The common welding methods of titanium alloy include argon arc welding, Submerged arc welding, vacuum electron beam welding, etc. Tungsten argon arc welding is used for thicknesses below 3mm, and consumable argon arc welding is used for thicknesses above 3mm. The purity of argon gas shall not be less than 99.99%, and the content of air and water vapor in argon gas shall be strictly controlled. Before welding, perform surface treatment such as degreasing, deoxidizing, and removing oxide film. Due to the high chemical activity of titanium and titanium alloys, which are easy to be polluted by oxygen, nitrogen and hydrogen, welding methods such as Arc welding, oxyacetylene (or oxypropane, etc.) gas welding, C02 welding, atomic hydrogen welding, etc. cannot be used.
When using argon arc welding to weld titanium alloy pipelines, it is usually done in all positions on site, which is much more difficult than welding stainless steel pipes. It is necessary to protect the weld area of titanium and its alloys above 400 ℃, as its chemical properties are particularly active and it is easy to absorb harmful gases, leading to a decrease in material performance and even the occurrence of cracks. So when welding, special attention should be paid to the following items:
1. When argon arc welding is used to weld titanium pipes and titanium alloy pipelines, the selection of welding materials is basically based on the principle of equal strength. When suitable welding materials cannot be found, the method of cutting from the base material can be used to solve the problem; The selection of argon gas will directly affect the hardness and toughness of the weld seam, and also have a significant impact on the generation of welding cracks. Therefore, it is required to use high-purity argon gas (99.99%), especially pure titanium.
2. The cutting and breaking of welded titanium pipes should be done using cold processing methods, and should not cause overheating and discoloration. Grind the inner and outer surfaces of the groove with a grinding wheel or stainless steel wire wheel within a range of no less than 20mm to achieve a metallic luster. Use a scraper to trim the burrs on the groove and then polish it smooth to avoid hanging cloth wires during cleaning.
3. The quality of cleaning during titanium alloy welding directly affects the generation of welding cracks and pores. Usually, acid cleaning (3% HF+35% HNO3+H2O) is used first, followed by rinsing with clean water, finally drying, and wiping with acetone or alcohol before welding. The cleaned welded junction must be welded immediately and must not exceed 4 hours, otherwise it needs to be cleaned again. Rubber gloves should not be used during cleaning, but white cotton gloves should be used.
4. The protection of the welding area during the welding of titanium pipes and titanium alloy pipes is crucial, similar to the welding of stainless steel pipes, but the requirements for gas protection are more stringent. The argon filling in the pipes should continue until the welding is completed, and the welding area temperature can only be stopped when it drops below 350 ℃; The argon protection on the outer surface of the pipe adopts a large diameter nozzle and an extended drag cover. In order to improve the protection effect, when the welding is completed, the drag cover does not end and must wait for the temperature to drop below 200 ℃ before stopping the air supply. Remove the drag cover.
The operation method for TIG welding of titanium alloy pipes is basically similar to that of stainless steel. While ensuring complete penetration, it is recommended to use a smaller welding line energy, such as fast welding with straight gun and filling welding wire. The welding operation mainly pays attention to the following points:
1. Before welding, a period of time should be passed for replacement. The welding joint can be inspected using the open flame detection method to confirm that the gas inside the pipe is completely argon before welding. Adopting short arc welding without swinging the welding gun and wire. When covering the surface, it is necessary to swing, but only slightly and slowly.
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