Requirements and test methods: 1 aluminum because of light weight, high strength, corrosion resistance properties, and non-magnetic, good formability and low temperature characteristics of a good function is widely used in various welding wlectrodes , the use of aluminum instead of steel materials welding, structural weight, reduce over 50%. Therefore, in addition a wide range of aluminum and aluminum alloy used in aviation, aerospace and electrical and other areas, but also more and more used in the petroleum chemical industry. Puyang Zhongyuan Dahua new air separation plant on the extensive use of aluminum-magnesium alloy (mainly: 5083,5183,5 A02 commensurate with the old brand of LF2, LF4). However, aluminum and welding alloys in the welding process, easily thrown oxidation, porosity, hot cracking, burn through and collapse and other topics. Such material is widely recognized that the welding is more difficult welding materials, in particular Small and Thin welding more difficult to grasp. Thus, the solution of aluminum and aluminum alloy seamless steel pipe welding defects, these must be resolved in the construction process is the subject. 2 Aluminium welding rod and aluminum alloy welding characteristics of the physical and chemical functions and 2.1 easily oxidized
Strong affinity for aluminum and oxygen. At room temperature, the aluminum surface can be oxidized to a thickness of about 0.1 ~ 0.2 m dense AL2O3 film. While this layer of aluminum oxide thin layer is compact and can prevent the oxidation of the metal of the inheritance of beneficial natural preservative, but it brought to the welding problem, because the aluminum melting point (2050 â„ƒ) far exceeds the melting point of aluminum (600 â„ƒ), the proportion of aluminum is about 1.4 times. In the welding process, will prevent the fusion between the metal, easy to form slag, and alumina films also absorbed more water, will lead to welding seam natural porosity. 2.2 The large thermal conductivity and specific heat
Thermal conductivity of aluminum is about four times the steel, so aluminum pipe welding, pipe welding to consume more than the heat, to get high-quality welded joint, the energy must be focused, high power heat source. 2.3 easy to form hydrogen gas hole
Welding aluminum and aluminum alloy main hydrogen stomatal pore. Aluminum absorption in the liquid and dissolved when a large number of hydrogen solubility in the molten state 0.0069ml / g, while in the state under high temperature solidification 0.00036 ml / g, a difference of nearly 20 times before. Large thermal conductivity of aluminum, the welding process in the same context, the cooling rate of 4 to 7 times the steel, the metal mold to speed up, weld pool in the rapid cooling process, the hydrogen solubility sharp decline in precipitation at this time a large number of supersaturated gas, hydrogen time to the formation of precipitation in the weld metal porosity. Therefore, when welding aluminum, weld a tendency to produce large holes angry. Easy to form a hot crack 2.4
Linear expansion coefficient of aluminum and steel mold shrinkage than about twice as easy to produce large welding deformation and stress, together with some impurities or adverse effects of alloying elements, in the rigid joints will lead to larger cracks. 2.5 burn and collapse
Aluminum and aluminum alloy from a solid into a liquid state. Because there is no significant color change, therefore, difficult to determine the bath temperature. Welding, often due to high temperature and can not easily be detected through a result of severe burn or collapse.
3 Preparation before welding 3.1 groove processing by mechanical processing method
After processing the groove surface should be smooth, without burrs and flash. Groove in the form of generally V-type, no blunt edge, bevel angle of 70 ~ 75 â„ƒ appropriate. Butt welding of different thickness of the transition section should be 14O. 3.2 Preparation before welding
Before welding wire, welded inside and outside the groove and the groove within the range of 30 ~ 50mm of oil and oxide film removed, remove the order and method are as follows: acetone or carbon tetrachloride and other organic solvents to remove surface oil, Singapore Clear on both sides inside and outside the scope of export shall not be less than 50mm. Remove oil, the wire by chemical method, mechanical method is easy groove, tubes are removed by chemical surface oxide film. Mechanical methods, the surface is available in and around the groove cut filing, scraping, milling or 0.2mm stainless steel wire brush to clean around to reveal metallic luster, clear on both sides of the groove from the edge of the range should be not less than 30mm, use tools Regular degreasing. Chemical method. Is about 70 â„ƒ 5 ~ 10% of the NaOH solution for 30 to 60 seconds, or at room temperature 5 to 10% NaOH solution, soak 3 minutes. Then with about 15% of the HNO3 (room temperature) after 2 minutes soaking with warm water. Or wash with cold water, then it is completely dry. Surface treatment that have been reliable, has not been oxidized or contaminated wire, no longer carry out such processing can be used directly. Clean good groove and wire, the former should not be tainted in the weld, in the absence of effective protective measures, should be within 8 hours of welding. Otherwise, should be re-cleaned. Pipeline Group, should be flush wall, no glitches, grain dust, the wrong side of the volume should be consistent with b ≤ 0.5mm. Lap weld without internal lining, requiring that the gap is zero as far as possible, especially in overhead position welding parts, the inner wall should be rounded down 1 ~ 1.5mm. 3.3 monitor the welder and other matters
AC TIG welding machine welding machine is required, with a steep drop of the external characteristics and adequate capacity. And there is no arbitrary parameters, regulating the use of flexible and reliable function, should also have the arc, arc stability and the elimination of DC component device, welder current, voltage measurement part of the table should be identified by the qualified welder in use before Check the ground is intact, cooling water and gas road is smooth, and its various functions to ensure that working. Welding areas shall be kept clean. In addition there should be wind, rain and snow prevention initiative measures, but also when the relative humidity to ensure welding ≤ 80%, ambient temperature> 5 â„ƒ.
4 Welding Technology 4.1 The selection of welding materials
Wire in principle, choose the same components with the base material of aluminum and aluminum alloy wire or strip. Argon purity> 99.95%, try to use large diameter wire. Al-Mg aluminum alloy in the arc, usually are recommended to use CB-AMr2, CB-AMr3, CB-AMr6, CB-AMr61, CB-AMr63, 1557,1577 electrode, on the Al-Cu aluminum alloy Recommended by 01201 and 01217. 4.2 points on the solid welding with.
Thermal conductivity of aluminum and aluminum alloy tubes for fast, fast crystallization of molten pool, so. Group without leaving gaps, blunt edge, force should be avoided to reduce the welding residual stress resulting from the larger, location for the weld length 10-15mm Easy. Positioning in the pipe welding positions 7, 9, 12 points. Positioning as a full seam weld often retained, so that subject should be a timely manner. Tack welding the surface before welding black powder, oxide film removal, and both ends of the Xiu gentle type. Weldment does not need to warm up. Welding welding trial before the trial panel, when the stomata after confirming that no formal welding. High frequency arc, the arc point should be about 20mm across the center line and remain stationary for about 2-3 seconds, shown in Figure 1. Then ensure that the case of penetration, the use of high current, fast welding. Wire does not swing, not to leave the tip of the wire argon protected areas. Leave the protected areas such as argon. Tip of the wire should be cut off. The angle between the wire and the weld surface should be in the 15O right. Torch and the weld surface angle should be maintained at between 80O ~ 90O, as shown in Figure 2. To increase the argon gas reserves and enhance the protection effect can be large-diameter ceramic gun mouth, increased flow of argon gas welding torch. There are significant obstacles when the nozzle argon gas flow when the fluid spatter adhesion. Necessary to remove or replace the nozzle spatter. When the Department of tungsten formed the extreme pollution, when the phenomenon of irregular shape. Must be repaired or replaced. Outside the nozzle should not be out of tungsten. Welding temperature control is mainly the size of the welding speed and welding current control. The results showed that high current, fast weld can effectively prevent the generation of holes. This is mainly because in the welding process to a more rapid rate of penetration in the weld, the molten metal heating time is short, less opportunities to absorb the gas. Crater, the attention to fill the crater and narrow melting pool, to avoid shrinkage, a combination of the end should be welded at over 20 ~ 30mm. Arc stop, stop gas for 6 seconds to delay. Rotating flat aluminum and aluminum alloy seamless steel pipe butt welding. Welding torch welding position should be in the slightly uphill. It is a good penetration. Thick-walled pipe welding the bottom. Do not fill in additional wires.
Welding lap joint and usually fold joint connector can be two or more ranges such as the thickness of the workpiece thickness or composition. Spot in the design of the structure, we must consider the accessibility of the electrode, the electrode must be able to easily reach the welding position of the workpiece. Should also be considered, such as margins, overlap volume, pitch, assembly gaps and joint strength of various factors.
The minimum margins depending on the type of welding wire , thickness, and welding conditions. For high yield strength of metal, thin pieces, or less desirable conditions with strong values.
Pitch is the center distance between two adjacent, the minimum and welded metal thickness, conductivity, surface cleanliness, and the nugget diameter.
Pitch minimum requirement is mainly on account of diversion, the use of high pressure conditions, and large electrode, the pitch may be appropriately reduced. Thermal expansion can be used to monitor or change the order of the controller of the point current, and effective compensation for diversion of the other device, dot pitch can be unlimited.
Assembly gap must be as small as possible because space will be consumed by the pressure to eliminate part of the electrode pressure, the actual welding pressure is reduced. Inhomogeneity of the gap and make the mig welding wires fluctuations, which led to the significant difference in joint strength, the gap is too large would cause serious flying, the allowable gap value depends on the workpiece stiffness and thickness, stiffness, thickness greater allowable gap smaller, usually 0.1-2mm.
Single solder joint shear strength depends on the junction of two plates nugget area, in order to ensure the joint strength, nugget diameter, in addition, the penetration rate and the indentation depth should also meet the requirements for the penetration rate of expression: η = h / δ-c × 100%. Two panel penetration rate is only allowed between the range of 20-80%. Magnesium alloy penetration rate is only the maximum allowed to 60%. The alloy is allowed to 90%. Welding of different thickness of the workpiece, each workpiece penetration rate of the minimum for thin joints in 20% of the thickness, the indentation depth should not exceed 15% of the thickness of plates, if the two workpiece thickness ratio greater than 2:1, difficult to close the site or in the welding, and using flat side of the workpiece electrode, the indentation depth can be increased to 20-25%. Figure 11-10 shows the low times of grinding on the nugget size.
The vertical direction of the panel by spot welding joints, the tensile load, strength, tensile strength is positive. As the nugget around sharp corners formed between the two plates can cause stress concentration, leaving the actual nugget strength decreased, which generally do not load the spot weld. Usually are tensile strength and shear strength of joints to determine the ductility ratio as an indicator, this ratio is larger, the better the ductility of joints.
Multiple solder joint strength will also depend on the formation of pitch and solder joint distribution. Pitch-hour joints be affected because of segregation strength, large dot pitch will limit the number of solder joints can be arranged. Therefore, we must take into account the pitch and the number of solder joints in order to obtain maximum joint strength, multi-column joints staggered and not the best arrangement for the rectangle.