What are the difficulties and roadblocks in developing an aluminum-based metal matrix composite via the casting route and how to overcome issues?
Dear Ziyauddin Seikh
Major roadblocks in producing the AMCs are achieving uniform dispersion of dispersoids throughout the Al matrix, inhibiting the formation of unwanted interfacial products at relatively high operating temperatures and most importantly accomplishing good interfacial bond strength. These factors also depend on processing methods used for blending/ mixing, density differences between matrix and reinforcements etc.
The major road block for MMC preparation via liquid metal route is wettability. Only when the liquid metal completely wets the reinforcement (fiber or particulates), the other issues (such as uniform distribution, orientation, interfacial reaction product etc.) become relevant. The wetting can be improved by decreasing the surface energy of the liquid metal and/or liquid metal/reinforcement interfacial energy, or increasing the surface energy of the dispersoid. Surface energy is a function of temperature. Coating of the reinforment helps with the wetting. If the dispersoids are in the particulate form with low density, moving solidification front can eject the particles out rather than engulfing them, thus causing distribution problem. There are several seminal papers available on this topic.
The problems mentioned already are the main reasons that aluminium MMC are not used very much. Like any composite you have to be clear about what it is that your are trying to achieve. Particulate ceramic will of course increase the elastic modulus and to various degrees, strength but usually at the cost of toughness and the other deficiencies of aluminium such as very poor wear resistance, little elevated temperature performance and very poor corrosion resistance remain. Powder metallurgy synthesis can offer better production route than casting and high stiffness SiC aluminium MMC is made successfully by high energy mixing and HIPing.