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CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

In this chapter, a review of the published literature related to the

machining of Inconel 718 is presented. The available cutting tool materials for

machining of Inconel 718 and their effects on surface roughness and wear

mechanisms have been discussed. The subsection reviews the previous

research done on the performance of the cutting tool and the effectiveness of

optimized machining parameters in machining Inconel 718.

2.2 MACHINABILITY STUDY OF INCONEL 718 IN TURNING

PROCESS

The most widely used source of machinability data is the

Machining data handbook published by Metcut Research Associates (1980).

The data was primarily grouped according to the machining process than

types of work piece material and its respective hardness. The handbook

provides the machining parameters such as cutting speed, feed and depth of

cut for particular tool-work piece combination. However, the handbook

approach has a number of limitations. The handbook data applies to a

particular machining situation and so it may not be suited either for the newly

developed tool or work piece material. The handbooks are manually input-

output oriented; hence lack of compatibility with advanced automated system.

The term machinability is used to refer to the ease with which a work material

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is machined under a given set of cutting conditions. It gives a prior knowledge

to the process planner and production machinist. Many researchers in the past

studied the machinability of different materials.

Trent (1991) outlined that tool life, cutting force, chip shape,

surface finish were all important parameters for the machinability assessment

of a material. Ezugwu et al (1991) reported about the tendency to form a built

up edge during machining and the presence of hard abrasive carbides in their

microstructure, which determines machinability. These characteristics of the

alloys cause high temperature (1000°C) and stresses (3450 Mpa) in the

cutting zone leading to accelerated flank wear, cratering and notching,

depending on the tool material and cutting conditions used.

Reen (1977) pointed out that the factors, namely, accurate rating of

machinability, tool life, surface finish and power consumed during cutting

must be considered for machinability assessment. Rahman et al (1997)

discussed the machinability of Inconel 718 subjected to various machining

parameters along with tool geometry using various coated carbide tool inserts.

Flank wear, surface roughness and the cutting force components have been

considered as performance indicators for tool life. His studies revealed that

tool life is significantly increased with increase of side cutting edge angle and

decreased with increase of cutting speed and feed. Choudhry and El-Baradie

(1998 a) reported that Inconel 718 stands out as the most dominant alloy in

production, accounting for as much as 45% of wrought nickel-based alloy

production and 25% of cast nickel-based products. Ezugwu et al (1998)

mentioned that significant proportion of published information is on the

machining of Inconel 718 in the last three decades. Alauddin et al (1999)

reported that Inconel 718 material possesses good strength, excellent

corrosion and creep resistance at high temperatures, which are responsible for

being difficult to machine.

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Ezugwu et al (2003), Ahmed et al (2006) discussed the

machinability of aero engine alloys and their properties and found that the

poor thermal conductivity of these alloys generated high temperature zone at

chip tool interface and that high temperature reduced the strength and

hardness of the cutting tool. In order to minimize the cutting zone

temperature, they suggested various cooling techniques such as high pressure

coolant, cryogenic cooling and minimum quantity lubricant system.

2.3 CUTTING TOOLS FOR MACHINING OF INCONEL 718

The different types of cutting tool materials generally used for

machining nickel-based super alloys are discussed in this section with

emphasis on Inconel 718. In addition, the advantages, drawbacks, cutting

conditions under which these cutting tools can be employed and their tool

wear are also discussed. HSS and cemented carbide cutting tools are the only

choice for the machining of exotic super alloys for several decades. HSS

steels are usually employed for intermittent cutting operations, whereas

cemented carbides are mainly used for continuous cutting operations.

Kramer (1987), Ezugwu et al (1999) have indicated that the

uncoated carbide tools are used to machine nickel-based alloys at cutting

speed range of 10-30 m/min and at feed rates up to 0.5 mm/rev for improved

productivity. Takatsu (1990) discussed the tungsten-based carbides used in

high feed-rate cutting and severe interrupted cutting. Because of their poor

thermo chemical instability, they cannot be used at high speeds. Coated

carbides on the other hand, have good wear resistance and strength. Brandt

et al (1990) pointed out the introduction of carbide tools which made it

possible to machine nickel based alloys at speeds of the order of 50 m/min.

Ezugwu et al (1991) reported that the carbide cutting tools, the oldest amongst

the hard cutting tool materials are used to machine nickel-based super alloys

in the speed range of 10–30 m/min. Cselle and Barimani (1995) estimated that

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40% of the cutting tools used in the industry are coated carbides and 80% of

them are used for machining purpose. Kitagawa et al (1997) suggested that

the economic achievable speed range for machining Inconel 718 using coated

carbide is 30 to 100 m/min. Ezugwu et al (2003) established that the carbide

tools have poor thermo-chemical stability and encounter pronounced

dissolution/diffusion of tool materials at the tool-chip interface into the

underside of the chip as it traverses the tool face causing tool wear, when

machining at speeds in excess of 30 m/min. Coatings are hard materials and

can provide good abrasion resistance thereby improves the performance of

coated carbides. Higher cutting speeds can be achieved using coated carbides.

Courbon et al (2009) contributed to a new insight that helps to

characterize and expand machining performance in high pressure jet assisted

machining of Inconel 718 with coated carbide tools across a useful region of

process operability. They also demonstrated that HPJA is an efficient

alternative lubrication solution by providing better chip breakability, and

reduction in cutting forces. The use of multi-layer (TiN + TiCN + TiN) coated

carbide tools produced by the PVD technique has also shown remarkable

improvement in the machining of Inconel 718.

Currently, some new ceramic tool materials such as Al2O3-TiC

mixed ceramics; Si3N4 ceramics (sialon) and the latest silicon carbide whisker

reinforced alumina have been used for machining Inconel 718. Whitney

(1974) explained that the whisker-reinforced alumina ceramics, have a low

coefficient of thermal expansion in addition to resistance to high temperature.

The range of cutting speed is from 200 to 750 m/min, with corresponding feed

rates of 0.18–0.375 mm/rev, depending on the hardness of the super alloy.

Anon (1979) reported that the mixed alumina is thermally tougher and retains

their hardness at high temperature. With cutting speed of 120–240 m/min,

these tools were first used around 1970, the speed range being almost ten

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times higher when compared to plain carbides. These mixed alumina tools

have been reported successful in machining nickel-base alloys because of

their improved thermal shock resistance properties.

Richards and Aspinwall (1989) recognized that the recent

development of ceramic tools is whisker reinforced, and imparts higher

tensile strength and fracture toughness. The thermal conductivity is also

increased by 40% than that of alumina, thereby reducing thermal gradients

and improving its ability to withstand thermal shock. Brandt et al (1990)

discussed the use of these super abrasive cutting tools which has enabled high

cutting speeds. Sialon ceramics have a low coefficient of thermal expansion

compared to that of alumina-based ceramics. High toughness together with

low thermal expansion coefficient has made sialon tools thermally shock

resistant.

Takatsu (1990) reported that, high fracture toughness of silicon

nitride based ceramics make them ideal for rough machining of nickel based

alloys at high speeds. Choudhury and El-Baradie (1998 a) revealed that the

silicon nitride ceramics tools can be employed to machine nickel-base alloys

at higher cutting speed and feed rate compared to those of mixed alumina

tools. The recommended speed range for CBN tools to machine Inconel 718

is from 120 to 240 m / min and the performance of these tools with high CBN

content was better because of their high hardness.

Chakoraborty et al (2000) reported about the inadequate fracture

toughness of ceramic tools which makes them susceptible to mechanical and

thermal shock during machining. Ceramics have high melting point and the

absence of secondary binary phase, like carbides, prevents them from

softening under high speed machining.

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2.4 TOOL WEAR

Most of the studies on tool wear mechanisms agree that there are

five basic causes of wear. These five causes of wear can occur in combination

with the others or singly. Nie et al (1998) quoted that “The causes of wear do

not always behave in the same manner, nor do they always affect wear to the

same degree under similar cutting conditions.” The five categories of wear are

• Abrasive wear

• Plastic deformation of the cutting edge

• A chemical reaction between the tool and the work piece at

elevated temperatures

• Diffusion between work and tool material and

• The welding asperities between work piece and the tool

Liao and Shiue (1996) analyzed the wear mechanism of two

cemented carbide tools namely K20 and P20 grades, in dry turning of Inconel

718. The feed rate and the depth of cut were 0.10 mm/rev and 1.5 mm,

respectively with the cutting speed of either 15 or 35 m/min. On the wear

surface of the K20 carbide, they observed a sticking layer very close to the

cutting edge. Built-up edge was formed at a cutting speed of 35 m/min with

chipping of the cutting edge. When P20 carbide was used, the sticking layer

could also be found, but comparatively, the wear was more irregular, the flank

wear length was larger and the groove was deeper.

Based on the economic consideration, uncoated carbide tools are

attracted and they give a better performance with respect to different cutting

speeds and feed rates. Choudhury and El-Baradie (1997) found that, no

significant difference in tool life was observed for the coated and uncoated

tools in the speed range of 26-48 m/min. Choudhury and El-Baradie (1998 b)

conferred that the uncoated carbide tools are better than the coated tools for

machining Inconel 718. Apparently, the coating does not improve the

performance of coated tools.

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Jindal et al (1999) analyzed that flank and nose wears are the

predominant failure modes, when machining nickel-based alloys with carbide

tools. The generation of extreme temperature (1100˚C) at the tool tip results

in erosion. Subsequently, it easily removes the coating layer(s) and ultimately

cutting forces raise, which cause rapid nose and flank wears.

Ezugwu and Bonney (2004) investigated the effect of varying

coolant pressure on tool performance, when machining Inconel 718 alloy with

coated carbide tools at high-speed conditions. Tool wear increased gradually

with prolong machining with high coolant pressures. Nose wear was the

dominating tool failure mode probably due to a reduction in the tool–chip and

tool–work piece contact length/area.

Sharman et al (2006) conducted a series of experiments for

evaluating the generation of residual stress in turning Inconel 718 using

uncoated and coated carbide tools. Klocke et al (2006) reported that, when

drilling, Inconel 718, the use of an ester, led to significant improvement in

tool life compared to the emulsion. As the results are verified, synthetic esters

represent an efficient alternative to conventional water-based lubricoolants,

even in difficult machining operations. Thamizhmanii et al (2007) concluded

that the super cobalt tool has given more life by minimum quantity lubricant

than dry milling. Muammer Nalbant et al (2007) suggested that the lowest

average surface roughness (0.806 µm) is obtained at the cutting speed of 15

m/min with single coated (TiN) cemented carbide inserts and minimum

average surface roughness is determined with single layer (TiN) coated

cemented carbide tools, while maximum average surface roughness is

observed with multicoated Al2O3 tools. Hsu et al (2008) suggested that the

NX2525 cermet can be a more suitable cutting tool for Inconel 718, which

produces almost no BUE and achieves the best surface roughness.

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2.4.1 Review on Tool Failure Modes and Wear Mechanisms

Severe flank wear and notching at the tool nose and/or the depth of

cut line are the dominant failure modes, when machining with carbide tools.

Kramer and Hartung (1981) reported that at the cutting speed above 30

m/min, carbide tools fail due to thermal softening of the cobalt binder phase

and subsequent plastic deformation of the cutting edge. Wang (1993)

recommended that tools should be replaced, when the flank wear on the tool

reaches the defined criterion. According to ISO 3685, the criteria most

commonly used for sintered carbide tools and ceramic tools are as follows

(ISO 3685, 1993):

• The maximum width of the flank wear land will be VB max. =

0.6mm, if the flank wear land is not regularly worn in zone B.

• The average width of the flank wear land will be VB max. = 0.3

mm, if the flank wear is considered to be regularly worn in zone

B, which is the criteria most commonly used for sintered

carbide tools Figure 2.1 shows the tool wear in turning process.

Figure 2.1 Tool wear in turning process

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Goh et al (1996) explained the results obtained from machining

steel using ceramics. Abrasion wear mechanism can be attributed to the

plastic deformation induced necking of asperities, referred to a superficial or

discrete plastic deformation. El-Baradie (1996) investigated the effect of

cutting speed on the wear of carbide tools when machining Inconel 718.

Ezugwu et al (1998) investigated that the rapid increase in notching occurs on

carbide tools at higher cutting speed. This usually leads to premature fracture

of entire insert edge. Premature fracture of cutting tools can be avoided by

employing the taper turning technique where the depth of cut is gradually

shifted during machining, thus shifting the notch wear along the entire flank

face of the tool. This will consequently lead to the generation of a uniform

flank wear on the cutting edge. Arunachalam et al (2004) explained that the

excessive flank wear can cause chatter and a poor finish on the machined

surface. Kilickap (2005) reported that the flank wear occurs on the relief face

of the tool and is generally attributed to (i) rubbing of the tool (friction) along

with the machined surface, thus causing adhesive and/or abrasive wear and

(ii) high temperatures, thus affecting tool-material properties as well as the

work piece surface. Because of the rigidity of the work piece, the worn out

area is parallel to the resultant cutting direction. Flank wear and micro

chipping are the predominant failure modes of cemented carbide tool insert

affecting tool performance and tool life (Thakur et al 2009). The geometry of

the tool plays a big part in controlling wear. It must allow for chip removal in

order to take the heat out with the chip (Basim A. Khidhir and Bashir

Mohamed, 2010).

2.5 MODEL DEVELOPMENT AND OPTIMIZATION

In machinability studies investigations, statistical design of

experiments is used quite extensively. Taguchi’s approach to design of

experiments is easy to adopt and apply for users with limited knowledge of

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statistics; it has gained a wide popularity in the engineering and scientific

community. There have been plenty of recent applications of Taguchi

techniques to manufacturing process for optimization (Yang and Tarng 1998,

Davim 2003, Aslan et al 2007, Muthukrishnan and Davim 2008). In

particular, it is recommended for analyzing metal cutting problems for finding

the optimal combination of machining parameters.

Montgomery (1997) referred the statistical design of experiments as

the process of planning the experiment so that the appropriate data can be

analyzed by statistical methods, resulting in valid and objective conclusions.

Design and methods such as factorial design, response surface methodology

and Taguchi methods are now widely used in place of one-factor-at-a-time

experimental approach, which is time consuming and exorbitant in cost.

Thomas et al. (1997) used a full factorial design involving six factors to

investigate the effects of cutting tool and process parameters on the resulting

surface roughness and on built-up edge formation in the dry turning of carbon

steel. El-Baradie (1998) used RSM and 23 factorial designs for predicting

surface roughness when turning high-strength steel. Taguchi method was used

by Yang and Tang (1998) to find the optimal cutting parameters for turning

operations. Thiele et al (1999) used a three-factor complete factorial design to

determine the effects of work piece hardness and cutting tool edge geometry

on surface roughness, and machining forces. The Taguchi method with

multiple performance characteristics was used by Nian et al (1999) in the

optimization of turning operations. A polynomial network was used by Lee

and Tarng (2000) to develop a machining database for turning operations. On

the other hand, Lin et al. (2001) used an abductive network to construct a

prediction model for surface roughness and cutting force. Optimization of

machining parameters is an important and challenging task to the process

planner in order to produce quality, quantity and cost effective manufacturing.

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Researchers have made an attempt to understand the relationship between the

parameters and the performance criteria.

Fischer and Elrod (1971), Lambert and Taraman (1973), Sundaram

(1978) , have developed mathematical models both theoretical and analytical

to predict the surface finish in fine turning of steel using carbide tools.

Surface roughness and dimensional accuracy have been considered as

important factors to predict machining performances of any machining

operation. Taraman (1974) developed mathematical models based on RSM

for cutting force, roughness, and tool life as a function of cutting speed, feed

and depth of cut in the turning of SAE 1018 cold-rolled steel. El-Baradie

(1998) established the machinability model by the functional relationship

between the input machining parameters (cutting speed, feed, and depth of

cut) and the output responses (tool life, surface roughness, and cutting force)

for turning. Choudhury and El-Baradie (1999) also emphasized the use of

RSM in developing machinability assessment model for turning Inconel 718

using uncoated and coated carbide tools.

Machining optimization problems, particularly turning process

optimization have been investigated by many researchers. Turning process

optimization is formulated as non-linear constrained multi-variable problems.

In order to solve this type of non-linear problems, several solution approaches

have been used. They include Geometric Programming (Ermer 1971), Goal

Programming (Sundaram 1978), Hooke-Jones Pattern Search (Mesquita1995),

Nelder Mead simple method (Agapiou 1992 a, b) and so on. Recently several

non-conventional techniques such as Simulated Annealing Algorithm (Chen

and Su 1996), Genetic Algorithm (Saravanan et al 2001, 2003), Ant Colony

Optimization (Vijayakumar et al 2003), Particle Swarm Optimization, Tabu

Search and Memetic Algorithms (Saravanan et al 2005) have been used. The

necessities of the complex manufacturing processes need to optimize several

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contradictory objectives simultaneously, whereas the single objective

optimization has a limited value to fix the optimal conditions. Many

researchers use a priori technique (decision maker has to combine different

objectives) in their multi-objective cutting parameters optimization (Lee and

Tarng 2000, Zuperl and cus 2003). Quiza Sardinas et al (2006) optimized the

multi-objective machining parameters based on posterior techniques by

decision makers presented with a set of Pareto optimal solutions (Abburi and

Dixit 2007, Abraham et al 2005). Jeyabal and Natarajan (2010) carried out the

tool wear optimization using nelder–mead and genetic algorithm with a single

objective function for minimizing the responses individually. Yang and

Natarajan (2010) attempted to solve multi-objective optimization problem in

turning by using multi-objective differential evolution (MODE) algorithm and

nondominated sorting genetic algorithm (NSGA-II).

Carl Leshock et al (2001) studied the surface temperatures

generated by plasma enhanced machining of Inconel 718 using numerical

modeling and comparison of numerical modeling with experimental results.

Noordin et al (2004) described the performance of a multilayer tungsten

carbide tool using response surface methodology, when turning AISI 1045

steel. Cutting tests were performed with constant depth of cut and at dry

cutting conditions. The factors investigated were cutting speed, feed and the

side cutting edge angle of the cutting edge. The main cutting force, namely

the tangential force and surface roughness were the response variables

investigated. The experimental plan was based on the face centered, central

composite design.

Ezugwu (2005) developed an artificial neural network model for

the analysis and prediction of the relationship between cutting tool and

process parameters during high-speed turning of nickel-based, Inconel 718

alloy. A very good performance of the neural network, in terms of agreement

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with experimental data, was achieved. The model can be used for the analysis

and prediction of the complex relationship between cutting conditions and the

process parameters in metal-cutting operations and for the optimization of the

cutting process for efficient and economic production.

Devillez et al (2007) proposed an optimization of the cutting

conditions and efficiency of the coatings. The results coming from uncoated

tools were compared with those obtained from coated tools under the same

conditions of machining. Harisingh et al (2007) predicted the values of the

tool life and surface roughness and concluded that the effect of process

variables on tool life and surface roughness can further be ascertained by

plotting the developed models.

Al-Ahmari (2007) proposed that RSM models are better than RA

models for predicting tool life and cutting force models. The developed

machinability models can be utilized to formulate an optimization model for

machining economic problem to determine the optimal values of process

parameters for the selected material. Elmagrabi et al (2007) show that the

response surface methodology, can be used in the design of experiments to

develop tool life models for several materials and the same approach can be

used for modeling other machinability performance measures (response) such

as surface roughness.

Machinability assessment of nickel based super alloy, Inconel 718

in turning operation has been carried out using few cutting tools under dry

cutting conditions alone. Hence, a complete study is carried out under wet

conditions and empirical modeling is conceded.

The literature review carried out for different types of cutting tool

materials and their performance in the machining of Inconel 718 are

discussed. In addition, the objectives, limitations and optimization of

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machining conditions under which these cutting tools employed and their

performance are also discussed.

2.6 SCOPE OF THE FUTURE WORK

• Machinability assessment, modeling and optimization of

machining parameters for Inconel 718 have not been focused by

the earlier researchers.

• In the earlier research works, a detailed study for

machinability of Inconel 718 using cermet cutting tool

material has not been made.

• No comprehensive tool wear and surface roughness analysis

were carried out to assess the performance of cermet tool in

turning operation.

• Optimization and statistical analysis of machining parameters in

finish turning were not conducted in the earlier research work.

2.7 OBJECTIVE OF THE RESEARCH

The main objective of this research is to select the machining

parameters for Inconel 718 material in finish turning process and to study the

tool wear and surface roughness. During the experimentation, surface

roughness and tool wear have been obtained by varying machining

parameters.

The performance study of low cost uncoated carbide, coated

carbide, ceramic and cermet cutting tools in finish turning process of Inconel

718 is to be conducted. The effect of machining parameters on the tool wear

and surface finish is to be analyzed.

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• Detailed analysis of the tool wear, wear mechanisms, and

surface roughness for the various combinations of cutting speed,

feed and depth of cut on the low cost uncoated carbide tools,

coated carbide tools, ceramic tools, and cermet tools is to be

done.

• Determination of the most significant machining parameters and

their percentage of contribution on the performance measure are

to be done.

• The machining data base for Inconel 718 is to be generated

which would be useful to researchers, process planners and tool

designers.