Welding technology of aluminum and aluminum alloy parts
Aluminum and its alloy materials have the characteristics of 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, defects often appear in welding due to improper selection of welding methods and welding process parameters. This paper introduces the welding process steps and the selection of welding parameters of this kind of metal parts.
Aluminum alloy; welding technology
Aluminum and aluminum alloy materials have the characteristics of low density, high strength, high thermal conductivity, strong corrosion resistance and good physical and mechanical properties, and are widely used in welding structures of industrial products. For a long time, due to improper selection of welding methods and welding process parameters, aluminum alloy parts are seriously deformed due to excessive stress after welding, or weld metal cracks or loose materials are caused by defects such as weld porosity, slag inclusion and incomplete penetration, which seriously affect product quality and performance.
Material characteristics of 1 aluminum alloy
Aluminum is a silvery white light metal with good plasticity, high electrical and thermal conductivity, oxidation resistance and corrosion resistance. Aluminum is easy to oxidize to form alumina film, and inclusions are easy to produce in weld, thus destroying the continuity and uniformity of metal and reducing its mechanical properties and corrosion resistance.
2 welding difficulties of aluminum alloy materials
(1) is easily oxidized. In air, aluminum is easily oxidized to form a dense alumina film (thickness about 0. 1-0.2μm) with a high melting point (about 2050℃), far exceeding the melting points of aluminum and aluminum alloys (about 600℃). 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, which hinders the fusion of basic metals during welding, and is easy to form defects such as porosity, slag inclusion and incomplete fusion, which leads to the decline of weld performance.
(2) It is easy to produce pores. Hydrogen is the main cause of porosity in aluminum and aluminum alloy welding. Because liquid aluminum can dissolve a lot of hydrogen, solid aluminum hardly dissolves hydrogen. When the temperature of the molten pool cools and solidifies rapidly, hydrogen cannot escape, and it is easy to gather in the weld to form pores. At present, it is difficult to completely avoid hydrogen holes. There are many sources of hydrogen, such as hydrogen in arc welding atmosphere and moisture in the air adsorbed on the surface of aluminum plate and welding wire. Practice has proved that even if the purity of argon reaches above 99.99% according to GB/T4842 standard, a large number of dense pores will appear when the water content reaches 20ppm, and when the relative humidity of air exceeds 80%, pores will obviously appear in the weld.
3 aluminum alloy material welding process
(1) Preparation before welding
Strictly clean the surface oxide film on both sides of the weld groove by chemical or mechanical methods.
Chemical cleaning is to clean the surface of the workpiece with alkali or acid. This method can not only remove the oxide film, but also remove the oil stain. The specific process is as follows: sodium hydroxide solution with volume fraction of 6% ~ 10% is soaked at 70℃ for 0.5min→ washed with water → soaked with nitric acid with volume fraction of 15% at room temperature/neutralized with kloc-0/min → washed with water → the surface of aluminum alloy is dull silver.
Mechanical cleaning can use pneumatic or electric milling cutter, scraper, file and other tools. For thin oxide film, the oxide film can also be removed by polishing with a 0.25mm copper wire brush.
Welding immediately after cleaning. If it is left for more than 4h, it should be cleaned again.
(2) Determine the assembly clearance and positioning welding spacing.
In the welding process, the aluminum plate expands when heated, which reduces the gap of weld groove. If the assembly gap before welding is too small, the grooves of the two plates will overlap during welding, which will increase the roughness and deformation of the welded plates. On the other hand, if the assembly gap is too large, it is difficult to weld and there is a possibility of burning through. Proper tack welding spacing can ensure the required tack welding spacing. Therefore, it is an effective measure to choose the appropriate assembly clearance and tack welding spacing to reduce deformation. According to experience, different plate thickness is a reasonable assembly process parameter of the joint.
(3) Select welding equipment
At present, there are many kinds of welding products on the market, and AC TIG welding (TIG welding) should generally be adopted. Under the protection of argon, a welding method is used to melt the base metal and filler wire by using the arc heat generated between the tungsten electrode and the workpiece. When the welding machine works, because the polarity of AC current changes periodically, in each cycle, half wave is DC positive connection and half wave is DC reverse connection. Tungsten electrode can emit enough electrons at positive half wave without overheating, which is beneficial to the stability of arc. The oxide film formed on the surface of the workpiece is easy to remove when the half-wave is reversed, and the weld with bright and beautiful surface and good shape is obtained.
(4) Selecting welding wire
Generally, 30 1 pure aluminum welding wire and 31/aluminum-silicon welding wire are selected.
(5) Choose welding method and parameters.
Generally, the left welding method is adopted, and the welding gun forms an angle of 60 with the workpiece. When the welding thickness is greater than 15mm, the welding gun forms a 90-degree angle with the workpiece.
When the welding wall thickness is greater than 3mm, a V-groove with an included angle of 60 ~ 70 and a gap of not more than 65438±0mm shall be formed, and multi-layer welding shall be completed. When the wall thickness is below 1.5mm, no groove, no gap and no filler wire will be added. When welding the butt joint of fixed pipe, when the pipe diameter is 200mm and the wall thickness is 6mm, the tungsten electrode with a diameter of 3 ~ 4mm should be used, the welding current is 220 ~ 240 a, and the filler wire with a diameter of 4mm should be used to complete the welding of 1 ~ 2 layer.
According to experience, when welding aluminum and aluminum alloys, appropriate welding parameters should be selected.