US2245477A - Permanent magnet and method of making same - Google Patents

Description

Patented June 10, 1941 UITED MS PATENT rrica Gottfried Bruno Jonas, Elndhoven, Netherlands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartford,

Conn., as trustee No Drawing.

Application February 13, 1937, Se-

rial No. 125,654. In Germany March 17, 1936 3 Claims.

My invention relates to permanent magnets, and to a method of producing same.

Although Ni-Ti alloys, as well as Ni-Co-Ti alloys are used for permanent magnets, the desired magnetic properties are generally not obtained.

manent magnet are not obtainable at all, Even if such alloys are improved by adding substances such as Cr, Mo, V, Al, As, or W, and are heattreated in the same manner, only in exceptional cases are the desired results obtained.

The object of my invention is to overcome the above difiiculties and to produce a permanent magnet having high m agnetic properties.

In accordance with my invention, I select a definite group of alloys from the entire known and practically infinitely large region of the above-mentioned magnet steel alloys, and heattreat these alloys in such a manner that, after magnetisation, particularly satisfactory values of remanence and coercive force are obtained.

More particularly, I use a steel alloy comprising 13-22% nickel, 8-30% cobalt, 8-0.5% titanium, 4-11% aluminium, up to 1.5% of impurities, and the remainder iron. In some cases I also use from 0.5-5% copper.

I form these constituents into an alloy by melting or sintering same, whereupon I cool the alloy from a temperature of about 1200 down to about 650 0., either directly from the melt or after the alloy has been re-heated to a high temperature, at a speed determined by the composition of the alloy, and at an average rate less than 8 C. per second. Thereupon I further cool the alloy to room temperature in any desired manner, for example in air.

The magnet bodies can be produced by casting I the alloy, or by intermixing the constituents in a finely divided state-preferably in the form of preliminary or intermediate alloys thereof-by compressing them more or less, and by homogenizing by a sintering process.

Although the addition of copper does not appreciably improve the magnetic properties of the above-mentioned alloys, it generally does increase the coercive force which is desirable for many purposes, in spite of the fact that the remanence is reduced.

I prefer to so select the quantities of titanium and aluminum that the total amount thereof is about 9 to 15%, preferably about 11%, because for the alloys under consideration this percentage produces magnets of very -favourable magnetic properties.

desired manner, for example in air.

Instead of adding titanium to form the 8-0.5% titanium content, I may add a suitable amount of ferrotitanium which is usually employed for such purposes and which may be assumed to contain from about 20 to 40% of titanium depending upon its quality. In the latter case however, those admixtures which in addition to iron and titanium are present in the ferro-alloy, for example Al, Si, Cu, and Mn, must be taken into account. Particularly the silicon which is nearlyalways present in ferrotitanium, should constitute a non-excessive percentage of the magnet alloy, i. e. a percentage less than about 1%, as otherwise it would deleteriously affect the magnetic properties of the alloys. The manganese present in the ferro-alloy does not substantially alter the magnetic properties of the alloys. In the same way I may add a suitable amount of ferrocobalt to form the 8-30% cobalt content.

I prefer not to use simultaneously in the same alloy nickel, cobalt and titanium in the maximum amounts of the above-stated ranges, as with such an alloy the magnetic maximum values are generally not attained by a technically simple thermal treatment. With the exception of the above-mentioned conditions as to the total percentage of titanium and aluminium,

and to the silicon content, there are no further restrictions in the choice of the percentage of the various components as long as they are within the prescribed ranges.

Magnet steel alloys obtained by the process of the invention may have, after magnetisation, a remanence of about 6,500-9,500 and a coercive force of about 900-300, whereas the majority of the magnets attain a BHmax value of over 1,500,000 gausses and some evenmore than 2,000,000 gausses.

The heat treatment according to the invention may be effected in difierent ways, and any magnet steel alloy having a composition within the ranges above set forth must be cooled at a definite rate according to its composition to obtain the highest values of remanence and coervice force. I generally carry out the heattreating by heating a body formed from an alloy of the above type, to a temperature which is as high as possible without reaching the melting point of the alloy-at least higher than 1150 and then cool the body to a temperature about 650 C. at a speed which is less than an average cooling rate of about 8 0. per second, and which has been definitely determined with the aid of test pieces. I then further cool the body in any By average cooling rate is meant the total decrease in temperature divided by the total time. In this hardening process, and particularly with bodies of comparatively large size, I prefer.

to use means to cause a very slowly-proceeding hardening, for example such as loamwater, compressed air, wet sand, non-circulating air, dry sand, or furnaces heated to a suitable temperature. In some cases, for example with bodies of medium size and of favorable shape, the bodies may be formed by merely pouring the molten alloy into a suitable mold of dry or wet molding sand in such manner that the particular cooling rate required is automatically obtained without further measures, and whereby subsequent hardening may be dispensed with.

In many cases a further improvement in the magnetic properties, particularly in the BHmax, may be obtained by annealing the body after the above-mentioned thermal treatment, for example for a few hours, and at temperatures between about 600 to 700 C and then cooling the same in any desired manner.

I have found that the very slowly-proceeding cooling makes the process according to the invention very suitable for bodies of larger dimensions as used in loudspeakers, e. g. for annular magnet-bodies having an outside diameter exceeding 70 mm., an inside diameter exceeding 40 mm. and a height exceeding 30 mm.

The magnetic properties of magnets produced in accordance with the present invention and by prior art method are indicated in Tables I and II below. Table I gives the magnetic properties of magnets produced in accordance with the invention and for four different alloys as indicated by Examples I to IV, whereas Table II gives the magnetic properties of magnets produced from the alloys of Example I to IV of Table I when these alloys are heat-treated by prior art methods.

In the following tables the bodies as originally formed were 10 mms. wide, 32 mms. thick and 100 mms. long, whereas prior to the annealing the bodies were divided into pieces having a width of 10 mms., a thickness of 12 mms. and a length of 30 mms. In the tables, Br denotes the residual flux density in gausses, whereas He indicates the coercive intensity in gausses.

TABLE I According to the invention TABLE II According to a usual prior art process Rapid cooling (from 1253 G for one minute in bath of After molten tin annealing at 650 C. for about and 6% hours further cooling Composition of alloy to room temperature From Tables I and II it is seen that in all the examples the magnets produced according to the invention have He values greatly exceeding the He values of magnets produced by the prior art method, whereas without annealing the Br values exceed those of the prior art magnets.

It should be noted that also in Example I, ferrotitanium is used from which the content of titanium indicated has been calculated. Besides, the aluminium in the ferrotitanium is present in the amount given therefore.

The following two examples illustrate the application of the invention to magnets for use in loudspeakers.

Example V.An annular magnet having an outside diameter of 74 mms., an inside diameter of 4'7 mms. and a height of 37 mms. (which is a size commonly used in present-day electrodynamic loudspeakers) was cast from a composition obtained by melting together an alloy containing about 16% Ni, 25% Co, 12.5% ferrotitanium (equivalent to about 3% Ti), 6.8% Al,

r 2.5% Cu and the remainder substantially iron.

Cooling in sand mold at rate of about 1 0. per Ex. Composition of alloy sec.

After re-heating After tel, to 1250 C. and being per see. more B, H, B, H, B, H, B.- H.

Rate of cooling (about 3 0.

per sec.) 16% Ni, 25% (-0, about 4% Ti, 7% Al,

remainder Fe 8000 440 8000 592 9900 350 9000 503 Rate of cooling about 3 0. per sec. ll 16% Ni, 25% Co, 13% FcrrotiL, (about 2.8% T1), 6.5% Al, 1% Cu 9000 400 8550 500 9700 324 9200 472 Rate of cooling about 3 0. per sec. III..- 10% Ni, 25% Co, 19% Ferrotit., (about 4% Ti) 5.1% Al, remainder Fe 8050 476 7800 032 9200 344 8550 524 Rate of cooling 7 0. per sec. IV. 17% Ni, 10% Co, 10% Ferrotit, (about 4% Ti), 7% Al, remainder Fe 6000 288 0050 320 7850 280 7800 350 in air for about 4.75 minutes to about 650' 0. Thus the average cooling rate was about 1.7 per second. After being magnetized, the resulting magnet had a BI value of about 9,000 gausseaand an H6 value of about 470 gausses.

Example VL-A ring of the same size as that of Example V was cast from a composition obtained by melting together an alloy containing about 16% Ni, 25% Co, 20% ierrotitanium (equivalent to about 4.5% Ti) 5% Al and the remainder substantially iron, After being re-heated, the

casting was cooled from a temperature or about 1250' C., in the presence of air in about 4 minutes and 10 seconds down to a temperature of about 650 C., i. e. at an average cooling rate of about 2 C. persecond. After magnetisation the resulting magnet had aBivalue 01' about 8,600 gausses and an He value of about 500 gausses.

While I have described my invention in connection with specific examples and applications I do not wish to be limited thereto, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is: 4

1. A permanent magnet comprising approxi mately 16% nickel, approximately 25% cobalt, approximately 2.8-4.5% titanium, approximately mately 1.7-3.0 C. per second, and cooling the' alloy to room temperature. I

3. Amethod of making a permanent magnet body comprising the steps of forming an alloy of approximately 16% nickel, approximately 25% cobalt, GPPI'OXIIIlfltC1Y1-2.8-4.5% titanium, approximately 5.1-7.0% aluminum, and the reminder substantially iron, cooling the alloy from a temperature lying between about 950 C. and

, the melting point to a temperature of about 650 C. at a cooling rate between approximately 1.7- 3.0 C. per second, annealing the alloy at a temperature 01' about 600 C. to 700 C., and cooling the alloy .to room temperature.

GQTIFRHH) BRUNO JONAS.