Laser welding is an advanced modern manufacturing technology. It is irradiated to the metal surface by a high-intensity laser beam. Through the interaction of the laser and the metal, the metal absorbs the laser and converts it into heat, which can melt and crystallize the metal to form a weld. Methods. In the automotive industry, laser welding is mainly used for the welding of body frame structures, such as the welding of the top cover and the side body. High-strength steel plates are mostly used for automotive body safety parts. Laser welding technology is used to reduce the joint width between workpiece joints, which reduces the amount of sheet metal used and the rigidity of the car body, and can meet the requirements of “lightweight” of automobile bodies. .

In actual laser welding, special clamping and equipment technology is required. The accuracy of this equipment is complementary to the quality of laser welding. The LCK-12×25 CNC precision laser processing machine used in the experimental research is a new type. Laser cutting equipment. In recent years, although a lot of work has been done on laser welding applications of high-strength steel at home and abroad, further research and exploration are still needed. The deep-melting welding process of 1.5mm thick high-strength steel plate was carried out, and the better welding effect was obtained. The performance evaluation of welded joints was also studied, and a set of effective control of welding plate quality was given. Process parameters.

1 Test conditions and methods 1.1 Test conditions The welding material is a high-strength steel (DOGAL800DP) produced in Sweden, and the workpiece size is 100 mm × 30 mm × 1.5 mm.

1.2 Test method The welded joint adopts the butt welding method, and the fixture is used to ensure the assembly precision of the welded part. Because there are many parameters affecting the high-strength steel laser welding process, if the comprehensive test design method of single-factor rotation is adopted, the test quantity is too large, and the interaction effect between various factors is not considered in the test design. Therefore, this study adopted an orthogonal test method to optimize the best welding procedure specifications for high-strength steel. Figure 1 is a schematic diagram showing the overall layout of a laser welded sheet test apparatus.

2 The main influencing factors of laser welding affect the laser welding factors such as laser power, welding speed, focus position, welding angle, shielding gas flow and plasma control, and preventing welding spatter.

2.1 Laser energy The energy density of the laser is closely related to the wavelength of the beam and the coupling ability between the beam and the material. It is a key parameter in laser welding. The premise of laser deep-fusion welding is that the focused laser focal spot has a sufficiently high power density. When laser deep-fusion welding, the penetration depth is directly related to the beam power density and is a function of the incident beam power and the beam diameter. The penetration depth h is approximately proportional to the 0.7 power of the laser power P, ie: h∞P 0.7 2.2 Welding speed The welding speed mainly affects the shape of the weld pool and the weld. As the welding speed increases, the flow pattern and size of the weld pool will Will change. Increasing the speed will make the weld penetration shallower, even if the workpiece is not completely penetrated; however, if the speed is too low, the material will be excessively melted, the weld will be widened, the surface will be sunken, and the workpiece will be welded through in severe cases. Therefore, there is a suitable range of welding speed for a particular material of a certain laser power and thickness, and the maximum penetration can be obtained at the corresponding speed value.

2.3 Focus Position The best focus position is determined by the geometry, type, orientation, clearance, misalignment and required weld strength, penetration, bead width and material offset. The maximum penetration is produced in the form of a butt joint, where the focus position is the best focus position.

The defocus amount refers to the distance between the optimal spot size and the workpiece. The defocus amount not only affects the size of the spot diameter of the workpiece surface, but also affects the incident direction of the beam, thus having a larger weld shape, penetration depth and cross-sectional area. influences. In order to maintain sufficient power density during soldering, the focus position is critical, and for most laser welding applications, the focus position is typically set at approximately 1/4 of the desired penetration depth below the surface of the workpiece.

2.4 Welding angle When the beam is incident at a certain inclination angle, it will increase the spot area of ​​the workpiece surface and reduce the power density. Even if the welding speed is constant, the deep melting will be reduced. Therefore, the beam should be prevented from being inclined when welding. If it is unavoidable, the angle of inclination should be minimized, and the minimum inclination should ensure that the focused beam does not ablate adjacent workpieces or fixtures.

2.5 Protective gas composition and flow The laser welding process uses inert gas to protect the molten pool. In most applications, gases such as helium, argon and nitrogen are often used. If there is too much plasma, the penetration becomes shallow and the surface of the weld pool becomes wider. The size of the plasma cloud varies with the shielding gas used, and the helium gas is the smallest, followed by nitrogen, which is the largest when using argon gas, that is, Ar→N 2→CO 2→He. Nitrogen is the most economical and most commonly used as a shielding gas. This test uses nitrogen as a shielding gas.

In terms of gas flow, if the gas flow is too small to drive off the plasma, the protection effect on the weld and the lens is not good; the gas flow is too large, taking away a lot of heat energy, and increasing the turbulence, the molten pool The agitation is intensified and it is easy to cause defects such as weld porosity.

2.6 Welding Splash and its prevention of splashing during the welding process will seriously affect the welding quality. As weld spatter and other debris accumulate on the focusing mirror and lens surface, the lens will absorb energy to cause thermal deformation, reduce weld quality, and reduce penetration.

To prevent welding spatter, an air knife can be installed on the focusing device. When the air knife is opened, a gas layer is formed in front of the focusing mirror to prevent splashing. In addition, a protective metal can be applied to the surface of the copper mirror to prevent welding spatter from harming valuable optical components.

3 Analysis and discussion 3.1 Mechanical properties of the weld test The test was carried out on a WDW-100 microcomputer-controlled electronic universal testing machine with a tensile speed of 0.5 mm/min and a collet spacing of 60 mm. The stress on the tensile test of the weldment should be. It can be seen that the weld weld strength is roughly the same as that of the base metal, and there is no softening phenomenon of the weld. Under the optimized process parameters, the weld strength is even higher than the base metal strength.

3.2 Weld Metallographic and Microhardness Tissue The microstructure and hardness of the laser-welded metal and heat affected zone are determined by the chemical composition and cooling rate. The hardened martensite is obtained in the weld zone and the heat affected zone. The reason for this result is related to the particularity of the laser welding and the properties of the material. The tendency of the joint hardening is generally reflected by the microhardness. The metallographic structure of the weld zone is upper bainite + low carbon martensite, and the metallographic structure of the heat affected zone is feathery upper bainite + low carbon martensite + ferrite.

Under the condition of a load of 400 g and a load time of 5 seconds, the microhardness of the weld was as follows: the heat affected zone was 441 HV and the weld zone was 438 HV. Comparing the microhardness (271 HV) of the base material area, the microhardness of the heat affected zone is about 1.7 times that of the base material zone. The joint microstructure obtained the lath-like low-carbon martensite, and the microstructure of the weld zone was refined, so the joint hardness was greatly improved compared with the base metal zone, and no crack was found in the metallographic observation.

MM-6 type metallographic microscope under different magnifications of metallographic and microhardness microstructure test Figure 3.3 SEM spectrum analysis of welded joints Scanning electron microscopy SEM (JSM-5610LV) and energy spectrometer-OXFORD for welded joints Energy spectrum analysis, the content mainly includes point scanning analysis of the changes and analysis of the weld zone relative to the composition of the parent metal.

It can be seen from the point scan results of the high-strength steel plate laser welded joint that the atomic percentages of the weld zone and the parent metal constituent elements and their elements have changed accordingly. From the weld to the substrate, the distribution of Mn, Cr, Si, S, Al, Zn and Fe elements showed different degrees of fluctuation, indicating that there are some differences in the composition of each phase.

3.4 Weld Sea Salt Corrosion Test and Analysis The test piece was ground with metallographic sandpaper to remove the rust products on the surface to expose the surface of the substrate. The surface of the sample was washed several times with acetone and ultrasonically cleaned in distilled water for 15 minutes. The above surface-treated sample was immersed in a prepared 3% NaC1 solution having a pH of 7.0, and was immersed for one day for seven days, and dried for 6 days.

Through experimental observation, the dry-wet ratio of the high-strength galvanized steel weldment was 6:1. After five weeks of observation, the corrosion of the welded test piece and the base metal was not much different. Even some weldments are better than the base metal in the weeks after the corrosion test. It can be seen that even if the corrosion test is continued, the situation is basically the same, indicating that the high-strength steel weldments have good corrosion resistance. The white corrosion products of the metallographic structure after welding of the test piece and the base material are mainly ZnCl2, Zn(OH)2 or NaC1.

3.5 Analysis of high-strength steel welding defects Porosity and cracks are the most common defects in laser welding. Weld hole is a direct result of deep-fusion welding. Because the weld is deep and narrow, the cooling rate is fast, and the gas generated during the welding process does not necessarily have enough time to escape from the melting zone. For non-penetrating welds, the problem is more serious, and it is easier to have dispersed pores at the root of the weld.

The cause of cracks in the laser welding process is the same as in the conventional welding process. Quenching cracks also limit the laser welding of high carbon steel and high alloy steel. Thermal cracking occurs when the weld is completely solidified and its strength is insufficient to withstand shrinkage stress. Therefore, those materials having a wide crystallization temperature range and having a high carbon, sulfur, and phosphorus content are liable to cause cracks. Through the process optimization, the appropriate welding process parameters are selected, and the welded test pieces are tested to show that there are no pores and cracks in the welded joint. In addition, it is also possible to reduce or even eliminate cracks by preheating, adding wire or adjusting welding parameters.

4 Conclusions (1) In the numerical control laser welding of high-strength steel plate, considering the main factors affecting the quality of laser welding, a set of optimized process parameters were obtained by orthogonal experiment, which was used as the final process parameter range for laser welding in this test.

(2) Through the process summary in actual processing, the tensile strength, metallographic structure, microhardness, composition segregation and weld corrosion test of high-strength steel welded joints show that when laser welding 1.5mm thick high-strength steel The optimum process parameters for ensuring weld formation, strength and quality are: with nitrogen as the shielding gas, the coaxial gas flow rate is 3.0m 3 / h, the laser power is 1300W, the defocusing amount is -0.4mm, and the welding speed is 0.90m. /min, the side blowing airflow is at an angle of 40° to the horizontal direction, and the side blowing air flow rate is 2.1m 3 / h, which can obtain a satisfactory weld seam and ensure the welding quality of the high-strength steel.

(Finish)

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