Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C3/00—Selection of compositions for coating the surfaces of moulds, cores, or patterns

Definitions

• Aluminum and magnesium castings have desirable properties for many applications where light weight, good corrosion resistance and reasonable strength are important.
• Various alloys of these metals have been developed to improve the strength and high temperature properties.
• beryllium alloys particularly beryllium-aluminum alloys, have high stiffness, low density and high melting points giving a desirable combination of properties.
• the higher casting temperatures and corrosiveness of these alloys caused substantial and unacceptable degradation of the core as well as poor metallurgical integrity of the casting. Reaction products formed from cast alloy and core become detrimental defects in the casting.
• core materials can be protected during melt casting of beryllium alloys by providing on the core (and optionally the mold) a coating selected to be substantially inert or non-reactive during the casting process.
• substantially inert or non-reactive is meant not reacting during the casting process sufficiently to detrimentally affect either core or casting.
• One object of the present invention includes the provision, in a process of casting beryllium alloys in which mold or core materials are reactive to a significant extent with the molten alloys, characterized in that the process comprises: utilizing at least one of a mold and a core coated with a protective coating which is selected to be substantially non-reactive during the casting process.
• the invention further includes a shaped mold or core or combination for beryllium alloy casting, the mold and/or core having a protective coating selected to be substantially non-reactive with the beryllium alloy being cast throughout the casting process.
• the invention includes, more particularly, a process of casting beryllium-aluminum alloys having from about 20 to about 80% by weight beryllium, comprising: providing a casting mold and at least one core yielding the desired shape; coating the core, and optionally the mold, with at least one of the group consisting of oxides, borides, nitrides and cermets selected to be inert to the molten alloy; casting the alloy while molten, into the mold and about the core; cooling, removing the mold, and recovering the cast part.
• the mold may be used to form further castings.
• the core can be re-used also.
• Some of the core coatings may serve as parting agents which facilitate core removal.
• Another object of the present invention is to provide a process for casting beryllium alloys wherein mold surfaces which contact and which react with the molten metal are coated similarly to the core surfaces.
• the benefits of this are reduced reactivity with molten metal and improved quality of the casting.
• a preferred embodiment is where the coating serves as a parting agent as well as a protective barrier, thus facilitating removal of both mold and core.
• Those coatings found to have parting agent properties are MgO, ZrO 2 , TiN and Al 2 O 3 .
• Particularly preferred as combined protective chemical barrier coating plus parting agent is ZrO 2 or Al 2 O 3 .
• the beryllium alloys are beryllium-aluminum alloys containing from about 20 to about 80% by weight beryllium and having additives to improve the microstructure, strength and ductility. More preferably, the alloys will have from about 50 to about 70% beryllium.
• the casting alloys may contain up to about 80% by weight beryllium.
• the beryllium alloys which are amenable to casting, include those containing from about 20 to about 80% by weight beryllium; from about 20 to about 75% aluminum, and the balance additives selected from silicon, silver, magnesium, copper, nickel, cobalt and impurities. All of these alloys melt at temperatures above about 1150°C (beryllium melts at 1277°C).
• a vacuum or an inert gas atmosphere e.g. argon, helium is maintained during casting.
• Mold materials commonly used in such casting techniques include sand-plus-binder, ceramics such as alumina (with binder), silica, alumina-silicate mixtures, zircon, sodium and potassium silicates, zirconia (with binder), gypsum, graphite, and magnesium/iron silicates. Any of the known processes for shaping the mold may be used. Many of these mold materials will react with molten beryllium alloys and can be coated similarly to the cores to protect against reaction.
• the cores may be formed of a) suitable metals (or alloys) which melt above the casting temperatures, b) suitable ceramics, for example alumina/binder or silica-base ceramics, and c) mixtures thereof.
• suitable ceramics for example alumina/binder or silica-base ceramics, and c) mixtures thereof.
• Such mixtures may comprise e.g. stainless steels + silica-base ceramics; titanium + A1 2 O 3 + binder and mixed ceramics.
• alumina is used with some form of binder and the binder has been found to be reactive with the molten alloy.
• metals useful in forming cores are various stainless steels, e.g. 304, 316 and 321; titanium and Ti-base alloys such as Tl6Al4V; nickel and Ni-base alloys such as IN-100.
• Such cores have been found to react with the beryllium alloys during casting.
• the core base may be shaped by any known metallurgical technique (in the case of metals) or ceramic molding technique (in the case of ceramics and mixtures).
• the cores may be hollow, e.g. as metal tube or slip-cast fired ceramic; or substantially solid, e.g. as metal rod or sintered ceramic powder.
• the core material should be susceptible to chemical dissolution or mechanical disruptions. These mechanical removal processes might include vibration, drilling, abrasion, and/or grinding. Depending on the process, residual core coating material might remain in the casting without detriment. If the protective coating also acts as a parting agent, it may be possible to remove the core as a unit or in several pieces.
• the coatings are selected from oxides, e.g. alumina, magnesia, beryllia, thoria, titania and zirconia; and borides, e.g. beryllium boride, aluminum boride, titanium boride; as well as nitrides, e.g. beryllium nitride, boron nitride, aluminum nitride and titanium nitride. Cermets may also be used e.g.
• Intermetallic oxides or borides or nitrides or cermets may be used e.g. Be-Ti boride; B-A1 nitride; beryllia-zirconia; alumina-thoria.
• the coating is formed on the core by any suitable technique, e.g. plasma spraying, vapour deposition, dipping, electro-deposition, injection around core body, brushing, spraying, impregnation, painting, and flow or gravity or cascade coating. Vaporization, melt or sintering temperatures will be reached in forming the coating, as required.
• the thickness of the coating should be selected to constitute an effective diffusion barrier during the entire casting process. Usually the thickness will be within the range of about 20 to 1000 microns, preferably about 50 to 200 microns. Multi-layer coatings may be used: examples include Al 2 O 3 under ZrO 2 and Al 2 O 3 under TiO 2 .
• a preferred coating is plasma-sprayed or physical vapour deposited alumina having a thickness of about 50-100 microns.
• Another preferred coating is ZrO 2 .
• the cast products have been found to be improved (when these coated molds and/or cores were used) in aspects such as smooth and defect free detailed passages, pockets and cavities.
• Coated molds and cores when able to be removed intact, can be re-used. If necessary a coating layer can be re-applied.
• a core constructed from a stainless steel tube (321) was plasma coated with 100 microns thickness of Al 2 O 3 .
• the core was located inside the internal cavity of a ceramic shell mold.
• the ceramic shell was preheated in the range of 900°C-1250°C (preferably 1200-1250°C) and then molten aluminum-beryllium 40:60 alloy in the range of 1200°C-1470°C (preferably 1400-1450°C) was poured into the shell, filling the internal cavity and surrounding the Al 2 O 3 -coated tube.
• an argon gas atmosphere was maintained. Once the casting was cool, it was cleaned and prepared in a manner similar to the usual procedure for aluminum and magnesium castings.
• the Al 2 O 3 -coated tube was found to be resistant to the molten alloy and to result in high quality castings.
• Example 2 All processing was the same as Example 1 except the core was coated by dipping in a ceramic slurry (a water-base slurry of beryllium oxide) followed by sintering.
• a ceramic slurry a water-base slurry of beryllium oxide
• the beryllia-coated tube was found to be resistant to the molten alloy and to result in high quality castings.
• Example 2 All processing was similar to Example 1 except the core was formed from a tube of titanium base metal and coated with aluminum boride by plasma spraying.
• the boride-coated tube was resistant to the molten alloy and resulted in high quality castings.
• Example 2 All processing was similar to Example 1 except the coating was plasma-sprayed thoria. Good quality castings resulted.
• Example 2 All processing was similar to Example 1 except the coating was vapour-deposited alumina-zirconia.
• the alumina-zirconia coated core was resistant to the molten alloy and resulted in high quality castings.
• Example 1 The procedures in Example 1 were repeated except the ceramic shell mold also was coated with 100 microns of plasma-sprayed alumina on all surfaces exposed to the molten alloy. Very high quality castings resulted when mold and core were removed.
• a SiO 2 -based ceramic core was coated with Al 2 O 3 to a thickness of 50 microns. Plasma spraying which produced a sound and chemically inert barrier, was used to provide the layer.
• the coated ceramic core was located inside the internal cavity of an investment casting ceramic shell so that part of the core would be exposed in the casting.
• the ceramic shell was preheated in the range of 900°C-1250°C and then molten aluminum-beryllium alloy of 65% beryllium at a temperature in the range of 1200°C-1450°C was poured into the shell, filling the internal cavity and surrounding the Al 2 O 3 -coated tube. Once the casting was cool, the casting was cleaned and prepared in a manner similar to known aluminum and magnesium casting procedures. The exposed ceramic core was then removed by leaching in a solution of hydrofluoric acid. A high quality casting resulted.
• Example 7 All processing was similar to that in Example 7 except the coating was plasma-sprayed magnesia.
• the magnesia-coated core was resistant to the molten alloy and yielded a high quality casting.
• Example 7 The procedures were similar to those in Example 7 except the core coatings were formed from the following ceramics: ThO 2 , ZrO 2 , MgO, TO 2 , AIN, BeN, BN, TiN. In each case, the coated cores were resistant to the molten alloy and yielded high quality castings.
• Example 7 The procedures were similar to those in Example 7 except the coating was derived from at least two layers of the different ceramic materials alumina and zirconia. Superior quality castings resulted.
• Example 7 The procedures were similar to those in Example 7 except the coating was derived from the cermet Be + alumina. Superior quality castings resulted.

Landscapes

• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Mold Materials And Core Materials (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=24630714

Family Applications (1)

Country Status (3)

Cited By (5)

Families Citing this family (1)

Citations (2)

• 1997
  • 1997-05-29 CA CA002206487A patent/CA2206487A1/en not_active Abandoned
  • 1997-05-30 EP EP97108688A patent/EP0810046A1/de not_active Withdrawn
  • 1997-06-02 JP JP14427697A patent/JPH1052733A/ja active Pending

Patent Citations (2)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events