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MACHINING SCIENCE AND TECHNOLOGY, 1 1 , 3 52 1997

mVIEW

ARTICLE

M CHINING OF FIBER REINFORCED

COMPOSITES

Ranga Komanduri

Mechanical and Aerospace Engineering,

Oklahoma State University, Stillwater, Oklahoma

Received

pril

18, 1996;

epted

January 30 1997

ABSTRACT

Since the introduction of glass fiber-reinforced polymer composites

in the early 1940s, composite materials development was driven by the

needs of space, defense, and aircraft industries where performance rather

than cost was the prime consideration. At the beginning, conventional

machining techniques were adopted to machine glass fiber-reinforced

composites for convenience as well as to keep the capital costs down.

This was followed by significant advancements in tool materials and tool

ing design. With the development of new and more challenging metal

matrix and ceramic-matrix composites, conventional manufacturing pro

cesses proved to be inadequate or even inappropriate to process them.

Need and opportunity, therefore, exists for alternate nontraditional ma

chining operations, such as laser machining, water jet WJ and abrasive

water jet AWJ cutting, electrical discharge machining, ultrasonic-as

sisted machining, and electrochemical spark machining. When composites

become more popular and are used in large volume in the civilian sector,

such as auto and other consumer industries, material and processing costs

will be the driving factors. A high degree of automation for the mass

manufacturing of composite parts will be required to bring the costs down

and compete with other materials. Advancements in the nontraditional

machining processes offer an opportunity to process these materials ec

onomically, thus realizing the full potential of the composite materials.

This paper gives a broad overview on the various issues involved in

machining conventional and nonconventional of fiber-reinforced com

posites. The field of composites, in general, and machining

of composites,

in particular, are so broad that it would not be possible to do justice by

Copyright 1997 by Marcel Dekker, Inc.


2/40

KOMANDURI

discussing each aspect of composite material machining without ending

up with a voluminous document This

review

therefore has to be limited

to a few aspects of composite materials and their machining techniques.

may also be pointed out that in this reviewcertain areas are dealt more

in-depth than others. Personal preferences and availability of material in

the open literature are some of the reasons for this nonuniformity in

coverage. Also, some areas are more actively pursued than others.

An

attempt is made to highlight some of the issues and opportunities in the

area of machining of composites.

INTRODUCTION

Our technologically advancing society is continually challenging the

limits of conventional materials and newer demands on performance. Extreme

and sometimes conflicting requirements are forcing us to engineer materials

not possible by conventional alloying methods. Composite materials come

under one class of engineered material developed specifically to meet this

challenge. Glass-reinforced resin matrix composites were first introduced in

the early 1940s. Since then, the use of composites is growing steadily in

various industries including aerospace, aircraft, automobile, sporting goods,

marine, off-shore drilling platforms, appliances, etc.

Composite materials form a material system composed of a mixture or

a combination

of

two or more macroconstituents that differ in form and chem

ical composition and are insoluble in each other 1). The matrix or the re

inforcing fibers can be inorganic e.g., ceramic or glass), organic polymers),

or metallic aluminum, titanium, etc.). While the term composite materials

could apply to any duplex alloy depending on the scale

of

reference), it is

commonly used to describe a material whose components do not form to

gether as an alloy during processing but have been separately manufactured

prior to the combining process

2-5).

This definition is extended today to

composites that can be directly cast from the melt by unidirectional solidifi

cation

of

certain alloys

of

eutectic compositions 6). However, for this ma

terial to be classified as a composite material it must be capable of supporting

higher stress levels than the matrix material, undergo larger strains than the

fibers, and the fibers have to be adequately bonded to the matrix. Professor

Dietz of MIT, a pioneer in the composites field, summed up the subject 7):

Science and technology, like literature and fine arts, have their fashionable

phrases and catchwords. One very much to the fore these days is

composite

materials

which has been coined to give dignity and renewed impetus to a

very old yet simple idea: putting dissimilar materials to work in concert so

as to achieve a new material whose properties are different in scale and kind

from those

of

any of the constituents. The objective here is to take advan

tage

of

the superior properties of both materials without compromising on

the weaknesses of either. In a sense, composites epitomize the art of com

promise at its best. These compromises are sought from the acceptable levels


3/40

MACHINING OF FIBERREINFORCED COMPOSITES us

strength, ductility, fracture toughness, oxidation and corrosion resistance,

modulus, density, creep, fatigue, weight, and cost. Neither the composite ma

terials per se nor the concept of engineering composites are new 8). Wood

is a natural composite consisting of cellulose fibers in a matrix lignin.

Cellulose fibers are strong in tension but flexible; lignin cements the fibers

and endows the material with stiffness. Bone is a composite of strong but

soft protein collagen and the hard but mineral apatite. The introduction

straw in bricks by the Egyptians in the days

the Pharaohs, or the incor

poration plant fibers into pottery in the days the Inca and Maya to

prevent their premature cracking are classical examples engineered com

posites 9). Many other materials such as paper, concrete, etc., have been in

existence for some time.

The successful development glass-reinforced plastics in the 1940s

gave yet another approach for the development new materials. The concept

of incorporating high-strength close to theoretical strength) fibers or whiskers

in a tough or ductile matrix to form a very high-strength composite has

opened up an exciting possibility 2,5). For example, it was discovered that

glass fibers have significantly more strength than that of the bulk by an order

magnitude), because the absence of defects. By combining strong glass

fibers in an epoxy matrix, a new composite material with the strength glass

fibers and ductility an epoxy is obtained. The main outcomes such a

combination include savings in weight, improvement in strength, and a de

crease in the cost of materials and fabrication.

In the fiber-reinforced composite, the fibers carry the bulk the load

and the matrix serves as a medium for the transfer load to the fibers. The

matrix can be a metal, polymer, or ceramic. The fibers likewise can be metal,

ceramic, glass, or polymers.

Some of the advantages of composites include high specific strength;

high specific stiffness or modulus; good dimensional stability; unusual com

bination of properties not easily obtainable with alloys; higher fracture tough

ness; higher oxidation and corrosion resistance; directional properties; good

resistance to heat, cold, and moisture; ease fabrication; and low cost.

Some the properties of com mon matrix materials and reinforcing

fibers and comparative metals are given in Table 1 10). Endowed with some

these features, composites are ideal candidates for a range applications

involving extreme conditions not possible with conventional alloying. Ex

amples include high-strength, lightweight applications space launchers and

vehicles), high operating temperatures gas-turbine engine parts), and resis

tance to severe destructive) conditions over a limited period

time rockets

and protection devices for vehicles and missiles re-entering the earth s at

mosphere). However, composites are not limited to extreme conditions as one

finds their applications under more conventional conditions because their

superior performance and/or affordable cost. Conventional machining prac

tices, such as turning, drilling are widely applied to the machining com

posites in view the availability equipment and experience in conven-


4/40

Table 1

Properties of Various Fibers, Composites, and Comparative Metals (10)

SPECIFIC SPECIFIC

DENSITY, MODULUS, MODULUS,

STRENGTH,

STRENGTH,

gjcm

GPa GPa cm /g GPa GPa cm /g

Group I Ceramic Whiskers

Graphite 2.2

700 320 20

9.1

Silicon nitride

3.2

400

125 7 2.2

Silicon carbide

3.2 500 155 7 2.2

Alumina

4.0 420

105

14

3.5

Group II: Glass, Ceramic, or Polymer Fibers

Carbon

RAE (type I)

2.0 460

230

1.7 0.9

RAE (type II)

2.0 260 130 2.9 1.5

Boron

2.5

420

170 2.5 1.0

Asbestos

2.5 190 76 6

2.4

Mica

2.7 230

85

3

1.1

Nylon-6,6 I.1

5

4.5 0.8

0.7

E-glass as drawn 2.5

60

24

3 1.2

Group III: Hard-Drawn Metal Wires

Piano (0.9

C

7.8 210

27

4 0.5

Stainless steel

7.9

200

25 2.4 0.3

Molybdenum 10.3 365 35

2.1

0.2

Tungsten 19.3 345 18 2.9 0.1

Group IV: Composites and Other Higher Strength Materials

Steel alloy-current 7.8 200 25 1.3 0.17

Al alloy

2.8

70

25 0.6 0.21

Ti alloy 4.5

115

26 1.0 0.22

Beryllium 1.8 300 170

0.5

0.28

0

Steel alloy-future 8

200

25 5

0.65

0 glass

I l

Fiber epoxy

c

Type 1

2

350

175

1.3 1.5

0.6 2.5

l

Type 2 2 200

100 2 . 3 5 1.2 2.5

Current and projected values for uniaxial layup.


5/40

MACHINING OF FIBER-REINFORCED COMPOSITES

117

tional machining. Although some of the fibers used in composites such as

glass, graphite, boron, alumina, and silicon carbide are hard sometimes equal

to or harder than the tool material and abrasive, conventional machining is

considered for these materials because these fibers are very brittle and ma

terial separation in machining is accomplished by brittle fracture rather than

plastic deformation ahead of the tool. However, the cutting tool materials are

chosen to minimize wear due to the hard abrasive constituents

of

the fibers

in many cases.

FIBER

REINFORCED COMPOSITES

Machining

of

a fiber-reinforced composite depends on the properties of

the fibers and the matrix as well as its response to the machining process. In

addition, the choice of the specific process depends upon the following fac

tors: type

of

machining operation, part geometry and size, finish and accuracy

requirements, number of parts, diversity

of

parts including the materials

of

the parts, availability

of

appropriate machine and cutting tools, availabilityof

in-house technology, current machining practice, manufacturing schedule,

capital requirements and justification for new equipment, and overall costs.

Certain machining operations may not be possible at all with some com

posite materials. For example, it is not possible to machine SiC whisker

reinforced alumina with a single-point cutting tool, even with a diamond tool.

may, however, be possible to shape it by diamond grinding or by some

of

the nonconventional machining processes, such as laser machining or ultra

sonic-assisted machining. In other cases more than one process may be can

didate processes and the specific process chosen depends on the factors out

lined earlier. Of course, the general tendency is to use current machining

practices by adopting existing equipment for this application. This is partic

ularly true of composite machining in small manufacturing shops in view of

capital limitations for procuring new machinery or new technology. This may

not be the case with large companies, such as Boeing, Lockheed, or General

Dynamics, where resources are normally available, if justified. Even a par

ticular process chosen now for machining composites may have to be aban

doned at a later date because

of

changes in the factors outlined earlier. For

example, the lot size may be increased significantly, necessitating a review

of alternate manufacturing processes, or new technology or a technology not

currently practiced in the plant may become available or be economically

attractive.

In the following text, some common fiber materials used in composites

are briefly discussed. Table 2 gives some of the mechanical properties of

these fibers. Table 3 gives the mechanical properties

of

resin matrix materials.

This will be followed by a brief description of the three types

of

composites,

namely, organic-matrix, metal-matrix, and inorganic-matrix composites. Ta

bles

6

give some

of

the mechanical properties of representative composite

materials.


6/40

KOMANDURI

able Z Mechanical Properties of Fibers

SPECIFIC

SPECIFIC

TENSILE TENSILE ELASTIC ELASTIC

DENSITY, STRENGTH, STRENGTH,

MODULUS, MODULUS,

FIBER g/cm ksi X 10

6

in.

X10

6

psi

10

K

in.

Kevlar-29 1.44 525 10.1

12

2.3

Kevlar-45

1.44 525 10.1 18

3.5

HT graphite

1.75 450 7.1 32

5.1

E-glass

2.55 350 3.8

10 1.1

S-glass

2.49 575 6.4 12.4

1.4

Boron

2.60 350

4.2

65 7.8

Boron

Filaments

Boron filaments cannot be formed exclusively. Instead they have to be

formed on a tungsten or carbon core using the chemical vapor deposition

CVD) technique. For example, BCI, and tungsten are reacted at

-2000F

resulting in the tungsten core transforming into tungsten boride on which

amorphous boron deposits. A typical 0.004-in.-diameter boron fiber contains

-0.0005-in.

tungsten boride core 11). To make a usable product, the fibers

are generally placed on a glass gauge tape preimpregnated with resin and

cured. Boron composites are also made in a metal matrix, generally aluminum

or titanium, to reduce weight, yet maintain high strength at room aluminum)

or elevated temperature titanium).

Glass

Fibers

Glass fiber-reinforced plastics GFRP) developed in the 1940s were the

first lightweight, high-strength, relatively inexpensive engineering compos

ites. The most comm on types

of

glass fibers are the E-glass electrical) and

the S-glass high strength). E-glass is a calcium alumina borosilicate with low

levels of sodium or potassium. Typical composition

of

an E-glass is

52-56

Si0

2

,

12-16 Al

20

3

,

16-25 CaO, and

8-13

B

20

E-glass has a tensile

strength of 500 ksi and a modulus of elasticity of 10.5 X 10

6

psi. S-glass has

higher specific strength and is more expensive than E-glass see Table 2).

Consequently, it is used primarily in military and aerospace applications for

able

3.

Mechanical Properties Polyester and Epoxy Resin Matrix 1)

PROPERTIES

Tensile strength, ksi

Tensile modulus of elasticity, psi X 10

6

Flexural yield strength, ksi

Impact strength notched-bar Izod test), ft

-Ib/in,

of notch

Density, g/cm

POLYESTER

6-13

0.30-0.64

8.5-23

0.2-0.4

1.10-1.46

EPOXY

8-19

0.41-0.61

18.1

0.1-1.0.

1.2-1.3


7/40

MACHINING OF FIBERREINFORCED COMPOSITES 119

reinforcing metals or ceramics. The inherently high cost of processing metal

or ceramic matrix composites makes it unattractive to use glass as a rein

forcing material; instead the choice is for the use of more expens ive high

performance fibers such as boron in the case of metal matrix composites

MMCs) and SiC in the case

of

ceramic matrix composites CMCs).

Aramid Fibers

Aramid is the generic name for aromatic polyamide. Aramid fibers were

introduced by du Pont in 1972 under the trade name

Kevlar

There are two

commercial types: Kevlar-29 and Kevlar-49. Kevlar-29 is a low-density, high

strength, low modulus fiber designed for such applications as ropes, cables,

armor shield, etc. Kevlar-49 is characterized by low density, high strength,

and high modulus.

is used in such applications as aerospace, space shuttle,

marine, automotive, and other industrial applications. The chemical structural

unit of aramid is shown in Fig. 1. The pol ymer chains are bonded together

by hydrogen bonding in the transverse direction. The fibers have high strength

in the longitudinal direction and low strength in the transverse direction. The

aromatic ring structures give high rigidity to the polymer chains, causing them

to have a rod-like structure.

Carbon Fibers

Carbon fibers have a combination of lightweight, very high-specific

strength and stiffness. Carbon fibers are produced mainly from two sources:

polyacrylonitrile PAN) and pitch. The tensile strength ranges from

450 650

ksi and the modulus

of

elasticity ranges from

28 35

X

10

6

psi. In general,

the higher modulus fibers have lower strengths and vice versa. The density

of

the fibers ranges from

1 7 2 1

g /cm . In view of the relatively high cost

of

carbon fibers, they are used mainly for high-tech applications where light

weight, specific modulus both at room and elevated temperatures are required.

They are also used in high value added products, such as tennis racquets,

golf clubs, and fishing rods.

Silicon Carbide Fibers

Silicon carbide fibers are produced by a vapor-phase decomposition pro

cess in which the fiber component is deposited from the vapor phase on a

o

0

11 0

0

C N

N

I I

H H

igur

1. Chemical structurat unit of an aromatic polyamide 1).


8/40

120

KOMANDURI

carbon substrate. They can also be formed by the decomposition

of

naturally

occurring substances such as rice hulls.

Modern composite materials, depending on the matrix material used, can

be classified as follows:

1.

Polymer organic) matrix composites PMC).

2. Metal matrix compo sit es MMC).

3. Ceramic in org anic) matrix compo sites CMC).

Theoretically, a multitude of materials can come under these categories.

In the following text a bri ef description of some of the composites used

industrially of the three previous categories are given.

PMC

The common types of fibers used to reinforce plastic materials are glass,

graph ite, aramid, and boron. Glass fiber reinforced plastics GFRP) are by

far the most commonly used materials in view of their relatively high specific

stren gth and low cost. The o th er materials p ro vide higher specific stren gth,

higher specific stiffness, and light weight. They are, however, expensive and

are used only for those applications w her e performance and not cost is the

major consideration. Aramid is used instead of graphite where strength, light

ness, and flexibility are major considerations, and stiffness and high-temper

ature p erforman ce are not. The commo n matrix materials used are p olyester

and epoxy resins. Polyester resins are lower in cost and are not as st ro ng as

the epoxy. Their use as co mp os ites include boat hulls, structural panels for

automobiles and aircraft, building panels, appliances, etc. Epoxy, in addition,

has a lower sh rink ag e after cure. is used co mmon ly in carbon and aramid

fiber composites. Tables 3 and 4 give some mechanical properties

of

polyester

and epoxy resins, GFRP polyester composites, respectively. Maximum-use

temperatures of polymeric matrix composites are relatively low, as the matrix

material is prone to softening or chemical decomposition or degradation) at

moderate temperatures. The same conditions apply for machining these ma

terials.

ble

4. Mec hanica l P roperties of GFRP Com posite s 1)

WOVEN CHOPPED SHEET-MOLDING

CLOTH

ROVING COMPOUND

Tensile strength, ksi 30 50 15 30 8 20

Tensile modulus

of

elasticity, Msi

1 5 4 5

0 80 2 0

Impact strength notched bar, Izod

5 0 30 2 0 20 0

7 0 22 0

Ib/in. of notch

Density, g/crrr

1 5 2 1 1 35 2 30 1 65 2 0


9/40

MACHINING OF FIBERREINFORCED COMPOSITES

MMC

121

MMCs are used for applications requiring higher operating temperatures

than are possible with PMCs. Most of these alloys are developed for the

aerospace industry, but new applications are found in the auto industry such

as in automobile engine parts. There are three types of MMCs depending on

the nature of the fibers used, namely, continuous, discontinuous, or particu

late.

Continuous fibers provide the highest stiffness and strength for MMCs.

Boron-aluminum composites are one of the earliest developed MMC mate

rials. is made by hot pressing layers of boron fibers between aluminum

foils, so that the foil deforms around the fibers and bond to each other 11).

By reinforcing with boron, the tensile strength can be increased by a factor

of 3-5 while the elastic modulus can be tripled. Silicon carbide, graphite,

alumina, and tungsten fibers are some

of

the fibers used in MMCs. To keep

the weight low, aluminum, magnesium, and titanium are the most commonly

used metal matrix materials. Table 5 gives some of the mechanical properties

of the MMCs. Applications of MMCs include use of boron-aluminum for the

fuselage of the space shuttle orbiter, SiC-AI for the vertical tail section of

advanced fighter planes, and SiC-titanium aluminide for hypersonic aircraft.

The discontinuous and particulate MMCs are low-cost MMCs that pro

vide higher strength and stiffness and better dimensional stability over rein

forced alloys. Small additions of the reinforcement -20 ) moderately in

crease the strength and stiffness.

They also increase the wear resistance and contribute toward the diffi

culty in machining these materials. These alloys are used for sporting equip

ment, automobile engine parts e.g., pistons), missile guidance parts, etc.

Table Mechanical Properties of MMC Materials 1)

TENSILE

ELASTIC

STRAIN

TO

STRENGTH, MODULUS, FAILURE,

ksi

psi X 10

6

Continuous-fiber MMCs:

AI 2124-T6 45 B) axial)

211

32 0.810

AI 6061-T6 51 B) axial)

205

33.6

0.735

Al6061-T6

45 SiC) axial) 212 29.6

0.89

Discontinuous-fiber MMCs:

AI 2124-T6 20 SiC) 94 18.4 2.4

AI 6061-T6 20 SiC)

70 17.7

5

Particulate MMCs:

AI 2124 20 SiC)

80 15

7.0

AI 6061 20 SiC)

72 15

5.5

No reinforcement:

AI 2124-F

66

103

9

AI 6061-F

45 10

12


10/40

122

CMC

KOMANDURI

CMCs are being developed mainly to improve fracture toughness. They

already possess higher specific modulus and elevated temperature mechanical

properties superior to metals. Continuous fibers, discontinuous fibers, or par

ticulates can be used as reinforcing materials. The common fiber materials

used are alumina and silicon carbide. Oak Ridge National Laboratories re

cently developed a SiC whisker-reinforced alumina. A 20 wt% of SiC whisk

ers to alumina can increase the fracture toughness from 4 to 8 ksi/in. . Such

an increase in toughness of a ceramic cutting tool will enable it to take heavy

cuts or to perform without fracture in interrupted cutting. Conventional

hot isostatic pressing (HIP) techniques can be used to consolidate the CMCs.

Table 6 gives some of the mechanical properties of CMCs. Other CMCs

include carbon/carbon composite in which high-strength carbon fibers are

embedded in a graphite matrix. The low density of carbon in combination

with the extraordinary strength of carbon fibers offers potential for the de

velopment of high specific-strength materials.

Figure 2 is the stress-strain behavior of various types of reinforcing

fibers with the slope giving the modulus of elasticity. Figure 3 is a plot of

specific strengths versus specific modulus for various types of reinforcing

fibers.

can be seen that aramid (Kevlar-49), graphite, and boron fibers have

outstanding specific strength/specific modulus compared to steel or aluminum.

Figure 4 is a plot of specific strength versus specific stiffness of various

composites with conventional metallic-matrix materials.

can be seen that

composites, in general, have higher specific strength/specific modulus over

conventional steel, AI, Ti, and Mg, and the MMCs have properties superior

to polymer-reinforced composites. Figure 5 shows the variation of specific

strength with temperature for various composites (12). Fiber-reinforced plas

tic have higher specific strength (tensile strength/density) at low temperatures.

For high-temperature applications, one needs to move from PMCs to MMCs

to CMCs.

ble 6. Mechanical Properties of SiC Whisker-Reinforced Ceramic-Matrix Composites at

Room Temperature (1)

FLEXURAL

FRACTIJRE

MATRIX

SiC WHISKER STRENGTIl, TOUGHNESS,

MATERIAL

ol % CONTENT ksi

s ~

Si,N.

0

60 95

4 6 6 4

10

60 75

5 9 8 6

30

50 65

6 8 9 1

Al

O

0

4.1

10

57 73 6.5

20

75 115

6 8 8 2


11/40

MACillNINGOF FIBERREINFORCED

OMPOSIT S

3

4137

600)

3447

500)

2758

400)

s 2068

[ 300)

il

1379

(200)

689

100)

Nomex

2

T U strain,

4

Figure 2. Stress-strain characteristics of various types of reinforcing fibers (from Kevlar 9

Data Manual

E.

I.

du Pont de Nemours).

8M

graphite

(hlg/l modulus)

lIqfon

lIT graphite

(hlg/l tensue)

K '1l 49

resin-Impregnated

str n s

1 2 3 4 5

Sped/Ie tensU. modulus, Ill In.

= _

Sgl. . . .

p

5

i

c

'

c

u

'

0

Figure 3. Specific tensile strength versus specific tensile modulus of various types of reo

inforcing fibers (courtesy E. I. du Pont de Nemours).


12/40

4

KOM NDURI

0.34GrlMg

e

6

GrlEpo.y

0.37GrlAI

.37Gr/AI

4SB I

iCffi

onventional

Steel.AI.T Mg

O 25SiC IAI

@o.SOGrlEpo,y

0.25

t

e se

z

~

Figure

4. Specific strength versus specific stiffness for various metal matrix composites.

Number in front of the composite represents the volume fraction

of

the reinforcement 12).

MACHINING OF

FI R

REINFORCED COMPOSITES

Machining of fiber-reinforced composites differs significantly in many

aspects from machining of conventional metals and their alloys 13). In the

machining of fiber-reinforced composites, the material behavior is not only

inhomogeneous, but it also depends on diverse fiber and matrix properties,

fiber orientation, and the relative volume of matrix and fibers. The tool en

counters continuously alternate matrix and fiber materials, whose response to

machining can be entirely different. For example, in an aluminum-boron com

posite, the tool continuously encounters a soft aluminum matrix and hard

boron fibers. Similarly, in a glass-epoxy composite, the tool encounters a low-

T l l q e r a ~ u r e F

Figure

5. Variation of specific strength with temperature 12).


13/40

MACIllNING OF FIBERREINFORCED COMPOSITES

125

temperature soft epoxy matrix and brittle glass fibers. is this diverse re

quirement

o

a cutting tool that makes composites somewhat unique and at

the same time difficult to machine. Therefore, machining

o

fiber-reinforced

composites impose special demands on the geometry and abrasion resistance

o the tool materials.

Conventional machining practices are generally applied to the machining

o composites in view o the availability of equipment and experience, in

spite of the fact that the response o composites to machining is entirely

different from metal machining. However, in some applications conventional

machining with a tool harder than the work material may not even be an

economical proposition. For example, in the machining o glass reinforced

with continuous fibers o SiC composite, no conventional cutting tool mate

rial, including polycrystalline diamond PCD , would last longer than another

14 . The need, therefore, arises for alternate material removal processes or

nonconventional machining processes, such as laser cutting, ultrasonic ma

chining, water jet or abrasive water jet cutting, or electrical discharge ma

chining. Of course, all of these processes have their own limitations. The

objective would be to choose a process that takes advantage o its unique

capabilities while exploiting the weaknesses o the work material, enabling

to be processed economically and yet meet the requirements

o

the part.

Since most fibers used in composites e.g., glass, boron, and carbon are

hard and abrasive, the tool materials recommended for use include cemented

carbides coated and uncoated , ceramics, cubic boron nitride cBN , and

diamond single crystal or polycrystalline . However, cemented carbide and

diamond are the commonly used materials. High speed steel HSS tools are

also used in some cases, but at the expense o rapid tool wear. In view o

high tool wear and the high costs

o

tooling that are experienced with con

ventional machining, noncontact material removal processes offer an attrac

tive alternative. This will also minimize the dust and noise problems. In

addition, extensive plastic deformation and consequent heat generation as

sociated with conventional machining

o

fiber-reinforced composites espe

cially with an epoxy matrix can be minimized. These processes include laser

machining, water j t with or without an abrasive , ultrasonic machining, and

electrical discharge machining. Each o these processes offers certain advan

tages and shortcomings compared to conventional machining and even com

pared among themselves. For example, electrical discharge machining EDM

requires that the composite material be electrically conductive. Laser ma

chining depends on the optical absorption and thermal properties o compos

ite. In the following text some o the issues involved in the conventional

machining o representative composite materials are discussed.

Conventional Machining

As pointed out earlier, conventional machining o composites is some

what difficult in view o the diverse fiber and matrix properties, fiber orien-


14/40

6

KOMANDURI

tation, inhomogeneous nature of the material, and the presence of high vol

ume fraction of hard, abrasive fibers in the matrix. Glass-, graphite-, and

boron-reinforced composites even polymer based are difficult to machine

because of rapid tool wear. Even cemented carbide tools wear rapidly and

one may have to resort to the use of diamond-impregnated tools. Several

advances have been made in the tool materials development including poly

crystalline diamond tools, diamond-plated tools, and diamond-impregnated

tools in various forms such as core drills, milling cutters, drills, and grinding

wheels. In the following text various issues involved in the machining of

some of these materials are presented as examples.

Machining of Boron-Titanium Composites

Boron is hard and abrasive, whereas titanium is chemically reactive with

most tool materials. The dissimilar and difficult machining characteristics of

boron-titanium composites makes the task of machining even more challeng

ing than other composites. Doron and Maikish 15 conducted tests on a

boron-epoxy/titanium laminate in a variety of machining operations including

drilling, reaming, countersinking, routing, milling, and sawing. They used

diamond-impregnated tools as well as plated tools. The following are typical

drilling condition recommendations: cutting speed from 2 6 SFPM sur

face feet per minute and a feed rate of 0.0005-0.005 in./rev. A tool life of

up to 400 holes with a 1/4 in. diameter drill was reported. The use of cutting

fluids was recommended to reduce heat buildup both in the tool and the part.

Protecting the machinery to prevent wear of machine elements by abrasive

boron dust was also highly recommended. Diamond-impregnated tools were

found to perform better than diamond-plated tools.

Machining of Aramid-Reinforced Plastic Composites

Aramid-reinforced plastic composite is an inherently tough material;

therefore, cutting tools should be sharp and clean. Tools should be cleaned

frequently to remove buildup of partially cured resins that can cause a loss

in cutting action. The requirements of tools for machining aramid-reinforced

plastics are different from those of glass or carbon fibers. In many respects

aramid-reinforced polymer resembles wood. The structure is characterized by

the presence of highly oriented fibrous material embedded in a matrix. The

best results are obtained when machining is processed in such a way that the

fibers are preloaded in tension and then cut with a shearing action.

Special cutters were specifically developed to address this problem 16 .

For example, Paige 17 developed a four-fluted spiral rotary carbide milling

cutter Fig. 6a with a unidirectional helix throughout much of its length, and

a reverse directional helix adjacent to the cutting edge. Chip breakers are

arranged along the lands with the notches at alternate lands aligned and the

notches of the other lands intermediately aligned. The notches are cut at an


15/40

MACHINING OF FIBER.REINFORCED COMPOSITES

a

b

127

d

igur

6. New tool designs specially made for the machining of aramid fiber reinforcing

polymers 17- 20 .

angle of about 20 deg from a line perpendicular to the axis of the cutting

tool. These cullers with angled chip breakers were designed to cut cleanly

the aramid fiber-reinforced plastic composites. They operate at production

speeds with minimal overheating. Figures

6b-6d

show some of the other

designs or cutters specifically made for drilling aramid fiber-reinforced resin

composites 18-20 .

Since aramid-reinforced polymers are not particularly hard, HSS tools

should give a reasonable tool life if care is taken to avoid overheating the

tool. Coated TiN HSS tools should prolong the life further and minimize


16/40

128

KOMANDURI

buildup on the tool. Cemented carbide tools provide longer tool life and

maintain sharper cutting action. These tools could easily handle single ar

amid reinforcing fiber composites. When

hybrid

composites containing

glass or graphite in addition to aramid have to be machined, tool wear will

be high. Diamond impregnated or plated tools are not recommended for ma

chining aramid-reinforced polymer composites 13 . Backup support is rec

ommended on both entrance and exit sides to avoid fuzz and delamination.

E. du Pont de Nemours and Co., that developed aramid Kelvar fibers, has

conducted extensive machining tests on this material in collaboration withlor

in support of its customers. The guidelines for machining Kevlar aramid

composites and should be consulted for details on the recommended cutting

conditions for various machining operations 21 .

Machining

of

GFRP Composites

In view of the high hardness and abrasiveness

of

glass fibers in GFRP

composites, cemented carbide and preferably diamond tools single crystal

and polycrystalline are recommended for machining these materials. While

HSS tools can be used to machine GFRP, they wear rapidly and cutting speeds

should be kept low enough not to overheat the tool. Maintaining the sharpness

of

the tool will be a real problem for both HSS and cemented carbide tools

because of the abrasive action on the cutting tool by the glass fibers. Some

consider the machining

of

GFRP to be similar to drilling a hole in a resin

bonded grinding wheel 22 . A dull tool can dissipate considerable heat into

the workpiece and damage the resin-based composites. Although alumina

based tools can be used because

of

their higher hardness, the possibility

of

a

chemical reaction between alumina and glass should not be overlooked.

Polycrystalline diamond tools are preferred, particularly in the case of GFRP

components with a high glass fiber content (-60 ) that have to be machined

to tight tolerances and with a good surface finish 23 . Of course, rigid ma

chine tools are preferred when machining GFRP with PCD to take advantage

of

PCD's

superior cutting performance capability. To prevent wear of ma

chine elements in relative motion because of the abrasive action of the glass

fibers, protective covers have to be incorporated in the machine tool. Some

times it is necessary to dedicate a machine tool for machining this material

if enough parts are to be fabricated on a continuing basis. When using PCD

it is preferable to clamp rather than braze the insert on the tool holder to

avoid softening the braze material. The final choice

of

the tool material, say,

between cemented carbide and PCD, depends on the economics

of

the ma

chining operation as well as the part requirements. Appropriate dust collection

and extraction systems should be in place when machining GFRP. Operator

safety should be a prime consideration and the use

of

masks, hand gloves,

an apron or lab coat are required to minimize the risks involved in the health

and safety of operating personnel because of loose glass fibers and dust.


17/40


129

Cutting speeds ranged from

100-150

SFPM for carbides and

500-1500

SFPM for PCD.

Machining of Carbon Fiber-Reinforced Plastic CFRP Composites

Although CFRP are generally fabricated to near-set-shape, there often

is a need to conduct some additional machining operations such as drilling

of holes, trimming of the edges, etc. High tool wear and delamination of the

composites are some of the concerns in machining. Koplev et al. 24 con

ducted orthogonal machining tests using a quick stop device to freeze the

cutting process and obtain the chip root. They conducted cutting experiments

parallel and perpendicular to the fibers. When cutting parallel to the fibers,

they found the surface to have visible fibers. They also found that nearly all

fibers were fractured perpendicular to their longitudinal direction. When the

composite was machined perpendicular to the fibers, they did not find the

surface with visible fibers; instead they found the whole surface to be coated

with a thin layer of the matrix material. They also found a layer of disturbed

material with cracks below the surface layer. Koplev et al. observed a rather

sharp notch with no cracks in front when machining perpendicular to the fiber

direction. In contrast, a crack was found in front

of

the notch when machining

parallel to the fibers. Based on these observations Kop1ev et al. pointed out

that during machining of CFRP perpendicular to the fibers, two separate ef

fects occur near the tool tip. As the tool moves forward, it presses on the

composite in front of it causing the composite to fracture and create a chip.

At the same time a downward pressure on the composite below the tool

produces fine cracks =0.01 in. deep into the specimen. When the composite

is machined parallel to the fibers, the tool applies pressure on the specimen,

resulting in chips; but a crack is often seen in front of the tool tip indicating

that this crack seems further or deeper than the current chip. At the surface

there are cracks that reach depths

of only one or two fiber diameters.

Friend et al. 25 conducted machining tests using conventional and

nonconventional machining methods. To reduce tool wear, the authors rec

ommended diamond tooling. For producing intricate shapes of high accuracy,

the authors recommended ultrasonic machining.

Fundamental Studies on Chip Formation Mechanics

Several fundamental studies were conducted to obtain insights into the

mechanisms of material removal in machining composite materials. For ex

ample, Bhatnagar et al. 26 investigated orthogonal machining of unidirec

tional CFRP composites. They studied the effect

of

fiber orientation with

respect to the cutting direction on the machining response. When the fibers

were oriented at + 10 deg in the

e

direction, the fibers were found to bend

to the underside of the tool edge and not get cut e is measured counter

clockwise from the machined surface . The resulting surface was full of fibers


18/40

130

KOMANDURI

not being cut and delaminated 27 . In contrast, when the fibers were oriented

at the

1

deg direction, they were found to break in tension, with the ma

chining surface produced as a consequence. Chips were produced ahead of the

cutting edge of the tool by the shearing of the matrix in a plane along the fiber

orientation. A model for predicting the cutting forces for composites with fibers

oriented in different directions to the cutting direction was also proposed.

Wern et al. 28 investigated the stress fields in the machining

of

fiber

reinforced plastics using a model material and photoelastic analysis. They

used polyester matrix with embedded copper wire, which is a ductile rein

forcement in a brittle matrix and the opposite of conventional fiber-reinforced

plastic where the matrix is ductile and the fibers brittle. For this particular

case, Wern et al. found that in the case of fibers oriented away from the

cutting direction, the fibers were machined by shearing and tensile fracture;

and in the case of fibers oriented toward the cutting tool, the fibers failed by

shearing and bending. They also found the fiber matrix debonding to be

maximum for fibers oriented at 45 deg to the tool path.

Arola et al. 29 investigated the chip formation mechanism in the or

thogonal trimming of a graphite-epoxy composite using polycrystalline cut

ting tools. Similar to Bhatnager et al. 26 , Arola et al. found the character

istics of chip formation to be primarily dependent on the fiber orientation. In

the O-deg fiber orientation they found that the chip formation mechanism

included failure along the fiber-matrix interface through cantilever bending

and fracture perpendicular to the fiber direction. In positive fiber orientations

up to 75 deg, chip formation involved compressive-loading-induced shear at

the tool nose. In the 90 deg and negative fiber orientations, the chip formation

mechanism was composed of out-of-plane shear with severe compressive

loading-induced intralaminar deformation.

Di IIio et al. 30 investigated the effect of drilling parameters on specific

energy when drilling various composites thermoset and thermoplastic matri

ces with aramid and glass fibers . They found that specific energy can be

expressed as a function of the feed rate and diameter of the drill, similar to

the relationship obtained when drilling metals. The coefficients for the func

tional relationship varies with the particular type of composite and the twist

drill used. They also found that the thrust force or feed force , which is a

critical parameter in the drilling of composites, is related to the cutting torque,

which in turn, is related to the specific energy. Since, when drilling compos

ites, one of the main concerns is the delamination which does not exist in

the case of metals , the correlation between torque and thrust force represents

an important finding that affects not only the machinability, but also the in

tegrity of the composite after drilling. With this relationship, it may be pos

sible to determine a value of thrust force low enough to avoid delamination.

Since the temperature during drilling of composites is a concern, both from

the point of drill life and the workpiece quality, Bella et al. 31 evaluated

the temperature generated in drilling composites both analytically and exper

imentally.


19/40

MACHINING OF FIBER-REINFORCED COMPOSITES 131

is well known that delamination is a major concern during drilling of

composites because it reduces the structural integrity and results in poor as

sembly tolerance.

can thus cause long-term performance deterioration. Ho

Cheng and Dharan 32 investigated the delamination during drilling in com

posite laminates. was found that drilling-induced delamination occurs both

at the entrance and exit during drilling. Ho-Cheng and Dharan analyzed this

problem using fracture mechanics and found an optimal thrust force as a

function of drilled hole depth. They proposed that optimum thrust force for

no delamination can be used for feedback control to maximize productivity.

Wear of Cutting Tools in Machining of Composites

In view of the hard, abrasive reinforcing fibers such as glass, carbon,

and boron used in most PMCs, tool wear in conventional machining is gen

erally a serious problem with most tool materials. Hasegawa et al. 33 con

ducted extensive studies on the characteristics of tool wear when machining

GFRPs. They found the tool wear to be predominantly abrasive in nature and

proportional to the contact pressure between a glass fiber and the tool under

a constant cutting length. They divided the tool wear with cutting speed into

three regions. At very low speeds Region I they found the tool wear to be

negligible and independent of cutting speed and dependent only on the length

of cut; the wear increasing linearly with the length of the cut. At intermediate

speeds Region II the tool wear was found to increase with cutting speed.

At higher speeds Region III tool wear was found to increase rapidly and

independently of speed. Hasegawa et al. developed a rheological model of

tool wear to explain the observed wear phenomenon in machining GFRP.

Ramulu et al. 34 investigated the machining

of

a graphite-epoxy com

posite with PCD tools. The wear behavior was characterized by small cracks,

rounded edges, and flank wear. was also found that the larger the diamond

grain size the coarser the PCD grade the better the wear resistance.,

Ho-Cheng and Puw 35 investigated the machinability of carbon fiber

reinforced thermoplastics ABS in contrast to the thermosetting epoxy in

drilling. They found the machinability to be good, wear on the HSS drills to

be minimal, and the quality of finished surface to be good.

Nonconventional chining

To overcome the rapid tool wear experienced in conventional machining

of some composites containing hard, abrasive, or refractive constituents, al

ternate material removal operations have been adopted. These include laser

machining, electrical discharge machining,

W

and

Wl

cutting, ultrasonic

machining, and electrochemical spark machining.

These are basically noncontact machining operations involving no cut

ting tools, and consequently, no cutting forces. In the following text some of

the issues involved in the processing of composites are outlined.


20/40

3

KOM N URI

Laser Machining

Laser machining is based on the interaction

the work material with

an intense, highly directional coherent monochromatic beam

light, from

which material is removed predominantly by melting and/or vaporization. In

the case resin matrix material it is removed by chemical degradation. The

type

laser to be used for the machining of a given composite depends upon

the following characteristics the beam and the work material properties

5,36): power density, wavelength

emission type

laser), interaction

time [continuous wave cw) versus pulse], polarization the beam, absorp

tion coefficient at the given wavelength, melting and vaporization tempera

ture, thermal conductivity, heat capacity, diffusivity, and heat of vaporization.

Several types of lasers are available for machining composites including

solid-state lasers and gas lasers. Fiber-reinforced plastics generally exhibit a

high absorption of infrared rays typical those produced by a CO

2

laser.

Since polymers do not generally exhibit a fusion reaction, the vapor column

is not surrounded by molten material as in metals. This together with the

thermal properties

the plastics are such that the vaporization process occurs

at much lower specific powers 10-

3 1 5

W/cm

2

) .

As previously noted, laser-beam machining is based on the interaction

an intense, highly directional coherent monochromatic beam of light laser

beam) with a work material from which material is removed by melting and

vaporization or chemical degradation. The physical processes involved in

laser machining are basically thermal in origin. When a laser beam impinges

on a work material several effects arise, including, reflection, absorption, and

conduction the laser beam. The amount by which the beam is reflected

depends on the wavelength of the laser radiation, and on the condition and

properties of the work material, such as roughness, degree oxidation, and

its temperature. The amount

laser energy absorbed by the surface

the

composite material depends on the optical as well as the thermochemical

properties

the material. Figure 7 shows the variation

reflectance for

several metals as a function of the wavelength of several laser lines 37). For

efficient lasing action, absorption should be as high as possible or

reflection as low as possible. Most metals absorb more readily at shorter

wavelengths, and hence, less power is required to machine these materials at

these wavelengths. Therefore, Nd:YAG with a wavelength

1.06 l m would

be more suitable for machining MMCs than CO

2

In contrast, some of the

organic resins and other compounds have a higher absorption at higher

wavelengths close to that

the CO

2

laser 10.6 urn), and hence, CO

2

would

be more appropriate for machining such materials e.g., aramid-resin com

posites). It should be noted that as melting begins or the material begins to

interact with its atmosphere, the absorption may change. Thus, the

absorption in drilling may change as the process continues. For example, the

absorption in part of the hole drilled could be different from its initial

value at the surface.


21/40

MACmNING OF FIBER-REINFORCEDCOMPOSITES

133

,

LEAD GLASS

, I TRAN SMIT TANC E)

\

\

\

I I

ARGON 48

52/ nl \

Nd Y G

2nd

I

/ HARMONIC

.53

\

C02

10.6

He-Nef.6]

1

I

UBY ,694

... Nd:GLASS Nd:YAG

11.061

0.5 1.0 2.0 5.0

W V

l N TH

J

m I

0.2

O L _

--lL-...lL_-- - _ J. J

0.1

w

1

0 0 1 - r - - i - ; : = = : : : : : : : : ~ = = = = ; ; ; ; ; ; ~ ~ ; 1

u

z

2

Z

0:

0:

o

u

Z

U

0:

igur 7. % Reflectance of several metals as a function of wavelength, with the wavelengths

of several laser lines indicated 41).

Generally, the longer the wavelength of the laser beam, the higher the

reflectivity of the metal workpieces. Similarly, the higher the thermal con

ductivity thermal diffusivity), the higher the reflectivity. The higher reflec

tivity of some materials [especially high conductivity even better high dif

fusivity) metals such as aluminum and copper] at higher laser wavelengths

.g., 10.6 u.m for CO

2

) renders them unsuitable or uneconomical for ma

chining. For wavelengths greater than 5 urn, most metals reflect over 90

of the incident radiation at low power densities. Consequently, low wave

length lasers e.g., Nd:YAG with a wavelength

of

1.06 urn) would be pref

erable for laser machining of high conductivity metals, provided there is

adequate power available for lasing. In contrast, nonmetals e.g., plastics,

glass, and ceramics) with low thermal conductivity are ideal candidates for

CO

2

laser machining reflectivity is inversely proportional to the thermal con

ductivity). The amount of reflectivity can, however, be reduced substantially

by modification of the surface conditions on the work materials. For example,

the reflectivity of copper at a wavelength of 694.3 nm Ruby laser) can be

reduced from 95 to less than 20 by oxidizing the surface. Similarly, re

flectivity can be reduced significantly once the material begins to melt.

Energy transfer from a laser to the work material may occur in two

ways. At low values

of

specific power i.e., below a threshold value), the

laser energy is absorbed in a superficial zone

of

the work material and heat

is transmitted into the material by conduction. Above the threshold power,

which is high enought to melt and/or vaporize the material, a vapor column

surrounded by molten material forms and energy is absorbed through the

entire thickness of the workpiece. The temperature reached by the material

produces changes in the mechanical and physical properties near the inter

action of the work material and the laser. The nature

of

these changes as well

as the magnitude

of

the heat affected zone HAZ) depends on the temperature


22/40

4

KOMANDURI

Table 7.

Typical Thermal Properties of Fiber-Reinforced Material Constituents

p, K C,

K,

T., H

MATERIAL

g em :

W

m K

J kg

K

(em s- )lO-

C

J g

Resin 1.25 0.20 1200

1.30 450 1000

Aramid Fibers 1.44 0.05 1420

0.24

950

4000

Carbon Fibers 1.85

50 710 380

3300

43000

Glass 2.55 1.0 850 4.6

2300 31000

reached in the vapor column and the thermal exchange coefficient between

vaporized and solid zones.

Tables 7 and 8 give some of the thermal properties of resin and various

reinforcing fibers, and various unidirectional composites, respectively. Note

the poor thermal properties of polymer resin that constitutes 50-60 of a

fiber reinforced plastic (FRP). The properties

of

aramid fibers are somewhat

similar to that of resin with minor differences in magnitudes. In contrast, the

properties of carbon and glass fibers are different from that of the resin matrix

material. As a result large differences exist between the thermal properties of

resin matrix and glass or graphite fibers while the difference are negligible

with aramid. The energy needed for vaporization for glass or graphite is also

very high compared to the matrix. The laser power requirements, therefore,

will be strongly dependent on the fibers used and their volume fraction and

not the matrix. However, too high a laser power may vaporize or chemically

degrade the polymer matrix.

The vapor column generation mechanism is strongly influenced by the

nature

of

the constituent materials

of

the composite (fibers and matrix), which

may exhibit very different properties. At high specific powers, the time to

vaporize the constituents of the composite is very short, but because of their

different thermal properties, fibers and matrix can exhibit very different values

of vaporization times. Theoretically, the time t that will elapse before the

vaporization condition is reached on the work material surface under a laser

beam source can be calculated (38) as follows:

K

t

v

-

where K is the thermal conductivity, is the vaporization temperature, F

o

is

Table 8. Typical Thermal Propert ies of Unidirectional Composites

COMPOSITE

Aramid/resin

Graphite/resin

Glass/resin

1.35

1.55

1.90

0.13

25

0.60

1300

950

1000

0.74

170

3.2


23/40


135

the specific power, and is the thermal diffusivity. Using this equation, it is

possible to calculate the minimum values of times needed to vaporize the

materials at a given specific power. Figure 8 shows these values for a typical

matrix material and three types

of

fibers 39 . is possible to observe two

limit conditions at constant specific power.

can be seen that both the fibers

and the matrix exhibit different vaporization times. However, they are closer

than glass or graphite fibers for aramid fibers in a resin matrix. is evident

that FRP with aramid fibers respond better with a laser, and hence, would be

a candidate for this material. This is fortuitous since difficulties such as sur

face delamination and fuzziness are experienced in conventional machining

of aramid fiber-reinforced resin matrix composites.

Laser-Cut Quality Parameters

Some of the defects observed in laser cutting include fiber pullout from

the matrix surrounded by large zones with a loss

of

matrix material, the

presence of craters and delamination, uneven kerf width and taper in the cut

surface, the presence

of

charred material, and thermal cracks. Tagliaferri 40

identified principal quality criteria for laser-cut surfaces as shown in Fig. 9.

The quality parameters include the kerf width at inlet Wi and outlet W

o

of the

laser beam; the size of the heat affected zone HAZ

W

d

which is character

ized by the presence of fiber debonding from the matrix or matrix recession;

and thermal degradation

of

the fibers, matrix, and slope

of

the cut surface

tan a from inlet Wi to outlet

o

for a given thickness s .

Influence Cutting Parameters

The quality of laser cutting depends only on the interaction time between

the beam and the material, and therefore, on the translation speed

of

the beam.

s the cutting speed increases the kerf width

Wi

and

W

o

the slope

of

the

cut surface tends to decrease and reaches a steady-state value Fig. 10 . The

lB

t.

5

igur

8. Limit conditions for the vaporization of some composite constituents versus spe

cific power of the beam,

F

o

and interaction time t

40 .


24/40

136

KOMANDURI

ut 41 12 11

t

h t 2 i ~

X-Z

x-z

DO

Slope

o f the

Hatl ix

cu t

surface

[ j

fJ

ccession

H

W

o

te

-25 -

H

y-z

W

o

I

V

I

raters

< >

l Z

W

d

x-z

Hcat

L

b

afrected

el ln t ion

tone

-

Figure

9. Terminology for quality of laser beam cut surfaces of FRP 40 .

limiting width

W;

is close to the laser beam spot diameter. In contrast, the

slope

of the cut surface decreases with speed reaching a minimum value and

the slope increases with a further increase in speed Fig. 11 . The minimum

value depends on the thickness of the sample, the thinner the sample the

higher the

minimum

value. The HAZ, similar to the kerf width, decreases

with increasing speed Fig. 12 . This may be explained in terms of the in

teraction time and thermal properties of the work material. The damage di

minishes when the energy input is lower, resulting in a shorter interaction

time.

Caprino and Tagliaferri 41 developed a one-dimensional thermal model

to correlate maximum cutting speed V

ma x

,

power P , material thickness r ,

and focal spot diameter

d . The maximum cutting speed is given by

P

V

ax t

1 5

.. S=2.0mm

c S=3.3mm

o

S=4.5mrT

E

V Vo

..

1.

e

0

:

I

~

0

0

0

8 5

0

0

i

0

8

e

00

0

8 8

8

2

4

6 8 10

V (mlmin\

Figure 10.

Width of the kerf at the inlet

W,

and outlet

W

o

of the laser versus cutting

speed;

P

=

800 W

40 .


25/40

MACHINING

O

FIBERREINFORCED COMPOSITES

B

137

o S=2.0mm

o S=3.3mm

S=4.5mm

6

:

x

4

T

B

2

o

\

a 0 00

\

/ 0

0

~

8

a A a

0_ 10

0-

A A

.-6._.

8

8 2

4 6 B

V m/min)

10

12

igure

11. Slope of the cut surfaces versus cutting speed;

P

=

800 W (40).

Figure 13 shows a linear vanatton

of

V

na x

d versus Pit for various

composites (AFRP, GFRP, and CFRP). The slope

of

the curve gives the value

of

the material constant

K

which is 3730 Jzcrrr for AFRP, 40,000 J/crrr for

CFRP, and 11,100 Jzcrrr for GFRP.

The performance of the lasers in machining can also be changed by the

introduction of a gas jet. For example, the efficiency

of

metal machining can

often be increased by oxygen-assisted cutting. The technique takes advantage

of

the additional energy released due to the exothermic chemical reaction

of

the work material with oxygen. Depending on the type

of

work material, laser

0.8

S=2.0mm

o S=3.3mm

E

B.6

S= 5mm

J

B.4

e

0

0

c

0

0

e

0.2

c

0

e

s

e

e

a

22

a

e

e

B.B

8

2

4

6 8

18

V m/min)

igure

12. Heat-affected zone size at the inlet of the beam versus cutt ing speed;

P

= 800

W

(40).


26/40

l> AFRP

o

GFRP

o CFRP

138

6

/

i

6

,

/ /

i e

0

0-

_

I I

4 8

p-

...

.lWtYnml

KOM N URI

Figure 13. Experimental values V

m

ex d are reported as a function of Pit. V

m

is the maxi

mum cutting speed above which a through cut cannot be obtained rnm/s ; d is the focal spot

diameter 11m ; I is the thickness of the machine material 11m ;

is the material parameter

(J/cm ).

For AFRP

=

3730

J/cm ,

for CFRP

=

40,000

Jlcm ,

and for GFRP

=

11,100

lcm

40 .

machining may be assisted by oxygen, inert gas N

2

or Ar , or air. For ex

ample, oxygen assist air and not nitrogen would be preferable for laser

assisted machining of titanium alloys from energy considerations. However,

to avoid oxidation of titanium, inert atmosphere may be preferred. In contrast,

nitrogen assist is preferable for machining nickel-based superalloys.

Several types of lasers are used for machining. The most commonly

used are gas

C

2

and Excimer lasers and solid-state Nd:YAG and Nd:

Glass lasers. These lasers can be operated either in a cw mode or a pulse

mode for machining. The important requirements of lasers for machining

include the following:

1. Adequate power available cw or pulsed .

2. Controlled focal intensity profile.

3. Reproducibility of power, mode, polarization, and stability.

4. Reliability.

5. Initial and scanning costs.

Table 9 gives characteristics

of

various lasers for manufacturing appli

cations 41 . Excimer lasers, such as xenon chloride and argon fluoride, are

currently being developed and should soon find exciting applications for the

machining of composites.

Important characteristics of laser beams:

1. Spatial profile.

2. Beam divergence.


27/40

C l

=

z

Ii

Table 9

Lasers Commonly Used for Materials Processing 41)

C l

0

WAVE-

l

LASER

LENGTH,

MODE OF PULSE REP. PULSE

:l

=

YPE

I om

OPERATION POWER, W RATE, pps

LENGTH APPLICATIONS COMMENTS

t l

p

Ruby 0.6943 Normal pulse

2

X 10 peak

Low

0.2 5

ms Large material removal

Often uneconomical for

in one pulse, drilling

multipulse applications

Z

diamond dies, spot

l

0

welding

=

Nd:Glass

1.06

Normal pulse

2

X

10

6

peak Low 0.5 10 ms Large material removal

Often uneconomical

C l

t l

in o ne p ulse

I:l

C l

Nd:YAG

1.06 Continuous 200

Welding

Compact; economical at

0

low powers

Nd:YAG

1.06 Repetitively 10

4

peak

5000

200 ns Resistor trimming elec-

Compact and economical

0

Q switched

10

average tronic circuit fabrica-

en

tion

CO, 10.6

Continuous 375 Cutting organic materials, Bulky at high powers,

en

oxygen-assisted metal very eco nomi ca l at

cutting, s cri bing brittle low powers

materials

CO,

10.6

Repetitively

75000

peak

400

50 200

ns Resistor tri mming

Bulky but economical

Q -s wi tc hed 1.5 average

CO, 10.6 Superpulsed 100 average

500

10-1001 -5

Welding, hole produc- Bulky but econom ical

tion, scribing


28/40

140 KOMANDURI

3. Focusing.

4. Temporal behavior.

5. Power continuous and peak .

Benefits of laser machining:

1.

Material waste kerf width is minimum.

2. Minimum set-up time.

3. No tools, hence, no wear or replacement.

4. Smooth edge cuts can be obtained.

5. Low total heat input, hence, low overall distortion or damage of the

part.

6. Parallel sided cuts possible.

7. Sharp contoured surfaces can be generated.

8. Independent of workpiece hardness or strength.

9. Cuts can be made without a starter or sloping.

Limitations of laser beam machining:

1. HAZ. High temperatures imparted to the workpiece by laser ma

chining at or near the last cut can cause metallurgical changes. This

can reduce the fatigue properties of the work material.

2. Quality of holes in deep hole drilling is poor.

3. Unable to drill blind holes to precise depth.

4. Lasers not efficient with highly reflective or highly thermal con

ducting materials.

5. Lasers not recommended for thick workpieces.

6. Lasers tend to char thick nonmetals with reradiation from decom

position products.

Water-Jet and Abrasive Water-Jet Cutting

High-pressure water-jet cutting either in unison or in consort with fine

abrasives is a candidate process for machining inhomogeneous materials that

are hard and abrasive, such as most polymer-matrix composite materials.

Water cools the workpiece, and hence, minimizes the thermal deformation

problems commonly experienced in the conventional machining of compos

ites. A narrow kerf, a minimum amount of dust and toxic fumes, and prac

tically no delamination effects are some of the salient features of this system

42 . Rapid tool wear commonly experienced in conventional machining of

composites is also not an issue in water jet or AWJ cutting. AWJ cutting,

however, is not without its limitations. Some of the shortcomings of this

process are high noise levels

80 100

dB , and consequently, the need for a

catcher at the exit; safety; low removal rates; inability to machine blind holes

or pockets; abrasive particles or high-pressure water jet that can damage the


29/40


machine elements; and the size of the AWl cutting system and associated

equipment. Removal rates, dimensional accuracy, and finish can still be lim

itations

of

AWl

cutting, depending on the material and its thickness. When

machining thick materials the jet stream tends to angle away from the direc

tion of the cutting, resulting in a tapered surface. This effect becomes more

pronounced as the thickness and/or feed rate increases.

Boron-epoxy, boron-polyester, fiberglass-epoxy, graphite-epoxy, and ar

amid-epoxy composites are some of the candidate materials for water-jet cut

ting. Some of the metal matrix and ceramic matrix composites can also be

machined by AWls but at reduced cutting rates. AWl cutting of composites

depends more on the matrix material than on the reinforcement. Straight

water-jet cutting, with no abrasives, is recommended for materials with a low

yield strength =10 15 ksi).

Korican 43) reported using an AWl t 35-ksi pressure, 100-grit Garnet

abrasive, 20 hp) to cut resin-impregnated graphite, aramid, and glass fibers.

The cutting rates changed from 63 to 5 in./min as the thickness increases

from 1/8 to 1 in. Of course, the optimum value depends on the composite

material, its thickness, abrasive type, size used, jet pressure, etc. Table 10

gives the AWl cutting traverse rates for some composites materials of interest

after Hashish 44).

Ramulu and Arola 45) investigated the cutting of unidirectional graph

ite-epoxy composite by Wl and AWl They repored the principal material

removal mechanism with Wl to be the failure associated with microbending

induced fracture and out-of-plane shear. In

AWl

the material-removal

mechanism included shearing, micromachining, and erosion. They found

AWl

to be more feasible because

of

its material-removal mechanism, higher

removal rates, and superior surface finish.

Table 1

Traverse CUlling Rates in/min) using AWl P: 50 ksi, abrasive: 80 mesh garnet)

46)

THICKNESS, in.

1/16

1/8

1/4

1/2 3/4

Organic matrix composites

Glass-epoxy 225

180

100

40 28.0

Graphite-epoxy 150

125 95 35 26.0

Carbon/carbon 75 52 31

18 9.5

Metal matrix composites

B.C-Mg 15 B.C)

71 35 24

9.5

SiC-Al 15 SiC)

40 24 12

SiC Al 25 SiC)

22 12

Al,O,-Al 15 Al,O,)

22 12

Ceramic Matrix Composites

TiB,-SiC 15 TiB,)

0.68

0.35

SiC-Al,O, 7.5 SiC)

6.4

3.30


30/40

142

KOMANDURI

Hashish 46 presented a technique for precision drilling

of

small di

ameter holes in composites with an Wl The technique involves computer

control of the jet pressure-time profile and the abrasive flow rate that in tum

controls the hole size, shape, and quality. On a ceramic matrix composite

Hashish reported drilling a hole 0.043 in diameter with an accuracy of 0.001

in.

EDM of Composites

The principles of EDM are well known and will not be discussed in this

paper. Only opportunities in machining composites will be explored. EDM

can make complex shapes with high precision.

t

is, however, a slow process,

but automation of the process can bring the cost of manufacturing down. The

prerequisite for the EDM process is that the work material be electrically

conductive. The electrical resistivity of the work material should be lower

than 100 O-cm; organic matrix composites are, therefore, not candidate ma

terials for EDM. These composites can, however, be made conductive by

impregnating them with metallic fillers Cu, AI, or Ag powders , but that can

defeat the purpose of composites for high strength, lightweight applications.

MMCs will be ideal candidates for EDM, especially where complicated

shapes and high accuracy are required. Only a few ceramic matrix composites

that are electrically conductive can be shaped by EDM. However, recent

improvements in mechanical properties, especially the fracture toughness and

strength of whisker-reinforced ceramics by improved processing technology

and starting materials, makes them ideally suited for high-temperature and

fatigue-resistant applications. Silicon carbide whisker-reinforced alumina. is

one example. The fracture toughness is nearly double that of the material

without the fibers. t is the same with silicon-nitride-based composites. These

materials are very hard but extremely difficult as well as costly to machine

or grind. f however, these materials can be made electrically conductive by

adding conductive refractory materials such as TiC or TiN without compro

mising on other properties, processing these components by EDM can be

come an economic proposition. The particle size and percentage of TiC or

TiN to be added to the matrix can be adjusted to make it sufficiently electri

cally conductive to carry out the EDM process, without compromising sig

nificantly on the ultimate properties and performance requirements of the

material.

Silicon-nitride-based composites can be made electrically conductive by

adding TiN and alumina-based composites by adding TiC. Martin et al. 47

found that for Si

3N

4 -ba se d

composites with TiN additions, EDM cannot be

performed at a conductivity lower than 2 10

2

o

cm

since no electrical

arc is produced between the workpiece and the tool Fig. 14 . However, a

preferred value of conductivity for good EDM practice is found to be 5 X

10

3

0 -

1

m

I

In contrast, for wire EDM of SiC whisker-reinforced, zirconia-


31/40


143

EDM LImit

..J

10

U

IE

1

B

U

W

..J

W

10

30

0

6

70

8

9

100

owl TIN

>

t

>

10

t

U

::I

C 10

z

o

o

igur 14. Variation of electrical conductivity of a Si,N

4

TiN composite versus the TiN

content for two grades of TiN 47 .

1

. .

800

M

LIMIT

Z

IE

l

600

III

..J

IE

Conducllvllr

40 0

::I

2 St,

enlllh

III

..J

200

2

I

S

i

i

10r---....,

JL

.......

;

o

z

8

;i

IE

U

10

III

10 20

30

40 00

.1 TIC

igur 15. Influence of TiC content on electrical conductivity and on three-point-bend test

of alumina-based composites 47 .


32/40

KOMANDURI

toughened alumina with the addition of TiC, the minimum conductivity value

was reported as 1 0

1

cm Fig. 15). With a

30

vol

TiC addition to the

alumina-based composite the bend strength was reported to be 125 ksi Fig.

16), while up to 50 vol of TiN particles could be added to the Si

3N

matrix

without reducing its fracture toughness. This is one example where difficult

to-machine materials such as ceramic composites can be tamed by making

them electrically conducting and processing them by EDM. Typical EDM

removal rates for Si

3N

TiN composites, steel, and cemented tungsten carbide

are given in Table 11.

Gadalla and Cheng 48) investigated wire EDM of various ceramic com

posites. By combining a nonconducting ceramic with a more conducting ce

ramic it was possible to lower the electrical resistivity. For example, by com

bining TiC to alumina, it could be made more conductive for EDM cutting.

Table 12 shows various composite materials along with relevant properties)

that were successfully cut by wire EDM by Gadalla and Cheng. The addition

of a more conducting phase may in many cases not be detrimental to its

performance and may actually facilitate it. For example, by combining

30

TiC to alumina, the strength, hardness, wear resistance, and fracture strength

of the cutting tool material will increase. At the same time, EDM can be used

for processing this material. A similar situation exists for SWON with TiN,

Si

3N

with TiN, and SiC with TiB

2

as shown in Table 12.

1000

900

II

l:

I:

800

e

Z

W

II:

70 0

II I

J

II

: l

600

x

W

J

L

50 0

0 10

20

30

40

50 6 0

70

l w I T iN

igur

16. Effect of TiN ratio on the three-point bend strength of a Si

3N

4

,

TiN composite

47).


33/40

MACIDNING OF FIBER-REINFORCED COMPOSITES

145

Si,N.-TiN

COMPOSITE STEEL

190 140 190

0.28 0.5 0.16 0.14

24 64 16 24

ve v e

ve

10 10

5

5

20

0.0037 0.0043 0.0019

4 6 0

6 6 20

0.0011 0.0013 0.0007

ble 11. Comparative Performance of Si,N. Composite, Steel, and WC in Wire EDM (47)

Wire EDM (0.010 in. brass wire)

Wire speed, in./min

Cutting rate, in./min

Finish (Ra), l in

Die Sink EDM

(Copper die and oil dielectric)

Roughing operation

Die polarization, 80 V

Intensity, A

Die consumption,

Removal rate, in. /min

Finishing operation

Intensity

Die consumption,

emoval rate in. /min

Ultrasonic-Assisted Machining of Composites

Ultrasonic-assisted drilling involves the use of a rotary tool where an

axial vibratory motion at high frequency is superimposed. A special adaptor

is required to transmit the vibration from a piezoelectric transducer to the

tool. Ultrasonic vibration can reduce friction, break chips, and reduce tool

wear. is a particularly useful technique when the matrix or the reinforcing

fibers are hard, brittle materials. Use of a core drill permits cutting fluids to

be passed through its center. Ultrasonic machining through a slow operation

can result in high finish and accuracy of intricate parts.

Garlasco (11) of Grumman conducted ultrasonic machining experiments

on boron fiber composites. Based on the analysis of costs, he found the

technique to be more expensive than diamond-abrasive cutting. He, however,

recommended this process as a potential for applications when intricate

shapes

of

high accuracy and finish are to be produced.

ble

12.

Ceramic Composites Cut by Wire EDM (48)

MATERIAL

A1,O,-TiC

Si-SiC

SiAlON-TiN

Si.,N.-TiN

SiC-TiB,

COMPOSITION

70/30

5/95

80/20

75/25

80/20

DENSITY,

g/cm

4.23

3.09

4.0

3.75

3.3

ELEcrRICAL

RESISTIVITY,

fl-cm

3.5e-3

1.43e-4

7.0e-4

1.5e-3

1.0


34/40

t

KOM NDURI

Doran and Maikish 14 conducted rotary ultrasonic machining. They

used a core drill at speeds up to 6000 rpm while simultaneously vibrating

the tool axially at

20

kHz with a peak-to-peak amplitude of approximately

0.001 in. They found a slight improvement of tool life ~ 1 O - 2 0 ) . However,

the savings resulting from the tool life improvement was much less than the

additional costs involved in the use of ullrasonic machining of the boron

epoxy composite. Therefore, this process was not recommended unless com

plex shapes of high finish and accuracy are needed.

Electrochemical Spark Machining

Jain et al. 49 proposed electrochemical spark machining as a plausible

solution for the cutting of fiber-reinforced plastics. They investigated Kevlar

fiber-reinforced epoxy and glass fiber-reinforced epoxy as work materials,

copper as the tool, and an aqueous solution of NaCI as the electrolyte. They

established the feasibility for specific machining operations and the optimi

zation of process conditions. Subsequently, Jain et al. 50 extended this con

cept to traveling wire electrochemical spark machining TW-ECSM of com

posites. Glass-epoxy and Kevlar-epoxy composites were used as the work

materials, copper wire as the traveling wire electrode, and NaOH as the elec

trolyte. Jain et al. showed that higher accuracy was feasible with this tech

nique.

S FETY ONSIDER TIONS

Composites contain fibers, which when machined, can release finer frac

tions of the fibers into the atmosphere. Also, in the case of polymer-based

composites, some of the chemicals released because of heat and thermal dam

age du

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