Note: Descriptions are shown in the official language in which they were submitted.
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A PROCESS FOR PRODUCING A METAL-MATRIX COMPOSITE OF SIGNIFICANT
LCTE BETWEEN THE HARD BASE-METAL AND THE SOFT MATRIX
FIELD OF INVENTION
The present invention relates to a process for producing a metal-matrix
composite of
significant ACTE between the hard base-metal and the soft matrix.
BACKGROUND ART
Recently metal matrix composites (MMCs) have received much attention due to
their
electrical and mechanical properties. Tungsten composite (W-bronze and W-Cu)
is an
example of MMC's materials that has gained great importance in many
applications.
Characterized by its high density, high strength, adequate fracture toughness,
hardness, high wear resistance and low thermal expansion, make it very a good
candidate as lead replacement materials in many military and industrial
applications.
They are suitable for fabrication of ammunition, center of gravity (CG)
adjusters,
gyroscope rotors and radiation shelters. Other applications are irrelevant to
lead
replacement like kinetic energy penetrators and jet vanes. Additionally for W-
Cu
composites and due to their excellent thermal conductivity, they found their
way to
various electrical and electronic applications, these include, electrical
contacts,
resistance welding electrodes, electro-discharge machining electrodes, heat
sinkers
and power packaging for microelectronic and optoelectronics applications.
Alloying of W with bronze alloy is difficult to cast. Consulting W-Cu
equilibrium phase
diagram shows that W metal and Cu are almost completely immiscible in both
solid
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and liquid phase. Cu heat of mixing with W is positive i.e.35.5kJ/mole. Energy
of
formation of W-Sn solid solution is positive as well i.e. 20kJ/mole..
Accordingly attaining
fully dense sintered compacts of these systems is not easy to handle. To
tackle this
problem the researchers in the recent three decades tried different techniques
and
approaches. These techniques comprise liquid phase sintering, sintering at
higher
temperatures, using finer elemental powders and incorporating sintering
activators (Fe,
Co, Ni and Pd). Other more sophisticated technique is to change the surface
morphology and the wettability of the W-base metal by thin film Ni coatings on
the W
particles prior to sintering and mechanical alloying (MA) of the W composite
elemental
powders to reduce the W-W spacing and to mechanically super induce inter
diffusion.
layer on the elemental powders.
So far all these techniques were not sufficient to attain fully dense net
shape compacts
out of these composite systems. Despite the success of all these approaches
and
techniques, they bring along negative impacts to at least one or two of the
physical
and mechanical properties of the sintered compacts like. conductivity, thermal
expansion and strength. Another method and the most widely used are to
infiltrate the
sintered and porous W skeleton preforms by the matrix melt (bronze or copper).
The
object of all these processes is to achieve homogeneous pore free MMC material
with
even distribution of the Cu. or bronze phase in the W composite structure.
Porosity is
deleterious to flexural strength, electrical and thermal conductivity of the
composite.
The pores and voids act as points of stress concentrations and reduce the
cross-
sectional area across which a load is applied and lead to a tremendous fall of
the
flexural strength. Air that is present in the pores has poor thermal and
electrical
conductivity and thus it deteriorates the overall thermal and electrical
properties of the
composite. Therefore, it is essential to avoid pores formation in the
composite during
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its manufacturing process. Currently there are many infiltration techniques
available for
producing metal or metal-ceramics composite materials in particular tungsten-
copper
or tungsten-bronze composites.
The main steps of any conventional infiltration process are as follow.
= Tungsten powder preparation with average size of 1-5 m.
=. Optional step: coating the powder with nickel. Total nickel content is
about0.04%.
= Mixing the tungsten powder with a polymer binder.
= Compacting the powder by a modeling method (metal injection molding, die
pressing, isostatic pressing). Compaction should provide the predetermined
porosity level (apparent density) of the tungsten structure.
= Solvent debinding.
= Sintering the green compact at 1200 C-1300 C in hydrogen atmosphere for
2hrs.
= Placing the sintered part on copper plate in the infiltration/sintering
furnace.
= Infiltration of the sintered tungsten skeleton porous structure with copper
at 1110
-1260 C in either hydrogen atmosphere or vacuum for 1 hour.
Infiltration of the sintered tungsten skeleton porous structure by the second
metal,
having a lower melting point, can be conducted under gas pressure (gas
pressure
infiltration), under the pressure of mold movable part i.e. ramming (squeeze
casting
infiltration) and under the die pressure (pressure die casting).
US Patent No 5963773 disclosed a method of fabricating tungsten skeleton
structure
comprising the step of forming a source powder by coating a tungsten powder
with
nickel Then admixing the source powder and a polymer binder, performing powder
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injection molding and obtaining a tungsten skeleton structure by removing the
polymer
binder. A copper plate is then placed beneath the tungsten skeleton structure
and
infiltration is carried out at temperature between 11502C andl2509C. This
method is
not viable for producing. complicated shapes.
US. Patent No. 5413751 describes a process for forming heat sinks and other
heat
dissipating elements by press-forming composite powders for metal components,
for
example tungsten and copper, to form pressed compacts and then sintering the
pressed compacts to achieve a homogenous distribution of the copper throughout
the
tungsten-copper structure.
US Patents No. 4942076, 4988386, 5563101, disclose procedures of improving
heat
sink properties of W-Cu composite material utilized in microwave devices by
maintaining even dispersion of tungsten particles having low thermal expansion
coefficient within a copper matrix having high thermal conductivity. This
process
improves the thermal conductivity of the W-Cu composite and modifies the
thermal
expansion coefficient to be correspondent to that of Gallium Arsenide (Ga.
As.)
Substrate utilized in microwave devices.
In fact almost all previous infiltration techniques utilized so far to
infiltrate the W porous
structure preforms, have been yielding composite structures which more or less
have
some weak points in their mechanical, electrical and micro structural
properties. These
points can be summarized as follow.
= The percentage of the infiltrated component cannot be easily controlled
through
infiltration process and going for higher infiltrant percentages is difficult.
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= Liquid copper is known to have high, contact angles with several oxides
which
impede the infiltration process and accordingly control of furnace atmosphere
is
very essential for successful infiltration process.
= In the thermal debinding cycle of the tungsten green compact to produce
porous
5 tungsten structure preform, formation of tungsten oxide (WO3) is very
common.
This oxide is known to be very volatile and hence it is very important that
it, is to
be avoided to prevent material loss.
= For optimum and maximum infiltration, specific pore length and pore diameter
(UD) are recommended and maintaining these recommendations in the tungsten
porous preform is not easy.
= Mismatch between W skeleton and the matrix increases resistivity and
deteriorates electrical specifications
= Lakes of matrix solidified melts within the W skeleton lead. to non
consistent
mechanical properties.
.
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SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for producing a metal-
matrix
composite of significant ACTE between the hard base-material and any element
type
of the soft metal matrix. The process includes the steps of sintering,
pressing and
enforced infiltration of the sintered compact wherein the process is driven by
differential thermal expansion coefficients between the shell and the core
materials.
The present invention consists of several novel features and a combination of
parts
hereinafter fully described and illustrated in the accompanying description
and
drawings, it being understood that various changes in the details may be made
without
departing from the scope of the invention or sacrificing any of the advantages
of the
present invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be fully understood from the detailed description
given
herein below and the accompanying drawings which are given by way of
illustration
only, and thus are not limitative of the present invention, wherein:
Fig. 1 is a schematic representation of the process for producing a metal-
matrix
composite of significant zCTE between the hard base-metal and the soft,.
matrix
according to the preferred embodiments of the present invention;
Fig. 2 is a 3D schematic diagram of the process of the present invention;
Fig. 3 (a) is an optical graph of W80wt.%-Cu18-Sn shell-on-core sintered
compact
cross section of as received elemental powder, the shell is bronze admix. The
compact
sintered at 1150 C for 3 hours under H2/N2 20/80 wt. ratio as protective gas
utilizing
the process of the present invention.
Fig. 3 (b) is an optical graph of shell-on-core sintered compact cross section
of
W50wt.%-Cu45-Sn sintered compact of two-steps ball milled powder. The shell is
copper wherein the compact is sintered at 1150 C for 3 hours under H2/N2 20/80
wt.
ratio as protective gas utilizing the process of the present invention;
Fig. 3 (c) is an optical graph of shell-on-core sintered compact cross section
of
W80wt.%-Cu18-Sn sintered compact. The compact is sintered at 1150 C for 3
hours
under H2/N2 20/80 wt. ratio as protective gas utilizing the process of the
present
invention
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Fig. 3 (d) is an optical graph shell-on-core sintered compact cross section of
W40wt%-
pre-alloyed bronze sintered compact. The compact is. sintered at 1150 C for 3
hours
under. H2/N2 20/80 wt ratio protective gas utilizing the process of the
present invention;
Fig. 4 is the micro hardness profile across the solidified Cu-Sn shell and the
W80wt.%-
Cu18-Sn sintered compact of 99% of its theoretical density.
Fig. 5 shows SEM micrographs of W 80wt.%-Cu18-Sn sintered compacts of as
received elemental powder sintered and densified by this invention, The last
micrograph represents sintered compact of similar composition sintered by the
conventional method of uniaxial compaction and sintering.
Fig. 6 (a) shows the optical micrographs of W80wt.%-Cu18-Sn compacts of as-
received powder sintered conventionally at 1150 C for 3 hours;
Fig. 6 (b) shows the optical micrographs of W80wt.%-Cu18-Sn compacts of as-
received powder sintered by the process of the present invention under similar
sintering conditions as in Fig. 6 (a).
Fig. 6 (c) shows the optical micrographs of W90wt.%-Cu18-Sn compacts of as-
received elemental powder sintered conventionally at 1150 C for 3 hours.
Fig. 6 (d) shows the optical micrographs of W90wt.%-Cu18-Sn compacts of
similar
composition to Fig. 6 (c) sintered by the process of the present invention
under similar
sintering conditions as in Fig. 6 (c).
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Fig. 7 shows a schematic representation of the process of the present
invention at its
final stage.
Fig. 8 (a) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball
milled powder sintered conventionally.
Fig. 8 (b) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball
milled powder sintered by the present invention wherein sintered density of
99%
theoretical density is achieved.
Fig. 8 (c) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball
milled powder sintered conventionally.
Fig. 8 (d) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball
milled powder sintered by the present invention technique wherein sintered
density of
98% theoretical density is achieved.
Fig. 9 (a) shows the EDX line scan across the shell/core boarder of W50wt%-
Cu45-Sn
sintered compact of ball milled powder sintered by the process of the present
invention.
Fig. 9 (b) shows the EDX line scan of sintered compact of similar composition
of
W50wt%-Cu45-Sn sintered compact of ball milled powder sintered by the present
invention technique but with pre-alloy bronze matrix. The shell is of Cu
element.
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Fig. 10 shows the alumina ceramics mould shows the cavity in where the W-
bronze
green compact placed and covered all around by the Cu-Sn powder mix shell
before
sintering.
5 Fig. 11 shows a typical thermal and infiltration sintering cycle program of
the present
invention wherein the sintering, heating and cooling temperature rates can be
altered
according to the sintered material specifications.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a process for producing a metal-matrix
composite of
significant ACTE between the hard base-metal and the soft matrix. Hereinafter,
this
specification will describe the present invention according to the preferred
embodiments of the present invention. However, it is to be understood that
limiting the
description to the preferred embodiments of the invention is merely to
facilitate
discussion of the present invention and it is envisioned that those skilled in
the art may
devise various modifications and equivalents without departing from the, scope
of the
appended claims.
The present invention is generally related to a method for producing a
composite
material comprising a matrix phase and a dispersed phase, in particular metal-
metal,
metal-ceramics/carbide or composite material, such as tungsten-bronze,
tungsten-
copper, AI-SiC and Al-AI203. The hard base component could be particulates,
dispersoids, cermets, short or continuous fibers, monofilament or whiskers
reinforcements of any material type having significantly lower CTE value than
that of
the matrix. The process includes hot pressing, sintering and infiltration,
acting
simultaneously in one stage to yield microstructure of even distribution of
the
dispersed phase in the metal matrix.
The "three in one densification process" relates to the sintering/infiltration
of metal-
metal or metal-ceramics/carbide composite material by one process comprises
sintering, hot pressing and enforced infiltration acting simultaneously in one
step. The
objective of this invention is to produce, pore free, homogeneous sintered
compact
structure with minimum defects at its near theoretical density. The hard metal
or
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ceramics reinforcements uniformly distributed in a softer metal matrix. The
first stage is
the preparation of the as received composite powder components, either by
mixing or
ball milling. Usually the composite consists of two or more components, i.e.
the hard
base metal component of higher melting point like tungsten and the matrix soft
component having lower melting point, like bronze or copper. The powders
mixture
consist of W base metal ranges between 50-90 wt percent and the balance. is
pre
alloyed bronze or admixture of Cu-SnlOwt%. The ball milled and the as-received
admixed powders are die-pressed separately under uniaxial pressure ranges
between
360-720 MPa preferably around 400 MPa to yield compact discs of 13mm diameter
and around 4 mm thickness. In case of utilizing the as received hard pre-
alloyed
bronze powder as the compact soft component i.e.. the matrix, it is necessary
to add
0.01 of zinc stearate as a binder to enhance the green compaction process,
while it is
not important to add any sort of binder if the compact soft component is Cu or
Cu-
Sn10wt% admixture. The green compact is then placed in a ceramic mold of
suitable
cavity, large enough to accommodate the green compact and then covered all
around
by Cu or bronze powder called here as the outer shell. In case of green
compact being
fabricated from ball milled and mechanically alloyed powders, the outer
covering shell
should be a single phase elemental powder; otherwise the sintered, compact
suffers
severe swelling induced by the divergency in diffusion pathways and powder
particle
size between the core and the covering shell. The composite system i.e. the
green
compact as a core and the covering shell is then sintered in a furnace
following a pre
designed sintering program comprises heating up stage at heating rate of 5-8
C/min
and isothermal sintering stage at a temperature of 1150 C- 1300 C for 2-3
hours. The
final stage is the cooling down stage. It is essential to pay great attention
to the cooling
down rate. Usually 4-8 C/min is suitable for the W composite system.
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Hereinafter the detailed description of the invention will be described in
accordance
with the accompanying drawings which will be referred either individually or
collectively, in any combination thereof, wherein:
Fig. 1 is a schematic representation of the process for producing a metal-
matrix
composite of significant ACTE between the hard base-metal and the soft matrix
according to the preferred embodiments of the present invention. This figure.
shows
that the process which is a "three in one densification process" on its
ongoing action.
As the skin of the outer shell starts to solidify and as the solidification
process
proceeds the outer shell shrinks and compresses continuously towards the
center, the
compression stress builds up gradually driven by the differential thermal
expansion
coefficients ACTE of the shell and the compact materials at the core.
Fig. 2 shows a 3D schematic diagram of the present invention technique wherein
the
core is representing the W-Cu-Sn compact encircled or surrounded by Cu-Sn
shell. As
the shell solidifies, it shrinks and exerts compression stress on the core
which
enhances the core densification process.
Fig. 3 (a) shows an optical graph of W80wt.%-Cu18-Sn shell-on-core sintered
compact
cross section of as received elemental powder, the shell is bronze admix. The
compact
sintered at 1150 C for 3 hours under H2/N2 20/80 wt. ratio as protective gas
utilizing
the process of the present invention.
Fig. 3 (b) shows an optical graph of shell-on-core sintered compact cross
section of
W50wt.%-Cu45-Sn sintered compact of two-steps ball milled powder. The shell.
is
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copper wherein the compact is sintered at 1150 C for 3 hours under H2/N2 20/80
wt.
ratio as protective gas utilizing the process of the present invention;
Fig. 3 (c) shows an optical graph of shell-on-core sintered compact cross
section of
W80wt.%-Cu18-Sn sintered compact. The compact is sintered at 1150 C for 3
hours
under H2/N2 20/80 wt. ratio as protective gas utilizing the process of the
present
invention
Fig. 3 (d) shows an optical graph shell-on-core sintered compact cross section
of
W40wt%-pre-alloyed bronze sintered compact. The compact is sintered at 1150 C
for
3 hours under H2/N2 20/80 wt ratio protective gas utilizing the process of the
present
invention
Fig. 4 shows the micro hardness profile across. the solidified Cu-Sn shell and
the
W80wt.%-Cu18-Sn sintered compact of 99% of its theoretical density.
Fig. 5 shows the SEM micrographs of W 80wt.%-Cu18-Sn sintered compacts of as
received elemental powder densified by the process, in which hot pressing,
sintering
and infiltration process acting together. The micrographs reveal sintered
compacts
having 99% theoretical density. The last micrograph represents sintered
compact of
similar composition sintered by the conventional method of uniaxial compaction
and
sintering.
Fig. 6 (a) shows the optical micrographs of W80wt.%-Cu18-Sn compacts of as-
received powder sintered conventionally at 1150 C for 3 hours;
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Fig. 6 (b) shows the optical micrographs of W80wt.%-Cu18-Sn compacts of as-
received powder sintered by the process of the present invention under similar
sintering conditions as in Fig. 6 (a).
5 Fig. 6 (c) shows the optical micrographs of W90wt.%-Cu18-Sn compacts of as-
received elemental powder sintered conventionally at 1150 C for 3 hours.
Fig. 6 (d) shows the optical micrographs of W90wt.%-Cu18-Sn compacts of
similar
composition to Fig. 6 (c) sintered by the process of the present invention
under similar
10 sintering conditions as in Fig. 6 (c).
Fig. 7 shows a schematic representation of the process of the present
invention at its
final stage. As the sintering process elapsed and the covering shell melt
cools down
and gradually solidifies, it undergoes a substantial contraction driven by its
high
15 thermal expansion coefficient and step by step it commences compressing and
tightening firmly the core which has less thermal expansion coefficient and
less
contraction. This action is hot-isostatic-pressing-like action and leads to
denser
sintered compacts of near its theoretical density.
Fig. 8 (a) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball
milled powder sintered conventionally.
Fig. 8 (b) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball
milled powder sintered by the present invention wherein sintered density of
99%
theoretical density is achieved.
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Fig. 8 (c) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball
milled powder sintered conventionally.
Fig. 8 (d) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball
milled powder sintered by the present invention technique wherein sintered
density of
98% theoretical density is achieved.
Fig. 9 (a) shows the EDX line scan across the shell/core boarder of W50wt%-
Cu45-Sn
sintered compact of ball milled powder sintered by the process of the present
invention..
Fig. 9 (b) shows the EDX line scan of sintered compact of . similar
composition of
W50wt%-Cu45-Sn sintered compact of ball milled powder sintered by the present
invention technique but with pre-alloy bronze matrix. The shell is of Cu
element.
Fig. 10 shows the alumina ceramics mould shows the cavity in where the W-
bronze
green compact placed and covered all around.by the Cu-Sn powder mix shell
before
sintering.
Fig. 11 shows a typical thermal and infiltration sintering cycle program of
the present
invention wherein the sintering, heating and cooling temperature rates can be
altered
according to the sintered material specifications.
So far, the most widely used method to attain fully dense W-bronze or W-Cu
composite compacts, is to infiltrate pre sintered, porous W-base metal
preforms by
bronze or Cu melts. This technique proves to be costly and time consuming as
it
comprises many stages. In this investigation, manipulation the powder
constituent
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physical properties of W composites has led to the implementation of
exclusively
novel "three in one densification invention" process includes sintering, in-
situ hot
isostatic pressing (HIP) and infiltration acting simultaneously at relatively
low
densification temperature of 1150 C under H2/N2 20/80 wt. ratio as protective
gas. Pilot
sintered/infiltrated compacts of = 99% theoretical density of different W (50,
80 and 90)
wt.% balance is Cu-10wt.%Sn compacts of as received W, Cu and Sri metal powder
precursors were produced. Other. sintered/infiltrated compact sets of W50wt.%
and
W80wt.%, balance is bronze 10wt.%Sn compacts of ball milled powder mixes gave
sintered density 95% of theoretical density. The compacts were subjected to
density
measurements, shrinkage and porosity characterization. Microstructure,
hardness and
densification mechanisms of the sintered/infiltrated compacts were evaluated
and
examined using scanning electron microscopy (SEM), energy dispersive x-ray
analysis
(EDX) and x-ray diffraction analysis (XRD).
During the heating up stage and the isothermal stage, sintering of the compact
at the
core is proceeding. The main sintering mechanisms at those two stages are,.
firstly by
solid state diffusion and as the liquid phase forms particles rearrangement
becomes
the dominant sintering mechanism. As the temperature exceeds the shell's
powder
melting temperature, the powder starts to melt and wet the compact surfaces
which
now become a concentric core within the covering shell and forming the so
called
shell-core system. The melted covering shell usually assumes spherical shape
under
its surface tension. At this stage, the melted. shell enhances the sintering
process of
the core. It improves core protection from furnace environment, reduces
oxidation and
prevents contamination. As the heating-up stage and the isothermal stage are
elapsed. The melted shell cools down and eventually starts to solidify. The
solidification process proceeds gradually from the outside towards the center.
Now if
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the density of the solidified shell is greater than that of its liquid, the
new solidified thin
shell will occupies less volume than the original liquid from which it
solidifies;
accordingly negative internal pressure builds up in the not-yet-solidified
metal melt
around the core. The magnitude of this negative internal pressure depends on
the
cooling rate, the solidified sphere diameter and its material physical
properties (CTE of
metal, metal thermal conductivity and the ratio of solid/liquid density). As
far as this
internal negative pressure is not counter balanced by an external force or by
outer
sphere surface buckling, it will experience vacuum-sintering-like effects and
leads to
the elimination of some voids and porosities from the core. As the
solidification of the
outer shell terminates, it undergoes a substantial thermal contraction due to
its high
CTE and starts compressing firmly and.isostaticly the compact at the core
which is not
solidified yet, firstly because of the heat transfer sequence and secondly
because it
contains tungsten particles having low heat dissipation rate and act as a
thermal
source. At this stage the strength modulus of the solidified covering shell is
substantially higher than that of the not-yet-solidified soft component (mushy
matrix)
within the compact at the core. The tensile strength of bronzel0%Sn is
temperature
dependant. As the temperature increases from room temperature to 3009Cthe
bronze
metal looses around 80% of its original ultimate tensile strength.
Accordingly, the
matrix component of lower strength within the core, yields under the applied
external
isostatic pressure and bring the W particles together and expel all voids and
residual
porosities out to the compact/covering shell interface leaving high dense
sintered core.
The sintered compact i.e. the core remains under compressive stress even after
its
own matrix entirely turns to solid and its temperature reaches room
temperature.
The main features of the "three in one densification invention" are.
= It is very simple and cost effective.
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The weight percentages of the sintered compact components can be preliminary
controlled. It is easy to increase the matrix weight percentage of the compact
which is important in improving the thermal conductivity of the product.
= No need for any sort of sintering additives which can lead to non
controllable
shrinkage during liquid phase sintering and have negative effects on thermal
conductivity of the sintered part.
= The covering shell shields the composite compact during sintering process
and
prevents compact oxidation and contamination.
= Post sintering/infiltration process, it is easy to salvage the outer shell
machined
metal for the next production batch usages.
An aspect of the present invention specifies a method for producing a
composite
material having matrix and dispesoid phase like W-bronze, where W is the
dispesoid
phase and the bronze is the matrix. The method comprises die pressing of the
composite elemental powders to yield a green compact and then to place this
compact.
in a ceramics mold of suitable cavity. The green compact is adjusted and
placed in the
cavity as a concentric core surrounded by the covering shell powder. The
weight of the
loose covering shell powder encircled the green compact is at least equal to
its weight.
The ceramics mold and its charge are then introduced into a furnace and the
green
compact with the covering shell powder sinter at temperature above the shell
powder
melting temperature for a certain time under protective gas, hydrogen or inert
gas to
prevent oxidation. The compact soft component metal, called here the matrix,
is
usually similar to the covering shell metal unless specified otherwise. Now as
the core
i.e. the composite green compact gets sintered and as the temperature exceeds
the
25. matrix melting temperature the liquid phase forms inside and outside the
core and
enhance the densification process. When the isothermal sintering stage
entirely
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elapsed, the charge starts to cool down. The differential thermal expansion
between
the core material and the covering shell material leads to a different degree
of
contraction depending on their materials thermal expansion coefficients. At
certain
stage, the completely solidified shell starts contracting and exerting
isostatic pressure
5 on the semi solidified core. This induced pressure and stress/strain reaches
its
maximum degree as the temperature drops to room temperature. The amount of the
exerted stress/strain on the sintered compact at the core depends on the
temperature
gradient, the volume fraction of the W hard component owing .to the lower
thermal
expansion coefficient and the bulk volume of the core and that of the outer
shell. The
10 coefficients of thermal expansion (commonly referred to as iCTE) of the W-
bronze
core are.
a`= VIb 4.5`10-6 +Vfm18.4`10-6 1
am=18.410"6
ac is the thermal expansion coefficient of the sintered compact named here as
15 the core
am is the thermal expansion coefficient of the compact matrix and frequently
of
the covering shell as well.
Vfb and Vfm are the volume fraction of the W-base metal and the bronze matrix
respectively.
During temperature drop from 8009C (the approximate lower solidification
temperature
of bronze) to 259C (the room temperature), the covering shell metal shrinks
and tightly
hold around the core compact, which its metal matrix and despite of its
similarity to the
covering shell metal, is still not yet entirely solidified, exerts compressive
stress/strain
on it resulting in densification and pore elimination. This process
incorporates, hot
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pressing, sintering and infiltration, acting simultaneously and resulting in
homogeneous
pore free sintered composite structure.
The process of the present invention generally relates.to a process for
producing a
metal-matrix composite of significant L\CTE between the hard base-metal and
the soft
matrix, the process includes the steps of sintering, pressing and enforced
infiltration of
the metal-matrix composite compact wherein the process is driven by
differential
thermal expansion coefficients between shell and core materials.
The materials are particulate composite materials or fiber composite materials
and the
metal-matrix composite is composite comprising tungsten, bronze and zinc
stearate
binder. The composite preferably includes 50 wt% of tungsten, 49 wt% of bronze
and 1
wt% of zinc stearate.
The tungsten component of the composite is ranging from 50 wt% to 90wt%. The
bronze component is ranging from 10 wt% to 50 wt% wherein the zinc stearate.
binder
of 1 wt% is added to the composite having pre-ally bronze component only.
The zinc stearate binder is intimately mixed with the tungsten and bronze
whereas the
bronze component of the composite can be pre alloyed bronze or Cu-Sn admixed
bronze. The weight of the Sn element in bronze is 10 % whether it is in the
pre alloyed
bronze or in the Cu-Sn admixed bronze.
During pre sintering of the tungsten-bronze, the green compact is set as a
core
covered by bronze powder shell whereas post sintering, the tungsten-bronze
composite compact becomes a concentric core in bronze solidified sphere shell.
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The composite material can be a ball milled tungsten-Cu-Sn powder or as
received
tungsten-Cu-Sn elemental powder. The composite could be a ball milled tungsten-
pre
alloy bronze as well.
The material of the covering sphere shell can be as received pre-alloy bronze
powder
or as received Cu-Sn admix powder or as received Cu elemental powder.
The weight of the covering shell should be at least equivalent to the weight
of the
compact.
The densification of the tungsten-bronze compacts is conducted at a
temperature
ranging from 1150 C-1400 C, preferably at 1200 C under H2 or H2/N2 protective
gas.
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Example
Tungsten-Cu-Sn composites
In order to apply the "three in one densification invention" for the
production of
W80wt%-Cu18-Sn sintered compacts, a suitable ceramic mold with a certain
cavity of
required shape was fabricated.
As received tungsten powder of 12pm particle size and 99.9 purity was admixed
with
copper powder of <45pm particle size, 99.5 purity and tin powder of <45pm
particle
size and 99.8 purity. The amount of tungsten was 80wt% and the balance was Cu-
SnlOwt. mix, no binder was used. The admixture was mixed manually in small
glass
container for 30 minutes to avoid any sort of segregation induced by the
variations in
particle size and density of the mixture components. Then admixture was die
pressed
uniaxialy under 360 MPa. The green compact disc produced was 5 gram weight of
13mm diameter and nearly 4 mm thickness and having 70% of its theoretical
density..
The green compact was then placed in the ceramic mold cavity and covered all
around
by 5 g weight as received Cu-Sn10 wt% powder mix. The covered green compact
was
sintered in alumina tube furnace. The heating up rate was 8 C/min, the
isothermal
sintering temperature was 1150 for 3 hours and the cooling down rate was 4
C/min.
The "three in one densification invention" sintering process was conducted
under
H2/N2 gas of 20/80 wt ratio. Post sintering, the.mold charge was dismantled
and. the
solidified covering shell was machined and grinded away to extract the
sintered
compact. The invention yielded sintered compact of 99% theoretical density
with even
dispersion of the W phase in Cu-Sn matrix, homogeneous, voids and cracks free
structure. The sintered compact had an average micro hardness value of 250
(Hv).
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The Cu-Sn volume fraction in the sintered compact was of 36% which is very
difficult
to be attained by the existing conventional infiltration technology. However,
by this
invention, controlling the volume fraction of the soft component in the
compact, which
is very important factor in electronic industry applications, is not a real
problem as it
can be designed prior to green compact production.
The three main mechanical and physical parameters governing the "three in one
densification invention" are:
= The 4CTE between the compact at the core and the covering shell.
The differential amount of the radial displacement (Ur) of the solidified
compact
and the surround covering shell.
= The compression stress or the radial stress (võ) induced by the differential
radial
displacement and becomes responsible for the compact densification.
Sintered compacts of 50-90 W wt% were produced successfully by this invention.
Sintered compacts of 80-90 W wt% showed the best results. Besides these
parameters, other factors like, connectivity, contiguity and the particle size
of the hard
component have great effects on "the three in one densification invention"
action.
