Aluminum alloys mainly have the following difficulties in welding:

　　 1. Aluminum alloy has a strong affinity with oxygen. It is easy to combine with oxygen in the air to form a dense and strong aluminum oxide film with a thickness of about 0.1μm and a melting point of 2050°C, far exceeding the melting point of aluminum and aluminum alloys, and the density is very high, about 1.4 times that of aluminum. During the welding process, the aluminum oxide film will hinder the good bonding between the metals and easily cause slag inclusion. The oxide film will also absorb moisture, which will promote the formation of pores in the weld during welding. These defects will reduce the performance of the welded joint. In order to ensure the quality of welding, the oxide on the surface of the weldment must be strictly cleaned before welding, and to prevent re-oxidation during the welding process, and effectively protect the molten metal and the metal at high temperature. This is an important aspect of aluminum and aluminum alloy welding. Features. Specific protective measures are: use mechanical polishing or chemical method D40 to remove oxides on the groove and surrounding parts of the workpiece before welding; use qualified shielding gas for protection (for example, 99.99% Ar) during welding.

　　 2. The thermal conductivity and specific heat of aluminum alloy are large. The thermal conductivity and specific heat capacity of aluminum and aluminum alloys are very large. During the welding process, a large amount of heat energy is quickly transferred to the interior of the collective metal. In order to obtain high-quality welded joints, a heat source with concentrated energy and high power must be used, with a thickness of 8mm and above. The board needs to adopt preheating and other technological measures to be able to realize the fusion welding process.

　　3. The linear expansion coefficient of the aluminum alloy car body is large. The linear expansion coefficient of aluminum and aluminum alloys is about twice that of steel, and the volume shrinkage rate during solidification reaches 6.5% to 6.6%, so welding deformation is easy to occur. The effective measure to prevent deformation is to select reasonable process parameters and welding sequence, it is also very important to use suitable welding tooling, especially when welding thin plates. In addition, when welding some aluminum and aluminum alloys, the tendency to form crystal cracks in the weld metal and the tendency to form liquefaction cracks in the heat-affected zone are both large, and they are often in the brittle temperature range due to excessive internal stress. Hot cracks occur, which is one of the most 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 the joint design, select reasonable welding process parameters and welding sequence, and use welding filler materials that adapt to the characteristics of the base metal.

　　4. Porosity is easy to form during welding of aluminum alloy parts. Porosity in welded joints is a defect that is easily produced during welding of aluminum and aluminum alloys, especially the welding of pure aluminum and rust-proof aluminum. Hydrogen is the main cause of porosity during welding of aluminum and aluminum alloys, which has been proven in practice. The source of hydrogen is mainly the moisture in the arc column atmosphere, the moisture absorbed by the welding material and the base metal. Among them, the adsorption of moisture on the surface of the welding wire and the base metal often plays a prominent role in the generation of weld pores. The liquid molten pool of aluminum and aluminum alloys can easily absorb pores. A large amount of gas dissolved at high temperature will have a sharp drop in solubility when solidified from a liquid state. During the cooling and solidification process after welding, the gas is too late to precipitate and gather in the weld. Stomata. In order to prevent the generation of pores and obtain a good welded joint, the source of hydrogen must be strictly controlled. The water content of the welding materials (including welding wire, welding rod, flux, and shielding gas) used must be strictly limited before welding. Strictly carry out drying treatment. After cleaning, the base material and welding wire should be welded within 2 to 3 hours, and no more than 24 hours at most. In TIG welding, use a large welding current to match a higher welding speed. In MIG welding, use a large welding current and a slow welding speed to increase the existence time of the molten pool.

　　5. The strength and plasticity of aluminum alloy at high temperature. Low aluminum has a strength of only 10MPa at 370°C. During welding, the weld cannot be supported due to the failure of the liquid metal, resulting in poor weld formation, even collapse or burn through. In order to solve this problem, backing plates are often used when welding aluminum and aluminum alloys.