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Structure and Properties of Light Metal Composite Materials
The structure of the composite materials is determined by the type and form of the reinforcement components, whose distribution and orientation are affected by the manufacturing processes. For composite materials, which are reinforced with long fibers, extreme differences result with different fibers. For multi-filament-strengthened composite materials as Figure 1 the fiber/fiber contacts and non-reinforced areas are recognizable as a result of the infiltration of fiber bunch preforms. Structure defects, like fiber/fiber contacts, pores and non-reinforced areas are visible, which have a substantial influence on the composite characteristics.

Figure 1 Structure of a unidirectional endless fiber reinforced aluminum composite material (transverse grinding).
Figure 2 shows typical structure images of short-fiber reinforced light alloys. With short-fiber reinforced composite materials a planar-isotropic distribution of the short fibers develops, due to the fiber molded padding production. The pressure-supported sedimentation technology leads to a layered structure. The infiltration direction is generally perpendicular to these layers. A reinforcement of light metal cast alloys by short fibers does not lead exclusively to an increase in strength, e.g. at room temperature, as the objective. It leads to a strength increase with increasing fiber content.

Figure 2 structure of formation of short fiber reinforced light metal composite materials
The cast particle-reinforced light alloys show typical particle distributions depending on the processing methods. Gravity die cast materials show non-reinforced areas due to the solidification conditions (Figure 3.a); while with pressure die cast materials the distribution of the particles is more optimal (Figure 3.b).
Even better results are reached after the extrusion of feed material (Figure 3.c). In powder metallurgically manufactured composite materials (Figure 3.d) the extremely homogeneous distribution of the particles is noticeable after the extrusion of powder mixtures. The possibility of combining particles and fibers to form a hybrid- reinforced composite material with the different effects of both reinforcement components is shown in Fig. 4.

Figure 3 Arrangement of typical structures of different particle reinforced light metal composite materials: (a) SiC-particle reinforced Al (mold cast), (b) SiC-particle reinforced Al (die cast), (c) SiC-particle reinforced Al (extruded powder mixture), (d) SiC- particle reinforced Al (cast and extruded).

Fig. 4 Structure of formation of hybrid reinforced light metal composite materials with C short fibers and Mg2Si particles.
With particle addition to light metals like aluminum, the hardness, the Young’s modulus, the yield strength, the tensile strength and the wear resistance increase and the thermal expansion coefficient decreases. The order of magnitude of the improvement of these characteristics depends on the particle content and the selected manufacturing process.