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Introduction
Metal composite materials have found application in many areas of daily life for
quite some time. Often it is not realized that the application makes use of composite
materials. These materials are produced in situ from the conventional production
and processing of metals. Here, the Dalmatian sword with its meander structure,
which results from welding two types of steel by repeated forging, can be mentioned.
Materials like cast iron with graphite or steel with a high carbide content, as
well as tungsten carbides, consisting of carbides and metallic binders, also belong to
this group of composite materials. For many researchers the term metal matrix
composites is often equated with the term light metal matrix composites (MMCs).
Substantial progress in the development of light metal matrix composites has been
achieved in recent decades, so that they could be introduced into the most important
applications. In traffic engineering, especially in the automotive industry, MMCs
have been used commercially in fiber reinforced pistons and aluminum crank cases
with strengthened cylinder surfaces as well as particle-strengthened brake disks.
These innovative materials open up unlimited possibilities for modern material
science and development; the characteristics of MMCs can be designed into the
material, custom-made, dependent on the application. From this potential, metal
matrix composites fulfill all the desired conceptions of the designer. This material
group becomes interesting for use as constructional and functional materials, if
the property profile of conventional materials either does not reach the increased
standards of specific demands, or is the solution of the problem. However, the
technology of MMCs is in competition with other modern material technologies,
for example powder metallurgy. The advantages of the composite materials are only
realized when there is a reasonable cost – performance relationship in the component
production. The use of a composite material is obligatory if a special property
profile can only be achieved by application of these materials.
The possibility of combining various material systems (metal – ceramic – nonmetal)
gives the opportunity for unlimited variation. The properties of these new materials are basically determined by the properties of their single components.
Figure 1.1 shows the allocation of the composite materials into groups of various
types of materials.
The reinforcement of metals can have many different objectives. The reinforcement
of light metals opens up the possibility of application of these materials in areas
where weight reduction has first priority. The precondition here is the improvement
of the component properties. The development objectives for light metal
composite materials are:
• Increase in yield strength and tensile strength at room temperature and above
while maintaining the minimum ductility or rather toughness,
• Increase in creep resistance at higher temperatures compared to that of conventional
alloys,
• Increase in fatigue strength, especially at higher temperatures,
• Improvement of thermal shock resistance,
• Improvement of corrosion resistance,
• Increase in Young’s modulus,
• Reduction of thermal elongation.
To summarize, an improvement in the weight specific properties can result, offering
the possibilities of extending the application area, substitution of common materials
and optimisation of component properties. With functional materials there
is another objective, the precondition of maintaining the appropriate function of
the material. Objectives are for example:
• Increase in strength of conducting materials while maintaining the high conductivity,
• Improvement in low temperature creep resistance (reactionless materials),
• Improvement of burnout behavior (switching contact),
• Improvement of wear behavior (sliding contact),
• Increase in operating time of spot welding electrodes by reduction of burn outs,
• Production of layer composite materials for electronic components,
• Production of ductile composite superconductors,
• Production of magnetic materials with special properties.
For other applications different development objectives are given, which differ
from those mentioned before. For example, in medical technology, mechanical
properties, like extreme corrosion resistance and low degradation as well as biocompatibility
are expected.
Although increasing development activities have led to system solutions using
metal composite materials, the use of especially innovative systems, particularly in
the area of light metals, has not been realised. The reason for this is insufficient process
stability and reliability, combined with production and processing problems
and inadequate economic efficiency. Application areas, like traffic engineering, are
very cost orientated and conservative and the industry is not willing to pay additional
costs for the use of such materials. For all these reasons metal matrix composites
are only at the beginning of the evolution curve of modern materials,
Reinforcements
Reinforcements for metal matrix composites have a manifold demand profile,
which is determined by production and processing and by the matrix system of the
composite material. The following demands are generally applicable [4]:
• low density,
• mechanical compatibility (a thermal expansion coefficient which is low but
adapted to the matrix),
• chemical compatibility,
• thermal stability,
• high Young’s modulus,
• high compression and tensile strength,
• good processability,
• economic efficiency.
These demands can be achieved only by using non-metal inorganic reinforcement
components. For metal reinforcement ceramic particles or, rather, fibers or carbon
fibers are often used. Due to the high density and the affinity to reaction with the
matrix alloy the use of metallic fiber usual fails. Which components are finally
used, depends on the selected matrix and on the demand profile of the intended
application. In Refs. [4, 5] information about available particles, short fibers, whiskers
and continuous fibers for the reinforcement of metals is given, including data
of manufacturing, processing and properties. Representative examples are shown
in Table 1.1. The production, processing and type of application of various reinforcements
depends on the production technique for the composite materials, see
Refs. [3, 7]. A combined application of various reinforcements is also possible (hybrid
technique) [3, 8].
Every reinforcement has a typical profile, which is significant for the effect within
the composite material and the resulting profile. Table 1.2 gives an overview of
possible property profiles of various material groups. Figure 1.5 shows the specific
strength and specific Young’s modulus of quasi-isotropic fiber composite materials
with various matrixes in comparison to monolithic metals. The group of discontinuous
reinforced metals offers the best conditions for reaching development
targets; the applied production technologies and reinforcement components, like
short fibers, particle and whiskers, are cost effective and the production of units in
large item numbers is possible. The relatively high isotropy of the properties in
comparison to the long-fiber continuous reinforced light metals and the possibility
Matrix Alloy Systems
The selection of suitable matrix alloys is mainly determined by the intended application
of the composite material. With the development of light metal composite
materials that are mostly easy to process, conventional light metal alloys are applied
as matrix materials. In the area of powder metallurgy special alloys can be applied
due to the advantage of fast solidification during the powder production.
Those systems are free from segregation problems that arise in conventional solidification.
Also the application of systems with oversaturated or metastable structures
is possible. Examples for matrix configurations are given in Refs. [7, 9–15]:
Production and Processing of Metal Matrix Composites
Metal matrix composite materials can be produced by many different techniques.
The focus of the selection of suitable process engineering is the desired kind,
quantity and distribution of the reinforcement components (particles and fibers),
the matrix alloy and the application. By altering the manufacturing method, the
processing and the finishing, as well as by the form of the reinforcement components
it is possible to obtain different characteristic profiles, although the same
composition and amounts of the components are involved. The production of a
suitable precursor material, the processing to a construction unit or a semi-finished
material (profile) and the finishing treatment must be separated. For cost effective
reasons prototypes, with dimensions close to the final product, and reforming
procedures are used, which can minimize the mechanical finishing of the construction
units.
In general the following product engineering types are possible:
• Melting metallurgical processes
– infiltration of short fiber-, particle- or hybrid preforms by squeeze casting, vacuum
infiltration or pressure infiltration [7, 13–15]
– reaction infiltration of fiber- or particle preforms [16, 17]
– processing of precursor material by stirring the particles in metallic melts, followed
by sand casting, permanent mold casting or high pressure die casting
[9, 10]
1.2 Combination of Materials for Light Metal Matrix Composites 7
• Powder metallurgical processes
– pressing and sintering and/or forging of powder mixtures and composite
powders
– extrusion or forging of metal-powder particle mixtures [11, 12]
– extrusion or forging of spraying compatible precursor materials [7, 18, 19]
• Hot isostatic pressing of powder mixtures and fiber clutches
• Further processing of precursor material from the melting metallurgy by thixocasting
or -forming, extrusion [20], forging, cold massive forming or super plastic
forming
• Joining and welding of semi-manufactured products
• Finishing by machining techniques [21]
• Combined deformation of metal wires (group superconductors).
Melting metallurgy for the production of MMCs is at present of greater technical
importance than powder metallurgy. It is more economical and has the advantage
of being able to use well proven casting processes for the production of MMCs.
Figure 1.6 shows schematically the possible methods of melting metallurgical production.
For melting metallurgical processing of composite materials three procedures
are mainly used [15]:
• compo-casting or melt stirring
• gas pressure infiltration
• squeeze casting or pressure casting.
Both the terms compo-casting and melt stirring are used for stirring particles into
a light alloy melt. Figure 1.7 shows the schematic operational sequence of this procedure.
The particles are often tend to form agglomerates, which can be only dissolved
by intense stirring. However, here gas access into the melt must be absolutely
avoided, since this could lead to unwanted porosities or reactions. Careful attention
must be paid to the dispersion of the reinforcement components, so that
the reactivity of the components used is coordinated with the temperature of the
melt and the duration of stirring, since reactions with the melt can lead to the dissolution
of the reinforcement components. Because of the lower surface to volume
ratio of spherical particles, reactivity is usually less critical with stirred particle reinforcement
than with fibers. The melt can be cast directly or processed with alternative
procedures such as squeeze casting or thixocasting. Melt stirring is used by
the Duralcan Company for the production of particle-strengthened aluminum alloys
[9, 10]. At the Lanxide Company a similar process is used, with additional reactions
between the reinforcement components and the molten matrix being purposefully
promoted to obtain a qualitatively high-grade composite material [16]. In
the reaction procedures of the Lanxide Company it may be desirable that the reinforcement
component reacts completely with the melt to form the component in
situ, which then transfers the actual reinforcement effect to the second phase in the
MMC.
In gas pressure infiltration the melt infiltrates the preform with a gas applied
from the outside. A gas that is inert with respect to the matrix is used. The melting
of the matrix and the infiltration take place in a suitable pressure vessel. There are
two procedure variants of gas pressure infiltration: in the first variant the warmed
up preform is dipped into the melt and then the gas pressure is applied to the surface
of the melt, leading to infiltration. The infiltration pressure can thereby be coordinated
with the wettability of the preforms, which depends, among other
things, on the volume percentage of the reinforcement. The second variant of the gas pressure infiltration procedure reverses the order: the molten bath is pressed
to the preform by the applied gas pressure using a standpipe and thereupon infiltrates
the bath (see Fig. 1.8). The advantage of this procedure is that there is no development
of pores when completely dense parts are present. Since the reaction
time is relatively short with these procedures, more reactive materials can be used
than e.g. with the compo-casting. In gas pressure infiltration the response times
are clearly longer than in squeeze casting, so that the materials must be carefully
selected and coordinated, in order to be able to produce the appropriate composite
material for the appropriate requirements.