Note: Descriptions are shown in the official language in which they were submitted.
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Casting of an Aluminium Alloy
The invention concerns a casting of an aluminium alloy with good heat
resistance.
s For thermally stressed components today normally AISi alloys are used, where
the
heat resistance is achieved by the additi~~n of Cu to the alloy. Copper,
however,
also increases the heat crack tendency and has a negative effect on the
castability. Applications in which particular heat resistance is required
normally
occur in the field of cylinder heads in automobile construction, see e.g. F.J.
Feikus,
~o "Optimisation of Aluminium Silicon Casting Alloys for Cylinder Heads",
Giesserei-
Praxis 1999, Vol. 2, pages 50 - 57.
WO-A-0043560 discloses an aluminium alloy with 2.5 - 7.0 w.% Mg, 1.0 - 3.0 w.%
Si, 0.3 - 0.49 w.% Mn, 0.1 - 0.3 w.% Cr, max. 0.15 w.% Ti, max. 0.15 w.% Fe,
~5 max. 0.00005 w.% Ca, max. 0.00005 w.% Na, max. 0.0002 w.°!°
P, other
contaminants individually max. 0.02 w.% .and aluminium as the remainder, for
the
production of safety components in diecasting, squeeze casting, thixoforming
and
thixoforging processes.
2o The invention is based on the object of preparing an aluminium alloy with
good
heat resistance suitable for the production of thermally stressed components.
The
alloy is particularly suitable for gravity diec:asting, low pressure chilled
casting and
sand casting.
2s Components cast from the alloy should gave a high strength in connection
with
high ductility. The desired mechanical properties of the component are defined
as
follows:
Yield strength Rp0.2 > 170 MPa
3o Tensile strength Rm > 230 MPa
Elongation at fracture A5 > 6%
Because of the applications, the corrosion tendency of the alloys should be
kept
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as low as possible and the alloy muss: have a correspondingly good fatigue
strength. The castability of the alloy should be better than that of the
AISiCu
casting alloys which are currently used, and the alloy should have no tendency
to
heat cracks.
The term "casting" includes, as well as the pure components produced solely by
casting, those cast as a premould and subsequently formed to the final
dimensions by hot or cold shaping.
o Examples of pure castings are those which are produced exclusively by sand
casting, gravity diecasting, low pressure chilled casting, diecasting,
thixocasting or
squeeze casting.
Forming operations performed on a cast premould by shaping are for example
~5 forging and thixoforging.
The object according to the invention is achieved by an aluminium alloy with
2 to 4 w.% magnesium
0.9 to 1.5 w.% silicon
20 0.1 to 0.4 w.% manganese
0.1 to 0.4 w.% chromium
max. 0.2 w.% iron
max. 0.1 w.% copper
max. 0.2 w.% zinc
25 max. 0.2 w.% titanium
max. 0.3 w.% zirconium
max. 0.008 w.% beryllium
max. 0.5 w.% vanadium
with aluminium as the remainder, with farther elements and production-induced
3o contaminants individually max. 0.02 w.%, total max. 0.2 w.%.
The following content ranges are preferred for the individual alloy elements:
Mg 2.5 to 3.5 w.%, in particular 2.7 to 3.3 w.%
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Si 0.9 to 1.3 w.%
Mn 0.15to0.3w.%
Cr 0.15to0.3w.%
Ti 0.05 to 0.15
w.%
Fe max. 0.15 w.%
Cu max. 0.05 w.%
Be 0.002 to 0.005
w.%
V 0.01to0.1w.%
Zr 0.1 to 0.2 w.%
The effect of the alloy elements can be characterised approximately as
follows:
Silicon in conjunction with magnesium leads to a corresponding hardening where
in particular thermal hardening is of interest. Preferred is heat treatment to
a state
T6 e.g. solution annealing at 550°C for 1 ~ hours with subsequent
artificial ageing
at 160 - 170°C for 8 to 10 hours.
The combination of manganese and chromium leads to good heat resistance at a
sustained temperature of up to 180°C.
Titanium and zirconium are used for grain refining. Good grain refining makes
a
substantial contribution to an improvement: in casting properties.
Beryllium in conjunction with vanadium reduces the dross formation.
A preferred area of application of the castings according to the invention is
thermally stressed components, in particular pressure vessels, compressor
housings and engine components su~;h as cylinder heads in automobile
construction. The components are preferably produced in the sand casting or
chilled casting process.
Further advantages, features and details cf the invention arise from the
description
below of preferred embodiment examples and the drawing which shows:
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Figs. 1 - 3 tensile strength, yield strength and elongation at fracture as a
function of temperature after 500 hours sustained temperature load
for an alloy according to the invention and a comparison alloy
according to the prior art.
An alloy according to the invention reference AIMg3Si1 MnCr and a comparison
alloy reference AISi7MgCu1 by F.J. Feilcus, "Optimisation of Aluminium Silicon
Casting Alloys for Cylinder Heads", Giesserei-Praxis 1999, Vol. 2, pages 50 -
57,
with the compositions given in table 1, were compared with regard to long-term
behaviour under sustained temperature load.
Table 1: Chemical Composition of Alloys (in w.%)
Alloy Si Fe Cu Mn Mg Cr Zn Ti Be V Zr
AISi7MgCu16.97 0.110.940.0050.;38 0.0080.03
AIMg3Si1MnCr1.10 0.070.0010.20 3.2 0.210.0020.120.0030.030.0005
The alloy according to the invention was cast in a trial rod mould according
to Diez
for round rods 16 mm diameter. The rnechanical properties of yield strength
(Rp0.2), tensile strength (Rm) and elongation at fracture (A5) were determined
on
the trial rods in state T6 (165°C/6 hours) after a sustained
temperature load of 500
2o hours at various temperatures. The corresponding values for the comparison
alloy
were taken from the above article by F.J. I=eikus. The results are shown in
fig. 1 in
diagram form.
The alloy AIMg3Si1 MnCr according to the invention admittedly does not reach
the
peak values of the comparison alloy AISi7MgCu1 with regard to yield strength
and
tensile strength, but in its temperature behaviour is "less changeable". This
changeability has a disruptive effect in operation insofar as slight changes
in
temperature can cause great changes in mechanical properties. The yield
strength
of the alloy according to the invention remains at around the same level up to
3o around 180°C, gradually falls away up to 200°C, and only
above around 200°C
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begins to decrease continuously. The continuous decrease takes place with a
lesser gradient than the alloy AISi7MgCu1.
With regard to the elongation at fracture, the alloy according to the
invention is
s characterised by an almost constant value up to 180°C. High
elongation values
give a favourable fracture/failure behaviour. A visible deformation precedes
the
break of the component. Above 180°C the elongation rises continuously.
In the
comparison alloy AISi7MgCu1, the cle~~r hardening effect can be seen. Low
elongation values cause an unfavourable failure behaviour i.e. the component
only
~o deforms slightly or not at all. Under loa~~ peaks the component breaks
without
warning.
