METHOD FOR CHEMICALLY PICKLING A CAST METAL PART WITH POROUS CERAMIC CORE(S)

Abstract

A method for chemically pickling a cast metal part, including a metal envelope which delimits an inner space in which at least one porous ceramic core is housed, and an outer space, the ceramic core being in fluid communication with the outer space, which method including: filling the pores of the ceramic core with a liquid; and then chemically pickling the cast metal part. This chemical pickling method may be implemented in a method for manufacturing a metal part by investment casting. This method is applicable at least to manufacture of turbine blades for turbomachines and, especially, for aircraft turbojet engines.

Claims

1. A method for chemically pickling a cast metal part, comprising a metal envelope which defines an inner space in which at least one porous ceramic core is housed, and an outer space, the ceramic core being in fluid communication with the outer space, which method comprises: filling the pores of the ceramic core with a liquid; and then chemically pickling the cast metal part.

2. The method of claim 1, wherein filling the pores comprises immersing the cast metal part in the liquid.

3. The method of claim 1, wherein the pores are filled with a mass of the liquid representing at least 90% of a mass of the liquid that the pores would contain if they were saturated with the liquid.

4. The method of claim 3, wherein the pores are filled with a mass of the liquid representing at least 95% of the mass of the liquid that the pores would contain if they were saturated with the liquid.

5. The method of claim 1, to wherein the liquid is an aqueous solution.

6. The method of claim 1, wherein the chemical pickling is an acid pickling.

7. The method of claim 6, wherein the acid pickling comprises immersing the cast metal part in a bath comprising hydrochloric acid.

8. The method of claim 1, wherein the cast metal part is a turbine blade for a turbomachine.

9. A method for manufacturing a metal part by investment casting, comprising: providing a cast metal part, comprising a metal envelope which delimits an inner space in which at least one porous ceramic core is housed, and an outer space, the porous ceramic core being in fluid communication with the outer space; and chemically pickling the cast metal part by implementing a method claim 1.

10. The method of claim 9, wherein the metal part is a turbine blade for a turbomachine.

11. The method of claim 3, wherein the pores are filled with a mass of the liquid representing 100% of the mass of the liquid that the pores would contain if they were saturated with the liquid.

12. The method of claim 5, wherein the liquid is water.

13. The method of claim 8, wherein the turbine blade is for an aircraft turbojet engine.

14. The method of claim 10, wherein the turbine blade is an aircraft turbojet engine.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0044] FIG. 1 illustrates, in a perspective view, one example of a turbine blade for a turbomachine likely to be manufactured by investment casting.

[0045] FIG. 2 illustrates, in a top view, the turbine blade of FIG. 1 as manufactured by investment casting, after the shell mould used in its manufacture has been shaken out but before the porous ceramic core used in its manufacture has been shaken out.

[0046] FIG. 3 illustrates the course of the mass of water, noted M and expressed in mg, absorbed by the porous ceramic core of a turbine blade of the type shown in FIG. 2 as a function of the immersion time, noted t and expressed in seconds, of this blade in water.

DETAILED DESCRIPTION OF ONE PARTICULAR IMPLEMENTATION

[0047] Reference is first made to FIG. 1 which illustrates an example of a turbine blade 10 for a turbomachine which can be manufactured by an investment casting method.

[0048] Such a blade comprises a metal structure 20 comprised of three elements, namely: [0049] a root 11 by which it is to be attached to a turbine disc (not represented), [0050] an airfoil 12 delimited by a shell 13 which twists about an axis EV known as the “span axis”, forming a lower surface 14 and an upper surface 15 connected into a leading edge 16 and a trailing edge 17, and [0051] an intermediate region 18, called the platform, which extends approximately perpendicular to the axis EV and connects the root 11 to the airfoil 12.

[0052] The metal structure 20 is a one-piece part, which includes an inner cavity used, in operation, to cool the blade by conveying fresh air through this blade. This inner cavity extends, for example, from a mouth formed at the base of the root (and therefore not visible in FIG. 1) to the holes 19, known as “dust removal holes”, that a wall 22 located at the top of the blade includes. This wall forms the bottom of an open cavity commonly known as a “bathtub”.

[0053] If the blade is made by investment casting, then the bathtub and the inner cavity of the blade are, for example, obtained by using a ceramic core in two parts: a first part forming the negative of the bathtub and a second part forming the negative of the inner cavity of the blade, these two parts being connected by ceramic bridges forming the negative of the holes 19, while the metal structure 20 of the blade is obtained by casting a molten metal or metallic alloy between the ceramic core and a shell mould of a refractory material, typically a ceramic, after dewaxing and sintering this mould.

[0054] After solidification of the metal or alloy, the shell mould is shaken out.

[0055] The blade illustrated in FIG. 2 is thus obtained, in a top view at the airfoil 12.

[0056] As is visible in this figure, the outer surface of the metal structure 20 of the blade 10 has been left exposed by the destruction—or shake-out—of the shell mould, but the ceramic core—of which the part, referenced 21, forming the negative of the bathtub is seen—is still present.

[0057] If the outer surface of the metal structure 20 of the blade is subjected to a chemical pickling by immersing this blade in a highly concentrated acid bath, the first and second parts of the ceramic core will be saturated with acid by capillarity, since this core has a porous structure, and will retain the acid as long as the ceramic core is itself not shaken out.

[0058] As a result, after the ceramic core has been shaken out, lacks of material can be observed at the inner surface of the metal structure 20, especially at the inner surface of the wall 22.

[0059] In order to prevent the occurrence of this incomplete die filling, it is therefore proposed according to the invention filling the pores of the ceramic core with a liquid, for example by immersing the blade in this liquid, preferably until these pores are saturated with liquid, before conducting the chemical pickling.

[0060] The liquid is preferably water.

[0061] As illustrated in FIG. 3, which represents the course of the mass of water, noted M and expressed in mg, absorbed by the porous ceramic core of a blade as shown in FIG. 2 as a function of the immersion time, noted t and expressed in seconds, of this blade in water, an immersion time of 5 minutes is sufficient to obtain a quasi-saturation of the ceramic core with water.