Proceedings of the International Conference on Mechanical Engineering and Mechatronics

Ottawa, Ontario, Canada, 15-17 August 2012

Paper No. 83 (The number assigned by the OpenConf System)

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A Critical Review on Tool wear in Titanium machining

Narasimhulu Andriya, Venkateswara Rao P, Indian Institute of Technology Delhi

Department of Mechanical Enginering, New Delhi-110016, India

narasimha.iitdelhi@gmail.com; pvrao@mech.iitd.ac.in;

Sudarsan Ghosh

Indian Institute of Technology Delhi, Department of Mechanical Engineering

Hauz Khas, New Delhi-110016, India

sudarsan.ghosh@gmail.com

Abstract - Titanium machining is attaining importance for machining of titanium alloys as it has several benefits

over other materials. The major concern while machining of Titanium alloys is the rapid tool wear that takes place in

almost all varieties of tool materials especially while machining at higher cutting speeds. There are several issues

regarding tool wear, which should be understood and dealt with, to achieve successful performance of the process.

Researchers have worked upon several aspects related to titanium machining. The present work is an effort to

review some of these works and to understand the key issues related to process performance. The review reveals the

effect of tool wear on the types of tool material and different types of coatings, cutting edge geometry and cutting

parameters. The present work also aims to find out certain areas that can be taken up for further research in

minimizing the tool wear during machining of Titanium alloys.

Keywords: Titanium alloys, tool wear, machining.

1. Introduction Titanium is a metal showing a high strength-weight ratio which is maintained even at elevated

temperatures, and it has exceptional corrosion resistance. These characteristics were the main reasons for

the rapid growth of the titanium industry over the last 70 years. The major application of the material is in

the aerospace industry, both in airframes and engine components manufacturing. Non-aerospace

applications take advantage mainly of their excellent strength properties, for example in steam turbine

blades, superconductor missiles etc., or corrosion resistance, for example in marine services, chemical,

petro-chemical, electronics industry, biomedical industry etc.,

However, despite the increased usage and production of titanium, they are expensive when compared

to many other metals, because of the complexity of the extraction process, difficulty of melting and

problems during fabrication and machining. Near net-shape methods such as casting, isothermal forging,

and powder metallurgy have been introduced to reduce the cost of titanium components. However, most

titanium parts are still manufactured by conventional machining methods. Virtually all types of machining

operations, such as turning, milling, drilling, reaming, tapping, sawing, and grinding, are employed in

producing aerospace components. For the manufacturing of gas turbine engines, turning and drilling are

the major machining operations, whilst in airframe production; end milling and drilling are amongst the

most important machining operations.

The machinability of titanium and its alloys is generally considered to be poor owing to several

inherent properties of the materials. Titanium is chemically very reactive and, therefore, has a tendency to

weld to the cutting tool during machining, thus leading to chipping and premature tool failure. Its low

thermal conductivity increases the temperature at the tool/workpiece interface, which affects the tool life

adversely. Additionally, its high strength maintained at elevated temperature and its low modulus of

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elasticity further impairs its machinability (Hong, Riga et al. 1993). The poor machinability of titanium and its alloys have led many large companies (for example Rolls-Royce and GE) to invest large sums of

money in developing techniques to minimize machining cost. Reasonable production rates and excellent

surface quality can be achieved with conventional machining methods if the unique characteristics of the

metal and its alloys are taken into account.

2. Basic problems in machining of Titanium and its alloys Machinability is defined as the ease (or difficulty) with which a material can be machined under a

given set of operating conditions including cutting speed, feed rate and depth of cut. It can also be

described as a measure of the response of material to be machined with a given tool material resulting in

an acceptable tool life and at the same time providing good surface finish and acceptable functional

characteristics of the components. Machinability rating depends on tool life, surface finish and power

consumed during the machining operation. Component forces and chip shape also provide a good

assessment of the machinability of material(Trent 2000). Machinability of any material is mainly assessed

by measuring the tool life, surface generated and the magnitude of various cutting force components

during machining operations. The favourable responses of all these measures lead towards the

improvement of machining performance by selecting the appropriate process parameters, cutting tool and

cutting fluid.

Reasons for poor machinability Many authors (Komanduri and Von Turkovich 1981; Komanduri and Reed Jr 1983; Ezugwu and Wang

1997; Jawaid, Che-Haron et al. 1999; Yang and Richard Liu 1999; Hong, Ding et al. 2001; Hong, Markus

et al. 2001; Ribeiro, Moreira et al. 2003; Li, Yang et al. 2004; Abele 2008; Jianxin, Yousheng et al. 2008;

G. A. Ibrahim 2009; Bermingham, Kirsch et al. 2011; Li, Zhao et al. 2011) claim individually some of the

following points as the reasons for the poor machinability of titanium:

a. The high strength is maintained even at elevated machining temperatures and this opposes the

plastic deformation needed to form a chip.

b. Titanium‘s chip is very thin with an unusually small contact area with the tool [one third that of

the contact area of steel at the same feed rate and depth of cut]. This causes high stresses on the

tool, although cutting forces are reported to be similar to steel and hence the power

consumption in machining is approximately the same.

c. It has a high ‗coefficient of friction‘ between the chip and the tool face, although it is more or

less comparable with that obtained during machining of many hardened steels.

d. There is a strong chemical reactivity of titanium at the cutting temperature (>5000C) with

almost all tool materials available.

e. Titanium chips are formed by the ‗adiabatic or catastrophic thermoplastic shear‘ process.

Titanium‘s low volumetric specific heat and relatively small contact area along with the

presence of a very thin flow zone between the chip and the tool [approximately 8 µm compared

with 50 µm when cutting steels under the same cutting conditions] cause high tool-chip

temperatures ranging around 11000C.

f. Normally in titanium alloy machining built up edge does not occur that predominantly but

some authors have confirmed its presence at low cutting speeds, and this could lead to a poor

surface finish in some operations.

g. Low modulus of elasticity which cause chatter, deflection and rubbing problems on the

workpiece.

h. Titanium alloys can have a tendency to ignite during machining because of the high

temperatures involved and appropriate care needs to be taken.

i. There is a high rate of work hardening, although some researchers like Zaltin and Child and

Dalton have reported that it work-hardens to a lesser extent than steel.

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3. Common tool materials, coatings and their geometry used for Titanium alloy machining

When it comes to cutting tools, coated indexable inserts are responsible for the major share of

metal removed throughout machine shops in modern times. These are the tools that, since 1970, have

continually increased manufacturing productivity and, consequently, material prosperity. With the advent

of different types of coating materials, the applicability of these coated tools increased. About 80 percent

of the inserts used in machining today are coated cemented carbide grades. These tool inserts earned and

maintain that growing market share because of their broad application for removing large amounts of

material while sustaining a long tool-life. The high material removal rates and long life of these tools are

achieved through a balance of wear resistance and toughness. Relatively thin coatings in the order of

0.001mm. to 0.0178 mm. protect inserts from the heat, corrosion and abrasion that normally shorten their

lives. The basic insert – the substrate – provides most of the required toughness for the cutting tool, while

the coating adds wear protection and increased hardness. Most coatings are ceramic by nature, and if an

insert was made solely of the coating material, it would be too brittle for general use. The advantage is

obtained by correctly combining the most suitable coating material and substrate material.

Coating materials: Most coating-materials used on today‘s inserts are classified as ceramic. Some of

the most common coating materials along with their main properties are listed below: Titanium carbo-

nitride - TiCN – has very good abrasive wear resistance and good adherence to tungstencarbide substrate;

Aluminum oxide (alumina) - Al2O3 – provides very good outer thermal and chemical protection for the

insert substrate; Titanium nitride - TiN –mainly used for the golden color which provides clear wear

detection; Zirconium oxide - ZrO2 – gives very good thermal and chemical protection for the substrate;

By using more than one layer of different coating materials it is possible to combine the benefits that each

provides. Many insert grades have three layers of coatings to ensure good adherence between the insert

substrate and coatings. Also, sometimes the laminating effect of the coatings provides added strength to

the inserts.

Coating processes: Two principal coating processes CVD (Chemical Vapor Deposition) and PVD

(Physical Vapor Deposition)) are used for indexable inserts to provide cutting edges with fundamentally

different properties for machining. The CVD method allows one to form coatings with high adhesion

strength to substrate, high density, and compositional uniformity. One characteristic structure of such

coatings is equiaxed grains, which adapt better to operations in continuous cutting conditions. However,

to perform the gas phase CVD processes, a relatively high temperature and time duration are required.

These conditions, in turn, become the reason for the softening of the coated tool and the decrease in their

efficiency, especially for heavily loaded cutting operations, intermittent processes, and in cutting tough

materials (Che-Haron, Ginting et al. 2007; Blinkov, Anikin et al. 2011). The best tool substrate and

coating for machining titanium alloys and super alloys is a submicron substrate that is combined with a

physical vapor deposition (PVD) TiAlN coating. The thin, smooth surface of the PVD coating, together

with sufficient residual stress, enhances tool resistance to chipping and notching wear, so PVD coatings

provide enhanced wear resistance, chemical stability and resistance to built-up edge. Machining problems

that were seen in the past that arose from earlier coatings, no longer exist with PVD coatings because of

the improved adhesion techniques and the uniformity of the coatings (Bouzakis, Michailidis et al. 2000;

Bouzakis, Michailidis et al. 2001; Bouzakis, Hadjiyiannis et al. 2003; Bouzakis, Hadjiyiannis et al. 2004;

Klocke, Michailidis et al. 2010; Jaffery and Mativenga 2011).

In titanium machining the cutting edges of the tools should be kept sharp without rounding in order

to avoid high cutting temperatures and forces. The tools should be replaced immediately when wear

occurs (Guimu, Chao et al. 2003; Che-Haron, Ginting et al. 2007). Due to the low plasticity of titanium

and the attenuation of the cutting edge by the additional grinding operation, chip breakers should not be

used at all. The references and recommendations regarding the wedge angles are not consistent. While the

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rake angles of -5° to 0° are recommended, Lopez et al. (López de lacalle, Pérez et al. 2000) showed that

clearance angles of less than 18° cause much friction and heat; clearance angles larger than 20° weaken

the cutting edge.

4. Various types of tool wear and their reasons

Different tool materials show different responses to different wear mechanisms when machining

titanium alloys with notching, crater wear, and flank wear, chipping and catastrophic failure being the

prominent failure modes (A.R. Machado 1990; Ezugwu and Wang 1997; Ezugwu, Bonney et al. 2003).

However, the crater and the flank wear of the uncoated WC/Co cutting tools result mostly from

dissolution-diffusion, while in the case of other tool materials, attrition is the major cause of flank wear

(Dearnly PA 1986)

Hartung and Kramer (Hartung, Kramer et al. 1982) have reported that most of the commonly used

cutting tool materials suffer from chemical instability leading to rapid tool wear when machining titanium

alloys and titanium from the chip material adheres to the tool hindering relative sliding at the chip-tool

interface. This leads to the formation of a boundary layer of titanium at the interface. Repeated tearing

and transport of this titanium layer by the chip underside results in tool material being pulled away

causing increased crater wear. According to Hartung (Hartung, Kramer et al. 1982) who conducted

experimental studies of tool wear in titanium machining, the main mechanism controlling the crater wear

of cutting tool materials in the machining of titanium alloys is fundamentally different from that in the

machining of steel and nickel-based alloys. It is suggested that tool wear is greatly reduced when

adhesion occurs between the tool and the chip, preventing relative sliding at the tool/chip interface. This

adhesion is promoted by chemical reaction at the interface. The thickness of the reaction layer is

determined by the balance between the diffusion flux of tool constituents through the layer and the

removal of tool constituents through chemical dissolution at the interface between the reaction layer and

the titanium chip. In this way, for given cutting conditions, a characteristic thickness of the reaction layer

is maintained and tool wear is limited by the rate of dissolution of the reaction layer into the titanium. The

existence of a stable reaction layer of TiC on diamond and WC-based tools (the two most wear resistant

tool materials) has been demonstrated, and the estimated diffusion flux correlates well with the observed

wear rate.

Jawaid et al (Jawaid, Che-Haron et al. 1999) have observed that the dominant wear mechanisms of

cemented WC-Co tools were dissolution, diffusion and attrition, causing the plucking and pulling-out of

carbide from the tool. Abrasion wear mechanisms dominated the wear type at the flank face and tool

nose. As the substrates of the PVD-TiN and CVD-TiCN-Al2O3 coated tools were of the same

composition, they are subjected to similar wear mechanisms throughout the cutting tests. Both tools

experienced a variety of wear mechanisms that accelerated the tool wear when face milling of the Ti-6Al-

4V. Some of the observed wear mechanisms are delamination of coating, adhesion, attrition, diffusion and

plastic deformation and cracks. Coating delamination, galling on the rake face and adhesion of work

material at the cutting edge were responsible for the initial wear mechanism for both of the coated tools.

Attrition and diffusion wear mechanisms were responsible for the flank and rake face wear of both of the

coated tools. Evidence of the diffusion of cobalt and tungsten into the adhered workpiece was found at the

flank and rake faces of the tools. The thermal cracks observed were thought to be responsible for the

severe chipping and/or flaking of the inserts at both the rake and flank faces (Jawaid, Sharif et al. 2000;

Nouari and Ginting 2006).

Su et al (Su, He et al. 2006) have observed that flank wear was the dominant failure mode under the

different cooling/lubrication conditions, including dry, nitrogen-oil-mist, CCNG(compressed cold

nitrogen gas) and CCNGOM (compressed cold nitrogen gas and oil mist) for the cutting conditions of

Cutting speed 400 m/min, Feed rate 0.1 mm/rev, Axial depth of cut 5.0mm, Radial depth of cut 1.0mm)

and they have also found that TiN/TiC/TiN coating was removed from the cutting edge under all the

cooling/lubrication conditions employed. Adhesion of workpiece material onto the flank face of the tool

was observed under all the cooling/lubrication conditions. Fig. 1 shows an example of adhered workpiece

material on the flank face of the tool, and it indicates that there is a strong bond at the tool workpiece

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interface. Many researchers have reported that the temperature at the cutting edge of carbide tool can be

above 8000 C when machining titanium alloy, even at moderate cutting condition (Hartung, Kramer et al.

1982; Narutaki, Murakoshi et al. 1983; Kitagawa, Kubo et al. 1997; Hong and Ding 2001; Zhang, Li et al.

2010). The high temperature and the close contact between the tool and the workpiece provided an ideal

environment for diffusion of tool material atoms across the tool-workpiece interface (Dearnly PA 1986;

Jawaid, Che-Haron et al. 1999).

Fig. 1. Adhesion of workpiece material on the flank face of tool under CCNG (00 C) condition

(Su, He et al. 2006).

Venugopal et al (Venugopal, Paul et al.1 2007; Venugopal, Paul et al.2 2007) have found that in

machining of titanium alloys crater wear occurred predominantly by adhesion–dissolution–diffusion wear

as had been reported earlier, but with cryogenic cooling reduction of such wear mechanism over the

domain of the process parameters employed was observed and they have also reported that Edge

depression of the cutting tool insert, a major problem in machining of titanium alloys has also been

significantly decreased due to effective control of cutting zone temperature by cryogenic cooling. The

appearance of crater surface is smooth but the extent of crater wear (crater depth) is rather high indicating

adhesion–dissolution–diffusion nature of crater wear. The flank wear region however was not smooth,

rather abrasive wear marks were visible. The cutting edges underwent micro and macro fracturing along

with plastic deformation. Such wear of cutting edge modified the effective tool geometry due to ―edge

depression‖ of the cutting edge. Jianxin (Jianxin, Yousheng et al. 2008) have studied the diffusion wear of

titanium machining under dry machining. Fig. 2 shows the width of flank wear of WC/Co carbide tools

in dry turning of Ti–6Al–4V at different cutting speeds. It can be seen that the flank wear of the carbide

tools is greatly increased when using high cutting speeds. The rake face of the WC/Co carbide tool was

worn more at the high cutting speed of 120 m/min as can be seen in Fig. 3.

Fig. 2. Flank wear of the WC/Co carbide tools in dry machining of Ti–6Al–4V alloy at different

cutting speeds (Jianxin, Yousheng et al. 2008)

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Fig. 3. SEM micrographs of the worn rake face of the WC/Co carbide tool after cutting (a)

V=200m/min (a) 3min, (b) 6min, (c) 9min, and (d) 12 min (test conditions: V = 80m/min, f = 0.1mm/r, ap

= 0.5 mm) (Jianxin, Yousheng et al. 2008)

G. A. Ibrahim et al (G. A. Ibrahim 2009) have studied the wear mechanisms of CVD coated carbide

tools in turning of Ti-6Al-4V with multilayer layer (TiN-Al2O3-TiCN-TiN) coated tool. Figure 3 shows

the flank wear, crater wear and chip welded on the cutting edge near the nose radius, when machining

titanium alloy at cutting speed of 55 m/min, feed rate of 0.25 mm/rev and depth of cut of 0.10 mm. The

flank wear that occurred near to the nose radius was due to low depth of cut. The low depth of cut caused

a small contact area between cutting tool and workpiece material. Turning at low cutting speed, will

generate low temperature.

Fig. 4. SEM micrograph shows the flank wear, crater wear and chip stick on cutting edge of CVD

coated insert at v = 55 m/min, f = 0.25 mm/rev., d = 0.10 mm and Cutting Time = 24,53 min (G. A.

Ibrahim 2009).

Ginting and Nouari (Nouari and Ginting 2006) also confirmed that the dominant tool wear of

uncoated and coated tool was localized flank wear. Abrasive and adhesive wear which occurred at the

cutting speed of 95 m/min, feed rate of 0.35 mm/rev and depth of cut of 0. 20 mm are shown in Figure 4.

The wear increases with cutting speed owing to increase in the slip distance and cutting temperature

during machining. Adhesion or welding of titanium alloy onto the flank and rake faces were also

observed. Adhesion of titanium alloy can be seen clearly, which clearly proves a strong bond (no

evidence of any gaps) at the workpiece-tool interface. Figure 5 shows the different types of wear that has

occurred while machining Titanium with a CVD coated tool.

According to Konig (1979), the adhesion wear took place after the coating had worn out or coating

delamination had occurred. The adhered or welded titanium will be swept away by the chip, and

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deposited on to the workpiece continuously, thus leading to the initiation of chipping, flaking and finally

the breakage of the carbide tool at the cutting edge.

Fig. 5. SEM Micrograph showing different types of wear while machining Titanium on cutting

edge of a CVD Coated Insert at V= 95 m/min, f = 0.35 mm/rev., d = 0.20 mm and Cutting time = 20, 15

min (G. A. Ibrahim 2009).

Flank wear, notching and built-up edge are the common types of tool wear when machining

titanium. Edge notching appears as a localized abrasive wear on both the flank and rake face, along the

line corresponding with the depth-of-cut parameter. This wear is caused partially by the presence of a

hardened layer that typically is formed by previous casting, forging, heat treating, or prior machining

operations. Chemical reaction between the cutting tool material and the workpiece also could lead to a

notching-wear mechanism. This occurs when machining temperatures exceed 800°C, and induce

diffusion between the tool and the workpiece.

5. Summary

Titanium-based alloys are classified as difficult to machine materials as they cause high tool wear.

The low thermal conductivity of titanium-based alloys causes higher rate of heat transfer to the tool that

leads to rapid tool deterioration. The current research into the chemistry of the materials transferred to the

tool flank land indicates that the presence of compounds with lower thermal conductivity at the tool

surface leads to lower tool wear. From the literature it is suggested that coating materials with thermal

conductivity lower than that of the workpiece material can be used to improve tool life for machining

titanium-based alloys. Based on the previous research results there is a need to develop suitable tool

coatings to improve the machinability of titanium based alloys. In addition to thermal conductivity, the

solubility and reactivity of coating materials with titanium is also an important factor in tool wear/life.

Further it is suggested that multi-layer thermal barrier coatings with improved surface properties, in terms

of reactivity with titanium may be newly developed that could be used effectively in the industry for

machining titanium-based alloys. Also it is desired that while designing such multilayered coatings, the

thermal expansion coefficients of the constituents should be appropriately taken into consideration so as

to minimize thermal shock and therby leading to better adhesion of the coating materials onto the

substrate.

Detailed review of tool wear while machining of titanium and its alloys has been presented in this

paper. Different types of tool wear have also been explained which was encountered by many researchers

while machining of titanium alloys. The coatings on inserts have been deposited by two main process

routes - chemical vapour deposition (CVD) and physical vapour deposition (PVD), each of which has its

own advantages,limitations, and have received wide application for the deposition of wear resistance

coatings on the cutting inserts. From the literature it has been reported that machining of titanium alloy

with PVD coatings during continuous and intermittent cutting resulted in high efficiency of these coatings

both for the turning and also for milling operations. The type of wear observed in such machining

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processes is crater wear, delamination of coating and diffusion on rake face and abrasion wear on the

flank face in both CVD and PVD coatings. Hence a proposal of a new coated tool with multi layer

coating for machining of titanium and its alloys to resist high wear resistance and at the same time having

a low thermal conductivity has been made in the current paper. In short, this paper gives a complete idea

about the different wear mechanisms which exist for major tool wear like crater and flank wear. At the

end a possible way to control such wear in cutting tools has been suggested.

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