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Analysis of ultrasonic-assisted drilling of Ti6Al4V
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 49 (2009) 500–508
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Analysis of ultrasonic-assisted drilling of Ti6Al4V J. Pujana a, A. Rivero a, A. Celaya b, L.N. Lo´pez de Lacalle b, a b
´n, Spain ´n Fatronik, Paseo Mikeletegi 7, 20009 Donostia-San Sebastia Fundacio ´nica de la Escuela Superior de Ingenieros de Bilbao, Universidad del Paı´s Vasco, Alameda de Urquijo s/n, E-48013 Bilbao, Spain Departamento de Ingenierı´a Meca
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 September 2008 Received in revised form 22 December 2008 Accepted 23 December 2008 Available online 20 January 2009
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.
Keywords: Ti6Al4V Ultrasonic-assisted drilling Cutting temperature Machining
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 the a+b phase 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.
Corresponding author. Tel./fax: +94 6014216.
E-mail address: [email protected] (L.N. Lo´pez de Lacalle). 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.12.014
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 materials, and although removal rates are not high, ultrasonic technology 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 vibroimpact 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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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 1 ¼ f j sinððW f =2ÞpÞj
(1)
where A indicates vibration amplitude, f feed per revolution and Wf the number of vibration cycles per tool revolution. Fig. 1 shows the chip-breaking area (above each set of points) and non-chipbreaking area (below) and curves generated with feeds ranging from 10 to 100 mm 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 catastrophic 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
501
consequence of adiabatic shear leading to a large strain concentration 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 prediction and recreation 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, discontinuities and allotropic transformations are not yet taken into account. Regarding tool wear, research indicates that ultrasonicassisted 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 100–200 mm 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, highspeed imaging of chip formation and temperature measurement of the drill will be studied in order to analyze the material behaviors.
Fig. 1. Chip-breaking limit view according to Eq. (1) at four feed values and calculated according to typical ultrasonic vibration frequencies.
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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 PolytecTM OFV-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 frequency values. Amplitudes achieved were of the range 3–9 mm depending on the tuning frequencies of the system. As shown in Fig. 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 (10–20 mm), a vibro-impact working regime can be reached according to Fig. 1.
2.2. Force measurements and chip formation Feed forces were measured using a Kistler 9046B4 dynamometer composed of four quartz charge cells. Table 1 shows 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 employed 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 geometrical distortion and chip breakage of chip when US was applied could be mentioned (see Fig. 4 right, the extreme case of difference).
Fig. 2. Experimental setup (a) of the machine, details of the sample clamping system (b) and FEM simulation of the axial vibration mode (c).
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Fig. 3. Vibration amplitude measurement employing laser vibrometry at rotation speed n 2000 rpm and feed rate, f, 150 mm/min.
Table 1 Data corresponding to different drilling experiments of Ti6Al4V. Test 1 2
Spindle speed, n (rpm) 700 700
Feed, f (mm/min) 70 70
US
Fz (N)
No Yes
374 289
3 4
1000 1000
70 70
No Yes
382 296
23
5 6
1500 1500
70 70
No Yes
431 350
19
7 8
2000 2000
150 150
No Yes
450 406
10
9 10
2000 2000
200 200
No Yes
347 300
14
Reduction (%) 23
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 measurement 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, NikonTM Laird-270 A with a Schottky-Barrier IR charge-coupled
device working in the 3–5 mm 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. 5 shows 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: Ll ¼ l Ll;b
(2)
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 l2 Z l2 l5 l l5 (3) dl ¼ dl c = l T c =lT 1 2 2 b 1 l1 e l1 e 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; l1 and l2 indicate the sensing shorter and longer wavelengths; and c2 is a constant with a value of 1.4388 102 m K. For temperature calculations according to the images acquired with the high-speed camera, we observed
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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.
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. 8 shows 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 USassisted 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 in Fig. 7 and above the line in Fig. 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 amplitude, 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 9 mm 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 9 mm 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
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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.
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
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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 USassisted 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 conventional drilling and ultrasonic-assisted drilling temperatures according to Johnson–Cook (Eq. (4)) and Zerilli–Armstrong’s (Eq. (5)) flow stress, s, expressions: m _ T T room s ¼ A þ Bn 1 þ C ln _ 1 (4) 0 T melt T room where A ¼ 782.7 MPa, B ¼ 498.4 MPa, C ¼ 0.028, n ¼ 0.28 and m ¼ 1 for _ 0 ¼ 105 s1 are the constants of Johnson and Cook’s material flow stress law according to Lee and Lin [40]; Tmelt and Troom indicate the melting temperature of the material employed
and the room temperature; e is the strain and _ ¼ 105 s1 is the supposed strain rate of the material in the primary shear zone. In the case of Zerilli–Armstrong flow stress expression, we have
s ¼ C 0 þ C 1 e½C3 C4 lnð_ ÞT þ C 5 n
(5)
where C0 ¼ 810 MPa, C1 ¼ 1800 MPa, C3 ¼ 0.009, C4 ¼ 0.0005, C5 ¼ 530 MPa and n ¼ 0.5 are specific constants of Zerilli–Armstrong’s relation for each material [21]; s indicates the flow stress value, and e indicates stress strain (0.8). As shown in Fig. 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 USassisted 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 Fig. 8. Thermograms of the drill where (a) there is no US assistance and (b) with US assistance at the sample exit.
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. 9. Relation between feed force, temperature and vibration amplitude when machining at 2000 rpm and f 200 mm/min.
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Fig. 10. Flow stress curves at different strains and temperatures according to Johnson–Cook [40] and Zerilli–Armstrong [21] models.
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 transformation, 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. References [1] R.R. Boyer, An overview on the use of titanium in the aerospace industry, Materials Science and Engineering A 213 (1996) 103–114. [2] L.N. Lopez de Lacalle, J. Perez, J.I. Llorente, J.A. Sanchez, Advanced cutting conditions for the milling of aeronautical alloys, Journal of Materials Processing Technology 100 (2000) 1–11. [3] R.P. Zeilmann, W.L. Weingaertner, Analysis of temperature during drilling of Ti6Al4V, Journal of Materials Processing Technology 179 (2006) 124–127. [4] R. Li, A.J. Shih, Spiral point drill temperature and stress in high-throughput drilling of titanium, International Journal of Machine Tools and Manufacture 47 (2007) 2005–2017. [5] I.L. Cantero, M.M. Tardio, J.A. Canteli, M. Marcos, M.H. Miguelez, Dry drilling of alloy Ti-6Al-4V, International Journal of Machine Tools and Manufacturing 45 (2005) 1246–1255. [6] O.V. Abramov, High-Intensity Ultrasonics—Theory and Industrial Applications, Gordon and Breach Science Publishers, Amsterdan, ISBN 90-5699-0411, 1998. [7] N.J. Churi, Z.J. Pei, C. Treadwell, Rotary ultrasonic machining of titanium alloy: effects of machining variables, Machining Science and Technology 10 (2006) 301–321. [8] C.S. Liu, B. Zhao, G.F. Gao, X.H. Zhang, Study on ultrasonic vibration drilling of particulate reinforced aluminium matrix composites, Key Engineering Materials 291–292 (2005) 447–452.
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Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Analysis of ultrasonic-assisted drilling of Ti6Al4V J. Pujana a, A. Rivero a, A. Celaya b, L.N. Lo´pez de Lacalle b, a b
´n, Spain ´n Fatronik, Paseo Mikeletegi 7, 20009 Donostia-San Sebastia Fundacio ´nica de la Escuela Superior de Ingenieros de Bilbao, Universidad del Paı´s Vasco, Alameda de Urquijo s/n, E-48013 Bilbao, Spain Departamento de Ingenierı´a Meca
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 September 2008 Received in revised form 22 December 2008 Accepted 23 December 2008 Available online 20 January 2009
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.
Keywords: Ti6Al4V Ultrasonic-assisted drilling Cutting temperature Machining
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 the a+b phase 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.
Corresponding author. Tel./fax: +94 6014216.
E-mail address: [email protected] (L.N. Lo´pez de Lacalle). 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.12.014
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 materials, and although removal rates are not high, ultrasonic technology 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 vibroimpact 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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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 1 ¼ f j sinððW f =2ÞpÞj
(1)
where A indicates vibration amplitude, f feed per revolution and Wf the number of vibration cycles per tool revolution. Fig. 1 shows the chip-breaking area (above each set of points) and non-chipbreaking area (below) and curves generated with feeds ranging from 10 to 100 mm 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 catastrophic 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
501
consequence of adiabatic shear leading to a large strain concentration 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 prediction and recreation 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, discontinuities and allotropic transformations are not yet taken into account. Regarding tool wear, research indicates that ultrasonicassisted 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 100–200 mm 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, highspeed imaging of chip formation and temperature measurement of the drill will be studied in order to analyze the material behaviors.
Fig. 1. Chip-breaking limit view according to Eq. (1) at four feed values and calculated according to typical ultrasonic vibration frequencies.
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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 PolytecTM OFV-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 frequency values. Amplitudes achieved were of the range 3–9 mm depending on the tuning frequencies of the system. As shown in Fig. 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 (10–20 mm), a vibro-impact working regime can be reached according to Fig. 1.
2.2. Force measurements and chip formation Feed forces were measured using a Kistler 9046B4 dynamometer composed of four quartz charge cells. Table 1 shows 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 employed 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 geometrical distortion and chip breakage of chip when US was applied could be mentioned (see Fig. 4 right, the extreme case of difference).
Fig. 2. Experimental setup (a) of the machine, details of the sample clamping system (b) and FEM simulation of the axial vibration mode (c).
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Fig. 3. Vibration amplitude measurement employing laser vibrometry at rotation speed n 2000 rpm and feed rate, f, 150 mm/min.
Table 1 Data corresponding to different drilling experiments of Ti6Al4V. Test 1 2
Spindle speed, n (rpm) 700 700
Feed, f (mm/min) 70 70
US
Fz (N)
No Yes
374 289
3 4
1000 1000
70 70
No Yes
382 296
23
5 6
1500 1500
70 70
No Yes
431 350
19
7 8
2000 2000
150 150
No Yes
450 406
10
9 10
2000 2000
200 200
No Yes
347 300
14
Reduction (%) 23
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 measurement 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, NikonTM Laird-270 A with a Schottky-Barrier IR charge-coupled
device working in the 3–5 mm 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. 5 shows 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: Ll ¼ l Ll;b
(2)
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 l2 Z l2 l5 l l5 (3) dl ¼ dl c = l T c =lT 1 2 2 b 1 l1 e l1 e 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; l1 and l2 indicate the sensing shorter and longer wavelengths; and c2 is a constant with a value of 1.4388 102 m K. For temperature calculations according to the images acquired with the high-speed camera, we observed
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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.
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. 8 shows 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 USassisted 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 in Fig. 7 and above the line in Fig. 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 amplitude, 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 9 mm 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 9 mm 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
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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.
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
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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 USassisted 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 conventional drilling and ultrasonic-assisted drilling temperatures according to Johnson–Cook (Eq. (4)) and Zerilli–Armstrong’s (Eq. (5)) flow stress, s, expressions: m _ T T room s ¼ A þ Bn 1 þ C ln _ 1 (4) 0 T melt T room where A ¼ 782.7 MPa, B ¼ 498.4 MPa, C ¼ 0.028, n ¼ 0.28 and m ¼ 1 for _ 0 ¼ 105 s1 are the constants of Johnson and Cook’s material flow stress law according to Lee and Lin [40]; Tmelt and Troom indicate the melting temperature of the material employed
and the room temperature; e is the strain and _ ¼ 105 s1 is the supposed strain rate of the material in the primary shear zone. In the case of Zerilli–Armstrong flow stress expression, we have
s ¼ C 0 þ C 1 e½C3 C4 lnð_ ÞT þ C 5 n
(5)
where C0 ¼ 810 MPa, C1 ¼ 1800 MPa, C3 ¼ 0.009, C4 ¼ 0.0005, C5 ¼ 530 MPa and n ¼ 0.5 are specific constants of Zerilli–Armstrong’s relation for each material [21]; s indicates the flow stress value, and e indicates stress strain (0.8). As shown in Fig. 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 USassisted 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 Fig. 8. Thermograms of the drill where (a) there is no US assistance and (b) with US assistance at the sample exit.
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. 9. Relation between feed force, temperature and vibration amplitude when machining at 2000 rpm and f 200 mm/min.
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Fig. 10. Flow stress curves at different strains and temperatures according to Johnson–Cook [40] and Zerilli–Armstrong [21] models.
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 transformation, 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. References [1] R.R. Boyer, An overview on the use of titanium in the aerospace industry, Materials Science and Engineering A 213 (1996) 103–114. [2] L.N. Lopez de Lacalle, J. Perez, J.I. Llorente, J.A. Sanchez, Advanced cutting conditions for the milling of aeronautical alloys, Journal of Materials Processing Technology 100 (2000) 1–11. [3] R.P. Zeilmann, W.L. Weingaertner, Analysis of temperature during drilling of Ti6Al4V, Journal of Materials Processing Technology 179 (2006) 124–127. [4] R. Li, A.J. Shih, Spiral point drill temperature and stress in high-throughput drilling of titanium, International Journal of Machine Tools and Manufacture 47 (2007) 2005–2017. [5] I.L. Cantero, M.M. Tardio, J.A. Canteli, M. Marcos, M.H. Miguelez, Dry drilling of alloy Ti-6Al-4V, International Journal of Machine Tools and Manufacturing 45 (2005) 1246–1255. [6] O.V. Abramov, High-Intensity Ultrasonics—Theory and Industrial Applications, Gordon and Breach Science Publishers, Amsterdan, ISBN 90-5699-0411, 1998. [7] N.J. Churi, Z.J. Pei, C. Treadwell, Rotary ultrasonic machining of titanium alloy: effects of machining variables, Machining Science and Technology 10 (2006) 301–321. [8] C.S. Liu, B. Zhao, G.F. Gao, X.H. Zhang, Study on ultrasonic vibration drilling of particulate reinforced aluminium matrix composites, Key Engineering Materials 291–292 (2005) 447–452.
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