Aluminium Casting Alloys - Aleris

Heat treatment of aluminium castings
Heat treatment gives users of castings
the possibility of specifi cally improv-
ing the mechanical properties or even
chemical resistance. Depending on the
casting type, the following common and
applied methods for aluminium castings
can be used:
Stress relieving
Stabilising
Homogenising
Soft annealing
Age-hardening.
The most important form of heat treatment
for aluminium castings is artifi cial
ageing. Further information is provided
below.
40
Aluminium Casting Alloys
Ageing time [h]
Metallurgy – fundamental principles
For age-hardening to take place, there
must be a decreasing solubility of a par-
ticular alloy constituent in the α-solid so-
lution with falling temperature. As a rule,
age-hardening comprises three steps:
In solution annealing, suffi cient amounts
of the important constituents for age-
hardening are dissolved in the α-solid
solution.
With rapid quenching, these constituents
remain in solution. Afterwards, the parts
are relatively soft.
Figure 11.1
Yield strength of
gravity die cast test bars (Diez die) in Al Si10Mg alloy
Yield strength R p0,2
[MPa]
280
240
200
160
120
0
0 2 4 6 8 10 12 14 16
160 °C 180 °C 200 °C
As-cast state
In ageing, mostly artifi cial ageing, precipitation
of the forcibly dissolved components
takes place in the form of small
sub-microscopically phases which cause
an increase in hardness and strength.
These tiny phases, which are technically
referred to as “coherent or semicoherent
phases”, represent obstacles
to the movement of dislocations in the
metal, thereby strengthening the previously
easily-formable metal.
The following casting alloy types are
age-hardenable:
Al Cu
Al CuMg
Al SiMg
Al MgSi
Al ZnMg.
Figure 11.2
Elongation of
gravity die cast test bars (Diez die) in Al Si10Mg alloy
Elongation A5 [%]
5
4
3
2
1
0
0 2 4 6 8 10 12 14 16
Ageing time [h]
160 °C 180 °C 200 °C
As-cast state