CHAPTER 9 STUDY ON SURFACE...

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CHAPTER 9

STUDY ON SURFACE TOPOGRAPHY

9.1 INTRODUCTION

This chapter deals with studies on surface topography. Surface

topography literally means ‘the study or detailed description of the surface

feature or a region’. Surface integrity involves the study and control of both

surface roughness or surface topography, and surface metallurgy. Both of

these factors influences the quality of the machined surface and subsurface,

and they become extremely significant when manufacturing structural

components that have to withstand high static and dynamic stresses. For

example, when dynamic loading is a principal factor in a design, useful

strength is frequently limited by the fatigue characteristics of materials.

Fatigue failures almost always nucleate at or near the surface of a component;

similarly, stress corrosion is also a surface phenomena. Therefore, the nature

of the surface from both a topographical and a metallurgical point of view is

important in the design and manufacture of critical hardware.

Surface texture is concerned with the geometric irregularities of the

surface of a solid material, which is defined in terms of surface roughness,

waviness, lay, and flaws.

1. Surface roughness consist of the finest irregularities of the

surface texture, including feed marks generated by the

machining process.

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2. Waviness consists of the more widely spaced components of

surface texture that may occur due to the machine or part

deflection, vibration, or chatter.

3. Lay is the direction of the predominant surface pattern.

4. Flaws are surface interruptions such as cracks, scratches, and

ridges.

Enhanced surface texture specifications are essential to improve

fatigue strength, corrosion resistance, appearance, and sealing. The quality of

surface finish affects the functional properties of the machined parts as

follows:

1. Wear resistance. Larger macro irregularities result in non

uniform wear of different sections of the surface where the

projected areas of the surface are worn first. With surface

waviness, surface crests are worn out first. Similarly, surface

ridges and micro irregularities are subjected to elastic

deformation and may be crushed or sheared by the forces

between the sliding parts.

2. Fatigue strength. Metal fatigue takes place in the areas of the

deepest scratches and undercuts caused by the machining

operation. The valleys between the ridges of the machined

surface may become the focus of concentration of internal

stresses. Cracks and microcracks may also enhance the failure

of the machined parts.

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3. Corrosion resistance. The resistance of the machined surface

to the corrosive action of liquid, gas, water, and acid depends

on the machined surface finish. The higher the quality of

surface finish, the smaller the area of contact with the

corrosive medium, and the better the corrosion resistance. The

corrosive action acts more intensively on the surface valleys

between the ridges of micro irregularities. The deeper the

valleys, the more destructive will be the corrosive action that

will be directed toward the depth of the metal.

4. Strength of interference. The strength of an interference fit

between two mating parts depends on the height of micro

irregularities left after the machining process

(Helmi&YoussefHassan 2008).

Machined surface characteristics such as surface roughness and

form as well as the sub-surface characteristics such as residual stress, granular

plastic flow orientation and surface defects (porosity, micro-cracks, etc.) are

important in determining the functional performance of machined

components. The quality of surfaces of machined components is determined

by the surface finish and integrity obtained after machining. Surface integrity

is defined as the inherent or enhanced condition of a surface produced during

machining or other surface operations. Metal removal operations lead to the

generation of surfaces that contain geometric deviation (deviation from ideal

geometry) and metallurgical damage different from the bulk material. The

geometrical deviation refers to the various forms of deviations such as

roundness, straightness etc. Typical metallurgical surface damage produced

during machining include micro-cracks, micro-pits, tearing (pickup), plastic

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deformation of feed marks, re-deposited materials, etc. High surface

roughness values, hence poor surface finish, decrease the fatigue life of

machined components (MikellGroover 2010). Surface defects also act as

weak spots for crack propagation, thereby accelerating the fatigue failure of

the component in service. It is therefore, clear that control of the machining

process to produce components of acceptable integrity is essential. Machined

components for aerospace applications subject to rigorous surface analysis to

detect surface damages that will be detrimental to the highly expensive

machined components.

9.2 EXPERIMENTAL PROCEDURE

AL7075-T6 aluminium alloy with a nominal composition as shown

in Table 9.1. Its hardness was measured with a hardness tester to ensure that a

nominal bulk hardness of 150HRC was achieved. A bandshaw cutting cutting

machine was used to cut the workpiece materials supplied in block sizes of

100mm × 50mm × 30 mm. Before beginning the machining tests, the blocks

were face milled and ground on the side, top and bottom surfaces to remove

any surface defects and to ensure flatness to prevent any bias to the results.

Up to 3mm thickness of material at the top surface of the work piece was

skimmed off in order to eliminate any surface defect that can adversely affect

the machining result. The tool material was a HSS end mill with different tool

geometry. The end mill used in the test was HSS 4-flute with a diameter of 12

mm, helix angle of 45° and different tool geometry of radial rake angle, nose

radius of end mills specially made for this experiment.

The following cutting conditions were employed in this

investigation as shown in Table 9.1

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Table 9.1 Experimental plan for surface topography

Ex No

Input Process Variables

1. Surface topography of the base metal ( ) R(mm) Vc(m/min) fz(mm/tooth) ap(mm)

2. 4 0.8 115 0.04 2.5

3. 16 1 135 0.03 2

4. 12 0.4 115 0.04 2.5

5. 12 1.5 115 0.04 2.5

6. 8 0.6 135 0.03 2

7. 20 0.8 115 0.04 2.5

8. 12 0.8 155 0.04 2.5

9. 12 0.8 115 0.02 2.5

10. 12 0.8 115 0.04 1.5

11. 12 0.8 115 0.04 2.5

12. 16 1 115 0.04 2.5

In the present work, a detailed microscopic examination has been

made to study the effect of process parameters on surface topography and heat

affected zone.

All cutting tests were performed on a CNC HAAS Vertical

machining center. The spindle rotational speed ranges from 1000 to 3500

RPM. The nominal chemical composition and some physical and mechanical

properties of the alloy are shown in Tables 1.1 and 10.1, respectively. All

machining tests were conducted dry condition. 2D surface topography image

of the machined surfaces was undertaken using a microscopes machine with

high resolution digital camera. A personal computer was used for capturing

surface topography images with the help of data acquisition system as shown

in Figure 9.1.

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Figure 9.1 Experimental setup for surface topography

9.3 RESULTS AND DISCUSSION

Optical micrographs showing surface topography of the base metal

and machined surface are shown from Figures 9.2-9.14.The micrographs

(50X,500x) were taken when the process parameters were kept at their

maximum and minimum levels.

Figure 9.2 Surface topography of the base metal

Figure 9.2 shows the surface topography of the base metal of

AL7075-T6 grade Aluminium alloy. Figure 9.2 shows the free from

resolidified material, inclusion, chatter mark and cutting tool feed mark on the

surface.

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Figure 9.3 Surface produced by end milling of Al7075-T6 with radial rake angle of 16º, nose radius of 0.6mm, cutting speed of 95m/min, cutting feed of 0.05mm/tooth and axial depth of cut 3mm

Figure 9.3 shows the surface topography of when machining with

radial rake angle of 16º, nose radius of 0.6mm, cutting speed of 95m/min,

cutting feed of 0.05mm/tooth and axial depth of cut 3mm. There are some

small amounts of dispersed resolidified material, inclusion, chatter mark, and

collar formation,cutting tool feed mark on surface of machined Al7075-T6 at

low cutting speed. This is due to a rise in BUE because that cutting

temperature decreases during low cutting speed.

Figure 9.4 Surface produced by end milling of Al7075-T6 with radial rake angle of 4º, nose radius of 0.8mm, cutting speed of 115m/min, cutting feed a feed of 0.04mm/tooth and axial depth of cut 2.5mm

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cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm. There are some

small amounts of dispersed BUE, seams, pits and splattered particles, uneven

chip flow patterns cause ‘ears’ on chip and cut surface on the machined

surface of Al7075-T6 at a low radial rake angle. It provides a rough surface

due to its lack of the chip to flow out from the workpiece as shown the photo

image.

Figure 9.5 Surface produced by end milling of Al7075-T6 with radial rake angle of 12º, nose radius of 0.8mm, cutting speed of 155m/min, cutting feed 0.04mm/tooth and axial depth of cut 2.5mm


the rake angle of 12º, nose radius of 0.8mm, cutting speed of 155m/min,

cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm. From the surface

topography it can be observed large amount of dispersed HAZ, redeposited

materials, inclusion, burrs and vibration marks on the machined surface of

Al7075-T6 with high cutting speed. Collar formation raised up on shoulder of

cut surface due to that chip flow more difficult. This is due to a rise in strain

and strain rate. When cutting speed accelerates beyond 155 m/min, surface

roughness increase at a rapid rate. This might be because that cutting

temperature increases with the increase of the cutting speed.

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Figure 9.6 Shows the surface topography of when machining with



topography it can be observed large amount of dispersed HAZ, laps and

seams, inclusion, burrs and occasional “ears” of ploughed work material dull

the surface,cutting tool feed marks on the machined surface of Al7075-T6 at

low cutting feed. It can also be observed from Figure 9.6 that, as the feed rate

decreases, surface roughness increases. As the feed decreases below

0.02mm/tooth, smaller values of undeformed chip thickness are obtained and

chip removal becomes more difficult. In such circumstances there is a higher

tendency towards elastic and plastic deformation of the workpiece surface.

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Figure 9.7 Shows the surface topography of when machining with



topography it can be observed small amount of dispersed pits, recast materials

and Collar formation, cutting tool feed marks on the machined surface of

Al7075-T6 at a low axial depth of cut. The reason is due to the inefficient

removal of material by rubbing rather than efficient cutting, as a result of the

insufficient unreformed chip thickness and chatter.

Figure 9.8 Surface produced by end milling of Al7075-T6 with radial rake angle of 12º, nose radius of 0.8mm, cutting speed of 115m/min, cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm

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the rake angle of 16º, nose radius of 1mm, cutting speed of 115m/min, cutting

feed of 0.04mm/tooth and axial depth of cut 2.5mm. From the surface

topography it can be observed small amount of dispersed pits, inclusion,

voids, burrs and redeposited material on the machined surface of Al7075-T6

at high axial depth of cut. It is evident from Figure9.8, the large amount of

burrs is found on the machined surface at an axial depth of cut 2.5 mm, above

which the acceleration amplitude increases.

Figure 9.9 Surface produced by end milling of Al7075-T6 with radial rake angle of 4º, nose radius of 0.8mm, cutting speed of 115m/min, cutting feed a feed of 0.04mm/tooth and axial depth of cut 2.5mm



cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm. The worst surface

texture is observed on the machined surface employing a feed direction. It

was attributed that higher tendency for cutter vibration to occur and the high

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wear rate due to the high cutting forces induced were the main reasons. The

major surface damages observed after machining AL7075-T6 alloy with HSS

end mill are deformation of feed marks,very dull surface from a large number

of “ears”, micro-pits and re-deposited work material (chip) on to already

machined surface. A micro - pit generation can be attributed to brittle

fractures of hard carbide inclusion within the immediate surface during the

shearing of the workpiece material by the tool.

Figure 9.10 Surface produced by end milling of Al7075-T6 with radial rake angle of 12º, nose radius of 0.8 mm, cutting speed of 115m/min, cutting feed a feed of 0.04mm/tooth and axial depth of cut 2.5mm

Figure 9.10 shows the surface topography ofwhen machining with

radial rake angle of 12º, nose radius of 0.8 mm, cutting speed of 115m/min,

cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm. The Surface

topography shows that minimum surface roughness values were generated.

This could be attributed to the gradual tool wear observed due to the

significant reduction in temperature at the cutting interface. This tends to

maintain the geometry of the tool cutting edge for longer periods, thus

ensuring minimal variations in recorded surface roughness values.

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Figure 9.11 Surface produced by end milling of Al7075-T6 with radial rake angle of 12º, nose radius of 0.4mm, cutting speed of 115m/min, cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm



cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm. The Surface

topography shows that nose radius marks occur on machined surface which results in rough surface roughness.

Figure 9.12 Surface produced by end milling of Al7075-T6 with radial rake angle of 12º, nose radius of 1.5 mm, cutting speed of 115m/min, cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm.

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Figure 9.12 shows the Surface topography of when machining with


cutting feed of 0.04mm/tooth and axial depth of cut 2.5mm. The surface

topography shows that nose radius marks very less occur on machined surface

results in fine surface roughness due to wiping effect.

Figure 9.13 Surface produced by end milling of Al7075-T6 with radial rake angle of 8º, nose radius of 0.6 mm, cutting speed of 135m/min, cutting feed of 0.03mm/tooth and axial depth of cut 2mm.

Figure 9.13 shows the Surface topography of when machining with


cutting feed of 0.03mm/tooth and axial depth of cut 2mm. The surfaces

generated consist of well-defined uniform feed marks running perpendicular

to the tool feed direction.

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Figure 9.14 Surface produced by end milling of Al7075-T6 with radial rake angle of 16º, nose radius of 1 mm, cutting speed of 115m/min, cutting feed of 0.03mm/tooth and axial depth of cut 2mm

Figure 9.14 shows smooth, regular reflective surfacewhen

machining with radial rake angle of 16º, nose radius of 1 mm, cutting speed of

115m/min, cutting feed of 0.03mm/tooth and axial depth of cut 2mm. It can

be seen that the surface becomes very smooth thus ensuring minimal

variations in recorded surface roughness values as shown surface profile Ra=.

19µm.

9.4 CONCLUSIONS

In this chapter the effect of process parameters on Surface

topographyof AL7075-T6 grade alloy in CNC end milling process were

studied. The study of the structure of the specimens revealed the influence of

process parameters on surface texture, grain coarsening in the machined

morphology. It also gave an idea about Micrographs of the machined surfaces

show that micro-pit and re-deposited work material were the main damages to

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the surfaces. Surface finish generated when machining AL7075-T6 with HSS

end mill tools are generally accepted and free of physical damages such as

tears, laps or cracks in all the cutting conditions investigated.

The surface becomes rougher in cutting direction at a low radial

rake angle, while it is smoothly at relatively high radial rake angle. Surface

roughness value in cutting direction initially increases with the cutting speed

minimum and then decreases with the further increased cutting speed. This is

because a higher cutting speed generates higher temperatures which tend to

induce thermal softening for countering the dominant strain hardening.The

feed effect on surface roughness value shows that it almost linearly increases

with the increased feed in both directions.

CHAPTER 9 STUDY ON SURFACE...

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