AMORPHOUS METAL FOIL AND METHOD FOR PRODUCING AN AMORPHOUS METAL FOIL USING A RAPID SOLIDIFICATION TECHNOLOGY

Abstract

Amorphous metal foil and method for the production of an amorphous metal foil using a rapid solidification technology is provided. An amorphous metal foil having a width of 2 mm to 300 mm, a thickness of less than 20 μm and a maximum of 50 holes per square metre is also provided.

Claims

1. An amorphous metal foil, having a width of 2 mm to 300 mm, a thickness of less than 20 μm and a maximum of 50 holes per square meter.

2. An amorphous metal foil according to claim 1, the amorphous metal foil having a width of 20 mm to 200 mm and/or a thickness of between 10 μm and 18 μm and/or fewer than 25 holes per square meter.

3. An amorphous metal foil according to claim 1, wherein the holes each have a diameter of up to 5 mm.

4. An amorphous metal foil according to claim 1, wherein the foil has a total area of at least 10 square meters and on average fewer than 50 holes per square meters.

5. An amorphous metal foil according to claim 1, wherein the thickness is the average thickness of the foil over a length of 2 km.

6. An amorphous metal foil according to claim 1, wherein the thickness is the average thickness over the width of the foil.

7. An amorphous metal foil according to claim 1, wherein the foil has a wheel side that was formed by solidification on the outer surface of a heat sink, and an opposing, air side, wherein the wheel side of the foil has a surface roughness with an arithmetic mean, Ra, of less than 0.8 μm.

8. An amorphous metal foil according to claim 1, wherein the wheel side has a surface roughness with a deviation of less than +/−0.2 μm over a length of at least 2 km and/or over a surface of at least 100 m.sup.2.

9. An amorphous metal foil according to claim 1, wherein the foil has a continuous length of at least 2 km.

10. An amorphous metal foil according to claim 1, wherein the amorphous metal foil is a nickel-based foil or a cobalt-based foil or a copper-based foil.

11. An amorphous metal foil according to claim 1, wherein the amorphous metal foil is an iron-based foil.

12. An amorphous metal foil according to claim 11, wherein the foil comprises (Fe,T).sub.aM.sub.b and up to 1 at. % impurities, where 70 at. %≤a≤90 at. % and 10 at. %≤b≤30 at. %, T is one or more of the elements Co, Ni, Mn, Cu, Nb, Mo, Cr, Zn, Sn and Zr and M is one or more of the elements B, Si, C and P.

13. An amorphous metal foil according to claim 11, wherein the foil comprises Fe.sub.aCu.sub.bM.sub.cM′.sub.dM″.sub.eSi.sub.fB.sub.g and up to 1 at. % impurities, M is one or more of the elements from the group of IVa, Va, VIa elements or the transition metals, M′ is one or more of the elements Mn, Al, Ge and the platinum elements and M″ is Co and/or Ni, where a+b+c+d+e+f+g+impurities=100 at. % and 0.01≤b≤8, 0.01≤c≤10, 0≤d≤10, 0≤e≤20, 10≤f≤25, 3≤g≤12 and 17≤f+g≤30.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Various embodiments are explained in greater detail below with reference to the drawings and examples.

[0096] FIGS. 1a-1d show a molten droplet, rotating casting roller, ceramic casting nozzle, metal droplet, metal strip, solid particle, and air inclusion during the formation of a hole in a thin amorphous foil due to a local wetting defect.

[0097] FIGS. 2a-2c show the molten droplet, rotating casting roller, ceramic casting nozzle, metal droplet, and metal strip during the formation of a hole in a thin amorphous foil due to a local wetting defect.

[0098] FIG. 3 shows a photograph of a grain of copper at the start of a wetting defect in a comparison strip.

[0099] FIG. 4 shows a photograph showing residues of molten copper at the end of a wetting defect.

[0100] FIG. 5 shows a photograph of a comparison strip with wetting defects of different sizes.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0101] When producing amorphous foils using rapid solidification technology (melt-spinning), a glass-forming metal alloy is melted in a crucible that is typically made substantially of oxide ceramic (e.g. aluminium oxide) and/or graphite. Depending on the reactivity of the molten mass, the melting process may take place in air, in a vacuum or in an inert gas such as argon or nitrogen, for example. Once the alloy has been melted down at temperatures well above the liquidus point, the molten mass is transported to a casting tundish and injected through a casting nozzle, which generally has a slit-shaped outlet opening, onto a rotating wheel or roller made of a copper alloy. To this end, the casting nozzle is brought very close to the surface of the rotating copper wheel at a distance of approx. 50 μm to 500 μm from it. The molten mass passes through the nozzle outlet and hits the moving copper surface where it solidifies at cooling rates of approx. 10.sup.4° K/min to 10.sup.6° K/min. Due to the rotational movement of the roller, the solidified molten mass is carried away as a continuous foil strip, detached from the cooling roller and the wound onto a winding device as a continuous foil strip.

[0102] The maximum possible length of the foil strip is, in principle, limited by the holding capacity of the crucible, which can range from a few kilograms to several tonnes depending on the size of the plant. When operating with a plurality of crucibles in parallel it is even possible to achieve an almost continuous supply of molten mass to the casting tundish. The scale of plant in which commercially available amorphous foils are manufactured typically has crucible sizes of more than 100 kg. In the alloy consisting of 82.8 wt. % Fe, 1.3 wt. % Cu, 5.6 wt. % Nb, 8.8 wt. % Si, 1.5 wt. % B, which is commercially available from Vacuumschmelze GmbH & Co. KG, Hanau, Germany under the trade name VITROPERM 500, for example, and with a foil width of approx. 100 mm and a foil thickness of 0.018 mm, this results in a strip length of approx. 8 km.

[0103] In some applications of these metal foils, the amorphous foil as produced is wound to form a core, which is then converted to a nanocrystalline state by means of appropriate heat treatment. In the nanocrystalline state this metal foil is completely brittle with a high degree of sensitivity to breaking. This complicates the handling of the foil.

[0104] Some applications of these foils require the use of very thin foil thicknesses of approx. 10 to 20 μm with foil widths of 20 to 200 mm and low surface unevenness and roughness that are very largely free of structural defects such as pimples and holes. In these applications the strip is first heat treated and then brought into the desired final form. The desired final form may be wound foil cores or flat shields in which a plurality of foil layers are laminated with adhesive layers to form a composite material.

[0105] Common to all these applications is the fact that the foils, which are amorphous in the cast state, are first converted to a nanocrystalline state by means of heat treatment, then subjected to tensile stress during further working, when they must also be able to tolerate low shear forces. Holes in the foil restrict its cross section and promote foil tears during final working, leading in turn to reduced productivity in the manufacturing process.

[0106] It has been established that large holes with a lateral extent in at least one direction of more than 3 mm in the foil can result in tears that significantly reduce the productivity of the laminating process. In laminated applications, the foils should occupy a small space. This means that surface roughness and unevenness need to be as low as possible to achieve a high lamination factor.

[0107] Various approaches are used to avoid holes and air bubbles in the foil.

[0108] Wear on the casting-wheel surface during the casting process leads to increased surface roughness of the casting wheel and, in turn, to the formation of cavities or structures that transport process gas into the molten metal droplets and cause larger gas bubbles in the contact region between the molten metal droplets and the casting wheel. When the molten metal solidifies, these gas bubbles are frozen into the amorphous strip and result in hole-like defects, particularly in thin foils. This increased roughness also results in unevenness in the strip and in a lower lamination factor.

[0109] In order to minimise wear on the casting wheel it is desirable to select a high-strength casting-wheel material. In the copper materials produced using melting techniques generally used, the properties of strength and thermal conductivity tend to act in opposite directions. A copper material with the maximum possible thermal conductivity will always have a lower strength than more highly alloyed copper materials. This is due to the physics of alloys produced using melting techniques. Higher alloyed copper materials are generally stronger but have lower conductivity. In order to produce Fe—Cu—Nb—Si—B foils such as the alloy with the trade name VITROPERM 500 produced by Vacuumschmelze GmbH & Co. KG, Hanau,

[0110] Germany, in particular, it is however necessary to use casting-wheel materials with relatively high conductivities in order to achieve sufficiently high cooling rates during foil production. If the cooling rates are not sufficiently high, the foils become brittle—or partly brittle—and so cannot be wound continuously in the casting process, or tear off during winding, resulting in undesirably lower productivity in foil production. It is desirable to use casting-wheel materials with a thermal conductivity greater than 200 W/mK. However, such materials have a hardness of less than 250 HV (HV30).

[0111] In an embodiment, therefore, a casting wheel (acting as a heat sink) made of a material with a thermal conductivity of greater than 200 W/mK and a Vickers hardness of less than 250 HV is selected. One example of a suitable material is a copper-beryllium alloy with less than 2% beryllium.

[0112] If these soft and highly conductive materials are to be used in the casting of amorphous foils in the long term, it is necessary to ensure that the contact surface between the molten mass/strip and the casting wheel is worked evenly even during foil production and to keep the roughness of the wheel surface at a constant and uniformly low level.

[0113] Here a non-abrasive, reshaping method based on the rolling of the casting wheel described in U.S. Pat. No. 9,700,937 B1 is used. In addition, however, the casting roller is protected against abrasive residues so that even the smallest particles are unable to reach the wheel surface, where they might lead to wetting defects and so to the formation of holes in the foil.

[0114] The description below refers to FIG. 1 and explains how local wetting defects can lead to holes in thin amorphous foils. FIG. 1a shows a rotating casting roller 1 and a ceramic casting nozzle 2 that is positioned approx. 50 to 500 μm from the casting roller. Molten metal at a temperature of approx. 1300° C. flows through this casting nozzle and forms a molten metal droplet 3 between the nozzle and the casting wheel 1 that solidifies on the casting-wheel surface and from which molten mass permanently solidified by the rotational movement of the casting wheel 1 is carried away in the form of a thin metal strip 4.

[0115] A solid particle 5 formed during continuous casting-wheel working, which may, for example, be an abrasive grain, a metal particle from the casting wheel or a piece of brush hair, is transported in the air layer on the casting wheel towards the molten metal droplet 3 and then strikes and enters the molten metal droplet 3 at high speed, as illustrated in FIG. 1b, where it results in a small air inclusion 51 due to the short wetting defect. The air inclusion 51 and, in certain circumstances, the particle 5 as well pass underneath the molten metal droplet 3, as illustrated in FIG. 1c, initially forming a bubble/cavity in the solidifying molten metal before manifesting itself as a hole 52 in the solidified amorphous metal strip on exiting the molten metal droplet.

[0116] One example of such particle residues can be seen in the photograph of a comparison strip in FIG. 3. It shows how a copper particle formed during continuous casting-wheel working is the starting point for a wetting defect that grows into a hole in the strip. FIG. 4 shows the end of a wetting defect, but residues of molten copper can even be identified on the underside of the strip. One explanation of this observation that the copper is no longer present in particle form is that particles from continuous casting-wheel working have penetrated the molten metal droplet and caused a cavity in the molten metal droplet. The particles melt in this cavity due to the high temperature of the molten metal above them and so adhere to the underside of the strip of metal foil as a thin film.

[0117] Referring to FIG. 2, it is explained how a liquid or solid particle of organic dirt or a deposit such as water, oil or grease, for example, located on the casting-wheel surface 6 can result in a wetting defect. This liquid or solid organic material has an evaporation temperature below the melting temperature of the molten metal mass 3 (see FIG. 2a). If this dirt enters the droplet of molten metal 3, as illustrated in FIG. 2b, it will evaporate immediately due to the high melting temperature 3 and result in a gas bubble under the molten metal droplet 3 that subsequently manifests itself as a hole in the solidified amorphous metal strip 4 when it exits the molten metal droplet, as illustrated in FIG. 2c.

[0118] The figures show that these small wetting defects can manifest themselves as holes, particularly in very thin strips of typically less than 25 μm. In metal strips with higher foil thicknesses the structure of these cavities can be retained, forming “air bubbles” on the casting-wheel side of the final metal strip.

[0119] FIG. 5 shows that smaller wetting defects do not result in cavities/bubbles deep enough to break through the foil thickness and create a hole. For this reason, the thicker the foil thickness, the less probable the occurrence of holes.

[0120] Holes and surface roughness are important factors in the further working of heat treated strips under tensile stress as far as the magnetic properties of the foil are concerned. The holes weaken the cross section of the foils and if located in the edge region form a starting point for notch effects and tears in the foils, which in turn leads to a undesired, significant reduction in productivity during further processing.

[0121] For this reason it is currently practically impossible to buy amorphous metal foils with a foil thickness of less than 20 μm that is largely free of hole-like defects over long lengths commercially. Thin foils with a foil thickness of less than 20 μm have more holes the thinner they are made. For example, strips with foil thicknesses and foil width of 16 μm×60 mm have a plurality of holes measuring up to 4 mm.

[0122] Table 1 shows an evaluation of hole distribution in samples of the commercially available materials. It shows that a typical number of holes in the foil is between 141 and 443 holes per square metre of foil.

TABLE-US-00001 TABLE 1 Foil roughness Number of holes by hole size (quantity/m.sup.2) Ra Wheel Ra Air 0-1 1-2 2-3 3-4 4-5 Total Sample Dimensions side (μm) side (μm) mm mm mm mm mm per m.sup.2 # 1 16 μm * 60 mm 0.9 1.2 110 189 110 — — 409 # 2 16 μm * 63 mm 1.2 1.5 58 50 33 — — 141 # 3 17 μm * 60 mm 0.9 1.1 158 126 47 31 — 362 # 4 14 μm * 53 mm 0.8 1.1 211 166 44 22 — 443

Example According to the Invention

[0123] An amorphous foil with a composition of 82.8 wt. % Fe, 1.3 wt. % Cu, 5.6 wt. % Nb, 8.8 wt. % Si and 1.5 wt. % B is produced on a casting wheel with a thermal conductivity of more than 200 W/mK. The surface of the casting track is worked during foil production by rolling. The roller used is designed such that it leaves no residues likely to cause wetting defects on the casting wheel. A foil with a thickness of less than 20 μm and fewer than 10 holes per square metre can be produced, as given in Table 2.

[0124] Table 2 provides a summary of the results of a comparison of various different casting-track working methods. It gives details of different online casting-track working methods for the alloy VITROPERM with a foil width of 66 mm and a foil thickness of 18 μm produced on a casting wheel made of a copper alloy. Material-removing processes using sandpaper and wire brushes result in foils with at least 100 holes per square metre and in an increased number of tears during casting. In contrast, the reshaping process of rolling causes few tears and few holes.

[0125] In the example according to the invention rolling was used as the working process and the casting wheel was protected from lubricant residues in order to reduce the number of holes per square metre to fewer than 10.

TABLE-US-00002 TABLE 2 Tears during Holes Roughness Polishing casting Holes Ra method (Qty. per km) (Qty. per m.sup.2) (Wheel side) Sandpaper .circle-solid. .square-solid. ≤0.7 Wire brushes .circle-solid. .box-tangle-solidup. ≥0.7 Rolling .diamond-solid. .box-tangle-solidup. ≤0.7 Rolling* .diamond-solid. .circle-solid. ≤0.7 .square-solid. = 100 to 1000 .box-tangle-solidup. = 10 to 100 .circle-solid. = 1 to 10 .diamond-solid. = <1 *According to the invention

[0126] By using specially encapsulated rotating part lubricants on special roller tools, the rotating components are lubricated in such a manner as to ensure that no problematic lubricant residues reach the surface of the casting wheel. This makes it possible to produce thin foils that have an even lower number of hole defects and low roughness even with long casting-track lengths of more than 10 km.

[0127] The invention thus makes it possible to produce thin foils (<20 μm) in long lengths without residues from continuous casting-wheel working resulting in holes in the foil. Using this method it is possible to produce wide, thin foils in long lengths that have few hole-like strip defects and a lamination factor of over 73%. With foils produced in this manner it is possible to further work the foils in the nanocrystalline state at high productivity levels without the disruption of tears in the foil.

[0128] Methods that leave residues that prevent the wetting of the molten metal on the roller or result in residues on or in the metal foil and so have a negative impact on the performance characteristics of the foil are not therefore used. Such residues include all organic components such as oils and polishes. However, small solid bodies such as metal dust, abrasive grits and brush hairs caught in the molten metal droplets can also lead to local wetting problems, holes, inclusions and imperfections in the strip.

[0129] The invention thus provides an amorphous metal foil that has a lower number of holes and can be produced in a width of up to 300 mm and in longer continuous strip lengths, e.g. up to 8 km. The small number of holes improves the mechanical properties of the amorphous metal foil because the number of probable break points is reduced since fewer holes means less reduction in cross section and less notch effect and makes it easier to avoid strip tears in production, further working and use. The magnetic properties of soft magnetic metal foils are improved because they are no longer adversely affected by holes in the metal foil. The cost effectiveness of the production of these amorphous metal foils in industrial-scale plants is therefore increased.