The car body shell is a complex structural component made of thin steel plates. A car body is assembled by hundreds of sheet metal stamping parts through positioning welding, gas shielded welding, brazing and bonding. The welding of automobile body sheet metal parts is carried out according to a certain sequence, which is obviously procedural. The process flow of car body production: the car body parts are transported to the station by the lifting conveyor roller bed. After they are in place, the car body side wall, roof beam and main structure are positioned and clamped by the car body welding positioning fixture, and then the robot performs positioning welding on the car body with a positioning welding gun. After welding, the clamp is opened, and the car body is transported to the next station by the lifting roller bed. Automatic lines are mostly used for welding in the car body section, and the production process is shown in the figure.

The car body section generally includes the left and right side sub-assembly lines, the front and rear sub-assembly lines and the main line. The inner panel sub-assembly of the side enclosure production line is provided to the inner panel sub-assembly of the side enclosure production line. The main line receives the floor sub-assembly from the floor section, and pre-installs it with the side enclosure sub-assembly, front and rear enclosure sub-assembly, the upper cover plate of the instrument panel and the outer panel of the rear closure plate by robots. The pre-installation process is shown in Figure 2-3.

In pre-installation, the left and right side walls are mounted on the bottom plate with buckles, and then transported to the manual loading station through the roller bed to assemble the roof beam. Finally, the preassembled car body frame is transported to the most critical station of the main line-the car body assembly station. The assembly parts of each part are accurately clamped and positioned by jigs at the assembly station, and the welding gun robot forms a car body skeleton by welding twice.

There are many welding spots in the body section, which can not be completed on one production line, so the body section generally contains many main lines. Main line 1 completes welding of car body frame, main line 2 repairs welding of car body frame and positioning welding and repairing welding of rear closing plate outer plate and load-bearing beam, and main line 3 completes welding of roof on car body, in which the overlapping area of side wall and bottom plate and the welding area of load-bearing beam can not be reached by welding gun, so CO2 gas shielded welding is generally used for welding. CO2 gas shielded welding uses the heat of arc generated between welding wire and workpiece to melt welding wire and workpiece to form weld seam, and CO2 gas is used as protection to isolate arc and molten pool from air. CO2 gas shielded welding is mainly used in two aspects in automobile body manufacturing process: one is the position where positioning welding cannot be performed, and the other is the position where the welding strength is higher than that of positioning welding.

In order to monitor the geometric accuracy of the car body in real time and comprehensively, online measurement is usually used in the main line. On-line measurement is to use laser technology to measure the geometry of the car body, compare the measured data with the standard data and feed back the comparison results. Thereby realizing 100% monitoring of the geometric accuracy of the car body. On-line measurement can be divided into robot type and fixed side head type. Robot type is suitable for collinear multi-vehicle type measurement, and fixed side head type has high accuracy, but it is generally only for one vehicle type.

Main line 3 mainly completes the welding of the roof. Some roofs are welded by resistance positioning welding, but the advanced technology is laser brazing. Brazing is a welding method, in which brazing filler metal with lower melting point than the metal to be welded is melted, then the gap of weld seam is filled by capillary action, and the brazing filler metal is diffused with wire to realize connection. The main difference between brazing and fusion welding is that only brazing filler metal melts, while the welded metal does not melt. Liquid brazing filler metal enters the gap between two weldments by wetting and capillary action, and atomic bonding is achieved by mutual diffusion between liquid brazing filler metal and solid metal.

In order to ensure the beautiful appearance of laser brazing weld, the main line III will set up a special automatic grinding station for weld, which can simultaneously carry out the repair welding of positioning welding and laser brazing grinding on the front and rear ends of the roof. A robot equipped with grinding blades on the left and right sides grinds the weld at the set track and speed. After grinding, the car body is sent to the next station for automatic inspection of the weld quality, and the inspection results are displayed on the display screen in the delivery area of the main line 3 car body. Workers can judge whether the weld needs to be repaired through the inspection results. At the same time, the workers will also visually inspect the exterior surface and laser brazing weld of the car body. For the car body with unqualified exterior surface, the workers will mark the car body to remind the employees of the adjustment line to repair it. If the laser brazing weld is unqualified, it must be repaired by manual TIG welding in the delivery area of the line body. There are serious defects in the exterior surface or laser brazing weld, and when the repair time is long, the vehicle body needs to be repaired off line. The qualified car body will be transported to the next production line.