Asian International Journal of Science and Technology in Production and Manufacturing (2008) Vol. 1, No.2, pp. 105-110
The Effects of
CBN
Cutting Tool
Grades
on
the
Tool
Life and
Wear
Mechanism When Dry Turning of Titanium Alloy Y. Burhanuddin, C.H. Che Haron, J.A. Ghani, A. K. Ariffin, G.A. Ibrahim, A. Yasir and N.H. El-Maghribi Deparment of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia 43600 Bangi, Selangor, Malaysia Abstract This study investigates the significant factors that affect the tool life, wear progression, and wear mode mechanism of the CBN cutting tool when turning Ti-6Al-4V. The effects of cutting variables are investigated by the application of partial factorial design method. The machining tests
were carried out under dry cutting
condition.
The cutting speeds selected were
180
and
280
m min-1. The
feed rates
were
0.
05
and 0.25 mm rev- 1.
The experiment used T-chamfered edge. Two types of CBN grade were used in the experiment: low and high content. The study found that both of
cutting speed and feed
were significantly affecting
the tool life. The
detailed study on worn out tool using SEM revealed that the wear were occurred
on both flank and rake faces of the
cutting edge. The wear mechanisms such as rubbing, abrasion, adhesion, diffusion-dissolution and fracture were observed. Keywords: Tool life, Fractional factorial, CBN, Dry turning, Titanium 6Al-4V, Wear mechanism, Chip formation 1 INTRODUCTION The use of titanium alloys is prevalent in most commercial and military aircrafts. Titanium is selected as materials of jet engine and airframe components due to the
high strength-to-weight ratio,
capable to withstand
the
strength at
the high
operating temperature and good corrosion resistance (Campbell, 2006). The
relatively high cost of titanium
machining
has hindered wider use, for example in automotive applications. To minimize the inherent cost problem, successful applications must take advantage of the special features and characteristics of titanium.
This requires a more complete understanding of titanium alloys as compared to other
competing
materials, including the interplay between cost, processing methods, and performance
(Lutjering and Williams, 2007). The machinability of titanium is limited by some characteristics. Some of these are low thermal conductivity, chemical reaction with the cutting tools materials, a relatively low modulus of elasticity (Ezugwu et al., 2003; Konig, 1979; Machado and Wallbank, 1990; Siekmann, 1955; Turkovich, 1982). Due to these characteristics, titanium is generally turned with uncoated carbide (Haron and Abdullah, 1999;
Jawaid et al., 2000),
CBN/PCBN (Brookes
et al.,
1991; Klocke
et al.,
1996; Narutaki et al., 1983) and Poly Crystalline Diamond (PCD) tools (Brookes et al., 1991).
Uncoated carbide tools are
suitable
at low speed machining conditions
while CBN/PCBN
are
employed at
high speed machining
The objective
of the
CBN
cutting tool
when turning study
is
to investigate the
tool
life and wear
mechanism
of
Titanium 6Al-4V.
The
study will investigate the wear progression, the significant factors that affect the CBN tool life and the failure mode of CBN cutting tool. Y. Burhanuddin et al. 2 DESIGN OF EXPERIMENT
Machinability is defined as the ease or
the
difficulty with which a material can be machined under a given set of
cutting
conditions including cutting speed, feed, and depth of cut.
Machinability of a work material
is accessed in terms of four factors: tool life, cutting forces, power requirements, and surface finish.
It is obtained through a set combination of machining tests between material and machining parameters. The common approach employed in manufacturing companies by many engineers last time
is One- Variable-At-a-Time (OVAT).
This approach will
vary one variable at a time
while
keeping all other variables in the experiment fixed. This
type of experimentation
requires large resources to obtain a limited amount of information about the process.
OVAT experiment often
is unreliable, inefficient, time consuming and
may
yield false optimum
condition
for the process
(Antony, 2003). In spite of using OVAT, the experimenters use a statistical design of experiment. The design of experiments is widely used in experiment involving several factors where it is necessary
to study the
combined
effect of
these
factors on
responses.
The
design of experiment can significantly reduce the total number of experiment. This study will consider the simultaneous variation of
speed, feed, depth of cut
and CBN content
on the tool life
as a response.
The
present study will analyze the effect of those parameters on tool life by using ANOVA. In addition to the effect of cutting parameters, the interaction among parameters also will be studied. A fractional factorial design at two- levels of half-fractions is used. A design has eight trials (Table 1).
Table 1: Level designation of different process variables Level V (m/min) F (mm/rev) D (mm)
CBN content
-1
(Low) 180 0.05 0.1 Low 1 (High) 280 0.25 0.5 High 3 EXPERIMENTAL DETAILS 3.1 Machine and cutting inserts
The machining trials were carried out on a
Colchester T2
CNC
Turning.
A
MCLNR 2020K09 tool holder was used to provide an 85o cutting edge angle and 5o rake angle. Two grades of CBN content investigated are KD081 and KD120 designated CNGA 120408S1020. All of the cutting experiments were conducted in dry condition. The machining was carried out at various time interval from 10 sec to 1 min and then
the flank wear of
the
insert was recorded. Flank
wear was
considered as the
main
criteria
of
tool failure. The
machining
was
terminated if the average flank wear exceed 0.30 mm or fatigue failure occured on the cutting edg. 3.2 Statistical Analysis of Tool Life Table 4 shows the results of tool life. The results were analysed using the Design-Expert 6.0 and transformed to half
-normal probability plot
as
in Fig
1.
The figure
shows
that the
all of the factors will affect the cutting tool life. The
feed rate is the most
affecting
factor
and
followed
subsequently
by
the cutting speed,
depth
of cut
and
CBN content. The
ANOVA was
performed
to
calculate
the main effects of cutting speed
(V),
feed rate (f), depth of cut (d)
and CBN content,
together with their
two
-level interaction effects on
the tool life.
The ANOVA output and the calculated F ratios are shown in Table
5
for each significant effect. The
5 per cent level
was used
for testing the significance of the main effects and
the
interaction.
Table 5 shows
the
“Prob>F” value of all factors is less than 0.05 and among these factors;
feed rate is the most significant.
Table 4: Experimental conditions and results
Run Cutting speed (m/min)
A
Feed rate (mm/rev)
B Cutting
depth (mm)
C CBN content D life (sec) F actors Tool 1 180 0.05 0.1 Low 1200 2 280 0.05 0.1 High 630 3 180 0.25 0.1 High 640 4 280 0.25 0.1 Low 60 5 180 0.05 0.5 High 1200 6 280 0.05 0.5 Low 150 7 180 0.25 05 Low 60 8 280 0.25 0.5 High 36
The Effects of
CBN
Cutting Tool
Grades
on
the
Tool
Life and
Wear
Mechanism When Dry Turning of Titanium Alloy 3.3 Tool wear mechanism Fig 2 shows flank wear progression curves of CBN tool at various cutting conditions. The curves are generally divided into three stages: the rapid initial progress, the relatively constant progress and the rapid-to-failure progress.
At the low speed and
low
depth of cut, the
tool worn out mostly rubbing and attrition. The wear occurred due to
the depth of cut
is less than
the tool nose radius.
At a depth of cut of 0.5 mm
this wear mechanism is not obvious. Figure 3a, 3b, 4a and 4b show clearly the rubbing and attrition wear mechanism. The wear progress tends to decrease when the cutting is continued for more than 200 seconds. The chip Figure 1: Half-normal probability plot of results flowing is partially stuck at the tool edge. The stuck chip is attributed to the compressive force and The two-factor interactions show a strong interaction increasing in temperature. exists between depth of cut and CBN content (CD). In dry environment, machining at the higher feed rate The tool life is long when the CBN content is kept at and/or at higher cutting speed will increase the high level and depth of cut is kept at low level. temperature rapidly. Whereas the tool life is short when the CBN content The increasing in temperature will induce the is kept at low level and depth of cut is kept at high titanium to weld to the tool and formed the built-up- level. This result agreeable with Ezugwu (2005) edge-like layer. The welded material will help the statement that high CBN content gives longer tool wear progression. It is very similar to built-up-edge lives because it can withstand the notch-wear. (BUE) formation mechanism. Table 5: ANOVA for selected factorial model The blunt tool and BUE will increase the cutting [Partial sum of squares] force. If the welded material to the tool receives the Source Sum of D Mean F Prob increased stress, eventually cause the tool to fracture Squares F Square Value > F and the CBN material to pluck out. Figure 4a and 4b Model 1,739,00 5 347,80 263.0 0.038 show the notch surface of CBN tool due to severe 0 0 5 failure. A 618,300 1 618,30 467.6 0.021 Despite of the above-mentioned mechanisms, the 0 8 crater wear was occurred on CBN cutting tool. The tendency of crater wear on titanium alloy is stronger B 710,400 1 710,40 537.3 0.001 because occurrence of the CBN is classified as the 0 9 9 cutting tool wears mainly by adhesion (Narutaki and C 146,900 1 146,90 111.1 0.008 Yamane, 1993) 0 1 9 The compressive stress is considered small when D 195,800 1 195,80 148.0 0.006 chip is flowing on tool rake face, even though the 0 9 7 friction forces exist. This caused chip to rub and CD 129,000 1 129,00 97.60 0.010 scrap the tool material. The temperature of chip 0 1 formation is more than 1000oC. At an elevated temperature, the titanium react more to certain tool Residu 2,644 2 1322 material. The combination both of friction force and al high temperature caused the material to be welded to Cor 1,741,00 7 the tool. The dissolution and diffusion phenomena Total 0 occurred at the welded cutting tool and material. The *5% level of significance dissolution-diffusion mechanism caused the crater formation in the chamfered edge land (Min and Y. Burhanuddin
et al. (a) (b) Figure 3:
SEM micrograph
of CBN tools
while machining
Ti-6Al-4V
at
cutting speed
180
m/min, feed rate 0.
05
mm/rev
(a)
depth of cut
0.1
mm,
(b)
depth of cut 0.5 mm. Flank wear,
VB (mm) 0.60 180-005-010-L 180-005-050-H 0.50 180-025-010-H 180-025-050-L 280-005-010-H 280-005-050-L 280-025-010-L 280-025-050-H 0.40 0.30 0.20 0.10 0.00 0 200 400 600 800 1000 1200 time (s) Figure 2: Flank wear curves at different machining conditions (a) (b) Figure 4: SEM micrograph of CBN tools while
machining Ti-6Al-4V at cutting
speed
280 m/min
and
feed rate 0.25 mm/
rev (a) at
depth of cut 0.
1
mm,
(b) at
depth of cut 0.5 mm
The Effects of
CBN
Cutting Tool
Grades
on
the
Tool
Life and
Wear
Mechanism When Dry Turning of Titanium Alloy Figure 5: EDAX analysis of CBN insert tool at 280
m/min, 0.25 mm/rev and
0.1
mm.
Youzhen 1988). Figure 4a and 4b show the crater obviously. EDAX analysis also support that the dissolution-diffusion mechanism occurred at the cutting edge and crater (Figure 5). 3.4 Metallurgical aspects
Figure 6:
SEM
micrograph of machined surface
Figure 6 shows the SEM micrograph of
machined
surface
at a cutting speed of
280
m/min, feed rate of 0.
05
mm/rev and depth of cut of 0.
1
mm.
The change of grain orientation occurred because of tool compressive force. The grain was elongated in cutting direction and caused the thinner shape. Even though the grain shape changed but the white layer does not exist in the machined surface. Therefore, both lower speed and higher speed can be applied in finish turning of Ti-6Al-4V. 4 CONCLUSIONS In this work, it has been shown that partial factorial design of experiments can be used in analyzing the effect of CBN cutting tool grades to
tool life when
turning Titanium
6Al-4V. The
design is very helpful in the running of expensive cutting tool-material combinations. The
feed rate
is
found to be the most significant
factor to tool life, followed by
cutting speed, depth of cut and
CBN grades. At
the
same
cutting
speed and feed rate, the lower CBN grade is more affected by changing the depth of cut. CBN tool shows three stages of wear progression: the rapid initial progress, the relatively constant progress and the rapid-to-failure progress. The wear mechanisms present were rubbing, abrasion, adhesion, diffusion-dissolution, and fracture. The cutting speed induced attrition wears, while fracture was due to high depth of cut. Whereas diffusion- dissolution was induced by increasing temperature during the turning operation. 5 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-
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