WO2014087031A1 - Method for the powder-metallurgical production of magnetic cores - Google Patents

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

 METHOD FOR THE PULVIMETALURGICAL MANUFACTURE OF NUCLEUS

 MAGNETICS

DESCRIPTION

 The object of the present invention is an alternative method of manufacturing material cores: (1) completely amorphous, (2) amorphous matrix with nanocrystalline regions, or (3) completely nanocrystalline, which allows to obtain blocks of material (not formed by bonding of tapes) with the definitive or quasi-definitive form, replacing the melt-spinning technique with a powder metallurgical route consisting of the amortization of dust by means of high-energy mechanical grinding and subsequent rapid consolidation by electric means (FAST techniques, Field Assisted Sintering Tech ñiques).

This invention falls within the scientific-technical area of "material technology" and more specifically to the manufacturing sector, from powders, of all types of parts intended for functions of magnetic core.

Background of the invention

 The energy losses of a magnetic nature in the transformer and electric motor cores are usually around 1.4%, and are transferred to the atmosphere in the form of heat. The figure is really important and results in cost overruns that could be significantly reduced using materials with better technical performance [1] [2].

In the context of magnetic applications, materials for transformer cores and electric motors are the most used by volume of raw material, and the most important, by percentage of global market. And within them, silicon steel in the form of sheets is used in 90% of transformer cores, which accounts for 60% of the total market volume of soft (sweet) magnetic materials [1, 3].

Today, the main base material in the production of transformer cores and electric motors is iron, due to its intrinsically sweet magnetic character. The introduction of other elements can improve your behavior. For example, iron with 6.5% silicon results in a very reasonable behavior: a high magnetic induction is maintained, but significantly reducing magnetic anisotropy, by compensating for the magnetostriction constant and magnetocrystalline anisotropy (a material is all the more soft the smaller its magnetic anisotropy). Further improvements of this basic material can be achieved through a series of thermomechanical processes or treatments, designed to induce certain textures that reduce hysterical losses. Likewise, rolling processes are carried out, in order to reduce losses at high frequencies [3,4].

The latest generations of sweet materials include structurally amorphous and nanocrystalline alloys, which are actually the softest materials in existence. In the case of structurally amorphous materials, magnetocrystalline anisotropy is practically nil. The reason for this is that the atomic disorder that characterizes its structure (similar to that of a liquid) entails the absence of grain boundaries (the main obstacles that hinder the movement of the walls of the magnetic domains). Due to the absence of grain boundaries, amorphous ferromagnetic materials have very narrow hysteresis cycles and very low energy products, which makes them magnetically very soft materials [1, 2, 5].

In the case of nanocrystalline alloys, these materials consist of small grains, of nanometric size, embedded in a matrix with amorphous structure. A compensation effect of the magnetostriction constant between the two crystalline and amorphous phases (of opposite signs to each other) occurs here and, on the other hand, magnetocrystalline anisotropy is averaged macroscopically [1, 2, 5, 6].

In both cases, the internal atomic disorder increases the electrical resistivity of the material (approximately an order of magnitude higher than the conventional polycrystalline alloy of the same composition). The high electrical resistivity of amorphous and nanocrystalline alloys is also associated with the reduction of losses due to eddy currents. Therefore, the use of amorphous or nanocrystalline metals in the motor cores or electrical transformers results in a more efficient operation. However, combining the savings derived from the better magnetic behavior 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 molding techniques inevitably provide polycrystalline, never amorphous, metal materials with typically micrometric, non-nanometric grain sizes.

The usual technique of manufacturing amorphous metal in relatively large quantities is called melt-spinning [2, 5] and consists essentially of solidifying a metal, from the liquid state, on the thermally very conductive surface and normally kept at a low temperature of a rotating wheel The severe cooling rate — up to one million degrees Celsius per second — imposed on the atoms of the liquid, prevents them from finding the positions of the crystalline state. The result is that the material solidifies, but not with its atoms placed in a perfectly ordered arrangement (crystalline state), but in complete disorder (amorphous state). To prevent the material from devitrifying at room temperature, it is often necessary to introduce a large number of non-metallic elements into the alloy composition that decrease the tendency to crystallize. Unfortunately, these elements damage, in general, the magnetic properties of the material.

From a technological point of view, the described process has the disadvantage that it only allows the production of very thin tapes (the maximum thickness should typically be less than 0.1 mm, and the maximum width reached so far is about 25 cm). To form a piece it is necessary to stack and join many of these tapes. Thus, the challenge of obtaining a block of amorphous material still persists. With this objective, various powder metallurgical techniques have been designed whose starting point should be the production of amorphous dust. The production in large quantities of amorphous metal powders has been demonstrated using variations of the rapid cooling method in which the liquid is sprayed in the form of very small drops that are cooled sharply (by thermal conduction) in a fluid. 'Spray spraying', 'high-velocity gas jet atomization' and some other methods have been successfully used for this purpose

[5]. Only when the droplet size is below 50 μηι, is it possible to achieve the necessary severe cooling rates. Of course, a simple method for the production of amorphous powder is to crush the amorphous tapes obtained by melt-spinning. Several companies have used this method to produce commercial quantities of amorphous powder.

Another recently explored method is that of mechanical grinding (or mechanical alloy) that has been revealed as a relatively inexpensive way to produce large amounts of amortized powders.

But powders are only the starting point; to obtain the final piece, there must be some consolidation method that retains the amorphous character of the powders. Several methods of consolidation of amorphous metal powders have already been successfully tested: 'shock wave consolidation', 'explosive forming', 'sintered sub Tg' (Tg is the temperature of glass transition), 'hot extrusion near Tg', and 'roller rolling near Tg', among some others [7-8].

Finding an efficient and industrially attractive method for consolidating amortized powders without significant loss of their magnetic properties is today a challenge of enormous technological and environmental interest. The manufacturing technique proposed here seeks to fulfill that objective.

References:

[1] Documents on technological opportunities: Magnetic Materials (vol 19), COTEC Foundation, March 2003. [2] N. DeCristofaro, Amorphous metais 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", Vol 2, ASM handbook,

ASM International, 1990, p. 761-781.

 [4] K.H. Moyer, Magnetic Materials and Properties for Powder Metallurgy Part

Applications, "Powder Metal Technologies and Applications", Vol 7, ASM

Handbook, ASM International, 1998.

 [5] W.L. Johnson, "Metallic Glasses", in "Properties and Selection: nonferrous alloys and special-purpose materials", Vol 2, ASM handbook, ASM

International, 1990, p. 804-821.

[6] NA Spaldin, "Magnetic Materials: Fundamentalis and Applications", 2 ^nd 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 (both for electric motors, such as transformers or polar parts) is a complex task that until now has required the manufacture of amorphous material in the form of thin tapes (by very severe cooling, melt spinning) and its subsequent stacking and / or folding for the formation of the final piece. The process can be expensive, and the properties of the piece often resent the fact of having too many borders. Although various methods for obtaining amorphous block materials have been tried, none, at the moment, is exempt from difficulties and is being exploited industrially.

The purpose of this patent is to show an alternative route of manufacturing material cores: (1) completely amorphous, (2) amorphous matrix with nanocrystalline regions, or (3) completely nanocrystalline, which allows to obtain blocks of material (not formed by joining tapes) with the definitive or quasi-definitive form, replacing the melt-spinning technique with a powder metallurgical route consisting of the amortization of dust by means of high-energy mechanical grinding and subsequent rapid consolidation by electric means ( FAST techniques, Field Assisted Sintering Techniques). This combination allows, in addition to obtaining massive pieces of amorphous material (or partially nanocrystalline) with the definitive or quasi-definitive form, to reduce the amount of metalloids present in the alloy, necessary to retain the amorphous character at room temperature. In principle, it is expected to be suitable for manufacturing small parts, but nothing prevents designing larger parts by assembling smaller attachable blocks.

More specifically, the manufacturing method proposed in the present invention consists of a novel powder metallurgical route consisting of two stages: (i) a first powder amortization by high-energy and long-term grinding and (ii) a second of formed of the piece by means of some form of electrical consolidation of the amortized powder, such as the so-called sintering by electrical resistance SRE, or the so-called consolidation by electric shock CDE, but not necessarily one of these.

The requirement of electrical consolidation is due to the fact that the conventional cold-pressed powder-sinking route in the furnace does not work in this case because during the sintering stage, the necessary high temperatures and the time during which they are maintained make the material devitrify, losing the amorphous character achieved by grinding.

The magnetic cores obtained by this procedure may have a completely amorphous, completely nanocrystalline character, or a combination of the above (nanocrystalline regions embedded in amorphous matrix).

In general, the technical problem solved by the present invention is to produce block amorphous cores (not constituted by joining tapes) by lowering their production cost and, eventually, improving some properties. The solution to this technical problem, as indicated, is to establish a powder metallurgical route consisting of the use of mechanical grinding as a form of dust amortization and electrical consolidation of the amortized powder, which due to its extraordinary speed and nature, inhibits the devitrification of the material. Because of its simplicity, the proposed method represents a simplification of the production process and implies cost reduction.

A possible variant of the proposed method, instead of non-amorphous powders, would use as a starting material, tapes previously amortized by any conventional method of amortization (for example, melt spinning). In this case, the belts should be crushed by mechanical grinding of short duration, prior to their electrical consolidation.

The possible uses of the invention are varied, including the manufacture of all types of cores of master material intended for applications of transformers and electric motors, as well as other sweet polar parts. The possible restriction to small parts can be overcome by assembling smaller, attachable parts, manufactured by the route proposed here.

Throughout the description and the claims, the word "comprises" and its variants are not intended to exclude other technical characteristics, additives, components or steps. For those skilled in the art, other objects, advantages and features of the invention will be derived partly from the description and partly from the 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. In addition, the present invention covers all possible combinations of particular and preferred embodiments indicated herein.

Brief description of the figures

A series of drawings that help to better understand the invention and that expressly relate to an embodiment of said invention which is presented as a non-limiting example thereof is described very briefly below. FIG1. It shows a scheme of an "attritor" type high-energy ball mill, where the mechanical grinding amortization stage that is part of the method object of the present invention can be performed.

 FIG2. It shows a scheme of the system where the amortized powder from the mill of Figure 1 is electrically consolidated and constitutes the second stage of the method recommended by the present invention.

Preferred Embodiment of the Invention

As indicated, the method for the powder metallurgical manufacture of magnetic cores, object of the present invention is characterized in that it comprises (i) a first stage of amortization of a mixture of magnetically soft powders by mechanical grinding; and (ií) a second stage of electrical consolidation of the amortized powder in the first stage.

The mechanical alloy is a process that involves the repeated deformation, fracture and continuous welding of the dust particles (metallic and non-metallic) by the constant action of the high energy grinding to which they are subjected. Thus, figure 1 shows a type of ball mill where high-energy mechanical grinding is performed, although the high-energy mill does not necessarily have to be this. This process has the advantage of obtaining true solid state alloys, since an intimate combination takes place at the atomic level.

But the importance of mechanical grinding lies not only in the possibility of alloying mechanically, but also in reducing the grain size of the powder, with the consequent improvement of its mechanical properties and its sinterizability, being able to become ultra-thin and even nanometric. . If the grinding duration is long enough, and the composition and grinding conditions properly chosen, the powder can reach amortization. This possibility of powder amortization is the one that is used in the present invention. The main advantage of this method over severe cooling techniques is its lower cost and greater flexibility for industrial production. However, it should not be ignored that, due to the particular way in which the structure Amorphous is achieved (due to very severe deformation / dislocation of the structure) the tendency to devitrify is smaller and the proportion of metalloids present can be diminished or even annulled, with the consequent improvement in the magnetic properties of the final pieces derived from it.

It should be stressed that, even though the possibility of amortization by mechanical alloy is known in the current state of the art, the technical problem of consolidating the amortized powder to obtain a consolidated block still remains without losing the amorphous character. This problem is solved by the second stage of the recommended method.

Electric consolidation techniques (FAST techniques) not only allow the cold pressing and sintering stages of the oven to be combined in a single stage, but also reduce their duration, so that the use of inert atmospheres is unnecessary ( the time in which the powder is exposed to high temperatures is too short for undesirable oxidation reactions to take place), and the process can be carried out in the air. The time reduction can be very considerable: if the joint cold pressing process (on matrix or isostatic) and sintered in the oven can take around 30-60 minutes, the electrical consolidation can take only a few seconds, or even less, depending on the specific modality used.

As an example, it should be said that the two modalities mentioned above, the SRE and the CDE, have characteristic durations around the second (~ 0.1 - 50 s) and the millisecond (~ 0.1 - 100 ms), respectively, and sources of electric power also different: in the SRE, a transformer that provides low voltage (~ 10 - 30 V) and high intensity (~ 5 - 20 kA), and in the CDE, a capacitor bank, capable of supplying voltages during discharge means (~ 50 - 300 V) and high intensities (~ 1 - 5 kA).

Since electrical consolidation techniques are actually a certain type of hot pressing, much lower working pressures (<100 MPa) are required than those used in cold pressing of the conventional route (around 700-1500 MPa). The cooling will be carried out thanks to the cooling (for example, thanks to a coolant) from which the benches of the machine in contact with the electrodes / punches must consist.

An outline of the electrical consolidation equipment, especially with regard to the details of the matrix 1 could be the one indicated in Figure 2 (but not exclusively like this):

The matrix 1 is electrically insulating (for example, made of natural rock, refractory concrete, ceramic tube and metal strip, etc.).

The electrodes 2 will be of some copper alloy with high conductivity (for example, Cu-Zr alloy). In order to achieve greater uniformity in the indoor temperature, it may be interesting to interpose between the powder 3 and the electrode 2 a wafer 4 of somewhat less conductive material, for example, a pseudo-alloy (heavy metal) of Cu-W, which will also provide resistance to the EDM

The power source 5 may consist of a welding transformer (in the case of the SRE) that provides current intensities in the range of 2 to 12 kA, either with grid frequency (50 Hz) or better still, with higher frequencies , in the range of the average frequencies (~ 1000 Hz). A second possibility (in the case of the CDE) could be the use as a power source of a capacitor bank, large capacity and load voltages in the range of 50 to 500 V. Another possibility is to operate with both types from sources, for example, in a sequential application thereof: first discharge by capacitors, and then intervention of the welding transformer. This last possibility may have the advantage of allowing larger sized parts to be approached, whose electrical resistance is too high to be produced solely by the SRE technique.

The mechanical device that exerts the pressure must be able to supply the force necessary to reach pressures around 100 MPa. In the case of figure 2, a refrigerated bench with a movable upper part 6 and a fixed bottom 7. Application example

It is based, for example, on a mixture of Fe and Ni powders in the atomic proportion of 65% and 35%, respectively.

Subsequently, the powder mixture is subjected to mechanical grinding in an attritor-type high-energy ball mill, as shown in Figure 1, rotating at 500 rpm and water-cooled (20 ° C). To control the milling process, ethylene-bis-stearamide micro-powder wax can be added in a proportion between 1.5% and 2% by weight. The loading ratio (= dough balls / dough mass) is set to 20: 1. The atmosphere of the grinding vessel will be argon gas. The duration of grinding is set between 30 and 40 hours.

In this non-limiting embodiment, the electrical consolidation process by SRE is carried out in the air, with nominal parameters of 80 MPa pressure, a current density of ~ 6.5 kA / cm ² , and a passage time of 70 cycles, of 0.02 s each cycle. (The SRE can use medium frequency electric current, around 1000 Hz.). The final density of the compact must be 90% or higher.

The compact is cooled in situ, due to the effect of electrodes that are cooled by water. Finally, the compact is extracted from the matrix. If the chosen parameters have been suitable for the mass and geometry of the compact, it will have retained the amorphous character of the base powder, or at least, it will be constituted by an amorphous matrix in whose bosom islands of nanocrystalline material could have arisen.

Claims

1. - Method for the powder metallurgical manufacturing of magnetic cores characterized in that it comprises (i) a first stage of amorphization of a mixture of magnetically soft powders by mechanical grinding; and (ii) a second stage of electrical consolidation FAST (Field Assisted Sintering Techniques) of the powder amortized in the first stage. 2. - Method for the powder metallurgical manufacturing of magnetic cores according to claim 1, characterized in that tapes previously amortized by any conventional method of amorphization are subjected to mechanical grinding. 3. - Method for the powder metallurgical manufacturing of magnetic cores according to any one of claims 1 and 2, characterized in that the electrical consolidation FAST is carried out by sintering by electrical resistance (SRE). 4. - Method for the powder metallurgical manufacturing of magnetic cores according to claim 3, characterized in that the SRE is carried out at a voltage of ~ 10-30 V and intensity between 5 and 20 kA with a duration of -0.1 - 50 s. 5. - Method for the powder metallurgical manufacturing of magnetic cores according to any one of claims 1 and 2, characterized in that the FAST electric consolidation is carried out by electric shock consolidation (CDE). 6. - Method for the powder metallurgical manufacturing of magnetic cores according to claim 5, characterized in that the CDE is carried out at a voltage between 50 and 300 V and intensity between 1 and 5 kA with a duration of ~ 0.1 - 100 ms . 7. - Method for the powder metallurgical manufacturing of magnetic cores according to any one of claims 1 and 2, characterized in that the FAST electrical consolidation is carried out by a combination of SRE and CDE. 8. - Method for the powder metallurgical manufacturing of magnetic cores according to any one of claims 1 and 2, characterized in that the electrical consolidation is carried out by a succession of FAST techniques. 9. - Method according to claims 1 to 8, characterized in that magnetic core pieces are manufactured by assembling smaller size attachable parts. 10. - Magnetic core, obtained according to the method of any one of claims 1 to 9, characterized by being completely amorphous, completely nanocrystalline, or a combination of the two above, that is, nanocrystalline regions embedded in amorphous matrix.