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With the development of laser technology and aluminum alloy research and development technology. Carry out basic research on aluminum alloy laser welding application technology. Develop new aluminum alloy laser welding technology, and more effectively expand the application potential of aluminum alloy laser welding structure. So as to understand aluminum alloy laser welding. The application status and development trend of technology are particularly important.
Laser Welding Method
High-strength aluminum alloy has high specific strength, specific rigidity, good corrosion resistance, processing performance, and mechanical properties. It has become an indispensable metal material in the lightweight manufacturing of aerospace, shipbuilding, and other transportation fields. Among them, aircraft are most used. . Welding technology has unique advantages in improving the utilization rate of structural materials. Reducing structural weight, and realizing low-cost manufacturing of complex and dissimilar material overall structures. Among them, aluminum alloy laser welding technology is a hot spot that has attracted much attention.
Compared with other welding methods, laser welding has the advantages of concentrated heating. Small thermal damage, the large aspect ratio of the weld seam, and small welding distortion. The welding process is easy to be integrated, automated, and flexible, and can achieve high-speed and high-precision welding. Suitable for high-precision welding of complex structures.
1. CO2 Gas Laser
The working medium is CO2 gas, and the 10.6μm wavelength laser is output. According to the laser excitation structure, it is divided into two types: transverse flow and axial flow. Although the output power of the cross-flow CO2 laser has reached 150KW, the beam quality is poor and not suitable for welding; the axial-flow CO2 laser has good beam quality and can be used for welding aluminum alloys with high laser reflectivity.
2. YAG Solid-State Laser
The working medium is ruby, neodymium glass, and neodymium-doped yttrium aluminum garnet, etc., and the output wavelength is 1.06μm laser. YAG laser is easier to be absorbed by metal than CO2 laser and is less affected by plasma. It is optical fiber transmission, flexible welding operation, and good weld position accessibility. It is currently the main laser for welding aluminum alloy structures.
3. YLR Fiber Laser
It is a new type of laser developed after 2002. It uses optical fiber as the matrix material, doped with different rare-earth ions, and the output wavelength range is around 1.08μm. It is also optical fiber transmission. The fiber laser revolutionizes the use of a double-clad fiber structure, which increases the pump length and improves the pump efficiency, thereby greatly increasing the output power of the fiber laser. Compared with YAG laser, although YLR fiber laser appeared later, it has the advantages of small size, low operating cost, high beam quality, etc., and the obtained laser power is high.
Features of Aluminum Alloy Laser Welding
Compared with conventional fusion welding, aluminum alloy laser welding has concentrated heating, a large weld seam aspect ratio, and small welding structure deformation, but there are some shortcomings. In summary, they are:
1. The small diameter of the laser focus spot leads to high requirements for the welding and assembly accuracy of the workpiece. Generally, the assembly gap and the amount of misalignment need to be less than 0.1mm or 10% of the plate thickness, which increases the difficulty of implementing a complex three-dimensional weld welding structure;
2. As the reflectivity of aluminum alloy to the laser is as high as 90% at room temperature. Laser deep penetration welding of aluminum alloy requires a higher power laser. Research on laser welding of aluminum alloy sheets shows that laser deep penetration welding of the aluminum alloy. It depends on the dual thresholds of laser power density and line energy. The laser power density and line energy together restrict the behavior of the molten pool during the welding process. And ultimately reflect the forming characteristics of the weld. Above, the process optimization of full penetration welds can be evaluated by the back-to-width ratio of the weld forming characteristic parameters;
3. Aluminum alloy has a low melting point and good fluidity of liquid metal. It produces strong metal vaporization under the action of a high-power laser. The metal vapor/photo plasma cloud formed by the small hole effect during the welding process affects the laser energy of aluminum alloy. The absorption of chromium causes the instability of the deep penetration welding process. The weld is prone to defects such as pores, surface collapse, undercut, etc.;
4. Laser welding has a fast heating and cooling speed. The hardness of the weld is higher than that of the arc. However, due to the burning of alloy elements in laser welding of aluminum alloy. Which affects the strengthening effect of the alloy, there is still a softening problem in the weld of aluminum alloy. Thereby reducing the joints of aluminum alloy welding. strength. Therefore, the main problem of aluminum alloy laser welding is to control weld defects and improve the performance of welded joints.
Defect Control Technology of Aluminum Alloy Laser Welding
Under the action of a high-power laser, the main defects of aluminum alloy laser deep penetration welding are pores, surface collapse, and undercut. Among them, surface collapse and undercut defects can be improved by laser wire filler welding or laser arc hybrid welding; Stomatal defect control is more difficult.
Existing research results show that: there are two types of characteristic pores in laser deep penetration welding of aluminum alloys. One is metallurgical pores, which are hydrogen pores caused by material pollution or air intrusion during the welding process, like arc fusion welding; the other is a process The pores are caused by the unstable fluctuations of the small holes inherent in the laser deep penetration welding process.
In the laser deep penetration welding process, the small hole often lags behind the beam movement due to the viscous effect of the liquid metal. Its diameter and depth are affected by the plasma/metal vapor and fluctuate. With the movement of the beam and the flow of the molten pool metal, the hole In penetration deep penetration welding, bubbles appear at the tip of the small hole due to the metal flow of the molten pool, while in full penetration deep penetration welding, bubbles appear at the waist in the middle of the small hole. Bubbles migrate and roll with the flow of liquid metal, or escape the surface of the molten pool, or are pushed back to small holes. When the bubbles are solidified by the molten pool and captured by the metal front, they become weld pores.
1. Pre-Welding Treatment Method
Surface treatment before welding is an effective method to control metallurgical pores in laser welding of aluminum alloy. Usually, surface treatment methods include physical and mechanical cleaning and chemical cleaning. In recent years, laser shock cleaning has also appeared, which will further improve the automation of laser welding.
2. Parameter Stability Optimization Control
3. Double Spot Laser Welding
Double-spot laser welding refers to the welding process in which two focused laser beams act on the same molten pool at the same time. In the process of laser deep penetration welding, instantaneous closure to seal the gas in the small hole in the molten pool is one of the main reasons for the formation of weld pores. When double-spot laser welding is used, due to the action of the two light sources, the large opening of the small hole is conducive to the escape of internal metal vapor, and it is also beneficial to the stability of the small hole, thereby reducing the welding seam porosity. Researches on laser welding of A356, AA5083, 2024, and 5A90 aluminum alloys have shown that double-spot laser welding can significantly reduce weld pores.
4. Laser Arc Hybrid Welding
Laser arc hybrid welding is a welding method in which laser and arc are applied to the same molten pool. Generally, the laser is the main heat source, and the interaction between the laser and the arc is used to increase the laser welding penetration and welding speed, and reduce the welding assembly accuracy. The use of filler wire to control the structure and performance of the welded joint, and the auxiliary effect of the arc to improve the stability of the laser welding hole, thereby helping to reduce the weld pores. In the laser arc hybrid welding process, the arc affects the metal vapor/plasma cloud induced by the laser process, which is beneficial to the material’s absorption of laser energy and the stability of the small hole. The research results of the welding seam of aluminum alloy laser arc hybrid welding also confirmed its effect.
5. Fiber Laser Welding
The pinhole effect of the laser deep penetration welding process originates from the strong vaporization of metal under the action of the laser. Metal vaporization steam power is closely related to laser power density and beam quality. Which not only affects the penetration depth of laser welding but also affects the stability of the small hole. The research on SUS304 stainless steel high-power fiber laser showed that the molten pool was elongated during high-speed welding. Spatter was suppressed, the fluctuation of the small hole was stable. There was no bubble generation at the tip of the small hole.
When the fiber laser is used for high-speed welding of titanium alloy and aluminum alloy. The same Welds without porosity can be obtained. Allen’s research on the shielding gas control technology for titanium alloy fiber laser welding. It showed that controlling the position of the welding shielding gas can prevent gas from being involved. Reduce hole closing time, stabilize the welding hole, and change the solidification behavior of the molten pool, thereby Reducing welding seam porosity.
6. Pulse Laser Welding
Compared with continuous laser welding, the laser output adopts pulsating output, which can promote the periodic and stable flow of the molten pool, which is conducive to the escape of bubbles in the molten pool and reduces the pores of the weld. The effect on the porosity and performance of welds of SUS 304L stainless steel and Inconel 690 high-temperature alloy shows that: for square wave pulse laser welding, when the base power is 1700w, as the pulse amplitude ΔP increases, the weld porosity decreases. Among them, the porosity of stainless steel was reduced from 2.1% to 0.5%, and the porosity of superalloys was reduced from 7.1% to 0.5%.
Composite Treatment Technology After Welding
In actual engineering applications, even if the strict surface treatment is carried out before welding, the welding process is stable, and aluminum alloy laser welding will inevitably produce weld pores. Therefore, it is very important to use post-weld treatment to eliminate pores. . This method is currently mainly modified welding. Hot isostatic pressing technology is one of the methods to eliminate internal porosity and shrinkage porosity in aluminum alloy castings. It is combined with the stress heat treatment after laser welding of aluminum alloy to form a composite process composed of hot isostatic pressing and heat treatment of aluminum alloy laser welding components. Which eliminates both The weld pores improve the joint performance.
Due to the characteristics of aluminum alloy, there are still many problems in the application of high-power laser welding that need to be studied in depth. The main problem is to control the weld pore defects and improve the welding quality. The engineering control of porosity in laser welding of aluminum alloy should comprehensively consider all aspects. It includes welding, welding process, and post-welding treatment, so as to improve the stability of the welding process. Many new technologies and processes have been derived from this, such as laser cleaning before welding. Optimization of welding process parameter back-width ratio control, dual-beam laser welding, laser arc hybrid welding, pulse laser welding, and fiber laser welding.