ARTICLE IN PRESS Optics and Lasers in Engineering 47 (2009) 161–168
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Wood engraving by Q-switched diode-pumped frequency-doubled Nd:YAG green laser C. Leone a,, V. Lopresto a, I. De Iorio b a b
Department of Materials and Production Engineering, University of Naples Federico II, P.le Tecchio 80-80125 Naples, Italy Department of Aerospace Engineering, University of Naples Federico II, P.le Tecchio 80-80125 Naples, Italy
a r t i c l e in fo
abstract
Article history: Received 14 May 2008 Received in revised form 26 June 2008 Accepted 26 June 2008 Available online 23 August 2008
Laser deep engraving is one of the most promising technologies to be used in wood carver operations. In this method, a laser beam is used to ablate a solid wood bulk, following predetermined patterns. The sculpture is obtained by repeating this process on each successive thin layer. Obviously, in order to achieve larger material removal rates, the process needs a controllable variation of the depth to carve a 3D (three dimensional) profiles. The degree of precision of the shape, the removal rate and the surface quality during the engraving process strictly depend on the materials properties, the laser source characteristics and the process parameters. The aim of this work is to investigate the influence of the process parameters on the material removal rates by engraving panels made of different types of wood using a Q-switched diode-pumped Nd:YAG green laser working with a wavelength l ¼ 532 nm. The examined parameters were: the mean power that depends on the pulse frequency, the beam speed and the number of laser scansions, also called repetitions. The working parameters and the engraved depth were related and an energy-based model was proposed in order to predict the latter. Experimental results showed that the Q-switched diode-pumped frequency-doubled Nd:YAG green laser can be successfully used to machine different types of wood, obtaining decorative drawing and 3D engraved geometries without burning. However, an accurate selection of the wood types and the process parameters is necessary in order to obtain deep engraving without carbonization and a homogeneous carving. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Q-switched Nd:YAG laser Laser engraving Wood Wood carver
1. Introduction In the past, lasers were widely used in cutting and welding operations. In recent years, thanks to laser sources characterized by pulse duration variable from nanosecond to femtosecond, they have been applied in other machining like marking, scribing, selective ablations and engraving on metallic, ceramic and polymeric materials [1–6]. The advantages of these lasers are their non-contact working, high repeatability, high scanning speed, a worked area comparable to the laser spot dimension, high flexibility and high automation. Wood engraving and marking by laser resembles face milling to some extent, with the beam ‘tool’ diameter much smaller than that of mechanical cutters. However, since the surface layers lying below those that are burnt away will remain on the object being
Corresponding author. Tel.: +39 0817682374; fax: +39 0817682362.
E-mail addresses:
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processed, care should be taken to avoid damaging them. This place a limitation on the temperature field and heating time associated with the laser power and beam characteristics. Furthermore it places a limitation at material removal rate and confinines the process to shallow and more or less uniform removal depths with low-power lasers. Therefore, it is no coincidence that laser engraving technology is quite widespread in the handicraft industry to mark and engrave decorative wooden items. It is, then, important to choose optimized process parameters like wavelength, spot diameter, pulse repetition frequency, scanning speed and beam power according to the material absorption and the required surface quality [7–9]. Another limitation in wood laser engraving comes from the inhomogeneity and anisotropy of the material. Wood materials, in fact, have a fibrous structure, characterized by different fiber dimension and density that will interact in different modes with the laser beam [7–11]. This makes it difficult to obtain smooth engraved surfaces or well defined net shapes. The aim of the present research was to study the features and the performance given by a Q-switched diode-pumped
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frequency-doubled Nd:YAG green laser, working at 532 nm wavelength, in the engraving operation of different types of wood. This laser was characterized by pulse duration of about 150 ns, pulse repetition frequency up to 35 kHz, maximum mean power of 5 W and beam focus diameter of about 80 mm. Removed material volume and surface quality were evaluated by engraving different kinds of wood by single and multiple laser scanning, varying the pulse frequency, the scanning speed and the number of scanning repetitions. The effectiveness of the laser was evaluated on different types of wood, while the removed material was estimated by measuring the depth of the engraved profile.
2. Equipment, materials and experimental procedures An industrial Q-switched diode-pumped frequency-doubled Nd:YAG green laser (Fly green from LASIT) working at the wavelength of 532 nm was used in experiments. This laser is characterized by a pulse duration of about 150 ns, pulse repetition frequency up to 35 kHz and single pulse energy up to 1.4 mJ. In the laser head the beam was first expanded, then directed towards two galvanometer mirrors (which determined the feature geometry to be produced) and finally focused by a ‘‘flat field’’ lens with a focal length of 160 mm onto the work piece. The focused beam diameter indicated by the manufacturer is about 80 mm. This kind of laser system is widely used in industrial production for the marking and the micro-processing of different materials [1–6]. The laser system is controlled through a PC by means of customized CAD-CAM software, which allows the generation of
4.0
1.5
10
1.2
8
Mean power (W)
0.9 2.0 0.6 1.0
0.0 0
10 20 30 Pulse frequency (kHz)
6 4
Pulse power (kW)
Pulse energy - Pulse power 3.0
Pulse energy (mJ)
Mean Power
the geometric patterns to be worked and the control of the working parameters. The working parameters are: the diode current used to generate the laser beam (A), the pulse frequency (F) and the beam speed (V). The average power (Pm), the pulse energy (E) and the pulse power (Pp) depend, for a fixed diode current, on the pulse frequency as reported in Fig. 1, where the average power is experimentally measured using a power meter (F150A-SH thermal head and a NOVA display from OPHIR). The pulse energy is calculated as the ratio of the average power and the pulse frequency, and the pulse power as the ratio of the pulse energy and the duration. Fig. 1 shows that an increase in frequency produces an increase in the mean power up to 6 kHz followed by a decrease, and at the same time a decrease in the pulse energy and in the pulse power. The mean power, the pulse repetition frequency, the pulse energy and the pulse power play important roles during laser engraving, since they control the interaction between the laser beam and the material, so high power density and high pulse repetitions frequency are necessary to ablate the material [12–14]. In order to evaluate the efficiency of the process, laser engraving trials were performed on samples obtained by 10 mm-thick sheets of different kinds of wood on the radial plane surfaces. The different types were selected after considering different properties like density, hardness, colour, grain presence and market diffusion. The selected types of wood were: exotic walnut, (Daniela walnut-Guibourlia ehie), mahogany (Suvitenia mahogani), poplar (Populus alba), chestnut oak (Quercus petraea) and pine (Pinus pinea). In Table 1, the different main properties of the wood are reported. Square cavities 5 5 mm2 in plane dimension were obtained on the samples by engraving a sequence of linear patterns. The cavities were obtained by varying the pulse frequency from 2500 to 35,000 Hz, the scanning speed (S) at the values of 10, 40, 70, 100 and 200 mm/s and the number of scanned geometry, indicated as repetitions (R), in the range 1–10. The distance between linear patterns, indicated as step (st), was fixed at the value of 70 mm. No less than five cavities were made for each testing condition. In Table 2, the matrix of the experimental tests are reported.
0.3
2
Table 2 Matrix of the experimental tests
0.0
0
Wood
40
Fig. 1. Measured average power and calculated pulse energy and pulse power as a function of pulse repetition frequency. The measurement refers to the maximum diode current value (55 A).
Walnut Mahogany Poplar Chestnut oak Pine
9 > > > > > = > > > > > ;
Frequency (kHz)
Speed (mm/s)
Repetitions
2, 4, 6, 8, 10, 15, 20, 25, 30, 35
10 40 70 100 200
1 1, 2, 3, 4 1, 3, 5, 7 1, 3, 7, 10 1
Table 1 Main property of the tested woods Wood
Walnut Mahogany Poplar Chestnut oak Pine a b
Density (kg/dm3)
0.576 0.690 0.407 0.564 0.684
Hardness shore Da Mean value
St.dev.
53.21 65.93 42.93 47.71 56.50
4.44 3.65 2.16 6.54 10.80
Measure accord to ASTM 2240, ISO/R 868, DIN 53505. Mean value from bibliography.
Compression strengthb (N/mm2)
Flexural strengthb (N/mm2)
Young modulusb (N/mm2)
41 50 34 60 48
120 107 68 110 95
12500 10000 8500 13000 12700
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The geometric profiles of the engraved cavities were measured with 3D Surface Profiling Systems Talysurf CLI 2000 from Taylor Hobson. This system adopts a non-contact 3D measurement optical gauge (chromatic length aberration gauge) characterized by an axis resolution (data spacing) of 2 mm, a vertical range of 3 mm and a vertical resolution of 100 nm. In order to analyse the measured profiles, a surface analysis software (Taly Map Universal) was adopted. The software allows for the acquisition of the profiles and the measurement of the depth of the engraving. No less than two profiles were analyzed for each of the cavities.
3. Results and discussion The engraving process on the wood surface involves complex phenomena from a changing of the wood colour (darkening) to ablation and then to burning, depending on the laser scansion speed, the frequency and the wood structure. For example, in Fig. 2, photos of some tested panels after one laser scanning (one repetition) are reported. The different squares were obtained at different frequencies (same column) and different speeds (same row). At a very low speed (10 mm/s), deep engraving was easily obtained on all the types of wood for all the tested frequencies, together with the carbonization phenomena. According to what is
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reported in [7], this is due to the fact that there is not enough time for the surface to cool down between pulses because of low thermal conductivity. On the other hand, at high speed value (200 mm/s), the laser beam material interaction produces only material darkening, that is a light carbonisation and only at low pulse frequencies, from 2 to about 6 kHz in correspondence to the maximum laser power output. Adopting beam speed values in the range 40–100 mm/s the burning phenomena are absent and only engraving phenomena can be obtained together with material darkening. In contrast with what is observed for 10 mm/s, in the range of 40–100 mm/s, the engraved depth is very small and a strong dependence on the pulse frequency is also visible. In Fig. 3, the engraved depth is reported as a function of the frequency for the scansion speeds of 10, 40 and 70 mm/s on the walnut wood. The error bars represent the standard deviations of five measurements. It is possible to observe that in all the cases the curve shows a similar trend, with an initial increase up to 4 kHz, where the maximum power output was obtained, followed by a decrease. However, a significant reduction of the depth occurs when the speed increases. A similar trend was also observed for mahogany and poplar. The previous observations about pulse frequency and speed interaction effect could be clearer if some of the well known typical phenomena in laser marking fields are taken into account. In marking processes, in fact, a certain threshold pulse power
Fig. 2. Test panels after one laser scansion (A) walnut; (B) mahogany; (C) chestnut oak; (D) poplar and (E) pine; the different squares are obtained at different frequency (columns) and different speed (rows).
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function, below which no satisfactory results can be achieved [15], is present. The pulse power threshold value also depends on the adopted frequency, the speed and the obtained overlapping percentage factor. For the adopted laser source, the higher the pulse frequency the lower the pulse energy and the pulse power. This, together with the use of high speed that determines low overlapping values, does not allow the engraving phenomenon to be obtained. It results in a frequency limitation range for engraving when the beam speeds are increased (Fig. 3, V ¼ 40, 70 mm/s). For chestnut oak and pine, the measured depths were difficult to perform because of their inhomogeneity. In fact, looking at the ablated surface in the case of the walnut wood, the mahogany and the poplar, almost flat ablated surfaces are possible; by contrast, in the cases of chestnut oak and pine, the ablated surface is very rough, which repeats the fibrous structure of those types of wood. This characteristic is due to the presence of strong discontinuity on the radial plane surfaces that depends on the seasonal growth of the wood. As is well known, during the spring wood growth produces wide and low density vessel, while during the summer the vessel dimension diminishes and the local density increases, Fig. 4 [16]. This phenomenon is actually more complex than described, since it also depends on the temperature, the water availability (rainfall), photosynthesis availability, air pollution and monthly variations [17–22]. Of course, the homogeneity of the material plays a fundamental role during the laser ablation [6]. This is more evident for the chestnut oak and the pine wood, which are very unlike to homogenous ablate thanks to their local large density variations. When the laser beam works through a low density to a high density wood area, it changes the material and then the material removal rate. In this case, deep surface discontinuity is observable. For examples, in Fig. 5, the comparison between the behaviour profiles of two kinds of wood, mahogany and chestnut oak, is reported. Looking at the figure, the different behaviour of the two kinds of wood is evident. What is shown suggests the importance of a preventive selection of process parameters related to the particular kind of wood in deep laser engraving operations. Obviously, homogeneous woods like walnut, mahogany and poplar are preferable to inhomogeneous ones like chestnut oak and pine. For this reason, both these types will be ignored in the following discussion. Pulse frequency is, of course, a useful parameter when the laser process parameters have to be set, but the fundamental parameters, in the light of what has previously been asserted, are the mean power and the scanning speed. In Fig. 6, the same
3
Depth (mm)
S = 10 mm/s, R = 1 S = 40 mm/s, R = 1 S = 70 mm/s, R = 1
Transverse plane A) Large and low density spring axial vessels Radial plane
Annual ring
B) Narrow and high density summer axial vessels Fig. 4. Schematic representation of the axial wood vessel seasonal growth: (A) during the spring the growth produces large and low density vessels and (B) during the summer the vessel dimension diminishes and the density increases. For sake of simplicity, in the picture the radial vessels are ignored.
data shown in Fig. 3 are collected as a function of the mean power for different beam speeds. The engraved depth shows a quasilinear dependence on the mean power, with the slope that decreases as the speed increases. Since the depth increases as the speed decreases and burning occurs, a strategy for an effective laser deep engraving would be the use of both high speed and multiple laser scanning (repetitions). In this case, it is important to know how many repetitions are necessary to obtain a required depth. In order to study a predictive model for the engraving process that takes the mean power, the beam speed and the repetitions effect into account, an easy model based on energy consideration, often used for laser cutting, was here applied to the engraving [2,23]. The model starts from the equation V ¼a
Pm ti Qv
(1)
which relates the volume V of the removed material to the beam piece interaction time, ti, the beam mean power, Pm, the coefficient of absorption of the materials at the emission wavelength, a, and the material vaporization energy for unit volume, Qv. From Eq. (1), it is possible to evaluate the maximum depth of the engraving considering that Depth ¼ d ¼ V=scanned area ¼ V=5 5
(2)
and that the laser material interaction time, ti, can be written as total number of repetitions 5 1 ¼R 5 (3) ti ¼ speed st S
2
where R is the number of repetitions, 5 is the length of a single scansion, st is the steep, 5/steep is the number of single scansions needed to completely cover the engraved area (5 5 mm2) and S is the beam speed. Combining the Eqs. (1)–(3) the following expression can be derived:
1
0 0
10
20 Frequency (kHz)
30
40
Fig. 3. Engraved depth as a function of frequency, for the scanning speed of 10; 40 and 70 mm/s. Tests performed on walnut wood panels.
d¼
a Q v st
R
Pm k Pm ¼ R st S S
(4)
Eq. (4) relates the engraved depth to the mean power (Pm), the beam speed (S), the repetition number (R), the distance between linear patterns (st) and the material properties (k ¼ a/Qv)U
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165
Alpha = 45°Beta = 60°
0.601 mm
4.32 mm
4.52 mm
5 mm
Alpha = 45°Beta = 60°
1.62 mm
4.49 mm
4.65 mm
mm 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 mm 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Fig. 5. Aspect of engraved surface (A-A0 ) photo and surface scanning for a homogeneous wood (mahogany); (B-B0 ) photo and surface scanning inhomogeneous wood (chestnut oak).
3
3 S = 10 mm/s, R = 1 S = 40 mm/s, R = 1 S = 70 mm/s, R = 1 Depth (mm)
Depth (mm)
2
1
0 0
1
2
3
Mean power (W)
S = 10 mm/s; R = 1 S = 40 mm/s; R = 4 S = 40 mm/s; R = 3 S = 40 mm/s; R = 2 S = 40 mm/s; R = 1
2
1
0 0
0.1 0.2 Number of repetition*Mean power/speed (W/mms-1)
0.3
Fig. 6. Engraved depth as a function of mean power, for the scanning speed of 10; 40 and 70 mm/s. Tests performed on walnut wood panels.
Fig. 7. Engraved depth as a function of the Repetitions Mean power/Speed parameter, for scanning speed of 10 and 40 mm/s. Tests performed on walnut wood panels.
In order to verify the validity of the suggested model expressed by Eq. (4), in Fig. 7, the depth obtained at the speed of 40 mm/s for different repetition numbers was compared with the one obtained at 10 mm/s, as a function of ratio RPm/S. From the figure, it is possible to observe that all the data converge to a single linear curve; this means that a good agreement between the experimental results and the proposed model was found.
It is important to underline that the proposed model does not consider the previously described threshold. For this reason, in order to select the right combinations of mean power-beam speed values to obtain engraving, in Figs. 8–10, the mean power-beam speed values to obtain wood darkening with only one laser scanning (R ¼ 1) were reported for the different types of wood. The dark dots indicate material darkening, the white ones the
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3
300
Walnut wood, S = 10
Walnut wood - Engraving regime
Walnut wood, S = 40
200
Depth (mm)
Beam speed (mm/s)
Walnut wood - No engraving regime
Maximum engraving speed
Walnut wood, S = 70
2
Walnut wood, S = 100
1
100
0 0
0 0
1
2
3
Mean power (W) Fig. 8. Typical engraving regime for walnut wood panels. The working conditions refer to one repetition.
0.1 0.2 Repetitions*Mean power/speed (W/mms-1)
0.3
Fig. 11. Engraved depth as a function of the Repetitions Mean power/Speed parameter, all data. Tests performed on walnut wood panels.
3 Mahogany wood, S = 10 Mahogany wood, S = 40
300 Depth (mm)
Mahogany wood - Engraving regime Beam speed (mm/s)
Mahogany wood - No engraving regime 200 Maximum engraving speed
Mahogany wood, S = 70
2
Mahogany wood, S = 100
1
100 0 0
0 0
1
2
3
0.1 0.2 Repetitions*Mean power/Speed (W/mms-1)
0.3
Fig. 12. Engraved depth as a function of the Repetitions Mean power/Speed parameter, all data. Tests performed on mahogany wood panels.
Mean power (W) Fig. 9. Typical engraving regime for mahogany wood panels. The working conditions refer to one repetition.
300 Poplar wood - Engraving regime Beam speed (mm/s)
Poplar wood - No engraving regime 200 Maximum engraving speed 100
0 0
1
2
3
Mean power (W) Fig. 10. Typical engraving regime for poplar wood panels. The working conditions refer to one repetition.
condition where no interaction was obtained. As it is possible to see, in all the cases the two regimes are well separated by a line, which represents the maximum speed for a data mean power to obtain engraving on the three materials. However, neglecting all the data corresponding to the combination of mean power and speed where no interaction is obtained, in the Figs. 11–13 the engraved depth was reported as a function of the Repetitions Mean power/Speed for all the other data points, respectively, for the walnut wood, the mahogany and the poplar. As it is possible to observe, even for the higher speed, a linear trend is still present and the model is verified. The straight lines in Figs. 11–13 represent the bestfitting lines. Of course, data scattering is present in the figures, but we have to consider the typical material inhomogeneity, the use of wood panels coming from different parts of the trunk and the variation of the beam focus during the engraving process: all these characteristics can produce variation in engraving behaviours. Finally, an attempt was made to consider the possibility of relating the different wood trends to the material properties, considering that the higher slope of best-fitting lines in Figs. 11–13 corresponds to the lower density (poplar wood) and vice versa. In order to investigate this, the slope, A, of the bestfitting lines in Figs. 11–13 was evaluated and its inverse (1/A) was plotted as a function of the material density. The results are
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Not all kinds of wood are suitable to be engraved. It depends on
3 Poplar wood, S = 10 Poplar wood, S = 40 Poplar wood, S = 70 2
Depth (mm)
167
Poplar wood, S = 100
1
0 0
0.1 0.2 Repetitions*Mean power/S (W/mms-1)
0.3
Fig. 13. Engraved depth as a function of the Repetitions Mean power/Speed parameter, all data. Tests performed on poplar wood panels.
0.20
the wood structure and the presence of high different density values due to seasonal growth of the wood, which does not allow for flat engraved surfaces. The surface carbonization depends on an incorrect selection of the process parameters and, for the adopted laser, it happens at beam speeds of up to 10 mm/s. For speed more than 40 mm/s, the engraved depth is very low and multiple laser scanning are required to obtain deep engraving. The engraved depth is strongly affected by the mean power, the pulse frequency, the beam speed and the number of repetitions. Increasing the speed is possible to obtaining engraving with a reduced frequency range around the value where the maximum output power is achieved. The maximum speed necessary to obtain engraving linearly depends on the mean power. A simple linear model, based on energy consideration, for the prediction of the engraved depth, was proposed and verified. It takes into account the laser mean power, the beam speed and the number of repetitions. A relation between the model coefficients and the wood’s density was found.
1/A (mm2/sW)
0.15 Acknowledgements
y = 0.2333x R2 = 0.9964
0.10
The authors are particularly grateful to TIBS Research Centre of the University of Naples ‘‘Federico II’’ for the laser facilities and the financial support to develop the present research work. Thanks also go to Professor V. Tagliaferri from the University of Roma ‘‘Tor Vergata’’ and his staff for the availability of measuring equipment and for their hospitality.
0.05
0.00 0
0.25
0.5 Density (kg/dm3)
0.75
1
Fig. 14. Dependence of the inverse of the slope of the best fitting lines of Figs 11–13 as a function of wood density. The reported data are the best fitting lines equation and the coefficient of correlation, R2.
reported in Fig. 14, where it is possible to observe that the three points were arranged along a line starting from zero. This means that, for the tested materials, the beam energy is similarly absorbed and used to ablate a mass quantity that is independent of the wood. Moreover, the higher the density, the higher 1/A and the lower the slope A. These conclusions are expected if the fact that the three kinds of wood are made of the same materials (lignin, cellulose, etcy) is taken into account, but they are unexpected because of the changing in coefficient of absorption due to the different colours of the woods.
4. Conclusion In this work, the features and the performances given by a 5 W of nominal power Q-switched diode-pumped frequency-doubled Nd:YAG green laser in the engraving of different kind of woods are discussed and the main conclusions are the following:
Q-switched diode-pumped frequency-doubled Nd:YAG green laser can be usefully used in deep engraving of different kinds of wood without carbonization of the surface.
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