The influence of titanium's properties on titanium welding one The influence of oxygen and nitrogen
Oxygen and nitrogen are interstitially solid-solved in titanium, causing distortion of the titanium lattice, increased deformation resistance, increased strength and hardness, and decreased plasticity and toughness. The presence of weld oxygen and nitrogen in the weld seam is unfavorable and should be avoided.
2. The influence of hydrogen
The increase of hydrogen will cause a sharp decrease in the impact toughness of the weld metal of titanium, while the plasticity decreases slightly. Hydrides will cause brittleness of the joint.
3. The influence of carbon
At room temperature, carbon is interstitially solid-solved in titanium, increasing the strength and decreasing the plasticity. However, it is not as obvious as oxygen and nitrogen. When the carbon content exceeds the solubility, TiC, which is hard and brittle, is formed and distributed in a network-like manner, which is prone to cracking. The national standard stipulates that the carbon content of titanium and its alloys should not exceed 0.1%. During welding, the oil stains on the workpiece and the welding wire can increase the carbon content. Therefore, it is necessary to clean them up during welding.
III. Analysis of the weldability of titanium
Titanium has good weldability. Due to its small thermal conductivity (0.041 Cal/℃·cm·s), titanium metal only melts within the arc burning range and has good fluidity. Moreover, it has a small thermal expansion coefficient (8.6×10-6/℃, much smaller than that of carbon steel), which greatly improves the weldability of titanium metal.
IV. Relationship between the color of titanium weld seams and welding quality
1. Color changes of titanium and titanium alloy titanium tube weld seams and the mechanism of defect generation
The defects and generation mechanism of titanium and titanium alloy titanium tube weld seams are as follows. When welding titanium tubes, the protective layer of argon gas formed by the argon arc welding gun can only protect the welding pool from the harmful effects of air, but has no protective effect on the weld seam and its adjacent area that have already solidified and are in a high-temperature state. In this state, the titanium tube weld seam and its adjacent area still have a strong ability to absorb nitrogen and oxygen from the air. From 400℃ to 600℃, oxygen is absorbed, and from 600℃ to 800℃, nitrogen is absorbed. The air contains a large amount of nitrogen and oxygen.
As the oxidation level gradually increases, the color of the titanium tube weld seam changes and the plasticity of the weld seam decreases. The rules are: silver-white (no oxidation), golden yellow (TiO, approximately at 250℃ titanium begins to absorb hydrogen), blue (Ti2O3 oxidation is slightly more serious), gray (TiO2 oxidation is severe).
2. The color of the titanium weld seam surface can be used to judge the quality of titanium welding
The tests for different colors and hardness of titanium weld seams are as follows:
(1) It has been proved through experiments that as the color of the weld seam deepens, that is, the increase of the oxidation degree of the weld seam, the hardness of the weld seam increases. Through experiments conducted by peers, the hardness of titanium metal increases, and harmful substances such as oxygen and nitrogen in the weld seam increase, greatly reducing the quality of welding.
(2) The weldability of titanium and its chemical and physical properties have a very important relationship. However, the key lies in that in high-temperature conditions, the high reactivity of titanium is easily affected by air pollution. During heating, its grains expand, and when the welded joint cools, it will form brittle phases. The melting point of titanium is very high, reaching 1668±10℃, which is more than the energy required for welding steel. At the same time, the chemical activity of titanium is relatively active, reacting with O and H much more easily than steel. At 600℃, it rapidly combines. At 100℃, it absorbs H and O in large quantities, and the ability to dissolve H is several tens of thousands times greater than that of steel, thereby generating hydrogenated titanium, causing a sharp decrease in toughness. The increase in gas impurities increases the tendency of cold cracking and delayed cracking, and increases notch sensitivity. Therefore, the purity of argon gas used for welding should not be lower than 99.99%, the humidity should not exceed 0.039%, and the hydrogen content of the welding wire should be below 0.002%. The heat transfer coefficient of titanium is 1/2 that of steel, and it undergoes a transformation from α to β at 882℃. At higher temperatures, β grains rapidly grow in leaps and bounds, and the performance deteriorates significantly. Therefore, the temperature should be strictly controlled, especially the duration of high-temperature stay in the welding heat cycle. When welding titanium, there are no problems of hot cracking and intergranular cracking, but there is a problem of porosity, especially when welding α+β alloys.
V. Precautions for Titanium Welding
1. During titanium welding, strict protection should be carried out for the welding area and the post-welding high-temperature area to prevent the entry of air into the welding and high-temperature areas, which would have a serious impact on the weld quality. Therefore, 99.99% pure argon and trailing protective covers are necessary.
2. The weld bevel should be machined mechanically (grinding is not allowed).
3. Spot welding should be avoided and high-frequency arc starting should be adopted.
4. Post-weld heat treatment should be avoided; if post-weld heat treatment is necessary, the heat treatment temperature should be less than 650℃.
