Method of forming and subsequently heat treating articles of near net shaped from powder metal

This invention relates to a method of forming and subsequently heat treating articles of near net shape from powder metal which includes the steps of producing a thickwalled container by forming a cavity of predetermined shape in a mass of suitable container material such that the walls of the container are of sufficient thickness so that the exterior surface thereof does not closely follow the contour of the cavity, filling the container with powder metal, applying heat and pressure to the container such that the container material acts like a fluid to apply hydrostatic pressure to the heated powder contained in the cavity thereby consolidating the powder metal to produce a densified compact, preparing the densified compact for heat treating by selectively removing portions of the container to form a jacket of container material around the densified compact, heat treating the densified compact and completing removal of the container material.

FIELD OF THE INVENTION 
This invention relates to a method of forming and subsequently heat 
treating articles of near net shape from powder metal. 
BACKGROUND OF THE INVENTION 
The use of powder metallurgical techniques has become popular with high 
alloyed materials due to the problems encountered in casting such 
materials, e.g., segregation and resulting loss of physical properties. 
For example, powder metallurgical techniques are used extensively with 
nickel, cobalt, and ferrous-base superalloys. These are high temperature 
-- high strength alloys used for making turbine discs, blades, buckets, 
and other components of jet engines which are subjected to high stress at 
mid-range or high temperatures. The very properties which make these 
alloys attractive for use in jet engines cause the consolidation of the 
powders to be difficult. Moreover, subsequent operations, such as forging 
and machining the resulting densified compact, to produce a final part are 
also difficult because of the high strength and toughness of these alloys. 
Due primarily to the difficulties encountered in post-consolidation 
processing, efforts have been made to produce "near net shapes". As used 
herein, a near net shape is a densified powder metal compact having a size 
and shape which is relatively close to the desired size and shape of the 
final part. Heretofore, crude preforms have been produced which require 
extensive forming and machining to produce the relatively complex final 
part. Producing a near net shape reduces the amount of post-consolidation 
processing required to achieve the final part. For example, in many 
instances subsequent hot forging may be eliminated and the amount of 
machining required may be significantly reduced. Since these materials are 
difficult to machine, a reduction in the amount of machining offers a 
marked savings in tool and labor costs. Additionally, these materials are 
quite expensive, therefore, a reduction in machining results in a savings 
in material costs. Obviously, eliminating or reducing the amount of hot 
forging also offers savings advantages. 
While the desirability of producing near net shapes has been recognized, 
many problems have been encountered in accomplishing this objective. The 
basic step of consolidating the metal powder to produce a powder metal 
compact having a near net shape has been a major obstacle. Once an 
acceptable near net shape is produced, other problems are presented. One 
of these relates to the heat treatment of the densified compact to achieve 
maximum physical properties. 
Due to the fact that a near net shape is being produced, the configuration 
of the densified compact is relatively complex. Hence, the section size of 
the densified compact may vary greatly. As is well-known in the heat 
treating art, variations in section size may cause distortion and internal 
stresses in the densified compact due to differences in the rates of 
heating and cooling. The rate of heating also affects time at temperature 
which is determintive of the physical properties of the heat treated 
compact. Thinner sections, which reach temperature first, will be 
subjected to a longer holding period at temperture than thicker sections. 
This may result in significant, and most likely undesirable, differences 
in physical properties in various sections of the compact. For example, in 
an alloy strengthened by age hardening, overaging may occur in the thinner 
sections. Relative cooling rates are also critical in achieving a 
relatively uniform microstructure. Additionally, where heat treat 
temperatures approach the fusion temperature of the lowest melting 
constituent, the densified compact will become subject to deformation 
under relatively low stresses. Therefore, the densified compact is easily 
distorted. This problem is particularly acute in thinner sections which 
may deform under their own weight. Other problems associated with heat 
treating parts of complex shape should be immediately apparent to those 
knowledgeable in the art. 
BRIEF DESCRIPTION OF THE INVENTION 
This invention is directed to a method of forming and subsequently heat 
treating articles of near net shape from powder metal which offers unique 
solutions to many of the problems heretofore encountered. Generally, the 
method includes producing a thick-walled container from a mass of fully 
dense and incompressible material which is capable of plastic flow at 
elevated temperatures. The thick-walled container employed is disclosed in 
a co-pending U.S. patent application of the inventor herein, Ser. No. 
692,310, filed June 3, 1976. A cavity of predetermined shape is formed in 
the mass of material such that the walls of the container are of 
sufficient thickness so that the exterior surface thereof does not closely 
follow the contour of the cavity. It has been found that this type of 
container is capable of producing near net shapes having surprisingly 
close dimensional tolerances with a minimum of distortion. 
The cavity of the container is then filled with powder metal of desired 
composition. In some cases, the container is evacuated prior to filling to 
place the cavity under a vacuum. The container is then sealed. Heat and 
pressure are applied to the filled and sealed container whereby the 
container material acts like a fluid to apply hydrostatic pressure to the 
heated powder metal contained in the cavity thereby consolidating the 
powder metal to produce a densified compact. The densified compact is then 
prepared for heat treating by selectively removing portions of the 
container. As a general rule, less container material is removed from the 
regions surrounding thin sections than from the regions surrounding 
thicker sections. In this manner, the mass of the thinner sections are, in 
effect, increased by the container material. In this manner, the rate of 
heating and cooling can be adjusted. The container material helps to 
physically support the thinner sections at elevated temperatures to resist 
deformation. The modified container and densified compact combination are 
appropriately heat treated. During heat treating, the container material 
serves as a protective barrier to prevent surface contamination of the 
densified compact. After heat treating, the remaining container material 
is removed from the densified compact thereby producing a near net shape.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will be described with respect to a part made from Astroloy 
powder, a precipitation hardened nickel-base superalloy. The specific 
configuration of the part shown in the flow diagram is not intended to 
depict an actual production part, but is shown by way of example to 
illustrate a near net shape of relatively complex configuration. Similar 
shapes, however, are encountered in actual practice. It is to be 
recognized that other types of metal powder as well as other complex 
shapes may be produced in the manner disclosed herein. 
As shown in Step 6 of the flow diagram, the desired near net shape, 
generally shown at 10, includes a disc-shaped body 11 having two annular 
rings 12 and 14, one of the rings extending from each side of the body. 
The upper ring 12 includes a radially inwardly extending flange 13 while 
the lower ring 14 includes a radially outwardly extending flange 15. It 
should be apparent that the annular flanges 13 and 15 define undercuts 
which are generally a source of serious forming problems. 
In order to produce a near net shape having this configuration, a 
thick-walled container for consolidating the powder metal is produced. 
Generally, the container should be made from a mass of fully dense and 
incompressible material which is capable of plastic flow at elevated 
temperatures. In the case of Astroloy powder and other related powders, a 
suitable container material is low-carbon steel, such as an SAE 1008 or 
1010 steel. Low-carbon steel offers the advantages of being relatively 
inexpensive, readily available, and easily removed from the densified 
compact by machining or pickling. Other considerations which make 
low-carbon steel a satisfactory material for the container are that 
Astroloy and low-carbon steel have reasonably close coefficients of 
thermal expansivity and no deleterious reactions will occur between the 
constituents of Astroloy and the low-carbon steel. 
Referring to Step 1 of the drawing, a practical method for producing the 
container involves providing two disc-shaped pieces of steel 16 and 18. 
Appropriately dimensioned cavities 20 and 22 are machined in the two 
pieces of steel by standard machining techniques. The dimensions of the 
cavities are, of course, larger than the dimensions of the desired 
densified compact 10 to take into account the predicted amount of 
shrinkage which occurs as the powder densifies. While a two-piece 
container is shown, more complex parts may be produced by employing 
containers having three or more interfitting pieces. The sections 20 and 
22 of the cavity are machined in the pieces of steel in a manner analogous 
to the fabrication of a closed die. Alternatively, the container may be 
cast using an expendable core to form the cavity. 
In accordance with the disclosure in the aforementioned U.S. patent 
application Ser. No. 692,310 the container is "thick-walled". By way of 
definition, the exterior surface of a thick-walled container does not 
closely follow the contour of the cavity. This insures that sufficient 
container material is provided so that, upon the application of heat and 
pressure, the container material will act like a fluid to apply 
hydrostatic pressure to the powder in the cavity. It has been shown that 
the use of a thick-walled container produces a near net shape having close 
dimensional tolerances with a minimum of distortion. 
As shown in Step 1 of the flow diagram, each of the container parts 16 and 
18 are machined to produce cavities 20 and 22 of predetermined complex 
shape. After machining the cavities, care is taken to fully remove all 
contaminants, such as cutting fluids, oil and the like. This precaution is 
taken to prevent the formation of a barrier between the powder and the 
container material. It has been found desirable that during consolidation 
the material of the container and the powder metal form one dense mass 
wherein the Astroloy and the low-carbon steel are actually fused together 
at their interface. Cutting fluids and other contaminants will prevent 
this fusion. 
As shown in Step 2, after the container parts 16 and 18 are machined and 
cleaned, they are joined together to form a complete container 24. This is 
done by a welding operation. Care is taken to produce a hermetic seal 
between the container parts 16 and 18 so that the container may be 
evacuated. Obviously, poor weldments produce leaks which would permit the 
introduction of contaminants into the container. Again, it is pointed out 
that this process is being described with respect to Astroloy powder, an 
alloy which is highly reactive to oxygen. Therefore, it is desirable 
throughout the processing that the Astroloy powder be maintained in an 
inert atmosphere and, finally, under a vacuum during densification. Other 
alloy powders, however, may not be as susceptible to contamination and 
hence these precautions may not be necessary. 
In the process of joining the container parts 16 and 18, the container 24 
is tubulated. This is done by drilling a hole in one of the container 
parts for positioning a fill tube 26 which communicates with the cavity. 
The fill tube 26 is joined to the container part by welding. Again, care 
is taken to produce a hermetic seal. The container is then evacuated by 
connecting the fill tube 26 to a vacuum pump (not shown). After the 
container has been pumped down to a vacuum level of generally less than 10 
microns, the container is filled with Astroloy powder. Prior to filling 
the container, the Astroloy has been degassed and maintained under a 
vacuum. During filling, the container 24 is rotated and vibrated to insure 
complete filling of the cavity to maximum tap density. After the container 
24 has been completely filled with powder metal, the container is leak 
tested. Leak testing is done by measuring the rate of loss of the vacuum 
in the container. A decrease in vacuum of only a few microns per hour 
indicates that the container is properly sealed. After leak testing, the 
container is sealed by crimping and welding the fill tube 26. 
At this point, the filled and sealed container is ready for the 
densification step. Densification of the powder metal is accomplished by 
heating and applying pressure to the container. Heat and pressure may be 
applied by using an autoclave or a hot forging press. Step 3 of the flow 
diagram is a schematic of an autoclave which includes a pressure vessel 28 
and heating coils 30. When using an autoclave, the container 24 and 
contents are heated to a temperature of approximately 2050.degree. F and a 
pressure of 15,000 psi is applied for 2 hours. Alternatively, the 
container 24 may be preheated in a furnace and transferred to a forging 
press. In order to apply pressure, the container is restrained in a 
restraining ring or cavity. In the case of either an autoclave or forging 
press, an isostatic pressure is applied to the exterior surface of the 
container 26. With regard to an autoclave, isostatic pressure is applied 
by the pressure medium, usually an inert gas, such as argon. Isostatic 
pressure is also produced in the forging press by employing the 
restraining ring or cavity. It is to be remembered that, at the 
densification temperatures employed, the low-carbon steel flows readily 
under the applied pressures. Hence, even though the ram of the press 
applies a one-directional force, the container material acts like a fluid 
and fills the retaining cavity and reacts with an essentially equal force 
against all sides, ignoring the weight of the container material which is 
small compared to the applied force. 
Applying heat and pressure to the container in the manner described causes 
the container material to act like a fluid thereby applying a hydrostatic 
pressure to the heated powder metal contained in the cavity. Since the 
powder contained in the cavity is not at full density, the size of the 
cavity will decrease. The decrease in size of the cavity can be compared 
to the behavior of a gas bubble in a liquid under pressure. As the 
pressure is increased, the hydrostatic pressure on the walls of the bubble 
causes the diameter of the bubble to decrease. As the bubble decreases in 
size the gas in the bubble is compressed. The powder in the cavity is 
analogous to the gas in the bubble. The powder is compressed until it 
reaches full density. At the temperatures and pressures involved, the 
container material will actually fuse with the powder thus producing a 
unitary mass. A small diffusion zone is produced at the interface between 
the container material and the densified compact. This diffusion zone is 
very small and is normally limited to two atomic diameters. 
After hot compaction, the container is removed from the autoclave 20 or 
forging press and allowed to cool. 
The next step, Step 4 of the flow diagram, involves preparing the densified 
compact for heat treatment. This is done by partially removing portions of 
the container material in a selective and predetermined manner. As is 
apparent in the drawing, the body 11 of the densified compact 10 has a 
significantly larger section size than the rings 12 and 14. As pointed out 
above, variations in section size causes problems during heat treatment 
not only due to distortion of the densified compact, but also in the 
attainment of uniform physical properties. By using the thick-walled 
container described a unique solution to the heat treating problems is 
offered. 
Selectively removing portions of the container facilitates the attainment 
of uniform physical properties and to reduce distortion. As a general 
rule, a greater amount of container material is removed from those regions 
adjacent thick sections than in those regions adjacent thinner sections. 
Hence, a jacket 32 of container material having varying thickness is 
retained on the densified compact. The jacket 32 of container material 
reduces the extent of variation in the section thickness of the densified 
compact by increasing the size of those sections. As a result, a heat 
treatable body 34 is produced which is a composite of the densified 
compact and the jacket of container material. Since the jacket of 
container material is expendable, attention is focused on achieving the 
desired physical properties in the densified compact without distortion 
due to internal stresses or sagging. 
In this manner, the container material can be employed as a metallurgical 
tool for reducing or eliminating many of the problems encountered in heat 
treating near net shapes. It should be apparent, that although the heat 
transfer properties of Astroloy and low-carbon steel are different, a 
proper balance can be arrived at to produce the required heating and 
cooling rates in the various sections of the densified compact. As a 
result, distortion caused by internal stresses created during nonuniform 
cooling can be eliminated. Additionally, a uniform microstructure can be 
produced throughout the densified compact. A result which heretofore has 
been impossible to achieve due to the difference in the rates of cooling 
between small and large sections. An additional advantage is that the 
jacket of container material physically supports thin sections to prevent 
sagging. 
The heat treatment is illustrated schematically in Step 5 which shows the 
heat treatable body 34 positioned within a furnace 36. By way of example, 
a typical heat treatment for a part made of Astroloy is described below. 
The densified compact is first solution treated. The solution temperature 
varies with the intended application of the part. However, a typical 
solution treatment includes an initial heating to 1975 - 2075.degree. F 
for four hours. This is followed by an oil quence. It is noted that a 
relatively severe quench can be employed due to the fact that the jacket 
of container material promotes a relatively uniform cooling rate 
regardless of the variation in section size in the densified compact. 
Heretofore, without the jacket of container material it would have been 
necessary to employ a slower quench, such as a molten salt quench, to 
avoid internal stresses in the densified compact. The densified compact 
then undergoes a stabilization heat treatment which involves heating to 
1600.degree. F for eight hours followed by an air cool and a second 
heating to 1800.degree. F for four hours followed by an air cool. The 
densified compact then undergoes a precipitation treatment by heating to 
1200.degree. F for twenty-four hours to precipitate a fine gamma prime 
phase (an A.sub.3 B compound where "A" is nickel, cobalt, or iron and "B" 
is aluminum, titanium, or columbium). This is followed by an air cool and 
a second heating to 1400.degree. F for 8 hours to coarsen some of the 
gamma prime phase. This heat treatment is then followed by an air cool. 
The jacket of container material offers a significant advantage during the 
critical cooling stages. Because the jacket of container material has 
eliminated large variations in section size, all sections of the densified 
compact cool at approximately the same rate. Hence, a relatively uniform 
microstructure is produced. A uniform cooling rate also prevents the 
development of internal stresses. Additionally, the jacket of container 
material protects the densified compact to prevent any possible 
contamination during the heat treat process. 
After heat treating, the jacket of container material is removed from the 
densified compact. This may be accomplished by etching in a suitable acid 
bath. The etchant removes the ferrous base metal, but will not attack the 
nickel base metal. After etching the densified compact may be grit-blasted 
to remove any residue. Alternatively, the jacket of container material may 
be removed by machining. 
While the container material is sacrificed in the process described, it is 
pointed out that the cost of low-carbon steel is a fraction of the cost of 
superalloy powder such as Astroloy. 
The near net shape shown in Step 6 is then ready for further processing, 
typically, final machining. It should be apprent, however, that a 
significant number of previously required intermediate steps have been 
eliminated by producing a near net shape. Moreover, problems associated 
with producing and heat treating a near net shape have been reduced. 
The invention has been described in an illustrative manner, and it is to be 
understood that the terminology which has been used is intended to be in 
the nature of words of description rather than of limitation. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is, therefore, to be 
understood that the invention may be practiced otherwise than as 
specifically described herein and yet remains within the scope of the 
appended claims.