EXPERIMENTAL STUDIES OF SHEAR ZONES DURING CHIP FORMATION IN METAL CUTTING

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EXPERIMENTAL STUDIES OF SHEAR ZONES

DURING CHIP FORMATION IN METAL CUTTING

O. Vingsbo

To cite this version:

O. Vingsbo.

EXPERIMENTAL STUDIES OF SHEAR ZONES DURING CHIP

FORMA-TION IN METAL CUTTING. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-371-C5-377.

�10.1051/jphyscol:1985547�. �jpa-00224778�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment a u n08, Tome 46, aoDt 1985 page C5-371

EXPERIMENTAL S T U D I E S OF SHEAR ZONES DURING C H I P FORMATION I N METAL C U T T I N G

0. Vingsbo

Department of MeehanieaZ Engineering, University of Houston-University Park, Houston, Texas 77004, U.S.A.

RSsum6 - Les gtudes des micromGcanismes de coupe rapide des mBtaux sont rda- lisdes b l'aide d'une technique fondGe sur le mouvement d'un pendule. Un outil tranchant fait une rayure de la forme d'un arc sur la surface de l'bchantillon et la structure interne des zones de cisaillement est fig6e 1 l'aide d'un sys- tsme d'arrtt rapide pendant le processus de formation du copeau. Une Squation reliant des paramPtres de coupe aux m6canismes de dsformation dans les zones de cisaillement est proposde.

Abstract - Dynamic studies of the micromechanisms of metal cutting are per- formed with a pendulum technique. A tool edge cuts an arcuate groove in the specimen surface, and the internal structure of shear zones during chip for- mation is frozen with the aid of a quick-stop arrangement. An equation rela- ting bulk cutting parameters to the deformation mechanisms in shear zones is suggested.

I

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INTRODUCTION

Dynamic studies of chip formation mechanisms during a metal cutting represent g r e a t technical difficulties. A variety of experimental techniques have been tried on a bulk scale. One approach is t o choose such a cutting geometry t h a t a n external specimen surface is normal t o and intersects t h e bottom of t h e groove along t h e grooving direction. It is then possible t o record the cutting process dynamically with high speed movie photography. If the specimen surface is chemically e t c h e d prior t o cutting, t h e internal microstructure and microstructural changes will be revealed. Generally, however, only macro photographic methods will be applicable, and t h e spatial resolution will be correspondingly limited. For higher resolution, therefore, when microscope imaging is necessary, i t seems impossible t o achieve dynamic recording. In this case, a n alternative approach is to use some quick-stop arrangement, with t h e aid of which t h e cutting process c a n be stopped in a presumedly instantaneous manner. Because of t h e high inertia involved in bulk cutting, however, high enough deceleration of t h e tool (or t h e specimen) is difficult t o reach, and often explosive techniques have t o be used. The present report describes a new quick-stop technique, based on pendulum single pass grooving. The pendulum grooving technique was primarily developed for studies of abrasive wear /1,2/. It has, however, also been used in basic investigations of m e t a l cutting mechanisms, irrespective of applications.

I1

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THE PENDULUM GROOVING TECHNIQUE

A commercial Charpy V type impact t e s t e r has been modified by attaching a cutting tool, radially protruding from t h e end of t h e hammer shaft. During a downswing of t h e pendulum a n a r c u a t e groove is c u t in t h e s u r f a c e of a specimen, ridgidly supported by a holder a t t h e lowest point of t h e swing (See Fig. I).

The cutting tool is made of cemented carbide in the shape of a square-based pyramidal tip, with a 90' apex angle, and truncated t o a 1 x 1 mrn f l a t top. It is oriented with one f a c e t normal t o t h e grooving direction, which gives a 45' negative rake angle. The groove size is controlled by t h e adjustable specimen level. All experiments reported here w e r e performed with a t i p entrance velocity of 5.6 m/s and a n entrance energy of 300 Nm.

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Pendulum H a m m e r

C u t t i n g T o o l

Fig. 1 Principles of t h e pendulum technique.

The single pass pendulum a r r a n g e m e n t makes i t e x t r e m e l y simple t o design a quick-stop facility: t h e specimen is a t t a c h e d t o t h e holder with t h e aid of break pins, and is released and ejected from t h e holder when hit by, for example, a heel, positioned a t t h e r e a r end of t h e hammerhead (see Fig. 2). The specimen ejection is practically instantaneous, and t h e "frozen- in" microstructure is representative of t h e s t a t e of deformation a t t h e moment of ejection.

Fig. 2 Quick-stop arrangement.

The specimen size c a n b e kept small in order t o f a c i l i t a t e preparations for metallographic studies. The external bulk appearance of t h e chip, being extruded a t t h e front end of t h e groove, c a n be imaged in a scanning electron microscope, a s demonstrated by Fig. 3a. The internal s t r u c t u r e c a n be studied by classical rnetallographical methods in any section through chip and/or groove (Fig. 3b), and by transmission electron microscopy of thin foils. In f a c t , t h e possibility oif systematic studies of t h e relation between cutting parameters, chip formation mechanisms and t h e resulting microstructure has proved t o be one of t h e most rewarding f e a t u r e s of t h e pendulum technique.

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Fig. 3 a ) Scanning electron micrograph of chip extrusion.

b) Light optical micrograph of e t c h e d longitudinal section through a chip.

I11

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EXAMPLES OF RESULTS

The basic mechanism'of m a t e r i a l removal in m e t a l cutting is t h e plastic shear in a zone in front of t h e propagating tool edge. Under appropriate conditions of rake angle, feed, c u t t i n g force, etc., a chip is formed and extruded, until i t eventually g e t s separated from t h e work piece. In t h e absence of dynamic methods of studying t h e a c t i v e micromechanisms of chip formation, t h e present technique of freezing t h e internal s t r u c t u r e a t selected s t a g e s of chip formation, provides a n indirect method of following t h e process. Light optical micrographs of e t c h e d longitudinal cross sections have proved particularly instructive, making i t possible t o deduce t h e e n t i r e chip formation mechanism from series of quick-stop experiments.

Fig. 3b is a typical light optical micrograph of a longitudinal section through a chip, demonstrating t h e principal image layout, used throughout this report. The c u t t i n g direction is from l e f t t o right, with a primary shear zone extending from t h e lower edge of t h e cutting tip (a f r a g m e n t of which often stays in t h e groove), and bending upwards, toward t h e s u r f a c e ahead of t h e groove. It c a n be seen, how t h e negative rake angle g e n e r a t e s t h e extrusion of m a t e r i a l between t h e primary shear zone and a secondary shear zone a t t h e r a k e face. A t e r t i a r y shear zone is c r e a t e d by t h e friction f o r c e a t t h e i n t e r f a c e between t h e tool flank and t h e bottom of t h e groove under formation. The secondary and t e r t i a r y shear zones represent tool/workpiece interfaces, and a r e of less interest from t h e materials point of view. The primary shear zones, however, a r e c h a r a c t e r i s t i c of t h e plastic deformation properties of t h e work piece material itself under t h e prevailing conditions of dynamic load, and will be studied in some d e t a i l below in a series of light optical micrographs.

Fig. 4 shows an early s t a g e of nucleation of a shear zone in a Hihg Strength Low Alloy (HSLA) high speed steel. The propagating tool causes a compressive strain ahead of t h e r a k e face, which appears t o b e mainly elastic. At t h e lower edge, t h e s t r e s s field gradients initiate t h e thin shear zone, in which virtually all t h e plastic deformation, necessary for t h e extrusion process, will t a k e place. The interpretation of t h e micrograph in t e r m s of deformation mechanisms requires an understanding of t h e relations between t h e micrograph constrast,

C5-374 JOURNAL DE PHYSIQUE

c r e a t e d by t h e topography of t h e etched cross section surface, and the corresponding internal microstructural details. According t o t h e results of extensive previous studies of this type of material 11-3/ t h e image c a n be interpreted a s follows. Because of t h e appreciable amount of deformation t h a t has t o be accommodated by t h e thin shear zone, t h e strain density a s well a s the strain r a t e locally a t t a i n very high values. Nevertheless, t h e structural changes and corresponding image constrast a r e not primarily associated with dislocation movements, but rather with phase changes. The high strain density will cause heating t o local temperatures in t h e austenite range, and most of t h e plastic deformation takes place in a n austenitic matrix. After t h e tool has passed (or a f t e r t h e quick-stop event), t h e shear zone is deactivated and rapidly cooled by conduction t o t h e surrounding material. The cooIing is equivalent t o a quench, and a martensitic transformation is triggered. The zone will be l e f t with a n extremely f i n e grained martensitic s t r u c t u r e t h a t may be as-quenched or annealed, depending on local coioling conditions. This kind of martensite is quite well-known in tribological applications, and o f t e n referred t o a s "friction martensite". It responds only weakly t o etching and therefore appears bright ("white etching") in metallographic images. In Fig. 4 t h e nucleating primary shear zone is revealed mainly by i t s content of friction martensite, which is found t o increase towards t h e tool edge, and t h a t also t h e t e r t i a r y zone is characterized by i t s friction martensite.

Fig. 4 Nucleation of a shear zone.

Fig. 5 is a n image montage, showing different stages of lamellar chip extrusion from a n HSLA steel. In Fig. 5a, a primary shear zone has just reached t h e e x t e r n a l surface, and t h e extrusion of a chip lamella i s beginning. Fig. 5b represents a slightly l a t e r stage, when t h e lamella is being extruded between t h e primary and secondary shear zones. The well developed friction martensite reveals t h a t both zones operated a t high local temperatures, typical of so called adiabatic shear. In Fig. 5c, t h e extrusion of a lamella is nearly completed. The primary and secondary shear zones have merged into one lamella stalk of typical adiabatic shear, which i s just about t o be c u t off by t h e lower edge of t h e propagating rake face. Simultaneously t h e next primary shear zone is being nucleated.

Fig. 5 Different stages of lamellar chip extrusion in a HSLA steel.

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Hadfield steels a r e characterized by a n austenitic structure, which is thermally m e t a stable at room temperature, and will undergo local, strain-induced martensitic transformation in regions of high stress concentrations. Fig. 6 demonstrates t h e propagation of a primary shear zone through t h e austenitic matrix of a Hadfield s t e e l specimen. The stress field initiates martensitic transformation in small s c a t t e r e d regions (hardly visible in light optical micrographs) within a volume around t h e leading tip of t h e shear zone. However, no extensive martensite formation can be observed within t h e core of t h e shear zone. This may be explained by t h e f a c t t h a t , if local plastic deformation t a k e s place at elevated temperatures, t h e austenitic phase will b e thermally stable, effectively preventing strain-induced martensitic transformations. After t h e passage of t h e tip, t h e t e m p e r a t u r e is again low, but t h e n t h e local s t r e s s field h a s also passed, and n o strain-induced transformation c a n b e initiated.

Fig. 6 Propagation of primary shear zone in a Hadfield steel.

IV

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DEFORMATION MECHANISMS IN SHEAR ZONES

The pendulum technique inherently o f f e r s t h e opportunity of defining and measuring cutting parameters, which c a n b e related t o t h e localized deformation mechanisms within, t h e shear zones in t e r m s of dislocation mechanics. Detailed descriptions c a n be found in e.g. references 1 and 2. A brief summary is given below with special emphasis on shear zones in m e t a l cutting applications.

1V:l

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Specific grooving energy

The energy E consumed in making a groove (the grooving energy) is experimentally determined from the difference between t h e downswing and upswing angles of t h e pendulum shaft, and c a n be read from a gauge, which is standard equipment on a l l impact testers. E has been found t o b e related t o t h e mass W of t h e chip (the specimen weight loss) by a power function

The energy e = E/W, consumed in removing t h e unit mass (the specific grooving energy) has been found to be a measure of t h e resistance t o grooving. Logarithmic plots of measured e values a s a function of W (i.e. for different groove sizes) for a material,

yield intervals of straight line segments, from which experimental values of k and q c a n be obtained for e a c h W interval. The k and q values c h a r a c t e r i z e t h e e-W relation of a material completely. Therefore, i t is of g r e a t interest t o r e l a t e these originally empirical parameters t o t h e deformation behavior of t h e material.

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IV:2

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Specific grooving energy related t o localized deformation in shear zones

The process of deformation by shear zones is mainly characterized by t h e pronounced spatial inhomogeneity of t h e deformation. In f a c t , because essentially all t h e deformation is concentrated t o t h e small volume within t h e shear zones, i t is t h e maximum local strain density, r a t h e r than t h e t o t a l strain, t h a t determines t h e dislocation mechanical behavior of t h e material. In a n experimental study of a

HSLA

steel, heat t r e a t e d t o four different hardness levels, measured k values were plotted versus t h e corresponding q values a s shown in Fig. 7.

Fig. 7 Experimental k(q)

plot for a HSLA steel, h e a t t r e a t e d t o four different hardness levels.

Each point corresponds t o one groove, and numbers 9-12 r e f e r t o successively lower hardness values of t h e steel. The q axis is subdivided into t h r e e intervals, I corresponding t o homogeneous deformation of t h e chip, 11 t o t h e formation of shear zones, and I11 t o fully developed adiabatic shear zones. Experimentally, these morphological variations were achieved by increasing t h e groove size (maximum chip thickness). The curve shape of Fig. 7 suggests t h e following dislocation dynamical interpretation.

k may b e related t o t h e local shear stress, acting on the dislocations in t h e most favorably oriented slide system in a shear zone. Similarly, q may be associated with t h e local strain density in t h e zone. The k(q) curve then represents a stress-strain curve, with a hardening peak in interval 11, describing a balance between work hardening and dynamic softening. In interval I t h e strain density is low, and work hardening is a predominating low-temperature characteristic. A softer material work hardens more easily, giving t h e 9-12 sequence in Fig. 7. For increasing strain density (q), however, t h e local temperature increases, and softening by dynamic recovery will play a n increasingly important role. At t h e transition t o interval I1 softening predominates, and t h e k(q) curves passes a maximum. Interval 111, finally, corresponds t o high t e m p e r a t u r e adiabatic conditions. Dynamic recrystallization has drastically reduced t h e e f f e c t of previous structural obstacles t o dislocation movements, such a s precipitates, .grain boundaries, etc. The result is a catastrophic softening, t h a t highly reduces t h e influence of t h e structural hardness differences, and t h e points corresponding t o the four different hardness levels a r e closely spaced in interval 111 of the diagram. Still, however, the s a m e sequence (9-12) is maintained.

V

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CONCLUSIONS

The single pass pendulum technique offers unique possibilities of quasi-dynamic studies of high- strain and high-strain-rate deformation mechanisms during m e t a l cutting. The development and operation of shear zones c a n b e followed in detail.

It is possible t o work o u t a n equation, relating t h e specific grooving energy t o t h e weight of a chip, produced by a pendulum stroke. This equation contains t w o measurable empirical parameters, which c a n be used for a dislocation mechanical description of t h e defor'mation within shear zones.

ACKNOWLEDGEMENTS

Most of t h e work, summarized in this report, was conducted in a cooperative e f f o r t with BOFORS AB, Wear P a r t s Division. The author is g r e a t l y indebted t o Mr. Lars Hellner of BOFORS AB for his invaluable support, and t o BOFORS AB for t h e permission t o publish t h e results. Dr. Sture Hogmark of t h e Department of Materials Science, Uppsala University, has been instrumental in developing t h e pendulum technique. Most of t h e project work was carried o u t by Dr. Ulf Bryggman, formerly with t h e s a m e department.

REFERENCES

/ I / Vingsbo, 0. and Hogmark, S., Wear

100

(1984) 489.

/2/ Bryggman, U., Hogmark, S. and Vingsbo, O., Mechanisms of Gouging Abrasive Wear, Investigated with t h e Aid of Pendulum Single Pass Grooving, in "Wear of Materials

I985", ASME (1985), in print.