Composite mold with fugitive metal backup

Abstract

Composite mold for use in casting a metal or alloy includes an inner mold region that includes a mold cavity and a fugitive metallic outer backup mold region residing on the inner mold region, wherein the metallic material of the backup mold region has such a melting temperature that the backup mold region melts from the inner mold region after the molten metal or alloy is cast and at least partially solidified in the mold. The inner mold region can comprise a non-metallic refractory or ceramic shell mold, while the backup mold region can comprise tin or other relatively low melting point metal or alloy.

Description

FIELD OF THE INVENTION

The invention relates to casting of metals and alloys and, in particular, to a composite casting mold having an inner mold region and fugitive metallic outer backup mold region on the inner mold region for use in casting high temperature melting metals and alloys.

BACKGROUND OF THE INVENTION

In the well known “lost wax” process of investment casting, a fugitive or disposable pattern, such as a wax, polystyrene or other commonly used fugitive pattern material, of the article to be cast is made by injection molding a fluid pattern material in a die corresponding to the configuration of the article to be cast. That is, the fugitive pattern is a replica of the article to be cast. In high production commercial investment casting operations, a plurality of fugitive patterns typically are attached to a central fugitive sprue and pour cup to form a gang or cluster pattern assembly. The pattern assembly then is invested in a ceramic shell mold by repeatedly dipping the pattern in a ceramic slurry having ceramic flour carried in a liquid binder, draining excess slurry, stuccoing the slurry layer while it is wet with coarser ceramic particles or stucco, and then drying in air or controlled atmosphere until a desired thickness of a ceramic shell mold is built-up on the pattern. The initial ceramic slurry and stucco layers (e.g. the initial several layers) form what is called a facecoat of the shell mold for contacting the molten metal or alloy to be cast. The numerous outer backup layers subsequently applied on the facecoat are selected to impart sufficient strength to the mold to withstand casting of molten metal or alloy in the mold. A conventional ceramic shell mold can have ten or more facecoat and backup layers. The build-up of a ceramic shell mold of substantial thickness to withstand casting of molten metal or alloy in the mold requires considerable time and processing steps.

Once a shell mold of desired wall thickness is built up on the pattern assembly, the pattern assembly is removed from the green shell mold typically by a thermal treatment to selectively melt out the pattern assembly, leaving a ceramic shell mold having one or more mold cavities with the shape of each fugitive pattern. One common pattern removal technique involves subjecting the green shell mold/pattern assembly to a flash dewaxing step where the green shell mold/pattern assembly is placed in an oven at elevated temperature to rapidly melt the wax pattern from the green shell mold. Another pattern removal technique involves positioning the green shell mold/pattern assembly in a steam autoclave where steam at elevated temperature and pressure is used to rapidly melt the pattern from the green shell mold. Following pattern removal, the shell mold is fired at elevated temperature to remove pattern residue and to develop appropriate mold strength for casting a molten metal or alloy. Both the investment casting process and the lost wax shell mold building process are well known, for example, as is apparent from the Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751 as well as numerous other patents.

The ceramic shell mold typically is cast with molten metal or alloy by pouring the molten material into a funnel-shaped pour cup of the shell mold and flowing the molten material by gravity down a sprue channel through gates and into the mold cavities. The molten metal or alloy solidifies in the mold to form the desired cast articles in the mold cavities. That is, the cast articles assume the shape of the mold cavities, which have the shape of the initial fugitive patterns. The cast articles are connected to solidified gates, sprue and pouring cup. The ceramic shell mold then is removed, and the cast articles are cut or otherwise separated from the solidified gates and subjected to one or more finishing and inspecting operations before being shipped to a customer. The above described lost wax investment casting process is in widespread use in casting components for use in gas turbine engine components, airframes, vehicles, internal combustion engines, and numerous other applications.

SUMMARY OF THE INVENTION

The present invention provides a composite mold for casting a metal or alloy, especially a high temperature melting metal or alloy, wherein the mold includes an inner mold region having a mold cavity and a fugitive metallic (metal or alloy) outer backup mold region residing on the inner mold region, wherein the metallic material of the backup mold region has such a melting temperature that the backup mold region melts from the inner mold region after the molten metal or alloy is cast and at least partially solidified.

In an illustrative embodiment of the invention, the inner mold region comprises a relatively thin non-metallic refractory or ceramic shell mold.

In another illustrative embodiment of the invention, the outer backup mold region comprises tin or other metal or alloy that melts at a temperature of about 1300 degrees F. or below that extracts heat from the inner mold region after the molten metal or alloy is introduced into the mold. The outer backup mold region can be cast and solidified in-situ on the inner mold region or otherwise formed on the inner mold region.

The present invention also provides a method of casting a metal or alloy comprising the steps of introducing a molten metal or alloy into a mold cavity of a composite mold having an inner mold region that includes the mold cavity and a fugitive metallic outer backup mold region residing on the inner mold region, and melting the backup mold region from the mold after the molten metal or alloy is cast and at least partially solidified. The molten metal or alloy can be introduced by any number of known gravity casting techniques or under pressure into the mold cavity; for example, by die casting or pressure casting into the mold cavity.

In an illustrative method embodiment of the invention, a method of casting titanium aluminide comprises introducing molten titanium aluminide into a mold cavity of a composite mold having a relatively thin-wall ceramic inner shell mold region that includes the mold cavity and a fugitive metallic outer backup mold region residing on the inner mold region, and melting the backup mold region away from the inner shell mold region after the molten titanium aluminide is cast and at least partially solidified in the mold. Melting of the outer backup mold region leaves a thin ceramic shell mold region which does not overstress the cast component as it cools, thereby avoiding hot cracking of the cast component.

The present invention further provides a method of making a casting mold comprising the steps of forming a refractory or ceramic inner shell mold region and forming a fugitive metallic outer backup mold region on the inner shell mold region. The outer backup mold region can be cast and solidified in-situ on the inner shell mold region.

The above features of the present invention will become more readily apparent from the following drawings taken with the following detailed description of the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a ceramic inner shell mold for use in practice of an embodiment of the invention.

FIG. 2A is a perspective view of the ceramic shell mold of FIG. 1 having its sprue and mold section positioned in a mold. FIG. 2B is a perspective view of a composite mold including the inner ceramic shell mold region of FIG. 1 encased in a fugitive metallic outer backup mold region.

FIG. 3 is a schematic side sectional view of a casting machine for practicing a method embodiment of the invention with the vacuum chamber shown broken away.

FIG. 4 is a perspective view of a portion of the casting machine having a plurality of composite molds of FIG. 2 positioned to communicate to a gating system of a die platen.

FIG. 5 is a perspective view of cast composite mold and cast turbocharger wheels connected to gating after removal from the casting machine.

FIG. 6 is a perspective view of a ceramic investment shell mold having a sprue portion and mold-cavity portion with the mold-cavity-portion in a container in which a metallic backup material is to be cast and solidified.

FIG. 7 is a perspective view of a composite mold formed after metallic mold backup material is cast and solidified in the container.

FIG. 8 is a schematic side sectional view of another casting machine for practicing another method embodiment of the invention with the vacuum chamber shown broken away.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a composite mold for casting a metal or alloy, especially high temperature melting metals or alloys such as including, but not limited to, nickel based superalloys, cobalt based superalloys, titanium alloys, titanium aluminide and other intermetallic alloys or compounds where the temperature of the molten metal or alloy being introduced into the casting mold is quite high. For example, nickel base superalloys typically are cast at temperatures of 2300 degrees F. and above. Titanium aluminide, such as gamma TiAl compound, typically are cast at temperatures of 2900 degrees F. and above. The invention is not limited to casting such high temperature melting metals and alloys and can be practiced to cast any molten metal or alloy.

The composite mold of the invention includes an inner mold region having a mold cavity and a fugitive metallic outer backup mold region residing on the inner mold region. The metallic material of the backup mold region has such a melting temperature that the backup mold region melts away from the inner mold region after the molten metal or alloy is cast and at least partially solidified. The metallic outer backup mold region comprises a relatively lower temperature melting metal or alloy material compared the metal or alloy to be cast to this end.

An illustrative inner mold region comprises a relatively thin ceramic shell mold formed by the “lost wax” process, although any mold making process can be used in practice of the invention. The thickness of the inner shell mold region can be much less than a conventional “lost wax” shell mold since the inner mold region is structurally supported by the metallic outer backup mold region during casting and solidification of the molten metal or alloy in the mold. For purposes of illustration and not limitation, a ceramic shell mold can have a mold wall thickness of about 0.1 inch or less, such as from about 0.04 inch to about 0.10 inch, and more particularly about 0.075 inch for an exemplary mold wall thickness.

The fugitive metallic outer backup mold region is selected to melt at a temperature substantially less than the metal or alloy to be cast in the mold so that heat from the metal or alloy cast in the mold is transferred through the inner mold region and melts the outer backup mold region away from the inner mold region after the molten metal or alloy is cast and at least partially solidified. For purposes of illustration and not limitation, the backup mold region comprises a metal or alloy that melts at a temperature of 1300 degrees F. or below. To this end, the outer backup mold region comprises a metallic material including, but not limited to, tin, indium, bismuth, lead, aluminum, or zinc, or alloys thereof one with another or with other metals. For example, tin has a melting temperature of about 450 degrees F., indium has a melting temperature of about 312 degrees F., lead has a melting temperature of about 621 F, bismuth has a melting temperature of about 520 degrees F., aluminum has a melting temperature of about 1220 degrees F., and zinc has a melting temperature of about 786 degrees F.

The outer backup mold region can be formed on the inner mold region by any appropriate process. For purposes of illustration and not limitation, molten outer backup material can be cast and solidified in-situ on the inner mold region. Alternately, the outer backup mold region can be formed by spraying molten backup material on the inner mold region or attaching a previously formed backup region to the thin shell.

Referring to FIG. 1, for purposes of illustration and not limitation, a thin-wall ceramic inner shell mold region 10′ is shown formed by the “lost wax” process to include a pour cup 10a′ having a solid handling collar 10b′, a sprue or runner 10c′, and a mold section 10d′ where a wax or other fugitive pattern is repeatedly dipped in ceramic slurry, drained of excess ceramic slurry, and stuccoed with coarse ceramic stucco particles, and dried, until the desired wall thickness is built-up. Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751 as well as numerous other patents describe the “lost wax” process for making a shell mold.

In FIG. 1, the mold section 10d′ has a turbocharger wheel-shape having a plurality of vane-shaped mold sections 10e′ disposed about the periphery a central hub mold section 10f′. The mold section 10d′ has a thin wall 10w′ with a relatively small wall thickness as described above to define turbocharger wheel-shape mold cavity MC′in the mold sections 10e′ and 10f′. The pour cup 10a′, collar 10b′, and sprue or runner 10′ can have a larger wall thickness than that of the mold section 10d′. The larger wall thickness of the pour cup 10a′, collar 10b′, and sprue or runner 10c′ is provided by using a preformed ceramic or metal shape in those areas or by adding additional layers of dipped ceramic in those areas.

For purposes of illustration and not limitation, the wall thickness of the mold section 10d′ typically is in the range of 0.04 to 0.10 inch. Such thin wall typically comprises from 3 to 5 layers of facecoat plus stucco, which is substantially less than the number of layers employed to make a conventional “lost wax” mold.

The ceramic shell mold can be formed of one or more appropriate ceramic or non-metallic refractory materials, or combinations thereof, such as including but not limited to zirconia, silica, alumina, and graphite (as a non-metallic refractory) selected in dependence on the particular molten metal or alloy to be cast in the mold 10′. Typically, the ceramic or refractory is selected so as to be substantially non-reactive with the molten metal or alloy to be cast in the mold, although in some casting applications reaction of the molten metal or alloy and the ceramic or refractory inner mold region can be tolerated or desired. As described in the Example below, when the molten alloy to be cast comprises titanium aluminide, such as gamma TiAl, the ceramic inner shell mold region 10′ can comprise aluminia, silica, yttria, or other ceramics.

After the ceramic inner shell mold region 10′ is formed, the fugitive metallic outer backup mold region 20′ is formed thereon. One illustrative embodiment of the invention shown in FIG. 2A involves placing the sprue or runner 10c′ and the mold section 10d′ of the shell mold region 10′ in a cylindrical or other shaped metal mold 31′ and introducing molten backup material (e.g. molten tin) in the mold cavity MM defined between the inner mold region 10′ and the mold 31′ to a level just below collar 10b′ and solidifying the molten backup material in-situ on the inner shell mold region 10′.

The mold 31′ then is removed to leave the composite mold 50′ pursuant to an illustrative embodiment of the invention comprising the inner mold region 10′ and the fugitive metallic outer backup mold region 20′ on the inner mold region, FIG. 2B.

The outer backup mold region 20′ is relatively thick compared to the inner mold region 10′. For purposes of illustration and not limitation, the wall thickness of the outer backup mold region 20′ typically is in the range of 0.15 to 0.4 inch when applied on the above-described thin wall inner mold region 10′ made by the “lost wax” process, although the thickness of the outer backup mold region 20′ may vary from location to location depending on the exterior configuration of the inner mold region 10′. The invention is not limited to any particular thickness of the outer backup mold region 20′ so long as the outer backup mold region forms a support body that is capable of supporting the inner mold region and molten metal or alloy in the mold cavity and permits the inner mold region 10′ to contain the molten metal or alloy when introduced in the mold cavity under pressure.

The composite mold of the invention can be used in practice of various casting processes. For example, the composite mold is especially useful in pressure casting of metals or alloys, especially high melting temperature metals or alloys wherein the molten metal or alloy is injected or poured under superambient pressure into the composite mold. A modified die casting method is described in more detail below for purposes of illustration and not limitation of the invention. Other pressure casting processes include, but are not limited to, squeeze casting and centrifugal casting. The invention is also applicable to typical non-pressure casting such as gravity casting.

Regardless of the casting process employed, molten metal or alloy is introduced into the mold cavity of the composite mold 50′ and solidified to form a cast component. After the molten metal or alloy is at least partially solidified in the mold, the fugitive metallic outer mold region 20′ is designed to melt and drain away from the inner mold region 10′ as result of extraction through the inner mold region 10′ of heat from the metal or alloy cast into the mold. The molten metal or alloy in the mold 50′ is at least partially solidified to an extent that the thin wall inner mold region 10′ is capable of confining the metal or alloy in the desired shape after the outer backup mold region 20′ is melted and drained away. Typically, the molten metal or alloy is substantially solidified before the outer backup mold region 20′ begins to melt and drain away from the inner mold region 10′.

The composite mold is especially useful for casting metals or alloys that are sensitive to stress during cooling in the mold after casting. For example, some alloys, such as gamma TiAl intermetallic compound, are weak and brittle at temperatures encountered during cooling in the mold to ambient temperature. A thick wall conventional ceramic shell mold can be strong enough to overstress the casting during casting, leading to cracking of the casting in the mold during cooling.

In contrast, the composite mold 50′ includes the inner mold region 10′ to contain the molten metal or alloy and the outer backup mold region 20′ that supports the inner mold region 10′ during casting and that subsequently melts and drains away from the inner mold region 10′ after the molten metal or alloy is at least partially solidified. This leaves the at least partially solidified metal or alloy confined in the inner mold region 10′, which lacks sufficient strength to overstress the cast component therein.

Moreover, the composite mold promotes rapid cooling of the molten metal or alloy in the mold cavity from the casting temperature to the metal backup melting temperature. As is known, the cooling rate of the molten metal or alloy after casting affects many properties of the casting such as grain size. The thin wall ceramic inner shell mold region 10′ described above is less of a barrier to heat flow than a conventional thick wall ceramic shell mold. The relatively cooler outer backup mold region 20′ acts as a chill or heat sink to extract heat from the molten or solidifying metal or alloy in the mold cavity. In addition, as the backup mold region 20′ melts, its heat of fusion is extracted from the molten or solidifying metal or alloy in the mold cavity.

For purposes of illustration and not limitation, FIGS. 3 and 4 illustrate placement of composite molds 50′ pursuant to the above illustrative embodiment of the invention in a modified die casting machine. The die casting machine is shown comprising a base 11 which includes a reservoir (not shown) therein for hydraulic fluid that is used by hydraulic actuator 12 to move the movable die platen 16 relative to the fixed (stationary) die platen 14 to open and close the die platens 14, 16. The platen 16 is disposed for movement on stationary guide rods or bushings 18. A die platen clamping linkage mechanism (not shown) is connected to the movable die platen 16 in conventional manner not considered part of the present invention.

The die casting apparatus also comprises a tubular, horizontal shot sleeve 24 having intermediate section that is received in the stationary die platen 14 and a mold-receiving member or plate 30 fastened to the platen 14 by bolts, clamps, and other fastening means. The shot sleeve 24 extends into a vacuum melting chamber 40 where the metal or alloy to be cast is melted under high vacuum conditions, such as less than 100 microns, in the event an oxygen reactive metal or alloy, such as titanium alloy, titanium aluminide alloy, superalloy, etc., is to be die cast.

The vacuum chamber 40 is defined by a vacuum housing wall 42 that extends about and encompasses or surrounds the charging end section 24a of the shot sleeve 24 and the plunger hydraulic actuator 25 having ram 25a. The chamber wall 42 is vacuum tight sealed about the stationary, horizontal shot sleeve and plunger support members 44. The vacuum chamber 40 is evacuated by a conventional vacuum pump P connected to the chamber 40. The base 10 rests on a concrete floor or other suitable support.

A cylindrical plunger 27 is disposed in the cylindrical bore of the shot sleeve 24 for movement by ram 25a between a start injection position located to the left of a melt entry or inlet 50 in FIG. 3 and a finish injection position proximate mold receiving member or plate 30. The melt inlet 50 comprises a melt receiving vessel 52 mounted on the shot sleeve 24. The melt receiving vessel 52 is disposed beneath a melting crucible 54 to receive a charge of molten metal or alloy therefrom for die casting. The invention is not limited to a hydraulic plunger as a means for introducing the molten metallic material under superambient pressure in the mold 50′. For example, superambient gas pressure may be applied at the end of the shot sleeve with or without the plunger present for introducing the molten metallic material under pressure into the mold 50′.

The melting crucible 54 may be an induction skull crucible comprising copper segments in which a charge of solid metal or alloy to be die cast is melted. The charge of solid metal or alloy can be positioned in the crucible 54 before a vacuum is established in chamber 40 and melted by energization of induction coils 56 after the vacuum is established. Alternately, the solid metal or alloy charge can be charged into the crucible 54 in evacuated chamber 40 via a vacuum port (not shown) and melted by energization of induction coils 56. Known ceramic or refractory lined crucibles also can be used in practicing the present invention. Any melting method such as arc melting, electron beam melting, and others may be employed in practice of the invention. The crucible 54 can be tilted to pour the molten metal or alloy charge into the melt receiving vessel 52, which is communicated to the shot sleeve 24 via an opening 58 in the shot sleeve wall. The molten metal or alloy charge is introduced through opening 58 into the shot sleeve 24 in front of the plunger 27.

The plunger 27 is moved from the start injection position to the finish injection position by conventional hydraulic actuator 25. Typical radial clearances between the shot sleeve 24 and the plunger 27 are in the range of 0.001 to 0.008 inch.

A die casting machine having the features described above is disclosed in U.S. Pat. No. 6,070,643 and in copending patent application Ser. No. 11/311,433 of common assignee herewith, the teachings of both of which are incorporated herein by reference.

The die casting machine 10 is modified or adapted to cast a molten metallic material under hydraulic pressure into one or more evacuated composite molds 50′ described above.

Referring to FIGS. 1-4, pursuant to one embodiment of the invention, mold-receiving member 29 is fastened to platen 16 and is adapted to mate with mold-receiving member 30 that is fastened to platen 14 via plate 15 to form a chamber C for receiving the composite molds 50′ and a mold gating system 35 therebetween when the members 29, 30 are abutted at a vertical parting plane. The gating system is formed by machined, replaceable gating inserts 40, 42 that are received in respective members 29, 30 and that are coplanar at their outermost surfaces with those of the respective members 29, 30 in which they are received. When the members 29, 30 are abutted at the parting plane, the gating inserts form a gating system that comprises runners R that communicate with a common passage CP that communicates with the end of the shot sleeve 24. A respective runner R extends from the passage CP to a respective mold 50′. The members 29, 30 as well as inserts 40, 42 typically are steel or other suitable permanent metal or alloy (metallic material) and are mounted on or connected to respective platens 14, 16 of the die casting machine.

An O-ring vacuum seal S1 is provided between the members 29, 30 for establishing a vacuum tight seal therebetween, FIG. 3. The vacuum seal S1 extends about and surrounds the gating system 35.

The composite molds 50′ are shown positioned in the chamber C with their pour cups 10a′ residing in complementary configured cylindrical shaped recesses 41a on a shelf or ledge 41 forming the bottom wall of the chamber C when the members 29, 30 are abutted at the parting plane. One-half of the ledge or shelf 41 as well as each recess 41a is shown formed on the member 30 and the other half is formed on member 29. The molds 50′ thereby are positioned vertically inverted such that their rear closed ends 10g′ face upwardly.

The end surface of each mold pour cup 10a′ sealingly engages the shelf or ledge 41 in the recesses 41a to prevent molten material from leaking out at the interface when the mold is clamped or pressed on the ledge as described below. A flat seal or gasket optionally may be used if needed between the pour cup end surface and the ledge 41 to this end. The molds 50′ are positioned relative to the runners R using a fixed positioning plate 51 having slots 51a formed between fingers 51b. Each mold 50′ is inserted in a respective slot 51a with the adjacent fingers 51b being received in the mold positioning groove G′ formed between the pour cup 10a′ and collar 10b′. One half of each mold pour cup 10a′ thereby is positioned to straddle a respective runner R to receive molten material therefrom. The positioning plate 51 is fixedly fastened to one of the members 29, 30 in a horizontal orientation such that the fingers 51b are received in the facing other of the members 29, 30 overlying the shelf or ledge of that member.

When so positioned, the pour cup 10a′ and the sprue passage 10c′ of each composite mold 50′ communicates to the shot sleeve 24 via the gating system for receiving molten metallic material from the shot sleeve 24 as pushed by the plunger 27. The shot sleeve 24 is sealingly received in the member 30.

Each mold 50′ is supported in position in the chamber C against upward force of molten metallic material introduced into the mold via the gating system. For example, the upwardly facing end 10g′ of each mold 50′ is abutted by a respective support plate 60. Support plates 60 are connected to shafts 62 each of which is mounted on a hinge 64 such that the plates 60 can be brought into position to abut the closed ends of the molds after they are positioned in positioning plate 51. The support plates 60 are pressed gently toward the end of the molds 50′ by a main shaft 66 connected to a respective hinge 64 and a pressing device 67, such as a spring, pneumatic cylinder, hydraulic cylinder, and/or mechanical clamp, to bias the shafts 66 downwardly.

The vacuum chamber 40 then is evacuated to a suitable level for melting the particular charge (e.g. less than 100 microns for titanium alloys such as Ti-6A1-4V alloy and titanium aluminide such as TiAl) by vacuum pump P. The composite molds 50′ in chamber C are concurrently evacuated to the same vacuum level through the connection to the vacuum melting chamber 40 via the shot sleeve 24 and by virtue of being isolated from surrounding ambient air atmosphere by the vacuum seal S1 between members 29, 30. Optionally or in addition, the molds 50′ can be evacuated using a separate vacuum conduits or lines communicated to the mold interior.

The composite molds 50′ typically are at ambient (room) temperature when they are placed in the chamber C. Alternately, the molds 50′ can be preheated to a suitable elevated temperature before being placed in the chamber C. Still further, heaters (not shown) can be provided in the chamber C to heat or maintain the temperature of the molds 50′.

The solid charge of the metal or alloy in crucible 54 is melted by energizing induction coil 56, the melt then is poured under vacuum into the shot sleeve 24 via the melt inlet 50 with the plunger 27 initially positioned at the start injection position of FIG. 3. The molten metal or alloy is poured into the shot sleeve 24 and resides therein for a preselected dwell time to insure that no molten metal gets behind the plunger 27. The melt can be poured directly from the crucible 54 via inlet 50 into the shot sleeve 24, thereby reducing time and metal cooling before injection can begin.

The plunger 27 then is advanced in the shot sleeve 24 by actuator 25 to inject the molten metal or alloy under hydraulic pressure through the gating system and through the mold pour cup 10a′ and sprue 10c′ into the mold cavity MC′ of each composite mold 50′. The plunger 27 is advanced by a hydraulic system described in copending patent application Ser. No. 11/311,433 incorporated herein by reference to this end.

The molten metal or alloy is forced at velocities, such as 10-120 inches per second for titanium alloys and titanium aluminides, down the shot sleeve 24 and into the evacuated molds 50′.

After the molten metal or alloy is at least partially solidified, typically mostly solidified, in the molds 50′, the fugitive metallic outer mold region 20′ melts and drains away from the inner mold region 10′ to a collection chamber or other region of the die designed to collect the fugitive alloy as result of extraction of heat from the cast and solidified metal or alloy through the inner mold region 10′.

After the molten metal or alloy is at least partially solidified, typically mostly solidified, the members 29, 30 are opened by movement of platen 16 away from platen 14 within a typical time period that can range from 5 to 30 seconds following injection to provide enough time for the molten metal or alloy to form at least a solidified surface on the cast component (s) in the inner mold regions 10′. The metallic material solidified in the inner mold regions 10′ typically is substantially solidified by the time the inner mold regions 10′ are removed from the chamber C. The inner mold regions 10′ and connected solidified runner 60′ then are removed from the chamber C and transported to a demolding station where the inner mold regions 10′ are removed from the cast component by conventional techniques forming no part of the invention. The solidified runner 60′ can be removed before or after the inner mold regions 10′ are removed. FIG. 5 shows an inner mold region 10′ and connected runner 60′ on the right hand side of that figure and turbocharger wheel castings 100′ on the left hand side of that figure after the inner mold regions 10′ have been removed. The castings then can be inspected visually and by techniques according to customer requirements.

In pressure casting titanium alloys, titanium aluminide, nickel base superalloys, and cobalt based superalloys, the shot sleeve 24 contacting the molten metal or alloy can be made of an iron based material, such as H-13 tool steel, or a refractory material such as based on Mo alloy, W alloy, or TZM alloy, ceramic material such as alumina, or combinations thereof that are compatible with the metal or alloy being melted and die cast. The forward plunger tip 27a can comprise a permanent or alternately a disposable tip that is thrown away after each molten metal or alloy charge is injected in the investment mold 50′. A plunger tip can comprise a copper based alloy such as a copper-beryllium alloy, or steel, graphite, or other appropriate material.

The particular casting parameters employed to cast a component will depend upon several factors including mold size, gating, pour weight, and the strength of the composite mold to the melt injection pressures involved. The injection pressure is selected to retain the molds 50′ intact (no mold cracking or bursting under pressure) while achieving a satisfactory fill of the mold cavity regions. The nominal weight of metal or alloy in the crucible 54 depends on the mold size and the number of components to be die cast in the mold.

The following EXAMPLE is offered to further illustrate the invention without limiting it.

EXAMPLE 1

Turbocharger wheels have been successfully pressure cast of gamma titanium aluminide (TiAl) alloys (melting temperature of greater than 2800 F degrees F.) in composite molds of the type shown in FIG. 2B using a casting machine of the type shown in FIG. 3. The composite molds comprised an inner mold region made of four dips of a wax pattern in aluminia and silica based ceramic slurry followed by stuccoing and air drying to provide a wall thickness of about 0.075 inch. An outer mold region of tin was applied to the inner mold region by casting to a thickness of greater than 0.1 inch such that the molds had a shape similar to that shown in FIG. 2B.

In general, the turbocharger wheels were made using casting parameters in the following ranges: melt injection pressure settings: 400-1800 psi, melt injection velocities (plunger speed): 10-120 in/sec; TiAl melt superheat: 0 to 75 degrees F.; no mold preheat; shot sleeve length and diameter: 17.38 inches and 2.80 inches; and limit switch 80a set to dump plunger fluid pressure when the plunger 27 carrying the switch actuator 80b is about 0.75 inch from its final injection position. The melted tin previously forming the outer backup mold region melted and drained from the inner mold regions by gravity for reuse.

Referring to FIGS. 6-7, for purposes of further illustration and not limitation, a thin-wall ceramic inner shell mold region 10″ is shown formed by the “lost wax” process to include a pour cup 10a″ having a solid collar 10b″, a sprue or runner 10c″, and a mold section 10d″ for use in a different casting machine shown in FIG. 8.

In FIGS. 6-7, the mold section 10d″ has a turbocharger wheel-shape having a plurality of vane-shaped mold sections 10e″ disposed about the periphery a central hub mold section 10f″. The mold section 10d″ has a thin wall 10w″ with a relatively small wall thickness as described above to define mold cavity MC″ in the mold sections 10e″ and 10f″. The pour cup 10a″, collar 10b″, and sprue or runner 10″ can have a larger wall thickness than that of the mold section 10d″. The larger wall thickness of the pour cup 10a″, collar 10b″, and sprue or runner 10c″ is provided by additional dips of ceramic during the shell build process.

For purposes of illustration and not limitation, the wall thickness of the mold section 10d″ typically is in the range described above for the embodiment of FIGS. 1-2A, 2B. The ceramic shell mold 10″ can be formed of one or more appropriate ceramic or refractory materials, or combinations thereof, such as including but not limited to zirconia, silica, alumina, graphite, and other ceramic materials selected in dependence on the particular molten metal or alloy to be cast as describe above.

After the ceramic inner shell mold region 10″ is formed, the fugitive metallic outer backup mold region 20″ is formed thereon. One illustrative embodiment of the invention shown in FIGS. 6-7 involves placing a portion of the sprue or runner 10c″ and the mold section 10d″ of the shell mold region 10″ in a cylindrical or other shaped metal (e.g. aluminum) pan 31″ and introducing molten backup material (e.g. molten tin) in the mold cavity MM″ defined between the inner mold region 10″ and the pan 31″ to a level just below the upper lip 31a″ of the pan 31″ and solidifying the molten backup material in-situ on the inner shell mold region 10″.

The composite mold 50″ pursuant to this illustrative embodiment of the invention thereby comprises the inner mold region 10″ and the fugitive metallic outer backup mold region 20″ on the inner mold region with or without the pan 31″, FIG. 7.

The outer backup mold region 20″ is relatively thick compared to the inner mold region 10″ for the reasons described above in connection with FIG. 1-2A, 2B. For purposes of illustration and not limitation, the wall thickness of the outer backup mold region 20″ typically is in the range of 0.25 to 1.0 inch when applied on the above-described thin wall inner mold region 10″ made by the “lost wax” process, although the thickness of the outer backup mold region 20″ may vary from location to location depending on the exterior configuration of the inner mold region 10″. The invention is not limited to any particular thickness of the outer backup mold region 20″ as described above.

FIG. 8 illustrates placement of the composite mold 50″ pursuant to the another illustrative embodiment of the invention in a modified die casting machine. The die casting machine of FIG. 8 is similar to that shown in FIG. 3 such that like features of the casting machines bear like reference numerals. The casting machines are similar with the exception that the casting machine of FIG. 8 differs in having a metal (e.g. steel) gas impermeable container 60″ between platens 14, 16 for receiving the composite mold 50″. The container 60″ comprises a tubular body 60a″ which can be circular, square or any other cross-sectional shape. The container 60″ includes a welded-on end closure 62″. The container 60″ also includes a removable end closure 64″ fastened to welded-on annular flange 66″ by fasteners 67″ in a manner to define an internal container chamber 68″ in which the mold 50″ is received for casting with the space about the mold 50″ filled with refractory particulates 71″. The removable end closure 64″ engages an O-ring vacuum seal S1″for establishing a vacuum tight seal between the end closure 64″ and the annular flange 66″ of the container 60″ when the end closure 64″ is fastened on the container using fasteners 67″. An O-ring vacuum seal S2″ is provided between the end closure 62″ and the base plate 30 for establishing a vacuum tight seal therebetween when the container 60″ is abutted to the base plate 30 as will be described below. The container 60″ is not permanently fastened to the base plate 30. The vacuum seals S1″, S2″ may comprise Viton material or other suitable high temperature sealing material.

The container end closure 62″ includes a passage that is adapted to receive the shot sleeve 24 in flow relationship to the pour cup 10a″ of the composite mold 50″ when the mold 50″ is clamped in position in the container 60″. In particular, the container 60″ includes internally thereof a plurality of elongated clamping fingers 60f″ that are tightened over the surface of the collar 10b″ to clamp the mold 50″ in fixed position against end closure 62″ in the chamber 68″. The clamp fingers 60f″ are tightened using fasteners 70″ which are threaded into the end closure 62″.

When so clamped, the pour cup 10a″ and the sprue 10c″ of the composite mold 50″ communicates to the shot sleeve 24 for receiving molten metallic material from the shot sleeve 24 as pushed by the plunger 27. The shot sleeve 24 is sealingly received in passage of end closure 62″. The collar 10b″ seals against leakage of molten metallic material from the shot sleeve and the mold pour cup. The container 60″ includes nipples 60n″ to permit loose (free flowing) refractory particulates 71″, such as alumina or zirconia ceramic back-up sand, about the mold 50″ in the container 60″. The nipples 60n″ then are closed by fittings to prevent the loose particulates 71″ from falling out. The container 60″ also includes hoist rings 60r″ to allow use of a conventional overhead hoist to lift and transport the container 60″ for mold loading and unloading purposes.

In practicing a method embodiment of the invention, a solid ingot of the metallic material to be die cast is charged into the crucible 54 in the vacuum melting chamber 40. The container 60″ with the composite mold 50″ therein is held between the base plate 30 and the platen 16 by movement of platen 16 relative to platen 14. The container 60″ is filled with the loose back-up particulates 71″ via nipples 60n″ before the container 60″ is held in position relative to the fixed platen 14 by movement of the movable platen 16. The loose particulates 71″ are introduced manually or by machine through the nipples 60n″ into the chamber 68″ and about the mold 50″.

The vacuum chamber 40 then is evacuated to a suitable level for melting the particular charge (e.g. less than 100 microns for titanium alloys such as Ti-6A1-4V alloy and titanium aluminide such as TiAl) by vacuum pump P. The container 60″ with the composite mold 50″ therein held between the base plate 30 and the platen 16 is concurrently evacuated to 'the same vacuum level through the connection to the vacuum melting chamber 40 via the shot sleeve 24 and by virtue of being isolated from surrounding ambient air atmosphere by the container vacuum seals S1″, S2″.

The composite mold 50″ typically is at ambient (room) temperature when it is placed in the container 60″. Alternately, the mold 50″ can be preheated to a suitable elevated temperature before being placed in the container 60″ or while it is in the container using cartridge or resistance heaters.

The solid charge of the metal or alloy in crucible 54 is melted by energizing induction coil 56, the melt then is poured under vacuum into the shot sleeve 24 via the melt inlet 50 with the plunger 27 initially positioned at the start injection position of FIG. 8. The molten metal or alloy is poured into the shot sleeve 24 and resides therein for a preselected dwell time to insure that no molten metal gets behind the plunger 27. The melt can be poured directly from the crucible 54 via inlet 50 into the shot sleeve 24, thereby reducing time and metal cooling before injection can begin.

The plunger 27 then is advanced in the shot sleeve 24 by actuator 25 to inject the molten metal or alloy under hydraulic pressure through the gating system and through the mold pour cup 10a″ and sprue 10c″ into the mold cavity MC″ of the composite mold 50″. The plunger 27 is advanced by a hydraulic system described in copending patent application Ser. No. 11/311,910 incorporated herein by reference to this end.

The molten metal or alloy is forced at velocities, such as 10-120 inches per second for titanium alloys and titanium aluminides, down the shot sleeve 24 and into the evacuated molds 50′.

After the molten metal or alloy is at least partially solidified, typically mostly solidified, in the mold 50″, the fugitive metallic outer mold region 20″ melts and drains away from the inner mold region 10″ to the lower region of the steel container as result of extraction of heat from the cast and solidified metal or alloy through the inner mold region 10″.

After the molten metal or alloy has been at least partially solidified, the base plate 30 and platen 16 are opened by movement of platen 16 away from platen 14 within a typical time period that can range from 5 to 30 seconds following injection to provide enough time for the molten metal or alloy to form at least a solidified surface on the cast component(s) in the mold 50″.

The container 60″ then is removed from the base plate 30 and transported by a hoist's engaging hoist rings 60r″ to an unloading station where the back-up particulates 71″ are removed by opening nipples 60n″. Then, the end closure 64″ is removed so that the inner mold region 10″ can be unclamped and removed from the container 60″. The metallic material solidified in the inner mold region 10″ typically is substantially solidified by the time the inner mold region 10″ is removed from the container. The inner mold region 10″ then is removed from the cast components by conventional techniques forming no part of the invention. The castings then can be inspected visually and by techniques according to customer requirements.

In die casting titanium alloys, titanium aluminide, nickel base superalloys, and cobalt based superalloys, the shot sleeve 24 and plunger tip 27a contacting the molten metal can be made of any appropriate material as described above.

Moreover, the particular casting parameters employed to cast a component will depend upon' several factors including mold size, gating, pour weight; and the fragility of the investment mold to the melt injection pressures involved. The injection pressure is selected to retain the composite mold 50″ intact (no mold cracking or bursting under pressure) while achieving a satisfactory fill of the mold cavity regions. The nominal weight of metal or alloy in the crucible 54 depends on the mold size and the number of components to be die cast in the mold.

The following EXAMPLE 'is offered to further illustrate the invention without limiting it.

EXAMPLE 2

Turbocharger wheels have been successfully die cast of titanium and titanium aluminide (TiAl) alloys in composite molds of the type shown in FIG. 7 using a casting machine of the type shown in FIG. 8. The composite mold comprised an inner mold region made of four dips of a wax pattern in a ceramic slurry followed by air drying and stuccoing to provide a wall thickness of about 0.075 inch. An outer mold region of tin was applied to the inner mold region by casting as described above to a thickness of greater than 0.2 inch such that the molds had a shape similar to that shown in FIG. 7.

In general, the turbocharger wheels were made using casting parameters in the following ranges: melt injection pressure settings: 400-1800 psi; melt injection velocities (plunger speed): 10-120 in/sec; melt superheat: 0 to 75 degrees-F.; no mold preheat; shot sleeve length and diameter: 15 inches and 3 inches; and limit switch 80a set to dump plunger fluid pressure when the plunger 27 is about 0.5 inch from its final injection position. The melted tin previously forming the outer backup mold region melted and drained from the inner mold regions by gravity for reuse.

Although the composite molds are illustrated above for making cast turbocharger wheels, the invention is not so limited and can practiced to make other components that include, but are not limited to, internal combustion engine valves, automotive or truck turbocharger compressor and turbine wheels, compressor and turbine blades and vanes for gas turbine engines, and medical components including hip stems, acetabular knees, tibial trays, and spinal components.

Claims

1. Composite mold for casting a metal or alloy, comprising an inner mold region having a mold cavity and a fugitive metallic outer backup mold region residing on the inner mold region, wherein metallic material of the backup mold region has such a melting temperature that the backup mold region melts from the inner mold region after the molten metal or alloy is cast and at least partially solidified.

2. The mold of claim 1 wherein the inner mold region comprises a shell mold.

3. The mold of claim 2 wherein the shell mold has a wall thickness of about 0.1 inch or less.

4. The mold of claim 1 wherein the backup mold region comprises a metal or alloy that melts at a temperature of about 1300 degrees F. or below.

5. The mold of claim 4 wherein the backup mold region comprises a metallic material comprising tin, indium, bismuth, lead, aluminum, or zinc, or alloys thereof one with another or with other metals.

6. The mold of claim 1 wherein the backup mold region comprises a metal or alloy that melts at a temperature of at least 500 degrees F. below that of the metal or alloy to be cast in the mold.

7. The mold of claim 1 wherein the backup mold region is cast and solidified in-situ on the inner mold region.

8. Composite mold for casting a metal or alloy, comprising a ceramic inner shell mold region having a mold cavity and a fugitive metallic outer backup mold region residing on the inner shell mold region, wherein metallic material of the backup mold region has such a melting temperature less than about 1300 degrees F.

9. The mold of claim 8 wherein the inner shell mold region has a wall thickness of about 0.1 inch or less.

10. The mold of claim 8 wherein the backup mold region comprises a metallic material comprising tin, indium, bismuth, lead, aluminum, or zinc, or alloys thereof one with another or with other metals.

11. The mold of claim 8 wherein the backup mold region is cast and solidified in-situ on the inner mold region.

12. Method of casting, comprising introducing a molten metal or alloy into a mold cavity of a composite mold having an inner mold region that includes said mold cavity and a fugitive metallic outer backup mold region residing on the inner mold region, and melting the backup mold region from the inner mold region after the molten metal or alloy is cast and at least partially solidified.

13. The method of claim 12 wherein the inner mold region comprises a shell mold.

14. The method of claim 13 wherein the shell mold has a wall thickness of about 0.1 inch or less.

15. The method of claim 12 wherein the backup mold region comprises a metal or alloy that melts at a temperature of about 1300 degrees F. or below.

16. The method of claim 15 wherein the backup mold region comprises a metallic material comprising tin, indium, bismuth, lead, aluminum, or zinc, or alloys thereof one with another or with other metals.

17. The method of claim 12 including casting and solidifying the backup mold region in-situ on the inner mold region before introducing the molten metal or alloy.

18. The method of claim 12 wherein the molten metal or alloy is introduced under pressure into the mold cavity.

19. The method of claim 18 wherein the molten metal or alloy is die cast into the mold cavity.

20. The method of claim 12 wherein the molten metal or alloy is gravity cast into the mold cavity.

21. Method of casting titanium aluminide, comprising introducing molten titanium aluminide into a mold cavity of a composite mold having a ceramic inner shell mold region that includes said mold cavity and a fugitive metallic outer backup mold region residing on the inner shell mold region, and melting the backup mold region away from the inner shell mold region after the molten titanium aluminide is cast and at least partially solidified.

22. The method of claim 21 wherein the inner shell mold has a wall thickness of about 0.1 inch or less.

23. The method of claim 23 wherein the backup mold region comprises a metal or alloy that melts at a temperature of about 1300 degrees F. or below.

24. The method of claim 23 wherein the backup mold region comprises a metallic material comprising tin, indium, bismuth, lead, aluminum, or zinc, or alloys thereof one with another or with other metals.

25. Method of making a casting mold, comprising forming a non-metallic refractory or ceramic inner shell mold region and forming a fugitive metallic outer backup mold region in-situ on the inner shell mold region.

26. The method of claim 25 wherein the backup mold region is cast and solidified in-situ on the inner mold region.