GB2046150A - Casting Charge, Method of Preparation and Use - Google Patents

Abstract

A method of preparing a charge of alloy material for use in metal casting. Thin wall tubes (10) consisting of one of the metallic elements of the alloy material, or an If an alloy tube is selected, all elements in the tube alloy must also be materials included in the overall formulation of the alloy material. The type and quantity of the materials contained in the tubes is deducted from the quantities set forth in the overall formula and the balance of the formula is melted and poured as a core in the tube using the tube as a mould. The resulting master charge is cut into unit charges which are then used in a subsequent casting operation by melting the entire unit charge to cast precision products of an alloy which consists of the mixed and alloyed materials of both the tube and the core. <IMAGE>

Description

SPECIFICATION Casting Charge, Method of Preparation and Use This invention relates to the casting of alloys and particularly (but not exclusively) it is useful in the casting of alloyed parts or articles which must be of a high degree of uniformity in alloy composition and cast to precise dimensions to reduce or eliminate the necessity for any significant amount of finish machining. It is particularly useful in systems in which the size of the charge provided for each mould is precisely the volume of alloy necessary to cast the finished part or parts. Such systems include ones using a melting unit capable of discharging molten metal through a controlled opening and certain directional solidification "DS" casting processes and single crystal processes.

Examples of the type of parts for which this particular system is useful are in the casting of alloyed turbine blades, and wheels for the blowers of superchargers and turbines therefor. The high centrifugal speeds of these products make it essential that the parts all be of uniform composition, shape, size and weight.

It has been conventional practice to prepare a master alloy charge for remelt casting of precise parts such as turbine blades by melting the various elements which form the composition of the alloy and then casting them in a pipe or similar mould. After the cast alloy has cooled and solidified, it is removed from the pipe. Frequently, the removal of the alloy from the pipe is a very difficult, slow and unsatisfactory procedure. Further, the cast product produced from the pipe is not uniform in cross section and, therefore, must be machined by either turning or grinding to render it uniform in cross section and to render it absolutely straight. This has been necessary also to give it a proper external finish whereby throughout its length the charge was of uniform characteristics.Machining is also necessary to remove all slag and scale which, if not removed, would modify the composition and thus the resulting material would not have the desired predetermined chemical composition. This procedure is expensive, time consuming and, unless great care is taken, lacks the desired uniformity for accurately charging the mould in the final casting procedure.

According to one aspect of the present invention, a charge for use in casting an alloy product comprises an exterior shell consisting of one or more of the elements of the alloy to be cast and a core within the shell comprising-elements such that the shell and core together provide the alloy composition required, the core being adhered to the shell. The shell preferably consists primarily of the least costly of the elements of the casting alloy. Alternatively the shell consists primarily of the most abundant of the elements of the casting alloy. The most abundant element is the element of which there is most in the charge or the element of which there is most in the world and which is included in the charge or the element of which there is most in the world in an extracted form and which is included in the charge.The shell and the core are preferably bonded at their interface boundary.

Preferably there is at least at one end of the charge a plane surface which is perpendicular to the axial centreline of the charge.

The charge preferably has a uniform alloy composition throughout its length with the total ratio of the materials in the shell and those in the core at each cross section of the charge being the same as that of the alloy after the charge has been melted, mixed and cast. The shell and the core preferably have a void-free interface boundary. The shell is preferably in the form of a thin-walled tube.

The invention also extends, according to a second aspect, to a method of preparing a charge of alloy material for use iri casting including the steps of selecting one of the elements or alloys to be included in the charge composition and providing a shell made of the element or alloy; closing one end of the shell; providing a core portion of the charge by introducing in molten condition the remaining elements required to form the alloy of the final charge to fill the interior of the shell; causing the shell and molten material to adhere at the boundary between the shell and molten material; and permitting the shell and molten material to cool and solidify. The shell is preferably in the form of a thin-walled tube. The core portion is preferably fused to the tube.The charge preferably has at least one end of the solidified charge trimmed on a plane perpendicular to the central axis of the charge.

The solidified charge is preferably reduced to the precise, predetermined weight required in the subsequent casting operation. The temperature of the shell is preferably controlled so that it does not lose its geometric integrity during the pouring and solidification of the core portion. It is preferred that the temperature of the shell is controlled within 1 40C above or below the melt temperature of the core alloy.

The element or alloy selected for the shell is preferably the least expensive of the materials included in the alloy of the charge. Alternatively the element or alloy selected for the shell is the most abundant of the materials included in the alloy of the charge.

The invention also extends, according to a third aspect, to a method of casting an article from an alloy in which a charge according to the first aspect of the invention is placed over the gate of a mould in which the article is to be cast and the charge is heated so that it melts and flows into the mould.

The invention eliminates the necessity for the use of a mould in preparing the alloy charge for the subsequent casting operation. Instead of the conventional mould, a shell or tube of one of the elements or of alloy of more than one of the elements which are to be part of the composition of the final alloy to be cast is used as the mould. The composition of the ailoying material which is poured into this tube represents the composition of the final alloy minus the materials contained in the tube. The result is a casting of the alloy with an exterior shell or tube which also provides a part of the composition of the final alloy as it will be used in the casting of the finished article.

The resulting casting may then be severed into two or more segments of a predetermined precise length, each one being of the exact amount necessary to cast one of the articles. In doing this, at least one end of each segment is preferably rendered perpendicular to the axis of the cast charge. This then becomes the unit charge which may be placed in a melting or pouring crucible or in a tundish over the gate of the mould for the final article where, under controlled conditions, the charge then being melted in such a manner that both the inner or core portion and the exterior shell are melted and the elements of both the core and the shell become intermixed and blended and, thus, uniformly alloyed before the charge flows into the mould.Thus, throughout the process, nothing is used except the elements which form the composition of the final alloy and the only mould involved in preparing the charge is an integral part of the composition of the casting alloy.

The invention may be put into practice in various ways and one specific embodiment of the charge, the way it is made and the way it is used to form the final product will be described by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional elevational end view of one of the tubes in which the charge is cast with the gate and plug installed; Figure 2 is a sectional view of the tube taken along the plane Il-Il of Figure 1; Figure 3 is a plan view of the tube illustrated in Figure 1; Figure 4 is a side elevational view of a device for pouring a plurality of the charges; Figure 5 is a plan view of the tundish used in filling the device illustrated in Figure 4;; Figure 6 is a schematic sectional view of a modified support for the tubes during pouring of the charge taken along the plane VI--VI of Figure 4; Figure 7 is a sectional view (cross-hatching omitted for clarity) of a typical unit charge made according to this invention; and Figure 8 is a schematic sectional view of a typical device used in a remelt, unit-charge or certain directional solidification or single crystal casting systems for casting the final product from the charge illustrated in Figure 7.

First it is necessary to select an alloy which will produce an end product of the desired characteristics. While this invention can be practiced with a wide range of alloys, it is particularly suitable for iron or nickel alloys, that is, alloys in which either iron or nickel is the most abundant element.

If the selected alloy is an iron alloy, the tube or shell 10 will be iron or an alloy of iron. If the alloy is a nickel alloy, the tube will be nickel or an alloy of nickel. Normally the tube will be a commercially available, seamless tube. The use of rolled, crimped of , or lock-seamed tubing is also acceptable. Welded tubing produced by resistance welding or by a method which introduces only a very minor quantity of welding alloy can also be used. Normally such commercial tubing is manufactured from an alloy. In selecting the tubing, the precise alloy must be known. It is important to this invention that the tubing be of uniform composition and of uniform wall thickness, not only cross sectionally but throughout its length. The tubing must be free of impurities. Since economic factors must be considered, the price of the tubing must be taken into account.If tubing of an alloy compatible with the alloy to be used in the final product is less expensive than a single element tube, all other factors being considered, it will be used and the balance of the material used in preparing the final alloy will be adjusted accordingly.

The tube having been selected, the next step is to fill the tube with the balance of the alloy. This is done by melting the remaining alloy materials and pouring them into the tube to form a core. In preparing the alloy for the core, the type and amount of each element present in the form of the tube are deducted from the formula of elements incorporated in the composition of the alloy which is poured to form the core. For example, if the tube is 100% nickel, the quantity of nickel represented by the tube is deducted from the quantity of nickel which will be incorporated in the alloy used to pour the core. If the tube is an alloy such as nickel-chrome, the quantity of nickel and the quantity of chromium are both calculated by weight and deducted from the quantities of the same element incorporated in the core alloy. The same is done in the case of a tube of iron or iron alloy.

While tubes of iron and nickel have been described, it will be recognized that tubes of other elements such as chromium or tubes made of alloys of known composition with high ratios of the primary element could be selected. The selection of the tube is governed not only by the type of final alloy to be created but also by economics. For example, tubes of chromium and other materials are frequently far more expensive than tubes of nickel or iron. Availability is also a factor in determining the selection.

The diameter and wall thickness of the tubing may be varied within a limited range depending upon availability and cost. Thick wall tubing, however, is frequently not usable either because of cost or because it would introduce into the final alloy an amount of one element in excess of the formula specifications. On the other hand, the tubing wall thickness must be sufficient to permit the core to be poured and without the heat of the molten core either melting through the tube wall or softening the wall to a point which will result in the tube's loss of geometric integrity. Experience has indicated that with either nickel or iron based tubing, a wall thickness of 1.65 mm (0.065 inch) thickness performs satisfactorily.

Having selected the particular alloy composition and the type of tubing to be used, a plurality of the tubes of a suitable length such as 1 metre (40 inches) are placed in a pouring rack 20 (Figures 4 and 6). The pouring rack is a conventional structure long used in the metal casting field. It can incorporate a number of different designs. Thus, the construction about to be described is merely exemplary. The pouring rack 20 consists of a frame having vertical corner members 21 and side members 22 or bands joined together to form a preferably square basket-like structure, closed at the bottom by a base 23. The top 24 of the rack is flared out to provide a seat for a tundish.

To prepare the individual tubes 10 for placement within the rack, each tube is sealed at its lower end by a chill plug 30 and at its upper end by a pouring nozzle 31 having a funnei-like gate 32. Both the plug and the pouring nozzle have a portion of a suitable diameter to be inserted in the ends of the tube 10. This portion of the chill plug has a sufficiently close fit to the tube to prevent leakage of molten metal. In the case of the pouring nozzle, a sleeve 33 of fibrous paper especially made for foundry casting operations is placed between the nozzle and the inside wall of the tube. Both the nozzle and the chill plug are normally made of a ceramic material specifically designed for metal casting operations.

Prior to insertion of the chill plugs and pouring nozzles, the tubes are inspected for cleanliness ad, if necessary, cleaned to remove all foreign matter.

A plurality of the tubes equipped with the chill plugs and the pouring nozzles are seated in the pouring rack 20 and are separated from each other and held in position by narrow strips 25 of a paper based material of the same type as that used to surround the pouring nozzle. These paper strips are wrapped back and forth between the tubes to hold each tube spaced from every other tube (Figure 6).

These strips are preferably 80 mm (3 inches) wide and 3 to 6.5 mm (1/8 to 1/4 inch) thick. The strips 25 are mounted in at least two and preferably three vertically spaced locations.

The tubes 10, with the strips 25 in place, are held together by spacer bars 26 and 27 which space them from the sides and ends of the pouring rack. Thus, the tubes are firmly held and positively spaced. It is important that the tubes be firmly held in a vertical position during the actual pouring.

The tubes 10 having been locked into the pouring frame 20, a tundish 40 is placed on the top of the frame and secured by suitable clamps 41 and 41 a (Figure 5). The tundish is a basin with a plurality of holes 42 in its bottom wall. The holes 42 are arranged in a pattern to align with the gates 32 of the pouring nozzles inserted in the tops of the tubes 10, one hole 42 being provided for each of the tubes.

Normally, tundishes of this type are of ceramic material and are a conventional product long used in the metal casting fieid.

The alloy composition from which the core is to be poured is melted and is then poured through the tundish to fill all of the tubes in the pouring rack 20. In this pouring operation, it is important that the temperature of the metal, as it enters the tubes be controlled within a relatively narrow range because it is essential that the metal of the core bond to the surrounding tubes eliminating all voids or separation lines between the tube and core. It is preferable that actual fusing occur at the boundary between the core and the tube whereby there is a certain degree of melting and fusing of the inner wall surface of the tube with the core material.At the same time, it is important that the heat of the core material is not sufficient to either melt through the walls of the tube or softent the tube to the extent that it starts to warp or distort, i.e., lose its geometric integrity, because this will cause the tube to collapse. Therefore, accurate temperature control in a narrow range is essential to successfully pour the core and produce a satisfactory product. In general, it has been found that maintaining the temperature within a range of 250 F. above or below a predetermined melt temperature for the core alloy will prevent overheating of the tube yet result in the desired fusing at the boundary. This, however, will vary from alloy to alloy depending on the characteristics of the alloy.

After the core has been poured, the tubes and core are allowed to cool permitting the poured metal to solidify.

While the melting of the core alloy and its pouring into the tubes can be done under normal atmospheric conditions, this invention is particularly concerned with the making of charges for casting highly specialized parts of sensitive alloys which must be done under controlled conditions such as under a vacuum or at least under a neutral atmosphere. In either case, the melting of a core alloy and the pouring of the core and the cooling of the tubes after the core has been poured is carried on in a suitable pressure vessel which is either evacuated or charged with a neutral gas such as argon or nitrogen. This is desirable to maintain metallurgical cleanliness of the composition and eliminate scaling which would have to be removed mechanically because it upsets the net composition of the alloy.

After the tubes have been poured and cooled, they form a master charge. No mould is used, the tubes 10 serving as the confining mould for the core and the exterior shell of the master charge. Thus, the problem is recovering the cast alloy material from the conventional mould is eliminated along with the step of machining the master charge to produce a rod-like unit of uniform cross section and free from surface contamination resulting from incorporation of material from the inside wall of the mould.

Further, the conventional moulds used for this purpose have a relatively thick wall and, thus, occupy a substantial area within the pressure vessel. This is important because these vessels are limited in size and unnecessary use of potential production space within them is expensive. This invention permits a substantially larger charge to be melted and poured in each batch than is possible using conventional moulds. The next step is to cut the master charge into segments of a precise weight. Each segment becomes a unit charge 50 to be used in making single or multiple castings (Figure 7). Each unit charge 50 must have at least one end which is square, that is, the plane of the end is perpendicular to the axis of the charge. The number of segments obtained from each tube will depend upon the size of the cross section of the unit charge and the weight of the final part to be cast.

Normally the cutting is done with an abrasive wheel. In the use of this type of wheel, it is important that the boundary between the core and the inner face of the tube has no gaps or spaces in which any of the abrasive material from the cutoff wheel can become lodged and, thus, ultimately become embedded in the final casting made from the unit charge.

A typical unit charge 50 is illustrated in Figure 7. The exterior of the charge is formed by the tube which forms a shell 51. The interior of the unit charge is occupied by the core 52. At the boundary between the shell or tube and the core is a thin fusion zone 53 formed by the fusing of a thin layer of the inside surface of the tube with the adjacent material of the core. This zone of fused material eliminates any voids in which foreign materials can become deposited during the cutting or subsequent handling of the unit charge 50. Each unit charge 50 has at least one square end and a precisely calibrated weight based upon the final weight of the part for which the unit charge is to be used. Since both the exterior tube or shell 51 and the core 52 form part of the alloy of the final casting, the entire unit charge will be used.Since the use of the tube as a mould eliminates machining or other surface dressing and size truing, cost both in labour and materials is significantly reduced. This is significant since may of the alloys used in the typeof products to which this invention is particularly addressed are very expensive.

The final step is to use unit charges 50 to pour the final product. For this purpose a mould 60 is provided with a mould cavity 61 and a pouring gate 62 (Figure 8). A crucible ortundish 63 is placed over the mould. The bottom of the crucible has a pouring opening 64 which is aligned with the gate 62 of the mould. The crucible is selected to have an internal opening which will receive without significant side gap, a unit charge 50 of the exact predetermined size and weight necessary to produce the cast part. The internal opening of the crucible is not significantly larger than the charge. The crucible is surrounded by an induction heating coil 65. The induction coil does not extend to the bottom of the crucible, so that the bottom end of the unit charge 50 is the last portion of the charge to melt.Thus, the upper portion of the unit charge 50 including the core 52 and the surrounding shell 51, that is the tube, are both melted and form a pool of molten alloy metal within the crucible. This permits the element or elements or the tube and those of the core to blend so that the melted content of the crucible becomes an alloy consisting of both the tube and the core as the alloy was originally formulated. Sufficient time occurs during the heating to permit thorough intermixing and blending of the elements because the lower end of the unit charge 50 remains solid providing a plug or dam preventing the molten charge above it from flowing into the mould cavity 61. Finally the plug melts and the entire charge flows into the mould cavity.It is important that the lower end of the unit charge form a dam or plug preventing discharge of the remaining molten portion of the unit charge from entering the mould until all the rest of the unit charge has been melted. It is therefore essential that the end of the unit charge 50 seating against the bottom 66 of the crucible be flat and perpendicular to the axis of the unit charge so there will be no gap through which the molten metal can prematurely escape into the mould cavity 61. The weight of the unit charge having been calibrated to the weight of the product to be formed in the mould cavity 61, the entire unit charge is used in filling the mould cavity. Thus, the entire unit charge flows into the mould, fills the mould cavity 61 and provides a short sprue extending up into the gate 62.

The invention can be further illustrated by the following examples.

EXAMPLE I An alloy of the following composition was selected: Percentage PPM Max. Min. (Max.) Carbon 0.16 0.14 Silicon 0.55 0.35 Manganese 0.40 0.25 Sulphur 70* Aluminium 3.9 3.7 Boron 0.7 0.3 Chromium 16.0 14.5 Iron 11.5 10.0 Magnesium 60* Molybdenum 5.5 4.7 Nickel Balance Phosphorus 0.015* Titanium 2.1 1.9 Nitrogen 50* Oxygen 20* Lead 10* Silver 5* Bismuth 0.5 Selenium 3 Tellurium 0.5 Thallium 5 * content to be as low as possible Twenty-five, 100% nickel, seamless tubes of 32 mm (1 1/4 inches) diameter, 1 metre (40 inches) long and having a wall thickness of 1.65 mm (0.065 inches) were selected. The weight of the complete charge was 1 53.8 kg (339 Ibs).The collective weight of the tubes was 33.6 kg (74 Ibs). Therefore, the weight of the charge poured to form the core was 120.2 kg (265 libs). The weight of the tubes was deducted from the weight of the nickel used to formulate the alloy to be poured as the core. The tubes were inspected for foreign substances and, where necessary, cleaned. Each was equipped with a chill plug on one end and a pouring gate onthe other end. The tubes were then placed vertically in a pouring rack and a tundish mounted on the top of the rack. The rack along with a crucible containing the materials for the core alloy were placed in a pressure chamber which was then evacuated.The core materials were melted, allowed to blend and when the core alloy's temperature had been adjusted to the correct range, the molten alloy was poured into the tundish from which it flowed into and filled all the tubes. During pouring, care was taken to avoid heating the tubes to a temperature which would result in melting through the tube wall or softening the tube wall to the extent it started to lose its geometric integrity. However, the temperature was maintained high enough to assure some surface melting of the inner face of the tube resulting in a boundary area between the tube and core in which the elements of the tube and core became fused.

After the resulting tube-core castings had cooled, they were removed. The product was a master charge each cross-sectional portion of which contained the precise alloy composition of the product to be cast from segments of the master charge. The chill plug and the pouring gates were removed and one end of the filled portion of the tube was cut square. The tube was then cut into segments each of a precise, predetermined weight. Each segment constituted a unit charge ready for casting a part which would become the final product.

EXAMPLE II An alloy of the same composition as Example I was selected. However, seamless steel tubes of 32 mm (1 1/4 inches) diameter, 1 metre (40 inches) long and having a wall thickness 0.89 mm (0.035 inches) were selected. The weight of the complete charge was to be 154.7 kgs (341 Ibs). The coilective weight of the tubes was 16.3 kg (36 Ibs). Therefore, the weight of the charge poured to form the core was 138.4 kg (305 Ibs). The weight of the iron and carbon in the tubes was deducted from the weight of these elements used to formulate the alloy to be poured as the core. The procedure was repeated as for Example I to produce a number of unit charges ready for casting a part which would become the final product.

The particular alloys to which this invention is applied are not part of the invention. The formulation of the alloys, their mechanical and chemical characteristics are known to or within the skill of the trained metallurgist. Thus, their melting temperatures and the temperatures at which the tubes will soften or melt are known and need not be set out here. Further, because the invention is applicable to a wide range of alloy materials, unit weights melting points and shrinkage characteristics will also vary in a substantial range. These are characteristics either known to metallurgists or reasonably readily available from existing sources.

It will be understood that the length, diameter and wall thickness of the tubes as well as their alloy composition can vary through a substantial range. Also the number of tubes included in a single pouring can vary depending upon the quantity of alloy to be produced and the capacity of the equipment available for its production.

Claims (28)

Claims

1. A charge for use in casting an alloy product comprising an exterior shell consisting of one or more of the elements of the alloy to be cast and a core within the shell comprising elements such that the shell and core together provide the alloy composition required, the core being adhered to the shell.

2. A charge for use in casting an alloy product as claimed in Claim 1 in which the shell consists primarily of the least costly of the elements of the casting alloy.

3. A charge for use in casting an alloy product as claimed in Claim 1 in which the shell consists primarily of the most abundant of the elements of the casting alloy.

4. A charge for use in casting an alloy product as claimed in Claim 1,2 or 3 in which the shell and core are bonded at their interface boundary.

5. A charge for use in casting an alloy product as claimed in Claim 1, 2, 3 or 4 in which the shell and core are characterized by a void-free interface boundary.

6. A charge for use in casting an alloy product as claimed in Claim 1, 2, 3, 4 or 5 in which at least at one end of the charge there is a plane surface which is perpendicular to the axial centreline of the charge.

7. A charge for use in casting an alloy as claimed in any one of Claims 1 to 6 in which the charge has a uniform alloy composition throughout its length with the total ratio of the materials in the shell and those in the core at each cross section of the charge being the same as that of the alloy after the charge has been melted, mixed and cast.

8. A charge as claimed in any one of Claims 1 to 7 in which the shell is in the form of a thinwalled tube.

9. A charge for use in casting an alloy product substantially as described herein with reference to Example I or Example II or Figure 7.

10. A method of preparing a charge of alloy material for use in casting including the steps of selecting one of the elements or an alloy of more than one of the elements to be included in the charge composition and providing a hollow shell made of the element or alloy; closing one end of the shell; providing a core portion of the charge by introducing in molten condition the remaining elements required to form the alloy material of the final charge to fill the interior of the shell; causing the shell and molten material to adhere at the boundary between the shell and molten material; and permitting the shell and molten material to cool and solidify.

11. A method of preparing a charge of alloy material as claimed in Claim 10 in which the shell is in the form of a thin-walled tube.

12. A method of preparing a charge of alloy material for use in casting as claimed in Claim 10 or 11, in which the core portion is fused to the shell.

13. A method of preparing a charge of alloy material as claimed in Claim 10, 11 or 12 in which the temperature of the shell is controlled so that it does not lose its geometric integrity during the pouring and solidification of the core portion.

14. A method as claimed in Claim 1 3 in which the temperature of the shell is controlled within 1 40C above or below the melt temperature of the core alloy.

15. A method as claimed in any one of Claims 10 to 14 in which the element or alloy selected for the shell is the least expansive of the materials included in the alloy of the charge.

1 6. A method as claimed in any one of Claims 10 to 14 in which the element or alloy selected for the shell is the most abundant of the materials included in the alloy of the charge.

17. A method of preparing a charge of alloy material as claimed in any one of Claims 10 to 1 6 in which at least one end of the solidified charge is trimmed on a plane perpendicular to the central axis of the charge.

18. A method of preparing a charge of alloy material as claimed in any one of Claims 10 to 17 in which the solidified charge is reduced to the precise, predetermined weight required in the subsequent casting operation.

1 9. A method of preparing a charge of alloy material for use in casting substantially as described herein with reference to Example I or Example II of Figures 1, 2 and 3 or Figures 4, 5 and 6 of the accompanying drawings.

20. A charge as claimed in any one of Claims 1 to 9 whenever made by a method as claimed in any one of Claims 10 to 19.

21. A method of casting an article from an alloy in which a charge as claimed in any one of Claims 1 to 9 and Claim 20 is placed over the gate of a mould in which the article is to be cast and the charge is heated so that it melts and flows into the mould.

22. A method as claimed in Claim 21 in which the charge including the shell and core are melted except for a thin layer at the gate of the mould to permit intermixing and alloying of the materials of the tube and core before the melted charge enters the mould.

23. A method as claimed in Claim 21 or 22 in which the charge is melted by induction heating.

24. A method as claimed in Claim 23 in which the charge is positioned vertically within an induction heating coil and the lower end of the charge is spaced below the lower end of the coil to retard its melting.

25. A method as claimed in any one of Claims 21 to 24 in which the solidified charge is trimmed to provide a charge of the exact weight necessary to fill the mould.

26. A method as claimed in any one of Claims 21 to 25 in which the temperature of the shell throughout its length during pouring of the core is maintained in a range that maintains the shell's geometric integrity so that it supports the molten core.

27. A method as claimed in any one of Claims 21 to 26 substantially as specifically described herein with reference to Figure 8.

28. A cast alloy article whenever made by a method as claimed in any one of Claims 21 to 27.