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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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Figure 9.4 shows the surface topography of when machining with
radial rake angle of 4º, nose radius of 0.8mm, cutting speed of 115m/min,
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
Figure 9.5 shows the surface topography of when machining with
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 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 0.02mm/tooth and axial depth of cut 2.5mm
Figure 9.6 Shows the surface topography of when machining with
radial rake angle of 12º, nose radius of 0.8mm, cutting speed of 115m/min,
cutting feed of 0.02mm/tooth and axial depth of cut 2.5mm. From the surface
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 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 0.04mm/tooth and axial depth of cut 1.5mm
Figure 9.7 Shows the surface topography of when machining 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 1.5mm. From the surface
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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Figure 9.8 shows the surface topography of when machining with
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
Figure 9.9 shows the surface topography of when machining with
radial rake angle of 4º, nose radius of 0.8mm, cutting speed of 115m/min,
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
Figure 9.11 shows the surface topography of when machining 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. 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
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. 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
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. 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.
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