EP2398609B1 - Casting method for aluminium alloys - Google Patents

Description

Field of the invention

The invention relates to the casting of aluminum alloys, especially the casting of alloys containing magnesium and / or lithium sensitive to oxidation.

State of the art

The oxidation of aluminum alloys in the liquid state has detrimental consequences on the foundry process. In furnaces and transfer channels, the oxidation of the metal first results in a net loss of metal, known as a loss on ignition. In addition, during casting, too much oxidation of the liquid metal causes defects on the surface of the cast ingot which adversely affect the use of the products. These problems are particularly pronounced in alloys containing magnesium and / or lithium.
A main defect is the vertical groove which is notably generated by folds of the oxide skin on the surface of the marsh. In some cases, especially during the casting of 7xxx alloys, this problem is particularly important because the furrows, especially when they are long and deep, easily initiate surface cracks. The grooves and slots must generally be removed before the ingots obtained during the casting process are processed. It is possible, for example, to eliminate defects by machining, which can be economically very disadvantageous both by the cost of the operation and by the significant loss of metal that results. In some cases, the presence of slit makes the ingot unusable and it is necessary to remelt it.

It has been known for a long time that the addition of certain elements makes it possible to limit oxidation and improve surface quality.
As early as 1943, the patent US 2,336,512 described the addition of very small amounts of beryllium to magnesium-containing aluminum alloys in order to limit the oxidation of the liquid metal surface.
International demand WO 02/30822 describes the substitution of beryllium by calcium for the same purpose of limiting oxidation.

Adding additions may, however, be the cause of other problems. Thus, beryllium has some toxicity which has led to its deletion in aluminum alloys used as food packaging. Calcium can cause shear cracking during hot rolling.
It has also been proposed to protect the surface of the liquid metal by various devices.
The patent US 4,582,118 proposes to use a non-reactive and non-combustible atmosphere, such as an argon, helium, neon, or krypton or even nitrogen or carbon dioxide atmosphere, for the casting of aluminum alloys. lithium. The implementation of such methods is however very expensive.
The patent application EP 0 109 170 A1 discloses the use of a baffle on the periphery of the casting loom for sweeping the liquid metal surface with an inert gas (usually nitrogen and / or argon with or without chlorine or other halogen). However, the implementation of these gases is delicate and significantly increases the cost of operations. The use of carbon dioxide or flue gas to limit oxidation is also known from CN Cochran, DL Belitskus and DL Kinosz, Metallurgical Transcations B, Volume 8B, 1977, pages 323-331 .
The patent application EP 1 964 628 A1 discloses a method for producing aluminum ingots in which at least one step of the process is conducted under an atmosphere containing a fluorinated gas. However, the implementation of fluorinated gases is delicate and creates significant risks to people.
The patent US 5,415,220 discloses the use of molten salts of lithium chloride and potassium chloride to protect the surface of aluminum-lithium alloys during casting. However, the use of molten salts has the disadvantage of the risk of contamination of the liquid metal impurities and the difficulty of implementation.
The patent US 7,267,158 describes the forced addition of a wet gas, containing more than 0.005 kg / m ³ of water, to the surface of the molten metal so as to improve the surface quality of the cast ingots. However, this method has the disadvantage of bringing water vapor and liquid aluminum into contact despite the explosion hazards associated with contact with water and liquid aluminum.
Moreover, it is known from the demand EP 0 216 393 A1 to use dry air in a liquid aluminum processing ladle to prevent the ingress of hydrogen into the molten metal when a process gas is injected into the liquid metal and causes the breaking of the oxide protecting its surface.

The document US-2005/000677 discloses a casting process of an aluminum alloy containing at least 0.1% by weight of Mg in which a liquid surface of said alloy is brought into contact during solidification with a dry gas comprising at least about 2% by volume of oxygen and whose partial water pressure has a dew point of 0 ° C.

The problem is to find a casting process suitable for the most oxidizable 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 free of surface defects and pollution, in complete safety.

Description of the invention

A first object of the invention is a method for casting an aluminum alloy containing at least about 0.1% Mg and / or at least about 0.1% Li in which contact is made during contact. solidification of a liquid surface of said alloy with a dry gas comprising at least about 2% by volume of oxygen and whose partial pressure in water is less than about 150 Pa.

A second object of the invention is the use in a casting installation of aluminum alloys containing at least about 0.1% Mg and / or at least about 0.1% Li of a dry gas comprising at least about 2% by volume of oxygen and whose partial pressure of water is less than about 150 Pa on a liquid surface of said aluminum alloy in order to minimize oxidation thereof.

Description of figures

• Figure 1 : general diagram of a semi-continuous vertical casting installation.
• Figure 2 : diagram of a vertical casting installation including a device for supplying a flow of dried gas.
• Figure 3 : diagram of a device for supplying a dry gas flow for the casting of plates.
• Figure 4 : diagram of the thermo-balance used in example 1.
• Figure 5 : Evolution of weight gain over time for the experiments carried out with the 7449 alloy in Example 1.
• Figure 6 : Evolution geometry of weight gain over time for the experiments carried out with AA5182 alloy in Example 1.
• Figure 7 : evolution of the weight gain over time for the experiments carried out with the alloy AA2196 in example 1.
• Figure 8 : photographs of the surfaces obtained after the tests N ° 7 ( Fig. 8a ) and No. 5 ( Fig. 8b ) of Example 1.

Detailed description of the invention

The designation of the alloys follows 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 the standard EN 573-3.
Unless otherwise stated, the definitions of the European standard EN 12258-1 apply. The term "casting plant" here refers to all the devices making it possible to transform a metal in any form into a semi-product of raw form via the liquid phase. A casting plant may comprise a number of devices such as one or more furnaces necessary for the melting of the metal and / or its maintenance in temperature and / or for operations of preparation of the liquid metal and adjustment of the composition, one or more several tanks (or "pouches") intended to carry out a treatment for removing impurities dissolved and / or suspended in the liquid metal, this treatment possibly consisting in filtering the liquid metal on a filter medium in a "filtration bag" or to introduce into the bath a so-called "treatment" gas that can be inert or reactive in a "degassing ladle", a device for solidifying the liquid metal (or "casting loom"), for example by vertical semi-continuous casting by direct cooling, horizontal casting, continuous casting of yarn, continuous casting of webs between rolls, continuous casting of webs between tracks, which may include ifs such as a mold (or "mold"), a liquid metal supply device (or "nozzle") a cooling system, these different furnaces, tanks and solidification devices being interconnected by channels called "chutes In which the liquid metal can be transported.

Surprisingly, the present inventors have found that, in contact with a dry gas comprising at least about 2% by volume of oxygen and whose partial water pressure is less than about 150 Pa, a surface of liquid aluminum s oxide that allows to achieve casting free of surface defects unacceptable. This result is surprising because it is commonly accepted that moisture contained in the air can limit the oxidation of aluminum alloys in the liquid state.
In a first embodiment of the invention, this surprising effect is implemented in a casting process.
The process according to the invention is useful for highly oxidizable aluminum alloys containing at least about 0.1% Mg and / or at least about 0.1% Li. The process according to the invention is particularly useful for the alloys of the families 2XXX, 3XXX, 5XXX, 6XXX, 7XXX or 8XXX, especially when these alloys do not contain a voluntary addition of beryllium and / or calcium. The process according to the invention is particularly advantageous for alloys containing 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 alloys for which the process according to the invention is particularly advantageous are, in the family of 2XXX alloys, alloys AA2014, AA2017, AA2024, AA2024A, AA2027, AA2139, AA2050, AA2195, AA2196, AA2098, AA2198, AA2214. , AA2219, AA2524 in the family of 3XXX alloys AA3003, AA3005, AA3104, AA3915 alloys in the 5XXX family of alloys AA5019, AA5052, AA5083, AA5086, AA5154, AA5182, AA5186, AA5383, AA5754, AA5911 and in the family 7XXX alloys the AA7010, AA7020, AA7040, AA7140, AA7050, AA7055, AA7056, AA7075, AA7449, AA7450, AA7475, AA7081, AA7085, AA7910, AA7975 alloys.
The dried gas must contain at least about 2% by volume of oxygen and have a partial water pressure of less than about 150 Pa, preferably less than 100 Pa and even more preferably less than 70 Pa. In one embodiment, the invention is particularly advantageous, the partial pressure of water is even less than 30 Pa, preferably less than 5 Pa and even more preferably less than 1 Pa. The partial water pressure of a gas is also known as vapor pressure. The partial pressure 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 the gas i if this gas occupied alone all the volume offered to the mixture. The dew point of a gas is the temperature at which, while keeping current barometric conditions unchanged, the gas becomes saturated with water vapor. It can also be defined as the temperature at which the vapor pressure would be equal to the saturation vapor pressure. A partial pressure of water of 150 Pa corresponds to a dew point of -17.9 ° C. and to a quantity of water of 0.0013 kg / m ³ at this temperature. A partial pressure of water of 100 Pa corresponds to a dew point of -22.6 ° C and a quantity of water of 0.0009 kg / m ³ at this temperature. Partial pressure of water 70 Pa corresponds to a dew point of -26.5 ° C and a water quantity of 0.0006 kg / m ³ at this temperature.
The dried gas also advantageously comprises at least one gas chosen from air, helium, argon, nitrogen, carbon dioxide, carbon monoxide, combustion products of natural gas, methane, ethane, propane, natural gas, organic fluorinated compounds, organic chlorinated compounds. The addition of carbon dioxide to the dried gas can in some cases improve the antioxidant effect. In one embodiment of the invention, the dried gas comprises between 1 and 10% by volume of CO _2. However, this effect being limited and this addition having a cost, the CO ₂ content of the dried gas is less than 1%. in volume or even less than 0.1% by volume in another advantageous embodiment of the invention. In an advantageous embodiment of the invention, said dried gas is essentially air dried by any appropriate means to achieve the desired partial water pressure.
According to the invention, the dried gas is brought into contact with a liquid surface of aluminum alloy during most of the solidification of said alloy. The contacting of the gas with the surface is preferably carried out so as to establish above this surface an atmosphere whose water content is substantially equal, generally less than 10% or 20%, to that of the dried gas, that is to say, to avoid a significant diffusion of water vapor from the ambient air in said atmosphere.
Thus, when the contacting is carried out using a dry gas flow, it is advantageous for this flow to be sufficient with respect to the liquid surface subjected to the dewatered flow so as to establish the said atmosphere, if this flow is too low, the composition of said atmosphere may be too influenced by the external atmosphere and its water content may no longer correspond to the desired content.
Furthermore, it is generally not necessary to bring into contact with the dried gas all of the liquid surface of aluminum alloy available, as illustrated by the figure 1 (14, 15), to achieve the advantageous effect on the surface quality of cast products. Advantageously, the liquid surface of the aluminum alloy brought into contact with the dried gas represents at least 10%, preferably at least 25% and even more preferably at least 50% of the total liquid surface of said aluminum alloy.

A liquid surface of the aluminum alloy is kept in contact with the dried gas during most of the solidification. Thus, if it is not necessary to bring a liquid surface into contact with the dried gas as soon as the liquid metal is introduced into the refrigeration machine, casting, it is better to realize it as soon as a steady state is established. For example, in the case of vertical direct-cooling semi-continuous casting, it is preferable to carry it out at least from the beginning of the descent of the false bottom or at least from the beginning of the casting of an area that will not be cut off during subsequent operations. It is possible to vary the flow rate of a dry gas flow during casting, especially if surface defects occur. Thus, an increase in the flow rate of a dry gas flow allows in certain cases to remove furrows in the cast product. The contact between the liquid surface and the dried gas may possibly be removed before the end of the casting, especially when an area is reached which will be cut off during the following operations. In general, a liquid surface of the aluminum alloy is kept in contact with the dried gas for at least 50% or even at least 90% of the solidification.

The present invention is applicable to various casting processes and preferably to a casting process selected from vertical direct-flow semi-continuous casting, horizontal casting, continuous casting of yarn, continuous casting of rolls between rolls, continuous casting of caterpillar belts ("belt caster").
The semi-continuous vertical casting process by direct cooling aluminum alloys, known to those skilled in the art in particular under its name in the English language "Direct Chill casting" or "DC casting", is a preferred method in the context of the present invention. In this method, an aluminum alloy is cast in a mold having a false bottom, vertically and continuously displacing the false bottom so as to maintain a level of liquid metal that is substantially constant during the solidification of the alloy, the solidified faces being cooled directly with water. The figure 1 illustrates this process. An aluminum alloy is fed via a conduit (4) into an ingot mold (3) placed on a false bottom (21). The aluminum alloy solidifies by direct cooling (5). The solidifying aluminum alloy (1) has at least one solid surface (11, 12, 13) and at least one oxide - capable liquid aluminum alloy surface, which is called "liquid surface" in the present description (14, 15). A descender (2) allows to gradually lower the alloy during solidification so as to maintain the vertical position of the liquid aluminum surface (14, 15) substantially constant.

The method according to the invention is particularly advantageous for the casting of plates and billets by vertical semi-continuous casting by direct cooling. The process according to the invention is particularly advantageous for the casting of large plates, in particular of section greater than 0.5 m ² .

Numerous devices allow contacting according to the invention of the dried gas with a liquid surface of aluminum alloy. In the case of vertical direct-cooling semi-continuous casting, the device may in particular be (i) integrated with or fixed to an ingot mold so as to introduce the dried gas from the periphery of the liquid surface towards its center, (ii) positioned above the liquid surface so as to introduce the dried gas substantially perpendicular to the liquid surface, iii) fixed around a liquid metal injector so as to introduce the dried gas from the center of the liquid surface to its periphery and / or from the periphery to the center, and / or iv) be constituted by any combination of these devices.

An advantageous device for the supply of gas in the case of vertical direct flow semi-continuous casting is illustrated by the figure 2 . In this advantageous embodiment, the dried gas is supplied by means of a device (6) fixed around the liquid metal injector (4) so that the dried gas flow (7) is oriented from the core of said liquid surface towards its periphery and / or from the periphery to the core in the injection zone of the liquid metal. Advantageously, the gas supply device can be fixed on a dam retaining the oxides ("dirt dam") which is positioned around the injection zone of the liquid metal. In this way, it is possible to obtain a greater effect of the dry gas flow in the zone where the oxidation is probably the highest, that is to say near the liquid metal injector, and in the zone located between the crushed dam and the mold, this area being precisely the one most likely to generate surface defects on the cast products. In addition, this configuration also makes it possible to limit the size of the device.

The dried gas of the casting process according to the invention can also be used in other parts of a casting plant on a liquid surface of aluminum alloys containing at least about 0.1% Mg and / or less about 0.1% Li, to minimize oxidation. A casting installation comprises several other devices in which liquid surfaces of aluminum alloy are in contact with the atmosphere. Thus, the dried gas can advantageously be used to limit the oxidation of the liquid surface of alloys in an oven, in particular of fusion or of maintenance, in a treatment tank such as a filtration bag or a degassing bag or in a transfer channel such than a chute. In these uses, it is preferable to use dry gas and / or an aluminum alloy composition conditions similar to those of the process according to the invention, in particular as regards the supply of the dried gas. Advantageously, in the process according to the invention, the dried gas is also used in at least one oven, in particular for melting or holding and / or in at least one treatment tank such as a filtration bag or a degassing bag. and / or in at least one transfer channel such as a chute.
The products obtained by a process according to the invention and / or by a use according to the invention may optionally be wrought in particular by rolling, spinning and / or forging, so as to obtain in particular sheets and profiles.
The invention makes it possible in particular to cast the most oxidizable aluminum alloys, in particular aluminum alloys containing magnesium and / or lithium, without using additives such as beryllium and / or calcium and without using any device and / or expensive gas while obtaining cast ingots free of surface defects and pollution, safely.

Examples Example 1

In this example, the oxidation of the liquid metal was measured by thermogravimetric analysis. In these tests, a crucible containing the liquid metal is kept at a controlled temperature. This crucible contains about 5 kg of metal, for a diameter of 100 mm. The significant size of these experiments which makes it possible to take into account macroscopic effects can explain differences with the experiments carried out on very small quantities often reported in the prior art. The mass of the sample is weighed continuously. Weight gain is due to the oxidation of the liquid metal. A diagram illustrating this experience is presented on the figure 4 .
The dried gas (7) is supplied to the surface of the liquid metal (14) by a metal tube (6) of internal diameter 4 mm, arranged obliquely with respect to this surface. The scale (92) continuously measures the weight of the crucible (93) and its contents in situ in the oven (91). The distance between the orifice of the metal tube and the surface of the liquid metal was 120 mm. The air used can be dried until a partial water pressure of less than 70 Pa is reached. 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 1. Test conditions with thermobalance testing alloy Gas flow (1 / min) Gas Partial water pressure of the injected gas (Pa) 1 AA5182 7.9 Dry air <70 Pa 2 AA5182 0 Ambiant air > 600 Pa 3 AA2196 7.9 Dry air <70 Pa 4 AA2196 0 Ambiant air > 600 Pa 5 AA7449 4.1 Dry air <70 Pa 6 AA7449 3.8 Ambiant air > 600 Pa 7 AA7449 0 Ambiant air > 600 Pa 8 AA7449 4.1 Dry air 180 Pa 9 AA7449 3.8 Dry air 600 Pa

The Figures 5 to 8 present the results obtained.
The figure 5 shows the results obtained with AA7449 alloy. Significantly lower weight gains are obtained for test 5 for which a very dry airflow has been achieved. Bringing a liquid surface into contact with dry air whose partial pressure is still 600 Pa (dew point of -0.2 ° C, test 9) or even 180 Pa (dew point) -15.6 ° C, test 8) do not significantly limit oxidation. Similarly, the ambient air does not limit the oxidation with or without flow (tests 6 and 7), which excludes a purely mechanical effect related to a gas flow.
The figure 6 shows the results obtained with AA5182 alloy. This alloy also shows a significantly lower oxidation in the presence of a very dry air flow.
The figure 7 shows the results obtained with AA2196 alloy. It is again found for this alloy a significantly lower oxidation in the presence of a very dry air flow.
The figure 8a is a photograph of the surface obtained after the test in the case of test 7 (ambient air). A very important oxidation is observed, leading to oxidation products in the form of dark-colored cauliflower. The figure 8b is a photograph of the surface obtained after the test in the case of test 5 (dry air). A uniform gray-gray surface corresponding to a thin film of oxide is observed.

Example 2

Rectangular section plates 446 mm x 2160 mm made of AA7449 alloy were vertically cast using a direct-cooling semi-continuous casting (DC-cast) system, using an AlTiC fixture. The length of the plates obtained was 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.
The figure 3 illustrates the gas supply device that was used to supply dry air during the casting of the plates. The device consists of 4 tubes (611, 612, 621 and 622) regularly pierced with orifices (63) for injecting 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, by two pipes (81) and (82). The length L and the width 1 of the device (L = 1285 mm, 1 = 300 mm, spacing between the orifices: 20 mm) represent less than about 70% of the length and the width of the mold, so that the subject surface the dry gas flow represents about 50% of the total liquid surface of aluminum alloy (total liquid surface area: 0.96 m ² , surface subjected to a dry flow: 0.58 m ² ).
The dried gas was dry air with a partial water pressure of 60 Pa, in some cases containing 5% by volume of CO ₂ .

Table 2 describes the conditions of the various tests carried out as well as the results obtained. Table 2. Condition of casting tests and results obtained. Trial Cast length [mm] dry air flow [m ³ / h] (casting length) % CO2 of dry airflow comments 21 917 No - Long vertical furrows (~ 200mm) and deep 22 2776 None (Start) - Long vertical furrows (~ 200mm) and deep 22 (1150 mm) 5% No path 23 3575 22 (Start) 0% Some short (-40 mm) and shallow vertical furrows 27 (1150 mm) 0% Some short (-40 mm) and shallow vertical furrows 32 (2500 mm) 0% No path

The effect of dry air has been demonstrated several times: thus in test 22, bringing a liquid surface into contact with dry air made it possible to make the deep grooves disappear. Similarly, in test 23, the presence of dry air made it possible from the start to obtain a satisfactory surface quality for the cast plates (a few short (-40 mm) and shallow vertical furrows). It is also noted for this test that the increase in the flow of dry air has made it possible to remove the furrows. The effect of the presence of CO ₂ in the dried gas on the surface quality is, if it exists, second order with respect to the effect of the partial water pressure. Thus for the test 23, a satisfactory result is obtained in the absence of CO ₂ .

Claims (15)

1. Casting process for an aluminum alloy containing at least 0.1% of Mg and/or at least 0.1% of Li in which a liquid surface of said alloy is put into contact with a dried gas including at least 2% of oxygen by volume and with a water partial pressure lower than 150 Pa throughout most of the solidification process.
2. Process according to claim 1 in which the water partial pressure of said dried gas is less than 100 Pa and preferably less than 70 Pa.
3. Process according to claim 1 or claim 2 in which the gas is brought into contact with said surface in order to establish, above said surface, an atmosphere whose water content is substantially equal to that of the dried gas.
4. Process according to any of claims 1 to 3 in which said liquid surface of aluminum alloy subjected to the dried gas flow accounts for at least 10%, and preferably at least 25% and preferably still at least 50% of the whole of the liquid surface of said aluminum alloy.
5. Process according to any of the claims 1 to 4 in which said aluminum alloy is an alloy of the family 2XXX, 3XXX, 5XXX, 6XXX, 7XXX or 8XXX.
6. Process according to claim 5 in which said aluminum alloy does not contain any deliberate addition of beryllium and/or calcium.
7. Process according to any of claims 1 to 6 in which said dried gas also includes at least one gas chosen from air, helium, argon, nitrogen, carbon dioxide, carbon monoxide, natural gas combustion products, methane, ethane, propane, natural gas, organic fluorinated compounds, organic chlorinated compounds.
8. Process according to claim 7 in which said dried gas is mainly air.
9. Process according to any of claims 1 to 8 in which the CO2 content of dried gas is less than 1% by volume and preferably less than 0.1% by volume.
10. Casting process according to any of claims 1 to 9 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.
11. Direct chill casting process according to claim 10 in which said gas is supplied using a device (6) fixed around the molten metal injector (4) so that the dried gas flow 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.
12. Casting process according to any of claims 1 to 11 in which said 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 transfer channel such as a transfer trough.
13. Use, in a facility for casting aluminum alloys containing at least 0.1% of Mg and/or at least 0.1% of Li of a dried gas including at least 2% of oxygen by volume and with a water partial pressure lower than 150 Pa on a liquid surface of said aluminum alloy in order to minimize oxidation of it.
14. Use according to claim 13 in at least one device chosen from a furnace, a filtration ladle, a degassing ladle and a transfer channel.
15. Use according to claim 13 or claim 14 in which the conditions of using the dried gas and/or the composition of aluminum alloy are according to any of claims 2 to 12.

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