As shown in Fig. 9.8, squeeze casting is a metal forming process in which solidification is accomplished under high pressure to help eliminate shrinkage porosity and reduce porosity by keeping gases dissolved in solution. Squeeze cast parts are usually fine grained with excellent surface finishes and almost no porosity.9 To produce metal matrix composites, porous preforms of reinforcement material are infiltrated by molten metal under pressure. Reinforcement
forms can be continuous fiber, discontinuous fiber, or particulate with aluminum or magnesium alloys. The volume fraction of reinforcement in the MMC can vary from 10 to 70 volume percent, depending on the particular application for the material. Pressure is applied to the solidifying system by a hydraulically actuated ram. This process is similar to conventional die casting, except that the ram continues to apply pressure during solidification and the pressures are higher (usually in the range of 1.5-15 ksi) and applied slowly. High pressure helps in increasing processing speed, producing finer matrix microstructures, and producing sounder castings by minimizing solidification shrinkage.8 To help minimize solidification shrinkage, the pressure is maintained until solidification is complete. If cold dies and reinforcements are used along with high pressures, chemical reactions between the liquid metal and reinforcement can be minimized due to the shorter processing cycles. Squeeze casting is one of the most economical processes for fabricating MMCs and allows relatively large size parts to be made.5
Preforms for infiltration are usually prepared by sedimentation of short fibers or particles from a suspension.1 A Saffil alumina preform is shown in Fig. 9.9. Binders are frequently required to maintain preform integrity for handling. Binders (5-10 weight percent) can either be fugitive that are burned off during casting or high temperature silica or alumina compounds requiring firing before casting.1 The binding agent is normally introduced through a suspension liquid, so that it deposits or precipitates out on the fibers, often forming preferentially at fiber contact points, where it serves to lock the fiber array into a strong network.
Composites produced by squeeze casting are usually porosity free; however, the high pressures used may damage the preform, especially during the early stages of infiltration. A major advantage of infiltration processes is that they allow for near net shaped parts with either total reinforcement or areas with selective reinforcement. A limitation is the need for the reinforcement preform to be self-supporting, either as a preform or a dense pack of reinforcement. Indeed, preforming is one of the highest cost areas for this process. In addition, preform deformation during infiltration can result in lower-than-desired mechanical properties. Tooling may also be expensive for complex part shapes.
A pressure-less metal infiltration process, called the Primex process, allows an aluminum alloy to infiltrate a reinforcement preform without the application of pressure or vacuum. The reinforcement level can be controlled by the starting density of the preform being infiltrated. As long as interconnected porosity and appropriate infiltration conditions exist, the liquid aluminum will spontaneously infiltrate the preform. Key process characteristics include an aluminum alloy, the presence of magnesium, and a nitrogen atmosphere. During heating to the infiltration temperature (1380° F), magnesium reacts with the nitrogen atmosphere to form magnesium nitride (Mg3N2 ). Magnesium nitride is the infiltration enhancer that allows the aluminum alloy to infiltrate the reinforcing phase without the necessity of applied pressure or vacuum. During infiltration, Mg3N2 is reduced by the aluminum to form a small amount of aluminum nitride
(AlN).2 The AlN forms small precipitates and a thin film on the surface of the reinforcing phase. Reinforcement loading can be as high as 75 volume percent, given the right combination of particle shape and size. The most widely used cast composite produced by liquid metal infiltration is an Al-10Si-1Mg alloy reinforced with 30 volume percent SiCp. The 1% magnesium in this alloy is obtained during infiltration by the reduction of the Mg3N2. The only restriction in the selection of an aluminum alloy is the presence of magnesium to allow the formation of the Mg3N2. For SiCp containing systems, silicon must also be present in sufficient quantity to suppress the formation of Al4C3.
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