Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-09-07
2001-05-29
Pyon, Harold (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S517000, C164S519000, C164S004100
Reexamination Certificate
active
06237671
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the casting of metals and alloys, especially titanium and its alloys, using ceramic mold facecoats in a manner to provide detectability of any sub-surface ceramic inclusions that may be present on the casting.
BACKGROUND OF THE INVENTION
Investment casting of titanium and titanium alloys and similar reactive metals in ceramic molds is made difficult by the metal's high affinity for elements such oxygen, nitrogen, and carbon. At elevated temperatures, titanium and its alloys can react with the mold facecoat that typically comprises a ceramic oxide. For example, at elevated temperatures during investment casting in a ceramic investment shell mold having a ceramic oxide facecoat, such as zirconia, a titanium alloy such as Ti—6Al—4V will react with the ceramic oxide to form a brittle, oxygen-enriched surface layer, known as alpha case, that adversely affects mechanical properties of the casting and that is removed by a post-casting chemical milling operation as described, for example, in Lassow et al. U.S. Pat. No. 4,703,806.
Moreover, ceramic oxide particles originating from the mold facecoat can become incorporated in the casting below the alpha case layer as sub-surface inclusions by virtue of interaction between the reactive melt and the mold facecoat as well as mechanical spallation of the mold facecoat during the casting operation. The sub-surface oxide inclusions are not visible upon visual inspection of the casting, even after chemical milling.
The manufacture of titanium based structural airframe components by investment casting of titanium and its alloys in ceramic investment shell molds poses problems from the standpoint that the castings should be cast to near net shape so as to require only a chemical milling operation to remove any alpha case present on the casting. However, any sub-surface ceramic inclusions located below the alpha case in the casting are not removed by the chemical milling operation and further are not visible upon visual inspection of the casting. There thus is a need in the art for a method of making such structural airframe components by investment casting of titanium and its alloys in ceramic investment shell molds in a manner that enhances detectability of any sub-surface ceramic inclusions that may be present below exterior surfaces of the casting.
An object of the present invention is to provide a method of making castings, such as for example, structural airframe component castings, by casting titanium and its alloys as well as other metals and alloys in contact with a mold facecoat that satisfies this need by providing for ready detectability of sub-surface ceramic inclusions that may be present below the exterior surface of the casting.
SUMMARY OF THE INVENTION
One aspect of the present invention involves a method of making a cast component by casting of a metal or alloy, especially titanium and its alloys, in a ceramic mold in a manner to provide x-ray, neutron-ray or other non-destructive detectability of any sub-surface ceramic inclusions that may be present below exterior surfaces of the casting. The present invention can be practiced in one embodiment by forming a ceramic shell mold having a facecoat (or other mold layer that may contribute to inclusions in the casting) including erbium bearing ceramic or other X-ray or neutron detectable ceramic material, casting a metal or alloy in the shell mold, removing the solidified casting from the shell mold, and subjecting the solidified casting to x-ray or neutron-ray radiography to detect any sub-surface inclusions below the exterior surface of the casting, which inclusions are not detectable by visual inspection of the casting.
In another embodiment of the present invention, titanium metal or a titanium alloy is cast in contact with a mold facecoat and/or back-up layer including erbium bearing ceramic or other x-ray detectable facecoat component, casting the titanium metal or alloy in the investment shell mold, removing the solidified casting from the mold, chemically milling the casting to remove any alpha case present on the casting, and subjecting the solidified, chemically milled casting to x-ray or neutron-ray radiography to detect any sub-surface ceramic inclusions present below the exterior surface of the casting.
A mold facecoat slurry in accordance with another aspect of the present invention comprises erbium bearing ceramic, preferably fused erbia powder, an optional inorganic binder, and an inorganic pH control agent present in an amount to provide a slurry pH of greater than 10 that is applied to a pattern of a component to be cast to form the mold facecoat. The inorganic pH control agent comprises ammonium or other hydroxide present in an amount to provide a slurry pH of about 10.2 to about 10.4. The slurry may further include one or more other ceramic particulates selected from the group consisting of zirconia, alumina, yttria, and silica particulates in combination with the erbium bearing ceramic particulates. The slurry typically is applied as one or more coatings to a fugitive pattern of the casting in the well known lost wax process for forming a ceramic shell mold.
The present invention is advantageous in that castings can be produced in ceramic investment molds in a manner that provides enhanced detectability of any sub-surface ceramic inclusions proximate and below the surface of the casting not detectable by visual inspection, especially those inclusions that may be located below an alpha case layer of a titanium based casting and that are not removed by a post-cast chemical milling operation. Moreover, since the practice of the invention does not promote further formation of alpha case on titanium based castings, conventional chemcial milling regimes can still be used to remove the alpha case from the casting.
The above objects and advantages of the present invention will become more readily apparent from the following detailed description.
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Certificate of Analysis, Auercoat 4/3 Erbiumoxide fused.
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Koziol Kelly A.
Lassow Eliot S.
Strabel George R.
Howmet Research Corporation
Lin I.-H.
Pyon Harold
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