A number of experiments were performed at the Air Force Research Laboratory to cut thick aluminum and steel with a chemical oxygen-iodine laser (COIL) [1,2]. A 10 kilowatt class device was used for cutting aluminum, stainless steel and carbon steel. Cut depths of 20 mm were obtained in aluminum, 50 mm in stainless steel, and 41 mm in carbon steel using an N2 gas assist and 5-6 kW of power on target. The same laser at the same power level produced a cut depth of 65 mm in carbon steel with an O2 gas assist; a low quality cut to a depth of nearly 100 mm in carbon steel was demonstrated. The photograph shows a 25 mm (1") thick section of stainless steel which was cut with a 6 kW beam at a rate of 0.23 m/min (9"/min) and a 10 mm (3/8") thick section of aluminum which was cut with a 9 kW beam at a rate of 0.3 m/min (12"/min). The pen in the foreground is 127 mm (5") long.
These experiments showed that COIL cuts carbon steel and stainless steel at approximately the same rate. When these data are compared with existing CO2 and Nd:YAG laser cutting data, it was found that for a given cut depth, power and spot size, COIL cuts steel approximately three times faster than a CO2 laser using an inert gas (N2) assist. COIL cutting speeds in carbon steel are improved by approximately a factor of three when a reactive O2 gas assist is used in lieu of an N2 gas assist. Nd:YAG lasers and COIL cut steel at approximately the same rate. A significant finding is that COIL cuts aluminum at approximately the same rate as a CO2 laser cuts steel with an inert gas assist.
A simple theoretical model of thick-section cutting was developed at Phillips Laboratory [1,2]. This model uses a lumped-parameter technique to relate the cutting kerf depth with various process parameters and can be used to predict scaled laser materials processing performance to very thick sections. Using thermophysical data for stainless steel, carbon steel and aluminum, this theoretical model produces excellent agreement with the COIL cutting data. The theory still needs to have a reactive term added to model O2 gas assist data.
The Nd:YAG laser is presently limited to approximately 5 kW of continuous power output, whereas the COIL has demonstrated scaling to the 40 kW level. The ability to transmit a COIL beam through a fiber optic at high power density was demonstrated in Japan [3]. The CO2 laser wavelength of 10.6 mm inhibits its beam from being transmitted through fiber optics. The potential ability to transmit a high power, short wavelength laser beam through a fiber optic is intriguing and suggests the potential for the COIL device to tackle high power laser industrial applications which other laser technologies may be incapable of handling. Possible industrial applications for which COIL may be well suited include the decontamination and dismantling of nuclear power facilities, ship building and repair, underwater welding, and heavy machinery manufacturing.
More recent experiments at AFRL were performed where a COIL was used for cutting aluminum, titanium, inconel and copper plates. In these recent experiments, the laser was operated with a stable resonator having an intracavity aperture to produce a circular COIL beam with very few transverse modes. The multimode focal spot diameter was calculated to be 0.24 mm. The new aluminum cut was of particularly high kerf edge quality. These COIL cutting data are compared with an existing theoretical laser cutting model [1,2]. Using thermophysical data for aluminum, titanium, inconel and copper, this theory agrees very well with the data. To test the versatility of the model, the effects of different assumptions are examined; different assumptions produced very little effect on model predictions at high cutting speeds and a small difference at very slow cutting speeds. Overall, the theoretical model provides good agreement with experiments for a wide variety of metals.
We give many thanks to the RADICL test team who assisted our efforts and made the experiments run smoothly. Testing was contributed by the Gas and Chemical Branch of the Air Force Research Laboratory, Directed Energy Directorate. The experiments were assisted by Air Force Office of Scientific Research (AFOSR) funding to the University of Central Florida and Air Force Small Business Technology Transfer (STTR) program funding to STI Optronics and the University of Illinois at Urbana-Champaign.
References:
1. A. Kar, J. E. Scott and W. P. Latham, "Theoretical and Experimental Studies of Thick-Section Cutting with a Chemical Oxygen-Iodine Laser (COIL)," J. Laser Applications, Vol. 8, No. 3, June, 1996.
2. D.L. Carroll and J.A. Rothenflue, "Experimental Study of Cutting Thick Aluminum and Steel with a Chemical Oxygen-Iodine Laser using an N2 or O2 Gas Assist," J. Laser Applications, Vol. 9, No. 3, 1997.
3. K. Yasuda, T. Atsuta, T. Sakurai, H.Okado, A. Hayakawa, and J. Adachi, "Study on material processing of Chemical Iodine Laser," in Proceedings of the 3rd JSME/ASME Joint International Conference on Nuclear Engineering, p. 1769, 1995.
[UIUC Aeronautical and Astronautical Engineering Department]