Impact of cutting parameters on the quality of laser cutting.

Cus, Franc ; Milfelner, Matjaz ; Zuperl, Uros 等

Abstract: The paper discusses the use of the laser system in production and the influence of laser cutting parameters on the quality of manufacture In order to reach high accuracies of laser cutting it is necessary to introduce new machining technologies, to automate the cutting process and to adapt the cutting process to different sizes and shapes of workpieces and to other requirements in machining. The paper describes the influence of various technological parameters on the quality of laser cutting for the [CO.sub.2] laser system. Initially, the cutting parameters (cutting speed, output power of laser beam, frequency, working mode, gas pressure, etc.) are important for the programmer of the laser system and in the second stage for the machine operator.

Key words: Laser cutting, cutting parameters, cutting surface.

1. INTRODUCTION

The laser as a manufacturing tool has proved to be useful also in production mechanical engineering, particularly because of its numerous technological advantages. The advantages of this manufacturing technology include very high quality, possibility of automation of the process and adaptability to different sizes and shapes and to different other requirements in manufacturing.

The starting point for analysis of the laser cutting is always quality of the cut. Therefore it is very important that the cut achieved s properly described with different characteristics of quality. The usual way of action is that in the first stage the obtained quality of cut (cut shape, waviness and/or roughness of the cut surface and presence of burr on the workpiece lower edge) is usually evaluated. Often, those evaluations are not accurate enough; therefore, measuring of the individual characteristics must be performed. For evaluating the quality usually the following geometrical characteristics of the cut are used (top and bottom width, unevenness on the top and bottom edge, height and width of burr on the lower part, measuring of the depth of the heat affected zone (HAZ)).

2. MEASURING OF TEMPERATURE IN LASER CUTTING

Successful control of the laser manufacturing process is closely connected to the familiarization with thermal conditions in the cutting zone and its environment. Therefore, on the basis of the knowledge of the temperature cycles, determined by measuring the temperatures by thermo--couples placed at a certain distance of the cutting front from the travelling source, it is possible to conclude what the conditions in the cutting front are (Rajaram et al., 2003). Due to the narrow laser beam and the small depth of the heat affected zone (HAZ) the temperature measuring is very exacting, since accurate placing of thermo--couples at the selected distance from the cutting front is extremely difficult.

Measurements showed that in case of great temperature gradients a thin layer of molten material in the cutting front and a small thickness of the heat affected zone are achieved, which, however, finally assures better quality of the cut (El-Kurdi et al., 2003).

For evaluation of the critical cutting speed generally the following findings apply:

--In case of small steel plate the thickness from 0,6 to 0,8 mm rather high mean values of temperatures in the cutting front occur, therefore, the critical cutting speeds are between 40 and 50 mm/s.

--For the steel plate thickness 1,0 and 1,5mm, with lower cutting speeds the mean values of temperatures are almost constant and only when the critical cutting speed is exceeded, the temperature swiftly drops. For the said steel plate thicknesses the critical cutting speeds are 30 mm/s and/or 35 mm/s.

3. INFLUENCE OF TECHNOLOGICAL PARAMETERS ON QUALITY OF LASER CUTTING

The paper describes the influence of different technological parameters on the quality of cutting by [CO.sub.2] laser systems. These are the parameters which, in the first stage, are important for the programmer of the laser system and in the second stage for the operator. Knowledge of the said parameters can considerably contribute to the utilization of the laser system (Pietro & Yao, 2003).

Cutting speed; (vL) has a very great influence and is determined in accordance with the laser cutting method itself. If long, straight lines are cut, a high cutting speed is selected; on the other hand, when small holes are cut or very precise cutting is required, a low cutting speed is selected. Of course, the cutting speed depends also on the type of material cut and its thickness (Haferkamp et al., 1998).

--The Laser beam output power (PL) is closely connected to the speed of laser cutting. When the put power is too high for the selected cutting speed, burning of the material will take place; in the opposite case, incomplete cutting and/or the so-called formation of holes in material will occur.

Frequency of electromagnetic waves significantly influences the excitation itself in the laser medium. Higher frequency of laser waves gives greater top output of the laser beam and, accordingly, greater output power fed to the material cut.

Generally, high frequency is used for the so-called high-speed cutting, whereas the low frequency is used for cutting at lower speeds. It means that for cutting small holes and/or details always low frequency of the laser beam is used. Thus, lower output power of the laser beam acts on the material (Steen, 1991).

Assistant laser gases. For laser cutting, usually, oxygen, nitrogen and compressed air are used assistant cutting gases. Oxygen is used to cut softer materials. As it causes oxidation of the surface it is very useful in cutting of thicker materials. With it, usually, the ordinary and zinc--coated steel plates are cut. Also the stainless steel plates can be cut with it, but during cutting an oxidation layer is left on the surface of material, which results in dark edges. For cutting stainless steel plates the so-called clean cut is required in most cases. In this case cutting does not cause oxidation, but the gas used is N2 at 7--8 bar pressure. Compressed air is used for cutting of Al, stainless steel plates and non--metallic materials; in this case the air pressure should be 7-8 bar. This method causes a greater oxidation layer on the surface of material, however, the cost of cutting are significantly lower.

The cutting gas pressure has a very great influence on the quality of laser cutting. It must be properly set according to the material type and thickness and shape of the product cut. For cutting of soft materials the oxygen pressure is about 1 bar, 2 bars for cutting of small holes and 3 bars for cutting of stainless steel plates. For cutting of thick materials the pressure should be 7 bar. Nitrogen is used for cutting of stainless steel plates and the gas pressure varies between 7 and 8 bar. For cutting by air the cutting pressure is between 7 and 8 bar. The air as cutting gas is used for cutting stainless steel and aluminium (Tonshoff et al., 2003).

The distance between nozzle and work piece is the distance between the laser nozzle top and the workpiece surface. Usually, the distance between the nozzle and workpiece is about 1,5 mm, however for cutting of Al and stainless steel plates by high pressure much smaller distances are used, i. e., between about 0,3 and 0,5 mm.

4. OPTIMISATION OF CUTTING PARAMETERS

The model of optimization by genetic algorithms based on natural biological evolution principle was selected for optimization of cutting parameters in laser cutting. If compared with conventional optimization methods, the genetic algorithms are more robust and universal and can be used in all research areas. From the performed measurements of the output and speed of laser cutting for different workpiece materials and thickness and from the analysis of the cut quality (cut shape, cut surface waviness and/or roughness and presence of burr on the workpiece lower edge) the data for optimization of the laser cutting process were obtained.

The focal distance changes depending on the type of lens (5" and 7.5") and depending on the type and thickness of the material cut. The optimization of cutting parameters by genetic algorithms was carried out on the basis of the measurement results and the interaction of the cutting parameters in laser cutting. The optimization can be carried out with one, two or three variables and/or cutting parameters. Here below, a practical example of the laser cutting optimization is presented. The example of laser cutting optimization for material X2CrNiMo17-12-2 for [CO.sub.2] laser cutting is presented. Figure 1 shows the photos of surface of the laser cut with optimized values of the laser cut and with recommended (non-optimized) values of cutting (smaller and higher cutting power) and the changes on the surface itself of the laser cut, taking place in case of deviation from ideal values. The optimization of cutting parameters by genetic algorithms was carried out on the basis of the measurement results and the interaction of the cutting parameters in laser cutting. The optimization can be carried out with one, two or three variables and/or cutting parameters. In order to confirm and/or verify the results obtained from the optimization process, the results had to be verified still in practice. Here below, a practical example of the laser cutting optimization is presented. The example of laser cutting optimization for material X2CrNiMo17-12-2 for [CO.sub.2] laser cutting is presented.

[FIGURE 1 OMITTED]

5. CONCLUSION

The paper describes the influence of various technological parameters on quality of laser cutting by [CO.sub.2] laser system. The cutting parameters (cutting speed, laser beam output power, frequency, mode of operation, gas pressure etc.) affect the utilization of the laser system and reduction of production costs. Presentation of applicability is based on the practical example of optimization from production by genetic algorithm method. Advantages of use of laser systems are saving of material, possibility of machining different materials of different thickness, high quality and productivity and reduction of production costs.

6. REFERENCES

El-Kurdi Z., Ahmed A. & Mathew P. (2003). Monitoring and control of laser cutting. Proceeding of the 17-International Conference on Production Research, pp. 4-7 August, Blacksburg, Virginia, USA.

Haferkamp H., Goede M., Busse A. & Thurk O. (1998). On line quality monitoring during laser beam cutting using a thermographic system. C-ICALEO, pp. 11-1, November, 1998, Orlanda, USA.

Pietro P. & Yao Y.L. (1994). An investigation in characterizing and optimizing laser cutting quality. Int. Journal of Machine Tools Manufacture, Vol. 34, No. 2, pp. 225-243.

Rajaram N., Sheikh-Ahmed J. & Cheraghi S.H. (2003). CO> laser cut quality of 4130 steel. International Journal of Machine Tools & Manufacture, Vol. pp. 43, 351-358.

Steen W.M. (1991). Laser Material Processing. Springer-Verlag London Limited, Germany.

Tonshoff H.K., Ostendorf A., Krai V. & Hillers O. (1999). Process and condition monitoring features incorporated in laser heads, ICALEO, pp. E109-E11815-18, November, San Diego, USA.

Table 1. Measuring system for measuring the temperature

 Recommended Optimized Greater
 Values values values

Workpiece thickness [mm] 3 3 3
Laser output P [W] 350 400 500
Cutting speed [mm/min] 1150 1400 1500
Gas pressure [kPa] 125 150 175
Focal distance of lens f [mm] 63,5 63,5 63,5
Distance of focus from 0 0 0
 workpiece surface [z.sub.f]
 [mm]
Nozzle to workpiece 2 2 2
 distance s [mm]
Nozzle diameter d [mm] 2 2 2