Trust Laser is a laser equipment manufacturer

With the development of laser technology and aluminum alloy research and development technology, further carry out basic research on aluminum alloy laser welding application technology, develop new technology for aluminum alloy laser welding, and more effectively expand the application potential of aluminum alloy laser welding structures, so as to understand aluminum alloy laser welding The application status and development trend of technology are particularly important.


High-strength aluminum alloys have high specific strength, specific rigidity, good corrosion resistance, processability, and mechanical properties. They have become an indispensable metal material for the lightweight manufacturing of structures in the aerospace, ship, and other transportation fields, among which aircraft are the most widely used. Welding technology has unique advantages in improving the utilization rate of structural materials, reducing the weight of the structure, and realizing the low-cost manufacturing of complex and heterogeneous materials. The aluminum alloy laser welding technology is a hot topic that attracts much attention.


Compared with other welding methods, laser welding has the advantages of concentrated heating, small thermal damage, large weld width-to-width ratio, and small welding deformation. The welding process is easy to integrate, automate, and flexible, and can achieve high-speed and high-precision welding. Suitable for high-precision welding of complex structures.


With the development of material technology, various high-strength and high-toughness aluminum alloys have been continuously introduced, especially the third generation of aluminum-lithium alloys and new high-strength aluminum alloys, which have put forward more and higher requirements for aluminum alloy laser welding technology. The variety of alloys also brings a variety of new laser welding problems, so these problems must be studied in depth to more effectively expand the application potential of aluminum alloy laser welding structures.



High power laser

Laser welding is a technology that radiates high-intensity laser light to the metal surface, and melts and cools the metal to form a weld seam through the thermal coupling between the laser and the metal. According to the thermal mechanism of laser welding, it can be divided into two types: thermal conduction welding and deep welding. The former is mainly used for packaging welding or micro-nano welding of precision parts; the latter often produces small holes similar to electron beam welding during the welding process. Effect, forming a deeper and wider weld. Laser deep penetration welding requires high laser power. There are four main types of high-power lasers currently used in laser deep penetration welding.




1CO2 gas laser

The working medium is CO2 gas, which outputs a laser with a wavelength of 10.6 μm. It is divided into two types of cross flow and axial flow according to the laser excitation structure. Although the output power of the cross-flow CO2 laser has reached 150kW, the beam quality is poor and it is 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.


2YAG solid-state laser

The working medium is ruby, neodymium glass, neodymium-doped yttrium aluminum garnet, etc., and the laser with an output wavelength of 1.06 μm. YAG laser is easier to be absorbed by metal than CO2 laser, and is less affected by plasma. It is fiber transmission, flexible welding operation, and good weld position accessibility. It is currently the main laser for welding aluminum alloy structures.


3YLR Fiber Laser

It is a new type of laser developed after 2002. It uses optical fiber as a matrix material and is doped with different rare earth ions. The output wavelength range is about 1.08 μm, which is also optical fiber transmission. Fiber laser revolutionarily uses a double-clad fiber structure, which increases the pump length and improves the pump efficiency, so that the output power of the fiber laser is greatly increased. Compared with YAG laser, although YLR fiber laser appeared relatively late, it has the advantages of small size, low running cost, high beam quality, etc., and the laser power obtained is high.



Application Research of Laser Welding Structure of Aluminum Alloy

Since the 1990s, with the development of science and technology, the emergence of high-power and high-brightness lasers, the integration, intelligence, flexibility, and diversification of laser welding technology have become increasingly mature. At home and abroad, more attention has been paid to laser welding in various fields of aluminum alloys. Application in structure. At present, some automobile manufacturers in China 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 the structure of armored vehicles in the future.




In order to achieve lightweight manufacturing, the application and research of laser welding of aluminum alloy sandwich structures in the construction of ships and high-speed trains is the current research focus. Aluminum alloy is an important metal structural material for aerospace structure. Therefore, in the developed countries such as Japan, the United States, the United Kingdom, and Germany, the research on laser welding technology of aluminum alloy is very important.


With the development of fiber laser welding technology, the fiber manufacturing laser welding and laser arc hybrid welding technology has been listed as the focus of aluminum alloy welding technology in the aviation manufacturing field of advanced countries, especially thick plate welding and welding of dissimilar metals, such as the US NALI The project is conducting research on fiber laser welding and laser arc hybrid welding technology for the structure of civil aircraft and JSF aircraft engine combustion chamber.



Characteristics of laser welding of aluminum alloy

Compared with conventional fusion welding, aluminum alloy laser welding has concentrated heating, large weld width-to-width ratio, and small deformation of the welding structure, but there are also some shortcomings.


(1) The small diameter of the laser focusing spot results in high welding assembly accuracy requirements. Generally, the assembly gap and offset should be less than 0.1mm or 10% of the plate thickness, which increases the difficulty in implementing complex three-dimensional weld welding structures.


(2) Since the reflectivity of the aluminum alloy to the laser is as high as 90% at room temperature, the laser deep-melt welding of aluminum alloy requires the laser to have a higher power. Research on laser welding of aluminum alloy sheet shows that: laser deep penetration welding of aluminum alloy depends on the double threshold of laser power density and line energy. Laser power density and line energy jointly restrict the behavior of the weld pool in the welding process, and finally reflect the forming characteristics of the weld. In the above, the process optimization for full penetration welds can be evaluated by the back width ratio of the characteristic parameters of the weld formation;


(3) Aluminum alloy has a low melting point, good liquid metal fluidity, strong metal vaporization under the action of high-power laser, and metal vapor / photoinduced plasma cloud formed by the pinhole effect during welding affects the aluminum alloy to laser The absorption of energy leads to the instability of the deep-melt welding process, and the weld is prone to defects such as porosity, surface collapse and undercuts;


(4) Laser welding has a fast heating and cooling rate, and the hardness of the weld is higher than that of the arc. However, due to the burning of alloy elements in aluminum alloy laser welding, which affects the strengthening effect of the alloy, the problem of softening of the aluminum alloy weld still exists, thereby reducing the aluminum alloy welding joint. Strength of. Therefore, the main problems of laser welding of aluminum alloys are to control weld defects and improve the performance of welded joints.


Laser welding defect control technology for aluminum alloy

Under the action of high-power lasers, the main defects of laser deep-melt welds of aluminum alloys are pores, surface collapse and undercuts, and the surface collapse and undercut defects can be improved by laser filler wire welding or laser arc hybrid welding; and welds Stomatal defect control is more difficult.


Existing research results show that there are two types of characteristic pores in laser deep-melt welding of aluminum alloys. One type is metallurgical pores. Similar to arc fusion welding, hydrogen pores caused by material pollution or air intrusion during the welding process; the other type is process. Stomatals are caused by the unstable fluctuation of small holes inherent in the laser deep penetration welding process.


In the process of laser deep-fusion welding, the small holes often lag behind the beam movement due to the viscous effect of liquid metal. Their diameter and depth are fluctuated by the influence of plasma / metal vapor. With the movement of the beam and the flow of metal in the molten pool, In deep penetration welding, bubbles appear at the tip of the small hole due to metal flow closure in the molten pool, and in deep penetration welding, bubbles appear in the narrow waist of the middle of the small hole. Bubbles migrate, tumble with the flow of liquid metal, or escape from the surface of the molten pool, or are pushed back to the 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 the surface treatment control before welding and reasonable gas protection during the welding process, and the key to process pores is to ensure the stability of the small holes in the laser deep-fusion welding process. According to the research of domestic laser welding technology, the porosity control of laser deep penetration welding of aluminum alloy should comprehensively consider all links before welding, welding process, and post-weld processing. In summary, there are the following new processes and technologies.


1 pre-weld processing method

Surface treatment before welding is an effective method for controlling metallurgical pores in laser welds of aluminum alloys. Generally, surface treatment methods include physical mechanical cleaning and chemical cleaning. In recent years, laser impact cleaning has also appeared, which will further improve the degree of automation of laser welding.


2Parameter stability optimization control

The technical parameters of the laser welding process of aluminum alloys are mainly laser power, defocus amount, welding speed, and the composition and flow of gas protection. These parameters affect not only the protection effect of the welding area, but also the stability of the laser deep penetration welding process, and thus the weld seam. It is found through laser deep-melting welding of aluminum alloy thin plates that the stability of small hole penetration affects the stability of the molten pool, which will affect the formation of weld seam defects, and the stability of laser deep-melt welding is related to the matching of laser power density and line volume. Therefore, determining reasonable and stable process parameters for welding seam formation is an effective measure to effectively control the blowhole of aluminum alloy laser welds. The results of the research on the characteristics of full penetration stable weld formation show that the ratio of the width of the back of the weld to the width of the surface of the weld (the width of the back of the weld) is used to evaluate the formation and stability 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 back width ratio of the weld can be guaranteed, and the blowhole of the weld can be effectively controlled.


3 double spot laser welding

Double-spot laser welding refers to the welding process in which two focused laser beams are simultaneously applied to the same molten pool. In the process of laser deep-penetration welding, the instantaneous closing to seal the gas in the small holes in the molten pool is one of the main reasons for the formation of weld pores. When dual-spot laser welding is used, due to the effect of two light sources, the opening of the small hole is large, which is conducive to the escape of the internal metal vapor, and it is also beneficial to the stability of the small hole, which can reduce the weld pores. Studies on laser welding of A356, AA5083, 2024, and 5A90 aluminum alloys have all shown that double-spot laser welding can significantly reduce weld porosity.


4 laser arc welding

Laser-arc hybrid welding is a welding method in which laser and arc are applied to the same molten pool. Generally, laser is used as the main heat source, and the interaction between laser and arc is used to increase the penetration depth and welding speed of laser welding and reduce the accuracy of welding assembly. The filler wire is used to regulate the microstructure and properties of the welded joint, and the auxiliary effect of the arc is used to improve the stability of the small holes for laser welding, which is conducive to reducing the porosity of the weld. 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 holes. The results of laser arc hybrid welding of aluminum alloys have also confirmed its effectiveness.


5 fiber laser welding

The pinhole effect of laser deep penetration welding originates from the strong vaporization of metal under the action of laser. Metal vaporization force 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 small holes. Seiji. Et al. ’S research on SUS304 stainless steel high-power fiber lasers shows that the molten pool is elongated during high-speed welding, which suppresses splashing, the small holes are stable, and no bubbles are generated at the tip of the small holes. , The same can be obtained without pores. Allen et al. ’S research on the control technology of shielding gas for titanium alloy fiber laser welding shows 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. Reduced weld air holes.


6 pulse laser welding

Compared with continuous laser welding, the laser output is pulsating, which can promote the periodic stable flow of the molten pool, which is conducive to the escape of bubbles in the molten pool and reducing the weld pores. TY Kuo and SL Jeng studied the laser power output method of YAG laser welding, and its effect on the porosity 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 As the pulse amplitude ΔP increases, the porosity of the weld decreases. The porosity of stainless steel decreases from 2.1% to 0.5%, and the porosity of superalloy decreases from 7.1% to 0.5%.


7 post-weld composite processing technology

In practical engineering applications, even if strict surface treatment is performed before welding, the welding process has good stability, and laser welding of aluminum alloys will inevitably produce weld pores. Therefore, it is important to use post-weld treatment to eliminate pores. . This method is currently mainly modified welding. Hot isostatic pressing is one of the methods to eliminate internal porosity and shrinkage of aluminum alloy castings. It is combined with aluminum alloy laser welding stress heat treatment to form a composite process of hot isostatic pressing and heat treatment of aluminum alloy laser welded components. Weld seam pores and improved joint performance.


Due to the characteristics of aluminum alloys, there are still many problems in the application of high-power laser welding that need to be further studied. The main problem is to control the defects of weld seams and improve the welding quality. The engineering control of aluminum alloy laser welding seam blowholes should comprehensively consider all links before welding, welding process and post-weld processing, so as to improve the stability of the welding process. As a result, many new technologies and processes have been derived, such as laser cleaning before welding, optimization of back-to-width ratio control of welding process parameters, dual beam laser welding, laser arc hybrid welding, pulse laser welding, and fiber laser welding.