Method Of Casting Metallic Objects

Abstract

The disclosure relates to a method of casting objects having small, thin and narrow sections from alloys or other metal-containing materials to produce completed objects properly filled with materials that solidify with relatively fine and uniform grain structures. The method includes the steps of controlling the temperature of the mold within a relatively narrow temperature range which range includes the melting temperature of the metal-containing material to be cast prior to the pouring of the molten material into the mold and also controlling the temperature of the metal to be cast within as narrow a superheat range above the melting temperature as practicable.

Description

BACKGROUND OF THE INVENTION

Prior methods of casting objects in an evacuated atmosphere have included removing the refractory mold from the furnace in which the mold has been heated for the purpose of removal of wax or other fusible pattern material and placing the heated mold in a vacuum chamber where the mold commences to cool for a period of time prior to being poured. The cooling period varies with the time required for closing and evacuation of the chamber and readying the melting and pouring crucible. The mold will cool hundred of degrees Fahrenheit during this period and the temperature of the mold when it is poured will vary substantially under commercial operating conditions. Castings with thin sections and with proper grain structure have not been made using these methods.

Other prior techniques have used heating means to control the temperature of a mold readied for pouring but none of these methods has provided a casting method capable of producing castings with thin sections in which the proper controlled relationship among the mold temperature, temperature of the liquid metal and the melting temperature of the metal maintained before the pouring of the metal to provide complete filling and a fine uniform grain size throughout the casting.

SUMMARY OF THE INVENTION

Broadly, the present invention includes a method for forming a casting with at least one thin section of substantial size and having a selected grain structure which comprises heating the mold, maintaining the mold temperature within a range having an upper temperature of approximately 150.degree.F above the melting temperature of the metal-containing material to be formed and a lower limit of approximately 200.degree.F below the melting temperature, controlling the temperature of the liquid material within as narrow a range of superheat as practicable and pouring the liquid material into the mold. The optimum practical range of superheat of the liquid material has been found to be about 50.degree. to 100.degree.F to produce complete filling and the preferred fine grain structure with most alloys used in producing engine parts and other objects having thin sections whether stressed during use or not.

The method of the present invention is particularly applicable to investment casting where refractory molds are poured in an evacuated, inert gas or other non-atmospheric condition.

It has been found that the controlling of temperatures of the metal and the mold within the ranges of this novel method can be practicably accomplished under commercial production conditions by using induction coil heating means. The practice of the present method is also effective in controlling grain structure where the patterns have been coated with a material which gives the mold grain nucleating properties.

It is a feature of the invention that castings, such as turbine wheel and impeller blade parts, which have thin sections may be cast without formation of detrimental columnar grains at the thin sections of the parts, such as in the outer rim of the wheel and between the blades. Since during use these parts are highly stressed especially at the above locations, the improved filling of the thin sections during pouring and the improved grain structure provides an acceptable part that resists the formation of continuous cracks and their propagation during service.

It has also been found that parts having thin sections which are not subject to stress may be made using the method, for example; engine exhaust manifolds which are thin walled and have complex intersections.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

The drawing illustrates apparatus that may be used in practicing the invention, in which a refractory mold 1 is positioned on an metal plate 2 which in turn is placed on an insulation base plate 3 made of any suitable insulating material.

A cylindrically-shaped section of graphite material or susceptor 4 is positioned on the plate 2 within an induction coil 6. The susceptor may also be made of tantalum, molybdenum or other suitable material. susceptor 4 is heated by the magnetic field produced in induction coil 6 and the susceptor in turn through radiation heats mold 1 positioned within it.

Refractory melting crucible 7, mold 1 and susceptor 4 are positioned within a vacuum casting furnace (not shown) in a proper relationship to each other, as shown, to permit heating of the mold and metal and the pouring of the melted metal into the mold. Pouring crucible 7 is heated by an independently operated induction coil 8 and is tilted and rotated by conventional tilting means (not shown) to pour the molten metal 9 into the mold 1. Other suitable heating means such as resistance heaters may be used to heat the mold and crucible provided that each is independently controllable and capable of providing sufficient heat to produce readily the required temperatures in varying sized molds and crucibles.

As an example of the practice of the method, a mold shaped for casting a multi-blade turbine rotor was removed from a burnout furnace in which it had been heated to about 1,800.degree.-2000.degree.F. The turbine blades to be formed had thin sections that are only about .015 inch in thickness and had in addition relatively thin blading sections having substantial length and area. The mold was placed within a susceptor and susceptor and mold were placed in a suitable vacuum chamber having facilities for mounting and pouring a molten metal.

A charge of a nickel based alloy having a melting temperature of about 2,300.degree.F was placed in pouring crucible 7 in the chamber and the chamber was closed and evacuated. The charge was then melted and superheated to a temperature of about 2,385.degree.F. The mold temperature was maintained through use of the susceptor at approximately 2,275.degree.F. Mold temperatures as low as 2,100.degree.F may be used but a temperature of about 25.degree. below the melting point is preferred.

Where a mold temperature in the upper end of the range of the invention is used, it is preferred that it be close to the melting temperature of the alloy and should not be more than 150.degree.F above that temperature. It is preferred that the method be commercially operated such that the temperature of the mold is most likely to be just below the melting temperature of the charge.

Temperatures within the desired ranges were accomplished using proper controls together with the judgment of experienced personnel in the casting field. The pattern from which the mold had been made was not coated with a material which imparted nucleating properties to the mold; however, such coatings may be used if desired.

The mold was poured and after a suitable period for solidification of the alloy the mold was removed from the chamber. The casting formed was completely filled and had a fine equiaxed grain structure.

The cooling rate of the casting after initial solidification may be controlled to further influence the formation of grain structures to provide the particular characteristics which are desired for the part to be made.

Claims

I claim:

1. A method of casting a metal-containing material to form an object consisting essentially of said metal-containing material, said object having a thin section and a substantially uniform fine grain structure throughout comprising:

a. placing a mold in a chamber that is capable of being evacuated,

b. placing a charge of material to be poured into a heatable crucible in the chamber,

c. evacuating the chamber,

d. heating the mold and controlling the mold temperature in the chamber within a range having an upper limit of about 150.degree.F. above the melting point of the metal-containing material to be poured and a lower range of 200.degree.F. below that melting point,

e. melting the charge in the crucible,

f. controlling the temperature of the melted charge in the range of approximately 50.degree.F. to 100.degree.F. above the melting point of the metal-containing material, and

g. pouring the charge into the mold.

2. The method of claim 1 in which the charge is a nickel alloy.

3. A method of casting a metal-containing material to form an object consisting essentially of said metal-containing material, said object having thin sections and a substantially uniform fine grain structure throughout comprising:

a. heating a mold having thin pouring spaces of substantial area,

b. transferring the heated mold to a vacuum casting furnace,

c. controlling the temperature of the heated mold in the casting furnace within a temperature range of 150.degree.F. above to 200.degree.F. below the melting temperature of the metal to be poured,

d. controlling the temperature of the metal to be poured within a range of about 50.degree.F. to about 100.degree.F. above its melting temperature,

e. pouring the metal into the mold, and

f. cooling the poured mold to solidify the casting therein under controlled conditions.

4. A method of casting a metal-containing material to form an object consisting essentially of said metal-containing material, said object having a thin section and a substantially uniform fine grain structure throughout comprising:

a. placing a mold in a chamber that is capable of being evacuated,

b. placing a charge of the material to be poured into a heatable crucible in the chamber,

c. evacuating the chamber,

d. heating the mold and controlling the mold temperature at approximately 25.degree.F. below the melting point of the metal-containing material to be poured,

e. melting the charge in the crucible,

f. controlling the temperature of the melted charge in the range of approximately 50.degree.F. to 100.degree.F. above the melting point of the metal-containing material, and,

g. pouring the charge into the mold.

5. A method of casting a metal-containing material to form an object consisting essentially of said metal-containing material, said object having a thin section and a substantially uniform fine grain structure throughout comprising:

a. heating a mold having thin pouring spaces of substantial area,

b. transferring the heated mold to a vacuum casting furnace,

c. controlling the temperature of the heated mold in the casting furnace at approximately 25.degree.F. below the melting temperature of the metal to be poured,

d. controlling the temperature of the metal to be poured within a range of about 50.degree.F. to about 100.degree.F. above its melting temperature,

e. pouring the metal into the mold, and

f. cooling the poured mold to solidify the casting therein under controlled conditions.