SiC Particle Reinforced Aluminum by Casting

Abstract

Progress, whether in the laboratory or the factory, often hinges on finding a better material for the task at hand. Inevitably, the choice of a material demands tradeoffs between such factors as strength and weight, or safety and bulk. In product design, the real progress is made by those who can extract the most performance from the least amount of material, at the lowest cost. The many space-age spin-offs have yielded a variety of new materials that have two things in common: outstanding performance and, unfortunately, high cost. The latter may be due to the rarity of the material itself, the complexity or labor-intensiveness of its production, or the need for entirely new manufacturing facilities and techniques. Among the most technologically promising of these premium materials are metal-matrix composites (MMC's), in which the high strength, stiffness, and wear resistance of ceramics are combined with the formability and toughness of metals. In this class of materials is Dural@) SiC-AI. The composite is made through a proprietary process which combines SiC particles with molten aluminum. The technique involves a special pretreatment that solves the problem of making the aluminum wet the SiC particles, without unwanted chemical reactions. Dissolved gases and included oxides are minimized, and an extremely uniform distribution of particles in the melt is achieved. This invention, for which patents are pending, enables Dural MMC to be economically made by ingot metallurgy. A characteristic microstructure is given in Figure 1. Most other MMC materials are made by more complex, and inherently more costly processes. Like aluminum or steel, metal matrix composites are not a single material, but a family of materials. One side of the family is the type of alloyed aluminum used as the basic raw material. The other side is the volume percentage of SiC (or boron carbide or alumina) particles, which typically range from nearly zero to about 30% (10-20% is typical). That Dural SiC-AI can be worked and formed as readily as unreinforced aluminum enables materials fabricators to use existing equipment in producing components. This is particularly advantageousin applications where the high specific stiffness and strength of SiC-AI enable it to replace titanium forgings. Savings in both material and fabrication costs can also be realized by substituting SiC-AI for graphite-epoxy, whose production requires expensive hand labor. Compared with other space-age materials, cast metal-matrix composites possess three inherent cost advantages: low cost of its raw materials, simplicity of processing, and ease of fabrication. The first two advantages relate to production. As volume increases in response to demand, the production cost will continue to decline, eventually leveling off at a small percentage above that of the corresponding aluminum alloy. The third advantage-ease of fabrication-will have no effect on the price of the MMC itself. It will, however, enable manufacturers who are already using aluminum to convert to MMC without having to modify their tooling and machinery.