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Nine welding characteristics of aluminum alloy sheet
A, strong ability of oxidation
Aluminum and oxygen affinity is very strong, easy to combine with oxygen in air to generate dense and strong AL2O3 film, thickness of about 0.1 mu m, melting point 2050 ℃, far more than the melting point of aluminum and aluminum alloy, and the density is very big, is about 1.4 times of aluminum. In the process of welding, aluminum oxide film will hinder the good combination between metal and slag and easy. Oxide film will moisture adsorption, prompt production weld porosity when welding. These defects, can degrade the performance of the welded joint. In order to ensure the quality of welding, welding must be strictly before the oxide on the surface of the weld, and prevent the reoxidation during the process of welding, the molten metal and metal under high temperature for effective protection, it is an important feature of aluminum and aluminum alloy welding.
The specific protection measures are:
1, with mechanical or chemical methods to remove the workpiece before welding groove and the surrounding oxide on the surface of the part and the wire;
2, the welding process with qualified protective gas protection;
3, when welding, using flux, constantly in the process of welding with welding wire prick oxide film on the surface of the molten pool.
Second, the thermal conductivity and specific heat, thermal conductivity is fast
Although the melting point of aluminium and aluminium alloy is far less than steel, but the coefficient of thermal conductivity of aluminium and aluminium alloy, the large heat capacity, more than twice than steel, large amounts of heat energy in the process of welding was quickly conduction into the base metal, in order to obtain high quality welded joint, must use the energy concentration, large power source of heat, sometimes with the preheating process measures, such as to achieve the fusion welding process.
Three, linear expansion coefficient
The linear expansion coefficient of aluminium and aluminium alloy is about 2 times of steel, when the volume of solidification shrinkage rate of 6.5% - 6.5%, so easy to produce welding deformation. Effective measures to prevent the deformation is in addition to choose reasonable technological parameters and welding sequence, adopt appropriate welding equipment is also very important, especially when the welding plate. In addition, some aluminium and aluminium alloy welding cracks formed in weld metal crystallization tendency and formed in the heat affected zone of liquefied crack orientation are larger, often due to excessive internal stress and to produce hot crack in the brittle temperature range. This is aluminium alloy, especially when high strength aluminium alloy welding one of the most common serious flaws. In the practical measures to prevent this kind of crack in the welding field is mainly improving the joint design, selecting rational welding process parameters and welding sequence, to adapt to the characteristics of the parent metal welding filler materials, etc.
Fourth, easy formation porosity
Welding joint of the porosity is easy to application of aluminium and aluminium alloy welding defects, especially pure aluminium and antirust aluminium welding. Hydrogen is generated when the aluminium and aluminium alloy welding air hole, the main reasons for this have been proved by practice. Hydrogen source, mainly is the water in the atmosphere of arc column and welding material and the adsorption of moisture in parent metal wire and parent metal surface oxidation film of moisture adsorption, to weld porosity, often has a prominent position.
Aluminium and aluminium alloy liquid molten pool can easily absorbed gas, into the large amount of gas at high temperature, when the liquid solidification, solubility fell sharply, separating out too late in the process of solidification cooling after welding and gathered in the weld formation porosity. In order to prevent the formation of porosity, to obtain a good welding joint, the hydrogen source should be strictly controlled, before welding must strictly limit the welding materials (including welding wire, welding rod, flux, protective gas) of water content, dry out before use. Clean up after the parent metal and the welding wire is best in 2 to 3 hours after welding, is no more than 24 hours. TIG welding, choose big welding current with higher welding speed. MIG welding, choose big welding current welding speed is slow, in order to improve the existence of the molten pool time. Al - Li alloy welding, strengthening protection of is on the back, with groove scraping, remove oxide film, which can effectively prevent the pores.
Five, the welded joint is easy to soften
When the welding heat treatment strengthening of aluminum alloy, because of the influence of the welding heat, heat affected zone in welding joint will be softening, namely intensity is reduced, the substrate metal near seam area of some mechanical properties deteriorate. For cold work hardening alloy, improves the performance of the joint, and the welding line energy, the greater the performance degradation process is also more serious. Aiming at such problems, the measures mainly is formulated in accordance with specific material welding process, such as restrictions on the welding condition, appropriate welding sequence, preheat temperature and interpass temperature control, heat treatment after welding, etc. For softening can't recover after welding of aluminum alloy, it is better to use annealing or solid solution in the state of welding, heat treatment after welding, if does not allow for heat treatment after welding, energy concentrated welding method should be adopted and string cable welding energy, to reduce the decrease of strength of joint.
Six, alloy elements evaporation and loss
Some aluminum alloy containing low boiling point of the alloy elements, these elements are easy to evaporation loss at high temperatures, which changed the chemical composition of weld metal, reduce the performance of the welded joint. In order to compensate for the loss, at the same time of adjustment process, often used with the boiling point elements content is higher than the parent metal wire or other welding materials.
Seven, aluminum in the high temperature strength and low plasticity
Strength of aluminium at 370 ℃ was only 10 mpa, cannot hold liquid metal welding, it makes poor weld, even collapse or burn through formation, in order to solve this problem, often used when welding of aluminium and aluminium alloy plate.
Eight, the corrosion resistance of welding joint is lower than the parent metal
Heat treatment to strengthen corrosion resistance of aluminum alloy (such as a hard aluminum) joint reduced obviously, joint organization is uneven, corrosion resistance, the more easily. The purity of weld metal or density affects joints corrosion resistance. More impurity, coarse grains and brittle phase precipitation, corrosion resistance will be decreased obviously, not only produce localized surface corrosion and intercrystalline corrosion often occurs, in addition to aluminum alloy, the existence of the welding stress is also an important factor to influence the corrosion resistance.
In order to improve the corrosion resistance of welding joint, mainly take the following measures:
1, improve the joint organization composition inhomogeneity. Mainly through the welding material make the weld alloying, fine grains and to prevent defects; At the same time, adjust the welding process to reduce the heat affected zone, and to prevent overheating, heat treatment after welding.
2, eliminate the welding stress, such as local surface tensile stress can use local hammer ways to eliminate.
3, protection measures, such as taking anodic oxidation treatment or coating.
Nine, no color change, give a welding operation difficult
Aluminium and aluminium alloy welded by the solid into a liquid, no obvious color change, so bring many difficulties to the operator in the welding process. Therefore, grasp the welding when the heating temperature for welder, as far as possible using flat welding, in lead (out) of arc plate (out) arc, etc.