Preform compaction powdered metal process

A process is disclosed for forming a pressed metal part in which a preform is inserted into a pressed metal mold. The mold is then filled with powdered metal. The powdered metal and preform are compacted to create a compacted metal part wherein the preform defines an adjacent volume next to the compacted metal part. The compacted metal part is ejected from the mold and sintered to create a sintered metal part. The preform is removed by the sintering step in such a way that the adjacent volume becomes a void region. The preform can be formed of copper so that, upon sintering, the preform is removed from the sintered metal part through infiltration. Alternatively, the preform can be formed of zinc so that, upon sintering, the preform is vaporized and thereby removed from the sintered metal part. The void region created by the removal of the preform can be an undercut, a taper, an annular groove, a thread or an internal cavity. In this way, the present invention eliminates the need for machining such surfaces as has been necessary using previous compaction methods.

BACKGROUND OF THE INVENTION 
The present invention is directed to the field of pressed and sintered 
powdered metal components. The present invention has particular 
applicability to pressed metal parts which require annular grooves, 
undercuts, internal cavities and the like. 
In recent times, powder metallurgy (P/M) has become a viable alternative to 
traditional casting and machining techniques for fashioning metal 
components. In the P/M process, powdered metal is added to a mold and then 
compacted under very high pressures, typically between about 20-80 tons 
per square inch. The compacted part is ejected from the mold as a "green" 
part. The green parts are then sintered in a furnace operating at 
temperatures of typically 2000.degree.-2500.degree. F. The sintering 
process effectively welds together all of the individual powered metal 
grains into a solid mass of considerable mechanical strength. The P/M 
process can be generally used to make parts from any type of metal and 
sintering temperatures are primarily determined by the temperatures of 
fusion for each metal type. 
P/M parts have several significant advantages over traditional cast or 
machined parts. P/M parts can be molded with very intricate features that 
eliminate much of the cutting that is required with conventional 
machining. P/M parts can be molded to tolerances within about 4 or 5 
thousandths, a level of precision acceptable for many machine surfaces. 
Surfaces which require tighter tolerances can be quickly and easily 
machined since only a very small amount of metal need be removed. The 
surfaces of P/M parts are very smooth and offer an excellent finish which 
is suitable as a bearing surface. 
The P/M process is also very efficient compared with other processes. P/M 
processes are capable of typically producing between 200-2000 pieces per 
hour depending on the size and the degree of complexity. The molds are 
typically capable of thousands of service hours before wearing out and 
requiring replacement. Since almost all of the powdered metal which enters 
the mold becomes part of the finished product, the P/M process is about 
97% materials efficient. During sintering, it is only necessary to heat 
the green part to a temperature which permits fusion of the metal powder 
granules. This temperature is typically much lower than the melting point 
of the metal, and so sintering is considerably more energy efficient than 
a comparable casting process. 
P/M parts are inherently somewhat porous. Due to the nature of the metal 
powder and the compaction process, there are inherently some voids where 
the metal powder particles are not completely compacted. These voids are a 
function of compaction pressures and powder particle geometry. 
Consequently, the voids (and hence the porosity) can be controlled to 
whatever degree desired. Structural parts can be produced that are 80-95% 
as dense as solid metal parts with comparable mechanical strengths. 
The porosity of P/M parts can be exploited to advantage. The voids 
essentially represent a "cavernous" network that permeates the 
microstructure of a P/M part. These voids can be vacuum impregnated with 
oil to create self-lubricated parts with properties that cannot be matched 
by conventional cast and machined parts. The porosity also creates 
significant sound damping which results in quieter parts that do not 
vibrate or "ring" during operation. Also, the pores can be filled with 
corrosion-resisting materials or "infiltrated" with molten metals to 
provide various material and metallurgical properties that could not be 
attained in conventional cast and machined parts. 
In spite of the many advantages of P/M parts, they have previously suffered 
from certain drawbacks. P/M parts are molded under high pressures which 
are attained through large opposing forces that are generated by the 
molding equipment. These forces are applied by mold elements which move 
back and forth in opposing vertical linear directions. The P/M parts 
produced thereby have previously necessarily had a "vertical" profile. 
Such conventional mold tooling and operation requirements do not allow the 
formation of transverse features which are indented or recessed between 
the ends of the molded part. An example of such a P/M element illustrating 
the vertical profile limitation is shown in FIG. 1. Also, P/M parts must 
necessarily have a vertical profile to facilitate their release from the 
mold. Since mold elements move back and forth in opposing vertical 
directions, P/M parts formed with transverse features, i.e. grooves, 
undercuts, crosscuts or threads would inhibit mold release. As seen in 
FIG. 2, such profile features had previously required a secondary 
machining step which adds greatly to the cost of the part, creating an 
economic disincentive to P/M fabrication. 
The conventional P/M process is also not suitable for fashioning elements 
that have steeply sloped surfaces. If a surface is too steeply tapered the 
mold pressures will force the powder from the mold, thus prohibiting the 
formation of a tapered portion. Thus, tapered members of this type also 
require secondary machining. 
Previous attempts have been made to provide P/M parts with other than a 
transverse profile. One such attempt is to use a split die. With this 
method a die is provided which has a transverse profile features 
incorporated onto the die surface. The die is vertically split into 
sections which reciprocate horizontally. After compaction by the vertical 
application of force, the split die opens horizontally to release the 
green part. This method is very limited. The transverse profile section 
cannot be too large or else it will interfere with powder fill. Also, a 
large profile could interfere with mold release, resulting in damaged 
green parts and equipment down time. Additionally, the transverse profile 
section cannot be too small or else the die section becomes prone to 
breakage under the compaction pressures. In general, the mechanics of 
split die compaction are very complicated and prone to difficulties. In 
view of the limitations and complications of this technique, split die 
compaction does not provide an economically viable alternative to the 
conventional P/M process. 
Another method of creating P/M parts with grooves, undercuts and the like 
is to sinter bond two green parts. As seen in FIG. 3, two parts with 
appropriately tapered surfaces are individually compacted and fitted 
together prior to sintering. Upon sintering, the two parts become bonded 
together to form an integral part with an appropriately placed groove or 
undercut. While this method is effective, a double compacting step is 
required since each part must be formed separately and then assembled 
prior to sintering. The sinter bonding process also requires two complex 
sets of tools as well as careful material considerations. Thus, this 
technique also fails to provide an economically viable alternative to the 
conventional P/M process. 
SUMMARY OF THE INVENTION 
In view of the above-noted disadvantages encountered in prior processes, 
there is a need for a process to produce a P/M part which has other than a 
vertical profile. 
There is also a need for a P/M process which reduces the need for secondary 
machining. 
There is also a need for a P/M process which provides a grooved, undercut, 
or internal surface with one compacting step. 
There is also a need for a P/M part which permits efficient machining 
without extensive removal of metal. 
There is also a need for a P/M process which reduces traditional 
engineering limitations. 
The above and other needs are satisfied by the present invention are 
realized in a process for forming a pressed metal part including the steps 
of inserting a preform into a pressed metal mold and filling the mold with 
powdered metal. The powdered metal and preform are compacted to create a 
compacted metal part wherein the preform defines an adjacent volume next 
to the compacted metal part. The compacted metal part is ejected from the 
mold and sintered to create a sintered metal part. The preform is removed 
by the sintering step in such a way that the adjacent volume becomes a 
void region. 
The preform can be formed of copper so that, upon sintering, the preform is 
removed from the sintered metal part through infiltration. Alternatively, 
the preform can be formed of zinc so that, upon sintering, the preform is 
vaporized and thereby removed from the sintered metal part. The void 
region created by the removal of the preform can be any manner of shape, 
including an undercut, a taper, an annular groove, a thread or an internal 
cavity. In this way, the present invention permits the creation of P/M 
parts having surfaces with other than vertical profile features such as 
have not been available through previous methods. 
The above and other features of the invention will become apparent from 
consideration of the following detailed description of the invention which 
presents a preferred embodiment of the invention as is particularly 
illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The present P/M process solves the problems of the previous system by 
providing a compaction technique using a removable preform which is used 
to create undercuts, annular grooves, internal cavities and the like. 
Referring now to FIG. 4, a P/M mold 100 is provided which uses a lower 
punch 102 and a die 104. In an optional preliminary first step, the mold 
100 is partially prefilled with an amount of powdered metal 106. This 
optional prefill can be lightly compacted to tamp the powder into an 
approximation of its final volume. 
Whether or not a prefill step is performed, a preform 108 is inserted into 
the mold 100. The preform 108 is preferably a compacted green part itself, 
formed by a previous compaction step. However, the preform can be casted 
or otherwise formed. The preform 108 is formed of a material which has a 
melting point lower than the temperature of fusion of the powdered metal 
to be sintered. For example, if the metal powder is a ferrous metal, 
having a fusion temperature of 2050.degree. F., the preform is made of 
copper or zinc, which have respective melting temperatures of 1980.degree. 
F. and 787.degree. F. 
In the preferred embodiment, after preform insertion, the mold 100 is fully 
filled with metal powder 110. The amount of metal powder 110 in the mold 
is important since the size of the finished product is determined by the 
amount of powder and the degree of compaction. After filling, the powder 
is compacted. An upper punch 112 is brought down into the mold 100 and 
large forces are applied between the upper punch 112 and the lower punch 
102 in order to create the tons per square inch pressures necessary for 
full compaction. After compaction, the compacted part 114 is ejected from 
the mold 100 with the preform 108 compacted therein. The preform defines a 
volume which lies along a surface adjacent to the compacted part 114. This 
volume corresponds to the shape of the desired feature (i.e. groove, 
undercut, etc.) 
After ejection, the compacted part 114 with preform 108 is sintered in a 
sintering oven 116. As the temperature of fusion is reached, the preform 
is melted off. In a ferrous part as according to the preferred embodiment, 
a copper preform would melt and be absorbed into the porous network of the 
compacted part 114. This absorption or "infiltration" results in a 
finished part with improved strength and metallurgical properties. The 
preform 108 can also be formed of a material such as zinc, which has a 
vaporization temperature of 1665.degree. F. As the fusion temperature of a 
ferrous part is approached, the zinc melts and then vaporizes to become 
part of the furnace atmosphere. In this way, no portion of the preform 108 
remains on the finished part. 
After sintering, a finished sintered part 118 remains. The perform 108 has 
been completely removed by the sintering process. The preform 108 is 
necessarily formed with a "mirror image," i.e. a reverse profile of the 
desired groove. As the preform is removed by sintering, a void region is 
left adjacent to the sintered part 118 which corresponds to the desired 
profile, i.e. a groove, undercut, thread or the like. In this way, 
complicated transverse P/M part profile features can be generated which 
were not previously possible without secondary machining. In eliminating 
these machining steps, P/M parts with such complicated profiles can be 
generated for between 1/3 to 1/10 of the cost of parts requiring secondary 
machining, representing a significant economic improvement over such 
previous methods. 
Examples of preforms and the parts made by the present process are shown in 
FIG. 5. As seen in FIG. 5A, a part 120 with a deep undercut can be made by 
first inserting the appropriate preform 122. FIG. 5B shows a crosshole 
member 130 formed using a cylindrical preform 132. FIG. 5D illustrates a 
piece 140 with a tapered surface having a reverse profile of that of the 
respective preform 142. FIG. 5D depicts a threaded member 150 by a 
threaded preform 152. 
Heretofore unconsidered P/M part designs can now be considered with the 
present process. As seen in FIG. 6A, proper preform design permits P/M 
parts with asymmetrical profiles 160 to be produced by creating an 
off-center preform 162. As shown in FIG. 6B, even parts 170 with a 
substantially large internal cavity 172 can be created using a preform 108 
which is removed to leave behind a hollow region within a part. As 
depicted in FIG. 6C, complicated parts such as hydraulic cylinders 180, 
with highly complex internal profiles 182 can now be P/M processed without 
secondary machining by using an appropriate preform 184. 
As shown in FIG. 6D, it can even be possible to create a part 190 with an 
internal part 192 inside an internal cavity by imbedding the internal part 
192 in the preform 194 prior to compacting. This internal part 192 can be, 
for example, an internal gear 192 which can ride within an internal gear 
profile 196 inside the internal cavity 194 with no apparent means for the 
ingress of the gear. As the potential of the present process is explored, 
P/M engineers will be able to design parts which exploit these advantages, 
thereby greatly expanding the potential for many types of future P/M 
products. 
The foregoing description of the preferred embodiment has been presented 
for purposes of illustration and description. It is not intended to be 
limiting insofar as to exclude other modifications and variations such as 
would occur to those skilled in the art. Any modifications such as would 
occur to those skilled in the art in view of the above teachings are 
contemplated as being within the scope of the invention as defined by the 
appended claims.