MICRO-ARC OXIDATION TREATMENT INSTALLATION FOR METAL PARTS

Abstract

Installation for micro-arc oxidation treatment of parts (3) extending along an axis Ap between two ends, comprising: - a tank suitable for containing an electrolytic bath, - a generator suitable for delivering an electrical signal to subject each part to said treatment, - a tool comprising a support (10) and at least one counter electrode (4) of shape, the support being equipped with means for assembling at least one part with a counter electrode, comprising means for fixing the two ends of each part, held together in two parallel planes by a rod (40) equipped with means for locking said parts in compression along their axis Ap, each counter electrode adopting the shape of a cylinder fixed so that its axis Ac is coincident with the axis Ap of the corresponding part, and that the surface of each part is at a determined radial distance De, constant or variable, from the counter electrode.

Description

Title of the invention: INSTALLATION FOR TREATMENT OF METAL PARTS BY MICRO-ARC OXIDATION

[0001] The present invention belongs to the field of electrochemical metal treatment techniques, and more particularly to the field of processes for producing protective coatings on the surface of metal parts.

[0002] It relates to an oxidation process for manufacturing a coating on a metallic substrate, in particular aluminum or aluminum alloy. It also relates to a treatment installation comprising dedicated tooling, enabling the treatment of solid or hollow parts.

[0003] It is known that aluminum, and more generally metals exhibiting valve properties, as well as their alloys, possess significant mechanical strength relative to their low weight. They are therefore widely used in the manufacture of parts subjected to high deformation stresses, as is the case in the mechanical, aerospace, and automotive industries, etc. However, these metals are not particularly hard or resistant to physicochemical attack, which necessitates protecting them with suitable coatings to enhance their hardness, corrosion resistance, and abrasion resistance.

[0004] Various coating processes based on electrolytic processes are known. The most widespread currently in industry is sulfuric anodizing, due to its low implementation cost (electrolyte, parameter control) and its wide range of applications. However, the coatings produced are amorphous oxides, limited in thickness and exhibiting only average hardness (approximately 500 Hv maximum). Furthermore, some electrolytes contain particularly toxic compounds, such as hexavalent chromium, which are now prohibited by European REACH legislation.

[0005] Anodic oxidation processes using micro-arc discharges are also known, which produce ceramic-type coatings offering protection against severe abrasion and corrosion of metal parts. Micro-arc oxidation (MAO, also called PEO for plasma electrolytic oxidation) consists of immersing a part to be treated in an electrolytic bath and applying a potential difference between the part (substrate) and a counter electrode. The applied electrical energy causes dielectric breakdowns in the oxide layer naturally covering the substrate and generates discharge channels into which the electrolyte rushes, producing rapid cooling and the formation of an oxide layer. The process is manifested by the appearance of micro-discharges or Luminescent micro-arcs are distributed across the entire surface of the substrate, along with gas release. The result is the formation of a coating that grows in both directions (inwards and outwards) from the surface of the substrate.

[0006] Since the 2000s, various processes have been developed playing on the treatment parameters, the main ones being the frequency of the electrical signal, the current density, the duration of immersion of the parts in the bath and the temperature, to obtain coatings that are more chemically and physically resistant, and to optimize the manufacturing conditions, in particular the speed of production or energy expenditure.

[0007] For example, FR 2 808 291 proposes a micro-arc coating process for a metal having semiconductor properties, in which a signal voltage of triangular or trapezoidal shape is applied to the electrodes, generating a current which is controlled in its intensity, its shape and its ratio between the positive intensity and the negative intensity, with in addition a variable frequency parameter.

[0008] FR 2 877 018 describes a micro-arc oxidation process for manufacturing a low-porosity coating on aluminum and aluminum alloy parts. To achieve this, the parts are immersed in an electrolytic bath at a temperature close to zero, and an alternating electric current with a voltage exceeding 600 V is applied. Melting and then compaction occur on the surface of the substrate.

[0009] In FR 3 014 912, niobium alloy parts are treated by micro-arc oxidation to improve their resistance to corrosion and oxidation at high temperatures (above 1000°C). The parts are subjected to a succession of current cycles comprising a phase where the current is constant and positive in intensity, and a phase where the current is constant and negative in intensity, and in which the quantity of positive charge and the quantity of negative charge are in a ratio of 0.8 to 1.6.

[0010] Aluminum and its alloys are known to be materials of choice for industry, particularly aerospace. Parts used in aircraft construction are subjected to severe operating conditions: acidic or alkaline environments that accelerate corrosion, increased abrasion from friction between parts due to an aggressive external environment caused by the presence of sand and dust, and significant temperature variations. The surface coating of these parts must therefore meet very strict specifications in terms of mechanical strength and chemical inertness. Hardness and wear resistance are required, along with high corrosion resistance, good cohesion, and perfect adhesion to the substrate. Withstanding a wide temperature range, from -100°C to +300°C, is essential for aerospace applications.

[0011] In recent years, to meet new technological challenges, manufacturers' requirements have increased. There is therefore a need to have parts made of aluminium or its alloys, with improved properties.

[0012] The object of the present invention is to meet this need by providing an electrolytic micro-arc oxidation process for obtaining a high-performance ceramic-type coating on the surface of a metal part, particularly aluminum or aluminum alloy. Another object of the invention is to provide equipment designed for the optimal implementation of this process on various types of parts. In particular, one objective of the invention is to enable the treatment of hollow parts on both their external and internal surfaces. Another object of the invention is to produce high-hardness layers with a high and homogeneous thickness across the entire treated surface. A further object of the invention is to reduce the growth time of the protective layer, while simultaneously decreasing the electrical energy consumed and production costs.In particular, a method is sought that eliminates the need for pre-treatment and post-treatment steps on the parts. Another objective of the invention is to provide an environmentally friendly process that complies with current regulations.

[0013] The work carried out by the applicant has led to the development of an electrochemical OMA treatment process for aluminum-based parts, involving electrical parameters of the micro-arc oxidation process combined with conformational parameters of the tooling, particularly those related to the arrangement of the electrodes. This allows for the definition and control of the formation conditions and the structure of the protective layer. It is thus possible to optimize the physicochemical reactions transforming the constituent metal of the part and to produce thick, high-hardness coatings with adjustable porosity. Summary of the invention

[0014] According to a first aspect, the present invention relates to an electrolytic process for surface treatment of aluminum or aluminum alloy parts by micro-arc oxidation, in which the parts to be treated act as the first electrode (the anode in this case), the process comprising the steps of: - to fix on a tool comprising a support at least one workpiece to be treated and at least one counter electrode of a specific shape at a determined distance De from each other, and to connect them electrically to the terminals of a generator capable of delivering an asymmetrical alternating electrical signal of trapezoidal shape, - Immerse the assembled tooling in an electrolytic bath consisting of an aqueous solution of an oxyacid salt of an alkali metal and an alkali metal hydroxide, - subject said at least one part to a series of current cycles organized into at least two successive sequences SI, S2, ..., Sn having respective durations T1, T2, ..., Tn, - each sequence comprising a series of identical current cycles within each sequence, - each cycle comprising a phase during which the part is traversed by a constant peak current of positive intensity IP, with respective values ​​IP1, IP2,..., IPn identical or different, and a phase during which the part is traversed by a constant peak current of negative intensity IN, with respective values ​​IN1, IN2,..., INn identical or different, - the ratio R = IN / IP having for each of the sequences SI, S2, ..., Sn respective values ​​RI, R2, ..., Rn such that RI < R2 < ... < Rn, and - the duration of each of the said sequences being such that T1 > T2 > .. .> Tn.

[0015] The parts being processed are first machined to the desired shape and approximate dimensions. They can have a wide variety of shapes, although they most often have an elongated geometry or extend along an axis of revolution. These are generally small or medium-sized parts, ranging, for example, from a few centimeters to several tens of centimeters in length, or even a meter, or more, provided that suitable equipment is available. When the parts are of moderate size, it is advantageous to process several of them simultaneously in the same process, using a single tool, as will be explained later. However, there is nothing preventing the processing of one part after another.

[0016] Each workpiece is associated with a counter electrode. They are placed opposite each other, with a predetermined gap between them, so that a micro-arc reaction can take place. The workpiece acts as the anode, while the counter electrode is the cathode. A shaped counter electrode is used; that is, an electrode whose shape and dimensions complement the workpiece being treated in order to create electric arcs as close as possible to the workpiece. Shaped counter electrodes are advantageously made of austenitic stainless steel, in the form of a solid sheet or a grid to facilitate gas release.

[0017] The process described herein is particularly well suited to parts made of aluminum. Many aluminum-based alloys can be chosen depending on the intended use, such as those containing copper (belonging to the 2000 series), magnesium (belonging to the 5000 series), or casting alloys (e.g., AU5NKZr, AS7G06). The process can be adapted to other metals such as titanium, magnesium, zirconium, etc.

[0018] After the parts to be treated are installed on the support, the complete tooling is immersed in an aqueous electrolyte. The parts are then exposed to alternating electrical energy produced by a generator. The treatment consists of n independent sequences performed successively, with an increasing ratio of the current intensity IN / IP and a decreasing duration. The number of sequences is defined according to the properties of the coating that one wishes to obtain. It can range from two to ten successive sequences, or even up to thirty sequences.

[0019] According to a particular embodiment of the process according to the invention, it comprises three sequences SI, S2 and S3 having a ratio R, of respective values ​​RI, R2 and R3, such that 0.50 < RI < R2 < R3 < 0.75.

[0020] According to a preferred embodiment of the method according to the invention, the values ​​RI, R2 and R3 of the three sequences SI, S2 and S3 can be such that 0.55 < RI < 0.60; 0.60 < R2 < 0.65; and 0.65 < R3 < 0.70.

[0021] The three independent sequences SI, S2, and S3 that make up the process play different roles. During sequence SI, growth of the initial layer is observed. In the inventive process, the thickness of this initial layer can be increased by lengthening the duration of sequence SI or by increasing the value of the positive intensity Ipl. During sequence S2, densification of the layer occurs, which increases the hardness of the coating. Finally, sequence S3 has the main effect of smoothing the layer and making the coating thickness homogeneous over the entire treated area. In analytical terms, the layer obtained after treatment is composed of two phases: a crystalline phase composed of oxide crystallized in various forms (alumina, [3,...); and an amorphous phase composed mainly of elements from the electrolyte. Indeed, during the process, two phenomena occur.Initially, the aluminum is oxidized, resulting in the formation of a crystalline phase; then, the fusion of elements from the electrolyte through chemical reactions produces an amorphous phase.

[0022] According to a particular feature of the process of the invention, the duration of each of the sequences SI, S2, ... Sn referred to the total duration of the treatment is such that 60% > Tl > T2 > ...> Tn > 10%.

[0023] Generally, the total treatment time will be defined according to the desired total thickness. For example, to obtain coatings with a thickness between 80 µm and 120 µm, the treatment lasts from 45 minutes to 2 hours depending on the different alloys.

[0024] According to a preferred embodiment of the process according to the invention, it comprises three sequences SI, S2 and S3 whose duration, respectively T1, T2 and T3, relative to the total duration of the treatment is such that 55% > T1 > 40%; 40% > T2 > 25%, and 25% > T3 > 15%.

[0025] For thin coatings (less than 100 pm), it is advantageous to favor the S1 sequence over the S2 sequence, with durations such as 55% > T1 > 45%; 35% > T2 > 25%, and 25% > T3 > 15%. Conversely, for thick coatings (greater than 100 pm), the S2 sequence can be relatively longer, with, for example, durations such as 50% > T1 > 40%; 40% > T2 > 30%, and 25% > T3 > 15%.

[0026] Originally, the process according to the invention is carried out at a single, identical frequency within each sequence, and also for all sequences. This contributes to obtaining a homogeneous and uniform coating in thickness and composition. The frequency imposed by the generator can range from 80 Hz to 400 Hz. Since the number of micro-arcs generated per unit area increases with frequency, higher frequencies are used when it is desired to create layers of high density and thickness. Thus, in the process according to the invention, the sequences SI, S2, ... Sn are preferably carried out at the same frequency, chosen within a range of 80 Hz to 400 Hz.

[0027] It is known that the reactions at work in the anodic oxidation process are highly exothermic, but that the temperature rise of the reaction medium is detrimental to the proper formation of the coating. Therefore, the electrolyte temperature is monitored regularly, preferably continuously, and a refrigeration system allows for strict regulation of the bath temperature throughout the entire treatment period to maintain it within the set range. Thus, according to one feature of the invention, the temperature of the electrolytic bath is maintained between 20°C and 30°C by a regulating device. Preferably, a temperature between 20°C and 25°C is maintained.

[0028] The process according to the invention uses an aqueous electrolyte with low ionic concentrations. It is environmentally friendly and recyclable. Indeed, it does not use chromium VI or any other pollutants. It is preferably composed of sodium silicate (Na₂SiO₃) with a concentration ranging from 1 g / L to 10 g / L, and potassium hydroxide (KOH) with a concentration ranging from 1 g / L to 5 g / L, with demineralized water used as the solvent. Sodium silicate is chosen for its role in promoting increased growth kinetics and the homogeneity of the forming oxide layer. Furthermore, due to its relatively low melting point, it acts as a sealant for porous anodic layers. Potassium hydroxide is chosen for its ability to ensure the electrical conductivity of the solution, as well as good dissolution of the substrate.The pH of the electrolyte can, for example, be between 10.0 and 12.0 and its conductivity can range from 2.0 mS / cm to 4.0 mS / cm.

[0029] Thus, according to a feature of the process of the present invention, the electrolytic bath is an aqueous solution of sodium silicate at a concentration of 1 g / L to 10 g / L and potassium hydroxide at a concentration of 1 g / L to 5 g / L. The use of a silicate-based electrolyte makes this process compatible with all current environmental standards.

[0030] The method according to the invention can be implemented for parts requiring a Protective treatment can be applied to the entire surface or only a portion of it. In the latter case, it is essential to isolate the areas to be treated from those that must remain untreated. Therefore, areas not to be treated must be properly protected using masks typically made of polymer materials. These masks also help to channel the micro-arcs created between the workpiece and the electrode, resulting in a homogeneous treatment of the area being treated. Another advantage of the masks is that they isolate and protect the electrical contacts in the area being treated from oxidation.

[0031] It is essential that the electrolyte be confined to the areas to be treated and not be able to seep between the part and the masking elements. Therefore, to ensure the masks are sealed against the electrolyte, silicone gaskets are placed near the junction areas between at least one part and the masking elements, in retaining grooves. Before mounting the complete tooling, each part will be fitted with suitably shaped masking elements, along with gaskets, to cover and enclose the areas of the part that are not to be treated. Once this step has been completed, the part and its mask(s) can be mounted on the support and in the treatment tooling. Retaining grooves may be provided on the masking element. Note that gaskets are also useful for sealing gaps between masking elements and the support, or between two structural parts of the support.In this case, a groove can be made on the masking element, on the support, or on both.

[0032] Thus, according to an advantageous feature of the invention, one or more areas not concerned by the treatment of said at least one part are covered by electrolyte-tight masking elements, before assembly on said support, with seals installed at the junction between said at least one part and said masking elements ensuring sealing with respect to the electrolyte.

[0033] Seals can be used in the form of plates, cords, or O-rings. They are precisely dimensioned in relation to the part, for example, by waterjet cutting. The seals, generally made of silicone, can be in various formats depending on their location and use. For example, flat seals with a thickness between 1 mm and 4 mm; cords with a torus diameter of 1 mm to 5 mm; or O-rings of a specific diameter can be used.

[0034] The O-rings and cords are inserted into grooves that are shallower than the O-rings themselves, so that the latter protrude from the surface of the mask (before assembly). They are therefore partially inserted, for example, 50% to 70% of their thickness, into the groove receiving them. It has been found, counterintuitively, that this allows the elements to be effectively pressed together. masking of the areas of the parts and, once assembled, to prevent any infiltration of electrolyte.

[0035] Thus, in an original way, the seals are chosen from among O-rings, cords or strips, and are mounted by inserting 50% to 70% of their thickness into grooves with which the said masking elements are provided.

[0036] Furthermore, care is taken to ensure that the area to be treated, even if it has a complex shape, is kept at a substantially constant distance from its associated counter electrode. In this way, the equipotential lines are aligned with the shape of the part, which promotes uniformity of the coating thickness and reduces current losses in the electrolyte. The efficiency of the process is thus increased because electrical consumption is limited by increasing the yield of the layer formation reaction.

[0037] Therefore, according to a feature of the process, said at least one part comprises a treatment zone whose surface is placed opposite said counter electrode of shape, which adopts a conformation complementary to that of said treatment zone, the surface of the treatment zone being at every point at a radial distance De from said counter electrode of between 5 mm and 50 mm, and even better between 5 mm and 20 mm.

[0038] The electrolytic process according to the invention makes it possible to efficiently treat parts of various shapes and dimensions, and in particular elongated, solid, or tubular parts. It is expressly agreed that the term "elongated parts" refers to parts whose geometry can be defined with respect to a longitudinal axis. Such parts are most often longer than their width or diameter, but it is also possible to treat parts with a diameter (or width) greater than their length. It is further specified that while the remainder of this description primarily concerns elongated parts, the process can treat all part geometries.

[0039] The parts concerned may include parts exhibiting rotational symmetry about their longitudinal axis, without necessarily being strictly cylindrical. Elongated parts may receive treatment of their external surface. According to the invention, it is then recommended to use cylindrical counter-electrodes in the center of which the parts to be treated are placed. It is then necessary to use counter-electrodes whose diameter is designed to maintain a certain distance from the surface of the part to be treated, which is placed coaxially inside said counter-electrode.

[0040] Therefore, in a variant of the process according to the invention in which the parts to be treated adopt a shape extending along a longitudinal axis, the counter electrodes adopt the shape of hollow cylinders, each surrounding one of said parts. A radial distance De, constant or variable, between 5 mm and 50 mm. If the part to be treated has a strictly cylindrical surface, with each part extending along a longitudinal axis coinciding with the longitudinal axis of its counter electrode, the distance De is constant. However, for parts with a more complex external surface, for example, curved, or with a recessed or protruding element, the distance De between this surface and the counter electrode will be variable, while remaining within the recommended range. It should be noted that this method applies to the treatment of the entire surface of the parts or a portion thereof, for example, their central section, while the end sections can be masked and remain untreated.

[0041] Advantageously, hollow tubular parts, i.e., parts with an axial recess, can also undergo surface treatment. Generally, these parts exhibit rotational symmetry about a longitudinal axis, without necessarily being strictly cylindrical. If the external surface is to be coated, the procedure described above will be followed. However, if it is desired to treat the internal surface defining said axial recess, the invention recommends using cylindrical counter electrodes installed in the center of the parts to be treated. It is then necessary to use counter electrodes with a diameter designed to maintain a certain distance from the internal surface of the part to be treated, within which the counter electrode is coaxially positioned.

[0042] Therefore, in a variant of the process according to the invention in which the parts to be treated are in the form of a hollow tube extending along a longitudinal axis, the counter electrodes are in the form of cylinders, each being surrounded by one of said parts at a radial distance De, constant or variable, between 5 mm and 50 mm. If the part to be treated has a strictly cylindrical internal surface, each part extending along a longitudinal axis coinciding with the longitudinal axis of its counter electrode, the distance De is constant. Otherwise, the distance De between this internal surface and the counter electrode is variable, while remaining within the specified range.

[0043] Once the parts to be treated are fixed to the holder, each opposite a counter electrode shaped as described above, the holder is assembled with the other components of the tooling. The holder is conveniently designed to ensure the electrical connection between the parts and the generator, using electrical contact elements made of conductive metal. Electrical continuity can be further ensured via a contact relay, if necessary, depending on the tooling configuration.

[0044] It is understood from the foregoing that the process according to the invention can be implemented automatically using a central unit comprising pre-programmed control and monitoring means, in particular with regard to the parameters of the cycles composing the different sequences, their number and their duration. Therefore, in the process described in the present invention, the parameters of each sequence SI, S2, ..., Sn are controlled and managed by a central unit connected to the generator. Furthermore, real-time monitoring of the process parameters is ensured. The generator advantageously features various programs designed to process all aluminum alloys, allowing the processing parameters to be varied according to the alloy grade, the geometry of the parts being treated, and the desired coating properties.

[0045] It is particularly recommended to periodically monitor the composition of the electrolyte to track the levels of its constituents as well as pollutants (aluminum, silicates, iron, and copper). This is done using appropriate analytical techniques known to those skilled in the art. Indeed, the efficiency of the process is affected when a threshold concentration of pollutants is exceeded. The pH and conductivity are also monitored. All methods known to those in the technical field can be used.

[0046] The micro-arc oxidation process just described is applicable to all aluminum alloys (wrought and cast) and to aluminum parts obtained by 3D printing or additive manufacturing techniques.

[0047] It may be accompanied by preparatory steps and subsequent steps enabling the complete manufacture of the part, constituting an operating sequence which may include for example: degreasing (optional), OMA treatment, rinsing and drying.

[0048] It can also be used as a step in a process comprising a series of treatments using different techniques, preferably also complying with current environmental standards. Among multi-treatment ranges, it is possible, thanks to the process according to the invention, to carry out a partial micro-arc oxidation treatment of parts, on well-defined and delimited areas, and to subject the areas not treated by MAO to another treatment, for example anodizing or chemical conversion, depending on the end use for which the part is intended.

[0049] Among the remarkable advantages of this technique, it should be emphasized that it requires no pre-treatment or post-treatment steps, which greatly simplifies its implementation. Unlike conventional anodizing, it does not require surface preparation. Surface activation by pickling is unnecessary to allow layer formation. However, for industrial application of the process, it is advisable to degrease the parts before treatment to remove cutting oil residues that could contaminate the bath. Particularly advantageous, micro-arc oxidation eliminates the need for layer sealing for thick coatings (greater than 80 µm) to achieve good corrosion resistance. It also reduces the use of products chemicals (especially those used during pickling for conventional anodizing) and water consumption (small number of tanks on the treatment chain).

[0050] Implementing the process according to the invention requires efficiently channeling the energy onto the areas to be subjected to micro-arc oxidation treatment, otherwise the required performance will not be achieved. As explained above, the electrochemical treatment of aluminum-based parts by MAO involves electrical parameters of the micro-arc oxidation process combined with conformational parameters of the tooling, particularly those related to the arrangement of the electrodes, allowing for the optimization of the physicochemical reactions transforming the constituent metal of the part and producing thick, high-hardness coatings with adjustable porosity. The electrodes must be mounted on a support that holds them fixed in a well-defined configuration, enabling a uniform and thick coating to be applied over the entire treated surface.A specially designed installation for implementing the electrolytic treatment process according to the invention has thus been developed, which meets the requirements related to the tooling and arrangement of the electrodes, but also to other parameters, such as the temperature during the process, which make it possible to define and control the formation conditions and the structure of the protective layer and which influence the quality of the coating obtained.

[0051] Thus, according to a second aspect, the present invention relates to an installation for the electrolytic treatment of aluminum or aluminum alloy parts by a micro-arc oxidation process in which the parts to be treated act as the first electrode, said installation comprising: - a tank suitable for containing an electrolytic bath consisting of an aqueous solution of an oxyacid salt of an alkali metal and an alkali metal hydroxide, - a tool comprising a support and at least one shaped counter electrode, said support being equipped with means for assembling at least one workpiece to be treated with one of said shaped counter electrodes at a determined distance De from each other, - means for maintaining said fixed assembly while immersed in the electrolytic bath, - a system for regulating the temperature of the electrolytic bath, - a generator comprising means for electrical connection to the electrodes of said assembly, said generator being capable of delivering an asymmetrical alternating electrical signal of trapezoidal shape and comprising means for subjecting said at least one part to a surface treatment by a process according to one of the preceding claims.

[0052] The installation according to the invention makes it possible to efficiently process parts of various shapes and dimensions, and in particular parts of varying lengths, which may be solid or hollow and require treatment of their external surface, or where applicable, their internal surface. The tooling used in the installation that is the subject of the invention must be adapted according to one or the other of these configurations.

[0053] In the following description, the installation is considered as it appears during operation. The tank contains the electrolytic bath in which the tooling is immersed. The parts to be treated are also placed on the support. Indeed, although the parts are not strictly speaking part of the tooling, it is essential to mention them since they constitute anodes that interact with the counter electrodes. Furthermore, for each set of parts to be treated, the tooling must be assembled with the parts and will be disassembled at the end of the process to retrieve them. The assembly, including the parts to be treated, is carried out in a specific order, which may differ from one type of tooling to another. Describing an installation with "empty" tooling, which has no practical application, would not allow for a clear understanding of the technical aspects of the present invention.

[0054] According to a first embodiment, the invention relates to an installation for treating the external surface of parts extending along a longitudinal axis Ap between a first and a second end, in which the tooling comprises: - a support comprising a base in which are provided fastening means suitable for receiving each the first end of a part, an upper frame in which are provided fastening means suitable for receiving each the second end of said part, the base and the frame being held together in two parallel planes by a series of rods equipped with means for locking said parts in compression along their longitudinal axis Ap, - counter electrodes in the form of hollow cylinders with longitudinal axis Ac, each capable of receiving a part in its central opening, each counter electrode being fixed to the base of the support so that its longitudinal axis Ac coincides with the longitudinal axis Ap of the corresponding part, so that each of said parts is surrounded by a counter electrode at a determined radial distance De from it, constant or variable, preferably between 5 mm and 50 mm, - means of electrically connecting said parts and said counter-electrodes to the generator.

[0055] The elongated parts that are installed in the tooling support can be formed from a solid bar exhibiting rotational symmetry about its longitudinal axis, with a circular or substantially circular cross-section, and a constant or variable dimension along the length of the part. When the installation is in operation, the parts to be treated are each axially fixed in the central opening of a cylindrical counter electrode.

[0056] The base and the frame are held together in two parallel planes by a series of rods, for example, threaded rods whose ends are engaged in threaded holes located opposite each other at various points along the edge of the base and the frame. The parts to be treated and the associated counter electrodes are deployed between the base and the frame of the support, such that the axes Ac and Ap of the electrodes intersect them orthogonally. The ends of the parts are fixed on one side to the base and on the other to the frame, while the counter electrodes are fixed only to the base. The rods include means for locking said parts in compression along their longitudinal axis Ap. For example, in the case of threaded rods, simple screwing can be used to compress the parts and lock them in their respective positions.

[0057] Various means of fixing the parts can be envisaged. According to a particular embodiment of the invention, the support comprises a base in which blind recesses are provided, each adapted to receive by sliding the first end of a part, and an upper frame having perforations opposite each recess, each adapted to receive the second end of said part, either directly or via a connecting element. This connecting element can also function as a mask.

[0058] In this configuration, when several parts are mounted on the support, it may be advantageous to provide the electrical connections according to a grouped scheme. To this end, in one embodiment of the installation according to the invention, the base is formed of an upper plate and a lower plate between which is sandwiched an electrically conductive metal plate. The upper plate has the housings, the bottoms of which are closed by the plate, and the lower plate is provided with electrical contact pads between the plate and the means for connecting to the generator. The plate then acts as a stop so that all the parts, once inserted into their respective housings, are in contact with the plate, which is itself connected to the generator, in accordance with known best practices.

[0059] According to a second embodiment, the invention relates to an installation for treating the internal surface of hollow tubular parts extending along a longitudinal axis Ap between two end edges, wherein the tooling comprises: - a support comprising a parallelepiped-shaped box formed of two side walls, two front walls, a base, and a cover, the internal faces of the base and the cover having circular recesses, each adapted to receive an end edge of a hollow tubular part, the base and the cover being held together in two parallel planes by at least one rod cooperating with means for clamping said parts in compression along their longitudinal axis Ap, - counter electrodes in the form of cylinders with axis Ac, each adapted to be placed in the lumen of one of said tubular parts, each counter electrode being fixed to the support so that its longitudinal axis Ac coincides with the longitudinal axis Ap of the corresponding part, so that said part surrounds the corresponding counter electrode at a radial distance De from it, constant or variable, preferably between 5 mm and 50 mm, and - means of electrically connecting said parts and said counter-electrodes to the generator.

[0060] The hollow tubular parts installed in the tooling housing can be formed from a sleeve of varying length exhibiting rotational symmetry about its longitudinal axis, with a circular or substantially circular cross-section, and a dimension that is constant or variable along the length of the part. When the installation is in operation, the parts to be treated are fixed to the support, each containing a cylindrical counter electrode positioned axially in its central slot. The end edges of the part bear against the inner face of the base and the cover, respectively, at a location provided for this purpose. This location can be a simple flat circular area, or it can be delimited by a suitable relief, such as a shoulder or a recess, so that the part is precisely positioned and cannot slide laterally during the various manipulations of the tooling.

[0061] The support casing is formed of four side and front walls, a base, and a cover defining a rectangular parallelepiped. The base and the cover are held together in two parallel planes by one or more rods cooperating with means for compressively locking said parts along their longitudinal axis Ap. For example, according to the invention, each circular location of the base and the cover can be traversed at its center by a rod, each having a lower end segment and an upper end segment extending on either side outside the casing and cooperating with said means for compressively locking said parts.

[0062] In this case, the locking means are external to the housing. According to an interesting embodiment, the locking means cooperating with each rod comprise a lower bar and an upper bar bearing against the respective external faces of the base and the cover, said bars having a tapped hole in which the threaded end segments of said rod are engaged.

[0063] This arrangement offers several advantages. Indeed, on the one hand, the bars can rest on the base and the cover at a distance from the rod that passes through them, while remaining attached to it. Consequently, the base and the cover can have relatively large openings around the axis of the rod (without, however, exceeding the diameter of the support locations of the parts to be treated). These openings then allow the electrolyte to circulate between the interior space of the parts and the bath, which is essential given the significant temperature rise occurring at the electrodes.

[0064] On the other hand, the arrangement of the rods centered on the circular locations intended to receive in support each an end edge of a hollow tubular part has as a corollary that the rods are placed coaxial with the electrodes (parts and counter electrodes), which can be used to associate with each part to be treated, a rod carrying a counter electrode.

[0065] Thus, in an advantageous embodiment, each rod has a threaded central segment, and each counter electrode has a tapped hole along its axis Ac into which the rod is screwed, so that the counter electrode is held coaxially in the central opening of a workpiece at a predetermined height. The counter electrode is then perfectly centered on the axis of the workpiece. The operator can also easily adjust the vertical position of the counter electrode on the rod so as to make it coincide perfectly with the area to be treated on the corresponding workpiece.

[0066] Regardless of the type of support used in the processing tooling and the internal or external orientation of the surfaces to be treated, according to a preferred feature of the invention, the counter electrodes each adopt a conformation complementary to that of the part to be treated or of an area of ​​the part to be treated, placed opposite said counter electrode, the surface of said part or area to be treated being at all points at a radial distance De, constant or variable, from said counter electrode of between 5 mm and 50 mm.

[0067] As previously mentioned, the bath temperature must be controlled and maintained throughout the treatment within a setpoint range, which, according to the invention, is intended to be between 20°C and 30°C. The electrolytic treatment system is therefore equipped with a cooling system comprising, conventionally, an external recirculation loop with a heat exchanger and a pump that circulates a portion of the electrolytic solution through said heat exchanger. The process is continuous so as to circulate a total volume of solution greater than that in the tank. Temperature probes are placed at various points in the loop, particularly at the inlet and outlet of the tank. It is preferable for the tank to be significantly larger than the treatment equipment to allow for gradual, smooth temperature regulation.However, experimental tests revealed that a significant temperature rise could occur in the reaction environment near the electrodes, which is detrimental to the coating quality. This phenomenon is even more pronounced when the parts and their associated counter-electrodes are close to each other and positioned in a configuration where the tooling creates a [missing information]. A physical obstacle to the movement of the electrolyte. It became necessary to remedy this problem, otherwise efforts to improve the technology would be futile. Therefore, the temperature control system is equipped with a device designed to actively cool the electrolytic solution, including in the vicinity of the electrodes within the tooling, whether the tooling is closed or open.

[0068] Thus, the electrolytic treatment installation according to the invention comprises a temperature control system for the electrolytic bath including an external recirculation loop equipped with a heat exchanger, temperature sensors and a pump capable of imposing the permanent circulation of a fraction of electrolytic solution via said heat exchanger, between an outlet made at the base of the tank near a first lateral wall of said tank, and a weir placed above the tank a short distance from a second lateral wall of the tank opposite said first wall, the weir including a horizontal distribution ramp having a plurality of nozzles through which tempered electrolytic solution can flow.

[0069] The bath heated by the OMA process is continuously cooled by the tempered solution fraction (whose temperature is regulated to a setpoint) which is continuously fed into it after passing through the heat exchanger. Temperature sensors (or probes), preferably located at the inlet and outlet of the tank, are connected to an automatic pump control system. The position of the solution outlet opposite that of the overflow induces fluid movement within the tank, or even agitation, which promotes a homogeneous bath temperature, including in the vicinity of the tooling. This effect is enhanced by a well-designed overflow configuration, in which the tempered solution is dispensed through several nozzles distributed along a distribution manifold, preferably horizontal.The boom may, for example, have five to ten nozzles, aligned and distributed along the boom, so that the tempered solution flows into an interval occupying at least 80% of the length of the second side wall.

[0070] When the tooling uses a box-shaped support, the above thermal regulation system does not allow for maintaining a temperature below 30°C near the electrodes, which significantly degrades the performance of the treatment process. However, in this type of tooling, designed for treating the internal surface of tubular parts, the opening of the parts to be treated (the internal space) does not communicate with the intermediate space located between the parts and confined between the walls of the box. This problem is overcome simultaneously by two concurrent arrangements.

[0071] On the one hand, the interior space of the rooms can be put into direct communication with the bath, by means of openings made in the base and in the hood, around each fixing rod. Indeed, thanks to all the arrangements described above, the enclosure may not be completely closed: the base and the cover can have openings around each fixing rod, allowing the electrolyte to circulate through the interior space of the parts. These openings can be relatively large, with a diameter almost as large as the diameter of the tubular parts.

[0072] With regard to the intermediate space, a device addresses the heating problem by specifically directing a portion of the cooled solution towards the chamber. Thus, according to an advantageous embodiment of the invention, the temperature control system for the electrolytic bath comprises a temperature control device in the chamber, which includes at least one inlet hole for the electrolytic solution in one wall of the chamber (including side walls, hood, and base), connected to a nozzle by a conduit, and at least one outlet hole located at a distance in another wall of said chamber. The number of inlet holes connected to a nozzle can be determined by the operator, based in particular on the number of parts installed in the tooling and the size of the parts.

[0073] According to a preferred embodiment, the chamber has at least one inlet hole in the lower part of the chamber and at least one outlet hole in the upper part of the chamber. The solution thus has forced and turbulent circulation, capable of cooling the reaction environment around the electrodes.

[0074] It is also advantageous for the tooling fitted with the parts to be treated to be immersed in the tank while being completely surrounded by the electrolytic bath, so as to promote both chemical and thermal exchange. Therefore, according to a preferred feature of the installation of the invention, the means for keeping the tooling immersed in the electrolytic bath comprise suspension arms fixed laterally to the support, said arms being fitted with hooks capable of engaging with a bar running above the tank.

[0075] Micro-arc oxidation, carried out using the process and tooling described above, makes it possible to obtain high-hardness coatings, in particular between 1000 Hv and 1900 Hv, with a wide range of possible thicknesses, from as low as 20 µm to as high as 200 µm, depending on the alloy being treated. The semi-crystalline metal oxide layers formed on the treated parts have properties comparable to those of ceramics, with low porosity and a hardness significantly greater than that of coatings obtained by conventional anodic oxidation. They are particularly effective with regard to wear resistance, corrosion protection, and electrical insulation.

[0076] Depending on the intended applications, the coating obtained in accordance with the present invention can be used in different ways. - It can be used raw to allow the adhesion of a post-treatment such as a paint thanks to the roughness. - It can be polished, sandblasted or undergo more or less advanced microblasting to remove the friable and superficial part of the layer. It can also be mechanically machined by honing or grinding to remove all of the friable part of the coating. A suitable tool (such as a diamond tool for grinding) must be used to work this particularly hard coating. A stone finish can be applied to further reduce the surface roughness (Ra less than 0.1).

[0077] The present invention will be better understood and relevant details will become apparent, in light of the description which will be made of different embodiments, in relation to the attached figures.

[0078] [Fig. 1] is a schematic representation of a micro-arc oxidation equipment.

[0079] [Fig.2] represents the shape of the electrical signal at the generator output, according the invention.

[0080] [Fig.3] is a diagram of the course of a treatment comprising three sequences within the framework of the process according to the invention.

[0081] [Fig.4] shows the structure of a layer formed on an aluminum alloy by a process according to the invention, seen in scanning electron microscopy.

[0082] [Fig.5] is a perspective view of a first tooling used in an installation according to the invention.

[0083] [Fig.6] is a cross-sectional view of the same tooling.

[0084] [Fig.7] is a perspective view of a second tool used in an ins Installation according to the invention.

[0085] [Fig.8] is a cross-sectional view of the same tooling.

[0086] [Fig.9] is a perspective view of a third tool used in an ins Installation according to the invention.

[0087] [Fig. 10] is a cross-sectional view of the same tooling.

[0088] [Fig. 11] is a schematic representation of a temperature control system for the electrolytic bath in an installation according to the invention.

[0089] EXAMPLE 1: General presentation of an OMA installation

[0090] An installation designed to implement the process of the present invention, intended for the surface treatment by micro-arc oxidation of aluminum or aluminum alloy parts, is shown in [Fig. 1]. It comprises a tank 1 suitable for containing an aqueous electrolyte 2 in which at least one part to be treated 3, acting as the anode, and at least one counter electrode 4, serving as the cathode, are immersed. The electrodes are electrically connected to the terminals of the generator 5, which typically includes a transformer and a voltage regulator. electrical energy. Generator 5 incorporates a central control and monitoring unit for the parameters of each sequence of the process.

[0091] Each electrode pair, consisting of a workpiece 3 and its associated counter electrode 4, forms a dipole which is assembled in a tool 100 and held fixed by means of a support 10 in a well-defined configuration allowing for a homogeneous and uniform coating over the entire treated surface. The tool 100 and the support 10 will be described in detail later.

[0092] The equipment also includes a device 6 for measuring and regulating the temperature of the electrolytic bath connected to a refrigeration production installation which will be described later.

[0093] EXAMPLE 2: Electrical parameters

[0094] The generator used according to the invention delivers a pulsed bipolar signal. It is controlled by a central unit containing various programs in which the processing parameters are set according to the alloy grade, the surface area of ​​the workpiece being treated, and the desired coating properties. Numerous parameters can be varied, including the signal frequency, the total processing time, the current applied to its shape, and the duration of its different phases. This flexibility in the electrical parameters allows for the processing of all aluminum alloys, whether wrought or cast.

[0095] The electrical signal is asymmetric alternating current, trapezoidal in shape. The shape of the electrical signal at the generator output is shown in [Fig. 2], with electrical parameters defining its shape as given in Table 1.

[0096] The signal remains identical throughout the duration of a given sequence, with a constant ratio R = IN / IP. In a process according to the invention, several sequences run one after the other, with an increasing value of R and a decreasing duration at each change of sequence. Figure 3 illustrates this process schematically.

[0097] [Tables 1] Parameters Definitions DI Positive current rise phase - Time required to reach the peak positive current value D2 Constant current phase - Duration of the positive peak current plateau D3 Positive current fall phase - Time required to reach a zero output current value D4 Zero constant current phase - Duration of the intermediate plateau D5 Negative current rise phase - Time required to reach the peak negative current value D6 Constant current phase - Duration of the negative peak current plateau D7 Negative current fall phase - Time required to reach a zero output current value D8 Zero constant current phase - Duration of the final plateau Ip Peak value of the positive output current In Peak value of the negative output current R Quotient between the negative and positive peak currents (IN / IP) F Output current frequency

[0098] EXAMPLE 3: Tooling for treating the external surface of parts

[0099] Figures 5 and 6 illustrate an example of tooling applied to the particular case of processing elongated solid parts 3, exhibiting a symmetry of revolution with respect to their longitudinal axis Ap, and of which only the middle part 31 must be covered with a protective layer to the exclusion of the ends 32, 33. This middle part is curved.

[0100] The tooling 100 is designed here to process eight parts simultaneously. It consists of the rectangular base support 10 and two laterally fixed suspension arms 101, each having a hook 102. The hooks 102 allow the tooling 100 to be suspended from a bar mounted above the tank 1.

[0101] The support 10 comprises a base 11 surmounted by a chassis 12, which are held together in two distant, parallel planes by a series of rods 40, spaced to accommodate the parts 3 to be held. These elements are made of synthetic polymers resistant to operating temperatures. For example, the base 11 and the chassis 12 can be made of PPH (polypropylene homopolymer) and rods 40 of epoxy resin.

[0102] The parts 3 to be treated are installed perpendicularly between the base 11 and the frame 12, spaced apart. The area to be treated is the central portion 31, which here represents 60% of the total length of the part (generally between 50% and 100%). Each part 3 is associated with a counter electrode 4, which is cylindrical in shape and completely surrounds the central portion 31 of the part 3, at a radial distance De varying between De' and De" along the longitudinal axis. The counter electrodes 4 have a diameter such that the distances De' and De" are between 5 mm and 20 mm. They thus act as shaped electrodes 4 to create the electric arcs as close as possible to the part 3. They are made of austenitic stainless steel with high mechanical strength, either solid sheet or grid to facilitate gas release.They are fixed by means of steel tabs 23 welded to their base, and held onto the base 11 by screws.

[0103] A first end 32 of the parts 3 is fixed at the level of the base 11. This is formed of two plates, a lower one 13 and an upper one 14, between which is inserted a plate 15 made of aluminium alloy, preferably of the 2000 series, the electrical conductivity of which is increased by the presence of copper in its composition. The plate 15 extends over most of the base 11, while remaining confined within it, with the plates 13 and 14 joined at the periphery of the base 11. The upper plate 14 has eight recesses 16 with a side wall dimensioned to fit the shape of the first end 32 of a part 3. The bottom of the recesses 16 is closed by the plate 15, so that the parts 3 come into contact with the plate 15 when their end 32 is engaged in a recess 16. Electrical relays 17 ensure contact between the plate 15 and the generator 5.The housings 16 act as a mask preventing oxidation of the first end 32 of the parts 3. To protect a larger surface from oxidation, a masking element can be added, for example in the form of a washer 24 of the desired thickness secured to the plate 14 at the base of each part 3. The masking element is made of a temperature-resistant polymer, for example polyacetal (polyoxymethylene).

[0104] The second end 33 of the parts 3 is fixed to the frame 12. The frame can be in the form of a plate or a grid, but is advantageously shaped here into two four-pronged crosses 18. The intersection and the terminal part of the prongs 18 each accommodate a threaded rod 40. Each prong 18 overhangs a part 3 and has a hole 19 in the axis of said part. A connecting element 20 provides a rigid connection between the second end 33 of the part 3 and the hole 19 while also acting as a masking element. It has a cylindrical cap 21 that fits exactly over the second end 33 of the part 3. The cap 21 has a The nipple 22 has a dimension complementary to the bore 19 into which it is fitted. The joining element 20 can, for example, be made of polyacetal.

[0105] The assembly also includes a number of seals (25) to ensure sealing and prevent electrolyte ingress, in order to protect from oxidation the non-reactive parts of the components, as well as the electrical components and contacts. Sealing is achieved conventionally using gaskets cut to the required dimensions and shape, made of silicone chosen here with a hardness between 60 and 80 Shore A. The use of temperature-resistant and completely non-conductive red silicone gaskets is essential. The gaskets 25 are inserted into grooves 26 provided for this purpose, ensuring that they protrude by approximately 1 / 3 of their thickness. This allows two elements to be pressed together during tooling assembly.

[0106] Once the parts 3 are installed in the tooling, the tooling is suspended from a bar above the tank 1 by the hooks 102, so that it is immersed in the electrolytic bath. The electrical relays 17 and the counter electrode are connected to the generator circuit, the bath temperature control system is switched on, and the treatment is initiated.

[0107] EXAMPLE 4: Tooling for treating the internal surface of a tubular part

[0108] Figure 7 illustrates an example of tooling applied to the specific case of machining a cylindrical tubular part 3 with longitudinal axis Ap and circular cross-section. The part 3 extends between a lower end edge 42 and an upper end edge 43. Only the internal surface of the part needs to be machined.

[0109] The tooling 100 is designed here to process a single part. It consists of the support 10 formed of a rectangular base box 110 and two laterally fixed suspension arms 101, each of which has a hook 102. The hooks 102 allow the tooling 100 to be suspended from a bar mounted above the tank 1.

[0110] The parallelepiped-shaped enclosure 110 comprises two side walls 112, two front walls 114 (front and rear by convention), a base 11, and a lid or cover 113. The base 11 and the cover 113 each have a centered, circular opening 115. They are held together in two parallel planes by the rod 40 with axis At, which passes vertically through the enclosure 110, extending outwards on either side. The rod 40 has a threaded lower end segment 44 and an upper end segment 45, fixed respectively to a lower bar 46 and an upper bar 47 at a tapped hole. The bars 46, 47 bear against the respective external faces of the base 11 and the hood 113. Their tightening thus makes it possible to lock in compression the front walls 114 between the base and the hood, but also to hold the part to be treated and the counter electrode, as explained below..

[0111] The inner faces of the base 11 and the cover 113 are provided with circular recesses, each designed to receive an end edge 42, 43 of part 3. The perimeter of the circular opening 115 of the cover 113 has a shoulder 116 that receives the upper end edge 43 of part 3. Similarly, the perimeter of the circular opening 115 of the base 11 has a shoulder (not shown) that receives the lower end edge 42 of part 3. This edge is thus held radially before the rod 40 is tightened onto the bars 46, 47. It should be noted that the height of the front walls 114 is determined according to the size of part 3, while the dimensions of the side walls 112 are not critical.

[0112] The tooling includes a cylindrical counter electrode 4 with longitudinal axis Ac, which is placed in the opening of the tubular part 3. The counter electrode 4 has a tapped hole 49, formed along its axis Ac, which can receive the rod 40. The rod has a thread on at least one median segment 48, so that it holds the counter electrode 4 in place by simple screwing. The operator can adjust the desired height by tightening or loosening the rod. The axis Ac of the counter electrode 4 thus coincides with the axis At of the rod 40, which itself coincides with the axis Ap of the part 3. The counter electrode 4 is therefore exactly centered and is at all points at a constant radial distance De from the inner surface of said part 3, here chosen to be between 10 mm and 20 mm.

[0113] The electrical connection of part 3 is provided here in the base 11. Aluminum contact elements 51 are attached to the base of part 3, for example at two locations near the end edge 42. The contacts 51 are configured to receive a connection pin 53, held in place by screwing. These pins 53 are connected to the generator circuit 5. The upper part of the contacts 51 is insulated from the electrolyte by a protective cover 52.

[0114] Once part 3 is installed in the tooling, the tooling is suspended from a bar above the tank 1 by the hooks 102, so that it is immersed in the electrolytic bath. The pads 53 and the counter electrode are connected to the generator's electrical circuit, the bath temperature control system is switched on, and the treatment is initiated.

[0115] EXAMPLE 5: Tooling for treating the internal surface of two tubular parts

[0116] Figures 9 and 10 illustrate an example of tooling applied to the particular case of processing two cylindrical tubular parts 3 with longitudinal axis Ap' and Ap", of circular cross section, of which only the internal surface must be processed.

[0117] The general arrangement is identical to that of the tooling shown in the previous example, with a rectangular base 110 equipped with two laterally fixed suspension arms 101, each of which has a suspension hook 102 to a bar mounted above the tank 1. The base 11 and the hood 113 of the box 110 each have two opposing circular openings 115, the perimeter of which is arranged to receive an end edge 42, 43 of a part 3 to be treated. Two rods 40 with axes At' and At" pass vertically through the housing 110 and are each fixed by lower 46 and upper 47 brackets. A counter electrode 4 is screwed onto each rod 40, as before. Aluminum contact elements 51 are attached to the base of each of the parts 3. Once the parts 3 are installed in the tooling 100, the tooling is suspended from a bar above the tank 1 by hooks 102, so as to be immersed in the electrolytic bath. The electrodes are connected to the generator's electrical circuit, the bath temperature control system is switched on, and the treatment is initiated.

[0118] EXAMPLE 6: Temperature control system in tooling

[0119] The electrolytic treatment installation in which the tooling described in the preceding examples is placed includes a temperature control system for the electrolytic bath 2, as shown in [Fig. 11]. It comprises an external recirculation loop 200 for circulating the electrolyte between an outlet 201 located at the base of the tank 1, near a first side wall of said tank, and a weir 202 positioned above the tank 1 a short distance from a second side wall of the tank 1, and opposite the first wall. A heat exchanger 230, for example a plate heat exchanger of a known type, is mounted on the loop 200. Two temperature sensors 221 are installed on the loop 200, one just after the outlet of the tank 1, and the other just before the weir 202.A pump 220 circulates a fraction of the electrolytic solution at a rate determined by a processor that receives temperature data from sensors 221, for example, the volume of solution in the tank that has circulated in 1 hour. The weir includes a distribution manifold 203 mounted horizontally above the tank 1. The manifold 203 is in the form of a tube closed at its ends and opening into a plurality of nozzles 204, distributed along its length, through which the tempered electrolytic solution can flow. This flow along the second wall of the tank 1 creates a fluid movement that promotes flow through the tank 1 and homogenizes the temperatures.

[0120] When the tooling 100 uses a housing 110, an additional device is used to force the circulation of the electrolyte in the enclosed space between the parts 3 and the walls of the housing 110. An inlet hole 205 is provided in the base 11 of the housing 110 and a discharge hole 207 is provided in the cover 113, away from the inlet hole 205. A conduit 206 connects one of the nozzles 204 to the inlet hole 205, so that some of the electrolyte returning to the bath is sent directly into the housing 110, causing directed circulation within the housing. This significantly reduces the need for electrolyte to circulate. catively the temperature gradient around parts 3.

[0121] EXAMPLE 7: Characteristics of coatings obtained on aluminium alloys

[0122] The micro-arc oxidation process comprising three successive sequences was implemented on aluminum-based alloys, varying the electrical treatment parameters. The coatings obtained on the different substrates were characterized.

[0123] Structures and properties of coatings

[0124] The coating thickness can vary from 20 µm to 200 µm depending on the applied processing parameters. It is typically formed half on the outside and half on the inside of the metallic substrate. Figure 4 shows the cross-section of a coating formed on an aluminum alloy substrate 500, with a dense functional layer 501, 100 µm thick, and a porous surface layer 502, 60 µm thick. It is organized into two layers with distinctly different structures: - closest to the substrate, a dense functional layer which occupies approximately 2 / 3 of the total layer and which consists of a crystalline phase composed of oxides crystallized in different forms. For example, an aluminum oxide alloy A12O3 crystallizes as corundum; - In the outermost position, a surface layer that occupies approximately 1 / 3 of the total layer. It is porous and brittle. It is mainly composed of amorphous phases made up primarily of elements from the electrolyte. It therefore contains less crystalline phase than the dense layer.

[0125] Due to the friability of the surface layer, the coating can easily undergo mechanical repair (approximately 50% of the total layer), resulting in a thin, amorphous layer with the same hardness as the dense layer. This property is desirable in certain applications, such as the manufacture of valve bodies, flanges, cylinder housings, etc.

[0126] The composition of the formed coating depends on the alloying elements of the substrate, the constituent elements of the electrolyte (nature and concentration), and the electrical processing parameters. Depending on the chosen processing parameters, the layer formation rate varies between 1 µm / min and 5 µm / min, and the final thickness varies between 20 µm and 200 µm. The ability to obtain significant thicknesses is a considerable advantage because it facilitates subsequent grinding operations of the parts and therefore makes it possible to meet tight dimensions.

[0127] Hardness: The process of the invention makes it possible to achieve very high hardnesses, on the order of 1000 Hv to 1900 Hv depending on the aluminum alloy treated, i.e. four times more than with hard anodizing (OAD).

[0128] Corrosion resistance: Corrosion resistance has also been tested. Resistance to salt spray has proven to be greater than 1000 hours (for coating thicknesses of 80 pm and depending on the aluminum alloys treated).

[0129] Wear: Tribological tests performed on a 95 µm thick (ground) coating showed no signs of wear (< 1 µm). For comparison, the same test was performed on a 35 µm thick (ground) coating produced by OAD, this technique being the one that currently offers the best performance in terms of friction and wear resistance in industry. At the end of the test, the coating's wear depth was 100%, reaching the aluminum substrate. This demonstrates the high friction and wear resistance of the coating obtained by the OMA process according to the invention compared to the reference process by OAD.

[0130] Adhesion: Good adhesion and cohesion of the layer on the substrate are observed.

[0131] Temperature resistance: The coating exhibits excellent temperature stability from -110°C to 300°C, which can be attributed to its crystalline structure. In particular, the coating is resistant and stable at high temperatures. Indeed, a flame test (fire resistance) showed that when the melting temperature of the aluminum substrate is reached, it degrades while the coating remains resistant. (The melting temperature of aluminum is the limiting factor here.)

[0132] Roughness: The roughness of the coating varies depending on the thickness, but also on the processing parameters and the electrolyte used. The greater the layer thickness, the higher the roughness. It can be modified by additional treatments, such as microblasting, honing, grinding, etc., which those skilled in the art know how to choose and implement according to the intended application. The value of the arithmetic mean deviation Ra, measuring the roughness, is, for example, for a 2618 A type aluminum alloy coated with a protective layer according to the invention: - for a raw layer of 80 pm, Ra = 4.5 pm - 5 pm - for a raw layer of 120 pm, Ra = 5.5 pm - 6 pm - after microblasting, Ra = 2.5 pm - 3.5 pm (for a raw thickness between 80 pm and 200 pm) - after rectification, Ra = approximately 0.15 pm and can go up to 0.2 pm (for a gross thickness between 80 pm and 200 pm).

[0133] EXAMPLE 8: Production of a thin coating on aluminium-copper alloy

[0134] The aim here is to develop a coating with a thickness of less than 100 pm on a Al-Cu alloy, of the type 2024 alloy (according to the Aluminum Association-USA notation) or Al Cu4Mgl (according to European standards), which include copper as the main alloying element. Their composition is Al: base; Copper: 4.4; Mg: 1.6; Mn: 0.6.

[0135] A 2024 aluminium alloy part has been subjected to an OMA treatment composed of three sequences SI, S2 and S3, applying the following parameters: - RI = 60%; R2 = 62%; R3 = 65% - T1 = 15 min; T2 = 10 min; T3 = 5 min (total treatment time = 30 min) - Frequency of the three sequences: 100 Hz

[0136] The coating obtained has a thickness of 50 µm ± 10% (average over 5 test pieces). The E / T factor, which is equal to the coating growth rate (total thickness obtained / total treatment time), is determined. It is approximately 1.7 µm / min.

[0137] After characterization, the coating exhibits the following properties: - a mass loss of less than 40% of the initial total thickness, which is low for an OMA coating. This is advantageous in terms of abrasion and wear resistance properties; - a hardness between 1300 Hv and 1600 Hv (average of 1456 ± 50 Hv across the 5 specimens). For comparison, the hardness after treatment by OAD is between 326 Hv and 348 Hv (with an average of 336 ± 8 Hv).

[0138] EXAMPLE 9: Production of a thick coating on Aluminium-Copper alloy

[0139] A coating thicker than 100 pm was developed on an Al-Cu alloy of the same type as previously described. A part was subjected to an OMA treatment consisting of three SI, S2, and S3 sequences, applying the following parameters: - RI = 60%; R2 = 62%; R3 = 65% - T1 = 40 min; T2 = 30 min; T3 = 20 min (total treatment time = 90 min) - Frequency of the three sequences: 100 Hz

[0140] The coating obtained has a thickness of 150 µm ± 10% (average over 5 test specimens). The water-to-air ratio is approximately 1.7 µm / min.

[0141] After characterization, the coating exhibits the following properties: - resistance to salt spray for more than 500 hours; - a mass loss of less than 40% of the initial total thickness. This is low for an OMA coating and particularly interesting for resistance and wear properties; - a hardness between 1500 and 1900 Hv, (average of 1697 ±115 Hv over the 5 specimens).

[0142] EXAMPLE 10: Production of a thick coating on Aluminium-Zinc alloy

[0143] This example involves developing a coating with a thickness greater than 100 µm on an aluminum-zinc alloy, of the type of alloys designated 7175 (according to the Aluminum Association-USA notation) or AlZn5.5MgCu (according to European standards) which have zinc as the main alloying element. Their composition is Al: base; Zn: 5.6; Cr: 0.23; Cu: 1.6; Mg: 2.5.

[0144] A part made of aluminum alloy 7175 was subjected to an OMA treatment consisting of three sequences, applying the following parameters: - RI = 60%; R2 = 62%; R3 = 65%; - T1 = 30 min; T2 = 20 min; T3 = 10 min (total treatment time = 60 min); - Frequency of the three sequences: 200 Hz.

[0145] The coating obtained has a thickness of 150 µm ± 10% (average over 6 test specimens). The water-to-air ratio is approximately 2.5 µm / min. After characterization, the coating exhibits the following properties: - resistance to salt spray for more than 500 hours; - a mass loss of less than 40% of the initial total thickness; - a hardness between 1500 Hv and 1800 Hv (average of 1623 ± 100 Hv over 6 test specimens).

Claims

Demands

1. Electrolytic treatment installation by micro-arc oxidation of elongated parts (3) extending along a longitudinal axis Ap between a first and a second end, each part (3) to be treated acting as a first electrode, the installation comprising: - a tank (1) suitable for containing an electrolytic bath (2) consisting of an aqueous solution, - a tool (100) comprising a support (10) and at least one counter electrode (4) of the shape, - a generator (5) comprising means for electrical connection to each part (3) and to each counter electrode (4), said generator being capable of delivering an electrical signal to subject each part (3) to a surface treatment, the installation being characterized in that the support (10) is provided with means for assembling at least one part (3) with a counter electrode (4) of the shape at a determined distance De from each other, said assembly means comprising a base (11) provided with means for fixing the first end of each part (3), and means for fixing the second end of each of said parts, said fixing means being held together in two planes parallel to each other by at least one rod (40) provided with means for locking said parts in compression along their longitudinal axis Ap, and in that each counter electrode (4) adopts the shape of a cylinder with longitudinal axis Ac,and is fixed to the base (11) of the support (10) so that its longitudinal axis Ac coincides with the longitudinal axis Ap of the corresponding part (3), so that the surface of each of said parts is at a determined radial distance De, constant or variable, from the counter electrode (4).

2. Installation according to claim 1, for treating the external surface of elongated parts extending along a longitudinal axis Ap between a first and a second end (32, 33), characterized in that the tooling (100) comprises: - a support (10) comprising a base (11) in which are provided fastening means adapted to receive each the first end (32) of a part (3), an upper chassis (12) in which are provided fastening means adapted to receive each the second end (33) of said part, the base (11) and the chassis (12) being held together in two parallel planes by a series of rods (40) equipped with means for locking said parts in compression along their longitudinal axis Ap, - counter electrodes (4) adopting the form of hollow cylinders with longitudinal axis Ac, each capable of receiving a part (3) in its central light, each counter electrode (4) being fixed to the base (11) of the support (10) so that its longitudinal axis Ac is coincident with the longitudinal axis Ap of the corresponding part, so that each of said parts is surrounded by a counter electrode (4) at a determined radial distance De from it, constant or variable, and - means for electrically connecting said parts and said counter electrodes to the generator (5).

3. Installation according to the preceding claim, characterized in that the support (10) comprises the base (11) in which are provided blind housings (16) adapted to receive by sliding each the first end (32) of a part (3), the upper frame (12) having perforations (19) opposite each housing (16), adapted to receive each the second end (33) of said part, either directly, or by means of a joining element.

4. Installation according to the preceding claim, characterized in that the base (11) is formed of an upper plate (14) and a lower plate (13) between which is sandwiched a plate (15) made of electrically conductive metal, the upper plate (14) having the housings (16) the bottom of which is closed by said plate so that said at least one part (3), once inserted into a housing (16), is in contact with the plate (15), the lower plate (13) being provided with electrical contact pads between said plate and the means of connection to the generator (5).

5. An installation according to claim 1 for treating the inner surface of hollow tubular parts extending along a longitudinal axis Ap between two end edges (42, 43), characterized in that the tooling (100) comprises: - a support (10) comprising a parallelepiped-shaped box (110) formed of side walls (112), two front walls (114), a base (11) and a cover (113), the inner faces of said base and said cover having circular recesses each adapted to receive as support an end edge (42, 43) of a hollow tubular part (3), the base (11) and the cover (113) being held together in two planes pa- parallels to each other by at least one rod (40) cooperating with means for locking said parts in compression along their longitudinal axis Ap, - counter electrodes (4) adopting the form of cylinders with axis Ac, each suitable for being placed in the light of one of said tubular parts, each counter electrode (4) being fixed to said support so that its longitudinal axis Ac is coincident with the longitudinal axis Ap of the corresponding part, so that said part surrounds the corresponding counter electrode at a radial distance De from it, constant or variable, and - means for electrically connecting said parts and said counter electrodes to the generator (5).

6. Installation according to the preceding claim, characterized in that each circular location of the base (11) and the hood (113) is traversed in its center by a rod (40), each having a lower end segment (44) and an upper end segment (45) extending on either side outside the box (110) and cooperating with said compression locking means of the parts (3).

7. Installation according to the preceding claim, characterized in that the locking means cooperating with each rod (40) comprise a lower bar (46) and an upper bar (47) bearing against the respective external faces of the base (11) and the hood (113), said bars having a tapped hole in which the threaded end segments (44, 45) of said rod are engaged.

8. Installation according to any one of claims 5 to 7, characterized in that each of the rods (40) has a threaded median segment (48), and each counter electrode (4) has a tapped hole (49) made along its axis Ac in which said rod is engaged by screwing, so that said counter electrode is held coaxially in the central light of a part (3) at a determined height.

9. Installation according to any one of the preceding claims, characterized in that the counter electrodes (4) each adopt a conformation complementary to that of the part (3) to be treated or of an area of ​​said part to be treated, placed opposite said counter electrode, the surface of said part or area to be treated being at all points at a constant or variable radial distance De from said counter electrode of between 5 mm and 50 mm.

10. Installation according to any one of the preceding claims, ca- characterized in that it includes a temperature control system for the electrolytic bath (2), comprising an external recirculation loop (200) equipped with a heat exchanger (230), temperature sensors (221) and a pump (220) capable of imposing the permanent circulation of a fraction of electrolytic solution via said heat exchanger, between an outlet orifice (201) made at the base of the tank (1) near a first lateral wall of said tank, and a weir (202) placed above said tank a short distance from a second lateral wall of said tank opposite said first wall, the weir comprising a horizontal distribution ramp (203) having a plurality of nozzles (204) through which tempered electrolytic solution can flow.

11. Installation according to any one of claims 5 to 8 in combination with claim 10, characterized in that the temperature control system of the electrolytic bath (2) further comprises a temperature control device in the box (110), which comprises at least one inlet hole (205) of electrolytic solution provided in a wall of the box, connected to a nozzle (204) by a conduit (206), and at least one outlet hole (207) provided at a distance in another wall of said box.

12. Use of the installation according to any one of the preceding claims, wherein the parts (3) to be treated acting as the first electrode are made of metal selected from aluminium, titanium, magnesium, or an alloy thereof.

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