These magnesium components serve as the right and left structural parts for the skeleton of a cordless nail gun, supporting all of the inter- nal and external components. Each casting weighs less than 0.25 lbs.
used for housings and enclosures.
• Dimensional Stability—Annealing, artifi cial-aging or stress-relieving treatments normally are not necessary to achieve stable fi nal dimensions. Metallurgical changes in the structure of some metals can aff ect dimensions after prolonged exposure to elevated temperatures, but this is not the case with magnesium alloys. As a result, there have been few problems associ- ated with the dimensional change of castings in assemblies. Shrinkage rates are more consistent and predict- able in magnesium than in other nonferrous metals. Components release from the die with minimal force and distortion, hence they have minimal residual casting stress.
• Impact & Dent Resistance—T e elastic energy absorption characteristics of magnesium result in good impact and dent resistance and energy manage- ment, which is one reason magnesium castings can be used for automotive safety-related applications, such as air bag systems. Portable tools and hand- held electronics also benefi t from this combination of properties, off ering me- chanical shock resistance.
• Anti-Galling—Magnesium alloys possess a low galling tendency and can be used as a bearing surface in conjunction with a shaft hardness above 400 Brinell.
Types
Magnesium alloys can be divided into two groups: sand casting alloys and diecasting alloys. Alloys also can be classifi ed as general purpose, high-ductility and high-temperature alloys. Most magnesium alloys are
produced as high-purity versions to re- duce potential corrosion problems asso- ciated with higher levels of iron, nickel and copper. Sand casting alloys can be produced with a fi ne 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. T e 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. T is eff ect led 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.
Produced for a racing motorcycle, this one- piece magnesium casting replaced a three- piece aluminum part. The component is 33% lighter than the original, which impacts the overhaul performance of the bike.
Some applications expose the casting to higher operating temperatures or continuous stress that leads to concerns about long-term deformation and creep. Castings for use in higher temperature service conditions 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 fi nely dispersed particles at the grain boundaries. Recent property and castability im-
provements have been shown with new magnesium creep-resistant alloys that use specialized rare earth elements, such as calcium or strontium, as the signifi cant alloying elements. T ese new alloys can produce cast components with superior mechanical properties at higher temperature ranges in service.
Design Considerations
When evaluating
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.
2013 CASTING SOURCE DIRECTORY
the various alloys and processes for a magne- sium casting, the end-use application, post-casting operations and tooling costs should be consid- ered to obtain a qual-
METAL CASTING DESIGN & PURCHASING 39
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