EP1031391A1

EP1031391A1 - Anti-swirl mold pour cup and casting method

Info

Publication number: EP1031391A1
Application number: EP00103502A
Authority: EP
Inventors: John D. Spurlock; Marc S. Hall; Christopher T. Keating; Richard K. Foran
Original assignee: Howmet Research Corp
Current assignee: Howmet Corp
Priority date: 1999-02-22
Filing date: 2000-02-18
Publication date: 2000-08-30
Also published as: JP2000237840A

Abstract

Casting mold (40) having a pour cup (30) thereon wherein the pour cup includes a receptacle for receiving a free, unconfined stream (S) of molten metal from a pouring vessel (15) and a plurality of anti-swirl ribs (33) circumferentially spaced apart about the periphery of the receptacle and extending toward a discharge opening (30b) to the mold (40) in a manner to reduce swirling of molten metal due to misalignment between the mold pour cup (30) and the pouring vessel (15).

Description

FIELD OF THE INVENTION

The present invention relates to casting of molten metal into a casting mold.

BACKGROUND OF THE INVENTION

In the conduct of a production vacuum casting campaign to produce a plurality of metal castings, ceramic investment shell molds are preheated to a suitable elevated mold preheat temperature for casting in a mold heating furnace separate from a vacuum casting chamber. The vacuum chamber includes a melting chamber where a melting/pouring crucible is disposed and a mold-receiving chamber below the melting chamber. The melting chamber can be separated or isolated from the mold-receiving chamber by a suitable isolation valve, such as sliding gate valve, that allows a vacuum to be maintained in the melting chamber. An individual charge of metal, such as an individual ingot, is melted under vacuum (subambient pressure) in the crucible in the melting chamber above the mold-receiving chamber. Then the charge is determined to be at an appropriate casting temperature, an operator calls for a preheated mold to be removed from the mold heating furnace and positioned in the vacuum chamber for casting. For example, a mold handler manually removes a preheated mold from the mold heating furnace and manually positions the preheated mold on a mold pan beneath the melting chamber sealed by the closed isolation valve. After the isolation valve to the melting chamber is opened, the preheated mold is raised by an elevator under the mold pan to a preselected height in the melting chamber below the crucible. The crucible then is pivoted in a manner to pour the molten metal as a free molten metal stream into a frusto-conical pour cup of the preheated mold positioned therebelow in the melting chamber. The pour cup has a simple frusto-conical receptacle to receive the stream from the crucible and direct it into the mold to fill same. After filling of the mold with molten metal, the mold is lowered on the mold pan by the elevator into the mold-receiving chamber, and the isolation valve is closed. The melt filled mold can remain in the mold-receiving chamber or removed therefrom for solidification of the molten metal therein. This cycle is repeated to cast a plurality of preheated molds one a time over the casting campaign.
In such casting campaigns, the manual placement or positioning of the preheated mold on the mold pan in the vacuum chamber results in variations in mold alignment relative to the crucible in the melting chamber. This alignment variation from one mold to the next typically is greatest at the beginning of a casting campaign and also when there is a change in the mold used and/or product (casting) being produced. As molds are cast in campaign, adjustments can be made by the mold handler until a near optimum preheated mold position is consistently achieved during the remainder of the campaign. However, as "just in time" manufacturing procedures are adopted, more numerous, shorter run casting campaigns are being used such that the problem of misalignment of molds relative to the melting crucible becomes more troublesome.
The alignment variation from one mold to the next during a casting campaign adversely affects the alignment of the molten metal stream poured from the crucible into the mold pour cup of respective molds. For example, such misalignment produces melt splashing because the pour cup backs up with molten metal due to the misalignment. Short pours and lower mold gating efficiency are observed as a result. Moreover, such misalignment produces molten metal swirling in the pour cup that increases dwell time of the melt in the relatively cooler mold pour cup, producing a loss of thermal energy and resultant cold shuts and chill grain defects in the casting, and that decreases axial momentum of the molten metal stream, producing variability in mold filling time and decreased laminar metal flow and increased metal turbulence within the mold. Turbulence within the mold can cause the flowing molten metal to form eddies, reducing metal pressure and velocity, and result in misrun in thin sections of the mold as well as oxide formation that leads to dross formation. Variations in the mold filling patterns from one mold to the next can increase porosity in castings produced during a particular casting campaign.
There thus is a need for a casting mold and method that overcome the above problems associated with variations in alignment from one mold to the next relative to the crucible in the melting chamber of the vacuum casting chamber.
An object of the present invention is to satisfy this need.

SUMMARY OF THE INVENTION

The present invention provides in one embodiment a casting mold having a pour cup thereon configured to reduce the above adverse effects of misalignment between the mold and a pouring vessel, such as a melting/pouring crucible. To this end, the pour cup comprises a wall defining a converging receptacle for receiving a free stream of molten metal poured from a pouring vessel and a lower opening through which the molten metal can flow out of the pour cup into the casting mold. The pour cup wall includes a plurality of anti-swirl ribs circumferentially spaced apart about the periphery of the receptacle and extending toward the opening in a manner to reduce swirling of molten metal poured in the pour cup as a result of misalignment between the mold pour cup and pouring vessel.
In a method embodiment of the invention for pouring molten metal from a pouring vessel into a mold during vacuum casting, inert gas casting, or air melt casting, the above described pour cup of a casting mold is positioned beneath the pouring vessel, a charge of metal is melted or otherwise provided in a pouring vessel, and a free molten metal stream of the melted charge is poured from the pouring vessel into the pour cup such that a misaligned stream of molten metal is redirected or deflected by the anti-swirl ribs toward the center of the pour cup to improve filling of the mold.
The above and other objects and advantages of the present invention will become more readily apparent from the following drawings taken in conjunction with the following detailed description.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a elevation view of a casting mold pursuant to an embodiment of the invention shown disposed on a mold pan in a vacuum chamber below a melting crucible in a melting chamber.
Figure 2 is plan view of a mold pour cup pursuant to an embodiment of the invention.
Figure 3 is diametral sectional view of the mold pour cup of Figure 2.
Figure 4 is a perspective view of a casting mold pursuant to an embodiment of the invention misaligned beneath a melting crucible illustrating the molten metal stream poured into the mold pour cup and redirected by anti-swirl ribs.
Figure 5 is a sectional view of a ceramic shell mold having a mold anti-swirl pour cup and mold sprue having anti-swirl ribs.

DESCRIPTION OF THE INVENTION

Figures 1 to 4 illustrate schematically vacuum casting apparatus for a effecting a casting campaign to produce a plurality of metal castings. A vacuum casting chamber 10 is shown including a melting chamber 12 where a melting/pouring crucible 15 is disposed and a mold-receiving chamber 14 below the melting chamber. The melting chamber 12 is separated or isolated from the mold-receiving chamber 14 by a suitable isolation valve 16, such as sliding gate valve, that allows a vacuum to be maintained in the melting chamber by a vacuum pump P1 via vacuum pump conduit 18a communicated to the melting chamber. An individual charge of metal, such as an individual ingot I, is melted under vacuum (subambient pressure) in the vertically oriented melting/pouring crucible 15 in the melting chamber 12 above the mold-receiving chamber 14. The ingot I can be placed in the crucible 15 through sealable access port 13. When the charge is melted and determined to be at an appropriate temperature for casting, an operator calls for a preheated casting mold 20 to be removed from an adjacent separate mold heating furnace 22 and positioned on a mold pan 24 via opening of a vacuum tight door 17 of the vacuum chamber 10. For example, a mold handler manually removes a preheated mold 20 from the mold heating furnace and manually positions the preheated mold 20 on the mold pan 24 disposed beneath the melting chamber 12 sealed by the closed isolation valve 16. The vacuum tight door 17 of the vacuum chamber 10 then is closed. The mold receiving chamber 14 then is evacuated by a vacuum pump P2 via vacuum pump conduit 18b. The preheated mold 20 on the mold pan 24 is raised by an elevator 26 under the mold pan after the isolation valve 16 is opened to a preselected height in the melting chamber 12 below the melting/pouring crucible 15 as shown in dashed lines in Figure 1. The crucible 15 then is pivoted about pivot 15a by an electric servomotor or other crucible pivoting actuator to a pour position shown in dashed lines in a manner to pour the molten metal into a frusto-conical pour cup 30 of the preheated mold 20 positioned therebelow in the melting chamber 12 as a free, unconfined stream S of molten metal, Figure 4. After filling of the preheated mold 20 with molten metal, the melt filled mold is lowered on the mold pan 24 by the elevator 26 into the mold-receiving chamber 14. The isolation valve 16 then is closed. The melt filled mold 20 can remain in the mold-receiving chamber 14 or removed therefrom for solidification of the molten metal therein. This cycle is repeated to cast a plurality of preheated molds 20 one at a time over the casting campaign.
Each casting mold 20 includes frusto-conical pour cup 30 thereon. The casting mold 20 typically comprises a shell mold 40 formed by the well known "lost wax" process where a fugitive pattern (e.g. wax) (not shown) of the casting to be made is repeatedly dipped in ceramic slurry, excess slurry is drained, and the slurry is stuccoed with ceramic stucco until a desired mold thickness is built up. The pattern is removed to leave the shell mold 40, which is fired at elevated temperature to impart suitable mold strength for casting. The pour cup 30 typically is preformed by conventional cold pressing a suitable ceramic material suited to the molten metal being cast and a binder with the cold pressed cup being fired at elevated temperature to develop pour cup strength. The pour cup 30 typically is attached to a conventional wax pattern assembly so as to become part of the shell mold 20 during the conventional lost wax shell investment process where the pattern assembly is invested in a ceramic shell mold material. In casting nickel or cobalt based superalloys, the pour cup 30 can be made of mullite or other suitable ceramic material.
In accordance with an embodiment of the invention, the mold pour cup 30 comprises a wall 30a defining a downwardly converging receptacle 31 for receiving the free stream S of molten metal poured from the melting/pouring crucible 15 (or other pouring vessel) and a lower frusto-conical opening 30b through which the molten metal can flow out of the pour cup into the shell mold 40, which includes a sprue 40a and one or more mold cavities 40b, Figure 5, connected to the sprue 40a so as to receive molten metal therefrom.
The pour cup wall 30a defines an uppermost uninterrupted smooth pour cup rim 30c and includes below the rim 30c a plurality of elongated anti-swirl ribs 33 circumferentially spaced apart about the periphery of the receptacle 31 below rim 30c and extending toward the opening 30b in a manner to redirect or deflect a misaligned free molten metal stream S, Figure 4, poured from the crucible 15 toward the center of the receptacle 31 and thereby reduce swirling of molten metal poured in the pour cup 30. The anti-swirl ribs 33 typically are formed integrally with the other pour cup features during molding of the pour cup 30.
The anti-swirl ribs 33 each have a first inner surface 33a and second inner surface 33b provided with compound angles. For example, the first surface 33a of each rib 33 begins at and is blended smoothly with the smooth pour cup rim 30c and converges downwardly toward a central longitudinal axis A of the pour cup 30 at a greater angle than the second surface 33b. The compound angle shown on each anti-swirl rib 33 provides maximum anti-swirl effectiveness with use of minimum rib material. The second surface 33b is blended smoothly into the opening 30b as best shown in Figure 4 to provide smooth molten metal flow through the opening 30b.
The anti-swirl ribs 33 are illustrated as being spaced apart 90 degrees, although the invention is not so limited since a greater or lesser number of anti-swirl ribs 33 may be used with different circumferential spacing therebetween.
For purposes of further illustration and not limitation of the invention, a particular pour cup 30 for use in practicing the invention to gravity cast a nickel base superallay includes uppermost smooth rim 30c with an upper diameter of 5.5 inches and lower diameter of 4.3 inches and converging taper of 30 degrees. The rim 30c transitions to ribbed receptacle 30d having a converging taper angle of 20 degrees. The ribbed receptacle 30d transitions to frusto-conical discharge opening 30b having anti-swirl ribs 33 and a lowermost diameter of 2.4 inches. The anti-swirl ribs 33 have a width in the circumferential direction of 0.3 inch and first upper converging taper angle of 30 degrees and second lower converging taper angle of 5 degrees that extends into the opening 30b. All of the converging taper angles are with respect to the central longitudinal axis A of the pour cup 30 in Figure 3 for example. Pour cup 30 includes outer annular groove 30e to receive mold material, Figure 5.
In a method embodiment of the invention, a mold handler manually removes a preheated mold 20 from the mold heating furnace 22 and manually positions the preheated mold 20 on the mold pan 24 disposed beneath the melting chamber 12 sealed by the closed isolation valve 16. The vacuum tight door 17 of the vacuum chamber 10 then is closed. The preheated mold 20 on the mold pan 24 is raised by elevator 26 under the mold pan 24 after the isolation valve 16 is opened to a preselected mold height in the melting chamber 12 below the melting/pouring crucible 15. The crucible 15 then is pivoted about pivot 15a to the dashed line position of Figure 1 to gravity pour the molten metal charge into the pour cup 30 of the preheated mold 20 as the free, unconfined stream S of molten metal. In the event the mold handler has inadvertently positioned the preheated mold 20 on the mold pan 24 out of alignment with the melting/pouring crucible 15, the free stream S of molten metal will not be poured into the center of the pour cup 30 and instead will impinge slightly laterally offset of the center of the pour cup, for example as illustrated in Figure 4. In such a situation, the pour cup anti-swirl ribs 33 will immediately redirect or deflect the misaligned free molten metal stream S, Figure 4, impinging on the pour cup 30 toward the center thereof in a manner to reduce swirling of molten metal in the pour cup 30 and reduce molten metal splashing in the pour cup since back up of molten metal is reduced. The anti-swirl ribs 33 substantially reduce molten metal swirling in the pour cup 30 in a manner that decreases dwell time of the melt in the cooler mold pour cup 30 and that increases axial momentum of the molten metal stream to provide improved laminar metal flow and decreased metal turbulence within the mold. A shown in Figure 5, the invention also envisions providing anti-swirl ribs 33' that can extend down the walls W of the sprue 40a of the mold 40 having mold cavities 40b to continue the effects of improved laminar metal flow for the reduction or elimination of misruns. The sprue 40a communicates to the pour cup 30 and to the mold cavities 40b to convey molten metal to the mold cavities. The sprue ribs 33' can extend from the pour cup ribs 33 typically in general axial registry therewith to lateral runners 25 that feed melt to the mold cavities, although the invention is not limited to such registry.
The invention overcomes the above discussed problems associated with variations in alignment of the molds 20 from one mold to the next relative to the crucible 15 in the melting chamber 12 during a casting campaign.
Although the invention has been described in detail above with respect to certain embodiments, those skilled in the art will appreciate that modifications, changes and the like can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

Casting mold and a pour cup on the casting mold, said pour cup comprising a wall defining a converging receptacle for receiving a free stream of molten metal poured from a pouring vessel and a lower opening through which the molten metal can flow out of said pour cup into the casting mold, said wall having a plurality of anti-swirl ribs circumferentially spaced apart about the periphery of the receptacle and extending toward said opening in a manner to reduce swirling of molten metal in the pour cup due to misalignment between the mold pour cup and the pouring vessel.
The combination of claim 1 wherein said wall defines an uppermost uninterrupted smooth rim above said ribs.
The combination of claim 1 wherein said ribs each have a first inner surface and second inner surface, said first surface converging toward a central longitudinal axis of said pour cup at a greater angle than the second surface.
The combination of claim 1 wherein the mold includes a sprue wall having a plurality of anti-swirl ribs circumferentially spaced apart about the periphery of the sprue wall.
Investment casting shell mold and a pour cup on the casting mold, said pour cup comprising a wall defining a converging receptacle for receiving a free stream of molten metal from a pouring vessel and a lower opening through which the molten metal can flow out of said pour cup into the casting mold, said wall defining an uppermost uninterrupted smooth rim above a plurality of anti-swirl ribs circumferentially spaced apart about the periphery of the receptacle and extending toward said opening to reduce swirling of molten metal in the pour cup due to misalignment between the mold pour cup and the pouring vessel.
The mold of claim 5 wherein said ribs each have a first inner surface and second inner surface, said first surface converging toward a central longitudinal axis of said pour cup at a greater angle than the second surface.
Method of casting molten metal from a pouring vessel into a mold, comprising:

positioning a pour cup of a casting mold relative to the pouring vessel,

providing a charge of molten metal in the pouring vessel, and

pouring a free, unconfined stream of the molten charge from the pouring vessel into a pour cup receptacle, including redirecting the stream by contact with a plurality of anti-swirl ribs circumferentially spaced apart about the periphery of the pour cup receptacle and extending toward a lower pour cup opening into the mold so as to reduce swirling of molten metal in the pour cup due to misalignment between the pouring vessel and mold pour cup.
The method of claim 7 including manually positioning the casting mold on a mold pan and raising the casting mold on the mold pan to a position beneath the pouring vessel.
The method of claim 7 wherein the charge is melted under vacuum in the pouring vessel.
The method of claim 7 including directing flow of the stream by contact with a plurality of anti-swirl ribs circumferentially spaced apart about the periphery of a mold sprue wall.