Q235B steel is one of the most common steel materials in China, with certain strength, elongation, and good toughness and castability, easy to stamping and welding. Medium plate Q235B steel is widely used in engineering machinery, bridge construction, high-voltage transmission towers, shipbuilding, heavy machinery and large pressure vessels and other industries. In recent years, along with the rapid development of the above industries, the demand for a diversity of changes in these industries, these industries in the parts of many specifications, diverse shapes, some of the higher precision requirements, the traditional processing methods to limit its development. Laser cutting has outstanding advantages such as easy operation, non-contact, high flexibility, high efficiency, energy saving and environmental protection, and is capable of manufacturing high-performance complex components and precision parts, and has been praised as a “universal processing tool” and a “common processing tool for future manufacturing system”, leading the development of advanced manufacturing industry and contributing to the development of industrial intelligentization. It has led the development of advanced manufacturing industry and had a profound impact on the progress of industrial intelligence [1]. The application of laser cutting on sheet metal is very mature, and it has become an important means of sheet metal processing [2]. With the rapid development of fiber laser technology, the laser output power has increased from the original 10kW to the more mature 20kW, which has greatly expanded the application of laser cutting of medium-thickness sheet metal. High-power laser cutting has been widely used in the aerospace, rail transportation, oil pipeline, engineering machinery, fitness equipment and shipbuilding industries. Fiber laser has a high conversion efficiency, high beam quality, high stability, maintenance-free, small size and good heat dissipation and other significant advantages, the domestic high-power fiber laser price is low, stable performance, high cost-effective, accelerated the pace of localization of laser cutting equipment, driving the rapid development of laser cutting technology [3]. In recent years, high-power laser cutting research has become a hot spot again, and most of the laser power is about 10kW. The best process parameters of laser power, cutting speed, auxiliary air pressure, nozzle diameter, nozzle height and defocusing amount were obtained through one-way test, and the microstructure of the material was analyzed after the laser thermal effect. The cutting process of 20mm and 30mm thick Q355 steel plates was investigated using a 10kW fiber laser, and the optimal process parameters of laser power, cutting speed, auxiliary air pressure, nozzle diameter, nozzle height and defocusing amount were obtained by controlling a single variable. However, there are few studies and reports on laser cutting above 10kW, so it is important to study 20kW high power laser cutting to improve cutting efficiency and quality.
The test
The test was conducted by using 20kW laser cutting equipment to cut 25mm thick Q235B steel in square, and the test was carried out by adjusting different process parameters to compare the influence of the main process parameters on the cutting effect. The laser power determines the thickness of the cut metal material, while the cutting speed, the amount of defocus, and the auxiliary gas pressure determine the quality of the cut. Oxygen is commonly used as the auxiliary gas in laser cutting carbon steel tests, however, in practical industrial applications, air is also often used as the auxiliary gas considering the cost of use. Therefore, oxygen and air are used as auxiliary gases in this test.
The auxiliary gas has several roles in the carbon steel cutting process as follows. 1) Used to blow off the molten state of the metal. (2) Oxidation and combustion reactions occur to further release heat. 3) To cool the area that has not reached the ignition temperature of the metal, reducing the heat-affected zone. Different cutting process parameters are used to compare the cutting profile. The requirements are no or little slag on the bottom of the cut carbon steel, small side stripes and the highest possible cutting speed. In the test preset oxygen as an auxiliary gas, the laser power is too high and prone to over-burning, so reduce the laser power appropriately, the laser power is set at 15kW, the diameter of the single spray 1.6mm; air as an auxiliary gas, the laser power is set at 20kW, the diameter of the pressurized nozzle is 5.0mm. 3 Results and Analysis 3.1 Auxiliary gases and air pressure on the cutting quality of the large number of pre-cutting tests, and the cut quality is very good, and the cut quality is very good. After a large number of cutting tests in the previous period, the preset parameters for air pressure cutting test are: laser power 15kW, cutting speed 1.2m/min, +17mm out of focus, oxygen air pressure in 0.5 ~ 1bar (1bar = 105 Pa) within the comparative test; laser power 20kW, cutting speed 2m/min, out of focus -13mm, air pressure in the range of 6 ~ 10bar within the comparative test. Cutting section morphology under different air pressure with different auxiliary gases. The auxiliary gas plays the roles of blowing off slag, combustion and exothermic and cooling the material at the edge of the slit in the cutting process. When the auxiliary gas pressure is 0.8 bar (oxygen) or 10 bar (air), the auxiliary gas pressure is sufficient to blow the slag away from the cutting seam, and the cut surface forms uniform and detailed stripes, with no obvious slag hanging on the bottom of the material and a small heat-affected zone. When the air pressure drops to 0.5 bar for oxygen or 6 or 8 bar for air, the auxiliary air pressure is no longer sufficient to blow the molten material away from the slit, resulting in visible slag on the bottom of the material and thicker stripes on the cut surface, especially when cutting with air, where the stripes are clearly layered.
From the analysis of the above test results, it can be seen that when using high-power laser for carbon steel plate cutting, the auxiliary gas selection of oxygen or air can obtain better cutting results, oxygen gas pressure is low, the cutting surface is more flat and smooth than the air cutting surface. In the test, when the air pressure of oxygen is 0.8 bar or air pressure is 10 bar, it is more suitable and the cutting effect is better. 3.2 Influence of cutting speed on cutting quality After a large number of cutting tests in the early stage, the preset parameters for cutting speed test are: laser power 15kW, defocus +17mm, oxygen air pressure 0.8bar, cutting speed 0.8-1.4m/min; laser power 20kW, defocus -13mm, air pressure 10bar, cutting speed 1.6-2.4m/min. 2.4m/min; laser power 20kW, defocus -13mm, air pressure 10bar, cutting speed 1.6~2.4m/min. Different auxiliary gases cut the section shape at different cutting speeds. The cutting speed determines the cutting efficiency, the faster the speed, the higher the efficiency. When the cutting speed of oxygen is 0.8m/min or air cutting speed is 1.6m/min, due to the low cutting speed, the heat input of the material is too high, and the melting amount of the material increases significantly, thus forming over-melting, and the stripe on the cut surface is obvious and uneven. When the oxygen cutting speed is 1.2m/min or air cutting speed is 2m/min, the cutting speed matches with the input material heat, and the cut surface is smooth and even. When the oxygen cutting speed is 1.4m/min or air cutting speed is 2.4m/min, the cutting surface is smooth and flat due to the faster cutting speed. The laser energy is not sufficient to melt the material completely, and the auxiliary gas can only partially blow away the molten metal, resulting in sloping, coarse streaks on the cut surface and a tendency for slag to hang up at the bottom. If the cutting speed is further increased, the material cannot be cut off. From the analysis of the above test results, it can be seen that the use of high-power laser cutting of carbon steel plate cutting speed, high efficiency, when the test oxygen cutting speed of 1.2m/min or air cutting speed of 2m/min is more appropriate, the cutting effect is better; speed is too slow, it is easy to form a melting; too fast, it is easy to hang slag or rough cutting surface. 3.3 Influence of defocusing quantity on cutting quality On the basis of the previous test, the preset parameters for defocusing quantity cutting test are: laser power 15kW, cutting speed 1.2m/min, oxygen air pressure 0.8bar, using the defocusing quantity of ＋13～＋17mm cutting test; laser power 20kW, cutting speed 2m/min, air pressure 10bar, using the defocusing quantity of -10～-16mm cutting test; laser power 20kW, cutting speed 2m/min, air pressure 10bar, using the defocusing quantity of -10～-16mm cutting test. -The laser power is 20kW, the cutting speed is 2m/min, the air gas is 10bar, and the defocusing amount is 10～-16mm for cutting test. The cutting section morphology of different auxiliary gases under different defocusing amount is shown in Fig. 6 and Fig. 7. The defocusing amount plays a role in changing the power density reaching the surface of the material to be processed during the cutting process. When the auxiliary gas is oxygen and the defocusing amount decreases from +17mm to +13mm, the spot diameter on the plate becomes smaller and smaller, the power density becomes higher and higher, and the cutting slit becomes narrower and narrower.
With the narrowing of the slit, the auxiliary gas into the interior of the plate decreases, when the amount of defocus exceeds a certain value, resulting in the lower part of the molten metal can not be blown off in time, thus making the cut surface streaks in the lower part of the more obvious. When the auxiliary gas is air, the amount of defocus increased from -10mm to -16mm, the plate focus gradually move to the bottom, the upper part of the plate spot diameter is getting bigger and bigger, the lower part of the spot diameter is getting smaller and smaller, when the focus moves to the plate thickness of about half the position, the upper and lower parts of the material on the laser absorbed energy is equivalent, the formation of the cut surface shows a fine uniformity. Formation of the cut surface presents small uniform stripes; when the focus continues to move down, the upper part of the spot diameter is getting bigger and bigger, cutting pattern is more and more obvious. From the analysis of the above test results, it can be seen that the use of high-power laser carbon steel plate cutting, oxygen cutting with positive defocus, air cutting with negative defocus; when the oxygen cutting defocus amount of +15mm, air cutting defocus amount of -13mm, the cutting effect obtained is better!
