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Titanium alloy has the characteristics of low density, high strength and corrosion resistance. As a new material, titanium alloy pipe is widely used in the field of aerospace, and the proportion of titanium alloy pipe in the aero-engine pipeline is increasing. In addition, titanium alloy is a very active metal, at high temperature has a great affinity for oxygen, hydrogen, nitrogen and other gases, and the ability to absorb and dissolve gases is very strong, especially in the welding process, this ability is accompanied by the rise of welding temperature, the performance is particularly strong, in welding the need to control the absorption and dissolution of oxygen, hydrogen, nitrogen and other gases, to avoid the scrapping of products, This brings great difficulty to the welding of titanium alloy pipe.
1.1 Weldability of titanium alloy conduits
(1) Embrittlement of welded joints
At room temperature, titanium reacts with oxygen to form a dense oxide film, which gives it good chemical stability and corrosion resistance. At high temperature, especially in the welding process, titanium alloy and oxygen, hydrogen, nitrogen reaction speed is very fast, when the molten pool into the oxygen, hydrogen, nitrogen and other harmful gases, the plasticity, toughness and surface color of the welded joint have obvious changes, especially at 882℃ above, the joint grain growth tendency is serious, the formation of martensite when cooling. The joint strength, hardness, plasticity, toughness, overheating tendency is serious, serious embrittlement of the joint. Therefore, when welding titanium alloy, the molten pool, molten drop and high temperature zone, whether positive or negative, should be comprehensive and reliable gas protection.
(2) stomata
Porosity is the most common defect of titanium and titanium alloy welding, which mainly appears near the fusion line. Hydrogen is the main cause of the formation of pores. During welding, titanium has a strong ability to absorb hydrogen (even stronger at high temperature), but the solubility decreases significantly with the decrease of temperature. Therefore, the hydrogen dissolved in liquid metal often has no time to escape and accumulates near the fusion line to form pores.
(3) Delay crack near the seam area
Titanium alloy after a period of time. Cracks (delayed cracks) tend to occur in the near seam area. The reason is that hydrogen diffuses from the high temperature molten pool to the low temperature heat affected zone. With the increase of hydrogen content, the amount of TiH2 precipitated increases, which makes the brittleness of the heat affected zone increase. In addition, the structural stress caused by the volume expansion of the precipitated hydride eventually leads to the crack.
1.2 Welding requirements and precautions for titanium alloy conduit
(1) As far as possible to set up a special welding workshop, indoor smoking is strictly prohibited, the environment to keep clean and dry, strict control of air convection.
(2) Welders wear clean work clothes and degreasing gloves when welding. It is strictly prohibited to touch parts with bare hands.
(3) The welding area and the surface of the welding wire should be degreased with acetone.
(4) Adopt high purity protection argon, purity is not less than 99.99%. During welding, the gas supply flow should be protected on the front and back of the welding passage according to the value specified in the process regulations.
(5) During the welding process, the flow of argon gas in the tube and the nozzle of the welding tool should be kept constant to prevent convex and concave phenomenon in the forming of the weld pool in the tube.
(6) Short arc welding should be adopted as far as possible during welding, and small welding line energy should be adopted.
(7) When spot welding the nozzle, the gap is less than 30% of the wall thickness. Every weld should be welded as once as possible.
(8) Welding, welding tools should not swing from side to side, welding wire melting end shall not move out of the gas protection zone. The arc should be pumped 10-15s in advance, and the welding torch cannot be lifted immediately when arc breathing. The air supply should be delayed 15-30s until the temperature drops below 250℃.
1.3 Welding Process
1.3.1 Clean up before welding.
The occurrence of welding defects is closely related to the cleanliness of weldment and wire surface. Before welding, clean the oil, water, oxide film and other dirt in the range of 15 ~ 20mm on the edge of the pipe joint and on the surface of the welding wire. Cleaning method can be used chemical method (pickling) or mechanical means (stainless steel brush removal) to remove the surface oxide skin. Before welding, acetone or alcohol should be used to scrub the welds. After cleaning, the welds must be finished within 24 hours. Otherwise, they need to be cleaned again. After pickling, the wire should be dehydrogenated by vacuum and degreased by acetone before welding.
1.3.2 Gas protection. During the welding of Titanium Pipe joints, in order to prevent the welding joints from being polluted by harmful gases and elements at high temperature, the welding seams must be protected by necessary argon gas, with the purity not less than 99.99%.
1.3.3 Selection of welding process parameters.
(1) Selection of welding wire. The grade of filler wire should be selected according to the base material. Generally, the principle of homogeneity with the base material is adopted. Sometimes, in order to improve the plasticity of the joint, the welding wire with slightly lower alloying degree than the base material can be selected. The diameter of the welding wire should be selected according to the thickness of the base metal.
(2) The choice of power supply and polarity. Dc manual tungsten argon arc power supply is generally used for welding titanium and titanium alloy, and the polarity connection method is DC direct connection.
(3) The selection of tungsten pole. The diameter of the tungsten pole is selected according to the thickness of the titanium alloy tube wall, which is generally between 1.0-3.Omm. The extreme part of the tungsten should be ground into a cone of 25° ~ 45°.
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