Superalloy foils by hot isostatic pressing

Dense superalloy foils are prepared by hot isostatically pressing a mixture of low melting alloy powders and high melting alloy powders at a temperature at least equal to or greater than three-quarters of the melting point of the low melting point alloy powder and below the melting point of the high melting point alloy powder, at a pressure of at least 10 thousand pounds per square inch for about one to five hours.

FIELD OF THE INVENTION 
The present invention relates to a method of forming superalloy foils by 
hot isostatic pressing. More specifically, it relates to hot isostatic 
pressing a mixture of metal alloy powders having different compositions 
into a full density foil. 
BACKGROUND OF THE INVENTION 
Foil, as defined in A Concise Encyclopedia of Metallurgy, by A. D. 
Merriman, MacDonald and Evans LTD. 1965, is a very thin sheet metal with 
no standard thickness, but it is generally regarded as being intermediate 
in thickness between "leaf" and sheet. A Glossary of Metallurgical Terms 
and Engineering Tables, published by The American Society for Metals, in 
1983, lists a definition for foil as "a metal in sheet form less than 0.15 
mm (0.006 inches) in thickness." As used herein, the term "foil" 
designates a thin layer of metal having a thickness range of about 
0.005-0.017 inches. 
A process for making foils is disclosed in U.S. Pat. No. 4,917,858. U.S. 
Pat. No. 4,917,858 teaches blending a powder of titanium with a powder of 
aluminum in preselected proportions and rolling the mixed powders to a 
predetermined thickness to form a foil of a titanium-aluminum alloy and 
then sintering the rolled foil followed by hot pressing the foil to 
densify the foil to near theoretical density. U.S. Pat. No. 4,917,858 also 
discloses the incorporation of a third metal powder as an alloying 
element. Niobium, molybdenum, vanadium, chromium, manganese, erbium, and 
yttrium are listed as candidate third powder additives. The drawback of 
this method is that several intermediate steps, such as rolling, vacuum 
sintering, and pressing, must be carried out in order to obtain thin 
foils. Further, U.S. Pat. No. 4,917,858 is limited to utilizing powders of 
single elements and their alloys to form a foil. 
Therefore, it is desirable to provide a method for making foils compatible 
with a variety of prealloyed compositions, preferably in powder form, and 
particularly with admixtures of prealloyed powders having different 
physical properties (e.g. different melting points and softening points), 
which provides greater improvement of alloy composition, thereby allowing 
low ductility alloy foils to be manufactured. 
Additionally, it is desirable to provide alloy foils that can be utilized 
in repair and joining operations. 
SUMMARY OF THE INVENTION 
The present invention provides a method for preparing a superalloy foil 
comprising at least two alloy compositions having different melting points 
or softening points. Herein, low melting point alloy is referred to as low 
melting powder or low melting particles. High melting point alloy is 
referred to as high melting powder or high melting particles. 
The invention comprises a method for producing a foil comprising the steps 
of admixing a low melting powder and a high melting powder; and hot 
isostatic pressing the admixture at a temperature of at least equal to or 
greater than three-quarters of the melting point of the low melting point 
metal powder and below the melting point of the high melting point metal 
powder, at a pressure of at least 10 thousand pounds per square inch, 
herein ksi, for a period of time sufficient to form a foil having a 
workable density and about 0.008 inches to about 0.017 inches thick. 
Hot isostatic pressing is defined in Metals Handbook, 9th Edition, Vol. 7, 
p. 419, as a "materials processing technique in which high isostatic 
pressure is applied to a powder part or compact at elevated temperatures 
to produce particle bonding. This process usually results in the 
manufacture of a fully dense body, although partially dense bodies also 
can be intentionally produced. During processing, the compact is subjected 
to equal pressure from every side." 
Plastic deformation of the low melting point powder during hot isostatic 
pressing allows the low melting particles to soften and squeeze between 
the high melting particles, thereby forming a matrix-particle composite, 
strengthened by bonding mechanisms, which can be mechanical or diffusional 
in character. During hot isostatic pressing, some interdiffusion of 
elements occurs between the high melting powder and low melting powder. 
After hot isostatic pressing, the cans are etched away from the foil. An 
example of an etchant is a solution of 50% nitric acid-50% water. The 
average thickness of the foils is about 0.012 inches to about 0.013 
inches, with a range of thickness of about 0.008 inches to about 0.017 
inches. The foils have a minimum workable density of about 80 to about 95 
percent theoretical density. 
It is therefore an object of this invention to provide a method for making 
an alloy foil comprising low and high melting powders that can be used to 
repair or join superalloy structures. This would include low ductility 
alloys, which are otherwise unfabricable. 
It is also an object of this invention to provide a one step process of 
making superalloy foils that is a simpler, faster, and more economical 
method of making foils. 
DETAILED DESCRIPTION OF THE INVENTION 
It has been found that mixtures of metal powders having varying 
compositions and different melting or softening points can be hot 
isostatically pressed directly into metal foils. Such foils can be used to 
join compatible metal surfaces by application of heat and pressure. 
In the execution of this technique, one or more of the powder compositions 
is such that it has a relatively low melting point. This allows the foil 
to be hot isostatically pressed at or around the low melting point of this 
constituent. The rest of the powder in the mixture may be of one 
composition, or more than one, depending on the final desired chemistry of 
the foil. This powder may have a high melting temperature. 
Foils made by this combination of low melting and high melting powders are 
herein referred to as reactive foils. The term "reactive foils" means 
foils formed by the reaction of low melting powders and high melting 
powders. 
After hot isostatic pressing at a low temperature, the reactive foil can be 
subjected to a high temperature processing step that ultimately results in 
homogenization of the foil. 
Alternatively, the low melt powder and high melt powder may have 
compositions such that homogenization of these powders results in a 
chemistry that is very brittle, such as an intermetallic. In such a case, 
when the alloys form a brittle foil that cannot be heat treated to 
homogenize, hot isostatic pressing of the powder mixture may be done at a 
temperature such that melting of the low melting powder and homogenization 
of the powders occurs during the hot isostatic pressing step. The hot 
isostatic pressing temperature would still be below the melting point of 
the high melting powder. 
The low melting powder and high melting powder can be prepared by 
conventional powder making processes known in the art, such as gas 
atomization techniques or mechanical alloying. It is preferred that the 
powders have a generally spherical shape. 
The low melting powder and high melting powder used to form the dense foil 
of the instant invention can have a particle size in the range of about 50 
microns to about 150 microns. The particle size influences the flowability 
of the powder during loading of the can. In comparison to the high melting 
particle, the low melting particle is less than or equal to the high 
melting particle size. This is so that the low melting particles will 
distribute as a network between the high melting particles. 
Powders should be well mixed prior to loading into the hot isostatic 
pressing can. This is accomplished by pouring the powders into a glass jar 
and rotating the jar for about thirty minutes on a set of rollers. Other 
suitable means of mixing the low and high melting powders can be utilized. 
The volume percent of the low melting powder mixed with the high melting 
powder is about ten to about 50 percent. The percent selected is based on 
the desired composition of the reactive foil and its later application. 
Subsequent to blending the high melting and low melting powders, the 
mixture is loaded into a can. The functional requirements of the can 
include control of shape and dimension prior to and during processing; 
maintenance of leak tightness against low and high pressure during 
evacuation, sealing, and densification; noncontamination of powder; and 
minimal interaction with powder by diffusion processes during the hot 
isostatic pressing cycle. 
During loading of the can with a mixture of high and low melting powders, 
the can is vibrated ultrasonically to facilitate even packing of the 
powder. The packing density of the powder mixture in the can is about 
sixty to about sixty-five percent. Powder packing density is defined as 
the ratio of apparent density of settled powder to 100% dense material. 
After the can loading step, hot isostatic pressing, sometimes referred to 
as HIPing, of the sealed can takes place. 
Suitable temperatures, for the practice of this invention, range from about 
480.degree. C. for low melting powders, such as aluminum or aluminum 
alloys, to about 1700.degree. C. for high melting powders, such as 
tungsten or tungsten alloys. In the practice of this invention, the 
temperature applied during hot isostatic pressing is dependent on the low 
melting powder composition. Generally speaking, the temperature-selected 
is equal to or greater than about three-quarters of the melting point of 
the low melting powder. 
In some alloy systems, a temperature may be selected so that the low 
melting powder softens during processing, without becoming completely 
molten. The resultant foil contains a high melting phase and a re-meltable 
low melting phase. 
In other alloy systems, a higher temperature for processing may be chosen, 
depending on the desired microstructure of the reactive foil. At higher 
temperatures, the foil has a homogeneous structure containing 
intermetallic phase particles. The higher temperature during hot isostatic 
pressing is always below the melting point of the high melting powder. 
In this invention, applied pressures are equal to or greater than about 10 
ksi, with 15 ksi being an overall average pressure utilized. It is to be 
understood that the pressure and temperature are increased simultaneously 
during the hot isostatic pressing process. The hot isostatic pressing time 
is typically about four hours, and can be varied between about one and 
five hours. 
After the completion of hot isostatic pressing, the reactive foil is 
removed from the can by chemical etching. Etchants are chosen so as not to 
attack the reactive foil. The instant invention utilizes a solution of 50% 
nitric acid and 50% water as an etchant when the reactive foil is a 
nickel-based superalloy.

Examples 1-3 illustrate the invention. 
Three reactive foils of nickel-base superalloys were made using mixtures of 
metal powders. Each foil utilized four different metal powder alloys in 
its composition. The metal powder alloys were selected from the following 
powders, given in weight percent: 
POWDER A: Ni--7.5Co--9.2Cr--1.5Mo--6.0W--4.0Ta--3.7Al--4.2Ti--0.5Nb 
POWDER B: Ni--20Cr 
POWDER C: Ni--60Al--1B 
POWDER D: Ni--36Ti--1B 
POWDER E: Al--11.6Si 
The sizes of powders A-E are given in Table 1. Powder size fractions were 
not optimized for powder flowability, but represent what was available for 
those compositions. Of these, the Ni--60Al--1B, the Ni--36Ti--1B, and the 
Al--11.6Si were low melts. 
TABLE 1 
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POWDER MESH SIZE MICRONS 
______________________________________ 
A +170 &gt;88 
B -140 + 270 53-105 
C -120 + 325 44-125 
D -20 &lt;841 
E -140 + 325 44-105 
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Table 2 gives the powder mixtures and their volume fractions for reactive 
foils 1-3. 
TABLE 2 
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POWDER MIXTURES FOR REACTIVE FOILS 
Foil No. 
A B C D E 
______________________________________ 
FOIL 1 85 wt/o 5 wt/o 5 wt/o 
5 wt/o 
-- 
FOIL 1 80 vol/o 4 vol/o 10 vol/o 
6 vol/o 
-- 
FOIL 2 60 wt/o 5 wt/o 20 wt/o 
15 wt/o 
-- 
FOIL 2 48 vol/o 4 vol/o 33 vol/o 
15 vol/o 
-- 
FOIL 3 80 wt/o 5 wt/o -- 5 wt/o 
10 wt/o 
FOIL 3 66 vol/o 4 vol/o -- 5 vol/o 
25 vol/o 
______________________________________ 
The powders were well mixed in a glass jar by rotating for 30 minutes, and 
then loaded into a hot isostatic pressing can. The can was vibrated 
ultrasonically during powder loading, to facilitate even packing of the 
powder. Hot isostatic pressing was carried out under argon at 15 ksi and 
875.degree. C. for four hours. Temperature and pressure were increased 
simultaneously during hot isostatic pressing. The foils were removed from 
the cans by etching in a solution of 50% nitric acid-50% water. The 
average thickness of the foils was 0.012-0.013 inches, with a range in 
thickness of 0.008-0.017 inches. 
Examination of the foils using an optical microscope shows that the 
unmelted powder, POWDER A and POWDER B, were held together in the foil by 
a network of low-melting powder, which had melted during the hot isostatic 
pressing process. There was little or no deformation of POWDER A and 
POWDER B, as would be expected at the low hot isostatic pressing 
temperature. There was very little porosity in the foils, and fairly 
extensive diffusion interaction between the unmelted powders, A and B, and 
the molten powders, C, D, and E.