article

ORIGINAL ARTICLE

Nano-second pulsed DPSS Nd:YAG laser cutting of CFRPcomposites with mixed reactive and inert gases

Reza Negarestani & Lin Li & H. K. Sezer &David Whitehead & James Methven

Received: 27 August 2009 /Accepted: 6 November 2009 /Published online: 25 November 2009# Springer-Verlag London Limited 2009

Abstract Superior structural capabilities and lightweight ofcarbon-fibre-reinforced polymer composites have madetheir applications increasingly noticeable particularly inthe aerospace and automotive industries for reduced fuelconsumption. Anisotropic and heterogeneous features ofthese materials, however, have been prohibiting theapplication of laser cutting on these materials in industrialscale. In the present study the thermal degradationcharacteristics in laser cutting of these materials areinvestigated with a nano-second pulsed diode pumped solidstate Nd:YAG. A statistical analysis was performed for theoptimisation of the process parameters. Furthermore,quality improvement was achieved by the use of lowoxygen content assistant gas simultaneously with the inertgas shield. The controlled presence of oxygen as a burningmechanism reduced the fibre pull out up to 55% at the sametime with a high processing rate.

Keywords Laser cutting . Carbon fibre composites .

Gas mixture . DPSS Nd:YAG laser

1 Introduction

Carbon-fibre-reinforced polymers (CFRPs), as any otherfibre-reinforced polymeric (FRP) composite, are consistedof higher strength fibres bonded within a weak and adhesivepolymer. The combination offers a high performance and

extremely light material. At the same time, however, thismixture remains heterogeneous since each of the constituentsretain their individual physical properties. Machining of thesematerials (especially cutting and drilling) is of interest toproduce intricate shapes with desirable tolerances mainly forfastening and profiling applications. Nevertheless, in con-junction with anisotropic properties of the material whichdepend upon the fibre orientation and laminar lay out, the highmechanical strength and thermal resistance of carbon fibreslead to significant challenges for conventional [1] andunconventional [2] machining of these materials.

Lasers as a non-contact, fast, precise and flexible toolhave been successfully used for cutting both metallic andnon-metallic materials. However, laser cutting of CFRPscomposites show particular difficulties due to the aniso-tropic and heterogeneous properties of these materials [3].The challenges are produced mainly by the variations inthermal expansion coefficients of carbon fibres in the radialand longitudinal directions [3] and considerable differencein thermal properties of the fibres and the polymer matrix[4] (see Table 1 [3, 5, 6]). Consequently, laser processing ofthese materials generally result in severe thermal damagessuch as heat-affected zone, pull out of fibres, laminardelamination and fibre end swelling (Fig. 1).

Ablation and photochemical reactions in the laserprocessing of composite materials using a UV beam havebeen reported to considerably reduce thermal damages [710]. However, low material removal rate and lack offlexibility are the common drawbacks for such systemslimiting their application in industrial scale. CO2 lasers (IRbeam) have also been used to investigate laser cutting ofCFRPs both in CW [4, 11] and pulsed mode [12]. However,pulsed Nd:YAG laser (as another IR beam laser) has beenreported to give less thermal damage due to pulse-offcooling [3]. Lau et al. [13] studied the quality factors in

R. Negarestani (*) : L. Li :H. K. Sezer :D. Whitehead :J. MethvenManufacturing and Laser Processing Group,School of Mechanical, Aerospace and Civil Engineering,The University of Manchester,Manchester, UKe-mail: [email protected]

Int J Adv Manuf Technol (2010) 49:553566DOI 10.1007/s00170-009-2431-y

response to the effect of different process parameters on thefeature quality in laser cutting of composite materials usinga pulsed Nd:YAG system. They demonstrated the effec-tiveness of pulse width and the cooling gas on the quality.Mathew et al. [14] provided optimised process parametersin the laser composite cutting process. Despite the notedimprovements in the previous studies, achieved machiningquality does not satisfy the required specifications given in[11], which suggests the acceptable extent of fibre pull outas less than 150 m and the kerf width close to the beam

spot diameter with no fibre swelling. Thus further improve-ment is required for any possible practical applications.Current work introduces a novel laser cutting approach toachieve this. The technique is based on controlling the heatinput simultaneously with in-process cooling mechanismsincorporated in the laser-cutting process using a mixture ofoxygen and nitrogen assisting gases as reported here for thefirst time. Active O2 gas enhances the decompositionprocess of the composite material by exothermal reactionresulting in higher material removal rates (MRR) while theinert N2 gas reduces the thermal damages by enhancingthermal cooling during the process.

Assistant gas is an important process factor and, it canaffect the processing results in Nd:YAG laser cutting ofCFRPs [13]. Controlled mixture of oxygen and nitrogengases carried out in this study aims to utilise the combinedpositive effects for improving the quality of the cut whilstkeeping the MRR at reasonable levels. Laser-cutting testswith the developed technology is carried out using a highpower diode pumped solid state (DPSS) Nd:YAG lasersystem. This system offers more reliability, higher efficiency,narrower frequency linewidths and higher peak powers ascompared to arc lamp pumped laser systems used in previousstudies [13].

Fig. 1 SEM images of typical quality defects in laser cutting of CFRP composites a large heat affected zone, b fibre pull out, c delaminationbetween two lamina, d fibre end swelling

Table 1 Thermal and physical properties of the CFRP compositeused in the study [3, 5, 6]

Fibre Epoxy

Volume fraction 60% 40%

Density (kg/m3) 1,800 1,200

Thermal conductivity (W m1 K1) 50 0.1

Specific heat (J kg1 K1) 710 1,884

Vaporisation temperature (K) 4,000 700

Thermal expansion coefficient (m m1 K1) 0.5 L 655 T

554 Int J Adv Manuf Technol (2010) 49:553566

2 Relationship between CFRP decompositionand assist gas

Typically, epoxy structures are produced by combining aresin and a hardener. During polymerization of the liquidresin and the hardener, the cross-links form amorphousstructures. Therefore thermosetting epoxies (being heavilycross-linked and amorphous) do not show true melting orviscous flow upon heating and once exposed to excessheating (e.g. during laser machining) they will decompose[15]. Generally, thermolysis (i.e. thermal decomposition) ofthe epoxy resin consists of preheating and decomposition.Decomposition usually starts with dehydration and thereafterchain scissions occur due to reduction in thermal stability ofother bonds e.g. CO and CN, with the heat absorption [16].The resulting thermolysis products contain light gases,various hydrocarbons and char.

Carbon fibres on the other hand are highly crystallinestructures consisting of high content of carbon e.g. above95%. Because of the high bonding energies (of variouscarbon atom to atom bonds) at low atmospheric pressures(such as the current study) carbon elements undergo directvaporisation (degrade directly from solid to the gas phase)at temperatures round about 4,000 K [17].

Control of thermal degradation and hence damage ofCFRPs in thermal processing is best observed in inertatmosphere [18]. The decomposition of the matrix occurs atrelatively much lower temperatures (around 700 K) ascompared to the fibre vaporisation temperatures (4,000 K).Considering higher thermal diffusivity of the carbon fibreand rapid heatingcooling rates involved in pulsed lasercutting, quicker but controlled decomposition of fibres inparticular, can lead in less thermal damage to the cut

surface through conduction. In oxidative medium, thedecomposition of fibres [18] as well as the epoxy matrix[19] would be enhanced with the heat released from theexothermic reactions. Oxidative decomposition of CFRPs ismainly influenced by the carbon fibre oxidation in the formof following two exothermal reactions [18].

C O2 ! CO2 1

C 12O2 ! CO 2

Figure 2 illustrates thermal gravimetric analysis (TGA)and derivative thermal analysis (DTA) for the material usedin this study (60% carbon fibre40% epoxy polymer) inoxidative and inert medium at two different heating rates.As it can be observed from Fig. 2a, the material decom-poses quicker in air (through oxidation of fibres) ascompared to nitrogen medium. From Fig. 2b, it is clearthat in nitrogen the decomposition shows only one weightloss peak representing devolatilisation (at around 673 K). Inair on the other hand, a different decomposition mechanismis evident through more stages of weight loss mechanism(i.e. devolatilisation, char oxidation and then fibre oxidationat around 1,073 K). It can also be seen from the figure that,although increasing the heating rate decreases the weightloss, the difference between oxidative and non-oxidativeenvironment is still valid. Therefore, the observed differencecan still be expected to be valid at even higher heating rates asin laser processing.

Therefore, although presence of reactive gas can deterioratethe quality (through excessive degradation) in laser process-ing, once controlled, can be useful for compromising effective

Fig. 2 a TGA and b DTA of the material used in experiments in inert and oxidative mediums at two different heating rates

Int J Adv Manuf Technol (2010) 49:553566 555

material removal and reduced thermal damages. Thereby,mixing oxygen into the inert nitrogen assist gas is investigatedin this work. Presence of oxygen as a reactive medium canenhance: (1) diffusion at elevated temperatures and (2)chemical decomposition i.e. oxidation, of fibres. The nitrogenis on the other hand more effective to dissipate the heat andhence reducing the thermal damages. Properties of the oxygenand nitrogen gases are provided in Table 2 [20, 21].

3 Experimental procedure

A 400-W Powerlase DPSS Nd:YAG laser was used in thisstudy. The beam is non-polarised with 1,064 nm wave-length and 350 m focused spot diameter. High repetitionrate of 3 to 15 kHz and short pulse duration (i.e. 2847 ns)distinguishes the system from the millisecond pulsed Nd:YAG lasers used in most of the previous CFRP laser cuttingstudies [13, 14].

The material used in the experiments was 1.2 mm thickfully cured 0/90/0/90/0CFRP lamina. The volumefraction of the carbon fibres (7 m in diameter) is 60%and the resin is E-765 Epoxy by nelcote. The sampleswere clamped on an Aerotech 3-axis CNC stage withmaximum transmitting speed of 200 mm s1. Multiple passstrategy was used for laser cutting of 18 mm slots and5 mm outside allowance was considered for the stageacceleration purpose. A schematic view of the experimentalset up is given in Fig. 3. The figure shows the double gasjet inlets on the laser head used to feed the oxygen andnitrogen into the nozzle. The assist gas flow is hencecoaxial to the laser beam.

3.1 Process parameters

As the first step, design of experiments (DoE) approachwas used to adopt optimum process parameters forconducting the tests on mixture of oxygen in the gasassisted. Response surface methodology (RSM) based oncentral composite design (CCD) was applied. The RSM is acollection of mathematical and statistical techniques used toestablish the relationships between a response of interestand the independent variables of the process. The CCD is

the most commonly used RSM to study the quadraticeffects besides main and the factor interactions. DesignExpert software was used to generate the CCD for threenumerical factors (i.e. pulse frequency, pulse energy,cutting speed) with three levels and three replicates foreach experiment. The design factors and the levels aregiven Table 3. The range of the process parameters wasconfirmed following a number of screening tests whichshowed multiple pass cutting with low energy pulsesprovides better quality as compared to high-power singlepass cutting. Number of passes for different parametercombinations was optimised so as to get through cut in allcases. Finally, the assist gas was nitrogen delivered at8 bars through a 1-mm exit diameter converging nozzle.The obtained optimum process parameters were used tostudy the effect of pressure in mixing oxygen in the assistgas.

3.2 Assist gas

The main objective of the current work is to optimise theburning rate by varying the oxygen partial pressure in theassist gas so that thermally induced damage in laser cuttingof CFRPs can be eliminated/reduced. Hence, following theconfirmation of the laser parameters, firstly the effect ofassist gas pressure was studied involving pure oxygen, purenitrogen and 50% O250% N2 assist gas at different

Fig. 3 Schematic view of the experimental set up

Table 2 Properties of oxygen and nitrogen [20, 21]

O2 N2

Density (kg m3) 1.30 1.14

Thermal conductivity (W m1 K1) 0.0268 0.0265

Viscosity (106 N s m2) 2.06 1.78

Specific heat capacity (J Kg1 K1) 920 1,042

Heat of vaporisation (kJ kg) 255.39 208.21

Table 3 Process parameters ranges in DoE

Name Unit Minimum Maximum

Frequency kHz 3 7

Pulse energy mJ 7 25

Cutting speed mm s1 50 200


pressures. In laser metal processing assisted with an inertgas, increasing the gas pressure increases the drag force bygas flow on the cut front. When cutting with oxygen gas,the exothermic-reaction-induced burning rate would also beinfluenced by the gas pressure besides the enhanced dragforce [20]. As the final stage of experimental work, theoxygen volume fraction in the assist gas was then analysed.

The results were evaluated by using optical microscopyto assess the kerf width and fibre pull out both at the beamentrance side (referred as top) and beam exit side (referredas bottom). This was carried out using Polyvar opticalmicroscope with PC interface via a 12 Mega pixel camerainto I-solution software. The MRR and taper angle whichare interrelated with these factors were also studied. Theanalysis of MRR was for the productivity interest whiletaper angle is a suitable quality response to study thevariation of the two other factors i.e. kerf width on top andbottom. The fibre pull out on the top surface is a benchmarkin recognising thermal damages to the material during laserprocessing. Ideally, this extent should reach zero tominimise the mechanical failure in service life. Hence, thequantitative quality criteria aimed in this study (as in [11])include reduction of top fibre pull out below 150 m andtop kerf width close to the beam spot diameter. Byanalysing the bottom thermal damages comparative ratiosbetween the fibre pull out and kerf widths, both on top andbottom, were defined and applied to analysing the variationof the thermal damage inside the kerf.

4 Results

4.1 DoE analysis

Statistical models were built using linear regressionanalysis to relate the quality responses (i.e. fibre pull outon the top and bottom surfaces, top and bottom kerf widths,taper angle and the material removal rate) to the designfactors given in Table 3. Quadratic model were chosen inall cases. The model for the given response (r) isrepresented as [22]:

hr b0 Xk

j1bjxj

Xk

j1bjjx

2j X

ij

Xbijxixj 3

Where 0 is the response at the centre of experiment, jis the coefficient of main effects, jj is the coefficient ofquadratic effects and ij is linear by linear interactioneffects. The regression coefficients given in the equationwere calculated using the least squares method and thenfinalised by stepwise regression technique. A completeanalysis of variance technique was used to identify the

significance of the coefficients. The order of the model wasadjusted to neglect the insignificant terms. Once the modelis suitably reduced the normal plot of residuals wasanalysed to ensure that the model assumptions are notviolated. In all cases, the residuals were found to follow anormal distribution, indicating the model was appropriatefor the data set.

Generally, the DoE analysis showed that the fibre pullout on the beam entrance (i.e. top surface) is the majorquality defect and mostly influenced by the pulse energy.Figure 4a shows the variation in the extent of top surfacefibre pull out with change in the pulse energy. On the otherhand, pulse frequency was identified as the most effectivefactor for the fibre pull out at the beam exit (i.e. bottom ofthe kerf; Fig. 4b). The kerf widths at the beam entrance andthe exit were the other quantitatively analysed qualityfactors, and also found mostly sensitive to change in thepulse frequency. Additionally, the cutting speed and pulseenergy were also found significant for the kerf widths atthe top and bottom surfaces, respectively. The 3D view ofthe combined factors effects are shown in Fig. 4c and dfor the kerf widths at the beam entrance and beam exit,respectively.

The MRR and the taper angle of the kerf walls were alsoconsidered in the model. MRR was calculated according tothe number of passes and the kerf geometry (Fig. 5) at theknown scanning speed by Eq. 4 and the taper angle wascalculated using Eq. 5 based on the kerf cross sectiongeometry (Fig. 5).

MRR Removed volume cm3

process time min WaWb

2 d lc 109nlc60V

4

q tan1 Wa Wb2d

5

Where, Wa (m) and Wb (m) refer to the kerf widths atthe top (beam entrance) and the bottom (beam exit) siderespectively, d (m) is the sample thickness, lc (mm) is thecut length, n is the number of passes cutting through,V (mm s1) is the scanning speed and (Rad) is the taperangle. The significance of the frequency effect was oncemore observed for both MRR and taper angle responses.The change in the material removal rate with the pulsefrequency is shown in Fig. 6a. Effect of the pulse frequencyand the scanning speed on the taper angle is presented inFig. 6b.

In addition to the above analysis, the overall thermaldegradation was characterised using the ratio of the extent


of fibre pull out and the cut width at the top and the bottomof the workpiece by defining two ratios as:

Ri fiWi i : a; bf g 6

and,

R' RbRa

7

Where Ri is the ratio, fi (m) is the fibre pull out,Wi (m) is the kerf width and a and b indices refer to topand bottom surfaces, respectively. R is the arbitrary ratiobetween the bottom and top ratios. Analysis then revealedthat the bottom ratio exceeded the top ratio (i.e. R1) inmost cases. This could be explained by excessive heataccumulation towards the beam exit. Hence, for optimisa-tion purpose, R was also incorporated in the statisticalmodel. Frequency yet again showed the most significant

Fig. 4 Modelled influence of significant factors (in assistance of 8 bars N2) affecting: fibre pull out on top (a) and bottom (b), Kerf width on top(c) and bottom (d)

Fig. 5 Kerf geometry used tocalculate the taper angle andMRR


influence on R. The interaction of the pulse frequency andthe energy that according to the model was of secondaryimportance is given in Fig. 7.

Based on these responses, the optimization was carriedout in the software for the thermal defects of the cut (i.e.minimising the fibre pull out on the top and bottomsurfaces) and the geometry and processing time (i.e.minimising taper angle and maximising MRR). Theoptimum solutions predicted by the software are given inTable 4. Therefore, by conducting further confirmative trailtests, pulses of 7 mJ energy duration delivered at 5 kHzfrequency and with 125 mm s1 scanning speed wasidentified as optimum process parameters. The optimisedparameters were used for the assist gas effect analysispresented in the following sections.

4.2 Gas effect

4.2.1 Gas pressure effect

The first series of experiments were conducted to confirmthe optimum pressure for the assist gas. Here, cuttingwith the mixture of O2 in the N2 assist gas at a constantratio (i.e. half by half proportions) was compared to purenitrogen and pure oxygen with three repetitions for eachexperiment. The effects on various quality factors areplotted in Fig. 8. It can be observed that the processperformance is improved at higher pressures in general. Thefibre pull out on both the top and the bottom surfaces wasconsiderably reduced at 8 bars (Fig. 8a and b) and the MRRwas maximum at this pressure for the assist gas (Fig. 8c). Itwas then decided to use high level gas pressure (i.e. 8 bars)in the following experiments that were conducted with arange of oxygen volume fractions in the nitrogen assist gas.

4.2.2 Effect of oxygen volume fraction in the assist gas

To investigate the influence of oxygen content in the inert gasshield, the oxygen and nitrogen gas mixture was studied overthe 0100% range at 12.5% intervals. Total pressure of theassist gas was kept constant at 8 bars in all cases andthe experiments were repeated for three time at each level. Thevarious measured quality factors at varying oxygen levels areplotted in Fig. 9. As it can be observed, a low content ofoxygen (i.e. in the range of 12.5%) leads to considerablereduction of fibre pull out both at the beam entrance and the exitshowed a minimum in this range. It can also be observed thatvariation of oxygen volume fraction in the assist gas influencesthe kerf width both on the top and the bottom surfaces.

Fig. 7 Modelled influence of significant factors on the bottom to topratio, R

Fig. 6 Modelled influence of significant factors on a material removal rate and b taper angle


Figure 10 illustrates calculated quality factors variation inresponse to the oxygen level. As shown in Fig. 10a, the MRRgenerally increased with the increase in oxygen volumefraction. From Fig. 10c where the extent of fibre pull out iscompared to the kerf width, a minimum is observed over thatoxygen content range. Although the taper angle showed anelevated value for this range according to Fig. 10b, this couldbe neglected since the overall kerf geometry was narrower asa result of a decrease in the bottom and top kerf widths(Fig. 9). Therefore (from Figs. 9 and 10), the optimumresults in terms of both quality and productivity of the lasercutting CFRP materials were obtained for the mixture of

12.5% oxygen and 87.5% of nitrogen assist gas with the totalpressure is 8 bars. The microscopic images of the result inthis set are compared to the pure oxygen and also purenitrogen gas streams in Fig. 11.

5 Discussions

5.1 Statistical analysis

As mentioned, the investigation was preceded by astatistical analysis aimed for finding the best possible

Fig. 8 Variation of differentresponses corresponding 50%O250% N2 assist gas and theirindividual usage atdifferent pressures a top fibrepull out, b bottom fibre pull outand c material removal rate

Table 4 Optimum solutions predicted by the statistical DoE analysis

Solution Process parameter Responses Desirability

Pulse energy(mJ)

Scanning speed(mm s1)

Frequency(kHz)

Thermal defects Geometry Processingtime

Fibre pull outat the beamentrance (m)

Fibre pull outat the beamexit (m)

Taperangle ()

MRR(cm3 min1)

1 7 125 5 197.92 146.54 2.45 0.134 0.88

2 10.25 200 5 250.83 173.19 2.13 0.129 0.86

3 7 50 5 273.95 88.54 2.61 0.183 0.81


system parameter combination for improved quality. Thesignificant process factor for beam entrance fibre pull outwas the pulse energy. From Fig. 4a, the fibre pull outincreases with the increase of pulse energy for a givenfrequency and scanning speed. The effect is directly relatedto the increased heat input which is conducted through thefibres (due to high thermal conductivity of fibres) leading to

matrix recession. On the bottom surface the frequency wasthe dominant factor. The bottom fibre pull out showed aminimum for the middle value i.e. 5 kHz of the studiedfrequency range. This can be explained by effective MRRwith sufficient pulse-off time i.e. thermal cooling interval.For lower frequencies since the MRR generally decreases(Fig. 6a) more passes are required to cut through thematerial. This increases the heat input as well as heataccumulation towards the beam exit side. The excessiveheat from the extra passes necessary for a through cutincreases the kerf width on the top surface rather thanextending the fibre pull out. Therefore, the taper angle andthe bottom fibre pull out show similar trends in response topulse frequency as in, Figs. 6b and 4b, respectively. For thehigher frequencies on the other hand, the bottom fibre pullout also increases due to increased power irradiance and thedecreased interval thermal cooling.

For the top kerf width the statistical analysis predictedthe frequency as the significant factor. The factor interac-tion between frequency and the scanning speed was alsofound significant in this case. As shown in Fig. 4c the topkerf width slowly decreases with an increase in pulsefrequency from 3 to 5 kHz. From the same figure it can beobserved that the top kerf width shows a proportional andslightly bending relation to the scanning speed. Here,interaction time increases due to the decreased MRR at

Kerf width at beam entrance sideKerf width at beam entrance sideKerf width at beam exit sideFibre pull out at beam entrance side

320

Fibre pull out at beam entrance sideFibre pull out at beam exit side

280

240

200

160

200

m)

120

160

(m

80

120

40

80

0

40

00 12.5 25 37.5 50 62.5 75 87.5 100

Oxygen volume fraction (%)

Fig. 9 Influence of oxygen content in 8 bars balanced assist gas withnitrogen on fibre pull out and kerf widths on the beam entrance andbeam exit

Fig. 10 Influence of oxygencontent in 8 bars balanced assistgas with nitrogen on a materialremoval rate, b taper angle andc fibre pull out to kerf widthratio on top Rt and bottom Rb


higher scanning speeds and subsequent increase in numberof passes for a through cut [23]. This together with the non-polarised beam of the system can be attributed to increasein the top kerf width at higher scanning speeds. The effectis in fact opposite for the single pass cutting where the topkerf width generally reduced with the increasing scanningspeed [14]. Overall, as observed from the contour lines, thetop kerf width shows a minimum value for the parametercombinations of 5 kHz frequency and 125 mm s1 scanningspeed. Pulse frequency and energy as well as theirinteractions were the significant factors for the bottom kerfwidth. The model for the bottom kerf width showed aminimum for lower bound of the interaction of these factors(i.e. 3 kHz and 7 mJ) with a slightly bending andcontinuous proportionally increase up to the higher bond(i.e. 7 kHz and 25 mJ) as in Fig. 4d.

Material removal rate showed a proportional relationshipwith the pulse frequency which was found to be asignificant process parameter (Fig. 6a). More effectivepulses that interact with the material as well as less pulse-off time elevate the MRR. Taper angle on the other hand,was also most sensitive to the pulse frequency. The secondinfluencing factor was the interaction between the pulsefrequency and the scanning speed. As depicted in themodelled relationship in Fig. 6b, more frequent laserinteractions at higher scanning speeds would result in lesstaper angle. This can be justified first by less number ofpasses that would be required due to increased power

irradiance with the increase in pulse frequency andsecondly by decreased interaction time due to high scanningspeeds. These effects together provide a mechanism in whichthe kerf widths on top and bottom get a closer extent to eachother that reduces the taper angle. However, this could notachieve optimisation as the kerf widths at the top and thebottom increase at higher frequencies (see Fig. 4c and d).From the contour lines in Fig. 6b it can be seen that the taperangle shows a reduction for the ranges adjacent to 5 kHzfrequency and 125 mm s1 scanning speed. The effect ismostly dominated with the top kerf width which showedsimilar trend to these two factors (Fig. 4c).

As mentioned earlier, the other quality response that wasstatistically analysed in the optimisation series of experi-ments was ratio R, which gives the relationship betweenthe fibre pull out to the kerf width ratio on the bottomsurface to that of the top surface (see Eq. 7). Pulse energyand the frequency were modelled to have the mostsignificant influence on R. As it can be observed fromFig. 7, R generally increased with reducing both the pulsefrequency and the energy. More gradient was pronouncedon the lower half of the studied ranges of these processparameters while from higher bound of these factors (i.e.7 kHz frequency and 25 mJ) the pulse energy towards themiddle of the graph the trend descended slightly. The upperhalf levels of these factors showed less R, due to a morepronounced thermal input (i.e. high pulse energy) withmore frequent pulses. This in turn increase the fibre pull out

Fig. 11 a Top view and b bottom view of the laser cut kerf at 5 kHz frequency, 7 mJ pulse energy and 125 mm s1 in assistance of: (i) 8 bars pureoxygen (66 passes), (ii) 8 bars 12.5% oxygen mixed with 87.5% nitrogen (72 passes) and (iii) 8 bars pure nitrogen (78 passes)


on the top and bottom surfaces (see Fig. 4a and b) which isnot desirable. In general low pulse energies at intermediatefrequency i.e. 5 kHz could achieve moderate values of R.

5.2 Thermal degradation development

The development and analysis of R ratio, Eq. 7, revealedthat for nearly 60% of the experiments, the fibre pull out tokerf width ratio at the bottom exceeded the top ratio i.e.R1. Detailed analysis of the degradation mechanismthroughout the process is illustrated in Fig. 12. FromFig. 12a it can be observed that an unsteady thermaldegradation development occurs throughout the process.This could be attributed mainly to the considerable beamdivergence inside the kerf since the focal plane was set onthe top surface. Beam divergence as well as the beamscattering inside the kerf reduce the effective beam intensityand hence increase the number of passes required to processthe lower section of the kerf. This is shown in Fig. 12bwhere about 80% of the kerf depth is processed in nearly55% of the whole process. After the first 70% of theprocess time, the beam approaches the bottom side;

however, it would take the whole remaining 30% of theprocess to open the bottom kerf width. The heat accumu-lation imposed by long processing period of the lower endof the kerf, would affect the bottom side thermal damage.The fibre pull out on the top surface as well as the kerfwidth are also affected during processing of the lowersection of the kerf. It can be reasoned from Fig. 12a thatnearly 55% of the top fibre pull out occurs during the last30% of the process. Furthermore, the difference in thermalexpansion behaviour of different lamina (depending on thedirection of fibres in each laminate [24]) causes delaminationeffect at the edge along the lamina in which the fibres lyetransversely to the beam path.

5.3 Gas pressure effect

The effect of gas pressure analysis showed that the top fibrepull out reduces considerably at high levels of the gaspressure (i.e. 8 bars) of the studied range (Fig. 8a). Oxygen,because of the added energy of the exothermic reactions,showed the highest fibre pull out on the top surface for thelower pressures. For 8-bar pressure, however, the top fibre

28

44 5668 78

0

400

800

1200

1600

0 10 20 30 40 50 60 70 80 90Number of passes

Cut k

erf d

epth

(m

)

Beam

Entran

ce D

epth of CutB

eam Exit

28 Passes 78 Passes 68 Passes 56 Passes 44 Passes

Delamination at the edge of cut kerf

200 m 200 m 200 m 200 m 200 m

200 m 200 m 200 m

360 m 360 m360 m 360 m 360 m

a

b

Fig. 12 a Thermal degradationdevelopment on beam entrance,kerf depth and beam exitsections; b kerf depthdevelopment, at 7 mJ pulseenergy delivered at 5 kHzfrequency and 125 mm s1

scanning speed and using 8 barsnitrogen gas


pull out in case of oxygen was reduced considerably andwas only slightly more than that of the nitrogen. Thedifference between the two can be attributed to higher heatcapacity of nitrogen as well as its inert behaviour whiletheir closeness is mainly due to their close thermalconductivities which would make them showing similarheat dissipation i.e. thermal cooling at elevated velocities. Italso implies that for given process parameters the increasein oxygen gas pressure beyond a certain level, does notenhance oxidation and hence the thermal damage. From thesame figure it is evident that the mixture of oxygen andnitrogen gases showed an acceptable effect to top fibre pullout trend of each individual gases. The top fibre pull outwas on a close trend to that with pure nitrogen, while thematerial removal rate had improved remarkably comparedto pure nitrogen (Fig. 8c). On the bottom side the fibre pullout showed (Fig. 8b) similar trends to that for the topsurface.

5.4 Oxygen content effect

A mixture of gases was found to improve the machiningquality due to controlled combustion, shear and cooling atthe cutting front. In oxidative decomposition, presence offree radicals such as O and OH and the exothermal heatreduce thermal stability of material which in turn reducesthe reaction temperature as well as the activation energy[19]. The oxygen content analysis (see Fig. 9) showed thatthe composition gas behaviour was more like pure activegas for the ranges between 50% to 100% (referred as upperrange) while the high content of nitrogen in 0% to 50% O2range (referred as lower range) resulted in responses withcloser behaviour to inert gas. As in Fig. 10a, theproportional increase trend of material removal rate showedslightly descending trend at 50% and 100% oxygen contentlevels. Since nitrogen has lower heat of vaporisationcompared to oxygen (see Table 2), their mixture is prone

to more effective thermal degradation control as comparedto pure oxygen case. This together with nitrification of theepoxy matrix in presence of nitrogen [19] could justify theslight descending trend in MRR at 100% O2 while at 50%O2 content the higher heat capacity of the equally weightednitrogen, partially absorb the exothermic heat and henceslightly reduce the oxygen effect.

As illustrated in Fig. 9, fibre pull out on the top and thebottom sides at 12.5% oxygen content was reducedremarkably compared to pure nitrogen content. This isbecause the reaction temperature decreases in presence ofoxygen [19] and hence more effective decomposition/vaporisation is observed. This particular trend at 12.5%oxygen content is in agreement with [19] where 10% O2content was reported to show an inferior devolatilisationtemperature (503 K) and a peak decomposition temperature(959 K) as compared to 5% and 20% oxygen content forthe epoxy.

Similarly from Fig. 9, the minimum bottom kerf width isobserved at 12.5% oxygen content level. The bottom kerfwidth generally increased proportional to the oxygen contenton a slight ascending trend. The taper angle was generallyshowing smaller values for the upper range of oxygen content(Fig. 10b) due to higher material removal in presence ofmore oxygen. Figure 10c, on the other hand, showed that thefibre pull out to kerf width ratio on top i.e. Ra, and bottomi.e. Rb, exhibit their minimum value at the 12.5% oxygencontent similar to the overall trends of Fig. 9. Ra and Rbgenerally showed descending trend for the upper oxygencontent range which could be contributed to the less numberof passes that are used in this region. It can also be observedfrom Fig. 10c that Rb values are always greater than the Ravalues that agrees with the modelled R trend (Fig. 7) wherefor 7 mJ pulse energy and 5 kHz frequency at 125 mm s1

scanning speed the Rb was predicted to exceed Ra.Oxidative decomposition/vaporisation of CFRPs occurs

at 9731,073 K [25] which is much less than the vapor-

Fig. 13 Influence of presenceof a pure nitrogen and b pureoxygen on decomposition ofmatrix on the beam exit side;scanning speed, 125 mm s1;pulse energy, 7 mJ; pulsefrequency, 5 kHz and gaspressure, 5 bars


isation temperature of carbon fibres in inert atmosphere i.e.4,000 K [17]. Since the oxygen content in the assist gas iscontrolled in the current approach, excessive thermaldamage to the material (by thermal conduction along thefibres) was prevented and hence the cutting qualityimproved. The low oxygen content in the assist gas (i.e.12.5%) balanced with nitrogen, benefits the acceleratedoxidation at a light and comparatively higher viscosity gasflow which exhibits good heat capacity (cooling effect).These embody an effective material removal mechanismwith sufficient heat transfer properties that would lead toconsiderable reduction on the thermal damages on the beamentrance and exit surfaces. As it can be seen from Fig. 11,the optimum mixture (i.e. 12.5% oxygen with 87.5%nitrogen), resulted in a very narrow heat-affected zone onthe top, i.e. 70 m, which represent a 54% improvementcompared to pure nitrogen case and a 55% improvementcompared to pure oxygen case. On the bottom side thequality improved is 47% (i.e. reduced fibre pull out) ascompared to pure nitrogen and 59% as compared to pureoxygen. Although top kerf width did not show remarkablevariation with pure oxygen or nitrogen, the bottom kerfwidth was improved by 19% and 41% as compared to purenitrogen and oxygen, respectively.

The influence of the presence of oxygen on accelerationof the decomposition of CFRPs was particularly visible onthe bottom surface of the cuts. Figure 13 presents acomparison between the assistance of pure oxygen andpure nitrogen at the same process parameters. As depictedin Fig. 13a, in presence of pure nitrogen due to the inertnessof the gas a large heat-affected zone is pronounced (darkercolour surrounding the fibre pull out) which represents thematrix that has been thermally affected but not enough tobe totally decomposed. In case of pure oxygen (Fig. 13b),however, this region is eliminated as the oxygen wouldaccelerate the decomposition of thermally affected matrix.This is in agreement with previous finding [19] that showedthe epoxy char residue can be reduced significantly (around2.1%) in presence of oxygen as compared to pure nitrogen(12.417.9%).

6 Conclusions

Large differences between the thermal properties of carbonfibres and polymer matrix has brought up major challengesin laser machining of CFRPs. High thermal conductivity offibres particularly leads to large extent of matrix recessionaround the cut path. Statistical analysis predicted low pulseenergy at the intermediate level of pulse frequency andmedium to high scanning speeds to provide the optimumpossible results. Furthermore, a monitored mixture ofoxygen into the inert gas was investigated in order to

accelerate the vaporisation/decomposition process to reducethe thermal damage since the oxidative thermolysis of thematerial occurs at much lower temperatures (9701,070 K)as compared to the vaporisation temperature of fibres(4,000 K). The analysis revealed that low volume fractionof oxygen (i.e. typically 12.5%) mixed with nitrogen gasand total assist gas pressure of 8 bars are the optimumparameter configurations to improve the machining qualityin laser cutting of CFRPs. The fibre pull out was reducedby nearly 55% resulting fibre pull out of 70 m on the topsurface. The involved complexities in the material properties,material thermolysis and other embodied features such asparamagnetic characteristic of the oxygen gas welcomefurther investigation into such a process.

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Nano-second pulsed DPSS Nd:YAG laser cutting of CFRP composites with mixed reactive and inert gasesAbstractIntroductionRelationship between CFRP decomposition and assist gasExperimental procedureProcess parametersAssist gas

ResultsDoE analysisGas effectGas pressure effectEffect of oxygen volume fraction in the assist gas

DiscussionsStatistical analysisThermal degradation developmentGas pressure effectOxygen content effect

ConclusionsReferences

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