Method for preparing a mould intended for sintering of preforms produced by 3D printing

Abstract

The invention relates to a method for preparing a mould (M) for sintering a porous preform (P), characterised in that the following successive operations are carried out in one or more sequences: - producing the preform by 3D printing; - pouring into the mould a first quantity of a slip (S) comprising a solvent and a pulverulent inert material based on ceramic particles in suspension, and performing a first heat treatment of the mould containing the slip in order to obtain, on the mould surface, a solid and homogeneous bed (L1) of sacrificial powder, the initial porosity of which is between 42% and 80%, and on which the preform is deposited; - pouring a second quantity of slip so as to fill the mould in order to immerse the preform therein and performing a second heat treatment to solidify the bed (L2) of sacrificial powder around the preform; - pouring a third quantity of slip into the mould, which is subjected to a third heat treatment in order to form, around the preform, a solid and homogeneous coating (L3) of sacrificial powder, the porosity of which is still between 42% and 80%, before sealing the mould and proceeding with sintering.

Description

[0001] DESCRIPTION

[0002] METHOD FOR PREPARING A MOLD FOR SINTERING PREFORMS PRODUCED BY 3D PRINTING

[0003] The invention relates to a method for preparing porous molds intended for sintering preforms.

[0004] The manufacture of metal, ceramic or composite parts by powder metallurgy is carried out by simultaneous densification operations using, on the one hand, one or more molds comprising porous sacrificial counterforms and, on the other hand, a powdered material. These preforms can be produced, in particular, by additive manufacturing (known as 3D printing), by casting, machining or pressing, etc.

[0005] In the case of sintering processes where the pressure is transmitted uniaxially (the sintering tools are generally cylindrical in shape), the final parts obtained (called “solids”) are then cylindrical in shape.

[0006] Furthermore, in foundry, the so-called "poteyage" method consists of preparing the mold by coating its surface with a protective material. This method allows, depending on the composition of the coating material, to ensure different functions such as preserving the mold against chemical or physical attack and erosion by the molten liquid metal, its cooling, its lubrication, the facilitated demolding of the finished part and the improvement of its surface condition.

[0007] The production of parts by sintering under load and more precisely by "Spark Plasma Sintering" (i.e. "flash sintering", known as SPS) is known, in particular, from application WO 2017 / 077028 A1. This document proposes placing the part to be densified in a counter-form of powdery or porous structure having at least one deformable interfacial layer in order to optimize the pressure applied to its surface, the powder being called sacrificial. This accompanies the sintering of the part without interacting with it. This is then referred to as a mobile interface between a part to be sintered and a sacrificial media. Furthermore, patent applications US2022032370A1 and US2022032498A1 describe improvements to this technique consisting of combining additive manufacturing and SPS sintering in order to enable the production of parts with complex shapes.Additive manufacturing allows the shaping of an object (porous or not) then participating in densification by sintering according to two alternative methods: either this object is produced by additive manufacturing and directly constitutes the preform to be sintered, or this object has a complementary shape to a preform in lost material obtained by additive manufacturing and then constitutes a counter-form forming an impression or a mold for sintering.

[0008] Thus, when a pre-densified or densified part called a preform is produced by 3D printing, its dimensions are previously determined in order to anticipate its dimensional shrinkage during sintering.

[0009] This preform is then placed in a mold surrounded by a sacrificial powder which will accompany its sintering. After sintering, the assembly (powder + preform) is removed from the mold and the part is recovered after mechanical or chemical demoulding.

[0010] However, in the context of densification and finishing of preforms of complex shaped parts by SPS sintering (or, in general, by pressure sintering) using porous ceramic molds, the preparation of the sintering mold is a task carried out by an operator and which involves the mechanical placement and maintenance of the preform within the sacrificial powder bed. However, this task encounters technical problems which are explained below and which must be overcome if we wish to avoid generating defects on the sintered part.

[0011] Currently, there are two main methods for preparing sintering molds. The first method consists of successively placing a lower piston in the mold, positioning the preform, adding the sacrificial powder until it is covered, topping up the powder to compensate for the future dimensional shrinkage of the part, and placing an upper piston to close the mold. Then, the mold is turned over, the piston is removed, an additional quantity of sacrificial powder is added to perfect the shrinkage tracking, and the piston is placed to close the mold again.

[0012] A second method consists of placing the lower piston in the mold, depositing a first bed of sacrificial powder in the lower part of the mold, positioning the preform in the mold by placing it on this first bed of powder, embedding it in a volume of sacrificial powder, depositing a second complementary bed of powder in the upper part of the mold to compensate for the dimensional shrinkage and placing the upper piston in place to close the mold.

[0013] However, one of the main problems raised by these methods concerns the control of the density of the different powder contributions on each bed and the axial centering of the preform in the mold. Indeed, the density of the powder beds acts directly on the deformations of the preform during sintering as well as on the stability of its balance and its mechanical maintenance due to the fact that the preform is a low-density part, therefore friable and fragile.

[0014] In particular, for preforms with walls, or fragile preforms whose handling is critical, if the density of the sacrificial powder beds is low, the preform will crack as soon as the stresses are applied during pressure sintering.

[0015] Furthermore, for preforms with an entangled structure and which include cavities or areas that are difficult to access, the flow of the sacrificial powder inside this structure is hindered, which does not ensure satisfactory mechanical support of the preform when it is compressed.

[0016] Furthermore, centering the preform on a bed of low-density powder is a delicate operation and its support on this bed therefore remains unstable during sintering. It also turns out that depositing specific, or even identical, quantities of powder in the mold, on either side of the preform, is a laborious operation which, if not carried out precisely, causes homogeneity defects during sintering.

[0017] Furthermore, the reliability and quality of the parts resulting from the implementation of these preparation methods vary depending on the operators, which directly impacts the repeatability and reproducibility of the manufacturing process for these parts. In particular, the successive manual tasks of packing the sacrificial powder beds cannot be repeated in a strictly identical manner and the level of packing is therefore different for each production operation. In addition, for industrial production of serial parts, it is necessary to establish a standardized, permanent and viable method, which is impossible with these two methods due to the divergences generated by the large number of tasks and operators involved.

[0018] US4428906A1 and US4547337A1 describe a sacrificial media composed of a material initially in the form of a ceramic powder which constitutes what is called the skeleton. However, upon increasing the temperature, this powder transforms into a glassy and viscous fluid phase which flows through the skeleton. The latter in turn fragments, thus forming a totally dense media around the preform.

[0019] This sacrificial media is therefore incompressible and non-porous because no porosity can be created during the cooling of such a composite material with a glassy structure. This media therefore remains dense and must be broken at the end of the consolidation process to extract the sintered part, which is problematic due to the risk of breakage.

[0020] In this context and in order to resolve all of the problems set out above, the invention sought to improve the density and homogeneity of the powder bed to ensure, at the same time, stable physical maintenance of the preform in the mold during sintering, better centering and to propose, ultimately, a robust and reproducible manufacturing method for sintered parts.

[0021] This aim is achieved, according to the invention, by means of a method for preparing a mold intended for the sintering of a porous preform, characterized in that the following successive operations are carried out in one or more sequences after production of the preform by 3D printing;

[0022] - pouring into the mold a first quantity of a slip comprising, on the one hand, a solvent and, on the other hand, an inert powdery material based on suspended ceramic particles, then a first heat treatment of the mold with the slip in order to obtain, on its surface, a solid and homogeneous bed of sacrificial powder whose initial porosity is between 42% and 80% and on which the preform is deposited,

[0023] - the pouring of a second quantity of slip so as to fill the mold to embed the preform and a second heat treatment to solidify the bed of sacrificial powder around the preform then,

[0024] - a third quantity of slip is poured into the mold which is subjected to a third heat treatment to form, around the preform, a solid and homogeneous coating of sacrificial powder whose final porosity is always between 42% and 80% before sealing the mold and then sintering.

[0025] According to a specific characteristic of the method of the invention, the powdered material has a melting temperature higher than the melting temperature of the material constituting the preform.

[0026] According to a specific variant of implementation of the method of the invention, a third quantity of said slip is further poured into the mold which is again subjected to a heat treatment to form a solid and homogeneous upper layer of sacrificial powder before sealing the mold.

[0027] According to another specific characteristic of the process of the invention, the ceramic particles suspended in the slip have a particle size greater than the average diameter of the pores of the preform.

[0028] According to an alternative embodiment of the method of the invention, the preform is previously covered with a layer of an inert material and the ceramic particles suspended in the slip then have a particle size smaller than the average diameter of the constituent particles of the preform. According to an advantageous characteristic of the implementation of the invention, the method provides that, prior to its filling, the inner wall of the mold is coated with a carbon sheet and positioned on a porous plate allowing the absorption by capillarity of at least a fraction of the slip. According to another characteristic of the method of the invention, the slip further comprises at least one dispersing agent. Preferably, this dispersing agent is a polymer.

[0029] According to yet another characteristic of the process of the invention, the solvent is volatile under normal temperature and pressure conditions and is advantageously chosen from the group comprising water, alcohols, ketones or a mixture of the latter and is, preferably, isopropanol (PrOH).

[0030] According to a specific variant of implementation of the method of the invention, the slip comprises from 10% to 40% by mass of suspended powdered material and from 60% to 90% by mass of liquid.

[0031] Still according to the method of the invention, the heat treatment comprises a phase of drying in the open air. Where appropriate, the heat treatment further comprises drying in an oven between 80°C and 140°C for 10 minutes.

[0032] According to another advantageous characteristic of the invention, the method provides for the optimization of the viscosity of the sacrificial powder suspension and the homogenization of the slip before it is poured into the mold by introducing zirconia balls and stirring.

[0033] According to a specific variant of implementation of the process, the inert powdered material is chosen from the group comprising ceramics, metal carbides and oxides, nitrides and whose particles have an average diameter of between 0.34 and 300 microns.

[0034] Generally, the quantity of this inert powdery material suspended in the slip is determined based on the dimensional shrinkage of the preform resulting from sintering.

[0035] Another object of the invention is a porous ceramic mold prepared according to the method defined above for pressure sintering of a preform arranged in the mold. Yet another object of the invention is a use of the mold as defined above for densification by pressure sintering of 3D printed preforms using a binder jet technique.

[0036] A specific use of the mold of the invention is intended for the densification by pressure sintering of aluminum preforms and is characterized in that the initial porosity rate of the bed of sacrificial powder coating the preform is between 42% and 53% and is maintained in this range even until the sintering of the preform.

[0037] This porosity rate of the sacrificial powder therefore does not vary depending on the successive beds and it only depends on the ceramic material chosen to form the sacrificial coating.

[0038] The method of the invention therefore consists in preparing a mold intended for the pressure sintering of preforms previously shaped, mainly by a 3D printing technique but, where appropriate, also by other techniques, such as pressing, machining or casting. The main objective of preparing the mold is to improve the density of the sacrificial powder bed in order to ensure stable physical support and better centering of the preform in the mold during sintering.

[0039] More precisely, suspending sacrificial powder particles in a slip filling the mold in which the preform is embedded and controlling the sedimentation of these particles makes it possible to optimize the granular rearrangement and thus improve the density of the bed.

[0040] Indeed, with a traditional process, measurements of the tapped density of the sacrificial powder in the mold show a maximum relative density of the powder bed of 47%. The same measurements carried out with a mold preparation according to the process of the invention by wet method (via the slip with the powdered material in suspension), make it possible to achieve a relative density of 58%. This gain of 11% is very significant and results from the increase in the granular rearrangement of the sacrificial powder.

[0041] According to the method of the invention, the sacrificial media consists of several beds of an inert powdery material based on ceramic particles suspended in a slip. These powder beds form a solidified but naturally friable and porous coating. Then, this coating of solidified powder, the porosity of which remains constant during the steps of the process, is separated from the sintered part by a slight mechanical shake-out and therefore without risk of damage to this part.

[0042] This unexpected advantage results specifically from the three successive stages of deposition in the mold containing the preform of a sacrificial powder in suspension which is kept in this porous form until the final stage of the process.

[0043] The preparation of the slip consists of mixing sacrificial powder with a solvent that is chemically inert to the preform. The quantities of each of the components of this solution are optimized by controlling its viscosity. The invention thus provides for the slip to penetrate all the interstices of the structure of the preform that this powder could not reach by flowing only in dry form.

[0044] Jointly, the invention proposes a robust and standardized method for carrying out repetitive, reproducible and reliable preparations of the molds and, consequently, for obtaining sintered parts of very good quality. Thus, the invention makes it possible to anticipate the geometry of the preforms and to use, on an industrial scale, molds in series with several preforms and always with a homogeneous density and porosity of the sacrificial powder bed.

[0045] The method of the invention also makes it possible to limit the deformation of the sintered objects and to preserve the integrity of the preforms during sintering, whatever their dimensional characteristics and the complexity of their internal structure.

[0046] Other characteristics and advantages of the invention will emerge from reading the following description of three variants of implementation of the method of the invention with reference to figures 1 and 2 explained below.

[0047] [Fig. 1] is a graph representing the particle size distribution of the powder of pulverulent material here in the form of silicon carbide (SiC) used in a preferred embodiment of the method of the invention. [Fig. 2] are diagrams a) to k) representing the successive operations of the method of preparing the mold according to the invention.

[0048] For clarity, identical or similar elements are identified by identical reference signs in the description and throughout the figures.

[0049] Naturally, the modes of implementation of the method of the invention illustrated schematically by the figures presented above and described below are given only as non-limiting examples. It is explicitly provided within the scope of the invention that different modes can be proposed and combined together to propose others.

[0050] In the context of the invention, a slip is prepared comprising, on the one hand, at least one solvent and, on the other hand, an inert powdery material based on suspended ceramic particles forming a so-called sacrificial powder. Preferably, the slip further comprises a dispersing agent chosen from polymers.

[0051] Several powdered materials were tested, mainly ceramic and / or composite ceramic powders (oxides, carbides, nitrides, etc.). The example of implementation of the method of the invention described below uses silicon carbide (SiC).

[0052] The powdered material as well as the liquid dispersing agent are chemically inert with respect to the material constituting the preform. The solvent is volatile under normal conditions of temperature and pressure and is selected from the group comprising water, alcohols, ketones or a mixture thereof and is preferably a solvent such as isopropanol (PrOH).

[0053] The particles of the sacrificial powder used preferably have a so-called "fine" particle size, as illustrated in Figure 1, with a D10 of 0.34 pm, a D50 of 2.54 pm and a D90 of 6.20 pm. The particle size distribution is bimodal with a majority of particles having a size greater than 1 pm. Another solution could consist of using a powder with a multiple particle size distribution to optimize the density of the powder beds. Table 1 below gives three examples of implementation of the method of the invention with different types of powder.

[0054] However, the invention provides that the powder particles suspended in the slip have a particle size greater than the average diameter of the pores of the preform so as not to penetrate into its structure. Preferably, the slip has a mass ratio of 40 / 60 between the powder and the liquid precursor agent (here silicon carbide SiC) / (isopropanol solvent PrOH).

[0055] According to an alternative implementation of the method of the invention, the preform may be previously covered with a layer of an inert material (graphite, boron nitride, etc.), for example, using a spraying, dipping (“deep coating”) or any other appropriate technique. In this case, the ceramic particles suspended in the slip will then have a particle size smaller than the average diameter of the particles constituting the preform.

[0056] A preliminary step in the mold preparation process consists of optimizing the viscosity (% by mass of powder) of the slip and homogenizing it before pouring it into the mold. For this purpose, zirconia balls with a ball / SiC mass ratio of 1 / 1 are added to the slip and the mixture is then stirred for approximately 10 minutes, for example, using a three-dimensional mixer. It is also possible, without departing from the scope of the invention, to perfect the distribution of the powder suspended in the slip by using additional vibration means.According to a specific and advantageous embodiment of the method of the invention, the inner wall of the mold is previously coated with a carbon sheet and positioned on a porous support plate (for example, plaster) allowing the absorption by capillarity of at least a fraction of the slip flowing by gravity through the porous structure of the mold, the additional fraction undergoing a heat treatment in order to evaporate the liquid phase.

[0057] Subsequently, the operations implemented are described below and illustrated by Figure 2 - steps a) to k).

[0058] The first operation consists of pouring into the mold M a first quantity (by volume) of the slip L (figure 2a) containing the powder suspension and then carrying out a first heat treatment of the entire mold containing the slip (figure 2b). At the end of this operation, a solid and homogeneous bed L1 of sacrificial powder is obtained on the surface of the mold and after evacuation of a fraction of the liquid phase, both by evaporation and gravity flow through the porous structure of the mold (figure 2c).

[0059] On the powder bed L1 thus formed, the preform P is deposited (figure 2c) then a second quantity of this same slip is poured into the mold so as to completely submerge the preform (figure 2d). A second heat treatment is then carried out, preferably by drying in the open air and / or in an oven, to solidify the powder around the preform by forming a coating L2 (figure 2e).

[0060] In the next operation, a third quantity of the slip is poured (figure 2f) and is again subjected to heat treatment to form a solid and homogeneous upper layer L3 of sacrificial powder (figure 2g) before closing the mold, for example, by means of upper and lower pistons C or a cover (figure 2h). After sintering the preform P, it is demolded (figure 2i) and then freed from its coating of sacrificial powder to obtain a final densified part D (figure 2k).

[0061] The volumes of slip and therefore the quantities of suspended powder poured into the mold at each of the three successive operations are determined based on the dimensional shrinkage of the preform resulting from sintering. This shrinkage is known from experience and / or from prior modeling.

[0062] The successive heat treatments include, in particular, drying in the open air and / or in an oven between 80°C and 140°C for 10 minutes in order to evaporate the solvent (here PrOH) or at least the fraction of the solvent which has not passed through the mold and absorbed by the porous support plate.

[0063] At the end of the implementation of the method of the invention, it is observed that the sintered preform is not cracked and that both its structure and its geometry are preserved, which was not the case with previous methods of preparing the mold.

[0064] The porosity rates of the sacrificial media (powder bed) before and after sintering of the parts have the following values, depending on the nature of this media.

[0065] Thus, for the sintering of aluminum parts (the sacrificial media is then a SiC powder), the porosity rate of the sacrificial media remains constant in the successive coating beds of the process of the invention and is between 42% and 53% before sintering, then between 40% and 50% after sintering.

[0066] For sintering steel parts (the sacrificial media is alumina powder), the porosity rate of the sacrificial media before sintering is between 60% and 70% and between 30% and 45% after sintering.

[0067] For sintering high temperature ceramic parts (the sacrificial media is boron nitride powder), the porosity rate of the sacrificial media before sintering is between 50% and 80% and between 20% and 50% after sintering.

[0068] For sintering high temperature ceramic parts (the sacrificial media is carbon powder), the porosity rate of the sacrificial media before sintering is between 50% and 65% and between 19% and 30% after sintering.

[0069] These porosity values ​​are derived from measurements taken on the sacrificial media before and after sintering during various tests carried out as part of the development of the process of the invention. The porosity ranges after sintering are also dependent on the maximum temperature used for sintering. The higher this temperature, the less porous the media is at the end of sintering, but it remains porous in all cases.

[0070] The porosity rate of the sacrificial media can be reduced during the process thanks to the use of slips, which makes it possible to produce parts with complex geometries.

[0071] In conclusion, the method of the invention allows a standardized and easy preparation of the mold intended for the pressure sintering of preforms and which is not dependent on the personnel carrying out the operations. This method also makes it possible to facilitate the centering of the preform in the mold on the lower bed of sacrificial powder, to limit the deformations of the structure of the preform during sintering whatever its geometry and complexity.

Claims

CLAIMS 1. Method for preparing a mold (M) intended for the sintering of a porous preform (P), characterized in that the following successive operations are carried out in one or more sequences after production of the preform by 3D printing; - pouring into the mold a first quantity of a slip (S) comprising, on the one hand, a solvent and, on the other hand, an inert powdery material based on suspended ceramic particles, then a first heat treatment of the mold with the slip in order to obtain, on its surface, a solid and homogeneous bed (Ll) of sacrificial powder whose initial porosity is between 42% and 80% and on which the preform is deposited, - pouring a second quantity of slip so as to fill the mold to embed the preform and a second heat treatment to solidify the bed (L2) of sacrificial powder around the preform then, - a third quantity of slip is poured into the mold which is subjected to a third heat treatment to form, around the preform, a solid and homogeneous coating (L3) of sacrificial powder whose final porosity is always between 42% and 80% before sealing the mold and then sintering.

2. Method according to claim 1, characterized in that said powdery material has a melting temperature higher than the melting temperature of the material constituting the preform.

3. Method according to one of the preceding claims, characterized in that the ceramic particles suspended in the slip have a particle size greater than the average diameter of the pores of the preform.

4. Method according to one of claims 1 to 3, characterized in that the preform is previously covered with a layer of an inert material and the ceramic particles suspended in the slip then have a particle size smaller than the average diameter of the constituent particles of the preform.

5. Method according to one of the preceding claims, characterized in that prior to its filling, the inner wall of the mold is coated with a carbon sheet and positioned on a porous plate allowing the absorption by capillarity of at least a fraction of the slip.

6. Method according to one of the preceding claims, characterized in that the slip further comprises at least one dispersing agent.

7. Method according to the preceding claim, characterized in that said solvent is volatile under normal temperature and pressure conditions and is chosen from the group comprising water, alcohols, ketones or a mixture of these.

8. Method according to claim 6 or 7, characterized in that the solvent is isopropanol (PrOH).

9. Method according to one of the preceding claims, characterized in that the slip comprises from 10% to 40% by mass of suspended powdered material and from 60% to 90% by mass of liquid.

10. Method according to one of the preceding claims, characterized in that the heat treatment comprises a phase of drying in the open air.

11. Method according to the preceding claim, characterized in that the heat treatment further comprises drying in an oven between 80°C and 140°C for 10 minutes.

12. Method according to one of the preceding claims, characterized in that the viscosity of the suspension of sacrificial powder is optimized and the said slip is homogenized before it is poured into the mold by introducing zirconia balls therein and stirring.

13. Method according to one of the preceding claims, characterized in that said inert powdery material is chosen from the group comprising ceramics, metal carbides and oxides, nitrides and whose particles have an average diameter of between 0.34 and 300 μm.

14. Method according to one of the preceding claims, characterized in that the quantity of inert powdery material suspended in the slip is determined as a function of the dimensional shrinkage of the preform resulting from the sintering.

15. Porous ceramic mold (M) prepared according to the method according to one of the preceding claims for the purpose of pressure sintering of preforms.

16. Use of the mold according to claim 15 for densification by pressure sintering of preforms (P) printed in 3D using a binder jet technique.

17. Use according to claim 16 for the densification by pressure sintering of aluminum preforms (P), characterized in that the sacrificial powder is a SiC powder whose porosity rate in the powder beds coating the preform is between 42% and 53% and remains in this range until the preform is sintered.

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