Determination of welding temperature: pure niobium recrystallization temperature between 980~1090℃, recrystallization of about 1200℃, niobium, tantalum and stainless steel welding temperature should be controlled at the corresponding recrystallization temperature; The recrystallization temperature of niobium and tantalum alloys increases with the addition of other elements, and the welding temperature can be increased during brazing.
Selection of atmosphere during welding: Niobium and tantalum are more sensitive to impurity gases such as nitrogen, oxygen and hydrogen at welding temperatures. More than 200 ℃ will be combined with oxygen strong oxidation; Above this temperature, niobium tantalum will also strongly absorb hydrogen and nitrogen, making the metal brittle and hard, and the mechanical properties of the welded joint deteriorate.
Surface treatment before welding: mechanical method is used to remove the thick oxide layer on the surface; Clean with dilute hydrochloric acid and hot chromic acid successively, plating copper or nickel on the premise of keeping the wet film on the surface of the metal parts, and carry out diffusion treatment above 1000℃. Choice of solder: Because silver or silver alloy is easy to dissolve in niobium-tantalum, making the base material near the niobium-tantalum weld brittle, so avoid using silver-based solder; Palladium-based, gold-based, nickel-based and other solders can be selected, and nickel-based solders can be selected for cost-sensitive workpieces. Nickel-based solder joints are brittle and hard compared with gold-based solders.
Niobium tantalum welding selection of high purity atmosphere or high vacuum environment for brazing, the selection of high purity gas, generally choose not less than 99.996% purity, dew point is not higher than -60℃ argon; When the vacuum atmosphere is selected, the vacuum degree should not be less than 8.8*10-3Pa.
