Investigations on Thermodynamic Behavior of CO2 Laser Cutting process

Investigations on Thermodynamic Behavior of CO2 Laser Cutting process

1Pawan Mishra, 2 Dr.Tasmeem Ahmad Khan

1M.Tech. student, AL-Falah School of Engg. and Tech. Faridabad,Haryana(India)

2Professor, M.E.Department, AL-Falah School of Engg. and Tech. Faridabad,Haryana(India)

ABSTRACT

In CO2 laser gas-assisted cutting process, modelling of the interaction mechanism is important. Consequently, the present study treats the complete problem of the interaction of the melting surface with the boundary layer and describes the behaviour of the melting layer. In the analysis, gas – liquid interface parameters are developed and relationships between the parameters influencing the cutting action are identified theoretically. To achieve this, effects of momentum and gas – liquid interface shear stress, due to the assisting gas jet, are considered. The approximate magnitude of the heat absorbed is estimated and melting layer thickness is predicted. An experiment is carried out and the theoretical predictions are compared with the experimental findings. First and second law efficiencies of the cutting process are predicted, which may, then, be used to improve the process. It is found that the assisting jet velocity increases the first and second law efficiencies of the CO2 laser cutting process.

Keywords: Laser cutting speed, gas jet velocity, first and second law efficiency

1. INTRODUCTION

   Medium and high-power industrial lasers can be used for cutting, welding, drilling, scribing, marking and many kinds of thermal surface modification. Laser processes are clean, rapid and usually involve low overall heat input to the work piece, so minimizing distortion. Processing forces are minimal and many laser techniques are ideal for industrial applications. If laser light hits a solid target with sufficient power intensity a complex process starts involving first the heating of the target while it is in the solid state, with subsequent melting, vaporization and ionization.

2. LITERATURE REVIEW

   It has been shown that this complex process affects the resulting end prod ucts. For example drilled holes, cuts, welds, etc.[1]. Many methods have been suggested and tried to improve the use of lasers for machining purposes [2]. However, it has also been shown that laser machining can be improved by controlling the physical parameters involved with the laser/work piece interaction process, such as the plasma absorption coefficient, ambient pressure, plasma electron number density and temperature. Previously, it has been shown that the effect of the surface plasma in laser cutting processes had a very significant effect on the cut quality[1]. Therefore, further studies into the surface plasma properties are essential.

   Alternatively, two different cutting techniques have been developed with the introduction of a gas in the cutting section [3]. (1) In laser fusion cutting the material is melted by the radiation energy of the laser and the melt is blown by a gas introduced from a nozzle. In this case shielding gas is required for combustible materials. (2) The material may be eroded by vaporisation, i.e. oxygen is used in the cutting section. In laser oxygen cutting, material is heated to ignition temperature by the focused laser beam. Consequently, a high temperature oxidation, exothermic reaction takes place during the cutting. This reaction

   provides extra energy in the cutting section. The oxygen provides: (a) increased absorption of incident laser energy by the work piece; (b) considerable additional energy through the exothermic reaction, increasing the cutting process; (c) cleaning of molten metal oxide from the cutting section; and (d) cooling and protection of the focusing lens from liquid and vapour emission.

   Considerable research studies were carried out to investigate the laser heating project [6- 13].The analysis of acoustic sensing for laser grooving and cutting was carried out by Sheng and Chryssolouris [14].They introduced the process control scheme for groove depth and beam breakthrough regulation for laser cutting process. Surface temperature due to a moving laser cutting beam was formulated by Romer Meijer[15].They presented two types of approximating functions relating the scanning speed to the maximum surface temperature. Three-dimensional conduction model in relation to laser machining process was introduced by Modest[16].He predicted the shape of a developing groove that was formed by ablation of material by a stationary as well as moving laser beam. Temperature field during laser forming of beam was examined by Ji and Wu[17].They employed finite element method when solving this governing equation of heat transfer. They showed that temperature grad ient in a depth below the surface increased with increasing of laser power and work piece thickness while it decreased by laser beam scanning velocity. A dual gas jet laser cutting process involving co- axial and off- axial oxygen gas flowing was investigated by Hsu and Molian [18].They showed the effectiveness of the dual gas-jet in achieving the maximum machining rate without deteriorating the cut quality. The turbulent boundary laser approach allowing chemical reactions for laser oxygen-assisted cutting process was introduced by Yilbas and Sahin[19].They showed that heat transfer to the liquid metal decreased with increasing work piece thickness. The thermal analysis of laser cutting process was studied by Yilbus [20]. He indicated that the mathematical model employed presented the physical phenomena well, with the limit of characteristic distance, striation frequency and striation width, as predicted, agreeing well with the experimental findings. A numerical study for laser cutting process was carried out by Yilbas [21].The melting front velocities at different laser output power and work piece thickness were predicted.

   In laser cutting process, the end product quality is the determining factor for the successful machining operation. Howeve r, some cutting quality deviations can be attributed to slow process drifts and disturbances. Many reasons can be associated for such drifts. Some of these may include work piece property variations, assisting gas pressure fluctuations, laser output power variation and optical integrity perturbations. Considerable research studies were carried out to assess the end product quality of laser cutting process. Investigation into instabilities in laser cutting was carried out by Simon and Gratzke [22].They inves tigated that cutting speed less than the speed of the moving molten layer, sideways burning occurred, which in turn resulted in the formation of striation patterns. Scultz and Becker [23] studied the kerf width formation during laser fusion cutting. They introduced a model predicting the self adjusting kerf width. Powell et al[24] investigated laser cut edge quality improvements through pulsing the laser beams. They indicated that the pulsation in the molten layer prior to it being blown down from the kerf could cause periodic striations. Kar et al. [25] introduced scaling law to predict the kerf width during laser gas assisted cutting process. They indicated that thick metal cutting performance could be improved by producing narrow widths. Laser cutting parameters were investigated experimentally by Yilbas [26].He showed that smoothness of kerf surface deteriorate once the cutting speed increased to critical speeds and beyond the limits cutting ceases. The Taguchi method for determining laser cut quality was introduced by Yilbas [23].He showed that the end product quality improved at certain combination of the levels of the cutting parameters.

   The operating cost of a laser system is high when operated insufficiently, i.e. efficient operation is desirable. Moreover, high material removal rate, high dimensional accuracy, good end product quality and high degree of process repeatability must be achieved to ensure the laser machining operation viable. Consequently, investigation into thermodynamic efficiencies (first and second law efficiencies) and quality assessment of the end product in laser machining operation is essential. In the present study, oxygen assisted laser cutting of sheet metal is considered. The kerf width is presented analytically using t he scaling laws [20,24] and the parametric study based on a factorial analysis is introduced to access the resulting cut quality.

3. THERM ODYNAMIC INVESTIGATION A ND MODELLING OF CUTTING PARAM ETERS

   To perform the calculation of laser cutting parameters ( liquid layer thickness and laser-power requirements), the flow field developed at the gas- liquid boundary needs to be considered. Flow equations derived in the previous study are used. Laminar model for the flow conditions is taken into account and the following assumptions are made:

   1. At any time the interface between the liquid layer and solid material is a straight line and is the melting isotherm.

   2. The melting isotherm moves into the solid material with a velocity Vm (melting velocity) and it does so normal to the melting isotherm.

   3. The velocity of the melting isotherm is considered as the cutting speed.

   4. The blowing jet is attached to the molten liquid layer at any instant and throughout the molten layer.

   5. The physical properties of the liquid layer ( ) and of the solid material ( ) are constant throughout the liquid layer and the solid material.

   6. The thickness of the liquid molten metal remains constant for a particular gas jet velocity and melting velocity at any instant.

   In the light of the above assumptions, the establishment of the liquid layer thickness may be formed through the following steps.

   It was shown previously that inverse of the liquid-gas interface shear stress is[1]

   where is the gas-liquid interface shear stress and can also be written as

   (2)

   (3)

   Combining eqns. (1) (3) results in

   Knowing that[1]

   And combining eqns. (4) and (5) yields

   Since and .

   It is also shown that[13]

   Where S is the Stanton number. Substituting eqn. (7) in to eqn. (6),yields

   Or

   )

   Where .

   Knowing ,the liquid layer thickness can be calculated. To determine the heat transfer rate at the gas-liquid interface, it becomes customary to calculate the temperature rise across the liquid layer. Consider the heat transfer to the solid material. This will be less than that transferred from the hot-gas boundary layer to the liquid metal by an amount equal to that absorbed by the molten layer. If the molten layer does not flow back along the surface, heat absorbed per unit area is at the rate of

   )

   Where is the melted surface temperature, the melting rate per unit area and the melting temperature. In practice, the melted layer does flow back down stream and absorbs more heat than it would do if it remained in place. This absorption by the melted layer can be approximated as [13]

   Consequently, the rate of heat transfer to the solid under the material is approximately

   Or

   Rearranging eqn (12) yields

   (12)

   Where

   Knowing , can be determined and, consequently, can be calculated. One of the explanation of the above analysis is that if a power intensity ql is applied at the gas-liquid interface, the melting isotherm at the liquid-solid interface will advance into the solid material with a velocity which is the cutting velocity deÞned as the speed at which the isotherm propagates into the solid material.

4. FIRST A ND SECOND LAW ANA LYSIS OF CUTTING PROCESS

   In laser cutting process, heat input due to laser radiation increases the temperature of the solid metal to liquid temperature, i.e. temperature of the base metal increases from to . The heat input required for this process is

   Assuming is constant,The actual heat input due to laser irradiation is given by eqn(12), which is

   Consequently, the first law efficiency becomes

   Noting that during the heat transfer process, temperature of the base material increases, first, from to , then from to .

   The second law efficiency (energy efficiency) includes the heat required and input as well as the temperatures at which the heat exchange take place.[14] Therefore, second law efficiency is a reasonable measure of the quality of heat transfer. Consequently, the energy required for the laser melting process is

   And the actual exergy input ( is

   However, the second law efficiency can be defined as

   In this case ,it yields

   Consequently, can be computing the variables in eq uation (19).

   Laser Head

   Mirror

   Photo Detector Unit

   Laser Beam

   Nozzle

   Lens Fibre Optic cable

   Computer Fibre Probe

   Workpiece Oscilloscope

   X-Y Table

   Fig.1. Experimental Setup

5. EXPERIM ENTA L SET UP

   The experimental setup is shown in Fig. 1 CO2 laser delivering 500W output power was employed to cut mild-steel samples of 0.8 to 2 mm thicknesses. 100 mm focal length lens was used to focus the laser beam. Oxygen/argon gas mixture of 5/1 pressure ratio was introduced co-axially with the laser beam through a convergent nozzle of about 0)7 mm exit diameter. To obtain clear boundaries of the resolidified melt zone, cross-sections of the keyhole was etched before taking the photographs. Optical microscope was used to measure the thickness of the resolidified melt zone taking place above the free surface. The experiment was conducted by altering the total oxygen/argon gas mixture pressure fro m 75 to 172.5 kPa.

6. DISCUSSION

   For the numerical appreciation of results the following parameters were considered, for the I & II law evaluation of laser cutting process. The effects of gas jet velocity, workpiece thickness and cutting speed have been observed on the performance parameters like liquid layer thickness, required power intensity, I and II Law efficiencies.

   Figure 2 shows variation of the liquid layer thickness with jet velocity for different cutting speeds and work piece thicknesses. It is evident that increasing gas jet velocity decreases the liquid layer thickness, i.e. a large gas jet velocity results in a finite liquid layer. A similar trend may be seen for higher cutting speeds, except that the liquid layer is thicker. In this case, the gas- liquid interface needs to be maintained at a higher temperature, which in turn increases the energy requirement. It is evident that thinning down of the liquid layer occurs with increasing gas jet velocity. This is due to the increase in the interface (gas- liquid) shear stress ,which in turn enhances the blowing rates of the molten metal.

   LIquid Layer thickness

   (micrometer)

   120

   100

   80

   60

   40

   20

   0

   Cutting Speed

   0 50 100 150 200 250 300

   1 cm/s

   2 cm/s

   3 cm/s

   Gas Jet velocity (m/s) (Workpiece thickness = 1.0 mm)

   (a)

   LIquid Layer thickness

   (micrometer)

   200

   150

   100

   50

   0

   Cutting Speed

   0 50 100 150 200 250 300

   1 cm/s

   2 cm/s

   Gas Jet velocity (m/s) (Workpiece thickness = 1.5 mm )

   (b)

   LIquid Layer thickness

   (micrometer)

   240

   200

   160

   120

   80

   40

   0

   Cutting Speed

   0 50 100 150 200 250 300

   1 cm/s

   2 cm/s

   3 cm/s

   Gas Jet velocity (m/s)

   (Workpiece thickness = 2.5 mm )

   ( C )

   Fig.2

   The variation of the required absorbed power intensity (q1), distributed over the cut surface at the gas- liquid nterface with gas jet velocity is shown in Fig. 3 for different gas jet velocities and cutting speeds. The power intensity requirement (q1) does not vary considerably with gas jet velocity at low cutting speeds. This is due to that the liquid layer thickness is very small (Fig. 5) and power dissipated in the liquid layer becomes very small. On the other hand, the power intensity requirement at the interface varies significantly with

   gas jet velocity at great thickness, since the liquid layer thickness and, consequently, interface temperature is high for thick targets. In other words, the interface must be maintained at a higher temperature for thicker materials, in order to maintain the cutting speed, which in turn results in increased absorbed power intensity (q1) requirement at the gas- liquid interface. It should also be noted that the liquid layer thickness is larger for thicker materials and for lower gas jet velocities. In this case, interface temperatures should be maintained at a higher values which consequently demand increased absorbed power intensity.

   Required Power Intensity

   (W/mm2)

   50 Jet velocity

   45

   40

   35

   30

   25

   20

   0 0.5 1 1.5 2 2.5 3

   Workpiece Thickness (mm)

   (Cutting speed = 2 m/s)

   250 m/s

   200 m/s

   150 m/s

   100 m/s

   (a)

   Required Power Intensity

   (W/mm2)

   180

   157.5

   135

   Jet velocity

   250 m/s

   112.5 200 m/s

   90

   (b)

   0 0.5 1 1.5 2 2.5 3

   Workpiece Thickness (mm)

   (Cutting speed = 3 cm/s)

   Fig 3

   150 m/s

   100 m/s

   Figure 4 shows the variation of absorbed power intensity (q1) with cutting speeds for various gas jet velocities and thickness. Q1 increases with increasing cutting speeds. The saturation of cutting speed with increasing absorbed power intensity requirement occurs at cutting speed of 8 cm/s. This may be due to that the momentum and hence interface shear stress is insufficient for the liquid- metal removal from the cutting zone at high cutting speeds, and as a result of this mechanism the liquid molten layer becomes thicker with the consequence that a very small fraction of q1 reaches the solid- liquid interface. A similar argument can be made for the high gas jet velocities with the fact that at high gas jet velocities q1 is less than that corresponds to low gas jet velocities for the same material thickness and cutting speeds. Figure 5 shows the variation of the liquid layer thickness with gas jet velocity for different work piece thickness, Increasing gas jet velocity decreases the liquid layer thickness. Therefore, the higher the gas jet velocity is there always exists a liquid layer thickness developing on the free surface of the work piece . This justifies the fluid- heating model adopted in this analysis.

   Required Power Intensity

   (W/mm2)

   50 Jet velocity

   45

   40

   35

   30

   25

   20

   0 0.5 1 1.5 2 2.5 3

   Workpiece Thickness (mm)

   (Cutting speed = 2 m/s)

   250 m/s

   200 m/s

   150 m/s

   100 m/s

   (a)

   (b)

   180

   Required Power Intensity

   (W/mm2)

   157.5

   135

   112.5

   90

   Jet velocity

   0 0.5 1 1.5 2 2.5 3

   Workpiece Thickness (mm)

   (Cutting speed = 3 cm/s)

   250 m/s

   200 m/s

   150 m/s

   100 m/s

   Fig.4

   Required Power Intensity (W/mm2

   0.3

   0.25

   Jet velocity

   0.2

   0.15

   0.1

   0.05

   250 m/s

   150 m/s

   100 m/s

   0

   0 1 2 3 4 5

   Cutting Speed (cm/s)

   (Workpiece thickness = 0.5 mm )

   (a)

   Jet velocity

   Required Power Intensity (W/mm2)

   0.5

   0.4

   0.3

   0.2

   0.1

   0

   1

   2

   3

   4

   0 1 2 3 4 5

   ( b )

   Cutting Speed (cm/s) Workpiece Thickness = 2.5 mm

   Fig 5

   It is evident that the liquid layer thickness is larger for greater thickness of materials for a corresponding gas jet velocity. This indicates that a higher power is required at the interface for thicker materials as discussed before. The thinning down of the liquid layer thickness with increased gas jet velocity is due to the increase in the gas- liquid interface shear stress, which in turn enhances the blowing rates of the molten metal.

   100

   Efficiency

   90

   80

   Efficiency ( %)

   First Law

   70

   Second

   60 Law

   50

   50 100 150 200 250 300

   Jet velocity/(m/s) Cutting Speed = 1 m/s

   Fig.6

   100

   95

   90

   85

   Efficiency ( %)

   80

   75

   70

   65

   60

   Efficiency

   50 100 150 200 250 300

   First Law

   Second

   Law

   Jet velocity (m/s) Cutting Speed = 2 cm/s

   Fig.7

   100

   95

   90

   85

   Efficiency ( %)

   80

   75

   70

   65

   60

   Efficiency

   50 100 150 200 250 300

   First Law

   Second

   Law

   Jet velocity (m/s)

   Cutting Speed = 3 cm/s

   Fig.8

   The variation of first and second law efficiencies (energy and exergy efficiencies) with jet velocity is shown in Figs 6-8 for different cutting speeds. It is evident that efficiencies show increasing trend with increasing jet velocities and cutting speeds. The increasing trend of the curves indicates that cutting efficiency, in general, improves at cutting speed of 3 cm/s and jet velocity of 250 m/s. This may again be due to the fact that increase in the jet velocity decreases the liquid layer thickness on the surface, which in turn results more incident laser energy reaching the target. O n the other hand, when cutting speed is low, the liquid layer temperature rises and this results unnecessary heat transfer from the liquid layer to surroundings. It should be noted here that the first and second law analyses are valid for the range of data employed in these analyses. Therefore, it is not always the case that at high as jet velocity and high cutting speed, the cutting efficiency is expected to improve, i.e. at gas jet velocities equal or more than speed of sound and for cutting speed more than 8 cm/s, the cutting ceases.

7. CONCLUSION

The conclusions derived from the present study may be listed as follows:

1. Increasing gas jet velocity increases the liquid layer thickness. This is due to the increase in gas-liquid interface shear stress.

2. Power intensity requirement at the gas-liquid interface varies significantly with gas jet velocity at great thickness. In this case, interface temperature must be maintained at higher temperature in order to maintain the cutting speed for thicker materials.

3. Saturation of cutting speed with increasing absorbed power intensity requirement occurs at cutting speed of 8 cm/s.

4. Liquid layer thickness measured experimentally is in good agreement with the theoretical predictions providing that little variation occurs in both results due to the experimental error, which was predicted within the range less than 10%, and assumptions made in the analysis.

5. The first and second law efficiencies of cutting process improve with increasing cutting speed and jet velocity for a given range o f data.

References

1. Yilbas, B.S., Yilbas, Z. Effects of plasma on CO2 laser cutting quality, Opt Lasers Eng 9, (1988) l-12

2. Yilbas, B.S. A study of parameters affecting the continuous CO2 laser cutting. J Chinese Inst Eng.10(5). (1987) 543-547

3. Hoffman, M., The laser as an industrial cutting tool, Metal Const., Jan. (1979) 88-9.

4. Cooper C., An investigation into parameters effecting the cutting efficiency of a CO2 laser, M.Sc. thesis, Mech. Eng. Dept., University of Birmingham, 1973.

5. Bolin, S., Laser welding, cutting and drilling Part I, Source Book of laser Applications ASM publication, 1981, pp. 30-34.

6. J.N. Gonsalves, W.W. Duley, Cutting thin metals with the CO2 laser, J. Appl. Phys. 43 (1972) 4684-4687.

7. K.A. Bunting, G. Cornfield, Toward a general theory of cutting: a relationship between the incident power density and the cut speed,ASME J. Heat Tranfer 97 (1975) 116-121.

8. F.O. Olsen, cutting front formation in laser cutting, CIRP 38 (1989)215-218.

9. K. Danisman,B.S. Yilbas,Z. Yilbas,Study of some characteristics of plasma generated during CO2 laser beam cutting process, Opt. Laser Technol. 24 (1992) 33-3

10. I. Belic,A method to determine the parameters of laser cutting,Opt. Lasers Eng. 21 (1989) 277-278

11. P.D. Pietro, Y.L. Yao, A new technique to characterize and predict laser cut striations, Int. J. Mach. Tools Manuf. 35 (1995) 993-1002.

12. S.L. Chen,The effects of gas composition on the CO2 laser cutting of mild steel, J. Mater. Process. Technol. 73 (1998) 234-242.

13. G.V.S. Prasad, E. Siores, W.C.K. wong, Laser cutting of metallic coated steels, J. Mater. Process. Technol. 74 (1998) 147-159.

14. P. Sheng,G. Chryssolouris, Investigation of acoustic sensing for laser machining processes. Part 2. Laser gooving and cutting, J. Mater. process Technol. 43 (1994) 145-163.

15. G.R.B.E. Romer, J. Meijer, Metal surface temperature induced by moving laser beams, Opt. Quant. Electron. 27 (1995) 1397-1406.

16. F.M. Modest,Three dimensional, transient model for laser machining of ablating/decomposing materials, Int. J. Heat Mass Transfer 39 (1996) 221-234.

17. Z. Ji, S. Wu,FEM simulation of the temperature field duing the laser formation of sheet metal, J. Mater. Process. Technol. 74 (1998) 89-95.

18. M.J. Hsu, P.A. Molian, Off-axis, gas-jet-assisted, laser cutting of 6.35 mm thick stainless steel,ASME J. Eng. Ind. 117 (1995) 272-276.

19. B.S. Yilbas, A.Z. Sahin, Turbulent boundary layer approach allowing chemical reactions for co2 laser oxygen-assisted cutting process,Proc. Inst. Mech. Eng. C 208(1994) 275-258

20. B.S. Yilbas, The analysis of CO2 laser cutting, Proc. Inst. Mech. Eng. B 211 (1997) 175- 180.

21. B.S. Yilbas, A study into laser cutting process, Heat mass Tranfer32 (1997) 175-180

22. G. Simon, U. Gratzke, Theoritical investigations of instabilities in laser gas cutting, in: Procedings of the conference SPIE89 on High power Lasers and Machining Technology, vol. 1132, Paris, France, 1989, pp.211-221.

23.J.Powell,T.G. K ing, I.A. Menzies, Cut edge quality improvements by laser pulsing, in: Proceedings of the Second international conference on lasers in Manufacturing, Birmingham, UK, 1985, pp. 37-45.

1. A. Kar, A.Rothenflue, P. Latham, Scaling laws for thick-section cutting with a chemical oxygen- iodine laser Appl. 9 (1997)279-286.

2. B.S. Yilbas, Experiment investigation into CO2 laser cutting parameters, J. Mater. Process Technol. 58 (1996) 323-330.

3. G. Van Wylen, R. Sonntag, C. Borgnakke, Fundamentals of Classical Thermodynamics, 4th ed., Wiley, New York, 1994

4. B.S. Yilbas,Z. Yilbas,Effects of plasma on CO2 laser cutting quality, Opt. Lasers Eng. 9 (1988) 1-12.

5. B.S. Yilbas, Effect of process parameters on the Kerf width during the laser cutting processes, Proc. Inst. Mech. Eng. B 215 (1994) 1357-1365