doi:10.1016/j.jmatprotec.2008.05.042
journal of materials processing technology 2 0 9 (2009) 24122420
journal homepage: www.elsevier.com/locate/jmatprotec Wear, cutting
forces and chip characteristics when dry turning ASTM Grade 2
austempered ductile ironwith PcBN cutting tools under nishing
conditionsK. Katuku , A. Koursaris, I. SigalasSchool of Chemical
and Metallurgical Engineering, University of the Witwatersrand, PO
Box 3, Wits 2050, Johannesburg, South Africaa r t i c l e i n f
o
a b s t r a c t Article history:Received 20 August 2007Received
in revised form19 May 2008Accepted 23 May 2008Keywords:ADI PcBN
Wear rateCutting forceShear localization
Experimental studies of wear, cutting forces and chip
characteristics when dry turning ASTM Grade 2 austempered ductile
iron (ADI) with polycrystalline cubic boron nitride (PcBN) cut-
ting tools under nishing conditions were carried out. A depth of
cut of 0.2 mm, a feed of0.05 mm/rev and cutting speeds ranging from
50 to 800 m/min were used. Flank wear and crater wear were the main
wear modes within this range of cutting speeds. Abrasion wear and
thermally activated wear were the main wear mechanisms. At cutting
speeds greater than 150 m/min, shear localization within the
primary and secondary shear zones of chips appeared to be the
key-phenomenon that controlled the wear rate, the static cutting
forces as well as the dynamic cutting forces. Cutting speeds
between 150 and 500 m/min were found to be optimum for the
production of workpieces with acceptable cutting tool life, ank
wear rate and lower dynamic cutting forces. 2008 Elsevier B.V. All
rights reserved.1. IntroductionThe automotive industry, which is
extremely competitive, is interested in austempered ductile iron
(ADI) because it offers properties similar to those of heat-treated
alloy steels. These include high strength, high hardness, excellent
toughness, high ductility, good fatigue properties and useful wear
char- acteristics at lower cost and reduced weight.Because of these
properties, ADI is difcult to machine in the austempered condition.
With regards to other engineering ductile cast irons, the
relatively high strength and hardness of ADI as well as the
inclination of its retained austenite to strain hardening lead to
short contact length and higher mechani- cal loads on the cutting
tools edge (Yamamoto et al., 1995). The relatively high ductility
of ADI favours its adhesion on the cutting tool and brings also
about higher temperature on Corresponding author. Tel.: +27 82 262
4035; fax: +27 11 435 3838. E-mail address: [email protected] (K.
Katuku).0924-0136/$ see front matter 2008 Elsevier B.V. All rights
reserved. doi:10.1016/j.jmatprotec.2008.05.042
the cutting tools edge (Klocke and Klpper, 2002). Because of
higher specic loads and higher temperatures that develop on the
cutting tools edge when machining ADI in its austem- pered
condition, cutting tools often suffer relatively high ank and
crater wears compared to hardened steels and other engi- neering
grey cast irons. The severe crater scar that develops very close to
the cutting tools edge exposes the latter to frac- ture damage
(Pashby et al., 1993). Of course, the higher cutting temperatures
as well as the relatively low thermal diffusivity and short contact
length of ADI could also expose the cutting tools edge to thermal
softening (Gekonde and Subramanian,1995).In these conditions,
cutting tools for machining ADI should fundamentally yield at the
same time: high hot hardness and strength, excellent hot chemical
inertia as well as high toughness. Such cutting tools are ideal and
do not rigorouslyspeaking exist in the present cutting technology.
However, they are the purpose of the continuous research undertaken
in the eld. Coating technology appears to be an alternative.
Nowadays, coatings on cutting tools are being used to improve the
tribological properties of cutting tools in this ideal way (Knotek
et al., 2001). Results show signicant improvements in some cases,
especially at relatively low cutting speeds. How- ever, the issue
of the mechanical stability (aking) of these coatings sometimes
reduces the expectations. Thus, it is a matter of nding a
compromise cutting tool material and or coating as well as optimum
machining parameters.Machining of ADI in its austempered condition
is highly desirable because it can yield the tight tolerances and
surface nishes generally required (Klocke et al., 2007), save
machin- ing time and thus reduce costs (Klocke and Klpper, 2002).
In depth fundamental understanding of interactions involved in this
particular machining of ADI in its austempered condition should
show the way to the optimum cutting tool material and or coating as
well as optimum machining parameters (productive cutting speeds and
feed rates, etc.). These out- comes would be among the last
obstacles to be overcome before intensifying the use of this
material in the automotive industry.This very complex issue
attracts great interest from the cutting tool industry since nearly
two decades. The compre- hensive research conducted hitherto on the
machinability of various grades of ADI is almost very little. It
has so far addressed few fundamental questions concerning the
cutting performance and wear mechanisms of various types of cutting
tools under various machining parameters and conditions.Pashby et
al. (1993) investigated on the wear of Al2 O3 , Al2 O3 TiC, Al2 O3
SiCw , and Si3 N4 Al2 O3 ceramic cutting tools when dry turning ADI
close to ASTM Grade 2 under condi- tions close to light roughing
(depth of cut: 2 mm; feed rate:0.18 mm/rev; cutting speed: 100450
m/min). They reported that ank wear was the main wear mode although
tool fracture occurred at the highest speed. Si3 N4 Al2 O3 ceramic
cutting tools suffered accelerated wear whereas Al2 O3 SiCw ceramic
cutting tools signicantly underperformed Al2 O3 and Al2 O3 TiC
ceramic cutting tools under most conditions. Fracture damage on the
tools cutting edge and chemical interaction between tool and
workpiece were identied as important wear mechanisms in controlling
tool life.Masuda et al. (1994) investigated on the cutting per-
formance and wear mechanism of P20 cemented carbide cutting tools,
Al2 O3 ZrO2 (5 wt.%), Al2 O3 ZrO2 (20 wt.%), Al2 O3 TiC (30 wt.%),
Al2 O3 SiCw ZrO2 , Al2 O3 SiCw TiC and Si3 N4 ceramic cutting tools
when dry turning ADI close to ASTM Grade 1 under conditions close
to light roughing (depth of cut: 1 mm; feed rate: 0.1 mm/rev;
cutting speed:50400 m/min). They reported that Al2 O3 TiC (30 wt.%)
inserts had the longest life at a low cutting speeds of about 100
m/min and less, and ZrO2 -toughened Al2 O3 inserts had a longer
tool life at cutting speeds of about 250 m/min or more. Al2 O3 SiCw
ZrO2 and Al2 O3 SiCw TiC ceramic cutting tools exhibited aking
fracture at 250 m/min whereas Si3 N4 ceramic cutting tools had no
wear resistance at all. Cemented carbide inserts had longer life at
very low cutting speeds. As cutting speed rose, the ank wear rate
increased slightly for Al2 O3 TiC (30 wt%) inserts. In contrast, it
decreased for ZrO2 -toughened
Al2 O3 inserts due to the monoclinic-to-tetragonal transforma-
tion of ZrO2 at high cutting temperatures.In order to elucidate the
mechanism of poor machinability of ADI, Yamamoto et al. (1995)
investigated on the turn- ing of ADI close to ASTM Grade 1 with Al2
O3 SiCw cutting tools under conditions close to light roughing
(depth of cut:1.5 mm; feed rate: 0.2 mm/rev; cutting speed: 6300
m/min). Their results showed at cutting speed lower than 36 m/min,
the strain-induced residual austenite to martensite transfor-
mation occurred in the chips as well as the damaged layer of the
machined surface. This strain-induced transformation was
responsible of the poor machinability of ADI. At higher cutting
speeds this strain-induced transformation occurred only in the
damaged layer of the machined surface and not in the chips.Wada et
al. (1998) investigated on the wear of coated cemented carbide
cutting tools, coated Al2 O3 ceramic cutting tools and coated Si3
N4 ceramic cutting tools in dry turn- ing of ADI close to ASTM
Grade 2 under conditions close to light roughing (depth of cut: 1
mm; feed rate: 0.2 and0.4 mm/rev; cutting speed: 30400 m/min). They
found that Ti(C,N)Al2 O3 TiN coated P10 carbide inserts had the
slowest ank wear progress with regard to TiCAl2 O3 TiN coated P20
and TiN coated K10 carbide inserts. TiN coated Al2 O3 ceramic
inserts had tool wear progress similar to Ti(C,N)Al2 O3 TiNcoated
P10 carbide inserts. Abrasive wear was observed on theank face of
TiN coated Al2 O3 and TiNAl2 O3 TiN coated Si3 N4 ceramic inserts
at relatively low cutting speeds. The ank wear of TiNAl2 O3 TiN
coated Si3 N4 ceramic inserts increased rather slowly at the high
feed rate of 0.4 mm/rev. On the other hand, the TiN coated Al2 O3
ceramic inserts had a tendency to fracture easily at this high feed
rate of 0.4 mm/rev.Klocke and Klpper (2002) investigated on the
turning of ADI close to ASTM Grade 1 with coated cemented carbide
cut- ting tools (Al2 O3 coated K10, Ti(C,N) coated K10, TiNAl2 O3
coated P15), Al2 O3 and Si3 N4 ceramic cutting tools under con-
ditions close to light roughing (depth of cut: 1 mm; feed rate:0.2
mm/rev; cutting speed: 120400 m/min) with and without cutting
lubricants. They pointed out that coated cemented car- bides
cutting tools could be successfully used in the range of low
cutting speeds. In the range of high cutting speeds, the use of Al2
O3 ceramic cutting tools was attractive. The perfor-mance of Si3 N4
ceramic cutting tools was very poor. Cuttinglubricants were very
effective in the reduction of the ank and crater wear scars of
cemented carbide tools, particularly at relatively high cutting
speeds.Goldberg et al. (2002) studied the dry interrupted facing of
an ASTM Grade 3 ADI with Al2 O3 TiC and Al2 O3 SiCw ceramic cutting
tools under conditions close to light roughing (depth of cut: 2 mm;
feed rate: 0.10.4 mm/rev; cutting speed:425 m/min) and nishing
(depth of cut: 0.5 mm, feed rate:0.10.4 mm/rev; cutting speed: 700
m/min). Their results indi- cated that Al2 O3 SiCw ceramic inserts
performed better than Al2 O3 TiC ceramic inserts both for rough
interrupted facing and nish interrupted facing at high cutting
speeds. The lack of overwhelming performance for Al2 O3 TiC ceramic
inserts in this very situation would be linked to their poor
thermal shock resistance. They reported that the tool wear
characteris- tic was exclusively ank wear which was a direct
consequence of adhesiveabrasive wear mechanism.There is strong
interest in extending the application of polycrystalline cubic
boron nitride (PcBN) cutting tools beyond the traditional machining
of hardened steels, ake graphite cast irons and steels produced by
powder metallurgy meth- ods (Chou et al., 2003). The machining of
ADI, at least under nishing conditions, is among the recent
prospects. Indeed, PcBN cutting tools appear to be an alternative
for the machin- ing of ADI at high cutting speeds and temperatures.
At these high cutting speeds and temperatures, cemented carbide
cut- ting tools do not maintain hardness, and Al2 O3 based cutting
tools lack to offer adequate toughness (Heath, 1989).In an earlier
investigation, Shintani et al. (1990) reported that form the
standpoints of surface roughness and tool life, cBN-TiC cutting
tools performed better than Al2 O3 TiC ceramic cutting tools for
the machining of ADI close to ASTM Grade 3 under nish
conditions.Kato et al. (1991) investigated on the wear performance
of PcBN cutting tool in the turning of ADI close to ASTM Grade 3
ADI under nishing conditions (depth of cut: 0.2 mm; feed rate: 0.05
mm/rev; cutting speed: 40300 m/min). They observed that from the
standpoints of surface roughness and ank wear rate, the optimum
cutting speed was 100 m/minaround which the cutting temperature was
827 C. They sug-gested that the cutting performance of PcBN cutting
tools was controlled by the size and volume fraction of cBN grains
as well as the thickness of the binder phase.The dependence of
cutting performance upon size and vol- ume fraction of cBN grains
was also corroborated by Goldberg et al. (2002) upon their study of
the dry interrupted facing of an ASTM Grade 3 ADI with PcBN cutting
tools.Klocke and Klpper (2002) investigated on the dry turn- ing of
ADI close to ASTM Grade 1 with PcBN cutting tools under conditions
close to light roughing (depth of cut: 1 mm;
These workpieces were characterized in terms of their hardness
and microstructure. The microstructure was rst studied with an
optical microscope followed by a more detailed examination in a
scanning electron microscope (SEM).
Dry nish turning experiments were designed so as to investigate
the wear (wear mode, wear mechanism, tool life, cutting length to
end of tool life, ratio of volume of metal removed per unit ank
wear, ank wear rate) of uncoated Seco CBN 100 PcBN cutting tools
when machining ADI workpieces at different cutting speeds,
according to the ISO Standard3685-1977(E) for single point turning
(International Standard,1977). In conformity with this standard,
the wear criterion used for all the machining experiments was 300 m
of maxi- mum ank wear.After a certain cutting distance (cutting
time), turning was stopped, the cutting tool insert removed from
the toolholder and the ank wear and crater wear scar morphologies
were assessed by means of microscopic examination on the optical
microscope. The maximum width of the ank wear scar was then
measured.The insert was then carefully replaced in the toolholder,
and the procedure repeated until the tool wear exceeded the
criterion. Experiments were repeated in order to measure the
cutting forces.Plots of maximum ank wear VBC ( m) against cutting
time t (s) as well as plots of volume of ank wear per unit of
engagement length V ( m2 ) against cutting length lc (m) were
produced.The volume of ank wear per unit of engagement length V
corresponding to the geometry of the cutting tools used in this
study has been shown to be (Barry and Byrne, 2001):2feed rate: 0.2
mm/rev; cutting speed: 160400 m/min). Theirresults pointed out that
the application of PcBN cutting tool was relatively acceptable for
cutting speeds in the range of
V = 0.05 (VBC )where VBC is the maximum width of the ank wear
scar.
(1)160200 m/min.Data accumulated so far on the machinability of
ADI in its austempered condition with PcBN cutting tools allowed
obtaining, although partially, fundamental understanding of
interactions involved. However, these database need to be extended
and consolidated in terms of tool life, wear rate, cutting forces,
surface nish and geometric accuracy of the machined components,
chip formation mechanisms, surface integrity of the machined
components, wear mechanisms of cutting tools, etc. with regards to
the recent improvements in the PcBN cutting tool processing
technology.This paper focuses on experimental studies of wear (wear
mode, wear mechanism, tool life, cutting length to end of tool
life, ratio of volume of metal removed per unit ank wear, ank wear
rate), cutting forces and chip characteristics when dry turning
ASTM Grade 2 ADI with PcBN cutting tools under nishing
conditions.2. Experimental proceduresThe ASTM Grade 2 ADI
workpieces used had the following chemical composition: 3.51% C,
2.61% Si, 0.19% Mn, 0.016% P,0.009% S, 0.002% Ni, 0.62% Cu, and
0.044% Mg.
Power regression was used to t the plots of VBC against t, to
estimate the cutting tool life corresponding to the wear criterion
and to determine the Taylor cutting tool life equation. The ank
wear rate used in this study was the slope of the plot of V against
lc .A tri-axial dynamometer mounted on the turrets lathe andcoupled
to a multi-channel amplier was used for the mea- surement of
cutting forces.The force signals acquired were analysed so as to
evalu- ate the static and dynamic cutting forces corresponding to a
time. The static cutting forces were estimated as the average of
the signals. The dynamic cutting forces were estimated as the
variation from the static cutting forces (Li and Low, 1994).Chips
were collected after each cut and examined visu- ally. Their
morphology and microstructure were investigated with an optical and
a scanning electron microscopes. The chip hardness was measured to
assess the interplay between strain hardening and thermal
softening.The morphology, microstructure, hardness and average
thickness of chips were investigated after mounting in cold resin
and metallographic preparation. Bright eld optical microscopy and
differential interference contrast (DIC) tech- niques were used.
The DIC technique was used to improve the contrast between the
primary and secondary shear zones.Transmission electron microscopy
was used to examine the nanostructure of the secondary shear
zone.Hardness tests of ADI workpieces were done on a ground surface
using a Leco V-100-A2 Vickers hardness-testing machine. Hardness
tests were done using a load of 30 kgf and a dwell time of 15 s.
The etching reagents used for the microstructural investigation of
workpieces were 3% Nital and4% Picral.Dry nish turning experiments
were carried out on a LA 200L Liouy-Hsing CNC Lathe rated at 14.72
kW. The uncoated Seco CBN 100 PcBN cutting tool inserts used
contained about 50% by volume of cBN, 2 m in grain size and TiC
binder (Seco, 2006). Their index specication was SNGN 090312 S
(nose radius1.2 mm, honed, chamfer 0.1 mm 20). They were mountedon
a toolholder described as CSDNN 2525M12C. The combina- tion of
insert and toolholder resulted in a rake angle of 26, a clearance
angle of 6 and an approach angle of 45.A Quartz 3-Component Kistler
Dynamometer Type 9257B was used for the measurement of cutting
forces. A multi- channel Kistler amplier Type 5070 A was coupled to
the dynamometer. Dynoware acquisition software Type 2825A-02Version
2.4.1.3 was used for data logging.The dry nish turning experiments
were carried out using a depth of cut of 0.2 mm which is within the
dimensions of the tool chamfer. The feed was constant at 0.05
mm/rev and cutting speeds ranged from 50 to 800 m/min. The sampling
rate of the analogical input force signals was 500 Hz. The force
signals acquired were analysed for a cutting time of 1 s.Chip
hardness measurements were made with a Leco M-400A microindentation
hardness-testing machine. A load of50 gf and a dwell time of 10 s
were used for chip hardness mea- surements. The average chip
thickness was measured using the image analysis software AnalySIS
5. Chip etching was car- ried out in 3% Nital.An Olympus BX41M
optical microscope coupled to an Olympus Camedia Camera was used to
measure the width of the ank wear scar and to obtain optical
micrographs. A Philips XL 30 ESEM-FEG XL series scanning electron
micro- scope was used to obtain SEM micrographs.Chip samples for
transmission electron microscopy were prepared with focused ion
beam (FIB) using 30 kV Ga ions in a dual beam Nova FIB with in situ
lift-out. A Philips 420 scan- ning transmission electron microscope
(STEM) using a LaB6 electron source at 120 kV was used for this
investigation. STEMimages were captured in bright eld mode.3.
Results3.1. Workpiece characterizationThe hardness of the ADI
workpieces was 312 HV30 . The principal micro-constituents were
ausferrite (ferrite needles and stringer-like retained austenite)
and islands of residual austenite and graphite (Fig. 1).Image
analysis of optical micrographs of unetched sam- ples gave a volume
fraction of graphite of 15%, a distribution of graphite particles
of 190 nodules/mm2 , a mean graphite nodule size of 30 m and a
graphite nodule spacing of72 m.
Fig. 1 Microstructure of the workpiece (ASTM Grade 2ADI).
Etched, Picral, SEM, secondary electron image.3.2. Tool wear and
cutting forcesDamage to the cutting tools over the entire range of
cutting speeds was mainly in the form of ank and crater wears.
Early formation of crater scar was noticeable at cutting speeds
greater than 150 m/min.The Taylor cutting tool life equation was
derived from the plot in Fig. 2 ast = 3 107 v1.9781 , with R2 =
0.9958 (2)where t is the tool life (s), and v the cutting speed
(m/min). R2is the regression coefcient.The ank wear rate and the
ratio of volume of metal removed per unit of ank wear are shown in
Fig. 3. The ratio of volume of metal removed per unit ank wear
decreased rapidly with increasing cutting speed up to a speed of
about300 m/min and more slowly with higher speeds. The ank wear
rate showed a rapid increase with increasing cutting speed up to a
speed of about 200 m/min. In the range of200550 m/min the ank wear
rate increased approximately linearly with cutting speed and more
rapidly for higher speeds.
Fig. 2 Effect of cutting speed on tool life of Seco CBN 100PcBN
cutting tools.
Fig. 3 Effect of cutting speed on ank wear rate and the ratio of
volume of metal removed per unit of ank wear of Seco CBN 100 PcBN
cutting tools.
Fig. 4 Effect of cutting speed on static cutting forces forSeco
CBN 100 PcBN cutting tools.The static and dynamic cutting forces
corresponding to a cutting time of 1 s are respectively shown in
Figs. 4 and 5.The static cutting forces were low. They increased
rapidly with increasing cutting speed up to a speed of about150
m/min. Between speeds of 150 and 200 m/min, they showed a dramatic
decrease. At speeds over 200 m/min, they increased at a slow rate.
For speeds of over 150 m/min, the static thrust force was higher
than the static tangential force.The dynamic cutting forces
decreased with increasing cut- ting speed up to a speed of about
200 m/min and increased at different rates at higher cutting
speeds. The dynamic thrust force increased at the highest rate.3.3.
Chip characteristicsBluing and chip oxidation started to be evident
at cutting speeds greater than 200 m/min. In fact, from 200 m/min,
chips
Fig. 5 Effect of cutting speed on dynamic cutting forces forSeco
CBN 100 PcBN cutting tools.started glowing in subdued light,
indicating a temperature of400 C or higher (Sizes, 2007).At a speed
of 50 m/min the surface nish of the workpiece was very poor and the
sliding surface of chips was very rough (Fig. 6). Some severely
strained material was evident on the sliding surface of chips as
was the case with the machined surface of the workpiece. The
heavily deformed material indi- cated excessive or erratic built up
edge (BUE) that periodically formed on the cutting edge resulting
in poor surface nish of the workpiece.At a cutting speed of 100
m/min the amount of strained material on the underside of the chip
decreased. However there always were big discontinuities in the
deformed material in areas occupied by graphite nodules.At speeds
of 150 m/min or more the streaks along the length of the sliding
side of chips, were smooth (Figs. 7 and 8). The extent of
discontinuities on the sliding side of chips decreased as the
cutting speed increased. Streaks and micro- pores were more evident
as the cutting speed increased as a result of softening and
probably partial melting due to sliding of the chip against the
rake face of the cutting tool (Farhat,2003).The chips were
continuous with occasional segmentation and highly coiled for
cutting speeds of up to 150 m/min. The deformation pattern revealed
that deformation occurred quite homogeneously in the entire chip.
The ow-pattern in the sec-
Fig. 6 Underside of chip obtained at 50 m/min. SEM, secondary
electron image.
Fig. 7 Underside of chip obtained at 150 m/min. SEM, secondary
electron image.
Fig. 10 Microstructure of a chip obtained at 700 m/min. SEM,
secondary electron image.
Fig. 8 Underside of chip obtained at 700 m/min. SEM, secondary
electron image.ondary shear zone, which showed the level of
shearing and frictional energy close to the toolchip interface, was
also vis- ible (Fig. 9).For cutting speeds between 150 and 200
m/min, the chip characteristics were intermediate between those of
continu- ous and segmented chips.At cutting speeds above 200 m/min,
the chips became more and more segmented and less coiled (Fig. 10).
The segmented
Fig. 9 Microstructure of a chip obtained at 150 m/min. SEM,
secondary electron image.
Fig. 11 STEM image of the secondary shear zone of chips obtained
at 800 m/min, bright eld image.chips consisted of individual
segments that were slightly deformed and joined by narrow heavily
strained bands. The high strain was localized essentially in the
primary and sec- ondary shear zones.Examination in the STEM of the
secondary shear zone in chips produced at a cutting speed of 800
m/min revealed nano-sized recrystallized grains (Fig. 11). The
massive defor- mation expected in the secondary shear zone was not
evident.Plots of average chip thickness and chip hardness against
cutting speed are shown in Fig. 12. It can be seen that the average
chip thickness decreased rapidly with increasing cut- ting speed up
to about 200 m/min and very slowly for higher speeds.Conversely
chip hardness increased for speeds of up to200 m/min and dropped
precipitously between speeds of 200 and 300 m/min. At higher
cutting speeds the chip hardness uctuated signicantly and did not
show a clear trend. These uctuations are probably the result of
actual variations in hardness within chips, due to variations in
the extent of strain hardening.
Fig. 12 Effect of cutting speed on average chip thickness and
hardness of ASTM Grade 2 ADI chips.4. Discussion of results4.1.
Tool wearThe value of R2 in Eq. (2) indicates good agreement
between the Taylor equation and the experimental tool life results
over the range of cutting speeds used.The absolute value of the
velocity exponent (1.9781) is much higher than 1 which is the
absolute value of this expo- nent in cases where abrasive wear is
the dominant wear mechanism (Arsecularatne et al., 2006). The
higher value of this exponent indicates that abrasion was not the
dominant wear mechanism throughout the range of cutting speeds
used.It is well known that in the Taylor equation, the cutting
speed inuences tool life through its effect on temperature
(Arsecularatne et al., 2006). It is then likely that thermally
acti- vated wear mechanisms were active over a wide interval of the
range of cutting speeds used.The plot of ank wear rate against
cutting speed (Fig. 3) showed signicant changes in slope for
cutting speeds of 200 and 600 m/min indicative of changes in the
dominant tool wear mechanism. The ank wear rate increased rapidly
for speeds of up to 200 m/min and slowly for speeds between 200 and
600 m/min. At speeds over 600 m/min there was a sub- stantial
increase in ank wear rate.When considering chip hardness and
average chip thick- ness (Fig. 12) interesting trends emerge with
regard to the interplay between strain hardening and thermal
softening. At speeds of up to 100 m/min chip hardness increased due
to strain hardening. The practically constant hardness of chips
produced at 100200 m/min was probably due to increased chip
temperature resulting in a balance between strain hard- ening and
recovery processes. At speeds between 200 and300 m/min, the
hardness dropped substantially probably due
to recrystallization in deformed chips. At speeds in excess
of300 m/min the hardness appeared to be constant indicating
recrystallization in the chips. This appears to be consistent with
the nearly constant chip thickness obtained at speeds of between
300 and 800 m/min (Fig. 12).When considering ank wear rate (Fig.
3), chip hardness and average chip thickness (Fig. 12) strong
indications emerge with regard to tool wear mechanisms.At speeds of
up to 200 m/min temperatures are low, the chip strain-hardens and
abrasion (a mechanical effect) would be expected to be the dominant
wear mechanism. At speeds between 200 and 300 m/min temperatures
are high and clear evidence of recrystallization emerges. Under
these conditions diffusion wear is expected to dominate. This
mechanism appears to remain dominant at speeds of up to about 600
m/min. The increased ank wear rate at speeds in excess of 600 m/min
can probably be attributed to further temperature increases which
facilitate diffusion but also accelerate oxidation which becomes a
signicant contributor to tool wear. Liquation at these high cutting
speeds can also be expected to enhance wear of the tool. Clearly
there are synergistic effects between the three phenomena.4.2.
Cutting forcesThe low feed (0.05 mm/rev), low depth of cut (0.2 mm)
and low contact length (contact area) explain the low values of
cutting force (Trent and Wright, 2000).Kinks on the curves of
static cutting force at cutting speeds lower than 150 m/min (Fig.
4) show the effect of the erratic or excessive BUE that probably
occurred. In fact, during the cutting operation, the BUE acts as an
extension of the cutting tool (Trent and Wright, 2000) and it
usually reduces abnormally the static cutting force by restricting
the contact of the chip with the cutting tool.The sudden drop in
static cutting forces between 150 and200 m/min could be the result
of the decrease in the contact area between the chip and the
cutting tool and of the thermal softening in the secondary shear
zone.Above 200 m/min the balance between the effects of strain
hardening and thermal softening resulted in a very slight increase
in static cutting force.For speeds of 150 m/min or more, the static
thrust force was bigger than the static tangential force because of
the largenegative rake angle (Poulachon and Moisan, 2000) (26)
andprobably due to a higher crater wear rate.At cutting speeds
lower than 150 m/min, the fragmentation of chips and the
instability of the BUE (formation and fracture) induced additional
dynamic cutting forces.The decrease in dynamic cutting force in the
range between50 and 200 m/min could be attributed to the decrease
in break- ing frequency of chips.Beyond 150 m/min chips were no
longer short but started to become segmented and the wear rate
increased signicantly. The segmentation of chips, the ank wear as
well as the crater wear imply an increase in dynamic cutting
force.Thus, in order to produce workpieces at lower dynamic cut-
ting force (better surface nish and dimensional accuracy), cutting
speeds in the range of 150500 m/min are indicated.4.3. Chip
morphologyThe gradual, continuous and asymptotic decrease in the
aver- age chip thickness (Fig. 12) was due to the gradual,
continuous and asymptotic increase of the shear angle that occurs
when the cutting speed increases. The increase in shear angle with
increased cutting speed is linked to the strain hardening in the
primary shear zone or according to Oxleys model, to the decrease of
ow stress (thermal softening) in the secondary shear zone
(Subramanian et al., 2002).The asymptotic decrease in average chip
thickness (Fig. 12) accords with the formation of segmented chips
(Barry and Byrne, 2002). Since ADI is a relatively ductile
material, the formation of segmented chips (Fig. 10) is related to
shear local- ization in the primary or/and secondary shear zones.
Shear localization was revealed metallographically in the primary
and secondary shear zones by the presence of shear bands whose
microstructure contrasted clearly with that of the chip segments
(Fig. 10).The nano-sized equiaxed grains that appeared in the sec-
ondary shear zone of chips at cutting speeds greater than150 m/min
(Fig. 11) were the result of dynamic recovery and recrystallization
that occurred in this zone. At cutting speeds greater than 150
m/min, shear bands in the secondary shear zone probably transformed
partially.The renement of the microstructure of the primary and
secondary shear zones may be expected to increase the dif- fusion
rates in the chip and enhance the diffusion of cutting tool
constituents into the chips.At cutting speeds greater than 150
m/min, friction or/and seizure at the toolchip interface gave rise
to high temper- ature that activated the onset of shear
localization in the secondary shear zone. The temperature in this
zone increased even further with the onset of shear localization.
The resul- tant high temperature which favoured transformation and
dynamic recrystallization in the secondary shear zone, also
favoured thermally activated wear on the crater face of the PcBN
cutting tools. This was evidenced by the early appear- ance of the
crater wear scar at cutting speeds greater than150 m/min.However,
shear localization in the secondary shear zone was nearly reduced
by the partial melting that occurred on the chip underside, through
its lubrication effect. In fact, the increase in average chip
thickness that should be favoured by the onset of shear
localization in the secondary shear zone (Subramanian et al., 1999)
was negligible above a cut- ting speed of 200 m/min (Fig. 12). This
conrms the formation of lubricating transfer layer that maintains
more or less unchanged the toolchip interface tribological
conditions and consequently a nearly constant average chip
thickness (Rech,2006).The partial melting that occurred on the chip
underside could be expected to increase the rate of thermally
activated wear of the crater face of the PcBN cutting tools.5.
Conclusions(1) Flank wear and crater wear were the main wear modes
for cutting speeds in the range 50800 m/min.
(2) The absolute value of the velocity exponent of the Tay- lor
cutting tool life equation suggested that abrasion wear and
thermally activated wear were the main wear mech- anisms. These
indications on tool wear mechanisms also emerged when considering
the ank wear rate, chip hard- ness and average chip thickness
curves.(3) At cutting speeds less than 150 m/min, abrasion wear was
the main wear mechanism. At these cutting speeds, the fragmentation
of chips and the instability of the BUE, con- trolled the dynamic
cutting forces.(4) At cutting speeds greater than 150 m/min, shear
local- ization within the primary and secondary shear zones of
chips appeared to be the key-phenomenon that con- trolled the wear
rate, the static cutting forces as well as the dynamic cutting
forces.(5) At cutting speeds greater than 150 m/min, the higher
tem- peratures subsequent to shear localization brought about
nano-sized equiaxed grains within the secondary shear zone, via
dynamic recovery, recrystallization and proba- bly partial phase
transformation. These nano-sized grains probably increased the
crater wear rate of the PcBN cutting tools via diffusion route.(6)
At cutting speeds greater than 150 m/min, the higher tem- peratures
subsequent to shear localization favoured the partial melting of
the chip underside. This partial melting could be expected to
increase the crater wear rate of the PcBN cutting tools.(7) At
cutting speeds greater than 600 m/min, the higher temperatures
subsequent to shear localization probably favoured the oxidation
wear of PcBN cutting tools.(8) Cutting speeds between 150 and 500
m/min were found to be optimum for the production of workpieces
with accept- able cutting tool life, ank wear rate and lower
dynamic cutting forces.AcknowledgementsThe authors would like to
express their thanks to the DST/NRF Centre of Excellence in strong
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