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[China Aluminum Industry Network] Aluminum and aluminum alloy materials have low density, high strength, high thermal conductivity, and strong corrosion resistance. They have excellent physical and mechanical properties, and are therefore widely used in welding structures for industrial products. For a long time, due to the improper selection of welding methods and welding technical parameters, the aluminum alloy parts were severely deformed due to stress concentration after welding, or due to defects such as pores, slag inclusions, and incomplete penetration of welding seams, resulting in metal cracks or raw materials of welds. Loosening has severely affected the quality and function of the product.
1 Aluminum alloy material characteristics
Aluminum is a silver-white light metal with excellent plasticity, high electrical conductivity and thermal conductivity, together with the ability to resist oxidation and corrosion. Aluminium is easily oxidized to produce a thin film of Al2O3, which causes inclusions to form in the welds. It then damages the continuity and uniformity of the metal and reduces its mechanical and corrosion resistance. The common chemical composition and mechanical function of the aluminum alloy base metal and welding wire are shown in Table 1.
2 Aluminum alloy welding difficulties
(1) Very simple oxidation. In air, aluminum is simply oxidized to produce a fine aluminum oxide film (thickness of about 0.1-0.2 μm) and a high melting point (about 2050°C) far beyond 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 impedes the fusion of the basic metal, and it is very simple to make up the disadvantages of porosity, slag inclusion, and non-fusion. Causes the weld function to decrease.
(2) Prone to blowholes. The primary cause of blowholes in the welding of aluminum and aluminum alloys is hydrogen, because liquid aluminum can dissolve many hydrogens, while solid aluminum hardly dissolves hydrogen, so when the bath temperature rapidly cools and condenses, hydrogen is too late to escape, simply Welds form a collection of pores. Hydrogen pores are difficult to completely avoid at present. The origin of hydrogen is many, there are hydrogen in the arc welding atmosphere, and the aluminum plate and welding wire absorb the moisture in the air. Practice has proved that even if the argon gas meets the GB/T4842 standard, the purity reaches more than 99.99%, but when the moisture content reaches 20ppm, many fine pores will appear. When the relative humidity of the air exceeds 80%, the weld will be significant. Air holes appear.
(3) The weld deformation and the tendency to form cracks are large. The coefficient of linear expansion and the rate of crystal reduction of aluminum are approximately twice as large as those of steel, and the internal stress of large welding deformation is prone to occur, 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 card/Cm.s. °C). About four times that of steel, welding aluminum and aluminum alloys requires more heat than welding steel.
(5) Transpiration loss of alloying elements. Aluminum alloy contains low-boiling elements (such as magnesium, zinc, manganese, etc.). Under high temperature arc, it is easy to transpire and burn, and then change the chemical composition of the weld metal, so that the weld function is reduced.
(6) High temperature strength and low plasticity. Aluminum has low strength and plasticity at high temperatures, which damages the forming of the weld metal, and sometimes simply constitutes the collapse of the weld metal and the appearance of the weld through.
(7) No color change. When aluminum and aluminum alloys change from a solid state to a liquid state, there is no noticeable color change, making it difficult for operators to grasp the heating temperature.
3 Technical methods for welding aluminum alloy materials
(1) Preparation before welding
Use chemical or mechanical methods to severely trim the outer oxide film on both sides of the weld bevel.
Chemical cleaning is the use of alkali or acid to clean the appearance of the workpiece, the law can remove the oxide film, but also remove the oil, the specific technical process is as follows: volume fraction of 6% to 10% sodium hydroxide solution, placed at 70 °C soak 0.5min → Washing → nitric acid with a volume fraction of 15% soaked for 1 min at room temperature for neutralization → water washing → warm water washing → boring. The appearance of the washed aluminum alloy is matt silver white.
Mechanical finishing can use pneumatic or electric milling cutters, and can also use scrapers, trowels, etc. For thin oxide films, a 0.25mm copper wire brush can be used to remove the oxide film.
Welding is done immediately after finishing, and if it takes more than 4 hours, it should be reorganized from scratch.
(2) determine the installation gap and tack welding distance
During the welding process, the aluminum plate is heated and swelled, causing gaps in the bevel groove of the weld to be reduced. If the gap before the welding is too small, welding will lead to the stacking of the grooves of the two plates during the welding process, and the unevenness and deformation of the plate after welding will be added. In contrast, if the installation gap is too large, welding is difficult and there is a possibility of burning. The proper tacking distance can ensure the required tack gap. Therefore, selecting the appropriate installation gap and tacking distance is an effective way to reduce the deformation. According to the experience, the installation technology parameters of different reasonable thickness for joints are shown in Table 2.
(3) Selection of welding equipment
There are many types of welding products on the market at present, and it is usually preferable to communicate with TIG welding (ie, TIG welding). It is a method of welding the base metal and the filler wire with an arc generated by the tungsten electrode and the workpiece under the maintenance of argon gas. When the welding machine is operating, since the polarity of the communication current is periodically changed, the half wave is a direct current connection and the half wave is a DC reverse connection in each cycle. In the half-wave phase that is connected, the tungsten electrode can emit satisfactory electrons without overheating, which is beneficial to the stability of the arc. The oxide film formed on the outer surface of the reversed half-wave period is simply trimmed to obtain a beautiful, well-formed weld.
(4) Select welding wire
Usually choose 301 pure aluminum welding wire and 311 aluminum silicon welding wire.
(5) Selection of welding methods and parameters
It is usually done by a left-side welding method. The torch and the workpiece are at an angle of 60°. When the welding thickness is 15mm or more, 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, open the V-shaped groove, the included angle is 60°~70°, the gap shall not be greater than 1mm, and the multi-layer welding will be completed. When the wall thickness is less than 1.5mm, the groove is not opened, no gap is left, and no filler wire is added. When welding fixed pipe butt joints, when the pipe diameter is 200mm and the wall thickness is 6mm, a tungsten electrode with a diameter of 3 to 4mm should be selected, with a welding current of 220 to 240A, and a filler wire of 4mm in diameter, with 1 to 2 layers. After welding.
According to experience, the welding parameters applicable to aluminum and aluminum alloy welding are shown in Table 3.
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