Influencing factors and defect causes of laser bra

Analyze the influencing factors of laser brazing quality and the causes of defects

Shanghai Volkswagen Co., Ltd. first introduced laser welding technology from Germany in passat-b5 project in 1998, and then this technology was widely used in polo compact car project in 2001 and Touran MPV car project in 2004. As a kind of laser welding technology, laser brazing has also been applied in these three models. From the rear cover of passat-b5 to the roof of polo, to the full body coverage of Touran MPV, the length of laser brazing is constantly increasing. However, in the actual use of Shanghai Volkswagen, the causes and effects of laser brazing quality defects. 2. The universal material testing machine body and dynamometer installation do not need special foundation response factors have been concerned by many technicians. The following is a comprehensive analysis of the factors affecting the quality of laser brazing and the causes of defects based on the experience of many actual users of laser brazing technology in recent years

factors affecting the quality of laser brazing

in order to realize the processing process of "welding with laser", many parameters need to work together. Compared with other processing methods, the tolerance range of each influential parameter in laser brazing is very small. Such demanding requirements are not only determined by the processing requirements of laser brazing itself, such as the need for small heat affected zone and fast brazing speed, but also because it is a complex three-dimensional weld processing and has high surface quality requirements. 1. The composition of the experimental platform

from Figure 1, we find some of the most important influential parameters, including laser equipment specific parameters: laser power, focusing position, focus size, processing speed; Unique parameters of brazing wire: brazing wire feed speed, brazing wire preheating current, angle of attack, welding wire prestress; Geometric parameters: welding plate clearance, required weld section, surface quality; Other effects: welding plate materials, shielding gas, robot guidance accuracy

Figure 1 welding process diagram of laser brazing

1 Influence of welding plate material

only a small part of the laser energy focused on the workpiece is absorbed by the welding plate material, and most of it is reflected. The relationship between the laser wavelength and the absorptivity of different plate materials is shown in Figure 2 (the lines given are obtained at room temperature). The absorptivity of laser waves generated by solid-state lasers is about 35% for steel materials and 4% for copper materials. This explains why it is necessary to preheat steel materials with current. With the increase of temperature, the absorption rate also increases

Figure 2 absorptivity of materials to different wavelengths of laser at room temperature

2 The angle of attack

forms a resistance through the current circuit at the transition between the welding wire and the component, and the heat of heating the welding wire is generated by this resistance. The welding wire must be in contact with the parts before being burned by the laser, so the welding wire has already touched the workpiece before moving to the laser focus. At this position, the laser energy is not enough to melt the welding wire, and then the welding wire is transported to the focus along the brazing weld, where it is melted. Thus, the welding wire can keep in contact with the focus center all the time. In order to bend the welding wire, it is necessary to maintain a large angle of attack at all weld positions. An angle of attack greater than 40 ° has been proved to be reliable

3. Preheating current of brazing wire

if spatter occurs in the process of laser brazing, it indicates that the heating current has reached the maximum limit and the welding wire melts too early. If the quality of the weld is poor, it means that the heating current is within the lower limit, that is, the weld surface is getting rougher and rougher. In principle, large current should be used as much as possible during processing

4. Focus

the laser energy must be concentrated on the processing point by focusing, so part of the laser energy is absorbed by the brazing wire and part by the workpiece. Practical experience shows that about 50% of the energy in the focus is used to heat the surrounding area. In addition, the maximum machining gap size should also be considered. When the diameter of the focus is too small, the weld is reddish, the surface is very uneven, there is a strong splash tendency, and there is no starting trace of the weld on the back of the machined part. Too much laser energy is concentrated on the welding wire, so the solder becomes overheated. At the same time, the side of the workpiece is not heated enough, so the solder is not easy to flow into the gap of the workpiece

5. Welding speed

in addition to the above factors, the laser brazing process is also affected by the moving speed of the welding mirror group and the wire feed speed. Therefore, the necessary amount of solder is determined by the required weld area and the weld length of the workpiece to be processed. The supply amount of solder per unit time is determined by the feed speed of the welding wire and the sectional area of the welding wire. These parameter values must be consistent with the moving speed of the welding mirror group, so as to ensure the necessary amount of solder. In practical use, such as Polo roof brazing, it is more in line with our requirements to obtain a higher welding wire feed speed than the moving speed of the welding mirror group. Because this will produce a small amount of excess solder, reduce the risk of pores caused by uneven welding wire speed, and also ensure the weld cross-sectional area that must be achieved

since the laser energy of laser brazing is limited, the moving speed and thermal deformation temperature of the welding mirror group are also limited compared with that of PA and PVC. In the case of high speed, the robot running deviation relative to the theoretical trajectory will also become larger; The faster the operation speed is, the more sensitive the processing process is to external interference

6. Other

because the brazing focus is relatively small compared with the diameter of the welding wire and the width of the joint, the accuracy of each component in the equipment is of great importance. Specifically, it includes the repetition accuracy of the clamping mechanism, the guidance accuracy of the robot, the accuracy of the mounting points of the components on the robot and the tolerance of each component, which should be considered in the realization of the industrialization and utilization of more than 70 kinds of key new materials

types and causes of laser brazing defects

due to the complexity of the laser brazing process and many influencing factors, when there is a decline in processing quality, it cannot be explained by one reason in most cases, but the beginning and end of the processing track are usually considered to be the most critical part. In order to meet the quality requirements when using laser brazing, each link of the processing process must be adjusted very carefully

1. Classification according to the size of welding defect area

in actual production, the size of defect affected area can be divided into different types:

(1) persistent defect: it exists in the whole process of laser brazing. In this regard, it is not that the whole section of the weld has defects, but that the defects appear repeatedly in the weld with an unknown law

(2) local defects: local defects occur repeatedly in the same weld position, and its influence range is limited

(3) problem prone areas: some areas of the weld, such as the beginning and end of the weld, as well as the inclined area on the plate, are particularly prone to problems

2. Classification according to the surface characteristics of defects

(1) micro pores: when the diameter of pores is less than 0.2mm, they are micro pores

(2) pores: the maximum diameter of normal pores (larger than micro pores) shall not exceed 1.0mm

(3) void/weld interruption: if the diameter of the pore is greater than 1.0mm, it is called void

(4) fusion weld: there is no solder in the weld, and the weld looks like a laser fusion weld

(5) poor solder connection: the brazing wire is not connected on the side of the workpiece. At the position where the weld is connected, the weld looks "scattered in strands"

(6) single side connection of solder: solder is connected with only one side

(7) sausage phenomenon: the processed parts are not connected, and the solder extends and accumulates straightly at the weld

(8) irregular weld: weld collapse or bulge

(9) scaly accumulation: the weld surface is not smooth and appears very rough

(10) beginning/end of weld: insufficient or excessive weld filling will occur at the edge of the workpiece, or unmelted welding wire residues are found on the track

3. Causes of quality defects

(1) wrong working distance: TCP (tool coordinate point) is wrong or the programming is wrong

(2) the position of the focus side is wrong: either the position of TCP perpendicular to the beam axis is wrong, or the programming is wrong

(3) brazing wire calibration: the welding wire does not pass through the focus center

(4) brazing wire temperature: the preheating temperature of welding wire is wrong

(5) brazing wire material: the alloy composition of brazing wire material has changed

(6) laser power: dirty protective glass or aging arc lamp in the laser will reduce the laser power

(7) air leakage: the flow of protective gas decreases or there is air leakage in the pipeline

(8) gap dimension: the gap dimension between components is changing

(9) brazing wire feed speed: the feed speed of welding wire is not constant or inconsistent with the moving speed of welding mirror group

(10) robot speed: the feed speed given by the program is wrong or the robot speed fluctuates

(11) time control: it may be the problem of calculation time, and the switching points of laser and welding wire feeding mechanism may not be consistent with the processing process

4. The corresponding table of welding defects and their causes (such as table) table comparison table of welding defects causes

this table can be used as an auxiliary tool for relevant technicians to consult when finding the causes of welding quality defects

laser brazing technology has been used in Shanghai Volkswagen Automobile Co., Ltd. for eight years. We hope to summarize the quality defects of laser brazing through this paper, so as to lay a solid foundation for continuously improving quality and process level in the future, and also hope to provide a reference basis for other peers who are using or will use laser brazing technology. (end)