Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
Aluminium–magnesium alloys, with Mg contents of 4–10%, are
characterized by a high resistance to corrosion. Only Al–10% Mg castings
respond to heat-treatment. Aluminium–zinc–magnesium alloys have a relatively
high eutectic melting point, which make them suitable for castings that are
assembled by brazing. The as-cast alloys respond to both natural and artificial
ageing.
Cast aluminium and its alloys may be grain refined by the addition of
suitable innoculants to the melt. For example, commercial additives based
on a master alloy of Al–Ti–B are widely used: these produce intermetallic
particles in the melt which act as centres of crystallization for the alloy.
Table 3.1 presents some properties of a selection of cast aluminium alloys.
Wrought aluminium alloys
About 85% of aluminium is used for wrought products, produced from cast
ingots by rolling, extrusion, drawing etc. An outline is given in Table 3.2 of
the International Alloy Designation System employed for these materials.
We will consider these alloys in two groups, namely those the properties of
which are not enhanced by heat-treatment, and those that are.
Non-heat-treatable alloys
There are two important families:
Aluminium–manganese alloys (3xxx series) contain up to 1.25% Mn, which
gives rise to solution-hardening. The further addition of magnesium gives a
further increase in strength, coupled with high ductility and excellent corrosion
resistance. These alloys are widely used for cooking utensils, as well as for
beverage cans.
Aluminium–magnesium alloys (5xxx series) contain up to 5% Mg. The
alloys owe their strength to work hardening, which occurs at a rate that
increases as the Mg content is raised. Over a period of time, the tensile
properties may decline due to localized recovery, but special tempers may be
Table 3.1 Typical mechanical properties of some cast aluminium alloys
Alloy Condition Density Young’s Proof UTS Elongation
(Mg m –3 ) modulus stress (MPa) (% in 50 mm)
(GPa) (MPa)
Al–11.5Si (LM6) Sand cast 2.65 71 65 170 8
Al–5Mg–0.5Mn Sand cast 2.65 71 100 160 6
(LM5)
Al–6Si–4Cu– Sand cast ~2.7 71 130 180 1
0.2 Mg–1Zn
(LM21)