Ceramic fibers boost magnesium's potential.
Lightweight, corrosion resistive, and possessing excellent mechanical strength, magnesium has been successfully used in a wide range of applications. But new magnesium composites, introduced by Dow Chemical Co, exhibit significantly improved wear resistance along with better tensile yield strength, compressive yield strength, elastic modulus and better high temperature properties.
Until recently, engineers considered the available methods for improving magnesium's wear resistance and strength impractical. Plating a material with a hard metallic coating such as chrome or nickel, for example, presents an extra fabrication step, increasing the cost of the finished part. Reinforcing a metal with continuous ceramic fibers is also expensive, both because of the processing and the cost of the fibers.
By utilizing less expensive reinforcing agents such as silicon carbide or aluminum oxide, Dow has made commercial use of magnesium composites economically practical for a wide variety of magnesium alloys.
Dow researchers developed a proprietary process which allows molten Mg alloy ingots to be used in traditional castng methods. The Dow process enhances the composite strength and stiffness and increases production economies.
Dow has actually designed two types of Mg composites: one containing a low volume (0-10%) ceramic and a high volume fraction (10-30%). While both effectively enhance wear resistance, the high-volume composites also exhibit improved mechanical properties.
Abrasive wear tests show that additions of as little as 1.0 wt. percent alumina to AZ91 (0.5% by volume) significantly increased the abrasive wear-resistance of the Mg alloy. To determine lubricated metal-on-metal sliding wear, researchers employed the pin-on-V-block test which measures both friction and wear. This test consists of a test pin rotating against two test V-blocks that apply a controlled amount of pressure to the pin. Test variables included pressure, V-block and pin composition, lubricant, temperature and testing time. The volume losses of the pin and V-blocks were then converted from weight loss.
Dow researchers evaluated Mg composite pins and V-blocks against 1095 steel, cast iron, 6061-T6 aluminum and 7075-T6 aluminum. Tests against various dissimilar metal pins revealed that pin wear was relatively low and insensitive to the amount of alumina in the V-block. One exception appeared in the cast iron pins which showed an increased volume loss when V-block alumina content was raised from zero to 3.3%. The composite V-block volume loss was significantly affected by both the alumina concentration of the V-block and the composition of the dissimilar metal pin.
Small alumina additions (3.1 wt percent) to the composite V-block resulted in an immediate drop in volume loss. Further improvements were obtained as the alumina concentration was increased beyond 3.1 wt. percent. When the alumina content reached 9-11%, volume loss approached zero. On the other hand, with steel pins, a V-block composition of greater than 7.4% alumina was needed to lower the block volume loss. The block volume losses of the Mg composite blocks tested against 7075-T6 aluminum pins were essentially zero for all alumina concentrations tested.
These tests showed alumina's role in reducing the composite V-block volume loss and increasing the composite's wear resistance.
A number of prototype Mg composite parts were cast by Dow researchers. The first part tested was a diesel engine camshaft cover plate machined from 4 in. diameter billet that was cast in a graphite book mold. The cover attaches to the rear of the engine block. The turning camshaft bore against a 1095 steel thrust washer, which in turn rotated against the cover. The cover was lubricated through the hollow camshaft; operating temperature was approximately 240F.
After more than 250 hr of engine operation, no visible wear of either the thrust washer or camshaft bearing was visible.
Another prototype part was an oil pump cover plate which acts as the bearing surface for the rotating teeth of the gear pump. After approximately 100,000 miles, the composite cover is still in use with no visible wear at the pump's bearing surface.
A prototype die cast chain saw cylinder, cog tooth timing sprocket and crankshaft poly-v-pulley are in various stages of testing.
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|Date:||Jul 1, 1989|
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