EP0351327B1 - Method for fabrication of thixotrope metallic products by continuous casting - Google Patents

Abstract

A method is disclosed for making thixotropic metal products by continuous casting by pouring liquid metal into a mold with a movable end and having upstream and downstream portions. The upstream portion has a wall made of a heat insulating material at least at its inner surface to form a hot zone, and the downstream portion has a wall made at least partially of a heat conducting material to form a cold zone. A movement is imparted to the solidifying liquid to cause circulation between the cold zone and the hot zone in order to bring about surface remelting of crystals formed in the cold zone and degeneration of dendrites.

Description

This invention relates to a process for the production by continuous casting of thixotropic metallic products according to the preamble of claim 1.

In what follows, the term “metallic products” means any product of elongated shape having a circular or polyhedral section constituted by a metal such as aluminum for example or one of its alloys.
By thixotropic metal product is meant any metal composition having a non-dendritic primary solid phase and more particularly a phase with dendrites degenerated to such an extent that it is in the form of substantially spheroidal particles.

These thixotropic products provide significant advantages over conventional products during their shaping. This is how the energy required for this operation is much lower, the cooling time is shorter, the shrinkage formed has reduced dimensions and the erosive action of the metal with respect to the dies or shaping molds. is significantly reduced.

Many patents teach means of obtaining such products. We can cite for example, US 3948650 and its French correspondent n ° 2141979 which describes a casting process consisting in raising the temperature of a metallic composition until it is in the liquid state, in cooling for cause a certain solidification of the liquid and vigorously stir the liquid-solid mixture until about 65% by weight of the mixture thus formed is in the form of a solid having individual degenerate dendrites or nodules.

This process was later perfected to become continuous in US 3902544.
Then, according to the above-mentioned process, we set out in US 4434837 to produce a suitable stirring device comprising a two-pole stator which creates a rotating magnetic field moving perpendicular to the axis of the mold and generates electromagnetic forces directed tangentially to the mold and such that they cause a shear rate of at least 500 sec⁻¹. We also carried out in US 4457355 a mold formed of two parts of different thermal conductivity and in EP 71822 a mold formed of a succession of insulating and conductive sheets.

In more recent patent applications, improvements have consisted in US 4482012 in using a mold formed by two chambers connected to each other by a non-conductive seal, the first of which acts as a heat exchanger and in the US 4565241 stirring conditions have been recommended such that the ratio of the shear rate to the solidification rate is between 2.10³ and 8.10³.

Admittedly, this route of obtaining thixotropic products by stirring casting has led to suitable products.
However, we have arrived in the prior art at devices using electrical inductors with rotating field responsible for printing metal during solidification at high rotational speeds in a plane perpendicular to the axis of the mold so to stir it and break the dendrites to give the crystals the shape of spheroidal particles, that is to say that the thixotropic structure is obtained by a mechanical effect.

In addition, as US 4482012 indicates, it is essential to be able to closely control the heat extraction from the mass during solidification. Hence the realization of fragile and complicated to regulate heat exchangers formed by a clever assembly of thermally conductive and insulating parts which bring the metal to a temperature as close as possible to the liquidus while avoiding solidification on the walls of the mold.

This is why the applicant interested in the manufacture of thixotropic products but seeking to overcome the contingencies of the techniques of the prior art has developed a casting process in which, according to the invention, the metal is poured liquid in a mold provided at one of its ends with a movable bottom and consisting of two adjacent parts of the same axis which, according to the direction of the pouring, form an upstream part called hot zone whose wall is made of a material insulating heat at least on its internal face and a downstream part called cold zone whose wall is made, at least partially, of a heat conducting material and where the external surface is cooled by a coolant so as to cause, by solidification within the liquid contained in said part, the appearance of crystals and the formation on contact with the internal surface of a solid crust sufficiently rigid to allow the gradual extraction of the product thus formed using of the movable bottom, this process being characterized in that a movement is imparted to the liquid in the course of solidification ensuring at least one transfer from the cold zone to the hot zone and vice versa of duration ≦ 1 second to cause reflow in surface of the crystals it contains and ensure degeneration of the dendrites.

Thus, the invention consists in introducing a liquid metal into a mold composed of an upstream part constituted by a material having heat-insulating properties at least as regards its wall in contact with the metal. This material can be, for example, of the type of those which are commonly used in foundry for the manufacture of chutes or nozzles. Due to the reduced heat exchanges which take place in this part, the metal maintains itself normally, that is to say without any external disturbance, at a temperature sufficient for no crystallization to occur. Hence the designation of this part by the expression "hot zone".
This upstream part is connected by means of a suitable joint to a downstream part which, unlike the previous one, is very good conductor of heat at least over a portion of its height located most downstream and which, because of its ability to easily evacuate the calories from the metal it contains to the outside is called the "cold zone". This part is the analog of the ingot mold in a conventional continuous casting and it is within it that the crystallization process is initiated and that from the wall cooled externally cooled by a coolant fluid a crystalline envelope sufficiently rigid to allow progressive extraction using the movable bottom of the cast product, while inside this envelope delimited by the "solidification front", surface having the general profile of a meniscus whose apex is oriented towards the 'downstream, a "swamp" is formed, consisting of a mixture of liquid and generally dendritic solid particles, particles which will gradually integrate into the solidification front and allow the solid part to develop and the flow to progress.
We thus have a hot zone-cold zone containing respectively a liquid and a liquid loaded with dendritic particles and it is to this liquid that a movement is imparted such that the particles are entrained towards the hot zone. Under these conditions, it is found that the particles lose at least part of their ramifications and tend to spheroidize. However, for this phenomenon to be significant enough, the transfer from one zone to another must take place quickly and in any case for a duration less than or equal to one second. The shorter the duration, the better the degeneration rate of dendrites. It is obvious that this movement from the cold zone to the hot zone is accompanied by a reverse movement so that the particles return to the original zone and can then carry out a new cycle. During these cycles, the particles are brought into contact with the solidification front and some of them cling to it so that the product obtained is formed at least in part from degenerate particles which will at least partially give it thixotropic properties.

Preferably, the movement of the particles takes place in at least loops, the assembly of which generates a torus with an axis substantially coincident with the axis of the mold. These loops are located in meridian planes of the mold, that is to say passing through its axis, and each is entirely contained in the half-plane limited by said axis. Preferably, the portion of the loop along which the liquid passes from the cold zone to the hot zone is closest to the axis, the portion corresponding to the return being close to the wall of the mold.

From this description, two fundamental differences can be seen between the process of the prior art and that of the invention. In the first, the circulation of the liquid takes place by rotation around the axis of the mold, that is to say in a plane perpendicular to said axis and the degeneration is obtained by breaking the crystals maintained at a substantially constant temperature. In the second, the main circulation of the liquid takes place parallel to the axis of the mold and the degeneration results from a thermal and not mechanical phenomenon. This makes it possible to get rid of the contingency of maintaining the crystals at a temperature always close to the liquidus and therefore of the use of sophisticated heat exchangers with a delicate adjustment and also to resort to means of producing much more movement. simple than rotating field generators.

Preferably, two types of means are used:
One of them is to pass a single-phase electric current of frequency less than or equal to the industrial frequency within the downstream part of the mold which is known to be made at least partially by an electrically conductive material . However, the wall of this part must have over its entire thickness and along at least one generator an insert of material insulating from electricity on either side of which are fixed current leads. Thus, this part plays the role of whorl and the current which crosses it generates a magnetic field which develops electromagnetic forces generating the desired movement. In addition, the internal wall of this part must be covered with an electrically insulating film so that there is no electrical continuity between said metal part and the cast metal because if this were the case it would cause a short circuit and would prevent the development of the magnetic field conducive to movement.
The electromagnetic forces being a function of the intensity of the current flowing in the turn, preferably used for making the downstream part of metals of low electrical resistivity but mechanical strength nevertheless compatible with the cast metal. It can be, for example, copper or aluminum and their alloys in the case where aluminum is poured.

But we have also found that we could use assemblies made of different materials in which the portion closest to the upstream part is made if not with an insulating material at least in a material less good conductor of electricity such as stainless steel, for example. Under these conditions, the movement of the liquid can be amplified.

As for the insulating film, it may consist of an oxide layer obtained by anodization in the case of aluminum or an enamel, or even a fluorocarbon resin for example. The thickness of this film is a function of the electrical voltage under which the wall is located relative to the cast metal. We can use an oxide thickness of 1 µm for a voltage of 100 volts.

The downstream parts thus formed can be fitted on their internal face with a graphite ring a few millimeters thick which plays the role of lubricant with respect to the cast metal and can amplify the role of a lubricating agent with which it is sometimes necessary to coat the inner wall of the downstream part to facilitate the casting of certain metals.

This ring can be divided according to its generators in at least two sectors to avoid not only any Joule effect in the zone where on the contrary it is desired to cool, but also a reduction in energy which would limit the movement of the metal.

In a very particular way, one can use a ring having an insert placed opposite the insert of the downstream part; in this case, the Joule effect is also avoided, but the ring can then be shrunk directly onto the internal wall of said part without the need for an intermediate insulating film.

The other means of producing the movement of the liquid within the mold consists in placing at the outside of the downstream part of the mold at least one metal coil of axis substantially parallel to the axis of the mold and having it pass through by a single-phase current of frequency less than or equal to the industrial frequency. This turn electrically insulated from the wall of said part in fact creates a magnetic field parallel to the axis of the mold which develops electromagnetic forces generating the desired movement. Admittedly, this movement is more or less wide and a function of the intensity admitted in the turn, but it also depends on other factors such as the composition of the material constituting the wall of the cold zone or the structure of said wall.

According to the first factor, it is preferable to use a material having a resistivity greater than 5 µΩ.cm. It may for example be a non-magnetic stainless steel or titanium or else a ceramic provided that it has sufficient thermal conductivity. In the case of casting aluminum, the best solution not to break with the habits of the profession is to use aluminum but in the form of an alloy containing by weight about 1.8% Mn; 0.25% Cr; 0.2% Ti and 0.1% V whose resistivity is equal to 9.3 µΩ.cm instead of less than 3 µΩ.cm for conventional alloys. This resistivity can however be increased by adding Mg up to 5% in which case, values are reached from 11 to 12 µΩ.cm. The addition of Li up to 1% or Zr up to 0.15% is also favorable.

Other solutions consist in using composite materials such as for example stainless steel coated internally with a thin layer of aluminum.

According to the second factor, to reduce the intensity necessary for movement, the wall of the cold zone is divided along its generators into at least two sectors separated from each other by an electrical insulator such as mica, said sectors being kept assembled together by means of stainless steel pins and insulating dowels.

All these types of embodiment of the downstream part can also be lined on their internal wall and in the vicinity of the hot zone with a coaxial graphite ring preferably shared along its generatrices in at least two sections, all these particularities always having the aim to improve the efficiency of the electric current in its transformation into electromagnetic forces generating movement.

All the turns which surround the downstream part of the mold are designed and mounted so as to be able to adapt to any form of downstream part and to best respond to obtaining both optimum current-force performance. and a distribution of force within the metal which ensures movement of the liquid over the entire section and the entire height of the mold in order to cause the greatest possible degeneration of the dendrites on the greatest possible number of crystals.

Thus these turns can be moved parallel to the axis of the mold or formed by an assembly of removable elements capable of circumscribing molds of any section equidistantly, or at different distances. These assemblies are perfectly suitable for products with a rectangular section.

Other particularities can be included in the invention always aiming to improve the efficiency of the movement of the metal such as the addition around the hot zone of at least one metallic turn traversed by an electric current, this or these turns being connected either to that (s) of the cold zone, or to a current generator of intensity, frequency and / or phase different from the current supplying the coil (s) of the cold zone.

In order to channel the magnetic field created by the coil (s), the cold zone may be surrounded by magnetic yoke elements formed from metal sheets electrically isolated from one another and situated in planes passing through it. mold axis.

The cooling of the cold zone is obtained as is known either by means of fluid boxes integrated into the external wall of said zone or by direct application of a peripheral fluid blade on said wall.

Depending on the desired degree of cooling and its location to develop more or less quickly in a given place the formation of crystals and their sending into the hot zone at a more or less large stage of evolution, the fluid is adjusted in flow and / or in temperature while modifying in the case of direct cooling the impact surfaces of the fluid blade.

The hot zone or at least its part closest to the cold zone can be surrounded by a sheath in which circulates a gas under pressure and chemically inert with respect to the cast metal because under these conditions it is found that the cast product then presents a better surface appearance.

The invention will be better understood using FIG. 1 which represents a vertical half-section passing through the axis of a mold applicable to the invention.
There are: an upstream part 1 made of a heat insulating material which encloses the liquid metal 2 and forms the hot zone, a downstream part 3 made of heat conducting material fitted internally with a graphite ring 4 and cooled externally by a film 5 of water from a supply box 6 which forms the cold zone.
Under the effect of cooling due to water, the metal solidifies along the front 7 to give the product 8 cast.
A coil 9 supplied with alternating current surrounds the cold zone and creates a magnetic field which induces electromagnetic forces so that the liquid metal moves along arrow 10 parallel to the axis of the mold towards the hot zone and returns to the cold zone along the wall of the mold according to arrow 11 causing in its movement the particles 12.
The invention can be illustrated using the following application examples:

Example 1

• la partie amont était formée d'un anneau en MONALITE de hauteur 50 mm
• la partie aval en aluminium était revêtue intérieurement d'une fine couche anodisée (5 µm) et d'une bague graphite sectorisée en 12 morceaux, et était fendue sur toute sa hauteur. Le courant circulait directement à travers la partie aval , à laquelle d'ailleurs deux amenées avaient été fixées de part et d'autre de la fente. La tension aux bornes de ces amenées était alors de 1,05V. La vitesse de coulée était de 200 mm/min, ce qui est classiquement utilisé pour ce diamètre de billette. Un exemple de structure obtenue à coeur de la billette examinée par micrographie (voir fig. 2 sous grossissement 50) permet de se rendre compte de l'efficacité du procédé à obtenir une structure à dendrites dégénérées.

A 70 mm diameter billet of aluminum alloy of the AS7G0.3 type (that is to say containing by weight%: Si = 7 and Mg = 0.3) was produced according to the method described above:

• the upstream part was formed by a MONALITE ring 50 mm high
• the downstream aluminum part was coated internally with a thin anodized layer (5 µm) and a graphite ring segmented into 12 pieces, and was split over its entire height. The current flowed directly through the downstream part, to which, moreover, two leads had been fixed on either side of the slot. The voltage across these leads was then 1.05V. The casting speed was 200 mm / min, which is conventionally used for this billet diameter. An example of a structure obtained at the heart of the billet examined by micrography (see fig. 2 under magnification 50) makes it possible to appreciate the effectiveness of the process in obtaining a structure with degenerate dendrites.

Example 2

An alloy 2124 (according to the standards of the Aluminum Association) was cast in the form of a billet with a diameter of 400 mm according to the method described. The overall design of the tool was similar to that described in the previous example, with the exception of the current flow; in this case, it was operated through a whorl independent of the downstream part. The casting speed was 40 mm / min, which is conventionally used for this billet diameter.
After micrographic examination, it was found that with the exception of a peripheral zone of the order of 15 mm, the structure of the grains was particularly rounded, practically without dendrite arms and of very fine size, from the 'around 70 µm.

Example 3

A casting of plates in 800 × 300 mm format in a 7075 alloy (according to the standards of the Aluminum Association) was carried out according to the process described. As in the case of the ⌀ 400 mm billet, a turn surrounded the external face of the downstream part, at a short distance (10 mm). This turn consisted in fact of 4 elements of copper bar, internally cooled by water, these elements being interconnected in 3 corners and connected to the current leads in the 4th. The casting speed was 60 mm / min.
Macrographic examination of the cast product revealed a homogeneous and fine structure, with the exception of the corners, which exhibited an even finer structure. By micrographic examination, one could note a notable modification of the morphology of the grains which took forms in "potatoes" instead of the traditional forms "in cauliflower". A selective attack intended to reveal the arms of the dendrites showed that they had almost completely disappeared.

Claims (21)

1. A method of making thixotropic metal products by continuous casting, particularly products made of aluminium alloys with degenerate dendrites in at least part of their structure, wherein the liquid metal (2) is poured into a mould fitted with a movable end at one extremity and made up of two adjacent coaxial portions, wherein said portions form an upstream portion (1) in the casting direction, described as the hot zone, with its wall made of a heat insulating material at least on the inner surface, and a downstream portion (3) described at the cold zone, with its wall made at least partially of a heat conductive material, and wherein the external surface is cooled by a heat exchanging fluid (5), so as to make crystals appear through solidification within the liquid contained in that portion, and so as to make a solid crust form on contact with the inner surface, the crust being rigid enough to enable the product (8) thus formed gradually to be extracted with the aid of the movable end, characterised in that movement is imparted to the solidifying liquid, at least transferring it from the cold zone to the hot zone (10) and vice versa (11) within ≦ 1 second, to make the crystals (12) contained in the liquid remelt at the surface and to make the dentrites degenerescent.

2. The method of Claim 1, characterised in that the movement takes place in loops located in meridian planes, which together generate a torus with its axis substantially identical with the axis of the mould.

3. The method of Claim 1, characterised in that the inner wall of the cold zone is covered with a lubricating agent.

4. The method of Claim 1, characterised in that the movement is obtained by passing a monophase electric current of a frequency no higher than industrial frequency within the downstream portion of the mould, that the wall of that portion has an insert of electrical insulating material right through its thickness and along at least one generatrix, with power leads fixed one on each of the inset, and that said portion is coated internally with an electrically insulating film.

5. The method of Claim 4, characterised in that the inner wall of the cold zone is covered with a graphite ring coaxial with the hot and cold zones, over its whole periphery and at least in the vicinity of the hot zone.

6. The method of Claim 5, characterised in that the graphite ring is divided into at least two sectors along its generatrices.

7. The method of Claim 1, characterised in that the movement is obtained by means of at least one metal winding placed outside the cold zone of the mould, with its axis substantially parallel with the axis of the mould, and that a monophase current at a frequency no higher than the industrial frequency passes through the winding.

8. The method of Claim 7, characterised in that the cold zone is made of solid material with a resistivity over 5 µΩ x cm.

9. The method of Claim 7, characterised in that the cold zone is divided along its generatrices into at least two sectors, which are separated by an electrical insulator.

10. The method of Claim 7, characterised in that the cold zone is made of a combination of different materials.

11. The method of Claim 7, characterised in that the inner wall of the cold zone is covered with a graphite ring coaxial with the hot and cold zones, over its whole periphery and at least in the vicinity of the hot zone.

12. The method of Claim 11, characterised in that the graphite ring is divided into at least two sectors along its generatrices.

13. The method of Claim 7, characterised in that the winding (or windings) are displaced parallel with the axis of the mould.

14. The method of Claim 7, characterised in that the distance between the winding (or windings) and the outer wall of the cold zone is adjusted.

15. The method of Claim 1, characterised in that the hot zone contains at least one metal winding which is supplied with electric current.

16. The method of Claims 4 and 15, characterised in that the winding is connected to the cold zone.

17. The method of Claim 4, characterised in that the cold zone is surrounded by laminated magnetic yokelk elements, with the individual sheets located in planes passing through the axis of these zones.

18. The method of Claim 1, characterised in that the cold zone is cooled by means of a refrigerating fluid with a variable flow rate.

19. The method of Claim 1, characterised in that the cold zone is cooled by means of a refrigerating fluid with a variable temperature.

20. The method of Claim 1, characterised in that the cold zone is cooled by means of a refrigerating fluid which cools the zone in specific locations.

21. The method of Claim 1, characterised in that a pressurised gas is injected at the level of the cold zone.

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