US2883724A - Casting processes - Google Patents

Description

April 28, 1959 R. H. HANCOCK 2,883,724 CASTING PROCESSES Filed March 18, 1955 o 1 o J7 r i 1. 16 '1 '1 /////1// o F O i IL ATTORNEY? United States CASTING PROCESSES Ralph Harold Hancock, Coxhench, England, assignor to Rolls-Royce Limited, Derby, England, a British com- P y Application March 18, 1955, Serial No. 495,308

Claims priority, application Great Britain April 1, 1954 2 Claims. (Cl. 22-196) This invention relates to casting processes and in particular to processes of the kind in which an expendable pattern is invested in a refractory material to produce a casting mould. Processes of this kind are commonly referred to as lost wax, the expendable pattern normally being of wax which is melted out of the mould after investment of the pattern. In place of wax other materials such as low melting point metals may be used.

In certain cases the article to be cast is of hollow form, and processes of the kind referred to have been used to produce hollow cast articles from hollow patterns, which are obtained by the provision of inserts in the pattern. In certain cases one or more inserts are positioned in a die in which the pattern is formed, such insert or inserts being removed from the pattern prior to investment. Thus metal inserts have been used which are supported in a die in which the pattern is formed, such inserts being of uniform section or slightly tapered to facilitate their withdrawal from the pattern. In this manner a pattern of hollow form is provided which when invested is filled with refractory material, whereby after melting out of the pattern the mould is formed so as to provide for the casting of an article reproducing the hollow formation.

Difficulty has been experienced in the production of hollow cast articles by methods such as described above, due to fragility of the investment material, giving rise to inaccuracies in the cast part.

The present invention seeks to overcome such difiiculties and to provide a method of producing hollow cast articles by casting processes of the kind referred to which will present accuracy in the finished part.

According to the present invention a-casting process of the kind referred to includes the step of supporting a preformed member of material having increased strength as compared with the investment material within a hollow formation in the pattern prior to the investment thereof, such member being supported with clearance from the wall of the hollow formation whereby the investment material flows into the space defined between the surface of the member and the internal surface of the hollow formation, said member being removable from the hollow formation after the part has been cast in the mould.

Preferably the preformed member is of a ceramic nature, for example being formed of silica. Such preformed member may if desired be hollow, facilitating its destruction where this is necessary to permit its removal.

One particular application of the invention is to the casting of blades of gas-turbine engines in high melting point alloys such as nickel-chromium alloys. Such blades commonly have hollow formations permitting the passage of cooling air in operation of the turbine, and when produced by casting it is desirable that the hollow formation is accurately positioned in relation to the outside walls to obtain the designed wall thickness. In this case the preformed member is supported at one end only in the hollow formation and with clearance from the wall of the hollow formation.

Where as in certain designs the hollow formation extends substantially throughout the length of a turbine blade, the preformed strengthening member used in accordance with the present invention may comprise a rod, for example of silica, of substantially uniform section.

An example of this invention is illustrated in the ac companying drawings in which,

Figure 1 is a plan view of a split die, with the top half removed, used for forming a wax pattern,

Figure 2 is a section on the line IIII of Figure 1 with both halves of the die in position,

Figure 3 is a section through the pattern and wax support with the strengthening insert in position,

Figure 4 is a similar section to Figure 3 showing the pattern and support invested in a container,

Figure 5 is a section on the line VV of Figure 4.

The wax pattern is formed in a die having an upper half 10 and a lower half 11 into which the wax is injected through a drilling 12 on the line of the split by injection means shown diagrammetically by the syringe 13. Before injecting the wax a metal insert 14 is inserted in the die supported therein at either end but leaving a space between itself and the pattern which is formed in the die and is shown at 15.

The metal insert 14 can be drawn out of the die by means of the handle 16 and will then leave a hollow formation in the wax pattern therein. The pattern is next mounted on a wax support 17 which is shaped to provide the hollow required in the eventual mould through which the metal will be poured in.

After being so mounted the pattern may be coated with a refractory coating by spraying or dipping in known manner. A suitable refractory coating may be prepared as follows:

270 cc. of potassium silicate solution having a specific gravity of 1.3 and a K O/SiO ratio of 1:20 by weight were mixed with 1300 cc. of a solution of dioctyl sodium sulphosuccinate, prepared by diluting 650 cc. of a 60% solution of dioctyl sodium sulphosuccinate with 650 cc. of water. 1300 cc. of a non-ionic wetting agent prepared by reacting ethylene oxide with octyl cresol, and sold under the British registered trademark of Lissapol as Lissapol NDB. were added to the mixture, followed by 1800 cc. of a soap solution prepared by mixing 56.5 gms. of hard soap flakes with 1890 cc. of hot water. The liquids must be added in the above order, and mixed for 5 minutes. 20 cc. of a mixture of high molecular weight liquid aliphatic alcohols sold under the British registered trademark of Alphanol as Alphanol 79, were now added, and mixing continued for a further 5 minutes. 21 lbs. of silica flour, all of which passed a 200 mesh (British Standard Specification 410; 1943) sieve were now added slowly, mixing being continued for a further 10 minutes. At the end of this time a further 20 cc. of Alphanol 79, were added, together with 40 cc. of carbon tetrachloride. The composition was mixed for 5 minutes after these additions before being used. If desired the Alphanol 79, may be replaced by 2 ethyl hexanol.

Other compositions suitable for forming the initial coating of finely divided refractory of the pattern are known, and may be used, such as those using ethyl silicate as a binding agent.

After the pattern is placed on the support 17 and coated with the refractory coating there is fastened to the bottom of the support 17 a silica rod or tube 18 which passes through the hollow formation in the pattern and is secured at one end to the wax support 17 by additional wax shown at 18A. This rod or tube forms the strengthening insert and is supported with clearance from the wall of the hollow formation in the pattern. The silica rod or tube may be prepared in known manner.

The usual method is to fuse the silica in an electric aseayrac furnace after which articles of fused silica may be blown or drawn in similar manner to glass. Rods can be cut to length by Carborundum or emery wheels and polished if desired on Wheels fed with Wet sand.

Pattern, support and insert are then placed on a base plate 19 onto which is placed a flask or container 20 secured to the bottom of the base plate by wax shown at 21. The container 20 may advantageously be extended in length by a sheet of waxed paper attached to it.

An investment material 22 is then poured in from the top of the container 20 until it completely fills both the container and the waxed paper extension as shown in Figure 4. The investment will fill the interior of the pattern between the inner walls thereof and the rod or tube 18. A suitable investment material may be prepared from a fireclay grog containing at least 40% of alumina, and having a particle size distribution such that not less than 26% by weight and not more than 30% by weight is retained on a 200 mesh British Standard 410, 1943, sieve, using as binding liquid an acid sodium silicate solution prepared by adding dilute nitric acid to sodium silicate, as described in British Patent No. 650,532, and containing as gelation accelerator 3.1% by volume of a hexamine solution prepared by dissolving 107 lbs. of hexamine in 30 gallons (British) of water. The investment slurry is prepared by mixing 400 lbs. of the fireclay grog with gallons (British) of the binding liquid.

The packing of the investment may be accelerated in known manner by placing the container on a vibrating table. When the refractory material has been packed round the coated pattern any fine material on the top of the mould is trimmed off level with the top of the container and the mould is allowed to gel.

The base plate 19 is then removed and the pattern and wax support melted out. At the same time the mould is dried.

The mould may then be fired to a temperature of between 1000 C. and 1100 C. in the course of 10 hours and held at this temperature for 2 hours to harden the same.

The mould is then inverted and sufficient metal poured in to fill the space therein previously occupied by the pattern.

After the cast has cooled the mould is removed by destruction, the investment being cleared from the hollow interior by sand blasting. The silica rod may be broken up during this process.

Adoption of the invention presents accuracy in the finished part, it being appreciated that during the casting operation the part of the mould providing the hollow formation is supported by the strengthening member, i.e., the silica rod, extending within its length.

The strengthening member may be provided by materials other than silica, such as alumina, mullite or metal rods such as stainless steel, or rod of a nickelchromium alloy, for example that sold under the British registered trade-mark Nimonic.

What I claim is:

1. A casting process for precision casting which includes providing a pattern of the article to be cast formed from an expendable material and having a hollow formation therein, supporting same on an expendable pedestal in a container, supporting a preformed strengthening member at one end from said pedestal within the hollow formation with a clearance from the wall thereof, investing the pattern simultaneously internally and externally with a refractory material to form a mould whereby said material simultaneously flows around the said pattern, into the space defined between said strengthening-member and the internal surface of said hollow formation, and about the opposite end of said strengthening-member forming a core integral with said mould, said strengthening member having increased strength compared with said investment material, causing the investment material to set, causing removal of the expendable pattern forming the mould, casting the article and removing the mould therefrom together with said strengthening member.

2. A casting process for a blade for an internal combustion turbine engine which includes casting in a die a pattern of the blade in expendable material, said die containing a removable insert corresponding to the hollow formation to be found in the blade, removing pattern from the die, supporting same in a container on an expendable base support corresponding to the hollow required in the mould to introduce the blade metal, supporting one end only of a preformed strengtheningmember from said base support within the hollow formation with a clearance from the wall thereof and with the opposite end extending beyond the hollow formation, simultaneously investing the pattern internally and externally with a refractory material to form a mould whereby said material flows around the said pattern and into the space defined between said strengthening-member and the internal surface of said hollow formation and around said extending end of said strengthening member, said strengthening member having increased strength compared with said investment material, causing the investment material to set, causing removal of the expendable pattern forming the mould, casting the article and removing the mould therefrom together with said strengthening member.

References Cited in the file of this patent UNITED STATES PATENTS 1,303,785 Finhayson May 13, 1919 1,663,455 Lukomski Mar. 20, 1928 2,517,902 Luebkman Aug. 8, 1950 2,749,586 Kohl .Tune 12, 1956 2,752,652 Hurley July 3, 1956 2,752,653 Emblem July 3, 1956 FOREIGN PATENTS 678,035 Great Britain Aug. 27, 1952 685,857 Great Britain Jan. 14, 1953 903,135 Germany -n. Feb. 1, 1954

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