Note: Descriptions are shown in the official language in which they were submitted.
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POWDER METAL COMPOSITION WITH ALUMINUM NITRIDE MMC
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of the filing date
of United States Provisional Patent Application No. 63/222,240
entitled "Powder Metal Composition With Aluminum Nitride MMC"
filed on July 15, 2021, which is hereby incorporated by
reference for all purposes as if set forth in its entirety
herein.
STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] This disclosure relates to powder metallurgy
formulations and sintered components made therefrom. In
particular, this disclosure relates to an aluminum alloy powder
composition including aluminum nitride (A1N) metal-matrix
composite (NC) additions.
BACKGROUND
[0004] The 6061 aluminum alloy is a precipitation-hardened
aluminum alloy containing magnesium (Mg) and silicon (Si) as the
main alloying elements. It exhibits good mechanical properties
and weldability along with excellent corrosion resistance. Due
to this combination of properties, it has become one of the most
widely used aluminum alloys. Aluminum 6061 has a wide range of
applications including in the automotive and marine sectors. As
used herein, the 6061 aluminum alloy composition should be
understood to mean, by mass percent, between 95.85% to 98.56%
aluminum, 0.8% to 1.2% magnesium, 0.40% to 0.8% silicon, 0.0% to
0.7% iron, 0.15% to 0.40% copper, 0.04% to 0.35% chromium, 0.0%
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to 0.25% zinc, 0.0% to 0.25% titanium, and 0.0% to 0.15%
manganese with the remainder being no more than 0.05% each in an
amount of no more than 0.15% total.
[0005] There are a large number of ways of forming metal
components and powder metal or "PM" processes represent one
class of production techniques for forming metal components.
Powder metallurgy generally involves producing or obtaining a
powdery metal material, compacting this powder metal material in
a tool and die set to form a green compact or preform having a
geometry approximating the desired end product, and then
sintering the green compact to cause the powder metal particles
to diffuse into to one another and to densify into a much more
mechanically strong body. Powder metallurgy is well-suited for
producing parts in large volumes and can offer the benefits of
low scrap costs and the ability to produce components which may
not require subsequent machining after being formed.
[0006] Although this is just general overview of the powder
metal production processes, what can be appreciated from this
description is that much of the powder metal processes can
typically happen in the solid state or with only a limited
amount of liquid being formed during the sintering process.
However, this also highlights some of the challenges in using
powder metal processes as, with sintering being a diffusion-
dependent process, the resultant microstructure and porosity is
a function of the powder formulation and processing conditions.
Thus, attempting to convert a cast alloy to a powder metal
formulation can present challenges in creating both a comparable
microstructure and providing comparable mechanical properties.
[0007] Returning now to the 6061 aluminum alloy, the powder
metal alloy comparative equivalent to 6061 is available
commercially, for example, as Alumix 321 which can be obtained
from ECKA Granules (Feurth, Germany). An example chemical
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composition of Alumix 321 can have 1.31 wt% Mg, 0.5 wt% Si,
0.32 wt% Cu, 0.10 wt% Fe, 0.01 wt% Bi, 0.03 wt% Sn, and 0.01 wt%
V with the balance being aluminum. While not truly within the
6061 specification, it can be seen that this powder formulation
is roughly comparable. As with the 6061 aluminum alloy, the
main alloying elements in this system are magnesium and silicon
and these two elements are the basis of the heat treatment of
this system. They form an intermetallic phase of Mg2Si which
improves the mechanical properties. Copper is also responsible
for improving mechanical properties. Iron exists as an impurity
and forms different intermetallic phases which can affect
corrosion and mechanical properties, while Sn, V, and Bi were
added to improve the sintering response of the powder.
SUMMARY
[0008] Disclosed herein is an improved powder metal
composition comparable to a 6061 aluminum alloy which further
includes an aluminum nitride (AIN) MMC additive instead of MMC
additions such as A1203 or SiC, which are comparably more
traditional. By altering the properties and morphology of the
powder metal composition and including AlN as an MMC additive, a
composition is provided which can have good corrosion
resistance, moderate to high strength, high ductility, and good
electrical and thermal properties. It can also offer improved
wear resistance and further is believed to produce only minimal
tool wear during compaction and sizing compared to other
comparable compositions with MMC additives such as A1203 or SiC.
[0009] According to one aspect, a powder metal includes an
aluminum (Al) powder metal, an aluminum-copper (Al-Cu) powder
metal, a magnesium (Mg) powder metal, a tin (Sn) powder metal,
an aluminum-silicon (Al-Si) powder metal, and aluminum nitride
(A1N) as a metal-matrix composite additive.
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[0010] In some forms, at least a portion of the aluminum (Al)
powder metal may be a fine aluminum powder metal. For example,
the portion of the aluminum (Al) powder metal that is the fine
aluminum (Al) powder metal may be 10 wt% of the elemental
aluminum (Al) powder metal in total. However, in other forms,
it is contemplated that the amount of fine aluminum powder may
be in a range of 5 wt% to 15 wt% of the total elemental aluminum
(Al) powder metal or, more broadly 5 wt% to 30 wt% of the total
elemental aluminum (Al) powder metal. In some forms, the
majority of the aluminum (Al) powder metal may be an ECKA-
Bahrain aluminum powder metal or comparable aluminum powder
metal having +60 / -250 mesh and the fine aluminum powder metal
may be an ECKA Al EF2 / fine EEG aluminum powder or comparable
powder.
[0011] In some forms, a total aluminum content provided by
the aluminum (Al) power metal, the aluminum-copper (Al-Cu)
powder metal, and the aluminum-silicon (Al-Si) powder metal may
be 95.2 wt% of the powder metal composition; the total copper
content provided by the aluminum-copper (Al-Cu) powder metal may
be 0.29 wt%; the total magnesium content provided by the
magnesium (Mg) powder metal may be 1.07 wt%; the total tin
content provided by the tin (Sn) powder metal may be 0.49 wt%;
and the total silicon content provided by the aluminum-silicon
(Al-Si) powder metal may be 0.49 wt%; the aluminum nitride (A1N)
may be 0.98% wt%; the powder metal composition may further
include a flow aid and the flow aid may be 0.02 wt%; and the
powder metal composition may further includes a lubricant and
the lubricant may be 1.46 wt%. All of these weight percentages
are based on the total weight of the powder metal formulation
including the various powder metals, the flow aid, and the
lubricant.
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[0012] In some forms, the aluminum (Al) powder metal may be a
majority of +60/-250 mesh powder and a portion of the aluminum
(Al) powder metal may be fine aluminum powder metal, the
aluminum-copper (Al-Cu) powder metal may be a -325 mesh, the
magnesium (Mg) powder metal may be a -200 mesh, the tin (Sn)
powder metal may be a -325 mesh, and the aluminum-silicon (Al-
Si) powder metal may be -325 mesh.
[0013] In some forms, the aluminum-copper (Al-Cu) powder
metal may be a 50A1-50Cu powder metal. The 50A1-50Cu powder
metal may be atomized and crushed.
[0014] In some forms, the aluminum-silicon (Al-Si) powder
metal may be an 88A1-12Si powder metal.
[0015] In some forms, the powder metal composition may
further include a lubricant, and, in some forms, the powder
metal composition may further include a flow aid such as, for
example, a fumed SiO2.
[0016] In some forms, the powder metal composition may have
even distribution of the various powders which can include an
even distribution of the fine aluminum powder. This even
distribution may be achieved by mixing the powders in a high
intensity mixer.
[0017] In some forms, the aluminum nitride may have a
specific surface area of less than or equal to 2.0 m2/g and may
have a particle size distribution of D 10% of between 0.4 and
1.4 pm, D 50% of between 6 and 10 pm, and D 90% of between 17
and 35 pm. This is similar to a grade AT aluminum nitride.
[0018] In some forms, the aluminum nitride may have a
specific surface area of between 1.8 and 3.8 m2/g and may have a
particle size distribution of D 10% of between 0.2 and 0.6 pm,
D 50% of between 1 and 3 pm, and D 90% of between 5 and 10 pm.
This is similar to a grade BT aluminum nitride which is
generally finer than a grade AT aluminum nitride.
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[0019] In some forms, the aluminum nitride may have a
hexagonal crystal structure and may be single phase.
[0020] In some forms, the powder metal composition may have a
flow rate of between 2.8 and 3.0 g/s.
[0021] In some forms, the powder metal composition may have
an apparent density of 1.21 g/cc.
[0022] According to another aspect, a green compact may be
formed from the powder metal composition (according to any of
the forms described above or herein) by compaction of the powder
metal composition such as in a tool and die set using uniaxial
compaction. According to still another aspect, a sintered
powder metal component formed from this green compact by
sintering the green compact.
[0023] These and still other advantages of the invention will
be apparent from the detailed description and drawings. What
follows is merely a description of some preferred embodiments of
the present invention. To assess the full scope of the
invention the claims should be looked to as these preferred
embodiments are not intended to be the only embodiments within
the scope of the claims.
DETAILED DESCRIPTION
[0024] A powder metal composition is disclosed here which is
comparable to those of a 6061 aluminum alloy. Below, a specific
exemplary powder metal composition is disclosed and some
variations to that powder metal will be discussed.
[0025] According to one specific composition formulation, the
powder metal composition is as follows in Table I below:
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TABLE I
Wt%
Raw Material Element
(nominal)
Al (ECKA-Bahrain, +60 / -250 mesh) Al(90%)
84.27%
Al (ECKA Al EF2, Fine EEG Al powder) Al(10%)
9.36%
50A1-50Cu (ECKA-Velden, fine -325 mesh, Cu
0.30%
atomized and crushed) Al
0.30%
Mg (-200 mesh, gas atomized) Mg
1.10%
Sn (-325 mesh) Sn
0.50%
Si
0.50%
88A1-12Si (ECKA-Veldon, fine -325 mesh)
Al
3.67%
Total (Powder Metal Only)
100%
[0026] Table I illustrates the percentages of just the
metallic component of that powder metal formulation, while Table
II further includes the addition of non-metallic powder metal
additions including aluminum nitride (A1N) as metal-matrix
composite (MC) additives, flow aid, and lubricant. The
composition of Table I is more reflective of the alloy
composition of the metallic composition, while Table II data is
normalized to further take into account approximately 2.5 wt%
that are further additions and very slightly impact the overall
weight percentages of the actual powder metal composition blend.
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TABLE II
Wt%
Raw Material Element
(normalized)
Al (ECKA-Bahrain, +60 / -250) Al(90%)
82.20%
Al (ECKA Al EF2, Fine EEG Al powder) Al(10%)
9.13%
50A1-50Cu (ECKA-Velden, fine -325 mesh, Cu
0.29%
atomized and crushed) Al
0.29
Mg (-200 mesh, gas atomized) Mg
1.07
Sn (-325 mesh) Sn
0.49%
Si
0.49%
88A1-12Si (ECKA-Veldon, fine -325 mesh)
Al
3.58
AlN Powder (BT Grade AlN
0.98%
Flow Aid
0.02%
Lubricant
1.46%
Total (Powder Metal + MMC + Flow Aid +
100%
Lubricant)
[0027] Notably, rather than having a single type of aluminum
powder, it can be seen that the aluminum composition has a
composition which includes, by approximately 10% by weight of
the elemental aluminum powder metal portion, fine aluminum
powder metal. However, it is contemplated that this could be a
different percentage of fines, for example, from between 5% to
15 by weight or 5% to 30% by weight of the aluminum powder
metal. As yet another point of variance, it is contemplated
that the percentage of the aluminum coming from the ECKA Al EF2,
Fine EEG Al powder, could be instead provided by equally
increasing the amount of ECKA-Bahrain, +60 / -250. For example,
the ECKA-Bahrain, +60 / -250 could be 93.63 wt% (normalized,
which is equal to 82.2% plus 9.13% from the separate powders).
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[0028] Moreover, that the total aluminum comes from a variety
of sources. Beyond just the elemental aluminum powder metals,
aluminum is provided as part of the 50A1-50Cu powder metal and
as part of the 88A1-12Si powder metal (collectively adding about
another 4 wt% aluminum in this fashion) which also provides some
of the alloying elements. The alloying and morphology in those
instances, however, can be to obtain the final desired
microstructure as well as to provide a chemistry which is
conducive to sintering in the manner desired. That is to say,
elemental copper and silicon have much higher melting and
effective sintering temperatures than aluminum and, further by
alloying these elements with aluminum, the starting powders have
a different structure with some amount of copper and silicon
already alloyed with the aluminum.
[0029] It should be appreciated that the particular mesh
sizes and the powder morphology can impact the sinterability and
the final microstructure and properties of the sintered part
coming from this powder metal formulation. So, the mesh sizes
and powder morphologies should be established to provide
suitable resultant properties and densities. While the
disclosed powders are workable, some variation to the powders
may also be workable without deviating from the scope and spirit
of the disclosed formulation.
[0030] For example, some of the alloying elements might be
provided in slightly different forms than those found in the
tables above. Likewise, the alloying composition might be
somewhat different than that disclosed. For example, in the
case of tin, tin at 0.5 wt% is believed optimal, but a range of
0.1 wt to 1.0 wt tin is believed to offer improved
densification during sintering. It is also contemplated that
some variation may be made to other elements from the exemplary
composition above. For example, it is contemplated that the
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amount of silicon could be in a range of 0.40 to 0.8 percent by
weight of the total metallic powder metal composition and the
amount of magnesium could be in a range of 0.8 to 1.2 percent by
weight of the total metallic powder metal composition as these
are roughly comparable to the amounts of silicon and magnesium
in a 6061 composition. Similarly, some amount range of copper
(0.04 to 0.35 wt%) might also be workable in the powder
composition and the 0.30 wt% copper of the exemplary composition
falls within this range.
[0031] The lubricant can be a wax such as Licowax0 (available
from Clariant of Muttenz, Switzerland), which can help maintain
the compacted green part together by keeping the powder
particles together and can further help in the removal of the
green part during ejection from the tool and die set after
compaction. The lubricant is typically burnt off during the
sintering process in the preheating zone.
[0032] The flow aid can be added to improve fill and particle
packing. In the exemplary composition, the flow aid is a fumed
silica (SiO2)
[0033] With respect to the aluminum nitride (A1N) MMC
additions, it is contemplated those aluminum nitride additions
might be, for example Grade AT aluminum nitride (an agglomerated
powder with broader particle size distribution) or Grade BT
aluminum nitride (which has a comparably fine particle size and
is a deagglomerated powder). Both grades can be used in the
disclosed powder metal formulation with the difference being in
response to processing and properties.
[0034] Both grades AT and BT aluminum nitride have a
hexagonal crystal structure and are single phase. For the sake
of chemically characterizing these aluminum nitride additions,
as mass fractions both Grade AT and BT have a minimum of 32.0%
N, a maximum of 0.15% C, and a maximum of 0.05% Fe. However,
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Grade AT has a maximum of 1.3% 0, while Grade BT has a maximum
of 1.5% 0. The Grade AT has a specific surface area of less than
or equal to 2.0 11(2/g while the Grade BT has between 1.8 and 3.8
m2/g. The particle size distribution of the two different grades
is illustrated in Table III below:
Table III
Particle Size
Grade AT Grade BT
Distribution
D 10% 0.4 - 1.4 pm 0.2 - 0.6 pm
D 50% 6 - 10 pm 1 - 3 pm
D 90% 17 - 35 pm 5 - 10 pm
[0035] Aluminum nitride as the MMC additive can improve the
wear, ductility and thermal conductivity properties of the
powder metal formulation. In comparison to more traditional MMC
additives such as A1203 or SiC, there is minimal tool wear.
[0036] The various powder metals, aluminum nitride, flow aid
and lubricant are blended together during powder preparation,
preferably in a high intensity mixer, in order to get an even
distribution of the various particles, especially the fine
particles, throughout the overall powder metal composition blend
and to avoid segregation.
[0037] In terms of powder response prior to compaction, the
flow rate of this powder was measured at 2.9 g/s on average and
the apparent density was measured at 1.21 g/cc on average.
[0038] This powder metal composition was compacted into bars
having a density of 2.50 g/cc. The green strength of the
compacted green bars was 8,382 kPa on average. In terms of
sintering response, over a set of three bars the average mass of
the bars decreased 1.41% (which roughly corresponds to lubricant
loss during sintering), the average sintered density was 2.69
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g/cc, and, in terms of dimensional shrinkage resulting from
densification, the average height dimensional change was a 3.93%
decrease, the average width ------------- change was a 2.65% decrease, and the
average length change was a 2.11% decrease. The average Ti
hardness taken from 18 different readings was 58.1 HRE (with all
data points falling between 55.4 HRE and 60.1 HRE) and the
average laser flash analysis of thermal diffusivity was 72.3
(recorded at room temperature).
[0039] Ti tensile testing indicated across a group of five
tests, an average Young's Modulus of 83.2 GPA, an average yield
stress of 83 MPa, and average ultimate tensile strength (UTS) of
193 MPa, and an average elongation of 13.9%. Of this
preliminary tensile data, it is worth noting that there was
fairly high variation in the Young's Modulus results, with those
results ranging from 54.2 GPa to 135 GPa, while the average
yield stress was within about 5 MPa of the minimum and maximum
measured amounts, the average UTS was within about 3 MPa of the
minimum and maximum measured amounts, and the average elongation
was within about 2% of the minimum and maximum measured amounts.
[0040] In comparison wrought 6061 with a Ti treatment profile
would have a UTS of 210 MPa, a yield stress of 110 MPa), and
elongation of 16%. Thus, although sintered powder metal, this
disclosed powder metal formulation has near-wrought properties.
[0041] Further assessment of a powder metal having a similar
composition was performed on samples subjected to a 18 heat
treatment (further detailed below) and with and without further
inclusion of 2 vol% AIN (targeted). This composition was made
from powder metals having the chemistry of Table IV below.
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TABLE IV
Powder Amount
Ecka Al (-250/+60 micron) 936.3 g
Ecka A1-12Si (-45 micron) 41.7 g
Elemental Mg (China) 11.0 g
Ecka Elemental Sn (-20
5.0g
micron)
Ecka A1-50Cu Master Alloy 6.0 g
Lico Wax C 15.2 g
[0042] Samples made from this powder composition will
hereafter be referred to as "PM6061" or "PM6061-A1N" in the
examples below, with the "-A1N" designation being used to
indicate samples mad from this composition but with 2 volume
percent targeted aluminum nitride MMC additions. It will be
appreciated that these compositions are not necessarily to the
6061 specification, but rather are targeted to be comparably
performing powder metal compositions to wrought 6061.
[0043] For each of PM6061 and PM6061-A1N, fifty transverse
rupture strength (TRS) bars, five Charpys, and five Falex pucks
(50mm OD x 12 mm OAL) were compacted from each blend targeting a
green density of 2.50 g/cc and then sintered. Initially,
fifteen TRS bars from each composition were sintered under
different thermal profiles and the dimensional change, mass
change, average hardness, and sintered density of all TRS bars
to identify optimal conditions (as, furnace to furnace, optimal
conditions could vary). All remaining TRS bars along with the
Charpys, and Falex pucks were then sintered under conditions
found to be optimal during the initial sintering runs and sample
testing of the fifteen TRS bars.
[0044] For those samples prepared under optimal sintering
conditions, those samples were then measured for their as-
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sintered dimensional change, mass change, average hardness, and
sintered density of five of the TRS bars from each of PM6061 and
PM6061-A1N.
[0045] Those as-sintered dimensional change, mass change,
average hardness, and sintered density results are found in
Tables V and VI below, along with comparative T8 hardness:
TABLE V
Powder Mass Sintered OAL Width Length
Composition Change Density Change Change Change
(S) (g/cc) (S) (S) (S)
PM 6061 -1.46 2.70 -4.41 -2.21 -
1.67
PM 6061-A1N -1.38 2.70 -3.77 -2.54 -
2.04
TABLE VT
Ti T8
Powder Hardness Hardness
Composition (HRE) (HRE)
PM 6061 56.2 96.6
PM 6061-A1N 54.1 96.9
[0046] All remaining samples were processed into the T8 heat
treatment (target 2-3% RIH), in which the 18 heat treatment
included solutionizing at 530 C (two hours at temperature),
quenching, sizing 2% reduction in AOL, and aging at 160 C for 18
hours. For the T8 samples in Table VI above, the average
hardness is provided for samples subjected to this T8 heat
treatment.
[0047] Charpys were machined into threaded-end tensiles and
then the Yield Strength, Ultimate Tensile Strength, Young's
modulus, and total elongation to fracture were measured for five
specimens for PM 6061 and PM6061-A1N, which can be found in
Table VII below:
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TABLE VII
Powder E Yield UTS Elongation
Composition (GPa) (MPa) (MPa) (58)
PM 6061 66.8 300 341 7.0
PM 6061-A1N 66.7 303 335 3.7
Again, these are mechanical properties of samples made from the
powder composition and subjected to the 18 heat treatment.
[0048] Samples of each T8-process composition were also
subjected to a 3-point bending fatigue staircase, which are
results are provided in Table VIII, below:
TABLE VIII
Bending Fatigue Performance
Powder
Composition ua,10 (MPa) ud,50 (MPa) ua,90 (MPa)
PM 6061 147.7 141.7 135.7
PM 6061-A1N 151.3 140 128.7
[0049] Additionally, thermal diffusivity was measured at room
temperature via laser flash analysis on each twice in which the
specimens were machined from TB IRS bars. These results are
found below in table IX:
TABLE IX
Powder Thermal Diffusivity
Composition (mITC/s)
PM 6061 76.0
PM 6061-A1N 79.3
[0050] It should be appreciated that various other
modifications and variations to the preferred embodiments can be
made within the spirit and scope of the invention. Therefore,
the invention should not be limited to the described
embodiments. To ascertain the full scope of the invention, the
following claims should be referenced.
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