Cavity formation in rolling profiled rings

Int. J. mech. Sci. Pergamon Press. 1975. Vol. 17, pp. 669-672. Printed in Great Britain

CAVITY

FORMATION

IN

ROLLING

PROFILED

RINGS

A. G. MA~ALm, J. B. HAWKYARD a n d W . JOHNSON Department of Mechanical Engineering, U.M.I.S.T., Manchester, England

(Received 19 July 1975) Summary--When rolling profiled (T-shaped) rings at large reductions of ring wall thickness, it is found that for certain values of feed-rate, groove and shape factor, a specific form of cavity formation arises which is similar to the onset of piping as known in extrusion. INTRODUCTION A WELL-XNOWN d e f e c t o c c u r r i n g on t h e r e a r e n d o f e x t r u d e d p r o d u c t s is t h a t o f a sinkingin o f t h e m a t e r i a l , i.e. o f t h e b e g i n n i n g o f p i p i n g or c a v i t y f o r m a t i o n , o n t h e b o t t o m o f t h e slug; it o c c u r s d u r i n g t h e final u n s t e a d y p h a s e o f e x t r u s i o n w h e n t h e slug t h i c k n e s s has become greatly reduced: A s o m e w h a t similar p h e n o m e n o n , i.e. c a v i t y f o r m a t i o n , h a s also b e e n o b s e r v e d w h e n rolling profiled rings. I n t h e cases e x a m i n e d , a T - s h a p e d r i n g w a s f o r m e d f r o m a n initial r e c t a n g u l a r b l a n k rolled b e t w e e n a p l a i n m a i n roll a n d a g r o o v e d m a n d r e l , see Fig. 1. F o r large r e d u c t i o n s in r i n g wall thickness, e.g. 6 0 % , it

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FIG. 1. Arrangement of rolls and specimen. 48

w a s o b s e r v e d t h a t t h e r i n g s t a r t e d t o lose its f o r m a t its o u t s i d e surface w h e r e earlier it w a s in c o n t a c t w i t h t h e m a i n roll, i.e. a s e p a r a t i o n o f t h e m a t e r i a l f r o m t h e m a i n roll o c c u r r e d a n d a c a v i t y b e g a n t o be f o r m e d . This p a p e r p r e s e n t s t h e results o f experim e n t a l o b s e r v a t i o n s m a d e o n rolled t e l l u r i u m lead rings in a n a t t e m p t t o i d e n t i f y s o m e o f t h e p a r a m e t e r s t h a t lead t o this. EXPERIMENTAL WORK Equipment The ring rolling mill used for the experimental work reported here has been described previously, see ref. (2). The ratio of roll diameters used was 3.27; the main roll diameter, equipped with a pin transducer, was 9 in. and the mandrel diameter 2.75in. Both rolls were ground to a smooth condition. The method used for measuring the normal pressure distribution over the arc of contact in ring rolling is well known as the pressure-sensitive pin technique and in this case a small pin was fitted radially into the roll, ending flush with the roll surface and making contact with the deforming material to pick up its force reaction. The force changes exerted on the pin were converted into voltage variations by precision-mounted resistance strain gauges; the output of the pressure transducer was amplified and displayed on an oscilloscope and photographed with a continuous feed camera. The pressure transducer assembly as mounted in the main roll together with associated circuitry is shown in Fig. 2; a detailed description of the technique used and the instrumentation required is given in ref. (3).

Test material and experimental procedure The test material was tellurium lead in the "as cast" condition which at room temperature is strain rate sensitive. Typical stress--strain curves for the whole of the strain rate range expected during ring rolling are given in Fig. 3. 669

670

A. G. MAI~IALIS, J . B. HAWKYARD a n d W. JOHNSON

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Fla. 3. T y p i c a l s t r e s s - s t r a i n c u r v e s a t v a r i o u s s t r a i n r a t e s for t e l l u r i u m lead.

C a v i t y f o r m a t i o n in rolling profiled rings

RESULTS AND DISCUSSION I n Fig. 5, p h o t o g r a p h s of a f o r m e d c a v i t y for various ring-rolled specimens are presented. I n Fig. 6(a)-(f) t h e pressure curves for t h e successive final revolutions of t h e m a i n roll are g i v e n for specimens of initial groove and shape factor e q u a l to 0.50 a n d 0.625, rolled w i t h a feedrate of 0.019 in/rev. I t can be seen t h a t a t a r e d u c t i o n of 58~o, (b) of Fig. 6, a considerable d r o p in t h e pressure has occurred as c o m p a r e d w i t h (a) of Fig. 6 a n d it t h e n continues to decrease thereafter, see (c) of Fig. 6. The shape of t h e pressure c u r v e in t h e final stages of rolling, (d)-(f) of Fig. 6, is due solely to t h e binding effect which exists b e t w e e n t h e p i n a n d its insert ;8 e v i d e n t l y t h e pin is n o t in c o n t a c t w i t h t h e rolled material, a n d t h e c a v i t y has been well formed. (It m a y be r e m a r k e d t h a t t h e roll force continues to increase however.) A t c a v i t y c o m m e n c e m e n t t h e ratio of ring wall thickness H to t h e w i d t h b of the root of t h e groove close to t h e flange, see Fig. 4 (corresponding to t h e w i d t h of t h e e x t r u s i o n hole), was found a t t h i s stage to be equal to n e a r l y 0.5. Values of H/b are also g i v e n in Fig. 5(a) and (c). U s u a l l y t h e ratio of H/b lies b e t w e e n 0.4 and 0-8 for t h e various rolling p a r a m e t e r s examined. I n t h e case o f post-stea~ly or u n s t e a d y plane strain extrusion w i t h c a v i t y formation, J o h n s o n a n d K u d o 1 h a v e e s t i m a t e d or predicted, b y t h e use of a simple u p p e r b o u n d analysis, t h e H/b ratio for c a v i t y f o r m a t i o n ; it was found t h a t it begins to be formed w h e n t h e ratio of t h e slug thickness to t h e w i d t h of t h e extrusion orifice is a b o u t e q u a l to 0.5 for a s m o o t h r a m a n d to 0.7 for a perfectly rough ram when extruding through

A f t e r being annealed in boiling w a t e r , each ring specimen was cleaned a n d degreased w i t h e m e r y p a p e r a n d t r i c h l o r o e t h y l e n e a n d slightly d u s t e d w i t h french chalk p o w d e r to t r y to keep t h e friction c o n s t a n t t h r o u g h o u t the tests. T h e initial r e c t a n g u l a r section of a ring was 1 in. axial w i d t h ; t h e two initial shape factors (Ho/Wo) were 1 and 0.625, see Fig. 4, and t h e outside ring d i a m e t e r was 5 and 4.25 in. correspondingly, The t w o groove factors explored (bo/Wo)were 0.25 and 0"50, whilst t h e values of feed-rate (f/N} were 0.007, 0.019 and 0 . 1 0 i n / r e v , w i t h t h e m a i n roll r o t a t i o n a l speed k e p t c o n s t a n t at 31 r e v / m i n .

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4m

Ii (c)

671

(b)

Fro. 4. C a v i t y f o r m a t i o n (radial-axial section). (a) Initial position, (b) final position. To a n t i c i p a t e t h e change in pressure distribution w h e n t h e c a v i t y begins to form, t h e p i n was located on t h e centre-line of the axial w i d t h of a ring, t h u s corresponding to the middle of t h e groove, see Fig. 4 ( × - × axis).

M a t e r i a l , Tellurium lead

____=j

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(d)

II

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,

(e)

-Rolling

(f)

direction

FIG. 6. Showing h o w t h e pressure distribution t h r o u g h t h e gap changes w i t h successive revolutions in one t e s t as c a v i t y f o r m a t i o n occurs. (a) R e d u c t i o n 54.7%, 17th rev., (b) r e d u c t i o n 58%, 18th rev., (c) r e d u c t i o n 61.2%, 19th rev., (d) r e d u c t i o n 6 7 - 2 0 , 21st rev., (c) r e d u c t i o n 70"8~o, 22nd rev., (f) r e d u c t i o n 77.3~o , 24th rev.

672

A . G . MAMALIS, J. B. HAWKYARDand W. JOHNSO~~

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Fro. 7. Rigid triangle velocity fields and their corresponding hodographs. square dies. These magnitudes are similar to those encountered in ring rolling as reported above, see refs. (2) and (3). During ring rolling the feedrate appears to have an effect on the cavity formation; generally, the cavity is formed for slow feedrates at smaller reductions than it is for fast feedrates. Accounting for the ring rolling phenomenon described with some quantitative estimates in the m a n n e r of piping in extrusion is certainly realistic. One difference between the two situations lies in the fact that lateral spread is prevented in plane strain extrusion by the container wall; lateral spread with consequent "fish-tailing" is evident in our ring-rolled specimens of Fig. 5. Conceived in plane strain compression terms, this flow could be accounted for by either of the plane strain sliding block deformation models shown in Fig. 7. A

stationary block C (or C') is assumed; the speed of A(w) is greater t h a n that of the top tool (u) and hence a cavity is formed over the central region adjacent to the tool. Sliding outwards of the material is achieved through blocks D (Fig. 7(a)) or E in Fig. 7(b).

REFERENCES 1. W. JOHNSON and H. KUDO, The Mechanics of Metal Extrusion. Manchester University Press, Manchester {1962). 2. J . B. HAWKYARD, E. APPLETON and W. JOHNSON, Int. Machine Tool Design and Research Conf., Birmingham, Paper 85 (1972). 3. A. G. MA•ALIS, Ph.D. Thesis, U.M.I.S.T. (1975).

(b)

(cl (a) Reduction 5. Cavity formation on flange: tellurium lead. f/N = 0.007 in/rev, H,/W, = 0.625, b,/W, = 0.25, H/b = 0.50. (b) Reduction f/N = 0.007 in/rev, Ho/W, = 0.625, ho/W, = 0.50. (c) Reduction 80.7:/,, 0.10 in/rev, Ho/W, = 1, b,/W, = 0.25, H/b = 0.67. (d) Reduction SO”;,. 0.10 in/rev, H,/w, = 0.625, ho/W, = 0.25. FIG.

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