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Laser drilling of stainless steel with nanosecond double-pulse
ARTICLE IN PRESS Optics & Laser Technology 41 (2009) 148– 153
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Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Laser drilling of stainless steel with nanosecond double-pulse X.D. Wang a,, A. Michalowski b, D. Walter c, S. Sommer c, M. Kraus c, J.S. Liu a, F. Dausinger c a
Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, 430074 Wuhan, China ¨ r Strahlwerkzeuge (IFSW), University of Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany Institut fu c ¨ r Strahlwerkzeuge mbH (FGSW) , Pfaffenwaldring 43, 70569 Stuttgart, Germany Forschungsgesellschaft fu b
a r t i c l e in f o
a b s t r a c t
Article history: Received 4 March 2008 Received in revised form 13 May 2008 Accepted 27 May 2008 Available online 11 July 2008
Nanosecond double-pulse laser drilling is reported in this paper. The double-pulse herein represents two closely conjoint pulses with 21 ns pulse duration and about 52 ns interpulse separation, which are acquired by temporal pulse shaping. Percussion drilling with such double-pulse is performed in stainless steel samples with different laser fluences, sample’s thickness, repetition rates and ambient pressures. The experimental results show that the drilling rates of double-pulse drilling are more than one order of magnitude higher than that of conventional single-pulse drilling in air. Differences in the processing results between single-pulse and double-pulse with various processing parameters are investigated. In addition the ablation mechanisms of the double-pulse drilling are discussed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Laser drilling Double-pulse Pulse shaping
1. Introduction Nowadays, short-pulse lasers have been recognized as an important tool for micro drilling. In order to improve processing efficiency, a considerable amount of research has been done to find the optimum processing strategy during the last decade, such as the influence of pulse width, wavelength, pulse energy, repetition rate, ambient gas, focal condition, plasma effect and a number of special technologies [1–4]. It has been found that melt ejection and material vaporization are two main mechanisms responsible for material removal in pulse laser drilling. The intense laser energy melts and subsequently vaporizes the material rapidly. The evaporation-induced recoil pressure expels the molten material out of the melt pool. The amount of melt ejection and material vaporization determines the velocity of laser drilling. Some theoretical models have been developed to characterize the dynamics of the laser drilling process [5–8]. In the case of percussion drilling without any assist procedure, the drilling efficiency is rather low. The problem is due to resolidification of the melt pool. Increasing the laser power does not work well [9]. Normally, a high pressure gas jet is used to assist the drilling of the material, which accelerates melt ejection and material vaporization diffusing [10]. A double-pulse technique is reported in this paper to accelerate material removal and consequently improve efficiency in pulse laser drilling. The double-pulse technique was originally applied
Corresponding author. Tel./fax: +86 27 87556664.
E-mail address: [email protected] (X.D. Wang). 0030-3992/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2008.05.021
in laser-induced breakdown spectroscopy (LIBS). The aim of the double-pulse approach in LIBS is to increase LIBS performance through better coupling of laser energy to the ablated material, leading to a more efficient production of analyte atoms in an excited state [11,12]. A detailed review of double-pulse LIBS is presented by Babushok et al. [13]. Some investigations in terms of LIBS have demonstrated that collinear double-pulse interacting with solid samples leads to not only increased LIBS performance but also increased material ablation. Stratis et al. proposed that the intensity enhancement of double-pulse-induced plasma was given by the stronger mass ablation from the sample through directly measuring the sizes of the craters produced by double-pulse [14]. Sattmann et al. studied the material ablation for single and collinear double-pulse as a function of the pulse energy. The increases in the ablated mass of steel by up to a factor of 8 with the use of double-pulse LIBS were found [15]. Peter and Noll used nanosecond double-pulse, with the energy of 60 and 120 mJ, pulse width of 20 ns and interpulse separation of 6 ms, to irradiate the steel foil to study the ablation characteristics in steel [16]. The maximum enhancement ratio of ablation rate for double-pulse to single-pulse was 6 times. All these results indicate the possibility of efficiency enhancement in laser drilling with double-pulse technique. On the other hand, the double-pulse technique or similar approach has also been introduced to laser drilling. In 1975, Fox used double-beam approach, which combined a cw CO2 laser with Q-switched Nd:glass laser pulses, to achieve a factor of two increase in the drilling efficiency of carbon steel [17]. The Q-switched pulse was responsible for the ejection of molten metal, resulting in a higher drilling efficiency. Lehane combined a
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long pulse (3.5 ms FWHM) and a followed shorter pulse (150 ms FWHM) [18] to drill stainless steel plate. This approach allowed the capability of drilling through 1/8 in thick stainless steel targets at a standoff distance of 1 m without gas-assist. They argued that the improvement in drilling is due to the recoil pressure generated by rapid evaporation of the molten material by the second laser pulse. Recently, Forsman et al. [19] reported the double-pulse to increase the rate of material removal in drilling metals with nanosecond laser (532 nm, 3 ns pulse duration, 20–200 J/cm2). The results have shown a significantly enhance (3–10 times) material removal rates induced by the double-pulse while minimizing redeposition and heat-affected zones. The optimum interpulse separations were in the 40–150 ns range (stainless steel, Al). Their work presented an attractive and efficient approach for machining high-aspect ratio holes (10:1) in metals. However, there are not many other publications dealing with the drilling characteristics of such nanosecond double-pulse with the interpulse separation in nanosecond scale. In this paper the further clarification of the influence for different processing parameters on collinear nanosecond doublepulse drilling of stainless steel is reported. A Q-switched laser pulse with the wavelength of 1047 nm and the pulse duration of 21 ns is split into two sub-pulses with the interpulse separation of 52 ns, which is applied to drill stainless steel plates with the thickness ranging from 0.4 to 1 mm. The number of pulses for drilling through the samples and the average drilling rates are investigated with different laser fluences, sample’s thickness, repetition rates and ambient gas pressures. Differences in the processing results between single-pulse and double-pulse and the possible mechanism responsible for the improvement induced by double-pulse are discussed.
divide one laser beam into two parts and adjust the energy of each part easily, which includes a half-wave plate and a polarization beam splitter that allows a maximal transmission of P polarization and a maximal reflection of S polarization. By rotating the half-wave plate, the energy proportion of the transmission part to the reflection part can be continuously varied. Consequently, one laser pulse can be divided into two parts, which travel along mutually perpendicular directions, respectively. In order to produce time delay between the two split parts, some mirrors are used to prolong the optical path of the reflection part. Nine mirrors are used in these experiments and 52 ns delay time between two parts is produced. Then, the two split parts are recombined by the second beam splitter and pass through a quarter-wave-plate, which can change a linearly polarized beam into a circularly polarized beam. The waveforms of the resultant double-pulse compared with the single-pulse are shown in Fig. 2. The pulse energy of the double-pulse hereafter represents the sum energy of the two sub-pulses within one double-pulse and the energies of each sub-pulse are adjusted to be equal throughout the entire experiments. The combined laser beam then is focalized with a 150 mm focal lens. The focus diameter for the transmission beam is 30 mm. Because of the deviation of adjustment for the beam expander, the diameter of the reflection beam reduces a little after the transmission of a longer path. Therefore the focus diameter for the reflection beam is slightly more than 30 mm. The focal position is 200 mm inside the samples. Stainless steel samples (X10CrNi18-8) with different thickness of 0.4, 0.6, 0.8 and 1.0 mm are placed in turn in a vacuum chamber on a motorized 3D translation stage. A high-speed photodiode (P) with the rise time of 500 ps (Soliton UPD500) and an oscillograph (HP 54542A, 500 MHz, 2 GSa/S) are used to monitor the drilling process.
2. Experimental setup
3. Results and discussion
The experimental setup is shown in Fig. 1, which includes a Qswitched Nd:YLF laser (TL 20-1 FQ, Trumpf, 1047 nm, maximal pulse energy 4 mJ, maximal repetition rate 15 kHz, pulse duration FWHM 21 ns at 4 kHz), a beam expander (T), two wave plates (l/2 and l/4), two beam splitters (BS1 and BS2), a focal lens (F), some mirrors (M1–M9), a vacuum chamber, a photodiode (P) and an oscillograph. A linearly polarized laser beam from the Nd:YLF laser passes through a beam expander. A variable-ratio beam splitter is used to
Figs. 3(a) and (b) show the number of pulses for drilling through stainless steel samples with different thickness using the single- and double-pulse as the function of pulse energy at 4 kHz repetition rate. The pulse numbers are calculated from time the hole just drilling-through, and the drilling-through time is recorded by the oscillograph. An average of five measurements was used for each point. As illustrated in Figs. 3(a) and (b), for 0.4 mm thickness steel, the curves of single- and double-pulse are nearly coincident. When the pulse energy is more than 0.25 mJ,
M9 M8 M7
M6
M5 M4 M3 M2
M1
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Vacuum Chamber
P Sample
F
λ/4
BS2
BS1
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Fig. 1. Experimental setup. Laser ¼ Q-switched Nd:YLF laser (1047 nm, maximal pulse energy 4 mJ, maximal repetition rate 15 kHz, pulse duration FWHM 21 ns at 4 kHz); T ¼ beam expander; l/2 ¼ half-wave plate; l/4 ¼ quarter-wave plate; BS1 and BS2 ¼ Beam splitter; M1–M9 ¼ mirrors; F ¼ focal lens (f ¼ 150 mm) and P ¼ photodiode.
ARTICLE IN PRESS X.D. Wang et al. / Optics & Laser Technology 41 (2009) 148–153
1.0
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Fig. 2. Pulse shapes of single- and double-pulse. (a) Single-pulse, pulse width is 21 ns (FWHM) and (b) double-pulse, the width of each sub-pulse is 21 ns (FWHM), interpulse separation 52 ns.
the ablation velocities of single- and double-pulse drilling do not increase any more. On the contrary, the ablation velocities drop rapidly when pulse energy is less than 0.25 mJ. Therefore, 0.25 mJ can be considered as a saturation point because of the saturation of ablation rates after this point. The saturation points can also be found for different thickness and are marked as S1–4 and D1–4 in Figs. 3(a) and (b), respectively. It is worth noting that S4 is a supposed saturation point due to the absence of the experimental results with the pulse energy more than 2.5 mJ. The major reason for the saturation of laser drilling can be plasma shielding effect, which was proposed by many previous works [20–22]. As we see, the same saturation behavior is observed for double-pulse in our experiment, but for the thickness of 0.6, 0.8 and 1.0 mm the pulse energy of the saturation points of double-pulse is lower than that of single-pulse. With the increase of thickness, the advantages of double-pulse drilling are becoming more and more remarkable. Comparing saturation points S4 with D4, it can be seen that only 430 doublepulses with 1 mJ pulse energy can drill 1 mm steel through; however, 800 single-pulses with 2.5 mJ pulse energy are needed to drill through the same sample. Another comparison is made between saturation point D4 and point S5. Using the same pulse energy of 1 mJ, 430 double-pulses and 13,000 single-pulses are needed, respectively, to drill through 1 mm steel. The enhancement ratio of double-pulse drilling to single-pulse drilling exceeds 30 times in this case. Fig. 4 shows the enhancement ratios of double-pulse to single-pulse for drilling velocity in different thickness steel drilling. The maximal enhancement ratio increases with the thickness and reaches its maximal value at 1 mJ pulse energy for 1 mm steel. Defining the average drilling rate as a thickness divided by pulse’s number for drilling-through, our results can be presented as the dependencies shown in Fig. 5. As is seen in Fig. 5, the drilling rate of single-pulse strongly depends on the sample’s thickness. The ablation rate reduces by 1–2 orders of magnitude with the thickness increasing from 0.4 to 1.0 mm. On the contrary, the drilling rate of double-pulse stays constant when the thickness is varied. The thick dependence of drilling rate for single-pulse was also observed in some investigations [23]. As we know, melt ejection is an important mechanism of material removal in the laser drilling of metals. It arises from the high pressure gradients generated by vaporization within the hole, which can expel surrounding molten material. As the hole
deepens, a large amount of molten material deposits on the sidewall of the hole since its kinetic energy is not enough to support its flowing out of the hole any more. This can result in the reduction of the drilling rate in single-pulse drilling. However, the drilling rate of double-pulse is independent of thickness that shows that the double-pulse configuration can improve melt ejection. To clarify the impact of the double-pulse on the melt flow, the morphology of the crater after one double-pulse irradiated is investigated. In the experiment, the two sub-pulses are adjusted to be partly overlapped. Fig. 6 shows the morphologies of the melt pools on the sample’s surface after the partly overlapped doublepulse irradiated with different interpulse separations. For the interpulse separation more than 10 s (a), two melt pools can be observed clearly, which are produced by the two sub-pulses, respectively. For the interpulse separation of 1 ns (b), the time delay is so short that the melt pool looks like produced by the two sub-pulses at the same time. However, a ridge of the melt flow in the melt pool can be seen obviously when the interpulse separation is 52 ns (c). The ridge is not just at the boundary of the melt pool produced by the second sub-pulse but spreads over a larger area. Comparing with the result of more than 10 s interpulse separation, the recast profile induced by the first subpulse cannot be found in the melt pool any more. This shows that the second sub-pulse can force the melt flow induced by the first sub-pulse back to the melt pool on one hand. On the other hand, the spreading ridge of the melt flow proves that the ablation zone is in melt state at 52 ns later after the beginning of the first sub-pulse. Thus, it can be expected that the second sub-pulse can accelerate the molten material to flow out of the hole if the pressure gradients generated by the two sub-pulses are coincident. Fig. 7 shows average ablation rates in drilling through 1 mm steel using single- and double-pulse as a function of pulse energy with different repetition rates. At the energy over 1.1 mJ, high ablation rate of 2.7 mm per pulse is obtained with double-pulse ablation, and the ablation rate is almost independent of the repetition rate. Even at the repetition rate of 100 Hz, the drilling rate is still at a high level. However, in single-pulse drilling, not only the drilling rate is much lower than that achieved by using double-pulse, but it also strongly depends on the repetition rate. The interpretation for this phenomenon can be that in singlepulse drilling the previous pulse has an effect on the succeeding
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Fig. 3. Number of pulses for drilling-through stainless steel samples (X10CrNi18-8) with different thickness as a function of pulse energy. Repetition rate is 4 kHz. (a) Drilling with single-pulse and (b) drilling with double-pulse (pulse energy of double-pulse is the sum energies of the two sub-pulses with 52 ns interpulse separation).
pulse and the influence of the previous pulse is expanded with the repetition rate increasing. But the influence induced by the previous pulse does not play a major role in double-pulse drilling. Drilling at low ambient pressure is also investigated in our experiments. The dependence of average drilling rate for drilling though 1 mm stainless steel samples with the repetition rate in different ambient pressures is presented in Fig. 8. Single-pulse drilling at the pressure of 8 mbar has a high ablation rate close to that with double-pulse laser drilling in the open air, and it is almost independent on the repetition rate. However, single-pulse drilling in the open air has a very low drilling rate, especially at the low repetition rate. These results may suggest that vacuum environment is a possible reason for the enhancement induced by double-pulse drilling. During pulse laser drilling, a shock wave
occurs at the interaction area [3], and the propagation of the shock wave is governed by the Sedov–Taylor model [24]. Sedov calculated the density distribution of the region inside the shock wave [25]. It has been found that the mass of matters among the shock wave concentrates near the shock wave front, and there is a low-density region inside shock wave. So the absorption of the second sub-pulse is intensified and the processing rate is improved. Some investigations also proposed that the first subpulse could deplete the atmosphere in the interaction region while generating a spherical shock wave [26]. Due to the localized transient reduction of the particle density in the vicinity of ablation spot, the sample could better absorb the energy of the second sub-pulse. On the other hand, Corsi et al. [27] observed the laser-induced plasmas in single- and double-pulse configuration.
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The expansion of the plasma plume induced by the second subpulse is sensibly faster than the one induced by the first subpulse. This situation of the double-pulse ablation is also similar to that of the laser ablation experiments in vacuum environment, where the hydrodynamic expansion of the plasma is fast, not contrasted by the counter-pressure of the buffer gas. Thereby, we suppose that pulse laser-induced transient vacuum environment
is a possible reason for the enhancement induced by double-pulse laser processing. According to experiment results and analyses presented above, the enhancement of the double-pulse drilling can be resulting from the following mechanisms. First of all, the second sub-pulse accelerates the melted material produced by the first sub-pulse to flow out of the hole. Secondly, the first sub-pulse depletes the 2.8 2.6
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Fig. 5. Average drilling rate develops with hole depth for single- and double-pulse.
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Fig. 8. Average drilling rate for drilling-through 1 mm stainless steel samples (X10CrNi18-8) with different ambient air pressures as a function of repetition rate.
Fig. 6. Effect of the second sub-pulse on the melt pool. Energy density of the first sub-pulse (on the right side) is 24 J/cm2; the second sub-pulse (on the left side) is 14 J/cm2. (a) Dt410 s; (b) Dt ¼ 1 ns; (c) Dt ¼ 52 ns. Material: stainless steel (X10CrNi18-8).
ARTICLE IN PRESS X.D. Wang et al. / Optics & Laser Technology 41 (2009) 148–153
atmosphere in the interaction region. The sample could better absorb the energy of the second sub-pulse and the expansion of the plasma plume induced by the second sub-pulse is faster without contrasted by the counter-pressure of the buffer gas. Thirdly, the first sub-pulse heated the ablation zone to a high temperature and created a molten layer on the surface with modified optical coupling properties, which reduce the ablation threshold and increase the coupling of the ablation pulse with the target. In addition, the pulse energy of the single-pulse is split up into two parts to produce double-pulse. Therefore the peak intensity of the double-pulse is halved comparing to the original single-pulse and the plasma shielding effect can be weakened. However, it still needs further work to clarify which mechanisms concerned above are primary for the enhancement of the double-pulse drilling.
4. Conclusion This paper reports a series of nanosecond laser drilling experiments with the double-pulse of 52 ns interpulse separation, which are compared with conventional single-pulse drilling. Firstly, the double-pulse drilling has the same saturation behavior of laser fluence as the single-pulse drilling, but the pulse energy of the saturation points of double-pulse is lower than that of singlepulse. Secondly, the drilling rate of single-pulse strongly depends on the sample’s thickness. However, the drilling rate of doublepulse stays constant when the thickness is varied from 0.4 to 1 mm. Thirdly, variation of laser’s repetition rate has no obvious effect on the ablation rate in double-pulse drilling. Furthermore, double-pulse drilling in air allows a similar, even higher drilling velocity than that in single-pulse drilling in vacuum. The experimental results show that double-pulse drilling can achieve a significant improvement of laser drilling efficiency compared with the conventional single-pulse in open air. The results are practical and have potential applications for pulse laser drilling.
Acknowledgments The authors would like to thank the China Scholarship Council and German Federal Ministry for Education and Research for financial support. References [1] Yilbas BS. Parametric study to improve laser hole drilling process. J Mater Process Technol 1997;70(1–3):264–73. [2] Jackson MJ, Neill WO. Laser micro-drilling of tool steel using Nd:YAG lasers. J Mater Process Technol 2003;142(2):517–25.
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[3] Breitling D, Schittenhelm H, Berger P, Dausinger F, Hu¨gel H. Shadowgraphic and interferometric investigations on Nd:YAG laser-induced vapor/plasma plumes for different processing wavelengths. Appl Phys A 1999;69(Suppl.): S505–8. [4] Klimentov SM, Kononenko TV, Pivovarov PA, Garnov SV, Konov VI, Breitling D, et al. Role of gas environment in the process of deep-hole drilling by ultrashort laser pulses. Proc SPIE 2003;4830:515–20. [5] Uglov AA, Kokora AN, Orekhow NV. Laser drilling of holes in materials with different thermal properties. Sov J Quantum Electron 1976;6(3):311–5. [6] Armon E, Zvirin Y, Laufer G, Solan A. Metal drilling with a CO2 laser beam. I. Theory. J Appl Phys 1989;65(12):4995–5002. [7] Olson RW, Swope WC. Laser drilling with focused Gaussian beams. J Appl Phys 1992;72(8):3686–96. [8] Kononenko TV, Klimentov SM, Garnov SV, et al. Hole formation process in laser deep drilling with short and ultrashort pulses. Proc SPIE 2002;4426: 108–12. [9] Ba¨uerle D. Laser processing and chemistry. Berlin: Springer; 2000. [10] Fieret J, Terry MJ, Ward BA. Aerodynamic interactions during laser cutting. Proc SPIE 1986;668:53–62. [11] St-Onge L, Detalle V, Sabsabi M. Enhanced laser-induced breakdown spectroscopy using the combination of fourth-harmonic and fundamental Nd:YAG laser pulses. Spectrochim Acta Part B 2002;57:121–35. [12] Scaffidi JM, Angel S, Cremers DA. Emission enhancement mechanisms in dualpulse LIBS. Anal Chem 2006;78(1):24–32. [13] Babushok VI, DeLucia Jr. FC, Gottfried JL, Munson CA, Miziolek AW. Spectrochim Acta Part B 2006;61:999–1014. [14] Stratis DN, Eland KL, Angel SM. Effect of pulse delay time on a pre-ablation dual-pulse LIBS plasma. Appl Spectrosc 2001;55(10):1297–303. [15] Sattmann R, Sturm V, Noll R. Laser-induced breakdown spectroscopy of steel samples using multiple Q-switch Nd:YAG laser pulses. J Phys D 1995;28(10): 2181–7. [16] Peter L, Noll R. Material ablation and plasma state for single and collinear double pulses interacting with iron samples at ambient gas pressures below 1 bar. Appl Phys B 2007;86:159–67. [17] Fox JA. A method for improving continuous wave laser penetration of metal targets. Appl Phys Lett 1975;26(12):682–4. [18] Lehane C, Kwok HS. Enhanced drilling using a dual-pulse Nd:YAG laser. Appl Phys A 2001;73(1):45–8. [19] Forsman AC, Banks PS, Perry MD, Campbell EM, Dodell AL, Armas MS. Doublepulse machining as a technique for the enhancement material remove rates in laser machining of metals. J Appl Phys 2005;98:033302. [20] Mao X, Russo RE. Observation of plasma shielding by measuring transmitted and reflected laser pulse temporal profiles. Appl Phys A 1997;64(1):1–6. [21] Tokarev VN, Lunney JG, Marine W, Sentis M. Analytical thermal model of ultraviolet ablation with single-photon absorption in the plume. J Appl Phys 1995;78(2):1241–6. [22] Zhang DM, Liu D, Li ZH, Hou SP, Yu BM, Li G, et al. A new model of pulsed laser ablation and plasma shielding. Physica B 2005;362(1–4):82–7. [23] Kononenko TV, Konov VI, Garnov SV, Klimentov SM, Dausinger F. Dynamics of deep short pulse laser drilling: ablative stages and light propagation. Laser Phys 2001;11(3):343–51. [24] Breitling D, Ruf A, Berger P, Dausinger F, Klimentov SM, Pivovarov PA, et al. Plasma effects during ablation and drilling using pulsed solid-state lasers. Proc SPIE 2003;5121:24–33. [25] Sedov LI. Similarity and dimensional methods in mechanics. Moscow: Mir Publishers; 1982. [26] Noll R, Sattmann R, Sturm V, Winkelmann S. Space- and time-resolved dynamics of plasmas generated by laser double pulses interacting with metallic samples. J Anal Atom Spectrom 2004;19(4):419–28. [27] Corsi M, Cristoforetti G, Giuffrida M, Hidalgo M, Legnaioli S, Palleschi V, et al. Three-dimensional analysis of laser induced plasmas in single and double pulse configuration. Spectrochim Acta Part B 2004;59(5):723–35.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Laser drilling of stainless steel with nanosecond double-pulse X.D. Wang a,, A. Michalowski b, D. Walter c, S. Sommer c, M. Kraus c, J.S. Liu a, F. Dausinger c a
Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, 430074 Wuhan, China ¨ r Strahlwerkzeuge (IFSW), University of Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany Institut fu c ¨ r Strahlwerkzeuge mbH (FGSW) , Pfaffenwaldring 43, 70569 Stuttgart, Germany Forschungsgesellschaft fu b
a r t i c l e in f o
a b s t r a c t
Article history: Received 4 March 2008 Received in revised form 13 May 2008 Accepted 27 May 2008 Available online 11 July 2008
Nanosecond double-pulse laser drilling is reported in this paper. The double-pulse herein represents two closely conjoint pulses with 21 ns pulse duration and about 52 ns interpulse separation, which are acquired by temporal pulse shaping. Percussion drilling with such double-pulse is performed in stainless steel samples with different laser fluences, sample’s thickness, repetition rates and ambient pressures. The experimental results show that the drilling rates of double-pulse drilling are more than one order of magnitude higher than that of conventional single-pulse drilling in air. Differences in the processing results between single-pulse and double-pulse with various processing parameters are investigated. In addition the ablation mechanisms of the double-pulse drilling are discussed. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Laser drilling Double-pulse Pulse shaping
1. Introduction Nowadays, short-pulse lasers have been recognized as an important tool for micro drilling. In order to improve processing efficiency, a considerable amount of research has been done to find the optimum processing strategy during the last decade, such as the influence of pulse width, wavelength, pulse energy, repetition rate, ambient gas, focal condition, plasma effect and a number of special technologies [1–4]. It has been found that melt ejection and material vaporization are two main mechanisms responsible for material removal in pulse laser drilling. The intense laser energy melts and subsequently vaporizes the material rapidly. The evaporation-induced recoil pressure expels the molten material out of the melt pool. The amount of melt ejection and material vaporization determines the velocity of laser drilling. Some theoretical models have been developed to characterize the dynamics of the laser drilling process [5–8]. In the case of percussion drilling without any assist procedure, the drilling efficiency is rather low. The problem is due to resolidification of the melt pool. Increasing the laser power does not work well [9]. Normally, a high pressure gas jet is used to assist the drilling of the material, which accelerates melt ejection and material vaporization diffusing [10]. A double-pulse technique is reported in this paper to accelerate material removal and consequently improve efficiency in pulse laser drilling. The double-pulse technique was originally applied
Corresponding author. Tel./fax: +86 27 87556664.
E-mail address: [email protected] (X.D. Wang). 0030-3992/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2008.05.021
in laser-induced breakdown spectroscopy (LIBS). The aim of the double-pulse approach in LIBS is to increase LIBS performance through better coupling of laser energy to the ablated material, leading to a more efficient production of analyte atoms in an excited state [11,12]. A detailed review of double-pulse LIBS is presented by Babushok et al. [13]. Some investigations in terms of LIBS have demonstrated that collinear double-pulse interacting with solid samples leads to not only increased LIBS performance but also increased material ablation. Stratis et al. proposed that the intensity enhancement of double-pulse-induced plasma was given by the stronger mass ablation from the sample through directly measuring the sizes of the craters produced by double-pulse [14]. Sattmann et al. studied the material ablation for single and collinear double-pulse as a function of the pulse energy. The increases in the ablated mass of steel by up to a factor of 8 with the use of double-pulse LIBS were found [15]. Peter and Noll used nanosecond double-pulse, with the energy of 60 and 120 mJ, pulse width of 20 ns and interpulse separation of 6 ms, to irradiate the steel foil to study the ablation characteristics in steel [16]. The maximum enhancement ratio of ablation rate for double-pulse to single-pulse was 6 times. All these results indicate the possibility of efficiency enhancement in laser drilling with double-pulse technique. On the other hand, the double-pulse technique or similar approach has also been introduced to laser drilling. In 1975, Fox used double-beam approach, which combined a cw CO2 laser with Q-switched Nd:glass laser pulses, to achieve a factor of two increase in the drilling efficiency of carbon steel [17]. The Q-switched pulse was responsible for the ejection of molten metal, resulting in a higher drilling efficiency. Lehane combined a
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long pulse (3.5 ms FWHM) and a followed shorter pulse (150 ms FWHM) [18] to drill stainless steel plate. This approach allowed the capability of drilling through 1/8 in thick stainless steel targets at a standoff distance of 1 m without gas-assist. They argued that the improvement in drilling is due to the recoil pressure generated by rapid evaporation of the molten material by the second laser pulse. Recently, Forsman et al. [19] reported the double-pulse to increase the rate of material removal in drilling metals with nanosecond laser (532 nm, 3 ns pulse duration, 20–200 J/cm2). The results have shown a significantly enhance (3–10 times) material removal rates induced by the double-pulse while minimizing redeposition and heat-affected zones. The optimum interpulse separations were in the 40–150 ns range (stainless steel, Al). Their work presented an attractive and efficient approach for machining high-aspect ratio holes (10:1) in metals. However, there are not many other publications dealing with the drilling characteristics of such nanosecond double-pulse with the interpulse separation in nanosecond scale. In this paper the further clarification of the influence for different processing parameters on collinear nanosecond doublepulse drilling of stainless steel is reported. A Q-switched laser pulse with the wavelength of 1047 nm and the pulse duration of 21 ns is split into two sub-pulses with the interpulse separation of 52 ns, which is applied to drill stainless steel plates with the thickness ranging from 0.4 to 1 mm. The number of pulses for drilling through the samples and the average drilling rates are investigated with different laser fluences, sample’s thickness, repetition rates and ambient gas pressures. Differences in the processing results between single-pulse and double-pulse and the possible mechanism responsible for the improvement induced by double-pulse are discussed.
divide one laser beam into two parts and adjust the energy of each part easily, which includes a half-wave plate and a polarization beam splitter that allows a maximal transmission of P polarization and a maximal reflection of S polarization. By rotating the half-wave plate, the energy proportion of the transmission part to the reflection part can be continuously varied. Consequently, one laser pulse can be divided into two parts, which travel along mutually perpendicular directions, respectively. In order to produce time delay between the two split parts, some mirrors are used to prolong the optical path of the reflection part. Nine mirrors are used in these experiments and 52 ns delay time between two parts is produced. Then, the two split parts are recombined by the second beam splitter and pass through a quarter-wave-plate, which can change a linearly polarized beam into a circularly polarized beam. The waveforms of the resultant double-pulse compared with the single-pulse are shown in Fig. 2. The pulse energy of the double-pulse hereafter represents the sum energy of the two sub-pulses within one double-pulse and the energies of each sub-pulse are adjusted to be equal throughout the entire experiments. The combined laser beam then is focalized with a 150 mm focal lens. The focus diameter for the transmission beam is 30 mm. Because of the deviation of adjustment for the beam expander, the diameter of the reflection beam reduces a little after the transmission of a longer path. Therefore the focus diameter for the reflection beam is slightly more than 30 mm. The focal position is 200 mm inside the samples. Stainless steel samples (X10CrNi18-8) with different thickness of 0.4, 0.6, 0.8 and 1.0 mm are placed in turn in a vacuum chamber on a motorized 3D translation stage. A high-speed photodiode (P) with the rise time of 500 ps (Soliton UPD500) and an oscillograph (HP 54542A, 500 MHz, 2 GSa/S) are used to monitor the drilling process.
2. Experimental setup
3. Results and discussion
The experimental setup is shown in Fig. 1, which includes a Qswitched Nd:YLF laser (TL 20-1 FQ, Trumpf, 1047 nm, maximal pulse energy 4 mJ, maximal repetition rate 15 kHz, pulse duration FWHM 21 ns at 4 kHz), a beam expander (T), two wave plates (l/2 and l/4), two beam splitters (BS1 and BS2), a focal lens (F), some mirrors (M1–M9), a vacuum chamber, a photodiode (P) and an oscillograph. A linearly polarized laser beam from the Nd:YLF laser passes through a beam expander. A variable-ratio beam splitter is used to
Figs. 3(a) and (b) show the number of pulses for drilling through stainless steel samples with different thickness using the single- and double-pulse as the function of pulse energy at 4 kHz repetition rate. The pulse numbers are calculated from time the hole just drilling-through, and the drilling-through time is recorded by the oscillograph. An average of five measurements was used for each point. As illustrated in Figs. 3(a) and (b), for 0.4 mm thickness steel, the curves of single- and double-pulse are nearly coincident. When the pulse energy is more than 0.25 mJ,
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Fig. 1. Experimental setup. Laser ¼ Q-switched Nd:YLF laser (1047 nm, maximal pulse energy 4 mJ, maximal repetition rate 15 kHz, pulse duration FWHM 21 ns at 4 kHz); T ¼ beam expander; l/2 ¼ half-wave plate; l/4 ¼ quarter-wave plate; BS1 and BS2 ¼ Beam splitter; M1–M9 ¼ mirrors; F ¼ focal lens (f ¼ 150 mm) and P ¼ photodiode.
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Fig. 2. Pulse shapes of single- and double-pulse. (a) Single-pulse, pulse width is 21 ns (FWHM) and (b) double-pulse, the width of each sub-pulse is 21 ns (FWHM), interpulse separation 52 ns.
the ablation velocities of single- and double-pulse drilling do not increase any more. On the contrary, the ablation velocities drop rapidly when pulse energy is less than 0.25 mJ. Therefore, 0.25 mJ can be considered as a saturation point because of the saturation of ablation rates after this point. The saturation points can also be found for different thickness and are marked as S1–4 and D1–4 in Figs. 3(a) and (b), respectively. It is worth noting that S4 is a supposed saturation point due to the absence of the experimental results with the pulse energy more than 2.5 mJ. The major reason for the saturation of laser drilling can be plasma shielding effect, which was proposed by many previous works [20–22]. As we see, the same saturation behavior is observed for double-pulse in our experiment, but for the thickness of 0.6, 0.8 and 1.0 mm the pulse energy of the saturation points of double-pulse is lower than that of single-pulse. With the increase of thickness, the advantages of double-pulse drilling are becoming more and more remarkable. Comparing saturation points S4 with D4, it can be seen that only 430 doublepulses with 1 mJ pulse energy can drill 1 mm steel through; however, 800 single-pulses with 2.5 mJ pulse energy are needed to drill through the same sample. Another comparison is made between saturation point D4 and point S5. Using the same pulse energy of 1 mJ, 430 double-pulses and 13,000 single-pulses are needed, respectively, to drill through 1 mm steel. The enhancement ratio of double-pulse drilling to single-pulse drilling exceeds 30 times in this case. Fig. 4 shows the enhancement ratios of double-pulse to single-pulse for drilling velocity in different thickness steel drilling. The maximal enhancement ratio increases with the thickness and reaches its maximal value at 1 mJ pulse energy for 1 mm steel. Defining the average drilling rate as a thickness divided by pulse’s number for drilling-through, our results can be presented as the dependencies shown in Fig. 5. As is seen in Fig. 5, the drilling rate of single-pulse strongly depends on the sample’s thickness. The ablation rate reduces by 1–2 orders of magnitude with the thickness increasing from 0.4 to 1.0 mm. On the contrary, the drilling rate of double-pulse stays constant when the thickness is varied. The thick dependence of drilling rate for single-pulse was also observed in some investigations [23]. As we know, melt ejection is an important mechanism of material removal in the laser drilling of metals. It arises from the high pressure gradients generated by vaporization within the hole, which can expel surrounding molten material. As the hole
deepens, a large amount of molten material deposits on the sidewall of the hole since its kinetic energy is not enough to support its flowing out of the hole any more. This can result in the reduction of the drilling rate in single-pulse drilling. However, the drilling rate of double-pulse is independent of thickness that shows that the double-pulse configuration can improve melt ejection. To clarify the impact of the double-pulse on the melt flow, the morphology of the crater after one double-pulse irradiated is investigated. In the experiment, the two sub-pulses are adjusted to be partly overlapped. Fig. 6 shows the morphologies of the melt pools on the sample’s surface after the partly overlapped doublepulse irradiated with different interpulse separations. For the interpulse separation more than 10 s (a), two melt pools can be observed clearly, which are produced by the two sub-pulses, respectively. For the interpulse separation of 1 ns (b), the time delay is so short that the melt pool looks like produced by the two sub-pulses at the same time. However, a ridge of the melt flow in the melt pool can be seen obviously when the interpulse separation is 52 ns (c). The ridge is not just at the boundary of the melt pool produced by the second sub-pulse but spreads over a larger area. Comparing with the result of more than 10 s interpulse separation, the recast profile induced by the first subpulse cannot be found in the melt pool any more. This shows that the second sub-pulse can force the melt flow induced by the first sub-pulse back to the melt pool on one hand. On the other hand, the spreading ridge of the melt flow proves that the ablation zone is in melt state at 52 ns later after the beginning of the first sub-pulse. Thus, it can be expected that the second sub-pulse can accelerate the molten material to flow out of the hole if the pressure gradients generated by the two sub-pulses are coincident. Fig. 7 shows average ablation rates in drilling through 1 mm steel using single- and double-pulse as a function of pulse energy with different repetition rates. At the energy over 1.1 mJ, high ablation rate of 2.7 mm per pulse is obtained with double-pulse ablation, and the ablation rate is almost independent of the repetition rate. Even at the repetition rate of 100 Hz, the drilling rate is still at a high level. However, in single-pulse drilling, not only the drilling rate is much lower than that achieved by using double-pulse, but it also strongly depends on the repetition rate. The interpretation for this phenomenon can be that in singlepulse drilling the previous pulse has an effect on the succeeding
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Fig. 3. Number of pulses for drilling-through stainless steel samples (X10CrNi18-8) with different thickness as a function of pulse energy. Repetition rate is 4 kHz. (a) Drilling with single-pulse and (b) drilling with double-pulse (pulse energy of double-pulse is the sum energies of the two sub-pulses with 52 ns interpulse separation).
pulse and the influence of the previous pulse is expanded with the repetition rate increasing. But the influence induced by the previous pulse does not play a major role in double-pulse drilling. Drilling at low ambient pressure is also investigated in our experiments. The dependence of average drilling rate for drilling though 1 mm stainless steel samples with the repetition rate in different ambient pressures is presented in Fig. 8. Single-pulse drilling at the pressure of 8 mbar has a high ablation rate close to that with double-pulse laser drilling in the open air, and it is almost independent on the repetition rate. However, single-pulse drilling in the open air has a very low drilling rate, especially at the low repetition rate. These results may suggest that vacuum environment is a possible reason for the enhancement induced by double-pulse drilling. During pulse laser drilling, a shock wave
occurs at the interaction area [3], and the propagation of the shock wave is governed by the Sedov–Taylor model [24]. Sedov calculated the density distribution of the region inside the shock wave [25]. It has been found that the mass of matters among the shock wave concentrates near the shock wave front, and there is a low-density region inside shock wave. So the absorption of the second sub-pulse is intensified and the processing rate is improved. Some investigations also proposed that the first subpulse could deplete the atmosphere in the interaction region while generating a spherical shock wave [26]. Due to the localized transient reduction of the particle density in the vicinity of ablation spot, the sample could better absorb the energy of the second sub-pulse. On the other hand, Corsi et al. [27] observed the laser-induced plasmas in single- and double-pulse configuration.
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The expansion of the plasma plume induced by the second subpulse is sensibly faster than the one induced by the first subpulse. This situation of the double-pulse ablation is also similar to that of the laser ablation experiments in vacuum environment, where the hydrodynamic expansion of the plasma is fast, not contrasted by the counter-pressure of the buffer gas. Thereby, we suppose that pulse laser-induced transient vacuum environment
is a possible reason for the enhancement induced by double-pulse laser processing. According to experiment results and analyses presented above, the enhancement of the double-pulse drilling can be resulting from the following mechanisms. First of all, the second sub-pulse accelerates the melted material produced by the first sub-pulse to flow out of the hole. Secondly, the first sub-pulse depletes the 2.8 2.6
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Fig. 8. Average drilling rate for drilling-through 1 mm stainless steel samples (X10CrNi18-8) with different ambient air pressures as a function of repetition rate.
Fig. 6. Effect of the second sub-pulse on the melt pool. Energy density of the first sub-pulse (on the right side) is 24 J/cm2; the second sub-pulse (on the left side) is 14 J/cm2. (a) Dt410 s; (b) Dt ¼ 1 ns; (c) Dt ¼ 52 ns. Material: stainless steel (X10CrNi18-8).
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atmosphere in the interaction region. The sample could better absorb the energy of the second sub-pulse and the expansion of the plasma plume induced by the second sub-pulse is faster without contrasted by the counter-pressure of the buffer gas. Thirdly, the first sub-pulse heated the ablation zone to a high temperature and created a molten layer on the surface with modified optical coupling properties, which reduce the ablation threshold and increase the coupling of the ablation pulse with the target. In addition, the pulse energy of the single-pulse is split up into two parts to produce double-pulse. Therefore the peak intensity of the double-pulse is halved comparing to the original single-pulse and the plasma shielding effect can be weakened. However, it still needs further work to clarify which mechanisms concerned above are primary for the enhancement of the double-pulse drilling.
4. Conclusion This paper reports a series of nanosecond laser drilling experiments with the double-pulse of 52 ns interpulse separation, which are compared with conventional single-pulse drilling. Firstly, the double-pulse drilling has the same saturation behavior of laser fluence as the single-pulse drilling, but the pulse energy of the saturation points of double-pulse is lower than that of singlepulse. Secondly, the drilling rate of single-pulse strongly depends on the sample’s thickness. However, the drilling rate of doublepulse stays constant when the thickness is varied from 0.4 to 1 mm. Thirdly, variation of laser’s repetition rate has no obvious effect on the ablation rate in double-pulse drilling. Furthermore, double-pulse drilling in air allows a similar, even higher drilling velocity than that in single-pulse drilling in vacuum. The experimental results show that double-pulse drilling can achieve a significant improvement of laser drilling efficiency compared with the conventional single-pulse in open air. The results are practical and have potential applications for pulse laser drilling.
Acknowledgments The authors would like to thank the China Scholarship Council and German Federal Ministry for Education and Research for financial support. References [1] Yilbas BS. Parametric study to improve laser hole drilling process. J Mater Process Technol 1997;70(1–3):264–73. [2] Jackson MJ, Neill WO. Laser micro-drilling of tool steel using Nd:YAG lasers. J Mater Process Technol 2003;142(2):517–25.
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