U.S. patent application number 12/706617 was filed with the patent office on 2011-09-01 for casting process for aluminum alloys.
This patent application is currently assigned to Alcan Rhenalu. Invention is credited to Guillaume BES, Robert REY-FLANDRIN, Olivier RIBAUD, Stephane VERNEDE.
Application Number | 20110209843 12/706617 |
Document ID | / |
Family ID | 41119645 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110209843 |
Kind Code |
A2 |
BES; Guillaume ; et
al. |
September 1, 2011 |
CASTING PROCESS FOR ALUMINUM ALLOYS
Abstract
The present invention relates to a casting process for an
aluminum alloy containing at least about 0.1% of Mg and/or at least
about 0.1% of Li in which a liquid surface of said alloy is put
into contact with a dried gas including at least about 2% of oxygen
by volume and with a water partial pressure lower than about 150 Pa
throughout most of the solidification process. The invention makes
it possible to cast the most oxidable aluminum alloys, in
particular aluminum alloys containing magnesium and/or lithium,
without using additives such as beryllium and/or calcium and
without using expensive devices and/or gases, to obtain cast ingots
generally free from surface defects and pollution, in complete
safety.
Inventors: |
BES; Guillaume; (Moirans,
FR) ; REY-FLANDRIN; Robert; (St Etienne de Crossey,
FR) ; RIBAUD; Olivier; (Renage, FR) ; VERNEDE;
Stephane; (Grenoble, FR) |
Assignee: |
Alcan Rhenalu
17, place des reflets La Defense 2
Courbevoie
FR
92400
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20100212855 A1 |
August 26, 2010 |
|
|
Family ID: |
41119645 |
Appl. No.: |
12/706617 |
Filed: |
February 16, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61/286,594 |
Dec 15, 2009 |
|
|
|
Current U.S.
Class: |
164/66.1 ;
164/122; 164/259; 164/462; 164/463; 164/47 |
Current CPC
Class: |
B22D 11/003 20130101;
B22D 11/04 20130101; B22D 11/117 20130101; B22D 11/12 20130101 |
Class at
Publication: |
164/066.1 ;
164/047; 164/462; 164/463; 164/122; 164/259 |
International
Class: |
B22D 23/00 20060101
B22D023/00; B22D 11/12 20060101 B22D011/12; B22D 11/06 20060101
B22D011/06; B22D 27/04 20060101 B22D027/04; B22D 27/00 20060101
B22D027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 20, 2009 |
FR |
0900780 |
Claims
1. Casting process for an aluminum alloy comprising at least about
0.1% of Mg and/or at least about 0.1% of Li comprising bringing a
liquid surface of said alloy into contact with a dried gas
comprising at least about 2% of oxygen by volume and with a water
partial pressure less than about 150 Pa throughout most of the
solidification process.
2. Process according to claim 1 wherein the water partial pressure
of said dried gas is less than 100 Pa.
3. Process according to claim 1 wherein the gas is brought into
contact with said surface in order to establish, above said
surface, an atmosphere having a water content substantially equal
to that of the dried gas.
4. Process according to claim 1 wherein said liquid surface of
aluminum alloy accounts for at least 10% of an entire liquid
surface of said aluminum alloy.
5. Process according to claim 1 wherein said aluminum alloy is an
alloy of the family 2XXX, 3XXX, 5XXX, 6XXX, 7XXX or 8XXX.
6. Process according to claim 5 wherein said aluminum alloy does
not contain any deliberate addition of beryllium and/or
calcium.
7. Process according to claim 1 wherein said dried gas comprises at
least one gas selected from the group consisting of air, helium,
argon, nitrogen, carbon dioxide, carbon monoxide, natural gas
combustion products, methane, ethane, propane, natural gas, organic
fluorinated compounds, and organic chlorinated compounds.
8. Process according to claim 7 wherein said dried gas is air.
9. Process according to claim 1 wherein the CO.sub.2 content of
said dried gas is less than 1% by volume.
10. Casting process according to claim 1 which includes direct
chill casting, horizontal casting, continuous casting of wire,
continuous casting of strips between cylinders, and/or continuous
casting of strips using a belt caster.
11. A process of claim 10 that is a direct chill casting process
wherein said gas is supplied using a device fixed around a molten
metal injector so that dried gas flow is directed from said liquid
surface towards an edge thereof and/or from an edge towards a
molten metal injection zone.
12. Casting process according to claim 1 wherein said dried gas is
also used in at least one furnace and/or trough.
13. A facility for casting an aluminum alloy comprising at least
about 0.1% of Mg and/or at least about 0.1% of Li using a dried gas
including at least about 2% of oxygen by volume and with a water
partial pressure lower than about 150 Pa on a liquid surface,
wherein said facility is capable of minimizing oxidation of said
alloy.
14. A facility according to claim 13 comprising at least one device
selected from a furnace, a filtration ladle, a degassing ladle and
a conveying trough.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to FR 0900780 filed Feb.
20, 2009 and U.S. Provisional Application 61/286,594 filed Dec. 15,
2009, the contents of which are incorporated herein by reference in
their entireties.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates generally to the casting of
aluminum alloys, in particular the casting of alloys containing
magnesium and/or lithium, sensitive to oxidation.
[0004] 2. Description of Related Art
[0005] The oxidation of aluminum alloys in a liquid state has
detrimental consequences on the casting process. In furnaces and
transfer troughs the oxidation of metal initially results in a net
metal loss, called loss on ignition. In addition, during casting,
too great an oxidation of the molten metal generates surface
defects on the ingot cast which have a detrimental effect when the
products are used. These problems are particularly marked in alloys
containing magnesium and/or lithium.
[0006] A main defect is the vertical fold which is caused by
crumpling of the oxide skin on the surface of the sump. In certain
cases, and in particular when casting 7xxx alloys, this problem is
particularly great because the folds, especially when they are long
and deep, easily cause surface cracks. Folds and cracks must
generally be eliminated before transforming the ingots obtained
during casting. The defects may, for example, be eliminated by
machining, which may be a very economically unsatisfactory
solution, in terms of both the cost of the operation and the
significant metal loss which occurs as a result. In certain cases,
the presence of cracking makes the ingot unusable and it has to be
remelted.
[0007] It has long been known that adding certain elements makes it
possible to limit oxidation and to improve the surface quality. In
1943, U.S. Pat. No. 2,336,512 described the addition of very small
quantities of beryllium to aluminum alloys containing magnesium in
order to limit oxidation of the molten metal surface. International
application WO 02/30822 described the substitution of beryllium by
calcium with the same aim of limiting oxidation.
[0008] But the use of additives may cause other problems.
Beryllium, for example, is to some extent toxic which has led to
its removal from aluminum alloys used for food packaging. Calcium
may lead to edge cracking during hot rolling.
[0009] It has also been proposed to protect the surface of the
molten metal by means of various devices. U.S. Pat. No. 4,582,118
proposes using a non-reactive and non-combustible atmosphere, such
as for example, an atmosphere of argon, helium, neon, krypton,
nitrogen or carbon dioxide, for casting aluminum-lithium alloys.
But such processes are very expensive to use.
[0010] Patent application EP 0 109.170 A1 describes the use of a
baffle on the edge of the casting device to sweep the molten metal
surface with an inert gas (usually nitrogen and/or argon with or
without chlorine or another halogen). But these gases are tricky to
use and significantly increase the cost of operations.
[0011] The use of carbon dioxide or combustion gas to limit
oxidation is also known by C. N. Cochran, D. L. Belitskus and D. L.
Kinosz, Metallurgical Transactions B, Volume 8B, 1977, pages
323-331. Patent application EP 1 964 628 A1 describes a method for
producing aluminum ingots in which at least one stage of the
process is carried out in an atmosphere containing a fluorinated
gas. However, fluorinated gases are tricky to use and carry large
safety risks.
[0012] U.S. Pat. No. 5,415,220 describes the use of molten salts of
lithium chloride and potassium chloride to protect the surface of
aluminum-lithium alloys during casting. But the drawback to using
molten salts is the risk of contamination of the molten metal with
impurities, as well as the difficulty of using them.
[0013] U.S. Pat. No. 7,267,158 describes the forced addition of a
wet gas, containing more than 0.005 kg/m3 water on the surface of
the molten metal in order to improve the surface quality of the
cast ingots. This process has, however, the disadvantage of
bringing the water vapor and liquid aluminum into contact with each
other, in spite of the dangers of explosion caused by contact
between water and liquid aluminum. In addition, it is known from
application EP 0 216 393 A1 that dry air can be used in a treatment
ladle for liquid aluminum to prevent hydrogen from penetrating into
the molten metal when a treatment gas is injected into the molten
metal and causes the oxide coating protecting its surface to
burst.
[0014] A problem was to find a casting process suitable for most
oxidable aluminum alloys, in particular aluminum alloys containing
magnesium and/or lithium, which does not have these disadvantages
and makes it possible to obtain cast ingots that are free or
virtually free from surface defects and pollution, in as complete
safety as possible.
SUMMARY
[0015] A first subject of the invention is directed to a casting
process for an aluminum alloy comprising at least about 0.1% of Mg
and/or at least about 0.1% of Li in which a liquid surface of said
alloy is put into contact with a dried gas including at least about
2% of oxygen by volume and with a water partial pressure lower than
about 150 Pa throughout most of the solidification process.
[0016] A second subject of the present invention is casting an
aluminum alloy comprising at least about 0.1% of Mg and/or at least
about 0.1% of Li in the presence of a dried gas including at least
about 2% of oxygen by volume and with a water partial pressure
lower than about 150 Pa on a liquid surface of said aluminum alloy
in order to minimize oxidation thereof. The present invention also
encompasses a facility capable of conducting a casting process of
the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1: general diagram of a semi-continuous vertical
casting facility.
[0018] FIG. 2: diagram of a semi-continuous vertical casting
facility including a device for procuring a flow of dried gas.
[0019] FIG. 3: diagram of a device for procuring a flow of dried
gas for casting plates.
[0020] FIG. 4: diagram of the thermobalance used in example 1.
[0021] FIG. 5: weight increasing with time for experiments carried
out with alloy 7449 in example 1.
[0022] FIG. 6: geometry weight increasing with time for experiments
carried out with alloy AA5182 in example 1.
[0023] FIG. 7: weight increasing with time for experiments carried
out with alloy AA2196 in example 1.
[0024] FIG. 8: FIG. 8: photographs of surfaces obtained after tests
N.sup.o 7 (FIG. 8a) and N.sup.o 5 (FIG. 8b) in example 1.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0025] The names of alloys follow the rules of The Aluminum
Association, known to those skilled in the art. The chemical
composition of standardized aluminum alloys is defined for example
in standard EN 573-3.
[0026] Unless otherwise specified, the definitions of European
standard EN 12258-1 apply. A "casting facility" and a "facility" is
here considered to be all the devices used to transform a metal in
unspecified form into a semi-finished product of rough form via a
liquid phase. A casting facility or "facility" may include any
number of devices such as one or more furnaces necessary for
melting metal and/or keeping metal at a given temperature and/or
for operations to prepare the molten metal and adjust the
composition. For example, a casting facility may comprise one or
more ladles designed to carry out treatment to eliminate impurities
that are dissolved and/or in suspension in the molten metal. Such a
treatment may involve filtering molten metal through a filter
medium in a "filtration ladle" and/or introducing into the bath a
"treatment" gas which may be inert or reactive in a "degassing
ladle." A casting facility also comprises a device for solidifying
the molten metal (or "casting device), for example, by direct chill
casting, horizontal casting, continuous casting of wire, continuous
casting of strips between cylinders, and/or continuous casting of
strips using a belt caster. Devices such as a mold (or "ingot
mold") and/or a device for procuring molten metal (or "nozzle")
and/or a cooling system can also be utilized in a casting facility.
These various furnaces, tanks and solidification devices of a
casting facility can be connected to each other, for example, by
troughs (or "transfer throughs") in which the molten metal can be
transported.
[0027] Surprisingly, the present inventors noted that when put into
contact with a dried gas including at least about 2% of oxygen by
volume and with a water partial pressure of less than about 150 Pa
or least than 150 Pa, a liquid aluminum surface undergoes little
oxidization. This then makes it possible to produce castings that
are free from unacceptable surface defects. This result is
surprising, inter alia, because it is commonly accepted that, on
the contrary, the moisture contained in the air makes it possible
to limit oxidation of aluminum alloys in liquid state.
[0028] In a first embodiment of the invention, this surprising
effect involves use in a casting process. A suitable process
according to the present invention is useful for highly oxidable
aluminum alloys, especially those comprising at least about 0.1% of
Mg and/or at least about 0.1% of Li. A process according to the
invention is particularly useful for alloys of the 2XXX, 3XXX,
5XXX, 6XXX, 7XXX or 8XXX families, especially when these alloys do
not include a deliberate addition of beryllium and/or calcium. A
process according to the present invention is particularly
advantageous for alloys comprising less than 3 ppm of beryllium or
even less than 1 ppm of beryllium and/or less than 15 ppm of
calcium or even less than 5 ppm of calcium. Examples of suitable
alloys for which the process according to the invention is
particularly advantageous are, in the 2XXX family of alloys, alloys
AA2014, AA2017, AA2024, AA2024A, AA2027, AA2139, AA2050, AA2195,
AA2196, AA2098, AA2198, AA2214, AA2219 and AA2524, in the 3XXX
family of alloys, alloys AA3003, AA3005, AA3104 and AA3915, in the
5XXX family of alloys, alloys AA5019, AA5052, AA5083, AA5086,
AA5154, AA5182, AA5186, AA5383, AA5754 and AA5911, and in the 7XXX
family of alloys, alloys AA7010, AA7020, AA7040, AA7140, AA7050,
AA7055, AA7056, AA7075, AA7449, AA7450, AA7475, AA7081, AA7085,
AA7910, and AA7975.
[0029] The dried gas advantageously comprises at least about 2% of
oxygen by volume and has a water partial pressure lower than about
150 Pa, preferably lower than 100 Pa and preferably still lower
than 70 Pa. In a particularly advantageous embodiment, the water
partial pressure is even less than 30 Pa, preferably less than 5 Pa
and even more preferably less than 1 Pa. The water partial pressure
of a gas is also referred by the "vapor pressure." The pressure
partial of a perfect gas "i" in a mixture of perfect gases of total
pressure "P" is defined as the pressure which would be exerted by
the molecules of gas "i," if this gas occupied on its own, the
total volume available to the mixture. The dewpoint of a gas is the
temperature at which, the gas becomes saturated with water vapor,
and wherever the current barometric conditions are unchanged.
Dewpoint may also be defined as the temperature at which the steam
pressure would be equal to the saturating vapor pressure. A water
partial pressure of 150 Pa corresponds to a dewpoint of
-17.9.degree. C. and to a quantity of water of 0.0013 kg/m.sup.3 at
this temperature. A water partial pressure of 100 Pa corresponds to
a dewpoint of -22.6.degree. C. and to a quantity of water of 0.0009
kg/m.sup.3 at this temperature. A water partial pressure of 70 Pa
corresponds to a dewpoint of -26.5.degree. C. and to a quantity of
water of 0.0006 kg/m.sup.3 at this temperature.
[0030] The dried gas also advantageously includes at least one gas
selected from air, helium, argon, nitrogen, carbon dioxide, carbon
monoxide, natural gas combustion products, methane, ethane,
propane, natural gas, organic fluorinated compounds, organic
chlorinated compounds. Adding carbon dioxide to the dried gas can,
in certain cases, improve the antioxidant effect. In one embodiment
of the present invention, the dried gas includes between 1 and 10%
of CO.sub.2 by volume. However, as this is a limited effect and as
adding CO.sub.2 costs money, the CO.sub.2 content of the dried gas
is preferably less than 1% by volume or even less than 0.1% by
volume in another advantageous embodiment of the invention. In an
advantageous embodiment of the invention, the dried gas is
primarily air dried by any suitable means to reach the desired
water partial pressure.
[0031] According to the present invention the dried gas is put into
contact with a liquid surface of aluminum alloy during most of the
solidification process of the alloy. The gas is preferably brought
into contact with the surface in order to establish an atmosphere
above this surface whose water content is substantially equal, that
is generally within 10% or 20%, to that of the dried gas, i.e. in
order to avoid significant diffusion of water vapor coming from the
ambient air in said atmosphere.
[0032] When contact is made using a flow of dried gas, it is often
advantageous for this flow to be sufficient in relation to the
liquid surface subjected to the dried flow in order to establish
the desired atmosphere. If flow is too low, the composition of said
atmosphere may be too greatly influenced by the external atmosphere
and its water content may no longer correspond to the desired
content. In addition, it is in generally not necessary to put all
the liquid surface of aluminum alloy available into contact with
the dried gas, as illustrated by FIG. 1 (14, 15), in order to
obtain an advantageous effect on the surface quality of the cast
products. Advantageously, the liquid surface of aluminum alloy
brought into contact with the dried gas accounts for at least 10%,
and preferably at least 25%, and preferably still at least 50% of
the entire liquid surface of the aluminum alloy.
[0033] A liquid surface of aluminum alloy is preferably kept in
contact with the dried gas during most of the solidification
process. Therefore, while it is not necessary to bring a liquid
surface into contact with the dried gas as soon as the molten metal
is introduced into the casting device, it is preferable to do this
as soon as a stationary mode is established. For example, in the
case of direct chill casting, it is preferable to do it at least at
the beginning of the descent of the dummy bottom, or at least at
the start of casting of a zone which will not be cut during later
operations. It is possible to vary the flow of dried gas during
casting, especially if surface defects appear. So an increase in
the flow of dried gas makes it possible in certain cases to make
folds in the cast product disappear. Contact between the liquid
surface and the dried gas can possibly be removed before the end of
casting, in particular when one reaches a zone which will be cut
during following operations. Generally a liquid surface of aluminum
alloy is kept in contact with the dried gas during at least 50% or
even at least 90% of the solidification process.
[0034] The present invention applies to various casting processes
and preferably to a casting process chosen from direct chill
casting, horizontal casting, continuous casting of wire, continuous
casting of strips between cylinders, and continuous casting of
strips using a belt caster.
[0035] The process known to one skilled in the art as "direct chill
casting" or "DC casting", is a preferred process within the context
of this invention. In such a process, an aluminum alloy is cast in
an ingot mould with a dummy bottom while moving the dummy bottom
vertically and continuously so as to maintain a substantially
constant level of molten metal during solidification of the alloy,
the solidified faces being directly cooled with water. FIG. 1
illustrates this process. An aluminum alloy is fed by a conduit (4)
into an ingot mould (3) placed on a dummy bottom (21). The aluminum
alloy is solidified by direct cooling (5). The aluminum alloy as it
solidifies (1) has at least one solid surface (11, 12,13) and at
least one aluminum alloy surface in liquid state which can be
covered with oxides, which is called "liquid surface" in this
description (14, 15). An elevator (2) makes it possible to
gradually lower the alloy being solidified in order to maintain the
vertical position of the liquid aluminum surface (14, 15)
substantially constant.
[0036] A process according to the present invention is particularly
advantageous for the casting of plates and billets by direct chill
cooling. The process according to the present invention is
particularly advantageous for the casting of large-sized plates, in
particular those having a section greater than 0.5 m.sup.2.
[0037] Many devices can be used to allow dried gas to be brought
into contact with a liquid surface of an aluminum alloy according
to the present invention. In the case of direct chill casting, the
device suitably may be, for example, i) integrated into an ingot
mould or fixed to the latter in order to introduce the dried gas
from the edge of the liquid surface towards its center, ii)
positioned above the liquid surface so as to introduce the dried
gas substantially perpendicularly to the liquid surface, iii) fixed
around a molten metal injector so as to introduce the dried gas
from the center of the liquid surface towards its edge and/or from
the edge towards the center, and/or iv) may be made up by any
combination or modification of these devices.
[0038] An advantageous device for procuring gas in the case of
direct chill casting is illustrated in FIG. 2. In this advantageous
embodiment, the dried gas is supplied using a device (6) fixed
around the molten metal injector (4) so that the dried gas flow (7)
is directed from the heart of said liquid surface towards its edge
and/or from the edge towards the heart in the molten metal
injection zone. Advantageously, the gas procurement device can be
fixed to a dam holding back oxides ("skim dam") which can be
positioned around the molten metal injection zone. In this way, a
greater flow of dried gas can be obtained in the zone where
oxidation is probably highest, i.e. near the molten metal injector,
and in the zone located between the skim dam and the ingot mould,
since this zone is often precisely the one likely to generate
surface defects on cast products. This configuration also makes it
possible to limit or affect the dimension of the device.
[0039] A dried gas from the casting process according to the
present invention can also be used in other parts of a casting
plant on a liquid surface of aluminum alloy containing at least
about 0.1% of Mg and/or at least about 0.1% of Li, in order to
minimize oxidation. A casting facility generally includes several
other devices in which liquid surfaces of aluminum alloy are in
contact with the atmosphere. The dried gas can therefore
advantageously be used to limit the oxidation of the liquid surface
of alloys, for example, in a furnace, in particular a smelting or
holding furnace, in a treatment tank such as a filtration ladle or
a degassing ladle or in a transfer trough such as a transfer
trough. For these uses, conditions of using the dried gas and/or a
composition of aluminum alloy similar to those described supra, in
particular concerning the procurement of dried gas, can preferably
be used. Advantageously, in the casting process according to the
present invention dried gas is also used in at least one furnace,
in particular a smelting or holding furnace and/or in at least one
treatment tank such as a filtration ladle or a degassing ladle
and/or in at least a trough such as a transfer trough.
[0040] Products obtained by a process according to the present
invention and/or a use according to the present invention can, as
an option, be wrought in particular by rolling, spinning and/or
forging, particularly in order to obtain sheets and sections.
[0041] The present invention makes it possible to cast oxidable
aluminum alloys, in particular aluminum alloys containing magnesium
and/or lithium, without using additives such as beryllium and/or
calcium and without using expensive devices and/or gases, to obtain
cast ingots free from surface defects and pollution, and in a
manner that is safe.
EXAMPLES
Example 1
[0042] In this example, oxidation of the molten metal was measured
by thermogravimetric analysis. In these tests, a crucible
containing the molten metal is held at a controlled temperature.
This crucible contains about 5 kg of metal, and has a diameter of
100 mm. The significant size of these experiments, which makes it
possible to take macroscopic effects into account, may explain
differences with experiments carried out on very small quantities
often reported in former art. The mass of the sample is
continuously weighed. The increase in weight is due to oxidation of
the molten metal. A diagram illustrating this experiment is
presented in FIG. 4.
[0043] The dried gas (7) is brought to the surface of the molten
metal (14) by a metal tube (6) of interior diameter 4 mm, placed
obliquely in relation to this surface. The balance (92) is used to
continuously measure the weight of the crucible (93) and its
contents in situ in the furnace (91). The distance between the
opening of the metal tube and the surface of the molten metal was
120 mm. The air used can be dried until it reaches a water partial
pressure of less than 70 Pa.
[0044] Three alloys were studied: alloys AA7449, AA2196 and AA5182.
The conditions of the various tests are summarized in table 1. In
all tests, the beryllium and calcium content were similar and less
than 1 ppm and 10 ppm respectively. TABLE-US-00001 TABLE 1 Test
conditions carried out with the thermobalance Gas flow rate Water
partial pressure of gas Tests alloy (l/min) Gas injected (Pa) 1
AA5182 7.9 Dry Air <70 Pa 2 AA5182 0 Ambient <600 Pa air 3
AA2196 7.9 Dry air <70 Pa 4 AA2196 0 Ambient <600 Pa air 5
AA7449 4.1 Dry air <70 Pa 6 AA7449 3.8 Ambient <600 Pa air 7
AA7449 0 Ambient <600 Pa air 8 AA7449 4.1 Dry air 180 Pa 9
AA7449 3.8 Dry air 600 Pa
[0045] FIGS. 5 to 8 show the results obtained.
[0046] FIG. 5 shows the results obtained with alloy AA7449.
Significantly smaller increases in weight are obtained for test 5
for which a flow of very dry air was used. Bringing a liquid
surface into contact with dry air whose water partial pressure is
still 600 Pa (dewpoint -0.2.degree. C., test 9) or even 180 Pa
(dewpoint -15.6.degree. C., test 8) do not make it possible to
significantly limit oxidation. In the same way, ambient air does
not make it possible to limit oxidation with or without flow (tests
6 and 7), which rules out a purely mechanical effect related to a
gas flow.
[0047] FIG. 6 shows the results obtained with alloy AA5182.
Significantly lower oxidation is again noted for this alloy in the
presence of a flow of very dry air.
[0048] FIG. 7 shows the results obtained with alloy AA2196.
Significantly lower oxidation is once again noted for this alloy in
the presence of a flow of very dry air.
[0049] FIG. 8a is a photograph of the surface obtained after the
test in the case of test 7 (ambient air). A large amount of
oxidation is observed, leading to oxidation products in the
characteristic dark-colored cauliflower shape. FIG. 8b is a
photograph of the surface obtained after the test in the case of
test 5 (dry air). A uniform light gray surface is observed,
corresponding to a fine oxide film.
Example 2
[0050] Plates of rectangular section 446 mm.times.2160 mm made of
alloy AA7449 were DC-cast using AlTiC refining. The length of the
plates obtained ranged between 900 mm and 4000 mm. The beryllium
content of the alloy was less than 1 ppm and the calcium content
was less than 15 ppm.
[0051] FIG. 3 illustrates the gas procurement device used to supply
dry air when the plates were being cast. The device consists of 4
tubes (611, 612, 621 and 622) regularly bored with openings (63)
used to inject the dried gas (7) on the liquid surface of the
aluminum alloy. The tubes are connected by screwed connections (9)
to form a rectangle. The tubes are supplied with gas by two of
these screwed connections, via two pipes (81) and (82). The length
L and the width l of the device (L=1285 mm, l=300 mm, space between
the openings: 20 mm) account for less than about 70% or less than
70% of the length and the width of the ingot mould, so that the
surface subjected to the dried gas flow accounts for about 50% of
the whole of the liquid surface of aluminum alloy (total liquid
surface: 0.96 m2, surface subjected to a dried flow: 0.58
m.sup.2).
[0052] The dried gas was dry air whose water partial pressure was
60 Pa, in certain cases containing 5% of CO.sub.2 by volume.
TABLE-US-00002 TABLE 2 Condition of the casting tests and results
obtained. flow of dry air % [m.sup.3/h] CO.sub.2 Length (length of
the dry Test cast [mm] cast) air flow observations 21 917 None --
Long (.about.200 mm) and deep vertical folds 22 2776 None -- Long
(.about.200 mm) and deep (Start-up) vertical folds 22 5% No fold
(1150 mm) 23 3575 22 0% A few short (.about.40 mm) (Start-up)
vertical and shallow folds 27 0% A few short (.about.40 mm) (1150
mm) vertical and shallow folds 32 0% No fold (2,500 mm)
[0053] The effect of the dry air was shown on several occasions:
during test 22, bringing a liquid surface into contact with dry air
made it possible to make the deep folds disappear. In the same way,
in test 23, the presence of dry air made it possible as of the
start of the test to obtain a satisfactory surface quality for the
plates cast (some short (.about.40 mm) vertical and shallow folds).
In addition, for this test it was observed that the increase in the
flow of dry air made it possible to make the folds disappear. The
effect of the presence of CO.sub.2 in the dried gas on the surface
quality is, if it exists, of second order as compared with the
effect of the water partial pressure. So for test 23, a
satisfactory result was obtained in the absence of CO.sub.2.
* * * * *