EP1231003B1 - Process for preparing an article of soft magnetic composite material - Google Patents

Description

The invention relates to a method for producing a molded part from a soft magnetic composite material, in particular for use as a magnetic core for a common rail injector, according to the preamble of the independent claims.

State of the art

Out EP 0 765 199 B1 are mixed with a thermoplastic resin mixed iron powder, which are particularly suitable for the production of magnetic cores. Specifically, there is provided to first treat an iron powder with phosphoric acid, then to mix this iron powder with a thermoplastic resin, to press this mixture at a temperature below the glass transition temperature or the melting point of the thermoplastic resin, to heat the pressed product to the thermoplastic Harden resin, and optionally to carry out a tempering of the resulting component to a temperature above the curing temperature of the thermoplastic resin.

It is also known from the thermoplastic material polyetherimide, which is known under the trade name Ultem®, as well as oligomers, as described in WO 95/33589 and sold under the trade names Orgasol 3501 and Orgasol 2001 by Elf Atochem, France.

In addition, in EP 0 765 199 B1 intended to mix the iron powder with a pressing aid or a lubricant, which may be a metal stearate, a wax, a paraffin, a natural or synthetic fat derivative or an amide-type oligomer. Specifically described as lubricants or pressing aids are the products Kenolube® from the company Höganäs AB, Sweden, H-wax® from the company Höchst AG, Germany, and Promold® from the company Morton International, Cincinnatti, USA, containing the iron powder in a proportion of preferably 0.2 to 0.8 weight percent are mixed.

Finally is off EP 0 765 199 B1 known to press this starting mixture at a pressure between 400 and 1800 MPa, and then in air at temperatures between 100 ° C and 600 ° C, preferably 200 ° C to 500 ° C, to anneal.

One according to EP 0 765 199 B1 powdered soft magnetic material produced under the trade name Somaloy ™ 500 by the company Höganäs AB, Sweden, and is more closely characterized in the company magazine SOMALOY ™ 500, SMC 97-1, pages 1-11, Höganäs AB, Sweden.

Furthermore, such soft magnetic composites are also described in Jan Tengzelius, "Soft Magnetic Composites for Electric Motors", Proceedings Hagen Symposium, 1.12.2000, pages 211 to 227 described.

In Scripture JP-A-2000/232014 describes a heat treatment at least two stages during sintering. There it is described to use certain concentrations of gas mixtures at certain temperature intervals. But there are always several steps necessary. A single step is not mentioned. Also, the time period is not discussed.

The object of the present invention was to provide a method by means of which a molded part made of a soft magnetic composite material which can be used in particular as a magnetic core for common rail injectors can be produced from a powder mixture containing an iron powder with improved mechanical and magnetic properties compared with the prior art.

Advantages of the invention

The inventive method have over the prior art has the advantage that the molded parts or magnetic cores for common rail injectors usual magnetic cores made of soft magnetic composites, for example, with mixtures of pure iron powder with polyamide binder, pure iron powder with polyphenylene sulfide binder or pure iron powder have been prepared with polyethylene binder, especially in terms of mechanical strength, density, saturation polarization, magnetic permeability, electrical resistivity, surface hardness and flexural strength are superior.

For example, the molded parts according to the invention in the form of magnetic cores to polyphenylene sulfide-bonded composite magnetic cores increased by at least 0.2 g / cm ³ density of more than 7.3 g / cm ³ , and they also have a significantly improved surface hardness and statistical flexural strength, which manifests itself in particular in the critical area of the pole faces in an improved edge crushing strength under continuous load. In addition, they are much less prone to material eruptions and it also penetrates less diesel fuel in the workpiece structure. Moreover, the molded parts according to the invention in the form of magnetic cores show a magnetic force of typically 95 N to 103 N, while corresponding molded parts made of polyphenylene sulfide-bonded composite only reach about 80 N.

Next, the molded parts according to the invention when used as a magnetic core in common-rail injectors compared with previously customary magnetic cores a significantly higher Switching dynamics, in particular a reduced by about 20 microseconds on-time, reduced energy requirements, by about 50% higher mechanical strength, better mechanical workability and greater insensitivity to manufacturing fluctuations in the production on.

In addition, they are cheaper to produce by using a cheaper raw material and the elimination of a previously required hot pressing, which also leads to less tool wear.

Thus, it has been found that in the gas atmosphere during annealing, especially in combination with temperatures between 380 ° C to 450 ° C, a certain minimum amount of oxygen is very advantageous to ensure sufficient oxide formation between the iron powder or on the surface, on the other hand but compared to the prior art significantly reduced amount of oxygen contained in the gas atmosphere used to significantly improved magnetic properties, such as a higher magnetic force, the resulting molded parts leads.

In particular, it is advantageous if the gas atmosphere during annealing is a gas mixture with an oxygen content of 2 vol% to 7 vol%, wherein a mixture of air and nitrogen or a mixture of air and a noble gas, the proportion of air between 40 vol% and 10% by volume, in particular 10% by volume to 30% by volume, is particularly easy and inexpensive to produce.

Moreover, it is advantageous if, after the tempering of the resulting molded parts in the form of magnetic cores, a mechanical processing, for example a careful grinding, is carried out, which compensates for differences in pole height and a leveling of pole faces, and by which the magnetic force of the moldings used, for example, as a magnetic core can be further increased to over 100 N.

A further improvement in the magnetic and mechanical properties of the molded parts obtained, in particular in terms of their density, is achieved when the annealing of the pressed moldings is carried out in a two-stage process, after pressing the starting mixture, the molding initially annealed at a relatively low temperature, thereafter pressed again in a die tool or by hot plan forming, and finally annealed again at a higher temperature.

Since the molded parts made of a soft magnetic composite material produced by the method according to the invention are preferably an oxide-bonded material, i. In the annealing process, one of the starting mixture, for example, added metal stearate decomposes to form a metal oxide, so that form by the presence of oxygen at grain boundaries iron oxide bridges that effectively improve the microstructure, are in the moldings according to the invention over those of polymer-bonded soft magnetic composites also contain at least almost no organic components. Thus, the moldings produced according to the invention, in addition to their higher density and a lower porosity, which leads to a significantly improved thermo-mechanical long-term stability, especially against hot diesel fuel.

Advantageous developments of the invention will become apparent from the measures mentioned in the dependent claims.

embodiments

In a first embodiment, to explain the background of a starting mixture with a pure iron powder and a pressing aid is assumed, as sold by the company Höganäs, Sweden, under the trade name Somaloy ^™ 500.

Specifically, the pure iron powder used is a high-purity iron powder with phosphated surface, the, as in EP 0 765 199 B1 described as lubricant a pressing aid selected from the group of metal stearates, waxes, paraffins, natural or synthetic fat derivatives or amide-type oligomers is added.
Preferably, the pure iron powder is used together with the known under the trade name Kenolube® pressing aids Höganäs AB, Sweden. For this purpose, the pressing aid Kenolube®, which contains essentially an amide wax and zinc stearate, in a proportion of 0.4 wt.% To 0.7 wt.%, Preferably 0.5 wt.% To 0.6 wt %, added to the pure iron powder and mixed with this to the starting mixture. Subsequently, the starting mixture is then pressed in a conventional die preferably at room temperature at a pressure between 600 MPa to 900 MPa, in particular 700 MPa to 800 MPa, for example in the form of a magnetic core for common rail injectors.

After pressing, the molding obtained is at temperatures between 380 ° C to 450 ° C, in particular at about 425 ° C, over a period of 10 min to 120 min, in particular 30 min to 60 min, in a nitrogen-air mixture or a noble gas-air mixture tempered, wherein the proportion of air between 50 vol% and 5 vol%, in particular 10 vol.% To 30 vol.%, For example, 20 vol.%. This is the added Pressing aids partly decomposed and partly converted into a binding oxide. Alternatively, it is also possible to use a mixture of an inert gas with oxygen, for example a nitrogen-oxygen mixture or an argon-oxygen mixture, which has an oxygen content between 1% by volume and 10% by volume, in particular 2% by volume to 7% by volume, contains.

The moldings obtained after annealing are preferably subjected to a final mechanical surface treatment, for example a grinding. This leads to improved mechanical properties and improved long-term stability of the resulting molded parts. In addition, it is achieved by the subsequent grinding that the measured magnetic force on such magnetic cores usually increases by about 5% to 10%.

In a second embodiment, the invention will be explained, wherein, unlike the above-described embodiment, after pressing the starting mixture to the molding initially a first annealing step at a temperature of 150 ° C to 400 ° C, in particular at temperatures between 230 ° C and 310 ° C, is made.

This first annealing step may be carried out in air or an inert gas atmosphere such as a noble gas atmosphere or a nitrogen atmosphere. However, it is preferably carried out, analogously to the tempering in the first embodiment, in a gas mixture of an inert gas and oxygen, the proportion of oxygen in the gas mixture being between 10% by volume and 1 volt.

Particularly preferably, the gas atmosphere in this embodiment again a mixture of air and nitrogen, wherein the proportion of air between 50 vol% and 5% by volume, in particular 10% by volume to 30% by volume, for example 20% by volume.

After the first annealing step, further compression is then carried out at room temperature for reshaping the pressed, annealed shaped part at a pressure between 600 MPa and 900 MPa, in particular 700 MPa and 800 MPa.

Alternatively, this post-forming can also be done by hot plan forming in a suitable die tool at elevated temperature, as for example in DE 100 05 551.6 is described.

After the illustrated post-forming takes place a renewed, second annealing of the resulting molded part, analogous to the first embodiment at temperatures between 380 ° C to 450 ° C, in particular 425 ° C, over a period of 10 min to 120 min, especially 30 min to 60 min, in a nitrogen-air mixture or a noble gas-air mixture, wherein the proportion of air between 50% by volume and 5% by volume, in particular 10% by volume to 30% by volume, for example 20% by volume ,

The moldings obtained after annealing are then preferably subjected to a final mechanical surface treatment, for example a grinding, analogously to the first exemplary embodiment.

Specifically, a molding made of a soft magnetic composite material of the phosphatized pure iron powder Somaloy 500 with 0.6% by weight of Kenolube according to the above embodiments, a statistical bending strength of at least 25 N / mm ² , determined on test bars according to ISO 3327, and a surface hardness HB 2, 5 / 31.25 of at least 70 on.

Further, on rings having an outer diameter of 40 mm, an inner diameter of 30 mm and a height of 5 mm, a magnetic polarization J ₁₀₀ of at least 1.4 Tesla at 100 A / cm, a saturation polarization J _s of at least 1.5 Tesla at 500 amperes / cm, a coercive force H _cB of at most 3.0 amps / cm, a maximum permeability μ _max . of at least 450 and a total loss ν _H + ν _W at 1 Tesla and 50 Hz of max. 8 W / kg measured. In general, even a saturation polarization of more than 1.7 Tesla and a maximum permeability of about 500 at a specific electrical resistance of about 10 μΩm is achieved.

The density of the molded parts obtained is at least 7.30 g / cm ³ , whereby an increase in density to about 7.5 g / cm ³ can be achieved by additional post-forming in a die tool or the additional Heißplanformen.

Claims (7)

1. Process for producing a moulding from a soft-magnetic composite material, having the following process steps: a) providing a starting mixture comprising an iron powder and a pressing aid, b) pressing the starting mixture to form a moulding, and c) heat treatment of the moulding, wherein the heat treatment is effected in a mixture of an inert gas and oxygen, wherein the oxygen content of the gas mixture is between 10% by volume and 1% by volume and the heat treatment is effected at temperatures of between 380°C and 450°C, in particular at 425°C, over a time period of from 10 min to 120 min, in particular of from 30 min to 60 min,
   characterized
   in that, after process step b), the pressed mouldings are firstly heat-treated at a temperature of from 150°C to 400°C, in particular of from 230°C to 310°C, are then post-formed, and are subsequently heat-treated again in accordance with process step c).
2. Process according to Claim 1, characterized in that, in step c), the oxygen content is between 7% by volume and 2% by volume, and in this case the gas mixture is, in particular, a mixture of air and nitrogen, or air and a noble gas.
3. Process according to Claim 1, characterized in that the pressing is effected at room temperature and at a pressure of between 600 MPa and 900 MPa, in particular of between 700 MPa and 800 MPa.
4. Process according to one of the preceding claims, characterized in that a starting mixture which contains phosphatized pure iron powder and a pressing aid selected from the group consisting of metal stearates, waxes, paraffins, natural or synthetic fatty derivatives and oligomers of the amide type, in particular Kenolube^®, is employed.
5. Process according to one of the preceding claims, characterized in that, after process step b) and before the heat treatment in accordance with process step c), the pressed mouldings are heat-treated in air, an inert gas atmosphere or a gas mixture of an inert gas and oxygen, wherein the oxygen content of the gas mixture is between 10% by volume and 1% by volume.
6. Process according to Claim 1 or 5, characterized in that post-forming is effected by carrying out further pressing at a pressure of between 600 MPa and 900 MPa, in particular of between 700 MPa and 800 MPa, or by carrying out hot plane forming.
7. Process according to one of the preceding claims, characterized in that the heat treatment in accordance with process step c) is followed by at least regions of the surface of the mouldings being machined, in particular ground.