At present, the application of laser technology in manufacturing industry is the research focus of all countries. With the development of industry, the need for high efficiency, environmental protection and automation, the application of laser technology has rapidly spread to many areas of manufacturing industry. On this basis, laser welding technology will become one of the important aspects of laser application. Laser welding is an important part of the application of laser processing technology. It is also the most attractive and promising welding technology in the 21st century. With the development of industrial manufacturing, efficient, agile and environmentally friendly processing technology will be favored. Laser welding can realize deep penetration welding, rapid welding and other difficult forms of welding technology in the process of welding, especially laser welding equipment collocation is flexible, real-time on-line detection technology is mature, which enables it to achieve high automation in mass production. At present, a large number of laser welding production lines have been put into industrial production. Practice has proved that laser welding has a wide range of applications in the processing industry. Basically, the traditional welding technology can be used in the field, laser welding can be competent, and welding quality is higher, processing efficiency is faster.
Laser welding is an effective welding process by using the radiation energy of laser. Its working principle is to stimulate the laser active medium (such as the mixture of CO2 and other gases, YAG Yttrium aluminium garnet crystal, etc.) to oscillate back and forth in the resonant cavity so as to form the stimulated radiation beam. When the beam contacts the workpiece, its energy is absorbed by the workpiece. Welding can be carried out when the temperature reaches the melting point of the material.
Laser welding can be divided into heat conduction welding and deep fusion welding. The heat of the former 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. Due to the small area of the focused spot generated by the laser, the heat affected zone around the weld is much smaller than the ordinary welding process, and the laser welding generally does not need to be filled with metal, so the surface of the weld 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, and the joint plasticity is relatively low. At present, joint quality can be improved by dual focus technology or composite welding technology.
Laser welding attracts so much attention because of its unique advantages: high quality joint strength and large aspect ratio can be obtained by laser welding, and the welding speed is faster. Because laser welding does not need vacuum environment, remote control and automatic production can be realized through lens and optical fiber. Laser has high power density, good welding effect for refractory materials such as titanium and quartz, and can weld different performance materials. Of course, there are also shortcomings in laser welding: the price of laser and welding system accessories is relatively expensive, so the initial investment and maintenance costs are higher than traditional welding process, and the economic benefits are poor. The conversion efficiency of laser welding is generally low (usually 5%~30%) because of the low absorption rate of solid materials to laser, especially after the appearance of plasma (plasma can absorb laser). Because of the small focus spot of laser welding, the equipment precision of workpiece joints is required to be higher, and a small deviation of equipment will result in a large processing error. With the widespread application of laser welding and the commercialization of laser production, the price of laser equipment has dropped significantly. The development of high-power laser and the development and application of new hybrid welding methods have improved the low conversion efficiency of laser welding. It is believed that in the near future, laser welding will gradually replace the traditional welding processes (such as arc welding and resistance welding) and become the main mode of industrial welding.
Most of the existing lasers are CO2 lasers, YAG lasers and semiconductor lasers, especially CO2 lasers and Nd:YAG lasers. Because of their early development and perfect technology, they have been widely used in various fields. Among them, CO2 laser belongs to gas laser, its laser active medium is a mixture of carbonate, nitrogen, helium and other gases, the wavelength of emission light is 10.6 um, generally working in a continuous mode, the electro-optical conversion efficiency is 10%-30%, its output power is generally 0.5-50 kW; Nd:YAG laser belongs to solid-state laser, its laser active medium is Nd-doped yttrium-aluminium-stone. Garnet (YAG) crystal has a wavelength of 1.06 micron and can be output by pulse and continuous mode. Its 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 Nd:YAG lasers are much lower than that of CO2 lasers, due to the shorter wavelength of the emitted light, the material has a higher absorption rate for its beam, and materials 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. UV and IR detectors are used to detect the UV and IR radiation of the plasma, and the UV and IR radiation signals are successfully linked to the quality of the weld, enabling on-line inspection of the welding process.
At present, the research results at home and abroad show that the signals that can be detected in real time by the laser welding process include acoustic signals, optical signals, electrical signals, ultraviolet/infrared radiation signals and ultrasonic signals.