The argon arc welding technology of titanium alloy TC4 titanium bar: argon tungsten arc welding​

TIG welding is the most commonly used method for welding titanium and titanium alloys. It is often used for welding titanium and titanium alloys with a thickness of less than 3mm. TIG welding can be divided into two types: open welding and in-box welding, which are divided into manual welding and automatic welding. Open welding is ordinary argon tungsten arc welding in an atmospheric environment. It uses a welding nozzle, a drag hood and a back protection device to pass an appropriate flow of argon or hydrogen-nitrogen mixture to separate the welding high temperature zone from the air. Prevent air intrusion and contaminate the metal in the welding area. This is a partial gas shielded welding method. When the structure of the weldment is complicated and it is difficult to realize the drag cover or back protection, the welding in the box should be used. The box body must be evacuated before welding, and then filled with ammonia or nitrogen mixture. The weldment is welded in the box body in an inert atmosphere, which is an overall gas shielded welding method.

Preparation before welding

The quality of TC4 titanium bar welding joints of titanium alloy forgings depends to a large extent on the pre-weld cleaning of the weldment and welding wire. When the cleaning is not thorough, a getter layer will be formed on the surface of the weldment and welding wire, and cause cracks in the welded joint. And stomata. Therefore, the groove and its surrounding area should be carefully cleaned before welding. Cleaning usually adopts mechanical cleaning and chemical cleaning.

Mechanical cleaning

Titanium alloy forging TC4 titanium bar adopts shearing, stamping and cutting of workpieces that require mechanical cleaning of the joint edges before welding. For weldments with low welding quality requirements or difficult pickling, you can wipe them with a fine emery cloth or stainless steel wire brush, or use a cemented carbide scraper to scrape the edge to be welded to remove the surface oxide film. The depth of 025mm can be used for gas welding. The thickness of the machining cutting layer should not be less than 1~2mm. Then use solvents such as acetone or ethanol, carbon tetrachloride or methanol to remove fingerprints on both sides of the groove, organic matter and oil stains on the surface of the welding wire. Use thick cotton cloth, hair brush or man-made fiber brush to scrub when degreasing.
For workpieces that have been heat-processed or heat-treated without shielding gas before welding, the surface needs to be shot blasted or sandblasted, and then chemically cleaned.

Chemical cleaning

Alkaline washing should be carried out first. During alkaline washing, soak the titanium plate in a concentrated alkaline aqueous solution containing 80% caustic soda and 20% sodium bicarbonate for 10-15 minutes, and the temperature of the solution is maintained at 40-50℃. After alkaline washing, take out and rinse, and then pickling. The formula of the pickling solution is: 55-60mL nitric acid, 340-350mL hydrochloric acid, and 5mL hydrofluoric acid per liter of solution. The pickling time is 10~15min (soaking at room temperature). After taking it out, rinse with hot water and cold water, wipe with a white cloth, and dry. The pickled weldments and wires should be welded within 4h, otherwise they must be pickled again. The welding wire can be stored in an oven at a temperature of 150~200℃, and it is ready for use. Wear clean white gloves to avoid contamination of the welding wire. Butt weldments should be covered with plastic cloth to prevent contamination, and the contaminated parts can be scrubbed with acetone or alcohol.

Preparation and assembly of the groove

Titanium alloy forging TC4 titanium bar in order to reduce the accumulation and gas volume of the weld, when choosing the groove form and size, the number of welding layers and the amount of filler metal should be minimized to prevent the decrease of joint plasticity. Lap joints are not used as much as possible due to the difficulty of back protection and poor stress conditions on the joints, and generally do not use permanent pads for butt joints. For butt joints with I-shaped grooves whose base material thickness is less than 25mm, filler wires may not be added for welding. For grooves processed by mechanical methods, there may be air left in the joints, which requires high joint assembly. It is best to use cold processing techniques such as wounding and milling when processing the groove of the titanium plate to reduce the increase in the hardness of the groove edge during hot processing and reduce the difficulty of mechanical processing.
Due to some special physical properties of titanium alloy forgings TC4 titanium bars, such as large surface tension coefficient and low viscosity in the molten state, the weldment must be carefully installed before welding. The solder joint spacing is 100-150mm, and the length is 100-150mm. It is 10-15mm. The welding wire, welding process parameters and shielding gas used in spot welding are the same as those in formal welding. When the arc is stopped at each spot welding point, the gas should be turned off after a delay. It is strictly forbidden to use iron to strike or scratch the surface of the workpiece to be welded during assembly

The choice of welding materials

• 1. Ammonia is suitable for welding titanium and titanium alloys. The gas used for welding is first-class hydrogen, with a purity of 9.9%, and the point is below -40℃. The total content of impurities must not be used to ensure the quality of the welded joints.
• 2. The composition of filler wire should generally be the same as that of the base metal. Commonly used grades are TA1 titanium bar, TA2 titanium bar, TA3 titanium bar, TA4 titanium bar, TA5 titanium bar, TA6 titanium bar and TC3 titanium bar. In order to improve the plasticity of the weld metal, a welding wire with a slightly lower strength than the base metal can be selected. For example, when welding titanium alloys such as TA7 titanium bars and TC4 titanium bars, in order to improve the ductility of the weld, pure titanium welding wire can be used, but it must be ensured that the impurity content in the welding wire should be lower than that of the base metal, only about half, such as 0≤0.12 %, N≤0.03%, H≤0.006%, C≤0.04%.

The welding wire is supplied in a vacuum annealed state, and there must be no defects such as burnt skin, cracks, oxide color, and non-metallic inclusions on the surface. The welding wire must be thoroughly cleaned before welding, otherwise the grease on the surface of the welding wire may become a source of contamination of the weld metal. When using welding wire without standard grades, narrow strips can be cut from the base metal to make welding wires, the width and thickness of the narrow strips are the same.

Gas protection measures

Because the titanium alloy forging TC4 titanium bar 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 the welding pool and the heat-affected zone whose temperature exceeds 350 ℃ The front and back are isolated from the air. Protective measures and scope of application for welding titanium and titanium alloys by argon tungsten arc welding.
The protective effect of the weld is not only related to the argon purity, flow rate, the distance between the nozzle and the weldment, and the joint form, but also related to the structure and size of the welding torch and nozzle. The thermal conductivity of titanium is small and the size of the welding pool is large. Therefore, the aperture of the nozzle should be increased accordingly to enlarge the area of the protection zone.
The structure can obtain an airflow layer with a certain degree of stiffness, and the diameter of the protection zone is about 30m. If the nozzle structure is unreasonable, there will be turbulent flow and low stiffness, both of which will cause air to enter the welding area. In order to improve the structure of the weld metal and improve the performance of the weld and heat-affected zone, the method of enhancing the cooling rate of the weld can be used, that is, air-cooled or water-cooled copper briquette is set on both sides of the weld or on the opposite side of the weld. The surface of the weld heat-affected zone that has left the nozzle protection zone but is still above 350℃ still needs to be protected.
A drag hood with argon flow. The length of the drag cover is 100~180mm, and the width is 30~40mm. The specific length can be determined according to the weldment shape, plate thickness, welding process parameters and other conditions, but the welding seam and the heat-affected zone metal with the temperature above 350℃ should be fully obtained. protection of. The four corners of the shell of the drag cover should be smoothly transitioned, and the dead corners should be minimized. At the same time, the drag cover should keep a certain distance from the surface of the weldment.
When welding long weld seams, when the welding current is greater than 200A, a cooling water pipe must be installed at the bottom edge of the drag hood to prevent the drag hood from overheating and even burn the copper wire and the shell. The drag hood for manual TG welding of titanium and titanium alloy thin plates is usually connected to the welding torch as a whole and moves simultaneously with the welding torch. When the pipes are connected, in order to strengthen the protection of the welding seam and the heat-affected zone of the front and rear ends of the pipes, a special annular drag cover is generally designed and manufactured according to the outer diameter of the pipe.
The back side of the titanium alloy forging TC4 titanium bar welding also needs to be strengthened. Usually, measures such as filling argon in a partially airtight cavity or the entire weldment, and adding an argon backing plate on the back of the weld. For flat plate butt welding, a copper backing plate with vent holes on the back can be used.
The argon gas flows out from the vent hole of the copper backing plate on the back of the weldment (small diameter m, hole distance 15-20m), and is temporarily stored in the small groove of the backing plate to protect the back of the weld from harmful gases. In order to enhance cooling, the backing plate should be made of red copper, and the depth and width of the groove should be appropriate, otherwise it is not conducive to the circulation and storage of argon. For titanium plates with a thickness of less than 4m, the size of the forming groove of the welding backing plate. If the back of the weld does not use a backing plate, a manually movable argon drag hood can be added. In mass pbaruction of titanium pipes, butt welding can be welded in an argon gas shield, and the pipe rotates and the torch does not move.
The choice of argon gas flow rate is based on achieving a good welding surface color. Excessive flow rate is not easy to form a stable air flow layer, and increases the cooling rate of the weld seam, and it is easy to appear titanium martensite on the weld seam surface. When the flow of argon in the drag hood is insufficient, the surface of the welded joint will show different oxidation colors; when the flow is too large, it will interfere with the air flow 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 weld on the front.
The surface color of the weld and the heat-affected zone is a sign of the protective effect. After the arc is applied to the titanium material, a thin oxide film is formed on the surface, and the color of the oxide film formed at different temperatures is different. It is generally required that the surface after welding is preferably silver-white, followed by golden yellow. The relationship between the surface color of industrial pure titanium welds and the cold bend angle of the joint. In the case of multi-layer and multi-pass welding, the protection effect of the welded joint cannot be evaluated solely by the color of the welding seam of the cover layer. Because if the bottom weld has been contaminated by impurities, and the protective effect is good when welding the top layer, the plasticity of the joint will still be significantly reduced due to the contamination of the bottom layer.

The selection of welding process parameters

Titanium alloy forgings TC4 titanium bar welding has the tendency of grain growth, especially β titanium alloy, and the grain growth is difficult to adjust by heat treatment. Therefore, the selection of welding process parameters for titanium and titanium alloys should not only prevent the weld seam from showing the tendency of grain coarsening under the action of the arc, but also avoid the formation of brittle and hard structures during the cooling process after welding. Welding should use a small welding heat input, so that the temperature is just above the minimum temperature required to form the weld. If the heat input is too large, the weld will be easily contaminated and form defects.
Tungsten argon arc welding generally uses a DC arc welding power source with constant current characteristics, and uses DC positive connection to obtain a larger penetration depth and a narrower penetration width. In multi-layer welding, the first layer generally does not add welding wire, and the second layer adds welding wire. The heated wire should be protected by gas. When multi-layer welding, the temperature between layers should be kept as low as possible, and the next weld should be welded after the previous layer has cooled to room temperature to prevent overheating.
Pulse argon arc welding is used for all-position welding of pure titanium and titanium alloy plates with a thickness of 0.1~2.0m, titanium alloys sensitive to welding thermal cycles, and thin-walled titanium tubes. This method can successfully control the formation of titanium welds, reduce the tendency of welded joints to overheat and coarse-grain, and improve the plasticity of welded joints. Moreover, the welding seam is easy to realize single-sided welding and double-sided forming, and a welded joint with high quality and small deformation can be obtained. Table 8 shows the process parameters of pulse automatic TIG welding of titanium plates with a thickness of 0.8~2.0m. Among them, the pulse current plays a major role in the penetration of the weld. The base current is used to maintain the stable combustion of the arc, and there is no need to re-ignite the arc when the next pulse is applied.
When the titanium and titanium alloy plates are very thick, the use of melting pole ammonia arc welding (MG) can reduce the number of welding layers, increase welding speed and pbaructivity, reduce costs, and reduce weld pores. However, G welding uses excessive fine particles, and the filler metal is more likely to be contaminated, so the protection requirements are stricter than TIG welding. In addition, the spatter of MG welding is relatively large, which affects the weld formation and protection effect. Short-circuit transfer is usually used for thin plate welding, and spray transfer is used for thick plate welding.
There are more filler wires in MIG welding, which requires a larger welding groove angle. For plates with a thickness of 15-25m, a 90° single-sided V-shaped groove can be selected. The drag hood of argon tungsten arc welding can be used for fusion electbare welding, but due to the high welding speed and long high temperature area of MG welding, the drag hood should be lengthened and cooled by running water. The selection of welding consumables for MG welding is the same as T welding, but the requirements for gas purity and wire surface cleanliness are higher, and the welding wire must be thoroughly cleaned before welding.

• 1. The purpose of annealing is to eliminate stress, stabilize the structure and improve mechanical properties. The annealing process is divided into two types: complete annealing and incomplete annealing. Alpha and beta titanium alloys (except TB2) are generally only subjected to annealing heat treatment. Due to the high heating temperature of complete annealing, in order to avoid air pollution on the surface of the weldment, it must be carried out in argon or vacuum. Incomplete annealing can be carried out in the air due to the low heating degree. The slight pollution of the air to the weld and the surface of the weldment can be removed by pickling.
• 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 rate, and then cooled in stages or directly air-cooled, and the temperature at which the air-cooling starts 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 joint after welding. However, due to the serious oxidation of high temperature heating, the deformation during quenching is difficult to correct, and the quenching treatment is not easy when the weldment is large, so it is generally rarely used and only suitable for pressure vessels with simple structure and small volume.

Source:  China TC4 Titanium Alloy Flange Manufacurer: www.titaniuminfogroup.com