US3498851A

US3498851A - Method for producing an anisotropic permanent magnet material

Info

Publication number: US3498851A
Application number: US419178A
Authority: US
Inventors: Toshimori Shuin; Takeo Sata
Original assignee: Nippon Gakki Co Ltd
Current assignee: Nippon Gakki Co Ltd
Priority date: 1964-12-17
Filing date: 1964-12-17
Publication date: 1970-03-03
Anticipated expiration: 1987-03-03

Description

March 3,1970 TOSHIMORI SHUIN ET AL 3,498,851

METHOD FOR PRODUCING AN ANISOTROPIC PERMANENT MAGNET MATERIAL Filed Dec. 17, 1964 United States Patent US. Cl. 148 101 4 Claims ABSTRACT O THE DISCLOSURE x A continuous method for obtaining a permanent magnet material composed of coarse crystal grains of (100) crystal orientation in the axial direction of the bar, by zone melting a bar of magnetic material.

The tarting material having a composition by weight of Al 69%, Ni 10-20%, Co -40%, Cu 0-8%, Ti 010%, and the balance Fe is cast into a bar of magnetic material, which is successively zone melted starting with the lower end. The molten zone is translated at a constant speed (5-30 mm. per minute) and the portions at a predetermined distance (30100 mm.) below the molten zone is cooled in a way that the temperature gradient along the solidifying zone will be -30 C. per millimeter.

The bar of magnetic material thus processed is solution treated at about 1250 C., and then is aged at about 600 C. There is produced a permanent magnet material having a magnetic characteristic of (BH) max.

7.5-85x10 gauss oersted.

This application is a continuation-in-part of our earlier application Ser. No. 200,298 filed June -6, 1962, and now abandoned.

- The present invention relates to a method for producing an anisotropic permanent magnet material composed mainly of Fe, Al, Ni and Co.

Alloys composed of Fe, Al, Ni and Co, or such alloys added with Cu, Ti, Nb and others are known as permanent magnet materials having excellent magnetic property. The alloy may be given magnetic anisotropy by subjecting it to heat treatment in the magnetic field and is, commercially used in the greatest amount. Magnetic property of a magnet alloy comprising 7.5 to 8.5% aluminum, 13.5 to 14.5% nickel, 23.0 to 25.0% cobalt, 2.0 to 4.0% copper, and the balance of iron, which is measured in the parallel direction to the magnetic field, is shown as follows:

Residual induction (Br) gauss 12,000 12,500 Coersive force Hc) oersted Maximum energy'product (BH) max.

gauss oerstedQ. 4".5-5.0 10

The easy magnetized direction and the direction of dendritic growth of this alloy accord with each other in the 100 direction. Therefore, when the molten alloy is so solidified that both terminals of the magnet body are cooled more than other parts and subjected to, so called, directional solidification, to obtain texture of columnar crystals, the product has such magnetic property as 12,500 to 13,500 gauss Br, 600 to 650-oersted .permanent magnet alloys used industrially.

Various improvements have been tried for the process of directional solidification. It is an important point of improvement of permanent magnet alloys to make coarse "ice directionalized columnar crystals thereby to make the deviation from the l00 direction as small as possible.

That is, it is described in Japanese patent publication No. 4,903/ 1953 that direction is given to cooling by a chill plate thereby to define the solidification direction in the case of casting those permanent magnet alloys, and the method has been developed with regard to manufacture and magnetic property. Patents of Japanese patent publication No. 2,754/ 1958, Japanese patent publication No. 6,706/ 1958, Japanese patent'publication No. 6,505/ 1959, and Japanese patent publication No. 461/ 1960 are concerned with such methods. I

All of these patents have the same object in regard to point of directional solidification, however,they are different mainly in the processof casting, i.e., position of a chill plate, form and a method of dividing a magnet.

On the other hand, such an' improvement as to form a magnet in the form of a bar having the texture of columnar crystals and to divide it for producing the product has been conducted apart from the above-described improvement. That is, according to the method of Japanese patent publication No. 5,258/ 1959, in an improvement of a process for producing an anisotropic permanent magnet material having texture of columnar crystals, the melt of a magnetic alloy is fed in a tube, and is withdrawn from the bottom terminal in the form of a solid bar. The distribution of temperature in the tube end the withdrawal speed of the solid bar are controlled,

whereby columnar crystalline structure is given and an excellent magnetic property may be obtained.

According to the method of Japanese patent publication No. 14,682/1961, the magnetic property is improved by casting a magnet alloy in form of a hive, and cooling the bottom of the moulding rapidly to form a magnet in form of a bar which has the texture of columnar crystals,

In spite of these improvements, columnar crystals perpendicular to the solidification direction are still contained in an amount of about 20 to 30% by volume. Then, a great amount of small crystal grains are formed even in the columnar crystals in the solidification direction, and these crystals are inclined to have great the deviation from the l00 direction. That is, deviation of direction of crystals and confusion of,magnetic block in the grain boundary are still present in-such alloys, which have an injurious effect on the magnetic property. In fact, it has been reported that the magnetic property of a single crystal of the alloy in the l00 direction are as follows: (BH) is 7.5 to 8.5 X10 gauss oersted. In case of adding Ti into the alloy, the crystals become fine, so that it is quite difiicult to obtain texture of columnar crystals and the effect of directional solidification is re duced.

It is known that as coercive force is increased effectively by addition of Ti, a single crystal of an alloy to which 5% of Ti is added shows the highes (BH) i.e. 110x10 gauss oersted, of any kinds of permanent magnet alloy. Accordingly, though the magnetic property of a single crystal of an alloy of the system is excellent, the property of a magnet which is obtained therefrom actually is deficient.

The present method resides in providing the re-orientation towards l00 direction of crystallization of permanent magnet alloy and crystal grain coarsening.

In the first stage of the study of the subject matter of the application, applicants adopted a process of melting a bar of a magnet alloy with a high frequency induction coil in a zone and moving the molten zone without quenching. In this case, with a travelling rate of less than 5 mm./min., there is obtained a columnar crystal 2-8 mm. in diameter and 50 mm. in length, which is a very coarse crystal grain, that is, developed remarkably in the axial direction, and this tendency was greater when the travelling rate decreases, but the orientation of the crystal grain toward 100 direction was insuflicient. In the next course of study, as a result of quenching the edge of molten zone by chill plate or oil, the crystal grain is elongated in the axial direction, namely, 3-7 mm. in diameter and 60-100 mm. in length and showed an orientation almost conforming with l direction. Furthermore, when the travelling rate of the-moltenzone is increased, that is, to 10-20 mm./min., in the next stage, the portion of the molten zone was cooled with water, oil and water chill plate, the crystal grain was '1-4 mm. indiameter and 50-80 mm. in length and the orientation to l00- direction of the crystal grain was improved remarkably. Thus, there wasdiscovered how to obtain industrial production of magnetic material having superior magnetic properties. 1

The present 'invention'relates to a method for producing an anisotropic permanent magnet material from an iron-base alloy containing aluminum, nickel and cobalt as the principal alloying elements,-which comprises the steps of: preparing an elongated bar of said alloy, heating a portionof said bar to a temperature above the melting point of said alloy to melt said portion and to form a molten-zone in said bar, moving the formed molten-zone in one direction along the axis of said bar at a predetermined travelling rate and cooling the region of said molten-zone adjacent to the rear boundary thereof by passing said region at the above said predetermined travelling rate through a cooling means having a predetermined temperature gradient to solidify the molten alloy so that it forms a bar of permanent magnet material having columnar crystal structure oriented in the axial direction of said bar.

A magnetic alloy to be used in the present invention as the starting alloy comprises Al 6-9%, Ni 10'-20%, Co 15-40%, Cu 08%, Ti 0-10% and the balance of iron, particularly preferably Al 7-8.5%, Ni 13,15 Co 20-38%, Ti 0-6%, Cu 2-4% and the balance of iron.

The temperature of the molten zone is selected depending upon the chemical composition of the magnet alloy employed as the magnetic material, and a temperature higher than the melting temperature by 50 to 100 C. is preferable. Accordingly, the temperature of a magnet alloy of Alnico 5, in which Ti is not contained, is 1450 to 1650 C. and in case of a magnet alloy of Alnico 12 containing Ti in an amount of about it is 1350" to 1550 C.

In the process of the present invention, the heating zone through which the magnetic alloy to be treated is passed in order to heat said alloy to the melting temperature thereof and to form the molten zone by zonemelting method and the cooling zone through :which the molten alloy in the molten zone is passed in order to solidify said molten alloy and to cool the solidified alloy at a predetermined cooling rate are so arranged as to be positioned on the opposite sides of said molten zone, respectively. The heating zone generally has an average temperature gradient of 20 C./mm. along the path of the alloy in the temperature range of 200C.- 1500 C., and the cooling zone has in general an average temperature gradient of -30 C. along the path of the alloy in the temperature range of 150 C.200' C.

The anisotropic permanent magnetmaterial produced industrially by the present invention is in the shape of bars having various sectional shapes (for example, circular, multiple, etc. and the sizes thereof are 1-"300'mm; in sectional'areas, 300-2000 mm. in length, in general those having 100-500 mm. in sectional areas and 500- 1000 mm. in length are suitable. The volume of molten alloy in the course of execution of the present invention is the volume of the portion in the zone formof the bar material and can be computed by the multiplication of the sectional area of the refractory tube (which Assuming hat the section is circular, in general the width of the molten zone corresponds to the diameter of the material. (However, when the sectional area is very small, or is very large, the relation changes and when the sectional area is 1-3000 mm. the width of molten zone becomes 3-50 mm. For-example, when obtaining a circular product of diameter 20 mm., the width of molten zone is about 20 mm., "therefore, the molten volume is about 45 grams (supposing the density is 7.3). Additionally, when the section of the refractory tube is other than a circle, theparameters correspond to the foregoing. 1

The holding time in the molten state in the course of the execution of the present invention can be determined bythe width of molten zone and the travelling rate. In a circular product of a diameter of 20 mm.,"when the width of molten zone is about 20 mm. and-the travelling rate is 10 mm.'/min., the holding time is 2 min. in all parts.

As above stated, the molten volume and the-holding time in eachalloy are varied by the-sectional shape oft the product and the latter-is further varied by the travelling rate. In addition, the size of alloy to be used as the starting material may be in any form or size which'is suitable'for the handling. t

In the case where the travelling rate of the molten zone is too high, the diameter and the length of the columnar crystals are short and the magneticproperty becomes inferior. While, in case where said rate is too low, quite excellent magnetic property which is similar to property of a single crystal may be obtained, but productivity is small and therefore it is expensive. Accordingly, the travelling rate is suitably selected according to the kind of the alloy and its use. Generally speaking, said travelling is conducted at the rate of 60 to 1200 mm./hr. In the present invention, the moving of the molten zone of the magnet alloy may be conducted more than twice to make the magnetic property thereof more similar to that of asingle crystal. By moving the molten zone several times, the travelling rate may be higher without spoiling the magnetic property, and besides its productivity may be improved. In the process of the present invention, the material charged in the tube is melted from one end by the high frequency induction coil in a zone and the molten zone is gradually moved. The portion passed through the molten zone becomes the cooling zone and the portion to be quenched subsequently becomes the heating zone. The cooling zone is not cooled by natural cooling, but is cooled 'by cooling water, cooling oil, a water-cooling jacket, or an air stream, whereby the portionof the coolingzone carrying out the forcible quenching becomes naturally the edge of the cooling zone. j

v The temperatures of respective portions in the present method are, 1350-1500 C. In the molten zone, 150?- 200 C. In the edge of the cooling zone, and the distance between them is 5-150' mm., but preferably 30- mm. I

According to the method of the present invention, the magnetic property of 'said allow is extremely improved and the productivity is greatly increased by conducting continuously the process for production of said alloy using starting magnetic alloy in different conditions, whereby a magnet having high performance maybe produced at low cost.

The starting alloys for producing magnets by thepresent method may be in any form of a bar, a fragment, powder or a melt, and therefore may be selected freely by the condition of starting alloys employed or the producing method. v

Furthermore, a magnetmaterial in the form ofv a bar having. anisotropic magnetic property maybe directly produced from the starting alloys, so that the productivity is extremely greater compared withthose of the conventional sand casting method and shell, casting m th d,

. Moreover, the operations of supplying the starting alloys and producing the product may be multiplied in asingle apparatus controlled by a smaller labor force by' 'automatic control of electric heating system. Accordingly, the productivity is quite high. Then, the magneticfieldcooling and annealing of the later process may be continuously carried out, which is very convenient'forthe pro ducing method. V i I As the heating method, a high frequency induction heating method is generally employed, however a metallurgical heating method, SL lCh as electron beam melting method, electric arc melting method, or electric resistance heating method may be utilized. Themolten zone is contaminated by oxygen in air. Therefore, it is desirable to. conduct the production'jin a non-oxidizing atmosphere, such asvacuuin, argon, orhydrogen atmosphere, where oxygen is absent or reduced'." I

The essence of the present invention is to mel't'a magnet alloy by a'high frequency induction coil in a 'zone' and to move't he molten Zone' and to "quench at once' the co mpletely molten portion. f

' The molten zone may be moved eitherIby' moving. a refractory tube which includes the material or bymoving the high frequency induction coil. Since the 'distance to the cooling zone is constant, there is'fnofditfere'nce of effect on'the quality of the'products and as to the moving mechanism,the known fine feeding apparatus can be applied without any difficulty. I

The producing method of the present invention is considered to be similar to the method described in Japanese patent publication No. 5,258/1 959. However, the former is quite different from thelatter inssubject matter -.and concrete processes. That is, the method of. the abovementioned patent publication No. 5,258/19591is a] kind of a continuous casting method, which comprises feeding tnolten alloys into a tube and solidifying it therein. Control of said alloy with regard 'to directional solidification can be conducted only in such'degree as;maintaini n'g the'temperature of the tube,vand; in connection with-it, control of withdrawal speed of the alloy in form of a bar is limited itself.

On the other hand, according to the present method, permanent magnet materials having desired magnetic properties may be produced by properly controlling the I heating system for positively melting and moving the molten Zone in the tube and-by controlling thetravelling rate of the molten alloys. a It has been found that forming of segregation which is a problem for the producing method of the present invention is not brought up in point of magnetic property, since the travelling rate of the molten zone is high. Magnet material in the form of a bar produced by the present invention is not contaminated by oxygen duringmelting, so that its surface is smooth and may be easily cut by a high speed cutter or a diamond cutter, or by subjecting it to discharge processing.

A particular characteristic of the present method resides in that the product has a great effect when it is used as a small-sized permanent magnet having high performance or a high performance magnet of cross sectionally different forms, and that the product has remarkably excellent magnetic property, compared with products by conventional methods.

Referring to the attached drawing, the present invention will further by explained hereinafter.

The sole figure of the drawing diagrammatically shows in cross-section an example of an apparatus used in an embodiment of the present invention.

Starting materials of a magnet alloy 1 is fed into a refractory tube 4 by feed rolls 2, preheated by heating wires 5, a portion of said materials being melted to form a molten zone 7 of belt form by heating said portion with a high frequency induction heating coil 6, while introducing inert gas into the tube from gas-introducing openings 3. The molten materials in said molten zone are cooled slowly bypassing them through heating Wires 8 and gradually solidified. A solidified magnet alloy 12 is subjected to forced cooling by passing it through a cooling zone formed by a 'water jacket 10 and a water bath 11 provided along the outer periphery of a pipe 9, and gradually taken out by leading rolls 13. Thus, the molten zone is moved gradually with respect to the materials of a magnet alloy along the axis thereof by the relative movement of the materials 1 and the coil 6. The molten zone may also be moved by stopping operation of rolls 2 and 13 and moving the coils 6.

The method for producing anisotropic permanent magnet materials ofthe present invention is illustrated in the following examples.

EXAMPLE 1 .Fragments of a magnetic alloy comprising by weight 8% Al, 14% Ni, 24% C0, 3% Cu and the balance of Fe are heated at a temperature ranging from 1450 C. to 14 80 C. in an argon atmosphere by high frequency induction coil, to melt them in form of a zone. Then, a magnet material in form of a bar is withdrawn by leading rolls in such way that the molten zone is moved at a travelling rate of 360 -mm./hr. and the portion where the molten zone is passing is cooled.

Distance from the lower part of the molten zone to the upper part of the water cooling zone is about 60 mm., the temperature of the side of the bar in the water cooling zone is 150 C. to 200 C. and therefore the temperature gradient between them is 20 to 23 C./mm. The thus produced magnet material is in the form of a bar of diameter 10 mm. is subjected to solution treatment at 1250" C. and subsequent aging at 600 C. for 6 hours, thereby to obtain an anisotropic magnet material. The magnetic property in the axial direction of said anisotropic magnet material is compared with those of magnet alloys comprising the same components which are produced by the conventional shell casting method and the side chill process, as follows:

Br, gauss BHc, oersted- (BH)max., g.0

Present: method. 14, 730 8. 4X10" Conventional methods. 13, 000 650 5. 9

EXAMPLE 2 A magnetic alloy in the form of a bar is produced from the same components withthose in Example 1 and fed into a tube of diameter 3 mm. made of refractory materials. Thereafter, the molten zone is moved at the rate of 360 mm./hr. by moving the high frequency induction coils. The temperature gradient from the molten zone to the cooling zone is 25 to 27 C./mm. Thethus produced magnet material in the form of a bar of diameter 3 mm. is subjected to solution treatment as defined in Example 1 to make an anisotropic magnet material. The magnetic property in the axial direction of this magnet material is compared with that of a magnet alloy comprising the same components by the conventional shell casting method, as follows:

A magnetic alloy is produced by melting 7% (by weight) A1, 13.5% Ni, 30% Co, 3% Cu, 5% Ti and the balance of Fe in a hydrogen atmosphere by high frequency induction coils, followed by melting them in form of a zone. Then, the molten zone is moved at the rate of 360 mm./hr. and the portion where the molten zone is passed is cooled in a water cooling device made of a copper jacket and a water tank to make magnet material in form of a bar of 15 mm. diameter.

The thus prepared magnet material is again melted in a zone in the same way as described above, and the molten zone is moved at the rate of 360 mm./hr. and cooled in the same Way as mentioned above, the produce a magnetic material. The magnetic material is subjected to the solution treatment defined in Example 1, thereby to give it anisotropic magnetic property in the axial direction. Said magnetic property is compared with properties of magnets of the same components produced by conventional shell casting method and the side chill process.

, What we claim is:

1. A method for producing an anisotropic permanent magnet material from an ion-base alloy containing aluminum, nickel and cobalt as the principal alloying elements, which comprises the steps of: preparing an elongated bar of said alloy having a length of about 300-2000 mm. and a cross-sectional area of about 1-3000 mm. heating a portion of said bar, to a temperature above the melting point of said alloy to melt said portion and form a molten-zone in said bar, said portion being heated above the melting point by an electrical heating means a temperature of 1350-1650 C. to form the molten-zone with a band width of about -50 mm., moving the thusly formed molten-zone in one direction along the axis of said bar at a travelling rate of about 3-30 mm./min., applying heat to the molten-zone after passage thereof through the electrical means at a temperature less than the melting point of the alloy to cause the alloy at least to partially solidify in said molten-zone at a slow rate, and thereafter forcibly cooling the now partially solidified zone by -80 mm. length of 3-8 mm. diameter having a crystal orientation or (1 00) substantially parallel to the axis of s aidbar.;l t J H 2. A method, as clai'rnedin claim '1 wherein said alloy consists by Weight of 6-9% of aluminum, 10-20% of nickel, ,15-40'% of cobalt, 0-8 of copper and the balance of iron, saidi portion of the alloy being heated at a temperature of between 1450 and 1650 with a high frequency induction heating coil to produce saidfrr'ioltenzondfthe'c'obling means, being at a temperaturev between -200? C.', the travelling, rate being 5-30 mm./rnin., and the. distance between the end of the induction coil and the, beginningjof. the cooling means being 5 to 150mm.

\ 3. AI method as claimed in claim 1 wherein said alloy consists by "weight. or 69% of aluminum, 10-20% of nickel, '1540,% OfICObZllI,,"O-8% of copper, 0-10% of titanium and the balance of iron, said portion of the alloy being heated at a temperature of between 1350'and 1550 C., With'a high frequency induction heating coil to produce said molten-zone, the coolingmeans being ata temperature between 150-200 C., the travelling rate being 3-15 mm./min. and the distance between the end of 'the' induction coil and the beginning of the cooling means being 5-150 mm. i

v 4. A .rriethod accordin'g a) claiinl, wherein the elongatedfbar of said, alloyis pass'ed in the heating zone through a cylindricalhollow sheathof refractory material.

References Cited 7 HYLAND BIZOT, Primary Examiner r Assistant Examiner ,t f jusifci. X.R. 148102, 103, 108'

Claims

7.5-8.5X10**6