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 for lightweight construction in the field of aerospace, naval vessels, etc. Among them, aircraft are the most widely 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, 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.
01 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.
02 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.
03 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 about 1.08μm. It is also optical fiber transmission. The fiber laser revolutionizes the use of a double-clad fiber structure, increases the pump length and improves the pump efficiency, so that the output power of the fiber laser is greatly increased. Compared with the YAG laser, although the 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.
Research on Application of Laser Welding Structure of Aluminum Alloy
Since the 1990s, with the development of science and technology and the emergence of high-power and high-brightness lasers, the integration, intelligence, flexibility, and diversification of laser welding technology has matured. At home and abroad, more attention has been paid to laser welding in various fields of aluminum alloy Application in the structure. At present, some automobile manufacturers in my country have adopted laser welding technology in some new models. With the development of laser welding technology for aluminum alloy thick plates, laser welding will be applied to armored vehicle structures in the future.
In order to achieve lightweight manufacturing, the application and research of laser welding of aluminum alloy sandwich structures is the current research hotspot in the manufacturing of ships and high-speed train structures. Aluminum alloy is an important metal structure material for aerospace structures, so developed countries such as Japan, the United States, the United Kingdom, Germany and other developed countries attach great importance to the research of aluminum alloy laser welding technology.
With the development of fiber laser welding technology, the aviation manufacturing field of advanced countries has listed fiber laser welding and laser arc hybrid welding technology as the focus of aluminum alloy welding technology, especially thick plate welding and welding of dissimilar metals, such as the United States NALI The project is conducting research on fiber laser welding and laser arc hybrid welding technology for civil aircraft and JSF aircraft engine combustion chamber structures.
Features of aluminum alloy laser welding
Compared with conventional fusion welding, aluminum alloy laser welding has concentrated heating, 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 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 sheet shows that laser deep penetration welding of aluminum alloy 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 low melting point and good fluidity of liquid metal. It produces strong metal vaporization under the action of high-power laser. The metal vapor/photoplasma 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, and the weld is prone to defects such as pores, surface collapse, undercut, etc.;
4) Laser welding has a fast heating and cooling speed, and 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 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 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.
Obviously, metallurgical pores are mainly controlled by pre-welding surface treatment control and reasonable gas protection during welding. The key to process pores is to ensure the stability of the small holes in the laser deep penetration welding process. According to the research of domestic laser welding technology, the porosity control of aluminum alloy laser deep penetration welding should comprehensively consider all aspects of welding, welding process, and post-welding treatment. In summary, there are the following new processes and new technologies.
01 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.
02 Parameter stability optimization control
The process parameters of aluminum alloy laser welding process usually mainly include laser power, defocusing amount, welding speed, and the composition and flow rate of gas protection. These parameters not only affect the protection effect of the welding area, but also affect the stability of the laser deep penetration welding process, thereby affecting the weld pores.
Through laser deep penetration welding of aluminum alloy sheet, it is found that the stability of small hole penetration affects the stability of the molten pool, which in turn will affect the weld formation and cause weld pore defects, and the stability of laser deep penetration welding is related to the matching of laser power density and line volume Therefore, it is an effective measure to effectively control the porosity of aluminum alloy laser welding seam to determine the reasonable and stable welding seam forming process parameters.
The results of the research on the forming characteristics of the full penetration and stable weld show that the ratio of the width of the back of the weld to the width of the surface of the weld (the ratio of the back width of the weld) is used to evaluate the forming and stability of the weld of the aluminum alloy sheet. When the laser power density of the thin plate laser welding is reasonably matched with the line energy, a certain weld back-to-width ratio can be ensured, and the weld pores can be effectively controlled.
03 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.
04 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.
05 Fiber laser welding
The pinhole effect of the laser deep penetration welding process originates from the strong vaporization of the 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. Seiji. et al.’s 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, and there was no bubble generation at the tip of the small hole. When the fiber laser was used for high-speed welding of titanium alloy and aluminum alloy , The same can be obtained without porosity welds.
Allen et al.’s research on the shielding gas control technology for titanium alloy fiber laser welding showed that by controlling the position of the welding shielding gas, it can prevent the gas from being involved, reduce the hole closing time, stabilize the welding hole, and change the solidification behavior of the molten pool, thereby Reduce welding seam porosity.
06 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. TY Kuo and SL Jeng studied the laser power output mode of YAG laser welding, and the effect on the pores and performance of SUS 304L stainless steel and inconel 690 superalloy welds. The results show that: for square wave pulse laser welding, when the base power is 1700w With the increase of the pulse amplitude ΔP, the porosity of the weld decreases. The porosity of stainless steel is reduced from 2.1% to 0.5%, and the porosity of superalloy is reduced from 7.1% to 0.5%.
07 Composite treatment technology after welding
In actual engineering applications, even if 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 pores and shrinkage of aluminum alloy castings. It is combined with the stress heat treatment after aluminum alloy laser welding to form aluminum alloy laser welded components. Combined with heat treatment, it not only eliminates welding seam porosity, but also improves 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 of 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.
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