Aluminum and aluminum alloy materials have low density, high strength, high thermal conductivity, strong corrosion resistance, and good physical and mechanical properties, so they are widely used in welding structures of industrial products. For a long time, due to the improper selection of welding methods and welding process parameters, aluminum alloy parts have serious deformation due to excessive stress concentration after welding, or because of weld porosity, slag inclusion, incomplete welding and other defects, resulting in weld metal cracks or loose materials, seriously affecting product quality and performance.

1 Aluminum alloy material characteristics

Aluminum is a silver-white light metal with good plasticity, high electrical and thermal conductivity, and also has the ability to resist oxidation and corrosion. Aluminum is easily oxidized to produce aluminum oxide film, which is easy to produce inclusions in the weld, thus destroying the continuity and uniformity of the metal, reducing its mechanical properties and corrosion resistance. The chemical composition and mechanical properties of common aluminum alloy base metal and welding wire are shown in Table 1.

2. Difficulties in welding aluminum alloy materials

(1) Easy oxidation. In the air, aluminum is easy to oxidize and form a dense aluminum oxide film (thickness of about 0.1-0.2μm), with a high melting point (about 2050 ° C), far more than the melting point of aluminum and aluminum alloys (about 600 ° C). 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 hinders the fusion of basic metal, and it is easy to form porosity, slag inclusion, non-fusion and other defects, resulting in the deterioration of weld performance.

(2) Easy to produce pores. The main reason for the porosity of aluminum and aluminum alloy welding is hydrogen, because liquid aluminum can dissolve a large amount of hydrogen, and solid aluminum almost does not dissolve hydrogen, so when the temperature of the molten pool is quickly cooled and solidified, hydrogen has no time to escape, and it is easy to gather in the weld to form pores. Hydrogen pores are difficult to completely avoid at present, there are many sources of hydrogen, hydrogen in the arc welding atmosphere, aluminum plate, welding wire surface adsorption of water in the air and so on. Practice has proved that even if argon is more than 99.99% pure according to the GB/T4842 standard, when the moisture content reaches 20ppm, there will be a large number of dense pores, and when the relative humidity of the air exceeds 80%, there will be obvious pores in the weld.

(3) The weld deformation and crack formation tendency is large. The linear expansion coefficient and crystallization shrinkage rate of aluminum are about two times larger than that of steel, which is easy to produce large internal stress of welding deformation, and will promote the production of hot cracks for the structure with large rigidity.

(4) The thermal conductivity of aluminum is large (pure aluminum 0.538 CAL/Cm.s. ° C). It is about 4 times that of steel, so when welding aluminum and aluminum alloys, more heat is consumed than welding steel.

(5) Burning loss by evaporation of alloying elements. Aluminum alloy contains elements with low boiling point (such as magnesium, zinc, manganese, etc.), which are easy to evaporate and burn under the action of high temperature arc, thus changing the chemical composition of the weld metal and reducing the performance of the weld.

(6) Low high temperature strength and plasticity. At high temperature, the strength and plasticity of aluminum are very low, which destroys the formation of the weld metal, and sometimes easily causes the weld metal to collapse and weld through.

(7) No color change. When aluminum and aluminum alloys change from solid to liquid, there is no obvious color change, making it difficult for the operator to master the heating temperature.

3 Process method for welding aluminum alloy materials

(1) Preparation before welding

Chemical or mechanical methods are used to strictly clean 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, the method can remove the oxide film, but also remove the oil, the specific process is as follows: the volume fraction of 6% to 10% of the sodium hydroxide solution, at about 70℃ soaking 0.5 min→ water → volume fraction of 15% of nitric acid at room temperature soaking for 1min for neutralization → water washing → warm water washing → drying. The surface of the washed aluminum alloy is a dull silver white.

Mechanical cleaning can be used pneumatic or electric milling cutter, can also be used scraper, file and other tools, for thinner oxide film can also be used 0.25mm copper wire brush grinding to remove oxide film.

Weld immediately after cleaning, and if the placement time exceeds 4h, it should be cleaned again.

(2) Determine the assembly gap and positioning welding spacing

During the welding process, the aluminum plate is heated and expanded, resulting in the reduction of the weld groove gap. If the assembly gap before welding is too small, the groove overlap of the two plates will be caused during the welding process, which will increase the roughness and deformation of the plate surface after welding. On the contrary, the assembly gap is too large, it is difficult to weld, and there is the possibility of burning through. Appropriate positioning welding spacing can ensure the required positioning welding gap, so selecting the appropriate assembly gap and positioning welding spacing is an effective measure to reduce deformation. According to experience, the more reasonable assembly process parameters for different plate thicknesses are shown in Table 2.

(3) Select welding equipment

At present, there are many kinds of welding products on the market, and it is generally appropriate to use AC tungsten argon arc welding (TIG welding). Under the protection of argon, it is a welding method that uses the arc generated by the tungsten electrode and the workpiece to melt the base metal and fill the welding wire. When the welding machine works, because the polarity of the AC current is changed periodically, half wave is DC positive connection and half wave is DC reverse connection in each cycle. The tungsten electrode can emit enough electrons without overheating during the positive half-wave, which is conducive to the stability of the arc. The oxide film generated on the surface of the workpiece during the reverse half wave is easily cleaned off to obtain a bright and beautiful surface and well-formed weld.

(4) Select the welding wire

Generally, 301 pure aluminum welding wire and 311 aluminum silicon welding wire are selected.

(5) Select welding methods and parameters

It is generally carried out by the left welding method, and the welding torch and the workpiece are 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 are at a 90° Angle.

When the welding wall thickness is more than 3mm, the V-shaped groove is opened, the Angle is 60° ~ 70°, and the gap is not greater than 1mm, and the multi-layer welding is completed. When the wall thickness is less than 1.5 mm, no groove is opened, no gap is left, and no filler wire is added. When welding the fixed pipe butt joint, when the diameter of the pipe is 200mm and the wall thickness is 6mm, the tungsten electrode with a diameter of 3 ~ 4mm should be used, and the welding current of 220 ~ 240A and the filling wire with a diameter of 4mm should be welded in 1 ~ 2 layers.

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