Metal Matrix Composite

A metal matrix composite (MMC) is a composite material made by strengthening a metal with ceramics or organic compounds. In this particular case ‘6061 Al/SiC fibre UD’ (For convenience this will be abbreviated to Alsic) will be considered. In the following it will be discussed what Alsic is composed of and how it can be made.

The first term in the name is ‘6061 Al’. This refers to a particular alloy of Aluminum. According to the aluminum association: “The 6xxx series are versatile, heat treatable, highly formable, weldable and have moderately high strength coupled with excellent corrosion resistance. Alloys in this series contain silicon and magnesium in order to form magnesium silicide within the alloy. [...] Alloy 6061 is the most widely used alloy in this series and is often used in truck and marine frames.”

The other main constituent in Alsic is Silicon Carbide. In particular it contains SiC fibres (as opposed to, for instance, grains). SiC is widely used for applications requiring high durability (such as abrasives or plated bullet-proof vests) as it is a very hard ceramic. It is also used in semiconductors that require increased durability.

Production methods can be split into two main categories [1]: Casting and powder metallurgy.

Cast MMCs can be blended (in simple terms: throw all constituents together, heat the mixture until the metal melts, then blend to get a homogeneous mixture). The alternative is squeeze casting. The ceramic will be made into a mold of the desired shape, and the metal will be ‘poured in’ under high pressure. This method is very similar to filling an ice cube tray with water, except now under pressure and at hundreds of degrees. A blend-cast MMC is generally isotropic, whereas squeeze-cast MMCs can be made in any configuration, thus also with a ‘grain’.

Powder metallurgic MMCs are based on standard forging methods. Powders of the metal and ceramic are blended to disperse the ceramic through the matrix and then compressed to form solid structures. Spray forming powder metallurgy can be used for less random MMC configurations. In this case a substrate is continuously sprayed with the metal matrix powder, whilst the ceramic particles are added gradually. This method enables the introduction of different types of ceramic particles in the same MMC and allows one to influence the chemical reactions between matrix and ceramic to increase bonding strength.

Although the production of MMCs comes with a few intricacies, it should be possible to make a large spherical MMC shell. Furthermore, the methods through which MMCs are produced may enable the production of a seamless hollow sphere. This, however, requires further research which is outside the scope of this project.

Alsic was considered here because Carl Ross suggested it in his book on pressure vessels. However, by no means is it the only MMC. For instance, one may consider using 7xxx Aluminum instead of 6xxx Aluminum. The aluminum association states that the 7xxx series of alloys produces “very high strength” alloys.