EP2929963A1

EP2929963A1 - Method for the powder-metallurgical production of magnetic cores

Info

Publication number: EP2929963A1
Application number: EP13859820.6A
Authority: EP
Inventors: Juan Manuel Montes Martos; Jesús CINTAS FÍSICO; Petr URBAN; Francisco GÓMEZ CUEVAS; Fatima TERNERO FERNÁNDEZ
Original assignee: Universidad de Sevilla; Universidad de Huelva
Current assignee: Universidad de Sevilla; Universidad de Huelva
Priority date: 2012-12-05
Filing date: 2013-12-05
Publication date: 2015-10-14
Also published as: WO2014087031A1; ES2473690A1; ES2473690B1; EP2929963A4

Abstract

The invention relates to a method for the powder-metallurgical production of magnetic cores, characterised in that it comprises (i) a first step of amorphisation of a mixture of magnetically soft powders by mechanical grinding; and (ii) a second step of FAST (Field Assisted Sintering Techniques) electrical consolidation of the powder amorphised in the first step.

Description

The object of the present invention is an alternative method for producing cores made of a material which is: (1) completely amorphous, (2) has an amorphous matrix with nanocrystalline regions, or (3) completely nanocrystalline, which makes it possible to obtain blocks of material (not formed by a linking of ribbons) with the definitive, or almost definitive shape, replacing the melt-spinning technique with a powder metallurgy path consisting of amorphisation of the powder by means of a high energy grinding, and subsequent fast consolidation via the electrical path (FAST, Field Assisted Sintering Techniques).
This invention falls within the framework of the scientific-technical field of "materials technology" and more specifically, that of the manufacture, based on powder, of all types of different parts intended to fulfil the functions of a magnetic core.

Background of the invention

Energy losses of a magnetic nature in the core of transformers and electric motors tend to be in the region of 1.4%, and are transferred into the atmosphere in the form of heat. This figure is actually considerable and gives rise to extra costs that could be significantly reduced by using materials with improved technical performances [1] [2].
In the context of magnetic applications, the materials for the cores of transformers and electric motors, are those used most widely by volume of raw material, and of most importance in terms of share of the global market. And within them, steel with silicon in the form of laminations is used in 90% of transformer cores, representing 60% of the total market volume of soft magnetic materials. [1,3].
As of today, the main base material in the production of cores for transformers and electric motors is iron, because of its intrinsically soft magnetic nature. The introduction of other elements can improve its behaviour. For example, iron with 6.5% of silicon gives rise to a very reasonable behaviour: it maintains a high magnetic induction, while notably reducing magnetic anisotropy, through compensation of the magnetostriction constant and magnetocrystalline anisotropy (a material is softer the lower its magnetic anisotropy). Additional improvements to this basic material can be achieved by means of a series of processes or thermomechanical treatments intended to induce determined textures which reduce hysteretic losses. At the same time, lamination processes are conducted, with the object of reducing losses at high frequencies [3,4].
The latest generation of soft materials include structurally amorphous and nanocrystalline alloys, which are really the softest existing materials. In the case of structurally amorphous materials, the magnetocrystalline anisotropy is practically zero. The reason for this is that the atomic disorder which characterises their structure (similar to that of a liquid) is accompanied by the non-existence of grain limits (the main obstacles which hinder the movement of the walls of magnetic domains). Due to the absence of grain limits, amorphous ferromagnetic materials present very narrow hysteresis cycles and very low energy products, which make them materials that are magnetically very soft [1, 2, 5].
In the case of nanocrystalline alloys, these materials are made up of tiny grains, of a nanometric size, embedded in a matrix with an amorphous structure. Here a compensation effect occurs of the magnetostriction constant between the two phases crystalline and amorphous (of opposite signs to each other) and, at the same time, the magnetocrystalline anisotropy is macroscopically averaged [1, 2, 5, 6].
In both cases, the internal atomic disorder makes the electric resistivity of the material increase (approximately one order of magnitude higher than the conventional polycrystalline alloy of identical composition). The high electric resistivity of amorphous and nanocrystalline allows is associated, moreover, to a reduction in the losses due to Foucault currents.
For all of the above, the use of amorphous or nanocrystalline metals in the cores of electric motors or transformers results in more efficient functioning. Moreover, with the addition of savings resulting from the improved magnetic behaviour and the considerable reduction of Foucault currents, it has been estimated that by replacing the cores of current power distribution transformers with amorphous materials, energy losses would be reduced by 75% [1, 2]. The difficulty lies in how to manufacture these materials, as conventional melting and moulding techniques provide unfailingly, polycrystalline metal materials, and never amorphous, and with grain sizes which are typically micrometric and not nanometric.
The usual technique for producing amorphous metal in relatively important quantities is known as melt-spinning [2, 5] and essentially involves making a metal solidify, from its liquid state, on a surface that is thermally highly conductive and normally kept at low temperature on a rotating wheel. The severe rate of cooling - of up to a million degrees centigrade per second - imposed on the liquid's atoms, prevent these from finding the positions inherent to the crystalline state. The result is that the material solidifies, but not with its atoms placed in a perfectly ordered arrangement (crystalline state), but rather in complete disorder (amorphous state). To prevent the material from devitrifying at room temperature, it is often necessary to introduce into the alloy's composition a large quantity of non-metallic elements which diminish the tendency to crystallize. Unfortunately, said elements, in general harm the magnetic properties of the material.
From a technological point of view, the described process has the drawback that it only allows ribbons with an extremely fine thickness to be produced (the maximum thickness must be typically less than 0.1 mm, and the maximum width reached until now is approximately 25 cm). To form one piece it is necessary to stack and join many of these ribbons. Therefore, the challenge to obtain a block of amorphous material still remains. With this objective, several powder metallurgy techniques have been developed whose starting point must be the production of amorphous powder.
The production in large quantities of amorphous metal powders has been demonstrated by using variations of the fast cooling method in which the liquid is ground in the form of minute droplets which are cooled abruptly (by thermal conduction) in the centre of a fluid. Spray atomisation, high velocity gas jet atomisation and certain other methods have been used successfully for this purpose [5]. Only when the size of the droplets is below 50 µm, is it possible to reach the severe cooling rate required. For certain, a simple method for producing amorphous powder is to crush the amorphous ribbons obtained by melt-spinning. Several companies have used this method to produce commercial quantities of amorphous material in powder form.
Another recently explored method is that of mechanical grinding (or mechanical alloying) which has been revealed as a relatively inexpensive way of producing large quantities of amorphised powders.
But the powders are just the starting point; to obtain the final part it is necessary to have some method of consolidation which retains the amorphous character of the powder. Various methods of consolidation of amorphous metal powders have already been tried out successfully: shock consolidation, explosive forming, sub Tg sintering (Tg is the temperature of vitreous transition), hot extrusion close to Tg, and lamination by rollers close to Tg, among others [7-8].
To find an efficient and attractive method from the industrial point of view, for the consolidation of amorphised powder without any significant impairment of its magnetic properties constitutes today an enormous challenge of technological and environmental interest. The producing technique proposed herein sets out to fulfil this objective.

References:

[1] Documentos sobre oportunidades tecnológicas: Materiales Magnéticos (vol 19), Fundación COTEC, marzo 2003. (Documents on technological opportunities: Magnetic Materials (vol. 19) COTEC Foundation, March 2003.
[2] N. DeCristofaro, Amorphous metals in electric power distribution applications, Materials Research Society, MRS Bulletin, Vol 23 (5), 1998, 50-56.
[3] D.W. Dietrich, "Magnetically soft materials", in "Properties and Selection: nonferrous alloys and special-purpose materials", .
[4] K.H. Moyer, Magnetic Materials and Properties for Powder Metallurgy Part Applications, "Powder Metal Technologies and Applications", .
[5] W.L. Johnson, "Metallic Glasses", in "Properties and Selection: nonferrous alloys and special-purpose materials", .
[6] N.A. Spaldin, "Magnetic Materials: Fundamentals and Applications", 2nd edition, Cambridge University Press, 2011, USA.
[7] C. Cline and R. Hopper, "Explosive Fabrication of Rapidly Quenched Materials", Scripta Metallurgica., Vol. 11 (12), 1977, p. 1137-1138.
[8] P. Shingu, "Metastability of Amorphous Phases and its Application to the Consolidation of Rapidly Quenched Powders", Materials Science and Engineering, Vol. 97, 1988, p 137-141.

Description of the invention

The manufacture of amorphous cores (for both electric motors, and transformers or polar parts) is a complex task which until now has required the manufacture of the amorphous material in the form of very thin ribbons (by means of very severe cooling, melt spinning) and their subsequent stacking and/or folding to form the final piece. The process can be costly, and the properties of the piece, are often impaired due to the fact of having too many edges. Although various methods have been tested to obtain amorphous materials in block, none for the time being is exempt from difficulty and is being industrially exploited.
The object of this patent is to show an alternative path for producing cores made of a material which is: (1) completely amorphous, (2) has an amorphous matrix with nanocrystalline regions, or (3) completely nanocrystalline, which allows blocks of material (not formed by linking ribbons) with the definitive, or almost definitive shape, replacing the melt-spinning technique with a powder metallurgy path involving amorphisation of the powder by means of mechanical grinding and subsequent fast consolidation through the electric path (FAST, Field Assisted Sintering Techniques). This combination moreover makes it possible to obtain massive pieces of amorphous (or partially nanocrystalline) material with the definitive or almost definitive shape, reducing the quantity of metalloids present in the alloy required to retain the amorphous nature at room temperature. In principle, it is envisaged as ideal for the manufacture of small sized pieces, but nothing prevents it from the design of larger pieces through assembly of smaller attachable blocks.
More specifically, the method of manufacture proposed by the present invention consists of a novel powder metallurgy path which consists of two steps: (i) a first step of amorphisation of the powder by means of high energy and long duration grinding and (ii) a second step of conformation of the piece by means of some modality of electrical consolidation of the amorphised powder, such as the one known as electrical resistance sintering ERS, or the so-called sintering by electrical discharge consolidation EDC, but not necessarily one of these.
The electrical consolidation requirement is due to the fact that the conventional powder metallurgy path of cold pressing plus sintering in an oven is not of any use in this case because during the sintering step, the high temperatures required and the time during which they are maintained, make the material devitrify, losing the amorphous nature achieved by means of the grinding.
The magnetic cores obtained by means of this procedure can be completely amorphous in nature, completely nanocrystalline or a combination of the above (nanocrystalline regions embedded in an amorphous matrix).
In general, the technical problem resolved by the present invention is to produce amorphous cores in block (not made up of a linking of ribbons) making their production cost cheaper and, eventually improving certain properties. The solution to this technical problem, as mentioned, is to establish a powder metallurgy path involving the use of mechanical grinding as a form of amorphisation of the powder and electrical consolidation of the amorphised powder, which given its extraordinary fastness and nature, inhibits the material from devitrifying. For its simplicity, the proposed method represents a simplification of the production process and entails a reduction in costs.
A possible variant of the proposed method, instead of non-amorphous powders, would use as the starting material, ribbons previously amorphised by any conventional method of amorphisation (for example, melt spinning). In this case, the ribbons must be crushed by mechanical grinding of a short duration, prior to their electrical consolidation.
The possible uses of the invention are very varied, including the manufacture of all types of cores made of amorphous material intended for applications in transformers and electric motors, as well as in other soft polar pieces. The possible restriction to small pieces can be overcome by assembling smaller attachable pieces, manufactured via the path proposed herein.
Throughout this description and the claims, the word "comprises" and its variants are not intended to exclude other technical characteristics, additives, components or steps. For persons skilled in the art, other objects, advantages and characteristics of the invention will be inferred in part from the description and in part from the putting into practice of the invention. The following examples and drawings are provided by way of illustration, and are not intended to be limiting of the present invention. Moreover, the present invention covers all possible combinations of particular and preferred embodiments indicated herein.

Brief description of the drawings

Next is a very brief description of a series of drawings which help to better understand the invention and which relate expressly to an embodiment of said invention presented as a non-limiting example of same.

Fig 1 Shows a scheme of a high energy ball grinding mill of the "attritor" type, in which it is possible to carry out the step of amorphisation by mechanical grinding which forms part of the method that is the object of the present invention.
Fig 2 Shows a scheme of a system in which the amorphised powder coming from the mill of figure 1 is electrically consolidated, and which constitutes the second step of the method disclosed in the present invention.

Preferred embodiment of the invention

As indicated, the method for the powder metallurgical production of magnetic cores, forming the object of the present invention is characterised in that it comprises (i) a first step of amorphisation of a mixture of magnetically soft powders by mechanical grinding; and (ii) a second step of electrical consolidation of the powder amorphised in the first step.
Mechanical alloying is a process which involves the repeated deformation, fracturing and continuous welding of the particles of powders (metallic and non-metallic) through the constant action of the high energy grinding to which they are subjected. Thus, figure 1 shows a type of ball mill in which the high energy mechanical grinding is carried out, although the high energy mill does not need to be this one necessarily. This process has the advantage of obtaining true alloys in solid state, as an intimate combination takes place at the atomic level.
However, the importance of mechanical grinding does not reside exclusively in the possibility of mechanical alloying, but also in reducing the size of the powder grain, with the ensuing improvement in its mechanical properties and its capacity to be sintered, as it may become ultrafine and even nanometric. If the duration of grinding is sufficiently long, and the composition and grinding conditions are appropriately chosen, the powder can arrive at amorphisation. This possibility of amorphisation of the powder is what the present invention uses. The main advantage of this method versus the severe cooling techniques is its lower cost and increased flexibility for industrial production. Nonetheless, one must not ignore that due to the particular way in which the amorphous structure is reached (through very severe deformation/dislocation of the structure) the tendency to devitrify is lower and the proportion of metalloids present can be diminished or even eliminated, with the ensuing improvement in the magnetic properties of the resulting end pieces.
It is appropriate to emphasise that, although the possibility of amorphisation by means of mechanical alloying is known in the current state of the art, the technical problem remains of consolidating the amorphised powder to obtain a consolidated block without it losing its amorphous nature. This problem is solved by means of the second step of the disclosed method.
Fast electrical consolidation techniques (FAST techniques) not only make it possible to combine the steps of cold pressing and sintering in an oven into a single step, but also manage to reduce its duration, in such a way that the use of inert atmospheres becomes unnecessary (the time during which the powder is exposed to the high temperatures is too short for undesirable rusting reactions to occur), and the process can be carried out in air. The reduction in time can be very considerable: whereas the entire process of cold pressing (in matrix or isostatic) and sintering in an oven can take approximately 30-60 minutes, electrical consolidation can take just a few seconds, or even less, depending on the specific modality employed.
By way of an example, it must be said that the two modalities mentioned above, ERS and EDC, have characteristic durations in the region of one second (~ 0.1 - 50 s) and one millisecond (~ 0.1 - 100 ms), respectively, and also different electrical power sources: in ERS, a transformer which provides low voltage (∼10 - 30 V) and high current (~5 - 20 kA), and in EDC, a capacitor bank, capable of supplying during its discharge medium voltages (~ 50 - 300 V) and high currents (~1 - 5 kA).
Given that the electrical consolidation techniques are in reality a certain type of hot pressing, much lower working pressures are required (< 100 MPa) than those used in the cold pressing of the conventional path (approximately 700-1500 MPa).
Cooling will be conducted through refrigeration, (for example, through a cooling liquid) that must be present in the banks of the machine in contact with the electrodes/punches.
A scheme of the electrical consolidation equipment, especially with regards to the details of the matrix (1) could be the one indicated in figure 2 (but not exclusively this one):
The matrix (1) is electrically insulating (for example, made of natural stone, refractory concrete, ceramic pipe and metal strapping, etc.).
The electrodes (2) will be made of some copper alloy with high conductivity (for example, Cu-Zr alloy). To achieve greater uniformity of the inner temperature, it may be interesting to interpose between the powder (3) and the electrode (2) a wafer (4) of a somewhat less conductive material, for example, a pseudo-alloy (heavy metal) of Cu-W, which will also provide resistance to electrical discharge machining.
The source of power (5) may consist of a weld transformer (in the case of ERS) which provides currents in the range of 2 to 12 kA, whether at a network frequency (50 Hz) or even better, at greater frequencies, in the medium frequencies range (~ 1000 Hz). A second possibility (in the case of EDC) would involve the use as a source of power of a capacitor bank, with a great capacity and voltages in the range of 50 to 500 V. Another possibility is to operate with both types of sources, for example, in a sequential application of same: first capacitor discharge, and then intervention of the weld transformer. This last possibility could have the advantage of allowing larger sized pieces to be tackled, whose electrical resistance is too high to be produced solely using the ERS technique.
The mechanical device that exerts the pressure must be capable of supplying the force required to reach pressures in the region of 100 MPa. In the case of figure 2, a refrigerated bank is shown with a mobile upper part (6) and an inner fixed part (7).

Example of application

The starting point is, for example, a mixture of powders of Fe and Ni in the atomic proportion, of 65% and 35%, respectively.
Subsequently, the mixture of powders is subjected to mechanical grinding in the high energy ball mill of the attritor type, such as the one shown in figure 1, rotating at 500 rpm and cooled by water (20°C). To control the grinding process it is possible to add micro powder wax ethylene bis stearamide in a proportion of between 1.5% and 2% in weight. The load ratio (= balls mass/powder mass) is established in the value of 20:1. The atmosphere in the grinding vessel will be of argon gas. The duration of grinding is fixed between 30 and 40 hours.
In this non-limiting example of embodiment, the electrical consolidation process by ERS is carried out in air, with nominal parameters of 80 MPa pressure, a current density of ∼6.5 kA/cm², and a cycle time of 70 cycles, 0.02 s each cycle. (The ERS can use medium frequency electrical current, of about 1000 Hz.). The final density of the solid must be 90% or higher.
The solid is cooled in situ, through the effect of the electrodes which are cooled by water. Finally, the solid is extracted from the matrix. If the selected parameters have been appropriate for the mass and geometry of the solid, the latter will have retained the amorphous nature of the base powder, or at least, will consist of an amorphous matrix in the middle of which islands of nanocrystalline material may have emerged.

Claims

Method for the powder-metallurgical production of magnetic cores characterised in that it comprises (i) a first step of amorphisation of a mixture of magnetically soft powders by mechanical grinding; and (ii) a second step of FAST (Field Assisted Sintering Techniques) electrical consolidation of the powder amorphised in the first step.
Method for the powder-metallurgical production of magnetic cores according to claim 1, characterised in that ribbons previously amorphised by any conventional method of amorphisation are subjected to mechanical grinding.
Method for the powder-metallurgical production of magnetic cores according to either of claims 1 or 2, characterised in that the FAST electrical consolidation is carried out by means of electrical resistance (ERS).
Method for the powder-metallurgical production of magnetic cores according to claim 3, characterised in that the ERS is carried out at a voltage of ∼10 - 30 V and a current between 5 and 20 kA with a duration of ∼0.1 - 50 s.
Method for the powder-metallurgical production of magnetic cores according to either of claims 1 or 2, characterised in that the FAST electrical consolidation is carried out by means of electrical discharge consolidation (EDC).
Method for the powder-metallurgical production of magnetic cores according to claim 5, characterised in that the EDC is carried out at a voltage comprised between 50 and 300 V and at a current comprised between 1 and 5 kA with a duration of - 0.1 - 100 ms.
Method for the powder-metallurgical production of magnetic cores according to either of claims 1 or 2, characterised in that the FAST electrical consolidation is carried out by means of a combination of ERS and EDC.
Method for the powder-metallurgical production of magnetic cores according to either of claims 1 or 2, characterised in that the electrical consolidation is carried out by means of a succession of FAST techniques.
Method according to claims 1 to 8 characterised in that pieces or magnetic cores are produced by means of the assembly of smaller sized attachable parts.
Magnetic core, obtained according to the method of any of claims 1 to 9, characterised in that it is completely amorphous, completely nanocrystalline, or a combination of the two, i.e., nanocrystalline regions embedded in an amorphous matrix.