Magnesium Alloys

Magnesium has a density two-thirds that of aluminum and only slightly higher than that of fiber-reinforced plastics and possesses excellent mechanical and physical properties. When coupled with the inherent advantages of the metalcasting process, magnesium alloys yield cost-effective solutions to product needs by allowing for part consolidation and weight savings over other materials and manufacturing methods.

Advantages of Magnesium
Magnesium alloy properties can provide a casting designer with several advantages over other lightweight alloys.

Weight—The lightest of all structural metals, magnesium preserves the light weight of a design without sacrificing strength and rigidity. This benefit is important when portability is a key element of the product design, such as with chainsaws, pneumatic nailers, circular saws, luggage, laptop computers and cellular phones. Automobiles and other transportation equipment continue to take advantage of magnesium’s low density in expanding application areas ranging from under-hood and driveline uses found in engine brackets and transfer cases to numerous interior parts, such as steering column components, pedal brackets, instrument panel supports and seating.

Damping Capacity—Magnesium is unique among metals because of its ability to absorb energy. Increased vibration absorption capacity provides for quieter operation of equipment when magnesium castings are used for housings and enclosures.

Dimensional Stability—Annealing, artificial-aging or stress-relieving treatments normally are not necessary to achieve stable final dimensions. Metallurgical changes in the structure of some metals can affect 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. Magnesium shrinkage rates are more consistent and predictable in comparison to other nonferrous metals. Components release from the die with minimal force and distortion, hence they have minimal residual casting stress.

Impact & Dent Resistance—The elastic energy absorption characteristics 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.

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 measurement.

Alloy Families
Magnesium alloys can be used in multiple applications, but they easily 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 associated with higher levels of iron, nickel and copper. Sand casting alloys often are 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. The 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. 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, including 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. Castings for use in higher temperature service conditions can be produced in alloys 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 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. These new alloys can produce cast components with superior mechanical properties at higher temperature ranges in service.

Casting Processes
Along with magnesium’s multiple alloys, the material can be cast by 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 different 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 volume of molten metal. Both hot chamber and cold chamber machines currently are used for magnesium. For optimum performance, 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 partially solidified alloys rather than liquid magnesium. Semi-solid molding commonly is used for smaller parts, such as those used in the electronics industry.

Design Considerations
When evaluating the various alloys and processes for a magnesium casting, a number of characteristics should be considered to obtain a quality, low-cost component. These include the end-use application, the post-casting operations and tooling costs.

High Stiffness-to-Weight Ratio—This characteristic is important where resistance to deflection is desired in a lightweight component.

Improved Die Life—Unlike molten aluminum, molten magnesium does not react with tool steels, resulting in longer die life and increased productivity. Because of low erosion and reduced heat input, which reduce the propensity for thermal fatigue (heat checking of the die), casting magnesium can achieve three to four times the die life than can be achieved with aluminum.

Machining—Magnesium is recognized as the easiest of structural metals to machine and is the standard of the cutting tool industry when comparing machinability of metals. The low power requirements for machining magnesium alloys permit the use of deeper cuts and higher feed rates, thus permitting fast and efficient machining when compared to other metals. Magnesium alloys also normally produce well-broken chips, which are easy to handle. 

--The American Foundry Society Technical Dept. 

This article was adapted with permission from materials prepared by the International Magnesium Assn., McLean, Va., and the North American Die Casting Assn., Wheeling, Ill.