With the development of laser technology and aluminum alloy 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 technology, and more effectively expand the application potential of aluminum alloy laser welding structure, 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, has become an indispensable metal material for structural lightweight manufacturing in aerospace, ships and other transport fields, among which aircraft are the most used. Welding technology has unique advantages in improving the utilization rate of structural materials, reducing the weight of structures, and realizing the low-cost manufacturing of complex and dissimilar materials, among which aluminum alloy laser welding technology is a hot spot.
Compared with other welding methods, laser welding has the advantages of centralized heating, small thermal damage, large weld depth to width ratio, and small welding deformation. The welding process is easy to be integrated, automated and flexible, and high-speed and high-precision welding can be realized, especially suitable for high-precision welding of complex structures.
With the development of material technology, a variety of high-strength and high-toughness aluminum alloys continue to be introduced, especially the third generation of aluminum lithium alloy, the emergence of new high-strength aluminum alloys, aluminum laser welding technology put forward more and higher requirements, while the diversity of aluminum alloys also brought a variety of new laser welding problems, so we must in-depth study these problems, In order to expand the application potential of aluminum alloy laser welding structure more effectively.
High power laser
Laser welding is a technology that radiates high-intensity laser onto the metal surface and makes the metal melt and then cool and crystallize to form a weld through the thermal coupling between the laser and the metal. According to the thermal action mechanism of laser welding can be divided into thermal conduction welding and deep penetration welding two kinds, the former is mainly used in precision parts package welding or micro and nano welding; The latter often produces a small hole effect similar to electron beam welding in the welding process, forming a deep and wide weld. The realization of laser deep penetration welding requires high laser power, and there are four types of high-power laser currently applied in laser deep penetration welding.
1, CO2 gas laser
The working medium is CO2 gas, output 10.6μm wavelength laser, according to the laser excitation structure is divided into transverse flow and axial flow two kinds. 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. Axial flow CO2 laser has better beam quality and can be used for welding aluminum alloys with high laser reflectivity.
2, YAG solid laser
The working medium is ruby, neodymium glass and neodymium-doped yttrium aluminum garnet, etc. The output wavelength is 1.06μm laser. YAG laser is more easily absorbed by metal than CO2 laser, and is less affected by plasma, for fiber transmission, flexible welding operation, good weld position accessibility, is the main laser for welding aluminum alloy structure.
3, YLR fiber laser
It is a new type of laser developed after 2002, using fiber as the matrix material, doping different rare earth ions, and the output wavelength range is about 1.08μm, which is also fiber transmission. The fiber laser revolutionary use of double-clad fiber structure, increase the pump length, improve the pump efficiency, so that the output power of the fiber laser is greatly increased. Compared with YAG laser, YLR fiber laser appears later, but it has the advantages of small size, low operating cost, high beam quality, and high laser power.
Application research on laser welding structure of aluminum alloy
Since the 1990s, with the development of science and technology, the emergence of high-power high-brightness lasers, laser welding technology integration, intelligent, flexible, diversified development is becoming more mature, more attention at home and abroad laser welding in various fields of aluminum alloy structure application. 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 plate, laser welding will be applied to armored vehicle structure in the future.
In order to realize lightweight manufacturing, the application and research of laser welding of aluminum alloy sandwich structure in ship and high-speed train structure manufacturing is the current research hotspot. Aluminum alloy is an important metal structural material for aerospace structures, so in Japan, the United States, the United Kingdom, Germany and other developed countries attach great importance to aluminum alloy laser welding technology research.
With the development of fiber laser welding technology, the aviation manufacturing field of advanced countries has listed fiber laser welding and laser arc composite welding technology as the focus of aluminum alloy welding technology, especially thick plate welding and dissimilar metal welding. For example, the NALI project in the United States is carrying out research on fiber laser welding and laser arc composite welding technology for the combustion chamber structure of civil aircraft and JSF aircraft engines.
Characteristics of laser welding of aluminum alloy
Compared with conventional fusion welding, aluminum alloy laser welding heating concentration, weld depth to width ratio is large, welding structure deformation is small, but there are some shortcomings, summed up as follows:
(1) The small diameter of the laser focusing spot leads to high requirements for workpiece welding and assembly accuracy, usually the assembly gap and the amount of wrong edges need to be less than 0.1mm or 10% of the plate thickness, which increases the difficulty of the implementation of the complex three-dimensional welding structure
(2) Because the reflectivity of aluminum alloy to the laser at room temperature is as high as 90%, the laser deep penetration welding of aluminum alloy requires the laser to have a high power. The research of laser welding of aluminum alloy sheet shows that the laser deep penetration welding of aluminum alloy depends on the double threshold of laser power density and linear energy, which jointly restrict the weld pool behavior in the welding process and finally reflect on the forming characteristics of the weld. The process optimization of the full penetration weld can be evaluated by the back width ratio of the welding seam forming characteristics.
(3) Aluminum alloy melting point is low, liquid metal flow is good, strong metal vaporization under the action of high-power laser, metal vapor/photoinduced plasma cloud formed with small hole effect in the welding process affects aluminum alloy absorption of laser energy, resulting in deep penetration welding process instability, weld prone to porosity, surface collapse, edge and other defects;
(4) The laser welding heating and cooling speed is fast, and the weld hardness is higher than that of the arc, but due to the burning of alloying elements in the laser welding of aluminum alloy, which affects the strengthening of the alloy, the aluminum alloy weld still has the problem of softening, thereby reducing the strength of the aluminum alloy welded joint. Therefore, the main problem of laser welding of aluminum alloy is to control weld defects and improve the performance of welded joints.
Laser welding defect control technology of aluminum alloy
Under the action of high-power laser, the main defects of aluminum alloy laser deep penetration welding joints are porosity, surface collapse and edge biting, which can be improved by laser wire filling welding or laser arc composite welding. It is difficult to control the porosity defect of weld.
The existing research results show that there are two types of characteristic pores in laser deep penetration welding of aluminum alloy. One is metallurgical pores, which are caused by material pollution or air intrusion in welding process like arc melting welding. The other type is the process porosity, which is caused by the unstable fluctuation of the small hole inherent in the laser deep penetration welding process.
In the process of laser deep penetration welding, the hole often lags behind the beam movement due to the viscous effect of 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 molten pool metal, bubbles appear at the tip of the hole in the non-permeated deep penetration welding due to the flow of molten pool metal, and bubbles appear at the narrow waist in the middle of the hole in the full penetration deep penetration welding. The bubbles migrate and roll with the liquid metal flow, 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, that is, become the weld porosity.
Obviously, the metal porosity is mainly controlled by the surface treatment control before welding and the reasonable gas protection during welding, and the key of the process porosity is to ensure the stability of the small hole during the laser deep penetration welding. According to the domestic laser welding technology research, aluminum alloy laser deep penetration welding porosity control should be considered before welding, welding process, welding post-treatment of various links, summed up as the following new processes and new technologies.
1, pre-welding treatment method
Surface treatment before welding is an effective way to control the aluminum alloy laser weld metallurgical pores, usually surface treatment methods have physical and mechanical cleaning, chemical cleaning, in recent years, there has been laser shock cleaning, which will further improve the degree of automation of laser welding.
2, parameter stability optimization control
Aluminum alloy laser welding process parameters are usually mainly laser power, defocus, welding speed, and gas protection composition and flow. 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 porosity of the weld. Through the laser deep penetration welding of aluminum alloy sheet, it is found that the stability of small hole penetration affects the stability of the weld pool, and then affects the weld formation resulting in weld porosity defects. Moreover, the stability of laser deep penetration welding is related to the laser power density and linear quantity matching, so determining reasonable process parameters of stable weld formation is an effective measure to effectively control the porosity of aluminum alloy laser weld.
The results 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 weld formation and stability of the aluminum alloy sheet. When the laser power density and line energy match reasonably, the weld back to width ratio can be guaranteed, and the weld porosity can be effectively controlled.
3, double spot laser welding
Double spot laser welding refers to the welding process in which two focused laser beams act on the same weld pool at the same time. In the process of laser deep penetration welding, one of the main reasons for the formation of porosity is that the gas in the small hole is enclosed in the weld pool by the instant closure. When double-spot laser welding is used, due to the action of two light sources, the large opening of the small hole is conducive to the escape of the internal metal vapor, and is also conducive to the stability of the small hole, so as to reduce the porosity of the weld. Studies on laser welding of A356, AA5083, 2024 and 5A90 aluminum alloys all show that dual-spot laser welding can significantly reduce weld porosity.
4, laser arc composite welding
Laser arc composite welding is a welding method in which laser and arc are applied to the same weld pool. Generally using laser as the main heat source, the interaction between laser and arc can improve the laser welding depth and welding speed, and reduce the welding assembly accuracy. The stability of laser welding holes can be improved by using filler wire to control the microstructure properties of welded joints, and the auxiliary effect of arc can help to reduce the porosity of weld.
In the laser arc composite welding process, the arc affects the metal vapor/plasma cloud induced by the laser process, which is conducive to the absorption of laser energy and the stability of small holes. The results of laser arc composite welding of aluminum alloy have also proved its effectiveness.
5, fiber laser welding
The small hole effect of laser deep penetration welding process is due to the strong vaporization of metal under the action of laser. The vapor force of metal vaporization 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. research on SUS304 stainless steel high-power fiber laser shows that the weld pool is elongated during high-speed welding, which inhibits spatter, the small hole fluctuation is stable, and the small hole tip has no bubbles. When the fiber laser is used for high-speed welding of titanium alloy and aluminum alloy, a porosity free weld can also be obtained. Allen et al. 's research on the control technology of protective gas in titanium alloy fiber laser welding shows that by controlling the position of welding protective gas, gas involvement can be prevented, the closing time of small holes can be reduced, the welding small holes can be stabilized, and the solidification behavior of the weld pool can be changed, thus reducing the porosity of the weld.
6, pulsed laser welding
Compared with continuous laser welding, the laser output adopts pulsating mode, which can promote the periodic and stable flow of the molten pool, which is conducive to the escape of the bubble in the molten pool and reduce the porosity of the weld. T Y Kuo and S L Jeng studied the influence of laser power output mode of YAG laser welding on porosity and properties of SUS 304L stainless steel and inconel 690 superalloy welds. The results show that: For square wave pulsed laser welding, when the base power is 1700w, the porosity of the weld decreases with the increase of pulse amplitude ΔP, and the porosity of stainless steel decreases from 2.1% to 0.5%, and that of superalloy decreases from 7.1% to 0.5%.
7, composite processing technology after welding
In practical engineering applications, even if the surface treatment is strict before welding and the welding process is stable, laser welding of aluminum alloy will inevitably produce weld porosity, so the use of post-welding treatment to eliminate porosity is very important. At present, the method is mainly modified welding. Hot isostatic pressing technology is one of the methods to eliminate the porosity and shrinkage of aluminum alloy castings, and it is combined with the stress heat treatment of aluminum alloy after laser welding to form a composite process of hot isostatic pressing and heat treatment of aluminum alloy laser welding components, which not only eliminates the porosity of the weld, but also improves the joint performance.
Due to the characteristics of aluminum alloy, there are still many problems in the application of high-power laser welding, the main problem is to control the porosity defect of the weld and improve the welding quality. In order to improve the stability of welding process, the engineering control of laser weld porosity of aluminum alloy should consider all aspects before welding, during welding and after welding. Thus, many new technologies and processes have been derived, such as pre-welding laser cleaning, welding process parameters back width ratio control optimization, double beam laser welding, laser arc composite welding, pulse laser welding and fiber laser welding.
