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Opportunities in laser cutting with direct diode laser configurations
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CIRP-1694; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
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CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Opportunities in laser cutting with direct diode laser configurations Gonçalo Costa Rodrigues, Joost R. Duflou (1)* Department of Mechanical Engineering, KU Leuven, Belgium
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser Cutting Direct diode lasers
This paper explores the opportunities associated with the use of well-designed direct diode laser configurations for cutting of metal sheets. Continuously improving diode stacking, and consequently greater brightness, allow the requirements for laser cutting to be met without the use of further brightness converters (diode-pumped solid state lasers). This contributes to a reduction in the number of optical components and increased energy efficiency. Furthermore, laser architecture related degrees of freedom for tailoring the wavelength, beam polarization and beam shape are instrumental to increase the cutting performance. Different stacking configurations, optical structures and related cutting strategies are discussed, with performance validation through experimental verification. © 2017 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction Laser technology for material processing has been improving at a fast pace. Less than a decade ago industrial cutting of metal sheets was fully dominated by CO2 gas lasers, mainly characterized by a long wavelength (10 mm) and poor efficiencies (5–10%) [1]. Today, solid state fiber lasers with a shorter wavelength (1 mm) and significantly better efficiencies (>30%), are becoming dominant for metal cutting applications [2]. Diode lasers are important building blocks for the majority of these laser systems, as they are used for pumping the active medium. Fig. 1 provides a schematic overview of the laser technology involved in building up a multikW solid state laser. Single diode laser emitters produce an asymmetric beam shape and deliver just a few Watt of output power. Nevertheless, they are typically cheap components that can be used in combination to obtain much higher laser powers. The first natural step to increase power, at the cost of beam quality, is a side-by-side combination of multiple emitters. This is typically done in diode bars that share cooling and controlling channels (step 1) and that are stacked (step 2) into high power diode modules, which are mostly used for pumping solid state fiber lasers. Direct Diode Laser (DDL) technology, that involves the direct use of a diode beam for material processing, is a recent achievement in cutting applications and emerges from developments in diode pump components such as higher single emitter power, improved coupling optics coatings and, more effective cooling strategies. The use of less optical components brings advantages in efficiency, compactness, reliability and price. Nowadays the technology is mainly limited to the availability of the above mentioned diode pump modules. As can be seen in Fig. 1, smart arrangement of emitters of strategically chosen wavelength and polarization
* Corresponding author. E-mail address: Joost.Dufl[email protected] (J.R. Duflou).
direction, allows power scaling at no cost to beam quality (step 3). While polarization multiplexing is limited to doubling the power, variants using different wavelengths are theoretically unlimited in achievable power range. In practice a limited number of available wavelengths (800–1100 nm) and their broad bandwidth (around 30 nm) narrow the possibilities [3]. Such beams are commonly coupled into guiding fibers (step 4), which deliver the laser from the resonator closer to the workpiece, and were, until recently, used only for less brightness demanding applications. Even though substantial research efforts still focus on further improving the brightness of DDLs [4–6], the associated costs are typically too high when evaluated against established brightness converting techniques, such as the fiber laser principle. Witte et al. [6] provide a more detailed description of the state of the art for brightness increasing techniques, while documenting a novel approach for increasing the number of possible wavelength multiplexing steps by wavelength stabilization of commercially available pump modules. Most recently, it has been demonstrated that state-of-the art DDLs already meet the requirements, in terms of achievable cutting speed and edge quality, for cutting certain material and thickness combinations [7]. This paper aims to discuss opportunities associated with innovative DDL based systems that are smartly designed for cutting applications. In the free beam concept, where the laser source is built into the cutting head, not only can further elimination of optical components (e.g. guiding fiber and related coupling optics) be achieved, but this also allows to tailor polarization and beam shape to the cutting process. Regarding the latter, Fig. 2 illustrates the potential of a beam tailored for cutting purposes: laser absorption in the cutting kerf is highly influenced by the incidence angle and laser polarization. The red dot represents the typical absorption in the material for randomly polarized solid state lasers. The beam shape is responsible for kerf formation and can be used to positively influence the incidence angle, (strategy represented by the yellow dot in Fig. 2). Laser polarization can also imply a significant gain in absorption when
http://dx.doi.org/10.1016/j.cirp.2017.04.136 0007-8506/© 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136
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Fig. 1. Different system stages from single diode laser emitter to laser beam delivery system.
Fusion cutting: achievable cutting speeds for DDL are always faster than for the equal power CO2 source. The fiber laser has a significant productivity advantage for thicknesses up to 4 mm. The edge quality criterion (ISO 9013) is not met for thicknesses greater than 4 mm. This is also the case for typical fiber laser cuts at these power levels. This performance overview creates a reference for comparative assessment of the opportunities proposed in the next sections and forms the basis for the discussion section. 3. Free beam opportunity Fig. 2. Laser material interaction: (a) Fresnel laser absorption curves for steel and (b) laser to kerf interaction scheme. Note that s- and p- are defined for an electrical field which is perpendicular and parallel, respectively, to the plane of incidence.
smartly controlled [8], as depicted by the green dot on the ppolarization absorption curves. 2. Cutting performance comparison: DDL vs CO2 vs fiber Currently CO2 and fiber laser sources dominate the industrial application landscape. A comprehensive study has been published by the authors in Ref. [7] that assesses a 2 kW Fiber Guided (FG-) DDL as compared to industrial cutting machines, as summarized in Fig. 3. This optimized performance comparison, that takes into account acceptable cutting edge quality requirements according to ISO 9013, leads to the following observations: Flame cutting: similar achievable cutting speeds for the different types of tested lasers, with exception of 1 mm sheets where DDL offers an advantage.
Fig. 3. Comparison of cutting performance for flame (O2) cutting of steel (left) and fusion (N2) cutting of stainless steel (right): the cut edges shown, feed rates and quality class provided were obtained for the diode laser setup. The bar graphs represent the relative cutting speed obtained with industrial fiber and CO2 machines for the same output power level.
3.1. Description In cutting machines the way the laser beam is guided from the resonator to the cutting optics is significantly different for different laser sources. CO2 resonators require a considerable amount of floor space and the laser beam is typically guided through a complex system of reflective optics requiring precise alignment and beam divergence compensation. With solid state fiber lasers and FG-DDLs the beam is typically guided through an optical fiber that simplifies the beam delivery process. The Free Beam (FB-) DDL concept consists of integrating the resonator directly in the laser head, thus avoiding the use of a guiding fiber. This is a unique opportunity for DDL sources due to their inherent compactness. Since less optical components are used, this architecture supports a higher energetic system efficiency and thus reduces both investment and operational costs compared to a conventional beam delivery system. 3.2. Experimental verification A FB-DDL system emitting 1,7 kW nominal output power at wavelengths of 920 and 960 nm was integrated, within the laser head, in a gantry platform, as shown in the left part of Fig. 4. In contrast with the FG-DDL setup and due to the need of less optical components, this configuration is able to deliver 350 W extra power per wavelength used, reaching wall plug efficiencies of 40% (15% more than the FG-DDL, including internal cooling [1]). The laser module was mounted on a Precitec LightCutter laser head with 100 mm focusing lens. With a weight of less than 20 kg, this configuration is able to withstand accelerations in the order of 5 m/ s2. A protective shutdown system, detecting excessive back
Fig. 4. FB-DDL setup (left) and beam measurement (Primes FocusMonitor) of FBDDL (middle) and FG-DDL (right).
Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136
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Fig. 5. Relative maximum cutting speed achieved for the FB-DDL (1,7 kW) when comparing to the FG-DDL (2 kW).
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Fig. 6. Comparison of flame cutting optimal results (S355 steel).
reflection, was implemented. Fig. 4 shows caustic beam measurements for both the used FB-DDL (middle) and the 125 mm focusing lens configuration of the FG-DDL (right). The 2D beam intensity profiles are given at different axial positions. Experiments of flame (S355 steel) and fusion (304L steel) cutting were realized using a similar optimization criterion and statistical analysis as described in Ref. [7] and are given in Fig. 5. Fig. 7. Comparison of fusion cutting optimal results (304L steel).
3.3. Analysis and discussion For FG- and FB-DDL configurations the achieved beam parameter product (bpp) is practically the same. A less homogeneous beam intensity distribution is visible for the FB-DDL in measurement planes further away from the beam waist. As for the cutting performance comparison, there are two aspects that need to be taken into account in the assessment: The available FB-DDL as tested had 300 W less output power ( 15%). A much higher power is technically possible for the same number of wavelength coupling stages, but could not be realized within the reported research program. The small magnification of the optical configuration used for the FB- setup generates a small beam waist, which favors thin sheet cutting, but results in a higher beam divergence, typically not optimal for thicker sheet cutting. Cutting speeds from the FGsetup resulted from an optimization procedure with two extra optical configurations, both with larger beam waist and lower beam divergence. Such optimization was not achievable with the experimental FB-setup. A detailed look at the cutting results in combination with these considerations reveal, not only that the FB- configuration is a viable solution, but also that extra potential for further optimization is present (e.g. through power scaling or favorable optical magnifications). 4. Laser polarization opportunities 4.1. Description The complex 3D kerf geometry generated during the cutting process results in a strong influence of laser polarization on the cutting efficiency. As depicted in Fig. 2, the absorbed energy at the first incidence can easily range from less than 10 to as much as 90% depending on the local conditions. It has been demonstrated that the cutting performance may be considerably improved in the polarization direction, both for 10 mm [9] and 1 mm [10] wavelength radiations. Geometrically favorable states of polarization, as the ones obtained with sectioned waveplates, may generate even better absorption in the cutting kerf, as demonstrated for radial [11] and stripped [12] polarization strategies. All these techniques rely on approximating the global absorption of the beam to the p- curves in Fig. 2, but require a defined polarization state to start from. While this is typically the output of CO2 resonators, the process of obtaining a defined polarization state for high power fiber lasers is inefficient (>50% loss) [10]. On the contrary, a FB-DDL concept offers the unique opportunity of achieving this polarization state by avoiding polarization coupling in the power scaling step. 4.2. Experimental verification A 750 W DDL module was built, without polarization coupling steps, and mounted on a Precitec Y52 laser head with 80 mm
focusing lens. It emits in the wavelengths of 920 and 960 nm with aligned polarization axes. A two position beam converter with 4 spatial sections, as described in Ref. [8], was added to achieve approximated radial or azimuthal states of polarization. Cutting tests were performed with linear (L-), radial (R-) and azimuthal (A) polarization strategies. Results were compared with the ones obtained with a DDL module, built to the same power but using a single wavelength strategy, 920 nm, and crossed polarization axes (C-). The maximum achievable speed was evaluated for acceptable cutting quality, according to the methodology defined in Ref. [8], for flame and fusion cutting respectively (Figs. 6 and 7). 4.3. Analysis and discussion The benefit of certain polarization strategies for the maximum cutting speed is clear. The L- configuration with the beam polarized in the feed direction was the most effective for the tested conditions. The beam converter is an interesting solution because it generates an approximately axis-symmetric characteristic, but the maximum speeds obtained with the L- strategy are still significantly higher: up to 20% for flame cutting and up to 14% for fusion cutting. Differences observed for the two processes are possibly related to the positive absorption effect being diluted when the exothermic oxidation reaction is present, responsible for around 50% of process energy. It is interesting to see that the Astrategy performs much worse for fusion cutting, which suggests that for flame cutting the exothermic reaction compensates the bad absorption characteristic. In general the performance of the Rpolarization is just slightly better than the base C- strategy: not more than 12% in both cases. No significant impact was found in edge quality for the tested conditions with the exception of flame cutting of steel where the Cstrategy provided the smoothest edges. 5. Beam shaping opportunities 5.1. Description For fusion cutting the kerf shape is largely defined by the beam geometry. In Ref. [8] a strong correlation between the position of the focal point and measurements of the kerf width at the top and bottom of the sheet was demonstrated. The kerf front is also strongly correlated with the beam shape, as shown by Ref. [13]. In this paper scanning optics are used in order to elongate the beam in the cutting direction, with a direct impact on cutting front shape and maximum cutting speed. Fig. 2 may be referred again to help understand the importance of beam shape control to improve the cutting efficiency. On one hand absorption can be maximized with an elongated cut front and, on the other hand, by keeping a small kerf width less material is removed and efficiency improves. This considerations naturally lead to non-symmetric beam shapes. A FB-DDL provides a unique opportunity to aim for non-symmetric beam shapes at the stacking stage (step 2).
Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136
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5.2. Experimental verification In order to test the influence of a non-symmetric DDL on the cutting process, a linearly polarized module, emitting 750 W at wavelengths of 920 and 960 nm, with an asymmetric beam shape, resulting from stacking, was used. Fig. 8 shows details about the beam shape and reveals an asymmetric bpp, with respectively a minimum and maximum of 17,1 and 25,9 mm mrad in two perpendicular directions. These results were obtained with a Ophir dual axis BeamWatch device. A motorized zero-order halfwave retarder was used to turn the polarization along with the measuring device. It is important to note that the intensity provided in Fig. 8b) results from an integration along the beam cross section, as detected by the camera. The higher peak intensity for the Profile X camera frame is thus related to a longer beam in the direction normal to the viewing plane. Four combinations of cutting experiments, resulting from main beam axes and polarization position directions, were realized: see Table 1. The material used was 2 mm thick 304L stainless steel and the experiment consisted of cutting straight lines at constant increments of cutting speed till failure.
conditions with no polarization alignment to the feed direction. With alignment of polarization the amount of dross is clearly less, but only completely avoided for alignment of elongation as well. 6. Integrated DDL system: discussion It is important to evaluate the proposed opportunities together. Both the polarization and beam shape possibilities come as unique characteristics of a FB-DDL concept, and thus only make sense in such configuration. Furthermore the clear interaction between them, as previously demonstrated in Table 1, points at a much stronger improvement in performance when combined. Taking into account the cost benefits implied by using a FB- configuration (e.g. simple architecture, higher efficiency, less floor space), a system that uses polarization and beam shape characteristics to increase cutting performance presents an updated view in the strengths of DDLs for cutting metal sheets. The drawbacks must be assessed as well, the most important one being the need for an extra NC-axis to perform steering of these non-symmetric beam characteristics in a 2D-flat bed machine. Different off-the-shelf coated optics may be used as rotators: waveplates for polarization and certain types of prisms for beam shaping. Challenges in cutting thin sheets at high speeds, especially small radii contours, may need to be overcome with stronger and faster motors. Depending on application, crash protection and back reflection preventive systems will be needed. 7. Conclusion
Fig. 8. Asymmetric bpp setup beam measurement: (a) camera pictures in the main axes of the beam; (b) intensity profiles as seen by cameras for an average waist position; (c) measured beam waist at different angles.
The given updated view on the strengths of DDL technology for metal sheet cutting is of relevance for system developers. Even though the investigated setups have a higher bpp than fiber and CO2 variants, the benefits of a FB- configuration and a process tailored beam are clearly leading to similar or better performances at lower investment costs. With the addition of a steady trend of improved brightness for DDLs over the past years, only the current uncertainty of relative cost evolution between technologies remains a limiting factor for DDLs to be considered a real alternative for cutting applications.
References Table 1 Cut experiments with asymmetric shaped DDL beam. Strategy Max. speed
Edge pic.
Strategy
Max. speed
800 mm/min Ref
1600 mm/min (+100%)
2100 mm/min (+163%)
2800 mm/min (+250%)
Edge pic.
Nozzle 1 mm, SOD 1 mm, 16 bar (N2), focus 1.5 mm below surface, 750 W.
5.3. Analysis and discussion A significant deviation from rotational symmetry could be demonstrated with the FB-DDL concept (Fig. 8c). The final beam is clearly elongated along the X direction. The slight astigmatism of 31 mm, visible in Fig. 8a and related to the mismatch of waist positions, is expected to have a negligible effect on the cutting experiments. The maximum cutting speeds are very different for the tested conditions, with the best condition being 2,5 times faster than the worst. For the tested material both polarization and elongation of the beam are favorable for maximum cutting speed when aligned to the feed direction. As expected from the theory presented in Fig. 2, the influence of elongation is also dependent on the polarization effect. Heavy attached dross is visible for the
[1] Kellens K, Costa Rodrigues G, Dewulf W, Duflou JR (2014) Energy and Resource Efficiency of Laser Cutting Processes. Physics Procedia 56:854–864. [2] Belforte D (2015) 2015 Industrial Laser Market Outperforms Global Manufacturing Instability. Journal of Industrial Laser Solutions 31. [3] Strohmaier S, Tillkorn C, Olschowsky P, Hostetler J (2010) High-power, Highbrighness Direct-diode Lasers. Optics & Photonics News 25–29. [4] Huang RK, Chann B, Glenn JD (2011) Ultra-high Brightness Wavelength-stabilized kW-class Fiber Coupled Diode Laser. Proceedings of SPIE 7918. 791810-(19). [5] Heinemann S, Fritsche H, Kruschke B, Schmidt T, Gries W (2013) Compact High Brightness Diode Laser Emitting 500 W from a 100 mm Fiber. Proceedings of SPIE 8605:1–7. [6] Witte U, Schneider F, Traub M, Hoffmann D, Drovs S, Brand T, Unger A (2016) kW-class Direct Diode Laser for Sheet Metal Cutting Based on DWDM of Pump Modules by Use of Ultra-steep Dielectric Filters. Optics Express 24:22917– 22929. [7] Costa Rodrigues G, Pencinovsky J, Cuypers M, Duflou JR (2014) Theoretical and Experimental Aspects of Laser Cutting with Direct Diode Laser. Optics and Lasers in Engineering 61:31–38. [8] Costa Rodrigues G, Duflou JR (2016) Into Polarization Control in Laser Cutting with Direct Diode Lasers. Journal of Laser Applications 28(2). 022207-(1-8). [9] Olsen FO (2017) Polarization effects in laser cutting: basics. in Ready John Fh , Farson Dave Fv , (Eds.) LIA Handbook of Laser Material Processing, Laser Institute of America Magnolia Publishing, Inc., United States of America 433–436. [10] Goppold C, Pinder T, Herwing P (2016) Experimental Investigation of the Linear Polarization State of High Power Fusion Cutting with 1 mm Laser Radiation. Journal of Laser Applications 28(3). [11] Weber R, Michalowski A, Abdou-Ahmed M, Onuseit V, Rominger V, Kraus M, Graf T (2011) Effects of Radial and Tangential Polarization. Physics Procedia 12:21–30. [12] Halo Project Newsletter 6. Available from: http://halo-project.eu/wp-content/ uploads/2016/06/HALO-Project-newsletter-6-Jun-2016.pdf. [13] Mahrle A, Beyer E (2009) Theoretical Aspects of Fiber Laser Cutting. Journal of Physics D: Applied Physics 42:175507.
Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136
CIRP-1694; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Opportunities in laser cutting with direct diode laser configurations Gonçalo Costa Rodrigues, Joost R. Duflou (1)* Department of Mechanical Engineering, KU Leuven, Belgium
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser Cutting Direct diode lasers
This paper explores the opportunities associated with the use of well-designed direct diode laser configurations for cutting of metal sheets. Continuously improving diode stacking, and consequently greater brightness, allow the requirements for laser cutting to be met without the use of further brightness converters (diode-pumped solid state lasers). This contributes to a reduction in the number of optical components and increased energy efficiency. Furthermore, laser architecture related degrees of freedom for tailoring the wavelength, beam polarization and beam shape are instrumental to increase the cutting performance. Different stacking configurations, optical structures and related cutting strategies are discussed, with performance validation through experimental verification. © 2017 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction Laser technology for material processing has been improving at a fast pace. Less than a decade ago industrial cutting of metal sheets was fully dominated by CO2 gas lasers, mainly characterized by a long wavelength (10 mm) and poor efficiencies (5–10%) [1]. Today, solid state fiber lasers with a shorter wavelength (1 mm) and significantly better efficiencies (>30%), are becoming dominant for metal cutting applications [2]. Diode lasers are important building blocks for the majority of these laser systems, as they are used for pumping the active medium. Fig. 1 provides a schematic overview of the laser technology involved in building up a multikW solid state laser. Single diode laser emitters produce an asymmetric beam shape and deliver just a few Watt of output power. Nevertheless, they are typically cheap components that can be used in combination to obtain much higher laser powers. The first natural step to increase power, at the cost of beam quality, is a side-by-side combination of multiple emitters. This is typically done in diode bars that share cooling and controlling channels (step 1) and that are stacked (step 2) into high power diode modules, which are mostly used for pumping solid state fiber lasers. Direct Diode Laser (DDL) technology, that involves the direct use of a diode beam for material processing, is a recent achievement in cutting applications and emerges from developments in diode pump components such as higher single emitter power, improved coupling optics coatings and, more effective cooling strategies. The use of less optical components brings advantages in efficiency, compactness, reliability and price. Nowadays the technology is mainly limited to the availability of the above mentioned diode pump modules. As can be seen in Fig. 1, smart arrangement of emitters of strategically chosen wavelength and polarization
* Corresponding author. E-mail address: Joost.Dufl[email protected] (J.R. Duflou).
direction, allows power scaling at no cost to beam quality (step 3). While polarization multiplexing is limited to doubling the power, variants using different wavelengths are theoretically unlimited in achievable power range. In practice a limited number of available wavelengths (800–1100 nm) and their broad bandwidth (around 30 nm) narrow the possibilities [3]. Such beams are commonly coupled into guiding fibers (step 4), which deliver the laser from the resonator closer to the workpiece, and were, until recently, used only for less brightness demanding applications. Even though substantial research efforts still focus on further improving the brightness of DDLs [4–6], the associated costs are typically too high when evaluated against established brightness converting techniques, such as the fiber laser principle. Witte et al. [6] provide a more detailed description of the state of the art for brightness increasing techniques, while documenting a novel approach for increasing the number of possible wavelength multiplexing steps by wavelength stabilization of commercially available pump modules. Most recently, it has been demonstrated that state-of-the art DDLs already meet the requirements, in terms of achievable cutting speed and edge quality, for cutting certain material and thickness combinations [7]. This paper aims to discuss opportunities associated with innovative DDL based systems that are smartly designed for cutting applications. In the free beam concept, where the laser source is built into the cutting head, not only can further elimination of optical components (e.g. guiding fiber and related coupling optics) be achieved, but this also allows to tailor polarization and beam shape to the cutting process. Regarding the latter, Fig. 2 illustrates the potential of a beam tailored for cutting purposes: laser absorption in the cutting kerf is highly influenced by the incidence angle and laser polarization. The red dot represents the typical absorption in the material for randomly polarized solid state lasers. The beam shape is responsible for kerf formation and can be used to positively influence the incidence angle, (strategy represented by the yellow dot in Fig. 2). Laser polarization can also imply a significant gain in absorption when
http://dx.doi.org/10.1016/j.cirp.2017.04.136 0007-8506/© 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136
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Fig. 1. Different system stages from single diode laser emitter to laser beam delivery system.
Fusion cutting: achievable cutting speeds for DDL are always faster than for the equal power CO2 source. The fiber laser has a significant productivity advantage for thicknesses up to 4 mm. The edge quality criterion (ISO 9013) is not met for thicknesses greater than 4 mm. This is also the case for typical fiber laser cuts at these power levels. This performance overview creates a reference for comparative assessment of the opportunities proposed in the next sections and forms the basis for the discussion section. 3. Free beam opportunity Fig. 2. Laser material interaction: (a) Fresnel laser absorption curves for steel and (b) laser to kerf interaction scheme. Note that s- and p- are defined for an electrical field which is perpendicular and parallel, respectively, to the plane of incidence.
smartly controlled [8], as depicted by the green dot on the ppolarization absorption curves. 2. Cutting performance comparison: DDL vs CO2 vs fiber Currently CO2 and fiber laser sources dominate the industrial application landscape. A comprehensive study has been published by the authors in Ref. [7] that assesses a 2 kW Fiber Guided (FG-) DDL as compared to industrial cutting machines, as summarized in Fig. 3. This optimized performance comparison, that takes into account acceptable cutting edge quality requirements according to ISO 9013, leads to the following observations: Flame cutting: similar achievable cutting speeds for the different types of tested lasers, with exception of 1 mm sheets where DDL offers an advantage.
Fig. 3. Comparison of cutting performance for flame (O2) cutting of steel (left) and fusion (N2) cutting of stainless steel (right): the cut edges shown, feed rates and quality class provided were obtained for the diode laser setup. The bar graphs represent the relative cutting speed obtained with industrial fiber and CO2 machines for the same output power level.
3.1. Description In cutting machines the way the laser beam is guided from the resonator to the cutting optics is significantly different for different laser sources. CO2 resonators require a considerable amount of floor space and the laser beam is typically guided through a complex system of reflective optics requiring precise alignment and beam divergence compensation. With solid state fiber lasers and FG-DDLs the beam is typically guided through an optical fiber that simplifies the beam delivery process. The Free Beam (FB-) DDL concept consists of integrating the resonator directly in the laser head, thus avoiding the use of a guiding fiber. This is a unique opportunity for DDL sources due to their inherent compactness. Since less optical components are used, this architecture supports a higher energetic system efficiency and thus reduces both investment and operational costs compared to a conventional beam delivery system. 3.2. Experimental verification A FB-DDL system emitting 1,7 kW nominal output power at wavelengths of 920 and 960 nm was integrated, within the laser head, in a gantry platform, as shown in the left part of Fig. 4. In contrast with the FG-DDL setup and due to the need of less optical components, this configuration is able to deliver 350 W extra power per wavelength used, reaching wall plug efficiencies of 40% (15% more than the FG-DDL, including internal cooling [1]). The laser module was mounted on a Precitec LightCutter laser head with 100 mm focusing lens. With a weight of less than 20 kg, this configuration is able to withstand accelerations in the order of 5 m/ s2. A protective shutdown system, detecting excessive back
Fig. 4. FB-DDL setup (left) and beam measurement (Primes FocusMonitor) of FBDDL (middle) and FG-DDL (right).
Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136
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Fig. 5. Relative maximum cutting speed achieved for the FB-DDL (1,7 kW) when comparing to the FG-DDL (2 kW).
3
Fig. 6. Comparison of flame cutting optimal results (S355 steel).
reflection, was implemented. Fig. 4 shows caustic beam measurements for both the used FB-DDL (middle) and the 125 mm focusing lens configuration of the FG-DDL (right). The 2D beam intensity profiles are given at different axial positions. Experiments of flame (S355 steel) and fusion (304L steel) cutting were realized using a similar optimization criterion and statistical analysis as described in Ref. [7] and are given in Fig. 5. Fig. 7. Comparison of fusion cutting optimal results (304L steel).
3.3. Analysis and discussion For FG- and FB-DDL configurations the achieved beam parameter product (bpp) is practically the same. A less homogeneous beam intensity distribution is visible for the FB-DDL in measurement planes further away from the beam waist. As for the cutting performance comparison, there are two aspects that need to be taken into account in the assessment: The available FB-DDL as tested had 300 W less output power ( 15%). A much higher power is technically possible for the same number of wavelength coupling stages, but could not be realized within the reported research program. The small magnification of the optical configuration used for the FB- setup generates a small beam waist, which favors thin sheet cutting, but results in a higher beam divergence, typically not optimal for thicker sheet cutting. Cutting speeds from the FGsetup resulted from an optimization procedure with two extra optical configurations, both with larger beam waist and lower beam divergence. Such optimization was not achievable with the experimental FB-setup. A detailed look at the cutting results in combination with these considerations reveal, not only that the FB- configuration is a viable solution, but also that extra potential for further optimization is present (e.g. through power scaling or favorable optical magnifications). 4. Laser polarization opportunities 4.1. Description The complex 3D kerf geometry generated during the cutting process results in a strong influence of laser polarization on the cutting efficiency. As depicted in Fig. 2, the absorbed energy at the first incidence can easily range from less than 10 to as much as 90% depending on the local conditions. It has been demonstrated that the cutting performance may be considerably improved in the polarization direction, both for 10 mm [9] and 1 mm [10] wavelength radiations. Geometrically favorable states of polarization, as the ones obtained with sectioned waveplates, may generate even better absorption in the cutting kerf, as demonstrated for radial [11] and stripped [12] polarization strategies. All these techniques rely on approximating the global absorption of the beam to the p- curves in Fig. 2, but require a defined polarization state to start from. While this is typically the output of CO2 resonators, the process of obtaining a defined polarization state for high power fiber lasers is inefficient (>50% loss) [10]. On the contrary, a FB-DDL concept offers the unique opportunity of achieving this polarization state by avoiding polarization coupling in the power scaling step. 4.2. Experimental verification A 750 W DDL module was built, without polarization coupling steps, and mounted on a Precitec Y52 laser head with 80 mm
focusing lens. It emits in the wavelengths of 920 and 960 nm with aligned polarization axes. A two position beam converter with 4 spatial sections, as described in Ref. [8], was added to achieve approximated radial or azimuthal states of polarization. Cutting tests were performed with linear (L-), radial (R-) and azimuthal (A) polarization strategies. Results were compared with the ones obtained with a DDL module, built to the same power but using a single wavelength strategy, 920 nm, and crossed polarization axes (C-). The maximum achievable speed was evaluated for acceptable cutting quality, according to the methodology defined in Ref. [8], for flame and fusion cutting respectively (Figs. 6 and 7). 4.3. Analysis and discussion The benefit of certain polarization strategies for the maximum cutting speed is clear. The L- configuration with the beam polarized in the feed direction was the most effective for the tested conditions. The beam converter is an interesting solution because it generates an approximately axis-symmetric characteristic, but the maximum speeds obtained with the L- strategy are still significantly higher: up to 20% for flame cutting and up to 14% for fusion cutting. Differences observed for the two processes are possibly related to the positive absorption effect being diluted when the exothermic oxidation reaction is present, responsible for around 50% of process energy. It is interesting to see that the Astrategy performs much worse for fusion cutting, which suggests that for flame cutting the exothermic reaction compensates the bad absorption characteristic. In general the performance of the Rpolarization is just slightly better than the base C- strategy: not more than 12% in both cases. No significant impact was found in edge quality for the tested conditions with the exception of flame cutting of steel where the Cstrategy provided the smoothest edges. 5. Beam shaping opportunities 5.1. Description For fusion cutting the kerf shape is largely defined by the beam geometry. In Ref. [8] a strong correlation between the position of the focal point and measurements of the kerf width at the top and bottom of the sheet was demonstrated. The kerf front is also strongly correlated with the beam shape, as shown by Ref. [13]. In this paper scanning optics are used in order to elongate the beam in the cutting direction, with a direct impact on cutting front shape and maximum cutting speed. Fig. 2 may be referred again to help understand the importance of beam shape control to improve the cutting efficiency. On one hand absorption can be maximized with an elongated cut front and, on the other hand, by keeping a small kerf width less material is removed and efficiency improves. This considerations naturally lead to non-symmetric beam shapes. A FB-DDL provides a unique opportunity to aim for non-symmetric beam shapes at the stacking stage (step 2).
Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136
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CIRP-1694; No. of Pages 4 G. Costa Rodrigues, J.R. Duflou / CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
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5.2. Experimental verification In order to test the influence of a non-symmetric DDL on the cutting process, a linearly polarized module, emitting 750 W at wavelengths of 920 and 960 nm, with an asymmetric beam shape, resulting from stacking, was used. Fig. 8 shows details about the beam shape and reveals an asymmetric bpp, with respectively a minimum and maximum of 17,1 and 25,9 mm mrad in two perpendicular directions. These results were obtained with a Ophir dual axis BeamWatch device. A motorized zero-order halfwave retarder was used to turn the polarization along with the measuring device. It is important to note that the intensity provided in Fig. 8b) results from an integration along the beam cross section, as detected by the camera. The higher peak intensity for the Profile X camera frame is thus related to a longer beam in the direction normal to the viewing plane. Four combinations of cutting experiments, resulting from main beam axes and polarization position directions, were realized: see Table 1. The material used was 2 mm thick 304L stainless steel and the experiment consisted of cutting straight lines at constant increments of cutting speed till failure.
conditions with no polarization alignment to the feed direction. With alignment of polarization the amount of dross is clearly less, but only completely avoided for alignment of elongation as well. 6. Integrated DDL system: discussion It is important to evaluate the proposed opportunities together. Both the polarization and beam shape possibilities come as unique characteristics of a FB-DDL concept, and thus only make sense in such configuration. Furthermore the clear interaction between them, as previously demonstrated in Table 1, points at a much stronger improvement in performance when combined. Taking into account the cost benefits implied by using a FB- configuration (e.g. simple architecture, higher efficiency, less floor space), a system that uses polarization and beam shape characteristics to increase cutting performance presents an updated view in the strengths of DDLs for cutting metal sheets. The drawbacks must be assessed as well, the most important one being the need for an extra NC-axis to perform steering of these non-symmetric beam characteristics in a 2D-flat bed machine. Different off-the-shelf coated optics may be used as rotators: waveplates for polarization and certain types of prisms for beam shaping. Challenges in cutting thin sheets at high speeds, especially small radii contours, may need to be overcome with stronger and faster motors. Depending on application, crash protection and back reflection preventive systems will be needed. 7. Conclusion
Fig. 8. Asymmetric bpp setup beam measurement: (a) camera pictures in the main axes of the beam; (b) intensity profiles as seen by cameras for an average waist position; (c) measured beam waist at different angles.
The given updated view on the strengths of DDL technology for metal sheet cutting is of relevance for system developers. Even though the investigated setups have a higher bpp than fiber and CO2 variants, the benefits of a FB- configuration and a process tailored beam are clearly leading to similar or better performances at lower investment costs. With the addition of a steady trend of improved brightness for DDLs over the past years, only the current uncertainty of relative cost evolution between technologies remains a limiting factor for DDLs to be considered a real alternative for cutting applications.
References Table 1 Cut experiments with asymmetric shaped DDL beam. Strategy Max. speed
Edge pic.
Strategy
Max. speed
800 mm/min Ref
1600 mm/min (+100%)
2100 mm/min (+163%)
2800 mm/min (+250%)
Edge pic.
Nozzle 1 mm, SOD 1 mm, 16 bar (N2), focus 1.5 mm below surface, 750 W.
5.3. Analysis and discussion A significant deviation from rotational symmetry could be demonstrated with the FB-DDL concept (Fig. 8c). The final beam is clearly elongated along the X direction. The slight astigmatism of 31 mm, visible in Fig. 8a and related to the mismatch of waist positions, is expected to have a negligible effect on the cutting experiments. The maximum cutting speeds are very different for the tested conditions, with the best condition being 2,5 times faster than the worst. For the tested material both polarization and elongation of the beam are favorable for maximum cutting speed when aligned to the feed direction. As expected from the theory presented in Fig. 2, the influence of elongation is also dependent on the polarization effect. Heavy attached dross is visible for the
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Please cite this article in press as: Costa Rodrigues G, Duflou JR. Opportunities in laser cutting with direct diode laser configurations. CIRP Annals - Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.136