MODELLING OF PERPENDICULARITY OF CUT IN HIGH POWER CO2 LASER CUTTING OF 5 MM THICK ALUMINIUM ALLOY

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1 Nonconventional Technologies Review Romania, June, Romanian Association of Nonconventional Technologies MODELLING OF PERPENDICULARITY OF CUT IN HIGH POWER CO LASER CUTTING OF 5 MM THICK ALUMINIUM ALLOY Miloš Madić 1, Miroslav Radovanović, Predrag Janković 3 and Srđan Mladenović 4 1 Faculty of Mechanical Engineering in Niš, University of Niš, Serbia, madic@masfak.ni.ac.rs Faculty of Mechanical Engineering in Niš, University of Niš, Serbia, mirado@masfak.ni.ac.rs 3 Faculty of Mechanical Engineering in Niš, University of Niš, Serbia, jae@masfak.ni.ac.rs 4 Faculty of Mechanical Engineering in Niš, University of Niš, Serbia, maki@masfak.ni.ac.rs ABSTRACT: Beside roductivity and costs, the cut quality is the third most imortant criteria in laser cutting. An imortant quality indicator of laser cutting is erendicularity of cut. In this aer, the effects of the laser ower, cutting seed and assist gas ressure on erendicularity of cut in CO laser cutting of 5 mm thick aluminium alloy was studied. In order to develo mathematical relationshi between laser cutting arameters and erendicularity of cut, full factorial design exerimentation, by varying each arameter at three levels, was conducted. Based on the obtained exerimental data, regression based mathematical model was develoed uon which the effects of considered laser cutting arameters on erendicularity of cut were analysed. KEYWORDS: CO laser cutting, aluminium alloy, erendicularity of cut, regression analysis 1. INTRODUCTION After steel, aluminium and its alloys are second most used materials in industry [1]. Such wide alication in automotive, aerosace, transort, marine, food and other industries is due to their favourable ratio of tensile strength and density, corrosion resistance, suitability for surface treatments and other unique roerties. The resence of magnesium as major alloying element in the 5xxx series (from 0.8 to u 6 wt %) leads to good formability and weldability and high corrosion resistance. Aluminium alloy 5083 has fair machinability, very good weldability and cold formability and excellent corrosion resistance esecially to seawater and industrial atmoshere. Tyical alications of aluminium alloy 5083 include welded structures, ressure vessels, marine alications, architectural use, aliances etc. []. In industry, for comlex contour cutting of aluminium and its alloys nonconventional machining technologies are redominantly used, articularly laser cutting and abrasive water jet cutting [1, 3]. In the case of laser cutting, Nd:YAG lasers are commonly used because wavelength of 1.06 µm is better absorbed by most of the reflective materials including aluminium and its alloys. On the other hand, although CO lasers can achieve higher owers at a lower cost [1], the use of CO lasers for cutting of aluminium and its alloys is not common [4], however, from the industrial oint of view, such a ossibility would be very welcome [3]. The use of laser cutting technology for cutting of aluminium and its alloys was investigated by a number of researchers and from different asects. The ioneered work on the use of use of CO lasers for cutting ure aluminium was made by Olsen [5]. Stournaras et al. [1] investigated the effect of laser ower, cutting seed, ulsing frequency and assist gas ressure on cut quality characteristics such as kerf width, surface roughness and the size of the heat affected zone (HAZ), while cutting mm thick AA5083 aluminium alloy sheet. Madić et al. [3] develoed mathematical relationshi between rocess arameters (laser ower, cutting seed and assist gas ressure) and kerf width obtained in CO laser cutting of aluminium alloy (AlMg3) sheet with thickness of 3 mm. With the use of discrete Monte Carlo method, the otimal values of laser ower, cutting seed and assist gas ressure for kerf width minimization were determined. Riveiro et al. [6] examined the effects of cutting arameters in cutting of an aluminium coer alloy. Regarding the surface roughness, the most influencing arameters were those related to the assist gas such as ressure, nozzle diameter and stand-off distance. Riveiro et al. [7] investigated the influence of different assist gases (argon, nitrogen, oxygen and air) on the edge quality and its surface chemistry during laser cutting of a tyical 04-T3 commercial aluminium-coer alloy, with a thickness of 3 mm. Araujo et al. [8] erformed microstructural examination of the HAZ while cutting 1.6 mm thick aluminium 04 sheet. Leone et al. [9] investigated laser cutting of 6061 T6 aluminium alloy through the emloyment of a 150 W multimode ulsed Nd:YAG laser. Dubey and Yadava [10] alied Taguchi quality loss function for simultaneous otimization of kerf deviation and kerf width in ulsed Nd:YAG laser beam cutting of 30

2 aluminium alloy sheet. Assist gas ressure, ulse frequency, ulse width and cutting seed were considered as inut arameters. The scoe of this aer is to develo mathematical model for the rediction of erendicularity of cut, which is considered as one of the most imortant quality characteristics, in CO laser cutting of aluminium alloy (AlMg3) sheet with thickness of 5 mm. To this aim, full factorial exerimental design was adoted while varying laser ower, cutting seed and assist gas ressure at three levels. The obtained exerimental data were used for the develoment of cut erendicularity mathematical model by using second order olynomial. In addition to modelling, the develoed mathematical model was otimized and laser cutting arameter values for minimization of erendicularity of cut were determined using exhaustive iterative search algorithm.. EXPERIMENTAL DETAILS Aluminium alloy (AlMg3) in sheet form of 5 mm thickness was used as workiece material. Laser cutting exerimental trials were erformed by means of Prima Industry laser cutting machine delivering a maximum outut ower of 4 kw at a wavelength of 10.6 μm, oerating in continuous wave mode. A focusing lens with focal length of 17 mm was used to erform the cut with a Gaussian distribution beam mode (TEM00). Nitrogen as assist gas with urity of % was sulied coaxially with the laser beam. The nozzle used has a conical shae with nozzle orifice diameter of mm. The nozzle-workiece stand-off distance was controlled at 1 mm. Laser beam was focused as the bottom of the workiece material (-5 mm). Rectangular secimens with dimensions 0 mm x 40 mm were cut in every exerimental trial and the cut quality was evaluated in terms of erendicularity of the cut (u). Perendicularity is defined as the distance between two arallel straight lines, which limit the uer and lower boundaries of the cut surface rofile at the theoretically correct angle of 90 o. For achieving high cut quality with close dimensional tolerances, it is imortant to obtain accurate erendicularity of cut edge, esecially when using sheet thickness over several millimetres [11]. The erendicularity of the cut edge was measured in accordance to ISO 9013 (00) standard [1] according to which the quality of the cut is classified into three classes. For 5 mm thick sheet, the erendicularity tolerances are as follows: class 1 (u mm), class (0.065 mm u mm) and class 3 (0.175 mm u mm). The laser cutting exeriment was lanned using full factorial design with three inut arameters which were varied at three levels. This design has all ossible combinations of all levels and all inut arameters, thus 7 exerimental trials were erformed. The main laser cutting arameters such as the laser ower (P), cutting seed (v) and assist gas ressure () were considered as variable inut arameters. Levels of variation of these arameters were selected by considering manufacturer's recommendation for arameter settings and ilot one arameter at time exerimentation. The exerimental matrix with exerimentally measured erendicularity of the cut is given in Table MATHEMATICAL MODEL FOR PERPENDICULARITY OF CUT Multile regression analysis is often emloyed for modeling a rocess, i.e. for deriving mathematical equations that relate inut and the outut arameters (rocess functions) based on some exerimental data. To this aim, different regression functions such as linear, curvilinear, logarithmic or other nonlinear are used. Based on the exerimental results from the full factorial design (Table 1) and the use of method of least squares, the second-order regression equation in terms of laser cutting arameters for the rediction of erendicularity of cut was obtained as follows: u P v P v 0.01 P 0.01 P v v (1) With the average ercentage error of about 5.6 % between exerimentally measured erendicularity of cut values and mathematical model redictions, it can be concluded that develoed model is adequate. 4. RESULTS AND DISCUSSION The develoed mathematical model can be used to redict erendicularity of cut within the covered exerimental hyersace. The main effects of the laser cutting arameters on the erendicularity of cut are given in Figure 1. These grahs are obtained by changing one arameter at a time, while keeing the other arameters constant at centre level. From Figure 1, it can be seen that the increase in the laser cutting arameter values results in an increase in erendicularity of cut values, however, the effect of the laser ower is dominant, followed by the cutting seed and assist gas ressure. Higher laser ower levels result in the higher heat inut during cutting oeration so as more material is being melted which 31

3 results in larger kerf widths on to and bottom of the workiece material. Table 1. Laser cutting exerimental design and results Exerimental Laser ower, P Assist gas ressure, Cutting seed, v Perendicularity of cut, u trial (kw) (bar) (m/min) (mm) a) b) c) Figure 1. Effects of laser cutting arameters on erendicularity of cut 3

4 For the urose of investigation of interaction effects of laser cutting arameters on erendicularity of cut, three 3-D surface lots were generated so as to cover all ossible interaction effects. To draw interaction lots, two arameters of interest were varied from its low level to its high level, while other third arameter was held constant at their central level (Figure ). a) b) c) Figure. Effects of laser cutting arameter interactions on erendicularity of cut As could be observed from Figure, a and b, for the given assist gas ressure or cutting seed, there exists an otimum value of laser ower where erendicularity of cut is minimal. Generally, an increase in the assist gas ressure increases erendicularity of cut, whereas the influence of the cutting seed is less ronounced. However, at low ower level, high assist gas ressure decreases erendicularity of cut (Figure, a). In the cutting seed and the assist gas ressure interaction lot (Figure, c), it can be seen that increase in cutting seed and assist gas ressure increases erendicularity of cut, whereas these effects are less ronounced at higher levels of cutting seed and assist gas ressure. As it could be seen from Figure, variation of laser cutting arameter values roduces different erendicularity of cut values, however, in most cases, only quality class 3 is obtained. In order to investigate whether there exist a set of the laser cutting arameters values which roduces lower values of erendicularity of cut, one needs to formulate otimization roblem in the covered threedimensional laser cutting arameter hyersace. Mathematically, the otimization roblem was stated as follows: Determine : P, v, to minimize : u P v P v 0.01 P 0.01 P v v Subject to :3. P 4 [kw] [bar] 1.6 v [m/min] () The otimization roblem in equation () was solved with exhaustive iterative search algorithm using the develoed software solution BRUTOMIZER which suorts the alication of exhaustive iterative search algorithm, continual and discrete Monte Carlo method for solving otimization roblems with and without constraints [13]. The iterative search algorithm was selected as it reresents a arameter free aroach that only requires erforming a large number of simulations that are, however, executed very fast. Moreover, one can take into account techno-technological ossibilities of laser cutting machine in terms of setting articular values for laser ower, cutting seed and assist gas ressure. As a result of the alication of exhaustive iterative search algorithm, the minimal value of erendicularity of cut of u = mm was obtained (class ). This solution corresonds to the following combination of the laser cutting arameter values: P 33

5 = 3.9 kw, = 10 bar, v = 1.6 m/min (Figure 3). These values can be easily set for erforming a given laser cutting oeration. Figure 3. Laser cut surface at P = 3.9 kw, = 10 bar, v = 1.6 m/min As could be seen from Figure 3, under these laser cutting conditions, clean, fine cut surface in the cut surface with regularly saced striations and dross-free edge was obtained. 5. CONCLUSIONS The CO laser cutting of aluminium alloy sheet with thickness of 5 mm was erformed in order to investigate the effect of laser ower, assist gas ressure and cutting seed on erendicularity of cut. Laser cutting exerimentation was based on the use of full factorial design. The obtained exerimental results were used for the develoment of erendicularity of cut rediction model, in terms of second order olynomial. Within the covered exerimental hyersace and obtained results, one can conclude that laser ower is the most significant arameter affecting the erendicularity of cut, followed by assist gas ressure and cutting seed. It has been observed that erendicularity of cut is inversely roortional to considered laser cutting arameters, however, in some cases, the effect of a given laser cutting arameter on erendicularity of cut is variable and must be considered through the interaction with other arameters. As a result of otimization, it was found that focusing the laser beam on the bottom surface of the sheet, using assist gas ressure of 10 bar at combination of laser ower of 3.9 kw and cutting seed of 3.6 m/min, roduced a minimal erendicularity of cut and fine cut surface. 6. REFERENCES 1. Stournaras, A., Stavrooulos, P., Salonitis, K., Chryssolouris, G., An investigation of quality in CO laser cutting of aluminum, CIRP Journal of Manufacturing Science and Technology, Vol., No. 1, , (009).. htt://aluminium.matter.org.uk/content/html/eng/ default.as?catid=&ageid=1 3. Madić, M., Radovanović, M., Kovačević, M., Modelling and otimization of kerf width obtained in CO laser cutting of aluminium alloy using discrete Monte Carlo method, Journal of Production Engineering, Vol. 18, No. 1,. 39-4, (015). 4. Cambell, F.C., Manufacturing Technology for Aerosace Structural Materials, Elsevier, Amsterdam/Kidlington, (006). 5. Olsen, F.O., Fundamental mechanisms of cutting front formation in laser cutting. In Euroto High Power Lasers and Laser Alications V ( ). International Society for Otics and Photonics, (1994). 6. Riveiro, A., Quintero, F., Lusquinos, F., Comesana, R, Pou, J., Parametric investigation of CO laser cutting of 04-T3 alloy, Journal of Materials Processing Technology, Vol. 10, No. 9, , (010). 7. Riveiro, A., Quintero, F., Lusquiños, F., Comesaña, R., Del Val, J., Pou, J., The role of the assist gas nature in laser cutting of aluminum alloys, Physics Procedia, Vol. 1, No. 1, , (011). 8. Araujo, D., Cario, F.J., Mendez, D., Garcia, A.J., Villar, M.P., Garcia, R., Jimenez, D., Rubio, L., Microstructural Study of CO laser machined heat affected zone of 04 aluminum alloy, Alied Surface Science, Vol , No. 1, , (003). 9. Leone, C. Genna, S., Caggiano, A., Tagliaferri, V., Molitierno, R., An investigation on Nd:YAG laser cutting of Al 6061 T6 alloy sheet, Procedia CIRP, Vol. 8, No. 1, , (015). 10. Dubey, A.K., Yadava, V., Robust arameter design and multi-objective otimization of laser beam cutting for aluminium alloy sheet, International Journal of Advanced Manufacturing Technology, Vol. 38, No. 3-4, , (008). 11. Madić, M., Radovanović, M., & Nedić, B., Modeling and otimization of CO laser cutting of stainless steel using RSM and GA. Nonconventional Technologies Review, Vol. 16, No. 4,. 9-14, (01). 1. BS EN ISO 9013:00(E). Thermal cutting Classification of thermal cuts Geometrical roduct secification and quality tolerances. International Organization for Standardization. Geneva. 13. htt:// ons 34