The titanium tube manufacturer will explain to you the problems and repair methods encountered during titanium tube welding:
Pores are a common issue encountered during titanium tube welding. The fundamental reason for the formation of pores is due to the effect of hydrogen. The formation of porosity in weld metal mainly affects the fatigue strength of the joint. Hydrogen is the main cause of cold cracking and pore formation. Because hydrogen has a low solubility in the alpha phase at temperatures below 300 ℃, with a solubility of only 0.002% at room temperature. When the weld or heat affected zone is cooled below 300 ℃ after welding, supersaturated hydrogen precipitates in the form of titanium hydride (γ phase).
The increase in volume and the generation of intergranular stress can lead to the development of intergranular microcracks, which will expand into cracks under external stress. When welding titanium pipes, the possibility of thermal cracking in the welded joint is very small. This is because the impurities such as S, P, and C in the titanium pipe are very low, and the low melting point eutectic formed by S and P is not easy to appear at the grain boundaries. In addition, the effective crystallization temperature range is narrow, and the shrinkage of the titanium pipe during solidification is small, so the weld metal will not produce thermal cracks. However, during the welding of titanium tubes, cold cracks may occur in the heat affected zone, characterized by delayed cracks occurring several hours or even longer after welding. During the welding process, hydrogen diffuses from the high-temperature deep pool to the lower temperature heat affected zone. The increase in hydrogen content leads to an increase in the precipitation of TiH2 in this zone, which increases the brittleness of the heat affected zone. In addition, the volume expansion during hydride precipitation causes significant tissue stress, and hydrogen atoms diffuse and aggregate towards high stress areas in this zone, resulting in the formation of cracks.
When welding materials such as titanium tubes, when the temperature is above 500-700 ℃, it is easy to absorb oxygen, hydrogen, and nitrogen from the air, which seriously affects the welding quality. Therefore, when welding titanium alloys, it is necessary to strictly protect the weld zone in all aspects of the molten pool and high-temperature areas (above 400-650 ℃).
Therefore, special protective measures must be taken during the welding of titanium pipes. Therefore, the argon arc welding method is adopted for treatment, and a larger welding torque is sprayed to expand the gas protection zone area. When the nozzle is not sufficient to protect the weld seam and high-temperature metal near the seam area, argon protection drag cover needs to be supplemented.
Correctly selecting welding process parameters to remove organic matter such as oxide scale and oil stains on the surface of the welded parts and welding wires. Control the flow rate and velocity of argon gas to prevent turbulence and affect the inflation protection effect. The method of using manual tungsten inert gas arc welding to treat cracks in titanium alloys is feasible and can achieve satisfactory results. The pre welding preparation and groove selection are as follows:
(1) Selection of welding materials. The purity of argon gas should not be less than 99.99%, with a dew point below -40 ℃ and a relative humidity of less than 5%. When the pressure in the argon cylinder drops to 0.981 MPa, it should be stopped from use. The filler wire is generally made of homogeneous material. To improve the plasticity of the joint, a welding wire TC3 with a slightly lower degree of alloy than the base metal can be used. The welding wire used for this welding is TC3.
(2) The surface quality of welded components and welding wires has a significant impact on the mechanical properties of welded joints. Before welding, the test piece and welding wire can be pickled. Rinse with clean water, dry and weld immediately. Wipe the titanium plate groove, both sides (within 50mm), the welding wire surface, and the parts where the tool holder contacts the titanium plate with acetone, ethanol, carbon tetrachloride, methanol, etc.
(3) Selection of welding equipment. The titanium tube argon arc welding should use a DC argon arc welding power source with descending external characteristics and high-frequency arc initiation, and the delayed gas delivery time should not be less than 15 seconds to avoid oxidation and pollution during welding. So WSM-315 IGBT inverter DC pulse argon arc welding machine is used.
(4) Selection of groove form. In principle, try to minimize the number of welding layers and welding metal. As the number of welding layers increases, the cumulative suction volume of the weld seam also increases, which affects the performance of the welded joint. Due to the large size of the welding pool during titanium tube welding, a single V-shaped 70-80 ° groove is required for the welded part.
