METHOD FOR IMPROVED MANUFACTURING OF A DUAL MICROSTRUCTURE PART

Abstract

A method for welding together at least two parts of green material, referred to as green parts, by means of co-sintering, comprising the following steps:assembling the at least two green parts at a junction zone of said parts so as to form a green one-piece assembly,de-binding the green one-piece assembly, andsintering the one-piece assembly so as to obtain a dense one-piece assembly forming a final part, characterised in that the two green parts (10, 12) each have a composition of different powder, so as to produce a final part (1) having at least two parts with different grain sizes.

Claims

1. Method for welding together at least two parts of green material, referred to as green parts, comprising the following steps: assembling the at least two green parts at a junction zone of said parts so as to form a green one-piece assembly, de-binding the green one-piece assembly, and sintering the one-piece assembly so as to obtain a dense one-piece assembly forming a final part, wherein the two green parts are each having a different powder composition, so as to produce a final part having at least two parts with different grain sizes.

2. Method according to claim 1, wherein at least two green parts have powder compositions with different granulometries.

3. Method according to claim 2, wherein the green parts comprise powders that have a D.sub.90 less than 16 m, 25 m or 45 m.

4. Method according to claim 1, wherein at least two green parts have different powder chemical compositions.

5. Method according to claim 1, wherein it comprises a step of adding a weld bead to the green parts in such a way that the weld bead hugs the shape of the junction zone, and in such a way as to form a homogeneous green one-piece assembly.

6. Method according to claim 5, wherein the weld bead is of a composition similar to that of the green parts.

7. Method according to claim 5, wherein the weld bead and the green parts to be assembled are of identical compositions.

8. Dense one-piece assembly forming a final part, comprising at least two green parts assembled by the method according to claim 1, wherein the at least two green parts have a different composition, in such a way that the final part has at least two parts with a different grain size.

9. Assembly according to claim 8, wherein the at least two green parts have a powder composition with a different granulometry.

10. Assembly according to claim 8, wherein the at least two green parts have a chemical composition.

Description

DESCRIPTION OF THE FIGURES

[0047] The invention will be better understood, and other details, characteristics and advantages of the invention will appear more clearly when reading the following description given by way of a non-limiting example and with reference to the accompanying drawings, wherein:

[0048] FIG. 1 is a perspective view of a part that has two portions the microstructure of which has to be differentiated,

[0049] FIG. 2 is a perspective view of the step of adding a weld bead between two parts to be welded according to a first alternative of the method claimed,

[0050] FIG. 3 is a perspective view of the step of adding a weld bead between two parts to be welded according to a second alternative of the method claimed,

[0051] FIG. 4 is a diagrammatic cross-section view of a homogeneous green one-piece assembly according to one or the other of the alternatives of FIGS. 2 and 3,

[0052] FIG. 5 is a diagrammatic cross-section view of a homogeneous green one-piece assembly after a step of machining according to one or the other of the alternatives shown hereinabove,

[0053] FIG. 6 is a diagrammatic cross-section view of a final part obtained by a conventional prior art method of co-sintering,

[0054] FIG. 7 is a diagrammatic cross-section view of a final part obtained by one or the other of the alternatives of the method claimed,

[0055] FIG. 8 is a perspective view of the part of FIG. 1 the differentiated microstructure of which has been shown diagrammatically, artificially enlarged.

DETAILED DESCRIPTION

[0056] In this application, the term feedstock or green material means a mixture according to:

[0057] at least one metal and/or ceramic material forming a part to be manufactured, and

[0058] one (or more) thermoplastic binders with a polymer base.

[0059] This mixture conventionally has the form of granules.

[0060] Moreover, in this application, the term green part means a part in the process of manufacture that has already been formed but that has not yet been de-bound. This green part therefore has the general shape of the final dense part but, as it has not yet undergone the sintering step, it does not yet have its final dimensions. The sintering step involves a phenomenon referred to as volume shrinkage, which is a phenomenon of dimensional contraction that entails a decrease in the dimensions of the part. This volume shrinkage depends on the initial composition of the feedstock and in particular the proportion of filler in said feedstock. In a formulation, a filler is a solid non-miscible substance dispersed in a matrix via a mechanical means. Thus the proportion of filler corresponds to the volume of powder in the feedstock.

[0061] As can be seen in FIG. 1, a user wishes to produce a final part 1 that comprises two parts (or parts 10, 12) with different microstructures.

[0062] For example, the user wishes to have a final part 1 that has small grains on the surface (in order to delay a crack initiation under fatigue for example), and in the rest of the part larger grains (in order to have resistance against creep for example).

[0063] The production technique according to the method presented here consists of forming the first green part 10 using a feedstock comprising a coarse powder granulometry, and the second green part 12 using a feedstock comprising a finer granulometry (see FIG. 8). The two parts 10, 12 are produced by means of a PIM method (or any other method for forming green parts of the 3D printing type for example) stopping at the step of moulding (or of forming) presented hereinabove.

[0064] The two green parts 10, 12 are therefore assembled at a junction zone 14 of these green parts 10, 12 in such a way as to form a green one-piece assembly. This assembly in the green state can in particular be done by the adding of a weld bead made of green material, as explained in detail hereinbelow:

[0065] As can be seen in FIGS. 2 and 3, a user wishes to weld two parts: for example, a plate 10 and a hollow cylinder 12. The two parts 10, 12 are so called green parts, i.e., as mentioned hereinabove, they have not yet passed the step of de-binding.

[0066] The two green parts 10, 12 are therefore put into contact at a junction zone 14 of these green parts 10, 12. A weld bead 16 itself in feedstock is added to the green parts 10, 12 in such a way that the weld bead 16 hugs the shape of the junction zone 14. The assembly then forms a homogeneous green one-piece assembly.

[0067] According to the embodiment shown in FIG. 2, the deposition of the weld bead 16 is done using an injection screw 18 a nozzle 20 of which points to the junction zone 14. The temperature at the outlet of the nozzle 20 is substantially the same as the injection temperature during the step of moulding of the green parts 10, 12. For example, for a feedstock Inconel 718 used today by the applicant, the temperature of the nozzle 20 is 190 C. For better control of the method, the injection screw 18 can be mounted on a robot arm 22, connected to a control unit 24 that allows for automation of the method.

[0068] According to the embodiment shown in FIG. 3, the adding of the weld bead 16 is carried out in two steps:

[0069] firstly, a strip of solid green material 26 is put into contact with the junction zone 14,

[0070] secondly, said strip of solid green material 26 is heated by means of a hot-air gun 28 in such a way as to soften the material of the strip 26 and to form the weld bead 16.

[0071] As for the preceding embodiment, the hot-air gun 28 can be fixed on a robot arm 24 connected to a control unit 24. This allows, as mentioned hereinabove, for a better control of the step of adding the weld bead 16.

[0072] For a green material of formulation Inconel 718 the temperature of the air at the outlet of the gun 28 must be greater than 100 C.

[0073] The two green parts 10, 12 can have identical compositions or, as explained hereinafter, different compositions. As with the weld bead 16: the latter can be of a composition similar to that of the two parts 10, 12 to be assembled or of a composition identical thereto (subject to the two parts 10, 12 themselves being of identical composition).

[0074] By similar composition is meant a composition that has:

[0075] the same proportion of filler as the composition of the other components 10, 12, 16 of the homogeneous green one-piece assembly in order to provide an identical (or substantially identical) volumetric shrinkage of each one of these components 10, 12, 16 during the sintering step,

[0076] an identical (or substantially identical) densification speed to that of the other components 10, 12, 16 of the homogeneous green one-piece assembly,

[0077] a sintering range compatible with that of the other components 10, 12, 16 of the homogeneous green one-piece assembly.

[0078] Following the adding of the weld bead 16, the method can comprise a machining step: the weld bead 16 being in the green state, it can be reworked immediately by machining, even before the de-binding step, in order to confer upon it directly a radius or a specific shape, as shown in FIGS. 4 and 5.

[0079] The advantage of a pre-debinding machining is that it requires less energy than a machining on a harder final part. Moreover, a machining error on a green part with less added value has less impact than a machining error on a final part with high added value.

[0080] Moreover, as can be seen in FIGS. 6 and 7, the method described in the present application makes it possible, in welding PIM parts, overcome the constraint imposed by the co-sintering known from the prior art which entails that the junction zone 14 is delimited by all of the initial surfaces 30, 32 in contact with the parts 10, 12 to be assembled (see FIG. 6). Thanks to the solution proposed by the method of the present application, it is the contact surfaces 34, 34, 36, 38 between the weld bead 16 and the parts to be assembled 10, 12 that constitute the junction zone 14 (see FIG. 7). There are therefore four contact surfaces 34, 34, 36, 38 where before there were only two, 30, 32. The direct advantage that results therefrom is that the mechanical strength of the junction zone 14 post-welding is improved.

[0081] Thus, following the adding of the weld bead 16, the homogenous single-piece assembly forms a final part 1 (see FIG. 5) integrally that comprises two parts 10, 12 and the weld bead 16. The junction zone 14 comprises at least four surfaces 34, 34, 36, 38 (a first surface 36 belonging to the first part 10, a second surface 38 belonging to the second part 12, third and fourth surfaces 34, 34 belonging to the weld bead 16): the first surface 36 cooperates with the third surface 34, and the second surface 38 cooperates with the fourth surface 34 during the sintering step and makes it possible to reinforce the mechanical strength of the final part 1.

[0082] Moreover, as mentioned hereinabove, the co-sintering of the surfaces 30, 32 of the green parts 10, 12 in contact is not necessary: as the junction is carried out via co-sintering of the surfaces 36, 38 of the green parts 10, 12 in contact with surfaces 34, 34 of the weld bead 16, added in a second step in such a way as to hug the shape of the junction zone 14, the contact between the different surfaces 34, 34, 36, 38 to be welded is satisfactory and does not require preparation upstream of the sintering step.

[0083] Following the adding and the possible machining of the weld bead 16, the homogeneous green one-piece assembly is de-bound, then sintered, in such a way as to obtain a homogeneous and dense one-piece assembly, which is a final part, as can be seen in FIGS. 7 and 8.

[0084] Thus an illustration of an example of the complete method of production of green assembled parts as described in the present application is, for example with parts made from Inconel 718:

[0085] injection of the parts to be assembled, for example two separate parts 10, 12;

[0086] assembly of the parts 10, 12 by the method claimed in the present application, by adding a weld bead 16 made of Inconel 718;

[0087] possible machining reworking in the junction zone 14 if needed;

[0088] de-binding according to a conventional protocol defined for Inconel 718,

[0089] sintering according to a conventional method defined for Inconel 718.

[0090] During sintering, the portion of the one-piece assembly that comprises powders of finer granulometry, corresponding to the second green part 12, has finer grains, while the portion of the final part 1 that comprises powders of greater granulometry, corresponding to the first green part 10, has larger grains (see FIG. 8).

[0091] As the two green parts 10, 12 are already of different granulometries, the sintered one-piece assembly can then follow a standard and homogeneous heat treatment: the dual microstructure is produced at the sintering step.

[0092] The same type of result can be obtained by varying the chemical composition of the powders of each one of the green parts 10, 12 rather than their granulometry, for example by using a superalloy with a nickel base with a variable carbon content. In this type of superalloy, the carbon precipitates in the form of carbide and this precipitated carbon content more or less opposes the enlarging of the grain during sintering.

[0093] In particular, by using a first feedstock (a first part 10) comprising a powder with chemistry no. 1 and a second feedstock (a second part 12) comprising a powder with chemistry no. 2. This can, for example, be an alloy of Ren 77 with a high rate of carbon as a powder with chemistry no. 1 and an alloy of Ren 77 with a low carbon content as a powder of chemistry no. 2. A sintered one-piece assembly is thus obtained that has a dual structure, thanks to the fact that the carbon content has, on the Ren 77, an influence on the enlarging of the grain during sintering.

[0094] Various embodiments make it possible to obtain final parts 1 that have different granulometries. For example with powders of Inconel 718 that have a D.sub.90 less than 16 m, 25 m or 45 m. It also possible to consider a case wherein the two powders that have a different chemical composition by taking, for example, Ren 77 containing 660 ppm of carbon or 160 ppm of carbon. These measured values of D.sub.90 relate to the granulometry of the powders used in the feedstock that forms each part taken separately.

[0095] The parameter D.sub.90, represents a point on the distribution curve of the sizes of particles that comprise a part. This particular point indicates what size 90% of the particles of the total volume of the part considered have. For example, if the D.sub.90 is 844 nm, then 90% of the particles of the part considered have a diameter less than or equal to 844 nm and 10% therefore have a larger size. This measurement can in particular be obtained via laser diffraction. Conventionally, in order to characterise the granulometry of a part, D.sub.10, D.sub.50 and D.sub.90 are measured. D.sub.10 is always smaller than D.sub.50 which is smaller than D.sub.90. The closer the values are, the more homogeneous the size of the particles of powder is.

[0096] The technical lock resides in forming a final part 1 in the green state using two different feedstocks (green parts 10, 12). Indeed, it is important that the two feedstocks have a proportion of filler (proportion of powder/binder) that is similar, which guarantees a volumetric shrinkage that is identical or substantially identical of each of the green parts 10, 12 during sintering.

[0097] It is also required that the sintering ranges of the green parts 10, 12 be compatible together.

[0098] It must also be ensured that the forming of the green parts 10 and 12 allows for obtaining a sound interface 14 at the junction between the two green parts 10, 12.

[0099] An illustration of the complete method of production of green assembled parts as described in the present application is, for example, carried out with parts made of Inconel 718:

[0100] injection of the parts to be assembled, for example two separate parts 10, 12 one of which would be of granulometry A and the other of granulometry B;

[0101] assembly of parts 10, 12 by the method claimed in the present application;

[0102] possible machining reworking in the junction zone 14 if needed;

[0103] de-binding according to a conventional protocol defined for Inconel 718,

[0104] sintering according to a conventional protocol defined for Inconel 718.