Weldability of titanium and titanium alloys
1. Effect of gas and impurity pollution on welding performance
At room temperature, titanium and titanium alloys are relatively stable. However, during the welding process, the wave droplets and molten pool metal have a strong ability to absorb hydrogen, oxygen, and nitrogen, and in the solid state, these gases have interacted with them. As the temperature increases, the ability of titanium and titanium alloys to absorb hydrogen and oxygen also increases significantly. It begins to absorb hydrogen at about 2509, oxygen from 400°C, and nitrogen from 600°C. These gases are After absorption, it will directly cause embrittlement of the welded joint, which is an extremely important factor affecting the quality of welding.
(1) Influence of hydrogen
Xenon is the most serious factor among gas impurities that affects the mechanical properties of titanium. Changes in the hydrogen content of the weld have the most significant impact on the impact performance of the weld. The main reason is that as the hydrogen content of the weld increases, the flake or needle-like Tih2 precipitated in the weld increases.
(2)The influence of oxygen
Oxygen has a higher solubility in both the α and β phases of titanium. In order to ensure welding performance, in addition to strictly preventing oxidation of the weld and the heat affected zone of the weld during the welding process, the oxygen content in the base metal and welding wire should also be limited.
(3) Effect of nitrogen
At high temperatures above 700', nitrogen and titanium react violently to form brittle and hard titanium nitride (TiN). The degree of lattice distortion caused by the formation of interstitial solid solution in titanium nitride is greater than the effect caused by an appropriate amount of oxygen. It is more serious. Therefore, nitrogen is more significant than oxygen in improving the tensile strength and hardness of industrial pure titanium welds and reducing the plastic properties of welds.
(4)The impact of carbon
Carbon is also a common impurity in titanium and titanium alloys. Experiments show that when the carbon content is 0.13%, the carbon is deep in α-titanium, the strength limit of the weld is somewhat increased, and the plasticity is somewhat decreased, but it is not as strong as the effect of oxygen and nitrogen. However, when the carbon content of the weld is further increased, network TiC appears in the weld, and its number increases with the increase in carbon content, causing the plasticity of the weld to drop sharply, and cracks are prone to occur under the action of welding stress. Therefore, the carbon content of titanium and titanium alloy base materials is not greater than 0.1%, and the carbon content of welds does not exceed the carbon content of the base metal.
2. Problems with cracks in welded joints
When titanium and titanium alloys are welded, the possibility of hot cracks in the welded joint is very small. This is because the impurities such as S, P, and (in titanium and titanium alloys) are very small, and the low melting point eutectic formed by S and P is not easy to appear in the welding joints. At the grain boundaries, coupled with the narrow effective crystallization temperature range, titanium and titanium alloys shrink little when they solidify, and the weld metal will not produce hot cracks. When titanium and titanium alloys are welded, cold cracks may appear in the heat-affected zone, which are characterized by cracks Delayed cracks that occur several hours or even longer after welding are called delayed cracks. Research shows that this type of crack is related to the diffusion of hydrogen and nitrogen during the welding process. During the welding process, hydrogen diffuses from the high-temperature deep pool to the lower-temperature heat-affected zone. The increase in hydrogen content increases the amount of TÌH2 precipitated in this area, increasing the brittleness of the heat-affected zone. In addition, due to the volume expansion during hydride precipitation, large tissue stress is caused. In addition, xenon atoms diffuse and aggregate to high-stress parts in this area. The main way to prevent the formation of cracks and prevent such delayed cracks is to reduce the source of hydrogen in the welded joints and perform vacuum annealing after the welding.
3. Porosity problem in welds
Porosity is a common problem when welding titanium and titanium alloys. The fundamental reason for the formation of pores is the result of the influence of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint. The main process measures to prevent the occurrence of pores include:
