Aluminum alloy welding mainly has the following problems

· Thermal stress

The coefficient of thermal expansion of aluminum alloy is higher and the modulus of elasticity is smaller. During the welding process, due to the large deformation and linear expansion coefficient of aluminum alloy, the volume shrinkage rate during solidification is about 6%, and the cooling speed and the rapid primary crystallization speed of the molten pool, the internal stress of the weld and the rigidity constraint of the welded joint are large, which is easy to produce large internal stress in the aluminum alloy joint, causing greater welding stress and deformation, forming cracks, wave deformation and other defects.

· Ablation evaporation

The melting point of aluminum is 660 ° C and the boiling point is 2647 ° C, which is lower than that of copper and iron. In the welding process, if the welding temperature is too high, it is easy to explode and form splashes, especially in high-energy beam welding, as shown in Figure 1. In addition, some of the alloying elements added to the aluminum alloy have a low boiling point, which is easy to evaporate and burn at the instantaneous high temperature of welding, and the splash generated by the explosion will also take away some droplets, which inevitably changes the predetermined chemical composition of the weld zone, which is not conducive to the performance control of the welded joint. Therefore, in order to compensate for high temperature ablation, welding wire or other welding materials with a higher boiling point element content than the base material are often used during welding. Aluminum tube aluminum square tube aluminum alloy material welding key technology

· Solid inclusion

The chemical properties of aluminum are very active and easily oxidized. During the welding process, the surface of the aluminum alloy oxidized to form a high melting point of Al2O3 (about 2050 ° C, while the melting point of aluminum is 660 ° C, the two are very different). The oxide is dense and has high hardness, and is included in the molten alloy liquid with low density in the molten pool area, which is easy to form a fine solid slag and is not easy to discharge, which not only affects the tissue formation of the weld, but also easily produces electrochemical corrosion, which will cause the mechanical properties of the welded joint to decline, and the Al2O3 is covered in the molten pool and groove, which seriously affects the welding of the alloy. Reduce the structural properties of welded joints.

· Stomatal collapse

The melting point of aluminum alloy is much smaller than its oxide, and its nature is active and easy to oxidize. During the welding process, the aluminum alloy is melted at high temperature to form a molten pool. The aluminum on the surface of the molten pool is oxidized to form an oxide film, which is covered on the molten pool in a solid form. Since the color of the oxide film after melting is not much different from the molten state of the aluminum alloy, and because the oxide film is covered in the welding process, it is difficult to observe the degree of melting of the aluminum alloy melt pool, it is easy to cause the temperature to be too high, causing the bulk collapse of the welding heat affected zone, and destroying the shape and performance of the weld metal.

Under the action of instantaneous high power of welding heat source, a large amount of hydrogen is dissolved in the alloy liquid. After welding, the solubility of the gas gradually decreases with the decrease of the temperature of the molten pool, which becomes the main reason for the formation of pores in the welding process. Because the solidification rate of aluminum alloy is too fast and the density is low, hydrogen pores of different sizes are formed during the rapid solidification of weld. These pores will continue to accumulate and expand during the welding process, and eventually form a large visible porosity, which reduces the organizational properties of the joint. Of course, the generation of pores is not necessarily formed in the welding process, due to the influence of the casting process technology, the base metal itself will also produce pores in the casting process. During welding, the constant change of heat input and internal pressure causes the original pores in the base material to expand by heat or combine with each other to form weld pores, which will increase with the increase of welding heat input. Therefore, in order to control the source of hydrogen, the welding material should be strictly dried before use. During welding, the current should be appropriately increased to extend the existence time of the molten pool and give enough time for hydrogen to precipitate, thus controlling the formation of pores.

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