Production, properties and industrial uses of magnesium and its alloys

Usage of magnesium alloys for aerospace applications has declined and consumption for nuclear power is static. Specialized alloy development for these markets has continued but at a slower pace. Consumption for general engineering structural applications has increased slowly, mainly as pressure die castings. Developments include fluxless melting, hot-chamber casting, and alloy development for improved creep resistance, castability, and electroplating characteristics.

The most notable increases in consumption have been for alloying into aluminum and for uses in metallurgical treatment such as production of nodular iron. The restricted use as an engineering material is ascribed to unfavorable economics rather than technical limitations. The most significant developments are therefore in extraction technology where new processes should reduce extraction costs and where the overall trend is towards larger units giving an economic advantage of scale.

Developments for general engineering applications are strictly conditioned by the dictates of economics and have tended to follow a separate pattern, although some overlap has occurred.

Castings

The sand casting technique (mainly for aerospace applications) involves use of small additions of SF₆ or CO₂ into air, but for aerospace castings the gas mixture has been more frequently applied to replace SO₂ while pouring and during heat treatment. Most of the other developments in foundry technology represent adoptions to magnesium practice of general foundry developments designed for such requirements as greater dimensional accuracy, production of thinner sections, more effective use of skilled labour, etc.

The majority of magnesium pressure die castings are made in alloys containing 8-9% Al. Alloy development to meet specific requirements includes use of AM60 (6% Al) to provide the better impact strength required for automotive wheel castings and AS41 (4% Al, 0,7% Si) for the improved creep strength required for later versions of the Volkswagen air-cooled engine.

AS21 (2% Al, 0,7% Si) provides still better creep strength but suffers, as do all these alloys with aluminum contents below about 8%, from reduced castability. Alternative alloys, involving the addition of calcium or rare-earth metals to the general purpose aluminum-containing alloys do not appear to have been adopted commercially, again probably owing to poor castability.

Magnesium-alloy castings are also produced in limited quantities by the gravity die process. The low-pressure process is used, notably in Italy for the production of automotive wheels.

An account has been given of a novel process designed to meet a specific needs. The requirement was for small cylindrical pieces of a magnesium alloy for administration to cattle. The density required was higher than that available from conventional magnesium-base alloys and the chemical and physical nature of these alloys were constrained by possible physiological effects in the cattle. The problem was solved by heating a magnesium-aluminium alloy to a temperature between its solidus and liquidus points, stirring in iron shot to give the required density and then moulding the mixture in a multicavity die by a simple pressing operation.

Wrought forms

Most of the stock for subsequent working is produced by the direct chill process but, compared with advances in other metals, the process has seen negligible development for magnesium in the past decade. Products are limited to cylindrical stock for extrusion, forging, conversion to powder and slab for rolling. As far as is known all products are made by the vertical drop technique. Melting is normally carried out in gas or oil-fired steel crucibles and the use of pumps or siphon devices to transfer metal to the mould is common.

For general engineering purposes alloy development has been primarily directed towards improvement in working characteristics with retention of medium strength. With ZM21 alloy (Mg2Zn1Mn) formation of the zinc-rich beta phase is suppressed by the addition of manganese and the resultant increase in solidus temperature enables hot deformation to be carried out significantly faster than, for example, with the more generally used AZ31 alloy. The improvement also extends to rolled forms where ZM21 alloy is less prone to edge cracking. Similar advantages are claimed for magnesium-zinc alloys containing calcium, rare-earth metals, and silicon.

The need to use elevated temperatures for deformation of magnesium has lead in the past to the production of complex pressed shapes reminiscent of those obtainable in superplastic alloys. The very fine structure obtainable by addition of zirconium has also appeared a good base for the superplastic phenomenon.

Powder-compacted and fibre-strengthened products

American work on atomized powder-compacted alloys which resulted in the extrusion alloy ZK60B (Mg-6Zn-0.6Zr) is described few years ago. The suppression of grain growth, ascribed partly to the cored structure of the powder and partly to the oxide coating on the particles, leads to high tensile strength coupled with high compressive strength, a combination difficult to achieve with more conventional wrought magnesium alloys.

Applications

Structural engineering applications. The variety of applications of magnesium in aerospace engineering has declined during the past decade for a number of reasons. In airframe construction the possible effects of corrosion, or the unacceptable cost of preventive maintenance, have generally limited use of magnesium to components which are readily accessible for inspection.

The increased power available from modern engines has reduced the need for weight reduction to some degree. For engine components such as compressor housings the development of increased power has resulted in operating temperatures beyond the capability of light alloys. Magnesium alloys are, however, still used extensively for the cooler engine components and notably for the massive gear-box castings required by helicopters. Many other structural components, including landing wheels, are still in regular production.

Wrought magnesium has been more widely applied to general engineering applications in the USA than in Europe and typical American applications include items such as step ladders, sack trucks, bakery delivery racks, etc.

More interest has focused on development of cast products, particularly those made by pressure die casting, notably in Germany and the USA. Magnesium wheels have been standard equipment on racing cars for many years but are now also used as standard or optional equipment by the car enthusiast and in more expensive models. Production of pressure die cast wheels has been established and many are produced by the low-pressure process. The total usage by Volkswagen has declined as their air-cooled engine has been phased out, but this company still remains the largest producer and user of magnesium structural castings, notably for gear boxes. Magnesium castings are used extensively in chain saws and some European manufacturers use them for electric drills.

Products and applications in chemical and metallurgical treatment. This broad category covers a variety of applications where magnesium is used as a chemical (in production of titanium, zirconium, as flares for production of light, etc.), for its electrochemical characteristics (cathodic protection, primary batteries), or for metallurgical treatments such as in nodular iron, steel desulphurization, etc. A number of specific products and compositions have been developed for these applications.

Powder and granulated forms. The majority of magnesium powder is produced by mechanical comminution, typically with a milling cutter. The initial product may be milled to produce a finer or more rounded particle. Magnesium powder is also produced by gas jet or centrifugal disintegration of molten metal. In the latter cases an atmosphere inert to molten magnesium must normally be used.

Additives for treatment of ferrous metals. The addition of magnesium to molten cast iron, to produce nodular iron, has been a routine practice for over 25 years. The majority of such iron is made by treatment with ferrosilicon containing 5 or 10% magnesium. Within the past decade considerable interest has developed in the use of magnesium for desulphurizing blast furnace iron. The increased scale of operation and limitations on the composition of the additive have required special developments.

Primary battery anodes. Magnesium alloys are used in seawater-activated primary batteries, usually with silver chloride cathodes. This system was developed to use the magnesium alloy AZ61 (Mg-6Al-1Zn). Although the theoretical open circuit EMF for this type of cell is 2.6 V practical working voltages are of the order of 1.1 V.

Uses in cathodic projection. Cast magnesium anodes are usually supplied in AZ63 (Mg-6Al-3Zn) and extruded shapes in AZ61 (Mg-6Al-1Zn) alloys. AM503 (1.25% Mn) is used for higher performance requirements.