Weldability Of Tungsten

Sep 03, 2020

Tungsten is a silver-white precious metal. It has a high melting point and is difficult to melt. The linear expansion coefficient of tungsten is very small, the elastic modulus and strength are very high, and the deformation of thermal expansion and contraction is also small. Tungsten has good electrical and thermal conductivity, low vapor pressure and strong corrosion resistance. Pure tungsten is not only used as an additive in the steel and non-ferrous metallurgy industries, but also plays an important role in many industries. For example, the manufacture of electrical contacts and high-temperature-resistant components (rocket engine nozzles, high-temperature furnace heating elements and reflective screens, etc.), nuclear industry cladding materials, and reactor high-temperature components.

The welding characteristics of tungsten and its alloys mainly include the following:

(1) The weld metal is prone to pores. Tungsten and its alloys easily absorb oxygen, nitrogen and other gases during the welding process. Oxidation occurs at around 300°C. After the metallurgical reaction of the arc, the formed oxides and nitrides will generate carbon monoxide and water. If they cannot be precipitated in time, pores will occur. Although tungsten and hydrogen cannot produce stable hydrides and will not produce hydrogen embrittlement, pores will occur when hydrogen is saturated. All of the above are from the gas pollution in the air. Therefore, inert gas should be used to protect the tungsten welding to prevent the generation of pores.

(2) The tendency of welding cracks is high. When welding tungsten and its alloys, welding cracks will occur. The main reasons for the cracks are: a. Tungsten and its alloys are brittle and have poor crack resistance. b. When the content of oxygen, nitrogen, etc. in the weld is high, nitrides and oxides will be produced, which are distributed at the grain boundary, which enhances its brittleness and at the same time increases the tendency of cracks. c. Tungsten and its alloys have low plasticity and high strength, and are prone to welding stress during welding, which increases the tendency of cracks.

The commonly used welding methods of tungsten and its alloys are as follows:

(1) Fusion welding tungsten argon arc welding and molten gas arc welding are two important methods for welding tungsten and its alloys, welding in a confined space, and using inert gas protection. In order to obtain good welding quality, do not fix the weldment during the welding process, which can effectively reduce the stress. At the end of the welding, it is best to use a lead plate. This can prevent crater cracks at the end of the weld. In air, TIG welding of tungsten and its alloys can also meet the performance requirements of joints. However, the quality of the joint is not very stable, and the DC positive connection method should be used for welding. After welding, the stress relief treatment should be performed below the recrystallization temperature of the base material.

(2) Electron beam welding of tungsten and its alloys For tungsten and its alloys, electron beam welding is the most ideal welding method. Vacuum before welding, the degree of vacuum is 1.33×10-4Pa. Compared with argon arc welding, electron beam welding can get larger penetration and narrower heat affected zone joints.