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Investigation of machining characteristics of hard-to-machine Ti-6Al-4V-ELI alloy for biomedical applications | Journal of Materials Research and Technology
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DOI: 10.1016/j.jmrt.2019.08.033
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Available online 10 September 2019
Investigation of machining characteristics of hard-to-machine Ti-6Al-4V-ELI alloy for biomedical applications
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Swasthik Pradhana, Sunpreet Singha, Chander Prakasha, Grzegorz Królczykb, Alokesh Pramanikc, Catalin Iulian Pruncud,e,
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c.pruncu@imperial.ac.uk

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a School of Mechanical Engineering, Lovely Professional University, Phagwara 144411, India
b Faculty of Mechanical Engineering, Opole University of Technology, 76 Proszkowska St., Opole 45-758, Poland
c Department of Mechanical Engineering, Curtin University, Bentley, Perth 6102, WA, Australia
d Mechanical Engineering, Imperial College London, Exhibition Rd., SW7 2AZ London, UK
e Mechanical Engineering, School of Engineering, University of Birmingham, Birmingham B15 2TT, UK
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Received 17 May 2019. Accepted 23 August 2019
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Table 1. Process parameter and experimental data.
Abstract

Dry machining of Ti-6Al-4V alloy was investigated using SNMA120408 grade inserts. The material studied is designed for orthopedic applications. The effects of main cutting speed (VC), at constant feed rate (F) and depth of cut (DC) on machining characteristics (Feed force (Ff), radial force (Rf), Tangential Force (Tf)) and surface integrity (i.e., tool-chip contact length, chip segmentation, surface roughness, and tool wear) were examined. Experimental data indicate the cutting speed as the major parameter with direct impact on the machining characteristics. Increasing of the cutting speed promotes higher tangential forces that allow a decrease of the chip contact length; a smaller contact length results in a lower surface roughness and flank wear rate, respectively. To gain further insight from the simulated turning process an advanced Finite Element (FE) model was developed. The numerical model was built on the DEFORM-3D commercial software by incorporating the experimental cutting parameters. The numerical simulations results agree very well with experimental outputs in terms of cutting forces (FCS), tool-chip (T-C) contact length. Therefore, it was possible to estimate with accuracy the effective stress (σE) and the cutting temperature (TC). Further, due to its high robustness, the numerical model developed can be implemented in solving the industrial challenge (i.e., biomedical field) for predicting formations of serrated chip segment, chip thickness, potential types of chips, types of fracture mechanism and tool wear mechanism/rate generated during machining process.

Keywords:
Titanium alloy
Serrated chip
Tool-chip contact length
Effective stress
Tool wear
Cutting temperature
DEFORM-3D
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1Introduction

Titanium and its alloys are very attractive for sensitive applications, especially for the biomedical industries due to their valuable properties. They are light weight, possess high strength, excellent fatigue performance and offer high resistance to aggressive environment [1,2]. Despite of their excellent properties, in practical applications, they prove major challenges during machining [3]. Ti alloys may present low thermal conductivity and high chemical affinity at the cutting-tool interfaces at elevated temperature. This leads to a bigger portion of heat being transferred into the cutting tool region that further generates relatively high-thermal load on tool. It, consequently, causes plastic deformation over the cutting edge and generates some hazards on the tool that further significantly reduce the tool life [4]. The titanium alloys have low elastic modulus and show spring-back phenomena while machining. The spring-back effects release the chattering on the machined surface that leads to have a poor surface quality of the machined materials [5]. The entanglement of chip with the cutting tool increases friction thus generating high temperature and chips welding progress. The welded chips layers may form continuously and stick to the workpiece which causes a catastrophic failure of the cutting tool. It then generate a severe damage to the workpiece during machining [6].

A significant number of methodologies were adopted by researchers in order to make the machining process of Ti-alloys more accessible [7]. Alexander et al. [5] studied the effect of cutting temperature on machining of Ti alloy that includes an automatic cutting fluid spray mechanism. They observed that the fluid penetrates dynamically inside the tool-chip (T-C) interface and therefore further improved the lubrication. Ojolo et al. [8] found that the cutting force (FC) may increase the T-C contact length thus reducing the cutting stability during machining of mild steel and aluminum alloy. Ramana et al. [9] investigated the effect of dry, flooded and minimum quantity lubrication (MQL) conditions along with different process parameters on the chip morphology of grade-5 Ti alloy. It was observed that MQL machining can reduce the temperature on the machining zone and the chip thickness ratio may become higher as compared to other machining conditions.

Anhai et al. [10] studied the effect of cutting velocity (VC) on the chips formation. Production of serrated chips becomes more evident at high cutting velocity. Guosheng and Zhanqiang [11] reported that the enlargement of tool-chip contact length along with the presence of a wedge shaped tool produce a change on the serrated chip that takes a trapezoid geometry. This is due to the chip side flow derived from high speed machining of pure Ti and its alloys. Khan et al. [12] examined the behavior of chip structure, surface roughness, and tool wear while machining AISI 9310 alloy. They found that the lower feed-rate (F) leads to generate mostly ribbon chips types whilst the higher F produces nearly tubular types of chips. The surface finish can be improved by tool wear reduction and controlling the damage near to the tool-tip. Ye et al. [13] identified the transformation of chip flow from continuous to serrated chip being dictated by the formation of repetitive shear bands within primary shear zone. Sun and Guo [14] observed during the milling of Ti alloy that the serrated tooth chips increase and the saw-tooth frequency decreases as a function of Vc. Venugopal et al. [15] studied the broadening of tool wear during machining of grade 5 Ti alloy. It was revealed that the adhesion, dissolution and diffusion mechanisms headed towards the formation of wear craters. Whilst, the abrasion and chemical attrition mechanisms led to the formation of flank wear. Sadik and Lindstrom [16] concluded that the chip-tool contact length decreases which helps to control the formation of flank wear, cutting temperature and cutting forces. Zhang et al. [17] and Rahman et al. [18] detected that the Vc and uncut thickness of the chips can have a great effect on the formation of strain in the serrated chip and temperature generation, whereas variation in the rake angle may have little impact on the serrated chips.

Ti-6Al-4V alloys are the most widely used commercially available materials for the fabrication of implants, orthopedic accessories, and surgical instruments. However, their surface characteristics/superficial layer obtained by machining process affect the implant performance. So, there is an urgent requirement in the investigation of the surface and machining characteristics of Ti-6Al-4V alloys.

In this study, experimental tests and FE analyses were performed simulating the machining of Ti-6Al-4V produced for biomedical applications. Micro-modeling of Ti-machining has been carried out in order to assess the effect of cutting speed, feed rate, depth of cut on cutting forces, cutting temperature, thermal stresses and T-C contact length on the surface morphology. The modeling, simulation and post-processing of Ti by machining have been conducted using the DEFROM-3D commercial software. The values determined for the T-C contact length and FC gathered from FE simulations were compared with the experimental ones. The results agree well, with only a minor error of 6–12%. Further analysis assessed the effects of cutting speed over effective stress, strain, cutting tool interface temperature and chip tool contact length. The methodology developed can be implemented in solving the industrial challenge for predicting the formations of serrated chip segment, chip thickness, potential types of chips, types of fracture mechanism and tool wear mechanism/rate generated during machining process.

2Experimental tests and finite element simulations2.1Experimentations

The experiment was carried out on the lathe machine (make HMT manufacturer, model no NH26). A biomedical alloy, Ti-6Al-4V grade-5, was used as workpiece material during the turning operation. The THM-08 cutting insert denoted by ISO SNMA120408 grade was chosen as counterpart [19]. The material of THM-08 cutting inserts was an uncoated carbide having good balance of material properties (i.e., higher hardness, good edge stability and great toughness). Fig. 1 depicts the Energy Dispersive Spectroscopy (EDS) analysis from THM-08 and Ti alloy. The diameter and length of the Ti alloy specimens used in the experiments was 60 and 500mm, respectively. Table 1 report the combinations of machining parameters considered in this investigation. Each experiment was replicated three times in order to have statistically significant results. Variations of output parameters were found to be below 0.05% of the confidence interval. The feed rate (F) and depth of cut (Dc) were kept constant of 1mm/rev and 0.5mm, respectively, while the cutting speed (Vc) was varied from 65 to 124m/min. The output responses measures during machining operation are the cutting forces (FCS) feed, radial and tangential forces in all three directions i.e. X, Y and Z axis; the machined surface roughness and cutting inserts flank wear. Table 1 presents the average of the cutting forces. The dynamometer with tool holder was mounted on the lathe carriage to record the cutting forces. The force indicator was connected to the dynamometer to display the FCS, as can be seen in Fig. 2. In this work, the surface roughness of the produced samples was measured by using Taylor Hobson Surtronic. The wear on the flank surface and length of contact in T-C interface were measured using the optical microscope. The micro-hardness of the machined surface was measured using a micro-hardness tester. In order to measure hardness from the top surface to bottom surface the specimen was cut from the machined surface and then a desired surface finish was achieved through polishing.

Fig. 1.

Details of EDS analysis made over the cutting tool and workpiece, respectively.

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Table 1.

Process parameter and experimental data.

Cutting speed (m/min)  Feed (mm/rev)  Depth of cut (mm)  Feed Force (N)  Radial Force (N)  Tangential Force (N)  Surface roughness (μm)  Flank wear (mm) 
65  0.10.587  84  262  0.973  0.087 
112  83  78  203  0.76  0.032 
124  103  81  304  0.893  0.067 
Fig. 2.

Schematic representation of experimental setup.

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Furthermore, chips morphology was analyzed through the scanning electron microscope (SEM). For surface characterization and its morphology the samples were polished by using different grades of sand paper and finally mirror finish was achieved through diamond polishing. Finally, etching was done to detect the metallographic structure (alpha-beta) of the titanium alloy using optical microscope and SEM.

2.2Finite element modeling and analysis

FE simulation were carried out on the DEFORM 3D software to reproduce the experimental machining operation. This was done according to the experimental cutting parameter’s value. The cutting inserts were modeled as the SNMA120408 specification and were created in Solidworks 2012 software. The STL file thus created was exported to the DEFORM 3D commercial software. Fig. 3 depicts the chip formation generated using the FEM, that is generated at the contact between the workpiece with the cutting tool.

Fig. 3.

Picture of formation of chip during simulation process.

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In this model, the cutting inserts and workpiece were considered as rigid and plastic materials, respectively. The workpiece and cutting inserts were meshed with 25,000 tetrahedral elements. Mesh size was determined via convergence analysis in order to obtain mesh independent solutions. A high density mesh of 0.001mm element’s size was applied at the interface between the cutting side of the insert and the workpiece. The finer mesh accurately simulates heat exchange with the environment. Furthermore, it allows suitable transmission of the heat between the cutting insert and chip, heat that is produced from the workpiece due to friction and plastic deformation progress [20]. The materials simulated in the FE model (insert: tungsten carbide (WC) and workpiece: Ti-6Al-4V alloy) are related to DEFORM-3D material library. The DEFORM-3D database also provided the thermal properties for both cutting tool and the workpiece at ambient temperature (˜23°C). The model includes the friction generation during machining that was built as per the constant shear stress hypothesis, that is, τ=m⋅k. Where, k is the shear flow stress and for dry cutting condition the (m) coefficient is usually set between 0.8 and 0.9 [21,22]. Simulations were performed using the incremental Lagrangian strategy, with direct iteration method of sparse solver. This simulation strategy was continuously run for a number of iterations until reaching the total cutting length. The following equation was used for the flow stress calculation of titanium alloy.

Where σ¯ is the flow stress of the workpiece, ε¯ is the effective plastic strain, ε¯. is the effective strain rate and T is the temperature. This model well describes material behavior due to its ability to predict the true flow stress value.

The cutting force values in X, Y and Z direction computed by FE simulation were validated against experiments. Fig. 4 depicts the results gathered from the simulation and experimental protocol for the FCS at different Vc of 65–124m/min. A rather small difference was observed between the simulated and experimental data, yet the agreement is very good with only an average error of 7.11%.

Fig. 4.

Comparison between simulated and experimentally measured cutting forces a) feed force, b) radial force and c) tangential force.

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3Results and discussion3.1Effects of Vc over output responses

The machining of Ti alloy using the THM08 cutting inserts was performed using different cutting speed Vc of 65m/min, 112m/min and 124m/min, respectively. The output responses received from the experiments was tool wear, FCS, surface roughness and the amount of flank wear as presented in Table 1. The feed force, radial force, and tangential force first decreased as cutting speed passed from 65m/min to 112m/min and then increased if cutting speed increased from 112m/min to 124m/min. The minimum feed force, radial force, and tangential force 83N, 78N, and 203N, respectively, were obtained at cutting speed of 112m/min. The surface roughness increased with cutting speed. This is due to the fact that with the increase in cutting speed the friction at the machining zone increased. Consequently, large burrs were produced on the machined surface, consequently deteriorating surface finish. The minimum (0.76μm) and maximum (0.973μm) value of surface roughness was obtained at cutting speed of 112m/min and 65m/min, respectively. The flank wear also increased with the increase in cutting speed owing to high friction at the cutting zone. The maximum flank wear (0.087mm) was obtained at high cutting speed. The best machining performance was obtained at cutting sped of 112m/min.

3.2Analysis of chip morphology

The geometry of formed chips and the way in which chips come in contact with the tool tip during machining process affect the output parameters. Variations in cutting parameters may affect the surface quality, heat generation and wear accumulation near the cutting edge with direct consequences on the life of the cutting inserts. When the continuous and discontinuous chips are generated at the contact between rake and flank interface (T-C), it may produce vibrations that later affect the surface quality of the machined part and finally lead to accentuated wear. The crater wear evolution and T-C contact length on the rake face of the cutting insert also depend on chip formation mechanisms. Therefore, it is important to recognize the nature of chip produced and chip morphology associated to the variation of cutting parameters. Here, chip thickness was found not to vary significantly, since the machining was performed at fixed levels of F and Dc (see details in Fig. 5). Fig. 5(a) through 5(c) shows the micrographs where the chip thickness reached at most 95 μm”.

Fig. 5.

Chip thickness during machining of Ti-6Al-4V at different cutting speed (a) 65m/min, (b) 112m/min and (c) 124m/min.

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Figs. 6(a) through 6(c) show typical images with the chips obtained during the Ti machining. It may vary slightly in shape and size as a function of Vc. Generally, snarled helical types of chips were generated and the shape of the chip remains almost same at different Vc[23]. A regular helical shape pattern was observed only for Vc=112m/min (Fig. 6(b)), but in case of other Vc an irregular pattern of the helix shape was observed. This variation may be produced because of the higher tool wear rate at Vc of 65m/min and 124m/min as compared to Vc of 112m/min.

Fig. 6.

Chips formed during machining of Ti-6Al-4V at different cutting speed (a) 65m/min, (b) 112m/min and (c) 124m/min.

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Serrations and free surfaces of chips generated at different Vc are shown in Fig. 7. Chip serrations increased with Vc (see Figs. 7(a) through 7(c). Formation of the chip segments at Vc of 65m/min is irregular, such as the height of the individual serrated chip is irregular as well as the sporadic shear initiation [24] is visible (see Fig. 7 (a)). Using a Vc of 112m/min, the serration of chip was more prominent and the chip segment shape presents also a regular pattern (see Fig. 6 (b)). At Vc of 124m/min, the formations of the chip segment were precise and compact. Segments were regularly arranged and look denser. Moreover, lamellae generated on chip free surface become denser indicate that by an increase of the Vc the lamellae produced on the free surface of chip was increased. Therefore, by increasing the Vc from 65m/min to 124m/min, the plastically deformed material becomes more intense due to the thermal softening effects. Fig. 8 confirms that plasticization is more significant in the end part of chip segment corresponding to the chip side flow [11].

Fig. 7.

Serrations and free surfaces of chips obtained for different cutting speeds: a) 65m/min; b) 112m/min; c) 124m/min.

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Fig. 8.

The main types of fracture observed that lead to the formation of chip segments.

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The cutting tool chips formation takes places while workpiece moves towards. Therefore, different chip segment-shapes form due to the deformation of the segment edge along the shear plane; and due to the compression of the segment length driven by the fracture and plastic strain [25]. The shape of the chip segment varies according to the change in Vc. At 65m/min, an irregular shape of the chip segment is visualized (as depicted in Fig. 7a). At 112m/min, the serration of the chip segment follows a regular pattern, but the segments of the chip are not in proper shape (Fig. 7b). Whereas, at 124m/min the serration of the chip segment was arranged in a regular pattern as well as a trapezoidal shape of the segment was observed (Fig. 7c).

The chip is divided in two segments, i.e., the first parts of the segment were associated to the shear plane and the second part of the segment was separated over the other segment. The chip segment was formed due to the fracture and plastic strain (see details of Fig. 8). Some variation was detected in the length of the fracture and plastic strain because of the presence of shear and normal fracture modes. If the separated part is longer than the attached part, i.e., the contribution of fracture is pronounced during the chip segment formation, the chip undergoes mode I fracture. However, when the segment formation is due to the facture under shear load is termed as mode II fracture [25]. The type of fracture observed during high speed machining is illustrated in Fig. 8.

3.3Analysis of tool wear

Tool wear plays an important role during machining. The wear out cutting edge of the cutting insert after machining process was evaluated using SEM and optical microscope. It was possible to identify the occurrence of various types of wear on the rake edge and flank edge of the insert due to variations of cutting parameters. The tool wear increases as, as the shear zone temperatures increase. This is in turn related with friction between T-C zone [26]. The deposition of chip material indicates potentially formation of the crater wear which is associated to the adhesion wear [15].

The tool wear generated by machining of Ti alloy was detected mainly on the flank and rake face of the cutting tool. As shown in Fig. 9 some difference may occur caused by different Vc. It has been found that with an increase of Vc from 65m/min to 112m/min, the flank wear was reduced from 0.087mm to 0.032mm. However, with a further increase to 124m/min the flank wear increased to 0.067mm. The T-C contact length measured on the rake face at Vc of 65m/min, 112m/min and 124m/min was 0.163mm, 0.213mm and 0.229mm, respectively. It can be observed that, least amount of wear was recorded during machining of Ti alloy when the speed is around 112m/min. This may be due to the smallest amount of contact between T-C interfaces whilst the cutting force exerted during machining is lower.

Fig. 9.

Tool wear during machining of Ti-6Al-4V at different cutting speeds: a) 65m/min; b) 112m/min; c) 124m/min.

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3.4Analysis of micro-hardness

In the case of Ti alloys, machined surfaces and sub-surface layers experience hardening. A report published by Ezugwu and Tang [27] highlighted the presence of compressive stress and pressure at the tool edge. Chou [28] revealed that cycling heating-cooling occurring during machining results in hardening. Zou et al. [29] found that the variation in Vc, F, Dc during machining contributes towards the evolution of micro-hardness.

Fig. 10 (a) shows the indentation patterns produced from the micro-hardness measurement on the machined surface. The Vickers pyramid number (HV) distribution was measured by applying a load of 300 gf with the dwell time of 10s. The average of three hardness values was recorded on a consecutive distance of 260μm. Micro-hardness was measured for each value of Vc in order to assess the corresponding variation trend. Fig. 10(b) shows that micro-hardness increases with Vc. By gradually increasing the Vc (65, 112 and 124m/min) during dry machining it elevates the hardening of the machined surface [30]. Fig. 10 (c) depicts the change in the micro-hardness with respect to distance from the edge of the machined sub-surfaces towards to the center of the machined sub-surface. The first indentation was made at 250μm from the edge and after that 6 consecutive indentations were made at a regular interval of 340μm. An average of 7 indents per specimen was taken. Fig. 10 (d) shows the variation of Vickers hardness values with respect to the distance from the edge towards the center of the machined sub-surface. A high value of micro-hardness was observed at the edge of the machined surface and a progressive reduction in hardness as we move towards the center. Since the outermost layer of the machined surface was exposed to the work hardening process, micro-hardness was higher [31].

Fig. 10.

Details of the Micro-hardness measurement.

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3.5Morphology of machined surface3.5.1Machined surface analysis

Microstructure observation of surface after machining reveals a fairly good quality. However, when machined samples are observed under Field Emission Scanning Electron Microscope (FE-SEM) or Scanning Electron Microscope (SEM), various types of surface damages can be observed on the machined surface such as deformation feed marks, feed marks irregularities, surface tears, micro-pits, chip fragments, flaw, built-up edge (BUE) and re-deposited of workpiece material [30,32,33]. Yellow circles, yellow arrows, red arrows, and white arrows represent the feed marks, adhered particles, BUE, chip fragments, respectively.

Figs. 11(a) through 11(c) shows details obtained from FESEM images of the Ti alloy machined surface generated with uncoated tungsten carbide inserts. The machined surface of the Ti alloy consisted of identical feed patterns along the lateral direction. This is because of the plastic flow of the work material. Hence, variations in the surface roughness and residual stress may occur due to the plastic flow of material over the machined surface [34,35]. It was observed that some of the chip adhered to the machined surface. The cause can be the generation of high temperature during machining of Ti-6Al-4V. Furthermore, the presence of built-up edge of the chip fragments was seen to deteriorate surface [33].

Fig. 11.

Surface imperfections of machined Ti alloy specimens (a) Cutting speed 65m/min, (b) Cutting speed 112m/min, and (c) Cutting speed 124m/min.

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3.5.2Sub-surface microstructure alteration

Metallographic samples of the machined sub-surface of Ti alloy were prepared to depict the microstructural micrographs at different Vc using scanning electron microscopy and optical microscopy. In order to view the microstructure of the machined sub-surface, the sample undergoes through several specimen preparation steps. The sample was mounted in order to handle it in an easier way. Polishing of the sample was carried out using finer and finer grits (i.e., 600–2500 grit paper mesh size), after that the sample was polished with diamond slurry. When a mirror polish was achieved, on the sub-surface of the sample, the etching of the sample was done with etchant “Kroll’s Reagent” to reveal the microstructure.

Figs. 12(a) through 12(c) show the optical microscopy images while Figs. 12(d) through 12(f) show SEM micrographs of the machined sub-surface at different Vc (i.e. 65, 112 and 124m/min, respectively). It is observed the occurrence of the widmanstatten microstructure (α+β) [36–38]. This remark is also known as basket weave structure. It reveals that the heat generated during machining of Ti alloys plays an important role in the microstructural changes on the sub-surface of manufactured surface. Due to the high heat generation, the prior β grains have transformed to acicular or needle or thin ribs like α lamellae in prior β grain boundaries and some newly α lamella were formed Figs. 12(a) through 12(c) and (f). The secondary region of the α lamella corresponds to the high temperature β phase region that is formed when the temperature generates during machining close to the β transition (see Fig. 12(b)). When the β phase is heated then the microstructure consists of complete martensite Fig. 12 (d). The retained prior α lamella are distorted in nature due to the variation of local stress during machining [39]. During machining of Ti alloy the cutting temperature at Vc=112m/min was lower than those corresponding to Vc=65 and 124m/min. Hence, the microstructure alteration in the sub-surface of the machined surface at 112m/min was less pronounced (see Fig. 12(e)).

Fig. 12.

Optical microscope images and FE-SEM images of the machined surface textured beneath at different cutting speed (a–b) 65m/min, (c–d) 112m/min, and (e–f) 124m/min.

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3.6Cutting temperature

During the machining, the temperature of the interaction area of the cutting tool and workpiece becomes high. Figs. 13(a) through 13(c) depict the variation of the cutting temperature with increasing Vc in the range of 65–124m/min. The results were gathered from FE simulations run using Ti alloy with uncoated tungsten carbide cutting tool. The general trend shows that the cutting temperature is proportional with the Vc for all types of cutting tools. This implies increasing of the deformation and strain rate that finally cause high tool wear [40–42]. Fig. 13 (d) shows the numerical value obtained from temperature variation with respect to the cutting speed Vc. It can be seen that cutting temperature does not increase monotonically with cutting speed. However, the highest cutting temperature (566°C) was reached at the very high cutting speed of 124m/min.

Fig. 13.

Evolution of the cutting temperature during simulation of Ti-6Al-4V machined specimens with different cutting speed values.

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3.7Stress distribution

The principal stress is the resultant normal stress of the axis which has zero shear stress. DEFORM-3D uses the Von mises stress or σE to analyze the characteristics of stress distribution. The σE is defined as follows:

where σ1, σ2, and σ3 are the three principal stresses [43].

Figs. 14(a) through 14(c) show the variation of maximum stress computed from FE simulations. The stress concentration around the surface of the chip was higher at low Vc and it decreases for higher values of Vc. The computed stress ranged between 1400 and 1450MPa (see Fig. 14 (d)).

Fig. 14.

Evolution of the effective stress during simulation of Ti-6Al-4V machined specimens with different cutting speed values.

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3.8Chip-tool contact length and tool wear rate

During machining process, the cutting tool is forced and the workpiece is being cut. This operation causes the formation of chip in the shearing zone that moves continually over the rake surface of the cutting inserts. An impression occurs on the rake face of the cutting tool, because of the continuous flow of the chips. The chips come in contact with the cutting inserts from the beginning of cutting edge to the area up to which its leaves the surface of cutting tool. T-C contact length defines the distance covered during the entering and leaving of the chips on the cutting tool [16]. If the T-C contact length is high a large amount of heat will be dissipated into the cutting tool. Generally, the T-C contact length varies according to the variation of the cutting parameters, workpiece materials, and geometry of the cutting tool. Normally, it should not be larger nor smaller, but appropriate. An un-standardized chip length will affect the thermal and mechanical characteristics produced during the cutting operation [44,45]. Hence, the types of the chip formed and the formation of the crater wear on the cutting tool also depend on the T-C contact length.

Figs. 15(a) through 15(c) compare the T-C contact lengths observed in the experiments for the different Vc values with their counterpart simulated via FE analyses. A good agreement between experimental and numerical results can be seen. In end, Fig. 15 (e), highlights that the tool wear rate increases with the Vc.

Fig. 15.

Tool-chip contact length obtained during simulation of Ti-6Al-4V confronted with experiment results.

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4Conclusions

The present study analyzed the mechanical response occurring in the machining process of Ti-6Al-4V alloy with regular SNMA120408 uncoated cutting insert. Furthermore, a comprehensive finite element model developed to solve the industrial challenge (i.e., biomedical field) by a better understanding of the mechanism/parameters that drive the hard contact between the tool and material surface when machining a hard-to-machine alloy as Ti-6Al-4V. The following conclusions can be drawn from the present work:

  • Results of simulations and experimental tests were in good agreement: for feed force, radial force and tangential force, average errors were 6%, 8% and 12%, respectively.

  • The formation of the chip segment during machining of Ti alloy revealed that the fracture and plastic strain lead to the formation of generic segment. The chip segment occurs due to the mode I and mode II type of fracture.

  • Tool wear detected in this study was mostly due to adhesion. It is developed by transferring the work material on the rake face. Besides, some wear craters were noted on the tools. T-C contact lengths determined experimentally also were in good agreement with FE simulations (average error was only 18%). The small variations in the T-C contact length led to non-uniformity amongst the resulted chips and crater wears.

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