Alternate Method for Inducing Superplastic Properties in Nonsuperplastic Metal and Alloy Powders

The disclosed invention is related to my commonly assigned co-pending application Ser. No. 361,279 entitled "A Method for Inducing Superplastic Properties in Nonsuperplastic Metal and Alloy Powders" filed concurrently herewith.

1. Field of the Invention

The invention is related to the field of powder metalurgy and in particular to a method for inducing superplastic properties in metal and alloy powders which have no superplastic properties.

2. Prior Art

Superplasticity is a property of certain alloys that allows them to be extensively deformed under appropriate conditions with very little stress. The prerequisite of superplastic alloys are defined by J. Wadsworth, T. Oyama and O. Sherby in their presentation "Superplasticity--Prerequisites and Phenomenology" at the Inter-American Conference on Materials Technology, Aug. 12-15, 1980, San Francisco, Calif., and by H. W. Hayden, R. C. Gibson and J. W. Broply in their article, "The Relationship Between Superplasticity and Formability", Metalurgical Society AIME, Plenum Press, 1971, pp. 475-497. Accordingly, for an alloy to exhibit superplasticity it should be of microduplex structure having a grain size of less than 10 micrometers, be either a eutectic or eutectoid composition, having a high strain rate sensitivity of flow stress and high angle grain boundaries.

A typical superplastic alloy is the nickel based alloy disclosed by Frecke et al in U.S. Pat. Nos. 3,702,791 and 3,775,101. Other superplastic alloys are described in the articles by J. Wadsworth et al and H. W. Hayden et al cited above.

Marya and Wyon, Proceedings of the 4th International Conference on the Strength of Metals and Alloys, Nancy France, Vol. 1, 1976, pp. 438-442 and Weill and Wyon, Proceedings of the 5th International Conference on the Strength of Metals and Alloys, Aachen, W. Germany, Vol. 1, 1979, pp. 387-392, have succeeded in making fine grained aluminum-gallium alloys superplastic at 50.degree. C. by rubbing gallium on an aluminum surface and heat soaking the wetted aluminum at 50.degree. C. for up to 50 hours. The invention is an alternative method for inducing superplastic properties in nonsuperplastic metal and alloy powders.

The invention is a method for inducing superplastic characteristics in nonsuperplastic metal and alloy powders. The method comprises adding to a nonsuperplastic material, metal or alloy in powder form, a pure metal capable of alloying with the nonsuperplastic material to form a eutectic having superplastic properties and melting temperature less than the nonsuperplastic material. The powder is then compacted under pressure and at a temperature either at or above room temperature to form a billet. The billet is heated to the temperature at which the eutectic forms then extruded or molded to the desired shape utilizing the superplastic properties of the formed eutetic. The formed article is subsequently heated to between 15.degree. C. and 30.degree. C. below the melting temperature of the formed eutectic to restore the grain boundaries of the nonsuperplastic alloy through the diffusion of the pure metal into the nonsuperplastic material.

The advantage of the disclosed method is that many nonsuperplastic metal or alloy powders can be made to appear as if they have a superplastic state. This apparent superplastic state permits these alloys to be formed into the desired shape using conventional extrusion and molding techniques at much lower temperatures and pressures. These and other advantages of the disclosed method will become apparent from a reading of the specification in conjunction with the drawings.

FIG. 1 is a flow diagram of the invented process.

Referring to the flow diagram of FIG. 1, a pure metal is added to a base metal powder made from a material not having superplastic properties as indicated in block 10. The material of the base metal powder may be either a pure metal or a metal alloy. The added pure metal should have the following characteristics:

(a) should be soluble and of high diffusivity in the base metal;

(b) should alloy with the base metal to form a eutectic having superplastic properties;

(c) should not significantly alter the properties of the base metal and/or cause embrittlement; and

(d) should not be contaminated by the processing environment.

Correspondingly, the formed eutectic should have the following characteristics:

(a) should have a grain size of less than 10 micrometers;

(b) should have a melting point less than the melting point of the base metal;

(c) should have high angle grain boundaries; and

(d) should have properties close to the properties of the added pure metal.

The pure metal may be added to the base metal powder as a thin coating applied to the external surfaces of the individual particles of the base metal powder or in powder form.

The individual particles of the base metal powder may be coated with the added pure metal by vacuum deposition, sputtering, ion beam milling, vapor deposition, electroplating or any other applicable method know in the art. The thickness of the coatings is selected such that the formed eutectic represents, preferably 6 to 8 percent or less of the total volume of each base metal particle. However, somewhat thicker or thinner coatings may be used.

When the pure metal is added in powder form, the quantity of added pure metal powder, again, is preferably selected so that the formed eutectic represents 6 or 8 percent by volume. The two powders are then thoroughly mixed using a shaker or ball mill to form a homogeneous mixture.

The base metal powder with the added pure metal is then compacted to form a billet as indicated by block 12. The compacting introduces strain energy into the system which acts as a driving force for the subsequent alloying and formation processes. The billet is then heat soaked at the melting point of the eutectic to be formed to allow the formation of the eutectic at the grain boundaries of the base metal particles as indicated by block 14. Soaking time may vary from 15 minutes up to a few hours depending upon the metals being alloyed and the billet size.

The billet is then formed to the desired shape utilizing the superplastic properties of the formed eutectic as indicated by block 16. The forming of the billet may be accomplished by conventional extrusion, molding, or other forming techniques known in the art at a temperature where the formed eutectic exhibits superplastic characteristics.

The forming of the billet may immediately follow the formation of the eutectic by lowering the temperature of the billet from the melting point of the eutectic to the temperature at which the eutectic has superplastic properties. Alternatively, after the formation of the eutectic the billet may be cooled to room temperature for storage and forming at a later time.

The formed billet is subsequently heat soaked at a temperature from 15.degree. C. to 30.degree. C. below the melting temperature of the formed alloy to further diffuse the pure metal into the base metal as indicated by block 18. By this process, most of the properties of the base metal grain boundaries are restored and the superplastic phase of the eutectic composition destroyed.

For nonsuperplastic ferrous metals and alloys aluminum may be added which will alloy with iron to form a eutectic having superplatic properties and a melting point of approximately 335.degree. C. For aluminum and aluminum alloys the added pure metals may be lithium forming a eutectic having superplastic properties at approximately 160.degree. C. or antimony forming a eutectic having superplastic properties at approximately 620.degree. C. For copper and copper alloys the added pure metal may be tin forming a eutectic having superplastic properties at approximately 200.degree. C.

Indium may also be used as the added metal for copper and coper alloys but no eutectic will form, however the resulting phase will have characteristics similar to those of superplastic materials. Additionally, aluminum may be used in combination with nickel and nickel alloys.

The advantages of this method are:

(1) Many nonsuperplastic metals and alloys can be made to appear as if they have superplastic properties.

(2) Nonsuperplastic metals and alloys can be formed at reduced temperatures and pressures thereby reducing tooling requirements.

(3) The base or nonsuperplastic material does not have to have ultra-fine grain sizes.

(4) Eliminates the problems encountered during superplastic forming of materials having thermodynamically unstable superplastic structures.

(5) Processing detail can be adjusted to obtain the high angle grain boundaries required for superplastic forming.

It is not intended that the invention be limited to specific examples disclosed and discussed herein. It is submitted that there are other pure metals which can be added to base metals and/or alloys to form a eutectic imparting superplastic properties to nonsuperplastic alloys within the scope of the invention as described herein and set forth in the appended claims.