- Email: [email protected]
Investigation of increased drilling speed by online high-speed photography
ARTICLE IN PRESS Optics and Lasers in Engineering 46 (2008) 705– 710
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Review
Investigation of increased drilling speed by online high-speed photography J. Dietrich , M. Brajdic, K. Walther, A. Horn, I. Kelbassa, R. Poprawe Department of Mechanical Engineering, Lehrstuhl fuer Lasertechnik, LLT, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany
a r t i c l e in fo
abstract
Article history: Received 7 February 2008 Received in revised form 6 May 2008 Accepted 7 May 2008 Available online 25 June 2008
Percussion drilling of through holes in stainless steel (grade 304/EN 1.4301; 8 mm) is carried out with temporal and spatial superposed Nd:YAG laser radiation of ms- and ns-pulsed laser sources. The drilling progress with superposed laser radiation is determined and compared with the drilling progress without superposed laser radiation. The process gases oxygen, helium, and argon are used. Coaxial and lateral high-speed photography is used to correlate the optical emission monitored with the drilling progress. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Laser Percussion drilling Increased drilling speed Optical emission High-speed photography
Contents 1. 2. 3.
4. 5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 3.1. Drilling depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 3.2. Optical emission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710
1. Introduction Drilling metals with laser radiation is used for manufacturing holes when processing with laser radiation is more cost-effective than other methods or if holes cannot be produced by conventional methods like mechanical or electro-chemical drilling. Depending on the geometrical and metallurgical specifications to be achieved, laser radiation with fs up to ms pulse duration is used. The achieved geometrical and metallurgical results, such as aspect ratio and recast layer thickness as well as the efficiency of the drilling process, such as productivity, depend on the chosen process layout (melt or vaporization dominated drilling) and on the suitable process parameters applied [1]. One application for laser drilling is the production of cooling holes in turbine blades.
Corresponding author. Tel.: +49 2418906479; fax: +49 2418906121.
E-mail address: [email protected] (J. Dietrich). 0143-8166/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2008.05.010
Cooling gas blown out of cooling holes reduces the thermal load of turbine blades by decrease of the surface temperature by forming a protective cooling gas film. The cooling holes with a diameter of 0.2 mm are drilled by laser radiation with high aspect ratios up to 50 and hole densities up to 100 hole per cm2 [2]. Due to the requirements for efficient laser drilling, much work is performed regarding a deeper process understanding on improving the underlying physical processes [3–17] and influence of process gases on the drilling process [18–20]. Percussion drilling with temporal and spatial superposed laser radiation exhibits an increase in drilling speed [21–23]. For understanding the physical processes the increased drilling speed is investigated for stainless steel with a thickness of 8 mm. Therefore, the laser radiations provided by a flash lamp pumped Nd:YAG laser radiation source and a Nd:YAG diode pumped laser radiation source are temporally and spatially superposed. The drilling depth and the optical emission are monitored by a novel lateral and coaxial high-speed photography.
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2. Experimental setup The laser radiations of two laser sources are superposed by a polarizer (Fig. 1). The laser radiations are provided by a flash lamp pumped Nd:YAG laser Slab FM 015-452 from Lasag and a diode pumped Nd:YAG laser Innoslab Boxtar (BX) from Edgewave. The superposed laser radiation is guided through a dichroitic mirror and focused onto the sample by a focusing lens (focal length 100 mm). The spatial intensity profile of the laser radiation is measured by photography using a camera based monitoring system (MicrospotMonitor Co. PRIMES GmbH) (Fig. 2, right). The laser radiations of both laser sources exhibit a nearly circular Gaussian spatial intensity profile with comparable focus diameters. The Slab- and InnoSlab-Laser sources are operated with constant laser parameters (Table 1). Using the same lens, the foci are displaced spatially at the propagation axis. The focus position of the laser radiation of the InnoSlab-Laser is located 2 mm below the Slab-Laser radiation focus (Fig. 2, left). A conical nozzle with an exit diameter of 1 mm is positioned 1 mm above the substrate surface. The total pressure for all process gases is 10 bar. The drilling depth is detected by a novel lateral observation method described in [23]. A hole drilled into the workpiece with a distance of approximately 100 mm from the polished front surface (Fig. 1) exhibits a deformation in the front surface caused by thermal stress [23]. The deformation is detected by high-speed photography (MotionXtra HG-100 K, Redlake) (Fig. 4 left and right) and analyzed with an image editing software. Laser sources
and high-speed camera are temporally synchronized by a pulse generator (Fig. 3). The optical emission induced by laser ablation is monitored adopting coaxial high-speed photography. The images are taken at a constant frequency (50 kHz), exposure time (8 ms), and are rendered into false color for a higher contrast (Fig. 4).
Fig. 2. True to scale scheme of the hole and beam caustics of the Slab- and InnoSlab-Laser radiations (left) and spatial intensity in focus (right).
Fig. 1. Scheme of the experimental setup.
ARTICLE IN PRESS J. Dietrich et al. / Optics and Lasers in Engineering 46 (2008) 705–710
3. Results 3.1. Drilling depth The drilling depth is determined by lateral process monitoring for the process gases oxygen, argon, helium, and constant laser parameters (Table 1). The drilling depth of holes produced with superposed laser radiation is compared with the drilling depth of holes produced without superposed laser radiation (Fig. 5). At the beginning of ablation (pulse 1–10, Fig. 5a) the drilling speed (depth per laser pulse) is larger in case of using the process gas argon than in case of using oxygen or helium. Thereby superposed laser radiation leads to an increased drilling depth. The process gases, helium or oxygen, do not affect the drilling speed during the first 20 pulses adopting superposed laser radiation. At a drilling depth of approximately 2 mm the drilling speed is reduced depending on the process gases (Fig. 5b). At this drilling depth the laser radiation is reflected by the hole wall the first time (Fig. 2). The spatial intensity profile is altered by reflection of laser radiation at the hole wall and changing ground geometry [11,24–27]. Further drilling speed is decreased for the process gases argon and helium, but less for oxygen. At approximately 100 pulses the drilling speed increases again when using the process gases argon and helium. The increased drilling speed can be caused by an amplified intensity profile due to multiple reflections at the hole wall [24–28].
Table 1 Technical specifications of the laser sources Laser
Slab FM 015-452
Innoslab Boxtar (BX)
Laser type Wavelength Pulse duration Repetition rate Pulse energy Beam quality Rayleigh length Polarization Focus diameter
Nd:YAG-Laser 1064 nm 500 ms 20 Hz 640 mJ 1.5 0.625 mm Linear 40.4 mm
Nd:YAG-Laser 1064 nm 4 ns 10 kHz 1.5 mJ 1.5 1.720 mm Linear 73.0 mm
Fig. 3. Trigger signals of lasers and high-speed-camera.
707
The stainless steel sample was not drilled through adopting the process gases argon and helium. The drilling process was stopped when no further increase of drilling depth was observed. The influence of superposed laser radiation on the drilling speed in case of the process gases argon and helium is negligible. Using the process gas oxygen the stainless steel is drilled through. Thereby, superposed laser radiation decreases the drilling time by approx. 22% which relates to 200 ms-pulses (Fig. 5c). 3.2. Optical emission The optical emission of the drilling process using superposed and non-superposed laser radiation is investigated by coaxial high-speed photography comparing photographs in false color. As the first 3 ms-pulses tend to be overexposed the 4th ms-pulse of the drilling process with the process gases oxygen (Fig. 6) and argon (Fig. 7) is discussed. As the optical emission of the drilling process with the process gas helium is comparable to argon, only photographs of the drilling process with the process gas argon are discussed. In case of drilling with non-superposed laser radiation the optical emission is detected at the beginning of the ms-pulse. After irradiation the optical emission is detectable for ulterior 100 ms in case of drilling with the process gas oxygen (Fig. 6, left), whereas the optical emission is detectable for less than 100 ms when using argon (Fig. 7, left). When drilling with superposed laser radiation optical emission is also detectable before the ms-pulse which is caused by a nspulse. After irradiation of the ms-pulse the intensity of the optical emission reduces until it is amplified by a ns-pulse again (Fig. 6 right, Fig. 7 right). In case of using the process gas argon optical emission of the drilling process caused by ns-pulses is only detectable during irradiation of the ns-pulse (Fig. 7, right). With the process gas oxygen the optical emission is detectable beyond each ns-pulse (Fig. 6, right).
4. Discussion The drilling speed in these experiments depends on the drilling depth, the used process gases, and the superposed laser radiation. The drilling speed decreases during processing which refers to reduced intensity due to the caustic of the laser beam. Furthermore, the drilling can be divided into two domains. The drilling depth in the first domain (pulse 1–20) is referred to a dominating ablation of the ms-pulsed laser radiation. In this domain superposed laser radiation or process gases do not affect the drilling speed as much as in the second domain. In the first domain drilling with the process gas argon is faster than with oxygen or helium. This effect is also reported by Horn in experimental studies on melt front velocities [29]. The second
Fig. 4. Scheme of the stainless steel sample with deformation (left) and photographs of the percussion drilling progress with non-superposed laser radiation in stainless steel (right).
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Fig. 5. Drilling depth in dependence on the number of ms laser pulses applied for different process gases.
domain (420 pulses) is characterized by thermal effects of the process gases. Unlike argon or helium, a reaction of oxygen with metal can cause a formation of oxide during laser drilling that can change the absorptivity of the surface as well as an increase of the temperature of the process [30,31]. The increased temperature is caused by an additional energy that is induced by an exothermal chemical reaction (combustion-energy) of oxygen with the material. The higher temperature-detectable as an increased intensity of the optical emission-improves the melt ejection resulting in a larger drilling speed. In case of drilling with superposed laser radiation additional optical energy is supplied to the drilling process. The effect of the optical energy on the drilling progress is negligible when using the process gases argon or helium as drilling speed is not improved. The drilling speed increases when drilling with the process gas oxygen as the absorbed optical energy causes an enhanced combustion-energy. The enhanced combustion-energy affects the temperature of the drilling process. One effect of the increased temperature of the drilling process is an extended drilling process as the optical emission is detectable for a longer time period. This leads to visible melt ejection after irradiation (Fig. 6, right, 1080–1120 ms). The total depth when drilling with the process gas argon is larger compared to helium which can be explained by different
thermal conductivities of the two gases. The larger thermal conductivity of helium leads to lower temperatures, a larger melt viscosity, and thus to smaller hole depths. Process gases do not penetrate inside the hole during processing as the pressure of vaporized metal is too large [18]. Therefore process gases especially oxygen have an influence onto the drilling speed between two laser pulses when the pressure of vaporized metal is degenerated. The drilling process continues after irradiation as optical emission and melt ejection is detectable by coaxial process monitoring for up to some hundred ms beyond a laser pulse. During this time process gas penetrates inside the hole and affects the drilling process which is detectable by residual oxide at hole walls when using the process gas oxygen [32].
5. Conclusion Percussion drilling of through holes in stainless steel with temporal and spatial superposed laser radiation is performed for different process gases (oxygen, argon, and helium) and constant laser parameters. The drilling depth and the optical emission are monitored and analyzed. The results of the experiments exhibit that drilling with superposed laser radiation improves the drilling
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709
Fig. 6. Optical emission induced by laser radiation during percussion drilling process in stainless steel with the process gas oxygen and non-superposed (left) as well as superposed (right) laser radiation (Photographs in false color of the 4th pulse).
Fig. 7. Optical emission induced by laser radiation during percussion drilling process in stainless steel with the process gas argon and non-superposed (left) as well as superposed (right) laser radiation (Photographs in false color of the 4th pulse).
speed when drilling with the process gas oxygen. The increased drilling speed is caused by additional optical energy supplied by the second laser source that amplifies and extends the exothermal reaction. The influence of superposed laser radiation on the drilling speed when drilling with the process gases argon and
helium is negligible as no further combustion-energy by chemical reaction is induced. Process gases are commonly used for two reasons: protection of optics and improvement of drilling quality [18]. Experiments show that process gases also affect the drilling speed and depth.
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Acknowledgments The authors are very thankful for the support by the EdgeWave GmbH for providing the InnoSlab-Laser. The authors gratefully acknowledge the financial support by the German Research Foundation (DFG) within the Collaborative Research Centre (SFB) 561 ‘‘Thermally highly loaded, porous and cooled multilayer system for combined cycle power plants’’. References [1] Liebers R, Du¨rr U, Trippe L, Schulz W. Drilling strategies for metals with pulsed YAG lasers. In:ICALEO 2006, 25th international congress on applications of lasers and electro optics. LIA, Orlando, FL; 2006. [2] Boyce MP. Gas turbine engineering handbook, 2nd ed (Incompressible flow turbomachines). Gulf Professional Publishing; 2001. [3] Ng GKL, Li L. Optimization of laser percussion drilling for improved reliability. In: Proceedings of the ICALEO; 2002 [Section E]. [4] Migliore L, Kardos G, Ozkan A, Derkach O, Schaeffer R, Dunsky C. Laser choices for micromachining: drilling speed and quality comparison. In: ICALEO; 2004. [5] Forsman AC, Banks PS, Perry MD, Campbell EM, Dodell AL, Armas MS. Doublepulse machining as a technique for the enhancement of material removal rates in laser machining of metals. J Appl Phys 2005;98. [6] Ostermeyer M, Kappe P, Menzel R, Sommer S, Dausinger F. Laser drilling in thin materials with bursts of ns-pulses generated by stimulated brillouin scattering (SBS). Appl Phys A 2005;81:923–7. [7] Lapczyna M, Chen KP, Herman PR, Tan HW, Marjoribanks RS. Ultra high repetition rate (133 MHz) laser ablation of aluminum with 1.2-ps pulses. Appl Phys A 1999;69:883–6. [8] Stoian R, Boyle M, Thoss A, Rosenfeld A, Korn G, Hertel IV. Laser ablation of dielectrics with temporally shaped femtosecond puses. Appl Phys Lett 2002;80(3):353–5. [9] Lehane C, Kwok HS. Enhanced drilling using a dual-pulse Nd:YAG laser. Appl Phys A 2001;73:45–8. [10] Schulz W. Diagnosis and modelling of nonlinear dynamics in laser cutting, welding and drilling. Warrendale, PA: Materials Research Society; 2005 (Materials Research Society Symposium Proceedings 850). ISBN:1-55859798-9; p. 65–78. [11] Ruf A, Berger P, Dausinger F, Hu¨gel H. Analytical investigations on geometric influences on laser drilling. J Phys D 2001;34:2918–25. [12] Low DKY, Li L, Byrd PJ. The influence of temporal pulse train modulation during laser percussion drilling. Opt Lasers Eng 2001;35:149–64. [13] Low DKY, Li L. Comparison of intra- and interpulse modulation in laser percussion drilling. Proc IMechE 2002;216B:167–71.
[14] Low DKY, Li L. Effects of inter-pulse and intra-pulse shaping during laser percussion drilling. Proc SPIE 2002;4426:191–4. [15] Klimentov SM, Garnov SV, Kononenko TV, Konov VI, Pivovarov PA, Dausinger F. High rate deep channel ablative formation by picosecond–nanosecond combined laser pulses. Appl Phys A 1999;69:633–6. [16] Breitling D, Ruf A, Dausinger F. Fundamental aspects in machining of metals with short and ultrashort laser pulses. Proc SPIE 2004:49–63. ¨ , et al. Investigation on laser micro ablation of [17] Hartmann C, Gillner A, Aydin U metals using ns-multi-pulses. J Phys 2007;59:440–4. [18] Schneider M, Fabbro R, Berthe L, Muller M, Nivard M. Gas investigation on laser drilling. Miami, Florida, USA; 2005. p. 1094–9. [19] Fieret J, Terry MJ, Ward BA. Aerodynamic interactions during laser cutting. Preprint CLM-P 780 Publ: Abingdon OX 14 3 DB, UK; 1986. p. 15. [20] Semak V, Matsunawa A. The role of recoil pressure in energy balance during laser materials processing. J Phys D 1997;30(18):2541–52. [21] Walther K, Brajdic M, Kreutz EW. Enhanced processing speed in laser drilling of stainless steel by spatially and temporally superposed pulsed Nd:YAG laser radiation. Int J Adv Manuf Technol 2006. [22] Kreutz EW, Trippe L, Walther K, Poprawe R. Process development and control of laser drilled and shaped holes in turbine components. Kyoto, Japan; 2006. [23] Walther K, Brajdic M, Kelbassa I. Increase of drilling velocity by use of superposed laser radiation. Mu¨nchen; 2007. p. 787–90. [24] Kar A, Rockstroh T, Mazumder J. Two-dimensional model for laser-induced materials damage: effects of assist gas and multiple reflections inside the cavity. J Appl Phys 1992;71(6):2560–9. [25] Milewski J, Sklar E. Modelling and validation of multiple reflections for enhanced laser welding. Modelling Simulation Mater Sci Eng 1996;4(3): 305–22. [26] Solana P, Negro G. A study of the effect of multiple reflections on the shape of the keyhole in the laser processing of materials. J Phys D 1997;30(23): 3216–22. [27] Wei PS, Ho CY. Beam focusing characteristics effect on energy reflection and absorption in a drilling or welding cavity of paraboloid of revolution. Int J Heat Mass Transfer 1998;41(21):3299–308. [28] Ki H, Mohanty PS, Mazumder J. Multiple reflection and its influence on keyhole evolution. J Laser Appl 2002;14:39. [29] Horn A, Weichenhain R, Albrecht S, et al. Microholes in zirconia coated Nisuperalloys for transpiration cooling of turbine blades. Proc SPIE 2000;4065: 218–26. [30] Low DKY, Li L, Corfe AG. Effects of assist gas on the physical characteristics of spatter during laser percussion drilling of NIMONIC 263 alloy. Appl Surf Sci 1999;154–155:689–95. [31] Low DKY, Li L, Corfe AG. The influence of assist gas on the mechanism of material ejection and removal during laser percussion drilling. Proc IMechE B 2000;214(7):521–7. [32] Walther K, Brajdic M, Dietrich J, et al. Manufacturing of shaped holes in multilayer plates by laser-drilling. In: PICALO, Beijng, China; 2008.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Review
Investigation of increased drilling speed by online high-speed photography J. Dietrich , M. Brajdic, K. Walther, A. Horn, I. Kelbassa, R. Poprawe Department of Mechanical Engineering, Lehrstuhl fuer Lasertechnik, LLT, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany
a r t i c l e in fo
abstract
Article history: Received 7 February 2008 Received in revised form 6 May 2008 Accepted 7 May 2008 Available online 25 June 2008
Percussion drilling of through holes in stainless steel (grade 304/EN 1.4301; 8 mm) is carried out with temporal and spatial superposed Nd:YAG laser radiation of ms- and ns-pulsed laser sources. The drilling progress with superposed laser radiation is determined and compared with the drilling progress without superposed laser radiation. The process gases oxygen, helium, and argon are used. Coaxial and lateral high-speed photography is used to correlate the optical emission monitored with the drilling progress. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Laser Percussion drilling Increased drilling speed Optical emission High-speed photography
Contents 1. 2. 3.
4. 5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 3.1. Drilling depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 3.2. Optical emission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710
1. Introduction Drilling metals with laser radiation is used for manufacturing holes when processing with laser radiation is more cost-effective than other methods or if holes cannot be produced by conventional methods like mechanical or electro-chemical drilling. Depending on the geometrical and metallurgical specifications to be achieved, laser radiation with fs up to ms pulse duration is used. The achieved geometrical and metallurgical results, such as aspect ratio and recast layer thickness as well as the efficiency of the drilling process, such as productivity, depend on the chosen process layout (melt or vaporization dominated drilling) and on the suitable process parameters applied [1]. One application for laser drilling is the production of cooling holes in turbine blades.
Corresponding author. Tel.: +49 2418906479; fax: +49 2418906121.
E-mail address: [email protected] (J. Dietrich). 0143-8166/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2008.05.010
Cooling gas blown out of cooling holes reduces the thermal load of turbine blades by decrease of the surface temperature by forming a protective cooling gas film. The cooling holes with a diameter of 0.2 mm are drilled by laser radiation with high aspect ratios up to 50 and hole densities up to 100 hole per cm2 [2]. Due to the requirements for efficient laser drilling, much work is performed regarding a deeper process understanding on improving the underlying physical processes [3–17] and influence of process gases on the drilling process [18–20]. Percussion drilling with temporal and spatial superposed laser radiation exhibits an increase in drilling speed [21–23]. For understanding the physical processes the increased drilling speed is investigated for stainless steel with a thickness of 8 mm. Therefore, the laser radiations provided by a flash lamp pumped Nd:YAG laser radiation source and a Nd:YAG diode pumped laser radiation source are temporally and spatially superposed. The drilling depth and the optical emission are monitored by a novel lateral and coaxial high-speed photography.
ARTICLE IN PRESS 706
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2. Experimental setup The laser radiations of two laser sources are superposed by a polarizer (Fig. 1). The laser radiations are provided by a flash lamp pumped Nd:YAG laser Slab FM 015-452 from Lasag and a diode pumped Nd:YAG laser Innoslab Boxtar (BX) from Edgewave. The superposed laser radiation is guided through a dichroitic mirror and focused onto the sample by a focusing lens (focal length 100 mm). The spatial intensity profile of the laser radiation is measured by photography using a camera based monitoring system (MicrospotMonitor Co. PRIMES GmbH) (Fig. 2, right). The laser radiations of both laser sources exhibit a nearly circular Gaussian spatial intensity profile with comparable focus diameters. The Slab- and InnoSlab-Laser sources are operated with constant laser parameters (Table 1). Using the same lens, the foci are displaced spatially at the propagation axis. The focus position of the laser radiation of the InnoSlab-Laser is located 2 mm below the Slab-Laser radiation focus (Fig. 2, left). A conical nozzle with an exit diameter of 1 mm is positioned 1 mm above the substrate surface. The total pressure for all process gases is 10 bar. The drilling depth is detected by a novel lateral observation method described in [23]. A hole drilled into the workpiece with a distance of approximately 100 mm from the polished front surface (Fig. 1) exhibits a deformation in the front surface caused by thermal stress [23]. The deformation is detected by high-speed photography (MotionXtra HG-100 K, Redlake) (Fig. 4 left and right) and analyzed with an image editing software. Laser sources
and high-speed camera are temporally synchronized by a pulse generator (Fig. 3). The optical emission induced by laser ablation is monitored adopting coaxial high-speed photography. The images are taken at a constant frequency (50 kHz), exposure time (8 ms), and are rendered into false color for a higher contrast (Fig. 4).
Fig. 2. True to scale scheme of the hole and beam caustics of the Slab- and InnoSlab-Laser radiations (left) and spatial intensity in focus (right).
Fig. 1. Scheme of the experimental setup.
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3. Results 3.1. Drilling depth The drilling depth is determined by lateral process monitoring for the process gases oxygen, argon, helium, and constant laser parameters (Table 1). The drilling depth of holes produced with superposed laser radiation is compared with the drilling depth of holes produced without superposed laser radiation (Fig. 5). At the beginning of ablation (pulse 1–10, Fig. 5a) the drilling speed (depth per laser pulse) is larger in case of using the process gas argon than in case of using oxygen or helium. Thereby superposed laser radiation leads to an increased drilling depth. The process gases, helium or oxygen, do not affect the drilling speed during the first 20 pulses adopting superposed laser radiation. At a drilling depth of approximately 2 mm the drilling speed is reduced depending on the process gases (Fig. 5b). At this drilling depth the laser radiation is reflected by the hole wall the first time (Fig. 2). The spatial intensity profile is altered by reflection of laser radiation at the hole wall and changing ground geometry [11,24–27]. Further drilling speed is decreased for the process gases argon and helium, but less for oxygen. At approximately 100 pulses the drilling speed increases again when using the process gases argon and helium. The increased drilling speed can be caused by an amplified intensity profile due to multiple reflections at the hole wall [24–28].
Table 1 Technical specifications of the laser sources Laser
Slab FM 015-452
Innoslab Boxtar (BX)
Laser type Wavelength Pulse duration Repetition rate Pulse energy Beam quality Rayleigh length Polarization Focus diameter
Nd:YAG-Laser 1064 nm 500 ms 20 Hz 640 mJ 1.5 0.625 mm Linear 40.4 mm
Nd:YAG-Laser 1064 nm 4 ns 10 kHz 1.5 mJ 1.5 1.720 mm Linear 73.0 mm
Fig. 3. Trigger signals of lasers and high-speed-camera.
707
The stainless steel sample was not drilled through adopting the process gases argon and helium. The drilling process was stopped when no further increase of drilling depth was observed. The influence of superposed laser radiation on the drilling speed in case of the process gases argon and helium is negligible. Using the process gas oxygen the stainless steel is drilled through. Thereby, superposed laser radiation decreases the drilling time by approx. 22% which relates to 200 ms-pulses (Fig. 5c). 3.2. Optical emission The optical emission of the drilling process using superposed and non-superposed laser radiation is investigated by coaxial high-speed photography comparing photographs in false color. As the first 3 ms-pulses tend to be overexposed the 4th ms-pulse of the drilling process with the process gases oxygen (Fig. 6) and argon (Fig. 7) is discussed. As the optical emission of the drilling process with the process gas helium is comparable to argon, only photographs of the drilling process with the process gas argon are discussed. In case of drilling with non-superposed laser radiation the optical emission is detected at the beginning of the ms-pulse. After irradiation the optical emission is detectable for ulterior 100 ms in case of drilling with the process gas oxygen (Fig. 6, left), whereas the optical emission is detectable for less than 100 ms when using argon (Fig. 7, left). When drilling with superposed laser radiation optical emission is also detectable before the ms-pulse which is caused by a nspulse. After irradiation of the ms-pulse the intensity of the optical emission reduces until it is amplified by a ns-pulse again (Fig. 6 right, Fig. 7 right). In case of using the process gas argon optical emission of the drilling process caused by ns-pulses is only detectable during irradiation of the ns-pulse (Fig. 7, right). With the process gas oxygen the optical emission is detectable beyond each ns-pulse (Fig. 6, right).
4. Discussion The drilling speed in these experiments depends on the drilling depth, the used process gases, and the superposed laser radiation. The drilling speed decreases during processing which refers to reduced intensity due to the caustic of the laser beam. Furthermore, the drilling can be divided into two domains. The drilling depth in the first domain (pulse 1–20) is referred to a dominating ablation of the ms-pulsed laser radiation. In this domain superposed laser radiation or process gases do not affect the drilling speed as much as in the second domain. In the first domain drilling with the process gas argon is faster than with oxygen or helium. This effect is also reported by Horn in experimental studies on melt front velocities [29]. The second
Fig. 4. Scheme of the stainless steel sample with deformation (left) and photographs of the percussion drilling progress with non-superposed laser radiation in stainless steel (right).
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Fig. 5. Drilling depth in dependence on the number of ms laser pulses applied for different process gases.
domain (420 pulses) is characterized by thermal effects of the process gases. Unlike argon or helium, a reaction of oxygen with metal can cause a formation of oxide during laser drilling that can change the absorptivity of the surface as well as an increase of the temperature of the process [30,31]. The increased temperature is caused by an additional energy that is induced by an exothermal chemical reaction (combustion-energy) of oxygen with the material. The higher temperature-detectable as an increased intensity of the optical emission-improves the melt ejection resulting in a larger drilling speed. In case of drilling with superposed laser radiation additional optical energy is supplied to the drilling process. The effect of the optical energy on the drilling progress is negligible when using the process gases argon or helium as drilling speed is not improved. The drilling speed increases when drilling with the process gas oxygen as the absorbed optical energy causes an enhanced combustion-energy. The enhanced combustion-energy affects the temperature of the drilling process. One effect of the increased temperature of the drilling process is an extended drilling process as the optical emission is detectable for a longer time period. This leads to visible melt ejection after irradiation (Fig. 6, right, 1080–1120 ms). The total depth when drilling with the process gas argon is larger compared to helium which can be explained by different
thermal conductivities of the two gases. The larger thermal conductivity of helium leads to lower temperatures, a larger melt viscosity, and thus to smaller hole depths. Process gases do not penetrate inside the hole during processing as the pressure of vaporized metal is too large [18]. Therefore process gases especially oxygen have an influence onto the drilling speed between two laser pulses when the pressure of vaporized metal is degenerated. The drilling process continues after irradiation as optical emission and melt ejection is detectable by coaxial process monitoring for up to some hundred ms beyond a laser pulse. During this time process gas penetrates inside the hole and affects the drilling process which is detectable by residual oxide at hole walls when using the process gas oxygen [32].
5. Conclusion Percussion drilling of through holes in stainless steel with temporal and spatial superposed laser radiation is performed for different process gases (oxygen, argon, and helium) and constant laser parameters. The drilling depth and the optical emission are monitored and analyzed. The results of the experiments exhibit that drilling with superposed laser radiation improves the drilling
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Fig. 6. Optical emission induced by laser radiation during percussion drilling process in stainless steel with the process gas oxygen and non-superposed (left) as well as superposed (right) laser radiation (Photographs in false color of the 4th pulse).
Fig. 7. Optical emission induced by laser radiation during percussion drilling process in stainless steel with the process gas argon and non-superposed (left) as well as superposed (right) laser radiation (Photographs in false color of the 4th pulse).
speed when drilling with the process gas oxygen. The increased drilling speed is caused by additional optical energy supplied by the second laser source that amplifies and extends the exothermal reaction. The influence of superposed laser radiation on the drilling speed when drilling with the process gases argon and
helium is negligible as no further combustion-energy by chemical reaction is induced. Process gases are commonly used for two reasons: protection of optics and improvement of drilling quality [18]. Experiments show that process gases also affect the drilling speed and depth.
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Acknowledgments The authors are very thankful for the support by the EdgeWave GmbH for providing the InnoSlab-Laser. The authors gratefully acknowledge the financial support by the German Research Foundation (DFG) within the Collaborative Research Centre (SFB) 561 ‘‘Thermally highly loaded, porous and cooled multilayer system for combined cycle power plants’’. References [1] Liebers R, Du¨rr U, Trippe L, Schulz W. Drilling strategies for metals with pulsed YAG lasers. In:ICALEO 2006, 25th international congress on applications of lasers and electro optics. LIA, Orlando, FL; 2006. [2] Boyce MP. Gas turbine engineering handbook, 2nd ed (Incompressible flow turbomachines). Gulf Professional Publishing; 2001. [3] Ng GKL, Li L. Optimization of laser percussion drilling for improved reliability. In: Proceedings of the ICALEO; 2002 [Section E]. [4] Migliore L, Kardos G, Ozkan A, Derkach O, Schaeffer R, Dunsky C. Laser choices for micromachining: drilling speed and quality comparison. In: ICALEO; 2004. [5] Forsman AC, Banks PS, Perry MD, Campbell EM, Dodell AL, Armas MS. Doublepulse machining as a technique for the enhancement of material removal rates in laser machining of metals. J Appl Phys 2005;98. [6] Ostermeyer M, Kappe P, Menzel R, Sommer S, Dausinger F. Laser drilling in thin materials with bursts of ns-pulses generated by stimulated brillouin scattering (SBS). Appl Phys A 2005;81:923–7. [7] Lapczyna M, Chen KP, Herman PR, Tan HW, Marjoribanks RS. Ultra high repetition rate (133 MHz) laser ablation of aluminum with 1.2-ps pulses. Appl Phys A 1999;69:883–6. [8] Stoian R, Boyle M, Thoss A, Rosenfeld A, Korn G, Hertel IV. Laser ablation of dielectrics with temporally shaped femtosecond puses. Appl Phys Lett 2002;80(3):353–5. [9] Lehane C, Kwok HS. Enhanced drilling using a dual-pulse Nd:YAG laser. Appl Phys A 2001;73:45–8. [10] Schulz W. Diagnosis and modelling of nonlinear dynamics in laser cutting, welding and drilling. Warrendale, PA: Materials Research Society; 2005 (Materials Research Society Symposium Proceedings 850). ISBN:1-55859798-9; p. 65–78. [11] Ruf A, Berger P, Dausinger F, Hu¨gel H. Analytical investigations on geometric influences on laser drilling. J Phys D 2001;34:2918–25. [12] Low DKY, Li L, Byrd PJ. The influence of temporal pulse train modulation during laser percussion drilling. Opt Lasers Eng 2001;35:149–64. [13] Low DKY, Li L. Comparison of intra- and interpulse modulation in laser percussion drilling. Proc IMechE 2002;216B:167–71.
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