The main components of the alloys are magnesium and silicon to form Mg2Si.
There is often an iron corrector such as manganese or chromium; occasionally small
amounts of copper or zinc to improve the strength without substantial loss of corrosion
resistance; boron in conductors to remove titanium and vanadium; zirconium or
titanium to control the grain size. Lead and bismuth are sometimes added to improve
machinability, but they are less effective than in magnesium-free alloys.
The composition range of the alloys now in use is:
up to 2.0%
up to 2.5%
up to 1.5%
B, Ti, Zr
up to 0.3%
up to 0.5%
up to 1%
The proper ratio for Mg2Si is Mg/Si = 1.73 but this is impossible
to achieve with ordinary operating tolerances; thus, most alloys have either
magnesium or silicon excess. Magnesium excess leads to better corrosion resistance
but lower strength and formability; silicon excess produces higher strength without
loss of formability and weldability, but some tendency to intergranular corrosion.
The structure of the alloys is relatively simple: the main constituent is
Mg2Si, which in the heat treated condition is in solution and to which
is due the age hardening after artificial aging. If sufficient copper and silicon are
present, it may be replaced at least partly by
Cu2Mg8Si6Al5, which will produce some
hardening also with natural aging.
Iron may be present as FeAl3, FeAl6, Fe2SiAl8
or FeMg3Si6Al8 in manganese-and chromium-free alloys;
in manganese- and chromium-bearing alloys it combines with them. Zinc is in solid
solution; boron, titanium and zirconium are seldom added in amounts sufficient to
produce visible compounds. Spheroidising of Mg2Si by additions of phosphorus,
sulfur, tellurium, iodine or ferric chloride is reported.
The lattice parameter of the commercial alloys is controlled by the magnesium and
silicon contents. Copper, manganese, chromium and zinc are usually present in amounts
too small to have a measurable effect; iron, titanium, boron, etc., have no substantial
effect. In most alloys the amount of elements other than magnesium and silicon is well
below 1% and the total is seldom above 3%. Thus, many properties do not differ
substantially from those of the ternary alloys or even of pure aluminum.
Surface tension, density, thermal expansion, length changes in aging, specific heat,
shrinkage in freezing and super conductive temperature are within error of
determination of the corresponding values for the high-purity alloys or pure
aluminum. Thermal conductivities are some 10-20% lower than for pure aluminum.
Electric resistivity is slightly higher than in the ternary aluminum-magnesium-silicon
alloys and of the order of 3,0-3,2 x 10-8Ωm (50-55% IACS) for alloys
with 0.4-0.5% Mg and a slight excess of silicon, in the artificially aged
condition. Higher magnesium, manganese and copper reduce this conductivity, which is
also lower in the naturally aged temper, but may reach values of 55-60% IACS in
annealed material. Alloys with lower magnesium and silicon contents have better
conductivity but lower strengths. Increasing the iron or the silicon from 0.2 to 2%
decreases the conductivity by some 10%. Chromium, manganese and especially titanium,
vanadium and zirconium markedly reduce the electric conductivity. Additions of boron,
which precipitates these elements as borides, are used to remove them in conductor
Mechanical properties of the commercial alloys depend on content of Mg,
Si, Cu and other alloying elements, treatment conditions (cold or hot
treatment) and heat treatment. Commercial alloys, especially if they contain manganese
or chromium, may show strengths some 10% higher. High strain rates lead to somewhat
better properties. Fully hardened alloys show some tendency to intergranular fractures
in tension testing, but manganese additions reduce this tendency. Silicon precipitates,
as platelets, may be responsible for this brittleness.
Compressive strength is practically the same as tensile even at elevated temperatures.
Shear strength is of the order of 70% of the tensile and is not substantially affected
by subzero temperatures or nuclear radiation. The modulus of elasticity is of the order
of 65 GPa. Notch sensitivity is very low.
Fatigue resistance values of the order of 80-120 MPa are reported for wrought products,
as usual strongly affected by testing conditions, corrosion, notches and surface
finish. Small amounts of cold work have no effect on fatigue resistance; larger
amounts reduce it.
At low temperature the strength is increased, to reach at 70K values up to 80% higher
than at room temperature, without appreciable loss of ductility or fracture toughness.
Higher temperatures decrease strength and increase ductility; the decline is less rapid
than in most alloys and approaches that of pure aluminum. Heating faster than the metal
can expand reduces the properties.
One important feature of this alloys, which is very valuable in their use as electrical
cables, is that they can be under-aged without loss of corrosion resistance and thus
are fairly insensitive to long exposure to temperatures slightly above normal or to
short exposures to temperatures in the range 350-600K. Thus, electric cables can be
overloaded and iced cables can be de-iced by a surge of current to heat them, without
risk of permanent loss of strength.
Creep resistance is slightly below the yield strength for zero creep at room
temperature, decreasing proportionally with increasing temperature. High creep
resistance with conductivities above 50% IACS are claimed for alloys containing
?Ö1% Mg2Si and small amounts of copper, nickel, manganese, beryllium and
Iron has a mild strengthening effect up to 0.8-0.9% but with a severe loss of
ductility, especially in castings; when corrected with manganese, some increase
in toughness may result. Manganese, chromium, titanium and zirconium, all slightly
harden the alloys. Copper increases appreciably the strength. Nickel does not
improve mechanical properties.
The commercial alloys corrosion resistance is very good for their strength, although
somewhat inferior to that of pure aluminum or aluminum-magnesium. Alloys with
more than 0.3-0.4% Cu or an excess of silicon, however, are occasionally
susceptible to intergranular and stress corrosion, especially in the artificially
aged temper; up to 0.3-0.4% Cu reduces the pitting depth with only a slight
increase of weight loss.
Iron and nickel reduce corrosion resistance by increasing pitting. Manganese and
chromium absorb the iron and reduce its effect; they also combine with excess
silicon and tend to decrease its effect; zinc, like magnesium, tends to make the
matrix more negative but in amounts up to 0.5% has little or no effect on corrosion
resistance. Aluminum-magnesium-silicon alloy rivets, used with aluminum-copper sheet,
are rapidly destroyed in corrosive conditions.
The few alloys of this type used for casting contain approximately 2% Si, less
than 1% Mg and occasionally manganese. Castability is poor, eutectic content
being too low for good feeding. Best castability is at 2% Si. Sensitivity to
casting conditions is low; overheating of the melt has little effect on the
properties; but coarse interdendritic spacing may reduce the response to heat
Hot shortness is highest along the Al-Mg2Si line and decreases with
either magnesium or silicon excess. It reaches a maximum at approximately 0.5%
Mg2Si. Homogenization of billets improves hot workability,
especially if followed by a slow cooling to precipitate and spheroidise
Mg2Si. Material with coarse Mg2Si precipitate particles
has better hot deformability than material in which a fine distribution of precipitate
hinders the movement of dislocations. Two-step homogenization with the second step
as high as 850K produces maximum homogeneity. Grain refinement leads to improved
surface of extrusions, but lower formability; a small interdendritic spacing
Hot workability is mainly a function of the magnesium content. Silicon, copper and
iron have little effect; manganese, chromium and lead reduce it slightly. Chromium,
manganese and especially zirconium tend to enhance the press effect and produce
higher strengths. In alloys containing Mg2Si hot rolling tends to produce
spheroidising and coagulation of the Mg2Si particles; the number of
particles decreases as the rolling temperature is increased.
The alloys used most extensively, especially for extrusions in which price is the
most important consideration, have a low magnesium and silicon content, and usually
no manganese, chromium, etc. Best iron content is of the order of 0.15-0.20%; lower
iron leads to coarse grain, higher to deterioration of reflectivity and surface
Annealing for recrystallisation is done in the range 600-700K. Generally speaking
the higher the temperature the shorter the time. However, at the higher temperatures
magnesium and silicon tend to dissolve and yield a less soft material. Grain size
is controlled more by nucleation than growth; for fine grain fast heating is more
effective than large amounts of deformation. Recrystallisation temperature and
resulting softening are also controlled by previous history; material solution
treated and quenched before cold working and annealing softens more slowly and
less than material previously annealed in the recovery range.
Heat treatment of the alloys is not too critical; in many alloys the temperature at
which all the soluble constituents are dissolved is well below that of the beginning
of melting. Heat treatment temperatures range from 720 to 850K. Solution treatment
of wrought products requires very short times, reportedly of the order of seconds.
High-temperature deterioration may result in dimensional growth.
Repeated heat treatments have no substantial effects on properties, although report
loss of magnesium with prolonged heat treatment. Alloys with a low Mg2Si
content are not too sensitive to quench rates, but as magnesium, silicon, copper,
manganese, chromium are increased, sensitivity increases. With architectural
extrusions and some rolled products which are not too critical, quenching after hot
working without solution treatment can produce acceptable mechanical properties
after artificial aging, provided that working is done at temperatures above 720K.
Quenching directly to aging temperature may be better than water quench.
Natural aging is very slow and produces only limited hardening, especially in the
copper-free alloys. Artificial aging in the range from 400 to 500K for periods of
from 100 to 4 hours produces maximum hardening. Cold working after aging can be
used to increase strength, but with a significant loss of ductility. Increases of
up to 40% in ultimate tensile strength and up to 60% in yield strength can be
obtained by 75% deformation, but the elongation may decrease by 80%. Copper
reduces the effect of the delay in aging; iron and zinc do not have an appreciable
effect. Manganese and chromium reduce grain boundary precipitation, thus reducing
embrittlement and susceptibility to intergranular corrosion.
If the proper technique and filler material are used, the alloys are easily welded
both by fusion and by pressure. Strength of the fusion weld zone may be 80-90% of
the annealed strength, unless the weldments are heat treated afterwards, in which
case strength and fatigue resistance very close to the maximum can be achieved.
Porosity, cracks and flux inclusions reduce substantially the strength of welds,
as also does repeated welding. Fast welding produces better properties. Pressure
welds have the same strength as the base metal. Fatigue strength of butt welds is
Corrosion resistance of the welds is very close to that of the base metal.
Precipitation at grain boundaries due to the welding heat may cause brittleness
near the weld. High silicon in the weld, as is obtained by using aluminum-silicon
filler material, substantially reduces cracking; dilution of magnesium in the weld
zone leads to cracking. The cracking tendency is at a minimum in alloys
with 1% Si, 1% Mg, 0.2-0.6% Fe.
Machinability of the alloys is good, especially in the aged tempers. Iron and
manganese in normal amounts have little effect on machinability; excess silicon
reduces it slightly. Additions of cadmium, bismuth and lead do not improve
machinability as well as they do in magnesium-free alloys; magnesium combines
with them to produce hard particles that act as chip breakers but increase wear
of the tools.
These alloys are used extensively for architectural purposes, and for these
applications bright etching or electro polishing and anodizing are often used.
Reflectivity and surface finish are therefore important. Reflectivities of the
order of 75-80% can be obtained in commercial alloys, electro-polished and anodized;
use of high-purity material may raise the reflectivity to 85%. Coring, other
segregation, uneven cooling, undissolved constituents and high iron, lead to uneven
brightness, streaks and other surface defects.