ã€China Aluminum Industry Network】 (1) Aluminum and oxygen have strong affinity in the air and can easily combine with oxygen to form a dense and strong aluminum oxide film with a thickness of about 0.1μm and a melting point of up to 2050°C, far exceeding that of aluminum and aluminum. The melting point of the alloy, and the density is very large, about 1.4 times that of aluminum. During the welding process, the aluminum oxide film can prevent good bonding between the metals and can easily cause slag inclusion. Oxide film also adsorbs moisture, which causes the weld to form pores during welding. These defects will reduce the performance of welded joints. In order to ensure the welding quality, the surface of the weldment must be strictly cleaned of oxides before welding, and to prevent reoxidation during the welding process, to effectively protect the molten metal and the metal at high temperatures, which is an important welding of aluminum and aluminum alloys Features. The specific protective measures are: using mechanical grinding or chemical method D40 to remove the oxide of the workpiece groove and surrounding parts before welding; use qualified protective gas for protection (such as 99.99% Ar) during welding.
(2) Aluminum has a high thermal conductivity and specific heat. Although the melting point of aluminum and aluminum alloys is much lower than that of steel, the thermal conductivity and specific heat capacity of aluminum and aluminum alloys are both large and more than twice as large as steel. A large amount of heat energy is rapidly transmitted to the inside of the collective metal. In order to obtain high-quality welded joints, heat sources with concentrated energy and large power must be used. Preheating and other technical measures are required for thick plates of 8 mm or more in order to achieve the fusion welding process.
(3) Linear expansion coefficient The coefficient of linear expansion of large aluminum and aluminum alloys is about 2 times that of steel, and the volumetric shrinkage ratio during solidification is 6.5% to 6.6%, so that it is easy to produce welding distortion. An effective measure to prevent deformation is to use a suitable welding tool besides choosing a reasonable process parameter and welding sequence. This is especially true when welding thin plates. In addition, certain aluminum and aluminum alloys have a tendency to form crystal cracks in the weld metal and a tendency to form liquefied cracks in the heat-affected zone when they are welded, often within the brittle temperature range due to excessive internal stress. Thermal cracking is one of the more common serious defects in aluminum alloys, especially high-strength aluminum alloys. The measures to prevent such cracks in the actual welding site are mainly to improve joint design, select reasonable welding process parameters and welding sequence, and adopt welding filler materials that are suitable for the characteristics of the parent material.
(4) Porosity in easy-to-form stomatal welding joints are defects that can easily occur during the welding of aluminum and aluminum alloys, in particular, the welding of pure aluminum and rust-proof aluminum. Hydrogen is the main reason for the generation of pores in the welding of aluminum and aluminum alloys, which has been proved by practice. The source of hydrogen is mainly moisture in the arc column atmosphere, the welding material and the moisture absorbed by the parent material. The moisture absorbed by the oxide film on the surface of the welding wire and the parent material often occupies a prominent position in the generation of pores in the weld. Aluminum and aluminum alloy liquid pools can easily absorb pores. A large amount of gas dissolved at high temperatures will rapidly decrease in solubility when solidified from the liquid phase. The gas will not precipitate in the process of cooling and solidification after welding, but will accumulate in the weld. Stoma. In order to prevent the generation of blowholes and to obtain good welded joints, the source of hydrogen must be strictly controlled. Before welding, the moisture content of the used welding materials (including welding wire, welding rod, flux, and shielding gas) must be strictly limited. Strictly carry out drying treatment. The cleaned base metal and welding wire are preferably welded within 2 to 3 hours, and more than 24 hours. During TIG welding, a large welding current is selected to match the higher welding speed. When MIG welding, use a large welding current slow welding speed to increase the existence of the molten pool.
(5) Strength and plasticity of aluminum at high temperatures Low aluminum has a strength of only 10 MPa at 370°C. When welding, it can cause poor weld formation, or even collapse or burn through, because it cannot support liquid metal. In order to solve this problem, pads are often used for welding aluminum and aluminum alloys.
(6) No color change, making welding operation difficult.
When the aluminum and aluminum alloys are changed from a solid state to a liquid state during welding, there is no noticeable color change, and therefore, the operator has many difficulties in the welding process. Therefore, the welder is required to have a good grasp of the heating temperature during welding, and to use flat welding as much as possible to induce (accept) the arc on the lead (receiving) arc plate.
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