Titanium Plate Welding Technology and Technological Measures to Prevent Pores
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) Choose an accurate argon gas flowmeter to control the gas flow. The choice of gas flow rate is subject to good protection effect. The flow rate of argon gas has a considerable influence on protection. Excessive flow rate is not easy to form a stable air flow layer, but instead forms turbulent flow in the protection area, causing harmful gases to immerse into the molten gas. pool, making the weld surface prone to micro-cracks. If the air flow is too small, the protection will not be in place, and the protection effect will not be achieved. When the argon flow in the towing hood is insufficient, the weld will show different oxidation colors.
(3) Strengthen weld protection. When welding, the end of the welding wire must not be moved out of the argon protection zone; when the arc is broken and the end of the weld is completed, the argon protection must be continued until the weld and the metal in the heat-affected zone cool down to below 100°C before removing the welding torch.
2. Cracks in welded joints
When titanium is welded, the possibility of thermal cracks in the welded joint is very small. This is because the impurities such as S, P, and C in Grade 5 titanium plate and titanium alloy are very small, and the eutectic with low melting point formed by S and P is not easy to form in the crystal. In addition, the effective crystallization temperature range is narrow, the shrinkage of titanium and titanium alloys is small when solidified, and the weld metal will not produce hot cracks.
During titanium welding, cold cracks may appear in the heat-affected zone, which is characterized by cracks occurring hours or even longer after welding. The research shows that this kind of crack is mainly caused by the influence of carbon and hydrogen and the excessive cooling rate. The way to prevent this kind of delayed crack is mainly to reduce the source of hydrogen and carbon in the welded joint, and to protect and clean the weld area before welding to prevent contamination by harmful impurities.
Secondly, the interlayer temperature should be strictly controlled. Under the premise of ensuring good fusion, welding with low heat input is used as much as possible, that is, the fusion ratio is reduced.
Adopt small-diameter welding wire, low welding current, narrow welding bead technology, and fast welding. It is best to control the cooling rate at about 100°C/s.
3. Porosity problem in weld seam
Pores are defects that are relatively easy to occur when welding titanium plates. The main reason is the result of the influence of hydrogen. The surface of plates and welding materials is not clean, the moisture and grease on the operator's gloves, the sand 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 technological measures to prevent porosity mainly include:
(1) The protective neon gas should be pure, and the purity should not be lower than 99.99%. The air guide tube should be reinforced plastic tube, and rubber tube should not be used.
(2) Thoroughly remove organic matter such as scale oil on the surface of the weldment and the surface of the welding wire.
(3) Apply good gas protection to the molten pool, control the flow and velocity of argon gas, and prevent turbulent flow from affecting the protection effect.
(4) Properly 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 porosity.
(5) Use small heat input during welding, preferably pulsed argon arc welding, which can not only improve the plasticity of the joint, reduce overheating and coarse grain, reduce deformation, but also increase the penetration depth and reduce the generation of pores.
4. Preparation before welding
In the welding site of Grade 5 titanium plate, it is best to separate a separate area, and 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 cotton gloves cannot be worn. , into the tank for welding, and shoes should be covered with shoe covers.
The inner and outer surfaces within 50mm on both sides of the groove 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 means such as a fine file austenitic stainless steel wire brush and milling cutter. The cleaning tools should be dedicated and kept Clean; the surface after mechanical cleaning should be degreased with sulfur-free acetone or ethanol before welding. It is strictly forbidden to use chloride solvents such as trichlorethylene and carbon tetrachloride, and it is not allowed to attach cotton fibers to the groove surface. According to the technical requirements, before welding, the "Feraline" test was done on the welding environment, and the "iron ion" pollution test was done around the weld, and the welding was carried out only after passing the test.
The quality of cleaning will directly affect the generation of welding cracks. When the cleaning quality is not good, a layer of off-white air-absorbing layer will be formed on the surface of the base metal and welding wire, resulting in welding cracks and pores, so pay attention when cleaning The following points:
① Use pickling first. The pickling solution is 2%~4%HF+30%~40%HN03+H20 (balance). Finally, rinse with clean water and dry, and scrub with acetone or alcohol before welding.
②The welding wire generally needs to be dehydrogenated after pickling, and then degreased with acetone before welding.
③ It is strictly forbidden to use oxides for degreasing.
④ Do not wear rubber gloves when cleaning, so as to avoid the reaction between the rubber and the degreasing solvent and cause pores in the weld.
⑤ Weld joint after cleaning
It must be welded immediately, and it should not be left for a long time (generally no more than 4 hours), otherwise it needs to be cleaned again according to the previous steps.
The protective cover used for titanium welding is very important. A skilled welder does not have a good protective cover. Even if the weld is beautifully welded, it will be completely oxidized and discolored after welding. This weld is still unqualified and will be scrapped.
5. Operation essentials of titanium manual tungsten argon arc welding
1) During manual argon arc welding, the minimum included angle (10~15°) between the welding wire and the weldment should be kept as far as possible. 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 protection zone.
2) During welding, the welding torch basically does not swing laterally. When it is necessary to swing, the frequency should be low, and the swing range should not be too large, so as not to affect the protection of argon gas.
3) When the arc is broken and the welding seam is finished, the argon gas protection should be continued until the welding seam and the metal in the heat-affected zone cool down to below 100°C 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 layer of off-white air-absorbing layer will be formed on the surface of the base metal and welding wire, resulting in welding cracks and pores.
6. Basis for selection of process parameters
1) If the welding current is high, the welding will be very smooth, but the grains of the weld seam are coarse and the performance is poor, and large residual stress is likely to be generated around the weld seam. Therefore, the upper limit of the welding current should be controlled. When determining the current of the fillet weld of the cover plate, the first time is self-melting, and the second time is adding wire. The welding current is 130-140A.
2) Arc voltage selection The arc voltage is actually to select the length of the arc. The commonly used argon arc welding machine does not have a voltage selection button, so the arc should be kept as short as possible to control the arc voltage between 14-16V.
3) Welding speed Titanium has poor thermal conductivity during welding, so the current and speed must be well matched, otherwise the fusion will not be good. After testing, when the welding current is 130-140A, the welding speed is 160-170mm/min, which is more suitable.
4) Determination of argon flow rate
Titanium must be cooled in an argon protective atmosphere within 100mm of the weld zone behind the molten pool (including the front and back of the weld and the heat-affected zone) during welding, otherwise the weld will be oxidized and discolored, which will degrade its performance. After field test and adjustment, the final determined argon flow rate is: 10-11L/min for the nozzle, 12-13L/min for the cover, and 3-5L/min for the back.
5) Grade 5 titanium plate argon arc welding uses a DC argon arc welding power source with falling external characteristics and high-frequency arc ignition. The end of the tungsten pole should be ground into a conical shape, and horizontal rotation flat welding is used. The welding torch should be supplied with gas in advance, and a current attenuation device and a 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 prevent the weld from being oxidized and polluted.
7. Assembly
In order to reduce welding deformation, tack welding is required before welding. The tack welds should use the same welding material 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 those of 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 the range of 20mm on both sides, and the welding should be performed by a qualified welder. 10-15mm, the spacing is 100-150mm, and the height should not exceed 1/3 of the wall thickness. Gap 0-1mm, blunt edge 0-1.0mm.
