US2946680A - Powder metallurgy - Google Patents

Description

July 26, 1960 w. R. RAYMQNT POWDER METALLURGY Filed Aug. 10. 1955 Ala/AN A? RAY/V047 Figure 2 is an isometric view of a porous refractory mold assembly for the preform of Figure 1 showing infiltrant metal particles for the infiltration operation of this invention.

Figure 3 is a vertical cross-sectional view alongthe line III-Ill of Figure 2 with the particles omitted from the mold cavity.

Figure 4 is a vertical cross-sectional somewhat diagrammatic view of the mold of Figure 2 mounted in a vacuum furnace for the heat treating operations.

Figure 5 is a view similar to Figure 3 and illustrating the assembly after the heat treatment in the furnace of Figure 3.

Figure 6 is a perspective view of a finished turbine bucket produced by the method of this invention.

As shown on the drawings:

In Figure 1 the die assembly -10 includes a pair of opposed punches or dies 11 slidably guided in a fixture 12 and having active faces cooperating to produce a preform 13 of the desired airfoil contours of fluid directing members.

The material used to form the preform 13 may be a powder of any of a variety of ceramic, intermetallic or other refractory compositions such as for example, alumina, titanium carbide, zirconium boride, tungsten carbide and the like. It is preferred that particle size of these powders be relatively small and distributed more or less uniformly over a desired particle size range. A particularly effective particle size distribution pattern for titanium carbide includes about 35 parts by weight having a maximum dimension of 3 microns, about 32 parts by weight having a dimension in the range of from 3 to 6 microns, about 30 parts by weight having a dimension in the range of from 6 to 12 microns, and a maximum of about 3 parts by weight having a particle size in excess of 12 microns.

While it is not absolutely essential that the refractory particles be pure, better results will be obtained if certain contaminants are held within reasonable limits. For example, the specifications for the titanium carbide which are employed this process are substantially as follows:

Table l Ingredient: Percent by weight Combined carbon minimum Free carbon ..maximum.. Iron do Oxygen d 1.2. Nitrogen do 0.25 Hydrogen do 0.03 Other impurities ..-do 0.75

The procedure for producing the preform may vary. One preferred procedure consists in mixing the refractory particles with a thermally depolymerizable binder such as polybutene, the binder constituting from about 5 to 35% by volume of the compact. Normally, the binder is added in solution in a suitable solvent such as xylene. The preform is then shaped cold in the dies 11 at pressures of about 0.5 to 25 tons per square inch and heated to a temperature sufficient to depolymerize the binder, and drive off the solvent. Processes of this type are fully described in US. Patent No. 2,593,507 to Eugene Wainer.

Another technique includes formation of a press block of the powder in the dies 11 at die pressures in the range-of 0.5 to 50 tons per square inch followed by presintering of the block in a vacuum furnace having a pressure of from 0.1 to 500 microns of mercury. The pre-sintering is conducted at temperatures of from 2000 to 2650 F. for a period varying from 5 minutes to 5 hours. The die block is then machined to the desired contour or alternately, of course, could be die shaped as accurately as possible.

According to this invention the preform 13 is to be infiltrated and heat treated in an inert porous mold which will impart finished surface characteristics to the blade. While a number of mold materials are useful, zirconium oxide, stabilized against crystallographic changes, is preferred. A heat stabilizer such as calcium oxide which reacts with zirconium oxide to form a stabilized crystallographic material, is used. Normally, about 1% or less of calcium oxide will be sufiicient to stabilize the zirconium oxide at any temperatures reached during the heat treatment and infiltration of the preform.

It is preferred to have the zirconium oxide particles more or less uniformly distributed in the range of from between 5 and 44 microns. To aid in shaping the mold, a lubricant such as calcium stearate or lead stearate in an amount of 1 to 5% of the total composition can be used together with a binder such as methyl cellulose. About 1 to 2% by weight of a methyl cellulose solution having a 5% concentration in water will normally be suflicient.

Pressures employed in shaping the mold in metal dies (not shown) may vary widely but usually pressures of about '1 to 5 tons per square inch will be satisfactory.

The green mold is fired at temperatures of from 2000 to 3000 F. for a period of from 30 minutes to 5 hours. Usually a 2-hour firing treatment at 2500 F. is preferred. As explained above, the firing can occur simlultaneously' with the sintering of the preform in the mo d.

As shown in Figures 2 and 3, the refractory mold is illustrated as a vertically split mold 14 composed of halves or sections 15 and 16 together cooperating to define a mold cavity 17 which snugly receives the vane portion of the preform 13 while a rounded head 13a of the preform projects into an enlarged cavity portion 17a. This cavity portion f17a communicates with a gate passage 18 projecting laterally from an end of the cavity 17a to a sprue cup 18a alongside the mold and having The mold sections 15 a cavity feeding the gatepassage. and 16 are held together in any suitable manner as by means of clamps, insertion in a sheath, or the like.

If desired, the preform 13 can be made directly in the mold. Thus the powder can be incorporated in a suitable slurry which is then slip cast into the porous mold which will drain off the liquid components of the slurry and confine the solids in the shape of the mold cavity. Alternately, the powder can be centrifugedin the mold to form the preform.

7 As shown in Figures 2 and 4, the mold 14 with the preform 13 therein has infiltrant metal particles 19 de-. posited in the sprue cup 18a and surrounding the preform end 13a. The mold assembly is now ready for the infiltration step and is placed in a sealed furnace 20 which can be evacuated or flooded with an inert gas such as argon or helium to maintain an inert atmosphere around the mold. The furnace is heated as by means of electrical heating elements 21 which surround the mold 14.

The infiltration is carried out at temperatures ranging from about melting point of the infiltrant metal to about 200 above that melting point. The infiltration step will be completed in a time period. from as little as 5 minutes to as much as 2 hours or more.

In addition to providing a sufficient amount of the corrosion resistant metal to impregnate completely the pores of the porous preform. 13, additional amounts of the infiltrant metal are provided to form a cast root 22 (Fig. 5) for the turbine blade, the root completely enveloping and bonded to the anchoring end 13a formed on the preform 13. The metal of the root 22 and the infiltrant of the pores of the preform 13 thereby provide a continuous, mono-metallic single phase system which not only provides the strength and corrosion resistance required in turbine blading or the like but also provides for a permanent bond between the vane portionand the root portion of the turbine blading.

Numerous difierent infiltration metals can be employed 5; to advantage in the present invention. resistant nickel-chromium.alloys-and thecobalt base alloys are particularly valuable for this purpose. The-commercial heat resistant alloys such as thoseof the In! cone and heat resistant nitridedstels (Nitralloy) may also be employed. A typical lnconel alloy :(fInconel X) has the following composition; l.

Table 11 Element: Percent by weight C 508 maximum Mn .05 to 1 Si .06 maximum C1 1 4-1 6 Al 0.5-1.0 Ti 2.0-2.6 C DEB-1L2 Fe 6-8' Ni Balance After the infiltration has been completed, the infiltrated comp-act can be further heat treated in the mold. For example, this can be accomplished byzmerelyftlroppjng the temperature of the assemblyfrorn the-infiltration temperature to a temperature which will normally he on :the order of 200 F. or so below the melting point of the infiltrant. Again, the heat treatment time will vary considerably depending upon the materials employed, the strength desired, the porosity, and similar factors, but ordinarily periods ranging from minutes to 2 hours will be employed. The heat treatment, like the infiltration, is carried out under non-oxidizing conditions, and preferably under vacuum conditions in which the absolute pressure is in the range from about 0.5 to 500 microns 'of mercury.

As shown in Figure 6, a turbine bucket 23 formed according to this invention has a vane portion 23a of airfoil shape and an enlarged massive anchoring root end portion 23b. The vane portion 23a is composed of a skeleton network or matrix of the refractory compounds with the pores of the network or spaces between the particles filled with the infiltrant metal in firmly bonded relation thereto. The root end 23b is composed of the infiltrant metal although it also has a core of the refractory compound surrounded by the infiltrant metal. All surfaces of the bucket 2-3 are smooth and have imparted thereto a finish of the walls defining the mold cavity. Since the mold material is not wet by the infiltrant metal and since the mo-ld is quite porous, the metal can freely flow to all surfaces of the bucket without causing the bucket to stick to the mold.

According to this invention it is also practical to form the mold in one piece around the preform and to simulaneously pre-sinter the preform and fire the mold. This will prevent relative shrinkage between the preform and h mold so that stresses are minimized. In this modification the preform can be completely enveloped by the mold. This modification has the advantage of eliminating separate firing and sintering steps and also eliminating the necessity for machining the end of the root which would otherwise be exposed in the sprue.

In order to further control the shrinkage, the particle sizes and relative densities of the preform and mold canbe regulated. For example, suitable shrinkage conditions are obtained by making a preform of titanium carbide powder of less than 5 micron particle size under a pressure of 1 ton per square inch at room temperatures. The powder can contain about 1% lubricant or plasticizer such as Sterotex or paraffin wax. The preform thus formed has a density of about 50%. i

The mold is formed from zirconium oxide powder of about 20 to 40 micron size, is pressed at room temperature at pressures of about 1 ton per square inch and may have a lubricant and a binder added. A suitable lubricant is about 3% by Weight Sterotex. A suitable binder is about 2% by weight of a 5% solution of methyl The rcorrosion causes a simultaneousshrinkage of. thenrold'randfpreform in amounts of 7 to 8% by volume and :theresulting parts will have .ailensity of about 623to=64-%-. .The-sint'ering and firing can occur simultaneously with the preform the mold or'separated from the mold.

.If the mold is formed around the preform, ,provision must be made to contact the preform with the infiltrant metal. The infiltration can then occur at-temperatures of about 2600 F. V

If shrinkage of the mold and preform are notcorrelated, the mold cavity should be shaped so that-itwill permit relative shrinkage without-stressing thepreform to a rupture point. The pre-sintering and pre firing of the mold will bring the parts down to a common remaining, shrinkage factor where they will shrink at substantially equal rates when heated-during the subsequent infiltrating and sinteringoperations.

Carrying out theinfiltration under vacuum conditions takes advantage ofthe improved properties of alloys melt: ed under vacuum conditions as comparedSWith air melted alloys. For :example, a typical corrosion resistant, alloy has a ductilityabont four times as great whemmeltedj'in vacuum as compared to its ductility whenmelted 'in, air.

The following physical properties were obtained by the process of the present invention employing titanium carbide particles as the matrix and Inconel X as the infiltrant metal:

Table III Density 6.2 gms./cc. Tensile strength, room temperature 50,000 p.s.i. Modulus of elasticity at 1800 F... 40,000 p.s.i.

Stress rupture strength:

hr. life at 1600 F 40,000 p.s.i. 100 hr. life at 1800 F 15,000 p.s.i. Thermal expansion, per F.:

From 70' to 1200 F 5.5 10- in./in. From 70 to 1800" F 6.0 10- in./in. Thermal conductivity 0.063 .cal./sec./cm.

C./cm. Electrical resistivity 1.37-1.43 10 ohm/ cm. Impact strength, unnotched Izod--. 8-10 ft. lbs. Hardness 55-58 Rc. Weight gain, after 100 hrs. in still air at 1800 F 20-30 mg./cm. Transverse strength:

Room temperature- 190,000-250,000 p.s.i. 1600 F 150,000-190,000 p.s.i. 1800" F 115,000-150,000 p.s.i.

It will be'evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

I claim as my invention:

1. The method of making corrosion resistant shapes from refractory compositions which comprises forming a refractory powder into a self-sustaining shaped preform, supporting said preform in aporous ceramic mold Which has a thermal shrinkage rate substantially the same as that of said preform, infiltrating said preform while in said mold with a molten corrosion resistant composition under non-oxidizing conditions, and heat treating the infiltrated preform while the same is still confined in said mold.

2. A method of making corrosion resistant shapes from refractory compositions which comprises forming refractory powder into a self-sustaining shaped preform, supporting said preform in a tight fitting complementary ceramic mold having a thermal shrinkage rate substantially the same as that of said preform, infiltrating said preform while in said mold with a molten corrosion resistant composition under vacuum conditions, and heat treating the infiltrated preform while the same is still confined in said mold.

3. The method of making improved corrosion resistant shapes from refractory compositions which comprises forming a refractory powder into a self-sustaining shaped preform, confining said preform in a tightly fitting ceramic mold having a thermal shrinkage rate substantially the same as that of said preform, infiltrating said preform in said mold with a molten corrosion'resistant infiltrant composition under non-oxidizing conditions, reducing the temperature after completion of infiltration to a temperature below the melting point of the infiltrant composition but high enough to heat treat the infiltrated mass, and heat treating said infiltrated mass at said temperature. 4. The method of making an infiltrated article having controlled surface characteristics which comprises compacting a.powder to provide a self-sustaining preform of desired shape, forming a complementary preform mold having a shrinkage rate substantially the same as the shrinkage rate of the preform at the infiltration temperature, assembling the preform in the mold, and infiltrating the preform at an elevated infiltration temperature with a material that will not wet the moldbut which is compatible with the preform, allowing the preform and mold to shrink at substantially the same rate, and continually ca supporting the preform in the mold whereby the mold finish will be imparted to the preform and the surface characteristics of the infiltrant material will be controlled by the mold finish. r

5. The method of making an infiltrated powdered metal article which comprises compacting a powdered metal to form a self-sustaining preform of desired shape, forming a preform mold for said preform, regulating the particle sizes and relative densities of the preform and mold to provide substantially the same shrinkage rate for the preform and mold, assembling the preform in the mold, contacting the preform with an infiltrant metal which is compatible with the preform but which will not wet the mold, heating the assembly above the melting point of the infiltrant metal to infiltrate the preform with the metal, allowing the preform and mold to shrink at substantially the same rate, and continually supporting the preform in the mold to thereby control the surface characteristics of the resulting article. V

References Cited in the file of this paten UNITED STATES PATENTS rea g-

Claims (1)

1. THE METHOD OF MAKING CORROSION RESISTANT SHAPES FROM REFRACTORY COMPOSITIONS WHICH COMPRISES FORMING A REFRACTORY POWDER INTO A SELF-SUSTAINING SHAPED PREFORM, SUPPORTING SAID PREFORM IN A POROUS CERAMIC MOLD WHICH HAS A THERMAL SHRINKAGE RATE SUBSTANTIALLY THE SAME AS THAT OF SAID PREFORM, INFILTRATING SAID PREFORM WHILE IN SAID MOLD WITH A MOLTEN CORROSION RESISTANT COMPOSITION UNDLER NON-OXIDIZING CONDITIONS, AND HEAT TREATING THE INFILTRATED PREFORM WHILE THE SAME IS STILL CONFINED IN SAID MOLD.

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