METHOD FOR DENSIFYING A METAL PART HAVING A COMPLEX SHAPE BY ISOSTATIC PRESSING

Abstract

A method for densifying a metal part, including the following steps: coating the metal part with a leak-tight material, compacting the coated metal part under an isostatic pressure of a fluid, removing the coating from the metal part, and performing final annealing of the metal part.

Claims

1. A method for densifying a porous metal part comprising at least one hollow portion, the method comprising the following steps: a preliminary step of filling said at least one hollow portion with a filler metal that is introduced in liquid form, then solidifying this filler metal, the filler metal having a melting point below the melting point of the metal part; coating the metal part with a leak-tight material; compacting the coated metal part under an isostatic pressure of a fluid; removing the coating from the metal part; removing the filler metal, and; and final annealing of the metal part.

2. The method according to claim 1, characterized in that the coating step comprises placing the metal part in a leak-tight pouch under vacuum.

3. The method according to claim 1, characterized in that the coating step comprises coating the metal part with a leak-tight metallic material.

4. The method according to claim 1, characterized in that the coating step comprises coating the metal part with a resin.

5. The method according to claim 1, characterized in that the filler metal has a melting point below 100 degrees C.

6. The method according to claim 1, characterized in that the metal part is made from copper and the filler metal is a tin-based alloy.

7. The method according to claim 1, characterized in that the compacting comprises pressurization an isostatic pressure that is at least 30% greater than the elastic limit pressure of the metal part.

8. The method according to claim 7, characterized in that the isostatic pressure is comprised between 2,000 and 10,000 bars.

9. The method according to claim 1, characterized in that it comprises a preliminary annealing step carried out before the coating step.

10. The method according to claim 1, characterized in that the isostatic pressurizing fluid is oil.

11. The method according to claim 1, characterized in that the isostatic pressurizing fluid is a gas.

12. The method according to claim 1, characterized in that the final and/or preliminary annealing comprises a phase of increasing temperature, a phase of maintaining the metal part at a solubilization temperature, and a phase of progressive cooling over a duration greater than the duration of the phase of increasing temperature.

13. The method according to claim 1, characterized in that the step of final annealing is followed by a step of cleaning by using hydrochloric acid.

Description

[0053] Other advantages and characteristics of the invention will become apparent on examining the detailed description of an embodiment that is in no way limitative, and the attached drawings, in which:

[0054] FIG. 1 is a diagram showing the notable steps of the method according to the invention;

[0055] FIG. 2 is a diagrammatic view of the processes of design and densification of a metal part;

[0056] FIG. 3 is a diagram showing a whole set of steps of the method according to the invention;

[0057] FIG. 4 is a simplified diagrammatic view showing a metal part in different phases of densification.

[0058] The embodiments that will be described below are in no way limitative, variants of the invention can be implemented in particular comprising only a selection of the characteristics described hereinafter, in isolation from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0059] In particular, provision is made for all the variants and all the embodiments described to be combined together in all combinations where there is no objection to this from a technical point of view.

[0060] Although the invention is not limited thereto, an embodiment of the method for densifying a metal part based on copper and 0.1% silver will now be described. The invention can be implemented for other types of metals with or without alloying.

[0061] In FIG. 1, an optional step of preliminary annealing can be seen, which can be implemented or not. The metal part according to the invention principally undergoes the steps B3, B4, B5 and C as shown in FIG. 1. These steps are described in greater detail below. As a minimum, they make it possible to have a perfectly densified metal part. They can be implemented for solid or hollow metal parts.

[0062] FIG. 2 shows a sequence of processes comprising the design of a metal part by means of a selective laser melting (SLM) machine. The metal part thus obtained has a porosity of approximately ten percent.

[0063] According to the invention, the densification is implemented by carrying out: [0064] a preliminary heat treatment to prepare the metal part for the following treatment, [0065] a compaction to reduce the porosity, [0066] then a final heat treatment to improve the conductivity of the metal part.

[0067] To take account more specifically of the hollow portions that would be present in a metal part, the present invention provides additional steps to maintain the geometric shape of the metal part.

[0068] FIG. 3 depicts a set of steps according to the invention. All or part of the steps in the figure contribute to the densification of a metal part while maintaining the geometric shape.

[0069] Step A1 relates to an annealing that can be carried out in a furnace under vacuum or in a neutral or inert atmosphere.

[0070] For example, the metal part has a complex shape based on copper with 0.1% silver (Cu0.1Ag).

[0071] The furnace is used for the annealing at a solution heat treatment temperature of approximately 950° C., which corresponds to a temperature to 20% below the melting temperature of copper. The metal part is placed in the furnace for a period of approximately one hour.

[0072] In step A2, the metal part is cooled progressively for approximately 8 to 15 hours before reaching the initial temperature. FIG. 4 shows such a metal part 1 with a high porosity. This part is shown in cross-section. It has a rectangular shape with a hollow portion 2 produced from the upper surface.

[0073] In step B1, the hollow portion 2 is filled with a filler metal in its liquid form. This is Field's metal, an alloy of tin having a very low melting point and a low wettability. In FIG. 4, it is noted that the filler metal 3 perfectly occupies all of the hollow portion, so that the assembly constituted by the metal part and the filler metal has a regular shape. The filler metal 3 is flush with the upper surface of the metal part.

[0074] In step B2, the filler metal is solidified by allowing it for example to return to ambient temperature, a state in which it is in a solid form.

[0075] In step B3, provision is made for a step of coating the metal part by placing it in an envelope of resistant polymer, then by creating a vacuum within the envelope so that the envelope hugs the whole of the metal part. According to the invention, provision is made for other coating methods, such as for example placing a resin or a metal in the form of a container completely enclosing the metal part 1. In particular, the use of bagging may be preferred when the geometric shape of the metal part allows the polymer envelope or pouch to press itself against the whole of the external surface of the metal part when the vacuum is created. A sarcophagus can be produced, in particular using resin, for example when the metal part has a geometric shape that is so complex that bagging would lead to the creation of air pockets when the vacuum is produced or when the metal part includes sharp projections that could pierce the pouch.

[0076] In FIG. 4, sub-step B31, the coating method by bagging is noted, i.e. the metal part 1 is placed in an envelope 4 or a pouch made of polymer. In sub-step B32, a vacuum is created in the envelope 4.

[0077] In step B4, compacting is carried out by placing the envelope 4 containing the metal part 1 in an oil bath 5, then isostatically compressing under very high pressure at approximately 4,000 bars for several minutes, for example one to 2 minutes. The metal part 1 is then densified and the porosity is reduced until reaching a value below 1%, for example down to 0.35%. Compacting is carried out uniformly in all directions to compress the metal part isostatically. The pressure applied is a function of the physical characteristics of the material constituting the metal part. For example, for copper or for other metals, account is taken of its elasticity. For copper, as its elastic limit value is 200 Mpa, provision is made to apply a pressure of 500 Mpa.

[0078] In step B5, the metal part is taken out of the bath and the coating envelope 4 is removed.

[0079] In step B6, the filler metal 3 is removed by heating the assembly to a temperature above the melting point of the filler metal but below the melting point of copper.

[0080] Step B7 consists of cleaning the metal part thus densified. To this end, hydrochloric acid is used.

[0081] In steps C1 and C2, a final annealing is carried out in a manner similar to the preliminary annealing. The objective of this final annealing is to improve the conductivity of the metal part thus densified. In FIG. 4, the final metal part 1 after compacting is noted in C2. Its size is reduced although its geometric shape remains the same. The dotted lines represent its initial size, for example in phase A2. Advantageously the porosity is reduced for example by 20 percent.

[0082] With the method according to the invention, the metal part has a leak-tightness and a conductivity comparable to those of the base material. The present invention combines a step of additive manufacturing with a step of isostatic pressing.

[0083] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without exceeding the scope of the invention.