Abstract:
Stripping is accomplished with a pulsed laser beam that targets the material and area to be stripped. The wavelength laser beam may be selected or adjusted to the absorption behavior of the material to be stripped without damaging the underlying base material. A control unit may operably adjust an optical system to guide the laser beam over the surface to be treated. a coating. This makes it possible to improve the surface stripping process.

Description:
[0001]     The invention relates to the application of specific laser treatment for the removal of electrically non-conductive surface protection layers of aircraft components so as to produce mass connection areas before establishing a connection with an electrical mass cable.  
         [0002]     Results of the tests carried out have shown that suitable laser treatment completely removes non-conductive surface layers, such as e.g. paint and oxide layers, from aluminium surfaces without thereby damaging the base material. The precipitation state and the corrosion resistance of the aluminium base material remain unchanged, and the electrical conductivity after laser mirroring is comparable to that of a non-coated aluminium surface. With controlled suction removal of dust residues that have not been vaporised directly, workplace health and safety regulations can be met significantly better than is the case with conventionally applied brushing or grinding processes. Furthermore, negative influences on the operator and on the materials can be considerably reduced.  
         [0003]     Below, the results are explained in the context of transverse sections, microhardness tests, corrosion tests (intercrystalline corrosion tests and salt-spray corrosion tests), REM and EDX analyses.  
       STATE OF THE ART/LITERATURE  
       [0004]     Aircraft are constructed in such a way that electrostatic charges do not result in any damage to or malfunction of electrical systems or of the structure. Good electrical conductivity both of the structure and of equipment components serves to bleed off electrical charges and to establish sound electrical mass connection of the electrical system. All the electrically conductive components such as e.g. the fuselage, wings, tail unit, flaps or tabs, landing gear, and engine fairings or cowlings have to comprise mass connection areas.  
         [0005]     In the past, electrical mass connections (mass connection areas) were produced by masking the respective aluminium surface by means of an aluminium foil prior to the application of a surface protection layer. After application of the surface protection layer, the protective foil was removed and, following cleaning of the surface, mass connection was established by means of a connecting shoe and a connecting cable. If during application of the surface protection layer non-conductive layers such as anodisation agent or paint found its way below the protective foil, these layers subsequently had to be removed by means of a fast-rotating brush or a grinding disc. These methods were time-consuming, and therefore uneconomical, and were associated with a disproportionate expenditure of money, time and energy; a factor which was also reflected in the quality, i.e. in the quantity of material that had to be removed, and in poor surface quality.  
         [0006]     Thanks to advances in laser methods over the past few years, laser-assisted processes are becoming increasingly interesting for applications such as e.g. the removal of layer systems. It is now possible to use lasers for cleaning processes or activation processes prior to the application of paint, or to use them for the removal of paint or layers in production, service and maintenance. Depending on the absorption behaviour of the layers to be removed, or of the surface impurities, excellent surface pre-treatment and quality can be achieved. Nonetheless, it is important in such applications to avoid any negative influence on the base material.  
         [0007]     Laser methods have the potential for use in the cleaning, activation and layer removal of aircraft components even in cases where the areas that have to be treated are larger than those that are necessary as electrical mass connection areas.  
       LITERATURE  
       [0008]     1. Schweizer, G.; Werner, L.: Industrial 2 kW TEA CO 2  laser for paint stripping of aircraft 10 th  Int. Conf. on Gas Flow and Chemical Lasers (GCL 94), Friedrichshafen Germany (1994)  
         [0009]     2. Bianco, M. et al.: Laser stripping of paints for aircrafts: A comparison between traditional and TEA superpulsed CO 2  lasers, Surfair XI, Cannes (1996)  
         [0010]     3. Kelley, J. D.: Flashjet coatings removal process—transition from development to production, Surfair X I, Cannes (1996)  
       OBJECT OF THE INVENTION  
       [0011]     For the purpose of clarifying the object of the invention,  FIG. 1  diagrammatically shows an electrical mass connection. The object is to produce a clean, flat, electrically conductive surface so as to ensure good contact with the mass connection cable.  
         [0012]     This object is met by a “laser mirroring” process for preparing electrical mass connection surfaces.  FIG. 2  shows the result of this, namely a mass connection surface.  
     
    
     DESCRIPTION OF THE INVENTION  
       [0013]     Tests have shown that with this process it is possible to remove non-conducting surface protection systems such as chromic acid anodising (CAA) layers or sulphuric acid anodising (SAA) layers, primers and topcoat paints. For comparison purposes, the ability to remove differently structured paint systems, e.g. solvent-based, water-based as well as chromate-containing and chromate-free systems, was tested.  
         [0014]     The base materials used for the investigations were conventional aluminium alloys (e.g. Al  2024 ) as used in aircraft engineering. The alloys tested were in plated and non-plated states and also in various commonly used heat treatment states. Tests were carried out both on suitable sheet material samples and on corresponding aircraft components.  
         [0015]     In order to experimentally determine the basic feasibility, various laser systems were used: a TEA-CO 2  and an Nd: YAG laser, both operated with pulsed laser beams. The laser processing parameters such as the average laser beam output, pulse repetition frequency, laser beam focus diameter on the component surface, scan parameters and process- and handling parameters were greatly varied in order to obtain an optimal result that was tailor-made for the aircraft components used.  
         [0016]      FIG. 3  diagrammatically shows the requirements which an electrical mass connection has to meet. In order to obtain a suitable surface for the mass connection, the non-conductive surface protection layer around the drill hole has to be completely removed. To ensure proper removal the non-conductive layer needs to have adequate capacity to absorb the laser radiation at the selected wavelength. In contrast to this it is advantageous if the base material has a high capacity to reflect the laser radiation.  
       RESULTS AND DISCUSSION  
       [0017]      FIG. 4  shows the treated surface following mechanical removal of the layer (mechanical mirroring) using a fast-rotating brush. This process not only removes the paint and the anodic oxidation but also the plating layer and the base material. By way of comparison,  FIG. 5  shows the surface and the transverse sections after treatment with TEA-CO 2  and Nd: YAG (Neodymium: Yttrium Aluminium Garnet) lasers. The figure clearly shows the excellent condition of the plating layer that is present without any damage following laser treatment. Even in microscopy examinations, both types of laser result in good plane removal of the paint without any visible damage to the plating layer. However, in the case of the CO 2  laser, residues were found directly at the transition between the treated and the untreated areas, which residues presumably are the result of optical interference which can occur with the use of a mirror-guided CO 2  laser. In comparison to this, the Nd: YAG laser showed a very good transition, without any residue, to the laser-treated area. Furthermore, the Nd: YAG laser provides the option of using a fibre coupling, which opens up the system&#39;s ability to mobile applications. For this reason an Nd: YAG laser with fibre coupling was selected for further tests.  
         [0018]      FIG. 6  shows scanning electron microscope images of the transition areas of mechanically-treated and of laser-treated components. Paint adhesion tests in the transition area did not show any impairment of paint adhesion around the area where removal had occurred.  FIG. 7  shows a transverse section of a fully laser-treated mass connection area. Again, an excellent surface finish and complete removal of paint is evident. The conductive plating layer (light-coloured area) remains completely undamaged. To prove that the very thin anodic layers of chromic acid (CAA) and sulphuric acid (SAA) respectively were removed by the laser, samples were made that were only coated with these anodic oxidation layers. Here again, the excellent removal of these layers by means of the laser and by means of correspondingly set laser parameters is evident (see  FIG. 8 ).  
         [0019]     By varying the laser radiation, beam scanning, as well as the process and handling parameters, a large spectrum of various removal results is obtained. From the point of view of economy, a low laser output of 120 watt and the option of extremely fast treatment and production of mass connection areas, combined with very good surface quality, are advantageous. Irrespective of the thickness of the layer, the absorption behaviour etc., it takes approximately 1 second to produce mass connection areas approximately 15 mm 2  in size.  
         [0020]     Laser mirroring is carried out by a pulsed laser beam that generates a specified size of the focal spot within a few nanoseconds. By way of an example,  FIGS. 9   a )  b ) and  c ) diagrammatically shows the variation of the tracking width and the spacing of the focal spot between the set focal spots. In addition to the tracking width, the spacing between focal spots, the number of passes, the direction of passes (horizontal and vertical) can be varied to treat layers uniformly and obtain a removal result which is optimised in relation to the components to be treated.  
         [0021]     Microhardness testing of the laser-treated materials shows no difference to virgin materials. To this effect, hardness impressions were made from the surface to the interior of the material on transverse sections ( FIG. 10 ).  
         [0022]      FIG. 11  shows the results of corrosion tests after a salt-spray corrosion test according to ISO 7253 lasting 168 h. While the mechanically mirrored samples showed clear signs of corrosion (both uniform corrosion and selective corrosion or pitting) due to the intact plating in the samples subjected to laser mirroring, no visible signs of corrosion were found.  
         [0023]     Based on the sensitivity of the precipitation state in the face of thermal influences, tests were carried out to determine the sensitivity to intercrystalline corrosion. These tests did not show any increased sensitivity of the materials to intercrystalline corrosion (i.e. no noticeable change in the precipitation state). Both samples, i.e. the laser-treated sample and a bright non-coated aluminium sample, show a comparable depth of intercrystalline corrosion of approx. 150 μm. These results were further confirmed in microscopic examinations at higher resolutions ( FIG. 12 ).  
         [0024]     EDX analyses, too, showed good removal of all surface impurities after completion of the laser mirroring process (compare  FIGS. 13   a  and  b ).  
         [0025]     The possibility of removing non-conductive surface layers was investigated in the context of producing electrical mass connection areas. The results of these investigations show that the production of electrical mass connection areas by means of a laser is feasible. Similar laser equipment can also be used for large-area removal of layer systems from various substrates. Similarly, plant engineering can be applied in the mobile use of an optical laser system, designed as a handheld system. Furthermore, with the use of a laser, the quantity of dust particles arising when compared to conventional brush or grinding methods is reduced, and controlled suction becomes possible. It should also be mentioned that no evidence was found of either physical or thermal damage to the components. Aspects such as surface quality and finish, productivity and environmental protection are additional beneficial factors.