Introduction

Titanium alloy is one of the strongest metals which is mostly used in automobile, railway, ship building, aircraft manufacture and construction industries. Titanium alloy has a low thermal conductivity and high weldability [1, 2]. Titanium alloys are related to metals that predominantly consisting of titanium (Ti) and supplementary chemical elements like aluminium and vanadium [3, 4]. They offer high toughness and tensile strength even at the critical temperature or elevated temperature. They are lightweight materials that can exhibit higher resistance to corrosion and are able to withstand higher temperatures. Titanium alloy (Ti64) is typically alloyed with calculated amount of aluminium and vanadium, usually with a weight percentage of 6 and 4 respectively [5]. The incorporation of these elements increases not only the strength but also the plasticity. The conventional tools wear out rapidly when employed in machining of Ti alloy continuously. Tool materials like cemented carbide and ceramics are usually found to be susceptible with Ti alloy at higher temperatures [6, 7].

The temperature attained during cutting will have immense effect on the tool life and surface integrity of the material being machined. The yield strength is also found to decrease with the increasing temperature during cutting. The increase in temperature isdue to transformation of cutting energy into heat [8, 9]. The heat liberated at the zone of cutting is found to be a prominent factor in deciding the performance of the tool. This is also related to the quality of the surface finish. When the temperature generated due to cutting is sufficiently high, it is found to be unfavourable to the work piece as well as tool. The low cutting temperature may be useful in improving the tool life.

Abhang et al. [1] estimated the temperature at the chip-tool interface during turning operation. They investigated cutting parameters such as cutting speed (v), feed rate (f), depth of cut (d) and tool nose radius and reported that increase inspeed, feed and depth of cut activates the temperature rise, while nose radius reduces the increase of temperature. Grzesik et al. [10] used thermocouples (K-type) to assess temperature during cutting of steel and reported that the temperature is found to be a decisive factor in surface integrity. Ghani et al. [11] investigated the fraction of heat being transformed to the tool at conventional speed as well as high speed and reported that undue increase in temperature rigorously affects the quality of the work piece and tool life. Petter Hagqvist et al. [12] adopted narrow bandwidth spot radiation pyrometer to estimate high temperature at cutting of titanium-based alloy. This method utilizes the spectral emissivity in estimating the temperature.

Heigel et al. [13] estimated the temperature at tool-chip contact zone using infrared sensor. Infrared camera was utilized to record dispersal of temperature, chip curl and rupture during machining. From their investigations at the cutting speed (v) rangesfrom 20 to 100 m/min, they found that 6-21% of variance in dissemination of temperature from one edge of the chip to the other, signifying that attention must be paid while executing thermographic calculations at the cutting zone. Machado et al. [14] explored machining of titanium alloy (Ti64) using tungsten carbide insert K10 and reported in their review article that the temperature is about 1000 K even at velocities which are relatively lower than 1 m/s and it shoots up to 1350 K for a speed of 3.33 m/s. The temperatures are usually around 250 K greater than the temperature whilst working with carbon steel. It is also reported that low cutting force is produced at elevated cutting temperatures, which are credited by lesser density and lower thermal conductivity of titanium alloys. The temperature gradient is indeed much rapid even at lower speeds and hence secondary shear area is associated in the cutting zone. Muller et al. [15] also reported that the absolute temperature is in the range between 1300 and 2000 K while working on titanium alloy. They reported further that temperature distribution at the bottom of the chip is initially seen transient and the later as quasi-stationary. The temperature behaviour at chips is seen asymptotic with growing speed. Though the intensified cutting force at higher speed increases the temperature, the asymptotic behaviour of chips limits the increasing temperature at the cutting zone [16].

Komanduri et al. [17] investigated the mechanism of chip formation and wear at low speed to a moderately high-speedmachining of Ti alloys. Turley et al. [18] used high speed camera to observe the chip formation and shearing during machining of Ti alloy and reported that catastrophic shear type chips are formed. Also revealed that greater size of shear strains is found in the shear bands. Friction and associated plastic deformation accelerate the heat at the workpiece-tool interface [19,20,21]. Ramesh et al. and Viswanathan et al. [22,23,24], Navneet Khanna and Davim [25] and Kechagias et al. [26] investigated the machinability of different grades of Ti alloys and magnesium alloy using design of experiments (DoE) and statistical method. They were intended to reduce the surface roughness of the machined part. Recently, Akkus et al. [27] utilized statistical methods to optimize the surface roughness, vibration and cutting energy during turning of Ti grade 5 alloy. Aydin et al. [28, 29] investigated the cutting zone temperature and applied finite element modelling (FEM) to analyse the chip formation. In recent research by Aydin [30], chip formation at conventional speed and high-speed machining was reported. It was also reported that saw tooth chips are formed at high-speed machining of Ti alloy due to ductile fracture. Optimization of cutting parameters are generally done to increase the surface finish and/or material removal rate [31,32,33] of the machined component.

The present investigation is intended to explore and evaluate the optimum parameters that influencing turning of Ti64 alloy and to attain the optimum cutting zone temperature to enhance tool life. The extended purpose of this research to improve the surface finish of the component.

Experiments and Measurement of Temperature

The turning experiments were performed on Titanium alloy (Ti64) with the use of coated carbide inserts. CNC turning lathe (Make: PROTECK, India) and high precision thermal imaging camera were employed in the experiments and measurement of cutting temperature with respect to different cutting conditions. Turning tools used were carbide tools with nose radius of 0.8 mm, namely PVD (CNMG 120408 WS 25 PT) and CVD (CNMG 120408 WS 25 PT).PVD coating is TiN coating, generally done by Physical Vapour Deposition technique. CVD is a Chemical Vapour Deposition which contains the coating materials of TiN, TiC, Ti (CN) and Al2O3. The PVD and CVD coated carbide tools are shown in Fig. 1 (left and the right side respectively). The composition of titanium alloy obtained from the Optical Emission Spectrometer is listed in Table 1. It evidences Grade 5 Titanium alloy that was being machined.

Fig. 1
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PVD and CVD coated carbide turning tools

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Table 1 Composition of Ti-6Al-4V alloy
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The material Ti64 is a furnace cooled metal that must have the annealed structure. The structure has elongated grains of bright alpha and inters granular beta structure in black colour. Figure 2 reveals the alpha beta structure of the Grade 5 Titanium alloy, where uniform grain sizes are also evidenced. The uniform grain size is achieved if rate of cooling is statically uniform. The bigger size of grains is appeared if there is a variant rate of cooling. At the higher magnification (200X), beta grains are evidenced between alpha grain boundaries.

Fig. 2
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Microstructure of titanium alloy (Ti64)

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Figure 3 shows the experimental setup that was being used for turning of Ti64 alloy and thermal image capturing. Table 2 unveils the specification of CNC lathe used in the experiments. It has a swing diameter of 450 mm and length of 1000 mm.

Fig. 3
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Experimental setup and thermal image capturing

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Table 2 Specification of CNC turning lathe being used in experiments
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Thermal imaging camera (Model: FLIR-E63900, T198547) employed in the thermal image capturing can quantify the cutting temperature with brightness. The quantification of cutting temperature is done with respect to the reference temperature sourced by lamp. It can show the cutting temperature changes in colour variations. The camera estimates the cutting temperature at an accuracy of ±5 °C. The current setup of measuring the thermal image complies with the safety standards ANSI B11.22-2002 (R2020). Prior to conducting the experiments, DoE was conducted with influencing independent parameters such as speed, feed and depth of cut. Table 3 reveals the range and levels selected for DoE (L16 orthogonal array). All experiments were conducted with PVD tool as well CVD carbide tool. No coolant (NC) was used in any experiment. The reason for dry machining is that the critical temperature at cutting can be investigated as NC does not take the advantage of coolant. Table 4 lists the cutting temperature generated at different cutting conditions, while the thermal images captured during experiments are presented and discussed in the next section. Figure 4 shows a turned Ti64 specimen with diameter of 85 mm and length of 125 mm.

Table 3 Levels used in DoE
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Table 4 L16 Orthogonal array depicting cutting condition and cutting temperature measured from experiments (1 refers to Level 1, 2 refers to Level 2, 3 refers to Level 3 and 4 refers to Level 4)
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Fig. 4
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Turned Ti64 sample

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Results and Discussion

Turning of Ti64 samples were done with both CVD and PVD carbide tools according to L16 orthogonal matrix. The results listed in Table 4 shows the machinability of Ti64 alloy with CVD and PVD carbide inserts and the cutting temperature generated. Nevertheless, the temperature generated by CVD tool and PVD tool are not the same. Thermal images captured while machining the Ti64 alloy using PVD coated insert is shown in Fig. 5, while Fig. 6 shows the thermal images captured during machining of the same material using CVD coated insert.

Fig. 5
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Thermal images during machining of Ti64 alloy using PVD coated carbide insert

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Fig. 6
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Thermal images during machining of Ti64 alloy using CVD coated carbide insert

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Turning is the subtractive process where materials are removed as chips. Chips are characteristic evidence of the machining process. The built-up edge and tool wear can be estimated by analysing the chip formation. The size and form of chips vary according to the cutting condition. The chips collected during turning of Ti64 alloy using CVD and PVD coated carbide inserts are shown in Tables 5 and 6 respectively. As for as PVD coated carbide tool is concerned, long and continuous chips as well as short chips were witnessed. Continuous chip formation is due to high cutting temperature at the cutting zone. There is a shear localization observed, which is common in titanium machining. We could observe some undeformed materials also at the tip of chips. The temperature induced would affect the tool life and surface finish of the machined parts. Short chips were also appeared when temperature is lower. In contrast to the above, short chips were observed during the machining of Ti64 with CVD tool. Short chips do not form any adverse effect in machining titanium alloy.

Table 5 Chip formation while machining with CVD coated carbide insert
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Table 6 Chip formation while machining with PVD coated carbide insert
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For analysisng the data, response surface methodology (RSM) is utilized. The regression analysis was done on the experimentally measured temperature data for two reasons. Firstly, to evaluate the performance of PVD and CVD coated inserts. Secondly to correlate the relationship between cutting parameters with cutting temperature.

$${T}_{PVD}=322.635+2.541v+251.9f+18d$$
(1)
$${T}_{CVD}=303.877+1.905v+287.6f+12.4d$$
(2)

Where as, TPVD and TCVD are temperatures (response parameters) in relation to PVD and CVD carbide inserts respectively.

Figure 7 compares the predicted result from TPVD and experimental result, whereas Fig. 8 compares the predicted result from TCVD and experimental result.

Fig. 7
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Main effect plot between actual and predicted result from TPVD

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Fig. 8
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Main effect plot between actual and predicted result from TCVD

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The effectiveness of the RSM model was measured using root mean square values. R2 value and adjusted R2 value of TPVDare 0.9516 and 0.9395 respectively. R2 value adjusted R2 value of TCVDare 0.9412 and 0.9265 respectively. Both these models are significant with p < 0.05. Further to the analysis, RSM surface plots were drawn for both TPVD and TCVD models as shown in Figs. 9 and 10 respectively. Surface plots were drawn by holding one of the independent parameters constant and varying other two independent parameters. In both models, speed is noticed as a dominant parameter that effects the temperature rapidly. To this end, it is concluded that both tools are performing almost same in machining of titanium alloy, but comparatively PVD carbide tool is better than CVD carbide tool in machining Ti64 alloy.

Fig. 9
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Response surface plots ofTPVD model

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Fig. 10
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Response surface plots ofTCVD model

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Optimization of independent parameters for achieving the low cutting temperature was also attempted. Table 7 reveals the optimized parameters predicted by TCVD model, while Fig. 11 depicts the thermal image captured during the validation experiments. It is noticed that the biasness of models is within acceptable limit (maximum error < 1.2%) (Fig. 11).

Table 7 Optimized parameters for machining Ti64 alloy using CVD coated insert
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Fig. 11
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Thermal image at optimized cutting condition using CVD coated carbide insert

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Table 8 shows TPVD model predicted temperature and its respected independent parameters. Validation experiment was conducted, and thermal image was captured to measure the actual temperature for the same cutting condition (27 m/min, 0.10 mm/rev and 1.0 mm). Figure 12 shows the thermal image captured at the validation experiment. The actual temperature during machining and the predicted value is almost similar with ±1.6% of error. It is concluded from this statistical analysis that both these models can be applied in machining of Ti64 alloy using PVD and CVD carbide tools.

Table 8 Optimized parameters for machining Ti64 alloy using PVD coated insert
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Fig. 12
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Thermal image at optimized cutting condition using PVD coated carbide insert

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Conclusions

  • Coated carbide inserts such as PVD and CVD were used for machining hard-to-machine material. Cutting speed is the prevailing parameter that effecting high cutting temperature. Feed rate and depth of cut have less effect on the cutting temperature as compared to speed.

  • PVD and CVD carbide tools are used  for machining Ti64 alloy, as they produces less temperature than HSS and other related tools.

  • The chips obtained for different temperatures are illustrated and presented in detail.

  • Optimal cutting condition for machining Ti64 alloy was also predicted and validated experimentally. Predicted results have good agreement with experimental results.

  • The temperature measured through this method gives only approximate value, the near-nett value can be obtained by using more sophisticated methods and tools.