American Machinist

Cast aluminum MMCs have arrived. (metal-matrix composites)

Cast aluminum MMCs have arrived

Get acquainted fast with processing these novel reinforced metals. Because of their performance advantages, they're here to stay

ALTHOUGH MOST metal-matrix composites (MMCs) are in various stages of development, preform-and particulate-reinforced cast aluminum alloys already have found some significant production applications, one literally accounting for a new auto engine.

With ceramic preforms in the cylinderbore regions of the four-cylinder aluminum-alloy block of the 1991 Prelude Si, Honda (Japan) has eliminated the need for cast-iron cylinder liners. This provides greater cylinder displacement (56 cc more) and, thus, greater output (140 hp vs 135 hp) relative to the Prelude 2.0 Si engine, without increasing engine size or weight, and providing better cooling.

The aluminum alloy, as reported by Tadayoshi Hayashi and colleagues at the 1989 annual SAE Congress, is ADC12, which contains by weight 9.6-12% silicon, 1.5-3.5% copper, and lesser amounts of other metals. The preform comprises 12% by volume alumina fiber for strength and 9% by volume PAN-type carbon fiber for lubricity.

Mass production techniques for the new block, which could number 20,000-30,000 for the current model year, were described by Masuo Ebisawa and colleagues at this year's SAE Congress. In brief, the 2-mm-thick preforms are preheated and positioned in the mold with a specially designed loader and fully infiltrated with the molten alloy during low-speed medium-pressure diecasting. The cylinder bores are then machined using polycrystalline-diamond (PCD) tools.

As shown on the opposite page, the preforms are made by an aspiration-press method. To prevent the short alumina fibers from entangling, causing lumps, they are first disseminated in a tub by agitation of a two-blade fan, then distributed by the distribution fan. The fibers are then collected on a resin-coated sand filter in an aspiration tub equipped with both rotating and fixed blades to ensure uniform distribution of the fibers on the outside of the filter. The rotating one generates turbulence in the peripheral area of the tub, causing laminar flow in the mid-section of the dispersion.

An inorganic binder is then incorporated in the fibers and the fibers are rubber-pressed by water pressure to within [+ or -] 0.2 mm of preform wall-thickness requirements. The preform is oven-dried, the filter thermally decomposed at 450-600 [degrees] C, and the binder is fired at 660 [degrees] C.

To cast the block, Honda chose a relatively new diecasting method, which two years ago already had produced some 450,000 conventional engine blocks. Unlike conventional diecasting, in which the molten metal is injected horizontally into the die rapidly (25-40 m/sec) and at high pressure (550-1000 kg/sq cm), the metal is drawn vertically into the die at only 0.3-0.8 m/sec, and low to moderate pressure (50-300 kg/sq cm) is applied. For the Prelude Si block: 0.8 m/sec and 24.5 MPa, or 3550 psi.

Casting temperature for the alloy is 710-750 [degrees] C. For proper penetration by the alloy, the preform is preheated in the 350-550 [degrees] C range. …

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