Sacrificial powder comprising a homogeneous mixture of magnetite powder with a titanium dioxide powder or an iron powder

Abstract

The present invention relates to a composition in powder form; to the method for producing same, the use thereof in a pressure sintering method, and a method for sintering the composition. The invention relates to a composition in powder form comprising a mixture of magnetite powder with a titanium dioxide powder or an iron powder; and to the method for producing same, the use thereof in a pressure sintering method, and a method for sintering the composition.

Description

DESCRIPTION Title: SACRIFICIAL POWDER COMPRISING A HOMOGENEOUS MIXTURE OF MAGNETITE POWDER WITH A TITANIUM DIOXIDE POWDER OR AN IRON POWDER Technical field of the invention

[0001] The present invention belongs to the field of manufacturing parts or objects from powder and more particularly to the field of manufacturing by implementing a sintering process. The invention relates to a composition in powder form, its manufacturing process, its use in a sintering process under load, as well as a process for sintering said composition.

[0002] The invention relates to a composition, in powder form, comprising a homogeneous mixture of magnetite powder with a titanium dioxide powder or an iron powder; as well as its manufacturing process, its use in a sintering process under load, and a process for sintering said composition. Prior art

[0003] Sintering is a manufacturing process that involves heating a powder without melting it. Under the effect of heat, the grains fuse together and form a part.

[0004] In the context of sintering under load, the uniaxial compression present in this type of process makes it difficult to manufacture complex three-dimensional shapes with large variations in thickness.

[0005] A method for overcoming this difficulty is known. The method involves assembling two powders separated by a deformable interface that forms a uniformly sintering cylinder during sintering under load. The first powder is a useful powder and will form the final complex-shaped part after densification (sintering), the second powder is a sacrificial powder and allows for homogenizing shrinkage on the different parts of the complex shape during uniaxial compression. The interface allows for the separation between the sacrificial part, corresponding to the sintered sacrificial powder, and the useful part, corresponding to the sintered useful powder, after co-sintering under load.

[0006] This method is based on the fact that the useful and sacrificial powders sinter with very similar densification behavior so that the assembly of powders behave as a homogeneous medium. If the behavior of the powders differs, then the final part may be deformed or partially densified.

[0007] Currently, the simplest way to ensure this homogeneity of behavior in the context of sintering under load is to use the same powder for the useful part and the sacrificial part.

[0008] However, this type of powder is often derived from high value-added materials that are often very expensive, difficult to produce or rare. Since the sacrificial part only serves as a homogeneous compressible pressure transmission medium and is not reusable at the end of the process, using the same useful powder as sacrificial powders would represent a significant economic and environmental loss.

[0009] For example, many processes such as "hot die compression" (also called "hot pressing"), "hot isostatic pressing", "forming", "forging", or some foundry processes use ceramic parts as sacrificial material, while ceramics have very different sintering temperatures than metals, which adds additional machining steps to the sintering processes under load, which are time-consuming and expensive.

[0010] By "sacrificial material" is meant, in the present invention, a material, for example a powder, possibly pre-sintered, having a shrinkage, i.e. a behavior in cold compression then in sintering under load, similar to a useful powder constituting a complex shape. This sacrificial material is arranged around the part to be manufactured so as to form an external geometry that is easy to press, for example a cylinder or a volume space with a rectangular section. According to the invention, the objective of the sacrificial material is to imitate the behavior in cold compaction then in sintering under load of titanium alloy powders.

[0011] Thanks to new approaches to co-sintering under load, the possibility of manufacturing complex metal parts is gradually emerging, particularly through coupling with three-dimensional printing. However, these processes require the use of inexpensive, easily detachable and / or fracturable sacrificial materials, with load sintering properties close to the metal powders used for manufacturing the parts.

[0012] Thus, there is a need for inexpensive, easily detachable and / or fracturable powders, having sintering properties under load close to the metal powders used for the manufacture of parts. Presentation of the invention

[0013] The present invention makes it possible to obtain a composition which surprisingly solves the drawbacks mentioned above. The composition, the subject of the present invention, is particularly effective in imitating the sintering behavior of the most well-known metals, alloys and superalloys and useful for applications ranging from automotive to aeronautics and space, while remaining inexpensive, easily detachable and / or fracturable and easily produced. In addition, to the inventors' knowledge, it does not present any health or environmental risk.

[0014] A first subject of the present invention is a composition, in powder form, comprising a homogeneous mixture of magnetite powder with a titanium dioxide powder or an iron powder.

[0015] Thus, according to a first embodiment, the composition in powder form may comprise a homogeneous mixture of magnetite powder and titanium dioxide powder.

[0016] Advantageously, the composition according to this first embodiment may comprise, by weight relative to the total weight of the mixture, from 20 to 30% of titanium dioxide powder, preferably 25%.

[0017] Advantageously, the composition according to this first embodiment may comprise, by weight relative to the total weight of the mixture, from 70 to 80% of magnetite powder, preferably 75%.

[0018] Advantageously, the composition according to this first embodiment may comprise, by weight relative to the total weight of the mixture, less than 5% of impurity(ies), preferably less than 1%.

[0019] By "impurity(ies)" is meant, in the present invention for this first embodiment, any compound other than titanium dioxide powder and magnetite powder. This may be, for example, silica or aluminosilicate compounds.

[0020] Advantageously, the composition according to this first embodiment can have, under a mechanical pressure of 45 to 55 MPa, a sintering temperature of 600°C to 1000°C, preferably 700°C to 1000°C.

[0021] By "sintering temperature" is meant, as used herein, the temperature at which the powder densifies (sinters) when sintering is carried out in a temperature-rise regime.

[0022] Advantageously, the composition according to this first embodiment can have, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 850°C.

[0023] By "sintering medium temperature" is meant, in the present invention, the temperature at which the part of the sintering under hot load is at 50% of the value of the total hot shrinkage in the case where the sintering is carried out in a temperature rise regime. For example, if a powder has a total shrinkage of 30% when hot, then for a temperature rise of 50K / min, the sintering medium temperature will correspond to the shrinkage temperature at 15% when hot, i.e. 50% of the value of the total hot shrinkage. This temperature makes it possible to indicate the average sintering temperature by a single value and the latter is dependent on the heating rate. The invention focuses on "spark plasma sintering" which allows high heating rates, such as 50K / min, and higher material performances than "hot die compression" (also called "hot pressing" in English).

[0024] Advantageously, the titanium dioxide powder may have a particle size (particles) of from 0.1 pm to 0.4 pm, preferably from 0.15 to 0.25 pm.

[0025] Advantageously, the magnetite powder may have a particle size (particles) of from 1 pm to 100 pm, preferably from 2 to 20 pm.

[0026] "Particle size" is used herein to refer to the statistical distribution of particle sizes. However, crystalline matter is rarely present in a single crystal state and is most often polycrystalline, i.e. composed of single crystals (crystallites) attached to each other. Thus, the particle size of a powder is based herein on the particles of said powder, whether they are polycrystalline or not. This characteristic can be measured, for example, by laser particle size analysis.

[0027] Advantageously, the titanium dioxide powder may have a grain size (crystallites) of 0.09 pm to 0.21 pm, preferably 0.15 pm.

[0028] Advantageously, the magnetite powder may have a grain size (crystallites) of 0.19 pm to 0.21 pm, preferably 0.20 pm.

[0029] "Grain size" means, as used herein, the size of the single crystals forming the particles of a powder. This characteristic can be measured, for example, by SEM imaging.

[0030] Advantageously, the composition according to this first embodiment may exhibit a sintering shrinkage, under a mechanical pressure of 45 to 55 MPa, within a range of 30 to 40%, preferably 35%.

[0031] "Sintering shrinkage" is used herein to mean a value reflecting the elimination of porosity from a composition as a function of the progress of sintering under the action of temperature and mechanical pressure. In the context of sintering under load, which is a high-temperature uniaxial compression process in generally cylindrical dies, the linear shrinkage of the material which densifies (sinters) is expressed in percent from the following formula: (L-Lo) / Lo, with L corresponding to the height of the compressed powder bed and Lo corresponding to the initial height.

[0032] Advantageously, the composition according to this first embodiment can exhibit a cold compaction shrinkage, under a mechanical pressure of 45 to 55 MPa, within a range of 0 to 10%, preferably 0 to 5%.

[0033] By "cold compaction" is meant, as used herein, compaction which is not thermally activated, but which is mechanically activated.

[0034] By "cold compaction shrinkage" is meant, as used herein, a value reflecting the elimination of porosity from a composition at a temperature of 20°C. The value of cold compaction shrinkage depends on several parameters such as the material, morphology, particle size and the applied load. For example, for a composition according to the invention, the smaller titanium particles naturally lodge in the interstices between the larger magnetite particles. This difference in particle size allows for a more compact composition and lower shrinkage during loading than that of the individual powders in the composition. Cold compaction shrinkage is calculated in the same way as sintering shrinkage: (L-Lo) / Lo, with L corresponding to the height of the compressed powder bed and Lo corresponding to the initial height.

[0035] According to a second embodiment, the composition according to the invention may comprise a homogeneous mixture of magnetite powder and iron powder.

[0036] Advantageously, the composition according to this second embodiment may comprise, by weight relative to the total weight of the mixture, from 40 to 60% of iron powder, preferably 50%.

[0037] Advantageously, the composition according to this second embodiment may comprise, by weight relative to the total weight of the mixture, from 40 to 60% of magnetite powder, preferably 50%.

[0038] Advantageously, the composition according to this second embodiment may comprise, by weight relative to the total weight of the mixture, less than 5% of impurity(ies), preferably less than 1%.

[0039] By "impurity(ies)" is meant, in the present invention for this second embodiment, any compound other than iron powder and magnetite powder. It may be, for example, silica or aluminosilicate compounds.

[0040] Advantageously, the composition according to this second embodiment can have, under a mechanical pressure of 45 to 55 MPa, a sintering temperature of 600°C to 1000°C, preferably 650 to 900°C.

[0041] Advantageously, the composition according to this second embodiment can have, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 775°C.

[0042] Advantageously, the iron powder may have a particle size (particles) of from 1 pm to 100 pm, preferably from 15 to 40 pm.

[0043] Advantageously, the magnetite powder may have a particle size (particles) of from 1 pm to 100 pm, preferably from 2 to 20 pm.

[0044] Advantageously, the iron powder may have a grain size (crystallites) of 1 pm to 50 pm, preferably 27 pm.

[0045] Advantageously, the magnetite powder may have a grain size (crystallites) of 0.19 pm to 0.21 pm, preferably 0.20 pm.

[0046] Advantageously, the composition according to this second embodiment may exhibit a sintering shrinkage, under a mechanical pressure of 45 to 55 MPa, within a range of 30 to 40%, preferably 35%.

[0047] Advantageously, the composition according to this second embodiment can exhibit a cold compaction shrinkage, under a mechanical pressure of 45 to 55 MPa, within a range of 0 to 10%, preferably 0 to

[0048] A second subject of the present invention is a method of manufacturing a composition according to the invention, comprising the following steps: - mixing titanium dioxide powder or iron powder with magnetite powder; - homogenization of the mixture obtained in the previous step, and obtaining the composition.

[0049] Advantageously, in the case of the first embodiment, the mixing step of the manufacturing process can be carried out with 20 to 30%, by weight relative to the total weight of the mixture, of titanium dioxide powder, preferably 25%.

[0050] Advantageously, in this same first embodiment, the mixing step of the manufacturing process according to the invention can be carried out with 70 to 80% by weight relative to the total weight of the mixture, of magnetite powder, preferably 75%.

[0051] Advantageously, in the case of the second embodiment, the mixing step of the manufacturing process can be carried out with 40 to 60%, by weight relative to the total weight of the mixture, of iron powder, preferably 50%.

[0052] Advantageously, in this same second embodiment, the mixing step of the manufacturing process can be carried out with 40 to 60% by weight relative to the total weight of the mixture, of magnetite powder, preferably 50%.

[0053] Advantageously, the composition according to the invention can be used during implementation by the following processes: sintering under load, additive manufacturing, hot quasi-isostatic pressing, forming or forging.

[0054] By “additive manufacturing” we mean, in this document, manufacturing processes by adding material.

[0055] "Quasi-isostatic hot pressing" means, as used herein, hot die pressing using powder or granules as the pressure transmission medium.

[0056] By "forming" is meant, as used herein, the forging of powder into deformable sealed containers.

[0057] By "forging" is meant, as used herein, the manufacture of a part by uniaxial compression which is not mechanically confined.

[0058] A third object of the present invention is the use of a composition according to the invention, as a sacrificial material in a vacuum sintering process loading of metal parts of complex shapes, preferably along the uniaxial compression axis.

[0059] Another object of the present invention is a method of sintering under load metal parts of complex shapes, preferably along the uniaxial compression axis, said method comprising a step of providing a composition according to the invention, as sacrificial material.

[0060] By "complex shape" is meant, as used herein, a type of three-dimensional shape having variations in thickness and not consisting of a simple protrusion of any 2D geometry.

[0061] Advantageously, the composition according to the invention can be used in the form of a preformed sacrificial sub-mold.

[0062] Advantageously, the composition according to the invention can be used as a filling material for a sacrificial zone of a hot compression mold.

[0063] By "an area" is meant, as used herein, one or more areas, preferably one, two or three areas.

[0064] By "hot compression mold" is meant, in the present, a hot pressing tool, preferably along the uniaxial compression axis, for example made of graphite, and which may comprise a useful zone and a sacrificial zone which may itself comprise a preformed sacrificial sub-mold.

[0065] By "useful zone" is meant, in the present invention, an area corresponding to the part to be sintered under load. According to the invention, this is a zone receiving a titanium alloy powder.

[0066] By "sacrificial zone" is meant, in the present invention, an area surrounding the part to be sintered allowing to constitute a regular powder assembly easy to press. According to the invention, it is a receiving area of ​​a sacrificial powder imitating the sintering under load of a titanium alloy powder.

[0067] The invention also relates to a preformed sacrificial sub-mold comprising a composition according to the invention or a filling material comprising a composition according to the invention.

[0068] Another object of the present invention is a method of sintering under load a metal part of complex shape comprising the following steps: - depositing a useful composition in a useful zone of a hot compression mold comprising a dry composition according to the invention or in a sub-mold preformed sacrificial material comprising a composition according to the invention; - cold pressing of the assembly obtained in the previous step; - first increase in temperature by a temperature delta AT1 of a value ranging from 550 to 650°C while maintaining the mechanical pressure at a minimum mechanical pressure between 0 MPa and 20 MPa; - second increase in temperature by a temperature delta AT2 of a value ranging from 100 to 200°C accompanied by an increase in mechanical pressure ranging from the minimum mechanical pressure to a maximum mechanical pressure of between 45 and 55 MPa; - third increase in temperature by a temperature delta AT3 of a value ranging from 250 to 350°C while maintaining the mechanical pressure at the maximum mechanical pressure and obtaining a sintered assembly; - removal of the sacrificial material from the sintered assembly and obtaining the complex-shaped metal part.

[0069] By "useful composition" is meant, as used herein, a composition comprising a powder constituting the part to be sintered. For example, the useful composition may comprise Ti-6AI-4V. This is one of the most commonly used titanium alloys in a wide range of applications where low density and excellent corrosion resistance are required. Preferably, the useful composition consists of Ti-6AI-4V.

[0070] By "dry composition" is meant, as used herein, a composition comprising less than 5% by weight of water, relative to the total weight of the composition.

[0071] In this document, the term "temperature delta" means a temperature difference between two measuring points.

[0072] By "sintered assembly" is meant, in this document, an assembly resulting from the sintering of a useful composition and a sacrificial material, the assembly also comprising an interface, separating the composition from the material, consisting of a thin (<0.5mm) non-sintering material, which allows the separation of the part(s), after sintering, resulting from the sacrificial material.

[0073] Advantageously, the sintering method according to the invention is implemented within a sintering device comprising at least one chamber.

[0074] Advantageously, obtaining the complex-shaped metal part is achieved by ejecting the sacrificial parts.

[0075] Advantageously, the minimum mechanical pressure is the contact pressure before sintering at high temperatures, it can be 10 MPa.

[0076] Advantageously, the maximum mechanical pressure is the contact pressure at the end of high temperature sintering, it can be 50 MPa.

[0077] Advantageously, the first increase in temperature may be an increase in a delta AT1 of a value ranging from 550 to 650°C, preferably 580°C and even more preferably an increase ranging from 20°C to 600°C.

[0078] Advantageously, the second increase in temperature may be an increase in a delta AT2 of a value ranging from 100 to 200°C, preferably 150°C and even more preferably an increase ranging from 600°C to 750°C.

[0079] Advantageously, the third increase in temperature may be an increase of a delta AT3 of a value ranging from 250 to 350°C, preferably 300°C and even more preferably an increase ranging from 750°C to 1050°C.

[0080] Advantageously, the first increase in temperature can be carried out over a period ranging from 0.4 to 2 hours, preferably 1 hour.

[0081] Advantageously, the second temperature increase can be carried out over a period of 0.2 to 0.4 h, preferably 0.3 h.

[0082] Advantageously, the third temperature increase can be carried out over a period of 0.1 to 0.3 h, preferably 0.2 h.

[0083] Advantageously, the third temperature increase step may comprise a temperature maintenance stage, at the maximum mechanical pressure, after a temperature sub-increase corresponding to a value ranging from 40 to 50% of the delta AT3, for a duration ranging from 30 minutes to 2 hours. For example, the third temperature increase step comprises a temperature maintenance stage at 875°C, at the maximum mechanical pressure, for a duration of one hour. Preferably, the third temperature increase step comprises a temperature maintenance stage when it is a process for sintering under load a fragile metal part of complex shape.

[0084] By "fragile metal parts" or "fragile parts" is meant, in this document, parts comprising a wall having a thickness, taken along the transverse axis, less than or equal to 3 mm.

[0085] Advantageously, the various stages of temperature increase can be carried out under vacuum.

[0086] By "vacuum" is meant, as used herein, that the atmospheric pressure in the chamber of the device allowing sintering is less than or equal to 50 Pa.

[0087] In the variant where the useful composition is deposited in a hot compression mold, the sintering process according to the invention may, in addition, comprise the following preliminary steps: - suspending a composition according to the invention in an aqueous composition and obtaining an aqueous suspension; - partial drying of the aqueous suspension and obtaining a wet suspension; - deposition of the wet suspension in a sacrificial zone of a hot compression mold; - complete drying of the wet suspension and obtaining a hot compression mold comprising a dry composition according to the invention.

[0088] By "wet suspension" is meant, as used herein, a suspension comprising 10 to 40% by volume, relative to the total volume of the suspension, of water. Preferably, 20 to 30% by volume of water.

[0089] Advantageously, the wet suspension can comprise 25% water and 75% composition according to the invention.

[0090] In the variant where the useful composition is deposited in a preformed sacrificial sub-mold comprising a composition according to the invention, the sintering method according to the invention may, in addition, comprise the following preliminary steps: - suspending a composition according to the invention in an aqueous composition and obtaining an aqueous suspension; - partial drying of the aqueous suspension and obtaining a wet suspension; - formation of a preformed sacrificial sub-mold from the wet suspension and obtaining a preformed sacrificial sub-mold.

[0091] Advantageously, the step of forming a preformed sacrificial sub-mold is implemented by a 3D printing method, for example stereolithography or direct ink writing (also called "Direct Ink Writing" or "DIW"), cold pressing, coupling in a mold, slip printing or slip casting.

[0092] Advantageously, the sintering method according to the invention may further comprise the following intermediate sub-step: - cooling the temperature of a delta AT4 (with AT4 = AT1 + AT2 +AT3); and - reduction of the mechanical pressure from the maximum mechanical pressure to the minimum mechanical pressure; the intermediate sub-step being implemented after the step of increasing the temperature from 750 to 1050 °C.

[0093] Advantageously, the temperature cooling step and the mechanical pressure reduction step may be simultaneous or sequential, preferably sequential. More preferably, the mechanical pressure reduction is carried out before the temperature cooling step for the manufacture of fragile parts. Brief description of the figures

[0094] [Fig. 1] Figure 1 represents (a.) a complex part, in the shape of helicopter turbine blades, made with a sacrificial powder in Ti-6AI-4V and (b.) and (c.) respectively represent a complex part, in the shape of helicopter turbine blades, made with a sacrificial powder according to the first embodiment of the composition of the invention (magnetite powder + titanium dioxide powder), and according to the second embodiment (magnetite powder + iron powder).

[0095] [Fig. 2] Figure 2 represents the particle size distribution of a titanium dioxide powder with an average particle size of 0.2 pm and a magnetite powder with an average particle size of 7 pm.

[0096] [Fig. 2bis] Figure 2bis represents the particle size of an iron powder with an average particle size of 27 pm and of a magnetite powder with an average particle size of 7 pm.

[0097] [Fig. 3] Figure 3 represents a frieze showing the main stages of manufacturing a complex part by hot pressing, which uses a sacrificial powder according to the invention.

[0098] [Fig. 4] Figure 4 represents the sintering curves, for a temperature rise of 50K / min and a mechanical pressure of 50 MPa, of the Ti-6AI-4V (Ti64) powder, marketed by AP&C, which is the target behavior to be imitated, of Fe3O4 (Magnetite), of TiC (Titanium dioxide), and of the sacrificial mixture (Composition OFNT25) which is in the same sintering temperature range as the Ti-6AI-4V powder. This figure shows in particular that the composition according to the first embodiment of the invention, OFNT25, has a behavior in swelling after 1000°C which generates an increase in porosity. The classic cycle therefore avoids going beyond 1000°C to imitate the behavior of the metal.

[0099] [Fig. 4bis] Figure 4bis represents the sintering curves, for a temperature rise of 50K / min and a mechanical pressure of 50 MPa, of the Ti-6AI-4V (Ti64) powder, marketed by AP&C, which is the target behavior to be imitated, of Fe3O4 (Magnetite), of Fe (Iron powder), and of the sacrificial mixture (Composition OFNFe50) which is in the same sintering temperature range as the Ti-6AI-4V powder. This figure shows in particular that the composition according to the second embodiment of the invention, OFNFe50, has a swelling behavior after 1000°C which generates an increase in porosity. The conventional cycle therefore avoids going beyond 1000°C to imitate the behavior of the metal.

[0100] [Fig. 5] Figure 5 is a microscopic scale photograph showing the presence of bubbles in a) the sacrificial material obtained from a composition according to the first embodiment of the invention, b) the sacrificial material obtained from a composition according to the second embodiment of the invention, once said compositions have sintered at 1000°C; the bubbles being due to a reaction involving a reduction and degassing of O2.

[0101] [Fig.6] Figure 6 represents (cycle 1) the sintering curves for thick parts (thickness>5mm), for a temperature rise of 50K / min, with an application of mechanical pressure from the beginning of the implementation of the sintering process and with an application of mechanical pressure when the temperature reaches 600 °C. It is clearly observed that the fact of applying mechanical pressure at 600 °C makes it possible to significantly limit the cold compaction of the sacrificial powders which become very close to the cold compaction of the metallic powders having a high initial compactness. This figure focuses on the powders of Ti-6AI-4V (Ti64) and sacrificial mixture according to the first embodiment of the invention, TiO2 / FeaO4 (OFNT25).

[0102] [Fig. 6bis] Figure 6bis represents (cycle 1) the sintering curves for thick parts (thickness>5mm), for a temperature rise of 50K / min, with an application of mechanical pressure from the start of the sintering process and with an application of mechanical pressure when the temperature reaches 600 °C. It is clearly observed that applying mechanical pressure at 600 °C makes it possible to significantly limit the cold compaction of the sacrificial powders which become very close to the cold compaction of the metal powders with high initial compactness. This figure focuses on Ti-6AI-4V (Ti64) powders and sacrificial mixture according to the second embodiment of the invention, Fe / FeaC (OFNFe50).

[0103] [Fig. 7] Figure 7 also represents (cycle 2), for Ti-6AI-4V (Ti64) powders and sacrificial mixture according to the first embodiment of the invention, an alternative cycle for sintering fragile parts (thickness < 3mm) with a temperature maintained at 875°C for 1 hour and a stop with hot pressure release, which allows the fragile structure of the parts to be preserved. For this new cycle, it can also be seen that there is no swelling and that the sacrificial powder always faithfully imitates the shrinkage of the titanium alloy powder with a slightly higher amplitude, which constitutes a safety margin to ensure complete densification.

[0104] [Fig. 7bis] Figure 7bis also represents (cycle 2), for Ti-6AI-4V (Ti64) powders and sacrificial mixture according to the second embodiment of the invention, an alternative cycle for sintering fragile parts (thickness < 3mm) with a temperature maintained at 875°C for 1 hour and a stop with hot pressure release, which allows the fragile structure of the parts to be preserved. For this new cycle, it can also be seen that there is no swelling and that the sacrificial powder always faithfully imitates the shrinkage of the titanium alloy powder with a slightly higher amplitude, which constitutes a safety margin to ensure complete densification.

[0105] [Fig. 8] Figure 8 shows different sintering cycles (a, b, c and d) for fragile parts, such as turbine blades. This figure allows in particular to compare the parts manufactured with the same useful powder but using a sacrificial powder of Ti-6AI-4V (reference) or a sacrificial mixture according to the first embodiment of the invention, TiC / FeaC (powder Y). It appears that the last cycle (d), the one comprising a long hold at a temperature (875°C for 1 h), is to be preferred. Indeed, this long hold of 1 h allows to finish the sintering without breaking the geometry of the fragile parts. The other cycles (a, b and c), more classic at the top of the figure do not pose any problem for more robust parts.

[0106] [Fig. 8bis] Figure 8bis represents different sintering cycles (a, b, c and d) for fragile parts, such as turbine blades. This figure allows in particular to compare the parts manufactured with the same useful powder but in using a sacrificial powder of TI-6AI-4V (reference) or a sacrificial mixture according to the second embodiment of the invention, Fe / FeaC (powder X). It appears that the last cycle (d), the one comprising a long hold at a temperature (875°C for 1 h), is to be preferred. Indeed, this long hold of 1 h allows sintering to be completed without breaking the geometry of the fragile parts. The other cycles (a, b and c), more classic at the top of the figure, do not pose any problem for more robust parts.

[0107] [Fig. 9] Figure 9 represents a classic sintering curve during spark plasma sintering (SPS) and its various characteristics studied. This curve allows in particular to measure the pressurization compaction, the sintering shrinkage (corresponding to the formula 1 -(h- ho) / ho, with h the height of the sample and ho the initial height) and the sintering medium temperature. The pressurization compaction can be read on the y-axis and corresponds to the shrinkage difference appearing before the shrinkage reaches a plateau. The sintering shrinkage can be read on the y-axis and corresponds to the shrinkage difference between two values ​​corresponding to two distinct shrinkage plateaus.The sintering medium temperature can be read on the x-axis and corresponds to the temperature at which the sintering shrinkage is at 50%, i.e. at 50% of the shrinkage between the end of the first shrinkage stage and the start of the second shrinkage stage. EXAMPLE

[0108] Other advantages, aims and particular characteristics of the present invention will emerge from the examples which follow, given for explanatory and in no way limiting purposes.

[0109] In the examples that follow, the different parameters were measured using the techniques detailed below: [001 10] Measurement of sintering temperature [001 1 1 ] The measurements are carried out from the SPS sintering curves obtained using the HPD25 marketed by FCT. The temperature ramp sintering curves are obtained by carrying out a sintering cycle with the application of a mechanical pressure of 50 MPa when cold and then heating at 50K / min until the powder densification stops. The displacement shrinkage during cold compression indicates the nature of the shrinkage by granular rearrangement, this aspect being very strongly impacted by the particle size of the powder mixture. The sintering temperature range is those where the compaction of the powder takes place.

[0112] Measurement of sintering medium temperature

[0113] The measurements are made from the SPS sintering curves obtained using the HPD25 marketed by FCT. The sintering medium temperature is the temperature at which the hot sintering shrinkage is 50% (see Figure 9).

[0114] Measurement of particle size

[0115] The measurements are carried out by laser granulometry using the Mastersizer 2000 marketed by Malvern Instruments.

[0116] Measurement of grain size (crystal I ites)

[0117] The measurements are carried out by scanning electron microscopy using the SEM 7200F-JSM marketed by Jeol with direct measurement of apparent grains.

[0118] Example 1: Production of a composition C1 according to the invention

[0119] Composition C1 comprises titanium dioxide powder and magnetite powder.

[0120] Titanium dioxide powder (titanium white pigment marketed by Moulin à Couleur) has an average grain size of 0.2 pm.

[0121] Magnetite powder (black iron oxide pigment marketed by Moulin à Couleur) has an average grain size of 7 pm.

[0122] The method of manufacturing composition C1, comprising the following steps: - mixture of 25% by weight of titanium dioxide powder with 75% by weight of magnetite powder; and - homogenization of the mixture obtained in the previous step.

[0123] Table 1: Composition of the C1 composition [Table 1] 00124] Example 2: Manufacture of a complex part by sintering under load using a composition (Cl) according to the invention

[0125] A complex part, in the shape of helicopter turbine blades, shown in Figure 1 b., was produced with the sacrificial powder composition C1.

[0126] Composition C1 was suspended in an aqueous composition comprising water. The absence of organic additives allows for better compactness after drying of the wet suspension.

[0127] Partial drying of the suspension composition was carried out, allowing a wet suspension to be obtained. Partial drying was carried out in air until an ideal proportion of around 30% by volume of water was obtained.

[0128] A useful composition was deposited in the useful areas of a hot compression mold (Ti-6AI-4V powder marketed by AP&C and having a grain size of 35 pm) while the wet suspension was deposited in the sacrificial areas of said hot compression mold.

[0129] Complete drying of the wet suspension was then carried out, resulting in a dry powder.

[0130] The dry powder and the useful composition were subjected to a process of sintering under load of a complex-shaped metal part comprising the following steps: - pressing at a temperature of 20°C; - increase in temperature from 20°C to 600°C while maintaining mechanical pressure at a minimum mechanical pressure of 10 MPa for a period of 15 minutes; - increase in temperature from 600°C to 750°C accompanied by an increase in mechanical pressure ranging from the minimum mechanical pressure to a maximum mechanical pressure of 50 MPa over a period of 3 min; - increase in temperature from 750°C to 1050°C while maintaining the mechanical pressure at maximum mechanical pressure for a period of 6 min; - obtaining a sintered assembly; - removal of the sacrificial material from the sintered assembly; and obtaining the complex-shaped metal part by post-sintering ejection of the sacrificial parts.

[0131] Co-sintering was carried out using a Spark Plasma Sintering SPS HPD25 machine marketed by FCT.

[0132] The sacrificial material removal step is performed during the SPS cycle.

[0133] This example confirms that composition C1 sinters, under load of 50 MPa, between 750°C and 1050°C, a temperature very close to the sintering temperature of many common industrial metal alloys.

[0134] Furthermore, it has been observed that during sintering under load, titanium dioxide and magnetite are reduced to ulvospinel. This reaction releases oxygen forming bubbles in the material when densification is sufficiently advanced and the pores have closed. The bubbles thus cause a significant swelling of the sacrificial material at the end of the thermal cycle (after 1000°C). The internal mechanical pressure of the C1 composition and the defects formed by the bubbles make the sacrificial material very easily destructible as soon as the load is removed. This characteristic is particularly interesting, because it appears at higher temperatures, but close to that of the end of sintering of the target metal powder. Thus, it is possible to facilitate the separation of the co-sintered parts by simply raising the temperature of the sintered assembly.

[0135] The other advantage of this high temperature swelling phase is that it allows quasi-isostatic compression conditions to facilitate the elimination of porosity at the end of the sintering cycle under load.

[0136] Example 3: Production of a composition C2 according to the invention

[0137] Composition C2 includes iron powder and magnetite powder.

[0138] Iron powder has an average grain size of 27 pm.

[0139] Magnetite powder (black iron oxide pigment marketed by Moulin à Couleur) has an average grain size of 7 pm.

[0140] The method of manufacturing composition C2, comprising the following steps: - mixture of 50% by weight of iron powder with 50% by weight of magnetite powder; and - homogenization of the mixture obtained in the previous step.

[0141] Table 2: Composition of the C2 composition [Table 2] 00142] Example 4: Manufacture of a complex part by sintering under load using a composition (C2) according to the invention

[0143] A complex part, in the shape of helicopter turbine blades, shown in Figure 1 c., was made with the sacrificial powder composition C2.

[0144] Composition C2 was suspended in an aqueous composition comprising water. The absence of organic additives allows for better compactness after drying of the wet suspension.

[0145] Partial drying of the suspension composition was carried out, allowing a wet suspension to be obtained. Partial drying was carried out in air until an ideal proportion of around 30% by volume of water was obtained.

[0146] A useful composition was deposited in the useful areas of a hot compression mold (Ti-6AI-4V powder marketed by AP&C and having a grain size of 35 pm) while the wet suspension was deposited in the sacrificial areas of said hot compression mold.

[0147] Complete drying of the wet suspension was then carried out, resulting in a dry powder.

[0148] The dry powder and the useful composition were subjected to a process of sintering under load of a complex-shaped metal part comprising the following steps: - pressing at a temperature of 20°C; - increase in temperature from 20°C to 600°C while maintaining mechanical pressure at a minimum mechanical pressure of 10 MPa for a period of 15 minutes; - increase in temperature from 600°C to 750°C accompanied by an increase in mechanical pressure ranging from the minimum mechanical pressure to a maximum mechanical pressure of 50 MPa over a period of 3 min; - increase in temperature from 750°C to 1050°C while maintaining the mechanical pressure at maximum mechanical pressure for a period of 6 min; - obtaining a sintered assembly; - removal of the sacrificial material from the sintered assembly; and obtaining the complex-shaped metal part by post-sintering ejection of the sacrificial parts.

[0149] Co-sintering was carried out using a Spark Plasma Sintering SPS HPD25 machine marketed by FCT.

[0150] The sacrificial material removal step is performed during the SPS cycle.

[0151] This example confirms that composition C2 sinters, under load of 50 MPa, between 750°C and 1050°C, a temperature very close to the sintering temperature of many common industrial metal alloys.

[0152] Furthermore, it was observed that during sintering a slight swelling of about 2% of the sacrificial material at the end of the thermal cycle (after 1000°C) was observed. This characteristic is particularly interesting, because it appears at higher temperatures, but close to that of the end of sintering of the target metal powder. Thus, it is possible to facilitate the separation of the co-sintered parts by simply raising the temperature of the sintered assembly.

[0153] The other advantage of this phase of slight swelling at high temperature is to allow quasi-isostatic compression conditions to facilitate the elimination of porosity at the end of the sintering cycle under load.

[0154] Example 5: Manufacturing of a comparative complex part by sintering under load using a metallic composition

[0155] A comparative complex part, in the shape of helicopter turbine blades, shown in Figure 1 a., was produced according to the method of Example 2 or 4 with a metallic composition of Ti-6AI-4V as sacrificial material.

[0156] This comparative example illustrates the ability of compositions C1 and C2 to imitate the sintering of a Ti-6AI-4V metal composition. The visible differences between the parts obtained according to the process of the invention (figure 1b example 2 and figure 1c example 4) and that of example 5 are solely due to filling defects which are not related to the composition of the sacrificial powder or to the sintering process under load.

[0157] In addition, the composition used in example 2 or 4 has the advantage of being less expensive than the reference composition used in example 5, while being easily detachable from the sintered assembly.

Claims

CLAIMS

1. Use of a composition, in powder form, comprising a homogeneous mixture of magnetite powder with a titanium dioxide powder or an iron powder, as a sacrificial material in a process for sintering under load metal parts of complex shapes, preferably along the uniaxial compression axis.

2. Use of the composition according to claim 1, wherein said composition comprises from 20 to 30% by weight of titanium dioxide powder, relative to the total weight of the mixture and from 70 to 80% by weight of magnetite powder relative to the total weight of the mixture.

3. Use of the composition according to claim 1, wherein said composition comprises 40 to 60% by weight of iron powder, relative to the total weight of the mixture and 40 to 60% by weight of magnetite powder, relative to the total weight of the mixture.

4. Use of the composition according to any one of claims 1 to 3, wherein said composition comprises less than 5% of impurity(ies) by weight relative to the total weight of the mixture,

5. Use of the composition according to any one of claims 1 to 4, wherein said composition has, under a mechanical pressure of 45 to 55 MPa, a sintering temperature of 600°C to 1000°C.

6. Use of the composition according to any one of claims 1, 2, 4 and 5, wherein said composition has, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 850°C.

7. Use of the composition according to any one of claims 1, 3 to 5, wherein said composition has, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 775°C.

8. Use of the composition according to any one of claims 1, 2, 4 to 6, wherein the titanium dioxide powder has a particle size of from 0.1 pm to 0.4 pm and the magnetite powder has a particle size of from 1 pm to 100 pm.

9. Use of the composition according to any one of claims 1, 3 to 5 and 7, wherein the iron powder has a particle size of from 1 pm to 100 pm and the magnetite powder has a particle size of from 1 pm to 100 pm.

10. Use of the composition according to any one of claims 1 to 9, wherein said composition exhibits a sintering shrinkage, under a mechanical pressure of 45 to 55 MPa, of 30 to 40%.

11. Use of the composition according to any one of claims 1 to 10, wherein said composition exhibits a cold compaction shrinkage, under a mechanical pressure of 45 to 55 MPa, of a range of 0 to 10%.

12. A method of sintering under load a complex-shaped metal part comprising the following steps: - depositing a useful composition in a useful area of a hot compression mold comprising a dry composition in powder form, comprising a homogeneous mixture of magnetite powder with a titanium dioxide powder or an iron powder or depositing in a preformed sacrificial sub-mold comprising a composition in powder form, comprising a homogeneous mixture of magnetite powder with a titanium dioxide powder or an iron powder; - cold pressing of the assembly obtained in the previous step; - first increase in temperature by a temperature delta AT1 of a value ranging from 550 to 650°C while maintaining the mechanical pressure at a minimum mechanical pressure between 0 MPa and 20 MPa; - second increase in temperature by a temperature delta AT2 of a value ranging from 100 to 200°C accompanied by an increase in mechanical pressure ranging from the minimum mechanical pressure to a maximum mechanical pressure of between 45 and 55 MPa; - third increase in temperature by a temperature delta AT3 of a value ranging from 250 to 350°C while maintaining the mechanical pressure at the maximum mechanical pressure and obtaining a sintered assembly; - removal of the sacrificial material from the sintered assembly and obtaining the complex-shaped metal part, preferably by ejection of the sacrificial parts.

13. A sintering method according to claim 12, wherein said composition comprises from 20 to 30% by weight of titanium dioxide powder, relative to the total weight of the mixture and from 70 to 80% by weight of magnetite powder relative to the total weight of the mixture.

14. A sintering method according to claim 12, wherein said composition comprises 40 to 60% by weight of iron powder, relative to the total weight of the mixture and 40 to 60% by weight of magnetite powder, relative to the total weight of the mixture.

15. A sintering method according to any one of claims 12 to 14, wherein said composition comprises less than 5% impurity(ies) by weight relative to the total weight of the mixture.

16. A sintering method according to any one of claims 12 to 15, wherein said composition has, under a mechanical pressure of 45 to 55 MPa, a sintering temperature of 600°C to 1000°C.

17. A sintering method according to any one of claims 12, 13, 15 and 16, wherein said composition has, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 850°C.

18. A sintering method according to any one of claims 12, 14 to 16, wherein said composition has, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 775°C.

19. A sintering method according to any one of claims 12, 13, 15 to 17, wherein the titanium dioxide powder has a particle size of 0.1 pm to 0.4 pm and the magnetite powder has a particle size of 1 pm to 100 pm.

20. A sintering method according to any one of claims 12, 14 to 16 and 18, wherein the iron powder has a particle size of 1 pm to 100 pm and the magnetite powder has a particle size of 1 pm to 100 pm.

21. A sintering method according to any one of claims 12 to 20, wherein said composition exhibits a sintering shrinkage, under a mechanical pressure of 45 to 55 MPa, of 30 to 40%.

22. A sintering method according to any one of claims 12 to 21, wherein said composition exhibits a cold compaction shrinkage, under a mechanical pressure of 45 to 55 MPa, of between 0 and 10%.

23. Sintering method according to any one of claims 12 to 22 in which the third temperature increase step comprises a temperature maintenance stage, at maximum mechanical pressure, after a temperature sub-increase corresponding to a value ranging from 40 to 50% of the delta AT3, for a duration ranging from 30 minutes to 2 hours.

24. A sintering method according to any one of claims 12 to 23, further comprising the following preliminary steps: - suspending said composition in an aqueous composition and obtaining an aqueous suspension; - partial drying of the aqueous suspension and obtaining a wet suspension; - deposition of the wet suspension in a sacrificial zone of a hot compression mold; - complete drying of the wet suspension and obtaining a hot compression mold comprising said dry composition.

25. A sintering method according to any one of claims 12 to 23, further comprising the following preliminary steps: - suspending said composition in an aqueous composition and obtaining an aqueous suspension; - partial drying of the aqueous suspension and obtaining a wet suspension; - formation of a preformed sacrificial sub-mold from the wet suspension and obtaining a preformed sacrificial sub-mold.

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