What do you do when you need to design energy or weight saving part for an aerospace, marine, automotive or robotics system and the perfect material is not available? Desirable properties not normally found in an off the shelf material. Properties for example low density, high strength, high modulus and low coefficient of thermal expansion (CTE). One answer is to select several materials that have the desired properties and mix them together to produce an engineered material with the combination of desired properties. This is primarily how composites are designed, including metal matrix composites (MMCs). One lightweight MMC system specifically aluminum-SiC MMCs is described in this article which combine a low density moderate strength ductile aluminum matrix with a low CTE, high strength silicon carbide reinforcement.
MMC history
Metal matrix composites using copper or aluminum as the metallic matrix were investigated in the 1950s and 1960s as a method to reduce weight while maintaining high mechanical strength. Boron carbide, aluminum oxide and silicon carbide were common ceramic reinforcement constituents. There have been waves of development and limited production over the following decades affected primarily by the economic environment. One consistent barrier was the relatively high cost to produce the materials or parts from the MMCs.
Several manufacturing methods to produce Al-MMCs were developed in attempt to address cost and manufacturability. These include stir casting and/or squeeze casting a slushy mixture of molten aluminum with SiC reinforcement. Some of the technical weaknesses of this process include a reaction between the melt and reinforcement, uniformity of the dispersion or high viscosity which tends to limit the amount of reinforcement loaded into the melt. This method among a few others could use fibers as the strengthening component. Another example of a MMCs process is to infiltrate a skeleton of hard reinforcement with liquid metal. The technical weaknesses of this process include the ability to form a handleable skeleton, uniformity of infiltration and a chemical reaction between the melt and reinforcement.
Powder metallurgy opens a processing dimension that operates in the solid state therefore minimizes a chemical reaction between the metal matrix and reinforcement ceramic. A simple and economical method is to premix metal powder with ceramic reinforcement to produce the Al-MMC composition prior to consolidation. Depending on the particle size of the powders and mixing method the weaknesses of this process include uniformity of the reinforcement material and weak adhesion of the reinforcement with the metal matrix. In some cases further thermomechanical metal working is necessary to improve the dispersion of the reinforcement material.
This article appeared in the July/Aug issue of Metal Powder Report.
