Metal-matrix composites and how they are made

A metal holding something harder

A metal-matrix composite has a metal or alloy matrix reinforced with particles, whiskers, fibres or hollow microballoons, added to raise strength, thermal stability and wear resistance. Composites are classified by their matrix, and for aircraft the common ones are based on aluminium, magnesium and titanium.^[1]

Aluminium-matrix composites typically use Al–Si, Al–Cu or Al–Si–Mg alloys reinforced with ceramics such as silicon carbide and alumina. Particle reinforcement gives more isotropic behaviour; fibres give the largest gains in strength and stiffness.^[1,2]

Three states of the matrix

Routes to metal-matrix composites are grouped by the state of the metal while the composite forms: liquid, as in stir casting and infiltration; solid, chiefly powder metallurgy; or vapour.^[1]

The liquid and powder routes share their difficulties. Ceramic particles may not be wetted by the molten metal, may sink or float because their density differs from the melt’s, and may react to form unwanted intermetallic phases.^[1]

Stir casting

In stir casting the reinforcement is stirred into molten metal through a vortex — the route is also called vortex or slurry casting — and the mixture is then cast. It is described as the most widely used and least expensive route to metal-matrix composites, and suits low-melting alloys such as aluminium and magnesium.^[1,2]

Every particle added thickens the melt and makes it harder to mix, and the particles can segregate as the casting solidifies. Alloying additions and preheating the reinforcement and mould are used to improve wetting.^[1]

Powder metallurgy

In the powder route the reinforcement is blended with metal powder, pressed and sintered, and often forged or extruded afterwards. Because the metal never melts, there is little reaction between matrix and reinforcement and the microstructure can be controlled closely; the reinforcement is usually discontinuous.^[1,2]

Its own difficulties are getting the powders to distribute evenly, avoiding clumps of reinforcement, bonding particle surfaces and removing porosity.^[1]

Infiltration and squeeze casting

Infiltration starts from a porous preform of the reinforcement and fills it with molten metal, which becomes the matrix as it solidifies. Squeeze casting is the pressure-driven version; it has been used, for example, for carbon-fibre-reinforced aluminium.^[1,2]

It makes near-net-shape parts with little reaction at the interface, but can leave porosity and reinforcement-rich patches. Temperatures depend on the system: one cited study infiltrated silicon carbide preforms with aluminium alloy, without pressure, at 790–810 °C under nitrogen.^[1,2]

Commonly misunderstood