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Titanium and titanium alloy welding process

Public number: Welding and Cutting Alliance Source: Time: 2020-02-21 02:03:26

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Classification and characteristics of titanium and titanium

There are three types of domestic industrial pure titanium: TA1, TA2, and TA3. The difference lies in the content of impurities containing hydrogen and nitrogen. These impurities strengthen industrial pure titanium, but the plasticity is significantly reduced. Although industrial pure titanium has low strength, it has excellent plasticity and toughness, especially good low temperature impact toughness; meanwhile, it has good corrosion resistance. Therefore, this material is mostly used in chemical industry, petroleum industry, etc. In fact, it is mostly used in working conditions below 350 ° C. According to the room temperature structure of the annealed state of titanium alloy, titanium alloy can be divided into three types: α-type titanium alloy, (α + β) -type titanium alloy and β-type titanium alloy. Among the α-type titanium alloys, the Ti-AI series alloys of the TA4, TA5, and TA6 types, and the Ti + AI + Sn alloys of the TA7 and TA8 types are widely used. At room temperature, the strength of this alloy can reach 931 N / mm2, and its performance is stable at high temperatures (below 500 ° C), and its weldability is good. β-type titanium alloy has less application in China, and its application range needs to be further expanded.

Weldability of titanium and titanium alloys

 

The welding properties of titanium and titanium alloys have many significant characteristics. These welding characteristics are determined by the physical and chemical properties of titanium and titanium alloys.

1. Influence of gas and impurity pollution on welding performance

At normal temperature, titanium and titanium alloys are relatively stable. However, in the test table, during the welding process, liquid droplets and molten pool metals have a strong absorption of hydrogen, oxygen, and nitrogen, and these gases have interacted with them in the solid state. As the temperature rises, the ability of titanium and titanium alloys to absorb hydrogen, oxygen, and nitrogen also rises significantly. It starts to absorb hydrogen at about 250 ° C, starts to absorb oxygen at 400 ° C, and starts to absorb nitrogen from 600 ° C. These gases After being absorbed, it will directly cause embrittlement of the welded joint, which is a very important factor affecting the welding quality.

   

(1) Hydrogen is the most influential factor in the mechanical properties of titanium in hydrogen impurities. The change of hydrogen content in the weld has the most significant effect on the impact performance of the weld. The main reason is that as the amount of hydrogen bomb in the weld increases, the amount of flaky or needle-like TiH2 precipitated in the weld increases. The strength of TiH2 is very low, so the effect of sheet-like or needle-shaped HiH2 is notched, and the combined impact performance is significantly reduced; the effect of changes in the hydrogen content of the weld on the improvement of strength and plasticity is not very obvious.

   

(2) Effect of oxygen Oxygen has a higher melting degree in both the α phase and β phase of titanium, and can form interstitial solid phase. The crystal wounds using right titanium are seriously distorted, thereby increasing the hardness of titanium and titanium alloys. And strength, but plasticity is significantly reduced. In order to ensure the performance of the welding joint, in addition to strictly preventing the main oxidation of the weld seam and the welding according to the heat affected zone during the welding process, the oxygen content in the base metal and the welding wire should also be limited.

   

(3) Effect of nitrogen At high temperatures above 700 ° C, nitrogen and titanium have a dramatic effect, forming brittle and hard titanium nitride (riN) and the degree of lattice distortion caused by nitrogen and titanium forming a gap solid solution, compared to The consequences caused by the amount of oxygen are more serious. Therefore, nitrogen has a more significant effect on improving the tensile strength and hardness of industrial pure titanium welds and reducing the plastic properties of welds than oxygen.

(4) The effect 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 weld strength limit is increased somewhat, and the plasticity is somewhat reduced, but less than oxygen. The effect of nitrogen is strong. However, when the carbon content of the weld was further increased, the mesh TiC appeared in the weld, and its amount increased with the increase of the carbon content, which caused the plasticity of the weld to decrease sharply and cracks easily occurred under the effect of welding stress. Therefore, the carbon content of the base material of titanium and titanium alloy is not more than 0.1%, and the carbon content of the weld does not exceed the carbon content of the base material.

2. Weld joint crack problem

When titanium and titanium alloys are welded, the possibility of thermal cracks in the weld joint is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is small. Grain boundary, plus effective crystallization temperature interval

Narrow, small shrinkage of titanium and titanium alloys during solidification, and the weld metal will not generate thermal cracks. Cold welding of titanium and titanium alloys can occur in the heat-affected zone on time, which is characterized by the occurrence of cracks several hours or more after welding, which is called delayed cracking. Studies have shown that this crack is related to the diffusion of hydrogen bombs during welding. During the welding process, hydrogen diffuses from the high-temperature deep pool to the lower-temperature heat-affected zone. Increasing the hydrogen content increases the amount of TiH2 precipitated in this zone, increasing the brittleness of the heat-affected zone. In addition, due to the volume expansion during the precipitation of hydride, larger tissue stress In addition, the hydrogen atoms diffuse and accumulate in the high stress parts of the region, so that cracks are formed. The method of preventing such delayed cracks is mainly to reduce the source of hydrogen in the welded joints. When invoices are also sent, the flames are suppressed.

    

3.Blowhole problem in weld

   

Porosity is a common problem when welding titanium and titanium alloys. The root cause of stomata is the result of the effects of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint. The main technological measures to prevent pores are:

(1) The protection of neon gas should be pure, and the purity should not be less than 99.99%

(2) Thoroughly remove organic matters such as scale oil on the surface of the welding piece and the surface of the welding wire.

(3) Apply good gas protection to the molten pool, control the flow and velocity of argon gas, prevent turbulence, and affect the protection effect.

(4) Correctly select the welding process parameters, increase the use of the deep pool residence time and the right to use the bubbles to escape, which can effectively reduce the pores.

      

3. TIG welding of titanium plate by manual tungsten plate

Titanium and titanium alloy welding production is the most commonly used tungsten plate argon arc welding, vacuum filled argon welding method is also very common. Under the protection and cooling of argon gas flow, the arc welding is more concentrated, the current density is higher, the heat affected zone is small, and the welding quality is higher.

  

When titanium and titanium alloys are welded, when the temperature is higher than 500 ℃ ~ 700 ℃, it is easy to absorb gas, hydrogen and nitrogen in the air, which seriously affects the welding quality. Therefore, when welding titanium and titanium alloys, the weld zone of the molten pool in full and high temperature (400 ℃ ~ 650 ℃) must be strictly protected. Therefore, special protection measures must be taken when welding titanium and titanium alloys. That is, the welding moment with a larger spray size is used to expand the area of the gas protection area. When the nozzle is not enough to protect the weld and high-temperature metal near the seam area, an argon protective cover must be attached. The color of the weld and near-seam areas is a standard wing for protection. Silvery white means the best protection, yellow is slightly oxidized and generally allowed.

Considering the practicability and efficiency of the project, we first prepared a simple trailer. Argon enters the distribution pipe from the air inlet, and directly enters the protection zone through the distribution pipe hole. With this drag cover, the welding protection effect is not very good, and the weld bead is dark blue. According to the analysis, the airflow enters the protection area directly from the distribution pipe. The air flow is not very uniform and stable, which makes the high-temperature weld bead protection difficult to be oxidized. Therefore, we have further improved the structure of the hood. Argon gas enters the distribution tube from the air inlet and returns to the top of the hood. After passing through the perforated plate, the perforated plate mainly functions as a gas sieve and distribution to make the argon flow more stable. Welding protection effect is better, the bead is silver or river yellow. Tow hood long charge L is 40 ~ 100mm and the material is brass. When argon arc welding of titanium and titanium alloys, we should also pay attention to the protection of the north side of the weld bead. In consideration of welding deformation, we use a slot to fix 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 to the bottom to make the argon gas evenly enter the protection zone through the copper tube holes. The protection effect is good, and the back of the bead is silver-white. Tig welding of manual tungsten plate and selection of parameters

(1) Preparation before welding

The surface quality of weldments and wires has a great impact on the mechanical properties of welded joints and must be cleaned strictly. Iron plate and titanium welding wire can be mechanically and chemically cleaned. 1) Mechanical cleaning butt welding with low quality requirements or difficult pickling can be wiped with fine sandpaper or stainless steel wire brush, but it is best to scrape the titanium plate with a yellow hard alloy to remove the oxide film. 2) Before chemical cleaning, the test piece and welding wire can be pickled. The pickling solution can be HF5% + HNO335% water melt. Rinse with clean water after pickling and apply welding immediately after drying. Or use acetone, ethanol, carbon tetrachloride, methanol, etc. to wipe the groove of the titanium plate and its two sides (within 50mm each), the surface of the welding wire, and the part where the jig is in contact with the titanium plate.

(2) Selection of welding equipment

TIG welding of titanium and titanium alloy gold-tungsten plates should use a DC argon arc welding power source with falling external characteristics and high-frequency arc initiation, and the delayed gas delivery time should not be less than 15 seconds to avoid welding from oxidation and pollution.


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(3) Selection of welding materials

The purity of argon gas should not be less than 99.99%, the dew point is below -40 ° C, and the total mass fraction of impurities & amp; lt; 0.001%. When the pressure in the argon cylinder drops to 0.981 MPa, it should be discontinued to prevent affecting the quality of the welded joint. In principle, a titanium wire with the same composition as the base metal should be selected. Sometimes in order to grasp the plasticity of the weld metal, a welding wire with a lower strength than the base metal can also be selected.

(4) Selection principle of bevel form

Minimize the number of welding layers and welding metals. With the increase of the number of welding layers, the cumulative suction set of the weld seam increases, which even affects the performance of the welded joint. Because the size of the welding pool is larger when welding titanium and titanium alloys, the test piece is V-shaped 70 ~ 80 ° bevel .

(5) Test piece butt welding and tack welding

In order to reduce welding distortion, tack welding is performed before welding. Generally, the tack welding distance is 100 ~ 150mm and the length is 10 ~ 15 mm. The welding wire, welding process parameters and gas protection conditions used for tack welding should be the same as those used for welding joints. Clearance 0 ~ 2mm, blunt edge 0 ~ 1.0mm.

(6) Selection of welding parameters

By comparing the performance of welded joints under different processes, we worked out a more suitable welding process specification.

Process (1), the welding current is 150A, 170A, 180A. According to this parameter, the surface of the welded joint is dark blue and gold, indicating that the joint is more oxidized and does not meet the technical requirements. This process is not desirable.

Process (2), the welding current is relatively reduced to 120A, 150A, 160A. According to this parameter, the surface of the weld appears golden purple, dark yellow, and X-ray flaw detection is not defective, but the mechanical property bending test is unqualified, indicating that the welded joint The plasticity is significantly reduced, which can not meet the technical requirements, and this process is also not desirable.

Process (3), the welding current is 95A, 115A, 120A. According to this parameter, the weld surface is silvery white and light yellow. There is no defect in X-ray flaw detection, but the mechanical properties are qualified for bending test and the tensile strength also meets the requirements. The joint performance meets the technical requirements, and this process is more suitable. Titanium and titanium alloys tend to have coarse grains when welding, which directly affects the mechanical properties of the welded joint. Therefore, the selection of welding process parameters should not only consider the oxidation of the weld metal and the formation of pores, but also the coarsening of the grains. Therefore, the welding heat input should be minimized. Processes (1) and (2), due to welding specifications Larger

Factors that cause joint oxidation to be more serious than process (3). And the results of microscopic metallographic experiments show that the degree of coarsening of joint grains is also more severe than that of process (3). Therefore, the mechanical properties of welded joints are poor.

    

The choice of gas flow rate is based on achieving a good protection effect. Excessive flow rate is not easy to form a stable laminar flow, and the cooling rate of the weld is increased, causing more α-phases to appear on the surface layer of the weld and even causing micro-cracks. When the flow of argon gas in the trailing hood is insufficient, the weld seams show different oxidation colors; when the flow is too large, it will interfere with the air flow of the main nozzle. The argon gas flow on the back of the weld should not be too large, otherwise it will affect the gas protection effect of the first layer of weld on the front.

(7) Essentials of Tungsten Arc Welding for Titanium and Titanium Alloys

1) During manual argon arc welding, the minimum angle (10 ~ 15 °) between the welding wire and the weldment should be kept as much as possible. The welding wire is smoothly and uniformly fed into the welding pool along the front end of the welding pool, and the end of the welding wire must not be moved out of the argon protection zone.

2) When welding, the welding torch basically does not make lateral swing. When swing is required, the frequency should be low and the swing amplitude should not be too large to prevent affecting the protection of argon.

3) When the arc is broken and the welding seam is closed, argon gas protection should be continued until the welding seam and the heat-affected zone metal is cooled below 350 ° C before removing the welding torch.

(8) Quality inspection

1) Visual inspection conforms to GB / T13149-91.

2) Deep ray injuries comply with JB4730-94.

3) The mechanical property test conforms to GB / T13149-91.

Fourth, the conclusion

   

1. The gas protection problem of titanium and titanium alloy welding is the primary factor affecting the quality of welded joints.

2. Titanium and titanium alloys should use as little heat input as possible when welding.

3. During TA2 Tungsten Arc Welding, the source of hydrogen should be strictly controlled to prevent the occurrence of cold cracks. At the same time, the generation of pores should be prevented.

4. As long as welding is performed in strict accordance with the welding process requirements and effective gas protection measures are taken, high-quality welded joints can be obtained.

Article source: Welding Empire

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