U.S. patent application number 10/956595 was filed with the patent office on 2006-03-30 for porous metallic product and method for making same.
Invention is credited to Khershed P. Cooper, Harry N. Jones, Chandra S. Pande, Bhakta B. Rath.
Application Number | 20060065330 10/956595 |
Document ID | / |
Family ID | 36097657 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060065330 |
Kind Code |
A1 |
Cooper; Khershed P. ; et
al. |
March 30, 2006 |
Porous metallic product and method for making same
Abstract
This invention pertains to a product and a method for making the
product. The product is a lightweight solid porous metallic product
containing small solid spheres having a coating of a primary alpha
phase thereon disposed in a solid metal alloy eutectic matrix. The
method includes the steps of mixing the hollow rigid spheres and a
metal alloy, which metal alloy can be preheated to render it
molten, in order to form a dispersion of the spheres distributed in
the molten alloy; initially cooling the dispersion to render it
semi-solid whereby the spheres are coated by a solid and the coated
spheres are disposed in the semi-solid mixture of the solid and
liquid; and finally cooling the sphere-containing semi-solid
mixture to a temperature at which the sphere-containing semi-solid
mixture becomes solid and the product is formed.
Inventors: |
Cooper; Khershed P.;
(Fairfax, VA) ; Rath; Bhakta B.; (Oakton, VA)
; Jones; Harry N.; (Alexandria, VA) ; Pande;
Chandra S.; (McLean, VA) |
Correspondence
Address: |
NAVAL RESEARCH LABORATORY;ASSOCIATE COUNSEL (PATENTS)
CODE 1008.2
4555 OVERLOOK AVENUE, S.W.
WASHINGTON
DC
20375-5320
US
|
Family ID: |
36097657 |
Appl. No.: |
10/956595 |
Filed: |
September 29, 2004 |
Current U.S.
Class: |
148/538 ;
148/400 |
Current CPC
Class: |
B22F 3/1112 20130101;
C22C 1/08 20130101 |
Class at
Publication: |
148/538 ;
148/400 |
International
Class: |
C22C 1/08 20060101
C22C001/08 |
Claims
1. A method for preparing a lightweight porous metallic product
comprising the steps of (a) mixing rigid, hollow and solid spheres
with a metal alloy in order to obtain a dispersion of the spheres
in a semi-solid alloy; and (b) cooling the semi-solid alloy
containing some liquid and the solid coated spheres to a solid.
2. The method of claim 1 which includes the step of heating the
metal alloy to a temperature at which it is in a liquid state and
the spheres remain solid, rigid and hollow.
3. The method of claim 2 wherein the alloy is composed of at least
one metal and at least one alloying component and the spheres are
selected from the group consisting of metallic and ceramic
spheres.
4. The method of claim 3 wherein size of the spheres is in the
range of 10-100,000 nm in diameter; the sphere coating thickness is
in the range of 1-100% of sphere radius; and volume fraction of the
spheres relative to the alloy is 10-90%.
5. The method of claim 3 wherein size of the spheres is in the
range of 100-10,000 nm in diameter; the sphere coating thickness is
in the range of 10-50% of average sphere radius; and volume
fraction of the spheres relative to said alloy is 30-70%.
6. The method of claim 5 wherein said spheres are inert to said
molten alloy.
7. The method of claim 6 wherein said alloy is an alloy of a metal
"A" and an alloying component "B" having a phase diagram
characterized by a liquid region at upper extremity thereof, a
primary a phase at the left thereof where composition of "A" is in
the majority, a semi-solid region below the liquid region above
eutectic temperature and defined by liquidus and solidus lines, and
a solid eutectic region below the eutectic temperature.
8. The method of claim 7 wherein the primary a phase at the left
side of the phase diagram is defined by solidus line at the upper
extremity and a solvus line at the right side thereof; and the
solid eutectic region is defined at its left side by the solvus
line, by another solvus line at its right side, and by the eutectic
temperature at its upper extremity; said initial and final cooling
steps are conducted along a vertical line passing through the
semi-solid region.
9. The method of claim 8 wherein the vertical line crosses over
less than one-half of the horizontal central extent in the
semi-solid region.
10. The method of claim 8 wherein the vertical line crosses over
less than one-third of the horizontal central extent in the
semi-solid region.
11. A method for preparing a lightweight, porous and net-shape
metallic product comprising the steps of (a) mixing rigid and
hollow spheres with a metal alloy in order to obtain a dispersion
of the spheres in the molten alloy; (b) initially cooling the
dispersion to render it semi-solid whereby the spheres are coated
with a solid phase and the coated spheres are dispersed in the
liquid metallic material; (c) filling a mold cavity with the solid
phase coated spheres dispersed in a molten metallic material; and
(d) finally cooling the solid phase coated spheres dispersed in the
molten material to a solid state in the mold vavity.
12. The method of claim 11 which includes the step of heating the
metal alloy to a temperature at which it is in a liquid state and
the spheres remain rigid, hollow and solid.
13. The method of claim 12 wherein the alloy is selected from the
group consisting of magnesium alloys, aluminum alloys, zinc alloys
and other lightweight alloys; and the spheres are selected from the
group consisting of metallic, ceramic and coated composite
spheres.
14. The method of claim 13 wherein size of the spheres is in the
range of 10-100,000 nm in diameter; the sphere coating thickness is
in the range of 1-100% of average sphere radius; and volume
fraction of the spheres relative to the alloy is 10-90%.
15. The method of claim 13 wherein size of the spheres is in the
range of 100-10,000 nm in diameter; and the sphere coating
thickness is in the range of 10-50% of average sphere radius.
16. The method of claim 15 wherein the spheres are inert to the
molten alloy.
17. The method of claim 16 wherein the alloy is an alloy of a metal
"A" and an alloying component "B" having a phase diagram
characterized by a liquid region at upper extremity thereof, a
primary a phase at the left thereof where composition of "A" is in
the majority, a semi-solid region below the liquid region above
eutectic temperature and defined by liquidus and solidus lines, and
a solid eutectic region below the eutectic temperature.
18. The method of claim 17 wherein the primary a phase at the left
side of the phase diagram is defined by solidus line at the upper
extremity and a solvus line at the right side thereof; and the
solid eutectic region is defined at its left side by the solvus
line, by another solvus line at its right side, and by the eutectic
temperature at its upper extremity; said initial and final cooling
steps are conducted along a vertical line passing through the
semi-solid region
19. The method of claim 18 wherein the vertical line crosses over
less than one-half of the horizontal central extent in the
semi-solid region.
20. The method of claim 18 wherein the vertical line crosses over
less than one-third of the horizontal central extent in the
semi-solid region.
21. A product comprising hollow rigid spheres having a solid
coating thereon disposed in a solid metal alloy eutectic.
22. The product of claim 21 wherein said solid coating on said
spheres is primary proeutectic alpha phase and volume fraction of
said spheres relative to said alloy is 10-90%.
23. The product of claim 22 wherein said alloy is composed of at
least one metal component and at least one alloying component.
24. The product of claim 23 wherein said metal alloy eutectic is
.alpha. and .beta. phases.
25. The product of claim 24 wherein said primary alpha phase
coating is featureless in the microscope and is of different
microstructure than is the .alpha.+.beta. eutectic mixture.
26. Lightweight porous net-shape product having physical properties
that are better than that of bulk product physical properties
comprising ceramic hollow spheres having a solid coating thereon
disposed in a solid metal alloy eutectic.
27. The product of claim 26 wherein said solid coating on said
spheres is primary proeutectic alpha metal alloy phase and volume
fraction of said spheres relative to said alloy is 30-70%.
28. The product of claim 27 wherein said alloy is selected from the
group consisting of magnesium alloys, aluminum alloys and zinc
alloys.
29. The product of claim 28 wherein size of said spheres is
100-10,000 nm; said sphere coating thickness is 10-50% of average
sphere radius; and volume fraction of said spheres relative to said
alloy is 30-70%.
30. The product of claim 29 wherein said alloy eutectic is .alpha.
and .beta. phases and wherein said primary alpha phase is of a
different microstructure than said eutectic mixture.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to the field of porous metallic
objects and preparation thereof that is characterized by the use of
small hollow spheres in a semi-solid mixture in a metal alloy.
DESCRIPTION OF THE RELATED ART
[0002] Lightweight or porous metallic materials and structures are
made in honeycomb and foam core forms. Other examples are linear
cellular metals and metal wire trusses. Most of these materials are
made in simple shapes such as two-dimensional panels and
one-dimensional bars and rods. Making these into complex
three-dimensional shapes is very difficult and cost intensive. This
is because of the nature of the processing routes for these
materials. These materials are expensive to fabricate because they
require several processing steps. Honeycomb structures are made by
epoxy joining or brazing together a honeycomb arrangement made by
folding thin metal ribbons. This forms the honeycomb core to which
are joined face panels to make a sandwich structure. Foam materials
are made through a powder metallurgy route, which involves mixing
volatile materials with the metallic powder and high temperature
processing. The foam core is also joined to face panels to make a
sandwich structure. Linear cellular metal structures are made by
hydrogen reduction of extruded oxide structures at high
temperature. Metal wire truss structures are made by liquid phase
sintering of the truss joints. To obtain the truss configuration, a
complex weaving operation is required. Some of these materials have
good stiffness but lack in other mechanical properties, such as
tensile and compressive strengths. Because of their high
fabrication cost, these lightweight materials and structures have
been targeted for high-end applications, such as aerospace,
propulsion and ballistic protection. Many of these materials are
made abroad and availability thereof is severely restricted.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
[0003] It is an object of this invention to make lightweight or
porous metallic products using small, hollow ceramic or metallic
particles to reduce density thereof.
[0004] It is another object of this invention to make net-shape
lightweight products that do not require substantial machining.
[0005] Another object of this invention is lightweight, porous
metallic products that retain greater than the expected amount of
their physical properties, i.e., lower density, higher relative
strength and higher relative stiffness.
[0006] Another object of this invention is a method for preparing
the lightweight metallic products, the method being characterized
by the use of semi-solid metal alloy.
[0007] Another object of this invention is a method for preparing
lightweight metallic products, the method being characterized by
cooling a liquid (L) metal alloy to a semi-solid or semi-liquid
state.
[0008] Another object of this invention is a method of making a
lightweight metallic product, the method being characterized by a
short holding period in the semi-solid or semi-liquid state in
order to stabilize the solid alpha (.alpha.) and the liquid
(L.sub.1) phases.
[0009] Another object of this invention is a method of using
semi-solid region in eutectic, peritectic and monotectic metallic
alloy systems.
[0010] These and other objects of this invention can be attained by
preparing a lightweight, net-shape product having better than
expected physical properties by mixing small ceramic and/or
metallic hollow spheres with a metal alloy, such as eutectic type,
heating the mixture to liquify the alloy whereby the spheres are
coated with a solid primary alpha (a) phase surrounded by liquid L,
phase, and solidifying the resulting mass whereby the hollow
spheres have a coating of the solid primary alpha phase surrounded
by the solid metal alloy of eutectic composition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a typical binary eutectic phase diagram for
metallic alloys.
[0012] FIG. 2 is a schematic illustration in cross-section of a
porous metal alloy product showing dispersed hollow spherical
particles having a solid coating of a primary or alpha phase on the
particles disposed in a solid metal alloy of eutectic
composition.
[0013] FIG. 3 is a graph of a volume fraction .phi. and scaled
separation distance showing separation distance between voids as a
function of void volume fraction.
[0014] FIG. 4 is a graph of volume fraction .phi. and Modulus of
Elasticity or stiffness, in MPa.
[0015] FIG. 5 is a graph of ratio of inside to outside diameter of
a hollow ceramic spherical particle and normalized density of a
sphere.
DETAILED DESCRIPTION OF THE INVENTION
[0016] This invention pertains to a method for making a lightweight
porous metallic product and to the product itself for eutectic,
peritectic and monotectic systems, although only the eutectic case
is discussed herein.
[0017] The invention is a novel approach for making under-dense or
lightweight or porous metallic materials, especially into a net
shape. The method involves mixing nanoscopic or microscopic hollow
spheres into a metal alloy of at least two components during
processing. The spheres are typically ceramic since most ceramics
have a higher melting point than a metallic alloy. The spheres,
however, can be non-ceramic, such as metallic, wherein the metal
the sphere is made of has a melting point that is higher, and
preferably substantially higher, such as several times higher, than
the melting point of the matrix metal alloy and is generally
immiscible in the matrix metal alloy. The hollow spheres can be
ceramic coated with a metal or they can be metallic coated with a
ceramic. This creates a dispersion of nanoscopic or microscopic
spherical voids or pores in a fully dense metal matrix thereby
reducing the effective density of the composite. To be effective,
the dispersion has to be uniform and clustering or agglomeration
has to be avoided. Thin-walled hollow nanospheres or microspheres
have a low specific gravity, preferably less than one. The bulk
density of the metallic alloy will be lowered because the hollow
spheres create free volume or space. Additionally, the uniformity
of distribution of the small rigid hollow spheres in the metallic
matrix, at the same time, maintains the integrity of the
product.
[0018] The molten metallic phase is of any metal component and any
alloying component. At least one metal and at least one alloying
component are used. Of particular interest here are the commercial
alloys, especially the alloys of the light metals such as
magnesium, aluminum and zinc, but not limited to these. Some of the
alloys of interest include, but are not limited to AZ-91-D,
AM-50-A, AM-60-B, AS-41-B, A356, A357, 319, A390, Zamak 3, and
ZA-8. The spheres are tiny, hollow but rigid particles and include
engineered and non-engineered particles, such as fly ash which is a
by-product of flue gas resulting from the coal-burning process. The
hollow nanospheres and microspheres can be ceramic or metallic and
typically range in size from 10 to 100,000 nm, more typically from
100 to 10,000 nm. Since majority of the spheres must remain solid
and hollow during processing, melting point thereof is in excess of
the melting point of the surrounding metallic matrix. In order to
avoid reactivity with the surrounding metallic matrix, the spheres
are typically inert thereto.
[0019] The invention is described as adding tiny hollow spheres to
a metallic alloy base material. The sphere-alloy mixture is
fabricated into a desired product by shaping the mixture with one
of several low cost, net-shape processing routes. Net-shape
processing of metallic alloys can involve metal casting or powder
metallurgy. Investment casting, die casting and semi-solid molding
are a few of the metal casting routes. Metal injection molding, hot
isostatic pressing and hot extrusion are a few of the powder
metallurgy routes. Solid freeform fabrication or layered
manufacturing is another near net-shaping technology and can
involve melting and solidification of sequential weld pools or
sintering and consolidation of powder particles. By the layered
manufacturing technology, very complex geometries, not possible by
conventional means, are fabricated. Net-shape products or parts
require little of no finishing. The advantages of net-shape forming
include, but are not limited to, low material waste, high
throughput, high volume and low fabrication cost.
[0020] Critical to the success of the invention is to ensure
uniform dispersion of the small hollow spheres and the base
materials, minimize problems due to density differences between the
spheres and the metallic phases, ensure integrity of the additives
during mixing that the hollow spheres do not collapse and remain
intact during processing, ensure uniform filling of the mold or any
other cavity by the mixture or slurry without gross segregation.
Finally, control of flow of the rheological mixtures is critical
and wetting, high temperature reactions and dissolution issues
should be considered.
[0021] The addition of hollow microspheres or nanospheres to the
metal matrix serves a dual purpose: it lowers the density and
increases the specific strength and stiffness of the product. The
porosity and voids created by the hollow additives reduce the
effective density of the composite aggregate. The hollow additives
also act as particulate reinforcements in which case, while the
absolute yield strength decreases due to the existence of pores,
the effective yield strength of the composite aggregate increases
due to constraint of plasticity flow offered by the hollow spheres.
This is the case if it is assumed that the hollow additives have
rigid, strong walls and do not deform under load. The same
reasoning that is applied to precipitation hardening or
strengthening can be applied to the spherical reinforcement, i.e.,
the reinforcements, whether as small, hard precipitates or small,
hollow spheres, impede dislocation flow thereby achieving improved
strength. Additionally, if the material of which the hollow
additives are made of has higher stiffness than the matrix
material, then the effective stiffness, calculated from the rule of
mixtures, of the composite aggregate should increase. Table 1,
below, gives the Elastic Modulus and density of several ceramics
which will affect properties of the final porous metal product.
Except for silica and boron nitride, most ceramics have a higher
Elastic Modulus than most engineering metals whose stiffness ranges
from 40 to 200 GPa. Compared to the lightest engineering metal,
i.e., magnesium, which has a Modulus of Elasticity of 45 GPa, all
ceramics listed in Table 1 have higher values. Hence, the specific
stiffness of the composite aggregate can be expected to increase
with the addition of ceramic microspheres or nanospheres.
TABLE-US-00001 TABLE 1 Physical Properties of Some Ceramics Ceramic
Density, g/cc Elastic Modulus, GPa silica 2.2 73 alumina 3.7 300
silicon carbide 3.1 410 aluminum nitride 3.3 330 boron nitride 1.9
74 silicon nitride 3.3 310
[0022] More specifically, in connection with an embodiment of this
invention, porous product and the method for making it can be
better described and understood by reference to FIG. 1, a eutectic
phase diagram of a two-component metal alloy of a metal component
"A" and an alloying component "B." Component "A" of the alloy is
the lighter component and is typically present in a weight amount
exceeding 50%. It is possible, however, for component "A" to be the
major component although in an amount below 50% if the alloy is one
of more than two components. The phase diagram is a typical graph
of weight composition "C" on the "x" axis versus temperature "T" on
the "y" axis. The phase diagram of FIG. 1, illustrates presence of
a liquid phase (L) above the liquidus lines 10, 11; presence of a
solid primary or proeutectic phase alpha (.alpha.) at the left side
of FIG. 1 bounded by solidus line 12 and solvus line 14; presence
of solid phase beta (.beta.) or proeutectic .beta. at the right
side of FIG. 1 bounded by solidus line 16 and solvus line 18;
presence of a semi-solid or semi-liquid region of solid alpha
(.alpha.) phase and liquid (L.sub.1) i.e., .alpha.+L.sub.1, at the
left side of FIG. 1 bounded by lines 10, 12 and 20, which segment
line 20 extends horizontally along the eutectic reaction
temperature T.sub.eu from point 22 to eutectic concentration point
24. The phase diagram of FIG. 1 also includes another semi-solid or
semi-liquid region, that is of less interest herein, of solid beta
(.beta.) phase and liquid L.sub.2, i.e., .beta.+L.sub.2, at the
right side of the phase diagram of FIG. 1 bounded by lines 11, 16
and 26, which segment line 26 extends from point 25 on the eutectic
reaction temperature T.sub.eu to the eutectic concentration point
24. Also depicted on the phase diagram of FIG. 1 is the solid
eutectic of alpha and beta phases (.alpha.+.beta.) defined by lines
14, 18 and the eutectic reaction temperature line formed by
segments 20, 26.
[0023] As illustrated in FIG. 1, the method typically includes the
steps of mixing the hollow spheres and metal alloy chips to form a
dispersion point "a", melting the metal alloy, initial cooling, and
final cooling. It should be understood that the method includes
variations or embodiments which do not strictly adhere to the given
steps. For instance, the mixing step can be carried out by mixing
solid hollow spheres with solid metal alloy chips, as shown at 28,
or else a melted alloy can be used and be mixed with the spheres in
order to uniformly disperse the spheres in the molten metal alloy
matrix or the spheres can be added at a later stage of the initial
cooling into the semi-solid region or in any other way. Whichever
way it is done, what is ultimately desired at this point is a
dispersion of the hollow spheres in the metal alloy, shown in FIG.
1. The step of melting can take place resulting in a molten alloy
of given composition, with the spheres dispersed therein, as shown
at point "b" on the phase diagram of FIG. 1, where a schematic of
the molten mixture is shown at 30. At temperature "b," the spheres
are in solid state. Typically, the mixture at point "b," is cooled
along vertical line 25 to point "c" within the semi-solid region
.alpha.+L.sub.1 where the spheres become coated with solid primary
or proeutectic alpha phase. At point "c," the status of the mixture
is illustrated by depiction 34 in FIG. 1 where the mixture is shown
as being composed of solid spheres 40 coated with the solid primary
or proeutectic alpha phase 42 disposed in the liquid metal L, 41.
Volume fraction of the liquid phase L, in the semi-solid region can
be determined by drawing a horizontal line 32 across vertical line
25 at point "c", which identifies the hypothetical cooling path of
the mixture. Line 32 is composed of segment 36, to the right of
vertical line 25 and segment 33, to the left of the vertical 25.
The ratio of line 36 to line 32, which is the sum of lines 33 and
36, gives volume fraction of the solid alpha phase, which forms
around the spheres, whereas the ratio of line 33 to line 32, yields
volume fraction of the liquid L.sub.1.
[0024] Of course it should be understood that as the mixture 30 is
cooled from point "b" along the vertical line 25 to point "c," some
solid alpha phase will appear as soon as liquidus line 10 is
crossed. At a temperature of T.sub.1, for instance, composition of
the alpha (a) phase is determined at point 51 and composition of
the liquid L.sub.1 is determined at point 52. So, as the
temperature of mixture 30 is reduced from point "b" to point "c,"
the composition of the alpha phase and the liquid phase in the
semi-solid region will change after crossing liquidus line 10 and
as the temperature is gradually reduced to point "c."
Hypothetically, one can visualize the process being conducted by
formation of a thin concentric coatings, of solid alpha phase
around the hollow spheres, with the coated spheres residing in the
liquid L.sub.1 phase. Composition of the alpha phase in the
concentric coating is that determined by the solidus line 12 and
composition of the liquid L.sub.1 phase is determined by the
liquidus line 10.
[0025] Above discussion is predicated on the assumption that
cooling proceeds along the path of the vertical line 25 and enough
liquid L, is present to facilitate filling of a mold cavity.
However, if cooling proceeds along a vertical line through the
eutectic composition point 24, it should be understood that there
will not be any liquid L, to facilitate filling of mold cavity.
[0026] Depiction 34 shows rigid spheres 40 coated with solid
primary or proeutectic alpha phase 42 disposed in the liquid matrix
L.sub.1. Not all spheres are coated with the alpha phase and the
coating thereon may not be of a uniform thickness. Also, some
primary of proeutectic alpha phase can form on impurity surfaces in
the liquid L.sub.1. It is desirable to have a uniform thickness of
alpha phase on the spheres since the alpha phase provides a
protective coating on the spheres and thus insulates them from the
molten metallic matrix, which may be reactive therewith. The
coating thickness can vary widely and can be uniform or
non-uniform. Depending if the spherical particle is nanoparticle or
microparticle and depending upon the selected alloy composition,
the coating thickness is typically within the range 1 to 100% of
sphere radius, more typically in the range 10 to 50% of sphere
radius.
[0027] Injection of the semi-solid mixture can be made into a mold
at temperature "b" and then cooling down along line 25 all the way
into the solid region .alpha.+.beta. to room temperature, or any
other desired temperature. This scenario is possible but is not
typical of processing since formation of the solid phase is brief
and its deposition on the spheres is not entirely adequate.
Deposition of the alpha phase on the spheres is governed by the
fact that the spheres provide nucleation sites for the alpha phase
and adequate time should be devoted for formation of the alpha
phase and its stabilization. In this sense, a holding period of
less than a few minutes, such as less than about 1 minute, is
typically provided to allow the alpha phase to grow and stabilize
on the sphere surface and form a coating.
[0028] So, whether the mixture is cooled slowly initially to a
point within the semi-solid region or from a point above the
liquidus line 10 quickly into the semi-solid region above the
eutectic reaction temperature, denoted .alpha.+L.sub.1 in FIG. 1,
what results is depiction 34 wherein the solid spheres are
surrounded by the primary proeutectic alpha phase disposed in a
liquid metal matrix. The spheres can have a continuous coating of
the primary phase thereon but the coating can also be
discontinuous. Coating thickness can vary greatly, depending on
many variables, including the relative proportion of the alpha
phase, proportion of the spheres, if a holding period is employed,
and others. Typically, coating thickness is in the range of 1 to
100%, more typically 10 to 50% of sphere radius.
[0029] Since too much solid alpha phase in the semi-solid makes it
difficult to push the semi-solid mixture into a mold cavity and
since too much liquid in the mixture weakens the product too much,
it has been estimated that the area of operation for purposes
herein, is about one half of the horizontal extent in the
semi-solid region between points 22 and 24, more typically about
one-third of the central region.
[0030] One of the final steps of the method is cooling to a low
temperature, such as room temperature, whereas the final product 36
is formed, shown in FIG. 1. As shown, the final product 36 is all
solid with the hollow spheres 40 coated with primary proeutectic
alpha phase 42 disposed in the eutectic 44 of phases .alpha. and
.beta.. This is its form at point "d" and remains that at below the
eutectic temperature T.sub.eu.
[0031] It should be realized that whether the mixture is cooled to
point "c" from point "b" and then it is moved into a mold cavity or
quickly cooled from point "b" to point "d" and from there it is
moved into a mold cavity, determines whether liquid will be present
to facilitate mold cavity filling. It should be apparent that it is
only in the semi-solid region of .alpha.+L.sub.1, that the mold
cavity can be filled, although the semi-solid region of
.beta.+L.sub.2 can also be used, depending on properties of the
alloying element "B." Cooling through point 24, as discussed, will
not be accompanied by any liquid, unless independently provided.
Although alpha and beta compositions in the eutectic .alpha.+.beta.
change slightly, as evidenced by the solvus lines 14, 18 on the
phase diagram of FIG. 1, the compositions of the alpha and beta
phases in the eutectic remain essentially that as determined by
points 22 and 25, respectively. It is only if cooling is extremely
slow that respective compositions are realized below the eutectic
temperature.
[0032] As should be apparent, as soon as the eutectic temperature
T.sub.eu is crossed, the mixture solidifies into depiction 36 which
has solid spheres 40 coated by the solid primary proeutectic alpha
phase 42 and the coated spheres are distributed in the solid
eutectic 44 of .alpha.+.beta. of average composition determined by
point 24. The primary proeutectic alpha phase is usually colorless
or featureless under microscope but .alpha.+.beta. eutectic is of a
different microstructure and usually appears lamellar under the
microscope. So, the product that is finally obtained, contains
solid spheres 40 coated with the solid primary proeutectic alpha
phase 42 of a composition determined by point 22 and the same alpha
phase of composition 22 and the beta phase by point 25 disposed in
the solid eutectic 44 of .alpha.+.beta..
[0033] When the above preparation procedure is followed, a product
is obtained that has physical properties that exceed properties
that can be predicted on the basis of the law of physical mixtures.
On the basis of the method discussed herein, the resulting product
has physical properties on the order of about 70% of the bulk
properties versus about 50% when considered on the basis of
physical mixtures containing a metal alloy and the small or tiny
metal or ceramic hollow spheres.
[0034] Although preparation of a porous metallic product rich in
component "A" has been demonstrated, it should be understood that a
porous metallic product can also be prepared rich in component "B"
by following a cooling path indicated by line 38 in FIG. 1. If this
is done, then component "B" should be in general, lighter than
component "A" and have other desired attributes.
[0035] Although, hypothetically, any rigid hollow spheres can be
used herein to lighten a metal alloy, realistically speaking, only
the lighter metal alloys of the lighter structural kind, such as
magnesium, aluminum, zinc and even titanium, can especially benefit
from the invention herein. Lightness of the starting metal is of
import since it is unlikely that one would gain much by lightening
a heavy metal.
[0036] While presently preferred embodiments have been described of
the novel lightweight porous metallic product and the method for
its preparation, persons skilled in this art will readily
appreciate that various additional changes and modifications can be
made without departing from the spirit if the invention as defined
and differentiated by the claims that follow.
* * * * *