Nowadays, more and more auto body parts use hot-formed parts. The production of hot-formed parts needs to go through various stages of mold forming, and the mold needs to be trimmed to form metal into parts. Because the hot-formed parts are very hard, it is almost impossible to repair them in the mold, and they need to be machined to form the correct shape, which not only increases the production cost, but also reduces the production efficiency. Since the laser beam does not exert any force on the workpiece and does not contact the cutting tool, this means that the workpiece has no mechanical deformation, no tool wear, nor the problem of tool replacement, and the cutting material does not need to consider its hardness, that is, the laser cutting ability is not affected by the hardness of the material being cut, and any hardness material can be cut. At present, laser cutting is still the most effective processing method for hole cutting and trimming of high-strength steel hot-formed parts.
 

In the process of laser cutting, for the cutting of complex surfaces, the problem of cutting burr will inevitably occur. The cutting burr of parts has a great impact on the product quality. The main purpose of this study is to analyze the causes of cutting burr and propose feasible solutions to ensure the quality and safety performance of parts.
After a lot of field practice and analysis, we summarized the following points.

1.Cutting corner height and focus influence

Generally, the focus position should be on the surface of the workpiece or slightly lower, but the requirements for different materials are different. When cutting carbon steel, the cutting quality is better when the focus is on the surface of the plate; When cutting stainless steel, the focus should be about 1/2 of the thickness of the plate.
After testing, when the power and speed parameters are the same, changing the height and focus will affect the size of corner burr. However, in the actual 3D workpiece cutting process, we found that the current equipment is generally auto-focusing, using capacitive height sensor, and the focus and height are basically in a stable state during the production process, so its impact on cutting burr is basically eliminated.

2.Impact of cutting nozzle and cutting air pressure

During laser cutting, the laser beam should pass through the gas flow field generated by the nozzle. The special structure of the supersonic nozzle can convert almost all the pressure of the auxiliary gas into kinetic energy, blow off the slag, and achieve a more perfect laser cutting surface. Changing the cutting nozzle type can improve the cutting burr when the cutting parameters are unchanged.

3.Influence of deviation between digital analog and actual workpiece

The production process of hot-formed parts is completed in the following three steps.

(1) Production of blanking sheet. The blanking sheet is the sheet with specific profile produced on the uncoiling and blanking line, and then transported to the hot forming line. The robot sends the blanking sheet to the coding machine through the vacuum sucker for coding, and then puts it on the conveyor line for transmission to the heating furnace.
(2) The metallographic structure is austenitized.
(3) Hot stamping.

In the actual production process of parts, affected by factors such as hot press rebound and feeding, the workpiece has different states, and there is certain error between the digital and analog programming and the actual workpiece, which is the cause of poor corner quality and burns.

After field production test, the same cutting parameters and the same batch of parts were measured for many times, and it was found that the impact on burr was small.

4.Influence between workpiece fixture positioning

The positioning between the workpiece and the fixture greatly affects the accuracy of the cutting part. During the cutting process, the cutting gas with high pressure participates in the cutting and blows the workpiece, which may be one of the reasons for the corner burr.

With the same cutting parameters, the same batch of parts can be measured several times, and the pressure clamp and backup face can be added, which can improve the stability of production dimension, but has little impact on the improvement of burr.

5.Effect of cutting power and corner speed

The size of laser power has a considerable impact on cutting speed, cutting seam width, cutting thickness and cutting quality. The required power is determined according to the material characteristics and cutting mechanism. After testing, it is found that changing the corner speed will affect the size of corner burr at the same speed.

In the laser cutting process, the cutting speed has a considerable impact on the quality of the cutting material. The ideal cutting speed will make the cutting surface appear relatively stable lines, and there will be no slag at the lower part of the material. When the cutting speed is relatively slow, the action time of the laser energy in the cutting seam is prolonged, resulting in the increase of the cutting seam width. When the cutting speed is too slow, the action time of the laser beam is too long, the difference between the upper and lower cutting seam of the workpiece will be large, the cutting quality will be reduced, and the production efficiency will be greatly reduced. With the increase of cutting speed, the action time of laser beam energy on the workpiece becomes shorter, which makes the heat diffusion and heat conduction effects smaller, and thus the width of the cut seam correspondingly smaller. When the speed is too fast, the cut workpiece will not cut through due to insufficient cutting heat input. This phenomenon belongs to incomplete cutting, and the molten material cannot be blown off in time. These molten substances will cause the cut seam to be welded again.

To sum up, we found that in actual production, changes in parameters such as power and speed have a great impact on burr. The 3D workpiece is not always cut in a straight line. There will be a lot of corners, bulges or changes in direction during the cutting process. The actual running speed of the machine tool is constantly changing. To this end, we collected real-time path speed, laser power, frequency, duty ratio and other parameter data, matched the best cutting parameters according to their changes, found their corresponding functional relationship, and realized that the output variable will knead the time-varying parameters into the motion of the machine tool, so as to achieve the efficiency not reduced, the cutting quality increased, or the efficiency slightly decreased, and the cutting quality improved. Write the formula into the system program and activate the function directly through the human-computer interface.

About HGTECH: HGTECH is the pioneer and leader of laser industrial application in China, and the authoritative provider of global laser processing solutions. We have comprehensively arranged laser intelligent machine, measurement and automation production lines, and smart factory construction to provide overall solutions for intelligent manufacturing.