The material characteristics and weldability of titanium and titanium alloy plate, and the weldability tests were carried out for the welding defects of titanium and titanium alloy which are easy to produce oxidation, crack and porosity. Through the continuous exploration of titanium and titanium alloy welding process specifications, as well as the reasonable analysis of the problems in the test process, the welding process characteristics and key points of titanium and titanium alloy are summarized.
Titanium and its classification and characteristics
There are three kinds of industrial pure titanium, TA1, TA2 and TA3, which are different in the content of hydrogen, oxygen and nitrogen impurities. These impurities strengthen the industrial pure titanium, but reduce its plasticity significantly. Although the strength of industrial pure titanium is not high, but good plasticity and toughness, especially has a good low temperature impact toughness; At the same time, it has good corrosion resistance. Therefore, this kind of material is mostly used in chemical industry, petroleum industry and so on, in fact, it is mostly used in working conditions below 350℃.
Titanium alloys can be classified into three types according to their annealed structure at room temperature:
Alpha titanium alloys, (alpha + beta) titanium alloys and beta titanium alloys.
Among alpha titanium alloys, Ti-AI alloy of type TA4, TA5 and TA6 and Ti+AI+Sn alloy of type TA7 and TA8 are more commonly used. The strength of the alloy can reach 931N/mm2 at room temperature, and it is stable at high temperature (below 500℃) with good weldability.
Titanium beta alloy is less used in China, and its application scope needs to be further expanded.
Weldability of titanium and titanium alloy
The welding properties of titanium and titanium alloys have many remarkable characteristics, which are determined by the physical and chemical properties of titanium and titanium alloys.
Influence of gas and impurity pollution on welding performance
Titanium and titanium alloys are stable at room temperature. However, in the test table, during the welding process, the liquid droplet and pool metal have a strong absorption of hydrogen, oxygen and nitrogen, and in the solid state, these gases have acted with it. With the increase of temperature, the ability of titanium and titanium alloy to absorb hydrogen, oxygen and nitrogen also increased significantly. It began to absorb hydrogen at about 250℃, oxygen at 400℃ and nitrogen at 600℃. When these gases were absorbed, they would directly cause embrittlement of welding joints, which was an extremely important factor affecting welding quality.
1.1 effects of hydrogen
Hydrogen is the most serious factor that affects the mechanical properties of titanium.
The change of hydrogen content in welding seam has the most significant impact on the impact performance of welding seam. The main reason is that with the increase of hydrogen bomb content in welding seam, the amount of flake or needle TiH2 precipitated in welding seam increases. The strength of TiH2 is very low, so the action of HiH2 in sheet or needle shape is notch, and the impact performance is significantly reduced. The effect of hydrogen content change on strength and plasticity is not very obvious.
1.2 the influence of oxygen
Oxygen in titanium alpha phase and beta have a higher degree of fusion, and can form a gap solid deep phase, the use of titanium crystal wound seriously distorted, so as to improve the hardness and strength of titanium and titanium alloy, so that the plasticity is significantly reduced. In order to ensure the performance of welding connection, in addition to the strict welding protection during welding and welding according to the heat affected zone of the main oxidation, but also limit the basic metal and welding wire oxygen content.
1.3 effects of nitrogen
Under the high temperature of 700 ℃ above, nitrogen and titanium play role, form a rigid titanium nitride (TiN) and nitrogen and titanium gap formation of solid solution caused by lattice slanting Noah, than is caused by the amount of oxygen is more serious, therefore, nitrogen of industrial pure titanium weld tensile strength and hardness, reduce the plastic properties of the weld is more significant than oxygen.
1.4 carbon impact
Carbon is also a common impurity in titanium and titanium alloy. The experiment shows that when the carbon content is 0.13%, the carbon is deep in alpha titanium, and the strength limit of welding seam is increased and the plasticity is decreased, but it is not as strong as oxygen nitrogen. However, when the carbon content of the weld was further increased, reticular TiC appeared in the weld. The number of TiC increased with the increase of carbon content, which made the plasticity of the weld drop sharply, and cracks were easy to appear under the action of welding stress. Therefore, the carbon content of titanium and titanium alloy base material is no more than 0.1%, and the carbon content of welding seam is no more than the carbon content of base material
Weld joint crack problem
Titanium and titanium alloy welding, the welding joint of hot cracking formation of possibility is very small, it is because the titanium and titanium alloy in the impurities such as S, P, C, and low melting eutectic formed by S, P is not easy to appear on the grain boundary, combined with effective crystallization temperature range is narrow small, titanium and titanium alloy solidification shrinkage small, weld metal does not produce hot crack.
During welding of titanium and titanium alloy, cold cracks may appear in the heat-affected zone, which is characterized by delayed cracks that occur several hours or even longer after welding. The crack is related to the diffusion of hydrogen bomb during welding. In the welding process, hydrogen diffused from the high temperature deep pool to the lower temperature heat affected area. The increase of hydrogen content increased the amount of TiH2 precipitated in this area, and increased the brittleness of the heat affected area. To prevent this delay in crack production, the main method is to reduce the source of hydrogen welding joint, invoice, also need to carry out the gas quench treatment.
3. Porosity in weld
When titanium and titanium alloy are welded, pore is the problem that often encounters. The root cause of stomatal formation is the result of hydrogen influence. The formation of porosity in weld metal mainly affects the fatigue strength of the joint.
The main technological measures to prevent the generation of air holes include:
3.1 protect neon gas to be pure, purity should not be lower than 99.99%
3.2 thoroughly remove the scale grease and other organic matters on the surface of welding parts and wires.
3.3 apply good gas protection to the molten pool, control the flow and velocity of argon gas, prevent turbulence and affect the protection effect.
3.4 proper selection of welding process parameters and increase the right to use the deep pool residence time for bubble escape can effectively reduce the porosity.
The gas protection of titanium and titanium alloy welding is the primary factor affecting the quality of welded joint.
Small heat input should be used when welding titanium and titanium alloy.
During TA2 manual argon tungsten arc welding, the source of hydrogen should be strictly controlled to prevent cold crack and porosity.
As long as the welding is carried out strictly in accordance with the requirements of the welding process and effective gas protection measures are taken, high-quality welded joints can be obtained.
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