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.