The application of laser technology in the manufacturing industry is the focus of research in various countries. With the need for high efficiency, environmental protection and automation in industrial development, the application of laser technology has rapidly spread to many fields of manufacturing. On this basis, the laser welding process will become one of the important aspects of laser applications.
Laser welding is an important part of the application of laser processing technology, and it is the most eye-catching and promising welding technology in the 21st century. As early as the end of the last century, European and American countries have fully applied laser welding to industrial production. While accelerating the research and development of laser welding technology, China has gradually established a development system combining production, learning and research. And there have been major breakthroughs in individual areas. With the development of industrial manufacturing, efficient, agile and environmentally friendly processing technology will be favored. Laser welding with its high-energy beam focusing method can achieve deep welding, rapid welding and other welding processes that are difficult to achieve in the welding process, especially the laser welding equipment is flexible, real-time online detection technology is mature, enabling it to Highly automated in mass production, a large number of laser welding production lines have been put into industrial production. Practice has proved that laser welding is widely used in the processing industry. Basically, the field of traditional welding technology can be used, laser welding can be competent, and the welding quality is higher and the processing efficiency is faster.
Laser welding is a process that uses the radiant energy of a laser to achieve effective welding. Its working principle is: to excite a laser active medium (such as a mixture of CO2 and other gases, YAG yttrium aluminum garnet crystal, etc.) in a specific way. The reciprocating oscillation in the cavity forms a stimulated radiation beam. When the beam is in contact with the workpiece, its energy is absorbed by the workpiece, and welding can be performed when the temperature reaches the melting point of the material.
Laser welding mode. Laser welding can be divided into heat conduction welding and deep fusion welding. The former heat is diffused to the inside of the workpiece through heat transfer, and only the surface of the weld is melted. The inside of the workpiece is not completely penetrated, and basically no vaporization occurs, and it is mostly used for low-speed thin wall. The welding of the material; the latter not only completely penetrates the material, but also vaporizes the material to form a large amount of plasma. Due to the large heat, a keyhole phenomenon occurs at the front end of the molten pool. Deep-fusion welding can thoroughly penetrate the workpiece, and has high input energy and fast welding speed. It is the most widely used laser welding mode.
Weld shape and microstructure properties of laser welding. Because the laser produces a small spot size area, the heat affected zone around the weld is much smaller than the ordinary welding process, and the laser welding generally does not need to fill the metal, so the weld surface is continuous and uniform, and the appearance is beautiful. Surface defects such as pores and cracks are very suitable for applications where the weld profile is critical. Although the area of focus is relatively small, the energy density of the laser beam is large (typically 103 to 108 W/cm2). During the welding process, the metal is heated and cooled very quickly. The temperature gradient around the molten pool is relatively large, so that the joint strength is often higher than that of the base metal. On the contrary, the joint plasticity is relatively low. At present, joint quality can be improved by dual focus technology or composite welding technology.
The advantages and disadvantages of laser welding, laser welding is so highly valued, it has its own unique advantages:
1.Laser welding can achieve high quality joint strength and large aspect ratio, and the welding speed is faster.
2.Because laser welding does not require a vacuum environment, remote control and automated production can be achieved through lenses and optical fibers.
3.Laser has a large power density, good welding effect on difficult-to-weld materials such as titanium, quartz, etc., and can weld different performance materials.
Of course, laser welding also has shortcomings:
1.Laser and welding system parts are more expensive, so the initial investment and maintenance costs are higher than traditional welding processes, and the economic benefits are poor.
2.Due to the low absorption rate of the laser by solid materials, especially after the plasma appears (the plasma has an absorption effect on the laser), the conversion efficiency of laser welding is generally low (usually 5% to 30%).
3.Due to the small focus spot of laser welding, the precision of the equipment for the workpiece joint is high, and the small equipment deviation will cause large machining error.
With the popularization of laser welding and the commercial production of lasers, the price of laser equipment has dropped significantly. The development of high-power lasers and the development and application of new hybrid welding methods have also improved the shortcomings of laser welding conversion efficiency. It is believed that in the near future, laser welding will gradually replace traditional welding processes (such as arc welding and resistance welding). Become the main way of industrial welding.
Most of the existing lasers are CO2 lasers, YAG lasers and semiconductor lasers, especially CO2 lasers and Nd:YAG lasers. Due to the early development and relatively complete technology, they have been widely used in various fields.Among them, the CO2 laser belongs to a gas laser, and the laser active medium is a mixed gas of carbonic acid gas, nitrogen gas, helium gas, etc., and the emitted light has a wavelength of 10.6 μm, generally works in a continuous manner, and the electro-optical conversion efficiency is 10% to 30%. The output power is generally 0.5~50 kW; the Nd:YAG laser is a solid-state laser, and its laser active medium is a neodymium-aluminum-garnet (YAG) crystal doped with neodymium (Nd), and the emitted light has a wavelength of 1.06 μm. It can be output in two ways: pulse and continuous. The electro-optical conversion efficiency is 3%~10%, and its output power is mainly 0.1~5 kW. Although the output power and electro-optical conversion efficiency of the Nd:YAG laser are much lower than that of the CO2 laser, due to the shorter wavelength of the emitted light, the material has a higher absorption rate for the beam, and the material with high reflectivity (such as aluminum alloy) It has good welding effect with copper alloy, etc. Especially Nd: YAG laser can be transmitted by optical fiber, which can be well matched with robot processing system, which is beneficial to remote control and automatic production, so it plays an important role in laser welding. status.
It is well known that the emergence of plasma is the biggest problem facing laser welding. The high energy density of the laser not only melts the metal, but also vaporizes the metal (when the energy density exceeds 106 W/cm2). When the vaporized metal contacts the laser beam in the air, ionization occurs. A large amount of plasma This is the result. The plasma not only absorbs and scatters the laser beam, but also refracts the laser, causing the spot to be out of focus, which seriously affects the laser welding effect. Therefore, reducing the occurrence of plasma is the most effective way to optimize laser welding.
No matter which welding process is used, waste products will be produced. At present, the control of product quality in industrial manufacturing is more based on real-time monitoring technology than post-weld processing technology. Therefore, real-time monitoring of the welding process has become the focus of automation in laser welding.