Aluminum-magnesium alloy has high mechanical properties, corrosion resistance, low temperature resistance, good plasticity, light weight and low price, so it is widely used in low temperature pipelines. Although aluminum-magnesium alloys have many of the above advantages, its weldability is more difficult to master than that of ferrous metals.
The main reason is: aluminum-magnesium alloy at room temperature. The surface is oxidized to form a refractory oxide film, which is dense and beneficial to natural corrosion, but is extremely unfavorable for welding. Although cleaning measures are taken before welding, a new oxide film will be produced within a few seconds. This layer of film brings great difficulties to welding. There is no difference in color when melting, and it is difficult to observe the degree of melting. It is even easy to cause the welding and heat-affected zone to collapse, and a large number of convex and concave phenomena are formed on the back, which damages The formation of weld metal. The strength is low at high temperature (the tensile strength is only about 1kg/cm at 370°C), and the melting point of the oxide film is very different from that of the base material, and the melting point of the oxide film is about 2050°C. The melting point of aluminum and magnesium is only about 600°C, which is a large temperature difference. Bring many difficulties to welding. Welding of aluminum-magnesium alloys mainly produces H2 porosity problems. H2 pores come from the moisture, and at the same time are related to factors such as air, oxide film, and argon.
The welded joint produces pores, which reduces the strength of the joint. Although the AC tungsten argon arc welding machine has the function of anode crushing, it cannot completely remove the pores, and there are many residues in the molten pool. H2 can be dissolved in a large amount of liquid aluminum. The specific gravity of aluminum and the viscosity of molten aluminum are large. According to the Stoff’s principle of floating, bubbles rise slowly in the molten pool. In addition, aluminum has strong thermal conductivity, fast condensation, and bubbles. Bubbles are formed before the bath is discharged before crystallization. To obtain high-quality welds, new welding processes must be adopted.
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