- Email: [email protected]
Laser ablation using a long-pulsed, high-fluence, CW single-mode fiber laser
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 22–30
journal homepage: www.elsevier.com/locate/jmatprotec
Laser ablation using a long-pulsed, high-fluence, CW single-mode fiber laser W.R. Harp, J.R. Dilwith, J.F. Tu ∗ Department of Mechanical and Aerospace Engineering, North Carolina State University, 3211 Broughton Hall, Raleigh, NC 27695, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
Laser ablation is commonly used to produce microsized holes that are difficult to produce
Received 22 October 2006
by conventional methods. To achieve non-thermal ablation, ultra-short-pulse lasers in the
Received in revised form
range of femtoseconds are often used. However, they are very expensive and their material
21 May 2007
removal rates are very limited due to the small amount of energy they can deposit and their
Accepted 15 June 2007
low repetition rates. In this paper, the feasibility of using a continuous-wave fiber laser to perform thermal ablation via long-period pulses is investigated. The fiber laser has excellent beam quality and can be modulated to operate at different pulse widths in the range of tens
PACS:
of microseconds with high repetition rates. Results show that a pulse of 18 s produces the
52.38.Mf
optimum hole characteristics. Also, the removal rate is superior to other lasers used for laser
42.55.Wd
ablation. A though-hole was generated in a piece of stainless steel 100 m thick using only
42.62.−b Keywords:
seven pulses.
Fiber laser
© 2007 Elsevier B.V. All rights reserved.
Laser ablation Thermal ablation Micro-drilling
1.
Introduction
Conventionally, laser systems tend to be application specific. For example, kilowatt continuous-wave (CW) high power lasers are used for high speed welding and cutting, while ultrashort-pulse lasers are used for laser ablation. It is, therefore, desirable to explore the feasibility of laser ablation using a CW single-mode Yb-doped fiber laser which was originally designed for macro cutting and welding to extend its processing capabilities. There are two main categories of laser ablation. The first is thermal ablation in which the laser interacts with the surface of the specimen creating a molten pool and vapour. The exiting vapour then creates a recoil pressure which pushes the melt radially outward from the center of the beam to create a cavity (Charschan, 1993).
∗
Corresponding author. E-mail address: [email protected] (J.F. Tu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.06.062
The second category of laser ablation is non-thermal or “cold” ablation. Non-thermal ablation requires extremely small pulse widths, usually 10−11 to 10−12 s and smaller. The short-pulse width is short enough to evaporate or “blast” away material before heat can be conducted through the material, thus no melting occurs. Since no melting occurs there is no recast layer and little to no heat affected zone (Steen, 2003). Three parameters are critical to laser ablation. The first is peak power density, in W/cm2 . A large peak power density is essential for ablation to occur. Typically the peak power density is in the GW/cm2 range for laser ablation. The second parameter is pulse duration. Typical ultra-short-pulse lasers have a fixed amount of energy they can deliver per pulse. If the pulse duration is lengthened the peak power produced will be significantly less, therefore short-pulse lasers use pulse durations in the nanosecond, picosecond and femtosecond ranges
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 22–30
23
that allow for large power. The third parameter is the repetition rate of the laser. Having a low repetition rate will increase the time needed for producing holes of desired depths. If the repetition rate is high enough the heat can be retained and the ablation will be more efficient (Friedrich, 1998). An additional parameter that is helpful in laser ablation is the beam quality of the laser beam. The beam quality affects the ability to focus the laser to achieve high power densities and affects the quality of the ablated hole. According to Rodden et al., when the beam quality is poor, a larger exit hole diameter is produced (Rodden et al., 2002).
2.
Initial feasibility check
To determine the initial feasibility of laser ablation using a 300 W single-mode CW fiber laser, the three ablation parameters (peak power density, pulse duration, and repetition rate) are examined. First, the pulse duration is examined. The laser used in this study is rated at 300 W CW output. Measurements using a photodiode sensor revealed that there is an initial transient spike of about 2 s that occurs at the beginning of a laser pulse before it stabilizes to the 300 W CW output power. According to the measurement, the spike is about 4.5 times the steady state. Fig. 1 shows the case in a 55 s pulse. The top curve is the signal that is sent to the laser to enable emission, while the second curve displays the laser output detected by the photodiode. The figure lists the CW output as 150 W, which is lower than the rated output, and this is explained below. Similarly, Kleine et al. (Kleine et al., 2002) measured the pulse shape of a 50 W Yb-doped fiber laser and also found that for a 100 s pulse, there was a 1–2 s spike that was of 2–3 times higher power than the CW rated output. There is a major difference in the spike produced by this fiber laser and a pulse produced by a short-pulse laser. In a conventional short-pulse laser, the pulse energy is specified first and then the pulse duration. Together, they determine the peak power of the pulse. However, in a fiber laser a spike with a predetermined peak power is produced. The total energy deposited by a fiber laser depends only on the pulse duration. If the fiber laser is modulated to produce a longer pulse, more
Fig. 1 – Pulse shape measurement. Input signal (top); initial spike and steady state signal (bottom).
Fig. 2 – Composite of various pulse widths (18, 17, 16, and 15 s).
energy is deposited. As shown in Fig. 1, a pulse longer than the spike duration will deposit energy at a higher peak power only during the spike duration and will deposit energy at the steady state power during the rest of the pulse. To enhance ablation, it is desirable to shorten the modulated pulse. However it was found that for the fiber laser used in this investigation, the pulse duration cannot be less than 15 s. As shown in Fig. 2, below 18 s, the spike decreased slightly for 17, 16, and 15 s and was nonexistent for 14 s. Compared with typical ultra-short-pulse lasers for laser ablation (range of fs and ps), the effective pulse width of 2 s is very long. As a result, the nature of laser ablation using this laser will be thermal ablation, similar to nano-second lasers. It is expected to see melt ejection and recast layers in the results. To determine the magnitude of the spike and the steady state output, a photodiode was used, with a 10 m pinhole over the sensor to prevent the sensor from being saturated. A steady state beam was generated and a portion was sampled by the sensor to gather reference data on the sensor’s output versus the laser power. Fig. 3 shows the conversion line for the photodiode sensor and there is seen a linear rela-
Fig. 3 – Conversion line of the photodiode sensor with a 10 m pinhole.
24
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tionship between the supplied power and the sensor output signal. Pulses were generated through an external circuit and the pulses were sent through an analog interface with the laser. A voltage was sent to the interface and the power was determined by the magnitude of the signal, with 10 V representing a programmed 300 W. Using Fig. 3 as the conversion guide, the actual laser output versus the programmed output is seen in Fig. 4. In the figure, the output power increased as the programmed power is increased, however, after 150 W programmed power, the steady state output remained constant at 130 W and the transient spike was constant at 530 W. This could be a result of how the laser is triggered by the external circuit. The peak power density depends on both the peak power and area of the focus spot. Using a 150 mm focusing lens, the smallest diffraction-limited spot size that can be achieved with this laser is 10.8 m, and the power density achieved is: Power density =
150 W /4(10.8 × 10−4 cm)
= 0.164 GW/cm2
(1)
Similarly, using a 100 mm lens, a diffraction-limited spot size of 7.2 m is achievable and using a 60 mm lens gives a 4.3 m spot size. The power densities available using those lens are 0.368 and 1.03 GW/cm2 , respectively. However, knowing that there is an initial spike in power of 530 W, the new power density for each of the lenses is 0.6, 1.35, and 3.78 GW/cm2 , respectively. These values are based upon diffraction limited spot sizes and the measured peak power. These peak power densities are large enough that ablation could result. The repetition rate of the laser is the third critical issue when considering ablation. A laser must be able to rapidly generate pulses in order for the process to be economical. This laser has a repetition rate of 25 kHZ, which yields a 40 s cycle time. This would allow for multiple pulses of 18 s to be generated. Table 1 below shows a comparison of the different parameters associated with the current laser, a nanosecond laser, a picosecond laser, and a femtosecond laser. This table is not exhaustive and was based on specifications from a few laser manufacturers. The process parameters of the nanosecond laser can be compared to the fiber laser more easily since they both are in the thermal ablation regime. Both the picosecond and femtosecond lasers are in the non-thermal ablation
Fig. 4 – Measured pulse output.
regime, therefore only their processing capabilities will be compared.
3.
Experimental apparatus
The laser used in these experiments was a 300 W CW Ybdoped single-mode fiber laser (IPG YLR-300). It has a M2 value of 1.04, and wavelength of 1075 nm. The overall experimental setup is shown below in Fig. 5. The aluminum structure and all associated equipment were located on a pneumatic optical table which isolated the equipment from vibrations in the environment. To focus the collimated laser, a 5× beam expander was used along with a 150 mm or 100 mm Gradium focusing optic. The diameter of the beam exiting the collimator was 4.5 mm and after expansion, was 22.5 mm. When focused through a 150 mm lens, the measured minimum spot size achieved was 11.5 m, and the measured minimum spot size for the 100 mm lens was 9.36 m. Using the peak power of 530 W, the power density for the 150 mm lens was 510 MW/cm2 , and the power density for the 100 mm lens was 770 MW/cm2 . A set of linear motors was used in an x–y configuration to move the specimens during experiments. A M6 tapped breadboard was mounted to the top of the linear motors for specimen fixturing. Specimens of 316 stainless steel with a
Table 1 – Typical laser specifications of different short-pulse laser systems Yb (100 mm lens) CW Pulse width (s) Energy (J) Peak power (W) Wavelength (nm) Beam mode (M2 ) Spot size (m) Fluence (J/cm2 ) Power density (GW/cm2 ) Removal rate Repetition rate
2.00E−06 1.10E−03 550 1075 1.04 7.2 2702 1.351 25 m/pulse 25 kHz
Nd:YAG Nano
Nd:YVO4 Pico
3.00E−08 3.00E−03 100,000 1064 <2 30 424 14.15 1.5 m/pulse 10 kHz
1.00E−11 4.00E−05 4.0E+06 1064 <1.2 40 3 318.3 0.3 m/pulse 50 kHz
Ti:sapphire Femto 1.50E−13 5.00E−04 3.3E+09 775 <1.3 150 3 18,863 0.1 m/pulse 5 kHz
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25
Fig. 6 – Mark diameters as a function of focal position (150 mm lens).
Fig. 5 – Experimental setup.
thickness of 0.95 mm were clamped to posts and held above the breadboard. The drilling process was controlled through a motion control program linked to a Dspace DS1104 R&D controller board that utilized Matlab for data acquisition and control. A Zeiss microscope was used to examine the specimens and images were taken from a camera port on the microscope. Imaging software was used to determine the diameter of the holes. Surface plots were also performed using two different instruments: a Talysurf profilometer to measure a 2D cross section of the holes and a Zygo NewView 500 Interferometric Microscope to render 3D analysis of the holes.
4.
Experimental procedures and results
4.1.
Optimal focusing tests for very long pulses
250 m increments, pulses of 50, 100, and 1000 s were fired at the specimen. Additional tests were also performed for multiple pulses at 50 s pulse widths. The results (Fig. 6) reveal that the diameter of the ablated mark increases for increasing pulse widths yet remains relatively constant for multiple 50 s pulses. These tests were repeated and verified using smaller 100 m increments and the minimum diameters for the 50, 100, and 1000 s pulses were 82.6, 99.2, and 170.8 m, respectively.
4.2.
A white light interferometric microscope was used to obtain 3D profiles of the ablated marks. Measurements (Fig. 7) revealed that the marks were not holes, but rather bumps were formed. This seems to be a result of the long (≥50 s) pulse width. Also, the power density may be too low to effectively ablate the material. Therefore, the 150 mm lens was exchanged for a 100 mm lens to obtain a higher power density and the pulse width was reduced below 50 s to investigate its effect on the process.
4.2.1. The initial objective of the experiments was to determine the optimal focusing position for ablation by finding the position where the diameter of the ablated mark was at its minimum. Using a 150 mm lens and adjusting the focal position in
3D specimen profile measurement
Long-pulse tests with 100 mm lens
The 150 mm lens was replaced with a 100 mm lens. The test to find the optimal focusing position was repeated for this lens and for pulse widths of 20, 30, and 40 s. The results are shown in Fig. 8 and show that the minimum diameters
Fig. 7 – Interferometric measurement of mark (50 s mark). Left: 3D image of mark; right: 2D profile of mark.
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 22–30
Fig. 8 – Mark diameters as function of focal position (100 mm lens).
for the 20, 30, and 40 s pulses were 42.8, 55.0, and 68.4 mm, respectively. Again, the increase in diameters with the pulse width is seen here. Fig. 9 shows the spot images for the 20 s test at various focal positions. The marks correspond to the curve above for
the 20 s test at +0.45, +0.3, +0.15, 0, −0.15, −0.3, −0.45, and −0.6 mm from the focus. The mark generated at zero focus was then tested using the interferometric microscope and the Talysurf profilometer. In both measurements (Figs. 10 and 11), it shows a blind hole that is approximately 20 m deep. Fig. 12 shows the partial results for 30 s pulses at various focal positions. The depth of the hole at zero focus was about 19 m while the diameter was approximately 54.0 m. The measured diameter from the microscope was close to this at 55 m. The measurement from the Talysurf profilometer revealed that the hole depth was a little over 18 m with a diameter of about 60 m. Finally, the 40 s pulsing test was examined and the mark produced at zero focus did not produce a hole, but at −0.15 mm focus, a hole was produced. Further testing would have to be performed to determine whether focusing into the plane is more effective, but this is most likely due to a problem in the fixturing not holding the workpiece perfectly flat. As can be seen from Fig. 13 the 40 s pulse width created larger holes than the 30 s, though the roundness of them was better than the smaller pulse width tests. It is expected though that after multiple pulses the smaller pulse width schemes will produce holes of equal roundness. Measurements were able to show that the depth of the hole to be 18 m and the diameter of
Fig. 9 – Spot images for 20 s test.
Fig. 10 – Interferometric measurement of mark created by 20 s pulse. Left: 3D image of mark; right: 2D profile of mark (lines added for clarity).
27
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Table 2 – Summary of successfully drilled holes Pulse width (m)
Diameter (m)
20 30 40
Fig. 11 – Surface profile for 20 s pulse width hole.
the hole to be around 42 m which was close to the microscope’s value of 45.0 m. A summary of the results is shown in Table 2. Information from the above table was used to infer that the shortest possible pulse width would be the best choice. The 20 s pulse produced a hole with the smallest diameter and the largest material removal rate (MRR). The results are most likely due to the shorter pulse time allowing for less conduction. It appears that the larger pulse widths allowed for more heat conduction through the surrounding area which caused melting instead of ablation, thus some of the molten material was re-deposited in the hole. The 20 s pulse showed that it produced the smallest diameter hole with the greatest depth, but produced the largest recast layer around the mouth of the hole. The 30 s pulse produced a larger diameter whole, smaller depth and had less of a recast layer. The smaller depth and recast layer suggests that the longer pulse allowed from more melting around the perimeter of the hole which reduced the recast layer and slightly enlarged the diam-
40.1 55.0 68.4
Depth (m) ∼24 ∼19 ∼18
eter. The 40 s pulse produced a shallow hole with less of a recast layer. Finally for pulse widths of 50 s no hole was produced because of the excessive melting that occurred, creating a pool of molten material. After cooling, this pool created an area of less dense material which resulted in a bump instead of a hole. Some of these trends have been observed by similar research in that thresholds were found to exist for the removal of material (Yashkir, 2002). The threshold for the current setup was found to be somewhere in between 50 and 40 s at which there will be minimal to no material removal. This study also showed the same trend in increasing crater depth with decreasing pulse time and confirmed that there is a limit to the effectiveness of reducing pulse width. The information in this section presented results that showed that pulses under 50 s would produce indentations rather than raised surfaces. The information in Figs. 1 and 2 provide a better insight into the cause of this situation. After the initial spike, the power falls to the CW wave output, and therefore for longer pulse widths, the energy deposited is used for melting and recasting. The following section shows how minimizing the pulse width yields results consistent with predictions based in Fig. 2.
4.2.2.
Sub 20 s pulse tests
A series of shots were fired at the optimally focused position using pulse widths from 20 s down to 10 s. Marks were found on the specimen for pulse durations of 15 s and larger. Looking at Fig. 14, the 15 s pulse produced a hole with a depth of 8 m, whereas the 16 s pulse produced a hole with a depth of
Fig. 12 – Spot images for 30 s test.
28
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Fig. 13 – Spot images for 40 s test.
20 m. This is evidence to the presence of a spike that is at its peak between 15 and 16 s. Fig. 15 shows the enlarged images of the holes created in the specimen. The blind holes that were produced were measured on the microscope and measured by the interferometric microscope. Their depths and diameters are compared in Fig. 16. The result for the 18 s pulse gives the largest aspect ratio and is assumed to be the optimum pulse width. From the plot it shows that both the diameter and the depth of the blind holes tend to decrease in size with lowering pulse width. Shots less than 15 s do not create any marks and measurements show that a 14 s pulse does not generate any output. Therefore, the initial spike is a large contribution to the ablation process. Though the highest depth was achieved at a 20 s pulse, the highest aspect ratio from Fig. 16 was found to be at 18 s. This pulse width had only slightly less depth than the larger
pulse widths, but had a signification reduction in diameter which helped to increase the aspect ratio.
4.3.
Multiple pulse drilling
From the above results for ablation, it was desirable to view the result of drilling with multiple pulses on a specimen of stainless steel 100 m thick. The pulse width used in this instance was the 18 s that was determined to be the width producing the best aspect ratio. Helium assist gas was used to remove material as it was ablated. The reason for using the assist gas was that again, the feasibility of drilling was being tested. Therefore, since ablation was achieved without the assist gas, using the assist gas should help in drilling. Five pulses were fired at the specimen, and then the lens was refocused half the thickness of the specimen, 50 m, and then another five pulses were fired. This gave a hole with a 44 m entrance diameter
Fig. 14 – Profile for pulses. Top row: 15, 16, and 17 s; bottom row: 18, 19, and 20 s (lines added for clarity).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 22–30
29
Fig. 15 – Enlarged images for sub 20 s pulses.
Fig. 16 – Plot of hole characteristics for various pulse widths.
Changing both of these parameters allowed for the creation of a crater with less than 50 m in diameter. The resulting hole also contained a very small heat affected zone even though the pulse time was substantially larger than that of ns, ps, and fs-short-pulse lasers. The addition of the 100 mm lens and using the shortened pulse allowed for the power density to approach that of the typical nanosecond laser. Because they have higher order modes, or lower beam qualities, the short-pulsed lasers minimum spot size is much larger than the fiber laser. This can limit the size of the holes that they can create. The MRR produced for the single pulse of 20 s was very high at a value of 24 m. The typical MRR for short-pulse lasers is two orders magnitude smaller than the value obtained with the CW laser. Though the pulse times are much smaller for the short-pulse lasers, the time needed in between pulses is larger than what is needed for the CW laser. Therefore since the MRR is so much lower for the short-pulsed lasers, it would take somewhere
and an 18 m exit diameter. A second attempt was performed where again the assist gas was used, but seven pulses were fired at the specimen without refocusing. This method produced a hole with a 38.4 m entrance diameter and a 14.4 m exit diameter (Fig. 17).
5.
Discussion
From the results given it was determined that the spot size and pulse width can greatly effect the outcome. With a spot size of 11.5 m and a pulse width of 50 s, it was not possible to produce holes with the available power. Changing the lens allowed for a smaller spot size which in turn raised the power density, a key factor when trying to ablate the surface of a material. Finally, decreasing the pulse width caused a greater percentage of the pulse energy to come from the initial spike and also reduced the conduction through the material.
Fig. 17 – Thru-hole generated by multiple pulse drilling. Left: entrance; right: exit.
30
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around 100 pulses to create the same depth as 1 pulse from the CW laser. The off time needed for these short-pulse lasers creates a bottleneck making the overall process take longer.
6.
Conclusions
A feasibility study was conducted to determine if a fiber laser designed for use as a continuous source could be used to perform pulsed ablation. It was seen that the particular fiber laser used in this study produces a pulse that consists of an initial spike in power. The results show that laser ablation occurs when a 100 mm lens is used with pulse durations at 40 s or below. At 18 s pulse duration, a blind hole of 43.6 m diameter and 23.5 m depth can be created with little heat affected zone. This performance is comparable to nanosecond lasers, but with much higher hole depth per pulse. It was also found that the pulse duration must be less than 50 s so that the ablating effect of the initial spike is not nullified. At longer pulse durations (50 s or more), raised surfaces are created instead of holes. This seems to be an occurrence of spot welding and forms the basis for further research. Areas of further consideration: • Determining the magnitude of the spike through further power measurements. • Investigate the spot welding versus micro-drilling phenomena. • Investigate multiple pulse drilling.
Acknowledgements The research is supported in part by NSF Grant # CMS-0402857 and NSF Grant # DMI-0355481. The assistance of the NCSU Precision Engineering Center is also acknowledged.
references
Charschan, S.S., 1993. Guide to Laser Materials Processing, 1st ed. Laser Institute of America, Orlando, pp. 40–41. Friedrich, C., 1998. Laser Micromachining–Laser Ablation, Michigan Technology University Notes. Kleine, K.F., Whitney, B., Watkins, K.G., 2002. Use of fiber lasers for microcutting applications in the medical industry. In: Proceedings of Laser Institute of America, ICALEO 2002, 21st International Congress on Applications of Lasers and Electro-optics, Scottsdale, AZ, October 14–17. Rodden, W.S.O., Kudesia, S.S., Hand, D.P., Jones, J.D.C., 2002. A comprehensive study of the long pulse Nd:YAG laser drilling of multi-layer carbon fibre compostites. Optics Commun. 210 (3–6), 319–328. Steen, W.M., 2003. Laser Material Processing, 3rd ed. Springer, London, pp. 335–341. Yashkir, Y., 2002. Laser micromachining with passively Q-switched intracavity harmonic generator, Technical Digest of SPIE Opto-Canada: SPIE Regional Meeting on Optoelectronics, Photonics, and Imaging, SPIE Volume TD01, pp. 462–466.
journal homepage: www.elsevier.com/locate/jmatprotec
Laser ablation using a long-pulsed, high-fluence, CW single-mode fiber laser W.R. Harp, J.R. Dilwith, J.F. Tu ∗ Department of Mechanical and Aerospace Engineering, North Carolina State University, 3211 Broughton Hall, Raleigh, NC 27695, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
Laser ablation is commonly used to produce microsized holes that are difficult to produce
Received 22 October 2006
by conventional methods. To achieve non-thermal ablation, ultra-short-pulse lasers in the
Received in revised form
range of femtoseconds are often used. However, they are very expensive and their material
21 May 2007
removal rates are very limited due to the small amount of energy they can deposit and their
Accepted 15 June 2007
low repetition rates. In this paper, the feasibility of using a continuous-wave fiber laser to perform thermal ablation via long-period pulses is investigated. The fiber laser has excellent beam quality and can be modulated to operate at different pulse widths in the range of tens
PACS:
of microseconds with high repetition rates. Results show that a pulse of 18 s produces the
52.38.Mf
optimum hole characteristics. Also, the removal rate is superior to other lasers used for laser
42.55.Wd
ablation. A though-hole was generated in a piece of stainless steel 100 m thick using only
42.62.−b Keywords:
seven pulses.
Fiber laser
© 2007 Elsevier B.V. All rights reserved.
Laser ablation Thermal ablation Micro-drilling
1.
Introduction
Conventionally, laser systems tend to be application specific. For example, kilowatt continuous-wave (CW) high power lasers are used for high speed welding and cutting, while ultrashort-pulse lasers are used for laser ablation. It is, therefore, desirable to explore the feasibility of laser ablation using a CW single-mode Yb-doped fiber laser which was originally designed for macro cutting and welding to extend its processing capabilities. There are two main categories of laser ablation. The first is thermal ablation in which the laser interacts with the surface of the specimen creating a molten pool and vapour. The exiting vapour then creates a recoil pressure which pushes the melt radially outward from the center of the beam to create a cavity (Charschan, 1993).
∗
Corresponding author. E-mail address: [email protected] (J.F. Tu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.06.062
The second category of laser ablation is non-thermal or “cold” ablation. Non-thermal ablation requires extremely small pulse widths, usually 10−11 to 10−12 s and smaller. The short-pulse width is short enough to evaporate or “blast” away material before heat can be conducted through the material, thus no melting occurs. Since no melting occurs there is no recast layer and little to no heat affected zone (Steen, 2003). Three parameters are critical to laser ablation. The first is peak power density, in W/cm2 . A large peak power density is essential for ablation to occur. Typically the peak power density is in the GW/cm2 range for laser ablation. The second parameter is pulse duration. Typical ultra-short-pulse lasers have a fixed amount of energy they can deliver per pulse. If the pulse duration is lengthened the peak power produced will be significantly less, therefore short-pulse lasers use pulse durations in the nanosecond, picosecond and femtosecond ranges
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23
that allow for large power. The third parameter is the repetition rate of the laser. Having a low repetition rate will increase the time needed for producing holes of desired depths. If the repetition rate is high enough the heat can be retained and the ablation will be more efficient (Friedrich, 1998). An additional parameter that is helpful in laser ablation is the beam quality of the laser beam. The beam quality affects the ability to focus the laser to achieve high power densities and affects the quality of the ablated hole. According to Rodden et al., when the beam quality is poor, a larger exit hole diameter is produced (Rodden et al., 2002).
2.
Initial feasibility check
To determine the initial feasibility of laser ablation using a 300 W single-mode CW fiber laser, the three ablation parameters (peak power density, pulse duration, and repetition rate) are examined. First, the pulse duration is examined. The laser used in this study is rated at 300 W CW output. Measurements using a photodiode sensor revealed that there is an initial transient spike of about 2 s that occurs at the beginning of a laser pulse before it stabilizes to the 300 W CW output power. According to the measurement, the spike is about 4.5 times the steady state. Fig. 1 shows the case in a 55 s pulse. The top curve is the signal that is sent to the laser to enable emission, while the second curve displays the laser output detected by the photodiode. The figure lists the CW output as 150 W, which is lower than the rated output, and this is explained below. Similarly, Kleine et al. (Kleine et al., 2002) measured the pulse shape of a 50 W Yb-doped fiber laser and also found that for a 100 s pulse, there was a 1–2 s spike that was of 2–3 times higher power than the CW rated output. There is a major difference in the spike produced by this fiber laser and a pulse produced by a short-pulse laser. In a conventional short-pulse laser, the pulse energy is specified first and then the pulse duration. Together, they determine the peak power of the pulse. However, in a fiber laser a spike with a predetermined peak power is produced. The total energy deposited by a fiber laser depends only on the pulse duration. If the fiber laser is modulated to produce a longer pulse, more
Fig. 1 – Pulse shape measurement. Input signal (top); initial spike and steady state signal (bottom).
Fig. 2 – Composite of various pulse widths (18, 17, 16, and 15 s).
energy is deposited. As shown in Fig. 1, a pulse longer than the spike duration will deposit energy at a higher peak power only during the spike duration and will deposit energy at the steady state power during the rest of the pulse. To enhance ablation, it is desirable to shorten the modulated pulse. However it was found that for the fiber laser used in this investigation, the pulse duration cannot be less than 15 s. As shown in Fig. 2, below 18 s, the spike decreased slightly for 17, 16, and 15 s and was nonexistent for 14 s. Compared with typical ultra-short-pulse lasers for laser ablation (range of fs and ps), the effective pulse width of 2 s is very long. As a result, the nature of laser ablation using this laser will be thermal ablation, similar to nano-second lasers. It is expected to see melt ejection and recast layers in the results. To determine the magnitude of the spike and the steady state output, a photodiode was used, with a 10 m pinhole over the sensor to prevent the sensor from being saturated. A steady state beam was generated and a portion was sampled by the sensor to gather reference data on the sensor’s output versus the laser power. Fig. 3 shows the conversion line for the photodiode sensor and there is seen a linear rela-
Fig. 3 – Conversion line of the photodiode sensor with a 10 m pinhole.
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tionship between the supplied power and the sensor output signal. Pulses were generated through an external circuit and the pulses were sent through an analog interface with the laser. A voltage was sent to the interface and the power was determined by the magnitude of the signal, with 10 V representing a programmed 300 W. Using Fig. 3 as the conversion guide, the actual laser output versus the programmed output is seen in Fig. 4. In the figure, the output power increased as the programmed power is increased, however, after 150 W programmed power, the steady state output remained constant at 130 W and the transient spike was constant at 530 W. This could be a result of how the laser is triggered by the external circuit. The peak power density depends on both the peak power and area of the focus spot. Using a 150 mm focusing lens, the smallest diffraction-limited spot size that can be achieved with this laser is 10.8 m, and the power density achieved is: Power density =
150 W /4(10.8 × 10−4 cm)
= 0.164 GW/cm2
(1)
Similarly, using a 100 mm lens, a diffraction-limited spot size of 7.2 m is achievable and using a 60 mm lens gives a 4.3 m spot size. The power densities available using those lens are 0.368 and 1.03 GW/cm2 , respectively. However, knowing that there is an initial spike in power of 530 W, the new power density for each of the lenses is 0.6, 1.35, and 3.78 GW/cm2 , respectively. These values are based upon diffraction limited spot sizes and the measured peak power. These peak power densities are large enough that ablation could result. The repetition rate of the laser is the third critical issue when considering ablation. A laser must be able to rapidly generate pulses in order for the process to be economical. This laser has a repetition rate of 25 kHZ, which yields a 40 s cycle time. This would allow for multiple pulses of 18 s to be generated. Table 1 below shows a comparison of the different parameters associated with the current laser, a nanosecond laser, a picosecond laser, and a femtosecond laser. This table is not exhaustive and was based on specifications from a few laser manufacturers. The process parameters of the nanosecond laser can be compared to the fiber laser more easily since they both are in the thermal ablation regime. Both the picosecond and femtosecond lasers are in the non-thermal ablation
Fig. 4 – Measured pulse output.
regime, therefore only their processing capabilities will be compared.
3.
Experimental apparatus
The laser used in these experiments was a 300 W CW Ybdoped single-mode fiber laser (IPG YLR-300). It has a M2 value of 1.04, and wavelength of 1075 nm. The overall experimental setup is shown below in Fig. 5. The aluminum structure and all associated equipment were located on a pneumatic optical table which isolated the equipment from vibrations in the environment. To focus the collimated laser, a 5× beam expander was used along with a 150 mm or 100 mm Gradium focusing optic. The diameter of the beam exiting the collimator was 4.5 mm and after expansion, was 22.5 mm. When focused through a 150 mm lens, the measured minimum spot size achieved was 11.5 m, and the measured minimum spot size for the 100 mm lens was 9.36 m. Using the peak power of 530 W, the power density for the 150 mm lens was 510 MW/cm2 , and the power density for the 100 mm lens was 770 MW/cm2 . A set of linear motors was used in an x–y configuration to move the specimens during experiments. A M6 tapped breadboard was mounted to the top of the linear motors for specimen fixturing. Specimens of 316 stainless steel with a
Table 1 – Typical laser specifications of different short-pulse laser systems Yb (100 mm lens) CW Pulse width (s) Energy (J) Peak power (W) Wavelength (nm) Beam mode (M2 ) Spot size (m) Fluence (J/cm2 ) Power density (GW/cm2 ) Removal rate Repetition rate
2.00E−06 1.10E−03 550 1075 1.04 7.2 2702 1.351 25 m/pulse 25 kHz
Nd:YAG Nano
Nd:YVO4 Pico
3.00E−08 3.00E−03 100,000 1064 <2 30 424 14.15 1.5 m/pulse 10 kHz
1.00E−11 4.00E−05 4.0E+06 1064 <1.2 40 3 318.3 0.3 m/pulse 50 kHz
Ti:sapphire Femto 1.50E−13 5.00E−04 3.3E+09 775 <1.3 150 3 18,863 0.1 m/pulse 5 kHz
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Fig. 6 – Mark diameters as a function of focal position (150 mm lens).
Fig. 5 – Experimental setup.
thickness of 0.95 mm were clamped to posts and held above the breadboard. The drilling process was controlled through a motion control program linked to a Dspace DS1104 R&D controller board that utilized Matlab for data acquisition and control. A Zeiss microscope was used to examine the specimens and images were taken from a camera port on the microscope. Imaging software was used to determine the diameter of the holes. Surface plots were also performed using two different instruments: a Talysurf profilometer to measure a 2D cross section of the holes and a Zygo NewView 500 Interferometric Microscope to render 3D analysis of the holes.
4.
Experimental procedures and results
4.1.
Optimal focusing tests for very long pulses
250 m increments, pulses of 50, 100, and 1000 s were fired at the specimen. Additional tests were also performed for multiple pulses at 50 s pulse widths. The results (Fig. 6) reveal that the diameter of the ablated mark increases for increasing pulse widths yet remains relatively constant for multiple 50 s pulses. These tests were repeated and verified using smaller 100 m increments and the minimum diameters for the 50, 100, and 1000 s pulses were 82.6, 99.2, and 170.8 m, respectively.
4.2.
A white light interferometric microscope was used to obtain 3D profiles of the ablated marks. Measurements (Fig. 7) revealed that the marks were not holes, but rather bumps were formed. This seems to be a result of the long (≥50 s) pulse width. Also, the power density may be too low to effectively ablate the material. Therefore, the 150 mm lens was exchanged for a 100 mm lens to obtain a higher power density and the pulse width was reduced below 50 s to investigate its effect on the process.
4.2.1. The initial objective of the experiments was to determine the optimal focusing position for ablation by finding the position where the diameter of the ablated mark was at its minimum. Using a 150 mm lens and adjusting the focal position in
3D specimen profile measurement
Long-pulse tests with 100 mm lens
The 150 mm lens was replaced with a 100 mm lens. The test to find the optimal focusing position was repeated for this lens and for pulse widths of 20, 30, and 40 s. The results are shown in Fig. 8 and show that the minimum diameters
Fig. 7 – Interferometric measurement of mark (50 s mark). Left: 3D image of mark; right: 2D profile of mark.
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Fig. 8 – Mark diameters as function of focal position (100 mm lens).
for the 20, 30, and 40 s pulses were 42.8, 55.0, and 68.4 mm, respectively. Again, the increase in diameters with the pulse width is seen here. Fig. 9 shows the spot images for the 20 s test at various focal positions. The marks correspond to the curve above for
the 20 s test at +0.45, +0.3, +0.15, 0, −0.15, −0.3, −0.45, and −0.6 mm from the focus. The mark generated at zero focus was then tested using the interferometric microscope and the Talysurf profilometer. In both measurements (Figs. 10 and 11), it shows a blind hole that is approximately 20 m deep. Fig. 12 shows the partial results for 30 s pulses at various focal positions. The depth of the hole at zero focus was about 19 m while the diameter was approximately 54.0 m. The measured diameter from the microscope was close to this at 55 m. The measurement from the Talysurf profilometer revealed that the hole depth was a little over 18 m with a diameter of about 60 m. Finally, the 40 s pulsing test was examined and the mark produced at zero focus did not produce a hole, but at −0.15 mm focus, a hole was produced. Further testing would have to be performed to determine whether focusing into the plane is more effective, but this is most likely due to a problem in the fixturing not holding the workpiece perfectly flat. As can be seen from Fig. 13 the 40 s pulse width created larger holes than the 30 s, though the roundness of them was better than the smaller pulse width tests. It is expected though that after multiple pulses the smaller pulse width schemes will produce holes of equal roundness. Measurements were able to show that the depth of the hole to be 18 m and the diameter of
Fig. 9 – Spot images for 20 s test.
Fig. 10 – Interferometric measurement of mark created by 20 s pulse. Left: 3D image of mark; right: 2D profile of mark (lines added for clarity).
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Table 2 – Summary of successfully drilled holes Pulse width (m)
Diameter (m)
20 30 40
Fig. 11 – Surface profile for 20 s pulse width hole.
the hole to be around 42 m which was close to the microscope’s value of 45.0 m. A summary of the results is shown in Table 2. Information from the above table was used to infer that the shortest possible pulse width would be the best choice. The 20 s pulse produced a hole with the smallest diameter and the largest material removal rate (MRR). The results are most likely due to the shorter pulse time allowing for less conduction. It appears that the larger pulse widths allowed for more heat conduction through the surrounding area which caused melting instead of ablation, thus some of the molten material was re-deposited in the hole. The 20 s pulse showed that it produced the smallest diameter hole with the greatest depth, but produced the largest recast layer around the mouth of the hole. The 30 s pulse produced a larger diameter whole, smaller depth and had less of a recast layer. The smaller depth and recast layer suggests that the longer pulse allowed from more melting around the perimeter of the hole which reduced the recast layer and slightly enlarged the diam-
40.1 55.0 68.4
Depth (m) ∼24 ∼19 ∼18
eter. The 40 s pulse produced a shallow hole with less of a recast layer. Finally for pulse widths of 50 s no hole was produced because of the excessive melting that occurred, creating a pool of molten material. After cooling, this pool created an area of less dense material which resulted in a bump instead of a hole. Some of these trends have been observed by similar research in that thresholds were found to exist for the removal of material (Yashkir, 2002). The threshold for the current setup was found to be somewhere in between 50 and 40 s at which there will be minimal to no material removal. This study also showed the same trend in increasing crater depth with decreasing pulse time and confirmed that there is a limit to the effectiveness of reducing pulse width. The information in this section presented results that showed that pulses under 50 s would produce indentations rather than raised surfaces. The information in Figs. 1 and 2 provide a better insight into the cause of this situation. After the initial spike, the power falls to the CW wave output, and therefore for longer pulse widths, the energy deposited is used for melting and recasting. The following section shows how minimizing the pulse width yields results consistent with predictions based in Fig. 2.
4.2.2.
Sub 20 s pulse tests
A series of shots were fired at the optimally focused position using pulse widths from 20 s down to 10 s. Marks were found on the specimen for pulse durations of 15 s and larger. Looking at Fig. 14, the 15 s pulse produced a hole with a depth of 8 m, whereas the 16 s pulse produced a hole with a depth of
Fig. 12 – Spot images for 30 s test.
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Fig. 13 – Spot images for 40 s test.
20 m. This is evidence to the presence of a spike that is at its peak between 15 and 16 s. Fig. 15 shows the enlarged images of the holes created in the specimen. The blind holes that were produced were measured on the microscope and measured by the interferometric microscope. Their depths and diameters are compared in Fig. 16. The result for the 18 s pulse gives the largest aspect ratio and is assumed to be the optimum pulse width. From the plot it shows that both the diameter and the depth of the blind holes tend to decrease in size with lowering pulse width. Shots less than 15 s do not create any marks and measurements show that a 14 s pulse does not generate any output. Therefore, the initial spike is a large contribution to the ablation process. Though the highest depth was achieved at a 20 s pulse, the highest aspect ratio from Fig. 16 was found to be at 18 s. This pulse width had only slightly less depth than the larger
pulse widths, but had a signification reduction in diameter which helped to increase the aspect ratio.
4.3.
Multiple pulse drilling
From the above results for ablation, it was desirable to view the result of drilling with multiple pulses on a specimen of stainless steel 100 m thick. The pulse width used in this instance was the 18 s that was determined to be the width producing the best aspect ratio. Helium assist gas was used to remove material as it was ablated. The reason for using the assist gas was that again, the feasibility of drilling was being tested. Therefore, since ablation was achieved without the assist gas, using the assist gas should help in drilling. Five pulses were fired at the specimen, and then the lens was refocused half the thickness of the specimen, 50 m, and then another five pulses were fired. This gave a hole with a 44 m entrance diameter
Fig. 14 – Profile for pulses. Top row: 15, 16, and 17 s; bottom row: 18, 19, and 20 s (lines added for clarity).
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Fig. 15 – Enlarged images for sub 20 s pulses.
Fig. 16 – Plot of hole characteristics for various pulse widths.
Changing both of these parameters allowed for the creation of a crater with less than 50 m in diameter. The resulting hole also contained a very small heat affected zone even though the pulse time was substantially larger than that of ns, ps, and fs-short-pulse lasers. The addition of the 100 mm lens and using the shortened pulse allowed for the power density to approach that of the typical nanosecond laser. Because they have higher order modes, or lower beam qualities, the short-pulsed lasers minimum spot size is much larger than the fiber laser. This can limit the size of the holes that they can create. The MRR produced for the single pulse of 20 s was very high at a value of 24 m. The typical MRR for short-pulse lasers is two orders magnitude smaller than the value obtained with the CW laser. Though the pulse times are much smaller for the short-pulse lasers, the time needed in between pulses is larger than what is needed for the CW laser. Therefore since the MRR is so much lower for the short-pulsed lasers, it would take somewhere
and an 18 m exit diameter. A second attempt was performed where again the assist gas was used, but seven pulses were fired at the specimen without refocusing. This method produced a hole with a 38.4 m entrance diameter and a 14.4 m exit diameter (Fig. 17).
5.
Discussion
From the results given it was determined that the spot size and pulse width can greatly effect the outcome. With a spot size of 11.5 m and a pulse width of 50 s, it was not possible to produce holes with the available power. Changing the lens allowed for a smaller spot size which in turn raised the power density, a key factor when trying to ablate the surface of a material. Finally, decreasing the pulse width caused a greater percentage of the pulse energy to come from the initial spike and also reduced the conduction through the material.
Fig. 17 – Thru-hole generated by multiple pulse drilling. Left: entrance; right: exit.
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around 100 pulses to create the same depth as 1 pulse from the CW laser. The off time needed for these short-pulse lasers creates a bottleneck making the overall process take longer.
6.
Conclusions
A feasibility study was conducted to determine if a fiber laser designed for use as a continuous source could be used to perform pulsed ablation. It was seen that the particular fiber laser used in this study produces a pulse that consists of an initial spike in power. The results show that laser ablation occurs when a 100 mm lens is used with pulse durations at 40 s or below. At 18 s pulse duration, a blind hole of 43.6 m diameter and 23.5 m depth can be created with little heat affected zone. This performance is comparable to nanosecond lasers, but with much higher hole depth per pulse. It was also found that the pulse duration must be less than 50 s so that the ablating effect of the initial spike is not nullified. At longer pulse durations (50 s or more), raised surfaces are created instead of holes. This seems to be an occurrence of spot welding and forms the basis for further research. Areas of further consideration: • Determining the magnitude of the spike through further power measurements. • Investigate the spot welding versus micro-drilling phenomena. • Investigate multiple pulse drilling.
Acknowledgements The research is supported in part by NSF Grant # CMS-0402857 and NSF Grant # DMI-0355481. The assistance of the NCSU Precision Engineering Center is also acknowledged.
references
Charschan, S.S., 1993. Guide to Laser Materials Processing, 1st ed. Laser Institute of America, Orlando, pp. 40–41. Friedrich, C., 1998. Laser Micromachining–Laser Ablation, Michigan Technology University Notes. Kleine, K.F., Whitney, B., Watkins, K.G., 2002. Use of fiber lasers for microcutting applications in the medical industry. In: Proceedings of Laser Institute of America, ICALEO 2002, 21st International Congress on Applications of Lasers and Electro-optics, Scottsdale, AZ, October 14–17. Rodden, W.S.O., Kudesia, S.S., Hand, D.P., Jones, J.D.C., 2002. A comprehensive study of the long pulse Nd:YAG laser drilling of multi-layer carbon fibre compostites. Optics Commun. 210 (3–6), 319–328. Steen, W.M., 2003. Laser Material Processing, 3rd ed. Springer, London, pp. 335–341. Yashkir, Y., 2002. Laser micromachining with passively Q-switched intracavity harmonic generator, Technical Digest of SPIE Opto-Canada: SPIE Regional Meeting on Optoelectronics, Photonics, and Imaging, SPIE Volume TD01, pp. 462–466.