Design type and quality evaluation standard of las

Since the 1980s, laser technology has been used in the field of automobile body manufacturing, mainly using laser to weld car bodies. There are two main types of lasers used in laser welding equipment: nd:yag solid-state lasers. The main advantage is that the generated beam can be transmitted through optical fibers; CO2 laser can work continuously and output high power

in the development of new laser welding technology, laser technology has experienced the development process of unequal thickness plate laser splicing technology, body laser welding technology and laser hybrid welding technology in the process of body manufacturing

compared with single laser fusion welding technology, laser hybrid welding technology has significant advantages: higher stability of arc welding during high-speed welding, greater penetration, welding ability of larger gap, better toughness of weld, adjustment of weld microstructure through welding wire, etc. With the development of laser welding technology, the design type of weld and the evaluation of weld standards are also constantly changing and improving, especially in recent years, the wide application of galvanized sheet, three-layer sheet and ultra-high strength steel plate has put forward higher requirements for the design type of joint, and the evaluation of weld standards is also continuously refined and optimized, which not only provides a guarantee for the manufacture of high-quality welded car bodies, It also provides a theoretical basis for weld repair

at present, FAW Volkswagen adopts laser cutting, laser fusion welding, laser hybrid welding and other advanced manufacturing technologies to varying degrees in the body manufacturing process of almost all brand models, such as Audi C6, golf A6, Bora, Sagitar, maiteng, model X and so on (see Table 1). Due to different welding positions, the types and evaluation standards of welded joints and the welding defects of welds are also different, resulting in certain differences in weld repair standards

take the FAW Volkswagen maiteng car body as an example, the total length of the laser weld on the car body is up to 42m, and the joint types of the weld involve butt joints during laser brazing of the top cover, lap type I joints on the upper edge of the front and rear windshield, lap fillet welds at the rear flume, and fillet welds at the front end. Due to the different types of welds, the welding methods, parameters, evaluation standards and post weld repair standards during laser welding are different (as shown in Figure 1)

Figure 1 top view of maiteng body laser weld

laser welding principle and equipment

laser welding uses laser as welding heat source and robot as motion system. The advantage of laser heat source is that it has extremely high heating capacity and can concentrate a large amount of energy on a small welding point. Therefore, it has the characteristics of high energy density, concentrated heating, fast welding speed and small welding deformation, which can realize the rapid connection of thin plates

when the power density on the laser spot is large enough (greater than 106w/cm2), the metal is heated rapidly under the irradiation of the laser, its surface temperature rises to the boiling point in a very short time, and the metal vaporizes. The metal vapor leaves the surface of the metal molten pool at a certain speed, producing an additional stress reaction on the molten metal, causing it to sag downward, creating a small bump under the laser spot. With the heating process, the laser can be directly shot into the bottom of the pit, forming a slender "hole". When the recoil pressure of metal vapor is balanced with the surface tension and gravity of liquid metal, the small hole will not continue to go deep. When the spot density is large, the small hole will penetrate the whole plate thickness to form a deep penetration weld. The metal melts in front of the small hole, bypasses the small hole and flows to the rear, and solidifies again to form the weld (as shown in Figure 2), so the laser weld has the characteristics of deep and narrow

Figure 2 principle of laser fusion welding

laser hybrid welding combines two arcs. The two arcs interact and support each other. The formed molten pool is larger than that of laser fusion welding (as shown in Figure 3), so the bridging ability is better and larger welding assembly gap is allowed. At the same time, the weld pool of laser hybrid welding is smaller than that of MIG welding

Figure 3 Comparison of laser welding, MIG laser hybrid welding and MIG welding joint formation

FAW Volkswagen laser welding joint type

maiteng's 42m long laser weld ensures the accuracy of its overall size, body strength and rigidity. Maiteng's laser welding is mainly in the connection of the top cover and side wall of the car body, the splicing of unequal thickness plates of the bottom plate, the splicing of unequal thickness plates of the door inner plate, the connection between the bottom plate and the side wall, the rear wall plate, the front and rear wind windows and the door frame

the commonly used welding materials for laser welding include cold-rolled steel plate, hot-rolled steel plate with less than 0.02% carbon equivalent, hot-dip galvanized steel plate and electro galvanized steel plate. Generally, the material of steel plate must meet the corresponding technical standards to achieve high-quality connection. To achieve high-quality connection mainly depends on three elements: design, processing and materials. Figure 4 shows the possibility of welding and the relationship between obtaining high-quality welded joints between joint design and processing and manufacturing. In addition to the general principles of design, such as the treatment of design end face, necessary pre welding preparation, description of welding joint properties, welding process, clamping technology, accessibility of laser beam, machining accuracy and post welding repair all play a corresponding role

Figure 4 Relationship between weldability and influencing factors

in order to obtain a good laser weld, the following principles must be followed: the joint is formed through the interconnection of the welding area, and the design requirements of the workpiece must be considered for each joint type. The form of weld finally depends on the design form of welding end face and the scope and type of processing preparation. For the use of metal coated steel plates, it should be noted that the connection area must be designed with an exhaust channel, and the gap design is generally selected between 0.05 ~ 0.2mm according to the thickness of the coating, or it can be solved by designing a certain plate angle. In addition, the clamping design and pretreatment of the workpiece must ensure smooth exhaust

quality evaluation standard of laser welded joints

1 The common internal quality evaluation standard of automobile body laser welded joints

the quality defects of welded joints are generally divided into surface defects and weld internal defects. The surface defects are generally visible, and the internal defects cannot be found visually. They must be observed and found by corresponding technical means, such as microscopic metallographic analysis

2. Common quality evaluation standards for the external surface of automobile body laser welding joints

the weld joint shall meet the following requirements when being repaired: pay attention not to affect the functional dimension changes and design requirements of the assembly, and the dimensional stability, better surface accuracy and better joint corrosion resistance of the body assembly can be guaranteed after repair. In addition, welding is not allowed in the glued area during repair, so as not to cause unnecessary welding deformation, which will affect the international market and the quality of the repaired joint. When the flange edge is short (less than or equal to 8mm) or the area cannot be drilled for repair, fillet weld shall be used at the lap joint, or MIG or plasma brazing shall be used at the butt weld. The length of the repaired weld must be at least equal to the length of the weld defect location. When using arc brazing (MIG, wig, plasma) and other means for repair, the weldability of the base metal must be analyzed in advance, such as analyzing its yield strength and fracture strength through drawing experiment, and analyzing the microstructure and morphology of its weld and weld and heat affected zone through metallographic means. In some special cases where it is impossible to repair, such as the body of the assembly deformed due to heat during connection, according to the standard, using this repair method that makes pingkuang group hope to wait until more government support for base metal heating is not allowed, and the cold bonding method or other cold processing methods must be considered