Materials and Manufacturing Processes

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CO2 Laser Cutting of Calcareous Stones R. M. Miranda ab; L. Quintino b a Universidade Aberta, Lisboa, Portugal b IDMEC, Lisboa, Portugal

Online Publication Date: 31 December 2004 To cite this Article: Miranda, R. M. and Quintino, L. (2004) 'CO2 Laser Cutting of Calcareous Stones', Materials and Manufacturing Processes, 19:6, 1133 - 1142 To link to this article: DOI: 10.1081/AMP-200035267 URL: http://dx.doi.org/10.1081/AMP-200035267

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MATERIALS AND MANUFACTURING PROCESSES Vol. 19, No. 6, pp. 1133–1142, 2004

CO2 Laser Cutting of Calcareous Stones R. M. Miranda1,2,* and L. Quintino2 1

Universidade Aberta, Lisboa, Portugal 2 IDMEC, Lisboa, Portugal

ABSTRACT Portugal is one of the major European producers of natural stones. In the last decade, transformation of stones has been privileged in most of the companies and the quantity of finished product for exportation increased with a major added value. New technologies and processes have been investigated. For example, CO2 laser has been used for cutting, marking, and drilling. The major advantage of this tool is its flexibility, and thus, it improved the working environment significantly. This article presents a report on the use of CO2 lasers in the cutting process of marbles and limestones. The cut quality was evaluated by adjusting the laser output power and assist gas type and pressure. The CO2 laser can be used as a feasible tool for cutting ornamental stones. Due to the economic reasons, it is specially adequate for cutting nonlinear shapes where conventional cutting tools, such as the diamond wires and saws, have limitations on both the shape and the dimensions to be cut. Key Words: quality.

Laser cutting; CO2 laser; Cutting stones; Natural stones; Cut

*Correspondence: R. M. Miranda, Universidade Aberta, R. Escola Politećnica 147, Lisboa 1269-001, Portugal; E-mail: [email protected]. 1133 DOI: 10.1081/LMMP-200035267 Copyright & 2004 by Marcel Dekker, Inc.

1042-6914 (Print); 1532-2475 (Online) www.dekker.com


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Miranda and Quintino

INTRODUCTION Ornamental stones are widely used in building, construction, and decoration. Portugal is one of the major European producers and exporters of ornamental stones, namely, granite, marble, and limestone. In the past, companies have invested mostly in the transformation technologies, allowing transformation of exploited blocks into finished products and adding value to the high-quality raw material. New technologies have been investigated to transform stones aiming at increasing productivity, flexibility, and working environments. High-power lasers were tested, especially due to the flexibility of the tool; lasers can cut, drill, machine, and engrave. Limitations were identified when processing granite, but marble and limestone can be processed under certain conditions. This article reports the capabilities of a CO2 laser to cut marble and limestone.

EXPERIMENTAL PROCEDURE From the large amount of calcareous stones transformed in Portugal, three different stones were selected and tested in this work, under the commercial marks Moca Creme, Rosa de Borba, and Branco Pardais. Moca Creme is a limestone, extracted from the Limestone Massif of Estremadura. It is widely used in interior facings due to its ornamental aspect, especially when it is cut normal to the rift plane. Rosa Aurora, usually called Rosa de Borba due to its occurrence in the Estremoz-Borba-Vila Viçosa anticlinorium, is a calcitic rosed white marble with darker rose tonalities, sometimes greyish or brownish veined, that has prevailing applications in both exteriors and interiors. Branco Pardais is a white marble from the Estremoz-Borba-Vila Viçosa anticlinorium, dated from lower Cambrian, also used in both indoor and outdoor applications.[1] Petrographic analysis of the stones was performed under optical microscopy and showed the following most relevant features.[2] Moca Creme is biomicritic with a coarsely clastic argilous matrix with a total porosity of about 9.66%. . Branco Pardais is a crystalline calcitic marble with about 98% calcite, a medium-grained rock with granoblastic texture and fine-grained zones and a total porosity of about 0.40%. Accessory mineral is quartz. . Rosa de Borba is a crystalline calcitic marble with about 99% calcite, a fine-grained rock with granoblastic texture with medium-grained zones. Accessory mineral is sericite. .

The hardness is a very simple way to measure the characteristic widely used in stone processing. The hardness was measured using a Vickers indenter under a load of 19.6 N.


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CO2 Laser Cutting of Calcareous Stones Table 1.

Laser cutting parameters.

Cutting parameter Laser power (W) Assist gas pressure (bar) Assist gas type

Moca Creme

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Values 500; 1,000; 1,500; 2,000; 2500 5; 20 Argon and compressed air

HV83 10

Branco Pardais HV103 8 Rosa de Borba HV100 4 The cutting experiments were performed with a Rofin Sinar CO2 laser excited by a radiofrequency delivering a maximum output power of 2.5 kW at a wavelength of 10.6 mm operating in a continuous wave mode. The laser beam has a transverse electromagnetic mode TEM20 and a beam diameter of 19 mm evaluated by deep burns in acrylic blocks. The laser power was monitored by means of a beam splitter and a power meter incorporated into the laser. The laser beam was focused onto the surface using a ZnSe lens with a focal length lens of 127 mm. Both compressed air and argon were used as coaxial gases, at pressures of 5, 10, 15, and 20 bar. The laser beam and the coaxial gas passed through a conical copper cutting nozzle with an aperture diameter of 3 mm. A computer numeric control X–Y table moved the stone plates in relation to the fixed laser head. Laser cutting parameters tested were varied as shown in Table 1. All cuts were performed on stone plates with varying thickness of 10, 20, and 30 mm. These are the common thickness used in the industry. The thinner is used for decoration and indoors applications, and the thickest ones are used for outdoor facings.

RESULTS AND DISCUSSION A systematic study was conducted on the influence of the most common operation parameters in the laser cutting process aiming at assessing the technical feasibility of CO2 laser for cutting and finding the optimum parameters. Each parameter was varied while keeping all others constant. The laser cutting involves a thermal process that consists of rapid and localized heating of the stone with the formation of a blue colored plasma at high energy densities in a molten shearing mechanism as observed by Steen.[3] Both plasma and particles resulting from stone decomposition absorb the energy radiation. The separation of the parts is produced when the assist gas injected coaxially with the laser beam blows off the plasma and the molten pool. Besides the effect of expelling the molten material, the assist gas provides a cooling effect on the cut surfaces that decreases the extension of the heat-affected zone.[4]


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Figure 1.

Visual aspect of the lower edge of a laser cut produced in Rosa de Borba.

Figure 2.

Visual aspect of a laser cut surface produced in Moca Creme.

When the plate thickness increases, the quantity of the molten material increases and it is more difficult to blow away, even at higher assist gas pressures. When the cutting speed decreases, the cut surface quality improves and the molten material is more efficiently removed, thus the adherent dross decreases.[5,6] In calcitic marbles, a vitrified dross was observed in the lower edge of the cut (Fig. 1). The limestone did not exhibited dross, and the cut surfaces were of industrially acceptable quality. Generally, the cut surfaces exhibited microscopic cracks and grain detachments in a narrow width affected by the thermal cycle imposed by the laser (Fig. 2). The extension of this layer was measured and seen to be less than 0.3 mm and is easily removed during finishing. Another interesting observation was that the cut surfaces exhibited a lighter color, and a color change along the plane of cutting was observed (Fig. 1). This is due to structural modifications of the rock under the laser radiation, which are currently under investigation. When investigating the effect of laser cutting operating parameters, it was seen that the maximum cutting speed increases with the laser beam power for the same cut quality evaluated visually. Figure 3 depicts the maximum cutting speed vs. the laser output power for the tested stones, keeping all other parameters constant.

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CO2 Laser Cutting of Calcareous Stones

Maximum cutting speed (mm/min)


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350 300 250 Moca Creme Rosa de Borba Branco Pardais

200 150 100 50 0 500

1000

2000

2500

Potência (W) Figure 3. Maximum cutting speed vs. the laser output power keeping all other parameters constant (thickness: 20 mm; assist gas: compressed air; pressure: 5 bar).

The cutting speed follows a linear relationship with the laser output power, which is in agreement with the theoretical analysis of the energy balance in a cutting process.[3,6] If the energy input increases, the interaction time can be reduced and the cutting speed increased in order to keep the process efficient, regarding the energy. The slope of the curve is smoother for the calcitic marbles and sharper for the limestone; that is, an increment in the laser power increases the cutting speed for Moca Creme, whereas in calcitic marbles it is absorbed in the dross itself. Another remark is that the slope of the curve is of the same order of magnitude for both marbles. In fact, Rosa de Borba and Branco Pardais have quite similar physical and mechanical properties, whereas Moca Creme has a higher thermal conductivity. Boutinguiza et al.[7] suggested a simple equation to describe the energy balance in the cut process, which is: P=tv ¼ A

ð1Þ

where P is average laser power, t is material thickness, v is cutting speed, and A is a constant that depends on the optical and physical properties of the material. This formulation assumes all the energy introduced in the interaction area laser/material is used to melt before heat losses occur. Using this relationship and plotting the average laser power to plate thickness ratio as a function of the maximum cutting speed as shown in Fig. 4, it can be observed that the tested materials very much agree with these assumptions. Constant A is different in Moca Creme from the other crystalline marbles, which behave similarly. This difference is due to the materials reflectivity and thermal conductivity, which are higher for Moca Creme.

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Average power/thickness (W/mm)


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1200 1000

y = 3.5892x R2 = 0.8822

800 600 400

y = 2.2255x 2 R = 0.8785

200 0 0

100

200

300

Maximum cutting speed (mm/min) rosa+branco

moca

Figure 4. Average laser power to plate thickness ratio as a function of the maximum cutting speed for the tested stones in three thickness.

Plotting the cutting speed for different plate thickness of tested stones, a sharp reduction in the cutting speed occurs when the plate thickness increases (Fig. 5). When the plate thickness is 10 mm, this relation tends to be parabolic as already reported by other authors for metallic and polymeric materials.[6] One interesting feature observed is that the cutting speed is more than halved when the plate thickness increases from 10 to 20 mm, whereas there is a minor difference when it increases from 20 to 30 mm. This suggests the existence of a critical thickness above which an increase of the laser power does not result in a significative increase of the cutting speed. This critical thickness should be around 10 mm, which, from an industrial point of view does not constitute a limitation because, for decorative purposes, tiles are around this thickness. When cutting Moca Creme, a sedimentary stone, the sedimentation direction was seen to drastically affect the cutting speed, especially for the thicker plates. The cutting is faster along the preferential orientation plane than along the plane perpendicular to the previous one, and the cutting speed can be increased by a factor of 2 (Fig. 6). When investigating the influence of the gas type on the cutting speed, tests were conducted with an inert gas (argon) and compressed air. Because the rocks under study have residual oxidizing elements, such as iron, it was considered that the effect of oxygen was very similar to compressed air.[6] The tests conducted have shown that the gas type has no significant influence on the maximum cutting speed and on the cut surface quality, so for economic reasons compressed air was used. When the gas pressure increases, the cutting speed increases. Figure 7 depicts the maximum cutting speed as a function of the assist gas pressure. It can be seen that

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800 600 Moca Creme Rosa de Borba Branco Pardais

400 200 0 10

20

30

Thickness (mm) Figure 5.

Maximum cutting speed vs. thickness for the tested stones. Laser power: 2000 W.

350 Maximum cutting speed (mm/min)


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S0 S1

150 100 50 0 1000

2000

2500

Power (W) Figure 6. Effect of sedimentation plane on maximum cutting speed for Moca Creme. S0, sedimentation plane; S1, perpendicular to sedimentation plane. Thickness: 20 mm.

for pressures within the range 15 to 20 bar, the cutting speed can be improved by almost 50%. However, this poses an operating problem, that is, the need of a special high-pressure focusing lens, which are expensive. The assist gas pressure affects the efficiency of the molten material removal[8] and, when its pressure increases, the molten material is more rapidly removed. This has two consequences: the cutting speed can be increased, and the cut quality can be improved. Stone hardness is a very used parameter in stone processing.[2] Plotting the maximum cutting speed as a function of the stone hardness, it was observed

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600 Maximum cutting speed (mm/min)


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10

15

30

Assist gas pressure (bar) Figure 7. Maximum cutting speed vs. assist gas pressure for Moca Creme, 10 mm thick. Laser power: 1000 W.

that there is a linear relationship between the stone hardness and the maximum cutting speed (Fig. 8): The harder the stone, the slower the cut. When cutting ornamental stones, specific energy is used to compare the efficiency of the cutting processes. The specific energy is defined as the energy required to remove a volume unit of material. The concept was first introduced by Teale[9] and used by other researchers for mechanical cutting.[10–12] Specific energy is highly related to not only mechanical properties such as compressive strength, fracture toughness, and elasticity modulus,[13] but also microstructural features such as porosity, grain size, and mineralogical composition.[14] Using the first concept, the specific energy, Esp, in a laser cutting process can be defined as: Esp ¼ Et=v

ð2Þ

where Et is total energy introduced and v is cutting speed. Et can be assumed as: Et ¼ P=A

ð3Þ

where P is emitted laser power, A is area of interaction between the laser beam and the material, and is the material absorption. Assuming an absorption coefficient of 0.5 for finished marbles under CO2 laser radiation, Esp can be calculated for the materials under study with varying thickness (Fig. 8). It can be observed that an increase in the plate thickness increases the specific energy. For the same plate thickness, the specific energy depends on the material characteristics, and it is possible to be correlated with the stone hardness (Fig. 9). It is lower for the soft stone and higher for marbles, but of the same order of magnitude. It is observed to be much higher than for diamond sawing. Neves[15] obtained values of 3 105 J/m3 and 12 105 J/m3 for Moca Creme and Branco Pardais, respectively, for diamond cutting of 20-mm-thick plates.

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700 600 500 400 300 200 100 0 80

95

110

Hardness HV Figure 8.

Maximum cutting speed vs. stone hardness. Thickness: 20 mm.

60 Specific energy (x1012 J/m3)


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40 80

95

110

Hardness HV Figure 9. Specific energy for CO2 laser cutting as a function of stone hardness. Thickness: 20 mm.

CONCLUSION In conclusion, it can be said that marble and limestone tiles can be cut with a 2.5 kW CO2 laser with a fairly good cut quality. The maximum cutting speed obtained increases with the laser output power and decreases with the plate thickness. For thicknesses higher than 20 mm there is an almost constant cutting speed, suggesting a critical thickness for cutting. Thus, the linear cutting speed observed with the laser is lower than the one achieved with diamond saws. When cutting complex shapes, some important advantages emerge from using laser cutting: The cutting speed is quite compatible, and the laser has no limitations in both shapes and dimensions. Another important advantage relies in the fact that the laser is a clean tool that does not produce noise or waste to be recycled.


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Assist gas type has no significant effect on the cut quality, but the pressure highly affects the cutting speed because the plasma and the molten material are more easily blown away from the cut front. The specific energy required for cutting is higher than the one observed for diamond saws, and it increases with the stone hardness and the plate thickness. REFERENCES 1. Catalogue of Portuguese Ornamental Stones; General Direction of Geology and Mines: Lisbon, Portugal, 1983 (available in English and Portuguese). 2. Miranda, R.M. Contribution to the Phenomenological Study of Portuguese Ornamental Stones Processing with Water Jet and Laser. Ph.D. thesis, Technical University of Lisbon, 1996 (only available in Portuguese). 3. Steen, W.M. Laser Materials Processing; Springer Verlag: London, 1993. 4. Livingstone, S.A.J.; Chua, K.L.; Black, I. Experimental development of a machining database for the CO2 laser cutting of ceramic tile. J. Laser Appl. 1997, 9, 211–214. 5. Yibas, B.S. A study into CO2 laser cutting. Heat and Mass Transfer 1997, 32, 175–180. 6. Powell, J. Laser Cutting; Springer Verlag: New York, 1989. 7. Boutinguiza, M.; Pou, J.; Lusquinos, F.; Quintero, F.; Soto, R.; Perez-Amor, M.; Watkins, K.; Steen, W.M. CO2 laser cutting of slate. Optics Lasers Eng. 2002, 37, 15–25. 8. Chen, S.L. The effect of high-pressure assistant-gas flow on high-power CO2 laser cutting. J. Mater. Proc. Tech. 1999, 88, 57–66. 9. Teale, R. The concept of specific energy in rock drilling. Int. J. Rock Mechn. Min. Sci. 1965, 2, 57–74. 10. Summers, D.A. Waterjetting Technology; E & F N Spon: London, 1995. 11. Vijay, M.M. Evaluation of Abrasive Entrained Waterjets for Slotting Hard Rocks, Proc. of the 5th American Water Jet Conf., Toronto, Canada, Aug 29–31, 1989; 333–347. 12. Brook, N.; Page, C.H. Energy Requirements for Rock Cutting by High Speed Water Jets, Proc. of the 1st Int. Symp. on Jet Cutting Techn., Coventry, UK, April 5–7, 1972; B1-1–B1-12. 13. Moodie, K.; Taylor, G. A Review of Current Work on Cutting and Fracturing of Rocks by High Pressure Waterjets, Proc. of the Conf. on Fluid Power Equipment in Minning, Quarrying and Tunneling I. Mechn. Eng., UK, Feb 1974; 41–48. 14. Agus, M.; Bortolussi, A.; Ciccu, R.; Kim, W.M.; Manca, P.P. The Influence of Rock Properties on Waterjet Performance, Proc. of the 7th American Water Jet Conf., Seattle, USA, Aug 28–31, 1993; 427–442. 15. Neves, A.P. Rock Cutting Geomechanics. Master’s thesis, Technical University of Lisbon, 1993 (only available in Portuguese). Received January 25, 2003 Accepted February 26, 2004


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