Aluminum-skinned aircraft are typically chemical conversion coated or anodized prior to application of coating systems or adhesive bonding. The environmental risks and costs of managing millions of gallons of spent chemical processing solutions, which often contain hexavalent chromium, other hazardous air pollutants, and/or cyanide as hazardous waste, need to be reduced.
The main objective of this project was to demonstrate the use of a one-step process at ambient conditions based on laser-interference structuring (LIS) techniques to replace the chemical conversion coating or anodization prior to the application of coating systems or adhesive bonding for Aluminum (Al) and Titanium (Ti) aircraft components. The goal of this project was to develop an understanding of the effects of LIS on the surface microstructure, topology, and physical mechanisms that would improve adhesion and corrosion protection of Al2024-T3.
The technical approach was based on the use of the LIS technique as a noncontact, i.e., without major solid/liquid medium application or abrasion, and non-chemical surface preparation method for aerospace coating systems. Specifically this project investigated the effect of LIS on: (a) enhancement of coating/paint adhesion and (b) corrosion resistance of the coated or bonded substrate. The laser-interference technique was used to structure surfaces of Al and/or Ti, creating periodic "rough" surfaces with pre-engineered series of ridges and valleys at submicron scale. The laser-interference power profile is created by splitting the beam and guiding those beams to the sample surface by overlapping each other with defined angles to each other.
Microstructure analysis indicates that the LIS was found to reduce the formation of CuMn-rich precipitates in Al 2024-T3 over a 500-800 nanometer depth from top surface. The precipitate dissolution is expected to lead to an increase in corrosion protection of the laser-interference treated surface as the localized corrosion would be reduced. The American Society for Testing and Materials (ASTM) D3359 X-cut and cross-hatch coating adhesion ratings indicate that the LIS specimens meet the performance requirements in the coating adhesion specifications by having a higher or identical ranking to those specimens prepared with current state-of-the-art chemical conversion or sulfuric acid anodizing. After the ASTM B117 corrosion exposure, it was found that the laser processed specimens exhibited only few blisters. It was found that the corrosion damage was minimized at a laser rastering speed of 4 millimeters/second, for which only 33% of specimens developed very minor corrosion damage. The ASTM D1654 creepage ratings, used to evaluate corrosion damage along the scribe lines, were found to be at least nine for all coated panels. These results indicate that the laser-interference technique with the additional acetone wiping has the potential to be further developed as a minor chemical surface preparation technique for chromate-containing epoxy primers coatings.
The single-step and non-chemical laser-interference processing has the potential to drastically reduce the environmental impact of chemical surface treatments used in the manufacture and maintenance of Department of Defense (DoD) weapons systems. Moreover, the environmental risks and costs of managing millions of gallons of spent chemical processing solutions as hazardous waste also will be greatly reduced. The wealth of data generated in this project provides the DoD community the basis for developing, optimizing, and transitioning these non-chemical, laser-based surface treatments. Specifically, the extensive microstructural characterization of surfaces/sub-surface, coating adhesion testing results, and corrosion resistance testing results which were obtained for a surface structure periodicity of 1.7 micron provides a strong scientific basis for understanding the specific chemical, morphological, and microstructural changes induced by the laser-interference that affect the surface adhesion and enhance corrosion protection. Additional work is needed to fully utilize the knowledge generated in this project by exploring other structure periodicities that that explored in this pioneering study using novel, much more powerful, and high-productivity +50kHz lasers.