Magnesium

Design Notes - Magnesium and Magnesium Alloys

1. Introduction:
Un-alloyed magnesium has too low a strength for any engineering use. Although magnesium alloys have a lower strength (160 to 365 MPa) than aluminium alloys, their low density (magnesium is the least dense of any structural metal) about 1750 kg/m³, means they have as high a strength to density ratio. The Young's modulus is 45 GPa.

The fatigue endurance strength of magnesium alloys is about 70 to 100 MPa for smooth specimens, but the presence of notches or small radii will greatly reduce it.

Magnesium and its alloys have comparatively high damping capacities compared to other metals, up to about 3 times that of cast iron and up to about 30 times that of aluminium.

2. Types of Alloys:
Three main groups of magnesium alloys are in common use:

i) Magnesium-manganese alloys - used mainly for sheet metal fabrications, they are readily weldable.
ii) Magnesium-aluminium-zinc alloys - used mainly for gravity die and for sand casting as well as extrusions and forgings. Can be heat treated by solution treatment and precipitation hardening.
iii) Magnesium with rare earths including zirconium (which imparts a fine grain structure for improved mechanical properties) are used in both cast and wrought form.

3. Codes Used to Specify Magnesium Alloys:
The British Standard uses the letter 'MAG' followed by a number. The ASTM uses two letters and numbers, firstly two letters indicate the major alloying elements (A - aluminium, B - bismuth, C - Copper, D - cadmium, E - rare earth, F - iron, H - thorium, K - zirconium, M - manganese, Z - zinc, etc.) which are followed by two numbers indicating the nominal percentage contents of these two elements. Where an alloy is hear treated, the letters and numbers are followed by a dash and a letter (and possibly a number indicating the temper).
In the Unified Numbering System (UNS) the numbers M10001 to M19999 have been reserved for magnesium and magnesium alloys.

4. Typical Applications:
Components for textile and printing machinery, vehicles (engine covers and sumps, seat frames, steering wheels), aircraft, hand held power tools, camera and mobile phone bodies, cases for portable computers.
Uses in automotive engine and driveline systems is limited by the tendency of many alloys to creep and lose clamping force at temperatures above about 100^oC. A lot of research is curently going into developing new alloys more resistant to creep.

5. Potential Hazard
In large sections magnesium is difficult to ignite (high thermal conductivity reduces this danger) however dust and fine chips are easy to ignite and suitable precautions should be taken.

6. Processing
Magnesium alloys are weldable by any of the processes that can be used on aluminium, eg: MIG, TIG, Laser Beam.
The machinability of magnesium and it alloys is excellent.
Magnesium alloys can not be cold worked, they work harden very rapidly at room temperatures.

7. Corrosion Resistance
Magnesium and magnesium alloys are vulnerable to corrosion. To minimise this impurity levels, e.g. iron, need to be kept to very low levels. Because some of the traditionally used corrosion protection coatings were unfriendly to the environment, more acceptable coatings and sealers are being developed.

For further information link to: MetalMart International Inc. (data in imperial units only!).

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David J Grieve, Updated: 16th March 2007, original: 13th December 2000.