Argon arc welding technology of titanium alloy titanium forgings TC4 titanium rod - tungsten argon arc welding
Argon tungsten arc welding is the most common method for welding titanium and titanium alloys, and is often used for welding titanium and titanium alloys with a thickness of less than 3mm. Argon tungsten arc welding can be divided into two types of open welding and box welding, which are respectively divided into manual welding and automatic welding. Open welding is a common tungsten argon arc welding in atmospheric environment, which uses the welding nozzle, drag cover and back protection device to pass the appropriate flow of argon or hydrogen nitrogen mixture to separate the welding high temperature area from the air to prevent air intrusion and contaminating the metal in the welding area. This is a local gas protected welding method. When the structure of the weldment is complex, it is difficult to achieve the drag cover or back protection, it should be used in the box welding. The box should be vacuumed before welding, and then filled with ammonia or nitrogen mixture, and the weldment is welded in the box under an inert atmosphere, which is a holistic gas protection welding method
1. Preparation before welding
The quality of welded joints of titanium alloy titanium forgings TC4 titanium rod depends to a large extent on the cleaning of welds and wires before welding, when the cleaning is not thorough, the getter layer will be formed on the surface of welds and wires, and lead to the formation of cracks and pores in welded joints. Therefore, the groove and its vicinity should be carefully cleaned before welding. Cleaning is usually done by mechanical cleaning and chemical cleaning.
1, mechanical cleaning
Titanium alloy titanium forgings TC4 titanium rods using cutting, stamping and cutting of the workpiece require mechanical cleaning of the joint edge before welding. For welding quality requirements are not high or pickling difficulties of the welds, you can use a fine abrasive cloth or stainless steel wire brush to wipe, or use carbide scraper to scrape the edge to be welded to remove the surface oxide film, scraping depth of 025mm for the use of gas welding cutting workpiece, the thickness of the machining cutting layer should not be less than 1~2mm. Then use acetone or ethanol, carbon tetrachloride or methanol and other solvents to remove the handprints on both sides of the groove, organic matter and oil on the surface of the welding wire. When removing oil, use a thick cotton cloth, brush or artificial fiber brush.
For the workpiece that has been hot worked before welding or heat treated in the absence of protective gas, the surface needs to be cleaned by shot blasting or sandblasting, followed by chemical cleaning.
2. Chemical cleaning
If the titanium plate has been pickled after hot rolling, but because of storage for too long and the formation of a new oxide film, the titanium plate can be lightly soaked in (2%~4%)HF+(30%40%)HNO+Ho solution 15-20mm at room temperature, and then washed with clean dry. For titanium plates that have not been pickled after hot rolling, the oxide film is thicker.
An alkali wash should be carried out first. When washing, the titanium plate is soaked in 80% caustic soda. In sodium bicarbonate 20% concentrated alkaline water solution for 10~15min, the temperature of the solution is maintained at 40~50℃. After alkali washing, remove and rinse, and then pickling. The formula of the pickling solution is: 55~60mL nitric acid, 340~350mL hydrochloric acid, 5mL hydrofluoric acid per liter of solution. Pickling time is 10~15min(soaking at room temperature). After removal, rinse with hot water and cold water, wipe and dry with a white cloth. The welds and wires after pickling should be welded within 4h, otherwise they should be pickled again. The welding wire can be stored in the oven at a temperature of 150~200 ° C, and is used as it is taken. The welding wire should be taken with clean white gloves to avoid contamination of the welding wire. The welding parts should be covered with plastic sheets to prevent contamination, and the stained parts can be scrubbed with acetone or alcohol.
2. Preparation and assembly of groove
In order to reduce the accumulation and gas volume of the weld, when selecting the groove form and size, the number of welding layers and the amount of filling metal should be minimized to prevent the decline of the joint plasticity. The groove form and size of titanium and titanium alloy are shown in Table 1. The lap joint is not used as much as possible due to the difficulty of its back protection and the poor stress condition of the joint, and the permanent pad butt joint is generally not used. For I-groove butt joints with base material thickness less than 25mm, welding can be performed without filler wire. For the thicker base material, it is necessary to open the groove and add the filler metal. Generally, flat welding should be used as far as possible. The bevelings processed by mechanical methods have high requirements for joint assembly because there may be air left in the joint. In the groove processing of titanium plate, it is best to use cold processing technology such as creation and milling to reduce the phenomenon of increased hardness of the groove edge during hot processing and reduce the difficulty of mechanical processing.
Due to some special physical properties of titanium alloy titanium forging TC4 titanium rod, such as large surface tension coefficient and small viscosity in molten state, the solder joint spacing is 100-150mm and the length is 10-15mm before welding. The welding wire, welding process parameters and protective gas used in spot welding are the same as those used in formal welding, and the gas should be delayed when the arc of each solid welding spot is stopped. Do not use iron to knock or scratch the surface of the workpiece to be welded during assembly
Third, the choice of welding materials
1, ammonia gas for titanium and titanium alloy welding gas is a hydrogen, its purity is 9.9%, the point is below -40℃, the total content of impurities <0.02%, relative humidity <6%, moisture ≤0.0010L during the welding process if the pressure of ammonia gas drops to w, should be stopped in use to ensure the quality of welded joints.
2, the composition of welding wire filling welding wire should generally be the same as the base metal composition. Commonly used grades are TA1 titanium rod, TA2 titanium rod, TA3 titanium rod, TA4 titanium rod, TA5 titanium rod, TA6 titanium rod and TC3 titanium rod. In order to improve the plasticity of the weld metal, the welding wire with slightly lower strength than the base metal can be selected. For example, when welding titanium alloys such as TA7 titanium rod and TC4 titanium rod, in order to improve the weld plasticity, pure titanium wire can be selected, but to ensure that the impurity content in the wire should be lower than the base metal, only about half, such as 0≤0.12%, N≤0.03%, H≤0.006%, C≤0.04%.
The welding wire shall be supplied in vacuum annealed state, and there shall be no defects such as burnt skin, crack, oxidized color and non-metallic inclusion on the surface. The welding wire must be thoroughly cleaned before welding, otherwise the oil on the surface of the welding wire may become a source of pollution of the weld metal. When no standard grade of welding wire is used, a narrow strip can be cut from the base metal for welding wire, and the width and thickness of the narrow strip are the same.
Fourth, gas protection measures
Since titanium alloy titanium forging TC4 titanium rod has a strong affinity for oxygen, nitrogen, hydrogen and other gases in the air, good protective measures must be taken in the welding zone to ensure that the front and back of the welding pool and the heat affected zone with a temperature of more than 350 ° C are isolated from the air. The protective measures and application scope of titanium and titanium alloys welded by argon tungsten arc welding are shown in Table 2.
The protection effect of weld is not only related to argon purity, flow rate, distance between nozzle and weldment, joint form and other factors, but also related to the structure and size of welding torch and nozzle. The thermal conductivity of titanium is small and the size of the welding solution pool is large, so the aperture of the nozzle should also be increased accordingly to expand the area of the protected area. The commonly used torch nozzle and drag cover are shown in Figure 1.
The structure can obtain a airflow layer with a certain stiffness, and the diameter of the protected area is about 30m. If the structure of the nozzle is not reasonable, there will be turbulence and little stiffness flow, both of which will mix air into the welding zone. In order to improve the microstructure of weld metal and improve the performance of weld and heat-affected zone, the method of enhancing the cooling rate of weld can be adopted, that is, air cooled or water cooled copper pressing blocks are set on both sides of the weld or on the opposite side of the weld. The surface of the weld heat affected zone, which has been removed from the nozzle protection zone but is still above 350 ° C, still needs to be protected. Usually adopted can pass.
A trailing hood with argon flow. The length of the drag cover is 100~180mm, the width is 30~40mm, the specific length can be determined according to the shape of the welding part, the thickness of the plate, the welding process parameters and other conditions, but the temperature is above 350 ° C and the metal in the heat affected zone is fully protected. The four corners of the shell of the drag cover should be smoothly transitioned, and the dead corners should be minimized, while the drag cover should be kept at a certain distance from the surface of the weldment.
When welding long welds, when the welding current is greater than 200A, the cooling water pipe should be set at the bottom cap edge of the drag cover to prevent the drag cover from overheating or even burning the copper wire and shell. Drag-covers for manual TG welding of titanium and titanium alloy sheets are usually attached to the torch and move simultaneously with the torch. In order to strengthen the protection of the weld seam and heat-affected zone at the back end of the front of the pipe, a special annular drag cover is generally designed and manufactured according to the outer diameter of the pipe, as shown in Figure 2.
Titanium alloy titanium forging TC4 titanium rod welding in the back also need to strengthen protection. Usually, argon is filled in a partially closed air cavity or the whole weldment, and argon is added to the back of the weld plate and other measures. For flat butt welding, a copper plate with a vent on the back can be used, as shown in Figure 3.
Argon gas flows from the copper gasket vent hole on the back of the weldment (the aperture is small m, the hole distance is 15~20m), and is briefly stored in the small slot of the gasket to protect the back of the weld from harmful gases. In order to strengthen the cooling, the gasket should be made of copper, and the depth and width of its groove should be appropriate, otherwise it is not conducive to the flow and storage of argon. For the titanium plate with a thickness of less than 4m, the forming slot size of the welding pad is shown in Table 3. If no backing plate is used on the back of the weld, an argon drag cover can be used to move by hand. For mass production of titanium tubes, the butt welding can be welded in the argon protective cover, and the tube will not move by turning the welding torch.
The choice of argon flow rate is to achieve good welding surface color, too large flow rate is not easy to form a stable gas layer, and increase the cooling speed of the weld, easy to appear on the weld surface titanium martensite. When the argon flow rate in the drag cover is insufficient, the surface of the welded joint shows different oxidation colors. When the flow is too large, it will interfere with the airflow of the main nozzle. Excessive argon flow on the back of the weld will also affect the gas protection effect of the first layer of the front weld.
The surface color of welding seam and heat affected zone is a sign of the protection effect. After the arc action of titanium material, a thin oxide film is formed on the surface. The color of the oxide film formed at different temperatures is different. Generally, the surface after welding is best silver white, followed by golden yellow. The relationship between the surface color of the industrial pure titanium weld and the cold bending Angle of the joint is shown in Table 4. In multi-layer and multi-pass welding, the protective effect of welded joint can not be evaluated solely by the color of the weld of the cover layer. Because if the bottom weld has been contaminated with impurities, and the protection effect is good when welding the cover layer, the result will still significantly reduce the plasticity of the joint due to the bottom pollution.
5. Selection of welding process parameters
Titanium alloy titanium forging TC4 titanium rod welding has a tendency of grain growth, especially β titanium alloy, but the grain growth is difficult to be adjusted by heat treatment. Therefore, the selection of titanium and titanium alloy welding process parameters should not only prevent the tendency of grain coarsening under the action of arc, but also avoid the formation of brittle and hard structure during the post-welding cooling process. Welding should use a small welding line energy, so that the temperature is just above the minimum temperature required to form the weld. If the line energy is too large, the weld is easy to be contaminated and form defects.
Table 5 and Table 6 are the technical parameters of manual and automatic TIG welding of titanium and titanium alloys, which are mainly applicable to butt long welds and girth welds. Table 7 shows titanium pipe hands
Process parameters of TIG welding.
Argon tungsten arc welding generally adopts DC arc welding power supply with constant current characteristics, and adopts DC direct connection to obtain larger penetration depth and narrower penetration width. In multi-layer welding, the first layer is generally not added welding wire, and the welding wire is added from the second layer. The heated wire should be protected by gas. When welding multiple layers, keep the interlayer temperature as low as possible and wait until the previous layer cools to room temperature before welding the next weld to prevent overheating.
For pure titanium and titanium alloy plates with a thickness of 0.1~2.0m, titanium alloys sensitive to welding thermal cycling and thin-walled titanium pipes, pulsed argon arc welding is recommended. The method can successfully control the forming of titanium weld, reduce the tendency of overheating and coarse crystal of welded joint, and improve the plasticity of welded joint. Moreover, the weld is easy to achieve single-sided welding and double-sided forming, and the welded joint with high quality and small deformation is obtained. Table 8 shows the process parameters of pulsed automatic TIG welding of titanium plate with thickness of 0.8~2.0m. The pulse current plays a major role in the penetration of the weld, and the function of the base current is to keep the arc burning steadily, so it is not necessary to re-start the arc when the next pulse is acted.
When the titanium and titanium alloy plate is very thick, the use of molten ammonia arc welding (MG) can reduce the number of welding layers, improve the welding speed and productivity, reduce the cost, but also reduce the weld porosity. However, the use of G welding is excessive fine particles, the possibility of contamination of the filled metal is large, so the protection requirements are more stringent than TIG welding. In addition, the splash of MG welding is large, which affects the weld forming and protection effect. Short circuit transition is usually used in thin plate welding, and jet transition is used in thick plate welding.
MIG welding is filled with more wire, which requires the welding groove Angle is larger, the thickness of 15~25m plate, can choose 90° single-sided V-shaped groove. Argon tungsten arc welding can be used for melting electrode welding, but due to the high welding speed and high temperature area of MG welding, the drag cover should be lengthened and cooled by water. The selection of welding materials during MG welding is the same as that of T welding, but the requirements for gas purity and wire surface cleanliness are higher, and the welding wire must be thoroughly cleaned before welding. Table 9 shows the process parameters of automatic MG welding of TC4 titanium alloy
1, The purpose of annealing is to eliminate stress, stabilize the organization, improve mechanical properties. The annealing process is divided into two categories: complete annealing and incomplete annealing. α and β titanium alloys (except TB2) are generally only annealed. Due to the high heating temperature of full annealing, in order to avoid air contamination of the weldment surface, it must be carried out in argon or vacuum. Due to the low heating degree, incomplete annealing can be carried out in the air, and the slight pollution of the air to the weld and the weldment surface can be removed by pickling method.
The cooling rate after annealing is not sensitive to a and B titanium alloys, but very sensitive to a+B titanium alloys. For this alloy, it must be cooled to a certain temperature at a specified speed, and then cooled in stages or direct air cooling, and the temperature of the initial air cooling should not be lower than the use temperature
2, quenching - aging treatment The purpose of quenching aging treatment is to improve the strength of the welded joint. However, due to the serious oxidation of high temperature heating, the deformation occurring during quenching is difficult to correct, and the welding part is not easy to be quenched when it is large, so it is generally rarely used, and only suitable for pressure vessels with simple structure and small volume.
1. Preparation before welding
The quality of welded joints of titanium alloy titanium forgings TC4 titanium rod depends to a large extent on the cleaning of welds and wires before welding, when the cleaning is not thorough, the getter layer will be formed on the surface of welds and wires, and lead to the formation of cracks and pores in welded joints. Therefore, the groove and its vicinity should be carefully cleaned before welding. Cleaning is usually done by mechanical cleaning and chemical cleaning.
1, mechanical cleaning
Titanium alloy titanium forgings TC4 titanium rods using cutting, stamping and cutting of the workpiece require mechanical cleaning of the joint edge before welding. For welding quality requirements are not high or pickling difficulties of the welds, you can use a fine abrasive cloth or stainless steel wire brush to wipe, or use carbide scraper to scrape the edge to be welded to remove the surface oxide film, scraping depth of 025mm for the use of gas welding cutting workpiece, the thickness of the machining cutting layer should not be less than 1~2mm. Then use acetone or ethanol, carbon tetrachloride or methanol and other solvents to remove the handprints on both sides of the groove, organic matter and oil on the surface of the welding wire. When removing oil, use a thick cotton cloth, brush or artificial fiber brush.
For the workpiece that has been hot worked before welding or heat treated in the absence of protective gas, the surface needs to be cleaned by shot blasting or sandblasting, followed by chemical cleaning.
2. Chemical cleaning
If the titanium plate has been pickled after hot rolling, but because of storage for too long and the formation of a new oxide film, the titanium plate can be lightly soaked in (2%~4%)HF+(30%40%)HNO+Ho solution 15-20mm at room temperature, and then washed with clean dry. For titanium plates that have not been pickled after hot rolling, the oxide film is thicker.
An alkali wash should be carried out first. When washing, the titanium plate is soaked in 80% caustic soda. In sodium bicarbonate 20% concentrated alkaline water solution for 10~15min, the temperature of the solution is maintained at 40~50℃. After alkali washing, remove and rinse, and then pickling. The formula of the pickling solution is: 55~60mL nitric acid, 340~350mL hydrochloric acid, 5mL hydrofluoric acid per liter of solution. Pickling time is 10~15min(soaking at room temperature). After removal, rinse with hot water and cold water, wipe and dry with a white cloth. The welds and wires after pickling should be welded within 4h, otherwise they should be pickled again. The welding wire can be stored in the oven at a temperature of 150~200 ° C, and is used as it is taken. The welding wire should be taken with clean white gloves to avoid contamination of the welding wire. The welding parts should be covered with plastic sheets to prevent contamination, and the stained parts can be scrubbed with acetone or alcohol.
2. Preparation and assembly of groove
In order to reduce the accumulation and gas volume of the weld, when selecting the groove form and size, the number of welding layers and the amount of filling metal should be minimized to prevent the decline of the joint plasticity. The groove form and size of titanium and titanium alloy are shown in Table 1. The lap joint is not used as much as possible due to the difficulty of its back protection and the poor stress condition of the joint, and the permanent pad butt joint is generally not used. For I-groove butt joints with base material thickness less than 25mm, welding can be performed without filler wire. For the thicker base material, it is necessary to open the groove and add the filler metal. Generally, flat welding should be used as far as possible. The bevelings processed by mechanical methods have high requirements for joint assembly because there may be air left in the joint. In the groove processing of titanium plate, it is best to use cold processing technology such as creation and milling to reduce the phenomenon of increased hardness of the groove edge during hot processing and reduce the difficulty of mechanical processing.
Due to some special physical properties of titanium alloy titanium forging TC4 titanium rod, such as large surface tension coefficient and small viscosity in molten state, the solder joint spacing is 100-150mm and the length is 10-15mm before welding. The welding wire, welding process parameters and protective gas used in spot welding are the same as those used in formal welding, and the gas should be delayed when the arc of each solid welding spot is stopped. Do not use iron to knock or scratch the surface of the workpiece to be welded during assembly
Third, the choice of welding materials
1, ammonia gas for titanium and titanium alloy welding gas is a hydrogen, its purity is 9.9%, the point is below -40℃, the total content of impurities <0.02%, relative humidity <6%, moisture ≤0.0010L during the welding process if the pressure of ammonia gas drops to w, should be stopped in use to ensure the quality of welded joints.
2, the composition of welding wire filling welding wire should generally be the same as the base metal composition. Commonly used grades are TA1 titanium rod, TA2 titanium rod, TA3 titanium rod, TA4 titanium rod, TA5 titanium rod, TA6 titanium rod and TC3 titanium rod. In order to improve the plasticity of the weld metal, the welding wire with slightly lower strength than the base metal can be selected. For example, when welding titanium alloys such as TA7 titanium rod and TC4 titanium rod, in order to improve the weld plasticity, pure titanium wire can be selected, but to ensure that the impurity content in the wire should be lower than the base metal, only about half, such as 0≤0.12%, N≤0.03%, H≤0.006%, C≤0.04%.
The welding wire shall be supplied in vacuum annealed state, and there shall be no defects such as burnt skin, crack, oxidized color and non-metallic inclusion on the surface. The welding wire must be thoroughly cleaned before welding, otherwise the oil on the surface of the welding wire may become a source of pollution of the weld metal. When no standard grade of welding wire is used, a narrow strip can be cut from the base metal for welding wire, and the width and thickness of the narrow strip are the same.
Fourth, gas protection measures
Since titanium alloy titanium forging TC4 titanium rod has a strong affinity for oxygen, nitrogen, hydrogen and other gases in the air, good protective measures must be taken in the welding zone to ensure that the front and back of the welding pool and the heat affected zone with a temperature of more than 350 ° C are isolated from the air. The protective measures and application scope of titanium and titanium alloys welded by argon tungsten arc welding are shown in Table 2.
The protection effect of weld is not only related to argon purity, flow rate, distance between nozzle and weldment, joint form and other factors, but also related to the structure and size of welding torch and nozzle. The thermal conductivity of titanium is small and the size of the welding solution pool is large, so the aperture of the nozzle should also be increased accordingly to expand the area of the protected area. The commonly used torch nozzle and drag cover are shown in Figure 1.
The structure can obtain a airflow layer with a certain stiffness, and the diameter of the protected area is about 30m. If the structure of the nozzle is not reasonable, there will be turbulence and little stiffness flow, both of which will mix air into the welding zone. In order to improve the microstructure of weld metal and improve the performance of weld and heat-affected zone, the method of enhancing the cooling rate of weld can be adopted, that is, air cooled or water cooled copper pressing blocks are set on both sides of the weld or on the opposite side of the weld. The surface of the weld heat affected zone, which has been removed from the nozzle protection zone but is still above 350 ° C, still needs to be protected. Usually adopted can pass.
A trailing hood with argon flow. The length of the drag cover is 100~180mm, the width is 30~40mm, the specific length can be determined according to the shape of the welding part, the thickness of the plate, the welding process parameters and other conditions, but the temperature is above 350 ° C and the metal in the heat affected zone is fully protected. The four corners of the shell of the drag cover should be smoothly transitioned, and the dead corners should be minimized, while the drag cover should be kept at a certain distance from the surface of the weldment.
When welding long welds, when the welding current is greater than 200A, the cooling water pipe should be set at the bottom cap edge of the drag cover to prevent the drag cover from overheating or even burning the copper wire and shell. Drag-covers for manual TG welding of titanium and titanium alloy sheets are usually attached to the torch and move simultaneously with the torch. In order to strengthen the protection of the weld seam and heat-affected zone at the back end of the front of the pipe, a special annular drag cover is generally designed and manufactured according to the outer diameter of the pipe, as shown in Figure 2.
Titanium alloy titanium forging TC4 titanium rod welding in the back also need to strengthen protection. Usually, argon is filled in a partially closed air cavity or the whole weldment, and argon is added to the back of the weld plate and other measures. For flat butt welding, a copper plate with a vent on the back can be used, as shown in Figure 3.
Argon gas flows from the copper gasket vent hole on the back of the weldment (the aperture is small m, the hole distance is 15~20m), and is briefly stored in the small slot of the gasket to protect the back of the weld from harmful gases. In order to strengthen the cooling, the gasket should be made of copper, and the depth and width of its groove should be appropriate, otherwise it is not conducive to the flow and storage of argon. For the titanium plate with a thickness of less than 4m, the forming slot size of the welding pad is shown in Table 3. If no backing plate is used on the back of the weld, an argon drag cover can be used to move by hand. For mass production of titanium tubes, the butt welding can be welded in the argon protective cover, and the tube will not move by turning the welding torch.
The choice of argon flow rate is to achieve good welding surface color, too large flow rate is not easy to form a stable gas layer, and increase the cooling speed of the weld, easy to appear on the weld surface titanium martensite. When the argon flow rate in the drag cover is insufficient, the surface of the welded joint shows different oxidation colors. When the flow is too large, it will interfere with the airflow of the main nozzle. Excessive argon flow on the back of the weld will also affect the gas protection effect of the first layer of the front weld.
The surface color of welding seam and heat affected zone is a sign of the protection effect. After the arc action of titanium material, a thin oxide film is formed on the surface. The color of the oxide film formed at different temperatures is different. Generally, the surface after welding is best silver white, followed by golden yellow. The relationship between the surface color of the industrial pure titanium weld and the cold bending Angle of the joint is shown in Table 4. In multi-layer and multi-pass welding, the protective effect of welded joint can not be evaluated solely by the color of the weld of the cover layer. Because if the bottom weld has been contaminated with impurities, and the protection effect is good when welding the cover layer, the result will still significantly reduce the plasticity of the joint due to the bottom pollution.
5. Selection of welding process parameters
Titanium alloy titanium forging TC4 titanium rod welding has a tendency of grain growth, especially β titanium alloy, but the grain growth is difficult to be adjusted by heat treatment. Therefore, the selection of titanium and titanium alloy welding process parameters should not only prevent the tendency of grain coarsening under the action of arc, but also avoid the formation of brittle and hard structure during the post-welding cooling process. Welding should use a small welding line energy, so that the temperature is just above the minimum temperature required to form the weld. If the line energy is too large, the weld is easy to be contaminated and form defects.
Table 5 and Table 6 are the technical parameters of manual and automatic TIG welding of titanium and titanium alloys, which are mainly applicable to butt long welds and girth welds. Table 7 shows titanium pipe hands
Process parameters of TIG welding.
Argon tungsten arc welding generally adopts DC arc welding power supply with constant current characteristics, and adopts DC direct connection to obtain larger penetration depth and narrower penetration width. In multi-layer welding, the first layer is generally not added welding wire, and the welding wire is added from the second layer. The heated wire should be protected by gas. When welding multiple layers, keep the interlayer temperature as low as possible and wait until the previous layer cools to room temperature before welding the next weld to prevent overheating.
For pure titanium and titanium alloy plates with a thickness of 0.1~2.0m, titanium alloys sensitive to welding thermal cycling and thin-walled titanium pipes, pulsed argon arc welding is recommended. The method can successfully control the forming of titanium weld, reduce the tendency of overheating and coarse crystal of welded joint, and improve the plasticity of welded joint. Moreover, the weld is easy to achieve single-sided welding and double-sided forming, and the welded joint with high quality and small deformation is obtained. Table 8 shows the process parameters of pulsed automatic TIG welding of titanium plate with thickness of 0.8~2.0m. The pulse current plays a major role in the penetration of the weld, and the function of the base current is to keep the arc burning steadily, so it is not necessary to re-start the arc when the next pulse is acted.
When the titanium and titanium alloy plate is very thick, the use of molten ammonia arc welding (MG) can reduce the number of welding layers, improve the welding speed and productivity, reduce the cost, but also reduce the weld porosity. However, the use of G welding is excessive fine particles, the possibility of contamination of the filled metal is large, so the protection requirements are more stringent than TIG welding. In addition, the splash of MG welding is large, which affects the weld forming and protection effect. Short circuit transition is usually used in thin plate welding, and jet transition is used in thick plate welding.
MIG welding is filled with more wire, which requires the welding groove Angle is larger, the thickness of 15~25m plate, can choose 90° single-sided V-shaped groove. Argon tungsten arc welding can be used for melting electrode welding, but due to the high welding speed and high temperature area of MG welding, the drag cover should be lengthened and cooled by water. The selection of welding materials during MG welding is the same as that of T welding, but the requirements for gas purity and wire surface cleanliness are higher, and the welding wire must be thoroughly cleaned before welding. Table 9 shows the process parameters of automatic MG welding of TC4 titanium alloy
1, The purpose of annealing is to eliminate stress, stabilize the organization, improve mechanical properties. The annealing process is divided into two categories: complete annealing and incomplete annealing. α and β titanium alloys (except TB2) are generally only annealed. Due to the high heating temperature of full annealing, in order to avoid air contamination of the weldment surface, it must be carried out in argon or vacuum. Due to the low heating degree, incomplete annealing can be carried out in the air, and the slight pollution of the air to the weld and the weldment surface can be removed by pickling method.
The cooling rate after annealing is not sensitive to a and B titanium alloys, but very sensitive to a+B titanium alloys. For this alloy, it must be cooled to a certain temperature at a specified speed, and then cooled in stages or direct air cooling, and the temperature of the initial air cooling should not be lower than the use temperature
2, quenching - aging treatment The purpose of quenching aging treatment is to improve the strength of the welded joint. However, due to the serious oxidation of high temperature heating, the deformation occurring during quenching is difficult to correct, and the welding part is not easy to be quenched when it is large, so it is generally rarely used, and only suitable for pressure vessels with simple structure and small volume.
Before stress relief treatment, the surface of the weldment must be thoroughly cleaned and then heat treated in an inert atmosphere. The process parameters of several kinds of titanium and titanium alloy post-welding heat treatment are shown in Table 10.
