A COMPARATIVE EXPERIMENTAL STUDY OF

A COMPARATIVE EXPERIMENTAL STUDY OF LASER FUSION CUTTING OF STEEL WITH 1 M AND 10 M LASER WAVELENGTHS Paper #502 1

Koji Hirano , Remy Fabbro 1

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Instrument System R&D Div., Process Research Laboratories, Technical Development Bureau, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan 2 PIMM Laboratory, CNRS-Arts et Métiers ParisTech, 151 Boulevard de l’Hôpital, Paris 75013, France the same, so it is reasonable to consider that the quality difference is caused by an effect of the laser wavelength such as laser beam absorption on kerf front, which is governed by the wavelength dependent Fresnel absorption law.

Abstract This study aims to contribute to a better understanding of the cut surface quality difference between a laser with 1 m wavelength (fiber or disc laser) and a 10.6 m CO2 laser for laser fusion cutting of thick steel plates. Laser cutting of SUS304 stainless steel with thicknesses of 3, 5 and 8 mm was performed with a multimode fiber laser and a CO2 laser with the same power and comparative beam focusing characteristics. As already reported in the literature, the fiber laser exhibits worse surface quality for thicknesses with 5 and 8 mm. The result shows that the degradation of the surface roughness occurs when the cutting velocity becomes smaller than 1.5 m/min. We measured also the inclination angle  of the kerf front and it is shown that this range of cutting velocity corresponds to  < 3 degrees. This supports results of our analyses that stronger disturbance of melt flow or less absorption on kerf sides, which are induced by the small angle , can be the reason for the laser wavelength dependence of the cut surface quality.

From theoretical side, a few mechanisms have been proposed to explain the wavelength dependence of the cut surface quality. Poprawe et al. [5] and Vossen et al. [6] discussed an unstable process of ripple formations on a kerf front based on a mathematical analysis of melt film ejection from the kerf. Petring et al. [7] proposed that in the case of 1 m, multi-reflections occur within the kerf and this destabilise lower part of kerf sides. We analysed dynamics of kerf front profile and showed that the kerf front in the case of a 1 m laser beam is more easily disturbed than the case of a 10 m laser beam for thick section cutting of steel [8]. We also pointed out that a lower absorption for a 1 m beam on discrete melt accumulations along kerf sides increases their viscosity and as a result surface roughness left on kerf sides [8]. Although none of these proposals have been verified experimentally, it should be noted that all of them are related to the wavelength dependence of the Fresnel absorption law. Thus experimental investigations on the mechanism of the wavelength dependence should be focused on this aspect. In particular, kerf front geometry seems to be the primary target, because the absorptivity of the laser beam at some point on the kerf front is highly dependent on the incidence angle of the laser beam. Comparisons of longitudinal kerf front profile [9] and transverse kerf side profile [10] between a disc or fiber laser and a CO2 laser have been reported. However, the results of these experimental studies could not be related to as far as the difference of surface roughness.

Introduction Laser cutting of steel is one of the most important applications in laser material processing. Most laser cutting machines still utilize conventional CO2 lasers, in spite of recent development of high brightness solid state lasers, such as fiber and disc lasers. A big reason is that the use of a fiber or disc laser, whose emission wavelength is situated around 1 m, leads to a worse cut surface quality than a 10.6 m CO2 laser when applied to cutting of thick steel plates. When the sample thickness goes over 4 or 5 mm, roughness of cut surfaces obtained with a fiber or disc laser becomes larger than the case of a CO2 laser [1-4]. Scintilla et al. [4] performed a comparative experiment with a disc and a CO2 laser using almost the same laser beam diameter and gas condition. Cut surface quality obtained by the disc laser was inferior to that obtained with the CO2 laser for thicknesses of 5 mm and 8 mm. The operating conditions for the two lasers were nearly

In this study, comparative cutting experiments are conducted with a fiber and a CO2 laser using almost the same operating parameters (laser power, focus diameter, Rayleigh length, focus position and assistgas pressure) for the two lasers. The object of this

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(a) Fiber

(b) CO2

Figure 1. Characteristics of the focused beams of the two lasers. experiment is not only to reconfirm the cut surface quality difference, but also to investigate effects of the inclination angle  of kerf front. It is well-known that the inclination angle  of kerf front varies with cutting velocity Vc. Thus the cutting velocity is varied in a wide range for each combination of laser cutting parameters.

fiber laser is still worse than the CO2 laser for thicker materials. The assist-gas was nitrogen with the pressure of 16 bar (relative value to the ambient pressure) at the reservoir. The nozzle diameter and the stand-off distance from the workpiece surface were 2.5 mm and 1 mm throughout the series of experiments.

Experimental Setup

The cutting speed was varied from 20% to 100% of the maximum cutting velocity for each sample thickness and laser wavelength. All of the other parameters such as laser power, focal position, and assist gas conditions were not changed.

Laser cutting of SUS304 stainless steel was performed by two standard industrial laser cutting machines with a 5 kW fiber laser and a 4 kW CO2 laser in Air Liquid Welding Company. The laser powers at the focus positions were adjusted to obtain the same level of 3.7 kW for both lasers. In the case of the fiber laser, output beam from a 150 m multimode fiber core was imaged with a collimation lens (fc = 150 mm) and a focusing lens (f = 250 mm). For the CO2 laser, collimated output from the laser oscillator was focused with a focusing lens of 7.5”. Focusing characteristics which were measured by a PRIMES Focus Monitor are shown in figure 1. The focus diameter was 250 m for both of the wavelengths. Also the Rayleigh lengths were approximately the same (3.1 mm for the fiber laser and 3.4 mm for CO2) due to the fact that product of L•M2 was almost the same (L: laser wavelength). The intensity distributions at the focus positions, however, were different; top-hat for the fiber laser and quasi-Gaussian for the CO2.

Results Maximum cutting velocity The maximum cutting speeds for different thicknesses and lasers are shown in figure 2. For thin sheets (h = 3 mm), the maximum velocity of the fiber laser is twice as fast as that of the CO2 laser. But this advantage of the fiber laser diminishes as the thickness increases and the maximum speed becomes about the same for the thicknesses of 5 mm and 8 mm. This result agrees Cutting Velocity Vc (m/min)

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Thicknesses of SUS 304 plates were 3, 5, and 8 mm. The focus positions were varied for each thickness. The focus positions were at 1, 4, and 5 mm below the surface (inside the plate samples) for the thicknesses of 3, 5, and 8 mm, respectively. These values were optimised for the fiber laser by preliminary cutting trials, and the same values were used also in cutting with the CO2 laser. In spite of this advantage for the fiber laser, we will see that quality obtained by the

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Figure 2. Maximum cutting velocities for each thickness.

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Figure 3 Averaged inclination angle of the kerf fronts (top) and examples of cut surfaces (bottom). front is approximated by a straight segment that connects two points on the top and bottom surfaces.

with what has been reported in the literature [1-4]. Inclination angle  of kerf front

As a global tendency,  increases with Vc for any thickness and wavelength. This agrees well with a theoretical prediction by 2D approximation that local is proportional to (Vc/A()IL) (A(): absorptivity as a function of ; IL: incident laser beam intensity). This relation suggests also that local is influenced by the local laser beam intensity and the angular dependence of A. As can be seen in the kerf front profiles shown in figure 3, rather straight kerf profile is obtained with the fiber laser, whereas the kerf profile in the case of the CO2 laser exhibits rounded profile in the lower part of the kerf, especially at a high velocity. This result can

The inclination angle  of the central part of the kerf front is shown in figure 3. The kerf front was obtained after a sudden stop of laser beam during cutting. The kerf front is hardly changed during this turn-off process because cooling rate of the melt film is very high [11]. The values shown in figure 3 were measured using an optical microscope either by observing from above of the kerf front (solid marks) or by observing a polished cut surface of the central plane of laser cutting (empty marks). The two methods give nearly the same angles. The values are averaged ones along the sample thickness, because in these measurements the kerf

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(a) Fiber, 6 m/min

(b) CO2, 5.5 m/min

(a) Fiber, 1.5 m/min

(b) CO2, 1.4 m/min

Figure 4 Cut surfaces of 3 mm thick samples. (a) Fiber, 2.8 m/min

(b) CO2, 2.7 m/min

Figure 6 Cut surfaces of 8 mm thick samples.

Figure 5 Cut surfaces of 5 mm thick samples. m in this region of . The maximum cutting speed, however, is the same (see figure 2). This suggests that for the 1 m kerf front is not straight but involves “steps”, which have locally larger inclination angle and thus increase the absorptivity and the energy consumption efficiency of the 1 m laser beam [11].

be attributed to the difference in the beam intensity distribution. Lower intensity of the periphery of the quasi-Gaussian distribution can lead to large  in a corresponding region of the kerf front. Moreover, in the case of the fiber laser, the increase rate of  with Vc is saturated with the increase of Vc. This may be a result from the Fresnel absorption characteristics: A() increases with  in the range of 0 <  < 10 (deg), for the case of the 1 m wavelength, so the increase of  with Vc can be restricted.

Cut surface quality Cut surfaces for different thicknesses are shown in figures 4 to 6, and surface roughnesses on each of the cut surfaces are plotted in figure 7. As already reported in the literature, the 1 m wavelength yields a poor surface quality for thicker samples. Figure 7 and figure 3 indicate that the poor quality for 1 m is obtained in the low velocity range less than 1.5 m/min which corresponds to the  region less than 3 degrees.

One of the important outcomes is that in the case of the maximum thickness 8 mm,  is lower than 3 degrees for the 1 m wavelength and around 3 degrees for CO2 laser. The Fresnel absorption law predicts that the absorption of the laser beam is less efficient for the 1

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Figure 7 Roughness of cut surfaces measured for each thickness h of the sample and for each depth (dep) from the top surface. along the kerf sides. The melt accumulations transfer a part of this power to the solid part. The contact area on the solid surface is locally melted and this melting continues down to the bottom of the kerf as the melt accumulations go down. This is considered to be the fundamental mechanism of the striation generation process. Our simple model calculations showed that an increase of the absorbed intensity on the melt accumulations can increase their temperature, decrease viscosity of melt accumulations, and as a result, the surface roughness. The small inclination angle in the central part of the kerf front implies also a small inclination angle on kerf sides. The lower absorptivity for very small inclination angle of the kerf sides can be the reason for the worse quality obtained for thick section cutting with a 1 m laser beam.

Discussions As shown by the above experiments, worse cut surface quality for the case of the 1 m wavelength appears when the inclination angle  becomes smaller than 3 degrees. This supports our theoretical analyses for the mechanism of the degradation for 1 m [11]. There are two factors which originate from small . The first factor is the stability of melt flow in the central part. When the central flow is destabilised, the dynamics of melt accumulations which slides down the wall is disturbed. Consequently, striations, which result from this downward displacement of the melt accumulations, are degraded. Our analysis of kerf front profile with the chain model, where the dynamics of the kerf front was expressed by displacement of a small segment of chain, indicates that the cutting front for a 1 m laser beam is disturbed much more strongly than that for a 10 m beam for a region with very small inclination angle such as  < 3 degrees.

Conclusions A comparative laser cutting of SUS304 with nitrogen assist gas was performed with a 3.7 kW fiber and CO2 laser beams that have very similar focusing characteristics. As has been shown by the previous studies, our experiments show that the fiber laser

The other factor is dynamics of melt accumulations

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yields a high cutting speed for a thin sample and worse cut quality for a thick sample. The measurement of revealed for the first time that the degraded cut surface quality for the 1 m wavelength appears for condition of low  ( < 3 degrees), which corresponds to a low cutting velocity range less than 1.5 m/min for conditions used in this study. This result supports our proposals that the degradation for 1 m wavelength is caused by the small angle  which induces destabilization of melt flow in the central part of the kerf front and decreases laser beam absorption on melt accumulations on kerf sides.

explanations for different surface quality in laser cutting with 1 m and 10 m laser wavelengths, Journal of Laser Applications 24, 012006. [9] Scintilla, L.D., Tricarico, L., Mahrle, A., Wetzig, A. & Beyer, E. (2011) Experimental investigation of the cut front geometry in the inert gas laser fusion cutting with disk and CO2 lasers, Proc. 30th Int. Congress on Applications of Lasers & Electro–Optics (Orlando, FL), 40-49. [10] Stelzer, S., Mahrle, A., Wetzig, A. & Beyer, E. (2013) Experimental investigations on fusion cutting stainless steel with fiber and CO2 laser beams, Physics Procedia 41, 392-397. [11] Hirano, K. & Fabbro, R. (2011) Experimental investigation of hydrodynamics of melt layer during laser cutting of steel, Journal of Physics D: Applied Physics 44, 105502.

Acknowledgement We would like to express our sincere thanks to Air Liquid Welding Company in France for having performed the comparative laser cutting experiment presented in this paper.

Meet the Author Since 2003 Koji Hirano worked on R & D of laser metal processing in Nippon Steel Corporation (Now Nippon Steel & Sumitomo Metal Corporation). He was engaged in PhD study at Arts et Métiers ParisTech from 2009 to 2011 and obtained PhD in 2012.

References [1] Wandera, C., Salminen, A., Olsen, F. O. & Kujanpää, V. (2006) Cutting of stainless steel with fiber and disc laser, Proc. 25th Int. Congress on Applications of Lasers & Electro-Optics (Scottsdale, AZ), 211-220. [2] Himmer, T., Pinder, T., Morgenthal, L. & Beyer, E. (2007) High brightness lasers in cutting applications, Proc. 26th Int. Congress on Applications of Lasers & Electro–Optics (Orlando, FL), 87-91. [3] Hilton, P. A. (2009) Cutting Stainless Steel with Disc and CO2 Lasers, Proc. 5th Int. Congress on Laser Advanced Materials Processing (Kobe, Japan), No. 306. [4] Scintilla, L. D., Tricarico, L., Mahrle, A., Wetzig, A., Himmer, T. & Beyer, E. (2010) A comparative study on fusion cutting with disc and CO2 lasers, Proc. 29th Int. Congress on Applications of Lasers & Electro–Optics (Anaheim, CA), 249-258. [5] Poprawe, R., Schulz, W. & Schmitt, R. (2010) Hydrodynamics of material removal by melt expulsion: Perspectives of laser cutting and drilling, Physics Procedia 5, 1-18. [6] Vossen, G., Schüttler, J. & Nießen, M. (2010) Optimization of Partial Differential Equations for Minimizing the Roughness of Laser Cutting Surfaces, in M. Diehl et al. (eds.) Recent Advances in Optimization and its Applications in Engineering, Springer-Verlag, 521-530. [7] Petring, D., Schneider, F., Wolf, N. & Nazery, V. (2008) The relevance of brightness for high power laser cutting and welding, Proc. 27th Int. Congress on Applications of Lasers & Electro–Optics (Temecula, CA), 95-103. [8] Hirano, K. & Fabbro, R. (2012) Possible

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