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7.2. Low and very low
temperatures
Contrarily to most other engineering
metals, the mechanical
properties improve at low temperatures
and especially the
elongation, which makes aluminium
an ideal metal for severe
winter conditions and even
cryogenic applications (see
Figure V.1)
Further examples can be found in
standard EN 12392 “Aluminium
and aluminium alloys - Wrought
products - Special requirements
for products intended for the production
of pressure equipment”.
CHANGE OF MECHANICAL CHARACTERITICS
AS A FUNCTION OF TEMPERATURE FOR
ALLOY 5086 O
FIGURE V.1
7.1. Elevated
temperature
The loss in strength at higher
than ambient temperatures is
negligible for temperatures up
to 100°C (short time exposure)
or 80°C (long time exposure).
When subjected to even higher
temperatures, then the loss in
mechanical properties is moderate
for non-heat treatable alloys
in the O/H111 temper and for
heat treatable alloys in the
T1/T4 temper.
The loss in mechanical properties
at temperatures above
100°C is very pronounced for
non heat treatable alloys in the
H12, H16 temper as well as for
heat treatable alloys in the
T5/T6 temper.
51
EUROPEAN ALUMINIUM ASSOCIATION
8.1. Work hardening of
non-heat treatable alloys
Hardening is achieved by cold
deformation, known as work
hardening, that improves the
physical properties and the hardness
of the metal. It also reduces
the metal’s capacity for deformation
and its ductility (Figure V.2).
The greater the deformation or
higher the work hardening rate,
the more pronounced is the
effect. It is also governed by the
composition of the material.
The 5083 alloy, for example,
which contains between 4 and
4.9% of magnesium, acquires a
great hardness but its capacity
for deformation is less than that
of the 5754 alloy which contains
between 2.6 and 3.6% Mg.
Work hardening is a general phenomenon
that takes place whatever
the method of deformation
used: rolling, deep drawing, folding,
hammering, bending, pressing,
etc. This means that it will
also occur during fabrication in
the workshop.
8. Influence of fabrication
on the properties of the alloys
MPa A%
Rm
A
Rp 0,2
Temper 0 H12 H14 H16 H18
30
20
10
Work hardening % 10 20 30 40 50 60 70 80
500
400
300
200
100
WORK HARDENING CURVE OF ALLOY 5083
FIGURE V.2
8.2. Softening
by annealing and recovery
It is possible to restore the ductility
of the work hardened metal by
heat treatment known as
“annealing” (partial or full
annealing). In this process, which
takes place at temperatures
between 150°C and 350°C, the
hardness and mechanical characteristics
of the metal slowly begin
to decrease: this is the recovery
phase [A-B] (Figure V.3). At lower
annealing temperatures this leads
to medium-strength material
properties. They then fall away
more rapidly at high temperatures
above 280 °C during recrystallization
[B-C] and eventually
attain a minimum value that corresponds
to the mechanical characteristics
of the fully annealed
metal [C-D].
Restoration and annealing are
accompanied by a change in the
texture and size of the grains of
metal observed under a microscope
with X50 magnification.
The texture can change from a
fibrous structure to a fully recrystallized
structure (Figure V.3).
The grain can grow in size during
recrystallization and annealing.
This growth is revealed during
subsequent working, e.g. folding,
by the rough “orange peel”
effect on the surface of the
metal. Grain growth above
around 100 microns reduces the
deformation capacity of work
hardening aluminium alloys.
The following conditions are
essential if a fine-grained annealed
structure is to be achieved:
• The metal must have undergone
a sufficient rate of deformation
corresponding to a rela-
T - t
Ta
52
ALUMINIUM IN COMMERCIAL VEHICLES CHAPTER V ALUMINIUM ALLOYS FOR COMMERCIAL VEHICLES 52 | 53
Hardness
Recovery
Recrystallization
Annealing
Time
A
C
B
D
O
HARDNESS CURVE DURING ANNEALING
FIGURE V.3
Micrographic views
tive reduction in section of at
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