Aluminum-magnesium alloys have high mechanical properties, corrosion resistance, low temperature resistance, good plasticity, light weight, and low cost, so they are widely used in low temperature pipelines. Although aluminum-magnesium alloy has many advantages above, its weldability is harder to grasp than the weldability of ferrous metals. The main reasons are:. The aluminum-magnesium alloy is oxidized at room temperature to form a layer of refractory oxide film. The oxide film is dense, which is beneficial to natural anti-corrosion, but it is extremely unfavorable to welding. Although the cleaning measures are taken before welding, it can be renewed within seconds. New oxide film will be produced. This film brings great difficulties to welding. There is no difference in color when melting, it is difficult to observe the degree of melting, and it is very easy to cause large-scale collapse of the welding and heat-affected zone. A large number of convex and concave phenomena are formed, destroying the formation of the weld metal. Low strength at high temperatures (tensile strength at 370°C is only about 1 KG/cm), the difference between the oxide film and the base metal melting point is very large, 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. There are many difficulties in welding. The welding of aluminum-magnesium alloys mainly produces H2 pores. The H2 pores come from the water and are related to air, oxide film, argon and other factors.
Welded joints produce air holes that reduce the strength of the joint. Although the AC tungsten arc welding machine has the function of anode crushing, it cannot completely remove the air holes and many remain in the bath. H2 can be dissolved in large quantities in liquid aluminum, the specific gravity of aluminum, and the viscosity of aluminum in the molten state According to the principle of the Steffi floatation, the bubble rises slowly in the bath, combined with the high thermal conductivity of aluminum, rapid condensation, and bubbles formed before the crystallization in the bath. To obtain high quality welds, new welding processes must be used.