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Experimental investigation into CO2 laser cutting parameters
Jourmd of
Materials Processing Technology ELSEVIER
Journal of Materials Processing Technology 58 (1996) 323-330
Experimental investigation into
CO
laser cutting parameters
2
Bekir S. Yilba Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Received 21 November 1994; accepted 20 July 1995
Industrial summary The quality of laser cuts is of the utmost importance in laser processing. As laser systems are becoming more demanding, the need for developments in the area of monitoring, diagnosis, regulation and modeling becomes essential to achieve and maintain a high-quality cutting process. Consequently, the present study examines the cutting parameters experimentally to achieve an understanding of the relationships between these parameters and the resulting cutting quality. The cutting parameters include workpiece thickness, assisting gas pressure, cutting speed and laser output power. The study is extended to include monitoring of the surface plasma developed during the cutting process, which in turn provides information on the effect of the surface on the formation of strations and cutting quality. This was achieved using the optical method. Scanning electron microscope (SEM) microphotography of the cut was achieved for detailed investigation of the effects of cutting parameters on the cut geometry. Keywords: Laser cutting; Carbon dioxide laser
1. Introduction Many attempts have been made to improve laser cut quality [1,2]. These include monitoring and controlling the laser output power, monitoring the laser-material interaction and modeling of the laser cutting process. Certainly, improvements in the stability and quality of the laser beam and optics will improve the materialprocessing quality. Modeling appears as an important tool for the pursuit of knowledge in the area of cut quality. However, those who have undertaken this task have almost exclusively looked at the steady-state behavior of the process [3-5]. Laser cutting is both dynamic and stochastic, with fluctuations in absorbed power, material composition and optical integrity. It is beneficial to develop steady-state modeling for obtaining the approximate order of magnitudes for various parameters, but there are clearly limitations when such models are used as controls. Consequently, experimental investigation into laser cut quality becomes essential for obtaining the actual control parameters. One of the deficiencies of the laser cutting process is the formation of striations, which strongly affect the quality of laser cutting. The mechanisms triggering the formation of stria are not well understood. Strias may develop due to the non-steady nature of the laser cutting process. Several explanations for stria occurrence exist, but there is still no general consensus on 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved S S D I 0924-0136(95)02094-3
this dynamic effect. One of the explanation is that at cutting speeds less than the sp6ed of the moving molten layer, sideways burning occurs and results in forming of a stria [6]. It has been also shown that pulsations in the molten layer prior to its being blown out of the kerf, cause periodic strias to form [7]. It was suggested that fluctuations in the absorbed laser power could cause both the thickness and temperature of the liquid layer to oscillate [8]. Absorbed power fluctuations can be caused by plasma formation, incident power variations or material absorptivity variations [9]. In light of the above arguments, the present study is designed to investigate the cutting parameters experimentally, which in turn provides improved cut quality, and to study the formation of strias through monitoring the light emission from the upper surface of the workpiece during the laser cutting process. I
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Fig. I. Experimental set-up.
324
B.S. Yilba)"/Journal (~/~Materials Processing Technology 58 (1996) 323 330
To determine the critical cutting s p e e d - - a t which cutting c e a s e s - - a variable-speed milling table has been developed. This enables the recording of power requirements at different cutting speeds and assisting gas velocities. To monitor the light emitted from the upper surface of the workpiece, a fiber-optic cable and a fast-response photodetector were used.
, o~2r2 + u2 + 2uo}cl tttting speed at any point P =.at speed rational speed
2. Experimental The experimental set-up is shown in Fig. 1. A CO2 laser of 500 W (CW) mean output power was used to cut the samples. To focus the laser beam, a ZnSe lens of 100 mm nominal focal length was employed in a lens bolder, the latter of which could move in the vertical direction so that different focus settings could be obtained. The spot diameter of the laser on the sample surface was measured and calculated by the Airy-disc equation [10], this diameter being found to be 270/Lm. Oxygen was introduced co-axially with a laser beam through a nozzle of about 0.8 mm in diameter at the workpiece surface. The variable-speed milling table was controlled by computer. A calorimeter was used to monitor the simultaneous laser output power. The cutting experiment was set to fit a circular disc on top of the linearly moving table so that the workpiece, which was mounted on the circular disc, had two directions of motion, one linear and the other circular. The table, which moved linearly, and the disc, which moved in a circular direction, could be operated independently by two separate stepping motors. With this arrangement spiral-shaped cuts, with continuously-variable velocity, were made by giving the workpiece a linear movement and a circular speed simultaneously (Fig. 2). The workpieces, which were of rectangular shape (120 × 80mm) were placed on the circular disc which was fitted on top of tile table, so that the laser beam hit the workpiece at its center. The table could be moved in three orthogonal directions, one along the direction of the linear cut, one perpendicular to it and the other in the vertical direction. The various spiral shapes were cut by moving the workpiece with this motion, the radius of the spiral being changed by moving the base table in the direction perpendicular to the direction of the linear cuts. By recording the laser parameters (laser output power, assisting gas pressure and thickness of the material) simultaneously, and calculating the cutting speed from the spiral curve, the relationships between these parameters and the cutting speed could be derived. To check the repeatability of the results obtained from the spiral cuts, circular shapes corresponding to a variety of spiral cut speeds were cut. To achieve this the workpiece was mounted on the circular disc, the latter being fitted on top of the table which could be operated
M
Fig. 2(a). Spiral cut.
Fig. 2(b). Photograph of spiral cut.
manually to alter the radius of the circular cuts. The rectangular workpiece, of a particular thickness, was set at the maximum radius possible from the center of the nozzle through which the laser beam, which was concentric with the gas jet, emerged. The motor which drove the circular disc was set at an arbitrary speed. The disc was set in motion and given a specific length of time to reach this steady speed. The laser and the assisting gas were switched on next and circular cuts obtained. If acceptable cuts were not produced at that
B.S. Yilba~ ..Journal ~/ Materials Processing Technology 58 (1996) 323-330
particular radius and speed of the revolving disc, the whole procedure was repeated after decreasing the radius of cut by traversing the table on which the circular disc was attached, whilst keeping the same rotary speed. This technique of decreasing the radius and keeping the power and angular speed of the circular disc constant was repeated until a clean circular cut on the workpiece was obtained. The radius of the cut, the thickness of the workpiece, the assisting gas jet velocity and the laser output power were recorded simultaneously in each case. A fiber-optic probe was used to monitor the surface plasma, the signal being displayed on a storage oscilloscope which was interfaced with the computer. The probe consisted of a polymer fiber cable of 1 mm in diameter and 1 m in length, and a fast-response photo detector. The polymer cable used had an attenuation of 2 0 0 d B k m -1 at a 6 6 5 n m wavelength, whilst the photodetector had a rise time of 1 ns and a spectral sensitivity of 0.55 A W -~ at an 850 nm wavelength. The photodetector was situated at one end of the probe, the latter being placed 0 . 5 m m above and 1 5 m m away from the irradiated spot and set at an angle of 45 ° with respect to the workpiece (Fig. 1). Once the laser was switched on, plasma appeared on the front surface of the workpiece and was detected by the probe. Mild steel sheets of different thicknesses were used as the workpieces. To obtain microphotographs of the laser cut cross-sections scanning electron microscopy (SEM) was used. The microphotographs of the resulting cuts were then studied to investigate the effects of cutting speed on the resulting laser cutting quality.
325
oxygen pressures. The maximum cutting speed was limited by the appearance of curved ridges on the lower surface of the cuts. The oxides produced during the cutting action tended to adhere to the underside of the plate, but were not fused to the metal and could be broken away easily, leaving a clean narrow underside to the cut. Experimental results indicated that, for a particular power level, self-burning occurs below a particular cutting speed and increases with increasing oxygen pressure, since oxidation is a chemical reaction and the rate of reaction is proportional to the local oxygen concentration. However, at high cutting speeds the heatspread from the cutting zone decreases, which increases the temperature gradients in the kerf and diminishes the tendency for the oxidation to become locally self-sustaining. For a given power level, an increase in the cutting speed demonstrated that the onset of curved striations, which appeared as a result of solidification of oxides near the under surface of the cut, provided an upper limit for the cutting speed. The limiting cutting speed (critical cutting speed) increased steeply with oxygen pressures up to 210kPa, above which it tended to become constant or to fall. This means that, for that particular power level, an increase in oxygen pressure
3. Discussion In general, for mild steel at very low cutting speeds, the cuts were of irregular width and contained holes of varying diameter spaced irregularly along the cut. This effect is referred to as self-burning and is shown for all of the workpieces in Fig. 3. An increase in cutting speed reduced the incidence of these holes until the cut became completely regular and was not more than 0.5 mm wide. In detail, the cut edge itself showed a series of raised ridges about 0.1-0.3 mm apart (Fig. 4). As the cutting speed was raised further, these ridges began to curve backwards (Fig. 5), i.e. away from the cutting direction at the lower edge of the metal. At still higher speeds, this curved portion extended up towards the top surface of the metal and the lower part of the cut became wider and less regular (Fig. 5). This is called the curved-striation effect. For a particular thickness, self-burning occurred only at very low speeds and the curved-striation effect only appeared at considerably high speeds, especially at high
Fig. 3(a). Laser cut at low speed--cutting speed 2 cm s -~, 02 gas pressure 140 kPa.
Fig. 3(b). Cross-section of laser cut at low speed (self-burningeffect).
326
B.S. Yilbas /Journal q/' Materials Processing Technology 58 (1996) 323 330
Fig. 4(a). Laser cut at moderate speed--cutting speed 4 cm s-~, 02 gas pressure 140 kPa.
Fig. 5(a). Laser cut at high speed--cutting speed 6 cm s - t , 02 gas pressure 140 kPa.
Fig. 4(b). Cross-section of laser cut at moderate speed. Fig. 5(b). Cross-section of laser cut at high speed (curving effect).
above 210 kPa does not enhance oxidation of the metal removed from the cut. Pressure above this level is therefore not required for oxidation and is presumably necessary only for the removal of the molten oxide from the cut. It should be noted that at some value of oxygen pressure the gas reached sonic velocity, but any further increase in pressure only decreased the pressure drop in the nozzle without increasing the jet velocity significantly. This effect also produces a leveling off and even a fall in the curves of cutting speed versus oxygen pressure. This usually occurs at oxygen pressures above 210 kPa. Figs. 6 - 9 show the influence of varying power intensity on the process. It can be seen that although an increase in power intensity rapidly increases the cutting speed initially, the process tends to saturate for all thicknesses of workpiece at power intensities in the region of 2 x 103 M W m 2. When examining the variation of cutting speed with assisting gas jet pressure, the dominating effect of the gas jet can be seen. In this case, the cutting velocity varies by more than a factor of two in the experimental range at constant incident power intensity. At jet pressure ratios in the critical region, the cutting mechanism saturates and, indeed, there is a positive downward trend at supercritical pressure ratios. It has been shown that this downward trend is due to the cooling effect of the jet [11], but this argument
cannot be explained by any gas dynamics considerations. Unstable shock-wave effects in the jet are thought to be the cause of reduced jet effectiveness, a view which is supported by the observation that although smooth curves can be drawn through the experimental results in the supercritical regions, the cutting process is quite unstable at high pressures.
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Fig. 6. Laser output power versus cutting speed for different workpiece thicknesses. Jet pressure = 70 kPa, U~ = 252 m s - ~.
B.S. Yilba~ /Journal o f Materials Processing Technology 58 (1996) 323-330
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Fig. 7. Laser output power versus cutting speed for different workpiece thicknesses. Jet pressure = 140 kPa, Ue = 297 m s -1.
Fig. 9. Laser output power versus cutting speed for different workpiece thicknesses. Jet pressure = 275 kPa, Ue = 387 m s - ~.
Even at moderate supercritical pressure ratios, an oblique chock is established in the kerf, downstream of which the jet flow is detached from the wall. Under these conditions the interface shear stress is reduced and the liquid layer must inevitably thicken, consistent with the findings of work [12]. A plane parallel nozzle was used in all of the experiments, hence the flow at the nozzle exit plane is mismatched. If a convergent-divergent nozzle had been used this mismatch could have been prevented and the flow expanded to higher Mach numbers. However, because of the small nozzle-workpiece gap there is no certainty that the flow can be started, and even if this is possible, strong shock interactions will still occur within the kerf. It is obvious, therefore, that operation above critical pressure ratios is not only expensive in terms of gas consumption, but positively a disadvantage as far as cutting performance is concerned.
The cutting speed has a maximum value at which stable cutting can be sustained under stated conditions. This does not imply that at the maximum cutting speed all the laser power is absorbed in the workpiece, but rather that the kerr cannot be kept open at higher speeds and the cut geometry, and hence the eventually favorable conditions for absorption and chemical reaction, collapses. It is worth pointing out here that even apparently stable cutting is accomplished through a sequence of transient processes as proved by the marks left in the kerf. This effect is particularly prominent when cutting at velocities much lower than the optimum and at low gas pressure, when the liquid boundary layer becomes thickened. When examining the variation of workpiece thickness with cutting speed, it is evident that at the cirtical jet pressure ratio (the maximum useful jet velocity) the cutting speed is almost independent of the workpiece thickness at power intensities near the saturation conditions (1750-200 MW m-2). The strength of this argument can be tempered by observing that whereas the results for a 0.75 mm thick workpiece show that fullysaturated conditions are achieved in the experimental range, this is not so in the 1.5 mm case. However, it remains true, for example, that the maximum cutting speeds at 2000 MW / m - 2 incident power intensity and 210kPa jet supply pressure are the same for both 0.75 mm and 1.5 mm thick workpieces. The implication is that even under optimum cutting conditions, only a relatively small part of the total laser power incident on the cut region is absorbed usefully. In Fig. 10 the optic probe response corresponding to the 0.75 mm thick sample is given. Spikes appear on the initial part of the probe response, which corresponds to keyhole formation. In this case, a high rate of exothermic oxidation reaction takes place. It is also evident from the previous study [7] that the amount of surface
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Fig. 8. Laser output power versus cutting speed for different workpiece thicknesses. Jet pressure = 210 kPa, U~ = 343 m s -~.
328
B.S. Yilba~" ~Journal (~/"Materials Processing: Technology 58 (1996) 323-330
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plasma is considerable, in turn emitting a high intensity of radiation which can be detected by the probe as spikes. Fluctuations in the probe response occur with time which may be due to the wavy behavior of the strias, as predicted in earlier studies [6,13]. In this case, once the size of the surface plasma increases, it absorbs the incident laser energy and as a result the temperature and pressure of the surface plasma increases. This plasma then expands rapidly, which in turn produces less plasma on the surface. This effect occurs periodically. It is evident from Fig. 11 that the probe response is relatively higher at high oxygen pressure (210 kPa) than that obtained at other pressures. In this case, a substan-
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Fig, 12(a), Fiber-optic probe response for 1 mm thick sample--cutting speed 4 cm s ], 02 gas pressure 140 kPa.
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.Fig. 12(b). Fiber-optic probe response for 1.5 mm thick sample cutting speed 4 cm s ~, O: gas pressure 140 kPa.
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tial amount of surface plasma occurs, which causes an increase in the erosion of the surface due to thermal effects. It is also possible that the contacting surface between the oxygen and the plasma increases, resulting in an increased exothermic reaction occurring in the plasma at the kerr. Fig. 12 shows the probe responses corresponding to different workpiece thickness. As the workpiece thickness increases, the amplitude of the probe responses
B.S. Yilba~'/Journal of Materials' Processing Technology 58 (1996) 323-330
increases, which shows that the size o f the surface plasma increases with increasing workpiece thickness. This m a y be caused by one, or all, o f the following: (i) As the workpiece thickness increases, the time required for full penetration o f the workpiece increases [14]. Consequently, the pressure developed inside the kerf (the recoil pressure [15]) increases, which in turn increases the mass removal rate. As a result, a substantial a m o u n t o f plasma is developed on the surface. (ii) The coupling effect o f plasma absorption and heating m a y differ in this case [16]. It is also evident f r o m Fig. 11 that immediately after keyhole f o r m a t i o n the a m o u n t o f surface plasma becomes small, but when cutting proceeds it increases in a m o u n t , eroding the workpiece surface. This erosion mechanism increases the kerf width and the m o m e n t u m o f the molten liquid, resulting in erosion on the kerf edge. Globules then develop along the kerf wall on the rear side o f the workpiece, consistent with the results o f earlier w o r k [17].
4. Conclusions The conclusions derived f r o m the present study m a y be listed as follows: (1) Self-burning results below a specific cutting speed and increases with increasing oxygen pressure. Heat-spread f r o m the cutting zone decreases at high cutting speeds, which in turn increases the temperature gradients in the kerf and diminishes the tendency for oxidation to become locally self-sustaining. (2) Curved strias develop near the undersurface o f the cut once the cutting speed increases to critical speeds. These appear due to solidification o f oxides in this region. (3) The critical cutting speed increases rapidly with oxygen pressure, providing that a further increase in oxygen pressure does not enhance the oxidation process, but increases the removal rate o f molten metal f r o m the cut. (4) Once the jet velocity reaches sonic velocity, the critical cutting speeds d r o p due to the cooling effect o f the jet and the possible development o f shock waves in the jet result in the cutting process being quite unstable. (5) At the critical gas pressure (giving the maxim u m useful jet velocity), the cutting speed is almost independent o f the workpiece thickness at high power intensities, which in turn indicates that only a relatively small part o f the total laser power incident on the cut region is absorbed usefully. (6) A substantial a m o u n t o f surface plasma occurs using thick workpieces at high oxygen pressure, this plasma causing an increase in the erosion o f the sur-
329
face due to thermal effects. In this case, the surface plasma m a y partially block the incident laser beam, resulting in less energy f r o m the laser beam reaching the surface. This plasma then expands due to the pressure differential in the plasma. As a result, m o r e incident energy reaches the target, which in turn increases the removal rate o f molten metal from the kerf, causing m o r e surface plasma. This process occurs periodically and leads to the development o f strias a r o u n d the kerf edge.
Acknowledgements The authors acknowledge the support o f King F a h d University o f Petroleum and Minerals, D h a h r a n , Saudi A r a b i a for this work.
References [1] H, Jorgensen and F.O. Olsen, Process monitoring during CO2 laser cutting, Gas and Metal Vapor Lasers and Applications, Proc. SP1E 1991, Los Angeles, CA, USA, Vol. 1412, pp. 1982O8 [2] B.S. Yilbas, R. Davies, Z. Yilbas and F. Begh, Study into measurement and prediction of penetration time during CO2 laser cutting process, Proc. Instn. Mech. Engrs. B, 204 (1990) 105 113. [3] B.S. Yilbas, R. Davies, and Z. Yilbas, Surface line and plug flow models governing laser produced vapor from metallic surfaces, Pramana J. Phys., 38(2) (1992) 195-209. [4] P. di Pietro, and Y.L. Yao., A new technique to characterize and predict laser cut striations, Int. J. Mach. Tools Manuf, 35(7) (1995) 993 1002. [5] W. Schulz and D. Becker, On laser fusion cutting: the self-adjusting cutting kerf width, High Power Lasers and Laser Machining Technology, Proc. SPIE '89, Paris, France, 1989, Vol. 1132, pp. 211 221. [6] G. Simon and U. Gratzke, Theoretical investigations of instabilities in laser gas cutting, High Power Lasers and Laser Machining Technology, Proc. SP1E '89, Paris, France, 1989, Vol. 1132, pp. 204 210. [7] B.S. Yilbas and Z. Yilbas, Effects of plasma in CO2 laser cutting quality, Opt. Lasers Eng., 9 (1988) 1 12. [8] B,S. Yilbas, R. Davies, Z. Yilbas, and F. Begh, Investigation into development of liquid layer and formation of surface plasma during CO~ laser cutting process, Proc. Inst. Mech. Eng., 206 (1992) 287-298. [9] K. Danisman, B.S. Yilbas, and Z. Yilbas, Study of some characteristics of plasma generated during a CO2 laser beam cutting process, Opt. Laser Technol., 24(1)(1992) 33 38. [10] A. Gorur, B.S. Yilbas and C. Ciftlikli, Focusing of laser hole drilling, Opt. Lasers Eng., 18 (1993) 349 369. [11] B.S. Yilbas and Z. Yilbas, A study of parameters affecting the continuous CO2 laser cutting, J. Chin. Inst. Eng., 10(5) (1987) 543 547. [12] B.S. Yilbas and A.Z. Sahin, Turbulent boundary layer approach allowing chemical reactions for CO2 laser oxygen assisted cutting process, Proc. Inst. Mech. Eng. C, J. Mech. Eng. Sci., 208 (1994) 275 284. [13] W.M. Steen, Laser Material Processing, Springer-Verlag, London, 1991.
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B.S. Yilba¢ /Journal of Materials Process&g Teehnology 58 (1996) 323 330
[14] B.S. Yilbas, R. Davies, Z. Yilbas, and F. Begh, Study into the measurement and prediction of penetration time during CO 2 laser cutting process, Proe. Inst. Meek. Eng. B, 204 (1990) 105 113. [15] B.S. Yilbas and Z. Yilbas, Plasma transients during laser drilling in subatmospheric pressure atmospheres of air, Opt. Lasers Eng.,
7 (1987) 1-13. [16] B.S. Yilbas, Measurement of plasma transmittance in relation to CO z laser cut quality of stainless steel, Proc. LASER 5 Conf., Birmingham, UK, 1989, pp. 245 248. [l 7] B.S. Yilbas, Study into transmittance of plasma during CO 2 laser cutting of mild steel, Arch. Metall., 33 (1988) 59 66.
Materials Processing Technology ELSEVIER
Journal of Materials Processing Technology 58 (1996) 323-330
Experimental investigation into
CO
laser cutting parameters
2
Bekir S. Yilba Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Received 21 November 1994; accepted 20 July 1995
Industrial summary The quality of laser cuts is of the utmost importance in laser processing. As laser systems are becoming more demanding, the need for developments in the area of monitoring, diagnosis, regulation and modeling becomes essential to achieve and maintain a high-quality cutting process. Consequently, the present study examines the cutting parameters experimentally to achieve an understanding of the relationships between these parameters and the resulting cutting quality. The cutting parameters include workpiece thickness, assisting gas pressure, cutting speed and laser output power. The study is extended to include monitoring of the surface plasma developed during the cutting process, which in turn provides information on the effect of the surface on the formation of strations and cutting quality. This was achieved using the optical method. Scanning electron microscope (SEM) microphotography of the cut was achieved for detailed investigation of the effects of cutting parameters on the cut geometry. Keywords: Laser cutting; Carbon dioxide laser
1. Introduction Many attempts have been made to improve laser cut quality [1,2]. These include monitoring and controlling the laser output power, monitoring the laser-material interaction and modeling of the laser cutting process. Certainly, improvements in the stability and quality of the laser beam and optics will improve the materialprocessing quality. Modeling appears as an important tool for the pursuit of knowledge in the area of cut quality. However, those who have undertaken this task have almost exclusively looked at the steady-state behavior of the process [3-5]. Laser cutting is both dynamic and stochastic, with fluctuations in absorbed power, material composition and optical integrity. It is beneficial to develop steady-state modeling for obtaining the approximate order of magnitudes for various parameters, but there are clearly limitations when such models are used as controls. Consequently, experimental investigation into laser cut quality becomes essential for obtaining the actual control parameters. One of the deficiencies of the laser cutting process is the formation of striations, which strongly affect the quality of laser cutting. The mechanisms triggering the formation of stria are not well understood. Strias may develop due to the non-steady nature of the laser cutting process. Several explanations for stria occurrence exist, but there is still no general consensus on 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved S S D I 0924-0136(95)02094-3
this dynamic effect. One of the explanation is that at cutting speeds less than the sp6ed of the moving molten layer, sideways burning occurs and results in forming of a stria [6]. It has been also shown that pulsations in the molten layer prior to its being blown out of the kerf, cause periodic strias to form [7]. It was suggested that fluctuations in the absorbed laser power could cause both the thickness and temperature of the liquid layer to oscillate [8]. Absorbed power fluctuations can be caused by plasma formation, incident power variations or material absorptivity variations [9]. In light of the above arguments, the present study is designed to investigate the cutting parameters experimentally, which in turn provides improved cut quality, and to study the formation of strias through monitoring the light emission from the upper surface of the workpiece during the laser cutting process. I
LASERHEAD ~___~RROR
PHOTODETECTOR UNIT t
LASER, '
BEAM i i COMPUTER k
J]
i
i'~j
FIBER-OPTIC/
, , LENS CABLE~
i
/
' ',,1"
WORKPIECE
/
"-r" o" PROBE ~ - - - - ~ - - ~ I '"
/ X-Y TABLE
?
I
CIRCULAROSCILLOSCOPE DISC
Fig. I. Experimental set-up.
324
B.S. Yilba)"/Journal (~/~Materials Processing Technology 58 (1996) 323 330
To determine the critical cutting s p e e d - - a t which cutting c e a s e s - - a variable-speed milling table has been developed. This enables the recording of power requirements at different cutting speeds and assisting gas velocities. To monitor the light emitted from the upper surface of the workpiece, a fiber-optic cable and a fast-response photodetector were used.
, o~2r2 + u2 + 2uo}cl tttting speed at any point P =.at speed rational speed
2. Experimental The experimental set-up is shown in Fig. 1. A CO2 laser of 500 W (CW) mean output power was used to cut the samples. To focus the laser beam, a ZnSe lens of 100 mm nominal focal length was employed in a lens bolder, the latter of which could move in the vertical direction so that different focus settings could be obtained. The spot diameter of the laser on the sample surface was measured and calculated by the Airy-disc equation [10], this diameter being found to be 270/Lm. Oxygen was introduced co-axially with a laser beam through a nozzle of about 0.8 mm in diameter at the workpiece surface. The variable-speed milling table was controlled by computer. A calorimeter was used to monitor the simultaneous laser output power. The cutting experiment was set to fit a circular disc on top of the linearly moving table so that the workpiece, which was mounted on the circular disc, had two directions of motion, one linear and the other circular. The table, which moved linearly, and the disc, which moved in a circular direction, could be operated independently by two separate stepping motors. With this arrangement spiral-shaped cuts, with continuously-variable velocity, were made by giving the workpiece a linear movement and a circular speed simultaneously (Fig. 2). The workpieces, which were of rectangular shape (120 × 80mm) were placed on the circular disc which was fitted on top of tile table, so that the laser beam hit the workpiece at its center. The table could be moved in three orthogonal directions, one along the direction of the linear cut, one perpendicular to it and the other in the vertical direction. The various spiral shapes were cut by moving the workpiece with this motion, the radius of the spiral being changed by moving the base table in the direction perpendicular to the direction of the linear cuts. By recording the laser parameters (laser output power, assisting gas pressure and thickness of the material) simultaneously, and calculating the cutting speed from the spiral curve, the relationships between these parameters and the cutting speed could be derived. To check the repeatability of the results obtained from the spiral cuts, circular shapes corresponding to a variety of spiral cut speeds were cut. To achieve this the workpiece was mounted on the circular disc, the latter being fitted on top of the table which could be operated
M
Fig. 2(a). Spiral cut.
Fig. 2(b). Photograph of spiral cut.
manually to alter the radius of the circular cuts. The rectangular workpiece, of a particular thickness, was set at the maximum radius possible from the center of the nozzle through which the laser beam, which was concentric with the gas jet, emerged. The motor which drove the circular disc was set at an arbitrary speed. The disc was set in motion and given a specific length of time to reach this steady speed. The laser and the assisting gas were switched on next and circular cuts obtained. If acceptable cuts were not produced at that
B.S. Yilba~ ..Journal ~/ Materials Processing Technology 58 (1996) 323-330
particular radius and speed of the revolving disc, the whole procedure was repeated after decreasing the radius of cut by traversing the table on which the circular disc was attached, whilst keeping the same rotary speed. This technique of decreasing the radius and keeping the power and angular speed of the circular disc constant was repeated until a clean circular cut on the workpiece was obtained. The radius of the cut, the thickness of the workpiece, the assisting gas jet velocity and the laser output power were recorded simultaneously in each case. A fiber-optic probe was used to monitor the surface plasma, the signal being displayed on a storage oscilloscope which was interfaced with the computer. The probe consisted of a polymer fiber cable of 1 mm in diameter and 1 m in length, and a fast-response photo detector. The polymer cable used had an attenuation of 2 0 0 d B k m -1 at a 6 6 5 n m wavelength, whilst the photodetector had a rise time of 1 ns and a spectral sensitivity of 0.55 A W -~ at an 850 nm wavelength. The photodetector was situated at one end of the probe, the latter being placed 0 . 5 m m above and 1 5 m m away from the irradiated spot and set at an angle of 45 ° with respect to the workpiece (Fig. 1). Once the laser was switched on, plasma appeared on the front surface of the workpiece and was detected by the probe. Mild steel sheets of different thicknesses were used as the workpieces. To obtain microphotographs of the laser cut cross-sections scanning electron microscopy (SEM) was used. The microphotographs of the resulting cuts were then studied to investigate the effects of cutting speed on the resulting laser cutting quality.
325
oxygen pressures. The maximum cutting speed was limited by the appearance of curved ridges on the lower surface of the cuts. The oxides produced during the cutting action tended to adhere to the underside of the plate, but were not fused to the metal and could be broken away easily, leaving a clean narrow underside to the cut. Experimental results indicated that, for a particular power level, self-burning occurs below a particular cutting speed and increases with increasing oxygen pressure, since oxidation is a chemical reaction and the rate of reaction is proportional to the local oxygen concentration. However, at high cutting speeds the heatspread from the cutting zone decreases, which increases the temperature gradients in the kerf and diminishes the tendency for the oxidation to become locally self-sustaining. For a given power level, an increase in the cutting speed demonstrated that the onset of curved striations, which appeared as a result of solidification of oxides near the under surface of the cut, provided an upper limit for the cutting speed. The limiting cutting speed (critical cutting speed) increased steeply with oxygen pressures up to 210kPa, above which it tended to become constant or to fall. This means that, for that particular power level, an increase in oxygen pressure
3. Discussion In general, for mild steel at very low cutting speeds, the cuts were of irregular width and contained holes of varying diameter spaced irregularly along the cut. This effect is referred to as self-burning and is shown for all of the workpieces in Fig. 3. An increase in cutting speed reduced the incidence of these holes until the cut became completely regular and was not more than 0.5 mm wide. In detail, the cut edge itself showed a series of raised ridges about 0.1-0.3 mm apart (Fig. 4). As the cutting speed was raised further, these ridges began to curve backwards (Fig. 5), i.e. away from the cutting direction at the lower edge of the metal. At still higher speeds, this curved portion extended up towards the top surface of the metal and the lower part of the cut became wider and less regular (Fig. 5). This is called the curved-striation effect. For a particular thickness, self-burning occurred only at very low speeds and the curved-striation effect only appeared at considerably high speeds, especially at high
Fig. 3(a). Laser cut at low speed--cutting speed 2 cm s -~, 02 gas pressure 140 kPa.
Fig. 3(b). Cross-section of laser cut at low speed (self-burningeffect).
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B.S. Yilbas /Journal q/' Materials Processing Technology 58 (1996) 323 330
Fig. 4(a). Laser cut at moderate speed--cutting speed 4 cm s-~, 02 gas pressure 140 kPa.
Fig. 5(a). Laser cut at high speed--cutting speed 6 cm s - t , 02 gas pressure 140 kPa.
Fig. 4(b). Cross-section of laser cut at moderate speed. Fig. 5(b). Cross-section of laser cut at high speed (curving effect).
above 210 kPa does not enhance oxidation of the metal removed from the cut. Pressure above this level is therefore not required for oxidation and is presumably necessary only for the removal of the molten oxide from the cut. It should be noted that at some value of oxygen pressure the gas reached sonic velocity, but any further increase in pressure only decreased the pressure drop in the nozzle without increasing the jet velocity significantly. This effect also produces a leveling off and even a fall in the curves of cutting speed versus oxygen pressure. This usually occurs at oxygen pressures above 210 kPa. Figs. 6 - 9 show the influence of varying power intensity on the process. It can be seen that although an increase in power intensity rapidly increases the cutting speed initially, the process tends to saturate for all thicknesses of workpiece at power intensities in the region of 2 x 103 M W m 2. When examining the variation of cutting speed with assisting gas jet pressure, the dominating effect of the gas jet can be seen. In this case, the cutting velocity varies by more than a factor of two in the experimental range at constant incident power intensity. At jet pressure ratios in the critical region, the cutting mechanism saturates and, indeed, there is a positive downward trend at supercritical pressure ratios. It has been shown that this downward trend is due to the cooling effect of the jet [11], but this argument
cannot be explained by any gas dynamics considerations. Unstable shock-wave effects in the jet are thought to be the cause of reduced jet effectiveness, a view which is supported by the observation that although smooth curves can be drawn through the experimental results in the supercritical regions, the cutting process is quite unstable at high pressures.
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B.S. Yilba~ /Journal o f Materials Processing Technology 58 (1996) 323-330
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Fig. 9. Laser output power versus cutting speed for different workpiece thicknesses. Jet pressure = 275 kPa, Ue = 387 m s - ~.
Even at moderate supercritical pressure ratios, an oblique chock is established in the kerf, downstream of which the jet flow is detached from the wall. Under these conditions the interface shear stress is reduced and the liquid layer must inevitably thicken, consistent with the findings of work [12]. A plane parallel nozzle was used in all of the experiments, hence the flow at the nozzle exit plane is mismatched. If a convergent-divergent nozzle had been used this mismatch could have been prevented and the flow expanded to higher Mach numbers. However, because of the small nozzle-workpiece gap there is no certainty that the flow can be started, and even if this is possible, strong shock interactions will still occur within the kerf. It is obvious, therefore, that operation above critical pressure ratios is not only expensive in terms of gas consumption, but positively a disadvantage as far as cutting performance is concerned.
The cutting speed has a maximum value at which stable cutting can be sustained under stated conditions. This does not imply that at the maximum cutting speed all the laser power is absorbed in the workpiece, but rather that the kerr cannot be kept open at higher speeds and the cut geometry, and hence the eventually favorable conditions for absorption and chemical reaction, collapses. It is worth pointing out here that even apparently stable cutting is accomplished through a sequence of transient processes as proved by the marks left in the kerf. This effect is particularly prominent when cutting at velocities much lower than the optimum and at low gas pressure, when the liquid boundary layer becomes thickened. When examining the variation of workpiece thickness with cutting speed, it is evident that at the cirtical jet pressure ratio (the maximum useful jet velocity) the cutting speed is almost independent of the workpiece thickness at power intensities near the saturation conditions (1750-200 MW m-2). The strength of this argument can be tempered by observing that whereas the results for a 0.75 mm thick workpiece show that fullysaturated conditions are achieved in the experimental range, this is not so in the 1.5 mm case. However, it remains true, for example, that the maximum cutting speeds at 2000 MW / m - 2 incident power intensity and 210kPa jet supply pressure are the same for both 0.75 mm and 1.5 mm thick workpieces. The implication is that even under optimum cutting conditions, only a relatively small part of the total laser power incident on the cut region is absorbed usefully. In Fig. 10 the optic probe response corresponding to the 0.75 mm thick sample is given. Spikes appear on the initial part of the probe response, which corresponds to keyhole formation. In this case, a high rate of exothermic oxidation reaction takes place. It is also evident from the previous study [7] that the amount of surface
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328
B.S. Yilba~" ~Journal (~/"Materials Processing: Technology 58 (1996) 323-330
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plasma is considerable, in turn emitting a high intensity of radiation which can be detected by the probe as spikes. Fluctuations in the probe response occur with time which may be due to the wavy behavior of the strias, as predicted in earlier studies [6,13]. In this case, once the size of the surface plasma increases, it absorbs the incident laser energy and as a result the temperature and pressure of the surface plasma increases. This plasma then expands rapidly, which in turn produces less plasma on the surface. This effect occurs periodically. It is evident from Fig. 11 that the probe response is relatively higher at high oxygen pressure (210 kPa) than that obtained at other pressures. In this case, a substan-
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tial amount of surface plasma occurs, which causes an increase in the erosion of the surface due to thermal effects. It is also possible that the contacting surface between the oxygen and the plasma increases, resulting in an increased exothermic reaction occurring in the plasma at the kerr. Fig. 12 shows the probe responses corresponding to different workpiece thickness. As the workpiece thickness increases, the amplitude of the probe responses
B.S. Yilba~'/Journal of Materials' Processing Technology 58 (1996) 323-330
increases, which shows that the size o f the surface plasma increases with increasing workpiece thickness. This m a y be caused by one, or all, o f the following: (i) As the workpiece thickness increases, the time required for full penetration o f the workpiece increases [14]. Consequently, the pressure developed inside the kerf (the recoil pressure [15]) increases, which in turn increases the mass removal rate. As a result, a substantial a m o u n t o f plasma is developed on the surface. (ii) The coupling effect o f plasma absorption and heating m a y differ in this case [16]. It is also evident f r o m Fig. 11 that immediately after keyhole f o r m a t i o n the a m o u n t o f surface plasma becomes small, but when cutting proceeds it increases in a m o u n t , eroding the workpiece surface. This erosion mechanism increases the kerf width and the m o m e n t u m o f the molten liquid, resulting in erosion on the kerf edge. Globules then develop along the kerf wall on the rear side o f the workpiece, consistent with the results o f earlier w o r k [17].
4. Conclusions The conclusions derived f r o m the present study m a y be listed as follows: (1) Self-burning results below a specific cutting speed and increases with increasing oxygen pressure. Heat-spread f r o m the cutting zone decreases at high cutting speeds, which in turn increases the temperature gradients in the kerf and diminishes the tendency for oxidation to become locally self-sustaining. (2) Curved strias develop near the undersurface o f the cut once the cutting speed increases to critical speeds. These appear due to solidification o f oxides in this region. (3) The critical cutting speed increases rapidly with oxygen pressure, providing that a further increase in oxygen pressure does not enhance the oxidation process, but increases the removal rate o f molten metal f r o m the cut. (4) Once the jet velocity reaches sonic velocity, the critical cutting speeds d r o p due to the cooling effect o f the jet and the possible development o f shock waves in the jet result in the cutting process being quite unstable. (5) At the critical gas pressure (giving the maxim u m useful jet velocity), the cutting speed is almost independent o f the workpiece thickness at high power intensities, which in turn indicates that only a relatively small part o f the total laser power incident on the cut region is absorbed usefully. (6) A substantial a m o u n t o f surface plasma occurs using thick workpieces at high oxygen pressure, this plasma causing an increase in the erosion o f the sur-
329
face due to thermal effects. In this case, the surface plasma m a y partially block the incident laser beam, resulting in less energy f r o m the laser beam reaching the surface. This plasma then expands due to the pressure differential in the plasma. As a result, m o r e incident energy reaches the target, which in turn increases the removal rate o f molten metal from the kerf, causing m o r e surface plasma. This process occurs periodically and leads to the development o f strias a r o u n d the kerf edge.
Acknowledgements The authors acknowledge the support o f King F a h d University o f Petroleum and Minerals, D h a h r a n , Saudi A r a b i a for this work.
References [1] H, Jorgensen and F.O. Olsen, Process monitoring during CO2 laser cutting, Gas and Metal Vapor Lasers and Applications, Proc. SP1E 1991, Los Angeles, CA, USA, Vol. 1412, pp. 1982O8 [2] B.S. Yilbas, R. Davies, Z. Yilbas and F. Begh, Study into measurement and prediction of penetration time during CO2 laser cutting process, Proc. Instn. Mech. Engrs. B, 204 (1990) 105 113. [3] B.S. Yilbas, R. Davies, and Z. Yilbas, Surface line and plug flow models governing laser produced vapor from metallic surfaces, Pramana J. Phys., 38(2) (1992) 195-209. [4] P. di Pietro, and Y.L. Yao., A new technique to characterize and predict laser cut striations, Int. J. Mach. Tools Manuf, 35(7) (1995) 993 1002. [5] W. Schulz and D. Becker, On laser fusion cutting: the self-adjusting cutting kerf width, High Power Lasers and Laser Machining Technology, Proc. SPIE '89, Paris, France, 1989, Vol. 1132, pp. 211 221. [6] G. Simon and U. Gratzke, Theoretical investigations of instabilities in laser gas cutting, High Power Lasers and Laser Machining Technology, Proc. SP1E '89, Paris, France, 1989, Vol. 1132, pp. 204 210. [7] B.S. Yilbas and Z. Yilbas, Effects of plasma in CO2 laser cutting quality, Opt. Lasers Eng., 9 (1988) 1 12. [8] B,S. Yilbas, R. Davies, Z. Yilbas, and F. Begh, Investigation into development of liquid layer and formation of surface plasma during CO~ laser cutting process, Proc. Inst. Mech. Eng., 206 (1992) 287-298. [9] K. Danisman, B.S. Yilbas, and Z. Yilbas, Study of some characteristics of plasma generated during a CO2 laser beam cutting process, Opt. Laser Technol., 24(1)(1992) 33 38. [10] A. Gorur, B.S. Yilbas and C. Ciftlikli, Focusing of laser hole drilling, Opt. Lasers Eng., 18 (1993) 349 369. [11] B.S. Yilbas and Z. Yilbas, A study of parameters affecting the continuous CO2 laser cutting, J. Chin. Inst. Eng., 10(5) (1987) 543 547. [12] B.S. Yilbas and A.Z. Sahin, Turbulent boundary layer approach allowing chemical reactions for CO2 laser oxygen assisted cutting process, Proc. Inst. Mech. Eng. C, J. Mech. Eng. Sci., 208 (1994) 275 284. [13] W.M. Steen, Laser Material Processing, Springer-Verlag, London, 1991.
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[14] B.S. Yilbas, R. Davies, Z. Yilbas, and F. Begh, Study into the measurement and prediction of penetration time during CO 2 laser cutting process, Proe. Inst. Meek. Eng. B, 204 (1990) 105 113. [15] B.S. Yilbas and Z. Yilbas, Plasma transients during laser drilling in subatmospheric pressure atmospheres of air, Opt. Lasers Eng.,
7 (1987) 1-13. [16] B.S. Yilbas, Measurement of plasma transmittance in relation to CO z laser cut quality of stainless steel, Proc. LASER 5 Conf., Birmingham, UK, 1989, pp. 245 248. [l 7] B.S. Yilbas, Study into transmittance of plasma during CO 2 laser cutting of mild steel, Arch. Metall., 33 (1988) 59 66.