Sintering tool with concave pistons

Abstract

The present invention relates to a sintering tool consisting of a press capable of applying uniaxial pressure to the powder being sintering that has been introduced into a die (1) by means of two opposing pistons (10, 20), characterized in that the proximal front surface of the pistons (10, 20) defining a sintering chamber (5) has a hemispherical shape (11, 21). It also relates to a method for manufacturing a part by sintering under load in a sintering chamber (5) defined by such a tool.

Description

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 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, via a hot uniaxial press.

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

[0005] The principle of the process involves the following steps: The desired part is first formed, for example, by molding, machining, or additive manufacturing of metal, ceramic, or composite powder, or by means of shaping raw powder without additives: Cold Isostatic Pressing (CIP), 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 assembly of sacrificial powder plus the shaped part (previously shaped 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 densify the base powder by compacting it, while allowing the sacrificial powder to transfer this pressure evenly, even onto complex shapes. Densification: Under the combined effect of heat and pressure, the base powder particles bond together to form a solid part.The sacrificial powder acts as a mold, transferring heat and pressure to help 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 tolerance and surface requirements, the resulting part may require finishing operations, such as machining, polishing, or other surface treatments to achieve final specifications.

[0007] This process allows for the production of parts close to their final dimensions, thus reducing the need for rework. It is particularly well-suited to producing parts with complex geometries that would be difficult or costly to achieve using 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] Prior art patent EP3437108B1 describes a process for the rapid fabrication of coated nuclear fuel particles. It involves the direct compaction and sintering of ceramic matrix powders around a fuel core, by simultaneously applying pressure and electric current. The invention optimizes the formation of double-shell structures for improved fission product retention and faster fuel pellet fabrication. In this patent, compaction is performed in flat presses between cylindrical or flat dies, by applying pressure and current. This configuration imposes a geometry strongly focused on lateral compactness and does not allow for optimal control of internal stresses for spherical parts.The piston-powder contact is planar, which induces: heterogeneous density gradients; poor adaptability to spherical shape; a risk of peripheral cracking during cooling.

[0010] Patent application DE102019111698A1 proposes a method for manufacturing ceramic balls by pressing and sintering a polymer / ceramic powder in a mold. The body is first preformed and then heated by pyrolysis to obtain a homogeneous microstructure. The invention improves the dimensional control and surface quality of the balls, with a particular focus on gripping and alignment devices for producing precision spherical balls. The solution relies on a spherical preform placed in a conventional mold, often with a flat bottom. Contact with the surfaces of flat or cylindrical molds induces stress concentration zones, especially at the poles of the balls. Furthermore: the densification is not homogeneous throughout the volume of the ball; the areas in contact with the tools exhibit surface defects; and the compaction kinematics do not conform to the sphericity of the final product.

[0011] Patent application JP2006045038A describes an annular cavity mold for improving part forming. It incorporates a ring between the upper and lower molds, acting as a guide and deformation limiter. This prevents material from seeping between the mold elements, reduces stress, facilitates demolding, and allows for more homogeneous densification of the molded part, while ensuring good dimensional accuracy at the periphery and in height. Although this solution improves densification by limiting material penetration between the molds, it maintains a flat contact surface between the pistons and the material. This results in radial material flow, which is poorly suited to spherical parts, poor reproducibility of the part core, and a strong dependence on the alignment accuracy of the molded parts.

[0012] Application DE102016004548A1 describes a manufacturing process for metallic or ceramic components using current- and pressure-assisted sintering (FAST / SPS). A special punch geometry creates controlled porosity: high at the center, low at the periphery. This gradient optimizes mechanical and functional properties for applications such as fuel cells. This solution offers specific temperature and pressure profiles to adjust the internal microstructure and conductive matrix sintering, with a standard piston geometry (flat or conical). Attempts are made to create areas of differentiated porosity, but without truly adapting the piston shape to the sphericity of the final object. This results in uneven pressure in internal areas, geometric complexity incompatible with the desired hemispherical shape, and the need for post-sintering machining to achieve a true sphere, thus increasing costs. Disadvantages of prior art

[0013] The prior art shows a predominance of devices with planar or cylindrical geometries, poorly suited to the sintering of spherical parts. Unevenly distributed mechanical and thermal stresses negatively impact the quality of the resulting product. The claimed invention proposes a conceptual change by adopting a hemispherical shape for the compaction surfaces, which allows for better structural homogeneity, reduced scrap, and a significant gain in production efficiency.

[0014] The main problem associated with sintering complex parts using prior art processes is controlling thermal and mechanical gradients that can lead to deformations or heterogeneities in the final part.

[0015] 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

[0016] To overcome these drawbacks, the invention proposes a method for manufacturing a part by load sintering having the characteristics of claim 1 and, for variants, of dependent claims. This method comprises: a step of preparing a preform of powder of the material to be sintered forming a solid monolithic part made up of non-porous and non-debinding particles; surrounding 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; placing said preform surrounded by said matrix in a tool comprising two opposingly moving pistons; applying a mechanical stress and an electric current through the sacrificial powder to induce a Joule effect and cause the sintering of the preform powder.

[0017] characterized in that the proximal end of each of said pistons has a hemispherical cavity.

[0018] According to one variant, the proximal front surface of said pistons has a transversely flared hemispherical shape.

[0019] Detailed description of a non-limiting example of implementation

[0020] The present invention will be better understood upon reading the following description, concerning a non-limiting example of embodiment, illustrated by the accompanying drawings where: A schematic view of a sintering equipment according to the invention is shown; A three-quarter top perspective view, in partial cutaway is shown; A cross-sectional view of the compaction chamber of the equipment according to the invention is shown; A perspective view of the volume of sacrificial powder in the tooling is shown.

[0021] General principle of the sintering tooling according to the invention

[0022] The invention relates to the manufacturing 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.

[0023] The sintering powder is placed in a die (1), usually made of graphite due to its resistance to high temperatures and good electrical conductivity. 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.

[0024] The assembly is placed in a vacuum chamber (3). The equipment is equipped 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.

[0025] Temperature control is crucial in the SPS process. The equipment is typically fitted with thermocouples placed near the die to accurately monitor and control the temperature. Cooling systems may also be included to manage heat dissipation.

[0026] 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.

[0027] An optional cooling system is integrated to allow rapid temperature control once sintering is complete, which is important for certain properties of sintered materials.

[0028] The entire process is controlled by a computerized system that allows for the programming of temperature, pressure and current cycles, as well as real-time monitoring of critical sintering parameters.

[0029] 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

[0030] 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 with a generally spherical shape, as illustrated in Figure 5. These hemispherical contact surfaces (11, 21) preferentially have a diametrical step to allow the workpiece to creep under pressure. At their proximal end (closest to the sintering chamber (5)), these pistons have a hemispherical cavity defining the contact surface (11, 21).

[0031] Preferably, this hemispherical cavity (11, 21) has a flared rim (12, 22) to allow the compaction powders (17, 18) to creep into the area where the two pistons (10, 20) meet.

[0032] Advantageously, the radius of curvature of the hemispherical surface (11, 21) decreases slightly when moving away from the axial direction and then reverses when moving closer to the transverse plane, with a circumferential zone of inflection (13, 23).

[0033] The circumferential inflection zone (13) is preferably located at an angle between 70° and 85° with respect to the axial direction.

[0034] 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.

[0035] 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.

[0036] The material for making the pistons (10, 20) or the inserts with the cavity is chosen to withstand a temperature range of 0-2200°C, while maintaining high mechanical compression, electrical conductivity, and thermal conductivity. Graphite is an example of such a material.

[0037] The design of these cavities (11) is key to addressing the problem of prior art tools, which were 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 to obtain a homogeneous stress field.

[0038] The tooling that is the subject 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.

[0039] General principle of the sintering process according to the invention

[0040] The process according to the invention comprises the following steps: Preparation of the material powder: The powder is selected and prepared to meet the requirements of the final part in terms of composition and particle size. Placement of the powder in the aforementioned tooling: Tooling consisting of a sintering press equipped with two pistons moving axially in opposing directions, including 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.Application of stress and current: Mechanical pressure, applied via two pistons each with a hemispherical cavity, is combined with an electric current to heat the powder by Joule heating, thus initiating sintering. Gradient control: Temperature and pressure are precisely controlled to minimize thermal gradients, thereby reducing the risk of part distortion. Part recovery and cooling: After sintering, the part is cooled in a controlled manner to prevent residual stresses and ensure a homogeneous structure.

[0041] The process principles involve using a sacrificial powder to control gradients, applying near-isostatic pressure to limit deformation, and integrating sensors for real-time monitoring of thermal conditions. These innovations enable the production of complex-shaped parts with superior quality, reducing material waste and manufacturing time. Preform preparation

[0042] The preparation of a preform (16) can be carried out in various 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, and ionic.

[0043] The preform (16) has a geometry that takes into account the deformation resulting from the sintering step, either through mathematical modeling or empirical adjustments. Preform densification stage

[0044] 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).

[0045] These pistons (10, 20) are part of a tooling system consisting of a hot uniaxial press, used for the SPS process. This process employs a heating method that results from the passage of a high-intensity current localized within the tooling (exploiting the Joule effect) under a controlled atmosphere (vacuum, inert gas). It allows for improved mechanical properties (hardness, Young's modulus, tensile strength, shear strength, ductility, etc.) through control of the microstructure obtained after sintering.

[0046] The powder transmits heat to the preform (16), causing the binder to carbonize and the powder in the preform (16) to sinter. Any gaseous effluents produced by heating the binder are vented through the network of pores created by the sacrificial powder surrounding the preform (16). The preparation of the sintering tooling involves 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 according to the chemical nature of the powders in the preform, its particle size, its geometric complexity, etc.Positioning of the preform (16) over the previous layer 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 layer of powder, but may also differ (in particular the quantity and the particle size);.

[0047] Pressure is then applied using the two pistons (10, 20), in conjunction with the passage of an electric current which will pass through matrix (1) and ensure the temperature rise to cause the sintering of the particles of the preform (16).

[0048] 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 realization.

[0049] Detailed description of a non-limiting example of implementation

[0050] The invention enables, 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, making it difficult to produce using traditional manufacturing methods.

[0051] 1. Preparation of the Powder Material

[0052] 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.

[0053] 2. Powder Arrangement in the Tooling

[0054] 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.

[0055] A sacrificial powder, composed of silicon carbide (SiC), is placed around critical areas of the YSZ powder to ensure uniform distribution of thermal and mechanical stresses. The YSZ powder is carefully loaded into the tooling, taking care to fill internal channels and thin-walled areas without leaving any voids.

[0056] 3. Application of Stress and Electric Current

[0057] The entire tooling is placed in an SPS chamber. A hemispherical piston is used to apply quasi-isostatic pressure to the YSZ powder. The initial pressure is 50 MPa, applied gradually to avoid premature cracking.

[0058] 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 within minutes, causing the powder to sinter while minimizing grain growth to preserve the material's mechanical properties.

[0059] 4. Control of Thermal and Mechanical Gradients

[0060] Temperature sensors are integrated at various points within the tooling to monitor thermal gradients in real time. Control software automatically adjusts heating and pressure parameters to maintain a homogeneous heat distribution, thus preventing overheating areas that could lead to deformation or internal defects.

[0061] 5. Cooling and Extraction of the Part

[0062] 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 to minimize residual stresses within the part.

[0063] After cooling, the die is disassembled and the removable inserts are taken out, allowing for easy extraction of the part. The sacrificial SiC powder is washed away, leaving the perfectly formed YSZ part.

[0064] 6. Characterization and Validation

[0065] The resulting part undergoes 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.

[0066] 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.

[0067] 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 thermal and mechanical conditions, makes it possible to obtain parts that meet the most stringent requirements of advanced industrial applications. Some applications of the invention

[0068] The process can be applied to manufacture complex ceramic parts, ensuring precise control of the material and geometric properties of the final part. It is particularly well-suited to the production of critical components. This includes the manufacture of complex-shaped parts in fields such as aerospace, medicine, energy, and luxury goods, where geometric precision and mechanical properties are essential. Furthermore, the process offers significant flexibility for manufacturing small batches or prototypes, enabling rapid optimization of complex designs.

[0069] The present invention introduces a sintering tool with pistons having a hemispherical front surface, thus defining a spherical compaction chamber. This structural choice leads to several significant improvements: Immediate geometric conformity: the shape of the pistons matches that of the final part, reducing or eliminating post-sintering machining steps. Homogeneous pressure distribution: the pressure application is distributed isotropically over the powder, promoting uniform densification and reducing the risk of cracking and localized porosity. Optimization of thermal and electrical transfer: the hemispherical contacts ensure better propagation of current and heat to the core of the material. Reduction of residual mechanical stresses: the hemispherical shape avoids the localized stress peaks that appear in planar or conical configurations.

Claims

- Pulsed current assisted sintering tooling consisting of a press capable of applying uniaxial pressure on the powder being sintered introduced into a matrix (1) by means of two antagonistic pistons (10, 20) characterized in that the proximal front surface of said pistons (10, 20) defining a sintering chamber (5) has a hemispherical shape (11, 21). - Pulsed current assisted 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). - Pulsed current assisted 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 closer to the transverse plane, with a circumferential inflection zone (13, 23). - Pulsed current assisted 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 - A method for manufacturing a part by pulsed current-assisted sintering in a sintering chamber (5) characterized in that said compaction chamber is defined by two proximal frontal surfaces of hemispherical shape (11, 21) of uniaxial antagonistic pistons (10, 20) defining said sintering chamber (5) and in that it comprises: 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, 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, placing said preform surrounded by said matrix in a tool comprising two antagonistic displacement pistons (10, 20) having at their proximal end a hemispherical cavity (11,21) 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, and in that step b) includes the use of a sacrificial powder disposed around the powder of material to be sintered to homogenize the thermal and mechanical gradients within the tooling.

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