Titanium alloy has the characteristics of low density, high strength, corrosion resistance, etc. Titanium alloy tubing, as a new type of material, is widely used in the field of aerospace, and the proportion of titanium alloy conduit in the aero-engine piping is increasing. In addition, titanium alloy is a very active metal, at high temperatures to oxygen, hydrogen, nitrogen and other gases have a great affinity for the absorption and dissolution of gases is very strong, especially in the welding process, this ability is accompanied by an increase in the welding temperature, the performance of the particularly strong, the need for oxygen, hydrogen, nitrogen and other gases in the welding of the absorption and dissolution of the control, to avoid product obsolescence, which gives titanium alloy tubing welding has brought about a great deal of difficulty. This brings great difficulties to the welding of titanium alloy tube.
1. Compliance with the safety system
Weldability of titanium alloy conduit
(1) Embrittlement of welded joints
At room temperature, titanium reacts with oxygen to generate a dense oxide film, which gives it better chemical stability and corrosion resistance. At high temperatures, especially in the welding process, titanium alloy and oxygen, hydrogen, nitrogen reaction rate is extremely fast, when the molten pool invasion of oxygen, hydrogen, nitrogen and other harmful gases, welded joints, plasticity, toughness and surface color, etc., there are obvious changes, especially in the 882 ℃ and above, the joints grain growth tendency is serious, the formation of martensitic organization cooling, resulting in joints, strength, hardness, plasticity, toughness decline, tendency to overheating Serious, serious embrittlement of joints. Therefore, when welding titanium alloy, the molten pool, molten droplets and high temperature zone, whether it is the front or back, should be comprehensive, reliable gas protection.
(2) Porosity
Porosity is the most common defect in titanium and titanium alloy welding, mainly in the vicinity of the fusion line. Hydrogen is the main cause of the formation of porosity. In welding, titanium absorption of hydrogen is very strong (stronger at high temperatures), but the solubility with the drop in temperature and a significant decline, so dissolved in the liquid metal in the hydrogen is often too late to escape on the fusion line gathered near the formation of pores.
(3) Delayed cracking in the near-seam area
Titanium alloy in the post-weld period of time. In the near-seam area is often prone to cracking (delayed cracking). The reason is that hydrogen from the high-temperature melt pool to the low-temperature heat-affected zone diffusion, with the increase in the amount of hydrogen, the precipitation of TiH2 increased, so that the heat-affected zone brittleness increases, coupled with the precipitation of the volume of the hydride expansion of the organizational stresses generated, ultimately resulting in cracks.
2. Prevent bad weather
We should pay attention to the weather forecast in a timely manner, and strictly prohibit risky operations in bad weather such as gale (including more than three winds), rainstorms, lightning, dense fog, etc., and strictly prohibit fishing vessels from operating at night.
Welding requirements and precautions for titanium alloy conduit
(1) Try to set up a special welding workshop, indoor smoking is strictly prohibited, the environment is kept clean and dry, and air convection is strictly controlled.
(2) Welders welding wear clean overalls, wear degreasing gloves, forbidden to touch the parts with bare hands.
(3) Welding area and wire surface should be degreased with acetone.
(4) The use of high-purity protective argon gas, purity is not less than 99.99%. Welding gas supply flow should be in accordance with the values specified in the process regulations on the front and back of the weld channel for protection.
(5) In the welding process, the argon gas in the pipe and the argon gas flow of the welding tool nozzle should be maintained at a constant level to prevent convexity and concavity of the weld pool molding in the pipe.
(6) Welding should be used as much as possible short arc welding, using a small welding line energy.
(7) The gap is less than 30% of the wall thickness when positioning spot welds on the receiver. Each weld should be welded as far as possible.
(8) When welding, the welding tool should not swing from side to side, the melted end of the wire shall not move out of the gas protection zone. Arc should be sent ahead of the gas 10-15s, the rest of the arc can not immediately lift the welding torch, should be delayed gas supply 15-30s, until the temperature drops below 250 ℃.
3.Strengthen safety education
To enhance safety awareness, the practitioners to carry out another safety skills training and warning education, enhance accident prevention and emergency response capabilities.
Welding process
1)Cleaning before welding.
Welding defects and welding parts, welding wire surface cleanliness has a great relationship. Welding should be welded before the edge of the pipe joint within 15 ~ 20mm and the welding wire surface of oil, water, oxide film and other dirt clean up. Cleaning methods can be used chemical methods (pickling) or mechanical means (stainless steel brush removal) to remove the surface oxide skin. Before welding should also be acetone or alcohol scrubbing, cleaning of the weldments must be welded within 24h, or need to be re-cleaned. Welding wire after pickling is best after vacuum dehydrogenation, degreasing with acetone before welding.
2) Gas protection. Titanium pipe joints in the welding, in order to prevent the welded joints at high temperatures by harmful gases and elements contamination, the weld must be necessary argon gas protection, purity is not less than 99.99%. Argon gas flow according to Table 2-1.
(3) the choice of welding process parameters.
(1) the choice of welding wire. Filler wire grade should be selected according to the base material, generally using the principle of homogeneity with the base material, sometimes in order to improve the plasticity of the joints, you can also choose a slightly lower degree of alloying than the base material of the wire. Selection of wire diameter should be based on the thickness of the base material to choose, see Table 2-1 shows.
(2) power source and polarity selection. Titanium and titanium alloy welding generally use DC manual tungsten argon arc power source, its polarity connection using DC positive connection.
(3) tungsten pole selection. The diameter of tungsten electrode is selected according to the wall thickness of titanium alloy tube, generally between 1.0-3.Omm, and the end of the tungsten electrode should be ground into a 25°-45° cone.
(4) Welding current and other parameters selection.
