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Journal of Materials Processing Technology 210 (2010) 1297–1303

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

ISI 304 stainless steel marking by a Q-switched diode pumped Nd:YAG laser

. Leonea,∗, S. Gennab, G. Caprinoa, I. De Ioriob

Department of Materials and Production Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125 Naples, ItalyDepartment of Aerospace Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125 Naples, Italy

r t i c l e i n f o

rticle history:eceived 5 June 2009eceived in revised form 19 March 2010ccepted 21 March 2010

a b s t r a c t

Laser marking tests were carried out on AISI 304 steel, using a Q-switched diode pumped Nd:YAG laser.The aim was to determine the correlation occurring between working parameters (i.e. pulse frequency,beam scanning speed, and current intensity) and resulting mark visibility. The latter was evaluated as thecontrast index measured from digital images of the marks. To characterize mark features, its width androughness were estimated, and analyses employing optical and scanning electron microscopy coupled

eywords:aser markingd:YAG lasertainless steel

with energy dispersive X-ray technique were carried out.From the experimental results, both surface roughness and oxidation increased as a function of fre-

quency, resulting in an improvement in contrast, up to a characteristic value, decreasing afterwards.Besides, the maximum visibility value was beneficially affected by lower scanning speeds and lower cur-rent intensities. An empirical model was built, and from it the best processing conditions for optimum

o acc

mark visibility, taking int

. Introduction

In the past decades, laser has been widely used in cuttingnd welding operations. Quite recently, this technology has beendopted in other industrial processes like marking, selectiveblation and machining of different materials, thanks to the intro-uction of laser sources characterized by short and ultra-shortulses having from nanosecond to femtosecond duration.

The main advantages of laser process are non-contact working,igh repeatability, high scanning speed, worked area comparableo the laser spot, high flexibility and automation. Furthermore, these of shorter wavelengths, which are better absorbed by the mate-ial, allows smaller feature sizes to be worked, while the use ofhorter pulses reduces heat-affected zone and opens new ways foranometric accuracy, as also reported in Meijer et al. (2002).

An extensive overview of the short and ultra-short pulse laserpplications, encompassing cutting, welding, drilling, scribing,leaning, selective ablation, and three-dimensional micromachin-ng, is reported in Meijer (2004), who highlighted the dependencef the worked material quality on laser, material, and processarameters.

Deep engraving of steel and aluminium using a 220 W Q-witched DPSS laser source was studied by Wendland et al.2005). Material removal rates of up to 20 mm3/min for steel and0 mm3/min for aluminium were found up to a maximum engraved

∗ Corresponding author. Tel.: +39 081 7682374; fax: +39 081 7682374.E-mail address: claleone@unina.it (C. Leone).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.03.018

ount the operating constraints of the laser system used, were drawn.© 2010 Elsevier B.V. All rights reserved.

depth of 1 mm, whereas the surface roughness was shown todepend on the worked material, besides pulse irradiance and rep-etition rate. The possibility to obtain three-dimensional geometryon difficult-to-machine materials is reported in Kaldos et al. (2004),where a 100 W pulsed Nd:YAG was used to machine a steel for coldworking tool, in order to evaluate its response in terms of surfacequality and material removal rate. Apparently, the beam–materialinteraction involved complex phenomena, with melting occurringfirst, followed by rapid heating up to vaporization, steam port for-mation and plasma shielding. The ejection of liquid material fromthe surface, forced by vapor and plasma pressure, was noticed.At the end of the pulse, the heat quickly dissipated at the mate-rial surface, leading to the formation of a recast zone. The authorsshowed that pulse frequency is still the most influential factor inlaser machining. The influence of operating parameters on the pro-ductivity and quality of the machined surface of aluminium alloy,stainless steel and titanium alloy was studied in Cicalà et al. (2008).From the experimental results, the material removal rate (MRR) andsurface quality mainly depend on frequency, while the latter wasalso beneficially affected by a reduction in sweep speed. A simplepolynomial models was used to predict the MRR and the rough-ness. The MRR ranged from 0.9 mm3/min for stainless steel up to1.3 mm3/min for aluminium alloy. Unfortunately, no data about themean power or pulse energy were given.

In Kurita et al. (2008), laser machining of ceramic mate-rial for making ceramic moulds for glass microlens forming wasinvestigated. For the same pulse energy (0.7 mJ), the materialremoval rate increased with increasing the scanning speed andremoval mechanisms involving melting, vaporization, and fracture

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298 C. Leone et al. / Journal of Materials Pr

rocesses were identified. Besides, the formation of an oxide film onhe worked surface was observed. The laser–material interaction

echanisms obviously change when organic materials are con-erned. (Leone et al., 2009) machined different wood types using aW Q-switched DPSS Nd:YAG. The mechanism of material removalssentially consisted of burning and evaporation. A simple modelased on energy balance, involving the beam mean power and

nteraction time, was able to predict the engraved depth as a func-ion of the process parameters. The maximum MRR, measured inhe range 60–90 mm3/min, was a decreasing function of the woodensity.

As previously shown, the material removal rates typically asso-iated with short pulse lasers is low if compared with otherechanical processes, rendering them only suitable for micro-scaleachining, like micromachining or marking operations. However,

aser marking for product identification and traceability purposess nowadays proving useful in many engineering fields, among

hich electronic components, plastic industry, food industry, andechanical engineering play a major role. For instance, Park et

l. (2007) investigated laser marking on a silicon wafer com-onent. A process monitoring system, based on the analysis ofhe plasma light generated during marking, was developed andpplied for quality estimation. A relationships between markinginewidth, plasma light, and process parameters was assessed by

eans of artificial neural networks. Shin et al. (2005) studied thecrylonitrile–butadiene–styrene (ABS) marking using both CO2 andd:YAG laser sources. The depth of the groove increased propor-

ionally to laser irradiation energy. In addition, the morphology ofhe heat-affected zone (HAZ) was influenced by laser source. Thisas explained considering that CO2 laser irradiation leads to a bet-

er cooling contraction effect, while Nd:YAG induces a better recoilressure effect. Furthermore, it was observed that the shape of the

aser marking could vary significantly, depending on the macro-olecule alignment due to the travelling path of molten plastic

uring injection moulding.Studies on laser scribing were performed by Chen et al. (2009a),

emonstrating that pulsed CO2 laser can be successfully used toark complex codes on eggshell surfaces. The mark visibility and

eometry critically depended on process conditions. In anotherork Chen et al. (2009b), the authors used FEM analyses to simulate

he mark geometry and temperature distribution on the eggshell.Laser marking tests on carbon steel were carried out by

angsombatsiri and Porter (2007). It was found that there arehree different modes to obtain a permanent mark: laser colouring,avoured by a low-power beam to give rise to surface oxidation;aser etching, requiring a more powerful laser to melt the surfacef the substrate; laser engraving, involving an even higher powero vaporize the material surface. The power threshold levels deter-

ining the change in interaction mechanisms strongly depend onhe substrate properties. Consequently, a power level activatingngraving in aluminium may result in laser colouring when otherubstrates (e.g., carbon steel) are considered.

Depending on the specific application, the criteria for mark-ng acceptation/rejection encompass various parameters, such as

idth and depth of the heat-affected zone, micro-crack presence,urability under harsh operational conditions. However, irrespec-ive of the application, mark visibility is an essential requirement.

Objectively measuring mark visibility involves many difficul-ies. Ng and Yeo (2000) used a spectrophotometer together withhree illumination modes (tungsten lamp, daylight, and fluores-ent lamp) to evaluate the difference in mark colour obtained when

arying laser scanning speed. They found that legibility is criticallyffected by the illumination mode. However, when the authors Ngnd Yeo (2001) fixed the illumination source, consistent results,llowing for the selection of the best scanning speed in stainlessteel, aluminium, and PBT marking, were given by both scotopic

ng Technology 210 (2010) 1297–1303

(low illumination) and photopic (normal illumination) viewing.Unfortunately, the method proposed by Ng and Yeo requires a quitesophisticated instrumentation. A simpler methodology for markcontrast assessment, based on the analysis of black-and-whiteimages acquired with a frame-grabber card and a charge-couple-device (CCD), was proposed by Qi et al. (2003).

In laser marking, three main process parameters, i.e. operatingcurrent intensity, pulse frequency, and scanning speed, can be inde-pendently set. All of them affect significantly groove geometry andmark visibility, as highlighted, for instance, by Qi et al. (2003), whoperformed extensive tests on stainless steel. Therefore, choosingoptimized process parameters to achieve satisfactory productivitytogether with the required surface quality is an important task.

In this work, laser marking tests were performed on AISI 304steel, using a 20 W Q-switched diode pumped Nd:YAG laser. Duringthe tests, the working parameters were suitably varied, to assesstheir effect on mark visibility. To quantify the latter, the methodadopted by Qi et al. (2003), was used. The heat-affected zone wasexamined by optical, as well as scanning electron microscopy, andits main geometrical features were recorded using a 3D surfaceprofiling system. The experimental data obtained were analyzed inorder to determine the correlation between process conditions andmark visibility.

2. Equipment, materials and experimental procedures

A LASIT Fly 20 industrial Q-switched diode pumped Nd:YAGlaser, working at the fundamental wavelength of 1064 nm, wasused in experiments. This laser is characterized by a pulse dura-tion of about 150 ns, pulse frequency up to 35 kHz, and single pulseenergy up to 4.5 mJ. The focused beam diameter indicated by themanufacturer is about 150 �m.

In the laser head the beam was first expanded, then directedtowards two galvanometer mirrors, and finally focused by a “flatfield” lens with a focal length of 160 mm onto the workpiecesurface. The laser was PC-controlled by means of a customized soft-ware allowing for the generation of the geometric patterns to bemarked and the control of the working parameters, i.e. operatingcurrent intensity used to generate laser beam, I, pulse frequency, f,and scanning speed of laser beam, V.

To assess the effect of current intensity and pulse frequencyon average laser power output, a power meter (F150A-SH thermalhead and OPHIR NOVA display) was employed.

The substrates utilized for marking tests were AISI 304 stainlesssteel sheets 2 mm in thickness. Before marking, the sample surfacewas mechanically polished by P1200 abrasive grinding paper andrinsed using acetone, to remove any oil and dust residues. Markingtests were performed at ambient temperature in air, setting theoperating current intensity at 35 and 45 A, the scanning speed at50, 100, 200 mm/s, and changing the pulse frequency in the range1–30 kHz. In each test, a single pass along a straight line 50 mm inlength was accomplished. At least five tests were carried out foreach experimental condition.

Jangsombatsiri and Porter (2007) have shown that the markcontrast response can be strongly altered by the presence of inco-herent oxides and particle debris produced during laser–materialinteraction. To avoid this effect, in this work the marked surfaceswere first carefully rinsed with acetone, then observed under anoptical microscope (Axioscope 40 from Zeiss) in reflected light at200× magnification and photographed by a digital camera (Nikon

Coolpix 4500, at 4.0 mega pixel resolution), fixing both the illu-mination conditions (halogen source with a 90◦ incident light) andthe camera parameters. The black-and-white images obtained wereanalyzed by Adobe Photoshop software, dividing the grey valuesinto 256 levels varying from 0 (black) to 255 (white). To quantify

ocessing Technology 210 (2010) 1297–1303 1299

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C. Leone et al. / Journal of Materials Pr

isibility, the following contrast index, C, previously proposed byQi et al., 2003), was utilized:

= 1 − Gm

Gv(1)

here Gm, Gv are the average grey values of the marked area and ofhe virgin surface, respectively. According to Eq. (1), a better markisibility corresponds to a higher C value.

The geometrical profile of each mark was measured perpendic-larly to the laser scanning speed, using a Talysurf CLI 2000 3Durface profiling system. From the profiles recorded, the relevanteometrical parameters of the heat-affected zone were obtained byTalyMap Universal surface analysis software.

To assess oxidation effects on mark visibility, selected spec-mens were observed using a Leica-Cambridge S440 Scanninglectron microscope (SEM), equipped with an INCA Energy 200icroprobe for energy dispersive X-ray (EDX) analysis.

. Experimental results and discussion

.1. Laser beam characterization

In Fig. 1, the measured average power Pm of the laser beam (fullymbols) is plotted against the pulse frequency, f, for the two oper-ting current levels adopted in this work. The open symbols in thegure represent the pulse energy, Ep, calculated from Pm.

Clearly, for a given I value, Pm increases at a gradually decreasingate with increasing f. Moreover, for a fixed pulse frequency, theverage power is the higher, the higher the current is.

The continuous lines in Fig. 1 graphically represent the followingelationship, correlating the laser operating parameters:

m = Ie · (˛ ln f + ˇ) (2)

ith:

e = I − Io (3)

The constants Io, ˛, ˇ in Eqs. (2) and (3) were calculated by besttting the experimental data in Fig. 1. The values Io = 28.6, ˛ = 0.168,= 0.197, yielding Pm (in W) when I (in A) and f (in kHz) are given,

ere obtained.

The quantity Ie, not to be confused with the operating current I,ill be labelled as “effective current” hereafter. In fact, for I = Io the

eam power is practically nil.

ig. 1. Measured average power, Pm , and pulse energy, Ep , against pulse frequency,, for different operating current intensities, I.

Fig. 2. Mark width, w, against pulse frequency, f, for different values of operatingcurrent intensity, I, and scanning speed, V.

3.2. Mark geometry

Two parameters, i.e. mark width, w, and surface roughness, Rt

(the difference between maximum peak height and maximum val-ley depth, measured according to ISO 1302:2002 standard), wereassumed for mark geometry characterization.

In Fig. 2, the mark width is plotted against pulse frequency, fordifferent values of operating current intensity and scanning speed.

As also found by Chen et al. (2009a) and Qi et al. (2003), withinthe range of I and V values adopted the mark width is negligiblydependent on current intensity and velocity, and mildly influencedby pulse frequency. In fact, irrespective of I and V, w roughly under-goes a 25% decrease in the field 1 to about 15 kHz; beyond this limit,it stays constant, becoming slightly smaller than the nominal laserbeam diameter, represented by the horizontal dashed line in Fig. 2.

The surface roughness (Fig. 3) increases with increasing pulsefrequency up to a given f value, monotonically decaying afterwards.The critical frequency for maximum roughness is relatively insen-sitive to the operating current intensity and scanning speed, beingcomprised in the range 4–8 kHz. Finally, for a given f, larger values ofRt are achieved, if lower scanning speeds and higher current inten-sities are used. The latter effect is qualitatively anticipated, since

decreasing V and increasing I results in a higher power per unitlength transferred from the laser beam to the worked material.

Fig. 3. Mark roughness, Rt , against pulse frequency, f, for different values of operat-ing current intensity, I, and scanning speed, V.

1300 C. Leone et al. / Journal of Materials Processing Technology 210 (2010) 1297–1303

Fs

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ig. 4. Contrast index, C, against pulse frequency, f, for different values of scanningpeed, V. Operating current intensity I = 35 A.

.3. Mark visibility

As specified previously, the mark visibility was quantifiedhrough the contrast index, C, defined according to Eq. (1). Theffect of pulse frequency and scanning speed on the contrast indexs shown in Figs. 4 and 5, where the variation of C is plotted as aunction of f for I = 35 A and I = 45 A, respectively.

The general trend of contrast index reveals some similaritiesith the one previously noticed (Fig. 3) in discussing Rt: a pre-

ise value of f is found, in correspondence of which the maximumark visibility is obtained; further, lower scanning speeds give rise

o better contrast, for a fixed pulse frequency. Under appropriateperating conditions C tends to become negative, physically indi-ating that the heat-affected zone turns out to be brighter than theurface of virgin material. In fact, a negative value of contrast wasctually measured for I = 45 A, f = 30 kHz, V = 200 mm/s (Fig. 6).

Increasing the operating current intensity mainly results in twoffects on the trend of mark contrast (Fig. 7): (a) the pulse frequencyor best C shifts through moderately higher values and (b) contraryo the trend of Rt, the optimum visibility value slightly decreases.he latter conclusion may appear unexpected, because larger cur-

ents imply higher average powers, as well as higher pulse energiesFig. 1). However, it must be noticed that the contrast index isnfluenced by many factors, among which mark texture and oxi-ation phenomena presumably play the major role. In fact, opticalicroscopy revealed a strict dependence of mark texture on pulse

ig. 5. Contrast index, C, against pulse frequency, f, for different values of scanningpeed, V. Operating current intensity I = 45 A.

Fig. 6. Mark exhibiting a negative contrast index C (C = −0.23). Operating currentintensity I = 45 A; pulse frequency f = 30 kHz; scanning speed V = 200 mm/s.

frequency and speed, as shown in Fig. 8, where some images ofmarks obtained setting different operating conditions are collected.

At low frequency and high scanning speed (Fig. 8a), the effectof each single pulse is clearly distinguishable as a dot along themark. Increasing frequency (Fig. 8b) gives rise to a higher density ofstriations, resulting from the overlapping of a larger number of dotsper unit mark length. Finally, decreasing speed further increases thedensity of striations, also causing a darker mark surface (Fig. 8c).

To gain some insight into the correlation occurring betweencontrast and roughness, the experimental data were arrangedaccording to the diagram in Fig. 9, where C is plotted against Rt.In the figure, the same symbol has been used for all the points,irrespective of the working conditions adopted.

Despite the scatter affecting the data, a clear tendency emergesfrom them: up to Rt ≈ 4 �m, C steadily increases with roughness;beyond this limit, relatively large variations in Rt result in negligiblevariations in visibility. This conclusion seems to be independent ofthe particular roughness index adopted: although not shown here,analogous results were obtained when the average roughness Ra

was considered, instead of Rt.

As previously specified, the worked surfaces of some specimens

were subjected to EDX. The analysis was limited to the case I = 45 A,and the samples selected are individuated by arrows in Fig. 5. Typi-cal EDX spectra of virgin and marked surfaces are shown in Fig. 10a

Fig. 7. Contrast index, C, against pulse frequency, f, for different values of operatingcurrent intensity, I. Scanning speed V = 50 mm/s.

C. Leone et al. / Journal of Materials Processing Technology 210 (2010) 1297–1303 1301

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Fig. 9. Contrast index, C, against surface roughness, Rt .

Fig. 10. EDX spectrum of (a) virgin surface and (b) marked surface. Working param-eters of the marked surface: pulse frequency f = 6 kHz, V = 50 mm/s and operatingcurrent intensity I = 45 A.

ig. 8. Images of marks obtained varying pulse frequency, f, and scanning speed,: (a) f = 2 kHz, V = 200 mm/s; (b) f = 6 kHz, V = 200 mm/s; (c) f = 6 kHz, V = 50 mm/s.perating current intensity I = 45 A.

nd b, respectively. It is seen that the peak O increases as a conse-uence of laser marking, implying oxidation.

In Fig. 11, the oxygen content measured by EDX is plottedgainst C. A linear correlation is found between C and O, as wit-essed by the best-fit straight line in the figure. Together with theata in Fig. 9, this suggests that roughness and oxidation are both

mportant in determining contrast. However, the role played byxidation is probably dominant at large C values, whereas the effectf roughness seems to prevail at low contrast. This statement is

upported by the following observations: (a) as highlighted previ-usly, the Rt–C curve (Fig. 9) flattens out at sufficiently high valuesf roughness, while the same does not occur for the Oxygen–C curveFig. 11); (b) the EDX analysis revealed that, when a negative C value

Fig. 11. Contrast index, C, against, oxygen content. Operating current intensityI = 45 A.

1 ocessing Technology 210 (2010) 1297–1303

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scanning speeds higher, and powers lower, than those selectedin the experimental campaign. Unfortunately, it is seen fromthe figure that the conditions for optimum visibility can fallout of the working window allowed by the system. Taking intoaccount this constraint, and fixing the minimum power available

302 C. Leone et al. / Journal of Materials Pr

as found (open circle in Fig. 11), the oxygen content was nearo that of the virgin surface; therefore, the contrast was conceiv-bly attributable to the difference in roughness between the virginRt = 1.1 �m) and the marked surface (Rt = 0.95 �m). Interestingly,he effect of laser marking in this case was a decrease in roughnessompared with the virgin material, a phenomenon uniquely foundhen the highest scanning speed was adopted.

.4. Best operating conditions

At the authors’ knowledge, no theoretical models are available athe time to select the optimum conditions for high quality marking.herefore, it was preferred in this work to use the data generatedo draw empirical formulae able to achieve the task.

The trends observed for both the roughness (Fig. 3) and con-rast (Figs. 4, 5 and 7) suggest the occurrence of two competitivehenomena, one of which governing the Rt and C evolution at lowrequencies (phase I), and the other in the high frequencies domainphase II). A physical interpretation can be attempted, assuminghat: (a) C is an increasing function of the energy actually pen-trating to the target; (b) the absorption capacity of the plasmalume formed during laser working, partially shielding the mate-ial surface, increases with increasing laser fluence; (c) for a givenhielding capacity of the plasma plume, the possibility for a sin-le pulse to impinge the material is the lower, the lower its energys. Under these hypotheses, for fixed current intensity and scan-ing speed, increasing f results in a higher average power availableFig. 1) and a larger mass removal rate; at the same time, the plumebsorption capacity is also improved, so that the process efficiencyrogressively decays, causing a decrease in contrast rate. When fre-uency is further increased, the energy of a single pulse becomesoo low to penetrate the plasma plume, and a smaller and smalleruantity of the power available is transferred to the material, driv-

ng the transition to phase II.On the basis of the previous observations, the experimental data

ere divided in two groups, one containing all the points associatedith phase I, and the other the remaining points (phase II). Since

he scope was correlating mark visibility with laser parameters, theaximum contrast index was assumed as the boundary between

hase I and phase II. The following empirical formulae:

I = 8.3955xI + 0.0969 (4)

II = −0.0162xII + 0.0734 (5)

ith

I = ˛ ln f + ˇ

V0.33I0.24e

(6)

II = V(

˛ ln f + ˇ)

− 0.48V − 38.3 (7)

ere found able to effectively fit the experimental trend of con-rast index in phase I (CI) and in phase II (CII), respectively. Theonstants appearing in Eqs. (4)–(7) were obtained by best fittinghe experimental data, and hold for I in A, V in mm/s, and f in kHz.

The curves in Figs. 4, 5 and 7 are a graphical expression ofhe empirical model found. The correlation with the experimentalesults is reasonable, and is judged satisfactory for practical pur-oses. Interestingly, the solid and dashed lines pertaining to phase

I in Fig. 7 are superposed, since, according to Eq. (7), C does notepend on current intensity in this phase. Further support to the

ffectiveness of the model is given by the data in Fig. 12, wherecomparison of the measured contrast index with its calculated

ounterpart, Cc, is carried out.Of course, since Eq. (6) includes effective current intensity, its

pplicability is strongly limited to the laser system used. Neverthe-

Fig. 12. Calculated values of the contrast index, Cc , against their measured counter-parts, C.

less, combining Eqs. (6) and (2), the following expression:

xI = (˛ ln f + ˇ)1.24

P0.24m V0.33

(8)

taking into account Pm, and having therefore a more general use-fulness, is easily drawn.

Putting CI = CII, and considering Eqs. (3), (7) and (8), the optimumPm value, Pmopt, resulting in maximum mark visibility, Copt, for agiven scanning speed was calculated, and plotted against f (thicklines in Fig. 13). The ordinate axis for Pm (on the right in the figure)is limited to 1–13 W, the range actually used in the experimentaltests. The thin lines in Fig. 13 represent Copt.

From Fig. 13, for an assigned V, the maximum visibility achiev-able increases with decreasing frequency. Since a decrease in finvolves lower Pmopt values, it is concluded that best visibilityimplies low frequencies and low powers. Further, in principle highscanning speeds should be preferred.

Fig. 13 suggests that C even better than the maximum value(C = 0.78) measured during this work could be achieved, adopting

Fig. 13. Calculated optimum average power, Pmopt , and optimum visibility, Copt ,against pulse frequency, f, for different scanning speeds, V.

C. Leone et al. / Journal of Materials Processi

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ig. 14. Calculated optimum operating current intensity, Iopt , and optimum visibil-ty, Copt , against pulse frequency, f, for different scanning speeds, V.

Pmopt = 1 W), the maximum theoretical C value (≈0.77) is obtainedor V = 50 mm/s, f ≈ 3 kHz; a slightly lower visibility (C ≈ 0.72)ertains to the condition V = 100 mm/s, f ≈ 4.2 kHz; finally, using= 200 mm/s and f ≈ 5.5 kHz results in Copt ≈ 0.68.

If the problem is the selection of the operating parameters for aiven laser system, Eq. (6) can be used, instead of Eq. (8), in solv-ng the relationship CI = CII. The result is plotted in Fig. 14, showinghe optimum operating current intensity Iopt resulting in Copt, forgiven frequency. Clearly, if the operating current intensity is in

he range 35–45 A, not all the operating parameters individuatedrom Fig. 13 can be actually set. For V = 50 mm/s, the highest C values about 0.73, achieved for f = 4 kHz, whereas, setting V = 200 mm/s,opt ≈ 0.57 with f ≈ 6.7 kHz. Therefore, the optimum visibility actu-lly obtainable is strongly dependent on the system characteristics.his highlights the importance of the laser features, when a markingperation requiring good legibility is under concern.

. Conclusions

Laser marking tests were carried out on AISI 304 steel, usingQ-switched diode pumped Nd:YAG laser, in order to determine

he best working parameters to obtain a given visibility. From theesults obtained, the main conclusions are as follows:

within the range of process parameters employed, mark width isonly moderately affected by operating conditions;

ng Technology 210 (2010) 1297–1303 1303

• mark contrast is affected by both surface roughness and oxida-tion, with the former probably prevailing at low contrast, and thelatter at high contrast;

• if the aim is obtaining good mark visibility, relatively low fre-quencies and average powers should be used;

• the best mark visibility achievable is strictly dependent on theoperating features of the particular laser system used.

Acknowledgements

The authors gratefully acknowledge the financial support pro-vided to this research by the Italian University and ResearchMinistry Council (MIUR), under the grant FIRB-RBIP069S2T 002 toTIBS Laboratory of the University of Naples “Federico II”. Dr. B. DeVito, of the University of Naples “Federico II”, is also acknowledgedfor the SEM/EDX analysis.

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