Abstract:
A method of manufacturing a protective edge for a blade, wherein a protective edge ( 30 ) made of an anodizable metal is provided, and that protective edge ( 30 ) undergoes a micro-arc oxidation electrolytic treatment. Protective edge ( 30 ) manufactured using said method.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a protective edge for a blade and a method for manufacturing said edge. The protective edge may be, in particular, a protective edge for a turbine engine blade. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of aeronautics, and more particularly in the field of airplane turbojet engines, decreasing the mass of the component elements of the turbojet engine is a constant concern. This concern has led to the development of fan or rectifier blades made of a composite material with an organic matrix, these composite blades being lighter than metal blades. 
         [0003]    The leading edge of composite blades is generally too sensitive to erosion and possible impacts (birds, grit, ice, sand, etc.) to be used without protection. It is therefore common to use a protective edge, made of metal or plastic, glued on the body of the blade. This protective edge then defines the leading edge of the blade. For example, patent document WO2013021141 describes such a protective edge. 
         [0004]    The adherence of the protective edge on the body of the blade is an essential aspect. To improve this adherence, according to one known method, a certain number of intermediate layers are deposited, e.g., a polyurethane film or an adhesion primer, between the body and the reinforcing edge of the blade. According to another method, the surfaces to be glued are prepared mechanically, for example by sanding or scratching. Although satisfactory, these methods are deemed too time-consuming, complex and/or expensive to manufacture. Furthermore, the aforementioned intermediate layers may contain certain substances considered to be harmful for the environment, and that are in the process of being phased out by international institutions (such as the REACH system in Europe). 
         [0005]    There therefore exists a need for a new type of protective edge that effectively protects the blade while being easy to secure on the body thereof. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    The invention relates to a method of manufacturing a protective edge for a blade, wherein a protective edge made of an anodizable metal is provided, and that protective edge undergoes a micro-arc oxidation electrolytic treatment. 
         [0007]    Micro-arc oxidation electrolytic treatment is a surface treatment technique, known in itself, that consists of submerging a substrate in an electrolytic bath and anodizing that substrate with a high voltage in order to reach the breakdown voltage of the layer of insulating oxide which is initially formed on the surface of the substrate in the first moments of the treatment. Micro-arcs are then initiated and move over the immersed surface of the substrate. This technique allows to create a coating on the surface of the substrate having a specific structure and specific physicochemical properties through the formation of an oxide layer which is made up of the component elements of the substrate, on the one hand, and the incorporation in the oxide layer of chemical species which are initially present in the electrolyte bath, on the other hand. At the end of the treatment, the obtained coating has a dense and hard structure in its core and a porous structure on its surface. This technique is also referred to in the literature as “micro-arc plasma oxidation treatment”, or more simply “micro-arc oxidation treatment”. 
         [0008]    Micro-arc oxidation is very different from the traditional anodization methods. It in particular differs from those methods by the large quantity of energy involved, which is necessary to reach the breakdown voltage of the oxide layer. 
         [0009]    The production of a surface coating by micro-arc oxidation leads to the formation of a specific coating, having a specific micro-structure quite different from that of a coating obtained by traditional anodization. 
         [0010]    Micro-arc oxidation does not require an input of metal to form the coating. The formed coating is obtained by oxidation of the metal (or metal species) present on the surface of the substrate undergoing the micro-arc oxidation treatment. 
         [0011]    In the proposed application, the protective edge is used as the substrate and the micro-arc oxidation treatment makes it possible to form, on the surface of the protective edge, a coating which is dense and hard in its core on the one hand, and porous on its surface on the other hand. The hardness of this coating allows effective protection of the blade against erosion or any impacts, while the surface porosity of the coating facilitates and improves the securing of the coating, in particular when it is secured by gluing, by providing good catching for the adhesive used. 
         [0012]    The protective edge may in particular be made of titanium, titanium alloy, aluminum or aluminum alloy. When the protective edge is made of titanium or titanium alloy, the coating formed on the protective edge is made up primarily of titanium oxide. 
         [0013]    To ensure protection for the blade, the protective edge is configured so that it can be glued on a blade body and thereby form the leading edge or trailing edge of the blade. 
         [0014]    In some embodiments, the protective edge has an outer face and an inner face opposed to each other, the outer face partially defining the aerodynamic surface of the blade, and the inner face being configured to be secured on the body of the blade. 
         [0015]    During the carrying out of the method according to the invention, the micro-arc oxidation treatment is normally applied on all of the external surfaces of the protective edge, and therefore on both the outer face and the inner face. 
         [0016]    However, the requirements in terms of surface properties are different on these two faces of the protective edge of the blade. 
         [0017]    On the outer face, the protective edge of the blade must preferably both be very smooth for aerodynamic reasons, and have a large resistance to wear in order to withstand impacts with the particles present in the atmosphere. 
         [0018]    The surface coating obtained by micro-arc oxidation generally has a certain roughness. Thus, a priori, such a coating does not seem at all appropriate for a surface having to constitute the leading edge or trailing edge of a blade. 
         [0019]    Or, at least, when this coating has the necessary roughness to allow gluing of a part, this roughness then generally tends to make the coating incompatible with aerodynamic requirements, which on the contrary demand that the surface be especially smooth. 
         [0020]    Consequently, in one embodiment, advantageously, the outer face of the protective edge is polished after the micro-arc oxidation treatment. This polishing then allows the outer face to achieve a roughness level compliant with a certain level of aerodynamic requirements. 
         [0021]    The polishing may in particular be done such that the roughness level of the outer face of the protective edge is less than 2 μm, and preferably less than 0.8 μm. 
         [0022]    Furthermore, after polishing, the dense and hard structure of the coating is on the surface. Advantageously, this part of the coating has a particularly high wear resistance. 
         [0023]    Conversely, on the inner face of the protective edge, the surface layer of the coating formed by micro-arc oxidation has surface porosity and inner face roughness qualities which are entirely adapted to the gluing of that face on the blade. Consequently, this surface layer may be retained in order to favor the physicochemical adhesion (e.g., the gluing) of the protective edge on the body of the blade. In particular, the inner face may preferably not be polished after the micro-arc oxidation treatment. The composition of the electrolyte bath used during the treatment may also be chosen so that the porous surface of the inner face incorporates specific chemical species favoring the physicochemical adherence. 
         [0024]    After the micro-arc oxidation step, optionally followed by a polishing step, the protective edge is glued on a blade body. The protective edge thereby forms the leading edge or trailing edge of the blade. 
         [0025]    In certain embodiments, the micro-arc oxidation electrolytic treatment comprises the following steps:
       the protective edge is immersed in an electrolyte bath, the protective edge forming a first electrode,   a second electrode (also called counter electrode) is immersed in the electrolyte bath, and   a voltage is applied to the first and second electrodes.       
 
         [0029]    Preferably, when this voltage is applied, a current is imposed. In other words, the micro-arc oxidation operation is controlled, in particular by regulating the intensity of the imposed current. 
         [0030]    Preferably, the applied intensity has pulses, in particular when the protective edge is formed of titanium or a titanium alloy. Preferably, the frequency of these pulses is lower than 50 Hertz. 
         [0031]    The second electrode may be arranged in the electrolyte bath facing the inner face of the reinforcement, so as to facilitate the treatment of that inner face, which generally has a geometry that is less favorable to the treatment than the outer face. Typically, the inner face of the reinforcement is concave (i.e., hollowed out), while the outer face is convex (i.e., cambered). 
         [0032]    The reinforcement may have a substantially U-shaped cross-section so as to be positioned straddling the body of the blade. This reinforcement may have a base which constitutes the thickest part of the protective edge. The outer face of this base may define the leading edge (or the trailing edge) of the blade. This base may be extended by two side flanks respectively situated on the pressure side and the suction side of the blade. The outer faces of these flanks respectively define, in part, the pressure side and suction side faces of the blade. In cross-section, the profile of these flanks may become thinner moving away from the base. The protective edge may be secured on the body of the blade, over all or part of the height thereof. 
         [0033]    The invention also relates to a protective edge manufactured using the method described above and a blade comprising such a protective edge. The blade may be a turbomachine blade, in particular an aeronautical turbomachine fan blade (e.g., of an airplane turbojet engine). The blade may also be a propeller blade or vane. 
         [0034]    The blade may comprise a body, or central part, made of an organic matrix composite material. The blade may, for example, be a composite blade obtained by draping a woven material. Still as an example, the composite material used may be made up of an assembly of woven carbon/plastic fibers and a resinous matrix (e.g., a matrix of epoxy resin, bismaleimide or cyanate-ester), the assembly being formed by molding, for example using a vacuum resin injection method of the RTM (Resin Transfer Molding) type. 
         [0035]    The protective edge may be secured on the central part, in particular by gluing, and may define the leading edge or trailing edge of the blade. 
         [0036]    The aforementioned features and advantages of the invention, as well as others, will appear upon reading the following detailed description of embodiment(s) of the invention. This detailed description refers to the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The appended drawings are schematic and are not to scale; their primary purpose is to illustrate the principles of the invention. 
           [0038]    In these drawings, from one figure (FIG) to another, similar elements (or parts of elements) are identified using the same reference signs. 
           [0039]      FIG. 1  is a side view of a blade comprising a protective edge. 
           [0040]      FIG. 2  is a partial view of the blade of  FIG. 1 , in sectional view along cross-sectional cutting plane A-A. 
           [0041]      FIG. 3  is a general view of a micro-arc oxidation electrolytic treatment installation. 
           [0042]      FIG. 4  is a schematic view showing the intensity pulses applied in one embodiment of the invention. 
           [0043]      FIG. 5  shows, schematically, in sectional view, the structure of an example coating created using the micro-arc oxidation electrolytic treatment on the substrate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]      FIGS. 1 and 2  show a turbomachine engine blade  10 . The blade may be a fan blade of an airplane turbojet engine. This blade  10  is intended to be situated in the stream of fluid flowing through the turbojet engine. Upstream and downstream are defined relative to the normal flow direction of this fluid. 
         [0045]    The aerodynamic surface  12  of the blade extends, from upstream to downstream, between a leading edge  16  and a trailing edge  18 , and along a longitudinal direction  20  between a shank  22  and an apex  24 . The blade  10  is secured by its shank  22  to a rotating support disc, which is shared by several blades. 
         [0046]    The suction  13  and pressure  11  faces are the side faces of the aerodynamic surface  12  of the blade that connect the leading edge  16  to the trailing edge  18 . 
         [0047]    The blade  10  comprises a body  9  on which a protective edge  30  is secured by gluing. This protective edge  30  extends over the entire height of the aerodynamic surface  12  of the blade  10 , along the longitudinal direction  20 . The protective edge  30  has an outer face  31  and an inner face  32  which are opposed to each other. The outer face  31  of the protective edge  30  defines the leading edge  16  and part of the suction  13  and pressure  11  faces. The rest of the suction  13  and pressure  11  faces and the trailing edge  18  are defined by the body of the blade  9 . The inner face  32  of the protective edge  30  is in contact with the body  9 . 
         [0048]    The protective edge  30  has a substantially U-shaped section and is positioned straddling the edge of the body  9 . This reinforcement has a base  39  that is the thickest zone of the reinforcement and that defines the leading edge  16 . This base  39  is extended by two side flanks  35  and  37  respectively situated on the pressure surface side and the suction surface side of the blade  10 . The flanks  35 ,  37  have, in cross-section (see  FIG. 2 ), a profile that becomes thinner going toward the trailing edge  18 . 
         [0049]      FIG. 3  schematically shows an example of a micro-arc oxidation electrolytic treatment installation  1 . This installation comprises a tank  2  containing an electrolytic bath  3  or electrolyte. For example, this electrolytic bath is made up of an aqueous solution of a hydroxide of an alkali metal (e.g., potassium or sodium) and of an oxyacid salt of an alkali metal. A substrate  5  forming a first electrode, made of metal or a metal alloy and having semiconductor properties, on the one hand, and at least one second electrode  4 , called “counter electrode”, on the other hand, are immersed into the electrolytic bath  3 . The installation also comprises a current supply source  6 , a voltage generator  7  and control means (for example, a microcomputer  8 ) controlling and monitoring the variable parameters depending on the treatment sequences. The control means  8  in particular allow to regulate the intensity I of the current flowing through the electrolytic bath. This installation allows to form a coating on the substrate  5  by transforming the metal constituting that substrate  5 . The general operation and control of such an installation being known from the prior art, they will not be described in further detail here. 
         [0050]    In the proposed method, the substrate  5  is formed by the protective edge  30 . For example, the protective edge  30  is made of titanium or titanium alloy. The usage ranges of the installation  1  for creating a coating on this protective edge  30  are, for example, the following:
       Current density: from 20 to 400 A/dm 2 ;   Voltage: from 200 to 1000 V, more particularly from 400 to 800 V;   Intensity pulse frequency: from 1 to 500 Hz, and preferably from 10 to 500 Hz;   Charge ratio q p /q n : from 0.4 to 1.8 (q p  being the positive charge and q n  the negative charge that are transmitted);   Treatment duration: from 10 to 90 min;   Temperature of the bath: from 5 to 40° C., and preferably from 10 to 40° C.;   pH of the bath: from 6 to 14;   Conductivity of the bath: from 100 to 1000 mS/m.       
 
         [0059]    The electrolyte bath  3  may comprise demineralized water and a mixture of potassium and/or sodium salts in various forms, such as hydroxides, silicates, aluminates, phosphates, thiosulfates, tungstates, thiocyanates or vanadate, with a composition comprised between 0.1 and 50 g/l. 
         [0060]    To facilitate the treatment of the inner face  32  of the protective edge  30 , the second electrode or counter-electrode  4  may be positioned facing that inner face  32 , and may in particular be positioned between the flanks  35 ,  37  of the protective edge  30  and/or have a geometry similar to the first electrode  5 . In the example of  FIG. 3 , two counter-electrodes  4  are used and positioned on both sides of the protective edge  30 , very close to the latter. 
         [0061]    The following measures are taken to increase the effectiveness of the micro-arc oxidation treatment: 
         [0062]    On the one hand, ultrasounds are diffused in the electrolytic bath  3  during the application of the treatment. 
         [0063]    On the other hand, the intensity applied between the substrate  5  and the electrodes  4  is not a direct intensity, but a variable alternating intensity, having periodic pulses applied at time intervals with a duration T. This duration T is chosen to be greater than 20 ms, for example 30 ms, so that the frequency of the pulses remains below 50 Hz. 
         [0064]    The variations as a function of time t of the intensity I are shown in  FIG. 4 . 
         [0065]    Each pulse is bipolar and includes:
       an intensity climb ramp, with duration T 1 , increasing the intensity from 0 to a positive plateau intensity I+;   a plateau with duration T 2  during which the intensity remains equal to I+;   an intensity descent ramp, with duration T 3 , decreasing the intensity from I+to 0;   a plateau with duration T 4  during which the intensity remains equal to 0;   an intensity descent ramp, with duration T 5 , decreasing the intensity from 0 to a negative plateau intensity I−;   a plateau with duration T 6  during which the intensity remains equal to I−;   an intensity climb ramp, with duration T 7 , increasing the intensity from I− to 0.   As soon as one pulse is complete, a new pulse is applied.       
 
         [0074]    The intensities I+ and I− of the positive and negative intensity plateaus are not necessarily equal. 
         [0075]    The positive and negative plateaus are separated by a short length of time with duration T 4  during which the applied intensity is zero. 
         [0076]      FIG. 5  shows schematically, in sectional view, the structure of an example coating  50  created using the micro-arc oxidation electrolytic treatment on the substrate  5 . In this example, the coating  50  has a porous surface structure  52  and a denser layer  51  situated between the porous layer  52  and the substrate  5 . The dense layer  51  is denser and harder than the porous layer  52 . As shown, the porous layer  52  has a relatively high porosity and a rough outer surface  53 . 
         [0077]    After the micro-arc of electrolytic treatment, the outer face  31  of the protective edge  30  may be polished, in particular by tribofinishing or by means of traditional abrasive silicon carbide discs having a particle size decreasing from 80 to 4000 grains per cm 2  or equivalent. For example, the desired roughness level for the outer face  31  may be approximately 0.6 microns, which corresponds to a certain level of aerodynamic requirements. This polishing step, in the case of the coating  50  of  FIG. 4 , amounts to eliminating the porous layer  52 , as diagrammed by the dotted line D. The dense layer  51  is thus on the surface and exposed. 
         [0078]    The inner face  32  of the protective edge, on the other hand, may remain raw, i.e., not be polished. In the case of the coating  50  of  FIG. 4 , this amounts to preserving the illustrated multi-layer structure. 
         [0079]    Thus, when the protective edge  30  has a coating  50  of the type shown in  FIG. 4 , the outer face  31  of the protective edge  30  is defined by the outer face of the dense layer  51 , defined by the dotted line D, while the inner face  32  of the protective edge  30  is defined by the outer face  53  of the porous layer  52 . 
         [0080]    The protective layer  30  thus has a hard and smooth outer face  31 , improving the protection of the blade  10  and compatible with a certain level of aerodynamic requirements, and a porous and rough inner face  32  promoting the adhesion of the protective edge  30  on the body  9  of the blade  10 . 
         [0081]    The examples or embodiments described herein are provided purely as an illustration and in a non-limiting manner, a person skilled in the art being easily able, in light of this description, to modify these embodiments or examples, or to consider others, while remaining within the scope of the invention. 
         [0082]    Furthermore, the different features of these examples or embodiments can be used alone or combined with one another. When they are combined, these features may be combined as described above or differently, the invention not being limited to the specific combinations described in this description. In particular, unless otherwise specified, a feature described in relation with one embodiment or example may be applied in an analogous manner to another embodiment or example.