IMPROVING THE GRINDABILITY

TICONAL ALLOYS

OF

A. A. B e z u g l o v a n d Z . A. R a k i t i n a UDC 669-154-176:669.018.5

Ticonal alloys often f r ac tu re in the grain boundaries or in the body of the gra ins during machining. Grain boundary f rac tu re during machining is the reason that most magnets are scrapped. The br i t t leness of these alloys resu l t s f rom precipi tat ion hardening and high-coerc iv i ty decomposition, leading to a biphase s t ruc ture (c~ + a ' ) . The two phases differ sharply in chemical composit ion and in physicomechanical p ro - per t ies [1]. The i ron - r i ch fe r romagne t ic a ' phase has high ductility, while the weakly magnetic a phase is r ich in nickel and aluminum, with low ductility. The low ductility of this phase is due to its enr ichment in aluminum in the p roces s of the a -- a + a ' t ransformat ion [2].

The aluminum content of the alloys must be s t r ic t ly control led to obtain high magnetic proper t ies . According to [2], the best magnetic proper t ies of alloy YuNDK35T5 are obtained with 7% A1. Thus, in- c reas ing the ductility of the alloy by reducing the aluminum content will impair the magnetic p roper t ies . Aluminum is a ve ry s trong a - f o r m i n g element. When the aluminum content is reduced to less than 7% the alloy is more susceptible to h igh- tempera ture decomposition, resul t ing in precipitat ion of 7 (a 7) phase, which impairs the magnetic p roper t i es . It follows, then, that to improve the ductility and thus the grind- ability of Ticonal alloys while retaining the magnetic proper t ies it is necessa ry to lower the aluminum con- tent and add another element with the same high a - f o r m i n g capacity. We selected silicon for this purpose.

We investigated the effect of silicon on the magnetic proper t ies and grindability of the Ticonal alloy containing 7% A1, 35% Co, 15% Ni, 3% Cu, 5% Ti, the remainder Fe, and also alloys with a low aluminum content.

Heats weighing 5 kg with 7% A1 and additions of 0.3, 0.6, 0.9, and 1.2% Si were melted in a high- f requency induction furnace with an acid crucible powered by the LPZ-67 tube genera tor . The charge m a - te r ia l s were added in o rde r of their melting points; silicon wrapped in aluminum foil was added before the metal was poured. The tempera ture of the metal was 1620-1650~ before casting of magnets 12 • 12 • 30 ram. I so thermal thermomagnet ic t rea tment (ITMT) was conducted in a bath of molten Silumin of eutectic composition in a magnetic field of 3500-4000 Oe. After ITMT, the samples were tempered at 650 ~ for 5 h + 550 ~ for 10 h.

The magnetic p roper t i e s were determined with the U-541 ball ist ic apparatus. Grindability tests were made in the 3G71 flat surfacing machine with E9A 25 SMIK disks. The relative grindability was de- termined by the formula:

K = �9 100%, p

where Y_2i is the total chipping, mm; P is the pe r ime te r of the sample, mm.

The cr i t ica l i sothermal holding tempera ture was determined for each of the alloys. It was found that silicon has no great effect on it (it was 790 • ~ for all alloys with holding for 10 rain). The magnetic proper t ies are shown in Fig. 1, where it can be seen that the addition of silicon increases the remanence slightly, although the coerc ive force and the maximum magnetic energy decrease .

The increase of the remanence is evidently due to the introduction of silicon into c~ phase. The in- c r ease in the saturation magnetization of a phase lowers the coercive force and consequently the magnetic

Novocherkassk Scient i f ic-Research Institute of Permanent Magnets. Trans la ted f rom Metallove- denie [ Te rmicheskaya Obrabotka Metallov, No.2, pp. 60-62, February, 1976.

�9 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy o f this article is available from the publisher for $15.00.

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&, CA/m (8/r

, 1 ~ w~/~

o o,3 o,s gqol:sg

K,%

5o

20' ~:~ 10:

Fig. 1 Fig. 2 Fig. 1. Effect of silicon on magnetic proper t ies of Ticonal alloy with 7%A1.

Fig. 2. Effect of silicon on grindabili ty of Ticonal alloy with 7% AI. !) After casting; 2) after ITMT (not ground); 3) after ITMT with p re l imin- ary grinding.

{SH),,= J/m 3

Ho,c~7/ i ' ~o % I

:,oq I P':"T I M I St', Wb/m ~ ',~ 1 N I ]--]. 2o t!

o ~ a l t I I I h I o 7 eA 6,6 g~ .s,t % a~ 7 g.g ~,g 6,~ 42 gO,At

Fig. 3 Fig. 4 Fig. 3. Magnetic p roper t i e so f Ticonal alloy with 0.8% Si in relation to aluminum content.

Fig. 4. Effect of aluminum on grindabili ty of Ticonal alloy with 0.8% Si. 1) Not ground; 2) previously ground.

energy, since the coercive force is proportional to the difference in the squares of the saturation magneti- zation of ~' and ~ phases.

Grindabilit3r tests showed that the addition of silicon sharply impairs the grindability of the Ticonal alloy with 7% A1 (Fig, 2). The samples cracked on the sharp edges and chipping was observed over the en- tire surface. Silicon evidently weakens the boundaries of the crystallites. The best results were obtained on samples subjected to preliminary grinding (Fig. 2). This confirms the fact that nonmetallic inclusions and other defects, acting as stress concentrators, collect on the surface of the sample in the so-called cast ing skin.

The tests showed that with the addition of 0.6-0.9% St the possibil i ty of the h igh- tempera ture c~ ~ ce + T t ransformat ion, which impairs the magnetic proper t ies , is completely excluded. Alloys with 7.0, 6.8, 6.5, 6.0, and 5.8% A1 were tested. All alloys contained 35% Co, 15% Ni, 5.5% Ti, 3.0% Cu, 0.8% 8i, the remainder Fe. The titanium content was increased to ra i se the coerc ive force .

The magnetic p roper t i e s after the complete cycle of ITMT and tempering are shown in Fig. 3. When the aluminum content is lowered to 6.8% the magnetic cha rac t e r i s t i c s improve, especial ly the mag- netic energy, which reaches its highest value: 21,000 J / m s. For individual samples the magnetic energy reached (23-24)103 J / m 3. The remanence and coerc ive force were fair ly high: 0.91 Wb/m 2 and 117-120 C A / m respect ive ly . With fur ther reduction of the aluminum content the remanenee remains almost un- changed, while the magnetic energy, decreas ing to 20,000 J / m 3 with 6.5~ A1, r emains unchanged when the aluminum content is reduced to 6.2%. The coerc ive force f i rs t dec reases to 105-108 CA/m with 6.5% A1 and then i nc r ea se s again, reaching 113-115 CA/m with 6.2% A1.

The anomalous change in the coerc ive force is evidently due to the fact that with 6.2 and 6.8% A1 the most favorable conditions are c rea ted for h igh-coerc iv i ty (~ --* c~ + ~ ' decomposition and precipitat ion

171

of magnet ic v~' phase, inc reas ing the amount of it. Elec t ron mic roscop i c analys is of these al loys sub- jec ted to the opt imal t r ea tmen t showed a high degree of shape anisot ropy for the a ' pa r t i c l e s , which also explains the higher coe rc ive fo rce . But the evenness and high densi ty of o~' p rec ip i t a t e s ensure higher magnet ic energy .

When the a luminum content is r educed to 5.8% all the magnet ic cha r ac t e r i s t i c s , espec ia l ly the co- e r c ive fo rce and magnet ic energy, dec rea se , which is due to the prec ip i ta t ion of fcc 7 phase resul t ing f r o m the h i g h - t e m p e r a t u r e o~ - - o~ + ? t r an s fo rma t ion . The p r e s e n c e of 0.8% Si is insufficient to p revent the t r ans fo rma t ion in this case .

Grinding t e s t s conf i rmed the fact that with lower a luminum concentra t ions the gr indabi l i ty improves . Coefficient K d e c r e a s e s continuously, reaching the min imum value at 5.8% A1 (Fig .4) . However , as a l - r eady noted, the magnet ic p r o p e r t i e s a re g rea t ly impa i red at this a luminum concentra t ion. The bes t c o m - bination of magnet ic p r o p e r t i e s and gr indabi l i ty is obtained with 6.2% A1.

The fa i r ly high magnet ic p r o p e r t i e s [B r = 0.9-0.91 W b / m 2, H c = 113-115 CA/m, 0BH)ma x = 20-21 �9 103 J / m 3] and the good gr indabi l i ty of the Ticonal alloy with 6.2% A1 and 0.8% Si lead us to r e c o m m e n d this alloy for m a s s product ion of pe rmanen t magne t s .

io

2.

LITERATURE CITED

Ya. M. Dovgalevski i , Cast ing of Magnets f r o m Magnlco Alloys [in Russian], Mashinost roenie , Moscow (1964). I. G. Lyapichev et a l . , '~KhTO alloy YuNDK35T5, " in: E lec t romechan ica l Ins t rument s and Sys tems . Elec t ronic In s t rumen t s [in Russian] , f~nergiya, Moscow (1967).

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