Aluminum alloy is the most widely used non-ferrous metal structural material in industry. It has been widely used in aviation, aerospace, automotive, mechanical manufacturing, shipbuilding, and chemical industries. The rapid development of industrial economy has led to an increasing demand for aluminum alloy welded structural components, which has led to in-depth research on the weldability of aluminum alloys. At present, aluminum alloy is the most widely used alloy.
(1) Extremely prone to oxidation. In the air, aluminum is easy to oxidize and combine, forming a dense aluminum oxide film with a thickness of approximately 0.1-0.2 μ m) The melting point is high (about 2050 ℃), far exceeding the melting point of aluminum and its alloys (about 600 ℃). The density of aluminum oxide is 3.95-4.10g/cm3, which is about 1.4 times that of aluminum. The surface of the aluminum oxide film is prone to adsorbing water, which hinders the fusion of the base metal during welding and easily forms defects such as pores, slag inclusions, and lack of fusion, leading to a decrease in weld performance.
(2) Easy to produce pores. The main reason for the formation of pores during the welding of aluminum and aluminum alloys is hydrogen. As liquid aluminum can dissolve a large amount of hydrogen, solid aluminum almost does not dissolve hydrogen. Therefore, when the molten pool temperature rapidly cools and solidifies, hydrogen cannot escape in time, and it is easy to accumulate and form pores in the weld seam. Hydrogen pores are difficult to completely avoid, and there are many sources of hydrogen, such as hydrogen in the arc welding atmosphere, and the oxide film on the surface of aluminum plates and welding wires adsorbing moisture in the air.
Practice has shown that even if the purity of argon gas meets the requirements of GB/T4842 standard and exceeds 99.99%, a large number of dense pores will appear when the moisture content reaches 20ppm. When the relative humidity of the air exceeds 80%, if heating and other measures are not taken, the weld seam will show obvious pores. At the same time, using low current slow welding, increasing the cooling time of the weld seam, and using the welding wire arc to stir the molten pool can effectively help gas discharge from the molten pool.
(3) The weld seam tends to deform and form cracks. The linear expansion coefficient and crystallization shrinkage rate of aluminum are about twice larger than that of steel, which is prone to generating significant internal stress due to welding deformation. This can promote the generation of hot cracks in structures with high rigidity.
(4) Aluminum has a high thermal conductivity (pure aluminum 0.538 calories/cm. s. ℃). About 4 times that of steel, therefore, welding aluminum and aluminum alloys consumes more heat than welding steel.
(5) The evaporation and burning loss of alloy elements. Aluminum alloy contains elements with low boiling points (such as magnesium, zinc, manganese, etc.), which are easily evaporated and burned under the action of high-temperature arc, thereby changing the chemical composition of the weld metal and reducing the performance of the weld.
(6) High temperature strength and low plasticity. At high temperatures, the strength and plasticity of aluminum are very low, which disrupts the formation of weld metal, and sometimes can also easily cause weld metal collapse and penetration.
(7) No color changes. When aluminum and its alloys transition from solid to liquid, there is no significant color change, making it difficult for operators to control the heating temperature.
