During product development it’s often difficult to make a functional prototype that behaves like the final part. This leaves you with some unappealing options for getting a test part: Either make expensive mockups using the part material, or predict the part’s behavior using a prototype with mechanical properties different from the final part. The next time you’re faced with these choices, consider a material called metal-matrix composite (MMC). It’s made by a special casting process and performs like steel and ductile iron but weighs half as much. The parts are produced by a process called advanced pressure-infiltration casting (APIC) and require little postmachining. The process has successfully produced various components such as brake rotors, connecting rods, and thermal planes for printed-circuit boards.
MMC parts consist of a reinforcement material and a metal. The molten metal is pressurized into the reinforcement material, forming a metal-matrix composite. Though a variety of materials can be used for both the reinforcement and the metal, the reinforcement is typically a ceramic, such as silicon carbide or alumina, and the metal is commonly an aluminum, magnesium, copper, or bronze alloy. Components currently being manufactured by die casting, permanent mold casting, or squeeze casting can be produced defect free using APIC.
Basic pressure-infiltration casting (PIC) uses a water-cooled autoclave with a furnace inside it. A graphite mold insert holds a preform covered by a filter. The preform consists of the reinforcement material in the shape of the final part. The required alloy in solid form is placed on top of the filter. The autoclave is evacuated and then heated to melt the metal. Next the autoclave is pressurized, forcing molten metal into the evacuated spaces between reinforcement particles. This setup, however, has limitations. First, the casting cycle lasts about 3 hr, which is impractical for producing parts in large batches. Also, housing the furnace and insulation within the pressure vessel takes up space that could be used to make bigger parts or multiple parts.
An improved PIC process that the inventor calls APIC increases usable space in the autoclave and reduces turnaround time from 3 hr to 15 min. This process, developed by Metal Matrix Cast Composites Inc. (MMCC), Waltham, Mass., replaces the furnace and vacuum pump with an external vacuum furnace and an inert-atmosphere furnace. A mold vessel with insulated sidewalls holds the preform in the mold cavity. The mold is preheated to near the metal’s melting point. A vent tube connected to a vacuum pump evacuates the air in the preform. Instead of placing solid metal on top of the preform and then melting it, molten metal is poured directly into the top of the mold vessel. The molten metal creates a seal between the preform and the atmosphere. It is then placed in an autoclave and pressurized to drive the molten metal into the preform, which takes about 1 min regardless of part size. The mold vessel is then placed on a chill surface, which provides controlled cooling from the uninsulated bottom of the mold.
When aluminum cools and solidifies it shrinks by about 6%. In processes such as die casting, metal is injected into a “cold” tool. The aluminum cools and solidifies from the outside in. As the part cools, however, it also shrinks. The front of unsolidified aluminum compensates for shrinkage until the center of the part cools. When the center hardens there is no molten aluminum to make up for shrinkage, creating porosity at the center of die-cast aluminum parts. PIC parts have zero porosity because they cool from the bottom and have a pool of metal on the top. As solidification ascends, excess molten aluminum corrects shrinkage.
Casting without finishing
Traditional casting methods often make rough-surfaced parts that require grinding or milling before they can be used. These finishing operations drive up part prices. MMC parts, however, come out of the mold at “near-absolute net shape,” meaning parts closely match the final shape and require little or no post machining. This is an important requirement for metal-matrix parts because the reinforcements, which are typically silicon carbide or alumina, are difficult to machine without using pricey cubic-boron nitride or diamond-cutting tools. MMC parts carry tolerances below 0.001 in., and can have surface finishes less than 10-rms µin., which even allows casting machine threads directly into parts. For most applications, gate removal is all the postmachining that’s required, with finish machining only necessary on high-tolerance surfaces like piston bores and wrist-pin bosses.
The variety of alloys and particulates that can be used for MMC lets engineers choose materials that specifically fit each application. Parts can be designed for high stiffness, light weight, high thermal conductivity or combinations of these properties. When manufacturing printed-circuit boards, for example, the boards are bonded to thermal planes, which require high thermal conductivity and low thermal expansion. To achieve this, engineers at MMCC combine graphite (Gr) fibers with an aluminum alloy (Al) to produce circuit boards with a surface coefficient of thermal expansion (CTE) of 5.8 ppm/C and a thermal conductivity of 250 W/m·K in the X-Y plane. Replacing conventional copper/tungsten (Cu/W) heat sinks with MMCs can reduce part densities from 16 to 17 gm/cc for Cu/W down to 6.1 gm/cc for Cu/Gr and 2.4 gm/cc for Al/Gr.
Lightweight parts can also replace ductile-iron parts without changing the part size and with minimal redesigning. The metal-matrix replacement parts are not only lighter, they’re also stiffer. Ductile iron, for instance, has an elastic modulus of 22 X 106 psi (152 GPa). A metal-matrix part with Al alloy and 55 vol. % alumina has an elastic modulus of 24 X 106 psi (165.5 GPa). And the density of the MMC is 44% less than that of ductile cast iron.
Brake systems, which often use ductile iron parts, can be improved with MMCs. Lightweight brake parts help reduce a vehicle’s unsprung weight (any weight not supported by the springs, such as wheels, tires, and brakes). Reducing unsprung weight decreases road shock and improves ride and handling. Various MMC parts can replace existing brake parts for improvements aimed not only at weight reduction but also at factors such as wet or dry friction performance and built-in lubricity. Reinforced-aluminum parts can be used on automobiles and bikes. Bronze-matrix composites, on the other hand, are a better choice for train and airplane brakes.
A brake rotor made for DeConti Industries Inc., optimized wear resistance and thermal conductivity while testing the APIC process on another front. The New Britain, Conn., company makes liquid-cooled brake systems using rotors with internal passages for coolant. Casting these parts in iron requires a binderized sand core that simply shakes out after the parts cool. Unfortunately APIC would push molten metal into the sand, making extraction nearly impossible. MMCC replaced the sand with a proprietary salt core that is not infiltrated by the metal and is water soluble for easy removal.
The rotors are being prepared for use on flight-line tow tractors for the Air Force and front-end rotors for the Army Demo III Combat Truck. The current parts have either a 13 or 17-in. diameter and are made of aluminum alloy for high thermal conductivity reinforced with 55 vol. % alumina particulate. This can be replaced with silicon carbide particulate, which provides high wear resistance because of its ceramic content. The liquid-cooled brake rotor is cast in low-cost tooling made from a carbon-matrix ceramic composite. The tools are pressed onto a pattern and can be used for APIC and for conventional casting and molding using metal or plastic.
Although the most common MMC combinations are aluminum/alumina and aluminum/silicon carbide, a wide variety of materials can be used. In fact, over 15 alloys can be combined with at least 30 different reinforcements in the form of either ceramic fibers, particulates, or whiskers. Tests on MMC parts confirmed the strengths of the aluminum/alumina combination. Alumina performed better than other reinforcements because of its low reactivity with aluminum. Particle size also impacts performance. The composite’s tensile strength increases with decreasing particle size. Fracture toughness, on the other hand, increases with particle size.
Particle size also contributes to the effects of heat treatment. For large particles (29.2 µm) the composite’s tensile strength is not affected by heat treatment. When alumina particles are small (12.8 µm), tempering to T6 or T7 increases tensile strength by 20% compared with similar materials tempered to T4. For small and large particles, fracture toughness hits a minimum at T6 tempering compared to T4 and T7 tempers. Heat treating, or tempering, involves exposing the material to specific temperatures for specific lengths of time. Guidelines for heat treatment methods have temper designations such as T4, T6, and T7, set forth by the Aluminum Association.
Alloy composition also has important effects on mechanical properties. Alumina particulate is available as abrasive-grade aluminum oxide (alundum) and high-purity aluminum oxide (alumina). Although alumina is more expensive, it is purer and therefore stronger. Silicon (Si) and magnesium (Mg), added to the matrix Al alloy in varying degrees, optimize the MMC’s mechanical properties. Adding 1.5% Mg increases tensile strength by about 10% but does not affect fracture toughness. Silicon, added alone, does not affect mechanical properties considerably. Combining Si and Mg with aluminum increases the composite’s tensile strength by about 25%. Fracture toughness, however, depends on Mg concentration. When Mg concentration is lowered to 0.5% and combined with Si, fracture toughness reduces. Fracture toughness returns and even increases when Mg is increased to 1.5%.