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Novel route to high quality ablation in a range of materials with a 400 w single mode continuous wave fiber laser
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Novel route to high quality ablation in a range of materials with a 400 w single mode continuous wave fiber laser

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Novel route to high quality ablation in a range of materials with a 400 w single mode continuous wave fiber laser white paper from JK Lasers.

Novel route to high quality ablation in a range of materials with a 400 w single mode continuous wave fiber laser white paper from JK Lasers.

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  • 1. Novel route to high quality ablation in a range of materials with a 400W single mode continuous wave fiber laser Mohammed Naeem, Steve Keen GSI Group, Laser Division, Cosford Lane, Swift Valley, Rugby, CV21 1QN, UK Abstract Low power (up to 500W) single mode Ytterbium fiber lasers operating at 1080nm with a very good beam quality (M2 ~ 1.10) are routinely being used for a range of micromachining applications The high brightness of the fiber laser enable high power densities even at modest power levels, which is sufficient for cutting a range of thin metals, capable of welding of various materials including high reflective material and also drilling small holes in metals including aerospace alloys. To date very little work has been carried out with these lasers for ablation applications. The laser ablation of metals is normally carried out with Q-switched pulsed lasers ranging from microsecond to femtosecond pulse durations, pulse frequencies up to 50 kHz and extremely high peak powers (MW). In this work, laser ablation of a variety of materials including TBC superalloys used for aerospace has been demonstrated with a single mode fiber laser up to 400W. The paper will investigate the material removal rates and ablation quality. Key words: Laser ablation, material removal rates, spot size, peak power, frequency, average power, scanning speeds, cracks, TBC superalloys, stainless steels, pulse shape. . flash-lamp, without reducing the lifetime of the device to an un-acceptable level. 1. Introduction The low power fiber lasers are very compact and robust and has an edge over lamp pumped Nd: YAG lasers in terms of beam quality and wall plug efficiency (approx 20%). Current investigations [1-3] show that the single –mode fiber laser is an efficient, reliable and compact solution for microcutting and micro joining. The diodepumped technology offers low maintenance cycles and high conversion efficiency. Theoretical pump- light conversions of more than 80% are possible [4] but typical optical conversion efficiencies for Ytterbium double- clad fiber lasers are 60-70% [5]. An area where there is a significant difference between lamp-pumped YAG and fiber laser performance is pulsed operation. Lamp-pumped lasers are capable of producing long, multi-ms, pulses with peak powers many times the rated average power of the laser, provided that the duty cycle is sufficiently low. This ability stems from the flash-lamp itself which is often more constrained by the maximum average thermal load than the peak power output. By contrast, while the semiconductor laser diodes used to pump a fiber laser can be on-off modulated over a wide frequency range as shown in Fig. 1 (from DC to tens of kHz in most industrial applications), they cannot typically be overdriven for long periods (multi-ms), in the same way as a 200 180 160 140 Power (W) 120 100 80 60 40 20 0 0 100 200 300 400 500 600 700 50 55 60 Tim e (m s ) 200 180 160 P ower (W) 140 120 100 80 60 40 20 0 25 30 35 40 45 Time (micro-s) Initial pulse spike expanded view duration around 1 microsecond Fig. 1: Fiber laser- Modulation characteristics 1
  • 2. From an applications perspective this regime can enhance laser material processing in terms of processing speed, weld penetration and cut quality [6]. This paper describes the use of different modulation regimes with the JKFL200 to achieve high material removal rates. The modulation regime can improve the removal rate by X8 compared to running the laser in a purely continuous mode (CW). Typical pulse with relaxtion oscillation 600 500 Power W 400 300 200 100 2. Experimental work 0 The ablation tests were carried out with a with a 400W single mode Ytterbium fiber laser (Fig.2) operating at 1080nm wavelength emits a gaussian beam with an M2 < 1.10 (Fig. 3). The laser was fitted with a GSI scanning head and the results reported are at an average power 200W. 30 35 40 45 50 55 60 -100 Time use cs Relaxation spike, 600nsecs duration, X4.2 pk enhancement CW level (continues constantly for as long as requested) Fig.5: Typical output waveform with relaxation pulse Period T Relax pulse X4-5 600nsecs Fig.2: GSI 400W Single mode fiber laser Ton 20usecs Fig. 6 shows energy calculations in the pulse waveform. Energy in relax pulse = peak power x duration = 600nsec X 800W = .48mJ Fig. 3: Beam profile of a 400W SM laser Energy in rest of the pulse = Ton x average power = Ton X 200 The laser was operated CW and modulated with a 2035usec off time at a frequency of 5 kHz + and typical drive waveform is highlighted in Fig. 4 and pulse is shown in Fig. 5. Fig. 7 shows the estimated percentage the relaxation pulse makes up of the entire waveform and the average power variation with frequency for a 20usec pulse. The graph shows that for frequency of 5 kHz the relaxation pulses only make up 1.3% of the total energy, whereas for 35 kHz frequency it is 28%. 20-35µsecs 28-200usecs (5-35 kHz) Fig.4: Turning the laser off produces a relaxation pulse typically X4-5 the CW level. 2
  • 3. Avergare pow er and percentage of total output for the modulated fibre laser 20usec off time 30 200 Relax pulse percentage 180 Total av e rage powe r 25 160 140 20 120 100 15 80 10 Percentage relax cf overall Average power 60 40 5 20 0 0 0 10 20 30 40 Fre que ncy kHz Fig.7: Average power and % of total power for the modulated fiber laser 20µsecs off time 76mm collimating lens Fiber output 100mm collimating lens Fig. 8: Optical set up for ablation Fig.8 shows the optical set up for the ablation experiment. A 4mm square was repetitively scanned with the pattern below: The spot sizes used during these tests are highlighted in Tab 1. Tab.1: Spot size and intensities used for the tests Output 28MW/cm2 10MW/cm2 Modulated 3 50µm spot 200W CW Speed 1250mm/sec Step 0.24 30µm spot 112MW/cm2 40MW/cm2
  • 4. 3. Results and Discussion Stainless steel The metal was exposed for 26seconds and weighed before and after to quantify the amount of material removal. Fig. 9 shows mass removed as a function of average power and modulation frequency respectively. 200W, CW, 26 seconds, 50 passes, and 10mg removed The square is very drossy, not clearly defined and full of reformed material Mass removed as function of frequency Mass of stainless removed vs frequency 90 80 70 M ass m g 60 50 40 30 20 165W average, 800W peak, 5KHz 165µsec on time and 80mg remove, much shaper defined edges compare to CW 10 0 0 5 10 15 20 25 30 35 Fre quency kHz Mass removed as function of average power Mass of stainless removed vs average power 90 80 M ass rem oved 70 60 50 40 165W average, 800W peak, 15KHz 55µsec on time and 80mg remove, the photograph doesn’t illustrate clearly but this was the best finish with the assist gas 30 20 10 0 200 165 160 165 150 150 138 126 Average powe r Fig.9: Mass removed in 2mm stainless steel Results show that mass of 80mg in 26 seconds, which corresponds to 184mg/min or given the density of stainless steel 8030kg/m3, 8.03mg/mm3. This gives material removal rate of 23mm3/min. The results also show that in the modulated regime the mass removed is 8x greater than that achieved with a pure CW output. The average power is decreasing due to altering the frequency of the off time of the laser. The increased material removal rate is due the high peak power in the relaxation pulse. Fig.10 shows photographs of the ablated surfaces at different laser parameters. 165W average, 800W peak, 15KHz 55sec on time and 80mg remove, the photograph doesn’t illustrate clearly but this was the best finish with argon shield gas Fig 10: Ablated surface of 304SS at different laser parameters 4
  • 5. TBC Coated materials Thermal barrier coatings are being widely applied in many types of engines and in aircraft's gas turbines. To increase temperature capability of the engine blades and vanes, a thin coat of a heat-insulating zirconia ceramics is applied on the surface of the blades as a thermal barrier coating. The cooling of the components causes a pronounced reduction of the metal temperature, which leads to a prolongation of the mechanical component's lifetime. Alternatively, the use of thermal barrier coatings allows raising the process temperature, obtaining thus an increased efficiency. Thermal barrier coatings usually consist of two layers (duplex structure). The first layer, a metallic one, is the so-called bond coat, whose function is, on the one side to protect the basic material against oxidation and corrosion and, on the other side, to provide with a good adhesion to the thermal insulating ceramic layer. Such a ceramic coating is mostly made of yttria partially stablilised zirconia (YSZ), since this material has turned out particularly suitable during the last decades. At present, there are two principle methods to apply thermal barrier coatings, one is plasma spraying and the other is electron beam physical vapour deposition (EBPVD). These methods have been studied excessively to avoid mechanical and adherence problems between coatings and substrate. 800W peak, 15 KHz, 26 seconds 800W peak, 15 KHz, 12 passes Current practice to drill TBC materials is to remove the coating with either a Q-switched pulsed Nd: YAG laser (short pulse width and high peak power) or use pulsed fiber laser. In this work a CW fiber laser was used to remove the TBC coating on aerospace alloy. Fig. 11 highlights the results on the coated and Fig 12. Shows uncoated aerospace materials 800W peak, 15 KHz, 20 passes Fig 11: Ablated surface of 2mm thick Haynes alloy with 0.5mm thick TBC at different laser parameters 200CW, 26 seconds, very massy with excessive cracking Fig. 12: 3mm HastalloyX alloy, 20 and 40 passes, 15 kHz, 800W peak 5
  • 6. 4. Summary Biography The ablation work with a 200W CW fiber laser has shown that: • Modulated regime can improve the material removal rate in stainless steel by X8 • Removal rate of 184mg/min equiv to 23mm3/min cf Powerlase paper which quotes 10mm3/min for a Q-switched laser • Better finish in the modulated regime • Dr. Mohammed Naeem is Materials Process Development Group Leader. He received an MTech degree in metallurgical quality control from Brunel University (UK) in 1981 and a Ph.D. in glass fibre composites from Loughborough University of Technology (UK) in 1985. He has over 18 years of experience in the support of industrial lasers with GSI Group, Laser Division and has published over 150 papers on laser material processing. He has previously served as Materials Processing Manager and held several Important Engineering Development roles. Removes TBC coating cleanly 5. References [1] Naeem M; Micromachining with a Single Mode 100W Fiber Laser, Conference Proceeding LAMP 2006, Kyoto Research Park, Kyoto, Japan; May 16-19, 2006 [2] Naeem M; Lewis S; Pulsed Nd: YAG laser or continuous wave fiber laser for micromachining? Proceedings of the Fourth International WLTConference on Lasers in Manufacturing 2007, Munich, June 2007 [3] Naeem M; Microwelding performance comparison between a low power (125W) pulsed Nd: YAG laser and a low power (100-200W) single mode fiber laser, Conference Proceeding PICALEO 2008, Beijing, China; April 2008 [4] Reichel, V, Unger, S, Hagemann, H, Auerbach, M (2000); “8W highly efficient Yb-dopped fiber laser, Proceeding of SPIE, Volume 3889 [5] Nilsson, J, Grudinin, A.B, Turner, P.W (2000); “Advanced pulsed and CW high- power fiber laser” CLEO Proceedings [6] Naeem M; Modulation techniques for enhanced material processing using a single mode fiber laser; Conference Proceeding ICALEO 2008, Temecula, CA, USA; October 20-23 October 2008 6