Roughness as parameter of cut quality during C[O.sub.2] laser cutting of high alloy steel for the special purpose.

Cekic, Ahmet ; Kulenovic, Malik ; Begic, Derzija 等

1. INTRODUCTION

In the world today, there is rapidly introducing of new technologies into manufacturing systems from more reasons such as: saving materials, precision of operation, short processing time, high accuracy of products and high flexibility. Nowadays laser technologies are increasingly used in manufacturing and automotive industries. In most of the use of laser technologies is the laser beam cutting. The laser beam cutting can be successfully used for the cutting different materials such as: metals, polymers, ceramics, composites, high alloy steels and others (Avanish & Vinod, 2008). The use of high alloy steels in car bodywork has spread in recent years. The AISI institute is forecasting a dramatic increase in the use of high alloy steels in the car industry (Lamikiz et al., 2005). In the literature, there are no more experimental studies of the laser cutting of high alloy steels. The exothermal reactions of iron and other alloy elements are complex and high alloy steels have a high content of alloy elements, which can lead to differences with regard to the cutting of common steels.

Among various types of lasers used for machining in industries, C[O.sub.2] (wavelength 10.6 um) and Nd: YAG (wavelength 1.06 [micro]m) lasers are most established in the automotive industries. In order to use advantages of this technology, especially by aspect of saving materials is necessary to evaluate the optimum process parameters for each kind materials, thickness and others process conditions to the achieving desired cut quality with maximum productivity. The setting of process parameters mainly depends of the material characteristics (density, specific heat, absorptivity, thermal conductivity, chemical composite, surface quality and etc) and thickness of materials. The most important process parameters are, such as: laser power input, mode of operation (pulsed mode and continuous wave), pulse frequency, pulse duration, kind and pressure of assistant gas, focus position, diameter and kind of nozzle. These parameters are usually determined so those insure the desired cut quality, and the productivity is less relevant. The setting of process parameters which would satisfy both the desired cut quality and high productivity is specific problem (Rajaram et al., 2003). The quality of the C[O.sub.2] laser cutting was evaluated by a standard process: measuring of surface roughness, kerf width and kerf deviation along the length of cut, kerf taper, size of heat affected zone, micro-hardness and metallographic and etc. Previous work in the literature elucidated the effects of oxidation dynamics and laser beam velocity on the quality of the laser cut (Avanish & Vinod, 2008). It suggested that there is an optimum range of cutting speeds for a given material, thickness and laser power. Surface roughness is an effective parameter representing the quality of machined surface. Some researchers (Nagels et al., 2007) shown that surface roughness value reduces on increasing cutting speed and frequency, and decreasing the laser power and gas pressure. Also nitrogen gives better surface finish than oxygen. In spite of an increase in available laser power during the last decade, the laser cutting process faces difficulties in cutting thicker parts. The main problems are losing the process stability and the deterioration of the cut quality.

2. EXPERIMENTAL PROCEDURE

The preliminary experiments were carried out with a laser system consisting of a 2800 W continuous C[O.sub.2] laser (Bystronic), a three axes CNC controlled table with work volume 2.0 m x 1.5 m x 0.5 m. Experiments were carried out on two different materials with three thickness of 5, 10, and 15 mm. The chemical compositions of the examined materials are given in Table 1.

Geometry of the sample shapes are given in Fig. 1. Sample I (EN 10083-3) was previous prepared for laser cutting by using the saw and milling machine. It was hot rolled square rod with dimensions of 140x140mm. Sample II (GX40CrNiSi22-10) is given from tube, produced by centrifugal casting.

The beam profile is nearly Gaussian ([TEM.sub.00]) with beam quality k [approximately equal to] 0.75. Others process parameters were kept constant throughout the experimentation: the laser power of 2520 W, the focal length of lens 127 mm, the assistant nitrogen gas pressure of 14 bar, nozzle diameter of 2.0 mm, focus position under bottom side of sample. Namely, previous experiments are shown that was not possibility to realize the cutting of tested materials when the oxygen used as assistant gas. In the previous experiments, the optimal value of assistant gas pressure and the focus position are obtained. In the experiments where the laser beam has been focused on the surface of the sheet and even above the sheet, the results show poor quality in cutting areas.

[FIGURE 1 OMITTED]

3. ROUGHNESS MEASUREMENT

Surface roughness as parameter of cut quality is analyzed. Roughness parameter [R.sub.a]--mean deviation is measured. The surface roughness is measured by using the device Perthometer Concept. DIN 2310 standard prescribed the measuring of surface roughness on the exactly determined depth. It depends of the material thickness. Because of the specific process and the obtaining of valid results, the measurements are performed at the five parallel lines which are uniform sorted along the depth of cut, Fig.2. Namely, the high of unevenness increase from the top to the bottom of cut along the length of cut. The exchange of conditions of the intersection laser beam and material is reason for that, as and the exchange of the local rate between the thermal conductivity and the coefficient of absorption (it depends of the temeparture exchange). Fig. 2 shows the increasing of surface roughness along the depth of cut in the depending of the process parameters as well as the kind and the thickness of material. During executing of experiments, the maximum deviation of 6.6 [micro]m is obtained.

[FIGURE 2 OMITTED]

In the aim of the obtaining of the valid results, the visual selections of samples are made. The samples with large dross were not taken in consediration.

4. RESULTS AND DISCUSSION

The experimental values of parameter roughness Ra for different cutting speeds during laser processing of GX40CrNiSi22--10 steel is given in Fig. 3 and in Fig.4 for EN 10083 -3 steel.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

It is observed that, the surface roughness increases by increasing the material thickness at keeping constant others process parameters during C[O.sub.2] laser cutting of both examined high alloy steels.

5. CONCLUSION

In this paper are carried out the experimental investigations the effect of the cutting speed and the thickness of material as well as the kind and the pressure of the assistant gas on the surface roughness during the C[O.sub.2] laser cutting two different materials.

Following conclusions can be drawn on the basis of results obtained:

* Once the correct value has been established, small variations in pressure do not determine the quality of the cut. Thus, assisting gas pressure can be maintained for different values of power, cutting speed and focal position as well as kind and thickness of tested materials.

* Maximum of the possibility cutting speed is depends of the kind and thickness of material for the given laser system.

Above this maximum there is no possibility to obtain the accepted level of cut quality.

* Better quality of cut and the large range of cutting speed is possibility during laser cutting of thinner materials.

In the future work, the influential process parameters during laser cutting of high alloy steels for the special purpose will be considered. These process parameters will be optimized with consideration of multi-performances characteristics of the cut.

6. REFERENCES

Al-Sulaiman, F.A.; Yilbas, B.S. & Ahsan M. (2006). C[O.sub.2] laser cutting of a carbon/carbon multi-lamelled plain-weave structure. Journal of Material Processing Technology, 173, (April 2006) page numbers (345-351), ISSN: 0924-0136

Avanish, K. D. & Vinod, Y. (2008). Laser beam machining-A Review. International Journal of Machine Tools and Manufacture, 48, (May 2008) page numbers (608-628), ISSN: 0890-6955

Lamikiz, A.; Lopez de Lacalle, L. N.; Sanchez, J. A.; Pozo, D.; Etayo.J. M. & Lopez. J. M. (2005). C[O.sub.2] laser cutting of advanced high strength steels (AHSS). Applied Surface Science, 242 (April 2005) page numbers (362-368), ISSN: 0169-4332

Nagels, E.; Dufloub, J. R. & Humbeeck Van, J. (2007). The influence of sulphur content on the quality of laser. Journal of Materials Processing Technology, 194, (November, 2007) page numbers (159-162), ISSN 0924-0136

Rajaram, N.; Sheikh-Ahmad, J. & Cheraghi, S. H. (2003). C[O.sub.2] laser cut quality of 4130 steel. International Journal of Machine Tools and Manufacture, 43, (March 2003) page numbers (351-358), ISSN: 0890-6955

Tab. 1. Chemical compositions of examined materials

Chemical composition of GX40CrNiSi22-10 steel

 C Cr Ni Si Mn S P

Min 0,40 18,0 12,0 1,80 ... 0,20 ...
Max 0,70 21,0 14,0 2,20 1,50 0,40 0,06

Chemical composition of EN 10083--3 steel

 C Cr Ni Si Mn S P

Min 0,42 1,10 0,05 0,17 0,50 ... ...
Max 0,49 1,40 0,25 0,24 0,80 0,05 0,05