Patent Grant
9221096
1. Field of the Technology
The present disclosure generally relates to equipment and techniques for centrifugal casting. The present disclosure more specifically relates to equipment and techniques for centrifugal casting of metallic materials.
2. Description of the Background of the Technology
Metallic casting generally includes supplying a volume of molten metallic material to a static or rotating mold and allowing the material to cool to produce a casting shaped by the mold. Castings may be cast in near net form or may be further modified in subsequent forging or machining applications to produce final components. Metallic materials shrink during phase transition from liquid to solid, which may result in castings comprising uncontrolled shrinkage porosity, especially in difficult to cast metallic materials such as, for example, titanium aluminide (TiAl) based alloys and other TiAl materials. Shrinkage porosity is inherent to the fundamental solidification mechanics and may negatively impact microstructure as well as casting yield. In general, minimized internalized porosity may be addressed by processing techniques such as hot isostatic pressing (HIP). However, uncontrolled internal porosity may result in surface distortions affecting surface quality of the casting and increase production costs. Uncontrolled internal porosity may also be exposed when castings are sectioned or separated from casting components. When porosity is surface connected, current processing techniques may be unsuitable for many casting applications. For example, surface treatment techniques designed to fill or enclose porosity may fail to maintain the continuity of the casting, which may detrimentally affect mechanical properties of the cast material. Material removal techniques such as machining to remove external porosity may also reduce casting yield and expose additional porosity.
Conventional casting techniques for casting various metallic materials, such as titanium aluminide based alloys, are incapable of controlling porosity such that the porosity is internalized away from both the surface of a casting and regions of the casting that may be subsequently sectioned. For example, others have described preparation of titanium aluminide sections using a series of static casting and vacuum arc remelting techniques. These static casting techniques, however, create significant porosity, which cannot be removed using HIP. Others have also described centrifugal casting techniques for preparation of titanium aluminide castings that require supplying molten material to the centrifuge before the centrifuge reaches rotational speed. Cooling rate and solidification, however, are difficult to control, as is evident by the requirement of a separate heating method and mold for each cast piece. Although various other centrifugal casting techniques have been reported, none are able to adequately control shrinkage porosity.
Given the drawbacks associated with conventional casting techniques for casting metallic materials, including centrifugal casting techniques, it would be advantageous to develop improved techniques for casting metallic materials.
According to one aspect of the present disclosure, a non-limiting embodiment of a centrifugal casting apparatus comprises a rotatable assembly configured to rotate about a rotation axis. The rotatable assembly comprises a sprue chamber positioned about the rotation axis and is structured to receive a supply of molten material. A first gate and a second gate are positioned to receive molten material from the sprue chamber in a general direction of centrifugal force. A first cavity and a second cavity are stacked and are respectively positioned to receive molten material from the first gate and the second gate in the general direction of centrifugal force.
According to another aspect of the present disclosure, an non-limiting embodiment of a centrifugal casting mold comprises a front face configured to receive a supply of molten material, a back face, a first cavity, and a second cavity. The first and second cavities each extend from the front face toward the back face and are defined by a sidewall and a back wall adjacent to the back face of the mold. The first and second cavities are stacked and are configured to receive molten material in a general direction of centrifugal force. The mold is structured to differentially insulate the first and second cavities such that a rate of heat extraction from the molten material is greater at the back walls than a rate of heat extraction at the sidewalls to promote directional solidification from the back wall generally toward the general direction of centrifugal force.
According to another aspect of the present disclosure, an non-limiting embodiment of a permanent centrifugal casting mold comprises a front face configured to receive a supply of molten material, a back face, and a first cavity extending from the front face toward the back face. The first cavity is defined by a sidewall and a back wall adjacent to the back face of the mold. A first gate defined in the mold is positioned between the front face and the first cavity.
According to another aspect of the present disclosure, a centrifugal casting method of producing a casting of a metallic material comprises positioning a rotatable assembly comprising a plurality of gates and a plurality of cavities positioned about a sprue chamber such that the plurality of gates and the plurality of cavities are positioned to receive molten metallic material from the sprue chamber in a general direction of centrifugal force. Each of the plurality of gates is coupled to one of the plurality of cavities and at least two of the plurality of cavities are stacked. The method further comprises rotating the rotatable assembly. The method further comprises delivering a supply of molten metallic material to a sprue chamber.
According to another aspect of the present disclosure, a method of assembling a centrifugal casting apparatus comprises positioning a wedge on a rotatable axis. The method also includes positioning at least two molds into sealing engagement with the wedge where each of the at least two molds comprises a front face and defines at least two cavities extending from the front face into the mold. The method further includes defining a sprue chamber structured to receive molten material where least a portion of the sprue chamber is defined by at least a portion of the front faces of the at least two molds.
Features and advantages of the apparatus and methods described herein may be better understood by reference to the accompanying drawings in which:
The reader will appreciate the foregoing details, as well as others, upon considering the following detailed description of certain non-limiting embodiments of apparatuses and methods according to the present disclosure. The reader also may comprehend certain of such additional details upon carrying out or using the apparatuses and methods described herein.
Metallic materials may generally include one or more metal elements, and in some cases also include one or more non-metal elements. Shrinkage porosity is inherent to the fundamental solidification mechanics of many such metallic materials when cast, which may negatively impact mechanical properties of castings. Present static and centrifugal casting techniques for various metallic materials, e.g., titanium aluminide based alloys, are incapable of controlling porosity in both the surface of a casting and in regions where the casting may be subsequently sectioned.
In various non-limiting embodiments, the present disclosure describes centrifugal casting apparatuses comprising rotatable assemblies and components thereof structured to control shrinkage porosity. For example, centrifugal force may be used to feed molten material, such as molten metallic material, into casting pores, thereby minimizing molten material starvation in the solidifying material. Controlled shrinkage porosity may generally include controlling the amount and/or location of shrinkage porosity within a casting such that it may be removed with subsequent processing. For example, controlled shrinkage porosity may include shrinkage porosity that is internalized, e.g., non-surface connected and/or minimized. In some non-limiting embodiments, shrinkage porosity may be internalized away from particular regions of castings such that the castings may be sectioned and/or removed from casting components or material without exposing internalized porosity to the atmosphere.
According to certain non-limiting embodiments, the disclosed centrifugal casting apparatuses and methods may streamline subsequent processing of various castings and eliminate standard production routes such as those used in investment casting. In contrast to conventional centrifugal casting devices, which often require assembly of sixty or more mold components, certain non-limiting embodiments of the centrifugal casting apparatuses disclosed herein comprise rotatable assemblies that may be assembled from fewer than a typical number of major components, significantly reducing setup time. In various non-limiting embodiments, castings may be heat treated and/or processed by HIP, for example. According to certain non-limiting embodiments, castings produced by the disclosed centrifugal casting apparatuses and methods may be suitable for subsequent use in forging or machining applications to produce final components for jet engines, turbochargers, or various high temperature components, for example.
The apparatuses and methods according to the present disclosure may be used in casting metallic materials. As used herein, metallic materials may comprise metal and metal alloys. Metallic materials include, for example, TiAl materials, which comprise, for example, TiAl based alloys. TiAl based alloys may include one or more alloying elements in addition to titanium and aluminum. In certain non-limiting embodiments, the present apparatuses and methods may be used to cast TiAl materials comprising titanium and about 25.0 to 52.1 atomic percent aluminum or about 14 to 36 weight percent aluminum. The disclosed centrifugal casting apparatuses and methods may be used to produce castings of TiAl materials comprising other percentages of aluminum and other alloying elements, without limitation of the above. It is also to be appreciated that while various non-limiting embodiments and beneficial features may be described herein in terms of TiAl based alloys and other TiAl materials, the disclosed apparatuses and methods are not so limited. Those having skill in the art will recognize that the disclosed apparatuses and methods may find wide application beyond TiAl materials, such as, for example and without limitation, metallic materials that suffer from shrinkage porosity or have other properties or characteristics similar to TiAl materials. While certain non-limiting embodiments may provide significant advantages over conventional casting techniques when applied to TiAl materials, it is to be understood that the apparatuses and methods disclosed herein may also be used to cast other metallic materials without limitation to benefits or advantages over conventional casting techniques.
TiAl materials have traditionally been cast using static investment casting techniques. More recently, various centrifugal casting techniques, including centrifugal investment casting, have been proposed for casting TiAl materials. The above techniques, however, may allow voids to form in deleterious locations within the final cast pieces and therefore may increase production costs, limit mechanical properties, and/or impair structural characteristics of the final cast pieces. These techniques are also limited in both the number of cavities and castings per cavity.
Direct gating differs from indirect gating in that the molten material is fed to the cavity generally in the direction of centrifugal force. Direct gating is not used in conventional centrifugal casting devices because indirect gating may reduce turbulence in the mold.
Referring to
Conventional centrifugal casting gating designs feed molten material to cavities through restricted paths, often including distinct choke points. For example, the diameter or cross-sectional area of the gates 8 in the device 2 shown in
Known centrifugal casting techniques for TiAl materials connect a single gate 8 to a cavity 10 to produce each final cast piece, as shown in
In various non-limiting embodiments, the rotatable assembly 12 comprises mold designs which may control the amount and location of shrinkage porosity such that it may be internalized to the material. The internalized porosity may then be removed through subsequent thermo-mechanical processing. In certain non-limiting embodiments, molds may be fabricated from materials comprising metallic materials, such as iron and iron alloys, e.g., steels, including semi-metallic materials such as graphite. According to one non-limiting embodiment, molds fabricated from such materials may comprise permanent casting molds, e.g., generally reusable casting molds. In various non-limiting embodiments, molds fabricated from the above materials may also reduce or eliminate contamination of the cast product by entrapped oxides. For example, molds used in investment casting are typically made of oxides. During casting, however, the oxide particles making up the mold invariably become entrapped in the investment cast product. The entrapped particles may subsequently react with the material of the cast product and provide a potential fatigue initiation site. Investment casting molds may be engineered to be inert to molten TiAl or the particular alloy being cast, and various chemical and machining methods may be available to partially remove the entrapped particles. Nevertheless, particle entrapment is unavoidable and the above stopgaps are not ideal, especially for castings used to fabricate end products intended for service in high temperature, high stress environments, such as turbines. In addition to reducing or eliminating contamination of the final product by entrapped oxides, molds comprising metallic materials may reduce or eliminate risk of contamination of the recycle loop due to entrapped oxides in scrap. For example, as described above, investment castings often include entrapped oxides and, therefore, scrap, e.g., revert, from investment castings may similarly include entrapped oxides. Consequently, products cast using this recycled scrap may also be contaminated with the entrapped oxides. However, scrap from castings produced in molds fabricated from the above metallic materials, do not have a potential for such inclusions and therefore may be recycled without risk associated with contamination of the recycling loop. Consequently, extensive cleaning of scrap before recycling may not be necessary, thereby saving time and reducing costs. Despite the above benefits, it is also contemplated that some embodiments may comprise molds fabricated with other materials. For example, in various non-limiting embodiments, molds may comprise expendable centrifugal casting molds. Such molds may be fabricated from expendable materials such as sand or oxides, for example.
In certain non-limiting embodiments, molds may be structured to control the solidification process by controlling the cooling rate of regions of the molten material. For example, molds may include insulation features configured to limit the amount and/or rate of thermal energy extraction from the molten material. Insulation features may generally comprise structural or material features associated with the mold and may be configured to modify the heat capacity of a region of the mold and/or rate of heat transfer from the molten material to the mold. In one non-limiting embodiment, the rate of heat transfer from the molten material may be at least partially controlled by the shape of the mold. For example, the thickness of one or more regions of the mold may be increased or reduced to increase or reduce the heat capacity of the region. In one non-limiting embodiment, the rate and/or amount of thermal energy that may be extracted by the mold may be controlled by the density or mass of a region of the mold. For example, in various non-limiting embodiments, one or more pockets (see, e.g.,
In various non-limiting embodiments, molds may be structured to control heat extraction from the molten material and, hence, control cooling of the material. For example, as introduced above, in certain non-limiting+embodiments, a mold may comprise insulation features configured to differentially insulate one or more portions of a cavity 18a-18f. Differential insulation features may beneficially modify the rate of cooling along one or more regions of the mold to, for example, control solidification of the molten material. For example, mold regions adjacent to the cavity 18a-18f may be structured such that molten material undergoes directional solidification. In one aspect, molds may be configured to modify cooling such that solidification is directional, e.g., generally toward the sprue chamber 14 or in a direction opposed to the centrifugal force. In this way, the mold may establish a solidification front within the cavity 18a-18f that generally progresses toward the gate 16a-16f and the sprue chamber 14. Thus, the centrifugal force generated by the rotation of the apparatus 12 may generally be opposed to the direction of solidification. For example, in certain non-limiting embodiments, molten material may be supplied to the solidification front to compensate for the shrinkage porosity. Additionally, casting pressure generated by the centrifugal force may force molten metal between dendrites forming near the solidification front to, for example, reduce molten material starvation and minimized shrinkage porosity. Consequently, in various non-limiting embodiments, the disclosed apparatuses and methods may avoid molten material starvation and overcome dendrite exclusion to produce denser castings having reduced shrinkage porosity compared to castings produced by conventional stationary and centrifugal casting techniques.
In various non-limiting embodiments, delivery of the supply of molten metallic material to the cavities 18a-18f is in-line with the cavities and the centrifugal force. For example, in one non-limiting embodiment, the cavities 18a-18f are coupled to the sprue chamber 14 via gates 16a-16f disposed between the sprue chamber 14 and the cavities 18a-18f. Various dimensions of the gates 16a-16f may be larger than corresponding dimensions of the cavities 18a-18f. The gates 16a-16f may further be in-line with both the cavities 18a-18f and the supply of molten metallic material in the sprue chamber 14, e.g., comprising a path generally in-line with the centrifugal force such that molten material may be accelerated toward and into the cavities 18a-18f by the centrifugal force. As a result, the sprue chamber 14 may act as a central riser for all the gates 16a-16f attached to it. In various non-limiting embodiments, this may eliminate the need for additional risers that may or may not be in-line with the cavities. Thus, such synergy between equipment design, volume of molten material, and available casting area may beneficially provide additional space for additional castings. For example, as stated above, multiple pieces may be cast within a single casting cavity 18a-18f.
A containment ring 40 is positioned adjacent to the first end 36 of the sprue chamber 28 and is structured to retain molten material within the sprue chamber 28. For example, in one non-limiting embodiment, the containment ring 40 comprises an extension to the sprue chamber 28, thereby increasing the volume of the sprue chamber 28 and/or the distance molten material must travel to exit the top end of the sprue chamber 28. The containment ring 40 defines a central diameter through which molten material may be supplied to the sprue chamber 28. The central diameter of the containment ring 40 is reduced relative to the diameter of the sprue chamber 40 such that the containment ring 28 forms an internal overhang 42 within the sprue chamber 28 to improve containment of the molten material. For example, in various non-limiting embodiments, the containment ring 40 may limit molten material from splashing or flowing out of the sprue chamber 28 during pouring and/or rotation. The containment ring 40 further defines an outer diameter comprising an external overhang 44 with respect to the sprue sections 30a, 30b. In the illustrated non-limiting embodiment, the top surface 46 of the containment ring 40 extends outward with respect to the rotation axis, beyond the sprue chamber 28, to thereby catch molten material about its top surface 46 that may splash out of the sprue chamber 28 during operation.
The second end 38 of the sprue is coupled to the table 26 via a wedge 48, as shown most clearly in
The first and second molds 22, 24 are each coupled to the first and second sprue sections 30a, 30b and extend generally radially from the rotation axis. Each mold 22, 24 comprises a front face 32a, 32b and an end face 56a, 56b. The front face 32a, 32b is posited along the sprue chamber 28 and defines entrances to the gates 60a, 60b. As shown in
According to various non-limiting embodiments, the gates 60a, 60b comprise a diameter and average cross-sectional area greater than the diameter and average cross-sectional area of the cavities 72a, 72b. For example, the diameter and cross-sectional area of each gate 60a, 60b adjacent to the material supply port 84a, 84b is greater than the diameter and cross-sectional area of the adjacent material supply port 84a, 84b. In various non-limiting embodiments, a volume of a gate 60a, 60b is greater than a volume of an equal length of a cavity 72a, 72b adjacent to the gate 60a, 60b. It is to be appreciated that while six stacked cavities 72a, 72b are shown, unless expressly stated otherwise, the present disclosure is not limited to stacked cavities or any specific number of cavities associated with each mold. For example, in various non-limiting embodiments, a mold may define only a single cavity. Similarly, while only two molds 22, 24 are shown in
In certain non-limiting embodiments, the first and second molds 22, 24 may be structured to control heat extraction from the molten metallic material and, hence, control cooling of the material. For example, the first and second molds 22, 24 may comprise various insulation features configured to produce directional solidification of the material toward the rotation axis. The thickness of the back walls 80a, 80b may be greater than the thickness of the sidewalls 76a, 76b. Thus, heat transfer from the molten material to the molds 22, 24 may be controlled by the heat capacity of the walls 76a, 76b, 80a, 80b defining each cavity 72a, 72b. For example, differential insulation features of the molds 22, 24 may include increased heat transfer at the back wall 80a, 80b compared to heat transfer at the sidewall 76a, 76b or region thereof. Accordingly, material adjacent to the back walls 80a, 80b may begin to solidify before material positioned adjacent to the gates 60a, 60b. In this way, a solidification front may generally progress within each of the stacked cavities 72a, 72b from the back wall 80a, 80b toward the gate 60a, 60b and sprue chamber 28. In addition to establishing a solidification front, in various non-limiting embodiments, the centrifugal casting force generated by the rotation of the molds 22, 24 about the rotation axis is generally opposed to the direction of solidification, thereby preventing molten material starvation and dendrite exclusion that may result in uncontrolled porosity in castings produced by conventional stationary and centrifugal casting techniques. For example, the sprue chamber 28, gates 60a, 60b, and portions of the cavities 72a, 72b located ahead of the solidification front may act as a reservoir to forcefully supply molten material to the solidification front to produce dense castings having controlled shrinkage porosity.
In certain non-limiting embodiments, the first and second molds 22, 24 are structured to control heat transfer from the molten metallic material to the mold while not detrimentally decreasing the cooling rate of the material. For example, the first and second molds 22, 24 may be structured to provide various levels of control over the solidification process while also providing increased solidification rates. As those having skill in the art will appreciate, an increased cooling rate may favorably decrease grain size, thereby benefiting mechanical properties of the casting at room temperature. Such an increased cooling rate in conventional designs, however, is difficult to control and results in uncontrolled shrinkage porosity. In contrast, in various non-limiting embodiments, the first and second molds 22, 24 are permanent molds and/or are fabricated from materials including metallic materials to provide increased solidification rates due to a high thermal conductivity that may be associated with the mold material, to thereby promote decreased grain size. For example, in one non-limiting embodiment, the first and second molds 22, 24 comprise a permanent steel mold. The first and second molds 22, 24 may also be structured to promote directional solidification, as described above, without sacrificing grain size due to, for example, a retarded cooling rate. That is, while certain portions of the molds 22, 24 may be differentially thermally insulated relative to other portions of the mold 22, 24, the overall cooling rate may be relatively fast. For example, the first and second molds may be configured to promote a differential cooling rate that is tightly defined, e.g., optimized to promote formation of a solidification front that rapidly progresses from the back wall 80a, 80b toward the sprue chamber 28.
While not shown in
The sprue chamber is in fluid communication with the stacked cavities 110a, 110e at the material supply ports 116a, 116e of each of the stacked cavities 110a, 110e via respective gates 118a, 118e. The stacked cavities 110a, 110e are each defined by a sidewall 120a, 120e and a back wall 122a, 122e. For brevity, various features of the rotatable assembly 100 may be described with respect to molds 102a and 102e. It is to be appreciated, however, that in various embodiments, the descriptions apply similarly to one or more additional molds 102b-102c, 102f-102h. For example, the six stacked cavities 110c, 110d of molds 102c and 102d may also be in fluid communication with the sprue chamber 106 at material supply ports 116c and 116d via gates 118c, 118d. The gates 118a, 118e comprise a diameter and average cross-sectional area greater than the diameter and average cross-sectional area of the respective stacked cavities 110a, 110e coupled to each of the gates 118a, 118e. For example, the diameter and cross-sectional area of the gates 118a, 118e adjacent to the material supply ports 116a, 116e are greater than the diameter and cross-sectional area of the material supply ports 116a, 116e or the cavities 110c, 110d. In various non-limiting embodiments, each gate 118a, 118e defines a volume greater than a volume defined by an equal length of the cavity 110a, 110e adjacent to the gate 118a, 118e.
In operation, the rotatable assembly 100 of the centrifugal casting apparatus utilizes centrifugal forces generated by the rotation of the rotatable assembly 100 to produce castings by centrifugal casting. In one non-limiting embodiment, the centrifugal casting apparatus comprises a vacuum arc remelting apparatus (not shown) configured to consume an electrode of metallic material to be supplied to a crucible, such as a water-cooled copper crucible. For example, the rotatable assembly 100 may be positioned within a vacuum environment such that when the electrode is consumed, the molten metallic material within the crucible may be supplied to the rotatable assembly 100. The rotatable assembly 100 may generally comprise the sprue chamber 106 positioned about the rotation axis and two or more stacked mold cavities 110a, 110e defined in one more molds 102a, 102e. While not shown in detail in
As the molds 102a, 102e extract heat from the molten metallic material, the material begins to freeze, producing shrinkage porosity. According to various non-limiting embodiments, heat extraction may be limited by the thickness of the walls 120a, 120e, 122a, 122e of the mold. For example, in one non-limiting embodiment, the thickness of the sidewalls 120a, 120e may be less than 1 inch. Accordingly, the thickness of the walls 120a, 120e, 122a, 122e may limit the ability of the mold 102a, 102e to absorb thermal energy from the molten material. As described above, in various non-limiting embodiments, the molds 102a, 102e are configured to control cooling of the material such that the material undergoes directional solidification from the back walls 122a, 122e generally toward the axis of rotation or the sprue chamber 106. The dimensions of the gates 118a, 118e leading to the cavities 110a, 110e are also large enough to prevent the supply of molten material in the sprue chamber 106 from being cut off from the shrinkage porosity. As a result, most of the porosity may be filled with molten material. When the material in the cavities 110a, 110e fully solidifies, the respective casting gates 118a, 118e also freeze, which closes off the molten material that may remain in the sprue chamber 106 from the casting cavities 110a, 110e. Accordingly, gates 118a, 118e may be fully dense upon freezing. When the solidified metallic material in the cavities 110a, 110b is sufficiently cooled to handle and no longer oxidize, the castings may be removed from the molds 102a, 102e, for example, by unbolting a first modular mold section from a second modular mold section, which may be similar to the arrangement described above with respect to modular mold sections 64a, 64b. The castings may be removed from the sprue chamber 106 at or near the position where the gates 118a, 118e meet the sprue chamber 106. Since the gates 118a, 118e are fully dense, any porosity inside the casting remains internal and may be removed by HIP, for example, to eliminate any internal porosity in the casting. When castings comprise multiple pieces, the fully dense casting may then be divided into the final pieces by machining equipment such as saws, cutting torches, abrasive water jet, or wire electro-discharge machining apparatuses, for example.
As introduced above, in various non-limiting embodiments, the gates 118a, 118e comprise a diameter or cross-sectional area greater than the largest diameter or cross-sectional area of the cavities 110a, 110e. In certain non-limiting embodiments, the increased size of the gates 118a, 118e prevents internal porosity from reaching the sprue chamber 106. For example, a gate 118a, 118e may be fully dense upon solidification, preventing internal porosity from connecting to the sprue chamber 106 where it may later become exposed when the casting is removed from the sprue chamber 106. Thus, gates 118a, 118e may form a density barrier to contain the internal porosity such that it may be addressed by processing, such as by HIP, for example. In various non-limiting embodiments, gates 118a, 118b may form a thermal barrier between casting cavities 110a, 110e and the sprue chamber 106. For example, the cooling rate of the molten metallic material in the sprue chamber 106 may be well below the cooling rate of the molten metallic material in the cavities 110a, 110e, resulting in a substantial temperature differential between the cavities 110a, 110e and the sprue chamber 106 well after an optimal cooling period for the casting has taken place. Consequently, grain size near the sprue chamber 106 may be increased. The gates 118a, 118e disclosed herein, however, may be configured to solidify closely following the casting, e.g., when the solidification front has extended through the casting, but still before the molten metallic material in the sprue chamber 106 has solidified. According to one non-limiting aspect, the solidified gates 118a, 118b, which may also be fully dense, thereby form a thermal barrier between the sprue chamber 106 and respective casting cavities 110a, 110e.
In various non-limiting embodiments, the rotatable assembly 100 comprises a plurality of vertically stacked cavities 110a, 110e positioned about a sprue chamber 106. The sprue chamber 106 may comprise a decreased radius compared to sprue chambers of conventional centrifugal casting apparatuses that are configured to feed a comparable number of cavities. In operation, according to one non-limiting embodiment, molten material may substantially simultaneously, e.g., continuously, fill the sprue chamber 106, gates 118a, 118e, and vertical cavities 110a, 110e. For example, molten material supplied to the sprue chamber 106 may begin to simultaneously fill the sprue chamber 106, adjacent gates 118a, 118e, and vertical cavities 110a, 110e from the bottom toward the top. Thus, as the molten material is poured into the sprue chamber 106, the molten material accumulates to form an increasing molten volume in the sprue chamber 106 that may be directly fed to the adjacent gates 118a, 118e and vertical cavities 110a, 110e without loss of superheat due to excessive travel and contact with various structures of the rotatable assembly 100. Thus, in various non-limiting embodiments, the sprue chamber 106 is configured to feed all the casting cavities 110a, 110e while promoting retention of superheat. For example, in operation, the sprue chamber 106 may be dimensioned to receive a single pour of molten material that completely fills a cavity of the vertical stacks of cavities 110a, 110e. For example, in one non-limiting embodiment, the sprue chamber is dimensioned to receive a single pour of molten material that completely fills at least the bottom cavity of each of the vertical stacks of cavities 110a, 110e. The volume of the single pour is preferably sufficient to also completely fill the gates 118a, 118e and the volume of the sprue chamber 106 adjacent to the completely filled cavities 110a, 110e. Thus, the rotatable assembly 100 may be configured to receive a volume of molten material that may be fed directly from the sprue chamber 106 into the cavities 110a, 110e without loss of superheat.
According to certain non-limiting embodiments, retaining superheat promotes production of cast pieces comprising improved surface quality. Titanium aluminide castings, for example, produced by conventional casting techniques suffer from poor surface quality. For instance, as stated above, when a thin layer of molten material must travel the radius of a large diameter sprue and subsequently climb various structures, such as sprue walls or gating, for example, to fill from the bottom of the mold cavities, the bulk of the molten material may be unable to retain superheat, resulting in poor surface quality. The poor surface quality may require producing castings several millimeters larger than the final piece so that the surface of the casting may be processed to produce a casting within the desired dimensions. In contrast, the rotatable assembly 100 may be configured to produce castings comprising improved smoothness and without surface defects commonly found in castings produced by conventional techniques. Consequently, castings may be produced with lower scrap rates and production costs.
Each mold defines five stacked cavities, wherein two of the cavities 320a, 322a comprise a decreased diameter compared to three larger diameter cavities 320b, 322b. The decreased diameter cavities 320a, 322a are positioned at intervals between the three larger diameter cavities 320b, 322b. As can be seen, multiple diameter cavities may increase flexibility with respect to casting sizes that may be produced in a single pour. For example, time and yield loss may be reduced by consolidating pours. The stacked cavities 320a, 320b, 322a, 322b are in fluid communication with the sprue chamber 308 through respective gates 324a, 324b, 326a, 326b. Each gate 324a, 324b, 326a, 326b comprises a diameter and cross-sectional area larger than the diameter and cross-sectional area of the cavity 320a, 320b, 322a, 322b in which it is coupled. In one aspect, the increased size of the gates 324a, 324b, 326a, 326b prevents full solidification of the gates 324a, 324b, 326a, 326b until after the material in the respective cavities 320a, 320b, 322a, 322b has fully solidified. That is, at least a portion of the material in the gates 324a, 324b, 326a, 326b may retain liquidity such that it may move into and fill portions of the solidifying metallic material in the casting cavity 320a, 320b, 322a, 322b. As summarized above, in various non-limiting embodiments, gates 324a, 324b, 326a, 326b comprise an increased dimension with respect to a dimension of the cavity. For example, according to certain configurations, optimal efficiency with respect to casting volume and yield may include a gate 324a, 324b, 326a, 326b comprising a cross-sectional area greater than the cross-sectional area of the cavity 320a, 320b, 322a, 322b, for example, between 100% to 150% of the cross-sectional area of the cavity 320a, 320b, 322a, 322b. Of course, in some non-limiting embodiments, gates comprising cross-sectional areas up to, for example, 400% or more of the cross-sectional area of the corresponding cavity, may also be used to produce castings having similar characteristics. Yield loss, however, may increase with increasing gate dimensions. According to various configurations of certain non-limiting embodiments, optimal gate lengths may comprise 50% to 150% of the largest dimension of the cross-section of the gate. Again, such lengths are merely optimizations of certain embodiments with respect to the number of castings that may be produced per volume of material supplied to the mold, and such examples are not intended to be limiting unless stated otherwise.
The first and second molds 304, 306 are structured to promote directional solidification generally toward the rotation axis or sprue chamber 308 such that centrifugal force continually presses molten material toward the solidification front of the casting to fill shrinkage porosity as it appears in order to produce a denser casting. The first and second molds 304, 306 comprise insulation features configured to promote directional solidification toward the sprue chamber 308. For example, the molds 304, 306 each comprise a side face 328, 330 defining two pockets 332a,b, 334a,b spaced apart and positioned proximal to the sprue 302. The pockets are configured to reduce the heat capacity of the mold along the corresponding portion of the mold. The molds 304, 306 further define a plurality of upper and lower pockets 336a,b, 338a,b extending along a portion of the molds 304, 306. The upper and lower pockets 336a,b, 338a,b are configured to insulate adjacent portions of the mold by limiting the heat capacity and rate of heat transfer through the mold. In addition to controlling heat transfer by altering heat capacity of portions of the mold via pockets or mass of mold walls, in various non-limiting embodiments, cavities may also be arranged to assist in controlling heat transfer.
Each cavity 410 comprises a molten material supply port 416 adjacent to a tapered or decreasing cross-section tapered from the material supply port 416 toward the back wall 414. In various non-limiting embodiments, the front face 406 may be configured to attach to a gate or plate, or directly to a sprue at the molten material supply port 416. For example, in some non-limiting embodiments, a mold 400 comprises a cavity 410 defining a decreasing cross-section over a portion of its length extending from the molten material supply port 416, which may be directly couplable to a sprue or sprue chamber. That is, the reduction in cross-section over an initial length of the cavity 410 may overcome the need for a gate. As such, castings may be produced with reduced yield loss and controlled shrinkage porosity. In various non-limiting embodiments, cavities 410 comprising decreasing cross-sections may define sidewalls 412 generally tapering in-line with the cavity 410, e.g., generally aligned with a centerline of the cavity 410, and may comprise a symmetrical taper with respect to adjacent sidewalls 412 of the cavities 412. In one non-limiting embodiment, a decreasing cross-section may be generally defined along the direction of centrifugal force and/or taper in a general direction opposed to the general direction of solidification. For example, in one non-limiting embodiment, a cavity defines a cross-section, such as a tapered section, that generally tapers away from the molten material supply port, e.g., toward a back wall 414 of the cavity 410.
In one non-limiting embodiment, the cavity 410 defines a decreasing cross-section comprising a tapered section that includes a first cross-section and a second cross-section. The second cross-section is less than the first cross-section and is located a greater distance from the rotation axis than the first cross-section. In operation, a solidification front may be formed and directionally advance generally from the back wall 414 toward first cross-section and the molten material supply port 416. Solidification of the material along the solidification front may result in dendrite formation within the solidifying material. According to various non-limiting embodiments, at least a portion of the molten material in front of the solidification front may remain molten for a period of time during which the material located at or near the second cross-section is subject to cooling and hence shrinkage. In this way, the molten material in front of the solidification front, e.g., at or near the first cross-section, may be accelerated by the centrifugal force such that it moves into and/or between the forming dendrites to fill shrinkage porosity as it arises to avoid formation of significant voids and thereby produce a dense casting. In this way, the portions of the mold in front of the solidification front, e.g., located more proximate to the sprue chamber, may act as a riser for the cavity 410. In various non-limiting embodiments, cavities may comprise multiple tapered sections. In certain non-limiting embodiments, the decreasing cross-section may prevent internal porosity from reaching the sprue chamber. In one non-limiting embodiment, the decreasing cross-section may form a density barrier to contain internal porosity such that it may be addressed by processing, such as by HIP, for example. For example, in use, at least a portion of the decreasing cross-section at or adjacent to the largest cross-section of the decreasing cross-section, e.g., at or adjacent to the molten material supply port 416, may be fully dense upon solidification, thereby preventing internal porosity from connecting to the sprue chamber where it may later become exposed when the casting is removed from the sprue chamber.
The mold 400 further includes insulation features comprising a plurality of pockets 418 defined in the sidewalls 412 defining the cavities 410. In various non-limiting embodiments, the sidewalls 412 of the mold 400 may also or alternatively comprise insulation features such as pockets similar to those illustrated in
It is appreciated that certain features of the centrifugal casting apparatuses and methods described herein are described in terms of illustrated embodiments. For example, for brevity and ease of understanding, only a limited number of variations with respect to the number and arrangement of molds and cavities are illustrated. As will become apparent to one of ordinary skill in the art after reading this document, the illustrated embodiments and their various alternatives may be implemented without confinement to the illustrated examples. The present disclosure is also not limited to the illustrated cavity or mold arrangements. For example, in various embodiments, molds may comprise multiple vertical stacks of cavities. Stacked cavities may comprise molds comprising multiple rows of stacked cavities. Stacked cavities may also comprise one or more cavities radially offset from the center of rotation. For example, a mold may comprise a stack of cavities wherein all the cavities are radially offset. In some non-limiting embodiments, stacked cavities may comprise multiple rows of stacked cavities. While the illustrated embodiments generally show stacked cavities where at least the material supply ports are aligned, in various non-limiting embodiments, cavities may be stacked such that one or more cavities are not aligned, e.g., cavities may be staggered or offset at uniform or non-uniform intervals.
It is to be appreciated that the configuration and number of molds may generally be related to the size and number of pieces to be cast and the volume of the sprue. For example, in various non-limiting embodiments, casting apparatuses may comprise a plurality of molds positioned about a rotation axis. The plurality of molds may each define a vertical stack of a plurality of cavities. Each of the plurality of cavities may define a plurality of linearly arranged cast pieces. Thus, depending on the configuration, various embodiments of the casting apparatuses may produce two to many hundreds of castings in a single casting run. That is, casting apparatuses comprising, for example, two to ten molds, each mold defining two to ten cavities, and each cavity defining two to six cast pieces, may produce between 8 and 600 cast pieces.
In the present description, other than in the operating examples or where otherwise indicated, all numbers expressing quantities or characteristics of elements, ingredients and products, processing conditions, and the like are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, any numerical parameters set forth in the following description are approximations that may vary depending upon the desired properties one seeks to obtain in the apparatuses and methods according to the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
This disclosure describes various elements, features, aspects, and advantages of various non-limiting embodiments of centrifugal casting apparatuses and methods thereof. It is to be understood that certain descriptions of the various non-limiting embodiments have been simplified to illustrate only those elements, features and aspects that are relevant to a more clear understanding of the disclosed embodiments, while eliminating, for purposes of brevity or clarity, other elements, features and aspects. It is appreciated that certain features, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately, in any suitable subcombination, or as suitable in any other described embodiment. For example, while the cavities are generally shown to extend along a horizontal operating plane, in various non-limiting embodiments, cavities may extend at positive and/or negative angles with respect to a horizontal operating plane. Additionally, certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.
Although the foregoing description has necessarily presented only a limited number of embodiments, those of ordinary skill in the relevant art will appreciate that various changes in the apparatuses and methods and other details of the examples that have been described and illustrated herein may be made by those skilled in the art, and all such modifications will remain within the principle and scope of the present disclosure as expressed herein and in the appended claims. Those having ordinary skill, upon reading the present description, will readily identify additional centrifugal casting apparatuses and methods and may design, build, and use additional centrifugal casting apparatuses and methods along the lines and within the spirit of the necessarily limited number of embodiments discussed herein. It is understood, therefore, that the present invention is not limited to the particular embodiments or methods disclosed or incorporated herein, but is intended to cover modifications that are within the principle and scope of the invention, as defined by the claims. It will also be appreciated by those skilled in the art that changes could be made to the non-limiting embodiments and methods discussed herein without departing from the broad inventive concept thereof.
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