Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF FORMING POWDER METAL COMPONENTS
HAVING SURFACE DENSIFICATION
FIELD OF THE INVENTION
[0001] The present invention relates to methods of manufacturing powder metal
components. More specifically, the invention provides a method of densifying
the core and
surface of a powder metal component to achieve a product having an evenly
densified
surface.
BACKGROUND OF THE INVENTION
[0002] Powder metal (PM) technology is a lower cost alternative for producing
components that could be made from wrought metal. The use of PM components is
precluded in many applications because of inferior mechanical strength caused
by residual
porosity. Therefore, in the manufacture of PM articles, the achievement of
high density,
close to that of wrought steel (generally assumed to be approximately 7.86
g/cc), is of
significant importance since the strength and durability of a PM article is
directly related to
its density. Typically, the basic steps involved in the manufacture of a
powder metal
component are: a) blending the desired metal powders; b) compacting the powder
to the
desired shape; c) sintering the compact; and d) forming the sintered compact
to the desired
final shape. The final step is used to impart the required dimensional
features of the article.
Following the forming step, it is also common to perform a heat treatment on
the fmished
article to impart, where desired, certain mechanical properties as known in
the art.
[0003] The final density of a PM article is dependent on the characteristics
of the
powders in the blend, sintering conditions, and the compressive forces applied
to the article
primarily during the compaction and forming steps. It is common in known
methods to
compact a powder blend to a moderate initial density, approximately 7.0 g/cc,
and further
densifying the compact during subsequent forming steps. Various compositions
of metal
powder blends are known in the art as are methods of compaction and sintering.
Examples of
known blends and methods are taught in U.S. Patent number 5,476,632
(incorporated herein
by reference).
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[0004] For manufacturing articles having a bearing surface and the like, it is
known to
increase the surface density at the desired locations to provide a densified
region that is
capable of withstanding the bearing forces. The prior art provides various
methods for
densifying surface portions of PM articles. For example, U.S. Patent number
5,540,883
(incorporated herein by reference), teaches a method of densifying a selected
surface portion
of a sintered article by applying rolling cylinders or the like to create a
bearing surface on an
article. The bearing surface, by virtue of having an increased density is
better suited to
withstand the physical stresses (i.e. rolling or sliding stresses) applied on
that portion of the
article. In the process taught in the aforementioned reference, a specific
section of the article
can be provided with a density that approximates the theoretical maximum value
while the
rest of the article has a density of approximately 90% to 98% of the
theoretical maximum
value. The reference is directed to providing bearing surfaces or bushings,
which are
inherently cylindrical and do not have a complex shape. Moreover, the
reference does not
teach any alteration of the core density during the cylindrical surface
densification step.
100051 Various other surface densification methods are known in the art such
as, for
example, in U.S. Patents: 6,168,754; 6,013,225; 5,884,527; and, 6,110,419 (all
of which are
incorporated herein by reference).
[0006] For example, U.S. patent number 5,884,527 teaches a method of roll
forming a
sintered gear comprising meshing a sintered pre-form in interference with a
rotating roll
forming gear die. This method is mainly suited for surface densifyiing pre-
forms of specific
geometries, such as gear teeth, that permit the design of a rolling die to
impart the necessary
combination of line contact and relative motion between the die and pre-form.
[0007] Furthermore, U.S. Patent number 6,168,754 teaches a method of
densifying the
contoured surface of a sintered pre-form comprising forcing said article at
ambient
temperatures through a series of dies having successively more interference
contact with the
surfaces to be densified. A disadvantage of this method is the need for
multiple dies and the
inherent complexity and cost.
[0008] Moreover, U.S. Patent number 6,013,225 teaches a method of surface
densifying a
pre-form utilizing a method of selectively heating the surface of said article
and forcing it
through a die. Inherent disadvantages of this method are the requirement of a
separate
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surface heating step, decreased tool life due to elevated die temperatures and
the
corresponding detrimental effect on dimensional accuracy.
[0009] In the known methods of surface densification, the densification step
is typically
conducted after the forming step, when the formed article has its final shape
and is close to
final core density. Moreover, the known methods are generally incapable of
surface
densifying contoured surfaces, require unduly complex dies, and/or involve
high process
costs. For example, in published U.S. applicationl0/767,014 (published under
number
2004/0177719), there is taught a method for surface densification. This
reference teaches a
process wherein a powder metal is compacted to the final form, sintered and
then surface
densified prior to sizing or forging. This reference stipulates that for any
fmal surface having
a complex or irregular shape, special densification processes are required
(such as peening).
Thus, the less expensive rolling process cannot be used for irregularly shaped
articles.
[0010] The present invention seeks to mitigate at least some of the
deficiencies in the
prior art powder metal manufacturing methods.
SUMMARY OF THE INVENTION
[0011] In one aspect, the present invention provides a method of producing
powder metal
components with high core and surface densities and with high precision on
contoured forms.
[0012] In another aspect, the invention provides a method of densifying a
sintered
powder metal article by first surface densifying a cylindrical surface on a
sintered perform
and subsequently forming the article to final core density and final shape in
a closed die
cavity, wherein the formed article has a compressed length of 5 to 30% less
than the original
sintered length.
[0013] In another aspect, the invention provides a method of making a sintered
metal
article comprising: blending one or more lubricants, carbon, alloys, and iron;
pressing the
blend to form a compact; sintering the compact to produce a sintered powder
metal article;
densifying the surface of the article at ambient temperature by relative
motion between the
article and a densification tool; and, forming the article in a closed die
cavity having a
clearance for movement of said article so as to allow the article to assume a
final shape and
final density.
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[0014] In another aspect, the invention provides a method of densifying a
sintered metal
article comprising: blending one or more lubricants, graphite, iron and one or
more of
ferromanganese, ferromolybedenum and ferrochromium; pressing the blend to form
a
compact; sintering the compact at a temperature of at least 1250 C; surface
densifying at least
one cylindrical surface of the sintered article by roller burnishing; forming
said article at
between 600 and 1300 MPa in a closed cavity so as to produce a final part with
core a density
of 90 to 98% of the theoretical density, a compressed length of 5 to 30% less
than the sintered
length and contoured densified surfaces.
[0015] In another aspect, the invention provides a method of making a sintered
metal
article by blending one or more lubricants, carbon, and iron powder pre-
alloyed with Mn,
Mo, Cr, Ni, etc; pressing the mixture, or blend, to produce a compact;
sintering the compact
at a temperature of at least 1100 C; surface densifying at least one
cylindrical surface of the
sintered article by roller burnishing; forming the article to a final core
density and shape in a
closed die cavity; wherein the formed article has a compressed length of 5 to
30% less than
the original sintered length.
[0016] In another aspect, the invention provides a method of making a sintered
metal
article by: blending one or more lubricants, carbon, and elemental or
substantially pure iron
and one or more of Mn, Mo, Ni, Cu, etc in elemental form; pressing the
mixture, or blend, to
produce a compact; sintering the compact at a temperature of at least 1100 C;
surface
densifying at least one cylindrical surface of the sintered article by roller
burnishing; and
subsequently forming the article to a final core density and shape in a closed
die cavity,
wherein the formed article has a compressed length of 5 to 30% less than the
original sintered
length.
[0017] In a further aspect, the invention provides a method of producing
overrunning
clutches or the like with high core density and densified contact surfaces.
[0018] Thus, in one aspect, the present invention provides a method for
producing a
powder metal article having a three dimensional shape and having at least one
densified
surface region, the method comprising:
a) providing a blend of powdered metals;
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b) compacting the blend to form a pre-form having a general shape of the
article, the
preform being generally cylindrically shaped at a region corresponding to the
at least one
densified surface region;
c) sintering the pre-form;
d) densifying the at least one surface region of the pre-form; and,
e) forming the pre-form to a desired final density and into a desired three
dimensional
shape of the article.
[0019] In another aspect, the present invention provides a method for
producing a powder
metal article having a three dimensional shape and having at least one
densified surface
region, the method comprising:
a) providing a blend of powdered metals;
b) compacting the blend to form a pre-form having a general shape of the
article, the
pre-form having a density of between 70% to 90% of the theoretical maximum
density and
being generally cylindrically shaped at a region corresponding to the at least
one densified
surface region;
c) sintering the pre-form;
d) densifying the at least one surface region of the pre-form to at, least 90%
of the
theoretical maximum density; and,
e) forming the pre-form to a desired final density and into a desired three
dimensional
shape of the article.
[0020] In another aspect, the invention provides a powder metal pre-form
having a
general shape of a desired article, the pre-form having a density of between
70% to 90% of
the theoretical maximum density and being generally cylindrically shaped at at
least one
surface region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] These and other features of the preferred embodiments of the invention
will
become more apparent in the following detailed description in which reference
is made to the
appended drawings wherein:
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[0022] Figure 1 is a top view of a die, in an open position, with the top
punch removed,
loaded with a pre-form.
[0023] Figure 2 is a cross sectional view of the die of Figure 1.
[0024] Figure 3 is a top view of a die, in a closed position, with the top
punch removed,
loaded with a pre-form.
[0025] Figure 4 is a cross sectional view of the die of Figure 3.
[0026] Figure 5 is a graph comparing the sub-surface density gradients of a
surface
densified pre-form and the final C-Mn-Mo one-way clutch outer race formed at
985 MPa.
[0027] Figure 6 is a graph of the formed core density of a C-Mn-Mo one-way
clutch
outer race.
[0028] Figure 7 is a graph of the formed closure of a C-Mn-Mo one-way clutch
outer
race.
[0029] Figure 8 is a graph of the formed radial movement of a C-Mn-Mo one-way
clutch
outer race.
[0030] Figure 9 is a graph comparing the sub-surface density gradients of a
surface
densified pre-form and the final formed shape for a C-Mo one-way clutch inner
race formed
at 925 MPa.
[0031] Figure 10 is a graph of the formed core density of a C- Mo one-way
clutch inner
race.
[0032] Figure 11 is a graph of the formed closure of a C-Mo one-way clutch
inner race.
[0033] Figure 12 is a graph of the formed radial movement of a C-Mo one-way
clutch
inner race.
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DETAILED DESCRIPTION OF THE INVENTION
[0034] In the following description, the terms indicated below will be
understood to have
the associated meanings:
[0035] "Metal powder" - a metal that is in a fine powder form. The metal may
be pure
(i.e. iron), pre-alloyed iron (i.e. iron alloyed with other metals such as,
but not limited to, one
or more of molybdenum, chromium or nickel), or an alloy of one or more metals
(i.e. ferro
manganese, ferro molydenum or ferro chromium).
[0036] "Powder metal article" - an article formed from a metal powder. The
metal
powder is compressed under high pressures in a die or mould having a desired
shape. The
compressed article may be subjected to other processes such as sintering etc.
to achieve
desired physical properties.
[0037] "Blend" or "Powder metal blend" - a blend of one or more metal powders
and
other additives such as lubricants, carbon (e.g. graphite) etc.
[0038] "Compacting" - the step of pressing a powder metal blend in a rigid die
until the
blend assumes a desired shape and density. The compacted shape may be the same
or similar
to that of the final article.
[0039] "Compact" or "pre-form" - the article resulting from compacting a
powder metal
blend.
[0040] "Sintering" - a process wherein a compact is subjected to high
temperatures and
selected atmospheres (e.g. a reducing atmosphere) to cause the compact to
become a coherent
mass by heating without melting. Sintering is normally performed on a powder
metal
compact to impart desired physical properties.
[0041] "Surface densification" or "Selective densification" - the step of
densifying a
select portion of a sintered compact, usually the outer surface or a portion
thereof. The
densification is preferably conducted by means of rollers and the like as
known in the art.
However, various apparatus for densifying surfaces will be apparent to persons
skilled in the
art after reviewing the following description. The densification step can be
conducted on the
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entire surface of the compact or on one or more cylindrical portions thereof.
Thus, as used
herein, the term "region" or "densified surface region" will be understood to
mean the entire
surface or a portion thereof. The densified surface region will typically be
densified to a
density of between 80% and 100%, and preferably at least 98%, of the
theoretical maximum
density. Further, the depth of the densified region would be at least 0.025mm
(or 0.001
inches) from the surface. In one embodiment, the densified region would extend
to a
thickness of about 1mm ( or 0.04 inches) from the surface.
[0042] "Theoretical maximum density" - refers to the density of the powder
metal
compact when processed until no pores exist. In the general case, the
theoretical maximum
density would be the density of wrought steel, i.e. 7.86 g/cc.
[0043] "Forming" - a process of providing a sintered compact with its final
shape. This
step is normally performed in a closed die or mould, wherein the sintered
compact is
subjected to pressure to result in the fmal dimensions and density of the
final article. The
forming step is known by various terms including: sizing, coining, repressing,
re-striking and
powder forging. In some cases, the sintered compact may also be heated prior
to the forming
step in order to improve the malleabilty of the material.
[0044] "Annealing" - a process of treating a surface densified or formed
sintered article
wherein the article is subjected to high temperatures in a select atmosphere
(e.g. protective
atmosphere, vacuum etc.) to anneal the article to obtain an advantageous
microstructure.
[0045] "Heat treatment" - a process of treating a formed sintered article
wherein the
article is subjected to high temperatures, select atmospheres (e.g. protective
atmosphere,
vacuum, carburizing, etc.), and rapid cooling to obtain desired mechanical
properties. Heat
treatment methods include, but are not limited to, through-hardening,
carburizing and
induction hardening which are typically followed with a tempering treatment
for optimum
properties.
[0046] In one embodiment, the present invention provides a method of surface
densifying
the contoured surfaces of a sintered powder metal article, such as, for
example, the cam forms
of a one-way clutch, by first surface densifying a cylindrical surface of a
lower density pre-
form and forming the article to final desired shape and density in a closed
die cavity. Thus,
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according to the invention, the surface densification step is performed prior
to the forming
step. Therefore, in summary, the steps of the invention are as follows: a)
mixing one or more
powder metals to form the desired blend; b) compacting the powder to create a
pre-form; c)
sintering the pre-form; d) performing a surface densification step on the pre-
form; and e)
forming the article to the desired shape and core density. After the forming
step, the formed
article may be further shaped and heat treated.
[0047] Formation of Powder Metal Blend
[0048] It will be appreciated by persons skilled in the art that a wide range
of powder
blends may be used in the method of the present invention. In one embodiment,
the present
invention utilizes low alloy steel compositions, where the carbon content is
less than 0.7%
and preferably below 0.3% by weight of the final sintered article.
[0049] In one embodiment of the invention, the powder compositions may
comprise low
cost iron powders, which are blended with calculated amounts of ferro alloys,
graphite and
lubricant such that the final desired composition is achieved following
sintering and the
powder blend is suited to compaction in rigid compaction dies. Examples of
these powder
blends are provided in U.S. Patent No. 5,476,632 (incorporated herein by
reference). The use
of substantially pure iron powder admixed with ferro alloys may be preferred
as such
powders are relatively highly compressible and are relatively inexpensive as
compared to pre-
alloyed powders. The powder blend of the invention may comprise elemental or
substantially
pure iron powder blends, fully pre-alloyed powder blends and partially pre-
alloyed powder
blends. It will be appreciated that any composition of powder metal blend may
be used in the
present invention. In one embodiment of the invention, alloys of iron, such as
ferro
manganese, ferro molydenum and ferro chromium may be used individually, or in
combination, as required to achieve desired performance requirements of the
final article.
For example, one, two, or three ferro alloys may be blended with the base iron
powder. A
wide range of alloy elements can be used in the process described herein,
depending on the
~ final product performance requirements, including: carbon, chromium, copper,
manganese,
molybdenum, nickel, niobium and vanadium. Alloy elements may be present either
singly or
in combination.
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[0050] The base iron powder will generally have a particle size distribution
in the range
of 10 to 350 m. This range includes the base iron powder particle sizes of
10, 20, 30, 40,
50, 60, 70, 80, 90, 100, 150, 200, 250, 300, or 350 m and any size there-
between. The
alloying additions typically will have a particle size distribution in the
range of 2 to 20 m.
This range includes the particle size of alloying additions of 2, 3, 4, 5, 6,
7, 8, 9, 10, 15, 20
m and any size there-between. Commercially available lubricant powder is added
to the
blend to facilitate compaction. Typical lubricants include zinc stearate,
stearic acid or
ethylene bistearamide. Various particle sizes, lubricants and other additives
will be apparent
to persons skilled in the art.
[0051] As a further alternative, the present invention may be used with pre-
alloyed
powder metals, some examples being molybdenum or chromium-molybdenum pre-
alloys
having between 0 to 1.5% of each alloy with the remainder being unavoidable
impurities.
With such powders, sintering can be conducted at temperatures of 1100 C to
1150 C, or
alternatively at higher temperatures greater than 1250 C. Typical commercial
examples of
prealloyed molybdenum powders are Quebec Metal Powder sold under the
trademarks QMP
AtometTM 4401, and Hoeganaes AncorsteelTM 85HP, both of which have
approximately
0.85% by weight molybdenum. The particle size of the pre-alloyed molybdenum
powder
metal is typically within the range of 45 to 250 m. Between 0 to 0.7% carbon
by weight
may be added. Compacting is facilitated by the addition of the lubricants
discussed
previously.
[0052] Compaction of Powder Metal Blend
[0053] The compaction step is performed in the known manner using powder
formulated
as discussed above, whereby the blended powder is pressed in a rigid die at
approximately
400 MPa to between 70 and 90% of the theoretical maximum density. Various
other
pressures and end product densities will be apparent to persons skilled in the
art. After
compacting, the shape and dimensions of the resulting compact, or pre-form,
are substantially
similar to the fmal article with the exception of allowances for size change
due subsequent
operations. However, in accordance with the present invention, one or more
surface regions
to be densified are maintained in a cylindrical form. These areas may, in the
final article, be
later formed into complex, non-cylindrical shapes, as will be discussed
further below. As
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will be made clear in the following description, by maintaining the surface
regions to be
densified in a cylindrical form, the surface densification of the compact is
facilitated.
[0054] Sintering of Compacted Pre-Form
[0055] Next, the compacted article, or pre-form, is then sintered using
methods
commonly known in the art. For example, the sintering process may be conducted
in a
reducing atmosphere or vacuum at a temperature in excess of 1250 C such that
oxides from
both the iron and alloy additions contained in the compact are reduced and
metallurgical
bonds are formed between contacting particles to impart strength and ductility
to the sintered
article. The chemical reduction process also allows for uniform diffusion of
alloying
elements throughout the iron particles resulting in a homogeneous
microstructure.
Particularly for higher carbon content materials, an isothermal hold or slow
cooling treatment
may also be utilised to maximize the ferrite content of said article as
described, for example,
in U.S. Patent No. 5,997,805 (which is incorporated herein by reference). As
will be
understood, such isothermal treatment serves to improve the malleability of
the sintered
article. Further, the isothermal treatment step can be included within the
cooling phase of the
sintering step (i.e. it can form a part of the sintering step) or can be
included as a separate step
following sintering. In the case of elemental powder blends and partially or
fully pre-alloyed
powder metal, sintering may take place at conventional sintering temperatures
of 1100 to
1150 C or at a higher temperature up to 1350 C. As known in the art, no
significant
densification occurs during the sintering process. As such, the density of the
sintered
compact will remain substantially the same as that of the compacted pre-form.
[0056] Surface Densification of Sintered Pre-Form
[00571 Densification of the surface, according to one aspect of the present
invention, is
generally performed using a plurality of small diameter rollers in a roller
burnishing tool to
cold roll the one or more cylindrical surfaces of the sintered compact.
Examples of such
tools and processes are provided in U.S. Patent No. 5,540,883; however,
various other tools
and methods known in the art may equally be used. As is known to persons
skilled in the art,
the application of a roller burnishing tool, for example, to a cylindrical
surface, compresses
the surface, collapsing the pores contained therein so that the surface of the
article has a
density approaching the theoretical maximum density.
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[0058] As mentioned above, the surface densification step of the present
invention is
conducted on one or more generally cylindrical surfaces (or surface regions)
of the pre-form.
Thus, as will be apparent to persons skilled in the art, the invention allows
the use of less
expensive (and easier to use) roller apparatus to achieve the desired
densification. Moreover,
by not initially forming (to the desired shapes) the surfaces to be densified,
the invention
makes it possible to achieve a uniform densification over the entire area
being densified.
Furthermore, the use of a roller densification apparatus, in accordance with a
preferred
embodiment of the invention, results in an optimum surface finish and
dimensional control,
which is not possible with known shot peening methods. This, therefore, offers
an important
advantage over other processes known in the art.
[0059] In the surface densification step, the surface regions being densified
are provided
with densities of at least 80% and up to 100% of the theoretical maximum
density. In a
preferred embodiment, the densified surface region has an approximate
thickness of between
0.025mm to lmm (i.e. 0.001 to 0.04 inches) below the surface. The core density
of the pre-
form is not significantly altered during the surface densification step and,
therefore, the core
of the pre-form remains the same as that resulting from the compaction step.
[0060] The article may subsequently be annealed, at temperatures between 800
and
1100 C in a protective atmosphere or vacuum, for the purpose of developing
proper
metallurgical bonding, re-crystallizing the densified surface material and
obtaining an
advantageous microstructure for forming or contact fatigue durability.
[0061] The surface region being densified by this step may comprise either or
both of the
inner and outer regions of the pre-formed article. This aspect is described,
for example, in
U.S. Patent numbers 5,540,883 and 5,972,132 (the entire contents of which are
incorporated
herein by reference)
[0062] Forming of Article
[0063] The selectively densified article is then subjected to a forming
operation to
achieve the desired final density, shape and dimensional requirements. The
forming step is
preferably carried out in a closed die and at ambient temperatures, although,
if required,
elevated temperatures may also be used. The final density is obtained and
closely controlled
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by the movement of the sintered material during forming and the dimensions are
controlled
by the rigid die set. Such dies are commonly known in the art. Where the final
dimensions
are not critical to component functionality, complete filling of the die
cavity may not be
required. The forming operation is alternatively referred to in the art as,
inter alia, sizing,
coining, repressing, forging or re-striking. These processes will be known to
persons skilled
in the art. All of the above mentioned processes involve the application of
pressure to a
sintered compact enclosed within a rigid die cavity. Conventional rigid dies
as used in
regular sizing/coining/repressing/restriking presses may be used in the
present invention to
achieve the fmal surface configuration and higher density of the fmal article
with precise
control. Forming is accomplished by the selection of the composition of the
sintered article,
by the selection of appropriate sintering temperature and furnace profile, by
the selection of
pressure used in the forming operation, and the selection of the forming tool
to provide the
necessary clearance between the tools and the sintered article for movement of
the sintered
compact to the final shape. The required choice of these parameters will be
known to persons
skilled in the art. After forming, the article will have a final core density
of between 90% and
98% of the theoretical maximum, and the densified surfaces will have assumed
the final
configuration with overall radial dimensions of the contoured form, differing
by 0.1 to 10%
, as compared to the diameter of the surface densified region of the pre-form.
Further, the final
article will normally have a length dimension that is approximately between 5
to 30% less
,than the same dimension measured on the sintered and surface densified pre-
form.
[0064] Generally, as mentioned above, the article resulting from the surface
densification
.step will have the approximate but not final shape of the desired article. As
such, the surface
densified article will typically not occupy the entire volume of the forming
die. Figures 1 to
4 illustrate a die having a punch or ram with walls 12 and 14 and an outer die
wall 16.
Typically, the outer wall 16 is stationary while the punch walls 12 and 14 are
designed to
move towards and away from each other. It will be understood that, in some
systems, one of
punch walls 12 or 14 may also remain stationary. The combination of these
elements (12, 14,
and 16) form a die cavity 20 into which a sintered pre-form 22 is inserted.
According to the
present invention, the surface of the pre-form will have a densified layer 23,
having a
generally cylindrical geometry are described above. As also described above,
the die cavity
is of the shape of the desired final product. As shown in Figures 1 and 2, the
pre-form is
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dimensioned to be smaller than the die cavity, thereby leaving a clearance 24
between the
pre-form 22 and the outer walls 16 and 18. As the punch walls 12 and 14 are
moved towards
each other, the pre-form is compressed and radially expanded until is fills,
and assumes the
shape of, the die cavity 20. The final punch position is illustrated in
Figures 3 and 4, which
also show the final formed article 26 with the densified surface 28 after
having assumed the
shape of the die walls 16 and 18.
[0065] Figures 1 to 4 only illustrate a die for the forming operation. It will
be understood
by persons skilled in the art that the actual shape and configuration of the
die will depend
upon the specific article being formed. For example, the die can include core
rods, moveable
outer walls or other configurations necessary to achieve the final article
shape. It will be
noted that Figures 1 to 4 serve to illustrate a forming operation conducted on
the outer surface
of a sintered pre-form. However, as will be understood by persons skilled in
the art, the
forming die can be used to provide the article with a desired outer and/or
inner shape also as
known in the art. Similarly, the forming operation can be used to form
multilevel parts, such
as an over-running clutch or other such articles as known in the art.
[0066] Heat Treatment of Formed Article
[0067] Subsequent to forming, the article may optionally be annealed, at
temperatures
between 800 and 1300 C in a protective atmosphere or vacuum and with suitable
cooling in
order to obtain proper metallurgical bonding and to fully develop the desired
mechanical
properties.
[0068] The fmal article is usually required to have high wear and fatigue
resistance. For
this reason, heat treatment such as carburizing, quenching and tempering,
etc., may be
applied to an article made from a blend with 0.4% or less carbon, while
through hardening or
induction hardening and tempering, etc., can be performed on a part containing
greater than
0.4% carbon. A prior through hardening treatment, either applied as a forced
cooling, or
quenching following annealing or as a separate heat treatment operation may be
applied to
increase the core yield strength. Both methods produce an article with a
hardened surface
case and a hard core that is resistant to wear and exhibits superior fatigue
performance.
Various other heat treatment methods will be known to persons skilled in the
art.
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[0069] The invention described herein relates to the surface densification of
a PM article
while it still has a simple cylindrical geometry (i.e. prior to final forming
of the article) and
utilizes the ductility of the pre-formed material to impart the final
contoured shape to the
densified surface. As will be appreciated by persons skilled in the art, the
invention provides
an improvement over previously known methods, which require surface
densification of the
final formed article. As will be understood by persons skilled in the art, one
of the key
advantages of the present invention lies in its ability to provide an
improved, efficient process
for producing a PM article having a complex shape and with specific surface
densification.
As indicated above, it is often very difficult or impossible to selectively
densify complex
surfaces since the densification apparatus known in the art can only
accommodate simple (i.e.
cylindrical bearings) or specific (i.e. gear teeth) shapes, or require
multiple passes through
different dies. As also described above, the prior art methods require complex
densification
methods and apparatus to achieve surface densification of irregularly shaped
objects.
Articles made according to the present invention may include any powder metal
article such
as gears, bearings, cams etc. as will be apparent to persons skilled in the
art.
[0070] The invention will now be described with reference to certain specific
examples.
It will be understood that the following examples are meant only to illustrate
the invention
and are not intended to limit the scope of the invention in any way.
[0071] Example 1- Carbon Manganese Molybdenum Outer Race
[0072] Iron powder, lubricant, graphite, ferromanganese and ferromolybdenum
were
blended to achieve a sintered composition of approximately 0.2% carbon, 0.9%
manganese
and 0.5% molybdenum. The powder was formed into rings, which were compacted to
a
density of 6.5g/cc (approximately 83% of the theoretical maximum) with a
pressure of about
350 MPa. The compacted rings were sintered at 1280 C for 20 minutes. A
nitrogen/hydrogen atmosphere was maintained throughout the cycle.
[0073] The bore of each sintered ring was surface densified by the method
described in
U.S. Patent No. 6,110,419 (incorporated herein by reference) thereby achieving
a local
surface density in excess of 99% of the theoretical maximum density, while the
core density
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remained at 6.5g/cc. This density profile is illustrated in Figure 5. The bore
surface
densified rings were formed in a closed die with a core rod having the
geometry of the final
cam form. The material exhibited remarkable ductility and densification. At
forming
pressures of 700 to 1050 MPa, core densities of 7.30 To 7.55g/cc were obtained
as illustrated
in Figure 6. Axial closures over the same pressure range were 15 to 18% of the
sintered
length (as illustrated in Figure 7), and radial movement of up to 4% of the
sintered outer
radius was achieved (as illustrated in Figure 8). Following the forming step,
the densified "
bore layer was intact (Figure 5), the core density was increased as indicated
above, the
surface had substantially assumed the final cam shape and the active cam form
exhibited both
excellent surface finish and dimensional stability.
[0074] Example 2 - Carbon Molybdenum Inner Race [0075] A blend with a sintered
composition of 0.6% carbon and 0.9% molybdenum was
prepared by combining iron powder, ferromolybdenum, graphite, and lubricant.
Rings of the
powder blend were compacted to 85% of theoretical maximum density with
pressure of
approximately 520 MPa. The rings were sintered in a nitrogen/hydrogen
atmosphere for 20
minutes at 1280 C followed by an isothermal hold (as described in U.S. Patent
No.
5,997,805, incorporated herein by reference) resulting in a malleable sintered
article. The
cylindrical outer surface of the sintered rings was selectively densified
using the roller
burnishing method (as described in U.S. Patent No. 5,540,883, incorporated
herein by
reference) to achieve a surface density of greater than 99% theoretical
maximum as
illustrated in Figure 9. As in Example 1, the material exhibited remarkable
ductility and
through-densification. Core densities of 7.20 to 7.45g/cc were achieved at
forming pressures
of 750 to 1050 MPa (as illustrated in Figure 10) resulting in axial closures
of 12 to 16% of
the sintered length (as illustrated in Figure 11). Radial movement of up to 4%
of the sintered
inner radius was obtained as shown in Figure 12. As in Example 1, after the
forming step,
the densified bore layer was intact (Figure 9), the core density was increased
as said, the
surface had substantially assumed the final cam shape and the active cam form
exhibited both
excellent surface fmish and dimensional stability.
[0076] Although the invention has been described with reference to certain
specific
embodiments, various modifications thereof will be apparent to those skilled
in the art
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without departing from the scope of the invention as outlined in the claims
appended hereto.
The disclosures of all references recited above are incorporated herein in
their entirety.
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