Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C9/00—Moulds or cores; Moulding processes
  • B22C9/02—Sand moulds or like moulds for shaped castings
  • B22C9/04—Use of lost patterns

Definitions

• the present invention relates generally to crucibles for casting. More particularly, the present invention provides a one piece integrated crucible and invested mold for use in the casting of reactive metals, particularly complex shapes thereof.
• Reactive metals such as titanium or titanium aluminid ⁇ s
• crucibles comprising of alumina, zircon, or silicates which are typically used in casting metals such as iron, nickel or aluminum.
• Turbocharger rotors are typically cast using nickel superalloy which can be easily melted and cast using invested mold having a zircon facecoat and an alumina-silicate crucible attached to the top ofthe mold. Molten titanium aluminide will rapidly react with the alumina-silicate crucible making casting impossible.
• Candidate crucible materials include yttria (Y 2 O 3 ), thoria (ThO 2 ), calcia (CaO) and other exotic rare earth oxides. Thoria is radioactive and suffers from poor thermal shock resistance. For this reason it has not been exploited industrially. However,
• yttria crucibles are very expensive, $50 to $ 100
• the alumina-silicate crucibles used for nickel superalloy castings are less than $ 1. Since titanium and titanium aluminide castings compete with nickel castings in a wide variety of applications, the high cost of yttria crucibles effectively prices titanium and titanium aluminide castings out of the market. This is particularly important for cost sensitive applications, for example automotive applications.
• Calcia is also a potential refractory material for titanium and titanium aluminides due to its thermal stability.
• US Patent No. 4,710,481 to Degawa et. al. discloses melting titanium and titanium alloys in calcia crucibles.
• calcia is highly hydrophilic and spontaneously hydrates in ambient levels of atmospheric moisture. The hydration is accompanied by volume changes which cause cracking and spallation.
• Calcia crucibles can spontaneously crack after only hours of exposure to atmospheric moisture. For this reason calcia is an impractical material for a crucible in commercial, industrial environments.
• the present invention provides a graphite crucible attached directly to the top of an invested mold in the shape of the complex molded part, a turbocharger rotor for the specif i c embodiments disclosed herein, to form an integrated unit.
• a method for producing the integrated crucible and mold is characterized by the steps of attaching an investment pattern to a cylindrical graphite crucible and building an investment mold shell around the pattern and a mating portion of the crucible.
• the mold shell is created by dipping the pattern and crucible mating portion in an inert ceramic facecoa t slurry, allowing the dipcoat layer to partially dry and/or cure, applying a layer of ceramic stucco over the dipcoat and laminating additional layers of dipcoat and stucco to achieve the desired thickness mold shell.
• the embodiment of the invention described in detail subsequently, provides a one piece crucible/mold system suitable for casting a metal part in vacuum at temperatures exceeding 1400°C .
• the method of employing the apparatus of the invention for cas t ing of complex components takes advantage of the rapid melting and casting of the ⁇ -TiAl to
• Carbon is a known contaminant of ⁇ - TiAl. At low levels, i.e. under 2000 ppm (wt%) it is often added to improve creep
• FIG. 1 is a section elevation view of the integrated graphite crucible and invested mold
• FIG. 2 is a pictorial view of an exemplary turbocharger rotor manufacturable from ⁇ -TiAl using the present invention.
• the integral crucible and mold includes a cylindrical graphite sleeve 10 having a bottom 12 adapted to accommodate attachment of a polystyrene invested mold pattern 14 having an attachment runner 15.
• the attachment runner is sealingly attached to the sleeve with wax 16.
• the sleeve is sized to accommodate a desired meitstock billet 17.
• An investment shell 18 is constituted by laminations of a facecoat slurry 20 and ceramic stucco 22 (shown only over a portion of the section for clarity), which extend over and adhere to a bottom mating portion of the graphite sleeve.
• the hardened shell laminate After firing, which results in vaporization of the pattern, the hardened shell laminate is fixedly attached to the graphite sleeve forming an integral crucible and mold for casting of the desired part. .
• An aperture 24 in the bottom face of the sleeve provides a flow path for the molten casting metal into the mold shell while the rim 26 retains the meitstock billet in the crucible in a position for induction heating.
• a method for creating and employing the apparatus of the present invention includes the steps of attaching the polystyrene turbocharger wheel pattern to the cylindrical graphite sleeve by dipping the- top of the polystyrene pattern in hot wax and inserting it into the recess in the bottom of the graphite sleeve.
• An investment shell is next built around the polystyrene pattern and the lower portion of the sleeve.
• the investment mold process begins with dipping the pattern made of wax or plastic having the shape of the desired casting in a facecoat slurry made from any ceramic inert to ⁇ -TiAl, for example, yttria or calcia.
• the crucible/invested mold assembly is then transferred while still hot from the furnace and placed in a casting chamber and evacuated.
• a cylindrical meitstock billet of the casting metal is inserted in the crucible.
• the metal is next rapidly melted by inductively heating the graphite crucible. Since graphite is a strong susceptor, the crucible is quickly heated to the melting temperature of the ⁇ -TiAl.
• the molten metal falls directly into the attached invested mold with the assistance of gravity with a minimum of superheat and hold time.
• the short hold time and low superheat prevents significant contamination by carbon from the crucible.
• the short drop distance attained as a result of the one piece design of the crucible/investment mold, allows full fill in thin sections. Greater drop distances, as would be required by two piece
• the graphite crucible/invested turbocharger mold assembly was then transferred while still hot from the furnace, charged with the casting metal meitstock billet, placed under a quartz bell jar and evacuated.

Landscapes

• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)
• Golf Clubs (AREA)
• Mold Materials And Core Materials (AREA)
• Adornments (AREA)
• Physical Vapour Deposition (AREA)
• Nitrogen And Oxygen Or Sulfur-Condensed Heterocyclic Ring Systems (AREA)

Applications Claiming Priority (5)

Publications (2)

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• 1998
  • 1998-01-27 WO PCT/US1998/001672 patent/WO1998032557A1/fr active IP Right Grant
  • 1998-01-27 AT AT98911363T patent/ATE216929T1/de not_active IP Right Cessation
  • 1998-01-27 ES ES98911363T patent/ES2176986T3/es not_active Expired - Lifetime
  • 1998-01-27 EP EP98911363A patent/EP0963262B1/fr not_active Expired - Lifetime
  • 1998-01-27 BR BR9807084-3A patent/BR9807084A/pt not_active IP Right Cessation
  • 1998-01-27 DE DE69805164T patent/DE69805164T2/de not_active Expired - Fee Related
  • 1998-01-27 JP JP53225898A patent/JP2001509083A/ja not_active Ceased
  • 1998-01-27 PT PT98911363T patent/PT963262E/pt unknown
  • 1998-01-27 CN CN98802006.8A patent/CN1101284C/zh not_active Expired - Fee Related

Non-Patent Citations (1)

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