CHAPTER 7 MILLING STUDIES OF ALUMINIUM ALLOY BASED METAL...

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CHAPTER 7

MILLING STUDIES OF ALUMINIUM ALLOY BASED

METAL MATRIX COMPOSITES

7.1 INTRODUCTION

The demand for low tolerances and better quality products have

forced the manufacturing industries to continuously improve quality control

and machining technologies. On e of the fundamental metal cutting processes

is end milling (Cevdet Gologlu and Nazim Sakarya, 2008). Milling is a

machining process in which a work is fed against the rotating multipoint

cutting tool. The tool rotates at a high speed and because of the multiple

cutting edges it removes metal at a very fast rate. The machine can also hold

one or more cutting tools at a time. This is why a milling machine finds wide

application in production work. It is superior to other machines as regards

accuracy, surface finish and is designed for machining a variety of tool room

work. The present market share of HSS milling cutters is 25%. The

advantages of HSS compared with carbide and other harder cutting tool

materials are the higher toughness and the lower cost. The schematic

illustration of an end milling process is shown in Figure 7.1.

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Figure 7.1 Schematic illustration of an end milling process

7.2 MILLING EXPERIMENTS

The machining experiments of the plain LM24 aluminium alloy,

aluminium alloy - alumina / silicon carbide composite are conducted in a

milling machine. End mills of M2 grade High Speed Steel (HSS) of 5 mm

diameter are used in the present work. The advantages of HSS compared with

carbide and other harder cutting tool materials are the higher toughness and

the lower cost. Because of their high toughness and resistance to fracture,

high speed steels are especially suitable for high positive rake angle tools and

for machine tools with low stiffness. The milling tests are carried at speeds of

10, 20 and 30 m/min and feeds of 0.1, 0.3 and 0.5 m/min with a constant

depth of cut of 0.5 mm. In the present work the end mills used with the

specification of ISO 1641: Part 1: 1978 and kerosene as a coolant. The surface

roughness Ra of the machined surface has been observed using a stylus type

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surface roughness tester. For the repeatability of the results, all the

experiments are conducted five times under the same machining conditions

and the average values are taken. The qualitative wear of the end mills is

measured using a toolmakers microscope.

7.3 SURFACE ROUGHNESS

The Mean (M), Standard Deviation (SD), Standard Error (SE) and

the upper and lower limits of Confidence Interval (CI) of the surface

roughness, Ra, in micron of the aluminium alloy and the aluminium alloy -

aluminium oxide / silicon carbide composite are presented in Tables from 7.1

to 7.9. The formulae used for the calculation are given as follows (Ronald et

al (2002) and David L Streiner (1996)).

Xi = Value of the ith sample.

M = Mean of i values = (Xi) / N

N = Sample size.

SD = [ i -M )2/(N-1)]1/2

SE = SD/(N)1/2

95% CI = M ± (1.96SE)

In all the conditions, the mean of surface roughness lies within the

respective upper and lower limits of confidence for the plain LM24

aluminium alloy and the aluminium alloy - aluminium oxide / silicon carbide

composites.

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7.4 EFFECT OF SPEED AND FEED ON SURFACE

ROUGHNESS

The ideal tool in machining is one which replicates its nose well on

the work surface. The surface quality, which largely depends on the stability

of the cutting nose and the dimensional accuracy, is controlled by flank wear

of tools (Sornakumar et al, 1993). The precision of a surface is usually based

on two criteria such as dimensional accuracy and surface roughness. Surface

roughness is the measure of the finer surface irregularities in the surface

texture. Surface roughness generally plays an important role in wear

resistance, ductility, tensile and fatigue strength for machined parts and

cannot be neglected in design. In end milling, the surface roughness depends

on rotational speed of end mill cutter, feed rate, depth of cut, mechanical

properties of the workpiece being machined, and the type of lubricant /

coolant used at the point of cutting. The effects of speed and feed on surface

roughness parameter Ra are presented in Figures 7.2 to 7.7.

At cutting speeds of 10, 20 and 30 m/min and feeds of 0.1, 0.3 and

0.5 m/min, the surface roughness decreases with the increasing speed and the

decreasing feed. The cutting speed and feed affect the surface roughness. The

decrease in surface roughness with the increase in cutting speed is attributed

to the increase in the contact between the tool and workpiece, thereby

resulting in increased cutting and burnishing effect between the tool and the

milled surface. The increase in surface roughness with the increase on feed is

attributed to the decrease in contact between the tool and workpiece, there by

resulting in decreased cutting and burnishing effect between the tool and the

milled surface.

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0.02

0.04

0.06

0.08

0.1

0.12

5 10 15 20 25 30Cutting speed, m/min

LM24 LM24 + 1%AluminaLM24 + 3%Alumina LM24 + 5%Alumina

Figure 7.2 Surface roughness vs Cutting speed at feed of 0.1 m/min for

aluminium alloy – alumina MMCs

0.04

0.06

0.08

0.1

0.12


LM24 LM24 + 1%AluminaLM24 + 3%Alumina LM24 + 5%Alumina



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0.06

0.08

0.1

0.12

0.14

0.16


LM24 LM24 + 1%Alumina

LM24 + 3%Alumina LM24 + 5%Alumina



0.02

0.04

0.06

0.08

0.1

0.12


LM24 LM24 + 1%SiCLM24 + 3%SiC LM24 + 5%SiC


aluminium alloy – SiC MMCs

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0.04

0.06

0.08

0.1

0.12

0.14





0.06

0.08

0.1

0.12

0.14

0.16





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7.5 EFFECT OF PARTICLE REINFORCEMENT ON SURFACE

ROUGHNESS

The surface roughness increases with the percentage weight

reinforcement of aluminium oxide / silicon carbide due to the increase in the

amount of particles and higher hardness. Defects such as voids and cavities

are formed on the surface due to tool - particle interactions resulting in pull-

out / fracture and debonding of particles during milling of a MMC, which

dominate the surface finish. The reinforcement particles in a MMC increase

tool - particle interactions and create greater damage on the machined surface,

which causes inferior surface finish of the aluminium alloy - aluminium oxide

/ silicon carbide composites.

The surface roughness of the aluminium alloy - aluminium oxide /

silicon carbide composite is higher than that of the plain LM24 aluminium

alloy due to particle reinforcement and higher end mill tool wear. The surface

roughness of the aluminium alloy - silicon carbide composite is higher than

that of the aluminium alloy - aluminium oxide composite. This is attributed to

the higher wear of the HSS end mill on machining the aluminium alloy -

silicon carbide composite due to the higher hardness of the silicon carbide

particles in the MMC.

The usage of coolant during machining prevents the formation of

built-up-edge of the end mill tool and results in an excellent surface finish

comparable to finish machining operations such as grinding or lapping.

7.6 EFFECT OF SPEED, FEED AND AMOUNT OF PARTICLE

REINFORCEMENT ON END MILL TOOL FLANK WEAR

Flank wear is the most important tool wear occurring in machining

operations. The flank wear is primarily attributed to the rubbing of the tool

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along the machined surfaces caused by abrasive, diffusive and adhesive wear

mechanisms (Sreerama Reddy et al, 2009). Among the different forms of tool

wear, flank wear is the significant measure as it affects the dimensional

tolerance of the workpiece. The dimensional accuracy of the workpiece is

controlled by flank wear of tools (Senthilkumar et al., 2003). Flank wear

occurs on the relief face of the cutting tool and is generally attributed to the

rubbing of the tool along the machined surface and thus affects the tool

material properties as well as the workpiece surface, due to high temperatures.

Abrasion, diffusion and adhesion are the main wear mechanisms in flank wear

(Senthilkumar et al, 2006a).

The optical microscopic studies are conducted qualitatively on the

HSS end mill tool. It is observed that the harder aluminium oxide / silicon

carbide in the MMCs abrades away the flank of the HSS end mill tool with

each cut. The flank wear increases with speed, feed and percentage weight of

aluminium oxide / silicon carbide particle reinforcement. The rise in

temperature adversely affects the wear resistance and hardness of the cutting

tool. Increased heat causes dimensional changes in the workpiece part being

machined and making control of dimensional accuracy difficult. The mean

temperature in machining is proportional to the cutting speed and feed as

follows: Mean Temperature va fb ------------------------------------ ( 7.1 )

where ‘a’ and ‘b’ are constants which depend on tool and workpiece

materials, ‘v’ is the cutting speed and ‘f’ is the feed of the tool (Kalpakjian,

1995).

The end mill wear is higher on machining the aluminium alloy -

aluminium oxide / silicon carbide MMCs, than that of machining the plain

LM24 aluminium alloy, which is attributed to the harder aluminium oxide /

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silicon carbide in the MMCs, and is harder than the HSS end mill tool. The

general mechanisms that cause tool wear are abrasion, diffusion, oxidation,

fatigue and adhesion. Most of these mechanisms are accelerated at higher

cutting speeds and results in higher machining temperatures. The flank wear

of an HSS end mill tool is higher on machining the harder aluminium alloy -

aluminium oxide / silicon carbide MMCs than that of machining the plain

LM24 aluminium alloy. The flank wear of the end mill increases with the

amount of percentage weight of particle reinforcement due to higher abrasion

of increasing the amount of aluminium oxide / silicon carbide particles in the

MMCs. The tool wear is higher on machining aluminium alloy - silicon

carbide composites than that of aluminium alloy - aluminium oxide

composites due to higher hardness of the silicon carbide particles.

7.7 MICROSCOPIC STUDIES OF MILLED SURFACES

The machined surfaces are analysed by using the optical

microscope, and the corresponding images are shown from the Figures 7.8 to

7.27.

Figure 7.8 Micrograph of milled surface of plain LM24 aluminium alloy

at cutting speed of 10 m/min and feed of 0.1 m/min

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Figure 7.11 Micrograph of milled surface of aluminium alloy - 1% weight

Al2O3 composite at cutting speed of 10 m/min and feed of 0.1 m/min



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SiC composite at cutting speed of 10 m/min and feed of 0.1 m/min

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The machined surfaces are also analysed by using a Scanning

Electron Microscope (SEM), and the corresponding images are shown in

Figures 7.28 to 7.30.

Figure 7.28a SEM image of milled surface of plain LM24 aluminium

alloy at cutting speed of 20 m/min and feed of 0.3 m/min

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Figure 7.28b SEM image of milled surface of plain LM24 aluminium

alloy at cutting speed of 20 m/min and feed of 0.3 m/min

Figure 7.29 SEM image of milled surface of aluminium alloy - 5% weight

Al2O3 composite at cutting speed of 20 m/min and feed of 0.3 mm/min

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Figure 7.30a SEM image of milled surface of aluminium alloy - 5% weight


Figure 7.30b SEM image of milled surface of aluminium alloy - 5% weight


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Figure 7.31 SEM image of milled surface of aluminium alloy-5% weight


Figure 7.32a SEM image of milled surface of aluminium alloy - 5% weight

SiC composite at cutting speed of 20 m/min and feed of 0.3 mm/min

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Figure 7.32b SEM image of milled surface of aluminium alloy - 5% weight

SiC composite at cutting speed of 20 m/min and feed of 0.3 mm/min

From the optical microscope and SEM images, the machined

surface morphology is found. The plastic deformation, debonding and

scratches made by the hard ceramic particles during machining of composite

are observed on the milled surface.

7.8 SUMMARY

The milling tests have showed that the surface finish improves with

increasing speed and decreasing feed. The surface roughness values of the

milled surface of the plain LM24 aluminium alloy are lower than that of the

aluminium alloy - aluminium oxide composite, which in turn is lower than the

aluminium alloy - silicon carbide composite. The surface roughness increases

with increase of particle reinforcement. The surface roughness of the entire

specimens is excellent and comparable to that of a finish machining processes

such as grinding or lapping, thus eliminating the costly finishing operations.

The end mill tool wears increases with the speed and feed. The end mill wear

on milling of the plain LM24 aluminium alloy is lower than the aluminium

alloy - aluminium oxide composite, which in turn is lower than the aluminium

alloy - silicon carbide composite.

CHAPTER 7 MILLING STUDIES OF ALUMINIUM ALLOY BASED METAL...

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