CHEMICAL POLISHING BATH FOR TITANIUM AND TITANIUM ALLOYS, AND METHOD USING SUCH A BATH

Description

technical field

[0001] The present invention relates generally to the field of surface treatment of titanium or titanium alloy parts, and in particular to the field of polishing titanium or titanium alloy parts. The invention is especially suited to polishing parts produced by additive manufacturing. State of the art

[0002] Additive manufacturing, or 3D printing, is a major focus of R&D in France and worldwide, enabling the production of three-dimensional polymer or metal parts from powders. Thanks to the successive melting of powders, additive manufacturing processes allow for the production of parts with complex geometries and also open the door to the production of new alloy grades inaccessible through traditional metallurgy, all while minimizing the amount of raw material required. Additive manufacturing is a particularly attractive technology for the aerospace, automotive, and naval industries, offering the potential to improve performance and reduce the weight of structures.However, powder bed fusion additive manufacturing processes inherently produce parts with a final surface finish that bears the hallmarks of powder-based manufacturing, particularly surface roughness (expressed by the Ra parameter) ranging from 5 to 25 µm depending on the process—significantly higher than the roughness of a machined part (on the order of a few microns). Furthermore, the produced parts are generally characterized by the presence of partially melted particles on their outermost surfaces, which can pose a risk of detachment during operation and thus degrade the performance of these parts (clogging of pipes, entrainment of particles in hydraulic systems, reduction of static and dynamic mechanical properties, etc.).

[0003] It is therefore necessary to reduce the roughness of additively manufactured parts, but also to eliminate surface irregularities and thus prevent the risk of particle / foreign body detachment from the surface during operation, which could generate functional problems related to the use of these parts. Improving the surface finish of the produced parts can also enhance certain performance characteristics in terms of fatigue and corrosion resistance, or even flow on surfaces for hydraulic applications.

[0004] Today, such a post-processing step represents a major technological barrier for the development of functional metal parts from additive manufacturing with controlled and reproducible characteristics.

[0005] Various polishing processes exist, such as electropolishing, plasma polishing, or laser polishing, but these processes require the use of electricity or even lasers, resulting in a high associated cost. Furthermore, the use of lasers or working with high electrical currents necessitates special operator protection. Finally, these complex processes require precise control of their parameters. Electropolishing baths are also difficult to manage because they are highly hygroscopic, requiring periodic dehydration.

[0006] Chemical polishing processes, which involve immersing the part to be treated in a bath, called a chemical polishing bath, containing acidic or basic compounds and oxidizing agents, without the application of an electric current or a laser, are simpler to implement.

[0007] An example of a chemical polishing process is described, for instance, in CN 105 297 026 A, which discloses the use of a bath containing at least one vegetable oil, two oxidizing agents in the form of nitric acid and hydrogen peroxide, and a complexing agent in the form of hydrofluoric acid. Besides its complexity, this bath has the disadvantage of using strong oxidizing and complexing agents for titanium, which rapidly attack the surface of a titanium part being polished, and the line between polishing and machining is blurred when using such a bath. Furthermore, it requires handling hydrofluoric acid, which is extremely corrosive and toxic, and therefore the implementation of such a polishing bath necessitates the implementation of specific protective measures for operators.

[0008] A chemical polishing process for polishing a part made of titanium or titanium alloy is described in each of the documents FR 1589844 A and JP S50 8689 B1.

[0009] The objective of the invention is to provide a polishing bath for titanium or titanium alloy parts, which does not have the disadvantages mentioned above and which makes it possible to obtain a surface with an average roughness (expressed by the parameter Sa) typically less than 4 or 5 µm and a limited material removal, typically less than 300 to 350 µm. General description of the invention

[0010] With this objective in mind, the present invention relates to a chemical polishing bath for titanium and titanium alloy parts, and a method using such a bath.

[0011] Chemical polishing aims to reduce the surface roughness of a metal part through the selective dissolution of a layer of material of varying thickness, ranging from tens of nanometers to several tens of micrometers. This material removal is achieved chemically, using reagents that can be acidic or basic, and may also possess oxidizing properties. It is important to note that the dissolution caused by the chemical polishing process has a direct impact on the part's dimensions, even if this is not the primary objective. This distinguishes it from chemical machining, whose sole purpose is to reduce the part's dimensions (by several tens or hundreds of microns) to achieve a target size.

[0012] The principle of chemical polishing also differs from that of chemical pickling, which is more akin to chemical machining. Indeed, chemical pickling aims to remove an oxide layer, a few microns thick, without affecting the dimensional characteristics of the part or its surface roughness, but rather to promote adhesion of the surface treatment applied subsequently.

[0013] A difference can also be noted compared to chemical degreasing / decontamination steps, the purpose of which is to solubilize chemical species adsorbed on the surface of the material being treated. No impact on the dimensional characteristics of the part is observed in this latter case.

[0014] Thus, in this text, polishing is understood to mean reducing the surface roughness of a part, while changing its dimensions as little as possible.

[0015] According to the invention, a chemical polishing bath for polishing a part made of titanium or titanium alloy, or a part thereof, comprises: water; a fluoride complexing agent capable of forming a complex with oxidized titanium, at an equivalent fluoride concentration of between 1.40 and 2.80 mol / L; phosphoric acid at a concentration of between 3.0 and 6.0 mol / L (i.e. between 290 and 590 g / L); and sulfuric acid at a concentration of between 0.5 and 1.5 mol / L (i.e. between 50 and 150 g / L).

[0016] The chemical polishing bath is therefore in the form of an aqueous solution with a high acid concentration. Thus, the pH of a bath according to the invention is less than 2.0, preferably less than 1.0, and particularly preferably a bath according to the invention has a negative pH. Advantageously, the concentrations of phosphoric acid and sulfuric acid are such that the pH of a bath according to the invention is negative and remains negative during use of the bath, even with the consumption of acids during the polishing of titanium or titanium alloy parts.

[0017] One of the merits of the invention is to have identified compounds, and their respective concentrations, which allow a significant reduction in roughness, without significantly increasing the discount, whereas these two effects generally require antagonistic parameters.

[0018] The solution according to the invention comprises two acids and a titanium oxide complexing agent. The bath density is controlled by the mixture of sulfuric and phosphoric acids. Indeed, in order to achieve a polishing effect, this bath generally has a higher viscosity, or density, than other chemical etching solutions, e.g., those used in chemical machining. Thus, the aggressiveness of this solution differs from conventional pickling or chemical machining solutions, even when using similar reagents.

[0019] Sulfuric acid, like phosphoric acid, possesses oxidizing properties, although these acids are not generally considered oxidizing agents, unlike, for example, nitric acid. The oxidizing properties of sulfuric and phosphoric acids are, however, sufficient to oxidize the surface of a titanium or titanium alloy part. Indeed, titanium is a metal highly susceptible to oxidation. Thus, advantageously, according to preferred embodiments, no oxidizing agent other than sulfuric and phosphoric acids is added to the polishing bath according to the invention.

[0020] The higher viscosity of the polishing solution slows the diffusion of chemical species from the solution to the surface of the workpiece, where they are consumed. The goal is to make diffusion slower than the reaction rate. The surface thus becomes depleted of etching agents (sulfuric or phosphoric acid) and complexing agents, as these are consumed faster than they reach the surface. Their arrival therefore occurs first near the crests of the raised areas of the surfaces to be polished, where their action is prioritized. Conversely, the recesses will not receive as many reactants (etching agents and complexing agents), as these have already been consumed by a reaction on the most prominent points of the workpiece. This phenomenon is thus responsible for a surface polishing effect through the selective dissolution of surface irregularities.

[0021] Within the framework of the invention, speaking of an increase in the viscosity of the bath is equivalent to speaking of an increase in its density, and vice versa.

[0022] According to the current understanding of the invention, the effectiveness of this chemical polishing bath relies on controlling the diffusion kinetics of sulfuric and phosphoric acids and complexing agents within the bath towards the surface of the part to be polished, as well as the reaction kinetics of the acids with the surface of the part to be polished. The operating principle of the chemical polishing bath according to the invention is as follows. An appropriate viscosity (higher than that of conventional chemical etching baths) of the chemical polishing bath slows down the diffusion of the active chemical species (here, the acids – sulfuric and phosphoric – and the complexing agents) from the solution towards the surface of the part. In the vicinity of the surface, the active species are consumed, resulting in a local depletion of the bath.To restore the balance of concentrations, the active species diffuse from the bath to the surface, reacting as soon as they come into contact with the surface to be polished. Since the most accessible areas are the peaks of the surface roughness, the attack on these peaks is favored, thus reducing the surface relief and therefore the overall roughness of the part.

[0023] Initial tests were conducted with the present polishing bath, using bath volumes ranging from 100 mL to 600 L, and proved its effectiveness. The chemical polishing bath according to the invention was specifically developed for polishing parts produced using additive manufacturing techniques, but can be applied to all types of titanium or titanium alloy parts, regardless of the manufacturing process. Furthermore, the dimensions and / or geometries can vary considerably. In addition, the titanium part to be treated can be part of a larger component. That is to say, the titanium part to be treated can be combined with another material, to which it is juxtaposed or superimposed.

[0024] The preferred fluoride complexing agents are in the form of a fluorinated salt. In other words, hydrofluoric acid itself is not used as a source of fluoride. However, it is not excluded from the present invention that hydrofluoric acid may form in the bath following the dissolution of these salts. Hydrofluoric acid is highly aggressive towards titanium, whereas the addition of a fluorinated salt serves only a complexing function. Furthermore, the use of a fluorinated salt reduces the hazards associated with assembling the solution. In other words, the fluoride complexing agent is advantageously added to the solution in salt form, rather than as pure hydrofluoric acid, because salts are less hazardous to handle.

[0025] Advantageously, the fluoride complexing agent added to the chemical polishing bath is selected from NH₄F, NH₄F·HF, NaF, NaF·HF, KF, KF·HF, H₂TiF₆, H₂SiF₆, or mixtures thereof. The role of the fluoride complexing agent in the invention is to combine with the oxidized titanium (in the form of hydrated titanium oxide, particularly as TiO₂, TiO(OH)₂, and / or Ti(OH)₄) that forms on the surface of the workpiece in the presence of sulfuric and phosphoric acids, or that may be in solution.

[0026] Preferred fluoride complexing agents are in the form of a fluoride salt and hydrofluoric acid. In other words, it is a weak acid in the presence of its anion, the fluoride ion or F⁻. Thus, the complexing agent used also has a buffering function, which makes it possible to control the pH of the chemical polishing bath.

[0027] Furthermore, fluoride ions (or F⁻) are capable of complexing a variety of metals, which is advantageous in the case of titanium alloy parts. Moreover, fluorides are extremely stable complexing agents in solution, and the risk of their degradation over time and the formation of uncontrolled byproducts is minimal.

[0028] Depending on the variant, the fluoride complexing agent is present in such a way that the fluoride concentration is between 1.74 and 2.46 mol / L, preferably between 1.80 and 2.20 mol / L.

[0029] Depending on the variant, phosphoric acid is present at a concentration between 4.0 and 5.5 mol / L, i.e. between 390 and 540 g / L, preferably between 4.5 and 5.0 mol / L, i.e. between 440 and 490 g / L.

[0030] Depending on the variant, sulfuric acid at a concentration between 0.50 and 1.25 mol / L, i.e. between 50 and 125 g / L, preferably at a concentration between 0.80 and 1.02 mol / L, i.e. between 80 and 100 g / L.

[0031] During use, the polishing bath becomes saturated with dissolved titanium due to the chemical attack on surface roughness. It is possible to regenerate such a bath after use by simply adding back the compounds consumed during polishing (i.e., acids and complexing agents), and in this case, the regenerated bath still contains titanium, as is also the case with traditional chemical polishing baths. However, traditional baths are destroyed when the titanium concentration becomes too high. The presence of titanium can indeed disrupt the bath's operation and impair polishing performance, notably by slowing the reaction kinetics, which could negatively impact the polishing process and compete with the diffusion of the reagents (sulfuric acid, phosphoric acid, and complexing agents).It is therefore preferable to define the permissible limit concentrations of titanium that do not degrade the chemical mechanism of polishing. Advantageously, titanium is present at a concentration ranging from approximately 2 to 35 g / L, or approximately 0.04 to 0.73 mol / L, preferably at a concentration ranging from approximately 10 to 30 g / L, or approximately 0.20 to 0.63 mol / L.

[0032] The accumulation of dissolved titanium in the bath during the use of this chemical polishing bath occurs along a concentration gradient from the surface of the part to be polished towards the bath volume. The higher titanium concentration near the surface of the part advantageously also increases the viscosity in this area. Thus, the viscosity of the bath increases as it approaches the surface of the part, promoting the removal of surface roughness peaks (i.e., the removal of valleys is slower than the removal of peaks). This phenomenon contributes to the polishing effect on the surface of the titanium or titanium alloy part by selectively dissolving surface asperities and minimizes depreciation.

[0033] Depending on the variants, the bath has a density ranging from 1.2 to 1.4 g / cm³.

[0034] Depending on the variant, the bath is essentially free of abrasive particles, in particular the bath is essentially free of metal oxide particles, especially silica (SiO₂) particles. In this text, "essentially free" means that no abrasive particles are added to the bath according to the invention. High concentrations of acids (sulfuric acid and phosphoric acid) as well as the presence of fluorides prevent the formation of oxide particles, especially titanium dioxide (TiO₂) particles. However, baths in which particles would form or fluorides would precipitate, for example, baths in which titanium fluoride (TiF₄) precipitates would form during the use of the bath in a chemical polishing process, also constitute baths according to the invention.Thus, a chemical polishing bath according to the invention can contain precipitates of metallic fluorides and be essentially free of abrasive particles within the meaning of the invention.

[0035] In some variations, no oil, particularly mineral or vegetable oil, is added to this chemical polishing bath (the bath is therefore typically free of these compounds). Advantageously, the high viscosity of the solution is due solely to the presence of sulfuric acid, phosphoric acid, and titanium in the bath.

[0036] According to a particularly preferred embodiment, the present polishing bath is aqueous and essentially consists of sulfuric and phosphoric acids, titanium, and a fluoride complexing agent. In other words, the present bath comprises less than 10 mol%, preferably less than 5 mol%, particularly preferably less than 2 mol%, of compounds that are not selected from water, sulfuric acid, phosphoric acid, a fluoride complexing agent (in particular fluorinated salts), titanium (in particular titanium salts, especially H₂TiF₆), or mixtures thereof.

[0037] In the context of the invention, the concentration ranges mentioned should be understood in the broadest possible sense, i.e., including the upper and lower limit values ​​of said concentration ranges. As used in this text, the term "approximately" means a range of values ​​within ±10% of the stated value.

[0038] In another aspect, the invention proposes a method for chemically polishing a part made of titanium or titanium alloy, or a part thereof, using the chemical polishing bath as described above.

[0039] The method according to the invention comprises at least the following steps: provide a chemical polishing bath as described above; immerse a titanium or titanium alloy part to be polished, or a part thereof, in the chemical polishing bath; remove the part from the bath after a predetermined immersion time.

[0040] To control the concentrations of reagents, particularly sulfuric and / or phosphoric acids, and complexing agents in the polishing bath, these concentrations can be determined by titration. The different chemical species can be titrated by acid-base titration in a non-aqueous medium, by thermotitration, or by spectrophotometry. Alternatively, the different reagents can also be titrated by ion chromatography or inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0041] The temperature of the polishing bath when the part to be polished is immersed is between 30°C and 60°C, preferably between 40°C and 50°C. The immersion time of the titanium or titanium alloy part in the polishing bath is between 20 and 60 minutes, preferably between 30 and 50 minutes. The bath temperature influences the reaction kinetics of the polishing process, and in particular the rate at which the titanium is etched. The higher the temperature, the faster the etching agents (especially sulfuric and / or phosphoric acid) and complexing agents act. The difference between the reaction rate of the reactants and their diffusion is accentuated, which improves the polishing process and the results obtained, i.e., a reduction in surface roughness while limiting the depreciation.

[0042] Thanks to the process according to the invention, which uses a polishing bath at a specific temperature and immerses the part or portion thereof to be polished for a particular time, it is possible to obtain a significant reduction in the surface roughness of the part without significantly increasing the discount. These two effects generally require opposing parameters: immersing a titanium or titanium alloy part for a longer period in the polishing bath promotes a reduction in roughness but increases the discount. Surprisingly, applying an immersion time of 20 minutes is sufficient to obtain a residual surface roughness (Sa) of less than 5 µm, and even less than 3 or 2 µm, while limiting the discount to approximately one hundred micrometers.In other words, in the present context, the discount observed for a part polished using a chemical polishing bath or a chemical polishing process according to the invention is less than 300 to 350 µm. The parameter Sa can be considered more relevant than Ra when quantifying the roughness, or a decrease in roughness, of a surface because it is an arithmetic mean roughness measured over a surface and not solely along a line like Ra.

[0043] Furthermore, titanium or titanium alloy parts polished using a chemical polishing bath according to the invention also advantageously exhibit a maximum roughness, or height, on a residual surface (Sz) of less than 50 µm, and even less than 40 µm.

[0044] Advantageously, a degreasing and / or chemical pickling step can be performed on the surface of the part to be polished before immersing said part, or the portion thereof, in the chemical polishing bath. Particularly advantageously, the chemical polishing bath is sufficiently aggressive to remove any residual oxide formed on the surface of the part to be polished, and a degreasing step is sufficient before treating the part with the process according to the invention. Such a surface preparation step cleans the surface, particularly to remove grease deposited on the surface of the part during handling, and notably facilitates access to the surface for acids (enabling, among other things, surface oxidation) and / or complexing agents that react with said surface during the chemical polishing process, thus increasing its effectiveness.

[0045] The chemical polishing bath according to the invention, or the process using such a bath, is particularly well suited to titanium alloys.

[0046] The present chemical polishing bath and polishing process have been developed in particular for the treatment of titanium alloy parts of allotropic form alpha, beta or biphasic alpha-beta or titanium alloy of type alpha, near alpha, beta, near beta or biphasic alpha-beta, in particular the titanium alloys can be TA6V, T40, TV10A3Fe2, Ti6242, β21, β21S, Ti 555-3, more particularly the titanium alloy is the TA6V or T40 alloy, commonly used in industry and in additive manufacturing.

[0047] Advantageously, the part to be polished in whole or in part using a chemical polishing bath according to the invention or the process using such a bath is a part made of titanium or titanium alloy obtained by an additive manufacturing process, or three-dimensional printing.

[0048] Thus, the present chemical polishing bath or chemical polishing process is particularly applicable for the treatment of parts produced for the aerospace, aeronautical, and automotive industries, to eliminate surface irregularities and thus prevent the risk of particle detachment from the surface of said parts in service, which could generate functional problems related to their use such as the clogging of pipes, or the reduction of their static and / or dynamic mechanical properties. Brief description of the figures

[0049] Other features and characteristics of the invention will become apparent from the detailed description of at least one advantageous embodiment presented below by way of illustration, with reference to the accompanying drawings. These show: [ Fig. 1 ] representative diagrams (points 1 to 6) of the operating principle of the present process; [ Fig. 2 ] confocal microscope images allowing comparison of the surface condition of a TA6V titanium alloy part after immersion in a chemical polishing bath according to a first embodiment of the present invention for 0 min (a), 1 min (b), 2 min (c), 10 min (d), 20 min (e) and 30 min (f); [ Fig.3 ] a graph representing the evolution of the roughness Sa (roughness - immersion time) for a sample immersed in a chemical polishing bath according to a first embodiment of the present invention for different durations; ] Fig. 4] a graph representing the evolution of the roughness Sz (roughness - immersion time) for a sample immersed in a chemical polishing bath according to a first embodiment of the present invention for different durations; ] Fig. 5 ] a graph representing the evolution of the discount (discount - immersion time) for a sample immersed in a chemical polishing bath according to a first embodiment of the present invention for different durations; and [ Fig. 6 ] a graph representing both the evolution of the roughness Sa, Sz and the discount (Sa, Sz, discount - titanium concentration) for different samples immersed in chemical polishing baths according to other embodiments of the present invention. Detailed description with examples

[0050] The operating principle of the polishing bath according to the invention will first be explained with reference to the figure 1As explained previously, the present invention surprisingly proposes a polishing bath that allows for a significant reduction in roughness without increasing the discount, whereas these two effects generally require opposing parameters.

[0051] The increased viscosity of the bath, controlled by sulfuric and phosphoric acids, slows the diffusion of chemical species from the solution to the surface of the part. This depletion of active species on the surface, due to their consumption near the surface, is responsible for a surface polishing effect through the selective dissolution of the surface texture.

[0052] The principle is represented at the Figure 1 : 1): attack and oxidation of the material by sulfuric and phosphoric acids and formation of a first passivation layer (metallic oxides from titanium and alloying elements where applicable); 2, 3): complexation of titanium oxides, and alloying oxides where applicable, by complexing agents to solubilize the oxides; 4, 5): slow diffusion of sulfuric and phosphoric acids (which allow among other things the oxidation of the surface) from the solution towards the surface of the alloy to be polished, the high density of the polishing bath allowing to slow down the arrival of the oxidizing compounds (sulfuric and phosphoric acids); 6): attack of the upper parts of the relief, leading to the reduction of the roughness. Examples:

[0053] We will describe below 9 examples of polishing titanium parts in 9 different baths, baths 1 to 9, according to the invention, as well as 4 examples of treating titanium parts in 4 different baths, baths 10 to 13, not according to the invention (comparative examples).

[0054] Thirteen TA6V titanium alloy parts, labeled A through M, produced using the same additive manufacturing process, each take the form of a 40x20 mm plate. The top face is the face to which the material layers are added sequentially during the manufacturing process, while the bottom face is the opposite face. The top face of each part typically exhibits a surface roughness (Sa) of approximately 30 µm and a surface roughness (Sz) of approximately 300 µm.

[0055] Parts A and J through M first underwent a degreasing step to prepare their surface, using a standard procedure well known to those skilled in the art. Parts B through I did not undergo any surface preparation. All 13 parts A through M were then partially covered with masking tape.

[0056] The partially masked titanium alloy parts A to I thus obtained were then subjected to chemical polishing processes using polishing baths of different compositions, referenced 1 to 9, according to 9 preferred but non-limiting embodiments of the invention. Furthermore, the partially masked titanium alloy parts J to M thus obtained were subjected to chemical polishing processes using comparative polishing baths of different compositions, not conforming to the present invention, referenced 10 to 13. The 13 different baths 1 to 13 were prepared in similar tanks, with solution volumes of 1 L; their compositions are detailed in Table 1. The baths corresponding to the counterexamples are identified in Table 1 with an asterisk (*).

[0057] All baths are maintained at a temperature of 40°C.

[0058] The titanium alloy part A was immersed in bath 1 for a variable duration between 1 and 30 minutes. In particular, observations of the surface of part A were carried out after immersion times of 1 minute, 2 minutes, 10 minutes, 20 minutes and 30 minutes.

[0059] Titanium alloy part B was immersed in bath 2, part C was immersed in bath 3, part D was immersed in bath 4, part E was immersed in bath 5, part F was immersed in bath 6, part G was immersed in bath 7, part H was immersed in bath 8, part I was immersed in bath 9, part J was immersed in bath 10, part K was immersed in bath 11, part L was immersed in bath 12 and part M was immersed in bath 13.

[0060] Each of the parts B to M were immersed respectively for 50 min in baths 2 to 13.

[0061] For each of the parts, the area covered by the masking tape is not in direct contact with the bath in which the part is immersed and does not undergo any change in its surface condition. [Table 1] Bath H₂SO₄ g / L (mol / L) H3PO4 g / L (mol / L) Titanium g / L (mol / L) NH4F.HF g / L (mol / L) Equivalent concentration of fluoride mol / L Piece Immersion time (min) Its sa (µm) Discount (µm) Bath 1 100 (1,02) 380 (3,9) 2 (0,04) 63 (1,11) 2,22 A Variable 4,57 ± 0,05 110 ± 20 Bath 2 100 (1,02) 380 (3,9) 3 (0,06) 63 (1,11) 2,22 B 50 4,2 ± 0,4 120 ± 3 Bath 3 100 (1,02) 380 (3,9) 5 (0,1) 63 (1,11) 2,22 C 50 4,7 ± 0,7 144 ± 20 Bath 4 100 (1,02) 380 (3,9) 7 (0,14) 63 (1,11) 2,22 D 50 3,8 ± 0,9 100 ± 10 Bath 5 100 (1,02) 380 (3,9) 10 (0,21) 63 (1,11) 2,22 E 50 3,5 ± 0,6 140 ± 30 Bath 6 100 (1,02) 380 (3,9) 15 (0,31) 63 (1,11) 2,22 F 50 3,3 ± 0,5 130 ± 20 Bath 7 100 (1,02) 380 (3,9) 20 (0,41) 63 (1,11) 2,22 G 50 3,2 ± 0,3 130 ± 10 Bath 8 100 (1,02) 380 (3,9) 28 (0,58) 63 (1,11) 2,22 H 50 2,9 ± 0,1 120 ± 20 Bath 9 100 (1,02) 380 (3,9) 34 (0,71) 63 (1,11) 2,22 I 50 2,95 ± 0,05 130 ± 2 Bath 10 * 98 (1) 0 (0) 5 (0,1) 34 (0,6) 1,2 J 50 6,6 ± 0,1 1200 ± 100 Bath 11 * 686 (7) 0 (0) 5 (0,1) 34 (0,6) 1,2 K 50 12,7 ± 0,5 600 ± 100 Bath 12 * 588 (6) 0 (0) 5 (0,1) 68 (1,2) 2,4 L 50 19,2 ± 0,5 400 ± 100 Bath 13 * 98 (1) 882 (9) 5 (0,1) 68 (1,2) 2,4 M 50 7,7 ± 0,4 280 ± 30

[0062] Within the framework of the present invention, the high viscosity of the polishing bath (typically the density is between 1.2 and 1.4 g / cm³) and the local inhomogeneities in concentration (due to the thickness of the diffusion layer) of the complexing agent are two parameters that allow for control of the polishing and promote the chemical etching of the peaks. The agitation of the bath when the workpiece is immersed in it is strictly controlled. For example, the bath can be agitated by recirculation at a rate such that the bath is renewed two to three times per hour.

[0063] The bath is preferably agitated at a lower rate when the workpiece is immersed (treatment phase) than during the preparation of the polishing bath (homogenization phase). This agitation rate is typically between 5 and 10 L / min during the treatment phase and around 100 L / min during the homogenization phases. These agitation rate values ​​are given by way of example only and do not constitute a limitation of the present invention. In particular, since the agitation rate influences the renewal of chemical species present in the liquid layer, called the diffusion layer, in the vicinity of the workpiece, a lower agitation rate allows the formation of a thicker diffusion layer, which slows the diffusion of the reagents and promotes polishing.

[0064] After immersion in the polishing bath, each treated part is rinsed and then observed under a confocal microscope to determine the reduction in its roughness and the discount. Specifically, for each part, the masking tape is removed, and an area comprising a treated surface and an untreated surface (previously masked by the tape) is observed using an Olympus DSX510 confocal microscope. The topography of each part (upper surface and, for some parts, the lower surface as well) is recorded and analyzed to determine the discount due to material removal and the variations in roughness parameters resulting from the polishing effect of the different baths, particularly the variations in Sa and Sz.

[0065] The surface roughness obtained after chemical polishing (treated surface) for titanium alloy parts A to I treated using baths according to the invention, as well as the reduction in the parts that occurred during this polishing, are reported in Table 1 and in Figures 2 to 6 . The surface roughnesses obtained at the end of chemical polishing (treated surface) for titanium alloy parts J to M treated using comparative baths which are not according to the invention, as well as the discount on the parts which took place during this polishing are reported in Table 1.

[0066] All parts A to M obtained by a substantially identical manufacturing process have a similar surface condition before polishing, with an Sa of the order of 30 µm and an Sz of the order of 200 µm.

[0067] We are initially only interested in part A, which was immersed for varying periods in bath 1. Fig. 2a is a snapshot of the surface before polishing. Fig. 2b is a snapshot of room A after 1 minute of immersion in bath 1, the Fig. 2c corresponds to part A after 2 minutes of immersion, the Fig. 2d to room A after 10 minutes of immersion, the Fig. 2e to room A after 20 minutes of immersion and the Fig. 2f to part A after 30 minutes of immersion. Microscopic observations of the Fig. 2 highlight the effectiveness of the polishing baths according to the invention in removing partially melted particles present on the surface of the samples. Indeed, no spherical particles are observed on their surface after treatment ( Fig. 2f , part A, 30 minutes of immersion), and this despite their initial size being on the order of ten microns in the case of the SLM manufacturing process (selective laser melting additive manufacturing process, or selective laser melting ) ( Fig. 2a ).

[0068] After polishing, part A underwent a reduction in size on each of its faces ( Fig. 5 ), and this from an immersion of just 1 minute in bath 1. The Fig. 3 And 4 indicate a significant reduction in the surface relief of the part treated by bath 1 according to the invention, and therefore a polishing of part A. Increasing the immersion time in the polishing bath improves the reduction of surface roughness, both on the upper and lower faces ( Fig. 3 And 4 ), however, this also increases the discount on each of these sides ( Fig. 5Furthermore, the average surface roughness Sa after chemical polishing is between 2 and 3 µm after 30 minutes of treatment, regardless of the initial roughness of the samples. It should be noted that the final roughness Sa appears to tend towards a limit of approximately 2-3 µm regardless of the manufacturing orientation (i.e., whether considering the top or bottom face), while the reduction continues to increase. It is not necessary to immerse the parts to be treated for excessively long periods in the polishing bath according to the invention to obtain the desired polishing effects. However, those skilled in the art will appreciate that the average rate of removal is 10 µm / min, resulting in a 300 µm reduction per face after 30 minutes of immersion in a bath according to the invention, which is half the reduction obtained with chemical machining processes.

[0069] The polishing bath developed here also makes it possible to significantly reduce the roughness parameter Sz ( Fig. 4 ). The evolution is similar to that of Sa with values ​​of less than 20 µm after 30 minutes of treatment.

[0070] If we now consider parts B to I, these were immersed for a roughly identical duration in baths differing from one another in the initial quantity of titanium they contained. The observations carried out focused solely on the upper surface and showed a significant reduction in the surface roughness Sa and Sz ( Fig. 6Indeed, the parts exhibited a Sa of approximately 30 µm before treatment and, after treatment, a Sa of less than 5 µm regardless of the bath (2 to 9) used, while the roughness Sz was reduced from approximately 200 µm to less than 70 µm. The reduction is limited to values ​​below 150 µm, which corresponds to a typical material removal during a polishing process, compared to a machining process. The results presented at the Fig. 6 indicate that the efficiency of a polishing bath according to the invention is optimal for titanium concentrations between 10 and 35 g / L.

[0071] Comparing the results obtained with bath 3 (according to the invention) and bath 13 (comparative example), it is observed that increasing the phosphoric acid concentration leads to an increase in the cut-off and a smaller reduction in surface roughness (Table 1). Comparing the results obtained with bath 3 (according to the invention) and baths 10 and 12 (comparative examples), it is also observed that a decrease in the phosphoric acid concentration, combined with an increase in the sulfuric acid concentration (bath 12) or a decrease in the fluoride complexing agent concentration (bath 10), also results in an increase in the cut-off and a smaller reduction in surface roughness (Table 1). The cut-off of each of parts J and L is more than twice that of part C treated with bath 3 (according to the invention). Baths 10 and 12 are chemical machining baths and not chemical polishing baths.

Claims

1. A chemical polishing bath for polishing a titanium or titanium alloy component, or a part thereof, said chemical polishing bath comprising: water, a fluoride complexing agent capable of forming a complex with oxidized titanium at an equivalent fluoride concentration of between 1.40 and 2.80 mol / L; phosphoric acid in a concentration of between 3.0 and 6.0 mol / L; and sulfuric acid in a concentration of between 0.5 and 1.5 mol / L.

2. The chemical polishing bath as claimed in claim 1, wherein the fluoride complexing agent is added to the bath in the form of NH4F, NH4F.HF, NaF, NaF.HF, KF, KF.HF, H2SiF6, H2TiF6 or mixtures thereof.

3. The chemical polishing bath as claimed in claim 1 or 2, wherein the fluoride complexing agent is present such that the concentration of fluoride is between 1.74 and 2.46 mol / L, preferably between 1.80 and 2.20 mol / L.

4. The chemical polishing bath as claimed in any one of preceding claims, wherein the phosphoric acid is present in a concentration of between 4.0 and 5.5 mol / L, preferably between 4.5 and 5.0 mol / L.

5. The chemical polishing bath as claimed in any one of preceding claims, wherein the sulfuric acid is present in a concentration of between 0.5 and 1.25 mol / L, preferably in a concentration of between 0.80 and 1.02.

6. The chemical polishing bath as claimed in any one of the preceding claims, wherein the bath has a pH of less than 2.0, preferably less than 1.0, particularly preferably the pH of the chemical polishing bath is negative.

7. The chemical polishing bath as claimed in any one of the preceding claims comprising titanium, present at a concentration ranging from approximately 2 to 35 g / L, namely between approximately 0.04 and 0.73 mol / L, preferably at a concentration ranging from approximately 10 to 30 g / L, namely between 0.20 and 0.63 mol / L.

8. The chemical polishing bath as claimed in any one of the preceding claims, said bath having a density ranging from 1.2 to 1.4 g / cm3.

9. The chemical polishing bath as claimed in any one of the preceding claims, said bath being essentially devoid of abrasive particles.

10. The chemical polishing bath as claimed in any one of the preceding claims, wherein no oil is added, in particular wherein neither mineral nor vegetable oil is added.

11. A chemical polishing method for polishing a titanium or titanium alloy component, or a part thereof, comprising the following steps: (i) providing a chemical polishing bath as claimed in any one of the preceding claims; (ii) immersing a titanium or titanium alloy component to be polished, or a part thereof in the chemical polishing bath which is preferably maintained at a temperature of between 30°C and 60°C, in particular between 40°C and 50°C; (iii) removing the component after a predetermined immersion time.

12. The chemical polishing method as claimed in claim 11, wherein the duration of immersion is between 20 and 60 minutes, preferably between 30 and 50 minutes.

13. The chemical polishing method as claimed in claim 11 or 12, wherein the titanium or titanium alloy component, or the part thereof, to be polished is subjected to a degreasing and / or chemical pickling step before being immersed in the chemical polishing bath.

14. The chemical polishing bath as claimed in any one of claims 1 to 10, or the chemical polishing method as claimed in any one of claims 11 to 13, wherein the component to be polished, or the part thereof, is made of titanium of allotropic alpha, beta or two-phase alpha-beta form or of titanium alloy of alpha, near-alpha, beta, near-beta or two-phase alpha-beta form, in particular the titanium alloys may be the alloys TA6V, T40, TV10A3Fe2, Ti6242, β21, β21S, Ti 555-3, more particularly the titanium alloy is TA6V alloy or T40 alloy.

15. The chemical polishing bath as claimed in any one of claims 1 to 10, or the chemical polishing method as claimed in any one of claims 11 to 13, wherein the titanium or titanium alloy component to be polished is obtained by an additive manufacturing method.