pujana2009 Analysis of ultrasonic-assisted drilling of Ti6Al4V

Ultrasonic assistance offered lower feed force and higher process temperatures as compared to conventional drilling.. Recently, ultrasonic vibration has been applied as a process assisti

Analysis of ultrasonic-assisted drilling of Ti6Al4V

J Pujanaa, A Riveroa, A Celayab, L.N Lo´pez de Lacalleb,

a

Fundacio ´n Fatronik, Paseo Mikeletegi 7, 20009 Donostia-San Sebastia ´n, Spain

b

Departamento de Ingenierı´a Meca ´nica de la Escuela Superior de Ingenieros de Bilbao, Universidad del Paı´s Vasco, Alameda de Urquijo s/n, E-48013 Bilbao, Spain

a r t i c l e i n f o

Article history:

Received 25 September 2008

Received in revised form

22 December 2008

Accepted 23 December 2008

Available online 20 January 2009

Keywords:

Ti6Al4V

Ultrasonic-assisted drilling

Cutting temperature

Machining

a b s t r a c t

In this study ultrasonic vibration was applied on the drilling of Ti6Al4V workpiece samples Several parameters of ultrasonic-assisted drilling were monitored, including feed force, chip formation by means of high-speed imaging, and temperature measurement on the drill tip by means of infrared radiation thermometry Ultrasonic assistance offered lower feed force and higher process temperatures

as compared to conventional drilling It has also shown higher force reductions and higher temperature increments when vibration amplitude was increased

&2009 Elsevier Ltd All rights reserved

1 Introduction

The widespread use of titanium alloys both in structural and

corrosion-resistant applications is well known There is growing

interest concerning the process ability of titanium alloys since

they exhibit a good compromise between density and yield

strength and also have good creep and fatigue resistance at mid

temperatures The Ti6Al4V alloy is inside thea+bphase alloys,

and it is most widely used among the different titanium alloys

employed in aerospace industry [1] Their characteristics allow

lightweight structures to be achieved at temperatures above

600 1C[2]

Nowadays, there has been a growing interest and tendency to

employ more friendly processing techniques In the machining

process, minimum use of lubricant and dry machining are

good solutions for reducing the wastage, but the lack or

reduction of cutting fluid tends to derive into problems associated

with heat generation and chip removal These problems become

more prominent when dealing with titanium Low thermal

conductivity and good thermal resistance make machining of

titanium, especially drilling[3,4] The fact being that temperature

is the major wear factor on coated tools being tested on dry

drilling experiments[5], the US-assisted drilling of Ti6Al4V alloy

is a prospective alternative to fluid-assisted cutting where

achieved temperatures will be lower than those achieved by

conventional drilling

The use of ultrasonic vibration in different manufacturing processes is well documented for more than 50 years [6] Ultrasonic machining has been mainly applied on brittle materi-als, and although removal rates are not high, ultrasonic technol-ogy suits very well this type of material Recently, ultrasonic vibration has been applied as a process assisting conventional machining operations (turning and drilling) instead of the vibro-impact regime of the ultrasonic movement being the main cutting mechanism This technique is called ultrasonic machining (USM)

or rotary ultrasonic machining (RUM) [7] Process assistance involves applying the ultrasonic technology in the machining of non-brittle and difficult-to-cut materials[8] Assisted ultrasonic machining has been proven to be an efficient technique for improving the machinability of several aeronautic materials such

as aluminum [9,10] or Inconel 718 [11] Chip breaking, burr generation, workpiece roughness, tool life or torque and cutting forces are some parameters studied with vibration applied in conventional cutting processes[12–14]

Although some researchers have observed chip fragmentation

in materials such as inconel[11,14] or aluminum [15,16] when ultrasonic vibration was applied in the drilling process, some others did not address the chip-breaking effect either in drilling

[8]or turning[17] However, the mechanism that produced chip segmentation has not been well explained Regarding chip segmentation and serrated chip formation, catastrophic shear failure and adiabatic shear forming mechanisms are considered the main causes[18] At this point, difficulties associated with the determination of appropriate constitutive equations[19,20]and the establishment of adequate failure modes [21] of titanium alloys in simpler laboratory and orthogonal cutting tests limit the

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International Journal of Machine Tools & Manufacture

0890-6955/$ - see front matter & 2009 Elsevier Ltd All rights reserved.



Corresponding author Tel./fax: +94 6014216.

E-mail address: implomal@bi.ehu.es (L.N Lo´pez de Lacalle).

understanding of more complex material behaviors encountered

in drilling operations

There might be two reasons for chip breaking when vibration is

superimposed on the drilling process The first one is purely

geometrical: due to the periodic nature of tool vibration and its

spin, chip breaking is dependent on tool vibration amplitude

calculated on the phase shift between vibratory motion and tool

spinning frequency Therefore, the vibration amplitude A which is

necessary to achieve segmented chips is given according to[22]

4A

f ¼

1

where A indicates vibration amplitude, f feed per revolution and

Wfthe number of vibration cycles per tool revolution.Fig 1shows

the chip-breaking area (above each set of points) and

non-chip-breaking area (below) and curves generated with feeds ranging

from 10 to 100mm have been drawn

In ultrasonic-assisted machining, it is almost necessary to work

in a resonant vibration state of the tool if high amplitudes have to

be achieved[23] When vibro-impact regimes are reached, there is

a non-linear type force acting on the tool and the system tends to

have unstable resonant states Auto-resonant control strategies

are used to tune phase shift in addition to frequency by means of a

closed-loop control system[24,25]

The second reason for segmented chip formation is the

strain–stress state of the material The analysis and study of chip

formation in Ti6Al4V has been long reported[26], but it is still not

clear which conditions generate serrated chip Several theories

have been formulated to explain the non-homogeneous chip

formation assuming different crack initiation criteria and different

crack initiation regions The first theory addressed the

cata-strophic shear instability in machining due to slope of the true

stress–true strain curve reaching of zero However, several of the

modern theories are based on adiabatic shear theory, a more

prominent thermal softening than the strain hardening effect of

the material, or crack initiation due to surface irregularities[27]

In the case of Ti6Al4V, segmented chip formation occurs as a

consequence of adiabatic shear leading to a large strain concen-tration in a narrow band [26] Due to the low thermal conductivity, all the heat generated concentrates on the shear band If temperature in the shear band is high enough, heat generation also might increase due to the possibility of allotropic transformation in titanium[18] Recently, the use of simulation by finite element method (FEM)-based software permits the predic-tion and recreapredic-tion of severely deformed shear bands [19] and serrated chip morphology[20] In this case, the results obtained are directly dependent on the employed material’s flow stress, but aspects such as surface cracks, phase transformations, disconti-nuities and allotropic transformations are not yet taken into account

Regarding tool wear, research indicates that ultrasonic-assisted machining yields longer tool lives There is evidence of maximum vibration amplitude over which tool life shortens

as a consequence of the impact regime reached [14] Similar working mechanism exists in modulation-assisted machining (MAM) [28] for particulate powder production This research group has also investigated the use of vibrating tools in drilling and turning operations where tools with high amplitudes

in the range 100–200mm were employed with notorious improvements in tool life especially concerning deep drilling operations This apparent contradiction might be due to the huge difference in the number of impact-cutting cycles between the tool and the workpiece on both techniques; while

in ultrasonic-assisted machining, working frequencies are of the order of 20 kHz, in the case of MAM, these are of the order

of 100 Hz

This study aims to analyze the effect ultrasonic assistance has

on the drilling of Ti6Al4V alloy To our knowledge, no study has been reported in the literature on applying ultrasonic assistance

to the drilling process in order to achieve more favorable cutting conditions Here, aspects such as measurement of force, high-speed imaging of chip formation and temperature measurement

of the drill will be studied in order to analyze the material behaviors

2 Experimental investigation

The experimental investigation of this work has been divided

into several aspects beginning with the construction of an

ultrasonic vibration device and then the monitoring of the drilling

process, including its feed force and temperature measurements

Titanium alloy Ti6Al4V was drilled in the aged condition, with

mechanical properties su 1100 MPa, hardness 41 HRC and

modulus of elasticity 114 GPa

2.1 Vibration system

In this study, ultrasonic vibration has been applied over

Ti6Al4V samples supplied by the Airbus aeronautic manufacturer

The samples were 3 mm thick with a diameter of 55 mm We

employed the two-flute uncoated tungsten carbide Gu¨hring drills

with a diameter of 4 mm, ref A2 2120 according to the DIN 338

standard

Ultrasonic vibration has been achieved by means of

a piezoelectric transducer, MPI 5020S-6PS, and a power

generator, MastersonicTM MSG-2000, which works in the

17.5–27.5 kHz frequency range The transducer was attached to

the aluminum cylindrical workpiece clamp The exact

vibration frequency yielding an axial vibration mode was

calculated using the FEM Nastran-PatranTM software

package The samples’ vibration control was measured using a

Polytec OFV 505 laser head and a PolytecTMOFV-5000 vibrometer

controller Fig 2 shows the US vibration system with details

of the clamping system and FEM modeling of the 17,688 Hz

axial vibration mode

According to the axial vibration analysis, FEM simulations

result in good agreement with experimentally measured

fre-quency values Amplitudes achieved were of the range 3–9mm

depending on the tuning frequencies of the system

As shown inFig 3, there are three vibrating stages In the first

stage, there is no tool–workpiece interaction In the second stage,

the drilling process starts, and due to the deflection of the

workpiece clamping system, a jump in the measured amplitude

can be observed in the graph In the third stage, the process stabilizes while the drill cuts the material In cases when the drilling feeds are of the order of the ultrasonic vibration amplitude (10–20mm), a vibro-impact working regime can be reached according toFig 1

2.2 Force measurements and chip formation Feed forces were measured using a Kistler 9046B4 dynam-ometer composed of four quartz charge cells.Table 1shows the values of feed force variation when a vibration frequency of 17,700 Hz was applied to the sample at different working conditions The ultrasonic vibration is not recorded by the Kistler plate because this frequency is 20 times higher than the device natural frequency Therefore, the cutting force reduction is related

to the chip formation mechanism, affected by the ultrasonic assistance

Feed force reductions of the order of 20% have been attained when ultrasonic-assisted drilling was employed Higher force reductions have been documented in the literature for different materials [23], but in the present experiments, the working regime has not reached a vibro-impact as might happen elsewhere

A high-speed camera, PhotronTM Ultima APX-RS, was em-ployed for the observation of in-process chip formation The recording rates employed have been of 9000 frames per second (fps) with a shutter frequency of 91,000 Hz halide lighting and a fiber optic system for light guiding have been employed in order

to ensure good image quality Fig 4 shows the images of conventional drilling (left) and ultrasonic-assisted drilling (right)

of Ti6Al4V at a spindle speed of 2000 rpm and a feed rate of

200 mm/rev

According to our images and further optic microscope analysis, slight difference was observed when ultrasonic-assisted and conventional drill produced chips were compared Some geome-trical distortion and chip breakage of chip when US was applied could be mentioned (see Fig 4 right, the extreme case of difference)

Regarding the aspects related to chip formation, burr height

has also been analyzed No difference was observed at this point

because these drilling tests have been based on new drills and

short machining time samples Due to this fact and to the very

little worn-out tool edges, burr height was inexistent when

drilling 3-mm thick Ti6Al4V samples using new tools

2.3 Temperature measurements

The thermal analysis of drilling was a key factor because it is

directly related to tool wear and tool life Temperature

measure-ment in drilling was itself a task [4,29–33] With respect to

temperature measurements, some attempts were made with

ultrasonic-assisted turning of Inconel 718[34], but no reference

has been observed in the drilling of Ti6Al4V

For the estimation of temperatures, a thermographic camera,

NikonTMLaird-270 A with a Schottky-Barrier IR charge-coupled

device working in the 3–5mm range, was employed Temperature measurements have been done in two zones The upper zone corresponded to the zone where the drill enters the sample and the lower zone corresponded to the zone where the drill tip exits the sample.Fig 5shows the disposition of the measuring setup of the tool, the sample and the IR camera

Spectral emissivity of the tool for different temperatures

in the normal direction of view was employed according to former investigations over uncoated WC tool samples carried out by Fourier transform infrared (FTIR) spectrometry

[35,36] Due to its null influence on temperature, a value of spectral emissivity between 0.35 and 0.45 was assumed for hard metal (tungsten carbide sintered with cobalt) in the spectral band corresponding to the camera sensor employed Fig 6

shows the curves of emissivity of WC as a function of temperature and wavelength According to these curves, the measurements of temperatures on WC were more accurate using near infrared sensors than using far infrared sensors because emissivity values at short wavelengths are much higher than at long wavelengths

Temperature can be calculated by equating the relation of radiances between the real body, Ll, and the radiances of the black body, Ll , b, multiplied by the spectral emissivity of the hot body,el:

Expressing the exitances of the hot body in integral form is possible once spectral emissivity and black body temperature, Tb, are known Then the definition of temperature of the real body, T,

is found by solving the next equation

Z l 2

l1

l5

ec 2 = l b1d ¼

Z l 2

l1

ll5

where the first integral expresses the exitance of the black body at

a known temperature, Tb, and the second integral expresses the exitance of the real body;l1andl2indicate the sensing shorter and longer wavelengths; and c2 is a constant with a value of 1.4388  102m K For temperature calculations according to the images acquired with the high-speed camera, we observed

Table 1

Data corresponding to different drilling experiments of Ti6Al4V.

Test Spindle speed,

n (rpm)

Feed, f (mm/min)

US Fz (N) Reduction

(%)

Fig 3 Vibration amplitude measurement employing laser vibrometry at rotation speed n 2000 rpm and feed rate, f, 150 mm/min.

that the chip did not interfere with the field of view of the drill.

The image acquisition rate of the thermographic camera was set

to a frequency of 30 Hz According to the emissivity variation due

to viewing angle at different points of the body clearance of the

drill, it was assumed that this value keeps constant and equals

normal emissivity values Aspects such as emissivity variation due

to grease and dust or machined material adhering to the drill have

not been considered here It is also important to note that the

measured temperature did not correspond to the machining

instant but corresponded to a time instant just after finishing the

machining process

The use a 1:1.2 lens that allowed enough spatial resolution in

order to measure temperatures on different areas of the drill

Fig 7 shows two thermographs of the upper view of the drill

where US-assisted and non-assisted drilling temperature fields

are shown

Fig 8shows the thermal field of the drill tip at the exit of the sample drilling process where it is clearly observed that temperatures achieved with US assistance are noticeably higher than that in the case of non-US assistance In conventional drilling, temperatures near 800 K are measured, whereas in US-assisted drilling, temperatures close to 1100 K are measured It makes a differential of 300 K higher when US assistance is employed In both Figs 7 and 8, the white line indicates the border between the sample and the air Consequently, a double image corresponding to the direct radiation and reflected radiation can be observed in both figures Reflected radiation corresponds to the area below the white light inFig 7and above the line inFig 8 Temperatures measured as the tool exited the work sample were higher than temperatures encountered on the tool as the tool entered the material Thus, as an approximation to real temperatures during the drilling process, it was assumed that temperatures at the exit of the tool are more accurate

When temperature measurements were compared to feed forces at different sample vibration amplitudes, it was observed that, whereas temperature increased with tool vibration ampli-tude, feed force decreased as vibration amplitude grew Fig 9

shows the double graph of measured average feed force, Fz, and maximum temperature at the tool tip exit of the sample against vibration amplitude variation

As observed in Fig 9, when no vibration was applied, feed forces lay in 350 N approximately, whereas amplitudes of vibration up to 9mm produced force reductions down to 170 N

In the case of temperatures, a contrary effect was observed When

no US vibration was applied, temperature of the drill tip at the exit

of the work sample was of the order of 750 K, whereas when US vibration of 9mm was applied, this temperature went beyond the limit of the camera filter, which is 823 K for black body radiation Although our results concerning the machining of aluminum were not treated in this work, it has been observed in experimental tests that aluminum 7075-T6 drilling yielded, as in the case of Ti6Al4V, lower feed forces and higher tool temperatures when US-assisted drilling was applied

3 Discussion of results Our results appear to describe the results in some points as confusing compared to those found in literature Temperature

Fig 4 Chip formation of Ti6Al4V without ultrasonic assistance (a, left) and with ultrasonic assistance (b, right).

Fig 5 Schedule of temperature measuring zones on the drill.

increment during the cutting process accelerates the diffusion of

the work material into the tool, decreases the hardness of the tool

making it more prone to abrasion and wear, and promotes

thermal softening either in the work or in the tool From one side,

temperature increments are directly related to reductions in tool

life[37, p 515], whereas from the other, some authors indicate

that the application of ultrasonic assistance promotes tool life

[10,11] With respect to the application of ultrasonic assistance on Ti6Al4V, our experiments indicate that a general reduction of feed force occurs parallel to an increase of temperatures, both variables being dependent on vibration amplitude Thus, higher vibration amplitudes yield higher feed force reductions and higher

Fig 6 Emissivity of WC as a function of wavelength (a) and temperature (b).

Fig 7 Thermograms of the drill where (a) there is no US assistance and (b) with US assistance captured at sample entering.

temperatures when Ti6Al4V is machined Temperature increment

observed with ultrasonic-assisted drilling is higher than when no

assistance is applied

This experimental evidence confirms the FEM-simulated

results found in the literature[34,38], where ultrasonic-assisted

machining over Inconel 718 test samples presented higher

temperatures than conventional machining samples When

US-assisted drilling was applied, FEM simulations carried out by

Mitrofanov have shown that the mean level of stress in the

primary shear zone was lower than when ultrasonic-assisted

machining was applied [39] Fig 10 shows the curves of flow

stress variation on Ti6Al4V material assuming different

conven-tional drilling and ultrasonic-assisted drilling temperatures

according to Johnson–Cook (Eq (4)) and Zerilli–Armstrong’s (Eq

(5)) flow stress,s, expressions:

s¼A þ Bn

1 þ C ln _

_

0

 

1  T  Troom

TmeltTroom

(4) where A ¼ 782.7 MPa, B ¼ 498.4 MPa, C ¼ 0.028, n ¼ 0.28 and

m ¼ 1 for _0¼105s1 are the constants of Johnson and Cook’s

material flow stress law according to Lee and Lin[40]; Tmeltand

Troomindicate the melting temperature of the material employed

and the room temperature;eis the strain and _¼105s1is the supposed strain rate of the material in the primary shear zone In the case of Zerilli–Armstrong flow stress expression, we have

s¼C0þC1e½C3C4ln _ ð ÞT

þC5n

(5) where C0¼810 MPa, C1¼1800 MPa, C3¼0.009, C4¼0.0005,

C5¼530 MPa and n ¼ 0.5 are specific constants of Zerilli–Arm-strong’s relation for each material[21];sindicates the flow stress value, andeindicates stress strain (0.8)

As shown inFig 10, a thermal softening effect that yields flow stress variations proportional to the observed force reductions is only achieved when it is considered that Ti6Al4V behaves corresponding to the Johnson–Cook model [40] and achieves temperature differences of more than 250 1C If material behavior corresponding to Zerilli–Armstrong is considered[21], variation of flow stress will be under 10% even when conventional and US-assisted drilling chip temperature differences are above 300 K According to these results, suppose that chip temperature in US-assisted drilling is notoriously higher than in conventional drilling, feed force reductions achieved cannot be explained by the thermal softening effect A new look to explain the force reduction

is found by Calamaz et al.[19]where a new model with a TANH (Hyperbolic Tangent) material law is proposed, in which a new term is added to the Johnson and Cook equation to model the strain softening effect

In future investigations, it would be desirable to have the analysis of chip–tool–workpiece contact and tool life analysis in order to better understand the temperature increment source and its effects on the economy and quality of machined parts

4 Conclusion After constructing a US-assisted workpiece holder, drilling of Ti6Al4V was carried out and different parameters were monitored

In situ chip formation was analyzed and no difference was observed with regard to chip geometry Similarly, when new tools were employed in drilling, burr formation was null

When ultrasonic-assisted drilling was applied, the feed force decreased by 10–20% on average, and the decrease in force was more notorious as the vibration amplitude was higher

Fig 8 Thermograms of the drill where (a) there is no US assistance and (b) with

US assistance at the sample exit.

 Tool tip temperature was higher when ultrasonic-assisted

drilling was applied In this case, the higher the vibration

amplitude, the higher the temperature variations at the

tool tip

 There exists a correlation between temperature variation and

feed force variation, but it cannot be explained due to the

thermal softening effect of Ti6Al4V only At this point, it is

necessary to carry out further research in quantifying the

existent heat generation mechanism and its effect on tool wear

and workpiece tensional state (residual stress, phase

transfor-mation, etc.)

Acknowledgements

This research was sponsored by the Basque Government

Project Advance Manufacturing Technologies and coordinated by

the marGUNE Cooperative Research Center Special thanks to

marGUNE researches working on US, especially O Gonzalo and

R Alberdi Thanks are also addressed to Prof Girot, for his valuable

suggestions

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