Application of Kunshan laser welding in automobile body manufacturing

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 welding of unequal thickness plate and body welding. Kunshan laser welding is mainly used for the welding of body frame structure, such as the welding of top cover and side wall

Kunshan laser welding is used in automobile, which can reduce the weight of car body and achieve the purpose of fuel saving; The assembly accuracy of the car body is improved, and the stiffness of the car body is increased by 30%, so as to improve the safety of the car body; Reduce the stamping and assembly costs in the process of car body manufacturing, reduce the number of body parts, and improve the degree of body integration.

1. Structure of laser welding

Kunshan laser welding application 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 lot of energy on a very small welding point, so 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.

(1) Laser source: there are two laser sources for laser welding: CO2 gas laser source and YAG solid 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 used in deep penetration welding.

(2) Photoconductive and focusing system: the photoconductive focusing system is composed of a circular polarizer, a beam expander, a reflector or an optical fiber and a focusing mirror, which can change the polarization state, direction, transmit and focus of the beam. The condition of these optical parts has an extremely important influence on the quality of laser welding. Under the action of high-power laser, the performance of optical components, especially the lens, will deteriorate, resulting in the decrease of transmittance and thermal lens effect. At the same time, the surface pollution will also increase the transmission loss.

(3) Welding robot: due to the difference of laser fusion welding, laser MIG hybrid welding technology and welding joint form, the assembly accuracy requirements of welding joint are also different. Laser welding of lap weld and laser brazing of fillet weld can use ordinary welding robot. For the laser brazing and laser welding of butt weld, it is necessary to use the welding robot with hinged arm, which is different from the conventional robot, and the automatic seam tracking and correction system is usually designed.

(4) Welding fixture: it can ensure the accurate positioning of the plate or assembly connected during laser welding, ensure the weld clearance and prevent welding deformation, so as to improve the quality of laser welded joint.

(5) Laser welding control system: the control system mainly includes video monitoring system of welding process, automatic seam tracking system and correction system of robot, wire feeding control system, etc. For different laser welding methods, the composition of the control system is the same. Laser welding does not need wire feeding system, automatic seam tracking system and trajectory correction system.

2. Advantages of Kunshan laser welding

The main characteristics of Kunshan laser welding are as follows:

(1) Small heat input, large weld depth width ratio and small heat affected zone lead to small shrinkage and deformation of the workpiece, so no post weld correction is needed;

(2) Weld strength is high, welding speed is fast, weld is narrow and surface condition is good, so cleaning after welding is avoided;

(3) Welding consistency, good stability, generally without filler metal and flux, and can realize partial dissimilar material welding;

(4) The beam is easy to control, the welding position is accurate, and it is easy to realize automation;

(5) Non contact processing, no need for pressure and surface treatment on the workpiece;

(6) Small solder joint, high energy density, suitable for high speed machining;

(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. It is especially suitable for processing high melting point, high hardness and special materials;

(8) No processing noise, no pollution to the environment;

(9) It can realize long-distance and multi-channel simultaneous or time-sharing welding for parts that are difficult to reach by common methods through optical fiber;

(10) It is easy to change the laser output focal length and solder joint position;

(11) It's easy to carry on automata and robots

(12) Compared with other welding methods, laser welding has higher initial investment;

(13) The assembly accuracy of the welded parts is high, and the accuracy of beam control is also required relatively;

(14) Due to the large spatter, the weld of penetration welding is coarser than that of brazing, but its strength is much higher than that of ordinary spot welding.

3. Application of Kunshan laser welding

There are two ways of laser welding used in car body: one is fusion welding, which does not need filling material, and the laser directly acts on the surface of the workpiece for welding; The other is filler welding, commonly known as brazing, which is mainly used in the welding of automobile roof.

3.1 fusion welding

According to the energy density of laser beam, fusion welding can be divided into heat conduction welding and penetration welding.

(1) Heat conduction welding

When the laser converges at a point, a high temperature (related to the energy density) will be produced. When the temperature reaches 1490 ℃, the steel plate will melt. Heat conduction welding is used to weld the steel plate. The process is as follows: firstly, the surface of the workpiece is heated to the melting point by the laser, and a hemispherical molten pool will be formed after the metal is melted, The radius and depth of the molten pool increase slowly. When the absorbed laser energy and the heat diffused around the molten pool reach a balance, the molten pool will not expand.

When the laser beam moves along the predetermined trajectory, the molten pool moves with it. The metal in front of the molten pool melts continuously, and the metal in the rear cools, thus forming a weld. The advantages of heat transfer welding are that the weld is smooth and less spatter, the speed is 1 ~ 3m / min, and the ratio of weld depth to weld width is less than 1. This welding method is mostly used for plate welding.

(2) Penetration welding

In the car body in white, because the welded steel plate has 2-4 layers, the thickness can reach 4mm, so the depth of the weld has higher requirements. At this time, the heat transfer welding can not meet the requirements of the process, which requires another welding method - penetration welding. Penetration welding has the characteristics of fast speed and deeper penetration, and its requirements for laser energy density are far higher than heat transfer welding.

When the laser acts on the surface of the workpiece, the metal vaporizes rapidly (the steel plate will vaporize into steam at 2590 ℃), diffuses out of the molten pool in the form of steam, and forms a steam channel. The absorption rate of the metal to the laser energy can be increased to 75% by multiple reflection of the laser in the channel, which is called "keyhole effect", When the steam pressure is not enough to diffuse out of the molten pool, the molten pool will not deepen, forming a stable welding state.

At the position where the molten pool passes, the molten metal flows around the steam channel, which makes the upper and lower plates fuse together. After the metal cools, a high-strength weld is formed. Compared with heat conduction welding, penetration welding has the advantages of deeper welding depth and faster welding speed. For 4mm thick low carbon steel plate, the welding speed can reach 5m / min; The disadvantage is that the spatter produced by the rapid vaporization of metal is easy to damage the workpiece and processing equipment.

3.2 filler welding (brazing)

Another kind of welding method is filling welding. This method does not melt the workpiece itself, but uses the thermal effect of laser to melt the welding wire and fill it between the two workpieces to be welded. Its advantage is that the weld is beautiful and the thermal deformation is small. This method is widely used in the welding of automobile roof.

The laser brazing technology of automobile top cover is the first laser technology applied to the body processing. Its principle is to use the laser to melt the welding wire (generally copper silicon alloy) and fill the gap between the top cover and the side wall workpiece, which not only plays the role of connection, but also can be sealed. The process of penetration welding is different from that of brazing. Laser penetration welding is carried out on two layers of steel plate without welding wire filling. The robot drives the lens to weld directly according to the predetermined trajectory without guiding device.

The laser power of penetration welding is 4kw, which is much higher than that of brazing. Because the workpiece needs to be melted during welding, the power ramp should be set at both ends of the weld, that is, at the beginning of welding, the power increases linearly from 1kW to 4kw within 30ms, and at the end of welding, the power decreases from 4kw to 1kW within 30ms.

The advantage of this method is to avoid welding through the steel plate and forming small holes during arc starting and arc ending, thus affecting the welding quality. The laser source adopts YAG continuous laser generator, and the maximum output can reach 4kw. Because the melting point of welding wire is lower than that of steel plate, the output power range of laser is (1.8 ~ 2) kW, which ensures that the top cover and side wall will not produce thermal deformation while melting the welding wire.

The laser is transmitted by flexible optical cable, which can separate the laser generator from the welding station and avoid damage to the equipment. According to the diameter of the welding wire, the optical lens is used to change the spot size of the laser focusing at the welding end to match each other. For example, if the diameter of the welding wire is 1.0 mm, the spot diameter of the laser focusing is adjusted to 1.2 mm to make the laser act on the welding wire to the maximum extent.

In the welding process, the robot drives the adaptive welding lens, the rotation axis drives the telescopic arm to rotate in the horizontal direction, and the telescopic arm itself can move in the vertical direction. Generally, the laser welding seam of car body is 42m long, which ensures the accuracy of overall dimension, strength and rigidity of car body. The roof with laser brazing has only one fine and smooth weld, which is directly used as the outer surface to increase the streamline visual effect of the car body, while the roof with traditional spot welding process has two rubber sealing strips.

Complete set of laser automatic welding equipment for car body in white roof

Nowadays, the application of laser welding in automobile body production has become a trend. Using laser welding can not only reduce the weight of the body, improve the assembly accuracy of the body, but also greatly enhance the strength of the body, so as to provide higher security for users while enjoying comfort.