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least 15%. This is “critical work
hardening”. If this condition is
not met, then heat treatment
must be restricted to restoration
without recrystallization,
• A rapid temperature gradient
of 20 to 60°C per hour,
• Temperatures over 350 to
380°C must be avoided,
• Holding times must be limited
to 2 hours maximum.
For 5000 series alloys, annealing is
usually performed between 320°C
and 380°C for 30 to 120 minutes.
Note: Non-heat treatable alloys in
the annealed temper (O, H111)
can only be brought to higher
strength by work hardening.
8.3. Heat treatable alloys
If some plastic deformation must
be done on products of heat
treatable alloys, it should be carried
out in the T4 temper; first the
allowable degree of deformation
is bigger than for the T6 temper
and second there is almost no
effect of work hardening. If for
the final product e.g. a bent
extrusion in a 6XXX alloy T6 temper
is needed, age hardening can
be carried out. Table V.8 gives an
indication how to proceed, using
typically a hot air furnace.
6000 SERIES ARTIFICIAL AGEING
Alloy Initial temper Artificial ageing Final Temper*
6060 T1 - T4 6 h at 185°C T5 - T6
or 8 h at 175°C
6005 T1 - T4 8 h at 175°C T5 - T6
6106 T1 - T4 8 h at 175°C T5 - T6
6061 T4 8 h at 175°C T6
6082 T1 - T4 16 h at 165°C
or 10 h at 170°C T5 - T6
or 8 h at 175°C
* T5, for an initial temper of T1, T6 for an initial temper of T4.
TABLE V.8 (typical)
53
EUROPEAN ALUMINIUM ASSOCIATION
8.4. Castings
Casting is the shortest path
from molten metal to the finished
product. It is recommended
for geometrically-complex
parts. It is advantageous to
involve the foundry from the
conceptual phase into the
design process. The expert of
the foundry, knowing the plants
equipment, the process of making
the mould, the flow of metal
into the mould, the cooling and
shrinking of the cast piece etc.
can be of great help during the
design phase. When the design
of a casting is optimized in view
of its production it is in most
cases possible for the foundry to
guarantee much better mechanical
properties than those listed
in standard EN 1706, especially
with respect to elongation.
The Table V.5 (see section 6.3)
merits some explanation.
The alloy 21100 needs very
careful design of the pieces with
respect to the casting process
and, in addition to that, the
metal treatment in the foundry,
especially the degassing of the
molten metal, must be carried
out very carefully in order to
minimize micro-porosity.
The index B or C in the column
“Machinability” has been put
because of the wearing of the
cutting tools due to the high silicon
content of the alloys.
The corrosion resistance of cast
pieces with the as-cast surface is
better than for machined surfaces
of the same piece due to
the much thicker oxide layer.
Design of casting parts
Generally speaking it is essential
to be aware of production possibilities
and limitations from the
initial development stage of a
new component, not just in
terms of the choice of alloy and
casting technique but also in
terms of design. There are a
number of basic rules which
designers should follow:
• Sections should be kept uniform
and thickness transitions
should be smooth, avoiding a
build-up of metal at intersections
so as to reduce the risk of shrinkage
porosity during cooling,
• For the same reason, isolated
bosses should be avoided and
walls must be correctly sized to
assist running,
• There should be a fillet at
every inside corner to avoid
cracking during the casting
operation (this is particularly
important for 21100 alloys),
• The filling design should be
somewhat asymmetrical to
ensure controlled solidification
and uniform feed,
• The number of intersections
and undercuts should be kept to
a minimum as they complicate
tooling and the casting operation
and hence increase the
cost. This applies equally to
deburring operations,
• The choice of dimensional
tolerances must allow for the
casting technique and any subsequent
heat treatment deformation
can occur during solution
treatment and quenching.
Flywheelhouse casting
for truck engine (Brabant Alucast)
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