Concave piston sintering tooling

Abstract

The present invention relates to a sintering tool comprising a press capable of applying uniaxial pressure to the powder being sintered, which is introduced into a die (1) by means of two opposing pistons (10, 20), characterized in that the proximal frontal surface of said pistons (10, 20), defining a compaction chamber (5), has a hemispherical shape (11, 21). It also relates to a method for manufacturing a part by load sintering in a compaction chamber (5) defined by such a tool. Abstract figure: Figure 3

Description

Title of the invention: Concave piston sintering tooling

[0001] The present invention relates to a method for manufacturing complex shaped parts by sintering using Spark Plasma Sintering (SPS) technology.

[0002] The process allows the production of parts with high density, homogeneous structure, and optimized mechanical properties, while retaining the initial geometric complexity, even for shapes that are difficult to produce with traditional sintering methods.

[0003] Spark Plasma Sintering is an advanced technique that combines the application of mechanical pressure and an electric current to cause the sintering of metallic or ceramic powders, by means of a hot uniaxial press.

[0004] A uniaxial hot press operates to produce complex-shaped parts close to the final dimensions ("near net shape") by a process called load sintering and the use of a sacrificial powder which acts as a deformable mold.

[0005] The principle of the process comprises the following steps: • The desired part is first produced, for example, by molding, machining or additive printing of metallic, ceramic or composite powder, or by means of shaping raw powder without additives: Cold Isostatic Pressure (in English "Cold Isostatic Pressure", pressing in a steel mold). • Another powder, called sacrificial powder, which can withstand high temperatures without chemically reacting with the base powder, is also prepared. This sacrificial powder is used to surround the base powder and acts as a deformable mold. • The sacrificial powder assembly plus the pre-shaped mold piece (pre-formed powder) is placed in the mold. The base powder already has the desired shape. This assembly ensures that the complex shape is well supported and that the forces applied during the process are distributed evenly. • Pressure Sintering: The assembly (base powder, sacrificial powder, and mold) is placed in a uniaxial press capable of applying both intense heat and pressure. The system is heated to a temperature high enough for sintering to occur; that is, the base powder particles begin to bond together at a temperature below their melting point. Uniaxial pressure (in one direction only) is applied while the system is heated. This pressure helps to densify the base powder by compacting it, while allowing the sacrificial powder to transfer this pressure evenly, even over complex shapes. • Densification: Under the combined effect of heat and pressure, the particles of the base powder bond together to form a solid part. The sacrificial powder acts as a mold that transfers heat and pressure, helping to maintain the desired shape while densification occurs. • Cooling and Mold Removal: After the sintering process, the assembly is slowly cooled to room temperature. Once cooled, the sacrificial powder is removed, either mechanically or by washing, leaving behind the complexly shaped sintered part.

[0006] Depending on the tolerance and surface requirements, the resulting part may require finishing operations, such as machining, polishing, or other surface treatments to achieve the final specifications.

[0007] This process allows the production of parts close to their final dimensions, thus reducing the need for rework. It is particularly well-suited to the production of parts with complex geometries that would be difficult or costly to obtain by other methods. Furthermore, it reduces material waste, as less material is removed during finishing.

[0008] This process is particularly useful in fields where the precision and complexity of parts are essential, such as aerospace, automotive, and biomedical applications. State of the art

[0009] Patent application WO2017120990Al relates to a process for preparing a transparent ceramic part of complex shape, precisely dimensioned, the process comprising: 1) the preparation of a raw ceramic body using an injection molding process; 2) the degreasing of the raw ceramic body using a two-stage degreasing process comprising water extraction degreasing and thermal degreasing in combination to obtain a treated body; and 3) The execution of flash sintering without direct pressure application to the treated body using a flash sintering device, followed by annealing and polishing to obtain the transparent ceramic part. The process for preparing a precisely dimensioned, complex-shaped transparent ceramic part is suitable for preparing a precisely dimensioned transparent ceramic part with a high precision, high density, high uniformity, high yield and complex component shape.

[0010] Patent application WO2022185009 is also known, describing a method for manufacturing a near-net-shape (NNS) part of complex shape by pressure sintering from a preform produced according to a first manufacturing method. This process for manufacturing a part by pressure sintering is characterized in that it comprises a first step of producing a preform by agglomerating particles with a D50 particle size of less than 15 µm in a bonding matrix. This preform forms a solid, non-porous, and unbound monolithic part, and a second step of heat-treating said solid, non-porous preform under pressure.

[0011] Patent application WO2021178588A1 describes another method for forming a consolidated element of complex shape, comprising the use of a first element having a first shape similar to the complex shape. The method involves placing the first element in a chamber and introducing a medium around the first element. The method further comprises applying pressure and heat and / or electricity in the chamber to treat the first element to form a consolidated element having the complex shape. Disadvantages of prior art

[0012] The main problem associated with sintering complex parts according to prior art processes is the control of thermal and mechanical gradients which can lead to deformations or heterogeneities in the final part.

[0013] To address this problem, the invention proposes a specific tooling allowing for quasi-isostatic application of pressure and rigorous control of thermal conditions within the powder. Solution provided by the invention

[0014] To overcome these drawbacks, the invention proposes a new method for manufacturing a part by pressure sintering, comprising: a. a step of preparing a preform in powder form of material to be sintered, forming a solid monolithic part made up of non-porous and unbound particles, b. surround said preform with a bonding matrix formed by a compaction powder having a melting point higher than the sintering point of said powder of said preform c. to arrange said preform, surrounded by said die, in a tool comprising two opposingly moving pistons d. Apply mechanical stress and an electric current through the sacrificial powder to induce a Joule effect and cause the preform powder to sinter,

[0015] characterized in that the proximal end of each of said pistons has a hemispherical cavity.

[0016] According to one variant, said proximal front surface of said pistons has a transversely flared hemispherical shape.

[0017] Detailed description of a non-limiting example of embodiment

[0018] The present invention will be better understood upon reading the following description, concerning a non-limiting example of an embodiment, illustrated by the accompanying drawings where: • [Fig. 1] Fig. 1 represents a schematic view of a sintering equipment according to the invention • [Fig.2] Fig.2 represents a three-quarter perspective view from above, in partial cutting • [Fig. 3] Fig. 3 represents a cross-sectional view of the compaction chamber of the equipment according to the invention • [Fig. 4] [Fig. 4] represents a perspective view of the powder volume sacrificial in the tooling.

[0019] General principle of the sintering tooling according to the invention

[0020] The invention relates to the manufacture of complex-shaped parts by Spark Plasma Sintering (SPS), also known as pulsed current-assisted sintering (FAST), used to densify powders into solid materials through the combined effect of mechanical pressure and the application of electrical pulses. The equipment for performing SPS sintering consists of a hydraulic or mechanical press used to apply uniaxial pressure to the powder being sintered. This pressure helps to compact the powder and promote densification during heating.

[0021] The sintering powder is placed in a die (1), generally made of graphite, due to its resistance to high temperatures and its good electrical conductivity properties. Pistons (10, 20), also made of a material with good thermal and electrical conductivity such as graphite, are used to apply pressure to the powder inside the die.

[0022] The assembly is placed in a vacuum chamber (3). The equipment is fitted with a power supply (2) capable of generating high-intensity electrical pulses (up to several kiloamperes) via electrodes (6, 7). These pulses create a plasma between the powder particles, which improves mass transport and facilitates sintering at lower temperatures and for shorter durations than traditional sintering methods.

[0023] Temperature control is crucial in the SPS process. The equipment is generally fitted with thermocouples placed near the die to monitor and control the temperature precisely. Cooling systems may also be included to manage heat dissipation.

[0024] The sintering chamber (5) is the enclosure where the process takes place. It can be under vacuum or under a controlled atmosphere (such as argon) to prevent oxidation of the materials at high temperature.

[0025] A cooling system is optionally integrated to allow rapid temperature control once sintering is complete, which is important for certain properties of the sintered materials.

[0026] The entire process is controlled by a computerized system allowing the programming of temperature, pressure and current cycles, as well as real-time monitoring of critical sintering parameters.

[0027] These components interact in a synchronized manner to enable the efficient sintering of powders into dense and homogeneous materials. SPS sintering is particularly suited to producing materials with fine and controlled microstructures. Piston shape

[0028] The equipment of the present invention is distinguished by the hemispherical shape of the proximal surfaces (11, 21) of the pistons (10, 20), defining a compaction chamber of overall spherical shape as illustrated by [Fig.3]. These hemispherical contact surfaces (11, 21) preferably have a diametrical step to allow for the finishing of the workpiece under pressure. These pistons have, at their proximal end (closest to the compaction chamber (5)), a hemispherical cavity defining the contact surface (11, 21).

[0029] Preferably, this hemispherical cavity (11, 21) has a flared rim (12, 22) to allow for the finishing of the compaction powders (17, 18) in the area where the two pistons (10, 20) come together.

[0030] Advantageously, the radius of curvature of the hemispherical surface (11, 21) is reduced slightly when moving away from the axial direction and then reversed when moving closer to the transverse plane, with a circumferential inflection zone (13, 23).

[0031] The circumferential inflection zone (13) is preferably located at an angle between 70° and 85° with respect to the axial direction.

[0032] This cavity can be formed directly in the end area of ​​each of the pistons (10, 20), for example by machining or forging. It can also be provided on an insert which is housed in the end of the piston, thus presenting a connecting area, for example of conical or frustoconical shape.

[0033] This piston configuration makes it possible to obtain a quasi-isostatic stress field, that is to say a homogeneous stress field in the 3 directions of space.

[0034] The material for manufacturing the pistons (10, 20) or the inserts having the cavity is chosen to withstand a temperature range of 0-2200°C: while maintaining high qualities in mechanical compression, electrical conductivity, and thermal conductivity. Graphite is an example.

[0035] The design of these cavities (11) is key to addressing the problem of prior art tools, which are limited to applying uniaxial stress. The hemispherical cavities (10) allow the initial uniaxial stress to be distributed in a quasi-isostatic field within the tool, and thus transfer part of the uniaxial stress to the centripetal radial plane in order to obtain a homogeneous stress field.

[0036] The tooling of the invention makes it possible to apply a quasi-isostatic stress field, that is to say a homogeneous stress field in the three directions of space.

[0037] General principle of the sintering process according to the invention

[0038] The method according to the invention comprises the following steps: 1. Material powder preparation: The powder is selected and prepared to meet the requirements of the final part in terms of composition and particle size. 2. Powder arrangement in the aforementioned tooling: Tooling consisting of a sintering press equipped with two pistons moving axially in opposing directions, comprising specific inserts and sacrificial powder to homogenize thermal and mechanical stresses. The invention relates in particular to the proximal shape (closest to the part to be sintered) of the pistons, which have a hemispherical cavity enabling the transformation of the axial pressure into a pressure comprising an axial component and centripetal components. 3. Application of stress and current: Mechanical pressure, applied via two pistons each having a hemispherical cavity, is combined with an electric current to heat the powder by Joule effect, thus triggering sintering. 4. Gradient control: Temperature and pressure are finely controlled to minimize thermal gradients, thus reducing the risk of distortion of the part. 5. Part recovery and cooling: After sintering, the part is cooled in a controlled manner to avoid residual stresses and ensure a homogeneous structure.

[0039] The principles of the process involve the use of a sacrificial powder to control gradients, the application of quasi-isostatic pressure to limit deformation, and the integration of sensors for real-time monitoring of thermal conditions. All of these innovations make it possible to produce complex-shaped parts with superior quality, reducing material waste and manufacturing time. Preform preparation

[0040] The preparation of a preform (16) can be carried out in different ways: The preform (16) can also be produced by a laser process by bonding grains in a fusible phase (two-phase material) using a powder coated with the fusible binder. During the laser transfer, the binder occupies the space between the metallurgical grains to form a solid, non-porous part. It can also be produced as a binderless material with good mechanical strength in its raw state, or by injecting the two-phase material or by depositing the binder onto the powder to produce the shape. The shaping methods are mechanical (injection or wire deposition), laser, or ionic.

[0041] The preform (16) has a geometry that takes into account the deformation resulting from the sintering step, by mathematical modeling or by empirical adjustments. Preform densification stage

[0042] The densification step consists of introducing the preform into a bed of electrically and thermally conductive sacrificial powder (17) with a particle size larger than that of the powders used to manufacture the preform. This sacrificial powder (17) completely surrounds the preform (16); it is subjected to pressure by two opposing pistons (10, 20).

[0043] These pistons (10, 20) are part of a tooling consisting of a hot uniaxial press, for the application of the SPS process, by a heating method resulting from the passage of a high-intensity current localized in the tooling (exploitation of the Joule effect) under a controlled atmosphere (vacuum, inert gas). It allows the obtaining of improved mechanical properties (hardness, Young's modulus, tensile strength, shear strength, ductility, etc.) by controlling the microstructure obtained after sintering.

[0044] The powder transmits heat to the preform (16), causing the binder to carbonize and the powder in the preform (16) to sinter. Any gaseous effluent produced by heating the binder is vented through the network of pores created by the sacrificial powder surrounding the preform (16). The preparation of the sintering tooling comprises the following steps: - Insertion of a quantity of compaction powder of a first type: The chemical nature, particle size and quantity of the powder are chosen by following the chemical nature of the powders, the preform, its particle size, its geometric complexity,... - Positioning of the preform (16) on top of the previous bed of sacrificial powder (17); - Insertion of a second quantity of sacrificial compaction powder (18) in order to cover the preform (16) previously introduced: the properties of this powder may be identical in every respect to the first bed of powder, but may also differ (in particular the quantity and the particle size);

[0045] Pressure is then applied using the two pistons (10, 20), in conjunction with the passage of an electric current which will pass through matrix (3) and ensure the temperature rise to cause the sintering of the particles of the preform (16).

[0046] The densified and sintered part (16) is then removed from the enclosure (16) and the sacrificial powder materials (17, 18) can optionally be recovered for a new embodiment.

[0047] Detailed description of a non-limiting example of embodiment

[0048] The invention allows, for example, the manufacture of a complex ceramic part intended for use in a heat exchanger system for aeronautical applications. The part in question has a complex geometry with internal channels, pronounced curves, and thin-walled areas, which makes it difficult to produce using traditional manufacturing methods.

[0049] 1. Preparation of the Material Powder

[0050] The chosen base material is yttrium-stabilized zirconium oxide (YSZ), known for its high-temperature resistance and excellent mechanical properties. The YSZ powder is prepared by high-energy grinding to obtain a fine and homogeneous particle size, typically less than 1 micron, to ensure optimal density after sintering. The powder is then sieved to remove agglomerates and guarantee a uniform particle size distribution.

[0051] 2. Arrangement of the Powder in the Tooling

[0052] Specific tooling is designed for the SPS process. This tooling includes a graphite matrix, resistant to high temperatures and mechanical stresses, with removable inserts adapted to the most complex areas of the part.

[0053] A sacrificial powder, consisting of silicon carbide (SiC), is placed around the critical areas of the YSZ powder to ensure a uniform distribution of thermal and mechanical stresses. The YSZ powder is carefully loaded into the tooling, taking care to fill the internal channels and thin-walled areas without leaving any voids.

[0054] 3. Application of Stress and Electric Current

[0055] The entire tooling is placed in an SPS chamber. A hemispherical piston is used to apply a quasi-isostatic pressure to the YSZ powder. The initial pressure is 50 MPa, applied gradually to avoid premature cracking.

[0056] An electric current of 12 kA is then applied through the graphite matrix, inducing a Joule effect that rapidly heats the YSZ powder. The temperature rises to 1350 °C in a few minutes, causing the powder to sinter while minimizing grain growth to preserve the mechanical properties of the material.

[0057] 4. Control of Thermal and Mechanical Gradients

[0058] Temperature sensors are integrated at various points in the tooling to monitor thermal gradients in real time. Control software automatically adjusts the heating and pressure parameters to maintain a homogeneous heat distribution, thus avoiding overheating areas that could lead to deformation or internal defects.

[0059] 5. Cooling and Extraction of the Part

[0060] Once sintering is complete, the part is cooled under a controlled argon atmosphere to prevent oxidation. Cooling is carried out gradually, reducing the temperature in stages, in order to minimize residual stresses in the part.

[0061] After cooling, the die is disassembled and the removable inserts are taken out, allowing easy extraction of the part. The sacrificial SiC powder is removed by washing, leaving the perfectly formed YSZ part.

[0062] 6. Characterization and Validation

[0063] The resulting part is subjected to a series of tests to validate its mechanical properties and internal structure. X-ray tomography analysis is performed to verify the absence of porosity or internal defects. Three-point bending tests reveal high mechanical strength, exceeding 900 MPa, while the measured density is 99.5% of the theoretical density of YSZ.

[0064] The internal channels of the part are inspected to confirm their integrity and geometric accuracy. Dimensional control shows that the tolerances are met with an accuracy of ± 0.05 mm, which is crucial for the intended application in the heat exchanger system.

[0065] This non-limiting example of implementation illustrates the ability of the Spark Plasma Sintering (SPS) process according to the invention to produce complex-shaped ceramic parts of exceptional quality. The use of specific tooling and sacrificial powder, combined with rigorous control of conditions thermal and mechanical processes allow for the production of parts that meet the most stringent requirements of advanced industrial applications. Some applications of the invention

[0066] The process can be applied to manufacture complex ceramic parts, while ensuring precise control of the material and geometric properties of the final part. It is particularly well-suited to the production of critical components. It is especially relevant to the manufacture of complex-shaped parts in fields such as aerospace, medicine, energy, and luxury goods, where geometric precision and the mechanical properties of the parts are essential. Furthermore, the process offers significant flexibility for the production of small series or prototypes, allowing for rapid optimization of complex designs.

Claims

Demands

1. - Sintering tooling consisting of a press capable of applying uniaxial pressure on the powder being sintered introduced into a die (1) by means of two antagonistic pistons (10, 20) characterized in that the proximal front surface of said pistons (10, 20) defining a compaction chamber (5) has a hemispherical shape (11, 21).

2. - Sintering tooling according to claim 1 characterized in that said proximal front surface of said pistons (10, 20) has a transversely flared hemispherical shape (11, 21).

3. - Sintering tooling according to claim 1 characterized in that the radius of curvature of said hemispherical surface (11,21) decreases when moving away from the axial direction and then reverses when moving towards the transverse plane, with a circumferential inflection zone (13, 23).

4. - Sintering tooling according to the preceding claim characterized in that said circumferential inflection zone (13) is located at an angle between 70° and 85° with respect to the axial direction

5. - Method of manufacturing a part by sintering under load in a compaction chamber (5) characterized in that said compaction chamber is defined by two proximal frontal surfaces of hemispherical shape (11, 21) of the uniaxial antagonistic pistons (10, 20) defining said compaction chamber (5).

6. - A method for manufacturing a part by load sintering in a compaction chamber (5) according to the preceding claim, characterized in that it comprises: a. a step of preparing a preform in powder form of the material to be sintered, forming a solid monolithic part made up of non-porous and non-debinding particles, b. surrounding said preform with a bonding matrix formed by a compaction powder having a melting temperature higher than the sintering temperature of said powder of said preform, c. placing said preform surrounded by said matrix in a tool comprising two opposing displacement pistons (10, 20) having at their proximal end a hemispherical cavity (11, 21).

7. d. apply mechanical stress and an electric current through said powder of material to be sintered to induce a Joule effect and cause the sintering of the powder of said preform. - Method of manufacturing a part by sintering under load in a compaction chamber (5) according to the preceding claim characterized in that step b) includes the use of a sacrificial powder arranged around the powder of material to be sintered to homogenize the thermal and mechanical gradients within the tooling.