Aluminum and aluminum alloy materials have low density, high strength, high thermal conductivity, strong corrosion resistance, and outstanding physical properties and mechanical functions, so they are widely used in the welding structure of industrial products. For a long time, due to improper selection of welding methods and welding technical parameters, severe deformation of aluminum alloy parts due to excessive stress after welding, or welding seam pores, slag inclusion, incomplete penetration and other shortcomings, resulting in weld metal cracks or The loose raw materials severely affected the quality and function of the products.
1. Features of aluminum alloy material
Aluminum is a silver-white light metal with outstanding plasticity, high electrical and thermal conductivity, together with the ability to resist oxidation and corrosion. Aluminum is easily oxidized to produce aluminum oxide film, and inclusions are simply generated in the weld, which then damages the continuity and uniformity of the metal, reducing its mechanical function and corrosion resistance. The chemical composition and mechanical functions of common aluminum alloy base materials and welding wires. Guang Yirong Copper and Aluminum Wholesale,
2. Difficulties in welding aluminum alloy materials
(1) Very simple oxidation. In the air, aluminum simply combines with oxidation to form a fine aluminum oxide film (thickness of about 0, 1-0, 2μm), with a high melting point (about 2050°C), far exceeding the melting point of aluminum and aluminum alloys (about 600 ℃ at the mercy). The density of alumina is 3, 95-4, 10g/cm3, which is about 1, 4 times that of aluminum. The surface of alumina film is easy to absorb moisture. When welding, it prevents the fusion of basic metals, and it is very simple to form pores, slag, and slag. Disadvantages such as lack of fusion, cause the function of the weld to be reduced.
(2) Stomas are prone to occur. The primary cause of pores in aluminum and aluminum alloy welding is hydrogen. Because liquid aluminum can dissolve a lot of hydrogen, while solid aluminum hardly dissolves hydrogen. Therefore, when the temperature of the molten pool is rapidly cooled and condensed, hydrogen is too late to escape. The pores are assembled in the weld. At present, it is difficult to completely avoid hydrogen holes. There are many sources of hydrogen, such as hydrogen in the arc welding atmosphere, and the surface of aluminum plates and welding wires absorb moisture in the air. Practice has proved that even if the purity of argon gas reaches 99% or more than 99% according to the requirements of GB/T4842 standard, when the moisture content reaches 20ppm, there will be many fine pores. When the relative humidity of the air exceeds 80%, the weld will be Significantly showing stomata.
(3) The welding seam has a large tendency to deform and constitute cracks. The linear expansion coefficient and crystal shortening rate of aluminum are about twice larger than that of steel. It is prone to large internal stress of welding deformation, and the structure with greater rigidity will promote the occurrence of hot cracks.
(4) The thermal conductivity of aluminum is large (pure aluminum 0, 538 cal/Cm, s, ℃). It is about 4 times that of steel. Therefore, welding aluminum and aluminum alloy consumes more heat than welding steel.
(5) Burning loss by transpiration of alloying elements. Aluminum alloy contains elements with low boiling points (such as magnesium, zinc, manganese, etc.). Under the effect of high-temperature arc, it is easily burned by evaporation, and then the chemical composition of the weld metal is changed to reduce the function of the weld.
(6) High temperature strength and low plasticity. At high temperatures, the strength and plasticity of aluminum are very low, which damages the formation of the weld metal, and sometimes simply forms the appearance of weld metal collapse and weld penetration.
(7) No color change. When aluminum and aluminum alloy change from solid to liquid, there is no significant color change, which makes it difficult for the operator to grasp the heating temperature.
3. Technical methods for welding aluminum alloy materials
(1) Preparation before welding
Use chemical or mechanical methods to strictly clean up the surface oxide film on both sides of the weld groove.
Chemical cleaning is the use of alkali or acid to clean the surface of the workpiece. This method can remove the oxide film and remove the oil stain. The specific technical process is as follows: the volume fraction of 6%-10% sodium hydroxide solution, soaked at 70 ℃ 0 , 5min→water washing→15% nitric acid volume fraction soaked at room temperature for 1min for neutralization treatment→water washing→warm water washing→drying. The appearance of the washed aluminum alloy is matt silver white.
For mechanical finishing, pneumatic or electric milling cutters can be used, scrapers, files, etc. can also be used. For thinner oxide films, 0 or 25mm copper wire brushes can also be used to remove oxide films.
Welding should be performed immediately after finishing. If it is placed for more than 4 hours, it should be sorted from the beginning.
(2) Determine the installation gap and tack welding distance
During the welding process, the aluminum plate is heated and expanded, which reduces the gap of the weld groove. If the installation gap before welding is left too small, the groove of the two plates will stack during the welding process, adding unevenness and deformation of the plate surface after welding. On the contrary, if the installation gap is too large, it will be difficult to weld and burn through. Appropriate tack welding distance can ensure the required tack welding gap. Therefore, choosing a suitable installation gap and tack welding distance is a useful way to reduce deformation. According to experience, the more reasonable installation technical parameters for butt joints of different plate thicknesses are shown in Table 2.
(3) Choose welding equipment
At present, there are many kinds of welding products on the market, and it is usually better to use communication tungsten arc welding (ie TIG welding). It is a welding method that uses the arc heat generated between the tungsten electrode and the workpiece to melt the base material and the filler wire under the maintenance of argon gas. When the welding machine is in operation, because the polarity of the communication current is changed periodically, in each cycle the half-wave is DC positive connection, and the half-wave is DC reverse connection. The half-wave time tungsten electrode of the positive connection can emit satisfactory electrons without overheating, which is conducive to the stability of the arc. The oxide film formed on the surface of the workpiece in the half-wave time of the reverse connection is simply sorted out to obtain a bright and beautiful appearance and excellent shape of the weld.
(4) Select welding wire
Usually use 301 pure aluminum welding wire and 311 aluminum silicon welding wire.
(5) Choose welding method and parameters
It is usually carried out by the left welding method, with the welding torch and the workpiece at a 60° angle. When the welding thickness is more than 15mm, the right welding method is used, and the welding torch and the workpiece form an angle of 90°.
When the welding wall thickness is more than 3mm, open the V-shaped groove, the included angle is 60°-70°, the gap should not be greater than 1mm, and it is completed by multi-layer welding. When the wall thickness is below 1, 5mm, no grooves, no gaps, and no filler wires are added. When welding fixed pipe butt joints, when the pipe diameter is 200mm and the wall thickness is 6mm, tungsten electrodes with a diameter of 3-4mm should be selected, with a welding current of 220-240A, and a filler wire with a diameter of 4mm, with a diameter of 1— —2 layers are welded.
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