METHOD FOR PRODUCING PARTS HAVING A COMPLEX SHAPE BY METAL POWDER INJECTION MOULDING

Abstract

A method for producing, by a metal powder injection moulding (MIM) technique, a part formed of at least one metal and/or at least one metal alloy including at least one internal cavity. A green core of a mixture of at least one powder of at least one ceramic and of a thermoplastic binder is used.

Claims

1. A method for producing, by a metal powder injection moulding technique, a part of at least one metal and/or at least one metal alloy including at least one inner cavity, comprising the following steps of: a) preparing a green core the shape of which corresponds to the shape of said cavity, said core comprising a mixture of at least one powder of at least one ceramic and of a thermoplastic binder; b) optionally coating the green core with a anti-adhesive layer; and then performing step c), or step d), or step e); c) placing said green core optionally coated with a anti-adhesive layer, into an injection mould replicating the external shape of the part to be produced; injecting a heated liquid mixture of at least one powder of at least one metal and/or at least one metal alloy constituting the part to be produced and of a thermoplastic binder, into said injection mould around the green core, and cooling said mixture to solidify it, whereby a green part of said solid mixture and comprising an inner cavity filled with the green core is obtained; d) coating said green core optionally coated with a anti-adhesive layer, with a heated liquid mixture of at least one powder of at least one metal and/or of at least one metal alloy constituting the part to be produced and of a thermoplastic binder; cooling said mixture to solidify it; and machining said mixture so that its external shape is the external shape of the part to be produced, whereby a green part consisting of said solid mixture and comprising at least one inner cavity filled with the green core is obtained; e) placing said green core optionally coated with a anti-adhesive layer between at least two green parts consisting of said solid mixture of at least one powder of at least one metal and/or at least one metal alloy constituting the part to be produced and of a thermoplastic binder; at the end of step c), or step d), or step e), successively carrying out following steps f), g), and h): f) simultaneously removing the thermoplastic binder from the green core and from the green part or from the green parts, whereby a core called a “brown” core and a part called a “brown” part or parts called “brown” parts are obtained; g) simultaneously sintering the brown core and the brown part to densify them, or simultaneously sintering the brown core and said at least two brown parts to densify them and so that said at least two brown parts are assembled together through diffusion welding around the brown core; h) removing the core, whereby the part to be produced is obtained.

2. The method according to claim 1, wherein said ceramic is selected from oxide ceramics such as alumina or zirconia.

3. The method according to claim 1, wherein the green core is prepared by injecting the heated liquid mixture of at least one powder of at least one ceramic, and of a thermoplastic binder into a mould the shape of which corresponds to the shape of said cavity, and then cooling said mixture to solidify it.

4. The method according to claim 3, wherein the green core is prepared by a ceramic powder injection moulding or “CIM” technique.

5. The method according to claim 1, wherein the green core is prepared by machining the mixture of at least one powder of at least one ceramic, and of a thermoplastic binder.

6. The method according to claim 1, wherein the green core is prepared by 3D printing shaping, using the mixture of at least one powder of at least one ceramic, and of a thermoplastic binder.

7. The method according to claim 1, wherein the metal or metal alloy which constitutes the part to be produced is selected from nickel and nickel-based alloys, titanium and titanium-based alloys, and stainless steels.

8. The method according to claim 1, wherein the part obtained at the end of step h) further undergoes one or several heat and/or mechanical treatments such as a hot isostatic pressing treatment.

9. The method according to claim 1, wherein the part to be produced is selected from parts of aeronautical turbomachineries including radial straighteners, axial straighteners, distributors, and centrifugal diffusers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0092] FIGS. 1 to 4 are schematic vertical cross-section views which illustrate the different steps of the method according to the invention.

[0093] FIG. 1 illustrates preparation of a green core.

[0094] FIG. 2 illustrates overmoulding of the green core.

[0095] FIG. 3 illustrates the debinding and sintering steps.

[0096] FIG. 4 illustrates the final step of dissolving the core.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0097] FIG. 1 shows the preparation of a core A, 1, (step a) of the method according to the invention), the shape of which corresponds to the shape of the cavity of the part to be produced, this core consisting of a mixture of at least one powder of at least one ceramic, and a thermoplastic binder. This core A, 1, may therefore be referred to as a “green core”.

[0098] The nature of the core powder, especially its composition and grain size, as well as the powder and binder distribution into the core, are to be adapted depending on the proprieties of the part to be produced, in particular depending on the sintering temperature of the metal or metal alloy that constitutes the part to be produced and on the expansion coefficient of the metal or a metal alloy which constitutes the part to be produced.

[0099] Thus, in the case where the core powder is alumina powder, its grain size may be such that it has a D.sub.90<30 μm. The core may comprise from 60% to 80% volume of powder and from 20% to 40% volume of binder.

[0100] FIG. 1 shows a first embodiment for preparing the green core A,1, wherein the core is prepared by injecting the heated liquid mixture of at least one powder of at least one ceramic, and a thermoplastic binder into a mould 2 the shape of which corresponds to the shape of the cavity of the part to be produced.

[0101] The heated liquid mixture is then cooled to be solidified.

[0102] According to a second embodiment (not represented) for preparing the green core A, the latter may be prepared by machining the mixture of at least one powder of at least one ceramic, and a binder.

[0103] According to a third embodiment (not represented) for preparing the green core A, the latter can be prepared by 3D printing the mixture of at least one powder of at least one ceramic, and a binder.

[0104] In other words and to sum up, the green core A may be made by moulding or by machining or by 3D printing in the green part condition.

[0105] Then, the thus prepared green core is optionally coated with a anti-adhesive layer (step b) of the method according to the invention).

[0106] This anti-adhesive layer may be made of any known anti-adhesive material used in this technical field.

[0107] Thus, this anti-adhesive material may be for example chromium oxide.

[0108] The green core coated with the anti-adhesive layer is then overmoulded by a mixture of at least one powder of at least one metal and/or at least one metal alloy constituting the part to be produced B and a thermoplastic binder. This mixture is generally called a “feedstock”.

[0109] FIG. 2 shows a first embodiment of this overmoulding step (step c) of the method according to the invention), wherein said green core A,1, coated with a anti-adhesive layer (not represented) is placed into an injection mould 3 replicating the external shape of the part to be produced; a heated liquid mixture of at least one powder of at least one metal and/or at least one metal alloy constituting the part to be produced and of a thermoplastic binder is injected into said injection mould 3 around the green core 1; and said mixture is cooled to be solidified, whereby a green part B,4, consisting of said solid mixture and comprising an inner cavity filled with the core A,1, is obtained.

[0110] In addition to the first embodiment of the overmoulding step shown in FIG. 2, this overmoulding step may be performed, according to a second embodiment (not represented) of this overmoulding step (step d) of the method according to the invention), by coating said core 1 coated with a anti-adhesive layer, with a heated liquid mixture of at least one powder of at least one metal and/or at least one metal alloy constituting the part to be produced and of a thermoplastic binder; by cooling said mixture to solidify it; and by machining said mixture so that its external shape is the external shape of the part to be produced. A green part consisting of said solid mixture and comprising at least one inner cavity filled by the core is thus obtained in the same way as in the first embodiment.

[0111] The core coated with a anti-adhesive layer may be coated with the feedstock by dipping the core coated with the anti-adhesive layer in liquid feedstock.

[0112] Machining may be performed by any appropriate known machining technique.

[0113] In other words, to sum up, overmoulding may be performed in a specific mould or by dipping in liquid feedstock and then the solid cooled feedstock is machined.

[0114] Instead of overmoulding the core A,1, coated with the anti-adhesive layer by the mixture of at least one powder of at least one metal and/or at least one metal alloy constituting the part to be produced B and if a thermoplastic binder (feedstock), it is possible according to another embodiment (step e) of the above-described method according to the invention to place, insert, integrate, said green core coated with a anti-adhesive layer between several (at least two) green parts consisting of said solid mixture (feedstock) of at least one powder of at least one metal and/or at least one metal alloy constituting the part to be produced and of a thermoplastic binder.

[0115] During sintering, the at least two green parts, which meanwhile have been debinded, thereby called “brown” parts will be assembled together by diffusion welding around the core itself sintered, in other words, during sintering, these at least two parts will bond to each other to thus surround the core also sintered and form the part to be produced.

[0116] The green parts may be obtained by any appropriate technique.

[0117] Thereby, these green parts may be obtained by powder injection moulding, by an additive manufacturing technique such as the Fused Deposition

[0118] Modeling or “FDM” technique or by green machining.

[0119] In other words, to sum up, the green part to be produced B may not be overmoulded on the core, and the green core may be inserted between at least 2 moulded or machined green parts.

[0120] FIG. 3 illustrates the debinding f) and sintering g) steps of the claimed method.

[0121] These steps are successively performed at the end of step c), step d) or step e).

[0122] During step f), which is therefore called a debinding step, are simultaneously removed the thermoplastic binder from the core (green core) and the thermoplastic binder from the green part or from the green parts.

[0123] Debinding may be performed by any known debinding technique.

[0124] This debinding technique is suitably selected according to the nature of the thermoplastic binder used, such as a resin or a polymer.

[0125] This debinding technique may be a catalytic, thermal, solvent, water, or supercritical fluid such as supercritical CO.sub.2 debinding technique.

[0126] At the end of this debinding step, a debinded green part called a “brown” part 5, consisting of said debinded solid mixture and comprising at least one inner cavity filled with the debinded green core 6 which may be called a “brown” core is therefore obtained.

[0127] Or, at the end of this debinding step, a “brown” core between several (at least two) debinded green parts, referred to as “brown” parts, is obtained.

[0128] After the debinding step, which is step f) of the claimed method, the “brown” core 6 and the “brown” part 5 are simultaneously sintered (step g) of the method according to the invention) to densify them, or the “brown” core and said at least two “brown” parts are simultaneously sintered to densify them and so that the at least two brown parts are assembled together by diffusion welding around the core thus forming the part to be produced.

[0129] Sintering is generally performed in a furnace where the “brown” core 6 and the “brown” part 5, or the “brown” core and said at least two “brown” parts are heated up to a temperature close to the melting point of the metal or of the alloy constituting part B.

[0130] The sintering temperature, duration and the atmosphere in the furnace are controlled so that the metal or alloy particles of part B bind together by diffusion. The pores of the “brown” core and the pores of the “brown” part(s) are gradually reduced and the brown part(s) densify during this sintering step. Densification generally leads to a shrinkage of the brown part or of the brown parts which is for example in the order of 10% to 20%. According to the invention, shrinkage of the brown part or of the brown parts on the one hand, and shrinkage of the brown core on the other hand are generally equal or close to each other, that is the shrinkage of the brown core and of the brown part(s) do not differ too much, for example not more than 10%, 5%, or even 1%, so that no deformation of the part occurs during sintering.

[0131] However, the value of the difference between shrinkage of the brown part or of the brown parts on the one hand, and shrinkage of the brown core on the other hand cannot generally be fixed since this value also depends on expansion coefficients. Furthermore, hightening stresses between the part and the core have also to be managed.

[0132] As a result, it is possible to have slightly different shrinkages enabling stresses due to differential expansion during cooling not to be created.

[0133] As is represented in FIG. 4, during step h) of the method according to the invention removing the soluble core 6 is performed by chemical, dissolution, etching to thus obtain a part 5 including an inner cavity instead of the core 6.

[0134] This chemical, dissolution, etching is generally performed using a base such as soda, for example in the case where the core is made of alumina.

[0135] Depending on the solidity condition of the core, other solutions for eliminating the latter can be contemplated such as shake-out.

[0136] Advantageously, the part 5 obtained at the end of step h) may undergo one or several heat and/or mechanical treatment(s) such as a hot isostatic pressing (HIP) treatment to further increase the density of the part.