Page 62 - Ship Construction.DJ Eyres 6Ed

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Aluminium Alloy 51
aluminium alloy superstructure, principally to clear a fixed draft over a river
bar with maximum cargo.
The total construction in aluminium alloy of a large ship is not consid-
ered an economic proposition and it is only in the construction of smaller
multi-hull and other high speed craft where aluminium alloys higher
strength to weight ratio are fully used to good advantage.

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Production of Aluminium
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
For aluminium production at the present time the ore, bauxite, is mined
containing roughly 56 per cent aluminium. The actual extraction of the
aluminium from the ore is a complicated and costly process involving two
distinct stages. Firstly the bauxite is purified to obtain pure aluminium oxide
known as alumina; the alumina is then reduced to a metallic aluminium.
The metal is cast in pig or ingot forms and alloys are added where required
before the metal is cast into billets or slabs for subsequent rolling, extrusion,
or other forming operations.
Sectional material is mostly produced by the extrusion process. This
involves forcing a billet of the hot material through a die of the desired shape.
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More intricate shapes are produced by this method than are possible with
steel where the sections are rolled. However, the range of thickness of sec-
tion may be limited since each thickness requires a different die. Typical
sections are shown in Figure 6.1.

ALUMINIUM ALLOYS Pure aluminium has a low tensile strength and is
of little use for structural purposes; therefore the pure metal is alloyed with
small percentages of other materials to give greater tensile strengths. There
are a number of aluminium alloys in use, but these may be separated into
two distinct groups, non-heat treated alloys and heat treated alloys. The
latter as implied are subjected to a carefully controlled heating and cooling
cycle in order to improve the tensile strength.
Cold working of the non-heat treated plate has the effect of strengthening
the material and this can be employed to advantage. However, at the same
time the plate becomes less ductile, and if cold working is considerable the
material may crack; this places a limit on the amount of cold forming
possible in ship building. Cold worked alloys may be subsequently subjected
to a slow heating and cooling annealing or stabilizing process to improve
their ductility.
With aluminium alloys a suitable heat treatment is necessary to obtain a
high tensile strength. A heat treated aluminium alloy which is suitable for
shipbuilding purposes is one having as its main alloying constituents mag-
nesium and silicon. These form a compound Mg Si and the resulting alloy
2
has very good resistance to corrosion and a higher ultimate tensile