When welding titanium alloys, 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, the weld area of the entire melt pool and high-temperature areas (above 400-650 ℃) must be strictly protected. Therefore, special protective measures must be taken when welding titanium and titanium alloys. 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. Special protective measures must be taken when welding titanium and titanium alloys!

Preparation before welding and selection of groove

(1) 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.

(2) Selection of welding equipment. Titanium and titanium alloy argon arc welding should use a direct current 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.

(3) 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.

(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 increases, which affects the performance of the welded joint. Due to the large size of the welding pool during the welding of titanium and titanium alloys, a single V-shaped 70-80 ° groove is required for the welded parts.

Correctly selecting welding process parameters to thoroughly 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.

Main defects and repair methods in titanium alloy welding

(1) When welding titanium and titanium alloys, the possibility of hot cracking in the welded joint is very small. This is because the impurities such as S, P, and C in titanium and titanium alloys 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 during solidification of titanium and titanium alloys is small, so the weld metal will not produce hot cracks. However, during welding of titanium and titanium alloys, cold cracks may occur in the heat affected zone, characterized by delayed cracking occurring several hours or 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 amount of TiH2 precipitated in this zone, increasing 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.

(2) Pores are a common issue encountered during welding of titanium and titanium alloys. 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 very low solubility in the alpha phase at temperatures below 300 ℃, with a maximum 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. Microcracks between crystals will propagate into cracks under external stress.