Magnesium Alloys

Magnesium alloy developments have traditionally been driven by aerospace industry requirements for lightweight materials to operate under increasingly demanding conditions. Magnesium alloys have always been attractive to designers due to their low density, only two thirds that of aluminum. This has been a major factor in the widespread use of magnesium alloy castings and wrought products.

A further requirement in recent years has been for superior corrosion performance, and dramatic improvements in this regard have been demonstrated for new magnesium alloys. Improvements in mechanical properties and corrosion resistance have led to greater interest in magnesium alloys for aerospace and other specialized applications, and alloys are now being specified on projects like the McDonnell Douglas MD 500 helicopter.

Key properties of magnesium alloys

• Lightweight
• Low density (two thirds that of aluminum)
• Good high-temperature mechanical properties
• Good to excellent corrosion resistance

Applications of magnesium alloys

Aerospace

For many years, RZ5 alloy has been the preferred material for helicopter transmission casings due to the combination of low density and good mechanical properties. However, more recently, the requirement for longer intervals between overhauls and hence improved corrosion properties has caused manufacturers to reconsider material choice.

In the past, RZ5 was generally used for gearbox casings, but many new projects will use WE43 instead, including the main rotor gearbox casings. For this application, an aluminum transmission would have been used but the exceptional corrosion resistance of WE43 makes it the preferred material. The Eurocopter EC120 and NH90 helicopters have also flown with WE43 transmission casings and WE43 is specified for the Sikorsky S92. Further applications for WE43 will go ahead in the future both on new programmes and also replace RZ5 on older helicopters.

RZ5, ZRE1, MSR and EQ21 alloys are widely used for aircraft engine and gearbox casings. This will continue although it is likely that WE43 will be used increasingly for its corrosion and high-temperature properties. Very large magnesium casings can be made, such as intermediate compressor casings for turbine engines. These include the Rolls Royce Tay casing in MSR, which weighs 130 kg, and the BMW Rolls Royce BR710 casing in RZ5.

Other aerospace applications include auxiliary gearboxes (F16, Eurofighter 2000, Tornado) in MSR or RZ5, generator housings (A320 Airbus, Tornado, Concorde) in MSR or EQ21, and canopies, generally in RZ5.

Magnesium alloy forgings are also used in aerospace applications, including critical gearbox parts for the Westland Sea King helicopter and aircraft wheels, both in ZW3. Forged magnesium parts are also used in aircraft engine applications. In the future, magnesium forgings are most likely to be used in higher temperature applications.

Automotive – motor racing

In motor racing, RZ5 is generally used for gearbox casings, although MSR and EQ21 alloys are also being used increasingly due to their superior ambient temperature properties or because of increased operating temperatures.

RZ5 wheels have been shown to have significantly better performance than Mg-Al-Zn alloy wheels under arduous racing conditions. Due to the high operating temperature of racing engines, WE54 casings have been used for a variety of Formula 1 engine parts and are used for engine components for a limited-edition road car. Forged WE54 pistons offer great future potential for motor racing and other applications will exist for other wrought products.

Bicycles

As mentioned above, the metal matrix composite Melram 072 is used in the bicycle industry due to its excellent stiffness and reduced weight compared to aluminum.