Welding technology of titanium and titanium alloy
Classification and characteristics of titanium and titanium alloy
Table of Contents
TA1, TA2 and TA3 are three kinds of domestic industrial pure titanium. The difference lies in the content of impurities containing hydrogen, oxygen and nitrogen. These impurities strengthen the industrial pure titanium, but reduce the plasticity significantly. Although the strength of industrial pure titanium is not high, its plasticity and toughness are excellent, especially its 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 ℃. According to the room temperature microstructure of the annealed titanium alloy, it can be divided into three types α Titanium alloy（ α+β), Titanium alloy and its application β Titanium alloy. α The Ti AI alloys of TA4, TA5 and TA6 and Ti + AI + Sn alloys of TA7 and ta8 are widely used. The strength of the alloy can reach 931n / mm ~ 2 at room temperature, and it has stable properties and good weldability at high temperature (below 500 ℃). β The application of Ti alloy is less 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.
The influence of gas and impurity pollution on welding performance
At room temperature, titanium and titanium alloy are relatively stable. However, in the test table, during the welding process, the liquid droplet and molten pool metal have a strong absorption of hydrogen, oxygen and nitrogen, and in the solid state, these gases have interacted with them. With the increase of temperature, the ability of titanium and titanium alloy to absorb hydrogen, oxygen and nitrogen also increases obviously. Hydrogen begins to be absorbed at about 250 ℃, oxygen begins to be absorbed at 400 ℃ and nitrogen begins to be absorbed at 600 ℃. After these gases are absorbed, the embrittlement of welded joint will be directly caused, which is a very important factor affecting the welding quality.
- (1) Hydrogen is one of the most important gas impurities affecting the mechanical properties of titanium. The change of hydrogen content in weld has the most significant effect on the impact properties of weld. The main reason is that with the increase of hydrogen content in weld, the amount of flake or needle like TiH2 precipitated in weld increases. The strength of TiH2 is very low, so the effect of flake or needle like hih2 is notch, and the impact property is significantly reduced; The effect of hydrogen content change on strength and plasticity is not obvious.
- (2) The influence of oxygen on the properties of titanium α Mutual harmony β Both of them have higher melting degree, and can form interstitial solid phase. The grain wound of titanium is seriously distorted, which can improve the hardness and strength of titanium and titanium alloy, but reduce the plasticity significantly. In order to ensure the performance of welding, in addition to strictly preventing the main oxidation in the weld and heat affected zone, the oxygen content in the base metal and welding wire should also be limited.
- (3) The effect of nitrogen is more serious than that of oxygen when nitrogen and titanium form interstitial solid solution. Therefore, nitrogen is more significant than oxygen in improving the tensile strength and hardness of industrial pure titanium weld and reducing the plasticity of weld.
- (4) The influence of 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 the titanium α In titanium, the strength limit of weld is increased and the plasticity is decreased, but the effect is not as strong as that of oxygen and nitrogen. However, when the carbon content of the weld is further increased, the network tic appears in the weld, which increases with the increase of carbon content, resulting in a sharp decrease in the weld plasticity and easy to crack under the effect of welding stress. Therefore, the carbon content of titanium and titanium alloy base metal is not more than 0.1%, and the carbon content of weld is not more than that of base metal.
Welding joint crack
When welding titanium and titanium alloy, the possibility of hot crack is very small. This is because there are few impurities such as s, P and C in titanium and titanium alloy, and the eutectic with low melting point formed by s and P is not easy to appear on the grain boundary
Narrow, titanium and titanium alloy solidification shrinkage is small, weld metal will not produce hot cracks. When titanium and titanium alloy are welded, cold cracks can appear in the heat affected zone, which is characterized by delayed cracks occurring several hours or even longer after welding. The research shows that the crack is related to the diffusion of hydrogen bomb during welding. During the welding process, hydrogen diffuses from the high temperature deep pool to the lower temperature heat affected zone. With the increase of hydrogen content, the amount of TiH2 precipitated in the zone increases, which increases the brittleness of the heat affected zone. In addition, due to the larger structural stress caused by the volume expansion of hydride precipitation, and the diffusion and accumulation of hydrogen atoms to the high stress part of the zone, cracks are formed. The main way to prevent this kind of delayed crack is to reduce the source of hydrogen in the welding joint, and also to carry out the fire suppression treatment in the dark air.
Porosity in weld
When welding titanium and titanium alloy, air hole is a common problem. The fundamental reason for the formation of pores is due to the effect of hydrogen. The porosity in weld metal mainly affects the fatigue strength of the joint. The technological measures to prevent blowholes are as follows:
- (1) The purity of neon should not be less than 99.99%.
- (2) Thoroughly remove organic matters such as oxide scale and oil on the surface of weldment and welding wire.
- (3) Good gas protection should be applied to the molten pool, and the flow rate and velocity of argon should be controlled to prevent turbulence and affect the protection effect.
- (4) The porosity can be effectively reduced by choosing the welding parameters correctly and increasing the residence time of deep pool.
Experiment on manual argon arc welding of titanium plate and tungsten plate
Argon tungsten arc welding is widely used in titanium and titanium alloy welding, and vacuum argon filling welding is also widely used. Under the protection and cooling of argon flow, the heat of arc is concentrated, the current density is high, the heat affected zone is small and the welding quality is high.
When welding titanium and titanium alloy, when the temperature is higher than 500 ℃ ~ 700 ℃, it is easy to absorb the gas, hydrogen and nitrogen in the air, which seriously affects the welding quality. Therefore, during welding of titanium and titanium alloy, the weld zone with full weld pool and high temperature (above 400 ℃ ~ 650 ℃) must be strictly protected. Therefore, special protection measures must be taken during welding of titanium and titanium alloy, that is, the welding moment with larger spray size is used to expand the area of gas protection zone. When the nozzle is not enough to protect the weld and high temperature metal near the weld, the argon filled protection hood is needed. The color of weld and near seam area is the standard of protection effect. Silver white indicates the best protection effect, while yellow indicates slight oxidation, which is generally allowed.
Considering the practicability and efficiency of engineering use, we first prepared a simple towing hood. Argon gas enters into the distribution pipe from the air inlet and directly enters the protection area through the hole of the distribution pipe. With this kind of cover, the welding protection effect is not very good, and the weld bead is dark blue. According to the analysis, the air flows directly into the reserve from the distribution pipe. The gas flow is not very uniform and stable, so that the high temperature weld bead is not well protected and oxidized. Therefore, we further improved the structure of the hood. Argon enters the distribution pipe through the air inlet and then goes down through the top of the hood; Through the porous plate, the porous plate mainly plays the role of gas screen and distribution, which makes the argon flow more stable, the welding protection effect is better, and the weld bead is silver or yellow. The length of the hood is 40 ~ 100 mm, and the material is brass. In argon arc welding of titanium and titanium alloy, attention should also be paid to the protection of the northern side of the weld bead. Considering the welding deformation, we adopt the method of slotting and fixing the copper backing plate for argon filling protection. In order to fully protect the back of the weld bead, a porous copper tube is added in the groove to make the argon enter the protection area evenly through the copper tube hole. The protection effect is good, and the back of the weld bead is silvery white. Selection of welding process and parameters for manual argon arc welding of tungsten plate.
(1) Preparation before welding
The surface quality of weldment and welding wire has great influence on the mechanical properties of welded joint, so it must be cleaned strictly. Mechanical cleaning and chemical cleaning can be used for iron plate and titanium welding wire. 1) For the weldments with low quality requirements or difficult pickling, fine sandpaper or stainless steel wire brush can be used for mechanical cleaning, but it is better to scrape the titanium plate with cemented carbide yellow to remove the oxide film. 2) Before chemical cleaning welding, the sample and welding wire can be pickled, and the pickling solution can be HF5% + HNO335% water solution. Rinse with clean water after pickling and weld immediately after drying. Or use acetone, ethanol, carbon tetrachloride, methanol, etc. to wipe the titanium plate groove and its two sides (within 50 mm each), welding wire surface, fixture and titanium plate contact parts.
(2) Selection of welding equipment
For TIG welding of titanium and titanium alloy gold tungsten plate, DC TIG welding power source with descending external characteristics and high frequency arc starting shall be selected, and the gas delivery delay time shall not be less than 15 seconds to avoid oxidation and pollution.
(3) Selection of welding materials
The purity of argon should not be less than 99.99%, the dew point should be below – 40 ℃, and the total impurity content should be less than 0.001%. When the pressure in the argon cylinder drops to 0.981mpa, it should be stopped to prevent affecting the quality of welded joint. In principle, titanium wire with the same composition as base metal should be selected. Sometimes, in order to improve the plasticity of weld metal, welding wire with slightly lower strength than base metal can be selected.
(4) Selection principle of groove form
Minimize the number of welding layers and welding metal. With the increase of the number of welding layers, the cumulative suction of the weld seam increases, which affects the performance of the welded joint. Because the weld pool size of titanium and titanium alloy is large, the specimen has a single V-shaped 70 ~ 80 ° groove.
(5) Specimen assembly and positioning welding
In order to reduce welding deformation, positioning welding is carried out before welding. Generally, the spacing of positioning welding is 100 ~ 150 mm and the length is 10 ~ 15 mm. The welding wire, welding process parameters and gas protection conditions used for positioning welding shall be the same as those used for welding joint. The gap is 0 ~ 2mm and the blunt edge is 0 ~ 1.0mm.
(6) Selection of welding parameters
By comparing the properties of the welded joints under different processes, we found out a more suitable welding process specification.
- Process (1), the welding current is 150A, 170a, 180A, according to this parameter welding, welding joint surface, showing dark blue, gold color, indicating that the joint oxidation is serious, does not meet the technical requirements, this process is not desirable.
- Process (2), the welding current is reduced to 120a, 150A and 160A. According to this parameter, the weld surface appears golden purple and dark yellow, and there is no defect in X-ray flaw detection, but the mechanical property bending test is unqualified, which indicates that the welding joint plasticity is significantly reduced and can not meet the technical requirements.
- Process (3), the welding current is 95A, 115a, 120a, according to this parameter welding, the weld surface is silver white, light yellow, X-ray flaw detection without defects, but the mechanical property bending test is qualified, tensile strength also meets the requirements, the performance of welded joint meets the technical requirements, this process is more appropriate. When titanium and titanium alloy are welded, there is a tendency of coarse grain, which directly affects the mechanical properties of welded joints. Therefore, the selection of welding process parameters should not only consider the weld metal oxidation and the formation of pores, but also consider the factors of grain coarsening, so the welding heat input should be as small as possible. Process (1) and process (2) are more serious than process (3) due to the larger factors of welding specification. The results of microscopic metallographic test show that the coarsening degree of joint grain is more serious than that of process (3). Therefore, the mechanical properties of welded joints are poor.
The selection of gas flow rate should be based on the good protection effect. Too large flow rate is not easy to form stable laminar flow, and increase the cooling rate of the weld, so that there are more cracks on the surface layer of the weld α Phase, and even cause microcracks. When the argon flow in the hood is not enough, the weld presents different oxidation color; When the flow rate is too large, it will interfere with the air flow of the main nozzle. The argon flow on the back of the weld should not be too large, otherwise the gas protection effect of the first layer of weld on the front will be affected.
(7) Operation essentials of manual TIG welding of titanium and titanium alloy
- 1) During manual argon arc welding, the minimum included angle (10 ~ 15 °) between welding wire and weldment should be kept as far as possible. The welding wire shall be fed into the molten pool smoothly and evenly along the front end of the molten pool, and the end of the welding wire shall not be moved out of the argon protection zone.
- 2) When welding, the welding gun basically does not swing horizontally. When it needs to swing, the frequency should be low and the swing amplitude should not be too large, so as to prevent affecting the protection of argon.
- 3) When the arc is broken and the weld is finished, the argon gas shall be continuously used for protection until the weld and the metal in the heat affected zone are cooled below 350 ℃.
(8) Quality inspection
- 1) The appearance inspection shall comply with GB/T 13149-91.
- 2) Deep X-ray injury was in accordance with JB 4730-94.
- 3) The mechanical property test conforms to GB/T 13149-91.
- 1. The gas protection of titanium and titanium alloy welding is the primary factor affecting the quality of welded joint.
- 2. When welding titanium and titanium alloy, small heat input should be used as far as possible.
- 3. During TA2 manual TIG welding, the source of hydrogen should be strictly controlled to prevent cold cracks and pores.
- 4. As long as the welding process requirements are strictly followed and effective gas protection measures are taken, high quality welded joints can be obtained.
Source: China Titanium Flange Manufacturer: www.titaniuminfogroup.com