US2398018A - Manufacture of permanent magnets - Google Patents

Description

April 9, 1946. A. UNI- EY TAL 2,398,018

MANUFACTURE OF PERMANENT MAGNETS Filed July 13, 1942 2 Sheets-Sheet l NoEn/Toes aeaffm in Ndmoegnenaamdm April 9, 1946. A. LINLEY ET Al.

MANUFACTURE OF PERMANENT MAGNETS Filed July 13, 1942 2 Sheets-Sheet 2 FIGA.

Patented Apr. 9, 1946 MANUFACTURE F PERMANENT MAGNETS Anthony Linley and Alan Tom, Sheield, England Application July 13, 1942, Serial No. 450,746 In Great Britain September 26, 1941 9 Claims. (Cl. 175-21) This invention relates to the manufacture of permanent magnets from alloys of the precipitation hardening type, particularly those made from an iron base alloy containing aluminum, nickel and cobalt as the principal alloying elements, its object being to provide an improved process for producing anisotropic properties therein.

More particularly the object of the invention is to provide an improved method of cooling a magnet alloy of the precipitation hardening type in such manner that not only is it subjected to the action of a magnetic field whilst cooling from an elevated temperature but such cooling is automatically YeHected at such varying rates through dierent ranges of temperature as are conducive to providing optimum conditions for promoting the desired metallurgical and magnetic changes whereby optimum anisotropic magnetic characteristics are imparted to the alloy.

Another object is to enable asubsequent ageing or tempering process to be either dispensed with or reduced or the time thereof to be considerably reduced.

Other objects of the invention will be apparent from a perusal of the following speciiication.

In the manufacture of magnetic iron and steel castings it has been proposed, for the purpose of making very dense castings of high magnetic conductance, to produce a flux of lines of force in a good magnetic conductor and to close the magnetic circuit by the molten iron or steel.

Anisotropic characteristics have however, hitherto been produced in permanent magnet alloys by a process such as is described, for example, in the British patent specication No. 522,731, in which the alloy is cast, reheated to a high temperature of 1250 C. or thereabouts and whilst cooling therefrom is subjected to the action of a magnetic iield. Such a process appears to involve a combination oi two physicai changes namely (a) a metallurgical change in which precipitation or dispersion hardening takes place due to the precipitation of the second phase in a controlled amount, and (b) a magnetic change in which the magnetic domains existing in the material are re-orlentatedin a definite direction whilst thematerial is in a condition of internal mobility, thus permitting anisotropic properties to be developed.

Both these changes must occur Itogether during heat treatment of permanent magnetic materials to give optimum anisotropic properties.

the alloy from a high temperature (1250 C. or thereabouts) at a definite rate of cooling, which is dependent upon the size of thel article. This rate of cooling should be of the order of 1-C.

to 5 C. per second on the average, over a cooling range of from 1200 C. to 600 C. Unavoidably, however, the second or magnetic change (b) must be carried out over a certain portion of the temperature range extending from above the Curie point to a temperature about C. below it. Now it is found that although the averagerate of cooling may be ofthe order of 1 C. to 5 C. per second, the cooling should be conducted in two stages. First, at a rate of say. 3 C. to 5 C. per second over the range 1250" C. or thereabouts down to the Curie point and then at a slower rate, preferably at l/2 C. to'3 C. per second or even slower. The most desirable rates of cooling are those which will give optimum dispersion hardening conditions in the first range and thereafter will allow a sufficient time for eilective application of the magnetic neld, in order to reorientate the domains.

In practice this differential cooling rate is difficult to obtain in a wide variety of sizes and shapes, particularly in conjunction with previously known methods of applying the magnetic field by means of electro-magnets or permanent magnets.

Further, because of the aforementioned dim- 1 culties in obtaining the requisite cooling rates, it is invariably found necessary, subsequently, in order to achieve optimum precipitation within the alloy, to subject the heat treated alloy to anageing or tempering process to bring about further metallurgical precipitation which has been suppressed by the too rapid cooling rate in the second range.

To obtain precise control of this further precipitation and in order not to disturb the magnetic anisotropy oi'the heat treated alloy, this ageing or tempering process has had to be carried out by-subiectin'g the alloy to long-periods of heating at comparatively low temperatures (say 500 C. to 600 C.) well below the critical temperature range of the alloy between the Curie point and 150 C. below it. The times of heating the alloy at such low temperatures in this ageing or tempering process may be anything up to 48 hours, depending upon the degree of precipitation already obtained in the previous combined heat and magnetic treatment. v

Permanent magnet alloys to which this treatment is applied are well known for their large grain size and consequent fragility. The treatments referred to above hitherto necessary for producing anisotropic properties in these magnetic materials, tend seriously to affect the mechanical strength and to impair the surface of such materials.

By the method of the present invention not only are the most desirable differential rates of cooling to produce anisotropic properties in a cast magnetbody of the precipitation-hardening type of alloy obtained and the time of an ageing process to give optimum results considerably reduced but the tendency hitherto experienced of such treatments to aect the mechanical strength and to impair the surface is very much diminished, thereby producing anisotropic magnets of better mechanical strength and form.

The present invention in its preferred embodiment also contemplates the provision of novel apparatus for casting a permanent magnet having the desired anisotropic properties. The apparatus preferably -includes a sand mould having moulding or casting spaces therein, and permeable metallic bodies closely adjacent certain of the faces of the casting space. These metallic bodies form pole-pieces of a magnetic circuit and the pieces at opposite sides of the casting space are of opposite polarity. Alternatively these pole-pieces may comprise bar magnets.

A relatively large runner or sprue opening is provided in the mould, so that the resulting runner will be of considerable mass so as to cool relatively slowly. The masses of the pole-pieces, which act as chills, are so chosen that the desired initial and subsequent rates of cooling of the magnet castings are attained.

Other objects and features of novelty will be apparent from the following specific descriptions when read in connection with 'the accompanying drawings in which certain embodiments of the invention are illustrated by way of example.

In the accompanying drawings:

Figure 1 is a sectional elevation of apparatus according to the present invention for producing anisotropic properties in permanent magnets.

Figure 2 is a section on the line 2-2 of Figure 1, being a plan of the lower portion of the moulding box.

Figures 3 and 4 are views similar to Figures 1 and 2` respectively and illustrate a modification.

Like reference numerals indicate like parts Y throughout the drawings.

The apparatus illustrated in Figures 1 and 2 is designed for the simultaneous manufacture of eight bar magnets and comprises a moulding box having an upper portion 5 and a lower portion 6. In the upper surface of the lower portion 6 of the moulding box two series of pole-pieces 1 of high permeability material are embedded and are arranged in two parallel rows or series as shown in Figure 2. A moulding space 8 for a bar magnet is formed in the sand between each two of the pole-pieces 1 in each row so that each polepiece presents one face to a moulding space 8. These polar faces (of the pole-pieces) may themselves form the ends of the moulding spaces 8 or a thin layer of sand or other material may intervene.

A casting runner 9 is formed in the upper portion 5 of the moulding box and a feeder channel I0 is formed between the two series of moulding spaces 8 with branches leading to each of them.

Each of the pole-pieces 1 has an extension II leading to the base of the lower portion 8 of the moulding box. The extensions II may be integral with the pole-pieces 1 or they may be provided by separate bodies of high permeability material. The extensions I I rest on a non-magnetic supporting sheet I2.

The two pole-pieces 1 which present faces to each one of the moulding spaces 8 are connected in a magnetic circuit so that these faces are of opposite polarity as indicated by the letters N and S and the moulding space forms an air-gap in the circuit. The various magnetic circuits are generated by electro-magnets each of which comprises a laminated core I3 and a winding I4. 'I'he electro-magnets are supported on a mild steel base plate I5 and they are disposed in two series, corresponding to the series of pole-pieces 1 with their several cores I8 registering with the several pole-piece extensions II. As shown in Figure 1 the cores I3 extend through apertures in the non-magnetic sheet I2 to make contact with the extensions I I. Such contact is, however, not essential provided the supporting sheet I2 is suiiiciently thin to enable a magnetic flux density of about 1,000 gauss to be produced across the air-gaps formed by the moulding spaces 8.

It will be appreciated that the mass of the polepieces 1 may be vvaried as desired by selecting bodies to form them of the requisite size and shape, the mass being such lin relation to that of the magnets to be cast in proximity thereto as to give the desired initial rate of cooling to the castings. Similarly the feeder channel I0 with its branches may be varied in capacity to slow down the subsequent cooling of the castings to the desired rate.

In the modification illustrated in Figures 3 and 4 the pole-pieces 1 which have polar faces presented to the moulding spaces 8 are energized by permanent magnets I6 or by electro-magnets I1 having windings I8. The magnets I6 and I1 are of the bar type and are vertically's embedded in the moulding sand as shown with the upper end 'of each magnet making contact with a pole-piece 1 and with the lower ends of all the magnets yoked together by a supporting base-plate I9 of mild steel. The pole-pieces 1, moulding spaces 8 and magnets I6 and I1 are arranged in parallel rows and adjacent magnets are oppositely disposed as regards polarity as indicated by the letters N and S so that each of the moulding spaces 8 forms an air-gap in a magnetic circuit.

In Figures 3 and 4 the right hand moulding spaces 8 are shaped for the manufacture of horse-shoe magnets and the remainder for bar magnets and it will be understood that other shapes of magnets maybe manufactured according to the invention by modifying the shape of the moulding space and, if necessary, that of the pole-pieces 1 and the disposition and shape of the energizing magnets I8 or I1, I8.

A suitable alloy for manufacturing anisotropic magnets according to the invention consists of an iron base containing 10 to 20% of nickel, 14 to 30% of cobalt, 6 to 11% of aluminium, together with copper up to 7% and titanium up to 5%. There may be present in the alloy small quantities of carbon, silicon and manganese as is usual in permanent magnet alloy, with or without small quantities of tungsten, chromium, columbium, molybdenum, tantalum, vanadium, boron, magnesium, selenium, zirconium and uranium. In the production of these alloy primary alloys of copper and cobalt may be used.

According to the present invention, therefore,

v anisotropic properties are produced in permanent cipitation-hardening type in contact with polar faces of bodies embedded in moulding sand and connected in a magnetic circuit so that such bodies act also as chills promoting a rapid initial rate of cooling in the solidifying alloy (of an average order of from 3 C. to 10 C. per second) and the surrounding sand and heat supply from the casting runner and feeder are utilized to promote a slower rate of cooling as the temperature of the casting falls to the region of the Curie i point and below.

The masses of the bodies embedded in the moulding sand, of the sand and of the casting runner and feeder are preferably such in relation to that of the casting itself that the rapid initial cooling rate is reduced to about 3 C. per second whilst the temperature of the casting is still well above the Curie point and that there-A after the retardation of the cooling rate is progressive so that it takes from one quarter to one half an hour or longer for the casting to cool to 600 C. and very much longer (four hours, for example) to cool to 500 C.

It will be appreciated that in all cases it will be desirable to wait a suicient period of time to permit the casting to cool to 600 C. or lower before removing it from the mould. Preferably the casting is allowed to cool to about room temperature before stripping it and in this manner the combined influence of thel favourable differential cooling rates obtained whilst the casting is cooling down to about 600 C. and the relatively much longer time and slower cooling rate involved between 600 C. and 500 C. or lower will render a subsequent ageing or tempering process unnecessary. In any case the length of time required in any subsequent ageing or tempering process will be drastically curtailed being inversely dependent on the degree of precipitation obtained in the mould.

In a particular example of manufacturing anisotropic permanent magnets according to the invention an iron base alloy containing 15% of nickel, 7.5% of aluminium, 25% of cobalt, 2.5% of copper and 1% of titaniumwas cast in the apparatus illustrated in Figures 3 and 4 and the magnets were removed from the moulds when cool.

On testing the magnets they were found to possess the following magnetic properties:

B. Rem gauss 11,500 Hc oersted 380 (BHhnax 2.4 10

After an ageing o r tempering process of one hour at 600 C. the results were:

B. Rem gauss- 10,400v Hc oersted 660 (BH) max 3.46 X 10 Material of the same composition when cast in the ordinary manner, reheated and cooled in a magnetic field from 1250 C. gave the following results:

B. Rem gauss 11,750 Hc oersted-- 310 (BH) max 1.8 X 10 After an ageing or tempering process of one hour at 600 C. the magnets possessed the following properties:

B. Rem gauss 10,800 Hc oersted 620 (BHhnax 3.18 X 10s After an ageing or tempering process of 15 hours at 600 C. the values were found to be:

B. Rem gaus's-- 11,1oo Hc oersted-- l650 (B1-nw 3.32 10 It will be seen therefore that by the employment of the present invention the period required in the ageing' or tempering process to give optimum results may be reduced from 15 hours to 1 hour.

What we claim is:I

1. The process of producing a permanent'magnet having desirable anisotropic properties, which comprises casting, with a.relatively large casting runner in a mould, a quantity of a magnet alloy oi' the precipitation-hardening type in contact with polar-faces of metallic bodies connected in a magnet circuit, whereby said bodies promote a rapid initial rate of cooling in the solidifying alloy and whereby the heat .if the runner is utilized to effect a slower rate of cooling as the temperature of the casting falls further.

2. The process of producing a permanent magnet having desirable anisotropic properties, which comprises casting, with a relatively largecasting runner in a sand mould, a quantity of a magnet alloy oi' the precipitation-hardening type in contact with polar faces of metallic bodies connected in a magnet circuit, whereby said-bodies promote a rapid initial rate of cooling in the solidifying alloy and whereby the heat of the runner and the insulating properties of the surrounding sand are utilized to eifect a slower rate of cooling as the temperature of the casting 'falls further.

3. The process of producing a permanent magnet having desirable anisotropic properties, which comprises casting, with a relatively large casting runner in a sand mould, a quantity of a magnet alloy of the precipitation-hardening type in contact with polar faces of metallic bodies connected in a magnet circuit, whereby said bodies promote a rapid initial rate of cooling in the solidifying -alloy and whereby the heat of the runner and the insulating properties ofthe surrounding sand are utilized to effect a slower rate of cooling as the temperature of the casting falls to the region of the Curie point and below.

4. The process of producing a permanent magnet having desirable anisotropic properties, which comprises casting, with a relatively large casting runner in a mould, a quantity of a magnet alloy of the precipitation-hardening type in contact with polar faces of metallic bodies connected in a magnet circuit, whereby said bodies promote a rapid initial rate of cooling in the solidifying alloy and whereby the heat of the runner is utilized to eil'ect a slower rate of cooling as the temperature of the casting follows further to the region of the Curie point and below,and thereafter subjectingthe casting to an ageing or tempering process in which it is held at a tempera- 'ture of from 500 C. to 600 C. for a period inversely dependent on the degree of precipitation obtained in the mould.

5. The process of producing a permanent magnet having desirable anisotropic properties, which comprises casting, with a relatively large casting runner in a mould. a quantity of a magnet alloy of the precipitation-hardening type consisting of an iron base containing aluminium, nickel, and cobalt as the principal alloying elements in contact with polar faces of metallic bodies connected in a magnet circuit, whereby said bodies 'promote a rapid initial rate of cooling in the solidifying alloy and whereby the heat of the runner is utilized to effect a slower rate of cooling as the temperature of the casting follows further to the region of the Curie point and below.

6. A permanent magnet possessing anisotropic properties and produced according to the process set forth in claim l.

7. Casting apparatus for producing permanent magnets having anisotropic properties comprising a mould having a moulding space therein, metallic bodies embedded in the mould and each presenting a face to the moulding space, means providing a magnetic circuit including the pole pieces, a relatively large runner space connecting the pouring opening to the moulding space, whereby an initial rapid rate of cooling of the casting is accomplished by the chill effect of the metallic bodies, and a subsequent slower cooling is eil'ected by the slower cooling of the relatively large casting runner.

8. Casting apparatus for producing permanent magnets having anisotropic properties comprising a sand mould having a moulding space therein, metallic bodies embedded in the sand mould and each providing opposite surfaces of the moulding space, means providing a magnetic circuit including the pole pieces, a relatively large runner space connecting the pouring opening to the moulding space, whereby an initial rapid rate of cooling ot the casting is accomplished by the chill effect of the pole pieces, and a subsequent slower cooling is etl'ected by the slower cooling of the relatively large casting runner within the sand mould.

9. Casting apparatus for producing permanent magnets having anisotropic properties comprising a sand mould having a moulding space therein, pole pieces of high permeability embedded in the sand mould and each providing opposite surfaces of the moulding space, means providing a magnetic circuit including the pole pieces whereby the latter are of opposite polarity, a relatively large runner space connecting the pouring opening to the moulding space, whereby an initial rapid rate of cooling of the casting is accomplished by the chill effect of the pole pieces, and a subsequent slower' cooling. is eected by the slower cooling of the relatively large castingrunner within the sand mould.

` ANTHONY LINLEY.

ALAN TORRY.

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