Titanium plate welding technology and process measures to prevent porosity

               
Update: 02-12-2020
               
Abst:

 1. Gas pollution problem   (1) Make preparations befor […]

 1. Gas pollution problem
  (1) Make preparations before welding. Strictly clean the surface of the weld to prevent the intrusion of hydrogen, oxygen, and nitrogen.
  (2) Use an accurate argon gas flowmeter to control the gas flow. The selection of gas flow rate is based on achieving a good protection effect. The size of the argon flow rate has a considerable impact on the protection. Excessive flow rate is not easy to form a stable gas flow layer. Instead, turbulence is formed in the protection zone, causing harmful gases to immerse into the melt. Pool, making the surface of the weld prone to microcracks. If the air flow is too small, the protection is not in place and the protection effect is not achieved. When the flow of argon in the drag hood is insufficient, the weld will show different oxidation colors.
   (3) Strengthen weld protection. During welding, the end of the welding wire must not be removed from the argon gas protection zone; when the arc is interrupted and the weld is finished, the argon gas protection should be continued until the weld and the heat-affected zone metal are cooled to below 100 ℃ before removing the welding gun.
  2, welded joint crack problem
When titanium is welded, the possibility of hot cracks in the welded joint is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is small, and the low melting point eutectic formed by S and P is not easy to appear at the grain boundary. In addition, the effective crystallization temperature range is narrow, the shrinkage of titanium and titanium alloys is small during solidification, and the weld metal will not produce hot cracks.
   During titanium welding, cold cracks can appear in the heat-affected zone, which is characterized by cracks occurring several hours or even longer after welding. Studies have shown that this kind of crack is mainly caused by the influence of carbon and hydrogen and the excessive cooling rate. The method to prevent this kind of delayed cracking is mainly to reduce the sources of hydrogen and carbon in the welded joint, and to protect and clean the weld area before welding to prevent harmful impurities from contamination.
  Secondly, the temperature between layers should be strictly controlled. Under the premise of ensuring good fusion, use low heat input as much as possible to weld, that is, reduce the fusion ratio.
   Use small diameter welding wire, low welding current, narrow bead technology, and fast welding. The cooling rate is best controlled at about 100°C/s.
  3. Porosity in the weld
   porosity is a defect that is relatively easy to produce in the welding of titanium plates, mainly due to the effect of hydrogen. The surface of the plates and welding materials is not clean. The moisture and grease on the gloves of the operator, the sand particles and flying dust from the angle grinder are all sources of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint.
  The main technological measures to prevent pores are:
  (1) The protection neon gas should be pure, and the purity should not be less than 99.99%. The air duct should be reinforced plastic tube, not rubber tube.
  (2) Thoroughly remove the oxide scale and oil stains on the surface of the weldment and the surface of the welding wire.
  (3) Apply good gas protection to the molten pool, and control the flow and flow rate of argon to prevent turbulence and affect the protection effect.
  (4) Correctly select the welding process parameters, increase the residence time of the weld metal in the molten pool, and make the bubbles escape, which can effectively reduce the pores.
  (5) Use small heat input when welding, preferably pulse argon arc welding, which can improve joint plasticity, reduce overheating and coarse crystals, reduce deformation, increase penetration depth, and reduce the generation of pores.
   4. Preparation before welding
It is better to separate an area at the welding site of titanium materials. Non-professionals are not allowed to enter to protect the cleanliness of the area. All welding personnel should wear clean work clothes, knitted gloves or thin sheepskin gloves, and cannot wear cotton gloves. Weld in the tank, and the shoes should be covered with shoe covers.
The groove and the inner and outer surfaces within 50mm on both sides and the surface of the welding wire should be cleaned of oil, and the oxide film, burrs and surface defects should be removed by mechanical methods such as fine-file austenitic stainless steel wire brushes and milling cutters. The cleaning tools should be dedicated and kept. Clean; the surface after mechanical cleaning should be degreasing treated with sulfur-free acetone or ethanol before welding. Chlorinated solvents such as trichloroethylene and carbon tetrachloride are strictly prohibited, and cotton fibers should not be attached to the groove surface. According to the technical requirements, before welding, the "Ferroline" test was performed on the welding environment, and the "iron ion" pollution test was done around the weld seam, and the welding was performed after the test passed.
The quality of cleaning will directly affect the generation of welding cracks. When the cleaning quality is not good, a gray-white gettering layer will be formed on the surface of the base metal and welding wire, resulting in welding cracks and pores. Therefore, pay attention to cleaning. The following points:
  ①Use pickling first. The pickling solution is 2%~4%HF+30%~40%HN03+H20 (the balance). Finally, rinse with clean water and blow dry, scrub with acetone or alcohol before welding.
  ②The welding wire generally needs dehydrogenation treatment after pickling, and then degreasing with acetone before welding.
  ③It is strictly prohibited to use oxide degreasing.
  ④ Rubber gloves should not be worn during cleaning to avoid the reaction of the rubber and the degreasing solvent to cause pores in the weld.
  ⑤The welded joint after cleaning
  The welding must be finished immediately, and it should not be put for a long time (generally not more than 4h), otherwise it needs to be cleaned again according to the previous steps.
   The protective cover used when welding titanium is very important. A skilled welder without a good protective cover, even if the weld is beautifully welded and all oxidized and discolored after welding, this weld is still unqualified and will be scrapped.
   5. The essentials of manual argon arc welding of titanium
  1) When manual argon arc welding, the angle between the welding wire and the weldment should be kept as small as possible (10~15°). The welding wire is fed into the molten pool smoothly and evenly along the front end of the molten pool, and the end of the welding wire must not be moved out of the argon gas protection zone.
  2) During welding, the welding gun basically does not swing laterally. When it is necessary to swing, the frequency should be low and the swing amplitude should not be too large to prevent affecting the argon protection.
  3) When the arc is interrupted and the weld is finished, the argon gas protection should be continued until the weld and the heat-affected zone metal are cooled to below 100 ℃ before removing the welding torch.
  4) The quality of cleaning will directly affect the generation of welding cracks. When the cleaning quality is not good, a gray-white gettering layer will be formed on the surface of the base metal and welding wire, resulting in welding cracks and pores.
  6, the selection basis of process parameters
   1) If the welding current is high, the welding will be very smooth, but the grain of the weld will be coarse, the performance will be poor, and large residual stress will easily occur around the weld. Therefore, the upper limit of the welding current should be controlled. When determining the current of the cover fillet weld, the first pass is self-melting, and the second pass is wire. The welding current is 130-140A.
  2) Arc voltage selection The arc voltage is actually to choose the length of the arc. Commonly used argon arc welding machines do not have a voltage selection button. The arc should be kept as short as possible to control the arc voltage between 14-16V.
  3) Welding speed Titanium material has poor thermal conductivity during welding, and the current and speed must be matched well, otherwise the fusion will not be good. After testing, when the welding current is 130-140A, the welding speed is 160-170mm/min.
  4) Determination of argon flow
  Titanium must be cooled in an argon atmosphere within 100mm of the weld zone behind the molten pool (including the front and back of the weld and heat-affected zone) during welding, otherwise the weld will be oxidized and discolored, which will reduce performance. After field test and adjustment, the final argon flow rate is: nozzle 10-11L/min, support cover 12-13L/min, backside 3-5L/min.
  5) For the argon arc welding of titanium plates, a DC argon arc welding power source with reduced external characteristics and high frequency arc ignition is used. The tungsten end should be ground into a cone shape and horizontal rotation flat welding is adopted. The welding torch should be supplied with gas in advance, and the current attenuation device and gas delay protection device should be used when the arc is extinguished. The delayed gas delivery time should not be less than 15 seconds to avoid oxidation and pollution of the weld.
  7, assembly
   In order to reduce welding distortion, tack welding is required before welding. The tack welds should use the same welding materials and welding process as the formal welding, that is, the welding wire, welding process parameters and gas protection conditions used should be the same as the welding joints. Before welding, remove the oxide layer, oil stain, moisture, rust, etc. on the surface of the welding wire, the welding groove and both sides of 20mm, and the welding should be performed by a qualified welder, and the tack welding should be on the base weld. The length of the weld should be 10-15mm, the spacing is 100-150mm, and the height should not exceed 1/3 of the wall thickness. The gap is 0-1mm, and the blunt edge is 0-1.0mm.
   Welding seams must not have defects such as cracks, pores, slag inclusion, oxidation and discoloration. When defects are found, they should be eliminated in time.

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