Welding Characteristics and Frequent Problems of Al-Mg Alloy

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, its weldability is harder to grasp than that of ferrous metals. The main reason is: aluminum-magnesium alloy at room temperature. The surface is oxidized 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, a new oxide film can be generated in a few seconds. This film has brought great difficulties for welding, there is no color difference when melting, it is difficult to observe the degree of melting, and even very easy to cause welding and heat-affected zone large collapse, the back of a large number of convex and concave phenomena, damage Weld metal formation. At low temperature, the strength is low (the tensile strength is only about 1kg/cm at 370°C). The melting point of the oxide film and the base metal is quite different. 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. Welding of aluminum-magnesium alloys mainly produces H2 pore problems. H2 pores come from moisture, and they 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 remains 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 are large. According to the principle of floating, the bubble floats slowly in the bath, and aluminum has a high thermal conductivity, rapid condensation, and bubbles. Bubbles are formed before the crystallization in the bath without waiting for discharge. To obtain high quality welds, new welding processes must be used.