Advanced Materials Research Vols. 264-265 (2011) pp 1211-1221 Online available since 2011/Jun/30 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.264-265.1211
THE EFFECT OF TOOL EDGE
GEOMETRY
ON TOOL
PERFORMANCE
AND SURFACE INTEGRITY
IN TURNING TI-6AL-4V ALLOYS Yanuar Burhanuddin, Che Hassan Che Haron, Jaharah A. Ghani Department of Mechanical and Materials Engineering Faculty of Engineering and Built Environment Universiti Kebangsaan Malaysia Email : yanuarb@eng.ukm.my, chase@eng.ukm.my, jaharah@eng.ukm.my KEYWORDS: surface integrity, Ti–6Al–4V, titanium alloys, turning ABSTRACT This paper focuses on the influence of cutting tool edge preparation,
cutting speed and feed rate on the tool
performance
and
workpiece’s surface integrity in dry turning of Ti-6Al-4V alloy using PCBN inserts. The parameters evaluated are tool life, wear rate, wear mechanisms, surface roughness and subsurface microstructure alterations.
The rate of wear growth of the insert was assessed by progressive flank wear using optical microscope by taking photographs after certain length of cut. The wear mechanism at the end of tool life was investigated in detail using scanning electron microscope (SEM)
and EDAX analysis.
The
results show, by increasing the cutting speed and feed rate resulted in tool life reduction. Cutting with honed edge insert at cutting speed of 180 m/min has shown very little wear, even after 20 min of cutting. The honed insert proved less sensitive to increases in feed rate than the chamfered insert. In general the honed insert showed a significant improvement in tool life. All inserts failed due to attrition wear and adhesion. No flank notch wear was observed, but some crater wear was occurred behind the cutting edge. Microstructure alteration was not encountered when machining using the different edge geometry. In these trials the subsurface micro structural
deformation caused by machining
consists
of deformed grain boundaries in the direction of cutting
and
elongation of grains.
Chip smearing
and
debris on the
surface
was also found. The hardness in the vicinity of machined surface (at distance <0.15 mm) was lower than the hardness in the deeper machined surface (at distance > 0.15 mm). The layer softening was occurred rather than the hard white layer. The machined surface hardness was not affecting by different type of edge geometry.
1. INTRODUCTION Titanium alloys are used
extensively
in the aerospace industry
for structural components and as compressor blades, discs, casings, etc., in the cooler parts of gas turbine engines. They have also found use in such diverse areas as the energy and chemical processing industries, offshore and marine applications, the automotive industry, medical implants and sporting equipment. Titanium alloys have excellent strength-to-weight ratios and good elevated-temperature properties. Consequently when operating temperatures exceed 1308 oC, titanium can be used as an alternative to aluminium or, at higher temperatures still, titanium can be used as a lightweight alternative to nickel-based alloys or steel (Ezugwu et al. 2003, Boyer 1996). Titanium also exhibits a higher
resistance to corrosion than
either
aluminium alloys
or
low-alloy
steels;
thus
it is
usually preferred to
these materials
in
corrosive
aerospace applications
(Ezugwu 2003). Ti–6Al–4V is a general- purpose grade
alloy. It is by far the most important and widely used titanium alloy, accounting for about 60
per cent
of
total
titanium
production (Boyer 1996). It is unfortunate that the inherent properties that make titanium such an attractive engineering material also ensure that it is classed as ‘difficult to machine’, regardless of the parameters being used to measure machinability. The main tool failure criteria reported are rake and flank face wear
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0.165-21/11/12,03:23:27) resulting
from two wear mechanisms: dissolution–diffusion and attrition, the former
producing
smoothly worn surfaces
and
the latter irregularly worn surfaces
(Ezugwu & Wang 1997). One of the main properties of titanium alloys that lead to problems during machining is
low thermal conductivity (about 7 W/m K).
The
high
temperatures generated during cutting cannot be dissipated into the bulk of the workpiece and as a consequence they are localized at the tool tip. Indeed, Narutaki and Murakoshi (1983) have shown that the average tool interface temperature reaches about 1000 oC at a surface cutting speed of just 60 m/min, with approximately 1400 oC being recorded at a speed of 210 m/min. Hartung (1982) found that
most potential tool materials either rapidly dissolve in or chemically react with titanium
under these conditions, thus leading to the presence of a built-up layer and premature failure due to diffusion and/or attrition. This problem is further aggravated as a result of the thinner chips obtained during titanium machining, which produces a chip–tool contact area around one-half that of steel (Siekmann 1955). As a result, cutting forces are concentrated
at the
very
edge of the tool
where
the
maximum cutting
temperature is
generated, leading to rapid wear and plastic deformation of tools at high cutting speeds (von Turkovich & Durham 1982).
To attain higher cutting speeds, it is necessary to suppress the cutting
temperature
as much as
possible
while dissipating
heat
quickly
(Narutaki & Murakoshi 1983). As a solution to this problem, Hong & Zhao (1999) found that tool life could be improved by up to three times when liquid nitrogen was applied as a coolant instead of conventional cutting fluid when turning Ti–6Al–4V. The improvement was attributed to a reduction in tool–workpiece interface temperature and the subsequent maintenance of tool hardness and reduction in chemical reactivity of the titanium alloy. However, expensive modifications to the machine tool are required to utilize liquid nitrogen. Similar improvements in tool life were obtained by Ezugwu et al. (2005) when machining Ti–6Al–4V
using high-pressure
coolant,
which improved coolant penetration to the cutting interface
and reduced cutting
temperature
and diffusion wear rate. However, the coolant use will harm the operator and imposed on the high coolant treatment cost. Therefore, applying high temperature resistant cutting tools (e.g. CBN and PCD insert) when machining in dry condition is one of the alternatives to solve the problem. 2. EXPERIMENTAL SETUP 2.1 Tool The experimental work detailed in this paper was undertaken
to evaluate the performance of
two different
inserts when
turning
Ti–6Al–4V.
The first insert was a chamfered insert (ISO classification CNGA 120408S1020). The second was an insert with an edge honed (ISO classification CNGA 120408E). TABLE 1 Chemical compositions
of Ti-6Al-4V C Fe N O
H
Al V
Sn - 0.216 0.0024 0.160 0.0033 6.28 4.27 -
TABLE 2
Physical
properties of Ti-6Al-4V
Characteristics
Ti-6Al-4V
Melting point (K) Density (x103 kg/m3) Young’s modulus (MPa) Modulus of rigidity (MPa)
Poisson’s ratio Specific heat
(KJ
/(kg.K) Thermal conductivity (W/(m.K)
1813 – 1923 4.42 113,190 44,100 0.3 – 0.33 0.56 7.54 2.2 Workpiece materials and equipment The
workpiece material used was a Ti–6Al–4V bar with 150 mm diameter and
300
mm
long. The
chemical composition
of Ti–6Al–4V
was
shown in Table 1.
All
machining
trials were conducted using a Colchester T2 CNC Lathe. Samples of the machined workpiece were cut from the bar using a hand saw.
These samples were used for microstructural and microhardness evaluation.
Samples
were hot mounted in bakelite, ground using SiC paper and
then
polished
using
diamond grit. After polishing they were etched
by immersing
in
Kroll’s
reagent for
about
10 s. Microhardness measurements were
carried out
using a
Vickers
indenter at a load of
10
g for 15 s. One of the problems associated with microhardness measurements concerns,
was
its sensitivity to hard particles just below the workpiece surface. To overcome this, series of measurements were taken
to obtain
an average
value.
Subsurface microstructural analysis was conducted
using
a Leica optical microscope up to a maximum of
50 times
magnification.
Surface roughness
measurements
were carried out using Perthometer Mahr. Measurements were conducted number of times at various positions on the bar. Tool wear was measured with a toolmaker’s microscope. 2.3 Experimental procedure The machining trials were carried out according to
ISO 3685 for tool-life testing with single point cutting tools.
Testing was carried out on
a
previously turned workpiece material, cuts
for
between
30 s
and
1 minute
were taken depending
upon
the level of wear experienced. Tests were stopped when either the
average
flank wear or maximum notch wear reached 0.
30 mm
or 0.5 mm respectively.
All inserts were held in a
tool holder corresponding to
the PCLNR
configuration. Tests were carried out to investigate the effect of
insert preparation,
cutting speed, feed rate and tool
edge condition
on
tool life/wear
and workpiece surface integrity.
Samples
from the workpiece were taken
from machined surface when cutting using a worn tool to about 0.30
mm average flank wear;
by doing
this, the effects of both cutting parameters and the level of tool wear could be examined. The detailed
cutting parameters
and
test matrix are given in Table 3. Both inserts tested were high content PCBN (grade KD081) with a -5o rake angle, 5o clearance angle, 85o side cutting edge angle and a 0.8 mm nose radius. TABLE 3 Level designation of different cutting and tool parameters Level V F D (mm) Edge Geo. (m/min) (mm/rev) -1 (Low) 180 0.05 0.1 Chamfered 1 (High) 280 0.25 0.5 Honed
3. RESULTS AND DISCUSSION 3.1 Tool life
and
wear
rate The flank wear progression curves for two different edge geometries of PCBN cutting tools when turning Ti-6Al-4V alloy is shown in Figure 1. For the test
at cutting speed of
180
m/min, feed rate of 0.05 mm/rev, depth
cut
of
0.1 mm and honed shape geometry the flank wear less than 0.2 mm even though the tool life has reached 20 minutes. The cutting test is stopped if one of the criteria has been fulfilled such as the cutting time elapsed more than 20 minutes, even though with the flank wear is less than 0.2 mm. The statistical ANOVA confirms that
cutting speed
had
the
largest
influence on tool
life and
followed by feed and depth of cut
as commonly reported by other researchers (Ezugwu 2003 et al., Che-Haron 2001, Nabhani 2001, Kahles et al. 1985). The curve seemed horizontal after the cutting elapsed time of 1.66 minutes
at cutting speed of
180
m/min, feed rate of 0.05 mm/rev, depth
cut
of
0.5 mm and honed edge. For both cutting speeds, an increase in feed rate resulted in reducing tool life for both inserts, therefore the chamfered insert proved far more sensitive to the parameters change. 3.2 Wear mechanism The wear mechanism for two different edge of PCBN cutting tool and two different CBN content are similar. Tools wear occurred mostly by adhesion and diffusion. The adherent layer was seen
welded to the tool surface on both the flank and the
crater
faces due to
the
high pressures and temperatures developed during cutting. This demonstrates
the
high chemical affinity of titanium
which
is
similarly reported by
other
researchers (Ezugwu 2003 et al., Ezugwu & Wang 1997, Yang & Liu 1999, Bhaumik et al 1995). However the attrition and adhesion occurred more often in both chamfered edge and honed edge (Figure 4). These mechanisms occurred especially at low feed rate and depth of cut. The dissolution-diffusion and catastrophic failure were occurred at high
feed rate and depth of cut
(Figures 5, 6).
At
the higher
cutting speed,
it may have been expected that the honed tool edge geometry would plastically deformed. Furthermore, the behaviors of chip formed at an accelerated rate were due to localized temperatures and stresses at the tool tip were not observed but surprisingly the honed inserts showed catastrophic fracture at
0.25 mm/rev feed
rate
and 0.5 mm depth of cut.
Figure 4 also show the Ti-6Al-4V smeared the tool rake face. The splashed titanium occurred when the burnt chips stuck both on the tool and workpiece during machining. The thin chip like a paper is easily ignited. The burnt chip during machining is unwanted because endangered the operator. The SEM micrograph and EDAX analysis (Figure 3) proved the splash occurrence with the existing of titanium and aluminum element. No evidence of notch wear
was observed on the flank face
of the inserts
as shown in
Figures 4, 5 and 6. 3.3 Surface Roughness The machined surface roughness values using two different edge PCBN tools are quite higher than using two different contents PCBN tools. At the
cutting speed of
280
m/min, feed of 0.25 mm/rev, depth of cut of 0.
5
mm
and chamfered
tool
had
a
surface roughness value of 3.5 µm. The maximum surface roughness value occurred at the end of machining run. The severe failure caused the irregular surface at the nose of tool. The irregular and rough tool nose surface resulted the rough surface and the abruptly raising the roughness value. The variation of machined surface roughness at 180 m/min cutting speed was almost similar to the machined surface roughness at 280 m/min cutting speed. The surface roughness was more influenced by feed rather than cutting speed. This is in accordance with the theory which stated that the surface roughness was a function of squared feed (Shaw 1986, Stepehenson & Agapiou 1996, Kalpakjian 2001). While the surface roughness from the different edge geometry view
can be seen in
Figure 7.
The machined surface roughness
was influenced by
the
feed and depth of cut rather than edge geometry. There were no considerable roughness differences at the different feed when using the honed edge. 3.4 Microstructure alterations and surface hardness Microstructure alteration was not encountered when machining using the different edge geometry. In these trials, however,
the subsurface microstructural deformation caused by machining consisted of deformed grain boundaries in the direction of cutting and elongation of grains
(Fig 8).
These types of defect are typically reported during the machining of titanium alloys
(Zlatin & Field 1973, Narutaki & Murakoshi 1983, Che-Haron 2001, Kahles et al 1985, Ezugwu & Wang 1997). The deformed grain boundary in the workpiece is very similar to other work using two different CBN content tools (Burhanuddin et al. 2008) as shown in Figure 8. Although plastic deformation is a phenomenon usually associated with abusive machining conditions, in these trials no discernible pattern could be found. The depths of deformation of about 0.15 mm were found (Fig. 8). It is suggested that a heavily deformed material layer could be formed at high temperatures generated during cutting without substantially altering the adjacent microstructure. However, the deformed grain boundary was caused by the excessive compression of cutting tool to the workpiece and exceeds the elastic limit. Figure 8 shows the deformed grain boundary. The deformed grain was not uniform beneath the machined surface. It shows the work piece surface was not fully circular so that the excessive compression or pressure occurred at certain areas. Chip smearing and debris on the surface were also found (Fig. 8). Chips were pulled back into the cutting zone and left rewelded to the surface as debris. It is important to note that no evidence of microcracks running in the bulk of the workpiece was observed in any of the operating parameters. Based on the Vickers hardness test to the machined surface samples there are some inferences should be considered. First, the hardness in the vicinity of machined surface ( <0.15 mm) less than the hardness in the deeper machined surface (> 0.15 mm). It showed that the softening layer was occurred rather than the hard white layer. A softer surface was obtained due to the ‘overageing’ of the titanium as a result of the very high cutting temperatures developed. Similar results have been reported for ground Ti–6Al–4V samples (Zlatin & Field 1973). Secondly, the effect of edge geometry difference was not affected the machined surface hardness as shown in Table 3. The hardness of 4B sample number (cutting condition: v = 180
m/min, f = 0.
05
mm/rev, d = 0.
1
mm
and honed edge)
is
relatively equal to
the
7B sample number (cutting condition:
v = 180 m/min, f = 0.
25
mm/rev, d = 0.5 mm
dan chamfered edge). Although a slight microhardness variation was found between the surfaces generated by the two tool types, the results only indicate a general trend and no conclusions can be deduced. The microhardness variation is not significant in any of the conditions examined. 4. CONCLUSIONS
An increase in cutting speed and feed rate
resulted in
tool life
reduction. Little wear was observed after 20 min of cutting using honed edge insert at cutting speed of 180 m/min. The honed insert proved less sensitive in increasing the feed rate than the chamfered insert. In general the honed insert showed a significant improvement in tool life compared to chamfered insert. All inserts failed due to attrition and adhesion wear; no flank notch wear was observed but some crater wear was observed behind the cutting edge. Microstructure alteration was not encountered beneath the machined surface when machining using the different edge geometry. However, in these trials, the
subsurface microstructural deformation caused by machining
consists
of deformed grain boundaries in the direction of cutting
and
elongation of grains.
Chip smearing
and
debris on the
surface
were also found. The hardness in the vicinity of machined surface ( <0.15 mm) less than the hardness in the deeper machined surface (> 0.15 mm). The layer softening was occurred rather than the hard white layer. The edge geometry difference was not very affecting to the increase of machined surface hardness ACKNOWLEDGEMENTS
The authors would like to thank the Malaysian Ministry of Science, Technology and Environment for sponsoring this work under project IRPA 03-02-02-
0062-EA122.
REFERENCES
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m/min;0.
05
mm/rev;
0.1
mm;
Chamfered
180 m/min;0.
25
mm/rev;
0.1
mm;
Chamfered 0.5 280 m/min;0.25 mm/rev;0.1 mm;Honed Flank Wear VB (mm) 180
m/min;0.05 mm/rev;0.5 mm;
Chamfered 0.4 280
m/min;0.
05
mm/rev;
0
.5 mm;
Honed
180 m/min;0.
25
mm/rev;
0.5
mm;
Honed 280
m/min;0.
25
mm/rev;0.5 mm;
Chamfered 0.3 0.2 0.1 0 0 200 400 600 800 1000 1200 Time (s) FIGURE 1 Tool wear progression curves 0.01400 0.01200 Wear Rate (mm/sec) 0.01000 0.00800 0.00600 0.00400 0.00200 0.00000 Honed 0.1 0.05 180.0 Honed 0.5 0.05 280.0 Honed 0.5 0.25 180.0 Honed 0.1 0.25 280.0 Chamfered 0.1 0.25 180.0 Chamfered 0.1 0.05 280.0 Chamfered 0.5 0.05 180.0 Chamfered 0.5 0.25 280.0 FIGURE 2 Tool wear rate FIGURE 3 SEM micrograph of honed-edge PCBN cutting tools after machining
at cutting speed of
280
m/min, feed of 0.
25
mm/rev and depth of cut of 0.
1
mm.
FIGURE 4 EDAX analysis of honed-edge PCBN cutting tools after machining
at cutting speed of
280
m/min, feed of 0.
25
mm/rev and depth of cut of 0.
1
mm.
FIGURE 5 SEM micrograph of honed-edge PCBN cutting tools after machining at
cutting speed of
280
m/min, feed of 0.05 mm/rev and depth of cut of 0..5 mm.
FIGURE 6 SEM micrograph of honed-edge PCBN cutting tools after machining at
cutting speed of
180
m/min, feed of 0.25 mm/rev and depth of cut of 0.5 mm.
2.00 1.80 Mean
Average Roughness ( µm) 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00
1.84 1.65 1.62 0.99 0.56 0.30 c 0.47 0.29 Honed Honed Honed Honed Chamfered Chamfered Chamfered Chamfered 0.1 0.5 0.5 0.1 0.5 0.1 0.1 0.5 0.05 0.25 0.05 0.25 0.05 0.25 0.05 0.25 180.0 180.0 280.0 280.0 180.0 180.0 280.0 280.0 FIGURE 7 Machined surface roughness variation FIGURE 8 SEM micrograph of deformed grain boundary in the cutting direction when machining using honed edge PCBN tool.
Advances in Materials and Processing Technologies II 10.4028/www.scientific.net/AMR.264-265 The Effect of
Tool Edge Geometry
on
Tool Performance and Surface Integrity in Turning Ti-6Al-4V Alloys 10.4028/www.scientific.net/AMR.264-265.1211 1212 Advances in Materials and Processing Technologies II Advanced Materials Research Vols. 264-265 1213 1214 Advances in Materials and Processing Technologies II Advanced Materials Research Vols. 264-265 1215 1216 Advances in Materials and Processing Technologies II Advanced Materials Research Vols. 264-265 1217 1218 Advances in Materials and Processing Technologies II Advanced Materials Research Vols. 264-265 1219 1220 Advances in Materials and Processing Technologies II Advanced Materials Research Vols. 264-265 1221
