Tantalum can react with high-temperature gases (except inert gases), and O2, N2, H2, etc. can penetrate into its interior, making it brittle. If it comes into contact with nascent H, it will also absorb hydrogen and become brittle. Therefore, tantalum material equipment must not come into contact with more active metals such as Fe, Al, and Zn, as it is easy to form a tantalum-iron (Al, Zn) primary cell. The hydrogen produced by the reaction of this primary cell will damage the tantalum cathode, causing the equipment to fail. If a very small piece of platinum (with an area approximately one ten-thousandth that of tantalum) with hydrogen overvoltage is connected to tantalum, all the hydrogen will be released on the platinum, thus avoiding the damage of hydrogen to tantalum.
Tantalum has excellent corrosion resistance, but it is expensive. Therefore, its main application forms are composite plates and linings. To reduce costs, the thickness of the tantalum layer is expected to be as thin as possible. As a result, the welding of composite plates or linings is very difficult because the melting points of tantalum and steel are quite different. (The melting point of tantalum is 2996℃.) The melting point of steel is 1400℃, and Fe and Ta will form a brittle intermetallic compound Fe2Ta at high temperatures. If measures are not taken properly, it is easy to cause weld cracking.
In the welding of thin-layer tantalum-steel composite plates or linings, the thickness of the cladding has a significant impact on its weldability. Figure 1 is a schematic diagram of the welding of Ta1/16MnR composite plates. The smaller the thickness h of the cladding, the higher the temperature T at the composite interface. When T > 1500℃, a melting zone will appear on the 16MnR at the interface. At 1460℃, Fe and Ta will undergo eutectic reactions, generating the brittle intermetallic compound Fe2Ta. Under the action of welding stress, cracks are prone to occur, and on the tantalum side of the interface, they will expand towards the tantalum weld pool. In severe cases, through cracks may occur. At this time, the molten iron at the base layer will diffuse through the through cracks into the tantalum weld pool. And it reacts with tantalum to form the brittle compound Fe2Ta, causing the weld seam to crack. The primary factor to prevent this phenomenon is to appropriately increase the thickness of the cladding or take other measures to reduce the interface temperature. For example, a layer of other metals with rapid heat conduction is pre-laminated on the interface to transfer the heat generated during welding to the surrounding areas. Regarding the relationship between the thickness of the cladding and the solderability of the composite board, after a large number of experiments, a model of the relationship between the interface temperature Ts and the thickness h of the cladding can be established.
