At present, automobile body welding mainly includes resistance spot welding, laser welding, MIG and MAG welding, among which laser welding technology is mainly used for tailor-welded and body welding of unequal thickness plate. Laser welding is mainly used for body frame structure welding, for example, welding of the top and side. The application of laser welding in automobile can reduce the weight of automobile body so as to save fuel; improve the assembly precision of automobile body, increase the rigidity of automobile body by 30%, thus improving the safety of automobile body; reduce the stamping and assembly costs in the process of automobile body manufacturing, reduce the number of automobile body parts, and improve the integration of automobile body. 1. Laser welding frame laser welding uses laser as welding heat source and industrial robot as motion system. The advantage of laser heat source is that it has a very high heating capacity and can concentrate a large amount of energy on very small welding points. Therefore, it has the characteristics of high energy density, concentrated heating, fast welding speed and small welding deformation, and can realize the rapid connection of thin plates.
(1) Laser source: There are mainly two kinds of laser sources for laser welding: CO2 gas laser source and YAG solid-state laser source. The most important performance of laser source is output power and beam quality. Considering these two aspects, CO2 laser source has more advantages than YAG laser source, and is the main laser source for deep penetration welding. (2) photoconductive and focusing system: the photoconductive focusing system consists of circular polarization mirror, beam expander, mirror or optical fiber and focusing mirror, which can change the polarization state, direction, transmission beam and focusing function. The status of these optical components has a very important impact on the quality of laser welding. Under the action of high-power laser, optical components, especially lenses, will deteriorate, resulting in a decrease in transmittance, thermal lens effect, and surface contamination will increase transmission loss. (3) Welding robot: Because of the difference of laser welding, laser-MIG hybrid welding technology and welding joint form, the assembling precision of welding joint is different. Laser welding of lap joints and laser brazing of fillet welds can be done by ordinary welding robots. For the laser brazing and laser welding of butt welds, a kind of welding robot with a winch arm, which is different from the conventional robot, must be adopted. (4) Welding fixture: it can ensure the accurate positioning of the plate or assembly connected by laser welding, ensure the gap between welds, prevent welding deformation, and thus improve the quality of laser welded joints. (5) Laser welding control system: the control system mainly includes the video surveillance system of welding process, the automatic seam tracking system of robot, the correction system, the wire feeding control system and so on. For different laser welding methods, the composition of control system is also the same. Laser welding does not require wire feeding system, automatic seam tracking system and track correction system. 2. The advantages of laser welding are as follows: (1) the heat input is very small, the weld depth-width ratio is large, the heat affected zone is small, resulting in shrinkage and deformation of the workpiece is very small, without post-weld correction; (2) high weld strength, welding speed, weld narrow and generally good surface condition, without post-weld cleaning and other work; (3) welding Welding consistency and stability are generally good without filling metal and flux, and can realize partial welding of dissimilar materials; (4) beam is easy to control, welding positioning is accurate and easy to realize automation; (5) non-contact processing, no pressure on the workpiece and surface treatment; (6) small solder joints, high energy density, suitable for high-speed welding. (7) Short time welding, not only has no thermal effect on the outside world, but also has small thermal deformation and heat affected zone of the material itself, especially suitable for processing high melting point, high hardness, special materials; (8) no processing noise, no pollution to the environment; (9) long distance, difficult to achieve by ordinary methods through optical fiber parts, multi-channel identical. Time-sharing welding; (10) It is easy to change the laser output focal length and solder joint position; (11) It is easy to mount on automatic and robotic devices; (12) Compared with other welding methods, laser welding has a large investment in the early stage; (13) The welded workpiece assembly accuracy is high, and the beam control accuracy is relatively high. Higher requirements; (14) Due to large spatter, the penetration weld is more rough than brazing, but the strength is much higher than ordinary spot welding.
3. The application of laser welding to automobile body can be divided into two ways: one is welding without filler material, and the other is filler welding, which is commonly called brazing, mainly used in automobile roof cover welding. 3.1 according to the energy density of laser beam, fusion welding can be divided into heat conduction welding and penetration welding. (1) Heat conduction welding (HTW) is a kind of heat conduction welding, because the laser converges at a point and produces a very high temperature (related to the energy density). When the temperature reaches 1490 C, the steel plate will melt. The heat conduction welding is carried out by using this heat effect. The process is as follows: firstly, the surface of the workpiece is heated to the melting point by laser, and the metal melts. After melting, a hemispherical molten pool will be formed, and the radius and depth of the molten pool will gradually increase. When the absorbed laser energy reaches a balance with the heat diffused around the molten pool, the molten pool will not expand any more. As the laser beam moves along a predetermined trajectory, the molten pool moves, the metal in front of the molten pool melts and the metal in the rear cools.
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