Processing of Titanium by Machining: A Closer Look Into Performance Metrics

in Bio-Fabrications
Mehdi Hourmand, University of Malaya, Kuala Lumpur, Malaysia
Mohammad S Uddin, University of South Australia, Adelaide, SA, Australia
Ahmed AD Sarhan, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia
r 2017 Elsevier Inc. All rights reserved.

1 Introduction 1
2 Types, Microstructure, and Mechanical Properties of Titanium Alloys 1
3 Tools Materials 3
4 Machining of Titanium Alloys 3
4.1 Chip Formations 4
4.2 Cutting Forces 5
4.3 Heat Generations 6
4.4 Tool Wear and Wear Mechanics 6
4.5 Surface Integrity (e.g., Roughness, Hardness, Residual Stress) 8
4.5.1 Surface defects 9
4.5.2 Microstructural alterations 9
4.5.2.1 Plastic deformation 10
4.5.3 Surface roughness 12
5 Mechanical Properties 13
5.1 Residual Stresses 13
5.2 Work Hardening Layer Formation and Microhardness 13
5.3 Fatigue 14
6 Effect of Machining on Biomechanical Performance 15
6.1 Corrosion Resistance 15
6.2 Biocompatibility 15
6.3 Cell Growth 16
7 Techniques to Improve Machining Performance 16
7.1 Vibration Assisted Machining 16
7.2 Cooling Systems 16
7.3 Laser Assisted Machining 20
8 Conclusion and Future Work 21
Acknowledgements 21
References 21

1 Introduction

As biomaterials have been developing tremendously over the recent decades, medical implantations in orthopedic, cardiovascular,
and pediatric applications are becoming the most successful surgical procedures. They offer decent solutions to damaged or fractured
bones and narrowed coronary arteries. For example, 54% of injury hospitalization rate is dedicated to bone fractures in Australia [1],
and this number is increasing all over the world due to the aging population [2,3]. At the time being, permanent metallic implants
made of stainless steel, cobalt–chromium, and titanium (Ti) alloys are used for fixing fractured bones [4–6]. A titanium plate for
cranioplasty is considered to be safe for implantation in humans, and it is one of the most widely used biomaterials for calvarial
fixation or reconstruction in Japan. Although the use of titanium implants in cranioplasty has recently become common, long-term
results of this strategy are sometimes difficult to manage. Involvement of plastic surgeons in initial considerations and inter-
disciplinary management of cranioplasties should be considered more routinely [7]. Moreover, low Young’s modulus Ti alloys and
titanium foam can be useful in practical applications like implant devices used for replacing failed hard tissue [8,9]. Meanwhile, some
types of pure titanium and titanium alloys are commercially available as dental implants [10–12].

2 Types, Microstructure, and Mechanical Properties of Titanium Alloys

Titanium (Ti) is a silvery white metal that was discovered in 1791 [13]. Researchers attempted to apply titanium for implant
fabrication in 1930 [14]. In the early 1950s, Ti and its alloys became backbone materials for biomedicine, energy, chemical, and

Reference Module in Materials Science and Materials Engineering doi:10.1016/B978-0-12-803581-8.10381-9 1

2 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

aerospace industries. The combination of corrosion resistance, high strength-to-weight ratio, excellent mechanical properties, good
biocompatibility (bio-inert), and maintaining its mechanical properties at relatively high working temperatures makes Ti an
excellent material for different critical applications [15,16]. However, the high cost of titanium is primarily due to two factors: high
affinity for interstitials such as oxygen, nitrogen, and carbon that decrease its ductility (i.e., need of particular and expensive
processing to limit their contents) and poor machinability as a consequence of its poor thermal conductivity [15,17].
Titanium alloys can be categorized to unalloyed titanium, alpha (a) alloys, beta (b) alloys, and alpha–beta (a–b) titanium
alloys [18]. Fig. 1 illustrates the phase diagram of the titanium alloys. Unalloyed commercially pure (cp) titanium are classified
into four grades that can be used for surgical implant applications [14]. Table 1 shows chemical compositions of unalloyed
Ti and Ti alloys.
Ti is an allotropic element that exists in various crystallographic forms. Ti has a hexagonal close-packed (hcp) crystal structure at
room temperature, which is referred to as its “alpha” phase. This structure transforms to a body-centered cubic (bcc) crystal
structure, called the “beta” phase, at 8831C (16211F) [18].
Alloying elements are categorized as a or b stabilizers. a stabilizers, like oxygen and aluminum, increase the temperature when
the a phase is stable. b stabilizers, like molybdenum and vanadium, at lower temperatures are responsible for stability of the b
phase. This temperature transformation from an a–b phase (or all-a phases) to all b phases is known as the b transus temperature.
The b transus is considered as the lowest equilibrium temperature at which the material is 100% b [18].
a alloys have essentially an all-a microstructure. b alloys are those alloys from which a small volume of material can be
quenched into ice water from above its b transus without martensitic decomposition of the b phase. a–b alloys contain a mixture
of a and b phases at room temperature. Within the a–b class, an alloy that contains less than 2–3% b, such as Ti–8Al–1Mo–1V,
may also be referred to as a “near-a” or “super-a” alloy. The b alloys can be further broken down into b and “near-b.” This
distinction is necessary, because the phase transformations that occur, the reaction kinetics, and the processing could be different if
the alloy is a near-b (lean) alloy, such as Ti–10V–2Fe–3Al, or a rich b alloy, such as Ti–13V–11Cr–3Al [18].
Ti6Al4V is one of the titanium alloys that is widely used to produce implants. Aluminum and vanadium are the main alloying
elements of Ti6Al4V as shown in Table 1. Fig. 2 demonstrates the microstructures of Ti–6Al–4V a–b alloy. Meanwhile, the
mechanical properties of Ti and its alloys are illustrated in Table 2.

Fig. 1 Phase diagram of the titanium alloys. Reproduced from Arrazola, P.J., Garay, A., Iriarte, L.-M., et al., 2009. Machinability of titanium alloys
(Ti6Al4V and Ti555.3). Journal of Materials Processing Technology 209 (5), 2223–2230.

Table 1 Chemical compositions of unalloyed Ti and Ti alloys

Element Grade 1 Grade 2 Grade 3 Grade 4 Ti6Al4V

Nitrogen 0.03 0.03 0.05 0.05 0.05

Carbon 0.10 0.10 0.10 0.10 0.08
Hydrogen 0.015 0.015 0.015 0.015 0.0125
Iron 0.20 0.30 0.30 0.50 0.25
Oxygen 0.18 0.25 0.35 0.40 0.13
Aluminum – – – – 6.00% (5.50B6.50)
Vanadium – – – – 4.00% (3.50B4.50)
Other elements – – – – 0.1% maximum or 0.4% total
Titanium Balance Balance Balance Balance Balance

Source: Reproduced from Park, J.B., Bronzino, J.D., 2002. Biomaterials: Principles and Applications. Boca Raton, FL: CRC Press.
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 3

Fig. 2 Microstructural feature of Ti–6Al–4V a–b alloy. Reproduced from Wang, Q., et al., 2017. Metallurgical-based prediction of stress-
temperature induced rapid heating and cooling phase transformations for high speed machining Ti–6Al–4V alloy. Materials & Design 119, 208–218.

Table 2 Mechanical properties of Ti and Ti alloys

Property Grade 1 Grade 2 Grade 3 Grade 4 Ti6Al4V Ti13Nb13Zr

Tensile strength (MPa) 240 345 450 550 860 1030

Yield strength (0.2% offset) (MPa) 170 275 380 485 795 900
Elongation (%) 24 20 18 15 10 15
Reduction of area (%) 30 30 30 25 25 45

Source: Reproduced from Park, J.B., Bronzino, J.D., 2002. Biomaterials: Principles and Applications. Boca Raton, FL: CRC Press.

3 Tools Materials

The properties required of tool materials for machining titanium are [19]:
1. fatigue resistance and toughness also to withstand the chip segmentation process;
2. high hardness at elevated temperatures to resist the high stresses involved;
3. chipping resistance especially because of the formation of segmented chip;
4. excellent compressive strength;
5. reduced tendency to react with titanium; and
6. high thermal conductivity to minimize thermal gradients and thermal shock on the tool.

Some of the previous researchers used the following cutting tools for machining of Ti alloys: uncoated carbide insert [20];
coated carbide insert with monolayer AlTiN, TiCN, TiAlN, and two layers TiN þ AlTiN and TiN þ TiCN [21]; uncoated carbide
“WC–Ti/Ta/Nb–Co” and multilayer chemical vapor deposition (CVD)-coated alloyed carbide “WC–Ti/Ta/Nb–Co þ TiN/TiC/TiCN”
[22]; coated carbide inserts “ZPMT 09T208 R with ISO JC5015 grade” [23]; cemented carbide with TiAlN [24]; WC cutting inserts
with a TiAlN ceramic coating on the top of a layer of titanium-nitride (CP500) [25]; DAEWOO ACE-V500 using coated carbide
cutting [26]; binderless cubic boron nitride (binderless CBN) [27]. Coated tungsten carbide tool has a better performance com-
pared to the uncoated one because of lower tool wear, longer tool life, producing smoother machined surface, and lower friction.

4 Machining of Titanium Alloys

Machining of Ti alloy is very challenging. Ti alloys are considered as difficult-to-cut materials because of [19,28–34] (1)
high-temperature strength; (2) low thermal conductivity; (3) low modulus of elasticity; (4) a relatively small contact area is created
that leads to high stress on the tool edges; (5) the strong chemical affinity of Ti to the cutting tool materials, including the coating,
leads to intensive adherence interaction to the chip/tool interface resulting in built up edge formation. The temperature generation
in Ti machining is much higher than those in machining of steel [32]. These phenomena are responsible for the rapid wear of
WC tools [32,34].
Fig. 3 shows that the chips are adhered and welded to machined Ti6Al4V after sild-milling using tungsten carbide end mill tool.
As can be seen in Fig. 3, the chips are adhered and welded to the wall of titanium alloy from built of egged on end mill. Built of
egged in milling tools is one of main problem in machining of Ti alloy.
4 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 3 The chips welded and joined to machined Ti6Al4V after sild-milling using tungsten carbide end mill tool.

Fig. 4 Type of chips, morphology of the chip undersurface and chip cross-sections for the (a–d) uncoated and (e–h) TiB2 coated WC–Co tools
during machining of Ti6Al4V alloy. Reproduced from Chowdhury, M.S.I., et al., 2017. Wear behaviour of coated carbide tools during machining of
Ti6Al4V aerospace alloy associated with strong built up edge formation. Surface and Coatings Technology 313, 319–327.

4.1 Chip Formations

Titanium chips are typically segmented and continuous. The segmentation is very significant in machining of Ti alloys. Narrow
bands of intensely sheared metal are being separated by broader areas and only lightly sheared. The intensely sheared layers are
thermoplastic shear bands, to which Ti is particularly sensitive due to its low thermal conductivity and thermal properties [35].
Each period of thermoplastic shear is short-lived and relieves the pressure in the “segmentation cycle.” Then, compressive strain
continues by dissociative movement till the following thermoplastic shear band is initiated. At the surface of tool, the flow-zone is
continuous and bonds strongly to WC or high speed steel (HSS) tools [35].
Fig. 4 shows type of chips, morphology of the chip undersurface, chip cross-sections for the (Fig. 4(a)–(d)) uncoated and
(Fig. 4(e)–(h)) TiB2 coated WC–Co tools during machining of Ti6Al4V alloy. Chip characteristics indicate improvement in
tribological performance of TiB2 coating versus uncoated tool. Chip thickness, chip compression ratio, and shear angle are better
for the TiB2 coating as compared to the TiAlN coating [36]. Basically, the undersurface morphology and the chip type are direct
indicators of frictional conditions at the interface between tool and chip [36,37]. Lower wear and friction lead to curlier chips and
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 5

Fig. 5 Back and free surfaces of chips during high-speed milling of Ti6Al4V. Reproduced from Kuram, E., 2016. The effect of monolayer TiCN-,
AlTiN-, TiAlN- and two layers TiCN þ TiN- and AlTiN þ TiN-coated cutting tools on tool wear, cutting force, surface roughness and chip morphology
during high-speed milling of Ti6Al4V titanium alloy. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering
Manufacture. Available at: http://dx.doi.org/10.1177/0954405416666905.

smoother chips' undersurface morphology. This was more prominent for the TiB2 coating (Fig. 4(e)) in comparison to the
uncoated tool (Fig. 4(a)). Chip cross-sections were studied as well. Chips formed for the uncoated tools have more intensive metal
flow at the tool–chip interface (Fig. 4(c)). This results in twinning and formation of the strain induced martensite plates. In
contrast, no martensite formation is observed for the TiB2 coated tools. This corresponds to the tribological data and observation
of undersurface morphology of the chips (Fig. 4) [36].
Fig. 5 illustrates different chip morphologies at the free and back surface of the chips during high-speed milling of Ti6Al4V.
It was observed that back surface of the chips was shinier and smoother than the free surface of the chips [21]. This result
was attributed to the combined effects of high contact pressures, frictional forces, and temperatures [21,38]. These shear stresses
and high contact occurred as the back surface slides over the rake face of the cutting tool. Also, rake face of the cutting
tool constrained the deformation of the back surface. These two constraints were synergistic and caused the smooth back
surface [21,39]. It was seen from Fig. 5 that the structure of the free surface was lamella. It was also observed from views of chips
that chip serrations occurred across the width of the chips in all coating materials. These teeth are named as “primary serrated
teeth.” At the upper or lower edge of the chips, serrated elements were observed. These larger elements are named as “secondary
serrated teeth” [21,40].

4.2 Cutting Forces

The machining characteristics of titanium so far were considered differently in several aspects from that of the other materials and
pure metals. The cutting forces and power consumptions are remarkably lower than the machining of copper, nickel, and iron.
This is a special case of the low speed range as shown in Fig. 6 [35].
Fig. 7 shows the effect of machining parameters on chip morphology, cutting force, and frequency of cyclic force in dry turning
of Ti6Al4V. It was reported that severe tool vibration happened with cutting speed larger than 60 m/min and feed less than
0.122 mm at any depth of cut. This force oscillation, which happened at high cutting speed and low feed rates, is attributed to the
high cutting temperature and low modulus of elasticity of Ti. This vibration produced chips with variety of width and thickness
(Fig. 8(a)) and led to chatter, cutting tool tip breakage, and rough machined surface [41]. If the frequency of these pulsating loads
does not happen in phase with the natural frequency of the machine, the variety of cutting forces can lead to self-excited chatter,
which causes the tool to break or pull out [28,29,42].
The cutting force augmented with the feed rate and depth of cut at constant cutting speed due to high material removal rate
(MRR). Cutting force increased by increasing of the cutting speed from 10 to 16 m/min and 57 to 75 m/min, which resulted in the
strain rate of hardening at low and high strain rates respectively. Due to the thermal softening of the material, cutting force
decreased with augmenting cutting speed outside these speed ranges [41]. Moreover, the cutting force augmented with augmenting
of the depth of cut and feed rate and decreasing cutting speed in high speed milling of Ti6Al4V [27].
In turning of Ti6Al4V, the formation of segmented chips produces a cyclic force and the frequency of force was similar to the
frequency of chip segmentation. The frequency of cyclic force is augmented linearly with cutting speed and reduced inversely with
feed rate [41]. The amplitude variation of the cyclic force with high-frequency is linked to the formation of segmented chip, which
is augmented with augmenting feed rate and depth of cut, and reduced with augmenting from 57 m/min cutting speed except at
the cutting speeds that harmonic vibration of the machine take places [41].
6 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 6 Comparison of cutting force vs. cutting speed in machining of nickel, iron, and titanium. Reproduced from Smart, E., Trent, E., 1975.
Temperature distribution in tools used for cutting iron, titanium and nickel. The International Journal of Production Research 13 (3), 265–290.

4.3 Heat Generations

Investigation on the nature of the heat source in the case of using cutting tool in machining of titanium is more challenging than
machining of iron. A tensile failure in the Ti in the area of the flow zone caused a thin layer of Ti chip to be welded to the surface of
cutting tool [43]. While cutting the length of Ti is completed, contact is much shorter than machining of iron, however, the most
significant difference relates to the very close region to the cutting edge, that the temperature increases far more quickly during
machining of Ti [43]. In situations of the high machining speed of Ti6Al4V the cutting temperature on the tool–chip interface
readily hit 10001C or even beyond that [44].
The major issue in machining titanium is the short tool life. The allowed rates of metal removal are low, even with low cutting
force. It is the high temperature and undesirable heat dispersion in the cutting tools used for machining of iron at similar speed
that leads to short tool life [35,43]. The temperature in the flow area is higher than during machining of iron at similar speed [35].
The highest temperature versus cutting speed connection, during machining of pure Ti with less carbon and nitrogen count, is
depicted in Fig. 9 [35].

4.4 Tool Wear and Wear Mechanics

Temperature, vibration, and stress in the cutting area are the three major factors that increase damage to the cutting tools [29,45].
Surface integrity, tool wear, and productivity are correlated and dependent on the machining parameters like feed, depth of cut,
cutting speed, cutting tool materials, existence of coolant, etc. [46]. Basically, the mechanisms of tool damage are chipping,
thermal diffusion, adhesion, abrasion, chemical reaction, plastic deformation, fracture, and fatigue [34]. Rise of temperature leads
to increase most of these malfunctions intensely. The presence and production of heat differ with instrument and material mixture
but most of all it relies on velocity [47]. The diffusion happens at high cutting temperature when work material elements and
cutting tool diffuse into each other's structure. The plastic deformation occurs when a cutting tool cannot endure the stress on its
cutting edge at high cutting temperature [35,48].
High cutting temperatures are produced during cutting of Ti alloys. The main reason for the rapid tool wear is the high
temperature at the cutting edge of the tool. A large amount (about 80%) of produced heat during Ti6AI4V is conducted into the
cutting tool as it cannot be taken out with the fast flowing chip or bed into the workpiece because of low thermal conductivity of
Ti alloys [28].
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 7

Fig. 7 Effect of machining conditions on cutting force, chip morphology, and cyclic force frequency in dry turning of Ti6Al4V. (a) Effect of feed
on the cutting forces. (b) Effect of feed on the amplitude variation of cutting force at 75 m/min cutting speed. (c) Effect of feed on the cyclic force
frequency and chip morphology at 75 m/min cutting speed. (d) Effect of cutting speed on the force variation, force amplitude, and frequency
variation at 0.280 mm feed and 1.50 mm depth of cut. Reproduced from Sun, S., Brandt, M., Dargusch, M.S., 2009. Characteristics of cutting
forces and chip formation in machining of titanium alloys. International Journal of Machine Tools and Manufacture 49 (7–8), 561–568.

Fig. 8 Chips made at a feed rate of (a) 0.054 mm (which had vibration) and (b) 0.280 mm and 75 m/min cutting speed. Reproduced from Sun,
S., Brandt, M., Dargusch, M.S., 2009. Characteristics of cutting forces and chip formation in machining of titanium alloys. International Journal of
Machine Tools and Manufacture 49 (7–8), 561–568.

Besides high mechanical pressure, high cutting temperatures, and high dynamic loads during cutting of Ti alloys, which lead to
rapid tool wear and/or plastic deformation, the cutting tool also suffers from the titanium’s high chemical reactivity. Ti and its
alloys react chemically with almost all available tool materials at cutting temperature more than 5001C because of their high
8 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 9 Temperature vs. cutting speed. Reproduced from Trent, E.M., Wright, P.K., 2000. Metal Cutting. Boston, MA: Butterworth-Heinemann.

Fig. 10 3D optical images of worn inserts after length of cut of 1000 m: (a) 3D image of uncoated insert; (b) 3D image of TiAlN-coated insert;
and (c) 3D image of TiB2-coated insert. Reproduced from Chowdhury, M.S.I., et al., 2017. Wear behaviour of coated carbide tools during
machining of Ti6Al4V aerospace alloy associated with strong built up edge formation. Surface and Coatings Technology 313, 319–327.

chemical reactivity. The tendency for welding the chips to cutting tools, extreme dissolution/diffusion wear, which increases with
rise of temperature, and other different characteristics that have already been mentioned, need more criteria in the selection of
material for cutting tool [28].
Dearnley and Grearson [49] declared that both crater and flank wear can happen on the cutting tools during the machining of
Ti alloys. But, only flank wear has been taken into account as an indicator for end of tool life, as it is always present and is the
easiest way to measure during the cutting.
Wear patterns obtained after turning of TiAl6V4 by optical 3D imaging are shown in Fig. 10. Intensive adhesive interaction with
the workpiece material as well as crater wear are observed on the rake surface of the tool with the TiAlN coating. This results in
intensive wear rate and substantial surface damage of the TiAlN coated insert (Fig. 10(b)). On the surface of uncoated insert the
wear intensity and surface damaging processes are less intensive (Fig. 10(a)). The TiB2 coated insert has the lowest wear intensity
(Fig. 10(c)) [36].
The TiB2 coating that showed the best result in short term wear studies was selected for further wear performance investigations
and compared to the uncoated tooling. The results of full scale wear studies (up to the uniform flank wear of 300 mm) are shown
in Fig. 11. The results showed that TiB2 coating strongly outperforms the uncoated tool. Flank wear is growing at a constant rate for
the uncoated insert. In contrast, flank wear is significantly more stable for the TiB2 coated insert (Fig. 11). Rake wear (cratering) is
stable for the uncoated insert up to length of cut of around 2000 m. [36].

4.5 Surface Integrity (e.g., Roughness, Hardness, Residual Stress)

Cutting processes generate and affect different surface integrity attributes on the produced parts. These can be categorized as follows:
(1) topography characteristics like surface roughness and textures waviness, (2) mechanical properties affected like hardness
and residual stress, (3) metallurgical phase like microstructure, grain size and shape, phase transformation, inclusions, etc. These
variations of the surface fall into five categories, i.e., thermal, metallurgical, mechanical, electrical, and chemical properties [50].
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 9

Fig. 11 Tool life of TiB2 coated and uncoated tools after full scale testing. Reproduced from Chowdhury, M.S.I., et al., 2017. Wear behaviour of
coated carbide tools during machining of Ti6Al4V aerospace alloy associated with strong built up edge formation. Surface and Coatings Technology
313, 319–327.

Fig. 12 Lay pattern produced after dry milling of Ti-6242S at f¼0.20 mm/tooth, Vc ¼ 125 m/min, ar ¼8.8 mm, aa ¼ 2.5 mm. Reproduced from
Ginting, A., Nouari, M., 2009. Surface integrity of dry machined titanium alloys. International Journal of Machine Tools and Manufacture 49 (3–4),
325–332.

4.5.1 Surface defects

Different forms of surface defects are reported in Section 4.5. As illustrated in Figs. 12 and 13, major forms are material pull-out or
cracking, surface drag, adhered material particles, feed marks, tearing surface [22], debris of microchips, chip layer formation,
surface cavities, surface plucking, laps (material folded onto the surface), slip zones, deformed grains, and lay patterns [22,51,52].
In traditional machining methods, the material is subjected intensely to high strain, temperature, and strain rate. Hence,
machined surface and cutting tool have to go through severe circumstances [24]. Ying-lin [53] reported high temperature in the
cutting zone can easily damage the finished surface of Ti alloys. This heat can melt Ti chips and increase the adhesion of chips to
the cutting tool and produced surface. Cutting fluids are generally accepted by the industry to remove heat-produced and induced
lubrication at the chip–tool and tool–workpiece interfaces during cutting [28,29].

4.5.2 Microstructural alterations

Fig. 14 shows the microstructure of Ti-6242S (titanium alloy) before machining. Moreover, Fig. 15 illustrates the microstructures
of machined surfaces were fabricated using uncoated tungsten carbide “WC–Ti/Ta/Nb–Co” and multilevel CVD-layered alloyed
carbide “WC–Ti/Ta/Nb–Co þ TiN/TiC/TiC.” To investigate alterations of microstructures, the microstructure of Ti-6242S (Fig. 14)
was compared to each microstructure of the top region of the machined surface (Fig. 15). It can be deduced that microstructures at
the top region down to plenty of micrometers under the produced surface tend to display plastic deformation [22].
10 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 13 Tearing surface produced after dry milling of Ti-6242S at (f ¼0.15 mm/tooth, Vc ¼ 100 m/min, ar ¼8.8 mm, aa ¼ 2.0 mm, when VB at the
initial cutting; tearing surface and feed mark) (b) mechanism of tearing surface. Reproduced from Ginting, A., Nouari, M., 2009. Surface integrity
of dry machined titanium alloys. International Journal of Machine Tools and Manufacture 49 (3–4), 325–332.

Fig. 14 Microstructure of Ti-6242S. Reproduced from Ginting, A., Nouari, M., 2009. Surface integrity of dry machined titanium alloys.
International Journal of Machine Tools and Manufacture 49 (3–4), 325–332.

The severity of plastic deformation on the subsurface under the machined surface can be instantly controlled using
suitable machining conditions and tool wear as shown in recorded images. The augmentation of machining condition from low to
high and the increase of tool wear from the initial wear to VB ¼ 0.3 mm produce fairly more significant plastic deformation and
fairly deeper microstructure alternation [22].
In the dry machining process, high cutting pressure and temperature lead to plastic deformation on the machined surface.
Typically, only the chip goes through plastic deformation in orthogonal metal cutting. High strain rate at the primary shear zone is
responsible for this plastic deformation. The above discussion reveals that the microstructure at the subsurface down to 50 mm
displays a thermal softening, which leads to lower microhardness of the produced surface. This is specified by the flow movement
in the direction of feed speed (Vf) [22].

4.5.2.1 Plastic deformation

Fig. 16 represents the impact of the high degree of subsurface plastic strain imparted on Ti-834 and Ti6Al4V during milling at 200
m/min speed. The plastic strain is mainly adapted by dislocation slip of a phase to 30 and 50 mm subsurface depths in Ti-834 and
Ti6Al4V, consequently. Concomitant distortion of the b phase toward the direction of cutting can also be seen in Ti6Al4V,
although only to approximately 10 mm depth. The surface integrity evaluation of damaged subsurface increasing from high speed
milling, which is only inferred from b phase deformity, while plastic deformation of a phase is not precisely seen because of
limitation of microscopy techniques. Fig. 16(b) shows that the subsurface destruction accumulation at the time of high-speed
milling of Ti-834 is not significant, as very little deformity of b phase is noticeable [54].
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 11

Fig. 15 Microstructural alternation after dry milling of Ti-6242S. Reproduced from Ginting, A., Nouari, M., 2009. Surface integrity of dry
machined titanium alloys. International Journal of Machine Tools and Manufacture 49 (3–4), 325–332.

Local annealing of the subsurface microstructure does not take place in high-speed milling as slip bands in a phase subsurface
were observed [54]. Even though the main structure for high cycle fatigue crack initiation is the intersection of slip bands with the
grain boundaries or sample surface, as was mentioned to take place in Ti-834 during uniaxial loading [55] and four-point bend
testing [56]. Meanwhile, the effect of slip bands during the milling on subsequent fatigue life of the material is unclear [54].
The depth to which slip take places is not uniform beneath the machined surface. It happens in an exact degree in Ti-834 as
shown in Fig. 16(b), although Fig. 16(c) illustrates the more proclaimed effect that has been seen in Ti6Al4V. It is obvious that the
reaction of adjacent a colonies to high-speed milling is contradictory and the depth to which slip takes place is significantly lower
in the adjacent colony 2 than colony 1 [54]. As each a colony can be regarded as a single alternative of the b-to-a transformation on
cooling, the a laths within each colony will have a same crystallographic orientation, and hence it behaves as a single effective
structural unit [54,57]. Considering the limited number of active slip systems, the crystallographic orientation of the a phase
would govern the ease with which slips happen at the time of the high-speed milling. This is alike the monotonic loading
of polycrystalline Ti where strain partitioning happens between “soft” and “hard” grains, though the pressure phase is less
12 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 16 Plastic deformation in the form of intense slip bands under the machined surface by high speed milling (a) Ti6Al4V and (b) Ti-834. (c)
Nonuniformity of deformation in Ti6Al4V. Reproduced from Thomas, M., Turner, S., Jackson, M., 2010. Microstructural damage during high-speed
milling of titanium alloys. Scripta Materialia 62 (5), 250–253.

Fig. 17 Effect of fz and vc on surface roughness: (a) fz Ra and (b) vc Ra. Reproduced from Yao, C.F., et al., 2013. Surface integrity and fatigue
behavior for high-speed milling Ti–10V–2Fe–3Al titanium alloy. Journal of Failure Analysis and Prevention 14 (1), 102–112.

complicated and the strain rates are noticeably slower than in high-speed milling. As a result, the postforging microtexture of Ti
alloy may have a pronounced effect on the level of accumulation of subsurface microstructural destruction and the subsequent
machinability of an element [54].

4.5.3 Surface roughness

Fig. 17 displays the effect high speed milling inputs on the surface roughness during machining of Ti–10V–2Fe–3Al. The effect of
feed per tooth (fz) on surface roughness is depicted in Fig. 17(a). The surface roughness increased from 0.51 to 0.96 mm with
increasing of fz from 0.08 to 0.22 mm/z. This is mainly because of the growth in feed per tooth, (1) the scallop and height
of machined surface escalates rapidly; (2) sudden shear deformation would increase in size with the growth in milling thickness
in order to extend the fracture zone; (3) friction and extrusion between workpiece and tool get more severe with an increase in
milling force [58].
Fig. 17(b) shows the effect of cutting speed on surface roughness. The surface roughness lowered significantly from 0.74 to
0.51 mm with accelerated cutting speed, which is from 60 to 100 m/min. In another case, the surface roughness would grow a bit
when cutting speed is increased from 100 to 140 m/min. Based on this, it can be concluded that the augmentation in cutting speed
would result in smoother surface. This conclusion can be made because the acceleration of cutting speed would cause the
following: (1) sudden shear deformation would be impaired without fracture tearing; (2) the friction and extrusion between
workpiece and tool would be shorter, even though the growth in milling force causes a growth in friction and extrusion [58].
Clearly the feed per tooth plays a more important role on the results of surface roughness rather than that of the cutting speed
based on these machining conditions [58].
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 13

Moreover, Hashmi et al. [23] performed wet high speed milling on Ti6Al4V using a tool having two tungsten carbide inserts
and Trimsol as cutting fluid. They reported that surface roughness decreases with decrease of depth of cut and speed as shown Fig. 18.
At low depth of cut, surface roughness decreased with augmentation of feed and cutting speed between 750 and 770 m/min [23].

5 Mechanical Properties

5.1 Residual Stresses

Fig. 19 shows the effects of feed per tooth (fz) and cutting speed (vc) on the residual stresses along the direction of the feed. The
machined surfaces are all in the status of residual compressive stress. Residual compressive stress rises from 390 to 620 MPa with
the rise of fz and vc. The reasons are as follows: (1) temperature and cutting force in the cutting deformation area rises with an
augmentation of vc and fz; however, the rise of cutting heat is basically on chip; (2) machined surface represents a specific degree of
softening because the cutting temperature rises. Hence, the burnishing effect by axial force on the machined surface becomes
significant, which leads to a intense plastic deformation and rise of the residual stress [58].

5.2 Work Hardening Layer Formation and Microhardness

Fig. 20 shows microhardness distribution with various fz and vc. In these experimental conditions, surface microhardness rose
slowly and quite linearly from 324 to 329 HV0.05 with the rise of vc and fz. This is commonly due to the machining surface

Fig. 18 Surface roughness of Ti6Al4V using wet high speed milling. Reproduced from Hashmi, K.H., et al., 2015. Optimization of process
parameters for high speed machining of Ti–6Al–4V using response surface methodology. The International Journal of Advanced Manufacturing
Technology 85 (5–8), 1847–1856.

Fig. 19 Effect of fz and vc on surface residual stresses: (a) fz  sr and (b) vc  sr. Reproduced from Yao, C.F., et al., 2013. Surface integrity and
fatigue behavior for high-speed milling Ti–10V–2Fe–3Al titanium alloy. Journal of Failure Analysis and Prevention 14 (1), 102–112.
14 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 20 Effect of fz and vc on surface microhardness: (a) fz  HV0.05 and (b) vc  HV0.05. Reproduced from Yao, C.F., et al., 2013. Surface
integrity and fatigue behavior for high-speed milling Ti–10V–2Fe–3Al titanium alloy. Journal of Failure Analysis and Prevention 14 (1), 102–112.

Fig. 21 Combined effects of surface roughness and residual stress on fatigue life. Reproduced from Yao, C.F., et al., 2013. Surface integrity and
fatigue behavior for high-speed milling Ti–10V–2Fe–3Al titanium alloy. Journal of Failure Analysis and Prevention 14 (1), 102–112.

temperature rising with the rise of vc and fz, which causes a certain degree of plastic deformation on the milling zone surface.
Quick-cooling of the milling zone surface leads to an increase of cold hardening of the machined surface. Moreover, high
temperature leads to the oxidation on the surface layer at the time of milling Ti alloy, which is one of the major reasons that causes
a rise of microhardness [58].

5.3 Fatigue
Fig. 21 depicts the combined effects of residual stress and surface roughness on fatigue life for Ti–10V–2Fe–3Al. The impact of
residual stress on fatigue life is higher compared to the surface roughness. This is caused by the fact that surface roughness on
fatigue life is generally depleted by different alteration of the surface circumstances. A smoother surface and a larger residual
compressive stress will result in a prolonged fatigue life [58].
The fatigue fracture of Ti–10V–2Fe–3Al, which is machined using high speed milling, is classified into fatigue crack propagation
area, fatigue fracture zone, and fatigue source area, and can be found in Fig. 22. With the rise of surface roughness, the area ratio of
and fatigue fracture area and fatigue crack propagation zone reduces, and the tool marks made by the cutting process on the edge
of fracture become clearer. Where stress in concentrated would be an origin for the fatigue crack, which then extends like a sector in
the form of corner crack. Fatigue striations are distributed obviously along the direction of crack propagation in the fatigue crack
propagation area, with secondary cracks being seen. Dimples can be observed in the fatigue area [58].
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 15

Fig. 22 Fracture morphology of fatigue of Ti–10V–2Fe–3Al (Ra ¼0.51 mm). (a) Whole morphology feature of fracture. (b) Edge morphology of
fracture. Reproduced from Yao, C.F., et al., 2013. Surface integrity and fatigue behavior for high-speed milling Ti–10V–2Fe–3Al titanium alloy.
Journal of Failure Analysis and Prevention 14 (1), 102–112.

6 Effect of Machining on Biomechanical Performance

6.1 Corrosion Resistance

As aforesaid, Ti and Ti alloys are popular in dental and biomedical applications. This is the result of corrosion resistance and high
stability, which are due to the native titanium dioxide film that preserves the Ti from more oxidation [59]. The facts show that Ti
has corrosion resistance and high stability in vitro [60,61]. However, there are studies representing the accumulation of Ti in tissue
adjacent to the implant [61,62] which remarks some degree of corrosion and metal release in vivo. Machining is a mechanical
process that can justify smooth or rough surface formed by subtraction method. The purposes of machining are to make particular
surface topographies, roughen and clean surface and enhance adhesion in bonding [63], and high corrosion resistance.
In vitro corrosion resistance and ion dissolution of pure Ti with various surface roughnesses were studied choosing constant
atomic absorption spectroscopy and potential meter. Ti samples are in the case of surface roughness (Ra) divided into five
categories: macrorough surface (53.06 mm), various two microroughness surfaces (10.12 and 24.82 mm), machined surface
(0.899 mm), and smooth surface (0.04 mm). Each group consists of three samples group beneath various treatments: “natural
oxidation” (24 h exposure to air), “oxidation under 4001C” (45 min thermal oxidation), and “oxidation under 7001C” (45 min
thermal oxidation) [64].
In Hanks corrosion media, comparative studies through constant potential anode polarization curves and Ti release rates
of the five groups of Ti samples show that “oxidation under 4001C” has the best grow corrosion resistance and decline ion
releases instantly, and “oxidation under 7001C” is better than “normal oxidation.” The corrosion resistance of titanium samples
with various Ra are good and high. Moreover, their corrosion resistance falls with the increasing of surface roughness [64].
In comparison with machined surfaces, microrough surfaces and macrorough surfaces have better corrosion resistance and a lower
ion release rate that are alike those of smooth surfaces. In addition, the corrosion resistance of machined surface Ti is the lowest.
It is assumed that surface treatment processes (like surface aging, surface thermal oxidation and so forth) will evolve the
corrosion resistance and decline the ion release rate of rough surface efficiently by augmenting the surface protection film
thickness, enhancing its structural uniformity, and facilitating the formation of ordered compact surface protection film [64].
However, the corrosion risk of Ti-based implants is higher under conditions of biological inflammation and high glucose
concentrations [65].

6.2 Biocompatibility
Biocompatibility is the material’s ability to perform with a proper host response in a specific application and the quality
of not having injurious or toxic effects on biological systems [66]. Ti and Ti alloys commonly have high biocompatibility
with humans [63]. They can be utilized in biomedical devices like orthopedic implants, dental implants, stents, and other
devices [51].
Ti6Al7Nb (Ti alloy) was used to find its high heat deformation behavior [67]. It was discovered that dynamic recrystallization
of the a-phase was found during high heat deformation of Ti6Al7Nb between 750 and 8501C and at low strain rates. By increasing
the strain rate and temperature, however, the material showed a flow localization and dynamic recovery in b-phase [67]. Nb as the
b-stabilizer was used because vanadium is a toxic material that is not allowed in biomedical devices [51].
16 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Table 3 Percentage of cell attachment to plastic (control) and titanium surface

Cells Control Polished Fine Rough

Periodontal ligament fibroblast 95.4 92.9 52.9 47.3

Epidermal keratinocytes 32.6 41.0 7.6 21.15
Gingival fibroblast 148.0 133.8 82.2 110.7

Source: Reproduced from Cochran, D.L., et al., 1994. Attachment and growth of periodontal cells on smooth
and rough titanium. International Journal of Oral & Maxillofacial Implants 9 (3), 289–297.

6.3 Cell Growth

Different surface treatments were developed to improve the clinical performance of Ti implants. Some of them were carried out in
in vitro tests on substrates with different surface topography for a complete understanding of osteoblasts. The surface roughness
(3.35, 1.21, 0.81, 0.31, and approximately zero) observation was made by Anselme et al. [68] and it must be considered, not only
in terms of organization but also in terms of amplitude. They tested adhesion and proliferation of human primary osteoblasts on
grooved Ti surfaces with different organizations and amplitude of topography with roughness scale below (microroughness) or
above (macroroughness) the cell size. It seems that surfaces with a low level of repeatability and relatively high microroughness
amplitude are preferred by cultured human osteoblasts [68].
The cell attachment behavior of rough, fine, and polished surfaces has been compared (in this case tissue culture plastic) in the
form of percent attachment in Table 3 [69]. Similarly, short- and long-term response of human bone marrow cells (HBMCs) and
protein adsorption on the surface of Ti6Al4V alloy have been studied by Deligianni et al. [70] as a function of surface roughness
(0.320, 0.490, and 0.874 mm). Increasing in surface roughness has been associated with increased cell attachment and pro-
liferation, whereas it did not affect the expiration of alkaline phosphatase (ALP) activity significantly. The enhanced adsorption
human serum albumin on smooth substratum was confirmed with that protein radiolabeling and X-ray photoelectron spectro-
scopy (XPS). On the other hand, XPS also revealed that the total protein (from culture medium supplied with 10% serum) and
fibronectin (ten times) were higher when compared with smooth surface. This differential adsorption of protein explains the
attachment behavior of cells on Ti6Al4V alloy substrate.

7 Techniques to Improve Machining Performance

To achieve a reasonable tool life, the cutting speeds in machining of titanium using tungsten carbide inserts are kept low [35] and
therefore, the productivity of machining is reduced. Thus, it is of utter importance to consider ways to improve the tribological
performance during cutting of Ti alloys [36]. Two major strategies are (1) application of coolant/lubricants that can be efficiently
supplied to the cutting zone; this could be solved through high pressure coolant supply to the cutting zone [71]; and (2)
machining with surface engineered tooling, mostly with physical vapor deposition (PVD)-coated tools [72]. Two major categories
of PVD coatings that are widely used for Ti machining are hard coatings [72] and self-lubricating coatings [72].

7.1 Vibration Assisted Machining

Fig. 23 shows a schematic diagram of the experimental setup and experimental setup for conventional turning (CT) and ultra-
sonically assisted turning (UAT). In the UAT process, the vibro-impact phenomenon at the workpiece–tool interface and the higher
temperature in the cutting zone (Fig. 24) could improve the machinability of Ti alloys with generating shorter chips (Fig. 25) as
compared with CT. Moreover, a significant decrease in cutting forces (Fig. 26) and improved surface roughness (Fig. 27) were
observed in UAT [25].

7.2 Cooling Systems

Improving the productivity in machining Ti alloys is very difficult, since the moderation is gained at cutting speed and the cutting
temperature is increased directly by feed rate [20,44]. The cutting temperature can be dropped till 30% by using the coolant [73,74].
It is more beneficial if the coolant penetrates into the tool–workpiece and tool–chip interfaces while the machining process is
going on [73]. Productivity of the heat removal process relies on the coefficient of heat transfer between the cutting zone and
coolant [29,73]. The coolant is also acting the same as a lubricant, which causes a longer tool life [44,53]. However, in high speed
end milling of Ti6Al4V alloy highly beneficial heat transfer was seen by flood cooling, which made chipping and thermal cracks
happen more often than in dry cutting [75].
Eliminating or reducing cutting fluid in cutting processes would significantly decrease both machining cost and environmental
hazards. Minimum quantity lubrication (MQL) process supplies a perfect solution by spraying a minuscule amount of lubricant in
a mist form close to tool–workpiece and/or tool–chip interface providing the necessary lubricity, decrease in friction, and cutting
energy, which is lacking in wet as well as dry machining [76–78].
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 17

Fig. 23 (a) Schematic diagram of the experimental setup; (b) experimental setup for CT and UAT. CT, conventional turning; UAT, ultrasonically
assisted turning. Reproduced from Muhammad, R., et al., 2014. Analysis of a free machining a þ b titanium alloy using conventional and
ultrasonically assisted turning. Journal of Materials Processing Technology 214 (4), 906–915.

Fig. 24 Cutting zone temperature in CT and UAT of Ti-6246 and Ti-676-0.9La at ap ¼0.2 mm under different cutting speeds. CT, conventional
turning; UAT, ultrasonically assisted turning. Reproduced from Muhammad, R., et al., 2014. Analysis of a free machining a þ b titanium alloy using
conventional and ultrasonically assisted turning. Journal of Materials Processing Technology 214 (4), 906–915.

Fig. 25 Various chip sizes and shapes in CT and UAT: (a) CT of Ti-6246; (b) UAT of Ti-6246; (c) UAT of Ti-676-0.9; and (d) CT of Ti-676-0.9.
CT, conventional turning; UAT, ultrasonically assisted turning. Reproduced from Muhammad, R., et al., 2014. Analysis of a free machining a þ b
titanium alloy using conventional and ultrasonically assisted turning. Journal of Materials Processing Technology 214 (4), 906–915.

MQL is one of the effective methods that can be used in machining titanium alloy. As can be seen in Fig. 28(a) and (b), chips are
adhered to milling tools during the milling process, which causes chips to adhere or weld to the wall of the machined surface.
Using MQL in milling of Ti6Al4V can help to decrease the temperature, prevent build up of edge and adhesion of chip on the tool
as shown in Fig. 28(c). Hence smooth surface can be produced without adhesion of chips (Fig. 28(d)). In addition, surface
roughness and tool wear can be enhanced by MQL.
In MQL machining, tool wear did not augment in spite of the augmentation in cutting speed. Up to a specific level, the
augmentation in MQL flow rate decreased tool wear and cutting force [20]. Increasing the spray pressure may not necessarily
18 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 26 Cutting forces (a) CT and UAT with different depths of cut. (b) ap ¼200 mm in CT and UAT with different cutting speeds. CT,
conventional turning; UAT, ultrasonically assisted turning. Reproduced from Maurotto, A., Muhammad, R., Roy, A., et al., 2013. Enhanced
ultrasonically assisted turning of a beta-titanium alloy. Ultrasonics 53 (7), 1242–1250.

Fig. 27 Interferometry scan on area of 0.53 mm  0.7 mm of machined surfaces with (a) CT and (b) UAT. CT, conventional turning; UAT,
ultrasonically assisted turning. Reproduced from Maurotto, A., Muhammad, R., Roy, A., et al., 2013. Enhanced ultrasonically assisted turning of a
beta-titanium alloy. Ultrasonics 53 (7), 1242–1250.

enhance tool life and surface finish. In fact, suitable spray pressures can cause a proper lubricant penetration at the tool–chip
interface, which eventually enhances tool life and surface roughness. MQL is associated with environmentally friendly machining
and productivity enhancement [79].
Nowadays, different nanoparticles (such as diamond and Al2O3, MoS2 exfoliated graphite nanoplatelets (xGnPs)) are mixed
with lubricant to enhance the MQL machining. In this method, a small concentration of nanoparticles in the mist oil increases
lubricity and thermal conductivity [20,80–82].
Many researchers claimed that the machinability of Ti alloys has been enhanced by spraying liquid nitrogen to the cutting zone
or cryogenically freezing the workpiece [83,84]. The nitrogen liquid has various advantages like being a clean, safe, and nontoxic
fluid that evaporates into the atmosphere leaving no mess and requiring no expensive disposal. However, the disadvantages of this
method are unavailability of a constant supply of liquid coolant and ice build-up on tool holders or tools [84,85].
By flowing liquid nitrogen to the flank face and tool rake at cutting speed of as high as 150 m/min the tool temperatures can be
decreased to less than 5001C. Hence the tool wear diffusion becomes negligible, which can make the cutting speed of conventional
emulsion machining double (usually it is limited to 60 m/min). Based on Hong et al. [84], cryogenic machining experiments show
that tool life augments up to five times in traditional machining, outperforming other machining processes. Another fact is that the
high pressure cooling augments the tool life by approximately three times basically by decreasing the machining temperature
during turning of Ti6Al4V [73].
Using industrial gases like carbon dioxide snow (CO2) in place of traditional lubricant and cooling fluids promises a rise in
productivity of turning of Ti–10V–2Fe–3Al. CO2 is made in a pressurized gas bottle and is fed to the tool tip through holes in the
tool holder’s clamping jaw. Comparing to flood emulsion cooling, tool life rises even at higher cutting speeds and the flank wear
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 19

Fig. 28 (a) and (b) Dry milling of Ti6Al4V, (c) and (d) milling of Ti6Al4V with using Minimum quantity lubrication (MQL).

Fig. 29 Average cutting forces in end milling of Ti6Al4V with various cooling conditions during (a) the first pass (b) the second pass.
Reproduced from Park, K.-H., et al., 2017. Milling of titanium alloy with cryogenic cooling and minimum quantity lubrication (MQL). International
Journal of Precision Engineering and Manufacturing 18 (1), 5–14.

spreads uniformly. The burr formation at the workpiece and the tool life limiting notch wear are suppressed [86]. Using
cryogenic cooling with liquid nitrogen in machining of g-titanium aluminides can improve the surface integrity and decrease the
tool wear [87].
Fig. 29 shows average cutting forces in end milling of Ti6Al4V under dry, wet, MQL, cryogenic, and MQL/cryogenic conditions.
The lowest cutting forces was achieved in MQL/cryogenic [20]. Fig. 30 illustrates image tool wear in dry, wet, MQL, cryogenic,
MQL/cryogenic, and Minimum quantity lubrication with nanoparticles (MQLN) using xGnPs conditions. The results of the MQLN
20 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Fig. 30 Tool wear after end milling of Ti6Al4V with various cooling conditions. Reproduced from Park, K.-H., et al., 2017. Milling of titanium
alloy with cryogenic cooling and minimum quantity lubrication (MQL). International Journal of Precision Engineering and Manufacturing 18 (1),
5–14.

Fig. 31 The respective placement of the laser beam, cutting tool, and workpiece in laser assisted turning (LAT): (a) front view and (b) side
view. Reproduced from Sun, S., Harris, J., Brandt, M., 2008. Parametric Investigation of laser-assisted machining of commercially pure titanium.
Advanced Engineering Materials 10 (6), 565–572.

experiment show the least tool wear among other methods. Minor flank wear happened at both high cutting speeds (100 and
120 m/min). This is due to the cutting oil vaporizing at high cutting temperature, and the xGnPs acted as a lubricant and reduced
between the workpiece material and tool [20].

7.3 Laser Assisted Machining

Rather than an improved version of cooling techniques, which has been focusing on enhancing the machinability of Ti alloys
for a long time, laser assisted machining (LAM) allows an alternate technique to enhance the machinability by decreasing
cutting force [74]. Fig. 31 shows the respective placement of the laser beam, cutting tool, and workpiece in laser assisted
turning (LAT).
Cutting forces in X, Y, and Z directions are decreased considerably by the help of a laser beam. The decrease of cutting forces is
dependent on depth of cut, cutting speed (respective laser energy input because of the time relation between beam and cutting
speed), laser spot size distance between beam and workpiece, angle of beam incident, and power of laser [88–90]. The prominent
impact of laser power on the decrease of force during LAT of economical pure titanium (which is ductile) can be seen at the
direction of beam incident [88].
While the laser beam is incident on the chamfer surface with its smaller angle along the direction of workpiece rotation, the
penetration of heat into the shear area is decreased because of the short beam interaction time. As a result, tool life is enhanced and
heating of the cutting tool is decreased, as the tungsten carbide tool’s strength, which is used for machining of Ti alloys, is low at
high temperature [90].
Critical cutting speed for the initial chip segregation is very low, while traditional machining is used. This leads to dynamic
cyclical forces on the tool that then cause cutting tool tip and chatter to break [28,41]. By preheating using a laser beam, the critical
cutting speed for the initial chip segregation augments [91]. At a consistent power of laser, the chip morphology changes from a
sharp saw-tooth morphology to a continuous chip and back to a saw-tooth shape with augmenting cutting speed. The saw-tooth
 Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications 21

chips are shaped at high and low cutting speeds and have various geometry ratios, that mention the variety of mechanisms of
formation. The continuous chip transition speed rises with power of the laser [92,93].
Tool life during LAM of Ti alloy is highly dependent on the cutting temperature. The best cutting temperature during LAM of
Ti6A14V would be 2501C where the balance is between the heat produced because of plastic deformation and the heat generated
from laser energy. The crater wear is controlled by cobalt diffusion, which is minimized at 107 m/min cutting speed [94,95].
Nonetheless, higher temperatures would cause shorter tool life in traditional machining as a result of the acceleration of cobalt
diffusion. At best cutting temperature, the tool life based on the volume of the removed material during LAM is less than using
traditional machining at more than 107 m/min cutting speed. The hybrid machining, where the reservoir cap is filled with liquid
nitrogen (LN2) is developed in order to decrease temperature of the rake face in the cutting tools, enhances the tool life
considerably at any cutting speed. Besides the local softening of workpiece via laser heating, the decrease in cutting energy during
hybrid machining is attributed to (1) the improved method of cooling of the cutting tool to keep its strength and hardness; (2) the
lower tool wear because of the decrease in tool–chip interface temperature. The less friction between the machined surface and
tool’s flank surface can greatly improve the surface roughness [95].
It was reported that tool life decreased during the milling of Ti6Al4V using plasma as the heat source due to the fact that the
cutting tool is exposed to higher temperatures and this ends in rapid tool degradation [96]. However, Ginta et al. [97] found that
the tool life rises with rising in temperature of workpiece up to 6501C by induction heating when PCD insert is used in end
milling. A longer tool life at high temperature is dedicated to the vibration amplitude and the decrease in cutting force. The
amount of built-up edge rises by preheating temperature up to 4501C based on the high chemical reactivity between cutting tool
and Ti alloy at increased temperatures and reduces with more augmentation in preheating temperature.
Other than that, LAM can produce smoother surface with smaller shorter deformation area and less grain pullout [88].
Nonetheless, LAM decreases the compressive residual stress at the machined surface or alters this stress into tension with growing
power of laser [89,98]. This can be seen significantly at low cutting speed and gets more insignificant when cutting speed reaches
54 m/min or higher. Higher tension or lower compressive residual stress decreases the fatigue resistance [74].
Even though, the heat supply should be controlled thoroughly to the heat affected layer thickness associated with formation of
Widmanstatten (needle-shaped) microstructure within the cutting area. This is very challenging to gain by plasma because of the
difficulty in prediction of the heat affected zone depth during heating by plasma [96]. The remaining Widmanstatten micro-
structure on the surface is dedicated to a decrease in its fatigue life [89,96]. Priority of laser beam to plasma is that the external heat
source is controllable in spot size and smaller [96] and there aren’t any observed Widmanstatten microstructures in the produced
subsurface after LAM [88,89,95].

8 Conclusion and Future Work

This study reviews extensively the machinability of Ti alloys in order to discuss and present the effect of machining parameters on
cutting force, heat generation, chip formation, tool wear, surface integrity (surface defect, microstructure, and surface roughness),
mechanical properties (hardness, residual stress, and fatigue) and biomedical performance (corrosion resistance, residual stress,
and cell growth). It was found that machining of Ti alloys is very challenging. Moreover, various cooling systems, vibration assisted
machining, and LAM can enhance the machinability of Ti alloys. MQLN has the best performance various cooling system.
Regarding future research directions, it is recommended that future works focus more to investigate the effect of new nanoparticles
with different sizes and percentages on machining of Ti alloy using MQLN. In addition, more fundamental and deeper works
are needed to investigate the effect of machining on biomedical performance of Ti alloys. This research can help the usage of
Ti alloys as implants without secondary operation or treatment, such as in coatings in the human body, leading to decrease in the
implants’ price.

Acknowledgements

The authors would like to thank both University Malaya and King Fahd University of Petroleum & Minerals for providing financial
and technical support.

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