and high-temperature alloys. Most magnesium alloys are produced as high-purity versions to reduce potential corrosion problems associated with higher levels of iron, nickel and copper. Sand casting alloys can be produced with a fine grain struc- ture due to small additions of zirconium. Aluminum is the prin-
cipal alloying element for many magnesium alloys, as it can improve the me- chanical strength, corrosion properties and castability of magnesium castings. The most widely used general purpose sand casting alloy is AZ91. In the alloy nomenclature, the letters A 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.
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. This effect 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. Some applications expose the casting to higher operating temperatures or con- tinuous stress that leads to concerns about long-term deformation and creep. Cast- ings for use in higher temperature service conditions can be produced in alloys such as the AS and AE series, based on the addi- tion of either silicon or rare earth elements (E), which promote the formation of fi nely dispersed particles at the grain boundaries.
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 over- haul performance of the bike.
Recent property and
castability improvements have been shown with new magnesium creep- resistant alloys that use specialized rare earth ele- ments, such as calcium or strontium, as the sig- nifi cant alloying elements. These new alloys can pro- duce cast components with superior mechanical properties at higher tem- perature ranges in service.
Casting Processes Along with magne-
sium’s multiple alloys, the material can be cast in a variety of methods,
including high-pressure diecasting, permanent mold casting, sand casting, and semi-solid and squeeze casting. Different alloys may be specified for these different processes, but in cases where the same alloy is used with dif- ferent casting processes, the properties of the finished castings will depend on the method. The most prevalent casting method for magnesium is diecasting. In this process, complex, thin-walled parts are produced at high production rates aided by the low-heat content per vol- ume of molten metal. Both hot chamber and cold chamber machines can be used for magnesium. For optimum perfor- mance, it is recommended that higher shot speeds are used for magnesium compared to aluminum, especially for thin-walled parts. Diecasting process variants (such as vacuum diecasting) can produce components with lower porosity and better properties than standard diecasting. Magnesium also is conducive to semi-solid casting methods, typically with magnesium alloy granules or par- tially solidified alloys rather than liquid magnesium. Semi-solid molding com- monly is used for smaller parts, such as those used in the electronics industry.
Design Considerations
These magnesium components serve as the right and left structural parts for the skeleton of a cordless nail gun, sup- porting all of the internal and external components. Each casting weighs less than 0.25 lbs.
38 METAL CASTING DESIGN & PURCHASING
When evaluating the vari- ous 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 compo-mp nent. Specifi cally, remember the following attributes of magnesium:
High Stiffness-to-Weight Ratio— 2010 CASTING SOURCE DIRECTORY
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