SACRIFICIAL POWDER COMPRISING A HOMOGENEOUS MIXTURE OF TITANIUM DIOXIDE POWDER AND MAGNETITE POWDER

Abstract

The present invention relates to a composition in powder form, its method of manufacture, its use in a sintering process under load, as well as a method of sintering said composition. The invention relates to a composition, in powder form comprising a mixture of titanium dioxide powder and magnetite powder; as well as its method of manufacture, its use in a sintering process under load, and a method of sintering said composition. Abstract figure: 0

Description

Title of the invention: SACRIFICIAL POWDER COMPRISING A HOMOGENEOUS MIXTURE OF POWDER OF TITANIUM DIOXIDE AND MAGNETITE 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 titanium dioxide powder and magnetite 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 process for manufacturing parts that involves heating a powder without melting it. Under the effect of heat, the grains weld 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. Said method implements an assembly of two powders separated by a deformable interface which 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 makes it possible to homogenize the shrinkages on the different parts of the complex shape during uniaxial compression. The interface allows 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 powder assembly behaves 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 guarantee 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" in English), "hot isostatic pressing" (also called "hot isostatic pressing" in English), "forming", "forging", or certain foundry processes use ceramic parts as sacrificial material, whereas ceramics have sintering temperatures very different from metals which adds, to the sintering processes under load, additional machining steps which are long and expensive.

[0010] By "sacrificial material" is meant, in the present, a material, for example a powder, possibly pre-sintered, having a shrinkage, that is to say 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, in particular by coupling with three-dimensional printing. However, these processes require the use of inexpensive sacrificial materials, easily detachable and / or fracturable, having 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. Statement 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 behavior to the sintering of the most well-known and useful metals, alloys and superalloys for applications ranging from automotive to aeronautics and space, while remaining inexpensive, easily detachable and / or fracturable and easily produced. In addition, it does not present, to the inventors' knowledge, any health or environmental risks.

[0014] A first object of the present invention is a composition, in powder form, comprising a homogeneous mixture of titanium dioxide powder and magnetite powder.

[0015] Advantageously, the composition according to the invention may comprise, by weight relative to the total weight of the mixture, from 20 to 30% of titanium dioxide powder, preferably 25%.

[0016] Advantageously, the composition according to the invention may comprise, by weight relative to the total weight of the mixture, from 70 to 80% of magnetite powder, preferably 75%.

[0017] Advantageously, the composition according to the invention may comprise, by weight relative to the total weight of the mixture, less than 5% of impurity(ies), preferably less than 1%.

[0018] By "impurity(ies)" is meant, herein, any compound other than titanium dioxide powder and magnetite powder. This may be, for example, silica or aluminosilicate compounds.

[0019] Advantageously, the composition according to the invention 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.

[0020] By "sintering temperature" is meant, as used herein, the temperature at which the powder densifies (sinters) when sintering is carried out in a temperature-increasing regime.

[0021] Advantageously, the composition according to the invention can have, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 850°C.

[0022] By "sintering medium temperature" is meant, in the present, the temperature at which the part of the sintering under hot load is at 50% of the value of the total shrinkage when hot 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 shrinkage when hot. This temperature makes it possible to indicate the average sintering temperature by a single value and the latter is dependent on the heating speed. The invention focuses on "spark plasma sintering" which allows high heating speeds. heating, such as 50K / min, and higher hardware performance than “hot die compression” (also called “hot pressing” in English).

[0023] Advantageously, the titanium dioxide powder may have a particle size (particles) of from 0.1 μm to 0.4 μm, preferably from 0.15 to 0.25 μm.

[0024] Advantageously, the magnetite powder can have a particle size (particles) of from 1 μm to 100 μm, preferably from 2 to 20 μm.

[0025] By "granulometry" is meant, in the present, the statistical distribution of particle sizes. However, crystalline matter is rarely present in the monocrystalline state and most of the time, it is polycrystalline, that is to say composed of monocrystals (the crystallites) attached to each other. Thus, the particle size of a powder is based, in the present, on the particles of said powder, whether they are polycrystalline or not. This characteristic can be, for example, measured by laser particle size analysis.

[0026] Advantageously, the titanium dioxide powder may have a grain size (crystallites) of 0.09 pm to 0.21 pm, preferably 0.15 pm.

[0027] Advantageously, the magnetite powder may have a grain size (crystallites) of 0.19 pm to 0.21 pm, preferably 0.20 pm.

[0028] By "grain size" is meant, herein, the size of the single crystals forming the particles of a powder. This characteristic can be, for example, measured by SEM imaging.

[0029] Advantageously, the composition according to the invention may exhibit a sintering shrinkage, under a mechanical pressure of 45 to 55 MPa, of between 30 and 40%, preferably 35%.

[0030] By "sintering shrinkage" is meant, as used herein, a value reflecting the elimination of the porosity of 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 process of uniaxial compression at high temperature 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.

[0031] Advantageously, the composition according to the invention 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%.

[0032] By "cold compaction" is meant, as used herein, compaction which is not thermally activated, but which is mechanically activated.

[0033] 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 the cold compaction shrinkage depends on several parameters such as the material, the morphology, the particle size and the applied load. For example, for a composition according to the invention, the smaller titanium particles naturally fit into the interstices between the larger magnetite particles. This difference in particle size makes it possible to obtain a more compact composition and a shrinkage during loading that is lower than that of the constituent powders of the composition taken individually. The cold compaction shrinkage is calculated in the same way as the sintering shrinkage: (L-Lo) / Lo, with L corresponding to the height of the compressed powder bed and Lo corresponding to the initial height.

[0034] A second subject of the present invention is a method of manufacturing a composition according to the invention, comprising the following steps: - mixing a titanium dioxide powder with a magnetite powder; - homogenization of the mixture obtained in the previous step, and obtaining the composition.

[0035] Advantageously, the mixing step of the manufacturing process according to the invention can be carried out with 20 to 30%, by weight relative to the total weight of the mixture, of titanium dioxide powder, preferably 25%.

[0036] Advantageously, 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%.

[0037] 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.

[0038] By “additive manufacturing” is meant, in the present, manufacturing processes by adding material.

[0039] By "hot quasi-isostatic pressing" is meant, as used herein, hot die pressing with the use of a powder or granules as a pressure transmission medium.

[0040] By "forming" is meant, as used herein, the forging of powder into deformable sealed containers.

[0041] By "forging" is meant, as used herein, the manufacture of a part by mechanically unconfined uniaxial compression.

[0042] A third object of the present invention is the use of a composition according to the invention, as a sacrificial material in a process for sintering under load metal parts of complex shapes, preferably along the uniaxial compression axis.

[0043] 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.

[0044] Advantageously, the composition according to the invention can be used in the form of a preformed sacrificial sub-mold.

[0045] Advantageously, the composition according to the invention can be used as a filling material for a sacrificial zone of a hot compression mold.

[0046] By "a zone" is meant, as used herein, one or more zone(s), preferably one, two or three zone(s).

[0047] 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.

[0048] By "useful zone" is meant, in the present invention, a zone corresponding to the part to be sintered under load. According to the invention, this is a zone receiving a titanium alloy powder.

[0049] By "sacrificial zone" is meant, in the present invention, an area surrounding the part to be sintered making it possible to constitute a regular powder assembly that is easy to press. According to the invention, this is a receiving area for a sacrificial powder imitating the sintering under load of a titanium alloy powder.

[0050] 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.

[0051] 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 preformed sacrificial sub-mold comprising a composition according to the invention; - cold pressing of the assembly obtained in the previous step; - first increase in temperature by an ATI temperature delta of a value ranging from 550 to 650°C while maintaining the mechanical pressure at a minimum mechanical pressure of 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.

[0052] 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-6A1-4V. 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-6A1-4V.

[0053] By "dry composition" is meant, in the present, a composition comprising less than 5% by weight, relative to the total weight of the composition, of water.

[0054] By "temperature delta" is meant, in the present document, a temperature difference between two measuring points.

[0055] By "sintered assembly" is meant, in the present, 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 (<0.5 mm) non-sintering material, which allows the separation of the part(s), after sintering, resulting from the sacrificial material.

[0056] Advantageously, the sintering method according to the invention is implemented within a sintering device comprising at least one chamber.

[0057] Advantageously, obtaining the metal part of complex shape is achieved by ejecting the sacrificial parts.

[0058] Advantageously, the minimum mechanical pressure is the contact pressure before sintering at high temperatures, it can be 10 MPa.

[0059] Advantageously, the maximum mechanical pressure is the contact pressure at the end of high-temperature sintering; it may be 50 MPa.

[0060] Advantageously, the first increase in temperature may be an increase in a delta ATI of a value ranging from 550 to 650°C, preferably 580°C and more preferably an increase ranging from 20°C to 600°C.

[0061] 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 more preferably an increase ranging from 600°C to 750°C.

[0062] Advantageously, the third increase in temperature may be an increase in a delta AT3 of a value ranging from 250 to 350°C, preferably 300°C and more preferably an increase ranging from 750°C to 1050°C.

[0063] Advantageously, the first increase in temperature can be carried out over a period ranging from 0.4 to 2 hours, preferably 1 hour.

[0064] Advantageously, the second increase in temperature can be carried out over a period ranging from 0.2 to 0.4 h, preferably 0.3 h.

[0065] Advantageously, the third increase in temperature can be carried out over a period ranging from 0.1 to 0.3 h, preferably 0.2 h.

[0066] Advantageously, the third temperature increase step may comprise a temperature maintenance stage, at the maximum mechanical pressure, after a sub-increase in the temperature 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.

[0067] By "fragile metal parts" or "fragile parts" is meant, in the present, parts comprising a wall having a thickness, taken along the transverse axis, less than or equal to 3 mm.

[0068] Advantageously, the different stages of increasing the temperature can be carried out under vacuum.

[0069] By "vacuum" is meant, in the present, that the atmospheric pressure in the chamber of the device allowing sintering is less than or equal to 50 Pa.

[0070] 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; - depositing 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.

[0071] 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.

[0072] Advantageously, the wet suspension can comprise 25% water and 75% composition according to the invention.

[0073] 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.

[0074] Advantageously, the step of forming a preformed sacrificial sub-mold is implemented by a 3D printing method, for example stereolithography, cold pressing, coupling in a mold, slip casting or slip casting.

[0075] Advantageously, the sintering method according to the invention may further comprise the following intermediate sub-step: - cooling of the temperature of a delta AT4 (with AT4 = ATI + AT2 +AT3); and - reduction of the mechanical pressure going from the maximum mechanical pressure to the minimum mechanical pressure; the intermediate sub-step being implemented after the temperature increase step from 750 to 1050°C.

[0076] Advantageously, the step of cooling the temperature and reducing the mechanical pressure can be simultaneous or sequential, preferably sequential. More preferably, the reduction of the mechanical pressure is carried out before the step of cooling the temperature for the manufacture of fragile parts. Brief description of the figures

[0077] [Fig-1] [Fig.l] represents (a.) a complex part, in the shape of turbine blades helicopter, made with a sacrificial powder in Ti-6A1-4V and (b.) one has a complex part, in the shape of helicopter turbine blades, made with a sacrificial powder according to the invention.

[0078] [Fig.2] [Fig.2] represents the particle size distribution of a titanium dioxide powder having an average particle size of 0.2 pm and of a magnetite powder having an average particle size of 7 pm.

[0079] [Fig.3] [Fig.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.

[0080] [Fig.4] [Fig.4] represents the sintering curves, for a temperature rise of 50K / min and a mechanical pressure of 50 MPa, of the Ti-6A1-4V powder (Ti64), marketed by AP&C, which is the target behavior to be imitated, of Fe3O 4 (Magnetite), TiO2 (Titanium dioxide), and the sacrificial mixture (Composition OFNT25) which is in the same sintering temperature range as the Ti-6A1-4V powder. This figure shows in particular that the OFNT25 composition has a swelling behavior 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.

[0081] [Fig.5] [Fig.5] is a microscopic scale photograph showing the presence of bubbles in the sacrificial material once the composition according to the invention has sintered at 1000°C; the bubbles being due to a reaction involving a reduction and degassing of O2.

[0082] [Fig.6] [Fig.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 sacrificial powders which become very close to the cold compaction of metallic powders with a high initial compactness. This figure focuses on Ti-6A1-4V (Ti64) and TiO2 / Fe3O4 sacrificial mixture (OFNT25) powders.

[0083] [Fig.7] [Fig.7] also represents (cycle 2) an alternative cycle for the sintering of 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.

[0084] [Fig.8] [Fig.8] 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 using a sacrificial powder of Ti-6A1-4V (reference) or a sacrificial mixture according to the invention TiO2 / Fe3O4 (powder Y). It appears that the last cycle (d), the one comprising a long plateau at a temperature (875°C for 1h), is to be favored. Indeed, this long plateau of 1h 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.

[0085] [Fig.9] [Fig.9] represents a typical sintering curve during flash sintering (spark plasma sintering (SPS) in English) and its various characteristics studied. This curve allows in particular to measure the pressurization compaction, the sintering shrinkage (corresponding to the formula l-(h-h0) / h0, with h the height of the sample and h0 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 stages. 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 beginning of the second shrinkage stage. EXAMPLE

[0086] 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.

[0087] In the examples which follow, the different parameters were measured by the techniques detailed below:

[0088] Measurement of sintering temperature

[0089] 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.

[0090] Measurement of sintering medium temperature

[0091] 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 [Fig.9]).

[0092] Measurement of particle size

[0093] The measurements are carried out by laser granulometry using the Mastersizer 2000 marketed by Malvem instruments.

[0094] Measurement of grain size (crystallites)

[0095] The measurements are carried out by scanning electron microscopy using the SEM 7200F-JSM marketed by Jeol with direct measurement of apparent grains.

[0096] Example 1: Manufacture of a composition Cl according to the invention

[0097] Composition Cl comprises a titanium dioxide powder and a magnetite powder.

[0098] Titanium dioxide powder (titanium white pigment marketed by Moulin à Couleur) has an average grain size of 0.2 pm.

[0099] Magnetite powder (black iron oxide pigment marketed by Moulin à Couleur) has an average grain size of 7 pm.

[0100] The method of manufacturing composition Cl, comprising the following steps: - mixing 25% by weight of titanium dioxide powder with 75% by weight of magnetite powder; and - homogenization of the mixture obtained in the previous step.

[0101] Table 1: Constitution of the composition Cl [Tables 1] Composition Cl % mass % molar % volume Titanium Dioxide 25 49 27 Iron Oxide 75 51 73

[0102] Example 2: Manufacture of a complex part by sintering under load using a composition according to the invention

[0103] A complex part, in the shape of helicopter turbine blades, shown in Figure 1b., was produced with the sacrificial powder composition CL

[0104] Composition Cl was suspended in an aqueous composition comprising water. The absence of organic additives allows for better compactness after drying of the wet suspension.

[0105] Partial drying of the suspension composition was carried out, making it possible to obtain a wet suspension. Partial drying was carried out in air until an ideal proportion of the order of 30% by volume of water was obtained.

[0106] A useful composition was deposited in the useful areas of a hot compression mold (Ti-6A1-4V 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.

[0107] Complete drying of the wet suspension was then carried out, making it possible to obtain a dry powder.

[0108] 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.

[0109] The co-sintering was carried out with a Spark Plasma Sintering SPS HPD25 machine marketed by FCT.

[0110] The step of removing the sacrificial material is carried out during the SPS cycle.

[0111] This example confirms that the composition Cl 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.

[0112] 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 significant swelling of the sacrificial material at the end of the thermal cycle (after 1000°C). The internal mechanical pressure of the Cl 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.

[0113] The other advantage of this high-temperature swelling phase is that it allows for quasi-isostatic compression conditions to facilitate the elimination of porosity at the end of the sintering cycle under load.

[0114] Example 3: Manufacture of a comparative complex part by sintering under load using a metallic composition

[0115] A comparative complex part, in the shape of helicopter turbine blades, shown in Figure 1a, was produced according to the method of Example 2 with a metallic composition of Ti-6AL4V as sacrificial material.

[0116] This comparative example illustrates the ability of composition Cl to mimic the sintering of a Ti-6A1-4V metal composition. The visible differences between the parts of Example 2 and Example 3 are only due to filling defects which are not related to the composition of the sacrificial powder or to the sintering process under load.

[0117] In addition, the composition used in example 2 has the advantage of being less expensive than the reference composition used in example 3, while being easily detachable from the sintered assembly.

Claims

Claims

1. A composition, in powder form, comprising a homogeneous mixture of titanium dioxide powder and magnetite powder.

2. Composition according to claim 1, comprising, by weight relative to the total weight of the mixture, from 20 to 30% of titanium dioxide powder.

3. Composition according to claim 1 or 2, comprising, by weight relative to the total weight of the mixture, from 70 to 80% of magnetite powder.

4. Composition according to any one of claims 1 to 3 comprising, by weight relative to the total weight of the mixture, less than 5% of impurity(ies).

5. Composition according to any one of claims 1 to 4 having, under a mechanical pressure of 45 to 55 MPa, a sintering temperature of 600°C to 1000°C.

6. Composition according to any one of claims 1 to 5 having, under a mechanical pressure of 45 to 55 MPa, a sintering medium temperature of 850°C.

7. A composition according to any one of claims 1 to 6, wherein the titanium dioxide powder has a particle size of from 0.1 pm to 0.4 pm.

8. A composition according to any one of claims 1 to 7, wherein the magnetite powder has a particle size of from 1 pm to 100 pm.

9. Composition according to any one of claims 1 to 8, having a sintering shrinkage, under a mechanical pressure of 45 to 55 MPa, of between 30 and 40%.

10. Composition according to any one of claims 1 to 9, having a cold compaction shrinkage, under a mechanical pressure of 45 to 55 MPa, of between 0 and 10%.

11. A method of manufacturing a composition according to any one of claims 1 to 10, comprising the following steps: - mixing a titanium dioxide powder with a magnetite powder; - homogenization of the mixture obtained in the previous step, and obtaining the composition.

12. Use of the composition according to any one of claims 1 to 10, as a sacrificial material in a process for sintering under load metal parts of complex shapes, preferably along the uniaxial compression axis.

13. 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 composition according to any one of claims 1 to 10 dry or in a preformed sacrificial sub-mold comprising a composition according to any one of claims 1 to 10; - cold pressing of the assembly obtained in the preceding step; - first increase in temperature by a temperature delta ATI of a value ranging from 550 to 650°C with maintenance of the mechanical pressure at a minimum mechanical pressure of 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 metal part of complex shape, preferably by ejection of the sacrificial parts.;

14. Sintering method according to claim 13, in which the third temperature increase step comprises a temperature maintenance stage, at maximum mechanical pressure, after a temperature under-increase corresponding to a value ranging from 40 to 50% of the delta AT3, for a duration ranging from 30 minutes to 2 hours.

15. A sintering method according to claim 13 or 14, further comprising the following preliminary steps: - suspending the composition according to any one of claims 1 to 10 in an aqueous composition and obtaining an aqueous suspension; - partially drying the aqueous suspension and obtaining a wet suspension; - depositing the wet suspension in a sacrificial zone of a hot compression mold; - completely drying the wet suspension and obtaining a hot compression mold comprising a composition according to any one of claims 1 to 10 dry.

16. A sintering method according to claim 13 or 14, further comprising the following preliminary steps: - suspending the composition according to any one of claims 1 to 10 in an aqueous composition and obtaining an aqueous suspension; - partially drying the aqueous suspension and obtaining a wet suspension; - forming a preformed sacrificial sub-mold from the wet suspension and obtaining a preformed sacrificial sub-mold.

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