Defense Articles

Background

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 aluminium. 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 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 speciality applications, and alloys are now being specified on programmes such as the McDonnell Douglas MD 500 helicopter.

Key Properties

·        Light weight

·        Low density (two thirds that of aluminium)

·        Good high temperature mechanical properties

·        Good to excellent corrosion resistance

Applications

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. More recently, however, 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 programmes will use WE43 instead including the main rotor gearbox castings. For this application, an aluminium transmission would have been used but for the exceptional corrosion resistance of WE43. The Eurocopter EC 120 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 to 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 castings can be made, such as intermediate compressor casings for turbine engines. These include the Rolls Royce Tay casing in MSR, which weighs 130kg 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 and 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 aero 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/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 castings 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.

Magnesium alloys are also used in many other engineering applications where having light weight is a significant advantage. Magnesium-zirconium alloys tend to be used in relatively low volume applications where they are processed by sand or investment casting, or wrought products by extrusion or forging. Zirconium-free alloys, principally AZ91 but also other alloys, are used in automotive and various other high volume applications.

Bicycles

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

Background

Magnesium was first recognised as an element in 1755 by Black however, it was not isolated until 1808 by Davy who evaporated the mercury from a magnesium amalgam made by electrolysing a mixture of moist magnesia and mercuric oxide, and was not prepared in its coherent form till 1831 by Bussy.

Occurrence

Commercial Production

Key Properties

Physico-Chemical Properties

Environmental Behaviour

Applications

Magnesium Metal

Magnesium in the metal form is used in:

·        An alloying element in aluminium, zinc, lead and other non-ferrous alloys.

·        Cathodic protection of other metals against corrosion.

·        Flashlight photography, photoengraving plates, flares and pyrotechnics (which includes incendiary bombs).

·        Airplane alloys and missiles construction.

·        As a reduction agent for the production of pure uranium from its salt.

·        Building construction as a flashing material.

·        Ammunition.

·        Its alloys are implemented in lightweight applications, auto parts, aerospace equipment, power tools, sporting goods, fixtures and materials-handling equipment.

Magnesium Compounds

Background

Macor is a tradename for a machinable glass ceramic grade from Corning.

Glass Ceramics

Composition

Key Properties

·        Key properties include:

·        Macor is white in appearance and looks very similar to porcelain

·        Continuous usage temperature limit of 800°C

·        Peak usage temperature of 1000°C

·        Has a similar co-efficient of thermal expansion to most sealing metals and glasses

·        It is non-wetting

·        It is non-porous

·        Excellent insulator at high voltages, various frequencies and high temperatures

·        Properly heat treated materials will not outgas in vacuum environments

·        It can be machined with high speed steel and carbide tools

·        Can be machined into complex shapes

·        Can be machined to tight tolerances (up to 0.005”)

·        Can be machined to a surface finish of 20µin

·        It can be joined and sealed to itself and other materials using processes such as metallising, sputtering, soldering and brazing, as well as adhesives such as epoxies.

·        It has no known toxicological effects, but exposure to dust should be avoided as much as possible as it can be an irritant

Applications

Macor has been used in the following areas:

·        Vacuum seals and in vacuum environments

·        Aerospace

·        Nuclear and radiation environments

·        Welding nozzles

·        Engineering components

·        Medical equipment.

Background

This Machineable Ceramic is a white ceramic which can be machined with ordinary steel or carbide tools. It has been used extensively in the Space Shuttle Orbiter. It can be used continuously up to 800°C and is a good electrical and thermal insulator. Outgassing in ultra-high vacuum environments can be eliminated, if degassed before use. MACOR® is non-wetting and can be bonded to itself as well as to various metals, if the MACOR® surfaces are first metallised.

Key Properties

Background

200 million tungsten bullets a year, using an ounce of tungsten each. That makes more than 5,500 t, or one eighth of existing annual tungsten consumption in the world.

Green Bullets

In 1999 the US Army began manufacturing “green” bullets. The bullets - which are used primarily for shooting practice during peace time - are as deadly to humans as their predecessors but less deadly to the Earth.

Lead bullets, which the Army currently uses, tend to bioaccumulate in the environment, often ending up in sediments, surface water, and groundwater, according to A Multimedia Strategy for the Management and Reduction of Lead Hazards released by the US Environmental Protection Agency Region 5. Accumulated lead can adversely affect wildlife and people who get their drinking water from a contaminated source, according to the report. Lead slugs are such a water quality hazard that the federal district court in New York ruled that spent lead shot is a “pollutant” as defined by the Clean Water Act.

The lead slugs the Army uses in traditional 5.56 mm bullets have been bioaccumulating at shooting ranges