Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
Duralumin and is still widely used for aircraft construction. The addition of
copper is deleterious to the corrosion resistance of aluminium, so that sheet
material in this group of alloys is often roll clad with pure aluminium or Al–
1% Zn to produce a corrosion-resistant surface layer. This soft surface layer
can lead to serious deterioration of the fatigue resistance, however.
Aluminium–magnesium–silicon alloys (6xxx series) are medium-strength
alloys, widely used in the form of extruded sections for structural and
architectural application, window frames being a typical example. The alloy
can be quenched as it emerges from the extrusion press, thus eliminating the
need for a separate solution-treatment operation. Optimum properties are
again developed by a final ageing treatment.
Aluminium–zinc–magnesium alloys (7xxx series) are the highest-strength
family of aluminium alloys. They are readily welded and find wide structural
application. An addition of copper is made to reduce the susceptibility to
stress-corrosion cracking (SCC), and the Al–Zn–Mg–Cu alloys are widely
used in aircraft construction. Alloy 7075 is the most widely known of these
and often rather complex heat-treatments have to be applied in order to
minimize the propensity to SCC.
A new family of age-hardening aluminium alloys containing lithium (8xxx
series) has been developed in recent years. These alloys have the advantage
of a lower density and a higher value of Young’s modulus than the 7xxx
series. They are designed to substitute for conventional aircraft alloys, with
a density reduction of 10% and a stiffness increase of at least 10%. Their
purchase price is several times that of existing high-strength aluminium
alloys, so their overall economic advantages have to be very carefully weighed
when considering a potential application.
Table 3.3 gives the mechanical properties of some wrought aluminium
alloys.
3.2.2 Magnesium alloys
Cast magnesium alloys
Up to 90% of magnesium alloys are produced as castings, widely used
in the aerospace industries. Magnesium–aluminium alloys contain 8–9%
Al with up to 2% of zinc to increase the strength and 0.3% Mn, which
improves the corrosion resistance. From the Mg–Al phase diagram, Fig.
3.10, it will be seen that increasing quantities of the β-phase (Mg 17 Al 12 ) will
be formed as the Al-content increases above 2%. This is accompanied by an
increase in proof stress and a decrease in the percentage elongation in the
material. Grain-refined castings are produced by adding zirconium and a
series of Mg–Zn–Zr alloys have been developed which also respond to agehardening.