Aluminum Alloys Magnesium Alloys
Corrosion Comparison - Aluminum and Magnesium Casting Alloys1
alloys, castings are the more prevalent product form than wrought products. One consequence of its rather low melting point is its susceptibility to creep at moderately elevated temperatures. However, alloys have been developed with improved creep performance.
Aluminum and zinc provide solid solution strengthening. Aluminum, in addition to providing strength and hardness, widens the freezing range and makes the alloy easier to cast. Aluminum in amounts greater than 6% promotes precipitation hardening. Zinc is the second most important alloying element. Zinc helps to refine the precipitate in aluminum-containing alloys. Zinc can also be used in combination with zirconium, rare earths (RE), or thorium to produce precipitation hardening alloys. Silver also improves the response to precipitation hardening. Zirconium is used in casting alloys for grain refinement. The powerful grain refining effect of zirconium is shown in Fig. 3.3. However, zirconium is not used in alloys containing aluminum because brittle compounds are formed. Manganese additions improve the corrosion resistance in sea water by removing iron from solution. Silicon increases fluidity for casting alloys but decreases the corrosion resistance if iron is present. Thorium and yttrium improve creep resistance; however, the use of thorium, which is mildly radioactive, has decreased due to increasing regulations on its use. Although much less soluble than aluminum and zinc, the RE elements are potent solid solution strengtheners. The rare earths are usually added as natural mixtures of either mischmetal or as didymium. Mischmetal contains about 50% cerium with the remainder as mainly lanthanum and neodymium, while didymium contains
approximately 85% neodymium and 15% praseodymium. The impurity elements nickel, iron, and copper must be held to low levels to minimize corrosion.
Magnesium alloys are produced in both the wrought and cast conditions. Some alloys are strengthened by cold working while others can be precipitation hardened by heat treatment. The alloys themselves can be divided into two broad classes: those that contain 2-10% aluminum with additions of zinc and manganese, and those containing zinc with additions of RE metals, thorium, silver, and zirconium for strength, creep resistance, and grain refinement. The tensile properties of magnesium alloys generally range from 10 to 50ksi yield strength and 20-55 ksi tensile strength with elongations of 1-15%.3
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