DE2944534A1 - CERAMIC MOLDING CORE FOR PLANT MOLDS AND METHOD FOR PRODUCING THE SAME - Google Patents

Description

PATHKTAKWAiT

.. ικλ. Κ. HOLSEB

βί> -3Ο AÜÖS73UKÖ

i '.- C: .7:? C3 Γ! 2 £ 76

χπ ^ l :: bccaes [- * c: i

R.1039

Augsburg, October 31, 1979

Rolls-Royce Limited, 65 Buckingham Gate, London SWlE 6AT, England

Ceramic mold core for investment casting molds and processes for the same

Manufacturing

The invention relates to a ceramic mold core with a mixed structure for investment casting molds.

For the investment casting of hollow metal parts, a ceramic mold core is known, the shape of the desired shape of the corresponds to the cavity to be produced in the casting, inserted before pouring the molten metal into the casting mold.

030020/0822

After the metal has cooled, the mold core must be removed from the finished casting, which is usually done by leaching out the Core material happens with the help of a suitable solvent.

So far, silicon dioxide has been widely used as a mold core material because it can withstand high temperatures and is with the help of a corrosive liquid, for example 50 to 60 # potassium hydroxide solution in water or molten potassium hydroxide, leached out of the finished casting.

However, it has been shown that in more recent developments, particularly in aeronautical engineering, silicon dioxide is used as the core material no longer meets the latest requirements of precision casting technology, especially with regard to directional control Solidification of castings. The use of directional solidification makes it very strong the mold cores required, while silicon dioxide is very brittle at high temperatures and also undergoes Deformed under pressure.

Refractory materials with high strength are known per se, but have not been used for other reasons, for example because of their lack of solubility in corrosive solvents, so that such materials

030020/0822

manufactured mold cores can not be removed from the finished castings, and because some of these materials, for example silicon nitride, react with the cast metal.

Attempts have been made to sinter a mixture of two different ceramic materials together, however the problem arises that during the sintering process the softer of the two materials first softens and is bound to a matrix, while the harder of the two materials fills the interstices of the matrix. At on Mold cores produced in this way can therefore not be more satisfactory in terms of the strength of the stronger of the two materials Way to be exploited.

The invention is therefore based on the object of developing a mold core suitable for investment casting which has the above the requirements explained better than the mold cores produced according to the previously known possibilities.

This object is achieved according to the invention by the in the characterizing Part of claim 1 specified core structure solved.

The term "matrix" refers to a network or To understand lattice-like ceramic structure, which during the

030020/0822

Casting process maintains its rigidity, but after leaching out the leachable material that penetrates it can be smashed and washed out after pouring.

It is of course important that the leachable material is inert to the casting material. It therefore forms a soluble protective layer over the outer surfaces during casting and the pore walls of the matrix material.

The invention thus enables a non-soluble refractory material of high strength to be used for manufacture the load-bearing structure of a mold core, whereby the removability after casting is ensured by a filler material that can be leached out which may be of lower strength.

The manufacture of the mold core according to the invention is preferably carried out in that initially the entire core from the formed insoluble material and this material is then partially converted into the leachable material will.

Alternatively, a porous matrix material is used, the pores of which are then filled with the leachable material which then forms a coating on the pore walls of the matrix.

030020/0822

One possible method of making the mixed structure of the present invention is to use a porous Manufacture body from the insoluble material of high strength and then to the extent a reactive medium through to pass through this body that a substantial part of its material in a soluble or leachable compound is transferred, while the body through the connection of the pore-enclosing, unconverted material retains its rigidity.

Accordingly, a method for producing a mold core according to the invention consists in that first of a ceramic Material of relatively high strength made a porous core and then this material in one is partially converted into a leachable ceramic material to such an extent that the core passes through after casting Leaching out the leachable material and then smashing the remaining core material from the finished casting can be removed.

The homogeneous core body to be produced first can, for example, consist of a nitride, namely, for example, of a covalent Silicon nitride or boron nitride or a nitride of a transition metal are formed, all of which are highly refractory, inert

030020/0822

Materials with high breaking strength are. The material is produced in porous form and then for example heated in oxygen or air, giving off nitrogen, and the oxide of the other involved in the compound Elements is created because most of the oxides are soluble in acidic or alkaline solutions. To a specific example to mention, silicon nitride has a very high breaking strength

2
of about 200,000 kN / m, and considerable oxidation to silicon dioxide can occur before the strength of the overall structure falls below the desired value. The final structure can contain 30 to 60 percent by weight silicon dioxide, which, as mentioned above, is soluble in caustic alkalis.

Alternatively, the mandrel can be made of some other refractory substance than the casting material is inert and insoluble or only poorly soluble, so that it is advantageous to partially convert this material into a better to convert soluble form. For example, the mold core can be made from a suitable silicide, boride, titanide or even from a stable carbide such as titanium carbide or silicon carbide can be produced, which is oxidizable with it Silicon dioxide or a structure containing soluble silicate, borate or titanate is formed.

030020/0822

The conversion of part of the material into a soluble compound does not have to take place through oxidation, but rather can in some cases, for example when using a cadmium compound, expediently by converting the Material can be made into a sulfide.

In any event, the conversion must of course be carried out so that the soluble component is within the whole Core structure is distributed so that the insoluble material is exposed to a sufficient extent that it is washed out or can be easily removed in some other way from the finished casting, while the matrix consisting of it, however retains a substantial part of the strength of the original material during the casting process. This can be done through this achieve that the core is made of a material with a porous structure, so that oxygen or another reagent used for conversion can easily penetrate into the core material. Accordingly, it has to be used for the production of a mold core silicon nitride used preferably has a porosity between about 20 and 30 percent by volume. The porosity increases also the rate of penetration of the liquid used for leaching into the mold core compared to a non-porous structure. Since silicon nitride reacts with some metals, for example nickel alloys, it is important that the converted Material layer of the mold core opposite to that to be cast

030020/0822

Metals is inert, and the converted material must also be physically compatible with the unconverted material, It is about a crumbling off of the protective transformed layer and thus an exposure of the reactive, non-transformed layer Materials to avoid.

To further explain the invention, an exemplary embodiment is given below:

For casting a hollow turbine blade for an aircraft engine using directional solidification, a mold core, the shape of which corresponds to the shape of the blade cavity, is formed from silicon nitride with a porosity of 30 percent by volume. The core body thus formed is then heated in air to a temperature of about 1000 ° C. until about 25 % of the silicon nitride has been converted to silicon dioxide, whereby a silicon dioxide content of about 30% by weight is obtained. This conversion to silicon dioxide does not result in a reduction in core strength below the desired lower limit value, which is required for directional solidification, and depending on the porosity of the structure, up to 50 % of the silicon nitride can be converted without falling below the desired limit strength.

After casting the turbine blade, the mold core is made with the help of an acidic liquid, preferably with

030020/0822

Potassium hydroxide, leached from the vane cavity. The potassium hydroxide can be used in molten form or as a solution in water. When using a water solution of potassium hydroxide, the leaching out of the mold core can be carried out at the boiling point of the potassium hydroxide solution or in an autoclave, if the leaching out is to take place at an even higher temperature of for example 350 C to 500 ⁰ C. The silicon dioxide dissolves in the liquid and exposes the silicon nitride matrix, which is then shattered and flushed out together with the resulting silicate solution.

Possible alternative processes for the production of leachable mold cores of high strength are, for example, initially the manufacture of a porous core body from silicon dioxide and the subsequent conversion of a sufficient part of the Silicon dioxide in silicon nitride by passing a suitable reagent through it, or making a honeycomb structure made of a high strength material, which is then filled with the soluble material.

030020/0822

Claims (1)

Λ Π '/ Λ 1 » ^Γ ^ * dipt .. n: o. Jl-, H ^, ^ - f. »/ ρ ΤΤ> »ΒϊΐίΓΓίΗκ-ν / :: ι .; r, C "? O O Λ" J "!. ι-_ ": ■. · :. C 7 Γ r.iiB «Γ.:.: ..: ι .: ■ .; .; -.-- ". a -X- 29Α4534 Claims 1. Ceramic mold core with a mixed structure for investment casting molds, characterized in that the core has a relatively rigid matrix of a refractory material has relatively high strength, which of a leachable further refractory material is interspersed. 2. Molding core according to claim 1, characterized in that the leachable material is silicon dioxide and the matrix-forming material is silicon nitride or silicon carbide. 3. Molding core according to claim 2, characterized in that the silicon dioxide content of the molding core is about 30 to 60 percent by weight amounts to. 4. Mold core according to one of claims 1 to 3, characterized in that the core has a porosity of about tO to 30 percent by volume. ! p. Process for the production of a mold core according to a of claims 1 to M, characterized in that initially the whole core is formed from refractory material of relatively high strength and that this material is subsequently formed 030020/0822 _e __ ORIGINAL INSPECTED to such an extent partially into a leachable refractory Material is converted that the core is after casting by leaching out the leachable material and subsequent smashing and washing out of the remaining material can be removed from the finished casting. 6. The method according to claim 5, characterized in that initially the entire core of porous silicon nitride or Silicon carbide is formed, which is then partially oxidized to silicon dioxide. 7. The method according to claim 6, characterized in that about 25 % to 50 % of the silicon nitride is converted to silicon dioxide. 030020/0822