EP3626028B1 - Levitation melting method using movable induction units - Google Patents

Description

This invention relates to a levitation melting method and a device for producing castings with movable induction units. The method uses induction units in which the respective opposite ferrite poles are designed to be movable with the induction coils and move in opposite directions. In this way, the induction units for melting the batches can be arranged close to one another in order to achieve an increase in efficiency of the induced magnetic field. When the molten charge is poured off, the induced magnetic field is reduced by increasing the distance between the ferrite poles and the induction coils and thus avoiding contact with the melt with the ferrite poles or the induction coils.

State of the art

Suspended melting processes are known from the prior art. So already revealed DE 422 004 A a melting process in which the conductive melting material is heated by inductive currents and at the same time is kept floating due to the electrodynamic effect. A casting process is also described there, in which the molten material is pressed into a mold by means of a magnet (electrodynamic press casting). The process can be carried out in a vacuum.

US 2,686,864 A also describes a method in which a conductive melting material z. B. is suspended in a vacuum under the influence of one or more coils without the use of a crucible. In one embodiment, two coaxial coils are used to stabilize the material in suspension. After melting, the material is dropped into a mold or poured off. With the method described there, a 60 g portion of aluminum could be kept in suspension. The molten metal is removed by reducing the field strength so that the melt escapes downwards through the tapered coil. If the field strength is reduced very quickly, the metal falls out of the device in the molten state. It has already been recognized that the "weak spot" of such coil arrangements lies in the middle of the coils, so that the amount of material that can be melted in this way is limited.

Also US 4,578,552 A discloses an apparatus and method for levitation melting. The same coil is used both for heating and for holding the melt, the frequency of the alternating current applied being varied to regulate the heating power, while the current strength is kept constant.

The particular advantages of suspension melting are that contamination of the melt by a crucible material or other materials that are in contact with the melt in other processes is avoided. Likewise, the reaction of a reactive melt, for example of titanium alloys, with the crucible material is excluded, which would otherwise force ceramic crucibles to escape onto copper crucibles operated using the cold crucible method. The floating melt is only in contact with the surrounding atmosphere, which is e.g. B. can be vacuum or protective gas. Since there is no fear of a chemical reaction with a crucible material, the melt can also be heated to very high temperatures. In contrast to cold crucible melting, there is also no problem that its effectiveness is very low because almost all of the energy that is introduced into the melt is dissipated into the cooled crucible wall, which leads to a very slow rise in temperature with high power input. In the case of levitation melting, the only losses due to radiation and evaporation are significantly lower compared to the thermal conduction in the cold crucible. Thus, with a lower power input, a greater overheating of the melt is achieved in an even shorter time.

In addition, especially in comparison to the melt in the cold crucible, the waste of contaminated material during the levitation melting is reduced. However, the floating melting has not become established in practice. The reason for this is that only a relatively small amount of molten material can be suspended in the levitation melting process (cf. DE 696 17 103 T2 , Page 2, paragraph 1).

Furthermore, to carry out a levitation melting process, the Lorentz force of the coil field must compensate for the weight of the batch in order to be able to keep it in suspension. It pushes the batch upwards out of the coil field. In order to increase the efficiency of the generated magnetic field, the aim is to reduce the distance between the opposite ferrite poles. The reduction in distance allows the same magnetic field that is required to hold a specific melt weight to be generated with a lower voltage. In this way, the holding efficiency of the system can be improved so that a larger batch can be levitated. The heating efficiency is also increased, since the losses in the induction coils are reduced. A device for levitation melting with rigidly arranged ferrite poles is known, for example, from US Pat DE 1565467 known.

The smaller the distance between the ferrite poles, the greater the induced magnetic field. However, as the distance decreases, the risk of contamination of the ferrite poles and the induction coils with the melt increases, since the field strength for the casting must be reduced. This not only reduces the holding force in the vertical direction, but also that in the horizontal direction. This leads to a horizontal expansion slightly above of the coil field levitating melt, which makes it extremely difficult to drop it into the casting mold positioned underneath through the narrow gap between the ferrite poles. Therefore, there is a practical limit to increasing the load capacity of the coil field by reducing the distance between the ferrite poles, which is determined by the probability of contact.

The disadvantages of the methods known from the prior art can be summarized as follows. Solid-state melting processes can only be carried out with small amounts of material, so that industrial application has not yet taken place. Casting in molds is also difficult. This applies in particular to the case where the efficiency of the coil field in the generation of eddy currents is to be increased by reducing the distance between the ferrite poles.

task

It is therefore an object of the present invention to provide a method and a device which enable the floating melting to be used economically. In particular, the method should allow the use of larger batches and improve throughput due to improved cycle field efficiency and shorten cycle times, while ensuring that the casting process continues safely without contact of the melt with the coils or their poles.

Description of the invention

• Versetzen der Induktionsspulenpaare in die Schmelzposition mit geringem Abstand,
• Einbringen einer Charge eines Ausgangsmaterials in den Einflussbereich wenigstens eines elektromagnetischen Wechselfelds, so dass die Charge in einem Schwebezustand gehalten wird,
• Schmelzen der Charge,
• Positionieren einer Gussform in einem Füllbereich unterhalb der schwebenden Charge,
• Abguss der gesamten Charge in die Gussform durch Bewegen der Induktionsspulen in mindestens einem Paar von der Schmelzposition mit geringem Abstand in die Abgussposition mit weitem Abstand,
• Entnahme des erstarrten Gusskörpers aus der Gussform.

The object is achieved by the method and the device according to the invention. According to the invention is a method for the production of castings from an electrically conductive material in the levitation melting method, wherein to bring about the levitation of a batch electromagnetic alternating fields are used, which are generated with at least one pair of opposing induction coils with a core made of a ferromagnetic material, the induction coils with their cores are movably arranged in each pair and move between a melting position at a short distance and a casting position at a large distance, comprising the following steps:

• Placing the induction coil pairs in the melting position with a short distance,
• Introducing a batch of a starting material into the area of influence of at least one alternating electromagnetic field, so that the batch is kept in a state of suspension,
• Melting the batch,
• Positioning a mold in a filling area below the floating batch,
• Casting the entire batch into the mold by moving the induction coils in at least one pair from the melting position at a short distance to the casting position at a large distance,
• Removing the solidified casting from the mold.

The volume of the molten batch is preferably sufficient to fill the mold to an extent sufficient for the production of a cast body (“filling volume”). After filling the mold, the mold is allowed to cool or is cooled with coolant, so that the material solidifies in the mold. The cast body can then be removed from the mold.

According to the invention, a “conductive material” is understood to mean a material which has a suitable conductivity in order to inductively heat the material and keep it in suspension.

According to the invention, a “floating state” is understood to mean a state of complete floating, so that the treated batch has no contact whatsoever with a crucible or a platform or the like.

The term “ferrite pole” is used synonymously with the term “core made of a ferromagnetic material” in the context of this application. The terms "coil" and "induction coil" are also used synonymously next to each other.

The efficiency of the generated alternating electromagnetic field can be increased by moving the induction coil pairs closer together. This enables even heavier batches to be levitated. However, when casting a batch, the risk of the molten batch touching the coils or ferrite poles increases with a decreasing free cross section between the coils. Such contaminations are to be strictly avoided, since they are difficult and expensive to remove again and therefore result in a longer downtime of the system. In order to take advantage of the closer spacing of the induction coil pairs as much as possible, without the risk of contamination during casting need to take, the induction coils with their cores according to the invention are movably supported at least in one pair. The coils of a pair preferably move in opposite directions in a centrosymmetric manner around the center of the induction coil arrangement.

The coils are pushed into the melting position to melt the batch. If the batch has melted and is to be poured into the casting mold, the coils are not simply switched off or the current level is reduced, as is customary in the prior art, but are moved according to the invention to a casting position. This increases the distance between the coils, which on the one hand creates a larger free diameter for the melt on its way into the casting mold and on the other hand continuously and in a controlled manner reduces the load-bearing capacity of the induced magnetic field. In this way, the melt is safely kept away from the induction coils and their cores as they pass through the coil plane and only slowly passes into the case because the field is already weakened in the center, but is still strong enough at the coils to meet the requirements To prevent contact. This prevents contamination of the coils and ensures a clean cast into the mold without splashing.

In a preferred embodiment variant of the invention, when the batch is cast, the current intensity in these induction coils is reduced simultaneously with the movement of the induction coils in the pairs of induction coils from the melting position into the casting position. As a result, the required displacement path of the induction coils can be reduced, since the induced magnetic field is no longer reduced only by the greater distance of the induction coils. However, it is important to ensure that the reduction in the current intensity is coordinated with the shifting of the coils in such a way that the field strength is always sufficiently high to be able to keep the melt away from the coils.

In one embodiment, the distance between the induction coils in the induction coil pairs from the melting position to the casting position is increased by 5-100 mm, preferably 10-50 mm. When determining the displacement path, it must be taken into account for which batch weights the system is to be designed and how large the minimum distance between the coils and the field strength that can be generated with them.

In a preferred embodiment, the electrically conductive material used according to the invention has at least one high-melting metal from the following group: titanium, zirconium, vanadium, tantalum, tungsten, hafnium, niobium, rhenium, molybdenum. Alternatively, a less high-melting metal such as nickel, iron or aluminum can be used. A mixture or alloy with one or more of the aforementioned metals can also be used as the conductive material. The metal preferably has a proportion of at least 50% by weight, in particular at least 60% by weight or at least 70% by weight, of the conductive material. It has been shown that these metals particularly benefit from the advantages of the present invention. In a particularly preferred embodiment, the conductive material is titanium or a titanium alloy, in particular TiAl or TiAIV.

These metals or alloys can be processed particularly advantageously because they have a pronounced dependence of the viscosity on the temperature and, moreover, are particularly reactive, in particular with regard to the materials of the casting mold. Since the method according to the invention combines contactless melting in suspension with extremely rapid filling of the casting mold, a particular advantage can be realized for such metals. With the method according to the invention, castings can be produced which have a particularly thin or even no oxide layer from the reaction of the melt with the material of the casting mold. And in the case of the high-melting metals in particular, the improved utilization of the induced eddy current and the exorbitant reduction in heat losses due to thermal contact have a noticeable effect on the cycle times. Furthermore, the load capacity of the generated magnetic field can be increased, so that even heavier batches can be kept in suspension.

In an advantageous embodiment of the invention, the conductive material is superheated during melting to a temperature which is at least 10 ° C., at least 20 ° C. or at least 30 ° C. above the melting point of the material. Overheating prevents the material from instantaneously solidifying when it comes into contact with the mold, whose temperature is below the melting temperature. It is achieved that the batch can be distributed in the mold before the viscosity of the material becomes too high. It is an advantage of levitation melting that there is no need to use a crucible that is in contact with the melt. The high loss of material from the cold crucible process on the crucible wall is avoided, as is contamination of the melt by crucible components. Another advantage is that the melt can be heated to a relatively high degree, since it can be operated in a vacuum or under protective gas and there is no contact with reactive materials. However, most materials cannot be overheated arbitrarily, otherwise a violent reaction with the mold is to be feared. The overheating is therefore preferably limited to at most 300 ° C., in particular at most 200 ° C. and particularly preferably at most 100 ° C. above the melting point of the conductive material.

In the method, in order to concentrate the magnetic field and stabilize the charge, at least one ferromagnetic element is arranged horizontally around the area in which the charge is melted. The ferromagnetic element can be arranged in a ring around the melting area, with not only circular elements but also under "ring" angular, in particular quadrangular or polygonal ring elements can be understood. So that the movement of the induction coils according to the invention is possible, the ring elements are divided into sub-segments according to the number of coils, between which the respective induction coils with their poles move in a form-fitting manner. The ferromagnetic element can furthermore have a plurality of rod sections which, in particular, project horizontally in the direction of the melting range. The ferromagnetic element consists of a ferromagnetic material, preferably with an amplitude permeability µ _a > 10, more preferably µ _a > 50 and particularly preferably µ _a > 100. The amplitude permeability relates in particular to the permeability in a temperature range between 25 ° C. and 150 ° C and with a magnetic flux density between 0 and 500 mT. The amplitude permeability is in particular at least one hundredth, in particular at least 10 hundredths or 25 hundredths of the amplitude permeability of soft magnetic ferrite (for example 3C92). Suitable materials are known to the person skilled in the art.

According to the invention is also a device for levitation melting of an electrically conductive material, comprising at least a pair of opposing induction coils with a core made of a ferromagnetic material for bringing about the levitation of a charge by means of alternating electromagnetic fields, the induction coils with their cores being arranged movably in relation to each other in each pair and move between a melting position with a small distance and a casting position with a large distance.

Brief description of the figures

• Figure 1 is a side sectional view of a mold below a melting area with ferromagnetic material, coils and a batch of conductive material.
• Figure 2 is a top view of an arrangement with two pairs of coils and a ferromagnetic annular element.

Figure description

The figures show preferred embodiments. They are used only for illustration.

Figure 1 shows a batch (1) made of conductive material, which is in the area of influence of alternating electromagnetic fields (melting range), which are generated with the help of the coils (3). Below the batch (1) there is an empty mold (2) which is held in the filling area by a holder (5). The casting mold (2) has a funnel-shaped filling section (6). The holder (5) is suitable for moving the mold (2) from a feed position into a casting position to lift what is symbolized by the drawn arrow. A ferromagnetic material (4) is arranged in the core of the coils (3). The axes of the pair of coils (3) are aligned horizontally, with two opposing coils (3) forming a pair. The drawing shows the melting position of the coil arrangement at a short distance.

The batch (1) is melted in the process according to the invention in suspension and poured into the casting mold (2) after the melt has taken place. For casting, the coils (3), as symbolized by the arrow shown, are separated from each other until the Lorentz force of the field can no longer compensate for the weight of the batch (1).

Figure 2 shows a plan view of an arrangement with two pairs of coils and a ferromagnetic annular element (7). The ring-shaped element (7) is designed as an octagonal ring element. Two coils (3) with their ferromagnetic material (4) lying on an axis A, B form a pair of coils. The coil axes A, B are arranged at right angles to each other. The figure shows the melting position of the coil arrangement with narrow distances between the coils (3). The ferromagnetic materials (4) positively seated in the annular element (7) then move together with their coils (3), as indicated by the double arrows, to pour the levitating melt outwards.

Reference symbol list

1

Batch

2nd

mold

3rd

Induction coil

4th

ferromagnetic material

5

holder

6

Filling section

7

annular element

Claims (5)

1. A method for the production of cast bodies made of an electrically conductive material with the levitation melting method, wherein for the realization of the levitation state of a batch (1) electromagnetic alternating fields are used which are generated with at least one pair of induction coils (3) being oppositely arranged and having a core made of a ferromagnetic material (4), wherein the induction coils (3) with their cores in each pair are arranged in a movable manner to each other and are moved between a melting position with low distance and a casting position with high distance, comprising the following steps of:

   - bringing the pairs of induction coils into the melting position with low distance,

   - introducing a batch (1) of a starting material into the zone of influence of at least one electromagnetic alternating field so that the batch (1) is hold in a levitation state,

   - melting the batch (1),

   - positioning a casting mold (2) in a filling zone below the levitating batch (1),

   - casting the whole batch (1) into the casting mold (2) by moving the induction coils (3) in at least one pair from the melting position with low distance into the casting position with high distance,

   - removing the solidified cast body from the casting mold (2).
2. The method according to claim 1, characterized in that during casting of the batch (1) contemporaneously with the moving of the induction coils (3) in the pairs of induction coils from the melting position into the casting position the current in these conduction coils (3) is reduced.
3. The method according to claim 1 or 2, characterized in that the distance of the induction coils (3) in the pairs of induction coils from the melting position to the casting position is increased by 5 - 100 mm, preferably 10 - 50 mm.
4. A device for the levitation melting of an electrically conductive material, comprising at least one pair of induction coils (3) being oppositely arranged and having a core made of a ferromagnetic material (4) for the realization of the levitation state of a batch (1) by means of electromagnetic alternating fields, characterized in that the induction coils (3) with their cores in each pair are arranged in a movable manner to each other and are moved between a melting position with low distance and a casting position with high distance.
5. The device according to claim 4, characterized in that the distance of the induction coils (3) in the pairs of induction coils from the melting position to the casting position is increased by 5 - 100 mm, preferably 10 - 50 mm.

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