WO1989002804A1

WO1989002804A1 - Forged body

Info

Publication number: WO1989002804A1
Application number: PCT/GB1988/000792
Authority: WO
Inventors: William Alfred Penny; Robert William George Rose; Ivor Rex Harris; Mufti Mohmed Ashraf; Ian Thomas; Neil Rowlinson
Original assignee: Penny & Giles Conductive Plastics Limited
Priority date: 1987-09-26
Filing date: 1988-09-26
Publication date: 1989-04-06
Also published as: GB8722689D0

Abstract

A permanent magnet has a body consisting of or comprising particles of a magnetic material which have been subjected to a rotary forging operation. The body may be formed by the rotary forging operation or may be formed using a polymer binder mixed with magnetic material particles which have previously been subjected to the rotary forging operation. The body may include up to 50 % by volume of a soft metal e.g. zinc, aluminium, copper, tin. Superconductive or magnetostrictive bodies can be produced in a similar way.

Description

"FORGED BODY "

This invention relates to compacts and is more _.particularly concerned with permanent magnets.

It has previously been proposed to form polymer-bonded magnets by melt spinning a neodymiu -iron-boron alloy, annealing the melt-spun alloy, milling the annealed and melt-spun alloy, and then compression moulding a mixture of polymer and the milled material to produce magnets containing up to 90% by weight of the milled material. Such polymer-bonded magnets have a maximum energy product (BH_max) of about 8 MGOe.

It is an object of the present invention to provide a permanent magnet which can have an improved maximum energy product, and a method for producing same.

According to one aspect of the present invention, there is provided a permanent magnet which has a body comprising or consisting of particles of a magnetic material which have been subjected to a rotary forging operation.

According to another aspect of the present invention, there is provided a method of producing a permanent magnet comprising the step of forming a body of particles comprising or consisting of magnetic material particles which have been subjected to rotary forging.

The magnetic material may be a ferrite or a rare-earth magnetic alloy eg SmCθ5 or Sπi2(Co,Fe,Cu,Zr ) ι^~j _r but in a preferred embodiment of the present invention, the magnetic material is based on the Fe.B.R system wherein R is at least one element selected from light- and heavy- rare earth elements inclusive of yttrium (Nd, Pr, La, Ce, Tb, Dy, Ho, Er, Eu, S , Gd, Tm, Yb, Lu and Y) and wherein the B content is 2 to 28 atomic percent, the R content is 8 to 30 atomic percent and the balance is iron, with the optional inclusion of at least one of Ti, Ni, Bi, V, Nb, Ta, Cr, Mo, , Mn, Al, Sb, Ge, Sn, Zr and Hf.

Preferred examples of alloys of the Fe.B.R. system are ^R15^Fe77^B8 (^{where R} i-^s preferably Nd or Nd 5__xD _x) and ^R13^Fe83^B4' (where R is preferably Nd or Nd]_3__xD _x, typically

, which may contain additions of aluminium and/or niobium.

The magnet body is preferably a rotary forged body comprising or consisting of particles.of the magnetic material, the particles having been subjected to rotary forging during formation of the body. However, it is within the scope of the present invention to subject the particles to rotary forging to impart or improve anisotropy,, and then form the particles (after comminution, eg milling, as riecessary) into a body eg a polymer-bonded body. Such a process may be useful in cases where it is difficult to produce the desired shape of the body directly by rotary forging or where reject or scrap rotary forged magnetic material is available.

The body may be formed substantially completely of the particles of magnetic material or may be composed predominantly of said particles. Such body may contain 0 to 50% by volume of soft metal eg zinc, aluminium, copper and/or ^"tin to improve its mechanical strength and/or it may contain polymer as a binder. Such soft metal may be in the form of particles which are mixed with the magnetic material before rotary forging or may be in the form of a coating on such magnetic material particles to be subjected to rotary forging.

Particularly in the -case of starting materials in the form of alloys of the system Nd_j_3 eg3B4 which have been produced by a melt spinning operation to produce a ribbon, followed by annealing for a short time at 700°C, and then milling, it has been found that high maximum energy products of 20 to 30MGOe can be achieved by rotary forging. This is particularly surprising since the milled material has a very fine grain size with totally random orientation of the magneto- crystalline axes. The rotary forging operation is normally effected at room temperature and the mechanics of rotary forging are such that little heat is generated during the forging operation. However, the rotary forging operation does produce a preferred orientation and, although the mechanism by which this takes place is not yet fully understood, it is believed to be due to recrystallisation as a consequence of the stored energy in the material. Further improvements may be achievable by using a starting material which has not been subjected to an intermediate annealing step.

It is within the scope of the present invention to effect rotary forging in a magnetic field in order to enhance the orientation effect produced by rotary forging.

Because the rotary forging operation is essentially a low temperature operation, this means that excessive grain growth, and thus coercivity degradation, which would occur at higher temperatures can be avoided. The milled ribbon typically has a very fine grain crystal structure (approximately 20 nm), the average size of the milled ribbon particles being approximately 100 micrometres by 25 micrometres thick. However, the present invention contemplates the use of other particle and crystal grain sizes and it is believed that the use of finer particle sizes will lead- to an improvement in the magnetic properties of the compact. The rotary forging operation is effected at such a low temperature that the Curie point of the material is not exceeded.

The rotary forging process changes the random magnetocrystalline orientation of the particles in the a-llo-y; to a highly preferred orientation with the c-axis of' th& tetragonal crystals aligned along the axis of load: application.

Rotary forging (which is sometimes called "orbital forging" or "rocking die forging") is preferably effected using a rotary forging machine of the type available from Penny & Giles Blackwood Limited of Blackwood, Gwent, NP2 2YD, united Kingdom. A machine of this type is disclosed in British Patent No. 2041268 and European Patent No. 0014570. Rotary forging machines are normally employed for the final shaping and densification of workpieces in the form of sintered compacts. Such machines generally comprise (1) a machine frame, (2) first and second platens disposed with their axes A_]_ and A2 at an angle o relative to each other, (3) a drive for effecting rotation of at Least: αne of the platens (and preferably both), and (4) eti. mechanism for urging at least one of the platens towards the other. The first platen has a conical, workpiece-engaging surface. The axes Aj_ and 2 are arranged to intersect at the apex of the conical workpiece - engaging surface which is coincident with the surface of a plastically deforming region of the workpiece. In the case where the surface of the workpiece being forged is planar and perpendicular to the axis A2, the first platen has a conical, workpiece-engaging surface with a semi-angle ofT^Yi-c in. relation to axis A_]_. For no slip to occur in the plane of instantaneous forging, it is necessary for ^V2/V^"l to equal cos <^~< , where Vj is the angular velocity about axis A and V2 is the angular velocity about axis

A₂-

This can be achieved (a) by keeping the second platen and workpiece fixed against rotation about axis A and "wobbling" the first platen so that it rotates at an angular velocity V about axis A_j_ whilst, at the same time, the axis A2 rotates at an angular velocity V2 about axis A2 or (b) by keeping the first platen fixed against rotation about axis A and "wobbling" the second platen and the workpiece so that they rotate at an angular velocity V2 about axis A2 whilst, at the same time, the axis A2 rotates about axis A_j at an angular velocity V_]_, or (c) by rotating the first platen at an angular velocity V_]_ about axis A_ and simultaneously rotating the second platen and workpiece at an angular velocity V2 about axis A2. Arrangement (c) is preferred and is employed in the rotary forging machine available from Penny & Giles Blackwood Limited.

It is usually preferred to provide an additional mechanism for varying the angle °< continuously in such a manner that the point of intersection of the axes Aj and A2 remains fixed.

In order to facilitate densification and removal of the forged magnets from the rotary forging machine, it is preferred to effect forging of the particulate material in a split and/or floating die assembly which is disposed between the platens and which is preferably also a floating die assembly. Such a die assembly is described and claimed in co-pending British Patent Application No. 8722690 entitled "Improvements in or relating to Rotary Forging Machines" in the name of "Penny & Giles Conductive Plastics Limited, of even date In the process of the present invention, it is convenient for the rotary forging machine to be operated with an angle -^~ of 2° or 3° angular velocities V-_ and V^"2 which are equal and about 12 to 200 rpm and a load (force urging platens together) of 5Machines" in the name of Penny & Giles Blackwood

- 70 tonnes, where , V_j_ and ^"2 are as defined above. However, it is possible to effect the rotary forging with up to plus or minus 15°, v-_ and V^"2 up to 300 rpm and a load of up to 100 tonnes. It is to be appreciated that, at small values ofc., operating the machine with Vj and ^"2 equal involves a certain amount of relative slip but this is found to be acceptable in practice and does not cause undue heating of the compact being forged. Preferably, the load is about 50 tonnes and is applied using a slow axial feed of eg about 0.25 mm/revolution. It is also preferred for the material being forged to contain a minor amount (e.g. greater than 0.005 wt%) of a lubricant (such as a fatty acid based lubricant eg zinc or calcium stearate, a fatty acid or a soft metal e.g tin, aluminium, copper, zinc) in order to assist densification.

using melt-spun alloys of the Nd.Fe.B type, permanent magnets having a remanence (Br) of 5.5 - 11.2kG, an intrinsic coercivity (He) of 15kOe and a maximum energy product (BH_max) of 4-30MGOe can be obtained, such properties depending strongly on the densification which can be achieved. For cylindrical and ring magnets having a diameter of 20-30mm, densities of 5.8

- 7.2 g.cm-- can be. achieved.

It is possible to mix various soft metals (eg Al, Zn, Sn, Cu) in the form of 5 - 100 μ powders, in amounts of 2 - 50% by volume, with melt-spun alloys of the Nd.Fe.B type. Rotary forging of such mixtures produces permanent magnets having a remanence (Br) of 3.6-9.0 kG, an intrinsic coercivity (H_c) of 15 kOe and a maximum energy product of 3-16 MGOe depending upon the amount of soft metal used. For cylindrical magnets of 20-30mm diameter, densities of 80 - 96% of the theoretical fully dense values have been achieved.

The present invention is also applicable to the manufacture of compacts by rotary forging particles comprising or consisting of a superconductive material e.g. ceramic superconductive material, or a magnetostrictive material.

The techniques described above for the manufacture of permanent magnets can be applied mutatis mutandis to the manufacture of such superconductive or magneto¬ strictive materials, including the use of a soft metal as described. In the case of superconductive materials, the inclusion of the soft metal is found to widen the superconductive transition temperature range. This is considered to be potentially useful in situations where a critical control of the temperature of operation is difficult. The rotary forging of superconductive material to produce a compact represents an effective way of densifying the material. At present it is believed to be advantageous in terms of the required superconductive properties to sinter the resultant rotary forged compact. However, if possible sintering is to be avoided because of the expense involved and because, in the case of ceramic superconductive materials, loss of oxygen occurs during sintering.

The invention is applicable, for example, to high temperature ceramic metal oxide superconductor materials of the Y-Ba-Cu-0 system (see Physical Review Letters, Vol 58, No. 9 pages 908 to 911) An example of such a material is YBa₂CU3θ7__x, where x is about 0.1. However, the invention is also applicable to other superconducting systems eg those based on rare earth- strontium- Cu-0 or Bi-Sr-Cu-O.

The ceramic superconducting powder can be formed by a per se known technique eg in the case of Y-Ba-Cu-0 materials by reacting a mixture of yttrium oxide, ^' barium carbonate and copper oxide in an oxygen atmosphere to give Ba2CU3θ7__x, where x is about 0.1. The reacted mixture can then be subjected to the usual annealing and grinding processes before being rotary forged either with or without soft metal binder (eg 0.5 - 50% vol soft metal such as Cu, Sn, Al or Zn) . Rotary forging with loads of 2-20 tonnes and at 10-200 rpm can give densities of 5.1 to 5.8 gcm^~3 with a typical Tc of 84-98K.

In the case of magnetostrictive materials, rotary forging represents an effective way of densification and enables sintering to be avoided. Heating to sintering temperatures could deleteriously affect the magnetostrictive properties.

The magnetostrictive materials for which the present invention is particularly useful are the relatively hard rare earth- iron alloys eg Tb-Dy-Fe alloys, which are difficult to compact by ordinary compaction techniques.

A particular example of such an alloy is

Tbg_#27^DYθ.73^Fe1.9• ^r-~'-^{~ raw} material may be pre-treated by crushing and then ball milling under cyclohexane before being rotary forged. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Fig. 1 is a schematic axial section through part of a rotary forging machine for producing a permanent magnet according to the present invention.

Fig. 2 is a plan view of a die used in the machine of Fig. 1,

Fig. 3 is a schematic axial section through part of another type of rotary forging machine for use in the present invention.

Fig. 4 is graph showing demagnetisation curves for two densities of rotary forged body (curves 2 and 3) and an isotropic, polymer-bonded body"(curve 1), each body having been formed using milled Ndi3Feg3B4 ribbon available under the designation MQ1 from General Motors Inc, curve 4 is the demagnetisation curve for a body of MQ1 alloy and 5% vol. zinc, and curve 5 is the demagnetisation curve for a body of MQ1 alloy and 10% vol. aluminium.

Fig. 5 is a graph in which Remanence Br (KG) is plotted against density p (g.c ^-3) for a rotary forged body formed of particles of Nd_]_3Fe83B4 alloy.

Figs. 6 and 7 are graphs plotting the magnitude of a superconducting transition (as measured by the deflection on a Meissner Balance) against temperature (K), and

Fig. 8 is a graph plotting contraction (Δ /L) against magnetic field for samples of magnetostrictive material Referring now to Figs. 1 and 2 of the drawings, the rotary forging machine which is partly illustrated therein is of the general type disclosed in GB-A- ^"2041268 and EP-A-0014570. However, the die part of such a machine has been modified to facilitate its use for the direct rotary forging of particles of magnetic material. The machine includes a first or upper platen

10 which is mounted in a machine frame (not shown) for rotation about axis A_. The machine further comprises a second or lower platen 11 which is mounted in the machine frame so as to be rotatable about axis A2 by a suitable mechanism. The manner in which the first platen 10 and the second platen 11 are mounted in the machine frame and rotated is as described in the above mentioned British and European Patent Publications and so will not be described in further detail herein. The second platen 11 is also., mounted for movement towards . and away from the first platen -10 by a further mechanism which is also not shown but which is shown

^* and described in the above mentioned British and European Patent Publications. . As can be seen from Fig. 1, axis A_]_ is inclined at a small angle c* of 2° or 3° relative to the axis A2. The first platen 10 has a conical lower surface having a semi-angle of ~7K -2.—(X.. Because angle o< is only very small, it is possible to arrange for the first and second platens 10 and 11 to be rotated in the same direction at the same speed. This does produce a certain amount of slip in operation, but not sufficient to cause any disadvantageously strong shear or heating effects which latter might raise the temperature of the material being compacted to levels at which grain coarsening can occur. If rotary forging is effected with greater than about 4°,then the first and second platens 10 and

11 can be rotated at different speeds such that V2/V1 = cos ot . Mounted on the second platen 11 for rotational and axial movement relative to axis A2 is a die assembly indicated by arrow 12. The die assembly 12 includes a base plate 13 bolted to the second platen 11, a retaining member in the form of an upstanding annular tool boss 14, a support mandrel 15 and an annular, hard metal die shell 16. The shell 16 is formed of three separate part-shells 16a, 16b and 16c which are held together by the tool boss 14 when in the position shown in Fig. 1. The outer diameter of the die shell 16 is a close sliding fit within the tool boss 14. Likewise, the mandrel 15 is a close sliding fit within the tool boss 14, but has a reduced diameter portion 15a at its upper end. This portion 15a of the mandrel 15 is a close fit within the die shell 16. The inner surface of the die shell 16 and the upper surface of the portion 15a of the mandrel 15 define respectively the peripheral side wall and^" base of an open-topped die cavity 17 in which powdered magnetic material 18 to be rotary forged is placed. The lower conical surface of the first platen 10 enters the die cavity to enable the rotary forging operation to be effected. Compaction occurs whilst rotation of the first and second platens 10 and 11 about their respective axes A_]_ and A2 takes place and whilst upward feed of the second platen 11 and die assembly 12 towards the first platen 10 takes place.

Once the required degree of compaction by rotary forging has taken place, the second platen 11 and die assembly 12 are retracted sufficiently to separate the platens and to permit an ejector pin 19 which extends through aligned bores in the second platen 11 and base plate 13 to engage the support mandrel 15 so as to cause it to move relative to the tool boss 14 so that the die shell 16 is moved completely out of the tool boss 14. At this stage, it is possible to remove the rotary forged material from the die cavity merely by stripping away the part-shells 16a to 16c. The part-shells 16a to 16c can then be re-assembled in the tool boss 14 after retraction of the ejector pin 19 in preparation for another rotary forging operation.

Referring now to Fig. 3 of the drawings, the rotary forging machine which is partly illustrated therein is similar to that described above in relation to Figs. 1 and 2 insofar .as the rotary forging operation is concerned. However, the die assembly employed is different. In Fig. 3, parts which are similar to those of Figs. 1 and 2 are accorded the same reference numerals but in the 100 series. In this embodiment, die assembly 112 includes base plate 113 and tool boss J14 which defines' a cylinder in which^" a piston 120 forming a retaining member for die shell 116 is* sealingly and slidably mounted. The piston 120 is formed of a lower annular part 120a of a hard, non-magnetic material and an upper ferromagnetic part 120b. The piston portion 120a is sealingly slidable on fixed mandrel 115 secured to base plate 113. The mandrel 115 and the base plate 113 are both formed of ferromagnetic material. The mandrel 115 has an air passage 121 at its lower end. The air passage 121 communicates with a compressed air inlet 122. The mandrel 115 is also provided with a bleed air passage 123 which serves to provide communication between the interior of the mandrel 115 and the annular chamber 124 which is defined under the piston 120 and between the mandrel 115 and the boss 114. The mandrel 115 is open at its upper end and has a stepped bore. A non-magnetic, hardened steel tube 125 is welded into the mandrel 115 so as to extend upwardly therefrom. The tube 125 and mandrel 115 together define a cylinder which slidably receives a solid piston in the form of a central pin 126.

The lower and upper parts 120a and 120b of the piston 120 are welded together and receive die shell 116 which is split into part-shells in a similar way to die shell 16 described above in relation to Figs. 1 and 2. However, in this embodiment, the piston 120 is internally stepped at 127 so as to support the part-shells which are correspondingly stepped on their outer peripheral surfaces. The tube 125 extends for a short distance into the die shell 116 from the lower end thereof. The die shell 116 is a close sliding fit over the tube 125. An annular die cavity 117 is defined between the die shell 116 and the central pin 126, the die cavity being closed at i.ts lower end by ' the tube 125 whose upper end surface defines the base - of the annular die cavity. *

The first or upper platen 110, unlike the platen 10, does not enter the die cavity 117 but overlaps it so as to engage against the die shell and, in this embodiment, the upper piston part 120b also.

In use, the interior of mandrel 115 and the chamber 124 are pressurised with air through compressed air inlet 122, air passing through air passage 121 and bleed air passage 123. The piston 120 and central pin 126 are urged into the positions illustrated in Fig. 3. A stop (not shown) is provided for limiting upward movement of piston 120.

In an argon environment, particles of magnetic material 118 (which may be mixed and/or coated with soft metal and/or polymer) to be rotary forged are introduced into the annular die cavity 117 and a rotary forging operation is effected in a similar manner to that described above by moving the second (or lower) platen upwardly along axis 2- This causes the particles of magnetic material to be axially compacted between the upper end surface of the tube 125 and the first platen 110. During the rotary forging operation, ^"the magnetic material 118 becomes compacted within the die cavity 117. To accommodate for this, the height of the die cavity is decreased as a result of upward movement of the mandrel 115 and tube 125 fixed thereto. The piston 120 and associated die shell 116 remain in the illustrated position in which they engage against the upper platen 110 because relative sliding movement between these parts and the tube 125 and tool boss 114 is permitted. Such relative movement occurs until the lower end of the die shell 116 is engaged by the upper end of the mandrel 115. Thus, it will be appreciated that, by reason of the piston 120, the die shell 116 is free to float within limits relative to the second platen. The same applies to the pin 126. After the mandrel 115 has engaged^' he die shell 116, rotary forging is stopped and axial pressure is gradually reduced to avoid cracking of the forged material as a result of sudden release of pressure. The piston 120 is moved downwardly using an ejection collar (not shown) relative to the die shell 116 until the latter is exposed completely above the piston 120. At this point, the rotary forged annular magnet can be removed by stripping the part-shells of the die shell 116 away in a similar manner to that described above in relation to Figs. 1 and 2. Finally, the central pin 126 is knocked out of the rotary forged magnet. For a further description and illustration of the die assembly of Fig. 3, attention is drawn to co-pending British Patent Application No. 8722690 entitled "Improvements in or Relating to Rotary Forging Machines" in the name of Penny & Giles Conductive Plastics Ltd., of even date

During the rotary forging operation described above in relation to Fig. 3, a magnetic field may be passed through the magnetic material 118, the ferromagnetic parts of the die assembly 112 assisting in providing the desired magnetic field. The ferromagnetic parts of the die assembly 112 are shown sectioned in full line alternating with dotted line, whilst the non-magnetic parts of the assembly 112 are shown sectioned in full line only. It is, however, within the scope of the invention to effect rotary forging using the assembly 112 without application of a magnetic field.

In a typical embodiment of producing a permanent magnet, a solid_/ cylindrical magnet having a diameter of 20mm is formed from particles of an Nd.Fe.B type melt-spun ribbon available from General Motors under the code MQ1. Rotary forging of this material is effected using die assembly 12 as described above in relation to Figs. 1 and 2 with the first and second platens 10 and 11 being rotated at a speed of 170 rpm about their respective axes A_j and 2, the anglec* between these axes being 2°. The second platen 11 is moved towards the first platen 10 at an axial feed rate of 0.25 mm/revolution so as to apply a load of about 50 tonnes during rotary forging. The resultant rotary forged magnet has a density of 7120 kgm^"3, a Remanence (BR) of 1120 mT, an Intrinsic Coercivity (He) of 1180 kAm^-1, and a Maximum Energy Product (BH_max) of 240 kJm"³. The material being forged is particulate material derived from melt-spun ribbon. Such particles have approximate dimensions of 100 microns across by 20 microns thick. In order to assist in ejection, molybdenum disulphide is used as a lubricant between the die shell 16 and the tool boss 14.

As can be seen from curve 2 and 3 in Fig. 4, improved results can be achieved from rotary forged bodies in accordance with the present invention as compared with an isotropic polymer-bonded magnet formed of the same magnetic material (curve 1). Curve 1 corresponds to a polymer-bonded magnet where the polymer content is 15 vol.% Curve 2 corresponds to a rotary forged body having a density of 5.7g.cm^~3. Curve 3 corresponds to a rotary forged body having a density of 7.0g.cm^~3. In each case, the starting magnetic material is a particulate material available from General Motors Inc.,- under the designation MQ1. Such material is ^" believed to be obtained by annealing and then milling a melt spun ribbon of _]_3Feg3B4 alloy. Curve 4 corresponds to a soft metal-bonded rotary forged body of density 7.0 gem^-3,using a load of 20 tonnes, the soft metal being zinc (5% by volume addition). Curve 5 corresponds to a rotary forged body consisting of 10% volume Al and MQI formed using a load of 15 tonnes (density of 6.3 gem"³)

In a typical embodiment of producing a ceramic superconductor body, yttrium oxide, barium carbonate and copper oxide (as supplied by Berkshire Ores Ltd) are reacted in a per se known manner in an oxygen atmosphere to give YBa2CU3θ7__x, where x is about 0.1. The reacted mixture is then rotary forged after being subjected to the usual annealing and grinding processes.

Curve 1 in Fig. 6 represents the behaviour of a section cut perpendicular to the forming axis of a YBa2CU3θ7__x compact which has been rotary forged (6 tonnes) and then sintered. Curve 2 in Fig. 6 represents the behaviour of a parallel section cut from the same compact. As can be seen, there exists a difference in transition temperature Tc which depends upon the orientation induced in the rotary forging process. This mechanism is not yet fully understood. The transition 'appears larger for the perpendicular section.

Curves 1 and 2 in Fig. 7 represent sections (perpendicular and parallel respectively) of a rotary forged sample of YBa2CU3θ7__x containing 5 vol % Cu formed with a load of 6 tonnes and then heat treated. The transitions for the Cu bonded sample are considerably broader and occur at a lower temperature than those observed for the YBa2Cu3U7__x only compacts.

In a typical embodiment of producing a magnetostrictive body, the magnetostrictive material Terfenol of preferred composition Tbø.27^DYθ.73^Pe1.9 (^as supplied by Rare Earth Products Ltd) is crushed to approx. 1 mm³ lumps and then ball milled under cyclohexane for 3 hours. The resultant powder is then rotary forged (with the optional addition of 0.2 to 50% volume additions of soft metal binder eg Cu, Al, Sn, Zn) .

Curve 1 in Fig. 8 shows the contraction perpendicular to an applied field of an as-cast rod of Terfenol D. Curve 2 corresponds to a rotary forged compact of Terfenol + 20 vol. % Cu with density 85% of the theoretical maximum, formed using a load of 10 tonnes.

20-30 mm diameter compacts of 84-90% of theoretical density have been produced using loads of 3-20 tonnes.

Claims

CLAIMS :

1. A permanent magnet which has a body comprising or consisting of particles of a magnetic material which have been subjected to a rotary forging operation.

2. A permanent magnet as claimed in claim 1, also comprising a soft metal and/or a polymer.

3. A permanent magnet as claimed in claim 2, wherein the soft metal is selected from zinc, aluminium, copper and tin.

4. A permanent magnet as claimed in claim 1, 2 or 3, wherein the magnetic material is based on the Fe.B.R system wherein R is at least one element selected from light- and heavy- rare earth elements inclusive of yttrium, with the optional inclusion of at least one of Ti, Ni, Bi, V, Nb, Ta, Cr, Mo, W, Mn, Al, Sb, Ge, Sn, Zr and Hf.

5. A method of producing a permanent magnet comprising the step of forming a body of particles comprising or consisting of magnetic material particles which have been subjected to rotary forging.

6. A method as claimed in claim 5, wherein the particles are mixed with a soft metal before being subjected to rotary forging.

7. A method as claimed in claim 5 or 6, wherein the particles are subjected to rotary forging during formation of the body.

8. A method as claimed in claim 5 or 6, wherein the particles are subjected to rotary forging before formation of the body.

9. A method as claimed in claim 8, wherein the rotary forged particles are formed into a body using a polymer binder.

10. A superconductive body comprising or consisting of superconductive or magnetostrictive material particles which have been subjected to rotary forging.