Titanium plate welding technology and technological measures to prevent porosity
The problem of gas pollution (1) Prepare for welding. Strictly clean the weld surface to prevent the intrusion of hydrogen, oxygen, and nitrogen. (2) Select an accurate argon gas flowmeter to control the gas flow. The choice of gas flow rate is based on achieving a good protection effect. The size of the argon gas flow rate has a considerable impact on the protection. Excessive flow rate is not easy to form a stable airflow layer, but instead forms turbulent flow in the protection zone, causing harmful gases to immerse in the molten metal. pool, making the surface of the weld prone to micro-cracks. If the airflow is too small, the protection is not in place, and the protective effect cannot be achieved. When the flow of argon gas in the drag hood is insufficient, the weld shows different oxidation colors. (3) Strengthen the weld protection. During welding, the end of the welding wire should not be moved out of the argon protection zone; when the arc is broken and the welding seam ends, the argon protection should be continued until the welding seam and the metal in the heat-affected zone cool down to below 100 ℃ before removing the welding torch.
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 S, P, C, and other impurities in titanium and titanium alloys is very 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. When titanium is welded, cold cracks can appear in the heat-affected zone, which is characterized by cracks occurring several 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 too fast a cooling rate. The main method to prevent such delayed cracks is to reduce the source of hydrogen and carbon in the welded joints and to protect and clean the weld area before welding to prevent contamination by harmful impurities. Secondly, the interlayer temperature should be strictly controlled. On the premise of ensuring good fusion, welding should be performed with low heat input as much as possible, that is, to reduce the fusion ratio. Adopt small diameter welding wire, low welding current, narrow bead technology, and fast welding. It is best to control the cooling rate at about 100°C/s.
The problem of pores in the weld The porosity is a defect that is relatively easy to produce in the welding of titanium plates, mainly due to the influence of hydrogen. The surface of the plate and welding material 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 porosity in the weld metal mainly affects the fatigue strength of the joint. The main process measures to prevent the generation of pores are: (1) The protective neon gas should be pure, and the purity should not be lower than 99.99%. The trachea should be made of reinforced plastic tube, not rubber tube. (2) Thoroughly remove organic substances such as oxide skin 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 gas to prevent turbulent flow and affect the protective effect. (4) Correct selection of 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 a small heat input during welding, preferably pulse argon arc welding, which can not only improve the plasticity of the joint, reduce overheating and coarse grains, and reduce deformation, but also increase the penetration depth and reduce the generation of pores.
Preparation before welding In the welding site of titanium materials, it is best to separate an 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, not cotton gloves. Welding in the tank, shoes should be covered with shoe covers. The inner and outer surfaces of the groove and its 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. Cleaning: The surface after mechanical cleaning should be degreased with sulfur-free acetone or ethanol before welding. Chloride solvents such as trichloroethylene and carbon tetrachloride are strictly prohibited, and cotton fibers must not be attached to the groove surface. According to the technical requirements, before welding, the “Ferroline” test was carried out on the welding environment, the “iron ion” pollution test was carried out around the weld, and the welding was carried out only after the test was qualified. The quality of cleaning will directly affect the generation of welding cracks. When the cleaning quality is not good, a gray-white getter layer will be formed on the surface of the base metal and welding wire, resulting in the generation of welding cracks and pores. Therefore, attention should be paid to cleaning. The following points: ①Pickling is used first. The pickling solution is 2%~4%HF+30%~40%HN03+H20 (balance). Finally, rinse with clean water and blow 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. ③It is strictly forbidden to use oxide degreasing. ④Do do not wear rubber gloves when cleaning, so as to avoid the reaction between the rubber and the degreasing solvent, which will cause pores in the weld. ⑤Welding joint after cleaning It must be finished immediately, and it should not be left for a long time (generally no more than 4h), otherwise it needs to be cleaned 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 welded seam is beautifully welded and all oxidized and discolored after welding, the welded seam is still unqualified and will be scrapped.
Operation essentials of manual tungsten argon arc welding of titanium 1) During manual argon arc welding, the minimum angle (10~15°) should be kept as far as possible between the welding wire and the weldment. The welding wire is smoothly and evenly fed into the molten pool along the front of the molten pool, and the end of the welding wire shall not be moved out of the argon protection zone. 2) During welding, the welding torch basically does not swing laterally. When swinging is required, the frequency should be low and the swinging amplitude should not be too large to prevent the protection of argon gas from being affected. 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 ℃ 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 getter layer will be formed on the surface of the base metal and the welding wire, resulting in the generation of welding cracks and pores.
Selection basis of process parameters 1) If the welding current is large, the welding is very smooth, but the grain of the weld is coarse and the performance is poor, and it is easy to generate large residual stress around the weld. 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 pass is self-melting, the second pass is wire, and the welding current is 130-140A. 2) Selection of arc voltage The arc voltage is actually the selection of the length of the arc. The commonly used argon arc welding machine does not have a voltage selection button, and the arc should be 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 should 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. 4) Determination of argon flow rate Titanium should be cooled in an argon protective atmosphere within 100mm of the weld area behind the molten pool (including the front and back sides of the weld and heat affected zone) during welding, otherwise, the weld will be oxidized and discolored, which will reduce the performance. Adjusted by the field test, the final argon flow rate is 10-11L/min for the nozzle, 12-13L/min for the cover, and 3-5L/min for the back. 5) Titanium plate argon arc welding should use a DC argon arc welding power source with lower external characteristics and high-frequency arc ignition. The welding torch should be supplied with gas in advance, and the current attenuation device and the gas delay protection device should be used when the arc is extinguished.
Assembly In order to reduce welding deformation, tack welding is required before welding, and the tack welding seam 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 and the welding groove and within 20mm of each side on both sides, and should be welded by a qualified welder, and the tack welding should be welded on the base welding seam. The length of the welding seam should be 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. Welds shall not have defects such as cracks, pores, slag inclusions, and oxidative discoloration. When defects are found, they should be eliminated in time.