U.S. patent application number 11/186006 was filed with the patent office on 2006-02-02 for method and device for producing a reinforced component by thixoforming.
Invention is credited to Rainer Gadow, Guenther Messmer, Klaus Siegert, Marcus Speicher, Peter Unseld, Konstantin von Niessen.
Application Number | 20060021728 11/186006 |
Document ID | / |
Family ID | 34626348 |
Filed Date | 2006-02-02 |
United States Patent
Application |
20060021728 |
Kind Code |
A1 |
Gadow; Rainer ; et
al. |
February 2, 2006 |
Method and device for producing a reinforced component by
thixoforming
Abstract
A method is disclosed for producing a reinforced component from
a composite material (MMC). A metallic matrix material is
reinforced by embedded fibers or particles, for which purpose a
semi-finished product is prepared that comprises fibers and
particles or both, together with a metallic matrix material.
Forming is effected by thixoforming in a mold at a temperature
above the solidus temperature and below the liquidus temperature of
the metallic matrix material. The method allows to manufacture
near-net-shaped products with excellent mechanical properties.
Inventors: |
Gadow; Rainer; (Aschau am
Inn, DE) ; Speicher; Marcus; (Saarbruecken, DE)
; von Niessen; Konstantin; (Weinstadt, DE) ;
Unseld; Peter; (Markdorf, DE) ; Messmer;
Guenther; (Duerbheim, DE) ; Siegert; Klaus;
(Sindelfingen, DE) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
34626348 |
Appl. No.: |
11/186006 |
Filed: |
July 20, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP03/12352 |
Nov 5, 2003 |
|
|
|
11186006 |
Jul 20, 2005 |
|
|
|
Current U.S.
Class: |
164/80 ; 164/113;
164/76.1; 164/900 |
Current CPC
Class: |
B22D 19/14 20130101;
C22C 1/005 20130101; B22F 2998/10 20130101; C22C 47/06 20130101;
C22C 47/12 20130101; B22D 17/007 20130101; C22C 47/04 20130101;
C22C 47/12 20130101; B22F 2998/10 20130101 |
Class at
Publication: |
164/080 ;
164/900; 164/076.1; 164/113 |
International
Class: |
B22D 23/06 20060101
B22D023/06; B22D 27/09 20060101 B22D027/09 |
Claims
1. A method for producing a component from a composite material
(MMC), comprising the steps of: providing a fibrous structure;
consolidating said fibrous structure with a metallic matrix
material to a semi-finished product; transferring said
semi-finished product into a mold; heating said semi-finished
product to a temperature above a solidus temperature and below a
liquidus temperature of the metallic matrix material; thixoforming
said semi-finished product to form a product; and cooling said
product down to room temperature.
2. The method of claim 1, wherein said semi-finished product is
produced by a process selected from the group formed by: laminating
layers of fibrous structures and metal sheets to form a prepreg;
coating a fibrous structure with a metallic matrix material;
laminating a plurality of coated fibrous structures to form a
prepreg; coating a fibrous structure with the metallic matrix
material by a screen printing process; coating a fibrous structure
with the metallic matrix material by electrostatic charging;
coating a fibrous structure with the metallic matrix material by
electrophoretic deposition from an aqueous suspension, at the aid
of an electric field; and coating a fibrous structure with the
metallic matrix material by thermal spraying.
3. The method of claim 2, wherein said fibrous structure is coated
by a thermal spraying process selected from the group formed by:
atmospheric plasma spraying, wire flame spraying and electric arc
spraying.
4. The method of claim 1, wherein a proportion by volume between
said metallic matrix material and a reminder of said semi-finished
product is selected to be in the range of 0.8 to 3.0.
5. The method of claim 2, wherein the fibrous structure during
thermal spraying is maintained under a tensile stress.
6. A method for producing a component from a composite material
(MMC), comprising the steps of: mixing a metallic matrix material
with a reinforcement material by a method selected from the group
formed by granulating and pelletizing; transferring a mixture
obtained from said mixing step into a mold; pressing said mixture
within said mold to form a semi-finished product; transferring said
semi-finished product into a die; heating said semi-finished
product to a temperature above a solidus temperature and below a
liquidus temperature of the metallic matrix material; thixoforming
said semi-finished product within said die to form a product; and
cooling said product down to room temperature.
7. The method of claim 6, wherein said reinforcement material is
formed by chopped fibers.
8. The method of claim 7, wherein said chopped fibers have a length
of between 0.5 and 20 mm.
9. The method of claim 6, wherein a proportion by volume between
said metallic matrix material and a reminder of said semi-finished
product is selected to be in the range of 0.8 to 3.0.
10. The method of claim 6, wherein said reinforcement material is
selected from the group formed by: pulverized particles, oxide
ceramics and carbides.
11. The method of claim 1, wherein said fibrous structure is
prepared with at least one layer made from long fibers, said long
fibers being defined by a characteristic selected from the group
formed by: a length of at least one millimeter; and an aspect ratio
(length-to-diameter ratio) of at least 50.
12. The method of claim 11, wherein a succession of layers is
laminated to form a prepreg, wherein said succession of layers
comprises at least two layers selected from the group formed by: a
fibrous structure comprising long fibers; a fibrous structure
coated with a metallic matrix material; a metallic matrix material
layer; a mixture of metallic matrix material and chopped fibers; a
mixture of metallic matrix material and reinforcement
particles.
13. The method of claim 6, further comprising the step of providing
at least one graded layer within said first mold.
14. The method of claim 1, wherein said metallic matrix material is
selected from the group formed by: an aluminum alloy; a copper
alloy; an alloy comprising aluminum, magnesium and silicon as main
components; an alloy comprising copper and tin as main components;
an alloy comprising zinc as main component.
15. The method of claim 1, wherein said metallic matrix material is
selected from the group formed by: an alloy of the type
AlMg4.5Mn0.4; an alloy of the type AlMgSil, an alloy of the type
AlSi7Mg, an alloy of the type AlSi3, an alloy of the type AlSi12,
an alloy of the type CuZn40A12 and an alloy of the type
CuSn13.5A10.3.
16. The method of claim 1, wherein said metallic matrix material
comprises embedded particles, said embedded particles being
selected from the group formed by oxide ceramics, carbides,
nitrides, metals, alloys and tribologically active materials.
17. The method of claim 1, wherein said fibrous structure comprises
fibers selected from the group formed by: carbon; silicon carbide;
aluminum oxide; mullite; a modification of a carbon fiber with at
least one component selected from the group formed by nitrogen,
titanium, boron, carbon and silicon; a modification of a silicon
carbide fiber with at least one component selected from the group
formed by nitrogen, titanium, boron; a modification of an aluminum
oxide fiber with at least one component selected from the group
formed by nitrogen, titanium, boron, carbon and silicon; a
modification of a mullite fiber with at least one component
selected from the group formed by nitrogen, titanium, boron, carbon
and silicon.
18. The method of claim 1, wherein said fibrous structure comprises
fibers selected from the group formed by: a fiber coated on its
surface; a fiber provided with a diffusion barrier; a fiber
provided with a protective layer; a fiber provided with a primer
layer.
19. The method of claim 1, wherein said fibrous structure comprises
fibers having a diameter of between 5 and 20 .mu.m.
20. A method for producing a component from a composite material,
comprising the steps of: preparing a mixture from a metallic matrix
material and a reinforcement material; providing a fibrous
structure; applying said mixture by a coating process onto said
fibrous structure and forming a semi-finished product there from;
transferring said semi-finished product into a die; heating said
semi-finished product to a temperature above a solidus temperature
and below a liquidus temperature of the metallic matrix material;
thixoforming said semi-finished product to form a product; and
cooling said product down to room temperature.
Description
RELATED APPLICATIONS
[0001] This application is a continuation application of copending
International Patent Application PCT/EP2003/012352 filed on Nov. 5,
2003, published in German, which is fully incorporated by reference
herewith.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for producing a
component from a composite material (MMC) using a metallic matrix
material which is reinforced by fibers or particles.
[0003] The invention further relates to such a composite material
and to the use of such a composite material.
[0004] The need to economize on primary sources of energy and to
reduce emissions gives light-metal materials ever increasing
importance in the automotive industry and in aeronautical and space
technologies. The use of MMC materials (metallic matrix composites)
and/or embedded high-strength reinforcing fibers is capable of
increasing the resistance to high temperatures and the rigidity of
such materials, which is insufficient for many applications. In
addition, it is desired to reduce the production costs by short
production cycles and net shape forming of components.
[0005] According to the prior art, embedding fibrous reinforcing
components leads to a significant increase in production costs. In
addition, insufficient resistance of the types of fibers suited for
this purpose to the chemically aggressive melts prevents the
strength of the fibers to be optimally utilized in conventional
production processes from the liquid phase. Conventional production
routes for MMC materials, from the liquid phase, mostly consist in
squeeze infiltration processes where a capillary resistance is
overcome by squeezing the melt into the porous fiber preform using
a piston ("squeeze casting"). The squeeze casting process may be
carried out also with vacuum support ("vacural process"). For
realizing complex geometries and reducing the mechanical stress
acting on such fiber pre-forms during the infiltration process, the
preform may be dipped into the melt in an evacuated autoclave and
may then be exposed to a gas pressure ("gas pressure process"). In
addition, powder-metallurgical forming by sintering, followed by a
pressing operation, for example by hot pressing or hot isostatic
pressing (HIP), is of course also known. However, sintering methods
are very time-consuming and costly and do not allow net shape
forming of components of complex geometries. And in most of the
cases, considerable residual porosity is encountered in the case of
sintering methods, which may have negative effects on the
properties.
[0006] For producing fiber-reinforced aluminum-based light metals,
one normally makes use of molten-metal and liquid-phase
impregnation and pressure-casting techniques. The long reaction and
contact times of molten metal phases with the reinforcing fibers
and the molding tools or mold surfaces lead, however, to
undesirable damaging effects. For example, solution and separation
processes and chemical interface reactions are encountered that
lead to problems such as adhesion of the part to the mold as the
part is removed from the mold, structural transformation by heat
transmission and, generally, to an extremely complex process
sequence. This results in complex production systems with
comparatively long cycle times for the molding operation. All
processes make use of liquid, chemically highly active light-metal
melts (mostly aluminum) so that additional protection for the
fibers used, in the form of a suitable protective coat, is
inevitable. When aluminum is to be reinforced by carbon fibers, the
formation of an aluminum carbide layer (Al.sub.3C.sub.4) at the
fiber-matrix interface is especially undesirable as given its
marked proneness to brittle fracture and its insufficient corrosion
resistance such carbide phase will lead to premature failure of the
bond. Coating of the fibers can generally be dispensed with if the
light-metal matrix is united with the reinforcing component in the
solid phase ("diffusion bonding"). This transforming process is
used to process laminates of a fibrous fabric and metal sheets by a
hot-pressing process. The economic efficiency of that process is,
however, prejudiced by extremely high cycle times. A summary of the
commonly used processes for producing fiber-reinforced aluminum
composite materials is provided in Talat Lecture 1402, Froyne, L.,
Verlinden, B., University of Leuven (Belgium), 1994, EAA European
Aluminium Association.
SUMMARY OF THE INVENTION
[0007] It is a first object of the present invention to disclose a
method of producing product from a metallic matrix composite
material (MMC) which avoids the drawbacks that arise in the prior
art.
[0008] It is a second object of the invention to disclose a method
of producing a near-net-shaped reinforced product from a metallic
matrix composite material which limits damage of any reinforcing
material caused by melting of the metallic matrix material.
[0009] It is a third object of the invention to disclose a method
of producing a product from a metallic matrix composite material
that is cost-effective and energy-saving.
[0010] It is a forth object of the invention to disclose a method
of producing a MMC product by thixoforming.
[0011] According to the invention these and other objects are
achieved by a method for producing a component from a MMC using a
metallic matrix material which is reinforced by embedded fibers or
particles, comprising the steps of
[0012] producing a semi-finished product containing the fibers or
particles and the metallic matrix material; and
[0013] thixoforming the semi-finished product in a mold at a
temperature above solidus temperature and below liquidus
temperature of the metallic matrix material.
[0014] Thixoforming has been generally known in the art for
transforming special alloys, especially aluminum-based or
copper-based alloys, where use is made of a two-phase or a
multi-phase range, in which a liquid-phase content exists, in an
especially selected temperature range above the solidus line and
below the liquidus line. For purposes of that hot transformation in
a partially solidified condition, a marked solidification interval
is required for the alloy to be transformed, i.e. a temperature
range in which both solid-phase and liquid-phase contents are
present one beside the other (compare FIG. 1). In that temperature
range, near net shape transformation can be reached by application
of pressure in a mold (thixotropy-shear rate destabilizing
behavior). Ideally, an alloy suited for thixoforming comprises a
globolithic structure, which forms a solid-phase framework that
ensures good maneuverability of the heated semi-finished product as
it is placed in a pressing mold. That solid-phase framework breaks
up the shear load, thereby reducing the yield stress so that the
flow distances in the mold are only small and near net shape
transformation can be reached.
[0015] By utilizing the thixoforming process, which is known as
such for light-metal alloys, for the production of composite
materials that are reinforced by embedded fibers or particles,
clearly improved properties can be achieved.
[0016] A first advantage consists in the substantially lower
temperature and the resulting reduced forming time, compared with
melting processes or melt infiltration processes. It is possible in
this way to do without coating the embedded fibers or particles, as
the embedded fibers will be hardly damaged by the short melting
process at a relatively low temperature.
[0017] Compared with a squeeze-casting process, clearly shorter
infiltration paths of the melt can be achieved by a structure of
alternating layers in the semi-finished products. As a result of
the shorter process times and the relatively short contact between
the required tool and the melt clearly improved mold times are
achieved, which leads to considerable cost savings compared, for
example, with the squeeze casting process. It is now possible to
achieve rapid process times in the millisecond range, compared with
several seconds in the squeeze casting process and several hours in
the diffusion bonding process. By carrying out the forming process
in a temperature range lower than that used for the squeeze casting
process (lower by approximately 100 K) energy savings in a
corresponding order are achieved. The reduced liquid phase content
during the forming process leads to shorter solidifying times in
the mold and, thus, to reduced cycle times. Due to suitable
preconditioning of the metallic matrix the entire melting system
(melting furnace, alloying furnace, degassing, alloy control,
casting furnace) can be omitted. And due to the modified mold
concept there is a clearly reduced amount of material to be
recycled, which must be molten again before it can be used once
more in the production process.
[0018] As no air inclusions are encountered in thixoforming,
especially in thixoforging, it is further possible to improve the
strength of the metallic matrix by a suitable heat-treatment,
especially by solution annealing and settling.
[0019] Further, thixoforging processes offer the possibility to
join the component to other components by a material bond.
[0020] In the context of the present invention the term
"semi-finished product" is understood to describe the preform which
is then formed into the final component by thixoforming in a
mold.
[0021] A semi-finished product may be a prepreg consisting of a
single or, preferably, a plurality of laminate layers, or a perform
produced from a granulated product by a pressing process. The term
semi-finished product when used in the context of the present
invention is therefore always used as a generic term, including
both a prepreg and other preforms prepared in other ways, that are
to be formed into a component by thixoforming in a mold.
[0022] According to a first variant of the process a prepreg is
produced by laminating layers of fibrous structures and metallic
matrix materials in the form of metal sheets.
[0023] This especially simple variant of the process consists in
joining layers of fibrous structures with the suitable sheets by,
preferably, alternating lamination in order to obtain the desired
form of the semi-finished product. Due to the thixoforming process,
providing the matrix material in the form of metal sheets will be
sufficient to guarantee short flow distances and yet uniform
wetting of the embedded fibers or particles.
[0024] According to an advantageous further development of that
variant of the process, the metal sheets are provided in the form
of cold-rolled sheets. During the heating process, the dislocation
density induced by the cold-rolling process leads to fine-grained
recrystallization with a globular structure, which has a favorable
effect on the resulting properties of the component.
[0025] According to another variant of the process according to the
invention, a fibrous structure is coated with a metallic matrix
material.
[0026] Such a process sequence permits improved homogeneity to be
achieved, compared with the use of individual layers of fibrous
structures and matrix materials.
[0027] According to a convenient further improvement of that
embodiment of the invention, a plurality of coated fibrous
structures are laminated to form a prepreg.
[0028] This permits a near-net-shape geometry to be approximated,
and selectively determined property characteristics to be
achieved.
[0029] According to a first variant of the process coating the
fibrous structure with a metallic matrix material may be effected
by screen printing.
[0030] Another variant of the process for coating a fibrous
structure with the metallic matrix material consists in applying
the coating by electrostatic charging.
[0031] Another variant of the process for coating a fibrous
structure with a metallic matrix material consists in
electrophoretic deposition (EPD) of the material from an aqueous
suspension, supported by an electric field.
[0032] In this case, the fibrous structure to be coated is arranged
as an electrically conductive electrode, and the charge-carrying
metal powder particles are deposited on the fiber and/or fabric
surface as a uniform layer under the effect of the electric field.
Such a process sequence is suited especially for fibrous structures
that are electrically conductive by nature, such as C fibers,
though other fibers, which are not electrically conductive by
nature, may also be processed if suitable intermediate layers are
used.
[0033] Such a process preferably uses metal particles having a
grain size of between 10 nm and 100 82 m in diameter, preferably a
grain size of between 100 nm and 10 .mu.m. The use of liquid or
dissolved surfactants permits electric charge distributions of the
solid particles in the suspension to be adjusted to ensure
concentration and field-strength controlled substance transport for
the deposition of layers. It is possible in this way to vary the
properties of the component, that is to be produced from that
material later, within very broad limits and to adapt them to the
requirements of the particular case.
[0034] According to another variant of the invention, a fibrous
structure is coated with a metallic matrix material by thermal
spray-coating. Processes that can be generally envisaged in this
connection are atmospheric plasma spraying (APS), arc wire
spraying, wire flame spraying or high-speed flame spraying.
Preferably, thermal spraying is effected by electric arc wire
spraying or powder plasma spraying, especially by atmospheric
plasma spraying.
[0035] Compared with the use of alternating layers of sheet metal
and fibrous structures, fibrous structures provided with thermally
sprayed metal layers offer the advantage of a finer-grained
structure. While the grain sizes of thixotropically transformable
Al--Si sheets, for example, are in the range of 2 to 20 .mu.m, the
dimensions of the different phases in thermally sprayed AlSi layer
structures are in the sub-micrometer range, due to the high
cooling-down speed during application of the layer. Thixotropic
transformation therefore permits improved impregnation of the
metallic phase into the fibrous framework.
[0036] The proportions by volume of matrix material and fibers is
preferably adjusted in the thermal spraying process to between 0.3
and 8.0, especially to between 0.8 and 3.0.
[0037] All process variants of the invention guarantee that when
producing the semi-finished products from layers the metallic
matrix and the embedded reinforcing layers are heated up to a low
temperature only. Even when applying the coat by thermal spraying,
it can be ensured by suitable process control that the
semi-finished products will be heated up to a temperature of
maximally 300.degree. Celsius for a few seconds only. That period
of time may even be reduced to a maximum of five seconds or to a
maximum of 2 seconds or even to a shorter period of time. These
features guarantee, according to the invention, that the fibrous
structure will not be degraded or chemically attacked during
production of the semi-finished product.
[0038] Preferably, the fibrous structure is cooled during thermal
spraying, especially using liquid carbon dioxide.
[0039] By applying suitable cooling measures, it is possible to
limit the heating-up temperatures of the fibrous structure during
the coating process to temperatures clearly below 300.degree.
Celsius, preferably to temperatures in the range of 100 to
200.degree. Celsius, and this even for short times.
[0040] According to a preferred further development of the
invention, the fibrous structure is kept under tensile stress on a
carrier device during the thermal spraying process.
[0041] It is thus possible to compensate any differences resulting
from the considerably greater expansion of the metallic matrix
material compared with the embedded fibers. It is especially
possible in this way to produce fibrous structures where embedded
long fibers are subjected to tensile stress when loaded, while the
matrix as such is not stressed before in an undesirable way.
[0042] For carrying out a coating operation at an industrial scale,
the carrier device may allow the fibrous structure to be conveyed
to a coating plane for thermal spray coating either continuously or
intermittently.
[0043] The fibrous structure may be delivered for this purpose via
a winding system.
[0044] If such a winding system is used, the fibrous structure may
first be coated on one surface and then be coated on the opposite
surface.
[0045] According to a preferred further development of the
invention, a spray distance of 50 to 200 mm is maintained between
the surface of the fibrous structure and the orifice in plasma
spray coating, while a spray distance of 80 to 300 mm is maintained
in electric arc spraying.
[0046] Using such process parameters, coating of fibrous structures
can be carried out in an especially favorable way so that on the
one hand uniform coating is achieved and on the other hand
aggressive thermal stresses acting on the fibrous structures are
avoided.
[0047] According to another variant of the process according to the
invention, a mixture of matrix material and chopped fibers is
granulated or pelletized.
[0048] Preferably, fibers having a length of between 0.5 and 20 mm,
preferably between 2 and 6 mm, are used.
[0049] The proportions by volume of matrix material and fibers are,
preferably, between 0.3 and 5, preferably between 1 and 2.
[0050] According to a further variant of the process, a mixture of
matrix material and pulverized particles is granulated or
pelletized.
[0051] The production of granulated products or pellets using
chopped fibers or pulverized particles can be utilized with
advantage for producing special graded layers or for producing
bearing materials, for example.
[0052] According to a first variant of the process, the granulated
or pelletized mixtures can be transformed to the semi-finished
product by cold pressing. If the matrix material used is
sufficiently ductile, the cold pressing process can be carried out
without the addition of binders. If, however, the material lacks in
sufficient ductility, it will be convenient to add suitable
pressing aids, such as paraffin.
[0053] According to another variant of the process, the mixture
obtained by the granulating or pelletizing process is applied on a
fibrous structure by a suitable coating process. This again may
consist of thermal spraying, a screen-printing process or any other
of the before-mentioned processes.
[0054] In order to allow the strength of the components to be
produced according to the invention to be improved in a selective
way it is preferred to use at least one layer with a long-fiber
composite structure in the production of a prepreg. The term long
fibers as used in this context describes fibers having a length of
at least 1 mm or an aspect ratio (length-to-diameter ratio) of the
fiber of at least 50, preferably at least 100, more preferably of
at least 150.
[0055] It is understood that for laminating prepregs the succession
of layers may be varied in a suitable way to selectively influence
the properties of the component to be produced. For example,
fibrous structures consisting of long fibers, that are coated with
a matrix material, can be laminated together in a suitable way. At
the same time, layers of a matrix material in the form of metal
sheets or films can be inserted. And there is also the possibility
to insert intermediate layers of granulated products of pellets.
Finally, a combination of coated fibrous structures with preforms,
produced from pelletized or granulated mixtures of matrix material
and chopped fibers or particles, is likewise imaginable.
[0056] According to another advantageous further development of the
invention, a prepreg, produced by laminating, is provided with an
outer layer of a matrix material.
[0057] It can be ensured in this way that the surface of the
component so produced is substantially free from embedded fibers or
particles.
[0058] The fibrous structures may be used in the most different
forms in order to guarantee specific properties of the component to
be produced. For example, fibrous structures may be used in the
form of oriented fiber arrangements consisting of unidirectionally
oriented long fibers (UD), or unwoven or woven fibrous structures
may be used in the form of 2D fiber composites, 3D fiber
composites, in the form of textured or knitted fabrics.
[0059] The semi-finished product may be produced for this purpose
from graded layers in order to selectively influence the properties
at heavily stressed locations of the component or in preferential
directions of stress. It is possible in this way to vary the
proportion of matrix materials and fibers selectively over the
cross-section of the component.
[0060] This feature is particularly advantageous for producing
components, that are subjected to especially high thermal and/or
mechanical local stresses, such as pistons or the like.
[0061] Generally, all materials that allow thixotropic reforming
are suited as matrix materials.
[0062] Preferably, the materials used for this purpose are aluminum
alloys or copper alloys, especially alloys composed of aluminum,
magnesium or copper as main components, or composed of copper and
tin or zinc as main components.
[0063] The matrix materials preferred in this connection are alloys
of the type AlMg4.5Mn0.4 (AA 5182), of the type AlMgSil (EN
AW-6082), of the type Alsi7Mg (EN AW-356, EN-AW-357), of the type
AlSi3 (AA 208, AA 296), of the type AlSi12 (AA 336, AA 384), of the
type CuZn40A12 or of the type CuSn13.5A10.3.
[0064] When copper alloys are used, the wear-reducing effect of
reinforcing fibers will be the main aspect. Such materials are
especially well suited for the production of advantageous bearing
materials, the use of a combination with carbon fibers or
particles, modified to approximate graphite, being particularly
well suited for this purpose because anti-seizure performance can
be achieved in this way.
[0065] According to another variant of the process, the metallic
matrix material as such is reinforced by embedded particles
consisting preferably of oxide ceramics, carbides, nitrides, metals
or alloys or tribologically effective substances.
[0066] For realizing the fiber reinforcement of the MMC, the
invention uses fibers consisting of carbon, silicon carbide,
aluminum oxide, mullite. And modifications of those fibers with
hydrogen, titanium, boron, carbon or silicon, and compounds
thereof, are likewise imaginable.
[0067] Although due to the shorter process times and the reduced
process temperatures coating of the fibers used generally will not
be required, the invention also provides the possibility, according
to another variant of the process, to use fibers with coated
surfaces, especially fibers provided with diffusion barrier or
protective layers, or fibers with primer layers.
[0068] It is thus possible to adapt the properties of the component
to be produced much more effectively to the particular
requirements. The use of primers, for example, may improve
interface adhesion between the embedded fibers and the metallic
matrix phase. At the same time, it is possible to embed fibers
which as such are less compatible with the matrix phase
employed.
[0069] Especially well suited for coating of the fibers are silicon
carbide, silicon nitride, titanium carbide, titanium nitride,
carbon or mixed phases or compounds thereof.
[0070] According to the invention the fibers used preferably have
diameters of between 0.5 and 150 .mu.m, more preferably between 5
and 20 .mu.m.
[0071] As has been mentioned before, the fibers may be used both in
the form of long fibers or endless fibers, and in the form of
chopped fibers.
[0072] It has been mentioned before that during the thixoforming
process, the semi-finished product is heated up to a given
temperature interval, depending on the particular matrix material
used, in which the matrix material comprises a defined liquid phase
content.
[0073] When using the alloy AlSi7Mg, for example, the semi-finished
product is heated up to a temperature of between 574 and
584.degree. Celsius for thixoforming, in which case a liquid phase
content of between 43 and 51 percent by volume will be reached.
When using the alloy AlMgSil, for example, the semi-finished
product is heated up to a temperature of between 635 and
645.degree. Celsius for thixoforming, in which case a liquid phase
content of between approximately 15 and approximately 35 percent by
volume will be reached. When using the alloy CuZn40A12, for
example, the semi-finished product is heated up to a temperature of
between 871 and 875.degree. Celsius for thixoforming, in which case
a liquid phase content of between approximately 20 and
approximately 40 percent by volume will be reached.
[0074] Thixoforming is preferably carried out as thixoforging in a
suitable die at controlled ram velocity and pressing force. The ram
velocity and pressing force are adjusted in this case according to
the particular requirements of the process. Ram velocities of up to
800 mm/s are possible. The velocity of impact of the upper die on
the workpiece is preferably adjusted to a rate of between 10 mm/s
and 300 mm/s as a function of the fiber-matrix ratio used, the
complexity of the component and the volume of the component.
[0075] In order to prevent premature solidification of the metallic
material, the tool is preferably heated up to temperatures of
between 100.degree. Celsius and 400.degree. Celsius.
[0076] Especially by increasing the ram velocity after contact,
rapid infiltration can be achieved at high pressure in a time range
clearly below 1 second. One thus obtains a short contact time
between the metallic melt and the fibrous structure, which results
in the fibers being subjected to reduced chemical attack by the
melt and, thus, in improved properties of the component so
produced.
[0077] Further, it is possible, for reducing the formation of
oxides, to carry out the thixoforming process in a protective-gas
atmosphere.
[0078] According to a first variant of the process, the
semi-finished product is pre-compacted in a mold for the
thixoforming process. The mold used for this purpose is preferably
the same that will be used later for thixoforming.
[0079] The pre-compacted semi-finished product may now be heated up
outside the mold, for example inductively, in a circulating-air
furnace, in a protective-gas atmosphere, by infrared radiators or
with the aid of lasers, and is then placed in the mold in preheated
condition for being thixotropically transformed, especially by
thixoforging.
[0080] The preferred matrix materials permit the semi-finished
product to be heated up to the temperature necessary for
thixotropic transforming, while still ensuring sufficient stability
of the semi-finished product to allow handling of the semi-finished
product when placing it in the mold, which may be effected
automatically, for example. The shear strength of the semi-finished
product gets lost only by the pressure applied by the ram during
the thixoforging process so that the material can be removed from
the mold in a minimum of time.
[0081] According to one variant of the process, the semi-finished
products are heated up in the mold to a temperature above the
solidus line, but below the liquidus line of the matrix material.
The layered material can be urged into contact with the mold wall
by slight pressure in this case. This will improve heat
transmission and, thus, reduce the heating-up time. Thixotropic
transforming is then carried out immediately thereafter, preferably
by forging.
[0082] With both variants of the process, the thixotropically
transformed component is preferably cooled down in a controlled
manner in the mold in order to achieve oriented solidification of
the metallic matrix.
[0083] Composite materials produced in this way can be used
according to the invention preferably as net-shaped highly
resistant construction components having high rigidity per unit or
a high specific modulus of elasticity. In addition, such materials
can be used with advantage as bearing materials.
[0084] It is understood that the features of the invention
mentioned above and those yet to be explained below can be used not
only in the respective combinations indicated, but also in other
combinations or in isolation, without leaving the scope of the
present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0085] Further features and advantages of the invention will become
apparent from certain preferred embodiments of the invention which
will be described hereafter with reference to the drawings in
which:
[0086] FIG. 1 shows an Al-Si phase diagram with selected ranges for
thixotropic transformation of the wrought alloy AlMgSi1 and the
casting alloy AlSi7Mg;
[0087] FIG. 2 shows a prepreg consisting of a layered composite
material composed of alternating layers of fibers and metal sheets
(films);
[0088] FIG. 3 shows a prepreg consisting of a sequence of fibrous
structures coated with a metallic matrix material;
[0089] FIG. 4 shows a globolithic structure of an Al--Si alloy;
[0090] FIG. 5 shows a cross-section of an Al--Si composite
material, infiltrated with a metallic matrix, with embedded C
fibers after a thixoforging process; and
[0091] FIG. 6 shows a diagrammatic representation of a winding
system for thermal spray coating of fiber fabrics at an industrial
scale.
PREFERRED EMBODIMENTS OF THE INVENTION
[0092] FIG. 1 shows a phase diagram of the preferred eutectic
system Al--Si (with admixtures of magnesium). The solid content in
the dark shaded area is .alpha. (Al). Wrought alloy Al--SiMi1 and
casting alloy AlSi7Mg are shown as alloys suited for thixotropic
transformation. The thixotropic transforming process requires a
narrow temperature range, which is defined, for the wrought alloy
AlMgSil, by the solidus line and the liquidus line and which is
represented in FIG. 1 by the rectangle marking the temperature
range between 635 and 645.degree. Celsius. The liquid content (L)
obtained in this range is between 15 and 35%. In contrast, the
temperature range required for thixotropically transforming the
casting alloy AlSi7Mg is indicated immediately above the eutectic
line and is again represented by a rectangle. That temperature
window ranges from 574.degree. Celsius to 584.degree. Celsius, and
the liquidity content (L) obtained in this window is between
approximately 43 and approximately 51%.
[0093] Such interrelations are generally known in connection with
the thixotropic transformation of light-metal alloys (compare
Siegert, K.; Messmer, G.; Baur, J.; Wolf, A.: "Thixoschmieden von
Aluminiumbauteilen" (Thixoforging of aluminium components), in:
Tagungsband zur 7. Sachsischen Fachtagung Umformtechnik, Chemnitz,
2000: Baur, J., Messmer, G.: "Automated Thixoforging Unit, in:
Proceedings to the 7. Int. Conf. on Semi-Solid Processing of Alloys
and Composites, Tsu-kuba, Japan, Sep. 24-28, 2002).
[0094] The thixoforming process for such light-metal materials was
developed especially for the production of net-shaped components.
In the temperature range in which thixoforming occurs the
respective semi-finished products still have sufficient strength to
allow handling of the semi-finished products (also known as "studs"
in thixoforging technology). Ideally, the respective alloys have a
globolithic structure that forms a solid-phase framework thereby
guaranteeing good handling properties of the heated semi-finished
products as the latter are being placed in the pressing die
(compare FIG. 4). The solid-phase framework breaks up under shear
load, thereby reducing the yield stress, so that high flow
distances in the mold and near net shape transformation are
guaranteed.
[0095] According to the invention, one now produces MMCs-with a
metallic matrix material, reinforced by embedded fibers or
particles, by preparing a semi-finished product that contains the
fibers or particles in the metallic matrix material, and forming
the semi-finished product by a thixoforming process in a mold at a
temperature above the solidus temperature and below the liquidus
temperature of the metallic matrix material.
[0096] For the above purpose, a loose bond between the fibers on
the one hand and the metallic matrix material on the other hand
will be sufficient to produce a high-quality composite material,
which is practically free from pores, by a thixoforming process in
a short transforming time.
[0097] Basically, three process lines are possible to obtain the
desired product.
[0098] According to a first process line, which is illustrated
diagrammatically in FIG. 2, a semi-finished product indicated
generally by reference numeral 10 is produced by lamination from
alternating layers of sheet metal and fibrous fabric. A fibrous
structure 12 consists, for example, of carbon fibers arranged in
the form of a fabric. For producing the semi-finished product 10,
fibrous composite layers 12 alternating with thin metal sheets 14
of the matrix material, for example of AlSi7Mg, are laminated. The
thickness d, of the fibrous structure 12 and d.sub.2 of the metal
sheets 13 is adjusted depending on the desired proportions by
volume of the fibers and the matrix material. Conveniently, the
semi-finished product 10 is enclosed by an outer layer 18 of the
matrix material to avoid that any fibers will get to the surface of
the component during the subsequent thixoforming process. A
semi-finished product or prepreg of that type is then placed in a
suitable die and is transformed in the latter by thixoforming in a
suitable temperature range using a ram.
[0099] During that process an initial pre-compacting step may be
carried out in the die, in cold condition of the semi-finished
product 10, followed by the step of heating up the semi-finished
product to the temperature required for thixoforming. This can be
achieved by heating up the pre-compacted semi-finished product
rapidly, either inductively or alternatively in a recirculating-air
furnace in a protective-gas atmosphere, or by high-power infrared
radiators or by laser. That step is then followed by rapid transfer
to the conveniently preheated die (100 to 400.degree. Celsius).
This operation may be carried out manually, semi- or
fully-automatically. Thereafter, the thixoforging operation is
carried out by the ram striking the surface of the semi-finished
product at a ram velocity of up to 800 mm/s, adapted to the
specific requirements of the process. The impact velocity of the
ram is adjusted, preferably, to between 10 mm/s and 300 mm/s,
depending on the fiber-to-matrix ratio, the complexity of the
component and the volume of the component. In order to prevent
premature solidification of the metallic matrix material the ram
conveniently is also preheated in a suitable way.
[0100] Alternatively, heating-up of the semi-finished product may
be effected also in the die. For this purpose, the layered
semi-finished product may be urged into contact with the wall of
the die at slight pressure whereby heat transmission can be
improved and the heating-up time can be reduced. Immediately
thereafter, the thixotropic transforming operation is then effected
by thixoforging.
[0101] If metal sheets (films) are used, cold-rolled sheets are
preferred as the high relocation density will later, as the
material is re-heated for thixoforming, lead to fine-grained
recrystallization with globular structure.
[0102] A second variant of the process for producing a
semi-finished product consists in coating individual fibrous
structures which are then arranged one on top of the other to
laminate a prepreg, and are again conveniently enclosed by an outer
metal sheet or film layer of matrix material. This variant of the
process will be described hereafter in more detail with reference
to FIGS. 3 and 6.
[0103] A third alternative of producing the semi-finished product
consists in preparing a mixture of chopped fibers or pulverized
reinforcing particles and metallic matrix powder. The mixture is
then formed by dry pressing, or is further processed by applying it
on a fibrous structure using a coating process.
[0104] In the case of the second variant of the process,
application by electrostatic charging, application by screen
printing processes or electrophoretic deposition (EPD) on the
fibrous structure are generally well suited for coating the fibrous
structure.
[0105] In the case of the EPD process, an electrically conductive
layer, or one that has been made electrically conductive, is
arranged as an electrode, and the charge-carrying metal powder
particles are deposited on the fiber and/or fabric surface as a
uniform layer under the effect of the electric field. The metal
particles in this case have a grain size of between 10 nm and 100
.mu.m in diameter, preferably a grain size of between 100 nm and 10
.mu.m. The use of liquid or dissolved surfactants permits the
electric charge distribution of the solid particles in the
suspension to be adjusted to ensure concentration and
field-strength controlled substance transport for the deposition of
layers.
[0106] An especially preferred coating method is the thermal
spray-coating method. Among these processes, the electric arc wire
spraying and powder plasma spraying processes, preferably the
atmospheric plasma spraying (APS) processes, are of main
importance.
[0107] During thermal spray coating of the fibers, a rise in
temperature of the fibrous structure is largely prevented by
selective cooling measures so that as a rule temperatures of
maximally 100.degree. Celsius can be maintained, and any damage to
the fibers can be excluded. Cooling can be effected locally by the
simultaneous use of coolant injectors and other suitable devices,
preferably using air and, if necessary, liquid carbon dioxide
(CO.sub.2).
[0108] Moreover, it is preferred in the case of that variant of the
process to apply long or endless fibers, or fibrous structures
produced from there from (oriented arrangements, fabrics or knitted
structures) on a suitable carrier device 30, in oriented or
prestressed fashion (compare FIG. 6), or to apply one or more
layers of the matrix metal by thermal spraying. Using a winding
system 30--illustrated in FIG. 6--the fibrous structure can be
wound off continuously or intermittently from a roll 34 containing
the fibrous structure layers to be coated, and can then be coated
in a coating plane (curved surface on the left side of FIG. 6) and
be wound up again on a roll 32 following the coating process. If
suitable dimensions and coating thicknesses are selected, a first
coating may be initially applied on a first upper surface,
hereafter the opposite surface may be coated as well.
[0109] A particular advantage resides in the option to selectively
prestress the fabrics in the coating plane, which prestress
additionally may be controllable to guarantee uniform permanent
prestressing. This then mechanically compensates for thermal and
convective stresses in the microstructure. Coating may be effected
using a 5-axes-robot-aided motion system adapted to move an arc or
plasma burner so that NC-controlled motion sequences can be
retrieved from previously stored programs and can be realized in a
reproducible, process-stable way for the purpose of achieving
uniform coating results. The spray distances between orifice and
fiber surface are 50 to 200 mm, preferably between 100 and 140 mm
for APS, and between 120 and 160 mm for electric arc spraying
applications.
[0110] As illustrated in FIG. 3, such layers 22 of fibrous
structures, coated with a matrix metal 24 on both sides, are
laminated to form a prepreg and are again preferably enclosed on
their outsides by a metal sheet or a film 18 of matrix metal to
produce a semi-finished product 20. The coating thickness d.sub.3
of the preceding coating process can be suitably controlled. This
can be done by applying one or more layers, or by influencing the
layer thickness by varying the actuating speed and/or the dwelling
time or the remaining spraying parameters. If larger layers of
matrix metal are desired, individual metal sheets and/or films may
be inserted additionally.
[0111] In the case of the third variant of the process, using
granulating and/or pelletizing operations, mixtures of matrix metal
powders and chopped fibers or reinforcing particle powders are
produced. Such granulated or pelletized mixtures can then by
compacted by cold pressing to form a green compact which is then
used as a semi-finished product. If the matrix metal has sufficient
ductility, cold pressing may be effected without any pressing aids.
If the ductility of the metal matrix material is low, suitable
binder aids, such as paraffin, may be used which will evaporate
readily during the subsequent heating-up process.
[0112] Another variant of the process consists in applying the
mixtures, formed by granulating or pelletizing, on fibrous
structures using a suitable coating process, for example a thermal
spraying process.
[0113] As has been mentioned before, special component properties,
adapted to particular local thermal or mechanical stresses, can be
obtained by the use of graded layers, for which purpose the entire
spectrum available in processing fibrous composite materials may be
utilized.
[0114] A particularly favorable distribution obtained in such a
composite material by an infiltration process resulting from a
thixoforging operation can be derived from FIG. 5.
* * * * *