Magnesium Alloy Castings

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Introduction to Magnesium Alloy Castings

Magnesium alloy castings can be produced using nearly all conventional casting methods, including sand, permanent and semi-permanent mold, shell, investment, and die-casting. When selecting a casting method for a specific part, manufacturers must consider multiple factors: the configuration of the proposed design, the intended application, required properties, total production volume, and the specific properties of the alloy.

These versatile castings have found applications across numerous commercial sectors, particularly where their exceptional lightness and rigidity provide significant advantages. Common applications include chain saw bodies, computer components, camera bodies, and various portable tools and equipment. In the aerospace industry, magnesium alloy sand castings are extensively used for their superior weight-to-strength ratio.

Designation System of Magnesium Alloy Castings

The codification method used to designate magnesium alloy castings follows ASTM Standard Practice B 275 (Table 1). This system provides an immediate, approximate indication of the chemical composition of an alloy, with letters representing the main constituents and figures representing the percentages of these constituents.

Magnesium alloy designations consist of three distinct parts:

Part 1: Indicates the two principal alloying elements using code letters arranged in order of decreasing percentage (or alphabetically if percentages are equal). These letters include: A-Aluminum, E-Rare Earth, H-Thorium, K-Zirconium, M-Manganese, Q-Silver, S-Silicon, T-Tin, and Z-Zinc.

Part 2: Indicates the amounts of the two principal elements using two whole numbers corresponding to the rounded mean percentage of each element.

Part 3: Distinguishes alloys with identical percentages of the two principal alloying elements. This consists of one of the following letters: A-First composition, B-Second composition, C-Third composition registered with ASTM, D-High-purity, and E-High corrosion resistance.

For example, in the designations AZ91A, AZ91B, and AZ91C:

• A represents aluminum, the primary alloying element
• Z represents zinc, the secondary alloying element
• 9 indicates that the rounded mean aluminum percentage falls between 8.6% and 9.4%
• 1 indicates that the rounded mean zinc percentage falls between 0.6% and 1.4%
• The final letters (A, B and C) signify sequential alloy developments with slightly different compositions that don't warrant changes to the basic designation

Major Casting Processes for Magnesium Alloys

Sand Casting

Magnesium alloy sand castings are prevalent in aerospace applications due to their significant weight advantage over aluminum and other materials. Extensive research and development have yielded remarkable improvements in general properties compared to earlier AZ-type alloys.

While a substantial volume of aerospace castings still use conventional AZ-type alloys, the industry is increasingly shifting toward newer zirconium-containing types. Although magnesium-aluminum and magnesium-aluminum-zinc alloys are generally easy to cast, they have limitations including microshrinkage when sand-cast and temperature limitations above 95°C.

The development of magnesium rare earth-zirconium alloys addressed these limitations. For instance, sand castings using EZ33A alloy demonstrate excellent pressure tightness. The greater oxidation tendency of zirconium-containing alloys is mitigated through specialized melting processes.

The two magnesium-zinc-zirconium alloys originally developed, ZK51A and ZK61A, exhibit high mechanical properties, but suffer from hot-shortness cracking and are nonweldable.

For moderate temperature applications (up to 160°C), ZE41A and EZ33A alloys are most widely used due to their excellent castability and ability to create complex components. These alloys also benefit from requiring only a T5 heat treatment (precipitation treatment).

For aerospace engine applications requiring superior mechanical properties at elevated temperatures (up to 205°C), thorium was substituted for rare earth metal content in ZE and EZ type alloys, creating ZH62A and HZ32. These thorium-containing alloys maintained good castability and welding characteristics but required greater care during melting and pouring due to increased oxidation tendencies.

Further development aimed at improving both room-temperature and elevated-temperature properties led to the QE22A alloy, where silver replaced some zinc. However, environmental concerns regarding thorium and price volatility of silver drove research toward alternative alloy compositions, resulting in the WE54A alloy containing approximately 5.0% yttrium combined with other rare earth metals.

Permanent Mold Casting

Generally, alloys suitable for sand casting work well for permanent mold casting, with one notable exception: magnesium-zinc-zirconium alloys exhibit strong hot-shortness tendencies, making them unsuitable for this process.

Die Casting

Die castings are primarily produced from magnesium-aluminum-zinc alloys, with AZ91A and AZ91B being the most common variants. The key difference between these two is the higher allowable copper impurity in AZ91B.

Advantages of Magnesium Castings

The most significant feature of magnesium castings is their lightweight nature, which has driven their use in aircraft and aerospace applications since World War II. More recently, energy conservation requirements in the automotive industry have increased the use of magnesium die-castings in vehicles.

Beyond weight advantages, magnesium offers several other important benefits:

• It is an abundantly available metal
• It is easier to machine than aluminum and can be machined much faster, preferably dry
• In die casting, it can be cast up to four times faster than aluminum
• Die lives are considerably longer than with aluminum alloys due to reduced welding onto die surfaces
• With proper protection against galvanic effects, it demonstrates very satisfactory performance

Modern casting technologies and protective coatings enable the production of complex components with thin-wall sections. The end products offer high stability combined with lightweight properties.

Melting and Processing Considerations

Furnaces for melting and holding molten magnesium casting alloys typically use indirectly heated crucibles, similar in design to those used for aluminum casting alloys. However, the different chemical and physical properties of magnesium alloys necessitate different crucible materials, refractory linings, and process equipment designs.

Molten magnesium tends to oxidize and can potentially explode without proper protection against oxidation. Unlike aluminum alloys, which form a continuous, impervious oxide skin limiting further oxidation, magnesium alloys form a loose, permeable oxide coating that allows oxygen to pass through and support combustion below the surface. This requires protection of the molten alloy using either a flux or a protective gas cover to exclude oxygen.

One advantage of working with magnesium is that it doesn't attack iron the way aluminum does, allowing for the use of steel crucibles. Particularly with larger castings, it's common practice to melt, process, and pour the molten magnesium alloy from the same steel crucible.