Davim J. Paulo (editor). Machining. Fundamentals and Recent Advances

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130 A. Pramanik et al.

near the particle occurs due to higher obstruction to plastic deformation; this

causes easier particle fracture. In addition, the coarse particles themselves may

have more defects, which will result in higher tool wear and worse surface finish

during machining of MMCs. A large quantity of harmful dust after machining

MMCs with larger particle sizes has been reported, although this disappears when

machining MMCs with a smaller particle size.

The effect of particles on processes parameters during machining of MMCs can

be explained explicitly by comparing machining outcomes and those of the corre-

sponding matrix material. These are described in the following sections.

5.2.1 Strength of MMC During Machining

The chip formation forces during turning depend on the strength of the material,

cutting conditions and tool geometry. Speed and feed influence the strength of the

workpiece material in the deformation zones through temperature, strain and strain

rate [29,

30]. The strength of the non-reinforced aluminium alloy is nearly insensi-

tive to strain rate at low strain and strain rate [31–33]. However, at higher strains

(more than 1) and strain rates (10

–1

or higher), i.e., as experienced during turn-

ing, the strength is considerably dependent on strain rate, increasing with increas-

ing strain rate [34–39]. In the following work, due to a lack of data, the effects of

strain, strain rate and temperature on shear strength are not considered explicitly.

However, it was found that the measured chip formation forces in the cutting and

thrust directions (F

and

, respectively), and shear angle (

) depend on the

cutting conditions. Hence, the experimental shear strength values, τ

, for both the

MMC and corresponding aluminium alloy at different machining conditions were

determined using Equation (5.1) following the procedure described in [29,

34].

φφ

−

cc tc

[(F cos ) (F sin )] sin

(5.1)

The shear strength values of MMC and non-reinforced alloy for different ma-

chining conditions are presented in Figure 5.1. It can be seen that the strength of

MMC is significantly lower than that of non-reinforced alloy for all the machining

conditions considered. At low feeds, the strength of MMC and non-reinforced

alloy decreases with increasing feed (Figure 5.1(a)). However, at higher feeds, τ

does not vary with feed significantly. Similarly speed does not influence the

strength of the MMC (Figure 5.1(b)). At lower speed ranges, the strength of non-

reinforced alloy increases with increasing speed, but after a certain speed it de-

creases with further speed increases.

At low feed (cut thickness), the cut area is small and the entire cut area may

have been work-hardened by previous tool passage. This will result in a higher

value at lower feeds than at higher feeds. Increased percentage of particle

fracture/debonding forces (as will be seen in Figure 5.25) indicates higher tool–

particle interactions at low feed for MMCs, which may be another reason for in-

Chip formation forces were calculated by subtracting the ploughing and particle frac-

ture/debonding forces from total machining forces.

Machining of Particulate-Reinforced Metal Matrix Composites 131

creased strength at low feed [19]. Consequently, higher strength of workpiece

materials is noted at low feeds. However, with increasing feed, work hardening

decreases and temperature increases, cancelling out the net variation of the strength

of the MMC and non-reinforced alloy.

Note that the strength of the two workpiece materials decreases with increasing

feed (at feeds below 0.2 mm/rev). However, the cutting forces increase due to

increasing area of cut.

For the non-reinforced alloy, at low cutting speeds, temperature generation is

low [40]; hence the increase of strength with cutting speed is likely to be due

to the influence of increasing strain rate. With further increases of cutting speed,

the machining temperature increases, and consequently thermal softening of

workpiece material occurs. However, the increase in strain rate will increase the

strength of the material [31]. It seems that, above a certain speed, thermal soften-

ing becomes dominant over strain hardening, resulting in a decrease in strength

[1,

31].

Similar to the non-reinforced alloy, work hardening of MMC increases with in-

creasing strain rate and decreases with increasing temperature [37]. Researchers

have found that composite materials may display considerably greater strain rate

sensitivity (i.e., increase in forces with cutting speed) than that of non-reinforced

material [41,

42]. However, the lower strength (Figures 5.1(a,b)) of MMCs during

machining may be a result of cracks generated due to presence of particles in the

shear plane and tool–chip interface [16,

22,

26].

5.2.2 Chip Shape

The types of chips formed are related to the material properties and cutting pa-

rameters such as speed, feed, etc. Compared to the non-reinforced alloy, chips of

different shapes are noted during machining of the MMC. The effect of reinforce-

ment particles on chip shape under different machining parameters is discussed in

the following sections.

Figure 5.1. Variation of the shear strength with: (a) feed (at a speed of 400 m/min and

depth of cut of 1 mm); (b) speed (at a feed of 0.1 mm/rev and a depth of cut of 1 mm)

132 A. Pramanik et al.

For MMCs, chip shape varies over the considered range of feeds as shown in

Figure 5.2. At a feed of 0.025 mm/rev, chips are very short and irregular in shape.

With increasing feed long chips were formed. At feeds of 0.05 and 0.1 mm/rev,

long spiral and straight chips, respectively, were observed. With further increases

of feed (0.2 and 0.4 mm/rev), all chips became short and of C-shape. Though at

medium feeds chips were very long, they did not entangle with the tool or work-

piece and were easily breakable. For non-reinforced alloys, in general, the chip

(a) (b)

(e)

Figure 5.2. Effect of feed on chip shapes of the MMC at a speed of 400 m/min and depth

of cut of 1 mm: (a) 0.025 mm/rev; (b) 0.05 mm/rev; (c) 0.1 mm/rev; (d) 0.2 mm/rev;

(e) 0.4 mm/rev

Machining of Particulate-Reinforced Metal Matrix Composites 133

shape did not change significantly with increasing feed (Figure 5.3). At all feeds,

chips were long, slightly twisted and had a tendency to entangle with the tool and

workpiece, which damaged the newly generated surface.

With variation of cutting speed, very long and brittle chips were formed for the

MMC (Figure 5.4). At lower speed (100 and 200 m/min) all the chips were of

spiral shape but at higher speeds (400 and 600 m/min) chips became straight. With

further increases of speed (at 800 m/min), some tightly curled chips were formed

together with long straight chips. For the non-reinforced alloy, at all cutting

speeds, chips were long and/or large spirals, which entangled with the workpiece

and/or the tool (Figure 5.3).

Continuous chips are forced to curl during formation due to unequal strain oc-

curring across the plastic zone [43]. This curl depends on the ductility/brittleness

of the chips. Chips of brittle materials have little or no tendency to curl whereas

those of ductile materials may form long spiral chips. Shapes of chips are influ-

enced by the uniformity of deformation and shear localization [44]. During defor-

mation of the MMC, stress concentrations and local deformations are experienced

due to the presence of the reinforcement particles [16,

19].

As the MMC experiences high strain while passing through the primary and

secondary shear zones, some particles are debonded, initiating cracks and work

hardening the matrix material [16,

22,

26,

45]. This makes chips brittle and easy to

fracture, resulting in the formation of short chips. At lower feeds, the deformation

(a) (b)

Figure 5.3. Effect of cutting conditions on shapes of the aluminium alloy at a depth of

cut of 1 mm: (a) feed 0.025 mm/rev and speed 400 m/min; (b) feed 0.4 mm/rev and speed

400 m/min; (c) speed 200 m/min and feed 0.1 mm/rev; (d) speed 800 m/min and feed

0.1 mm/rev

134 A. Pramanik et al.

of the chips is more homogeneous across its thickness which may lead to the for-

mation of longer chips. However, it seems that, if the feed is very low, chips be-

come very thin and may break due to failure of the highly strained particle–matrix

interfaces. On the other hand, at higher feeds, considerable non-homogeneous

deformation occurs due to higher cut/chip thickness, which contributes to the

generation of shorter chips. Similarly, at low cutting speed, the strain rate effect is

prominent, which may cause inhomogeneous deformation, resulting in the forma-

tion of spiral chips. However, with increasing speed, thermal effects may reduce

these inhomogeneous deformation and increase the ductility of the matrix [46],

which produces straight chips.

(a) (b)

(c)

(d)

(e)

Figure 5.4. Effect of speed on chip shapes of the MMC at a feed of 0.1 mm/rev and

a depth of cut of 1 mm: (a) 100 m/min; (b) 200 m/min; (c) 400 m/min; (d) 600 m/min;

(e) 800 m/min

Machining of Particulate-Reinforced Metal Matrix Composites 135

All the chips formed during machining of the non-reinforced alloy were long

and less breakable because of its high ductility and deformation without formation

of cracks due to absence of particles.

A harder material generally exhibits better chip disposability, i.e., shorter chips

with brittle fracture which may, however, affect the machined surface. On the

other hand, ductile material produces very long chips with poor disposability.

Long chips can damage the newly generated surface. Ductile cutting with short

chips are normally desired to obtain an undamaged surface [47]. It seems that hard

reinforcement particles in the MMC introduce disposability to highly ductile ma-

trix material.

5.2.3 Surface Integrity

Surface roughness, residual stress and hardness are three important measures to

describe integrity of a machined surface. It is well recognised that the surface

quality depends on cutting parameters and workpiece material properties.

5.2.3.1 Surface Roughness

During machining of an MMC, defects such as voids and cavities are formed on the

surface due to tool–particle interactions and resulting pull-out/fracture and debond-

ing of particles [16,

48–50]. The tool–particle interactions are influenced by feed.

The surface roughness of machined MMC surface is influenced by both feed and

tool–particle interaction. When the particle size is approximately equal to the cut

thickness (feed) then particle pull-out/fracture and debonding dominate the surface

finish. Otherwise it is controlled by feed as well as particle pull-out/fracture and

debonding. Hence, for an MMC, surface roughness decreases with increasing feed up

to a certain limit, but with further increases of feed, it increases [21,

49]. The increase

of the volume percentage and diameter of the reinforcement particles in an MMC

increases tool–particle interactions and creates greater damage on the machined

surface, which causes inferior surface finish [50,

51]. At cutting speeds of 10–100

m/min and low/medium feeds (0.1–0.48 mm/rev), surface roughness decreased with

increasing speed but the decreasing trend was less marked at higher speeds [52].

Figure 5.5. Effect of feed on surface roughness at a speed of 400 m/min and a depth of cut

of 1 mm: (a) R

and (b) R

max

136 A. Pramanik et al.

The profile of surface generated can be considered as successive movements of

the tool cutting edge profile at intervals of feed. Figures 5.5(a,b) show the varia-

tion of the measured surface finish (R

and R

max

) with feed. As expected, surface

roughness is low at low feeds and increases with increasing feed for both rein-

forced and non-reinforced materials. At low feeds, roughness for the MMC is

higher than that for non-reinforced alloy, but above a feed of 0.3 mm/rev the re-

verse trend is observed. The theoretical roughness values are lower than the ex-

perimental values for both materials, though the deviation of the experimental

from the theoretical roughness value is much smaller at low feeds.

The machined MMC surfaces in Figure 5.6 show that the feed marks are not

noticeable at lower feeds and that the surface texture is very irregular likely due to

(a) (b)

(e)

Figure 5.6. Effect of feed on machined surface of the MMC at a speed of 400 m/min

and a depth of cut of 1 mm: (a) 0.025 mm/rev; (b) 0.05 mm/rev; (c) 0.1 mm/rev;

(d) 0.2 mm/rev; (e) 0.4 mm/rev

Machining of Particulate-Reinforced Metal Matrix Composites 137

the presence of particles. On the other hand feed marks are very clear on the non-

reinforced alloy surface at all feeds and some material build-up is clearly visible at

the higher feed (0.4 mm/rev, Figure 5.7).

No noticeable influence of speed on the machined MMC and non-reinforced alloy

surface roughness values is noted for the range of speeds considered (Figure 5.8).

For the MMC, the absence of feed marks at low feed may be due to pull out

and fracture of particles from the machined surface and indentation by particles.

These are considered to be dominant factors that influence the texture of the newly

generated surface [16,

23,

48,

46,

53]. For a given length of cut, at low feed, the

distance between two successive tool paths is less and hence a greater number of

tool–particle interactions will occur than at higher feed. The relatively high parti-

(a) (b)

(e)

Figure 5.7. Effect of feed on machined surface of aluminium alloy at a speed of 400 m/min

and a depth of cut of 1 mm: (a) 0.025 mm/rev; (b) 0.05 mm/rev; (c) 0.1 mm/rev;

(d) 0.2 mm/rev; (e) 0.4 mm/rev

138 A. Pramanik et al.

cle fracture/debonding force at lower feed (as will be seen in Section 5.3.1) also

indicates higher tool–particle interactions. These will cause greater surface dam-

age at low feed. In the case of non-reinforced alloy no such damage is expected at

low feeds, which accounts for its superior surface finish. However, at higher feeds,

the crest (due to side flow of material) on feed mark ridges of the surface is likely

to exist due to its high ductility (Figure 5.7, at the feed of 0.4 mm/rev). In the case

of the MMC, those may not exist due to its lower ductility and tendency to frac-

ture (Figure 5.6). These may cause higher roughness of the non-reinforced alloy

surface compared to that of the MMC.

This can be further investigated using the profiles of machined MMC and non-

reinforced alloy surfaces which, at various feeds, are given in Figures 5.9 and 5.10,

Figure 5.8. Effect of speed on surface roughness (at feed 0.1 mm/rev, depth of cut 1 mm):

(a) R

; (b) R

max

Figure 5.9. Effect of feed on machined surface profile of the MMC at a speed of

400 m/min and a depth of cut of 1 mm: (a) 0.025 mm/rev; (b) 0.05 mm/rev; (c) 0.1 mm/rev;

(d) 0.2 mm/rev

Machining of Particulate-Reinforced Metal Matrix Composites 139

respectively. It can be seen that the MMC surface profile is very irregular at low

feeds but, with increasing feed, the feed marks are clearly recognised in the surface

profile. On the other hand, for the non-reinforced alloy, very smooth surface pro-

files are noted at low feeds. However, surface profile is dominated by feed marks at

higher feeds. For the MMC, it is noted that at low feeds (0.025–0.1 mm/rev) the

magnitude of R

max

varies from 7 to 12 µm (Figure 5.5(b)), which is in the range of

the particle sizes (6–18 µm). It appears that the surface roughness of the MMC is

influenced by particle size at low feeds.

5.2.3.2 Residual Stress

An important parameter of a machined component’s surface integrity is the resid-

ual stress distribution, which determines the fatigue life, etc. Residual stresses are

related to the incompatibility between a surface layer and the bulk material which

is generated by any mechanism that generates a variation in the geometry of the

surface layer [54]. These stresses depend on the workpiece material and machin-

ing parameters such as the cutting speed and feed. Only a few studies on turning-

induced residual stress of monolithic (non-reinforced) materials have been re-

ported to date [54–56]. These suggest that both mechanical and thermal effects are

responsible for the generation of residual stresses on the machined surface. Con-

sidering that the machining and deformation mechanisms of an MMC are more

complicated and different to those of a monolithic material, the mechanisms of

residual stress generation are also likely to be more complex. As a result, the ef-

fects of machining parameters on surface residual stress may not be similar when

the reinforcement particles are present. The effects of reinforcement particles and

varying machining parameters on the residual stress are compared and discussed

in the following sections.

Figure 5.10. Effect of feed on machined surface profile of the non-reinforced alloy at

a speed of 400 m/min and a depth of cut of 1 mm: (a) 0.025 mm/rev; (b) 0.05 mm/rev;