Laser removal of aircraft coatings

James J. Arthur, Thomas A. Naguy, and Debora A. Naguy

Commercial technology and rigorous testing improve processes

The U.S. Department of Defense (DoD) oversees major laser coating removal operations for depot maintenance facilities that repair, test, and remanufacture most of the weapon systems and support equipment possessed by the U.S. military. The current coating removal methods utilized by these depots are costly, time consuming, labor-intensive, and result in undesirable environmental conditions. Because of these concerns, all branches of the DoD involved in coatings removal operations are concerned with the identification of alternative methodologies focused primarily towards the elimination of chemical paint strippers, media blasting, and hand sanding. In response to this need, the U.S. Air Force Research Laboratory Environment and Energy Program Office (AFRL/RXSC) identified laser coating removal (LCR) as a potential replacement technology based on its ability to eliminate the use of chemicals and to substantially reduce waste streams.

Small area applications

AFRL/RXSC began investigation into LCR during a collaborative project with the Environmental Security Technology Certification Program (ESTCP) to test the ability of portable laser coating removal systems to remove organic coatings from small areas of components. Three handheld systems – 250W TEA CO₂, 40W Nd:YAG, and 120W Nd:YAG – were evaluated for removing a variety of coating combinations from aluminum, steel, honeycomb, and composite substrates. Engineering tests were conducted to assess coating removal rate, physical substrate damage, and impact on paint adhesion. The results of these tests were compared to baseline data and data available in literature for conventional coating removal techniques. Results showed the three systems' techniques were comparable to the coating removal techniques currently in use by the DoD. A full report on the testing conducted is available from the ESTCP (www.estcp.org). Based on these results, 120W and 500W Nd:YAG systems have been implemented at all three Air Force Air Logistics Centers (ALCs) for use in small area operations on components removed from aircraft during depot operations. FIGURE 1 shows the 500W Nd:YAG laser in use at an ALC.

Large area applications

The successful outcome of the handheld program allowed AFRL/RXSC to take the next step in the evolution of the technology and apply it to large components processed separately from the aircraft during depot maintenance. In order to accomplish this, the robotic laser coating removal system (RLCRS) project was initiated as a joint project with AFRL/RXSC, ESTCP, and the Oklahoma City ALC (OC-ALC) at Tinker AFB. This project involved development of a fully automated system for processing components such as ailerons, rudders, and aircraft doors.

This new system, shown in FIGURES 2 and 3, integrated advanced LCR technology with a fully automated robotic system. An important aspect considered in the design of this system was the desire of the Air Force to use commercially available components that had a demonstrated ability to be operated in industrial applications. Individual components that were selected for use in the RLCRS include a 6 kW CO₂ slab laser from Rofin-Sinar (www.rofin.com), a large gantry from PAR Systems (www.par.com), a beam delivery system developed by American Laser Enterprises (www.a-l-e.net), a laser scanner from Scanlab (www.scanlab.de), and a TEKA (www.teka.me) particulate capture system.

FIGURE 2. RLCRS system.

Another important aspect of this system is the non-contact contour-following system developed by BRIC Engineered Systems (www.bricengineeredsystems.com). This contour-following system allows the robot to automatically process any part that fits within the operating envelope of the gantry without performing any prior path programming or specific part set-up. Sensors placed slightly ahead of the coating removal end effector scan the surface to develop a three-dimensional map of the part surface in real time and adjust the robotic motions accordingly. Additionally, parts are placed on a semi-automated cart that allows for components up to 30 feet in length to be indexed through the operating envelope of the gantry robot.

Validation of this system involved engineering evaluation of test coupons to ensure that the process did not damage the part substrate. The results of these tests were compared to baseline data and demonstrated that laser coating removal using the RLCRS causes no degradation in physical properties to the aerospace substrates of interest. Additionally, testing performed on a variety of condemned KC-135 components (ailerons, rudders, landing gear doors, elevators, and flaps) validated the system's ability to effectively process the various geometries encountered in production operations. Almost all wastes associated with the current chemical removal process were eliminated by the implementation of this technology. The only wastes remaining are the removed coating itself, which is captured in filters, waste water from rinsing the parts after coating removal, minor masking materials, and personal protective equipment. This system has been installed at OC-ALC and is awaiting final engineering approval for production usage. A full report on the testing and results is available from the ESTCP (www.estcp.org).

Laser automated decoating system II

The successful development of the RLCRS system and its use of commercially available components were then leveraged by the Ogden ALC (OO-ALC) at Hill AFB to enhance their coating removal capabilities. As part of the depot maintenance performed at this facility, it is necessary to de-paint the nose radome prior to inspection, repair, repainting, and recalibration procedures. OO-ALC has used several different methods to de-paint these radomes. Chemical stripping was traditionally used, but was later replaced by the original laser automated decoating system (LADS). The LADS was a custom laser coating removal system designed and built by a small specialty manufacturer specifically to de-coat radomes. This system was installed at OO-ALC in 1995 and proved to be an effective coating removal tool, but eventually it became outdated and expensive to maintain. This system was ultimately decommissioned in 2008 and all radome processing was returned to chemical stripping methods.

FIGURE 3. RLCRS system stripping a KC-135 rudder.

AFRL/RXSC was tasked by OO-ALC to develop a replacement system for the LADS. The requirements for the replacement system were that it must be able to process A-10, F-16. and C-130 radomes. and, in order to maximize system usage, it was required to be capable of processing various other parts that are removed from the aircraft during depot maintenance. Additionally, the system was required to use only commercially available, industrially proven components to minimize long-term maintenance costs and repair times.

Similar to the RLCRS, LADS II, shown in FIGURE 4, consists of several subsystems integrated into a fully automated system. The individual components include an 8 kW CO₂ Rofin-Sinar laser, an American Laser Enterprises beam delivery system, a Scanlab laser scanner, and TEKA effluent removal systems. A custom hollow robotic arm with three pivoting axes and two rotating axes was designed and manufactured by BRIC Engineered Systems for this application. The robotic arm is mounted on a 30-foot long rail to allow for processing of long parts. The innovative BRIC Engineered Systems contour-following technology originally developed for the RLCRS was also implemented into this robotic system.

Since implementation at OO-ALC in April 2009, the LADS II has proven to have numerous advantages over the original LADS and chemical stripping. When compared to chemical stripping, the LADS II reduces ESOH concerns by eliminating chemical strippers and their hazardous wastes. Additionally, this system is able to fully strip an F-16 radome in 30 minutes, a process that could take up to 16 hours to perform chemically.

When compared to the original LADS system, the LADS II demonstrated improvements in power consumption and efficiency, gas consumption, and maintenance costs. The LADS II also demonstrated significantly faster strip times than the original system, which took six hours per radome. The drastically improved strip times over either of the previous methods will allow for a reduction in total processing time for radomes and for more parts to be processed through the system. Because the LADS II was designed to process parts as well as radomes, it is expected to have a full workload versus the 20% workload the original LADS system was able to process. The savings will multiply rapidly as additional radome and part workload is transitioned to the LADS II system.

Full aircraft laser coating removal

After the successful implementation of these previous systems into USAF operations, AFRL/RXSC has initiated a project to design, demonstrate, and validate an automated system capable of selective removal of coatings from the F-16 aircraft. This system will replace the current plastic media blasting method used to strip these aircraft. This system will be assembled and demonstrated at Concurrent Technologies Corp. (www.ctc.com) in Johnstown, PA, on a condemned F-16 and then will be transitioned to OO-ALC for production usage. This project will advance the work that has been done in prior AFRL/RXSC laser coating removal efforts and use it in the application where the technology has the potential to provide the largest benefit in environmental savings as well as depot processing efficiency. This project will build upon the principles that have been previously demonstrated, but will utilize the most recent advancements in laser technology, process control, and robotic manipulation to provide a state-of-the-art system.

FIGURE 4. LADS II system.

The aircraft coating removal system that will be developed will not be specific to the F-16 platform and will be scalable to other larger aircraft. The capabilities of the robotics will be designed specifically so that processing of larger aircraft will be possible with the goal being eventual transition to the C-130 platform. This project is currently in the final stages of system design.

Summary

The AFRL/RXSC laser coating removal program has been successful in transitioning laser technology into depot operations by utilizing a step-wise approach based on combining commercially available technologies with rigorous testing and validation. By applying these principals to every step of the transition process, AFRL/RXSC has been successful in moving laser technology into depot operations where it has not been traditionally applied. This transition to laser coating removal has provided the USAF with a reduction in the environmental burden and costs associated with coating removal operations as well as an improvement in overall process times.

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