Welding characteristics of aluminum and aluminum alloys

Welding characteristics of aluminum and aluminum alloys

(1) Aluminum is easily oxidized in the air and during welding, and the resulting alumina (Al2O3) has a high melting point, is very stable, and is not easy to remove. It hinders the melting and fusion of the base metal, and the specificity of the oxide film is large, it is not easy to float out of the surface, and it is easy to generate defects such as slag inclusion, incomplete fusion, and incomplete penetration. The oxide film on the surface of aluminum and the adsorption of a large amount of water will easily cause pores in the weld. Strict surface cleaning should be carried out by chemical or mechanical methods before welding to remove the oxide film on the surface. Enhance protection during welding to prevent oxidation. During argon tungsten arc welding, an AC power source is used to remove the oxide film through the "cathode cleaning" function. For gas welding, use a flux that removes the oxide film. When welding thick plates, the welding heat can be increased. For example, the heat of the helium arc is high, and the shielding of helium or argon-helium mixed gas is used, or the large-scale melting electrode gas shielded welding is used. In the case of direct current connection, the "cathode clean up".
(2) The thermal conductivity and specific heat capacity of aluminum and aluminum alloys are about twice that of carbon steel and low alloy steel. The thermal conductivity of aluminum is ten times that of austenitic stainless steel. During the welding process, a large amount of heat can be quickly transferred to the inside of the base metal. Therefore, when welding aluminum and aluminum alloys, in addition to the energy consumed in the molten metal pool, more heat needs to be consumed in other parts of the metal. The consumption of this kind of useless energy is more significant than that of steel welding. In order to obtain high-quality welded joints, energy with concentrated energy and high power should be used as much as possible. Sometimes preheating and other process measures can also be used.
(3) The linear expansion coefficient of aluminum and aluminum alloy is about twice that of carbon steel and low alloy steel. When aluminum solidifies, the volume shrinkage rate is relatively large, and the deformation and stress of the weldment are relatively large. Therefore, measures to prevent welding deformation must be taken. Shrinkage cavities, shrinkage porosity, thermal cracks and high internal stress are prone to occur when the aluminum welding pool solidifies. In production, measures to adjust the welding wire composition and welding process can be adopted to prevent the occurrence of thermal cracks. Where corrosion resistance permits, aluminum alloys other than aluminum-magnesium alloys can be welded with aluminum-silicon alloy wires. When the aluminum-silicon alloy contains silicon, the thermal cracking tendency is greater. As the silicon content increases, the crystallization temperature range of the alloy becomes smaller, the fluidity is significantly improved, the shrinkage rate decreases, and the thermal cracking tendency also decreases accordingly. According to production experience, when the silicon content is 5%~6%, no hot cracking will occur, so the use of SAlSi (silicon content) welding wire will have better crack resistance.
(4) Aluminum has a strong ability to reflect light and heat, and there is no obvious color change when solid and liquid transitions, making it difficult to judge during welding operations. The strength of high-temperature aluminum is very low, it is difficult to support the molten pool, and it is easy to weld through.
(5) Aluminum and aluminum alloys can dissolve a large amount of hydrogen in the liquid state, and hardly dissolve hydrogen in the solid state. During the solidification and rapid cooling of the weld pool, hydrogen has no time to overflow, and hydrogen pores are easily formed. The moisture in the arc column atmosphere, the moisture absorbed by the oxide film on the surface of the welding material and the base metal are all important sources of hydrogen in the weld. Therefore, the source of hydrogen should be strictly controlled to prevent the formation of pores.
(6) Alloying elements are easy to evaporate and burn, which reduces the performance of the weld.
(7) If the base metal is strengthened by deformation or solid solution aging, the heat of welding will reduce the strength of the heat-affected zone.
(8) Aluminum is a face-centered cubic lattice, without allotropes, and there is no phase change during heating and cooling. The grains of the weld are easy to be coarse, and the grains cannot be refined through phase change.