Patent Application
20070137827
The present invention relates to casting of metals and alloys and, more particularly, to casting of metals, alloys, and intermetallics under pressure in a non-metallic investment mold.
Titanium based alloys (e.g. Ti-6Al-4V) and intermetallics (e.g. TiAl) are used as cast components in the aerospace industry. Many such cast components are made by the well known gravity investment casting process wherein an appropriate melt is cast into a preheated ceramic investment shell mold formed by the lost wax process.
Although commonly used, investment casting of complex shaped components of such titanium based materials can be characterized by relatively high costs and low yields. Low casting yields are attributable to several factors, in particular inadequate filling of certain mold cavity regions such as especially thin wall mold cavity regions. For example, filling of thin wall mold cavity regions having a thickness of less that 0.050-0.060 inch with a titanium alloy or titanium aluminide melt is difficult due to melt fluidity, low superheat, and out-gassing problems.
In attempts to improve filling of the ceramic investment shell mold, high mold preheat temperatures (e.g. above 700 degrees F.) have been tried. However, such an approach is disadvantageous in that the molten titanium alloy or titanium aluminide can react with the mold at such high temperatures to form deleterious phases on the surface of the castings produced in such shell molds. This phase must then be removed by chemical treatment. Moreover, it is oftentimes necessary to increase the thickness of one or more thin wall mold cavity regions when the mold is being made in order to subsequently achieve satisfactory mold filling during casting. The result is an oversize casting that must then be mechanically and/or chemically treated to reduce cast dimensions to print dimensions for the particular component involved.
U.S. Pat. No. 6,070,643 describes vacuum die casting of oxygen reactive alloys such as titanium alloys, nickel based superalloys, and cobalt superalloys in metal molds. Drawbacks of using metal die casting molds include the high cost of the metal dies and the presence of die parting line between the metal dies, which parting line sometimes limits the complexity of the die cavity and thus the casting that can be die cast. A further drawback of using metal dies when die casting titanium aluminide material is the generation of cracking in the casting as a result of rapid solidification of the melt in the metal mold.
Permanent mold casting of reactive metals and alloys such as titanium and titanium and nickel based alloys using permanent, reusable, multi-part metal molds based on iron and titanium is described in Colvin U.S. Pat. No. 5,287,910. Casting of aluminum, copper, and iron based castings using permanent metal molds is described in U.S. Pat. No. 5,119,865.
An object of the present invention to provide method and apparatus for die casting metals and alloys, especially titanium alloys, titanium aluminides and others, under pressure in an investment mold in a manner that overcomes the above-discussed disadvantages and drawbacks.
The present invention provides method and apparatus for casting a metallic material involving the steps of providing a non-metallic mold in a mold-receiving chamber disposed in at least one of relatively movable first and second members, communicating a mold cavity in the mold to a gating passage disposed in at least one of the first and second metallic members, communicating the gating passage to a shot sleeve, and flowing metallic material under pressure from the shot sleeve to the gating passage and into the non-metallic mold.
In an illustrative embodiment of the invention, the first and second members are relatively movable by being mounted on respective first and second platens of a die casting machine.
In another illustrative embodiment of the invention, the metallic material is flowed by movement of a plunger in the shot sleeve. The plunger is moved in response to hydraulic pressure to flow the metallic material, and the hydraulic pressure is vented to a sump a preselected distance before the end of the injection stoke of the plunger.
Casting apparatus pursuant to another embodiment of the invention comprises relatively movable first and second metallic members at least one of which includes a mold-receiving chamber and at least one of which includes a gating passage for receiving metallic material from a shot sleeve, a non-metallic mold disposed in the chamber in a position to communicate to the gating passage, and means communicated to the shot sleeve for flowing the metallic material through the gating passage into the non-metallic mold.
In a further illustrative embodiment of the invention, a ceramic investment shell mold is held in position in the chamber to communicate to the gating passage, while the mold is supported in the chamber against force of molten metallic material introduced into the mold via the gating passage. For example, a support plate abuts the closed end of the mold and is biased thereagainst. The mold can include an open pour cup or other feature that communicates to the gating passage. The mold also typically includes a sprue passage between the pour cup and the one or more mold cavities to convey molten metallic material thereto. The mold cavities of the shell mold can be evacuated to subambient pressure, such as less than 100 microns, to die cast a reactive metal or alloy such as a titanium alloy, titanium aluminide, nickel base superalloy, cobalt base superalloy, and others. The mold can include a pour cup or other feature that communicates to the gating passage. The mold also typically includes a sprue passage between the pour cup and the one or more mold cavities to convey molten metallic material thereto.
The invention is useful in casting molten metals or alloys, such as titanium alloys and titanium aluminide alloys, that otherwise have difficulty in filling thin wall mold cavity regions using conventional investment casting processes. For example, the invention is useful in casting titanium alloy or titanium aluminide turbocharger compressor and turbine wheels having multiple airfoil vanes of thin wall thickness (e.g. 0.025 to 0.200 inch wall thickness) disposed about a hub. The invention likewise is useful in casting compressor blades and turbine blades having thin-wall airfoils (e.g. 0.025 to 0.35 inch wall thickness.)
Moreover, the invention can be used to cast complex investment molds having backlocks, undercuts, or other features that can not be cast using metal dies. Practice of the invention permits complex mold geometries to be cast, without the need for significant chemical and/or mechanical rework of the die cast component.
Further details and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.
Referring to
An embodiment of the invention will be described below with respect to vacuum casting of titanium alloy turbocharger wheels having multiple airfoil vanes of thin wall thickness disposed about a hub. Each non-metallic investment shell mold 31 includes a mold cavity-forming region 36 that defines a respective turbocharger wheel shape therein. Although only one mold cavity-forming region 36 is shown, each mold 31 can include one or more mold cavity-forming regions 36.
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 extension 14a 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 die 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 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 member 44. The vacuum chamber 40 is evacuated by a conventional vacuum pump P connected to the chamber 40. The base 11 and the vacuum housing wall 42 rest 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
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 of common assignee herewith, the teachings of which patent are incorporated herein by reference.
Pursuant to an embodiment of the invention, the die casting machine 10 is modified or adapted to cast a molten metallic material under hydraulic pressure into one or more evacuated non-metallic (e.g. ceramic) investment shell molds 31, which can be made by the well known lost wax process. Such an investment shell mold 31 is made by repeatedly dipping one or more fugitive patterns (such as wax or plastic patterns) of the component to be cast connected as part of a pattern assembly in a ceramic flour slurry, draining excess ceramic slurry, and applying a coarse ceramic stucco on the wet slurry followed by air or oven drying until a shell mold of desired wall thickness is built up on the patterns. The one or more patterns then are selectively removed by steam autoclaving, flash dewaxing, and other conventional pattern removal techniques, leaving an empty ceramic shell mold with one or more cavities where the one or more patterns formerly resided. The ceramic shell mold then is fired at elevated temperature to develop adequate mold strength for casting. Manufacture of ceramic shell molds using the lost wax process is well known and described in U.S. Pat. Nos. 4,966,225; 5,983,982; 6,749,006 and many others. For purposes of illustration and not limitation, the invention can be practiced using conventional colloidal silica-bonded or sodium silicate-bonded investment shell molds, although other investment shell molds can be used.
The particular ceramic flours and stucco materials from which the investment molds 31 are made depends on the metal or alloy to be die cast in the mold as well as the parameters of casting, such a melt superheat, mold preheat temperature and others.
Referring to
The pour cup 33 forms a first annular lip L1 that is axially spaced form a second annular lip L2 formed as part of each mold to define a peripheral positioning groove G about the pour cup. The invention envisions providing a ceramic core (not shown) in the turbocharger wheel-shaped mold cavity-forming region 36 of each mold 31 in order to produce an internal cavity in the cast turbocharger wheel at selected location(s). The ceramic core can be configured to produce the desired internal cavity in the cast turbocharger wheel, or other casting produced in the mold 31. The invention also envisions providing a reinforcement material or preform, porous or solid, in the mold cavity 36 so as to be incorporated in the cast component.
Referring to
An O-ring vacuum seal S1 is provided between the members 29, 30 for establishing a vacuum tight seal therebetween,
The molds 31 are shown positioned in the chamber C with their pour cups 33 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. In
The end surface of each pour cup 33 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 31 are positioned relative to the runners R using a fixed positioning plate 51 having slots 51a formed between fingers 51b. Each mold 31 is inserted in a respective slot 51a with the adjacent fingers 51b being received in the mold positioning groove G to this end. One half of each mold pour 33 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 33 and the sprue passage 32a of each mold 31 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 31 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 closed end of each mold 31 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 31e of the molds after they are positioned in positioning plate 51. The support plates 60 are pressed gently toward the closed end of the molds by a main shaft 66 connected to a respective hinge 64 and a pressing device 63, 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-6Al-4V alloy and titanium aluminide such as TiAl) by vacuum pump P. The investment mold 31 in chamber C 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 vacuum seal S1 between members 29, 30. Optionally or in addition, the mold 31 can be evacuated using a separate vacuum conduit or line communicated to the mold interior.
The investment mold 31 typically is at ambient (room) temperature when it is placed in the chamber C. Alternately, the mold 31 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 mold 31.
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
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 into the mold cavity 36 via the mold pour cup 33 and sprue 32. 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 mold cavity 36 in the investment mold 31.
The plunger 27 is advanced in the shot sleeve 24 using a hydraulic system shown in
Components of the hydraulic system of
After the molten metal or alloy has been injected, 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 25 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 cavity 36. The mold 31 then is removed from the chamber C and transported to a demolding station where the investment mold 31 is removed from the cast component by conventional techniques forming no part of the invention. The metallic material solidified in the mold cavity 36 typically is substantially solidified by the time the mold 31 is removed from the chamber C. The casting 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 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 31. 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 die 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 investment mold 31 intact (no mold cracking 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.
Turbocharger wheels have been successfully die cast of titanium and titanium aluminide (TiAl) alloys in conventional lost wax investment shell molds 31 by practice of the invention. 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., mold preheat: room temperature to 600 degrees F., lost wax investment shell mold wall thickness: 0.20 to 1.0 inch, 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 is about 0.75 inch from its final injection position.
Although the mold 31 is 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.
Further, while the invention has been described in terms of specific embodiments thereof, it is not intended to be thereto but rather only to the extent set forth in the following claims.
