With the development of laser technology and aluminum alloy development technology, further carry out basic research on aluminum alloy laser welding application technology, develop new aluminum alloy laser welding process, 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.High-strength aluminum alloys have high specific strength, specific stiffness, good corrosion resistance, processability and mechanical properties, and have become an indispensable metal material for lightweight manufacturing of structures in aerospace, ships and other transportation fields, among which aircraft are the 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, the laser welding beam energy density can reach 109W/cm2, and at the same time, it has the advantages of concentrated heating, small thermal damage, large welding seam aspect ratio, small welding deformation, etc. The welding process is easy to integrate, automate and flexible. , It can realize high-speed and high-precision welding, and the welding process does not require a vacuum environment and does not generate X-ray, which is especially suitable for high-precision welding of complex structures. The proportion of this technology in the industry has become an important indicator to measure the level of a country’s industrial processing. For example, Airbus A380 aircraft and Audi A8 car use the application of laser welding aluminum alloy structure as a sign of its advanced nature. Therefore, laser welding is an important development direction of aluminum alloy welding technology.
With the development of material technology, various high-strength and high-toughness aluminum alloys have been introduced continuously, especially the emergence of the third-generation aluminum-lithium alloy and new high-strength aluminum alloy, which has put forward more and higher requirements for aluminum alloy laser welding technology. The diversity of alloys has also brought about a variety of new laser welding problems, so these problems must be studied in depth in order to more effectively expand the application potential of aluminum alloy laser welded structures.
High Power Laser
Laser welding is a technology that irradiates a high-intensity laser to the metal surface, and through the thermal coupling between the laser and the metal, the metal is melted and then cooled and crystallized to form a weld. According to the heat mechanism of laser welding, it can be divided into two types: thermal conduction welding and deep penetration 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 weld with a large depth-to-width ratio. Laser deep penetration welding requires high laser power. There are four main types of high-power lasers currently used in laser deep penetration welding.
The working medium of CO2 gas laser is CO2 gas, and it outputs 10.6μm wavelength laser. 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 is not suitable for welding; the axial-flow CO2 laser has better beam quality and can be used for welding aluminum alloys with high laser reflectivity, and the commercial laser power also reaches 25kW. The A318 aircraft aluminum alloy lower wall panel adopts CO2 double beam laser welding.
The working medium of YAG solid-state laser is ruby, neodymium glass and neodymium-doped yttrium aluminum garnet, etc. 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 accessibility of weld position. It is currently the main laser for aluminum alloy structure welding. The optically pumped Nd:YAG solid-state laser produces most of the heat loss due to the input energy of the pump lamp during high-power operation, resulting in an increase in the temperature of the laser cavity and the laser thermal lens effect, resulting in low YAG laser power and photoelectric energy conversion efficiency. In recent years, semiconductor pumping and laser pumping have replaced optically pumped solid-state lasers, not only the beam quality has been improved, but also the energy conversion rate and the service life of the pump lamp have been significantly improved. For example, the Yb:YAG Disc laser that appeared in recent years, Its laser energy conversion efficiency reaches 20%, and the beam quality is comparable to that of fiber lasers.
YLR fiber laser is a new type of laser developed after 2002. It uses optical fiber as the 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. The fiber laser revolutionary adopts the double-clad fiber structure, which increases the pumping length and improves the pumping efficiency, thereby greatly improving the output power of the fiber laser. Compared with the YAG laser, although the YLR fiber laser appeared late, it has the advantages of small size, low operating cost, high beam quality, and high laser power; as shown in Figure 1, the laser power can reach 50kW. It can be seen that the appearance of fiber laser has significantly improved the welding penetration and welding speed. The results of various laser welding comparison studies conducted by German BIAS for AA6082 aluminum alloy show that: using a welding speed of 5m/min, the penetration depth of 8kW YLR fiber laser is 9mm, the welding penetration depth of 6kW YAG laser is 5mm, and the 4kW YAG laser welding penetration is 5mm. Welding penetration is 3mm. Using 17kW YAG fiber for welding, the welding speed is 6m/min, the actual weld penetration is 12mm, and the 3mm thick plate is welded, and the maximum welding speed reaches 20m/min, and when the coordination limit between power and beam quality is reduced. , close to the characteristics of electron beam welding, and can realize the welding of 2000 and 7000 series difficult-to-weld aluminum alloys. According to the survey statistics of the British Laser Association (Dr. J. Powell and Prof. WMSteen), fiber laser welding and aluminum alloy thick plate laser welding are the hot spots of laser welding research in the United Kingdom. The German BAM Institute (20 kW), the BIAS Institute ( 17 kW) and IPG (30 kW) are also conducting research on laser welding of aluminum alloy thick plates, and the laser deep penetration welding can reach 25mm.
The Application Research Of Aluminum Alloy Laser Welded Structure
Since the 1990s, with the development of science and technology and the emergence of high-power and high-brightness lasers, the integrated, intelligent, flexible and diversified development of laser welding technology has become more and more mature. application in structure.
German high-end cars, such as Audi, Golf, Passat and other brands, all use aluminum alloy roof laser welding structure. Since 1999, this technology has been expanded to automobile chassis and body, and based on the comprehensive utilization of laser welding and laser arc hybrid welding technology, This technology solves the problem of deformation during welding of structures with small skin thickness and the defect control problem of variable section and end welding in automobile manufacturing. At present, laser welding technology has become a standardized process in automobile manufacturing, covering various sheet metal structures such as the roof, body, and side frames of automobiles. BMW and GM use laser welding on the top of the frame, and Mercedes-Benz in Germany uses laser welding for transmission parts. They relate to Al-Mg-based, Al-Mg-Si-based and Al-Mg-Zn-based aluminum alloys, respectively. 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 the structure of armored vehicles in the future.
In order to achieve lightweight manufacturing, in the structural manufacturing of ships and high-speed trains, the application and research of laser welding of aluminum alloy sandwich structures is a current research hotspot. The high-speed train car wall panel sandwich aluminum parts manufacturing structure developed by the British Welding Institute for the Japanese high-speed rail manufacturer Nippon Sharyo uses 3mm thick AA6063 rolled plate, and uses fiber laser arc hybrid welding to achieve various forms of joint welding. It is an IPG 10 kW fiber laser, the filling material is ER5356 welding wire, and the welding laser power is 4~5kW. AlCAN-Transrapid’s high-speed train aluminum alloy roof structure also uses laser welding instead of MIG welding, using a YAG laser with a power of 4kW, a welding speed of 5m/min, and a weld length of 20m.
Aluminum alloy is an important metal structural material for aerospace structures. Therefore, in developed countries such as Japan, the United States, the United Kingdom, and Germany, the research on aluminum alloy laser welding technology is highly valued. The British Welding Institute successfully used 4kW Nd:YAG laser filler wire welding to carry out double-beam YAG laser welding of AA2014 aluminum alloy ribbed wall panels, using ER2319 aluminum alloy welding wire with a diameter of 1.2mm, the quality of the welded joint is good, and the welding The deformation is smaller than TIG welding. Gobbi et al. studied the CO2 laser welding of the aluminum alloy fuel tank under the aircraft wing. The joints have serious pores, and high-quality welded joints can be obtained by using YAG laser welding. Airbus has cooperated with BIAS, Fraunhofer and other research institutes. After 10 years of hard work, in 2003, the A318 passenger aircraft aluminum alloy lower wall structure double-beam CO2 laser wire filling welding and YAG laser wire filling were realized in the Airbus component production plants in Germany and France. Welding, the welding speed is 4~10m/min. This welding instead of riveting structure will reduce the weight of the aircraft by nearly 20% and reduce the manufacturing cost by about 20%. It has been applied to the wall structure of A318, A380 and A340 series aircraft. welding, as shown in Figure 2. In the future, it will also be applied to the welding of A350 aircraft panels, the number of panels will reach 18, and the total length of the weld will reach 1000m. 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 carrying out research on fiber laser welding and laser arc hybrid welding technology for civil aircraft and JSF aircraft engine combustion chamber structures.
Characteristics Of Aluminum Alloy Laser Welding
Compared with conventional fusion welding, aluminum alloy laser welding has concentrated heating, large aspect ratio of weld seam, and small deformation of welding structure, but there are also some shortcomings, which can be summarized as follows:
- The small diameter of laser focusing spot leads to the requirement of 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 seam welding structures;
- The reflectivity of aluminum alloys to laser light at room temperature is as high as 90%. %, so the laser deep penetration welding of aluminum alloy requires the laser to have a higher power. The research on laser welding of aluminum alloy sheet shows that the laser deep penetration welding of aluminum alloy depends on the dual thresholds of laser power density and line energy.
- The melting point of aluminum alloy is low, the liquid metal has good fluidity, and strong metal vaporization occurs under the action of high-power laser. The metal vapor/photoplasma cloud formed with the pinhole effect during the welding process affects the absorption of laser energy by the aluminum alloy, resulting in unstable deep penetration welding process, and the weld is prone to defects such as pores, surface collapse, and undercut;
- The heating and cooling speed of laser welding is fast, and the hardness of the weld is higher than that of the arc. However, due to the burning of alloy elements in the laser welding of aluminum alloys, which affects the strengthening effect of the alloy, the aluminum alloy weld still has a softening problem, thereby reducing the aluminum alloy welded joint. Strength of. Therefore, the main problem of aluminum alloy laser welding is to control weld defects and improve the performance of welded joints.
The Defect Control Technology Of Aluminum Alloy Laser Welding
Under the action of high-power laser, the main defects of aluminum alloy laser deep penetration welds are porosity, surface collapse and undercut, of which surface collapse and undercut defects can be improved by laser wire filler welding or laser arc hybrid welding; Stomatal defect control is more difficult. The existing research results show that there are two types of characteristic pores in laser deep penetration welding of aluminum alloys. One is metallurgical pores. Like arc fusion welding, hydrogen pores are caused by material pollution or air intrusion during the welding process; the other is process. The porosity is caused by the unstable fluctuation 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, and its diameter and depth fluctuate due to the influence of the plasma/metal vapor. In penetration deep penetration welding, bubbles appear at the tip of the small hole due to the closure of the molten pool metal flow, while in full penetration deep penetration welding, bubbles appear at the waist in the middle of the small hole. The bubbles migrate and roll with the flow of the 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, the metallurgical pores are mainly controlled by the surface treatment control before welding and the reasonable gas protection during the welding process, and the key to the process pores is to ensure the stability of the pores 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 pre-welding, welding process and post-welding treatment, which can be summed up as the following new processes and technologies.
- Pre-welding treatment method. Pre-welding surface treatment is an effective method to control metallurgical pores in aluminum alloy laser welds. Usually, the 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.
- Stability optimization control of welding process parameters. The process parameters of aluminum alloy laser welding process usually mainly include laser power, defocusing amount, welding speed, and the composition and flow of gas shielding. These parameters affect both the protection effect of the welding area and the stability of the laser penetration welding process, thereby affecting the weld porosity. Through laser deep penetration welding of aluminum alloy sheets, it is found that the penetration stability of small holes affects the stability of the molten pool, which will affect the weld formation and cause 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 and stable welding seam forming process parameters is an effective measure to effectively control the porosity of aluminum alloy laser welds. The results of the study on the forming characteristics of the full penetration 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 to the width of the weld) is used to evaluate the weld forming and stability of the aluminum alloy sheet. When the laser power density and line energy of thin plate laser welding are reasonably matched, a certain back-to-width ratio of the weld seam can be guaranteed, and the porosity of the weld seam can be effectively controlled.
- 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 dual-spot laser welding is used, due to the action of the two light sources, the opening of the small hole is large, which is conducive to the escape of the internal metal vapor and the stability of the small hole, thereby reducing the welding seam porosity. Studies on laser welding of A356, AA5083, 2024 and 5A90 aluminum alloys all show that double-spot laser welding can significantly reduce weld porosity.
- 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 improve the laser welding penetration and welding speed, and reduce the welding assembly accuracy. The filler wire is used to control the microstructure and properties of the welded joint, and the auxiliary action of the arc is used to improve the stability of the small holes in laser welding, which is beneficial to reduce the welding seam porosity. 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 absorption of laser energy by the material and the stability of the small hole. The results of the research on the aluminum alloy laser arc hybrid welding seam also confirmed its effect.
- Fiber laser welding. The pinhole effect in the laser deep penetration welding process originates from the strong vaporization of the metal under the action of the laser. The vapor force of metal vaporization is closely related to the 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 shows that the molten pool is elongated during high-speed welding, the spatter is suppressed, the fluctuation of the small hole is stable, and no bubbles are generated at the tip of the small hole. When the fiber laser is used for high-speed welding of titanium alloys and aluminum alloys , a porosity-free weld can also be obtained. Allen et al.’s research on shielding gas control technology for titanium alloy fiber laser welding shows that by controlling the position of the welding shielding gas, the gas can be prevented from being involved, the hole closing time can be reduced, the welding hole can be stabilized, and the solidification behavior of the molten pool can be changed. Reduce weld porosity.
- Pulse 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 reduces the porosity of the weld. TY Kuo and SL Jeng studied the laser power output mode of YAG laser welding, and the effect on the porosity and performance of SUS 304L stainless steel and inconel 690 superalloy welds. The results show that: for square wave pulsed laser welding, when the base value power is 1700w , with the increase of the pulse amplitude ΔP, 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%.
- Post-weld composite treatment technology. In practical 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 porosity of aluminum alloy castings. It is combined with the stress heat treatment after laser welding of aluminum alloys to form a composite process of hot isostatic pressing and heat treatment of aluminum alloy laser welded components, which not only eliminates the Weld pores, and improve joint performance.
Concluding Remarks
Due to the advantages of laser deep penetration welding, laser welding of aluminum alloys has received extensive attention abroad, and has become an important research and development direction for the structural manufacturing technology of vehicles such as aerospace, vehicles, ships, etc., especially fiber laser and Disc laser. The development of new high-brightness and high-power foundations has further expanded the application prospects of aluminum alloy laser welding structures. However, due to the characteristics of aluminum alloys, there are still many problems in the application of high-power laser welding to be further studied. The main problem is to control the porosity of the weld and improve the welding quality. The engineering control of porosity in aluminum alloy laser welds should comprehensively consider all aspects of pre-welding, welding process and post-welding treatment, 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 welding process parameters back-to-width ratio control, dual-beam laser welding, laser arc hybrid welding, pulsed laser welding and fiber laser welding.