cutting force of metal matrix composite in drilling process

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Engineers, Part E: Journal of ProcessProceedings of the Institution of Mechanical

http://pie.sagepub.com/content/215/2/177The online version of this article can be foundat:

DOI: 10.1243/0954408011530334

215: 1772001ceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

J P Davim and A Monteiro BaptistaCutting force, tool wear and surface finish in drilling metal matrix composites

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Cutting force, tool wear and surface nish indrilling metal matrix composites

J P Davim1* and A Monteiro Baptista

2

1Department of Mechanical Engineering, University of Aveiro, Aveiro, Portugal2Department of Mechanical Engineering, Faculty of Engineering, University of Porto, FEUP, 4200-465 Porto,

Portugal

Abstract: In this paper the evolution of the cutting force, tool wear and surface nish, measured when

drilling the metal matrix composite A356/20/SiCp-T6, is presented. The experimental work was

developed through the continuous measurement of the torque with an appropriate piezoelectric

dynamometer and the results were used to derive the cutting power (ranging from 0.1 to 0.4 kW) and thespecic cutting pressure, Ks (17003500 N/mm

2). The tool wear type was identied and its evolution

with cutting time was measured for different sets of cutting conditions, using polycrystalline diamond

drills. The holes surface nish was evaluated and very good results, exceeding standard values for

drilling, were obtained (Rabetween 0.25 and 1.2 mm).

Keywords: drilling, metal matrix composites (MMCs), polycrystalline diamond (PCD), tool wear,

cutting forces, surface nish

1 INTRODUCTION

The expression metal matrix composites (MMCs) covers

a very wide range of materials, from relatively simple

reinforcement of castings with a low cost refractory wool

to complex continuous bre lay-ups in exotic alloys.

Clearly the applications will also vary widely to reect

the costproperty relationships offered by each type of

MMC.

The properties of the resulting composite are generally

controlled by three critical components: the matrix, the

reinforcement and the interface. Several considerations,

which arise with respect to fabrication, processing and

service performance of composites, relate to processesthat are taking place in the interfacial region between the

matrix and reinforcement [1,2].

Among modern composite materials, particle-rein-

forced MMCs are nding increased application owing to

their very advantageous properties, including good

mechanical properties and good wear resistance. SiC-

reinforced aluminium is among the most common, and

several compositions for the matrix are available

commercially [3].

An ongoing problem with MMCs is that they are

difcult to machine, owing to the hardness and abrasivenature of the SiC or other reinforcing particles. The

particles used in MMCs are harder than tungsten carbide

(WC), the main constituent of hard metal, and even than

the majority of the cutting tool materials. Polycrystalline

diamond (PCD) is an exception, as it is 34 times harder

than SiC. This is why PCD is recommended by many

researchers [47] who have studied the drilling of these

materials.

An especially abrasive composite has been chosen for

this study, following the authors previous work [810].

2 MATERIALS AND EXPERIMENTAL

PROCEDURE

The composition of the work material is aluminium with

7.0 per cent silicon and 0.4 per cent magnesium, rein-

forced with 20 per cent by volume silicon carbide (SiC)

particles, having an average dimension of about 20 mm.

The material was T6 heat treated, solutionized and aged,

5 h at 154 C.

A typical microstructure of the aluminium matrix

composite tested, A356/20/SiCp-T6, obtained by con-

tinuous casting, is shown in Fig. 1. Randomly distributed

angular reinforcement particles, with some clustering, are

E01100 IMechE 2001 Proc Instn Mech Engrs Vol 215 Part E

177

The MS was received on 29 February 2000 and was accepted afterrevision for publication on 27 November 2000.*Corresponding author: Department of Mechanical Engineering,University of Aveiro, Campus Santiago, 3810193 Aveiro, Portugal.

Technical Note

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clearly visible (black). Fine precipitates in the matrix are

also present, but more difcult to see, as they are smaller

in dimension and grey coloured, slightly darker than the

matrix.

A series of drilling experiments were carried out on a

vertical miller with Heidenhain CNC controller (con-

tinuous speed control up to 3500 r/min and 2 kW spindle

power). The drills (according to DIN 338) with PCD

inserts were used to cut holes of diameter 5 mm in 15 mm

thick MMC discs. The drill geometry is 130point angle

and 20 helix angle. Table 1 presents the drilling con-

ditions used during the cooled tests (emulsion Alusol-B8 per cent).

A Kistler piezoelectric dynamometer with the appro-

priate signal conditioner, including charge amplier, has

been used. Several different programs for data acquisi-

tion, based on the software LabVIEW, have been

developed and used. They allow direct and continuous

recording and simultaneous graphical visualization of the

evolution of the torque and the feed force.

The cutting tool wear (width of the lip wear land at 1/4

the tool radius apart from the corner of the drill, as

presented in the schema of Fig. 2) was measured by

means of a Mitutoyo shop microscope with 30 mag-nication and 1 mm resolution.

The holes surface nish was evaluated (according to

ISO 4287/1) with a prolometer (Homeltester T500) in

the axial direction of the hole, using a 0.8 mm cut-off.

3 RESULTS AND DISCUSSION

To evaluate the wear resistance of the PCD drills when

machining the continuous casting composite, the surface

of the tool was observed and measured at regular

intervals, after each set of 10 holes.

The dominant wear type in drilling has been identied

by these direct observations of the worn surface of thetool. It is an abrasive form of ank wear in the drill,

characterized by scratching in the sliding direction,

which can be measured as Vbmax

. An example of this

wear type in a drill is presented in Fig. 2. The wear land

width is not uniform along the edge. It is smaller near the

axe and it grows very sharply near the corner. To obtain a

consistent measure of the tool wear, a precise position of

the measuring point has been chosen, as shown in the

schema of Fig. 2, and the average of the wear of the two

edges was used.

In the machining of MMCs the predominant wear

mechanisms are two-body abrasion and three-body

abrasion [4]. The resistance to abrasion of the cutting

tools material depends directly on the relative hardness

of the materials involved. The silicon carbide used as

Fig. 1 Microstructure of the aluminium matrix composite

tested, A356/20/SiCp-T6

Table 1 Drilling conditions with PCD

Rotationalspeed (r/min)

Cutting speed(m/min)

Feed(mm/rev)

1910 30 0.12546 40 0.13183 50 0.13183 50 0.053183 50 0.153183 50 0.2

Fig. 2 The wear land on the tip of a PCD tool. Drill lip, after

300 holes (cutting time 15 min),Vc 40 m/min andf 0.1 mm/rev

Proc Instn Mech Engrs Vol 215 Part E E01100 IMechE 2001

178 J P DAVIM AND A MONTEIRO BAPTISTA



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reinforcement in this MMC is harder than tungsten

carbide and than the generality of the materials for

cutting tools, except, for instance, PCD. However, PCD

inserts employed in drills, as well as the genuine PCD,

have cobalt as cement and still tungsten carbides, which

hardness is lower than that of the SiC reinforcement

particles. Therefore, the connection between the cobalt

and tungsten carbide can be damaged by the hardparticles, degrading the tool. The disintegration of the

insert material occurs next to the cutting edge. The

mechanical damage to the cutting tool material comes

from the kinetic energy transferred from (or to) the

reinforcement particles to (or from) the cutting edge and

depends mainly on the particles dimensions and the

relative cutting speed [5].

The predominant kind of wear in the tools when

machining these composites having been identied, wear

curves were obtained under different cutting conditions

(speed and feed).

These curves, obtained with the drills with PCD inserts

for several cutting speeds and feeds, can be seen in Fig. 3.

The increase of the cutting speed (with the same feed,

f 0.1 mm/rev) gives increased wear, although this is nota very signicant and clear result in the range of the

tested speed. For example, at a cutting speed of 40 m/min,

it is possible to perform the drilling for 14 min, corre-

sponding to 300 blind holes (approximately 12 mm deep)

to a wear Vbmax

0.15 mm. When the sliding distance iscompared for the same extent of wear, for example

Vbmax

0.1 mm, the highest value is observed for the

lowest cutting speed Vc 30 m/min (225 m correspond-ing to 7.5 min), but the opposite occurs for Vc40 m/min (160 m, 4 min) and Vc 50 m/min (200 m, 4min). The narrow variation of the cutting velocity, which

has been imposed by experimental limitations (maximum

rotational speed of the machine and drill diameter), does

not lead to a clear conclusion. Similar work by the present

authors, for the turning operation with the cutting

velocity ranging from 250 to 700 m/min, clearly shows

the increase of tool wear with this parameter [10].

The open symbols in Fig. 3 show the effect of the feed

at the same cutting speed (Vc 50 m/min). With thesingle exception, f 0.1 and f 0.15 mm/rev appearingin reversed positions, the smaller feed leads to higher

wear. This observation agrees with other experimental

results concerning MMC machining but has no easyexplanation. In fact, for almost all the test piecetool

material combinations, increasing the feed increases the

wear. This is the normal and expected behaviour that

results from the increasing severity of the contact

conditions. The particular trend observed with the

composites may result from the fracture of the hard and

brittle reinforcement particles, leading to reduced abra-

sion on the tool. However, no clear evidence of fractured

particles has been found in the chips, and the work

surface has not been observed by scanning electron

microscopy. Another explanation could be based on the

increasing plasticity of the aluminium matrix. Could the

matrix plastically deform over the reinforcement parti-

cles preventing, to some extent, its abrasive action on the

tool? This possibility is consistent with the very good

surface nish of the holes referred to later in this text.

On the basis of the measured torque values the cutting

power, P, has been evaluated through the following

expression, where M is the torque and o is the angular

velocity:

P Mo 1

In Fig. 4 the evolution of this function with time is

presented for different drilling conditions. A slightly

growing trend can be observed in the majority of the

situations. This is an expected result associated with the

degradation of the cutting edge and the wear of the tool.

For the same feed, increasing cutting speed increases the

power as a general observation. Some exceptions are

Fig. 3 Wear curves for PCD drill


CUTTING FORCE, TOOL WEAR AND SURFACE FINISH IN DRILLING METAL MATRIX COMPOSITES 179



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observed for Vc 40 and 50 m/min at 5 and around12 min of cutting time, but they are not signicant and

can be considered as a result of a reasonable experimental

dispersion. For the same cutting speed, Vc 50 m/min,the cutting power increases with feed.

Another very useful parameter in machining opera-

tions is the specic cutting pressure, Ks. This physical

quantity is widely used in machining databases and

reference books, and for the drilling operation it can be

obtained from the following equation, where M is the

torque, fis the feed and d is the diameter of the drill:

Ks 8M

fd2 2

A graphical representation of the specic cutting pressure

against cutting time is presented in Fig. 5. The time

dependence of Ks (a growing trend in almost all

situations) is decidedly more evident than that observed

for the cutting power. In this case the lower values

correspond to the highest feed, and vice versa, as usually

observed.

For the drilling investigations of the machining

quality, two aspects must be considered: the surface

nish and burr formation. In Fig. 6 the evolution of the

roughness parameters Ra (average roughness) and Rt(maximum peak-to-valley height), measured every 10

holes obtained with PCD inserts, is presented. The Ra

values, for the entire set of tested cutting conditions,varied approximately between 0.25 and 1.2 mm and theR tvalues between 4 and 13mm. Those results can be

considered very good since the best values indicated in

different national standards for expected Ra in drilling are

about 0.8 mm.

Fig. 4 Evolution of the cutting power with cutting time

Fig. 5 Evolution of the specic cutting pressure,Ks, with cutting time

Proc Instn Mech Engrs Vol 215 Part E E01100 IMechE 2001

180 J P DAVIM AND A MONTEIRO BAPTISTA



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As could be expected for geometrical reasons, the

increase of the feed determined the increase ofRaand R tvalues. For the same feed, the increase of the cutting

speed should diminish these values, as usually observedin machining operations. However, the present results do

not agree with this assertion. The best surface nish is

obtained (with 0.1 mm/rev) for the lower cutting speed

(30 m/min) and the worst is obtained for the intermediate

one (40 m/min).

A positive result of the drilling with PCD is the

absence of burr formation at the entrance of the holes.

Figure 7 illustrates the clean appearance of the cut. For

other types of tool materials, K10/20 cemented carbide

and especially TiN-coated high speed steel, burr forma-

tion started with the very rst holes.

Short chips, such as those presented in Fig. 8, are

formed when drilling the A356/20/SiCp-T6 composite.

From the machinability point of view, since short chips

Fig. 6 Evolution of the surface nish parameters with cutting time: (a) arithmetic mean roughness (Ra);(b) maximum peak-to-valley height (R t)

Fig. 7 Appearance of the aspects of three holes (diameter of5 mm) on the surface of the tested MMC, A356/20/

SiCp-T6. After 16 holes (cutting time 1 min), Vc40 m/min and f 0.1 mm/rev





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4 CONCLUSIONS

The following conclusions can be drawn from the results

of this study:

1. The predominant wear in drilling this type of SiC-

reinforced aluminium composite is developed in the

ank face of the tools.2. The abrasive wear mechanism is predominant in the

machining of these materials. Occasionally, some

adhesions are also observed.

3. When drilling MMCs, smaller feed leads to higher

wear of the tools.

4. The holes surface nish of the drilled samples

deteriorates with increasing feed at a constant cutting

speed, but its correlation with cutting speed is not well

dened.

5. The short type of chips produced in drilling aluminium

matrix composites renders this material well suited for

continuous operation.

REFERENCES

1 Taya, M. and Arsenault, R. Metal Matrix Composites,

1989, pp. 19 (Pergamon, Oxford).

2 Clyne, T.andWithers, P.An Introduction to Metal Matrix

Composites, Cambridge Solid State Science Series, 1995,

pp. 110 (Cambridge University Press).

3 Chadwick, G. A. and Heat, P. Machining metal matrix

composites. Metal Mater., 1990, 6, 7376.

4 Cronjager, L.andMeister, D.Drilling of bre and particle

reinforced aluminium. In Composite Material Technology ,

Vol. 37, 1991, pp. 185189 (American Society of Mechani-

cal Engineers, New York).

5 Lane, G. The effect of different reinforcements on PCD

tool life for aluminium composites. In Proceedings of the

Machining of Composite Materials Symposium, ASM

Materials Week, Chicago, Illinois, 1992, pp. 315.

6 Jawaid, A., Barnes, S.and Ghadimzadehs, S. Drilling of

particulate aluminium silicon carbide metal matrix compo-

sites. In Proceedings of the Machining of Composite

Materials Symposium, ASM Materials Week, Chicago,

Illinois, 1992, pp. 3645.

7 Morin, E., Masounove, J. and Laufer, E. Effect of drill

wear on cutting forces in the drilling of metalmatrix

composites. Wear, 1995, 184, 1116.

8 Baptista, A. M. and Davim, J. P. Drilling aluminiummatrix composites. In Tenth International Conference on

Composite Materials, ICCM-10, Vancouver, Canada, 1995,

pp. 581588.

9 Baptista, A. M.andDavim, J. P.Wear of tooling materials

when turning and drilling aluminium matrix composites. In

Wear of Materials 97, San Diego, California, 1997, poster.

10 Davim, J. P. and Baptista, A. M. Relationship between

cutting force and PCD cutting tool wear in machining

silicon carbide reinforced aluminium. J. Mater. Processing

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cutting force of metal matrix composite in drilling process

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