Chip formation, acoustic emission and surface white layers in hard machining

Byrne I , Department of Mechanical Engineering, University College Dublin, Ireland De Beers Industrial Diamonds Division, Ireland 1 Abstract In hard machining, sawtooth chip formatio

Chip Formation, Acoustic Emission and Surface White Layers in Hard Machining

J Barry I, G Byrne ( I ) , Department of Mechanical Engineering, University College Dublin, Ireland

De Beers Industrial Diamonds Division, Ireland

1

Abstract

In hard machining, sawtooth chip formation is due to initiation of adiabatic shear within the lower region of the primary shear zone Catastrophic failure within the upper region of the shear zone occurs through either of two different mechanisms and results in the rapid release of elastic strain energy This periodic release of strain energy is the dominant source of acoustic emission during sawtooth chip formation In addition to adiabatic shearing in the primary and secondary shear zones, there is evidence to suggest that it occurs in the tertiary shear zone also; namely the surface white layer

Keywords: Chip formation, Acoustic emission, Surface white layer

1 INTRODUCTION

The great utility of steels as engineering materials arises

in part from the wide range of mechanical properties

which may be obtained through the austenite-martensite

(hardening) transformation and subsequent tempering

processes The hardening operation involves heating the

material to a sufficiently high temperature such that the

lower temperature body centre cubic (BCC) a ferrite and

cementite (Fe3C) phases transform to austenite, an

interstitial solid solution of Carbon in face centred cubic

(FCC) y Iron On rapidly cooling an austenitic steel to

below its martensite-start temperature, the interstitial

Carbon atoms are retained in solution despite the reverse

transformation of the unit cell from FCC to a distorted

body centred tetragonal (BCT) structure [ I ] The strains

arising from the distortion of the unit cell result in either

heavily dislocated or heavily twinned microstructures

which in comparison to softer pearlite/ferrite structures of

the same composition, are relatively unstable [2]

When hard martensitic steels are deformed at sufficiently

high strain rates, the plastic strain is accommodated in

narrow localised adiabatic shear bands [3] It is shown

below that it is this phenomena of adiabatic shear band

formation which results in the formation of the sawtooth

chip and the surface white layer in hard machining The

influence of the chip formation mechanisms on acoustic

emission (AE) is also discussed, for both sawtooth and

continuous type chips

2 EXPERIMENTAL

Two grades of steel were employed in the current study; a

BS817M40 steel, and a low alloy tool steel, similar to the

AlSl L grades The composition of these steels are given

in Table 1 Both steels were oil quenched and tempered

to various hardness levels, depending on the particular

tests being undertaken The structure of both steels was

lath martensite The low alloy tool steel also contained

micron sized spheroidal alloy carbides

Commercially available mixed ceramic cutting tools were

used for most of the cutting tests This material contained

BS817M40 0.40 0.16 1.17 1.37 0.28 Tool steel 0.80 0.19 1.71 0.14 0.41 Table 1 : The composition of the steels used for machining

tests, in wt%

71% a-Al203, 28% TIC and 1% MgO and had an average grain size of 2 pn The standard tool geometry was of I S 0 (1832:1991) designation; TNGA 160412

T02020 The average edge radius, rp, was 7 pn

Table 2 summarises the different cutting configurations employed in the study, the respective cutting conditions, work materials and hardness levels and tool materials and geometries The turning, facing, orthogonal and nonoverlapping cutting tests were undertaken using constant surface speed control on a CNC lathe equipped with a platform dynamometer Acoustic emission was measured with the sensor mounted on the underside of the tool holder To obtain the AERMS signal, the raw signal was integrated with a time constant of TRMS = 12 ms The technique employed for quickstop testing is similar to that used by Betz [4] This orthogonal cutting technique involves setting the tool trajectory at a small angle (1 - 2')

to the free surface of the workpiece such that there is a gradual increase in the cutting force with tool path Once the cutting force exceeds a critical value, the chip formation zone fractures from the body of the workpiece

at a predefined (notched) location It is estimated that the deceleration of the tool is of the order of 5.106 m/s and that during the period of arrest, the tool travels no more than 4% of the undeformed chip thickness

The nonoverlap ping cutting technique developed during the course of this investigation is similar in principle to the planing arrangement used by Brinksmeier et al [5] in which the trajector of a round nosed tool is set at a very slight angle (0.006 ) to the flat surface of the workpiece Once the tool engages the workpiece, there is a gradual and continuous increase in both the depth of cut and the undeformed chip area In order to achieve higher cutting

IY

Cutting Cutting Speed, Feed, t pn Work material & Tool material &

Configuration vc, m/s Depth of cut, ap, pn hardness (HRC) geometry

f = 100 pn BS817M40, 52 A120fliC, a = 6',

ap = 200 pn

Turning &

Facing

Orthogonal f = 50, 100 pn BS817M40, 52 A120fliC, a = 6'

Cuttina Width of cut, b = 2 mm

Tool steel, 52, 58, 60 Tool steel, 45, 51, 54, 59

y=-22', r E = 1.2 mm

Y = -6' -22' -30' -40'

2.94, 4.17 '67' 3'33' 5'0

W C/8 %CO,

0

0.55, 1.42, h = 0 - 100 (cont.) BS817M40, 52 2.5, 4.7 b = l m m Tool steel, 45, 49, 56, 60 a = 6 ' , y = - 2 2 Quickstop

Non-overlapping 1.67, 3.33 h,,, = 0 - 100 (cont.) BS817M40, 39, 45, 48, 52, 54

Tool steel, 46, 51, 55, 60, 62

A120fliC, a = 6', y=-22', r E = 1.2 mm Table 2: The range of cutting conditions, work material hardness levels and tool materials and geometries for the different cutting tests undertaken (ha, denotes the maximum instantaneous undeformed chip thickness, see Figure 1) speeds in the current study, a nonoverlapping facing

arrangement was employed in which the tool follows a

spiral path with simultaneous feed in the axial direction

Figure 1 shows the manner in which the depth of cut, ap,

the undeformed chip area, A and the engagement length,

L, varies with tool path

3 CHIP FORMATION MECHANISMS

Hard steels are one class of materials from which

sawtooth chips are produced, Figure 2 (In addition to the

hardness of the steel, higher cutting speeds and depths of

cut favour the transition from continuous to sawtooth chip

formation [6], [?'I) Since this chip form was first identified

in the machining of Titanium alloys by Shaw in 1954 [8],

many researchers have sought to understand its

mechanism of formation

In brief, the different theories may be classified into those

which propose the cyclic instability to be based on the

initiation and propagation of cracks [7, 9, 10, 111, usually

from the free surface of the workpiece, and those which

are based on adiabatic shear initiation [6, 12, 131, a

thermoplastic instability common in materials of limited

strain hardening capacity when deformed at high strain

rates and/or to large plastic strains [3]

0.01

0.08

0.06

0.04

0.02

0

0 0.33 m 1 .o

Tool path, I,

1 .o

2

mm -

5

0.6 5

0.4 f

0)

c

C

0

0)

C

0.2 %

W

0

Figure 1 : The variation in depth of cut, undeformed chip

area (note cross section of undeformed chip area), and

engagement length in nonoverlapping cutting tests

While it may seem peculiar that such extremes in material

behaviour can be both considered of relevance for the

same process, it is worth noting that the crack theories of

chip formation are usually qualified by the suggestion that

subsequent sliding between the fracture surfaces results

in the formation of the white etching bands which run

along the underside of each 'tooth' in the chip Such

white etching bands are also characteristic of localised

adiabatic shear deformation and hence their presence

indicates little of their mechanism of formation

3.1 The Mechanisms of Sawtooth Chip Formation

The quickstop tests undertaken in the present study offer clear insight into the underlying metallurgical instability responsible for sawtooth chip formation The optical micrograph in Figure 3 shows a quickstop specimen produced from the low alloy tool steel of 56 HRC with a cutting speed of 85 m/min It is evident that cutting has been interrupted at a stage in the cyclic process when the incipient segment has undergone considerable upsetting (or compression deformation) Also, a thin white etching band is seen to extend from the tool tip partway along the primary shear zone That such a band did not form as a

Figure 2: Sawtooth (a) and continuous (b) chip types result of sliding friction across fracture surfaces, is evident from the fact that the cutting force at fracture is maximal (note the description of the quickstop technique in Section 2; see also reference [4]) If a crack had propagated down through the primary shear zone, a decrease in the cutting force would be expected Rather, this white band

is a localised deformation band formed as a result of adiabatic shearing Based on the fact that its thickness is greater at the tool tip, it is assumed that shear localisation initiated there and propagated towards the tool tip The absence of shear localisation in the upper region of the primary shear indicates that the material there is deforming under strain hardening conditions

While shear localisation contributes towards the destabilisation of the chip formation process, there is evidence to suggest that its occurrence does not guarantee failure within the primary shear zone Figure 4 shows a quickstop specimen produced from the low alloy tool steel of 60 HRC with a cutting speed of 33 m/rnin Several distinct white shear bands are evident along the (left) tool side of the chip

I

20 prn

Figure 3: Quickstop specimen obtained from the low alloy

tool steel of 56 HRC with cutting speed, v, = 1.42 m/s

Figure 4: Quickstop specimen obtained from the low alloy

tool steel of 60 HRC with cutting speed, v, = 0.55 m/s

The leftmost band, running along the edge of the

specimen was formed within the secondary shear zone

The remaining white bands were formed within the

primary shear zone, however, it is clear that only for every

second occurrence of shear localisation within the primary

shear zone, was a discrete segment formed For the

conditions under which the specimen in Figure 4 was

formed, it appears as though the decrease in shear stress

within the lower region of the primary shear zone (where

shear localisation occurs), was offset by an increase in

shear stress within the upper region of the shear zone

Under most cutting conditions, however, shear localisation

within the lower region of the primary shear zone is

sufficient to result in catastrophic failure over the whole of

the shear zone

Two mechanisms of failure have been observed within the

upper region of the primary shear zone during sawtooth

chip formation Under what may be described as severe

cutting conditions, such as higher work material hardness

and higher cutting speeds, ductile fracture occurs Figure

5(a) shows the underside of a chip segment produced in

the orthogonal cutting of the low alloy tool steel of 60 HRC

with cutting speed, v, = 3.3 m/s and feed, f = 100 pn

The dimple structure is typical of ductile fracture in which

voids nucleate, grow and coalesce [ I ] These dimple

structures were evident only near the tip of the segment

The underside of chip specimens produced from softer

steels at lower cutting speeds do not exhibit evidence of

ductile fracture The specimen shown in Figure 5(b)

reveals a structure which is similar to the finely spaced

lamellae on the free surface of continuous chips This

similarity suggests the mechanism of failure within the

upper region of the primary shear zone during sawtooth

chip formation (under moderate cutting conditions) is the

same as is operative during continuous chip formation

P

c

-

Figure 5: Mechanisms of failure in the upper region of the primary shear zone; (a) ductile fracture, (b) large strain thermoplastic deformation (See text for details)

3.2 Continuous Chip Formation

It is known that continuous chips do exhibit evidence of shear localisation on their free surface, albeit on a scale at least one order of magnitude less than that on sawtooth chips [14], [15] A more important distinction between the nature of the shear localisation on continuous and sawtooth chips is the relationship between the shear front spacings, D and d in Figure 2, and the undeformed chip thickness, h, On the free surface of continuous chips, the shear front spacing (equivalent to the lamella thickness) is largely independent of the undeformed chip thickness and

is typically 2 - 3 pn For sawtooth chips, the shear front spacing is similar to the value of the undeformed chip thickness; generally within +/- 50% [12]

Figure 6(a) shows the lamellae which characterise the free surface of most continuous chips; this particular chip was produced in the orthogonal cutting of the low alloy

tool steel of 45 HRC with cutting speed, v, = 1.67 m/s and

feed, f = 100 pn As the work material hardness and/or the values of cutting speed and undeformed chip thickness are increased, such that the onset of sawtooth chip formation is imminent, a transition in the structure of the free surface of the continuous chip occurs, from the lamellar structure, to what has been termed a 'fold'-type structure (This term is used as the structures are similar

to the folds formed in loose fabric) Figure 6(b) shows the folds on the free surface of a continuous chip produced in the orthogonal cutting of the low alloy tool steel of 49 HRC

with v, = 1.67 m/s and feed, f = 100 pn On comparison

to the underside of the sawtooth chip shown in Figure 5(b), it is clear that the mechanisms of failure in the upper region of the primary shear zone are the same for both chip types

Before discussing the mechanisms of fold formation, it is necessary to first consider the mechanisms of lamella formation When a lamellar-type continuous chip is sectioned, Figure 6(c), it is seen that the material has undergone fairly uniform shear strain as indicated by the reorientation of the fine martensite laths in a direction parallel to the shear stress (Note the random orientation

of the martensite laths in the as-tempered work material (45 HRC), Figure 6(e)) It is reasonable to assume therefore, that lamellae form as a result of cleavage due

to dislocation pileup and/or strain incompatibility at carbide or inclusion boundaries

In contrast to this fracture mechanism, Ramalingam and Black [I61 and Black [14], [I71 proposed a localised thermal softening model in order to account for Lamella formation The validity of such a mechanism is not disputed, however, it is thought that in the machining of hardened steels, it more accurately describes the formation of folds

Noting that the lamella to fold transition occurs with

increases in work material hardness and/or increases in

cutting speed and undeformed chip thickness, and that as

each of these parameters is increased, thermal softening

assumes a greater significance in the primary shear zone

(as indicated by the eventual initiation of adiabatic shear),

it is suggested that the formation of folds is due to very

localised thermal softening which suppresses cleavage

and the generation of new fracture surfaces Note, that in

Figure 6(d), even though a void nucleated at the large

carbide particle (in the right of the field), this does not

appear to have resulted In cleavage The distinction

between lamella and fold formation is illustrated in Figure

6(f) and 6(g)

Figure 6: Lamellae (a) and ‘folds’ (b) on the free surface of

continuous chips Sections through electroless Ni coated

lamellar-type chip (c) and fold-type chip (e) As-tempered

structure of the low alloy tool steel (45 HRC) Illustration of

the mechanisms of lamella (f) and fold (9) formation,

4

It is widely accepted that any physical phenomena which

results in the rapid release of elastic strain energy is a

potential source of acoustic emission (AE) Quantitative

models of AE in metal cutting processes where

continuous chips are produced, assume the energy of the

signal to be proportional to the square root of the

combined work rate of plastic deformation within the three

shear zones (Figure 2) and the work rate of sliding friction

over the rear of the tookhip contact length and on the

flank wear land [18], [19] The validity of such ‘continuous

chip’ AE models for the machining of hardened steels is

ACOUSTIC EMISSION AND CHIP MORPHOLOGY

discussed below When sawtooth chips are produced, an additional source of AE is operative

4.1 AE During Sawtooth Chip Formation

During the formation of a sawtooth chip, it is clear that the material ahead of the primary shear zone is periodically elastically strained; the instantaneous elastic strain energy being proportional to the integral of the product of the shear stress at the primary shear zone boundary and the elastic displacement of the shear zone boundary Within any given segment formation cycle, once shear localisation has destabilised the primary shear zone, failure occurs within the upper region of the primary shear zone, resulting in the release of the elastic strain energy

As the frequency of segment formation is typically greater than 10 kHz, it is beyond the response of conventional dynamometers Using a thin film sensor sandwiched between the insert and the shim of the toolholder, Davies

et al [I21 measured a periodic variation in the cutting force of the same frequency as segment formation, The release of strain energy is by far the dominant source

of AE when sawtooth chips are produced This is best illustrated in the data from the nonoverlapping cutting tests, Figure 7 Each of the five charts in Figure 7 corresponds to a workpiece of a particular hardness The

trace in each chart shows a median AERMS signal as a function of depth of cut; the grey envelope indicates the level of scatter (all traces acquired lay within this envelope) The inset charts have a reduced vertical scale

so as to more clearly show the data For each of these tests, the chip sample produced was examined in order to determine the approximate tool path after which the transition from continuous to sawtooth chip formation occurred For the low alloy tool steel with vc = 1.67 m/s,

sawtooth chips were produced only from the workpieces

of 59.6 and 61 HRC, at estimated depths of cut of 25 and

30 pn, respectively This clearly corresponds to the marked increases in AERMS at these points

In comparison to the AERMS during continuous chip formation, which is between 0.05 - 0.1 V, the AERMS produced during sawtooth chip formation is at least one order of magnitude greater

Figure 7: AERMS versus depth of cut in the nonoverlapping cutting of the low alloy tool steel with cutting speed, vc =

ranges from 0 - 100 pn

1.67 m/s In each of the five charts, the depth of cut

4.2 AE During Continuous Chip Formation

With regards to the sources of AE during continuous chip formation, the data in Figures 7(c - e) is insightful In each of these charts, the AERMS signal is seen to increase rapidly once the tool engages the workpiece In Figure7(c), following this initial rapid increase, the AERMS signal increases at a much lower rate for the remainder of the cut In Figure 7(d), the AERMS signal remains relatively

constant following the initial increase while in Figure 7(e),

the AERMS signal actually decreases with increasing depth

of cut before eventually levelling off Similar relationships

were noted for the BS817M40 steel

Noting that for each of the tests conducted, both the

cutting force and the thrust force increased near-linearly

with depth of cut, it is clear that the data in Figure 7(c - e)

do not obey many of the models developed to date for AE

in metal cutting If as suggested in references [18], [I91

that the plastic deformation and/or friction are the

dominant sources of AE in metal cutting, one would

expect a monotonic increase in AERMS with depth of cut

Clearly, the increasing depth of cut results in an increase

in the volume of material undergoing plastic deformation

and an increase in the area of sliding friction

One phenomena which may partly account for the

relationships evident in Figure 7(c - e) is that of lamella

formation on the free surface of the chip Just as failure

within the primary shear zone during sawtooth chip

formation results in the rapid release of elastic strain

energy, failure along the shear fronts between lamellae on

continuous chips also results in the release of strain

energy However, an important distinction here between

the two chip types relates to the extent to which the

localised failure propagates through the shear zone

It was noted above that the average thickness of the

lamellae on the free surface of continuous chips increases

with undeformed chip thickness, up to a limiting value of

2-3 pn, beyond which the lamella thickness remains fairly

constant, irrespective of depth of cut (see also reference

[14]) This behaviour suggests that cleavage cracks or

‘shear fronts’ between lamellae do not extend across the

thickness of the chip to the tool rake face, for chips

formed at depths of cut greater than approximately 10 pn

Shaw’s model of plastic deformation in the primary shear

zone [I51 is useful here; in particular the suggestion that

the normal stress over the shear zone is maximal in the

vicinity of the tool tip, monotonically decreasing to zero

where the shear zone meets the free surface of the

workpiece Near the free surface, cleavage cracks could

initiate due to the lower normal stress there, whereas the

propagation of such cracks towards the tool tip would be

impeded by the increasing normal stress

In the nonoverlapping cutting tests performed in this

study, a round nose tool was used such that there was a

continual variation in the undeformed chip thickness along

the engagement length; ranging from (nominally) zero at

the extremities of the engagement length to a maximum in

the middle (see Figure 1) If the cleavage cracks,

responsible for lamella formation, can only propagate a

finite distance through the shear zone, towards the tool

tip, it follows that only the cleavage cracks at the

extremities of the engagement length can propagate as

far as the tool rake face In the central region of the

engagement length, where the undeformed chip thickness

is greater, cleavage cracks would be rewelded due to the

higher normal stress nearer the tool tip

This theory may partly account for the relationships

between AERMS and depth of cut during continuous chip

formation It is suggested that cleavage during lamella

formation is a significant source of AE and that only when

the chip thickness is sufficiently small, do the cleavage

cracks extend to the tool face such that the elastic strain

energy released contributes to the measured AE For

thicker chips, it is thought that the elastic strain energy

released during cleavage is absorbed within the primary

shear zone While such a model cannot fully account for

the trends in Figure 7(c - e), it does provide a mechanism

which allows for a nonproportional relationship between AERMS and the energy dissipated in the shear zones

5

It will be evident from the discussion on sawtooth chip formation that adiabatic shear localisation is an important metallurgical process when cutting hardened steels The characteristic features generated by adiabatic shearing are the distinctive white bands observed in the primary and secondary shear zones The surface white layer is another such localised shear band

Thin foil specimens of machined surfaces were prepared for transmission electron microscopy (TEM) examination such that the viewing direction was normal to the machined surface The specimens were thinned to electron transparency using a single jet electropolishing unit The machined face of each specimen was protected from attack using a methanol reservoir Specimens of surfaces machined with unworn and worn cutting tools, in addition to specimens of the as-tempered steels, were examined in a TEM using an acceleration voltage of 200

kV (Reference to a worn cutting tool indicates the tool was used for a period of time equal to the average tool life, T, For the BS817M40 steel, T, = 712s, VBC = 255

pm, for the low alloy tool steel, T, = 791s, VBC = 125 pn) The structure of the surfaces of both steels, machined with unworn and worn cutting tools, consisted of very fine, misoriented cells The cell size generally ranging from 10

- 100 nm, however, for the BS817M40 steel machined with a worn tool, a number of coarser cells, up to 250 nm, were observed Apart from ascertaining the cell size, no other microstructural features of the white layers could be resolved in transmission mode In electron diffraction mode, considerably more detail could be resolved

Figure 8(a) shows an indexed selected area electron diffraction pattern from the as-tempered BS817M40 steel

of 52 HRC The selecting area aperture was of 1 pn projected diameter All diffraction patterns from both as- tempered steels were indexed as (cubic) martensite, a,

and cementite, 0

Figure 8(b) shows a diffraction pattern from a BS817M40 surface machined with an unworn cutting tool In this

THE STRUCTURE OF SURFACE WHITE LAYERS

Figure 8: Indexed diffraction patterns from an as- tempered BS817M40 steel (a) and BS817M40 surfaces machined with unworn (b) and worn (c) cutting tools

particular pattern, martensite/ferrite, austenite and

cementite reflections are present (Note, the continuous

rings are composed of many thousands of discrete

reflections; thus indicating a great number of reflecting

cells) It is noted, however, that the austenite reflections

are extremely faint and in a number of other patterns from

surfaces machined with unworn cutting tools, austenite

reflections were not present Also, in contrast to the

discrete cementite reflections in the as-tempered steel,

the cementite reflections from machined surfaces are in

the form of faint continuous rings This indicates a great

refinement in the size of the Iron carbide particles

Figure 8(c) shows a diffraction pattern from a BS817M40

surface machined with a worn cutting tool As for the

surface machined with an unworn tool, the pattern is

composed of discontinuous rings, but is considerably

spottier This indicates a much coarser cell structure

Also, the austenite (y) reflections are quite intense and

were noted in all patterns from surfaces machined with

worn cutting tools This clearly attests to an increase in

the volume of retained austenite with increased levels of

tool wear, in agreement with the findings of Tonshoff et al

[20] and Chou and Evans [21] In the low alloy tool steel

specimens, similar observations were made; namely, an

increase in the volume of austenite and a coarsening of

the cell structure with increased levels of tool wear Note,

the patterns in Figure 8(b) and 8(c) are typically of those

obtained from adiabatic shear bands

The increase in the volume of austenite in surfaces

machined with worn cutting tools is thought to arise from

the greater degree of completion of the reverse

transformation in the incipient surface during machining

In turn, this is most likely due to the increased energy

input to the incipient surface and the correspondingly

higher temperatures (Force measurements in orthogonal

cutting tests indicate the average shear and normal

stresses on the flank wear land to remain relatively

constant, irrespective of wear land width, the respective

values being, T~ = 400 MPa and os = 3 GPa)

6 CONCLUSIONS

The instability resulting in sawtooth chip formation is

adiabatic shear Prior to the onset of adiabatic shear, a

finer scale thermoplastic deformation mechanism is

evident This results in a transition in the free structure of

the continuous chip, from the familiar lamellar structure to

a 'fold'-type structure

For both chip types, a dominant source of AE is the

release of elastic strain energy, which in the case of the

sawtooth chip, arises from failure across the primary

shear zone, and in the case of the continuous chip, arises

from the fine cleavage cracks which define the shear

fronts between lamellae

The surface white layer is a nanocrystalline layer of ferrite

and austenite with extremely fine Iron carbides Both the

austenite content and the cell size increase with increased

levels of flank wear

7 ACKNOWLEDGEMENTS

This work was funded by Enterprise Ireland under their

Basic Research Grants Scheme and was undertaken

while Dr John Barry (co-author) was employed by the

Mechanical Engineering Department, University College

Dublin, Ireland

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