Method for improving the magnetic properties of a compacted and heat treated soft magnetic composite component

FIELD OF THE INVENTION The present invention relates to a method of improving the magnetic properties of a compacted and heat treated soft magnetic composite component produced from electrically insulated iron- based powders. Especially the method provides means for increasing electrical resistivity and consequently decreasing AC- losses. The present invention relates also to a soft magnetic composite component produced by the method.

BACKGROUND

Soft magnetic materials can be used for applications such as core materials in inductors, stators, and rotors for electrical machines, actuators, sensors, and transformer cores. Traditionally, soft magnetic cores, such as rotors and stators in electric machines, are made of stacked steel-sheet laminates. However, in the last few years there has been a keen interest in so called Soft Magnetic Composite (SMC) materials. The SMC materials are based on soft magnetic particles, usually iron based, with an electrically insulating coating on each particle. By compacting the insulated particles, optionally together with lubricants and/or binders, using the conventional powder metallurgy process, the SMC parts are obtained. By using the powder metallurgical technique it is possible to produce materials having a higher degree of freedom in the design of the SMC part compared to using steel- sheet laminates, as the SMC material can carry a three dimensional magnetic flux and as three dimensional shapes can be obtained with the compaction process.

Soft magnetic materials may generally be defined as materials that are easily magnetised and demagnetised. They typically have an intrinsic coercivity of less than 1000 Am ^"1 .

As a consequence of the increased interest in the SMC materials, improvements of the soft magnetic characteristics of the SMC materials is the subject of intense studies in order to expand the utilisation of these materials. In order to achieve such improvement, new powders and processes are continuously being developed.

Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. In particular, it is generally desirable to provide a soft magnetic composite component having improved magnetic properties regarding core losses. The magnetic permeability of a material is an indication of its ability to become magnetised or its ability to carry a magnetic flux. Magnetic permeability is defined as the ratio of the induced magnetic flux to the magnetising force or field intensity. When a magnetic material is exposed to an alternating electric field, energy losses occur due to both hysteresis losses and eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome retained magnetic forces within the iron core component and is proportional to the frequency of e.g. the alternating electrical field. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions and is proportional to the square of the frequency of the alternating electrical field. A high electrical resistivity is then desirable in order to minimise the eddy currents and is of special importance at higher frequencies, such as for example above about 60 Hz. In order to decrease the hysteresis losses and to increase the magnetic permeability of a core component it is generally desired to heat-treat a compacted part whereby the induced stresses from the compaction are reduced. As described above, a known method for decreasing the hysteresis losses, the DC losses, of a compacted iron- based soft magnetic component is heat treatment in various atmospheres. However, the maximum temperature during heat treatment is restricted to the temperature when the electrically

insulating coating begins to lose its insulating effect. This means that for every combination of used base powder and applied electrically insulating coating there is a limitation of maximum stress releasing temperature that can be used. If a higher stress releasing temperature is applied, lower DC- loss is obtained but the AC- losses may increase due to decreasing electrical resistivity causing an increasing total core loss.

In order to completely stress release a compacted component based on iron- based soft magnetic particles a stress releasing temperature of about 600 ⁰ C to 65O ⁰ C is often required, but temperatures up to 75O ⁰ C may be applied. However, known coatings such as phosphate- based coatings described in for example US patent 6 348 265 will lose its insulating properties at a temperature of about 55O ⁰ C, thus limiting the maximum stress releasing temperature to about this temperature.

Even though electrically insulating coatings and various combinations of soft magnetic particles and coatings to be used for producing compacted and heat treated soft magnetic composite articles have been described, it remains a problem to elevate the applied heat treatment temperature without worsen the AC- losses for any given combination of base particles and insulating particle coatings.

In addition to the soft magnetic properties, sufficient mechanical properties are desirable for many applications of SMC materials. High mechanical strength is often a prerequisite to avoid introducing cracks, laminating, and break-outs and to achieve good magnetic properties of compacts, which after compaction and heat treatment also may be subjected to machining operations. By impregnating a heat treated soft magnetic composite component with various inorganic resins, water glass, liquid polymers such as thermoset resin, thermo- plastics or anaerobic acrylics high mechanical strength can be obtained. Furthermore, the impregnating substance may add

lubricating properties which may also increase the lifetime of machining tools considerably.

US Patent 6 485 579 describes a method of increasing the mechanical strength of SMC components by heat treating the component in the presence of water vapour. Higher values for the mechanical strength are reported compared to articles heat treated in air, however, increased core losses are obtained. A similar method is described in WO2006/135324 where high mechanical strength in combination with improved magnetic permeability is obtained provided metal free lubricants are used. In order to be able to expand the utilisation of SMC articles based on compacted and heat treated electrically insulated iron- based powders, lower total core losses and enhanced mechanical properties are desired. Hence, there is a need of performing the stress relieving heat treatment of a component at higher temperatures allowing a thorough stress release and lower DC- losses without also inducing elevated AC- losses. In many cases there is also a need to improve the mechanical strength of the finished component.

SUMMARY OF THE INVENTION Embodiments of the method disclosed herein provide an increase of the electrical resistivity, and consequently a decrease of the eddy current loses, also denoted AC- losses, and core losses, of a compacted and heat treated soft magnetic component produced from electrically insulated iron- based soft magnetic particles.

Furthermore, embodiments of the method described herein allow elevated heat treatment temperatures to be applied, thus reducing the coercive force and hysteresis losses, i.e. .DC- losses, without any detrimental effect on the electrical resistivity or magnetic properties such as permeability, induction or coercivity.

Furthermore, some embodiments of the method described herein improve the mechanical strength of a heat treated soft magnetic component.

BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 show comparative results of electrical resistivity of different components and as a function of the duration of the controlled mechanical stress operation.

DETAILED DESCRIPTION OF THE INVENTION Method for producing soft magnetic composite parts:

Described herein is a method for producing soft magnetic composite parts. Embodiments of the method comprise the step of providing a compacted and heat-treated soft magnetic component, produced at least from electrically insulated iron-based powder particles having an electrical resistivity, e.g. by compacting an electrically insulated iron- based powder, optionally comprising a lubricant either as discrete particles or as a coating on the insulated iron- based particles, heating the compacted component to a temperature allowing desired stress release and, in case of a lubricant present, above the vaporisation temperature of the lubricant such that the lubricant substantially is removed from the compacted component, in a non reducing, inert, or oxidizing atmosphere.

In embodiments of the method described herein, the compacted and heat- treated component is further subjected to stress or force in the compacted and heat-treated component for a period long enough to increase the electrical resistivity of the compacted heat-treated soft magnetic component.

Mechanical stress may be imparted on the component as a direct mechanical force by another body or device, or via a physical field or wave, such as an electromagnetic field/or wave a sound wave, etc. The force may act on the

component as a whole and/or on parts of the component, e.g. on specific sections where higher electrical resistivity is desired.

The stress may be induced in the heat-treated component for the duration of the treatment, preferably causing elastic strain only, i.e. without introducing permanent strain or stress in the component. Accordingly, this process may be described as a controlled mechanical stress operation. Embodiments of this process may induce movement of the individual iron- based particles within the component; preferably in a controlled manner without causing a destruction of the component.

The magnitude and/or direction of the stress may vary over time, e.g. when the component is subjected to cyclic stress, e.g. by subjecting the component to a cyclic force causing a mechanical vibration of the component, an interaction with a wave or cyclically varying field, or another suitable physical interaction that causes a cyclically varying stress in the heat-treated component. The terms cyclic stress and cyclic force are intended to refer to any stress and force, respectively, where at least one property of the stress or force (e.g. its magnitude and/or direction) varies in a generally cyclic or recurrent manner; this may be in a periodic or other uniform manner, or in an irregular or even discontinuous manner.

In some embodiments subjecting the compacted heat-treated soft magnetic component to stress comprises subjecting the compacted heat-treated soft magnetic component to at least one of a fluctuating stress, a repeating stress, a static stress or a non-fluctuating stress or a combination thereof.

In some embodiments a fluctuating stress or a repeating stress comprises at least one of a mechanical vibration, a varying magnetic field causing magnetostriction or a vibration induced by a sound wave or a combination thereof.

In some embodiments a static stress or a non-fluctuating stress comprises an elastic deformation caused by a non- destructive mechanical loading such as a pressing operation, a rolling operation or a bending operation or a combination thereof.

Examples of stress imparted by interaction with a wave or varying field include magnetostriction, e.g. by subjecting a magnetostrictive component to a magnetic field. Other examples include a vibration induced by sound waves, etc. The stress-inducing process may also include an elastic deformation of the component caused by a non destructive mechanical loading such as a pressing operation, a rolling operation or a bending operation.

It will be appreciated that, in addition to the duration for which the component is subjected to stress, additional parameters of the process may be controlled, e.g. the magnitude of the stress applied and/or the direction relative to the geometry of the component along which the stress is applied; and/or a frequency of the cyclic stress, e.g. a frequency of the wave or varying field to which the component is subjected, etc. The magnitude of the controlled mechanical stress operation may e.g. be controlled by controlling a frequency and/or amplitude of an induced vibration, by the strength and/or frequency of a field or wave to which the component is subjected, by controlling a bending load, a rolling pressure, or in any other suitable way.

It will be appreciated that the choice of parameter(s), e.g. the duration of the controlled mechanical stress operation, may depend on the composition and/or other properties of the iron-based powder and/or on the properties, e.g. the shape and size, density, of the compacted component, and/or on the compacting and/or heating process. In particular, it will be appreciated that

the duration of the stress to be applied required for a given increase in electric resistivity for a given component may depend on the magnitude of the stress to which the component is subjected. Hence, the compacted heat- treated soft magnetic component may be subjected to stress having a magnitude large enough and for a period long enough so as to increase the electrical resistivity of the compacted heat-treated soft magnetic component. For stress of larger magnitude, the required time may be shorter. The magnitude of the stress may e.g. be determined as a maximum or average stress applied during a period, e.g. during a cycle of a cyclic operation.

Consequently, the method may further comprise determining, e.g. for a particular type of compacted, heat-treated soft magnetic component, at least one parameter indicative of a property of the controlled mechanical stress operation, and controlling the mechanical stress operation based on the determined parameter. For example, for a given magnitude of the stress, the process may include determining a duration of the controlled mechanical stress operation. In other embodiments, for a given duration, a required magnitude may be determined, or both the magnitude and the duration may be varied. In yet further embodiments, additional parameters may be determined such as a direction of stress relative to the component, a wavelength/frequency of a varying field, etc.

The at least one parameter may be determined so as to provide a predetermined change in electrical resistivity of the heat-treated component, e.g. a maximum increase in resistivity and/or a change in resistivity above a predetermined threshold, and/or a resulting resistivity above a predetermined threshold. For example, for a given controlled mechanical stress operation and for a given type of component, the duration of the controlled mechanical stress operation may be determined such that a further increase of the duration does not result in a further increase (or at least no further significant increase) in electrical resistivity. For example, the duration may be

determined such that a further increase of the duration only results in an increase in resistivity per unit increased duration smaller than a predetermined threshold slope. Alternatively, the duration may be determined such that the electrical resistivity is increased by a predetermined relative amount or factor, e.g. by a factor of at least 10 or at least 100. It will be appreciated that the parameter may be determined so as to provide the desired change in electrical resistivity of the heat-treated component without damaging or even destroying the heat-treated component or causing other undesired effects, such as an inelastic deformation of the component.

The at least one parameter may be determined by preparing a series of heat- treated sample articles and subjecting them to the controlled mechanical stress operation, controlled based on respective values of the at least one parameter, by measuring and comparing the resulting electrical resistivities, and selecting the parameter value based on the comparison.

The electrical resistivity may be measured by any suitable technique, as only a change in resistivity needs to be detected, e.g. by measuring the resistivity between the same points before and after the controlled mechanical stress operation. For example, the resistivity of the component may be measured before and after the controlled mechanical stress operation by the four-point method for measuring resistivity. Alternatively, the electrical resistivity may be measured by various conductivity measurements. Yet alternatively, the electrical resistivity may be measured indirectly, e.g. by measuring the total core loss of the component using a hysteresis graph, and therefrom determining the Eddy current loss (AC- loss) relative to the hysteresis loss (DC- loss) and calculating the resistivity using Steinmetz equation.

The controlled mechanical stress operation for increasing the resistivity of the component may be performed at any time after the heat-treatment, e.g. before, during, and/or after cooling the heat-treated component. In some

embodiments, the component may be subjected to a further heat-treatment after being subjected to the resistivity-increasing process, e.g. by subjecting the component to two or more cycles of heat-treatment and subsequent resistivity-increasing treatment.

The component may further be subjected to a process which enhances mechanical strength by, for example, impregnation, infiltration, sealing, casting or moulding with a suitable organic or inorganic compound such as water glass, polymers, resins etc., an superficial oxidation of the component for example a steam treatment such as described in US 6 485 579, coating the component with a suitable substance or embedding the component in a suitable support. For example, by using liquid polymer composites comprising nanometer sized or micrometer sized reinforcement structures, a high mechanical strength may be obtained. The process for enhancing mechanical strength may be performed during or after the stress-treatment described herein.

In some embodiments subjecting the compacted heat-treated soft magnetic component to stress comprises: - subjecting the compacted and heat-treated soft magnetic component to a pressing operation comprising applying a pressure substantially lower than a compaction pressure, where the compaction pressure is the pressure applied for providing the compacted and heat-treated soft magnetic component.

Furthermore, the pressing operation pressure may be applied to the soft magnetic component, when the component is placed outside a mould used for preparation of the compacted soft magnetic component.

In some embodiments the pressing operation pressure is less than 20% of the compaction pressure.

It has surprisingly been found that the application of a pressure smaller than the pressure used for compaction of the soft magnetic component, such as a pressure less than 20% of the compaction pressure, provides an improvement of the magnetic properties of the soft magnetic component without destroying the component mechanically, thus the pressing operation pressure is a non-destructive mechanical loading.

The present invention relates to different aspects including the method described above and in the following, and corresponding methods, devices, uses and/or product means, each yielding one or more of the benefits and advantages described in connection with the first mentioned aspect, and each having one or more embodiments corresponding to the embodiments described in connection with the first mentioned aspect and/or disclosed in the appended claims.

In particular, disclosed herein is a soft magnetic composite component produced by the method described herein.

In some embodiments the soft magnetic composite component is an inductor core.

Examples of SMC parts which may be produced by the method are inductor cores or stator or rotor cores in electrical machines such as motors or generators. Of special interest are inductors working at high frequencies.

Iron- based powder

The iron- base powder is preferably a plain/substantially pure iron- powder but could also be an iron- powder alloyed with suitable alloying elements such as Si, Al, Ni, Co, Cr providing sufficient soft magnetic properties are

obtained. The carbon content may preferably be below 0.1 % by weight in order to obtain sufficient compressibility and soft magnetic properties. The iron-based powder may be a water-atomized iron-based powder or a sponge iron powder having irregular shape. There are no special requirements on the particle size distribution as long as the iron- based powder particles are suitable to be compacted, for example the particle size should have a distribution such that the iron-based powder particles are suitable to be compacted by uniaxial compaction. This means that the particle size should be less than 1000 μm in order to ensure complete filling of thin cavities and above 10 μm in order not to give rise to galling caused by fine particles trapped between the die and punch. For example, the mean particle size may be between 45 μm and 500 μm.

An electrically insulating coating may be applied on the surface on the iron- based powder. This coating may be organic or inorganic. An example of a coating is described in the US patent US 6348265.

Particle size and mean particle size can be measured and calculated by a conventional sieve operation as described in SS EN 24497 or by laser diffraction as described in SS ISO 13320-1.

The term mean particle of a powder is defined as the size where 50 % by weight of the particles have a size less than the mean particle size and 50 % by weight of the particles have a size above the mean particle size.

Thus in some embodiments the mean particle size of the iron-based powder particles is between 10 μm and 1000 μm, such as between 45 μm and 500 μm.

Furthermore, in some embodiments the mean particle size is thus measured according to the standard SS EN 24497.

Lubricants

Lubricants used are chosen from the group of organic or inorganic substances having a lubricating effect. Examples of organic lubricants are waxes, polymers, oligomers, stearates such as metal stearates or other fatty acid derivates having a lubrication effect. Example of inorganic lubricants is MoS ₂ .

Heat treatment process The heat treatment may be performed in a inert, non- reducing, or oxidizing atmosphere. In some embodiments, the maximum heat treatment temperature applied during the heat treatment process may be the temperature where complete or partial stess release of the component has been obtained. In particular, the maximum applied heat treatment temperature may be established for every combination of iron- based powder and electrically insulating coating. For example, the temperature may be established such that a further increase in temperature does not result in a further decrease in coercivity Hc. The actual temperature may depend on a number of parameters such as purity, particle size and shape, chemical composition, etc. For example, the maximum applied heat treatment temperature may be above 55O ⁰ C, e.g. above 600 ⁰ C , for example between 600 ⁰ C and 75O ⁰ C.

EXAMPLES

Example 1

An insulated soft magnetic iron- based powder, Somaloy®700, available from Hδganas AB, Sweden, containing essentially pure iron powder insulated with a thin phosphorus containing coating as described in US patent 6 348 265, the particle size of the iron- powder being between 45 μm and 425 μm, was

mixed with 0.2 % by weight of a particulate lubricant (Kenolube®, available from Hδganas AB, Sweden).

The obtained mixture was transferred to a compaction die and compacted into magnetic toroid samples having an inner diameter of 45 mm, an outer diameter of 55 mm and a height of 5 mm, at a compaction pressure of 1100

MPa. The temperature of the die was 8O ⁰ C.

One sample, A, was heat treated at 55O ⁰ C in nitrogen atmosphere for 30 min and another sample, B, was heat treated at 65O ⁰ C in nitrogen atmosphere for

30 min.

The magnetic properties were measured on the toroid samples with 100 drive and 100 sense turns using a hysteresis graph MPG 100D from Brockhaus

Messtechnik. The total core losses were measured at 1 Tesla and 400 Hz and at 1 kHz

Maximum permeability at an applied electrical field of 4 kA/m was measured.

The specific electrical resistivity was measured on the toroid samples by a four point measuring method.

The following table 1 shows the results of the measurements. Table 1.

As can be seen from table 1 the sample, B, heat treated at a higher temperature compared to sample A, shows a lower coercive force, however, the electrical resistivity is deteriorated giving higher eddy current-, AC- loss. The total core loss is therefore of the same magnitude as for example A.

Example 2

A new sample, C, based on the same mixture as in example 1 , was compacted and heat treated according to sample B in example 1. After heat treatment sample C was placed in a vibrator, COROB™ PF EMIX, available from CPS COLOR™ Equipment S.P.A. Italy, for 12 seconds. The so called "clamping force" was set to 250 kg and the frequency to 620 rpm. The magnetic and electric properties were evaluated as in example 1.

The following table 2 shows the result from the measurements compared to sample B in example 1.

Table 2

Table 2 reveals that sample C has surprisingly achieved a much higher resistivity compared to sample B, thus the AC- loss has decreased considerably, about 60 %, compared to what was obtained for sample B. Furthermore, also other magnetic properties such as permeability, induction at 4 000 A/m and at 10 000 A/m and the coercive force are surprisingly not effected. This means that the shape and density are maintained, the compacted component has not been distorted or affected on a macro level.

Example 3 In this example the influence on the resistivity and AC- losses of the vibration time and size of the samples were determined. Two toroid samples, D and E, were prepared with the material and according to the process described in

example 1 except that sample E was compacted at a compaction pressure of 600 MPa. The samples were heat treated at 65O ⁰ C in nitrogen atmosphere for 30 min.

After heat treatment the samples were subjected to a vibration process according to example 2 with the exception that the electrical resistivity, measured according to the description in example 1 , was measured after defined time periods of vibration.

The results of the resistivity measurements are presented in figure 1. Figure 1 shows that maximum resistivity is obtained for sample D after about 40 seconds and a further vibration of another 10 seconds causes a slight drop in resistivity. For sample E the maximum resistivity is obtained after 50 seconds of vibration, another 10 seconds of vibration causes as well a slight drop, but less than for sample D. This example indicates that the optimum vibration time has to be established for each type of component depending on e.g. the composition of the component, shape and density.

The example was repeated with samples F and G corresponding to samples D and E, respectively, with the only exception that the height of the toroid samples F and G was larger, namely 10 mm.

The following figure 2 shows that for these relatively larger samples the vibration time has to be prolonged, up to 10 minutes in order to reach maximum resistivity. The examples indicates that the optimum vibration time, frequency, and amplitude have to be established by a man skilled in the art for each type of component depending on e.g. the composition of the component, shape and density.

The results of the resistivity measurements are presented in figure 2.

Example 4

In this example an alternative controlled mechanical stress operation without causing a destruction of the component was tested as an alternative to vibration, namely a weak elastic bending.

The same type of materials as in example 1 were compacted into toroid samples at a compaction pressure of 1100 MPa and at a die temperature of 8O ⁰ C. The dimensions of the toroid samples were as in example 1 except that the height was 10 mm. After compaction the samples were subjected to a heat treatment at 65O ⁰ C in nitrogen atmosphere for 30 min. Measurements of magnetic and electric properties were performed as in the previous examples. The sample denoted F was not subjected to any further processing after the heat treatment whereas the sample denoted G was subjected to the alternative stress operation (a weak elastic bending) prior to the measurements.

Table 3 shows the result of the measurements.

Table 3.

A remarkable improvement of the electrical resistivity of sample H was obtained due to the weakly elastic bending. As the maximal permeability or coercive force was not affected, the reduction of total core losses was substantial, about 75 %.

Other magnetic and physical properties were not affected indicating that the component has been subjected to a truly elastic stress operation (bending). It

may thus be concluded that no distortion, crack introduction, or change of shape of the component, has occurred.