GB1602027A - Method for removing cores - Google Patents

Description

(54) METHOD FOR REMOVING CORES (71) We, GENERAL ELECTRIC COMPANY, a corporation organised and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady, 12305, State of New York, United States of America do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to improvements in investment casting and in particular to the rapid removal of cores employed in the casting process.

The production of directionally solidified (DS) metal eutectic alloys and superalloys for high pressure turbine (HPT) airfoils with intricate internal passageways for air cooling requires that the core and mold not only be dimensionally stable and sufficiently strong to contain and shape the casting but also be sufficiently weak to prevent mechanical rupture (hot cracking) of the casting during solidification and cooling. The DS process requirements of up to 1875"C for a 16 hour time period imposed severe constraints on materials which may be used for the mold or core.

The currently available core materials do not possess the chemical stability required for casting eutectic alloy and superalloy materials. The prior art appears to be mostly limited to the use of silica or silica-zircon core and mold materials. At temperatures greater than 1600"C the silica based materials fail from the standpoint of both mechanical integrity and chemical incompatibility with the advanced alloy compositions.

Aluminum oxide (Al203) by itself, without a chemical or physical binder material, has been identified as a potential core and mold material based on both chemical compatibility and leachability considerations.

In our co-pending patent applications 23163/78 (serial No. 1602025) 23164/78 and 23168/78, (Serial No. 1602026 and 1602030), alumina material compositions are disclosed for making improved cores for D.S. casting. In particular, the cores have a porosity exceeding 20% by volume and have excellent crushability characteristics and are resistant to metal-core reaction. Depending upon processing techniques, the core may or may not have an integral dense alumina outer layer to prevent metal penetration. These cores made from the material compositions must be removed from the castings without any deleterious effect on the surface of the casting. However, Awl203 and other dense advanced core materials such as LaAIO3, MgAlO4 and Y3AI5O,2 are not easily attacked by the conventional autoclave techniques used for SiO2.

In our U.S. Patent 3,073,662, autoclave leaching of magnesia doped alumina is disclosed and claimed. It is the belief that the addition of the divalent alkaline earth cations into the trivalent cation lattice of Awl203 introduces lattice defects which enhance the kinetics of the dissolution of alumina.

In accordance with this invention there is provided a method for removing a ceramic material from a casting of an advanced superalloy or eutectic material.

The ceramic material is either alumina or a magnesia doped alumina wherein the magnesia content is between 1 and 20 mole percent.

The microstructure of the ceramic material of the core, in general, is characterized by a grain morphology characteristic of grains which have undergone vapor transport action and a network of narrow interconnecting bridges connecting adjacent grains. The porosity content is in excess of 20% by volume and is continuous throughout at least a central portion of the core. An integral layer of dense alumina may be disposed about this central portion.

The microstructure of the magnesia doped alumina is characterized by a matrix comprising an interconnecting network of magnesia doped alumina defining a plurality of interstices in which the material magnesium aluminate spinel is deposited.

The ceramic material is removed by an autoclave leaching process. A solution of either KOH or NaOH comprises the leaching solution. The solution is heated to a temperature of at least 2000C and may range up to 3500C and higher. A preferred temperature for leaching is about 290"C. The leaching solution attacks the interconnecting alumina network thereby separating the larger particles from each other. These large undissolved particles are subsequently removed from the casting by agitation of the caustic solution. The leaching rate has been found to be dependent upon the total porosity content of the core microstructure. NaOH is the preferred leaching agent.

Advanced superalloys, such as NiTaC-13 (one of a family of composite eutectic alloys consisting of a Ni-base matrix with an aligned eutectic reinforcing fibrous phase (consisting of Ta carbides embedded in the matrix), are not attacked by the core material or the leaching solution.

In the accompanying drawings:- Figure 1 is a scanning electron micrograph showing the morphology of the alumina grain structure of a fired compact at 500x.

Figure 2 is a photomicrograph of a portion of another fired compact of alumina taken at 50x.

Figure 3 is a scanning electron micrograph at 500x of the compact of Figure 2 showing the morphology of the alumina grain structure.

With reference to Figure 1 there is shown the microstructure of fired ceramic compact 10 made of alumina. The microstructure shows that the porosity is continuous throughout the compact 10 and that the grain morphology is characteristic of grains 12 which have undergone vapor phase transport action. The vapor transport action involves the evaporation or formation of a gaseous suboxide from a portion of material of one grain at high surface energy regions of the grain and the transportation of the material to'low surface energy regions of the grain, where it condenses or is oxidized. By this action the grains 12 become rounded.

Additionally, aluminum suboxide gaseous species are transported out of the compact 10 whereby the compact 10 registers a net weight loss. The vapor transport action results in a network of narrow connecting bridges 14 between the alumina particles or grains 12.

The fired compact 10 is suitable for use as a core in investment casting of directionally solidified eutectic and superalloy materials. The preferred material for the compact 10 is alumina or magnesia doped alumina because casting temperatures are in excess of 1600"C and directional solidification is practiced for 16 hours or more. Therefore, the preferred method of forming the compact 10 in an unfired state is by injection or transfer molding.

Referring now to Figures 2 and 3 there is shown a portion of another fired compact 50 made from the same alumina ceramic material composition embodying a reactant fugitive filler material empolyed in making the compact 10.

A different firing cycle results in the different structure. The compact 50 is made of alumina material fired in a controlled atmosphere to form a layer 52 of alumina as an outer portion encompassing and integral with a central portion 54 of alumina.

The alumina of the layer 52 is dense, that is, any porosity therein is discontinuous.

The structure of the material of the central portion 54 is porous and the porosity is continuous throughout.

As better illustrated with reference to Figure 3, the microstructure of the central portion 54 of the fired compact 10 has a porosity which is continuous throughout. Additionally, examination of the alumina grains 56 clearly indicates a grain morphology which is characteristic of alumina grains which have undergone vapor phase transport action. The vapor transport action involves the evaporation and/or formation of a gaseous suboxide of a portion of material of one grain at high surface energy regions of the grain and the transportation of the material to low surface energy regions of the same or another grain where it condenses. By this action the alumina grains 56 become rounded. Additionally, alumina suboxide gaseous species are transported away from the center portion 54 of the compact 50 where at least some of the species is oxidized at the outer surface thereof to form the integral outer portion layer 12 of alumina. As a result of the chemical reaction producing this vapor transport action, the fired compact registers a weight loss.

Further, the vapor transport action results in a network of narrow connecting bridges 58 between the alumina particles or grains 56.

Each of the fired compacts 10 and 50 is suitable for use as a core in investment casting of directionally solidified eutectic and superalloy materials. It is desirable for the cooling passages of the turbine blade to have a complex configuration.

Therefore, it is necessary for the cores to have a complex shape. The preferred method of forming either one of the compacts 10 and 50 in an unfired state is by injection or transfer molding. The preferred material for each compact 10 and 50 is alumina or magnesia doped alumina because casting temperatures are in excess of 1600"C and as high as 18500C concomitant with directional solidification times in excess of 16 hours.

Each of the alumina compacts 10 and 50 is easily removed from the casting by leaching in a KOH or NaOH solution in an autoclave. The leaching rate, however, is dependent upon the porosity of the compact 10 or 50. With a porosity content of from 50 percent to 60 percent by volume, a vary significant increase in the leaching rate of the compact 10 or 50 can be obtained. Additionally, the fired compact 10 or 50 and thereby increase the porosity content further. Suitable fugitive filler thickness is about 0.060 inch or less since it will have good crushability characteristics.

To increase porosity in the fired compact a reactant fugitive filler material is desirable. The reactant fugitive filler material provides, along with the alumina material, the total solids content necessary for injection molding. Upon a subsequent firing at an elevated temperature, the reactant fugitive filler is "burned" off in a suitable manner to increase the porosity content of the compact 10 or 50. A desirable reactant fugitive filler material is one which will also react with the alumina to eliminate or remove a portion thereof from the compact 10 or 50 and thereby increase the porosity content further. Suitable kgitive filler materials are those which will react at an elevated temperature with alumina to form aluminum suboxide. Preferred reactant-bearing materials are graphite, aluminum, aluminum carbide, aluminum oxycarbide, boron and boron carbide.

Suitable organic materials may also be employed as reactant materials as a carbon source.

The particle size of the alumina is important. It is desirable that the size of the pores in the compact, particularly at the outside surfaces which contact the cast metal, be small enough to prevent any significant metal penetration. It is desirable that metal penetration of the compact surface be minimized in order to obtain the best surface possible for the casting. The integral outer portion 52 of alumina functions as a barrier layer to prevent metal penetration into the core structure.

Suitable alumina material is obtainable as fused alumina powder from the Norton Company and as aggregate free alumina powder from the Meller Company. Suitable alumina powders are (a) Norton-400 alundum (aluminum oxide obtained by fusing bauscite in an electric furnace) wherein the particle size distribution is typically as follows: Particle Size Weight percentage O5,u 15% 5u--10u 13% 1 0-20 64% 201--30FL 7% > 30,a 1% (b) Norton-320 alundum wherein the particle size distribution is typically as follows: Particle Size Weight percentage 0-10M 3% l0-20 53% 20u--30u 36% 30,u37y 7% > 37,u 1% (c) Norton 38-900 alundum wherein the particle size distribution is typically as follows: Particle Size Weight percentage e-5p 55.5 34.0 5fL-10jE 34.0 > lO,u remainder (d) Meller 0.3y aggregate free alumina Various possible ceramic mixtures include 80 weight percent Norton-400, balance Meller 0.3p; 70 weight percent Norton-400, balance Meller 0.3, 100 weight percent Norton-320; 80 weight percent Norton-320, balance Norton 38900, and 100 weight percent Norton 38-900.

Alumina doped with at least 1 mole percent magnesia is also suitable as a ceramic material for making either of the compacts 10 and 50. It is believed that the addition of the divalent alkaline earth cations into the trivalent cation lattice of Awl203 introduces lattice defects which enhance the kinetics of the dissolution of alumina during autoclave caustic leaching.

The magnesia may be present in amounts from 1 mole percent up to 20 mole percent. It has been discovered that as the magnesia content decreases, the volume fraction of the magnesia doped alumina phase increases. The magnesia doped alumina phase encases the spinel phase. The spinel phase therefore provides either an interconnected network defining a plurality of interstices in which the magnesia doped phase is found or a dispersion of paricles within a matrix of magnesia doped alumina.

Above 20 mole percent magnesia, the magnesia doped alumina network begins to become discontinuous. Dissolution of the alumina network by autoclave KOH or NaOH processing therefore begins to require an excessive increase in processing time. The decrease in dissolution is attributed to the fact that autoclave leaching must occur by intergrannular attack which at a magnesia content of about 25 mole percent is almost an order of magnitude slower than for a magnesia content of 20 mole percent content.

Two methods of fabricating compacts with magnesia doped alumina may be employed. In one instance a mechanical mix of alumina powder of the desired particle size content and the appropriate amount of magnesia is prepared. This mechanical mixture is then added to the melted wax.

In the second instance, the same mechanical mixture is prepared and calcined at a temperature of 1500"C+200"C for 1 to 4 hours to form a two phase product of spinel and magnesia doped alumina. The calcined product is then crushed and ground to a particle size of from l,um to 40,us.

One or more waxes can be employed to provide adequate deflocculation, stability and flow characteristics. Despite the addition of deflocculent, large particle size, of the order of > 50y, can settle at a rather rapid rate in the wax and can change the sintering behavior of the remainder of the material mix of the molding composition material. The rate of settling of large particles is adjusted by varying the viscosity of the liquid medium, wax. To this end aluminum stearate is added to the wax to increase viscosity by gelling. Increased viscosity also has the additional benefits of preventing segregation of the wax and solids when pressure is applied and reducing the dilatancy of the material mixture.

The plasticizing vehicle system preferably consists of one or more paraffin type waxes which form the base material. A purified mineral wax ceresin may also be included in the base material. To 100 parts of the base wax material additions of oleic acid, which acts as a deflocculent, white beeswax, which acts as a deflocculent and aluminum stearate, which acts to increase the viscosity of the base wax, are added. A preferred plasticizing vehicle has the following composition: Binder: Material Part By Weight P-21 paraffin (Fisher Scientific) 33 1/3 P-22 paraffin (Fisher Scientific) 33 1/3 Ceresin (Registered Trade Mark) 33 1/3 (Fisher Scientific) Total 100 parts Additives: Material Range Preferred Typical oleic acid 0--12 6-8 8 beeswax, e-12 3-5 5 white aluminum 0--12 34 3 stearate The preparation of the material compositions and molding of the core shapes are not a part of this application. For a further discussion of material compositions and processes for firing of the compacts, attention is directed to our copending applications 23163/78 and 23168/78, (Serial No. 1602025 and 1602030).

After a fired compact 10 or 50 has been employed as a core in making a casting, and the casting has solidified thereabout, the core is removed from the casting by autoclave leaching employing either a KOH or a NaOH solution. A solution of from 10 weight percent of either KOH or NaOH in water up to saturation, about 70 weight percent in water, has been found to be satisfactory. The autoclave temperature is preferably greater than 200"C and may range upwards to 3500C and higher. The temperature preferably should not exceed 290"C. The pressure in the autoclave leaching process may be of the order of from 200 psi to 1250 psi.

The caustic leaching agent, during the autoclave leaching process, attacks the ceramic material of the core. In a core made from compact 10, the leaching agent dissolves the alumina of both the grains 12 and the interconnecting bridges 14.

When the bridges 14 have been dissolved, the remainder of the core material, mostly grannular material, is physically washed out of the core cavity by agitation of the caustic solution within the autoclave. Any remaining material may be removed by mechanical agitation after removal from the autoclave by such suitable means as ultrasonics. In the same manner, the caustic agent attacks and dissolves the alumina of the grains 56, the layer 52 and the network of narrow interconnecting bridges 58 to remove the core 50 from the castings.

After autoclave leaching process has been conducted to remove the core 10 or 50, the casting is removed from the autoclave, washed in water and dried in a warm oven. The casting is then stored for further use or processed further as required.

We have also discovered that the porosity of the core has a decided influence on leaching rate of the cores 10 and 50 by the novel autoclave caustic leaching process. As the porosity of the core increases, or with decreasing percent of theoretical density for the same, the leaching rate increases logarithmically.

Examination of castings of directionally solidified eutectic alloy and superalloy materials such, for example, as NiTaC-13, cast with the cores 10 and 50, revealed no apparent attack on the material. The surface finishes of the castings are acceptable regardless of whether the leaching agent is KOH or NaOH and regardless of the strength of the solutions of the same. The KOH and the NaOH have no deterimental effect on the finish or integrity of the superalloy casting.

WHAT WE CLAIM IS: 1. A method for removing a core of porous ceramic material from a casting, at least a central portion of the core having a continuous porosity exceeding 20 percent by volume and a grain morphology characteristic of grains which have undergone vapour phase transport action such that a network of narrow interconnecting bridges connects adjacent grains and particles, the ceramic material being either alumina or alumina doped with between 1 and 20 mole percent magnesia, the method comprising placing the casting in an autoclave which contains a leaching solution of NaOH or KOH, heating the casting, ceramic material and leaching solution to a temperature of at least 2000C such that substantially all of the interconnecting bridges are dissolved due to chemical attack by the leaching solution, and removing at least some of the undissolved ceramic material from the casting by agitation of the leaching solution.

2. A method according to Claim 1 wherein the temperature in the autoclave is no greater than 350"C.

3. A method according to Claim 1 or Claim 2 wherein the composition of the leaching solution is from 10 to 70 by weight hydroxide and the balance water.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. Additives: Material Range Preferred Typical oleic acid 0--12 6-8 8 beeswax, e-12 3-5 5 white aluminum 0--12 34 3 stearate The preparation of the material compositions and molding of the core shapes are not a part of this application. For a further discussion of material compositions and processes for firing of the compacts, attention is directed to our copending applications 23163/78 and 23168/78, (Serial No. 1602025 and 1602030). After a fired compact 10 or 50 has been employed as a core in making a casting, and the casting has solidified thereabout, the core is removed from the casting by autoclave leaching employing either a KOH or a NaOH solution. A solution of from 10 weight percent of either KOH or NaOH in water up to saturation, about 70 weight percent in water, has been found to be satisfactory. The autoclave temperature is preferably greater than 200"C and may range upwards to 3500C and higher. The temperature preferably should not exceed 290"C. The pressure in the autoclave leaching process may be of the order of from 200 psi to 1250 psi. The caustic leaching agent, during the autoclave leaching process, attacks the ceramic material of the core. In a core made from compact 10, the leaching agent dissolves the alumina of both the grains 12 and the interconnecting bridges 14. When the bridges 14 have been dissolved, the remainder of the core material, mostly grannular material, is physically washed out of the core cavity by agitation of the caustic solution within the autoclave. Any remaining material may be removed by mechanical agitation after removal from the autoclave by such suitable means as ultrasonics. In the same manner, the caustic agent attacks and dissolves the alumina of the grains 56, the layer 52 and the network of narrow interconnecting bridges 58 to remove the core 50 from the castings. After autoclave leaching process has been conducted to remove the core 10 or 50, the casting is removed from the autoclave, washed in water and dried in a warm oven. The casting is then stored for further use or processed further as required. We have also discovered that the porosity of the core has a decided influence on leaching rate of the cores 10 and 50 by the novel autoclave caustic leaching process. As the porosity of the core increases, or with decreasing percent of theoretical density for the same, the leaching rate increases logarithmically. Examination of castings of directionally solidified eutectic alloy and superalloy materials such, for example, as NiTaC-13, cast with the cores 10 and 50, revealed no apparent attack on the material. The surface finishes of the castings are acceptable regardless of whether the leaching agent is KOH or NaOH and regardless of the strength of the solutions of the same. The KOH and the NaOH have no deterimental effect on the finish or integrity of the superalloy casting. WHAT WE CLAIM IS:

1. A method for removing a core of porous ceramic material from a casting, at least a central portion of the core having a continuous porosity exceeding 20 percent by volume and a grain morphology characteristic of grains which have undergone vapour phase transport action such that a network of narrow interconnecting bridges connects adjacent grains and particles, the ceramic material being either alumina or alumina doped with between 1 and 20 mole percent magnesia, the method comprising placing the casting in an autoclave which contains a leaching solution of NaOH or KOH, heating the casting, ceramic material and leaching solution to a temperature of at least 2000C such that substantially all of the interconnecting bridges are dissolved due to chemical attack by the leaching solution, and removing at least some of the undissolved ceramic material from the casting by agitation of the leaching solution.

2. A method according to Claim 1 wherein the temperature in the autoclave is no greater than 350"C.

3. A method according to Claim 1 or Claim 2 wherein the composition of the leaching solution is from 10 to 70 by weight hydroxide and the balance water.

4. A method according to any one of the preceding claims wherein the

temperature in the autoclave is about 290 C.

5. A method according to Claim I and substantially as herein described with reference to any one of the examples.