Study On Three-dimentional Laser Cutting And Lap-welding Of Auto Body-in-white Parts

Laser processing technology has been widely used in auto body-in-white and auto body parts manufacturing, especially the laser cutting and welding technology has gradually become standard process of auto body-in-white manufacturing in overseas. Although laser welding and cutting technology have grown rapidly in mainland, there still exists a great gap in the application of car manufacturing compared with foreign countries. In current situation, the complete laser processing equipments, production line and processing technology are mainly introduced for welding and cutting of tailor-welded blanks and auto body components from overseas. These hindered the promotion of laser technology application and reduced the level of the laser industry. In order to enhance our advanced technology and capabilities of auto manufacturing and accelerate the development of laser processing technology in auto manufacturing industry at a deeper level and a broader scope, first, the basic theory and technology of laser cutting and welding of auto body parts should be enhanced, and then the research of complete laser processing equipments should be accelerated.In this paper, the research situation at home and abroad was reviewed. From the application of laser cutting and welding in auto body, the key issues were detected as shown the following:in the 3D laser cutting process of auto body, the cutting head in narrow spatial location easily collides with the workpiece, the workpiece corner is easy to be over-burnt, and the workpiece deformation causes over-travel alarm etc. In the laser welding process of auto body, the welding head in narrow spatial location also easily interferes with the workpiece or the fixture, two parts of auto body can't be welded to connect as a result of large gap due to workpiece deformation or not enough clamping force at overlap location, the laser length and space of welding seam at auto body welding position are determined with no description, and a variety of new materials used for auto body make more difficult to weld and so on. In order to find practical solutions to these problems, a comprehensive research from theory and experiment was carried out in this paper.First, with the door and side panel as an example, the structure and process were analyzed taking flexibility and security into consideration, the overall layout of laser cutting and welding production unit of auto body was designed and the main types of equipments were chosen. The problems during the 3D laser cutting process, such as collision, deformation of the workpiece, the corner over-burn were analyzed and solved. The selection principle of laser form (pulse or CW laser) was proposed. The influence of 3D laser cutting process parameters such as laser power, cutting speed, assistant gas pressure, pulse frequency, the beam incident angle and the focus position on cutting quality was studied by testing. And the cutting quality from the terms of the kerf width, cutting roughness, cutting waviness and heat affected zone width and so on was evaluated. Through analysis of test results, the rules of process parameters on cutting quality were summarized.Secondly, the characteristics of laser welding pool were numerical calculated, and the influence of process parameters on welding pool shape and temperature distribution was analyzed. The high-power fiber laser and CO2 laser welding experiments on automotive dissimilar materials such as high-strength galvanized steels, galvanized steels and common cold rolled automotive plates were carried out, and then their welding performance were compared and analyzed. The high-power fiber laser lap-welding joint forming properties and mechanical properties of heterogeneous materials were studied, so as to control and optimize the craft quality of fiber laser welding of auto body. The results show that when applying fiber laser and CO2 laser to weld respectively, the morphology and properties of welding joint have some differences, and the welding performance is much better under fiber laser welding condition. In addition, when welding the lap joint of dissimilar plates under the same parameters, the relative position between two plates has a certain effect on the welding performance.Then, for the structural characteristics of laser Welding of auto body-in-white and the difficulties of lap welding of galvanized sheets, the influence of incident angle of laser beam, the plates space and the welding layout on the mechanical properties of the welded joint were analyzed. And the critical incident angle and the limit welding plates space in different lap joints and other effecting factors condition were studied. The results showed that plates thickness, weld width have a certain impact on the critical beam incident angle and the limit plates space. When plates thickness is large, the beam incident angle should not be too large, but the plates space may be increased appropriately. And when the weld width is narrow, the beam incident angle can be properly increased, but the plates space can't be too large. In addition, the plates space and the beam incident angle are mutually influenced. When plates space is small, the beam incident angle should not be too big. When the size of workpiece and the total length of welding seam are constant, the surface morphology and the mechanical properties of the lap joint arranged according the weld seam layout principle "short length and multi section" are good.Finally, the performance of laser welding and resistance spot welding were compared. For the same plate's lap joint, designing different length of laser seam and through comparing the mechanical properties of weld joints with different seam length, one considerable welding seam was obtained. The influence of laser welding seam length on the mechanical properties of the lap joint was summarized. And the influence such as different thickness, different plates, different weld pool width, and relative position of heterogeneous plates and so on also were analyzed. The results showed that when the plate thickness and weld pool width are not the same, the laser weld length is different. For the lap joint of heterogeneous plates with similar thickness and material, their relative position has little impact on determining the shortest length of welding seam. In addition, based on the door parts of a vehicle, a comparison test between laser welding and resistance welding was carried out. Results found that under laser welding condition, the welding speed and the overall static stiffness of the door were higher, and the laser welding process reflected a higher flexibility. Simultaneously, results also showed that compared with the resistance spot welding joint, the microstructure of laser welding joint is more even and smaller, the hardness is higher, and the corrosion resistance is better.