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 Journal of Materials Processing Technology 209 (2009) 4385–4389

Contents lists available at ScienceDirect

 Journal of Materials Processing Technology

 j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

A comparative study of the cutting forces in high speed machining of Ti–6Al–4V

and Inconel 718 with a round cutting edge tool

N. Fang∗, Q. Wu

College of Engineering, Utah State University, Logan, UT 84322-6000, USA

a r t i c l e i n f o

 Article history:

Accepted 5 October 2008

Keywords:

High speed machining

Ti–6Al–4V

Inconel 718

Cutting forces

Round cutting edge tool

a b s t r a c t

Titanium alloy Ti–6Al–4V and nickel-based superalloy Inconel 718 have been widely employed in mod-

ern manufacturing. The published literature on high speed machining (HSM) of the two materials often

involves different machining set-up, which makes it difficult to directly apply the research findings from

one materialto theother to select themost appropriate tool geometry and cutting conditions. A compar-

ative experimental study of HSMof Ti–6Al–4Vand Inconel 718 is conducted in this paper using thesame

machining set-up. The scope of this study is limited in high speed finish machining, where the tool edge

geometry plays a significant role. The experimental set-up and the methods of measuring the cutting

forces and the tool edge radius are introduced. A total of 40 orthogonal high speed tube-cutting tests

were performed, involving five levels of cutting speeds and four levels of feed rates. Based on extensive

experimental data, the similarities and differences between HSM of Ti–6Al–4V and Inconel 718 are quan-

titatively compared and qualitatively explained in terms of four quantities: (1) the cutting force  F c , (2)

the thrust force F t , (3) the resultant force R, and (4) the force ratio F c /F t . A total of 12 empirical regression

relationships are obtained.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Among the most effective and efficient modern manufacturing

technologies, high speed machining (HSM) is employed to increase

productivity while simultaneously improving product quality and

reducing manufacturing costs. Depending on work and tool mate-

rials as well as tool life requirements, the cutting speed used in

HSM is often 2–50 times higher than those employed in traditional

(relatively low speed) machining. Due to its high material removal

rate and short product cycle time, HSM has received steadily grow-

ing applications in recent years in many industrial sectors, such

as defense, aerospace, aircraft, automotive, and die- and mould-

making.

Research on HSMinvolves a wide variety ofworkmaterials rang-

ing from easy-to-cut aluminum alloys (Schulz et al., 2001; Siemset al., 2000) to difficult-to-cut hardened steels (Quan et al., 2004;

Behrens et al., 2004) and advanced aerospace materials (Ezugwu

and Bonney,2003). Among advanced aerospacematerials, two have

been extensively studied: titanium alloy Ti–6Al–4V (Su etal.,2006;

Molinari et al., 2002; Baker et al., 2002; Komanduri and Hou, 2002;

Barry etal.,2001;Bayoumi andXie,1995) and nickel-based superal-

loyInconel718 (Nalbant et al., 2007; Ezugwuet al., 2005; Dudzinski

∗ Corresponding author . Tel.: +1 4357972948; fax: +1 4357972567.

E-mail address: nfang@engineering.usu.edu (N. Fang).

andDevillez,2004; Coelho et al.,2004; Narutakiand Yamane, 1993).

Due to their exceptionally high strength-to-weight ratio, excellent

mechanical properties (especially high temperature performance),

and superior corrosion resistance, Ti–6Al–4V and Inconel 718 have

receivedgrowingapplications in making critical parts, components,

and structures. For example, titanium alloy components make up

20–30% of the dry weight in a jet engine. However, because of 

their high strength and low thermal conductivity (Nabhani, 2001;

Arunachalam and Mannan, 2000), HSMof these two materialsoften

cause numerous problems in modern manufacturing.

The publishedliteratureundoubtedlyadvances thefundamental

understanding of various aspects of HSM processes. However, lit-

tle literature is available to compare HSM of Ti–6Al–4V and Inconel

718 while keeping all the other machining set-up (such as the cut-

ting conditions, toolgeometry,and toolmaterial) the same.The vastmajority of the published literature focuses on either Ti–6Al–4V (Su

et al., 2006; Molinari et al., 2002; Baker et al., 2002; Komanduri and

Hou, 2002; Barry etal., 2001; Bayoumi andXie, 1995) or Inconel 718

(Nalbant et al., 2007; Ezugwu et al., 2005; Dudzinski and Devillez,

2004; Coelho et al., 2004; Narutaki and Yamane, 1993) alone, often

involving different machining set-up. The research findings from

HSM of one material might not be directly applicable to HSM of the

other material. A comparative study that uses the same machin-

ing set-up not only improves the scientific understanding of the

effect of different aerospace materials in HSM, but also makes it

feasible to extend the research findings from one material to the

0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmatprotec.2008.10.013

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otherso as toselectthe most appropriate tool geometryand cutting

conditions.

Moreover, HSM is significantly different from traditional

machining in many aspects, such as the mechanism of chip for-

mation and the generation of cutting forces and temperatures. The

tool cutting edge in HSM is subjectedto extremely high mechanical

and thermal loads. The failure of the tool edge (such as severe plas-

tic deformation, severe wear, and breakage) often results in early

tool failure. Compared to extensive study on the tool edge prepara-

tion in traditional machining andothermachining processes (Liu et

al., 2007; Fang and Wu, 2005; Fang, 2003; Shirakashi and Obikawa,

1998; Roth and Oxley, 1972; Usui and Hoshi, 1963; Albrecht, 1960),

research on how the tool cutting edge affects HSM processes lacks

largely behind.

To address the two above-described issues, this paper conducts

a comparative study of HSM of both Ti–6Al–4V and Inconel 718,

while simultaneously taking into account the effect of tool edge

geometry as well. The scope of the study is limited in high speed

finish machining, where small feed ratesthat areon thesame order

of magnitude as the tool edge dimension are commonly used. As

the first step of our ongoing systematic study, the cutting forces

are employed in this paper as the major criterion for compari-

son. The cutting forces significantly affectthe cutting temperatures,

tool wear and tool life, machining dynamics, the machined surfaceintegrity, and so on.

An experimentalapproach is taken in this study. First,the exper-

imental set-upand themethodsof measuringthe cutting forcesand

the tool edge radius are introduced. Then, similarities and differ-

encesbetweenHSM of Ti–6Al–4Vand Inconel 718 arequantitatively

compared and qualitatively explained in terms of four force-related

quantities: (1) the cutting force  F c , (2) the thrust force F t , (3) the

resultant force R, and (4) the ratio  F c /F t  of the cutting force to the

thrust force. Next, a total of 12 empirical regression relationships

are obtained using extensive experimental data. Finally, the major

research findings made from this study are summarized.

2. Experimental set-up and methods of measurements

 2.1. Experimental set-up

Table 1 summarizesthe experimentalset-up, where a totalof 40

orthogonal high speed tube-cutting experiments were performed

on a CNC turning center (HAAS SL-10). The cutting experiments

involvedTi–6Al–4V and Inconel 718, coated carbide tools, five levels

of cutting speeds, and four levels of feed rates. No cutting fluids or

coolants were employed in order to facilitate the collection of the

cutting force data.

As shown in   Table 1, the employed cutting speeds

(58–174 m/min) were at least two times higher than those

used in traditional machining of the two work materials. Small

feed rates (0.075–0.12 mm/r) comparable to the magnitude of tool

 Table 1

Experimental set-up.

Category Value

Work material (tube) Ti–6Al–4V and Inconel 718

Tube outer diameter 50 mm

Tu be wall thickness 1. 4 mm fo r Ti–6Al–4V

1.2mm for Inconel 718

Tool insert TPG 432 (Kennametal Inc.)

To ol mater ial Ce me nte d car bide (KC 80 50 ) with TiC/TiN/ TiCN

coating

Tool working rake angle 5◦

Tool edge radius 0.06 mm

Cutting speed 58, 87, 116, 144, 174 m/min

Feed rate 0.075, 0.09, 0.105, 0.12 mm/r

 Table 2

Chemical composition (%) of Ti–6Al–4V.

Element % Element %

C <0.08 V 3.5–4.5

Al 5.5–6.75 N <0.05

Fe <0.4 H <0.01

V 3.5–4.5 O <0.2

Ti Balance

 Table 3

Chemical composition (%) of Inconel 718.

Element % Element %

C 0.08 Co 1.0

Ni 50–55 Al 0.2–0.8

Cr 17–21 Si 0.35

Nb 4.75–5.5 Mn 0.35

Mo 2.8–3.3 Cu 0.3

Ti 0.65–1.15 Fe Balance

edge radius (0.06mm) were used. All feed rates were at least 10

times smaller than the width of cut (i.e., the wall thickness of the

workpiece tube) to ensure plane-strain deformation conditions in

orthogonal cutting.Tables 2 and 3 show the chemical compositions of the two work

materials tested in this study. As seen, Inconel 718 contains a sig-

nificant amount of Ni and Cr.

 2.2. Methods of measurements

As shown in Fig. 1, the tool edge radius was measured using a

Mitutoyo type-SV602 fine contour measuring instrument, which

was equipped with a diamond stylus with the tip radius of 5m.

The tip radius of the diamond stylus was taken into account when

measuring the tool edge geometry. The portion of the tool edge

that had the most uniform distribution of edge radius (which was

0.06 mm) was employedin thecuttingtests.The tool edge geometry

was measured again after each cutting experiment to ensure thatno significant tool-edge wear (i.e., the wear of the tool cutting edge

before it is fully worn away (Wu and Fang, 2006)) had occurred, so

the experimental results were comparable.

The cutting forces were measured with a Kistler 9257B three-

component dynamometer, a Kistler 5814B1 multi-channel charge

amplifier, and a computer data acquisition system (Labview). The

measurement system frequency was far more than two times than

the frequency of the cutting forces. The sampling frequency was

1 kHz. An analog anti-alias filter was used. After the force signals

were collected from the dynamometer, MATLAB was employed to

Fig. 1.   The measurement of tool-edge radius.

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filter the high-frequency noise signal. The digital filter employed in

MATLAB was Butter (1, 0.015).

For eachwork material, 20 cutting experiments were conducted.

Most cutting tests were repeated three times, with the variation of 

measurements generally within 5% range. The average value of the

cutting force measurements was taken as the experimental data.

3. Comparative study of the experimental results

 3.1. Force components: the cutting and thrust forces

To make the results comparable, normalized cutting forces

(N/mm) – the cutting forces along an unit width of cut (w) – are

employed. Furthermore, the scales of axes in relevant figures are

kept the same when plotting relationships between relevant quan-

tities.

Figs. 2 and 3 show the variations of the normalized cutting force

F c /w and thrust force F t /w with the cutting conditions (i.e., the cut-

ting speed andthe feed rate)for each work material.From these two

figures, the similarities and differences in machining Ti–6Al–4Vand

Inconel 718 are observed and compared as follows:

(1) For both materials: either an increase in cutting speeds or adecrease in feed ratescauses a decrease in both thecutting force

and the thrust force. These varying trends are consistent with

what occurs in traditional (relatively low speed) machining.For

example, an increase in cutting speeds increases the cutting

temperatures and hence reduces the cutting forces.

(2) Under the same cutting conditions, the cutting force and the

thrust force in machining Inconel 718 are higher than those in

Fig. 2.  The cutting force vs. the cutting conditions: (a) Ti–6Al–4V and (b) Inconel

718.

Fig.3.   Thethrustforce vs.the cutting conditions: (a)Ti–6Al–4V and (b)Inconel718.

machining Ti–6Al–4V. This phenomenon is under expectation

because Inconel 718 has higher strength than does Ti–6Al–4V.

The typical shear strength is 860 MPa for Inconel 718 and550 MPa for Ti–6Al–4V (Ezugwu and Bonney, 2003).

(3) The variation of the thrust force with the feed rate is smaller

in machining Ti–6Al–4V than that in machining Inconel 718,

especially at the lower cutting speeds as clearly shown in Fig. 3.

This phenomenon can be explainedfromthe effect of tool edge

radius in machining as well as the interactions among tool edge

geometry, cutting conditions, and work material constitutive

behavior. Therecentwork (Fang, 2003) shows that thetooledge

radius significantly affects the thrust force. The cutting forces

can be analytically predicted based on the given tool geometry,

cutting conditions, and a work material constitutive model. The

quantitative predictive modeling of the cutting forces is beyond

the scope of this study.

 3.2. The resultant force

Fig. 4 shows the variation of the normalized resultant force R/w

with the cutting conditions for each work material. As seen from

the figure, the resultant force decreases with increasing cutting

speeds or with decreasing feed rates for both materials. However,

the resultant force in machining Inconel 718 is higher than that in

machining Ti–6Al–4V. Again, this is because Inconel 718 has higher

shear strength than does Ti–6Al–4V.

 3.3. The force ratio

The force ratio  F c /F t   determines the direction of the resultant

force and hence affects the machining vibrations, tool wear, and

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4388   N. Fang, Q. Wu / Journal of Materials Processing Technology 209 (2009) 4385–4389

Fig. 4.   The resultant force vs. the cutting conditions: (a) Ti–6Al–4V and (b) Inconel

718.

machined surface integrity. Fig. 5 shows the variation of the forceratio with the cutting conditions for each work material.

It can be seen from Fig. 5 that for both materials, the force ratio

increases as increasing cutting speeds or feed rates. This varying

trend is very consistent for both Ti–6Al–4V (Fig. 5a) and Inconel

718 (Fig. 5b) under the employed cutting conditions.

A particular observation made from Fig. 5a is that at the small

feedrate of 0.075mm when machiningTi–6Al–4V, thecutting force

ratio F c /F t  can be less than 1.0, which means the cutting force  F c  is

less than the thrust force  F t . This unique phenomenon is due to

the magnified effect of the tool edge radius (Fang, 2003) under the

small feed rate conditions. Work material properties also play a role

in affecting the value of  F c /F t .

4. Empirical regression relationships

4.1. Determination of the cutting forces

The experimental data included in Figs. 2–5 was further used to

generate a set of empirical regression equations to quantitatively

relate the cutting conditions to the four force-related quantities.

The generated regression equations are listed in the following para-

graphs, where the subscripts Ti64 and In718 represent Ti–6Al–4V

and Inconel 718, respectively.

The cutting force F c  can be determined as

F c   Ti64  = 103.52V c −0.155 f 0.784w   (1)

F c   In718

  = 103.81V c 

−0.153 f 0.894w   (2)

The thrust force F t  can be calculated as

F t   Ti64  = 103.02V c −0.257 f 0.127w   (3)

F t   In718  = 103.41V c −0.216 f 0.495w   (4)

The resultant force R can be calculated as

RTi64  = 103.44V c −0.202 f 0.483w   (5)

RIn718  = 103.80V c −0.175 f 0.746w   (6)

The force ratio F c /F t  can be determined as

F c   Ti64/F t   Ti64  = 100.508V c 0.101 f 0.657 (7)

F c   In718/F t   In718  = 100.394V c 0.0635 f 0.400 (8)

The R2 values (the coefficient of determination) for Eqs. (1)–(8)

are 0.977, 0.992, 0.810, 0.968, 0.930, 0.990, 0.897, 0.939, respec-

tively. These equations represent reasonably good fits with the

experimental data.

From the positive or negative exponents of the cutting speed

V c  and the feed rate  f  in the above equations, two common vary-

ing trends in machining the two materials can be found. The two

common trends, which are also in consistent with the observations

made from Figs. 2 to 5, are described as follows:

(1) As the cutting speed increases, the cutting force, the thrust

force, and the result force all decrease (Eqs. (1)–(6)); however,

the force ratio increases (Eqs. (7) and (8)).

(2) As the feed rate increases, all the four force-related quantities

increase (Eqs. (1)–(8)).

Fig. 5.   The force ratio vs. the cutting conditions: (a) Ti–6Al–4V and (b) Inconel 718.

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4.2. Comparison of the cutting forces

To quantitatively study how higher the cutting forces are in

machining Inconel 718 than those in machining Ti–6Al–4V, Eqs.

(1)–(8) are further employed to derive the following relationships:

F c   In718/F c   Ti64  = 100.29V c 0.002 f 0.110 (9)

F t   In718/F t   Ti64  = 100.39V c 0.041 f 0.368 (10)

RIn718/RTi64  = 100.36V c 0.027 f 0.0263 (11)

F c   In718/F t   In718

F c   Ti64/F t   Ti64= 10−0.114V c 

−0.0357 f −0.257 (12)

Eqs. (9)–(12) reveal that thecutting forces andthe force ratioare

governed not only by work materials, but also by the  interactions

among work materials, the cutting speed, and the feed rate.

5. Conclusions

A comparative experimental study of high speed machining of 

two major aerospace materials – titanium alloy Ti–6Al–4V and

Inconel 718 – has been performed. Based on extensive experimen-

tal data generated from 40 orthogonal high speed tube-cutting

tests that involved five levels of cutting speeds and four lev-

els of feed rates for each work material, the similarities and

differences in machining the two materials are summarized as

follows:

(1) For both materials: as the cutting speed increases, the cutting

force, the thrust force, andthe result force alldecrease;however,

the force ratio increases.

(2) For both materials: as the feed rate increases, the cutting force,

the thrust force, the result force, as well as the force ratio all

increase.

(3) Under the same cutting conditions, the cutting force and the

thrust force in machining Inconel 718 are higher than those in

machining Ti–6Al–4V.

(4) The variation of the thrust force with the feed rate is smallerin machining Ti–6Al–4V than that in machining Inconel 718,

especially at the lower cutting speeds.

In the final analysis, the cutting forces in machining Ti–6Al–4V

and Inconel 718 aregovernedby the interactions among work mate-

rials, tool geometry, and the cutting conditions.

 Acknowledgement

The support of thisstudy by the U.S. NationalScience Foundation

under Grant No. 0620792 is greatly appreciated.

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