Aluminum-magnesium alloy has high mechanical properties, corrosion resistance, low temperature resistance, good plasticity, light weight and low cost, so it is widely used in low temperature pipelines. Although aluminum-magnesium alloy has many of the above advantages, its weldability is more difficult to grasp than the weldability of ferrous metals. The main reason is that the aluminum-magnesium alloy is at normal temperature. The surface is oxidized to form a refractory oxide film. The oxide film is dense and beneficial to natural corrosion protection, but it is extremely unfavorable for aluminum-magnesium alloy. Although it is removed before welding, it can generate new oxidation in a few seconds. Membrane, this film brings great difficulty to the welding. There is no difference in color when melting. It is difficult to observe the degree of melting, and it is easy to cause large collapse of the welding and heat affected zone, and a large amount of convex and concave phenomenon on the back surface. Destroy the formation of weld metal. The strength is low at high temperature (the tensile strength is only about 1 kg/cm at 370 ° C), and the melting point of the oxide film and the base material are very different, 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. It brings a lot of difficulties to welding. Welding of aluminum-magnesium alloys mainly causes H2 porosity problems. H2 pores come from moisture and are related to air, oxide film, argon and other factors.
The welded joint creates pores that reduce the strength of the joint. Although the AC TIG welder has the function of anode crushing, it does not completely remove the pores, and many remain in the molten pool. H2 can be dissolved in liquid aluminum in a large amount, the specific gravity of aluminum, and the viscosity of aluminum in molten state is large. According to the principle of Stoff's floating, the floating speed of bubbles in the bath is slow, and the thermal conductivity of aluminum is strong, the condensation is fast, and the bubbles are fast. Air bubbles are formed without evaporating before the crystallization of the bath. In order to obtain high quality welds, a new welding process must be used.