Abstract : This paper discusses the characteristics of laser welding process, the application and research status of laser welding in the automotive industry, microelectronics industry, biomedicine, etc., the intelligent control of laser welding, and discusses the issues that laser welding needs further research and discussion. >> Into the colorful world of laser welding Â
Key words: laser welding; hybrid welding; welding device; application field
introduction
Laser welding is one of the important aspects of the application of laser processing material processing technology. In the 1970s, it was mainly used for welding thin-walled materials and low-speed welding. The welding process is heat-conducting type, that is, laser radiation heats the surface of the workpiece, and the surface heat is diffused to the inside through heat conduction. By controlling the parameters such as the width, energy, peak power and repetition frequency of the laser pulse , the workpiece is melted to form a specific molten pool. As a high-quality, high-precision, low-deformation, high-efficiency and high-speed welding method, laser welding, along with high-power CO2 and high-power YAG lasers and fiber transmission technology, metal molybdenum welding beam objective lens, etc. It has been successfully developed, and its application in the fields of machinery manufacturing, aerospace, automotive industry, powder metallurgy, biomedical microelectronics industry is more and more extensive. The current research focuses on the theory of C02 laser and YAG laser welding of various metal materials, including laser-induced plasma spectroscopic, absorption, scattering characteristics and intelligent control of laser welding, composite welding, laser welding and small hole behavior. Welding mechanism and prevention methods, welding and welding of nickel-based heat-resistant alloys, aluminum alloys and magnesium alloys, modeling and numerical simulation of welding phenomena, joining of steel materials, copper, aluminum alloys and dissimilar materials, laser joints Some studies have been done on performance evaluation [1].
First, the quality and characteristics of laser welding
Principle of laser welding: Laser welding is to radiate a high-intensity laser beam to the metal surface. Through the interaction between the laser and the metal, the metal absorbs the laser and converts it into heat, which can melt and crystallize the metal to form a weld. Figure 1 shows the evolution phase of the melting process at different radiant power densities [2]. There are two mechanisms for laser welding:
1, heat conduction welding
When the laser is irradiated on the surface of the material, part of the laser is reflected, part is absorbed by the material, and the light energy is converted into heat energy to be heated and melted. The heat of the surface layer of the material continues to be transmitted to the depth of the material in a heat conduction manner, and finally the two weldments are welded. Together.
2, laser deep penetration welding
When a laser beam with a relatively high power density is irradiated onto the surface of the material, the material absorbs light energy into heat energy, and the material is heated and melted to vaporize, generating a large amount of metal vapor, and the molten metal is generated under the reaction force generated when the steam exits the surface. The liquid is squeezed around to form a pit. As the laser continues to illuminate, the pit penetrates deeper. When the laser stops, the melt around the pit flows back, and the two weldments are welded together after cooling and solidification.
These two welding mechanisms are selected according to the actual material properties and welding needs, and different welding mechanisms are obtained by adjusting the welding process parameters of the laser. The most basic difference between the two methods is that the surface of the former is kept closed, while the latter is penetrated by the laser beam into holes. Conductive welding has less disturbance to the system because the radiation of the laser beam does not penetrate the material to be welded. Therefore, the weld is not easily invaded by gas during the conduction welding process, and the continuous closing of the small hole can cause the pores during deep fusion welding. Conductive and deep-welding methods can also be converted to each other during the same welding process. The transition from conduction to small hole mode depends on the peak laser energy density and laser pulse duration applied to the workpiece. The time dependence of the laser energy density can cause the laser welding to change from one welding method to another during the laser-material interaction, that is, the weld can be formed in the conduction mode during the interaction process, and then Then turn into a small hole way.
1. Laser welded weld shape
For high-power deep-welding, due to the molten metal at the weld pool, a deep-through circular cavity is formed due to the instantaneous vaporization of the material, and the metal around the small hole is continuously melted as the laser beam and the workpiece move relative to each other. Flowing, sealing, solidifying to form a continuous weld, the weld shape is deep and narrow, that is, it has a large penetration ratio, and the aspect ratio can be up to 5:1 and up to 10 when welding high-power devices. :1[2]. Figure 2 shows the comparison of the weld cross-section shapes of the four welding methods on 316 stainless steel and DUCOLW30 steel. The conclusions of the comparison are as follows: (1) The main advantages of laser welding and electron beam welding are similar to those of TIG and plasma welding: welding The slit is narrow, the penetration is deep, the weld is parallel on both sides, and the heat affected zone is small; (2) TIG and plasma welding have less investment, and have been widely used for many years with more experience; (3) Laser welding and electron beam welding in terms of high productivity The advantage is much greater. However, electron beam welding must be carried out in a vacuum chamber or in a partial vacuum. Can also be in the air, but the penetration ability is worse than laser welding; (4) laser welding and electron beam welding, the weld is narrow and the heat affected zone is small, so the deformation is minimal.
2. Microstructure and properties of laser welded welds
When using high-power laser beam welding, due to its high energy density, the workpiece to be welded is subjected to thermal cycling of rapid heating and cooling, so that the weld and heat-affected zone is extremely narrow, and its hardness is much higher than that of the parent metal [3]. Therefore, the plasticity of this area is relatively low. In order to reduce the hardness of the joint area, corresponding process measures such as pre-weld preheating and post-weld tempering should be taken. Laser tempering is a new process that uses a non-focused, low-energy-density beam to perform multiple scans of the weld bead after laser welding to reduce the weld hardness. The microstructure and hardness of the laser weld metal and heat affected zone are determined by the chemical composition and cooling rate. In laser welding, current welding processes generally do not require filler metal. In this case, the microstructure and hardness of the weld are mainly determined by the chemical composition of the steel sheet and the laser irradiation conditions. Laser welding with filler wire can select the welding wire of any alloy composition as the best weld transition alloy, so as to ensure the best performance of the joint of the two base metals [4]. It is possible to weld heterogeneous or same metal materials with high melting point, high thermal conductivity and physical properties [5], and to obtain welds with no pollution and less impurities. The laser welding heating speed is fast, and the welding molten pool is rapidly cooled, which is quite different from the ordinary conventional welding in the metallographic structure.
Second, the application field of laser welding
1. Manufacturing applications
Tailired Bland Laser Welding technology has been widely used in the manufacture of foreign cars [6]. According to statistics, in 2000, more than 100 laser tailoring production lines for blanking blanks were produced worldwide, with an annual output of tailor-welded blanks for car components. 70 million pieces and continue to grow at a higher rate. Domestically introduced models such as Passat, Buick, and Audi also use some cut blank structures. Japan used CO2 laser welding instead of flash butt welding to join the steel rolling coils in the steel industry. In the study of ultra-thin plate welding, such as foils with a thickness of less than 100 microns, it is impossible to weld, but through a special output power waveform. The success of YAG laser welding shows the broad future of laser welding. Japan has also successfully developed YAG laser welding for the maintenance of steam generator thin tubes in nuclear reactors for the first time [6]. In China, Su Baorong also carried out gear laser welding technology [7].
2. Powder metallurgy field
With the continuous development of science and technology, many industrial technologies have special requirements for materials, and materials manufactured by the smelting method cannot meet the needs. Due to the special properties and manufacturing advantages of powder metallurgy materials, traditional smelting materials are being replaced in some fields such as automobile, aircraft and tool cutting tools. With the development of powder metallurgy materials, its connection with other parts. It has become increasingly prominent, which limits the application of powder metallurgy materials [8]. In the early 1980s, laser welding entered the field of powder metallurgy material processing with its unique advantages, opening up new prospects for the application of powder metallurgy materials, such as welding diamonds commonly used in the bonding of powder metallurgy materials. The strength is low, and the heat-affected zone width is particularly incapable of adapting to high temperature and high strength requirements, causing the solder to melt and fall off. Laser welding can improve the welding strength and high temperature resistance [9-10].
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