Aluminum alloy laser welding technology

With the development of laser technology and aluminum alloy research and development technology, it is particularly important to further carry out basic research on aluminum alloy laser welding application technology, develop new aluminum alloy laser welding processes, and more effectively expand the application potential of aluminum alloy laser welding structures, so as to understand the application status and development trend of aluminum alloy laser welding technology.
High-strength aluminum alloy has high specific strength, specific stiffness, good corrosion resistance, processing performance and mechanical properties. It has become an indispensable metal material for lightweight manufacturing of structures in aerospace, ships 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 structural weight, and realizing low-cost manufacturing of complex and heterogeneous materials. Among them, aluminum alloy laser welding technology is a hot topic that has received much attention.

Compared with other welding methods, laser welding has the advantages of concentrated heating, less thermal damage, large weld depth-to-width ratio, and small welding deformation. The welding process is easy to integrate, automate, and be flexible, and can achieve high-speed and high-precision welding, which is particularly suitable for high-precision welding of complex structures.
With the development of material technology, various high-strength and high-toughness aluminum alloys are constantly introduced, especially the emergence of the third-generation aluminum-lithium alloy and new high-strength aluminum alloy, which puts forward more and higher requirements for aluminum alloy laser welding technology. At the same time, the diversity of aluminum alloys also brings a variety of new problems for laser welding. Therefore, 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 lasers to the metal surface, melts the metal through the thermal coupling between the laser and the metal, and then cools and crystallizes to form a weld. According to the thermal action mechanism of laser welding, it can be divided into thermal conduction welding and deep fusion welding. The former is mainly used for packaging welding or micro-nano welding of precision parts; the latter often produces a small hole effect similar to electron beam welding during the welding process, forming a weld with a larger depth-to-width ratio. Laser deep melting welding requires high laser power. Currently, there are four main types of high-power lasers used in laser deep melting welding.

1CO2 gas laser
The working medium is CO2 gas, and the output is 10.6μm wavelength laser. According to the laser excitation structure, it is divided into transverse flow and axial flow. Although the output power of transverse flow CO2 laser has reached 150kW, the beam quality is poor and it is not suitable for welding; axial flow CO2 laser has good beam quality and can be used for welding aluminum alloys with high laser reflectivity.

2YAG solid laser
The working medium is ruby, neodymium glass and neodymium-doped yttrium aluminum garnet, etc., and the output wavelength is 1.06μm. 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 aluminum alloy structure welding.

3YLR 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 has an output wavelength range of about 1.08μm. It is also fiber-optic transmission. Fiber laser revolutionized the use of double-clad fiber structure, increased the pump length, and improved 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, and high laser power.

Application research of aluminum alloy laser welding structure
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 have become increasingly mature, and more attention has been paid to the application of laser welding in aluminum alloy structures in various fields at home and abroad. At present, some automobile manufacturers in my country have adopted laser welding technology in some new models. With the development of aluminum alloy thick plate laser welding technology, 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 in the manufacturing of ship and high-speed train structures is currently a hot topic. Aluminum alloy is an important metal structural material for aerospace structures. Therefore, developed countries such as Japan, the United States, the United Kingdom, and Germany 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. For example, the US NALI project is conducting research on fiber laser welding and laser arc hybrid welding technology for the combustion chamber structure of civil aircraft and JSF aircraft engines.
Characteristics of aluminum alloy laser welding
Compared with conventional melting welding, aluminum alloy laser welding has concentrated heating, large weld depth-to-width ratio, and small deformation of the weld structure. However, there are also some shortcomings, which can be summarized as follows:
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(1) The small diameter of the laser focus spot leads to high requirements for workpiece welding assembly accuracy. Usually, the assembly gap and misalignment need to be less than 0.1mm or 10% of the plate thickness, which increases the difficulty of implementing complex three-dimensional weld structures;

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(2) Since the reflectivity of aluminum alloy to laser is as high as 90% at room temperature, aluminum alloy laser deep melting welding requires the laser to have a higher power. Research on laser welding of aluminum alloy thin plates shows that deep penetration laser welding of aluminum alloy depends on the dual thresholds of laser power density and line energy. The laser power density and line energy jointly restrict the molten pool behavior during the welding process and are ultimately reflected in the weld formation characteristics. The process optimization of full penetration welds can be evaluated by the weld formation characteristic parameter back-width ratio;

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(3) Aluminum alloys have low melting points and good liquid metal fluidity. Under the action of high-power lasers, strong metal vaporization occurs. During the welding process, the metal vapor/photoplasma cloud formed by the pinhole effect affects the absorption of laser energy by aluminum alloys, resulting in instability in the deep penetration welding process. The weld is prone to defects such as pores, surface collapse, and undercuts;

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(4) Laser welding has fast heating and cooling speeds, and the weld hardness is higher than that of arc welding. However, due to the burning of alloy elements in aluminum alloy laser welding, the alloy strengthening effect is affected, and the aluminum alloy weld still has a softening problem, thereby reducing the strength of the aluminum alloy welded joint. Therefore, the main problems of aluminum alloy laser welding are to control weld defects and improve weld joint performance.
Aluminum alloy laser welding defect control technology
Under the action of high-power laser, the main defects of aluminum alloy laser deep penetration welds are pores, surface collapse and undercut. Among them, surface collapse and undercut defects can be improved by laser wire welding or laser arc hybrid welding; while weld pore defect control is more difficult.
Existing research results show that there are two types of characteristic pores in aluminum alloy laser deep penetration welding. One type is metallurgical pores, which are hydrogen pores caused by material contamination or air intrusion during the welding process, just like arc melting welding; the other type is process pores, which are caused by the unstable fluctuation of small holes inherent in the laser deep penetration welding process.

In the process of laser deep penetration welding, the small hole often lags behind the movement of the beam due to the viscosity of the liquid metal, and its diameter and depth fluctuate under the influence of plasma/metal vapor. With the movement of the beam and the flow of the molten pool metal, bubbles appear at the tip of the small hole due to the closure of the molten pool metal flow in the incomplete deep penetration welding, and bubbles appear at the thin waist in the middle of the small hole in the full penetration deep penetration welding. Bubbles migrate and roll with the flow of liquid metal, or escape from the surface of the molten pool, or are pushed back to the small hole. 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, and the key to process pores is to ensure the stability of the small holes during laser deep melting welding. According to the research on domestic laser welding technology, the control of pores in aluminum alloy laser deep melting welding should comprehensively consider all aspects of pre-welding, welding process, and post-welding treatment. In summary, there are the following new processes and technologies.
1 Pre-welding treatment method
Pre-welding surface treatment is an effective method to control the metallurgical pores of aluminum alloy laser welds. Usually, the surface treatment methods include physical mechanical cleaning and chemical cleaning. In recent years, laser shock cleaning has also appeared, which will further improve the degree of automation of laser welding.
2 Parameter stability optimization control
The process parameters of aluminum alloy laser welding process usually include laser power, defocus, welding speed, and gas protection composition and flow rate. These parameters affect not only the protection effect of the welding area, but also the stability of the laser deep penetration welding process, thus affecting the weld porosity. Through laser deep penetration welding of aluminum alloy thin plates, it was found that the keyhole penetration stability affects the stability of the molten pool, which in turn affects the weld formation and causes weld porosity defects. Moreover, the stability of laser deep penetration welding is related to the matching of laser power density and line quantity. Therefore, determining reasonable process parameters for stable weld formation is an effective measure to effectively control the porosity of aluminum alloy laser welds.

The results of the study on the formation characteristics of full penetration stable welds show that the ratio of the back width of the weld to the surface width of the weld (weld back width ratio) is used to evaluate the weld formation and stability of aluminum alloy thin plates. When the laser power density and line energy of thin plate laser welding are reasonably matched, a certain weld back width ratio can be guaranteed, and the weld porosity can be effectively controlled.
3 Dual-spot laser welding
Dual-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, the instantaneous closure of the keyhole to seal the gas in the molten pool is one of the main reasons for the formation of weld porosity. When dual-spot laser welding is used, due to the effect of two light sources, the opening of the small hole is larger, which is conducive to the escape of internal metal vapor and the stability of the small hole, thereby reducing weld porosity. Studies on laser welding of A356, AA5083, 2024 and 5A90 aluminum alloys have shown that dual-spot laser welding can significantly reduce weld porosity.
4 Laser arc hybrid welding
Laser arc hybrid welding is a welding method that applies laser and arc to the same molten pool. Generally, laser is used as the main heat source, and the interaction between laser and arc is used to improve the laser welding penetration and welding speed, and reduce the welding assembly accuracy. Filler wire is used to regulate the microstructure and properties of the weld joint, and the auxiliary effect of the arc is used to improve the stability of the laser welding small hole, which is conducive to reducing weld porosity.

In the process of laser arc hybrid welding, the arc affects the metal vapor/plasma cloud induced by the laser process, which is conducive to the material’s absorption of laser energy and the stability of the small hole. The results of the study on aluminum alloy laser arc hybrid welding welds also confirmed its effect.
5 Fiber laser welding
The pinhole effect in the laser deep melting welding process is due to the strong vaporization of the metal under the action of the laser. The metal vaporization steam force is closely related to the laser power density and beam quality, which not only affects the melting depth of laser welding, but also affects the stability of the pinhole. Seiji et al.’s study on high-power fiber laser for SUS304 stainless steel showed that the molten pool was elongated during high-speed welding, spatter was suppressed, the pinhole fluctuation was stable, and no bubbles were generated at the tip of the pinhole. When the fiber laser was used for high-speed welding of titanium alloy and aluminum alloy, a pore-free weld could also be obtained. Allen et al.’s study on the protective gas control technology for titanium alloy fiber laser welding showed that by controlling the position of the welding protective gas, the gas can be prevented from being involved, the pinhole closing time can be reduced, the welding pinhole can be stabilized, and the solidification behavior of the molten pool can be changed, thereby reducing the weld porosity.

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6 Pulsed laser welding
Compared with continuous laser welding, the laser output is output in a pulsating manner, 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 the reduction of weld porosity. T Y Kuo and S L Jeng studied the influence of YAG laser welding laser power output mode on the weld porosity and performance of SUS 304L stainless steel and inconel 690 high-temperature alloy. The results showed that for square wave pulse laser welding, when the base power is 1700w, the weld porosity decreases with the increase of pulse amplitude ΔP, among which the porosity of stainless steel decreases from 2.1% to 0.5%, and the porosity of high-temperature alloy decreases from 7.1% to 0.5%.
7 Post-weld composite treatment technology
In actual engineering applications, even if strict surface treatment is performed before welding and the welding process is stable, aluminum alloy laser welding will inevitably produce weld porosity. Therefore, it is very important to use post-weld treatment to eliminate porosity. This method is currently mainly for decorative welding. Hot isostatic pressing technology is one of the methods to eliminate internal pores and shrinkage in aluminum alloy castings. It is combined with stress heat treatment after aluminum alloy laser welding to form a composite process composed of hot isostatic pressing and heat treatment of aluminum alloy laser welding components, which can not only eliminate weld porosity but also improve joint performance.
Postscript
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 porosity defects and improve the welding quality. The engineering control of aluminum alloy laser weld porosity should comprehensively consider all aspects before welding, during welding, and after welding, 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 back-width ratio control of welding process parameters, dual-beam laser welding, laser arc hybrid welding, pulsed laser welding, and fiber laser welding.