METHOD FOR MANUFACTURING A COMPLEX-SHAPED NEAR-NET-SHAPED PART (NNS) BY PRESSURE SINTERING

Abstract

The invention relates to 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, said preform forming a solid, non-porous, and unbound monolithic part, and a second step of heat-treating said solid, non-porous preform under pressure. Figure 1

Description

Title of the invention: METHOD FOR MANUFACTURING A COMPLEX-SHAPED NEAR-NET-SHAPED PART (NNS) BY CHARGE SINTERING Scope of the invention

[0001] The invention relates to a method of manufacturing a near net shape (NNS) part of complex shape by sintering under load from a preform made according to a first manufacturing method, for example by an additive technique or by molding.

[0002] To manufacture a complex-shaped part with high mechanical hardness and toughness, the most obvious solution is to machine a solid piece. This solution allows us to take advantage of the intrinsic qualities of the material.

[0003] Such a part can also be produced by casting or molding. However, these techniques limit the achievable geometries, unless complex multi-drawer molds are used. Furthermore, it is difficult to maintain isotropic characteristics.

[0004] Another solution involves producing a part by sintering under load. Sintering corresponds to the thermal consolidation of a powdered material from its constituents. It is one of the most delicate and often the most expensive operations in the preparation of ceramics. During the thermal cycle, the microstructure is established through the transport of material between grains in order to minimize excess interfacial energy, which is generally accompanied by a decrease in porosity. This decrease manifests itself macroscopically as a shrinkage compared to the "raw" part.

[0005] The raw material for a sintered component is generally a metal or ceramic powder. The characteristics of the part to be obtained determine the chemical composition of the powder.

[0006] In particular, hot isostatic pressing (HIP) is a process for producing a high-quality metallic component such that it can be used in many applications, for example aerospace.

[0007] HIP is a heat treatment method that uses high temperatures and pressures to sinter material particles, resulting in a component with improved structural properties on forged or cast objects.

[0008] The HIP process subjects a material, either in solid form or in powder form, to both a high temperature and an isostatic gas pressure in a high-pressure containment vessel.

[0009] The material can, for example, be a metallic powder.

[0010] For example, HIP can be used to densify existing metallic components with internal voids, or to join two components together.

[0011] Metal powders can be either pure metal powders (iron, copper) or alloy powders (bronze, brass, steel, etc.). The different nature of the powders (spongy, irregular, spherical, laminar) gives the component different properties.

[0012] Finally, we know of additive printing techniques, which make it possible to create parts of complex shapes (hollow, curved, interlacing) by even integrating functionalities within the same part.

[0013] There are a variety of families of layer addition manufacturing processes:

[0014] - the fusion of wire through a heated nozzle (FDM or FFF process), - the projection of binder onto a powder-type substrate (3DP), - the projection of material droplets (Polyjet), - the assembly of layers from cut sheets or plates (Stratoconception) - the polymerization of a resin under the effect of a laser or a UV source (Stereolithography), - the solidification of a powder bed under the action of a medium to high power energy source (laser) (SLS) and the projection of a powder stream into a laser energy stream (CLAD).

[0015] The mechanical qualities of the parts thus obtained are generally quite poor and are degraded by the presence of binder.

[0016] It has been proposed to improve the qualities of parts obtained by additive manufacturing through an additional hot isostatic pressing (HIP) step. HIP / CIP serves to eliminate pores and cavities, thereby increasing the material properties. Under typical pressures between 400 and 2,070 bar and temperatures up to 2,000 °C, the materials can reach 100% of their maximum theoretical density. HIP / CIP increases the ductility and fatigue resistance of parts obtained by additive manufacturing. State of the art

[0017] Patent application WO2016189312 describes a method for forming a three-dimensional object which comprises the steps of:

[0018] - depositing successive layers of powder onto a surface; - selectively bind the powder to form a three-dimensional object; - to envelop the three-dimensional object with a particulate material; - and push the particulate matter towards the object in order to exert pressure directly or indirectly on it, and apply heat directly or indirectly on the object so that the object is densified.

[0019] Prior art patent application WO2020070107 is also known, describing a method for manufacturing a complex-shaped part by successive layer deposition using a 3D additive manufacturing and pressure sintering technique. This prior art method comprises the following steps:

[0020] - a preliminary step in creating a model from a material chosen between porous or powdery materials based on a metallic alloy, a ceramic, a composite material and a lost material by the formation of successive layers deposited according to the digitally controlled three-dimensional (3D) additive printing technique from a material chosen from among a porous or powdery material based on a metallic alloy, a ceramic, a composite material and a lost material, followed

[0021] - from a step of introducing a preform made of porous or powdery material to densifier deduced from the model in a mold filled with sacrificial material in porous or powdery material in addition to the preform, an interface layer isolating the porous preform from the sacrificial material

[0022] - uniaxial pressure densification sintering is then applied to the mold in order to to form the part which is finally extracted from the mold. The sacrificial material is a material which can be chosen from among a ceramic, a silica, a metallic silicate and a composite material.

[0023] A debinding step of the preform is performed at the output of 3D additive printing by heat treatment at temperatures between 200 and 600°C and heating rates between 0.1 and 1°C / min, depending on the material of the counterform. In this prior art solution, the purpose of this step is to eliminate, before introduction into the sintering equipment, the organic compounds introduced into the material during the fabrication of the counterform.

[0024] We also know of patent application WO2016030654A1 which proposes a method for forming a component in a hot isostatic press, comprising the steps of obtaining a feed material, constructing a mold by obtaining a three-dimensional pattern, forming a ceramic shell around said three-dimensional pattern, removing said pattern from inside said ceramic shell, so that a cavity is left inside said ceramic shell, and covering said ceramic shell with a metallic layer, wherein an opening is defined through said metallic layer and said ceramic shell, filling the cavity in said mold with said feed material via said opening, sealing the opening defined by said metallic layer, so that the mold is impermeable,placing said mold inside a hot isostatic press and subjecting said mold to a high temperature and isostatic gas pressure, by removing said mold from the hot isostatic press, and removing the finished component from said mold.

[0025] Disadvantage of the prior art

[0026] The solution described in patent WO2016189312 is based on the SPS sintering of a porous preform, which is then highly densified in a mold filled with a sacrificial material. This results in a volume loss ranging from 40% to 80%, which is not homothetic and whose evolution is difficult to model. The shape and volume of the resulting part are very different from the shape and volume of the preform, and this process is completely unsuitable for manufacturing parts requiring a precise and predetermined geometry. This prior art process also requires an additional debinding treatment before the sintering process, which complicates the manufacturing process and introduces denaturation of the part before the SPS step by making it porous due to the extraction of the binding phase.

[0027] Furthermore, the porosity or powdery nature of the preform leads to fragility of the preform during the sintering stage, amplifying the deformation phenomenon of the part between introduction into the sintering mold and its densification.

[0028] Finally, the use of a sacrificial material leads to a high cost of the process.

[0029] The solution based on patent WO2016030654A1 avoids this problem, but the transmission of pressure by a gas leads to insufficient heat transfer, limiting the range of materials eligible for the process.

[0030] Solution provided by the invention

[0031] In order to overcome the drawbacks of the prior art, the present invention, in its most general sense, relates to a method for manufacturing a part by pressure sintering, characterized in that it comprises:

[0032] - a first step in producing a preform by agglomerating particles of particle size D50 less than 15 pm in a bonding matrix, said preform forming a solid, non-porous, and unbound monolithic piece,

[0033] - a second step of heat treatment under pressure of said preform solid and non-porous, consisting of:

[0034] • Prepare a thermal sintering tool with the following steps: • Placement of a lower piston in a graphite matrix, • Insertion into said lower piston of a quantity of a first reusable compaction powder, with a particle size larger than that of said bound particles of the preform and having a melting point higher than the sintering point of said particles of said preform • Positioning of said solid and non-porous preform directly above said compaction powder bed • Insertion of a second quantity of a second reusable compaction powder, with a particle size larger than that of the said bound particles of the preform and reusable, directly onto said preform • Installation of the upper piston in the graphite matrix, • Applying axial pressure to said pistons and heating said particles of said preform to a sintering temperature.

[0035] According to the invention, unlike the prior art, the binder is not removed before the SPS stage, and as a result the part has a homogeneous non-porous state, the interstice between the grains being filled by the bonding matrix, which is then extracted during the sintering stage, and not before the heat treatment stage under pressure jointly ensures the effect of surface welding of the grains and densification.

[0036] Preferably, said preform is manufactured by additive manufacturing of a powdery material having a D50 particle size between 1 and 10 pm.

[0037] Advantageously, said sintering powder has a D50 particle size between 15 and 30 pm.

[0038] According to a particular embodiment, said first and second compacting powders are identical.

[0039] Preferably, the said compacting powder(s) are non-metallic powders.

[0040] Advantageously, the said compaction powder(s) are carbon graphite powders or an oxide ceramic.

[0041] Detailed description of a non-limiting example of embodiment

[0042] 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:

[0043] [fig. 1] [fig. 1] represents a schematic view of a tool for implementing the invention

[0044] [fig.2] Fig.2 represents a partial schematic view of the tooling during the densification phase

[0045] [fig.3] the [fig.3] represents a schematic view of the decapsulation of the sintered part.

[0046] General principle

[0047] The process that is the subject of the invention is broken down into two phases:

[0048] - the preparation of a solid, non-porous preform from metallic or ceramic particles held together by a binder - the positioning of the preform within a volume of compaction powder arranged between two pistons - A post-processing technique designed to increase the densification rate and simulate- The elimination of the binder is achieved by applying pressure to the pistons and heating to the sintering temperature to cause debinding, with the binder diffusing into the compaction powder and simultaneously sintering the metallic or ceramic particles of the preform.

[0049] The process is similar to a Hot Isostatic Pressing (HIP) process which allows the use of no special tooling to apply pressure although the load is not applied in a perfectly isotropic manner, more so than an SPS process which provides a perfectly uniaxial action.

[0050] According to the HIP process known in the prior art, the preform is placed in an enclosure into which a neutral gas (argon, nitrogen) is injected under an applied pressure of approximately 100 MPa, homogeneously in all directions including in the incoming corners.

[0051] According to the invention, it is not a neutral gas that transmits the pressure, but a volume of powder that transmits the pressure and heat to the preform. Detailed description of the invention

[0052] The first step of the invention consists of manufacturing a preform made of metallic or ceramic particles, or more generally, of a sinterable material. These particles must have a D50 particle size of less than 15 µm, meaning that half of the grains have a cross-sectional area of ​​less than 15 µm, and half have an area greater than 15 µm. Preferably, the particle size distribution is small, and less than 1% of the particles have a cross-sectional area that differs by ±10% from the median cross-sectional area.

[0053] Preform preparation

[0054] The preparation of a preform can be carried out in different ways.

[0055] One method involves manufacturing a part using a "Bound Metal Deposition" type process developed for metal based on the polymer fused deposition modeling (FDM or FFF) process, from a wire, rods or granules that will be extruded. These are composed of metal powder and a binder, or the "Metal Binder Jetting" technique.

[0056] The metal powder is for example 316L steel with a sintering temperature above 600°C and a mass percentage of binder of around 15%, in the form of metal wire with a binder coating.

[0057] The resulting part is a solid, non-porous part, formed by a combination of ceramic or metal and plastic.

[0058] The preform can also be manufactured using 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 fills the spaces between the metallurgical grains to form a solid, non-porous part.

[0059] The fusible binder is generally made of polymers. The major drawback of these processes is that the resulting object is not of relevant quality and that functional tests cannot be carried out under real-world conditions.

[0060] The preform has a geometry that takes into account the deformation resulting from the sintering step, by mathematical modeling or by empirical adjustments.

[0061] Preform densification step

[0062] The densification step consists of introducing the preform into a bed of powder with a particle size larger than that of the powders used to manufacture the preform. This powder completely surrounds the preform; it is subjected to pressure by two opposing heated pistons. The powder transmits heat to the preform, causing the binder to carbonize and the preform powder to sinter. Any gaseous effluent produced by the heating of the binder is vented through the grains of the powder bed surrounding the preform.

[0063] The preparation of the sintering tooling comprises the following steps:

[0064] - Placement of the lower piston in the graphite matrix, - Implementation of a system for releasing the lower part (graphite paper) or graphite spray, or any other system known to a person skilled in the art), - 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 of the preform, its particle size, its geometric complexity,... - Positioning the part on top of the previous powder bed - Insertion of a second quantity of compaction powder, in order to cover the preform 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); - Implementation of a system for releasing the upper part (graphite paper or graphite spray, or BN, or any other system known to those skilled in the art), - Placement of the upper piston in the graphite matrix, - Preliminary compaction of the assembly before insertion into the press.

[0065] The part is positioned on the compaction powder bed in such a way as to optimize densification while reducing shrinkage in certain directions. Prior to this, a deformation study and a principal pressing axis are identified through modeling or an empirical approach. The positioning optimization is carried out either through numerical analysis, but more often empirically.

[0066] The quantity of powder is chosen so as to absorb by capillary action the gaseous / liquid phases of binder forming during this sintering step.

[0067] In general, the compaction powder allows the transfer of charge from the compression pistons while accommodating the shape of the part. The characteristics of the compaction powder are such that the sintering temperature is significantly higher than that of the printed powder constituting the part to be densified, or else chemically compatible and allowing the use of deburring processes after densification.

[0068] The part is dimensioned (local overthickness) so that it is ultimately "near net shape" (close to the final shape), which minimizes the machining finishing steps required to obtain the shape in a "net shape" state. The part may be hollow, and the interior is also filled with sintering powder.

[0069] Tooling description

[0070] The tooling consists of an upper piston (10) and a lower piston (20) actuated in opposite directions along a longitudinal axis (1) by cylinders (11, 21) exerting an opposing thrust on the pistons (10, 20). The pistons (10, 20) are made of an electrically conductive material and are connected to a pulsed current generator (30).

[0071] The piston (10) is extended by a connecting block (12) called a "spacer," which is also electrically conductive and has an upper matrix (13) which, together with a lower matrix (23), defines a treatment chamber (15). The lower matrix (23) forms the end of a connecting block (22) called a "spacer," which is also electrically conductive and extends the piston (20).

[0072] The part (16) obtained by additive manufacturing is placed in this chamber (15), on a layer (17) of compaction powder and then covered with a second layer (18) of compaction powder. The compaction powder will be, for example, graphite carbon or an oxide ceramic.

[0073] Pressure is then applied using the two pistons (10, 20), in conjunction with the passage of an electric current which will pass through the enclosure (15) and ensure the temperature rise to cause the sintering of the particles of the preform (16).

[0074] The densified and sintered part (16) is then removed from the enclosure (16) and the powdery materials (17, 18) can be recovered for a new embodiment.

Claims

Demands

1. - A method for manufacturing a part by pressure sintering, 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, said preform forming a solid, non-porous, and unbound monolithic part; • a second step of heat-treating said solid and non-porous preform under pressure, consisting of: - Preparing a thermal sintering tool with the following steps: • Placing a lower piston in a graphite matrix; • Inserting into said lower piston a quantity of a first reusable compaction powder.of a particle size greater than that of said bonded particles of the preform and having a melting point greater than the sintering point of said particles of said preform • Positioning of said solid and non-porous preform directly above said compaction powder bed • Insertion of a second quantity of a second reusable compaction powder, of a particle size greater than that of said bonded particles of the preform and reusable, directly onto said preform • Placement of the upper piston in the graphite matrix, - Applying pressure to said pistons and heating to a sintering point of said particles of said preform.

2. - A method for manufacturing a part by sintering according to claim 1, characterized in that the preform is manufactured by additive manufacturing of a powdery material having a D50 particle size between 1 and 10 pm.

3. - A method for manufacturing a part by sintering according to claim 1 characterized in that said compaction powder has a D50 particle size between 15 and 30 pm.

4. - A method for manufacturing a part by sintering according to claim 1 characterized in that said first and second compaction powders are identical.

5. - A method for manufacturing a part by sintering according to claim 1 characterized in that it comprises a step of setting up a system for releasing the upper part deposited between the compaction powder and the piston.

6. - A method for manufacturing a part by sintering according to claim 1 characterized in that the compacting powder(s) are non-metallic powders.

7. - A method for manufacturing a part by sintering according to claim 1 characterized in that the compacting powder(s) are carbon graphite powders or an oxide ceramic.