1. Welding characteristics of aluminum and its alloys
(1) aluminum is easily oxidized in air and during welding, and the generated alumina (Al2O3) has a high melting point and is very stable and difficult to remove. It hinders the melting and fusion of the base metal, and the oxide film has a large proportion, which is not easy to float out of the surface, and it is easy to produce defects such as slag inclusion, incomplete fusion and incomplete penetration. The oxide film on the aluminum surface and the adsorption of a large amount of water are easy to cause pores in the weld. Before welding, strict surface cleaning should be carried out by chemical or mechanical methods to remove the oxide film on its surface. Strengthen protection during welding to prevent oxidation. In TIG welding, AC power supply is selected and oxide film is removed by "cathode cleaning". In gas welding, flux for removing oxide film is used. Welding heat can be increased when welding thick plates. For example, helium arc has high heat, so it should be protected by helium or argon-helium mixture, or gas shielded welding with large metal electrode. In the case of DC connection, "cathode cleaning" is not needed.
(2) The thermal conductivity and specific heat capacity of aluminum and its alloys are more than twice that of carbon steel and low alloy steel. The thermal conductivity of aluminum is ten times that of austenitic stainless steel. In the welding process, a large amount of heat can be quickly conducted into the base metal. Therefore, when welding aluminum and aluminum alloys, more heat is unnecessarily consumed in other parts of the metal except the metal molten pool. This useless energy consumption is more obvious than steel bar welding. In order to obtain high-quality welded joints, concentrated energy and high-power energy should be used as much as possible, and sometimes technological measures such as preheating can be adopted.
(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 is large, and the deformation and stress of weldments are large. Therefore, measures to prevent welding deformation are needed. Shrinkage, porosity, thermal crack and high internal stress are easy to occur when aluminum welding pool solidifies. In production, measures such as adjusting welding wire composition and welding process can be taken to prevent hot cracks. Al-Si alloy welding wire can be used to weld aluminum alloys other than Al-Mg alloy if corrosion resistance allows. When the content of silicon in Al-Si alloy is 0.5%, the tendency of hot cracking is greater. With the increase of silicon content, the crystallization temperature range of the alloy becomes smaller, the fluidity is significantly improved, the shrinkage rate is reduced, and the hot cracking tendency is correspondingly reduced. According to the production experience, when the silicon content is 5%~6%, there will be no hot cracking, so the welding wire containing SAlSi strip (silicon content is 4.5%~6%) will have better crack resistance.
(4) Aluminum has strong reflection ability to light and heat, and there is no obvious color change when it is in solid-liquid transition state, so it is difficult to judge when welding. The strength of high-temperature aluminum is very low, so it is difficult to support the molten pool and easy to weld through.
(5) Aluminum and aluminum alloys can dissolve a lot of hydrogen in liquid state, but hardly dissolve hydrogen in solid state. In the process of solidification and rapid cooling of welding pool, hydrogen is too late to overflow, which is easy to form hydrogen holes. The moisture in the arc column atmosphere, the moisture absorbed by the welding material and the oxide film on the surface of the parent 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) Alloy elements are easy to evaporate and burn, which reduces the welding performance.
(7) When the parent metal of the parent metal is deformed or solution aged, the welding heat will reduce the strength of the heat affected zone.
(8) Aluminum is a face-centered cubic lattice with no isomorphism, and there is no phase change during heating and cooling, so the weld grain is easy to be coarse, and it is impossible to refine the grain through phase change.
2. Welding method
Almost all kinds of welding methods can be used to weld aluminum and aluminum alloys, but aluminum and aluminum alloys have different adaptability to various welding methods, and each welding method has its own application occasions. Gas welding and covered electrode arc welding equipment is simple and easy to operate. Gas welding can be used for repair welding of aluminum plates and castings with low requirements on welding quality. Covered electrode arc welding can be used for repair welding of aluminum alloy castings. Inert gas shielded welding (TIG or MIG) is the most widely used welding method for aluminum and aluminum alloys. Aluminum and aluminum alloy sheets can be welded by tungsten alternating current argon arc welding or tungsten pulse argon arc welding. Thick aluminum and aluminum alloy plates can be welded by tungsten-helium arc welding, argon-helium mixed tungsten gas shielded welding, MIG welding and pulsed MIG welding. Metal arc welding and pulsed metal arc welding are widely used (argon or argon/helium mixture).