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

Wuhan National Laboratory for Optoelectronics,School of Optoelectronic Science and Engineering,Huazhong University of Science and Technology,Luoyu Road 1037,430074Wuhan,China b

Institut fu

¨r Strahlwerkzeuge (IFSW),University of Stuttgart,Pfaffenwaldring 43,70569Stuttgart,Germany c

Forschungsgesellschaft fu

¨r Strahlwerkzeuge mbH (FGSW),Pfaffenwaldring 43,70569Stuttgart,Germany a r t i c l e i n f o

Article history:

Received 4March 2008Received in revised form 13May 2008

Accepted 27May 2008

Available online 11July 2008Keywords:Laser drilling Double-pulse Pulse shaping

a b s t r a c t

Nanosecond double-pulse laser drilling is reported in this paper.The double-pulse herein represents two closely conjoint pulses with 21ns pulse duration and about 52ns interpulse separation,which are acquired by temporal pulse shaping.Percussion drilling with such double-pulse is performed in stainless steel samples with different laser ?uences,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.

1.Introduction

Nowadays,short-pulse lasers have been recognized as an important tool for micro drilling.In order to improve processing ef?ciency,a considerable amount of research has been done to ?nd the optimum processing strategy during the last decade,such as the in?uence 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 subse-quently 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 deter-mines 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 ef?ciency is rather low.The problem is due to resolidi?cation 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 ef?ciency in pulse laser drilling.The double-pulse technique was originally applied

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 ef?cient 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 8with the use of double-pulse LIBS were found [15].Peter and Noll used nanosecond double-pulse,with the energy of 60and 120mJ,pulse width of 20ns and interpulse separation of 6m s,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 6times.All these results indicate the possibility of ef?ciency 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 CO 2laser with Q-switched Nd:glass laser pulses,to achieve a factor of two increase in the drilling ef?ciency of carbon steel [17].The Q-switched pulse was responsible for the ejection of molten metal,resulting in a higher drilling ef?ciency.Lehane combined a

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Optics &Laser Technology 41(2009)148–153

long pulse (3.5ms FWHM)and a followed shorter pulse (150m s FWHM)[18]to drill stainless steel plate.This approach allowed the capability of drilling through 1/8in thick stainless steel targets at a standoff distance of 1m 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 (532nm,3ns pulse duration,20–200J/cm 2).The results have shown a signi?cantly enhance (3–10times)material removal rates induced by the double-pulse while minimizing redeposition and heat-affected zones.The optimum interpulse separations were in the 40–150ns range (stainless steel,Al).Their work presented an attractive and ef?cient 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 clari?cation of the in?uence for different processing parameters on collinear nanosecond double-pulse drilling of stainless steel is reported.A Q-switched laser pulse with the wavelength of 1047nm and the pulse duration of 21ns is split into two sub-pulses with the interpulse separation of 52ns,which is applied to drill stainless steel plates with the thickness ranging from 0.4to 1mm.The number of pulses for drilling through the samples and the average drilling rates are investigated with different laser ?uences,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.

2.Experimental setup

The experimental setup is shown in Fig.1,which includes a Q-switched Nd:YLF laser (TL 20-1FQ,Trumpf,1047nm,maximal pulse energy 4mJ,maximal repetition rate 15kHz,pulse duration FWHM 21ns at 4kHz),a beam expander (T ),two wave plates (l /2and l /4),two beam splitters (BS1and 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

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 polariza-tion and a maximal re?ection of S polarization.By rotating the half-wave plate,the energy proportion of the transmission part to the re?ection 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 re?ection part.Nine mirrors are used in these experiments and 52ns 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 150mm focal lens.The focus diameter for the transmission beam is 30mm.Because of the deviation of adjustment for the beam expander,the diameter of the re?ection beam reduces a little after the transmission of a longer path.Therefore the focus diameter for the re?ection beam is slightly more than 30m m.The focal position is 200m m inside the samples.Stainless steel samples (X10CrNi18-8)with different thickness of 0.4,0.6,0.8and 1.0mm are placed in turn in a vacuum chamber on a motorized 3D translation stage.A high-speed photodiode (P)with the rise time of 500ps (Soliton UPD500)and an oscillograph (HP 54542A,500MHz,2GSa/S)are used to monitor the drilling process.

3.Results and discussion

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 4kHz 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 ?ve measurements was used for each point.As illustrated in Figs.3(a)and (b),for 0.4mm thickness steel,the curves of single-and double-pulse are nearly coincident.When the pulse energy is more than 0.25mJ,

M 8M 9

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Fig.1.Experimental https://www.360docs.net/doc/cc5823374.html,ser ?Q-switched Nd:YLF laser (1047nm,maximal pulse energy 4mJ,maximal repetition rate 15kHz,pulse duration FWHM 21ns at 4kHz);T ?beam expander;l /2?half-wave plate;l /4?quarter-wave plate;BS1and BS2?Beam splitter;M1–M9?mirrors;F ?focal lens (f ?150mm)and P ?photodiode.

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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 0.25mJ.Therefore,0.25mJ 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 S1–4and D1–4in Figs.3(a)and (b),respectively.It is worth noting that S4is a supposed saturation point due to the absence of the experimental results with the pulse energy more than 2.5mJ.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 0.6,0.8and 1.0mm 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 https://www.360docs.net/doc/cc5823374.html,paring saturation points S4with D4,it can be seen that only 430double-pulses with 1mJ pulse energy can drill 1mm steel through;however,800single-pulses with 2.5mJ pulse energy are needed to drill through the same sample.Another comparison is made between saturation point D4and point https://www.360docs.net/doc/cc5823374.html,ing the same pulse energy of 1mJ,430double-pulses and 13,000single-pulses are needed,respectively,to drill through 1mm steel.The enhance-ment ratio of double-pulse drilling to single-pulse drilling exceeds 30times in this case.Fig.4shows 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 1mJ pulse energy for 1mm steel.

De?ning 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 1–2orders of magnitude with the thickness increasing from 0.4to 1.0mm.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 ?owing 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 con?guration can improve melt ejection.

To clarify the impact of the double-pulse on the melt ?ow,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.6shows the morphologies of the melt pools on the sample’s surface after the partly overlapped double-pulse irradiated with different interpulse separations.For the interpulse separation more than 10s (a),two melt pools can be observed clearly,which are produced by the two sub-pulses,respectively.For the interpulse separation of 1ns (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 ?ow in the melt pool can be seen obviously when the interpulse separation is 52ns (c).The ridge is not just at the boundary of the melt pool produced by the second sub-pulse but spreads over a larger https://www.360docs.net/doc/cc5823374.html,paring with the result of more than 10s interpulse separation,the recast pro?le induced by the ?rst sub-pulse cannot be found in the melt pool any more.This shows that the second sub-pulse can force the melt ?ow induced by the ?rst sub-pulse back to the melt pool on one hand.On the other hand,the spreading ridge of the melt ?ow proves that the ablation zone is in melt state at 52ns later after the beginning of the ?rst sub-pulse.Thus,it can be expected that the second sub-pulse can accelerate the molten material to ?ow out of the hole if the pressure gradients generated by the two sub-pulses are coincident.

Fig.7shows average ablation rates in drilling through 1mm steel using single-and double-pulse as a function of pulse energy with different repetition rates.At the energy over 1.1mJ,high ablation rate of 2.7m m 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 100Hz,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 single-pulse drilling the previous pulse has an effect on the succeeding

1.00.80.60.4I n t e n s i t y (n .u .)

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Fig.2.Pulse shapes of single-and double-pulse.(a)Single-pulse,pulse width is 21ns (FWHM)and (b)double-pulse,the width of each sub-pulse is 21ns (FWHM),interpulse separation 52ns.

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pulse and the in?uence of the previous pulse is expanded with the repetition rate increasing.But the in?uence 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 1mm stainless steel samples with the repetition rate in different ambient pressures is presented in Fig.8.Single-pulse drilling at the pressure of 8mbar 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 intensi?ed and the processing rate is improved.Some investigations also proposed that the ?rst sub-pulse 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 con?guration.

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1.0mm 0.8mm 0.6mm 0.4mm

Double-pulse, Steel, 4k Hz

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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 4kHz.(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 52ns interpulse separation).

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The expansion of the plasma plume induced by the second sub-pulse is sensibly faster than the one induced by the ?rst sub-pulse.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 ?rst sub-pulse to ?ow out of the hole.Secondly,the ?rst sub-pulse depletes the

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Fig.4.The enhancement ratio of double-pulse to single-pulse for drilling velocity in different thickness steel drilling.

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double-pulse.

Fig.6.Effect of the second sub-pulse on the melt pool.Energy density of the ?rst sub-pulse (on the right side)is 24J/cm 2;the second sub-pulse (on the left side)is 14J/cm 2.(a)D t 410s;(b)D t ?1ns;(c)D t ?52ns.Material:stainless steel (X10CrNi18-8).

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Fig.7.Average drilling rate for drilling-through 1mm stainless steel samples (X10CrNi18-8)with single-and double-pulse as a function of pulse energy with different repetition rates.

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Fig.8.Average drilling rate for drilling-through 1mm stainless steel samples (X10CrNi18-8)with different ambient air pressures as a function of repetition rate.

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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 ?rst sub-pulse heated the ablation zone to a high temperature and created a molten layer on the surface with modi?ed 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 of52ns interpulse separation, which are compared with conventional single-pulse drilling. Firstly,the double-pulse drilling has the same saturation behavior of laser?uence as the single-pulse drilling,but the pulse energy of the saturation points of double-pulse is lower than that of single-pulse.Secondly,the drilling rate of single-pulse strongly depends on the sample’s thickness.However,the drilling rate of double-pulse stays constant when the thickness is varied from0.4to 1mm.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 signi?cant improvement of laser drilling ef?ciency 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 ?nancial support.

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