ALLIAGES ALUMINIUM-SILICIUM POSSEDANT DES PROPRIETES MECANIQUES AMELIOREES

ALUMINUM-SILICON ALLOYS HAVING IMPROVED MECHANICAL PROPERTIES

Abrégé français

L'invention concerne un procédé de traitement thermique d'objets constitués d'alliages majoritairement Al-Si contenant une phase eutectique. L'invention concerne également des objets constitués de ces alliages. Selon l'invention, pour améliorer la ductilité du matériau et augmenter les valeurs d'allongement à la rupture, les objets sont soumis à un traitement de recuit, en particulier un recuit par choc consistant à chauffer rapidement les objets à une température de recuit comprise entre 400 et 555 ·C, à maintenir les objets à cette température pendant une durée de 14,8 minutes au maximum, puis à soumettre les objets à un refroidissement forcé entraînant un durcissement de ceux-ci par précipitation. Les objets selon l'invention comprennent des dépôts de silicium à sphéroïdisation dans la partie de phase eutectique, ces dépôts présentant une superficie en coupe moyenne A¿Si? inférieure à 4 µm?2¿ et/ou une distance moyenne .lambda.¿Si? entre les particules de silicium inférieure à 4 µm et/ou une densité de sphéroïdisation moyenne .xi.¿Si? supérieure à 10.

Abrégé anglais

The invention relates to a heat treatment method for articles composed of
substantially Al-Si alloys that contain a eutectic phase, and to articles that
consist of these alloys. In order to improve the ductility of the material or
to increase the elongation after fracture, an annealing process is carried out
in the form of shock annealing, said process comprising the following steps:
rapidly heating the material to an annealing temperature of from 400 to 555
~C, maintaining it at this temperature for a period of not more than 14.8
minutes, force-cooling it, and then aging the article. The inventive article
comprises spheroidized silicon precipitations in the eutectic phase portion
with an average plane of section ASi of less than 4 µm2 and/or an average
distance between the silicon particles .lambda.Si of less than 4 µm and/or
an average spheroidization density .xi.Si of greater than 10.

Revendications

10
We claim:
1. A thermal-treatment process for improving the ductility of articles
consisting of
a cast or wrought alloy with a eutectic phase, which contains preferably
refined
or purified aluminum-silicon or optionally other alloys and/or impurities,
said
articles being subjected to an annealing treatment and subsequent aging,
characterized in that the annealing is effected as shock annealing comprising
rapid heating of the material to an annealing temperature of 400°C to
555°C,
maintaining it at this temperature for a holding period from preferably at
least
1.7 minutes to at most 14.8 minutes, and subsequently force cooling it to
essentially room temperature.
2. The process as defined in claim 1, characterized in that the shock
annealing is
effected with a holding time of less than 6.8 minutes, preferably with a time
span
from at least 1.7 minutes to a maximum of 5 minutes if necessary.
3. The process as defined in any one of claims 1 or 2, characterized in that
the
aging of the article, which follows the shock annealing, is effected as
thermal
aging at a temperature in the range between 150°C and 200°C with
a period from
1 to 14 hours.
4. The process as defined in any one of claims 1 to 3, characterized in that
the aging
of the article that follows the shock annealing is effected as cold aging at
essentially room temperature.
5. An article of an aluminum-silicon alloy, optionally containing other alloys
and/or
impurities such as magnesium, manganese, iron and the like, with a eutectic
phase, consisting essentially of an .alpha.Al matrix and silicon exudates,
characterized
in that the silicon exudates are spheroidized in the eutectic phase and have
an
average cross-sectional area A Si of less than 4µm2.

11
<IMG>
A Si = average surface of the silicon particles in µm2
A = average surface of the silicon particles per image, in µm2
n = number of images sampled
6. The article as defined in claim 5, characterized in that the silicon
exudates in the
eutectic phase are spheroidized and have an average cross-sectional area of
less
than 2µm2.
7. An article of an aluminum-silicon alloy, optionally containing other alloys
and/or
impurities such as magnesium, manganese, iron and the like, with a eutectic
phase, consisting essentially of an .alpha.Al matrix and silicon exudates,
characterized
in that the average free path length between the silicon particles .lambda.Si
in the
eutectic phase defined as the root of a square measured surface divided by the
number of silicon particles contained within it is of as a size that is less
than
4um.
<IMG>
wherein .lambda. Si; = average spacing between of the silicon particles in
µ2
A Quadrat = square reference surface, in µm2
N Silicon = number of silicon particles
n = number of images sampled
8. The article as defined in claim 7, characterized in that the average free
path
lengths are of a size that is less than 3µm, and preferably less than
2µm.

12
9. An article of an aluminum-silicon alloy, if necessary with an enriching
element
and optionally with other alloys and/or impurities such as magnesium,
manganese, iron and the like, with a eutectic phase, consisting essentially of
an
.alpha.Al matrix and silicon exudates, characterized in that the average
spheroidization
density .zeta.Si, defined as the number of spheroidized eutectic particles per
100µm2
is greater than 10.
<IMG>
.zeta.Si = Mean spheroidization density of the eutectic Si particles
N Silicon = Number of silicon particles
A = Reference surface in µm2
n = Number of images sampled
10. The article as defined in claim 9, characterized in that the average
spheroidization density is greater than 20.
11. The article as defined in any one of claims 5 to 10, produced according to
the
method set out in any one of claims 1 to 4, characterized in that this is
manufactured by the thixocasting method.

Description

CA 02465683 2004-04-30
Aluminum-Silicon Allovs Having Improved Mechanical Properties
Field of the Invention
The present invention relates to a method for improving the mechanical
properties of
aluminum-silicon alloys. More specifically, the present invention relates to a
thermal-
treatment process for improving the ductility of articles consisting of a cast
or wrought
alloy with an eutectic phase, which contains preferably refined or purified
aluminum-
silicon or optionally other alloys and/or impurities, said articles being
subjected to an
annealing treatment and subsequent aging.
Background of the Invention
Further, the present invention relates to an aluminum-silicon alloy that
contains at least
one processing element, optionally magnesium, as well as additional alloying
and/or
contaminating elements with an eutectic phase consisting essentially of an a-
Al matrix
and silicon exudates.
With silicon, aluminum forms a simple eutectic system, the eutectic point
being at a
silicon concentration of 12.5%-wt and a temperature of 577°C.
By alloying magnesium, which can be dissolved in the a-AI matrix up to a
content of at
most 0.47%-wt at a temperatufe of 550°C, it is possible to achieve a
considerable
increase in the strength of the material by means of thermal treatment and the
MgZSi
exudates formed thereby.
When an Al-Si-Mg smelt cools, the residual smelt can harden eutectically, when
silicon
in it separates out in a coarse, lamellar form. For a considerable time, it
has been part of
the prior art that sodium or strontium be added to alloys of this kind and
thereby impede
the growth of the silicon crystals during hardening; this is referred to as
enrichment or
refinement and always results in improvement of the mechanical properties, in
particular
to an improvement of elongation at fracture.
The mechanical properties of semiproducts or of objects of aluminum alloys can
be
greatly influenced by thermal treatment methods, and the thermal-treatment
states are

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2~
defined in European Standard EN 515. In this Standard, the letter F stands for
"production state" and T stands for "thermally treated to stable states." The
particular
thermal treatment state is characterized by the number that is associated with
the letter T.
In the description appended below, the thermal-treatment states of the
material are,
indicated by the following short forms:
F - production state
TS - quenched from the production temperature and thermally aged
T6 - solution quenched and thermally aged
T6x - thermally treated according to the present invention
T4x - thermally treated according to the present invention
On the one hand, the properties of the material and, on the other hand, the
costs or
economic factors involved in production are important for marketing or the
industrial
use of objects that are of Al-Si alloys, since in particular long annealing
treatments at
high temperatures and the straightening processes that may be necessitated by
so-called
gravitational creep during protracted annealing are themselves costly.
In principle, it can be said that an Al-Si alloy in State F has for the most
part a low
material strength Rp and a relatively high value for elongation at fracture A.
At a thermal treatment state T5, which is to say quenched from the production
temperature and thermally aged, for example at 155°C to 199°C
for a period of 1 to 12
hours, higher strength values Rp will be achieved, but at a low elongation at
fracture
number A of the samples.
At a thermal-treatment state corresponding to T6, with solution annealing at a
temperature of, for example, 540°C for a period of 12 hours and
subsequent thermal
aging, it is possible to achieve a significant increase in the strength of the
material at an
almost equally great elongation at fracture of the samples, or ductility of
the material as
compared to State F. The long duration of the solution annealing permits
advantageous
diffusion of the magnesium atoms in the material, for example, whereby after
quenching

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3~ '
and thermal aging of the article fine, evenly distributed Mg2Si exudates are
formed in the
a-Ai matrix, and these exudates result in a decisive increase in the strength
of the
material.
As discussed heretofore, solution annealing at high temperatures and for
protracted
periods entails the disadvantages of gravitational creep and costly
temperature-time
treatment. For reasons of economy, very frequently achieving great strength
and good
ductility of the material by T6 is abandoned and a treatment state T5 is
selected for the
article. The lesser strength that results from T5 must if necessary be
compensated for by
making design changes to the component in question.
Summar~of the Invention
It is the objective of the present invention to create a new, cost-effective
method of
thermal treatment, with which the ductility of the material can be greatly
increased
without causing major losses of material strength as compared to T6, or with
which
significantly greater ductility and greater material strength can be achieved
in
comparison to T5.
It is also an objective of the present invention to describe a microstructure
of an article
of the type described in the introduction hereto, which results in
advantageous
mechanical properties of the material.
The objective of this method is achieved in that the solution annealing is
conducted as
shock annealing comprising rapid heating of the material to an annealing
temperature of
400°C to 555°C, maintaining it at this temperature for a period
of at most 14.8 minutes,
and subsequent forced cooling, essentially to room temperature.
The advantages that are obtained are that the highest ductility values are
achieved for the
material by a simple high-temperature brief annealing. In addition, the so-
called shock
annealing causes little or no component deformation or warping of the article,
so that
there is no need to straighten it. The short-time annealing treatment is very
economical
and can be incorporated very easily into a production sequence, for example by
using a
continuous heating furnace. Material strength can be adjusted by an adapted
thermal

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4~
aging technology. With the majority of Al-Si alloys, the greatest increase
will be
achieved if, as can be provided for, the shock annealing is effected with a
holding time
of less than 6.8 minutes, preferably for a time span ranging from 1.7 up to at
most 5
minutes.
If the article is thermally aged after the shock annealing, it is advantageous
to do this at a
temperature in the range between 150°C and 200°C, for a period
ranging from 1 to 14
hours.
It can also be advantageous from the material standpoint if the aging of the
article that
follows shock annealing be effected as cold aging, essentially at room
temperature.
The additional objective of the present invention is achieved in that the
silicon exudates
are spheroidized in the eutectic phase and have a cross-sectional area As;, of
less than 4
~m2.
The formula for determining the cross-sectional area is shown below, the
factors being:
n
AS; _ ~ ~ A 5 4 wm2
n ka,
As; = average surface of the silicon particles in ~.mz
A = average surface of the silicon particles per image, in p.m2
n = number of images sampled
The advantages of a microstructure of this kind are essentially that crack
initiation in the
material caused by spheroidization of the Si exudates and by their fineness is
significantly reduced and ductility of the material is improved. In other
words, the
spheroidization and small size result in a favourable morphology of the
brittle eutectic
silicon and lead to significantly higher values for the material's elongation
at fracture. In
the case of mechanical loading, the stress peaks on the Si-Al phase boundary
surface are

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5'
reduced. A transcrystalline break was also found during tests, and this
indicates the
highest ductility of the material.
From the method standpoint, but also for high values for elongation at
fracture, it can be
advantageous if the silicon exudates in the eutectic phase be spheroidized and
have an
average cross-sectional area of less than 2~.mz.
If, as was shown during development, the solution. according to the present
invention is
realized in that the average free path length between the silicon particles
~,5; in the
eutectic phase defined as the root of a square measured surface divided by the
number of
silicon particles contained within it is of as a size that is less than 4pm,
preferably less
than 3pm, and in particular less than 2 Vim, an especially homogeneous stress
distribution at low stress peak values will be achieved in the material that
is stressed,
since the spacing between the small-surface silicon particles essentially
effects the flow
behaviour of the material in a corresponding stress state. Determination of
the distance
between the silicon particles ~,s;, is shown formally below.
n
AQuadrat ~ 4
~ N s;iicon
wherein ~,s; = average spacing between of the silicon particles in ~m2
A Quadrat = square reference surface, in ~.m2
Ns;,;~o" = number of silicon particles
n = number of images sampled
Although solution annealing as in the prior art which is effected as long-time
annealing
for 2 to 12 hours for diffusion of the alloying components that are effective
for
hardening and their enrichment in the mix crystal entails spheroidization of
the silicon
particles as a secondary effect, the particles are very large and distributed
unevenly as a
result of the long annealing time; this can have a deleterious effect on the
material's
behaviour at fracture. It was most surprising that a eutectic silicon network
can be
spheroidized according to the present invention by brief shock annealing for a
short time
span of just a few minutes, whereby an advantageous microstructure of the
material can

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6
be achieved. In this connection, it is important that the temperature for the
shock
annealing be a high as possible, although below the lowest smelting phase,
preferably 5
to 20°C below this.
With increasing annealing times, the silicon particles are subjected to a
diffusion-
controlled growth, the initially favourable high spheroidization density ~s;
becoming
smaller.
In one solution to the task of the present invention, the highest ductility of
an article of
an Al-Si alloy was found if the mean spheroidization density ~s;, defined as
the number
of spheroidized eutectic silicon particles per .100~m2 , has a value that is
greater than 10,
and preferably greater than 20.
n
~S;=n~A'x100>_ 10
~s; = Average spheroidization density of the eutectic Si particles
Nsn;~o" = Number of silicon particles
A = Reference surface in ~m2
n = Number of images sampled
The following is once again a formal specification of the formula.
Work has shown that essentially each AI-Si alloy that contains the eutectic
can be
provided with a structure according to the present invention, when the
articles formed
therefrom has high material-ductility values. The improvement of the goods and
an
improvement of elongation at fracture are particularly efficient if the
article is
manufactured by the thixocasting method.
Brief Description of the Drawings
The present invention will be described in greater detail below on the basis
of test results
and drawings as appended hereto. The drawings show the following:

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7 .
Figure 1: Bar chart showing mechanical values for material as a function of
thermal
treatment
state;
Figure As in Figure 1
2:
Figure REM image of a cut
3:
Figure As in Figure 3
4:
Figure Mean surface of the Si exudates
5:
Figure As in Figure 5
6:
Figure Mean free path length between
7: the Si particles
Figure Mean spheroidization density
8:
Figure 9: Bar chart showing material mechanical properties of various Al-Si
alloys
Table 1: Numerical values for Figure 9.
Detailed Description of the Preferred Embodiments
In Figure l, a bar chart shows the Rpo.2 limiting values and the values for
elongation at
fracture A of samples manufactured from a test component produced from an
AlSi7Mg0.3 alloy, said component having been produced by the thixocasting
method.
The values for thermal treatment state T6 (12 hours 540°C + 4 hours
160°C) of the
material are compared to those that were achieved with the T6x method
according to the
present invention after shock annealing far 1 minute (T6x1), after 3 minutes
(T6x3) and
after 5 minutes (T6x5) at a temperature of 540°C. All the samples were
hot-aged (4
hours) at a temperature of 160°C. The results of the tensile test show
that the samples
display significantly higher values for elongation at fracture after shock
annealing, the
T6x3 effecting an increase of A by approximately 60% as compared to T6.
In Figure 2, using identically produced samples, the state values F, T4x3, T5,
T6x3 and
T6 are compared in a bar chart with respect to Rp o,2 and elongation at
fracture A. When

CA 02465683 2004-04-30
compared, they display marked increases of the values for elongation at
fracture. As can
be seen from Figure 2, the material can be aged cold (T4x3) or hot (T6x3)
after shock
annealing for 3 minutes in order to obtain the superior elongation at fracture
characteristics according to the present invention.
Figure 3 and Figure 4 show raster electron microscope images of Si exudates.
With
respect to the imaging and evaluation method, it must be noted that it is
essential to have
binary images available in order to permit quantitative evaluation. The images
were
captured with a raster electron microscope for an annealing period of 2 hours
inclusive,
after which the cut was etched for 30 seconds using a solution of 99.5% water
and 0.5%
liquid acid. After annealing for 4 hours, the cut was etched with Keller
solution and the
images could be captured by optical microscope. All the images were processed
digitally using Adobe Photoshop 5, and evaluated with the Leica QWin V2.2
image
analysis software; the minimal detection surface amounted to 0.1 pmt. Figure 3
shows
the AlSi7Mg0.3 after a normal T6 annealing time of 12 house , using an REM
image.
Figure 4 shows the microstructure of the same material after shock annealing
for three
minutes. It is clear that even after a very short time there is
spheroidization of the silicon
exudates (Figure 4) and the diffusion-controlled growth of these can be seen
after long
annealing times (Figure 3).
Figure 5 and Figure 6 show the mean cut surface As; of the silicon particles
during cut
testing as a function of the annealing time at 540°C. The increase of
average cross-
sectional area of the silicon particles, which characterizes the size of the
particles, can be
clearly seen from the details of Figure 4 with the logarithmic time axis. The
increase of
the average silicon surface within the first 60 minutes, which is governed by
diffusion,
can be clearly seen from Figure 6. The average size of the silicon particles,
which
increases with annealing time, is to a large extent dependent on the initial
size of the
silicon particles in the eutectic. Since an extremely well refined and finely
divided
silicon is present in this particular case, in some cases that involve silicon
particles that
has not been refined so well which is to say with initially larger silicon
particles the time
in which a critical average silicon surface As; of approximately 4p.m2 can be
achieved
can be shorter.

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9
The change of the average distance between the silicon particles as a function
of
annealing time is shown in Figure 7, using test results. The increase in the
average
spacing of the silicon inclusions can be clearly seen.
Finally, Figure 8 shows the decrease of the average spheroidization density
~;, is shown
as a function of annealing time. The sharp decrease of the average
spheroidization
density begins as soon as at 1.7 minutes and drops from a value of <10 for ~s;
to a
pronounced loss of ductility. At higher annealing temperatures, the value can
be reached
after 14 to 25 minutes, when a density value of greater than 20 has to be
provided for a
much higher value for elongation at fracture.
The bar chart at Figure 9 shows the measured values for limit of elongation
and
elongation at fracture that follow from Table 1, for eight different Al-Si
alloys. In all of
these alloys, an increase in the ductility of the material is achieved
according to the
present invention.

Dessin représentatif