• Anti-Galling—Magnesium alloys pos- sess a low galling tendency and can be used as a bearing surface in conjunction with a shaft hardness above 400 Brinell.
• Impact and Dent Resistance—Te elastic energy absorption character- istics of magnesium result in good impact and dent resistance and energy management, which is one reason magnesium castings can be used for automotive safety-related applications, such as air bag systems. Portable tools and handheld electronics also benefit from this combination of properties, offering mechanical shock resistance.
treatments normally are not necessary to achieve stable final dimensions. Metallurgical changes in the structure of some metals can af- fect dimensions after prolonged exposure to elevated temperatures, but this is not the case with magnesium alloys. As a result, there have been few problems associated with the dimensional change of castings in assemblies. Shrinkage rates are more consistent and predictable in magnesium than in other nonfer- rous metals. Components release from the die with minimal force and distortion, hence they have minimal residual casting stress.
Types Magnesium alloys can be divided
into two groups: sand casting alloys and diecasting alloys. Alloys also can be classified as general purpose, high- ductility and high-temperature alloys. Most magnesium alloys are produced as high-purity versions to reduce potential corrosion problems associ- ated with higher levels of iron, nickel and copper. Sand casting alloys can be produced with a fine grain structure due to small additions of zirconium. Aluminum is the principal alloying
element for many magnesium alloys, as it can improve the mechanical strength, corrosion properties and castability of magnesium castings. Te most widely used general purpose sand casting alloy is AZ91. In the alloy nomenclature, the letters A and Z denote the major alloying elements, aluminum and zinc. However, not all properties improve with aluminum and zinc additions. Ductility and fracture toughness are gradually reduced when more aluminum is added. Tis effect led
2016 CASTING SOURCE DIRECTORY
Castings for use in higher temperature service condi- tions can be produced in al- loys such as the AS and AE series, based on the addition of either silicon or rare earth elements (E), which promote the formation of finely dispersed particles at the grain boundaries. Recent property and
Fig. 1. Magnesium’s light weight has made it a useful alloy for a number of new markets and applications, such as the automotive, power tool, computer and electronics industries.
to the introduction of a series of alloys with reduced aluminum content and the addition of manganese (the AM series), which is used extensively for automotive safety-related components. Manganese is added to control the iron content of the alloys. Several alloys, such as AM60 (6% aluminum, 0.05% manganese), have found widespread application in parts like instrument panel supports, steering wheel armatures and seat parts. Some applications expose the casting to higher operating temperatures or continuous stress that leads to concerns about long-term deformation and creep.
castability improvements have been shown with new magnesium creep-resistant alloys that use specialized rare earth elements, such as
calcium or strontium, as the significant alloying elements. Tese new alloys can produce cast components with superior mechanical properties at higher tem- perature ranges in service.
Design Considerations When evaluating the various alloys
and processes for a magnesium casting, the end-use application, post-casting operations and tooling costs should be considered to obtain a quality, low-cost component. Specifically, remember the
following attributes of magnesium: • High Stiffness-to-Weight Ratio—
R
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Meehanite call us or visit our website Lodi Iron Lodi, CA 95421
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St. Louis Precision Casting Company St Louis, MO 63128 Tel: 314-849-4080
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www.meehanitemetal.com For more info about Meehanite Metal,
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© 2016 Meehanite Metal Corporation METAL CASTING DESIGN & PURCHASING 39
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