machining of composite materials - state of the art

8/7/2019 Machining of composite materials - State of the art

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MACHINING OF COMPOSITE MATERIALS:

STATE OF THE ART

11. Basic Definitions

Many structural applications require the use of materials combining,

simultaneously, superior strength and stiffness with low weight. Composite

materials are excellent candidates for fulfilling these requirements because of their

high specific properties. In this scenario, one of the most interesting aspects is the

fact that the material itself is also a structure, which consists of two or more

phases on a macroscopic scale, as shown in Figure 1-1[1].

Figure 1-1: Phases of a composite system (after [1])

A structural composite is designed with the following purpose in mind: the

properties and mechanical performance of the composite material are superior to

those of the constituent materials when acting independently.

The matrix is the less stiff and weaker phase and is a continuous medium. The

reinforcement is usually discontinuous, stiffer and stronger. Needless to say, the

properties of a composite structure depend on the properties of the constituents,


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geometry and phase distribution. The homogeneity of the material system

depends on the more or less distribution of the reinforcement. Composite

materials are, therefore, rather anisotropic in their nature. This fact implies that

the materials properties, at a certain point, vary with direction or depend on the

orientation of the reference axes (Figure 1-2).

Figure 1-2: Unidirectional ply and principal coordinate axes (after [1])

A laminate is made up of several unidirectional plies stacked together with

various orientations as shown in Figure 1-3 [1]. Since the principal material axes

vary from ply to ply, it is desirable to analyse laminates using a common fixed

system of coordinates (x,y,z). The orientation of each ply is given by the angle

between the reference x-axis and the major principal material axis (fibre

orientation) of the ply, measured in a counter clockwise direction on the x-y

plane.

12. Applications of Composite Materials

Composites have unique characteristics that make them perfect material choices

for several applications, such as: high strength, high stiffness, long fatigue life,

low density and great adaptability to a specific function.


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Figure 1-3: Multidirectional laminate and reference coordinate system (after [1])

The aerospace and aeronautical industries have been major users of composite

technology in the last decades. From small parts to fairly large structures, weight

savings while preserving high material properties has always been an issue in

commercial aircrafts, such as the Boeing 777 shown in Figure 1-4 [2].

Figure 1-4: Boeing 777 commercial airliner (after [2])


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Another example is a speedbrake structure of a military aircraft, the Vought A7,

composed of several composite parts, Figure 1-5 [3].

Figure 1-5: A7 speedbrake structure (after [3])

The examples of composite applications shown above are only a small part of

many hundreds of other similar applications, not only on the aerospace and

aeronautical industries.

13. Trends in major civil aircraft manufacturers

After the terrorist attacks of September 11, 2001, a negative financial fallout

occurred with many air carriers. As of 2005, a slow recover took place and the

two main commercial aircraft carriers and rivals, The Boeing Co. (Seattle,

Washington) and Airbus Industrie (Toulouse, France) developed significantly

different views regarding the future of commercial air travel.Airbus reasoned that the number of non-stop flights between large number of

paired cities would decrease, although populations continue to concentrate in and

around major metropolitan areas. Based on these predictions, Airbus developed


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the super-jumbo A380, capable of carrying 555 to as many as 890 passengers to

fly at lower per-passenger costs.

Boeing, on the other hand, expected the population distribution to contribute to

an increase in new non-stop flights and growth in the number of trips between

paired cities. According to Boeings calculations, large aircraft like the B747 and

the A380, will be only 4% (790) of the total commercial jet fleet, while almost

4300 are expected to be regional jets over the next 20 years. Consequently,

Boeing came up with the 7E7 (E means efficiency) and then renamed 787

Dreamliner. Being this a mid-sized jet, belonging to the same category as the

B767, and taking into account that this category makes up the greatest number

of in-service commercial jets, airlines began to look more at fuel efficiency,

especially with rising fuel prices.

In the quest for more efficient aircraft, both aircraft manufacturers turned their

attention once again to composite materials.

14. General Materials Information on the B787

The B787 Dreamliner will be the first full size commercial aircraft with

composite wings and fuselage (Figure...).


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Figure 28-B787 Dreamliner fusellage section

Composites on the B787 (Figure...) will account for 50% of the aircrafts

structural weight. Aluminium will comprise only 12% of the mentioned weight.

Titanium will make up a greater percentage than aluminium, namely 15%. Steel

will comprise 10% and other metals 5%.


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The material to be used on the B787s primary structures, such as the wings and

fuselage, is Torays 3900-2 prepreg material (comprised of intermediate-modules

T800 carbon fibre and a toughened 350F-cure epoxy), used in both unidirectional

tape and woven formats.

15. Special Materials for the Aerospace and Aeronautical

Industries

Aluminium-Lithium Alloys

Among the new aircraft materials, aluminium lithium alloys are particularly

attractive because of their weight-saving potential. When aluminium is alloyed

with lithium, for every 1% addition of lithium, there is approximately a 3%

reduction in alloy density and an increase in stiffness of about 6%. The

commercial alloys typifIed by 2090, 2091, 8090, and 8091 contain from 1.9-2.7%

lithium. Therefore, they offer up to about 10% density advantage over the 2000

and 7000 series alloys. They also have correspondingly higher stiffness and offer a

25% advantage in specifIc stiffness. With alurninum-lithium alloys, weight saving

in aircraft structures of up to 10% is possible in strength-critical structures and of

up to 18% in stiffness critical structures. In addition to being light and stiff, the

alloys are strong, damage tolerant, and corrosion resistant. However, their

properties are strongly sensitive to processing conditions and, therefore, product

quallty is more diff1cult to control than for conventional alloys. Other short

comings include high anisotropy of unrecrystallized products caused by the strong

crystallographic textures developed during processing, low short-transverse

properties of thick plates, lack of thermal stability of some products, limited

experience with manufacturing requirements, and limited amounts of design data.

Aluminium-lithium alloys were introduced more than 30 years ago by Alcoa as

alloy 2020 for use on the RA-5C Vigilante military aircraft. Outside of the

U.S.s.R., where several alloys were developed in the 1960s, the technology


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appeared to lay dormant until the mid-1980s, when Alcoa, Alcan, and Pechiney

introduced alloys 2090, 8091, and 2091, respectively, and Alcan and Pechiney

joint1y introduced alloy 8090. During the late 1980s, a mechanically alloyed

powder metallurgy product known as AI-905XL (formerly IN 9052) was also

introduced by IncoMAP and, more recently, a cast Al-Li-Cu alloy known as

Weldallte 049 (now registered as alloy 2095) was developed by Martin Marietta

Laboratories. The latter material has excellent weldability, superior to that of the

2000series alloys, including alloy 2219, and is a strong contender as fuel tank

material for NASA'sspace shuttle because of the material's excellent cryogenic

properties. The 2000 and 8000 series AI-Li alloys are available commercially in a

variety of forms and tempers which can be selected to meet the specific design

requirements of either high strength (e.g., 209O-T8X, 8091-1'8), medium strength

combined with corrosion resistance and damage tolerance (e.g., 8090T8XXX,

2091-T8X), or high damage tolerance (e.g., 2091-T8XXX).

Applications of AI-Li alloys are not widespread to date. Alloy 8090-T83 is used in

limited quantities by Airbus 1ndustries, for the D-nose skins of the leading edge

of the A330/340 aircraft wing. Alloys 2090-1'83 and 2090T62 are used by

McDonnell Douglas for some flooring sections fi the C-17 air lifter craft. The new

Boeing 777 aircraft makes only limited use of AI-Li alloys. In contrast, Westland-

Agusta, U.K./Italy is unique in making extensive use of 8090 forgings and sheets

and 2090 and 2091 sheets for the EHI0l helicopter. The alloys are also being

tested for a variety of new applications, inc1uding lower wing skins and fuselage

applications (panels and doors).

Aluminium Bases Metal Matrix Composites

The 2000 series alloys offer strength and damage tolerance, the 7000 series alloys

offer higher strength potential' and the 6000 series alloys are conducive to good

corrosion resistance and improved machinability, whereas the Al-Fe-X (8XXX)

alloys provide opportunities for high-temperature performance


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Meanwhile, a matrix based on AI-Li provides a unique combination of high stiff-

ness and low density. Composite properties are also strongly influenced by the

type of reinforcing medium. Fibbers provide the highest stiffness, strength, and

toughness combination. Particulate reinforcement is often used for wear-

resistance applications and offers high stiffness but only low strength and low

toughness, whereas whisker reinforcement offers high stiffness, medium strength,

and low toughness. Several reinforcing mediums have been used for AMCs,

including alumina, carbon, and Sialon fibbers, but SiC is the most common

reinforcing medium.

Hybrid Composites

Hybrid composites are FRP-metal sandwich laminates consisting of alternating

layers of high-strength aluminium alloys and fibre reinforced epoxy adhesive.

This hybrid structural material, illustrated schematically in Fig. 9, was developed

in the late 1970s at Delft University in the Netherlands and Fokker Aircraft and

was later commerciallzed in collaboration with Alcoa and Akzo. Two categories

of hybrid composites are available commercially today, the ARALLi!!) and

GLARE@ laminates, which differ in the type of f:1ber used for reinforcement.

ARALL laminates (for aramid reinforced aluminium laminate) use 50% fibre

volume of adhesive prepreg of high-modulus aramid fibres. GLARE laminates (for

glass reinforcement) are unidirectional or bi-axial reinforced with 60% fibre

volume of high strength glass fibres. GLARE laminates are a more recent

development, complementing the original ARALL product through provision of

higher compression strength.

Both ARALL and GLARE laminates come in different configurations ranging

from two layers of aluminium with one FRP layer in between, to f:1ve layers of

aluminium with four interspaced FRP layers. In GLARE laminates, the glass

fibres can be layed up in a cross-ply configuration. Also, both ARALL and

GLARE laminates can be fabricated with different aluminium alloys. This allows

laminate properties to be c10sely tailored to component design requirements. The


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laminates are produced by curing in a heated platen press. After curing, ARALL

laminates can be stretched to eliminate undesirable residual stresses.

FRP-metal laminates have the ability to impede and self-arrest fatigue crack

growth, which makes the materials highly damage tolerant. As cracks develop in

the aluminium face sheets, fibre bridging across a propagating crack causes the

unbroken fibres to carry increasing portions of the load, which may decrease the

stress intensity at the crack tip to the point where the cracks cease to grow. This

makes the material particularly well suited to applications requiring good fatigue

resistance. GLARE laminates are particularly well suited for f1rewall applications

because of a high bum-through resistance.

Figure 29-A380 Materials overview


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16. Machining of Composite Materials

Although composite parts may have the advantage to replace many mechanical

fastened pieces, and so a significant reduction in the number of fastener holes

leading to cost reduction, the need to unify components by mechanical fastening

is still considerable.

New problems may arise during the machining of composite materials when

compared to the machining of traditional materials like metals, in the basic

operations of drilling or milling. Fibre reinforcement in composites is usually very

abrasive which leads to rapid tool wear and deterioration of the machined

surfaces. Delaminations can be introduced into the workpiece and several types of

damages can be observed on the workpiece surfaces. Parts must be very well

supported to resist force applied by the tools during machining, which requires

expensive fixtures to be built.

Drilling composite Materials

Generally, the thrust and torque applied on a drill bit depend on speed, feed rate,

tool geometry and tool wear. Experimental testing showed that as drilling

progresses, thrust increases steadily until a nearly constant value corresponding

to steady drilling through the thickness of the laminate is reached, after which it

drops sharply when the tool exits the laminate on the opposite side (Figure).


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A sharp decrease in normal force as the bit enters the workpiece is always

associated with the introduction of delamination by mechanical action of the tool

peeling up the top layer of the laminate. Delamination of the top layer can also

be produced by high thermal stresses generated by drilling, but in that case no

discontinuities are observed in the normal force history. Delaminations near the

exit side are introduced because the tool may act like a punch, separating the

thin uncut layer from the rest of the laminate. This phenomenon is associated

with an almost instantaneous drop in normal force from its steady-state value to

zero. Delaminations can be greatly reduced or eliminated by reducing feed rates

near the exit and using backup plates to provide support and prevent

deformations leading to exit side delaminations.

The oscillations observed during the steady portion of the drilling process are

related to the different ply orientations of the laminate. With unidirectional (UD)composites, the amplitude of such oscillations is rather large. For cross-ply or

quasi-isotropic laminates, the amplitude is much lower. Maximum normal force

and maximum torque both increase significantly with the number of holes drilled


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due to chipping and wear of the cutting surfaces. Figure shows the increase in

the maximum axial force with number of holes for an 8 mm split point carbide

bit drilling 4.5 mm thick graphite-epoxy laminates with a speed of 1900 RPM

and a feed rate of 0.015 mm/min.

17. General Tool Design

Because glass and carbon fibre s are very abrasive, drill bits made out of high-

speed steel (HSS) fail after drilling just a few holes in composite materials.

Tungsten carbide tools possess adequate life, particularly when sub micrometer

carbide is used, because its resistance to rupture is 50% higher and it is harder

than the standard C2 grade carbide. A much higher number of holes can be

drilled with tungsten carbide tools coated with polycrystalline diamond (PCD).

PCD-coated tools can be easily chipped, particularly when used with portable

drills, and cannot be sharpened. Another approach involves grinding the pointangle into carbide tipped blank. PCD veins are then sintered in that groove, and

the drill point geometry and flutes are ground in. These diamond veined drills

can be sharpened. For graphite-epoxy or glass-epoxy, best results are obtained

with a solid tungsten carbide dagger drill or with PCD-coated twist drills.

Another factor to consider in the selection of a drill bit is cost; PCD tipped drills

are typically 20 times as expensive as solid carbide drills [28]. It must bedetermined if the longer tool life and improved hole quality balance the higher

cost.

MilIer tested nine types of 4.85 mm diameter drill bits in 6.35 mm thick graphite-

epoxy at 20,000 rpm and feed rates 0.025 mm/rev, using portable self-feed air

motors. Among those tested were the carbide tipped chisel point bit, the eight-

facet bit, the jodrill, and the four-flute tapered straight flute bit shown in Fig. 6.A solid carbide drill with an eight-facet split point and a jodrill with 30 helix

geometry produced the maximum numbers of holes with a tolerance of +0.051

and -0.025 mm. The success of the eight-facet split point bit is attributed to the


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long taper angle at the shoulder, which tends to minimize fibre breakout. The

same effect is obtained in the jodrill using a step at the drill shoulder. Solid

carbide tools must be handled with care to prevent breakage and chipping of the

cutting edges. A micro grain carbide grade is recommended to provide as much

toughness as possible.

The helix angle is not so important for machining graphite-epoxy composites

because the chips are in powder form and are continually suctioned off. Positive

rake angles (Fig. 7) are needed to generate the least amount of heat during

cutting. However, the more positive the rake angle, the more fragile the cutting

edge becomes. A small chisel angle is the second element of good tool geometry

and serves to improve the penetration rate.

Figure 30-Examples of drill bits used with composite materials (a) carbide tipped chisel point; (b)

eight facet; (c) jodrill; (d) four flute, tapered, straight flute


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Composites are often bonded to aluminium or titanium parts, and the hole to be

drilled must go through both the composite and the metal. Drilling aluminium

with a tool designed for drilling graphite-epoxy calls for a tool of a very different

shape in order to remove the long stringy aluminium chips. Drilling titaniumwith the same tool as is used for composites is difficult because tool wear makes

drilling through titanium difficult. It is recommended to drill the composite first,

remove the tool, and drill the titanium part with a different tool. Then, because

the titanium chips damage the surface of the hole in the composite section as

they are removed, the hole must be reamed. This procedure, involving three tools

and three operations, is expensive. Ways are sought to provide combination tools

or to design drills that automatically change operating conditions as the drill bit

enters different materials.

The machining of materials containing aramid fibre s requires special tooling.

Because of their low compressive strength, aramid fibre s have a tendency to

recede within the matrix instead of being sheared off. Frayed fibre s will protrude

from the hole surface to create what is commonly known as fuzz. To drill this

type of material, fibre s should be pulled from the periphery of the hole toward

the center and then sheared. This can be accomplished by tools with protruding

peripheral cutting edges and positive axial and radial rake angles. Rake angles

and relief angles (Fig. 7) in the range of 29-35 and 12-30, respectively, were

found adequate. Self-centering drills were designed so that around the

circumference, serrations are ultimately oriented upward and downward (Fig.

8a), introducing a cutting action in both directions to cut unsheared aramid

fibres. A serrated spade drill (Fig. 8) for drill press operation and a serrated

countersink are also recommended. For these three types of drill bits, serrations

are designed to trap the uncut fibres and shear them.

Drilling of composite materials with boron fibre reinforcement requires the use of

diamond impregnated tooling. The major problem encountered with boron-epoxy

laminates comes from the heat generated. Material thinner than 3.17 mm can be

drilled without fluid cooling. Above that thickness, coolant must be supplied

through the tool at pressures up to 75 psi to prevent damage to the workpiece.


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When a layer of boron-aluminium is bonded to one piece of titanium, a two-tool

approach is preferable. A multilayered section would require too many tool

changes and must be drilled with one tool, using a reduced feed rate when

cutting through a titanium layer. Tool wear was shown to depend on prior heat

treatment when drilling boron-aluminium composites. Although ultrasonically

vibrating the tool was not effective when drilling boron-epoxy, it reduced friction,

tool wear, and the tendency of the aluminium to accumulate on the tool when

drilling boron-aluminium. Drilling with an ultrasonically vibrated tool was also

successful in producing holes in boron-aluminium /titanium laminates.

Graphite-Bismaleimide Titanium

To minimize the positional errors and to obtain tight tolerances during

manufacturing, composite panels and structural parts are typically drilled

together in a stack and assembled. However, drilling of dissimilar material like

graphite composite and Ti metal in a stack is a challenging task to

manufacturing engineers because of the different machining properties for each

material.

The main problems encountered with regard to the quality characteristics of

drilled composite Ti stacks inc1ude severe tool wear, heat induced damage, hole

size, roundness, shape, surface texture, and presence of titanium burrs.

Drilling Graphite-Bismaleimide Titanium Stacks

The fibre reinforced plastic (FRP) material used in this study was a multi-

directional Gr/Bi composite consisting of IM-6 graphite fibre s and a 3501-6 ther-

moset matrix with a ply orientation of [45/90/-45/0/-45/ 0/45/0/-45/90/-

45/0/45/0/45/90/-45/90/90], which was acquired from The Boeing Company.

The Gr/Bi thickness was 7.62 mm with a ply thickness of 0.2 mm. Ti 6AI-4V

alloy sheets with 3.1 mm thickness were used in the experiment. It was decided

that the most efficient stacking sequence would be to rest the Gr/Bi on top of

the Ti alloy. This approach in stacking sequence would result in the least amount

of exit delamination. Drilling experiments were performed with water soluble syn-


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thetic coolant on a commercial vertical mill, which was retrofitted with a CNC

control and drive unit. Two types of drill materials with a standard drill

geometry were selected for this investigation based on their availability and

widespread use in industry: high-speed cobalt (HSS-Co) and carbide.

Figure 31-Hole quallty features of Gr/Bi-Ti stacks

Tool Wear

Tool wear was measured at the flank face on the drills. Flank wear, occurring at

the outer cutting edges of the drill, was more notable than wear on chisel edge

and cutting lips. Tool wear was measured by viewing the drills underneath amicroscope.

Drill wear was measured in various feeds and speeds to characterize the wear characteristics of each drill bits

in drilling GriBi-Ti stacks. Tool wear on the HSS-Co drills occurred rapidly when

drilling the Gr/Bi- Ti stacks. Flank and crater wear was observed on HSS-Co

drills. Some of the more extreme tool wear was observed on the helical cutting

edges of the HSS-Co drills as well. Minor flank wear was beginning to form onthe carbide drills. The least amount of wear occurred at the tip of the point angle

for all the drills tested.


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The largest number of holes that failed were drilled using carbide drills. In the

case of HSS-Co drills, the time to failure of tool was determined either by the

CNC machine stopping because a maximum thrust limit(~3700 N) was reached,

or visually by noticing worn cutting edges and smoke. Due to difficulties of

measuring the wear length of failed HSS-Co drill bits, the wear length was

designated I mm. Fig. 8 present the tool wear at various feed and speed. For the

HSS-Co drills, slow feed was detrimental to tool life because of the long tool

engagement periods. This seems be contrary to normal cutting conditions of the

HSS-Co tools. However, Ti alloys heat up rapidly during drilling and do not dissi-

pate the heat quickly because of the low thermal conductivity. Increasing the

spindle speed created more heat generation due to friction in the cutting zone of

the Ti plate, decreasing the tool life. Therefore, the longest HSS-Co tool life was

achieved at the combination of high feed and low speed (0.25 mm/rev and 660

rpm). Heat generation was the primary concern when drilling Gr/Bi- Ti stacks

because high temperatures in the drilling region were detrimental to tool life and

allowed for increased matrix degradation. As a result, fewer holes were produced

when high spindle speeds and slow feeds were used for HSS-Co drilling.

Figure 32-Effect of speed and feed on the hole production for HSS and HSS-Co drills: (a) constant

speed; (b) constant feed


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Figure 33-(a) Drill flank wear versus hole number with different tool materials. The effect of (b) feed

and (c) speed on the carbide drill flank wear at the 20th hole

Hole Quality Parameters

Hole quality in Ti alloys and composite material were evaluated in terms of ma-

terial integrity, hole diameter and roundness, Ti burrs, and surface finish on the

first hole drilled at each feed and speed. Fig. 1 presents hole quality features of

Gr/ Bi- Ti stacks. The first holes made by fresh drills at each drilling condition

were used to verify the effect of feed, speed, and tool materials on hole quality

parameters. The hole diameter error can be readily calculated as the difference

between the actual hole diameter and the specified drill diameter. Hole geometry

and diameter were measured with a coordinate measuring machine (CMM)

mounted with a 1 mm Renishaw ruby tipped, spherical probe, and a Control

Console, which displays all system functions and measurements. At least 40

points were measured to obtain the least square diameter and roundness at a


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given depth in the hole. For each hole, both diameter and roundness of a hole

were measured at every 10-15 Ilm on the entire stack hole. Optical microscopy

was utilized to observe the drilled hole quality. Surface formation and topography

were characterized in terms of surface describing parameters from the surface

profile measurements. The surface roughness across the depth of the hole walls

were me asured on a Surfanalyzer System 4000. Surface roughness measurements

were performed on both Gr/Bi and Ti alloy materials. Profilometer operation

parameters consisted of a 0.80 mm cut-off length and 0.25 mm/sdrive speed. The

surface roughness parameters such as, average surface roughness ((RRaa)),, average root

mean square (Rq), average peak-to-peak height (Ry), and average roughness

value of lO-point height (Rz), were measured and recorded. ln order to measure

the burr height, an electronic digital height gage was used. Unlike HSS-Co drills,

carbide drills showed the least amount of tool wear because carbide has a greater

hot hardness. The tool-life criterion is defined as the carbide drill flank wear

length exceeding a value of 400 flm. It is obvious that the carbide drill wear

increases exponentially with increase in the feed and spindle speed. The thrust

forces produced by a carbide drill did not increase drastically with increase in

number of holes, as HSS-Co did. This is due to the greater hot hardness of

carbide. Consequently, high speeds and high feeds had negative effect on the too

1 life of carbide drilIs, because heat generation was not a major factor on the

carbide tool wear.

Selection of Drilling Conditions

To select the feed and speed range, preliminary experiments were performed at

0.03-0.25 mm/rev and 325-2750 rpm, respectively. If a drill made at least one

hole on the stack at a given condition, the condition was passed (P). If not, the

condition was failed (F). Fig. 3 present the passed or failed conditions for both

drills. At the combination of low speed and high feed, both drills failed to make a

single hole. HSS-Co drills were not successful at making holes over 2720 rpm.

Selected feeds were 0.08, 0.13, 0.20, and 0.25 mm/rev.


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Hole Diameter and Cylindricity

The average hole diameter, which defines the size tolerance, was measured by the

CMM at a given depth of an entire hole. A roundness (circularity) criterion

specifies a tolerance zone bounded by two concentric circles within which each

circular element of the surface must lie and applies independently at any plane.

Cylindricity is a surface of revolution in which all points of the surface are

equidistant from a common axis.

Fig. 4(a) and (b) show average values of the typical stack hole diameter error,

which is a subtraction of the actual diameter from the drill diameter, by both

drills with various feeds and speeds. Generally, HSS-Co drills have a tendency to

produce undersize holes, whereas holes made by carbide drills tend to be larger

than the drill size. The amount of oversize increases with increasing feed and

speed in carbide drilling. This phenomenon could be due to vibrations induced at

higher feed and speed. Fig. 5 show the typical hole profiles measured at the

machined stack hole for both drills. Generally, variation in the profiles tends to

be higher for the Gr/Bi holes than Ti holes. So, the hole roundness of Gr/Bi- Ti

stacks is dependent on the Gr/Bi plate. li can be seen that the variation of the

profiles of holes by HSS-Co drills is higher than carbide drills. Speed has an effect

on cylindricity of drilled holes in that higher speeds produce larger cylindricity.

Feed is negatively proportional to cylindricity, because roundness deviations of

Gr/Bi holes were reduced on higher feed for both drills.

Workpiece Damage

HSS and HSS-Co drills produced workpiece damage inc1uding fibre exposure on

the Gr/Bi- Ti interfaces and Ti burrs. By placing the Gr/Bi on top of the Ti

alloy, the exit plies of the composite were supported. Thus, as the drill exited the

composite there is very little delamination of bottom plies.

Two types of material damage were present at the exit surface of the Gr/Bi. The

first damage was the discoloration ring located around the hole and the second

can be referred as a damage ring, which was induced by both heat and metal

chips. The discoloration zone may be due to heat generated during drilling,


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however, there was no fibre breaking and fibre s were not exposed at the

discoloration ring. Fig. 8 shows micrographs showing damage at the interface

surface of Gr/Bi specimens. The Gr/Bi damage was noticed when the HSS and

HSS-Co drills were used, but minimal damage took place when carbide drills were

used, at least for the amount of holes produced using carbide drills. During

drilling, damage rings, located iu exit plies of the composite at the Gr/Bi and Ti

alloy interface, were formed. The damage radial distance was measured, where 0

is the diameter of the damage obtained by subjective averaging and, D is the hole

diameter (Fig. 8(a)). The radial distance of both the discoloration rings and

damage rings increased as the number of holes drilled increased when both HSS

and HSS-Co drills were used. In Fig. 8(b), the damage revealed that the fibre s

were exposed as a result of the matrix overheating, causing a defect in the

material. Also, as shown in Fig. 8(c), delamination induced by both heat and the

metal chips can be observed at the exit ply. Therefore, the damage was minimal

when the carbide drills were used because carbide had a higher hot hardness com-

pared to HSS and HSS-Co drills, thus the cutting edges remained sharper,

shearing away material more efficiently and generating less heat.


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Figure 34-(a) Gr/Bi damage region at the Gr/Bi-Ti interface at 0.08 mm/rev and 1750 RPM (The

Gr/Bi damage radial distance, t, is specified as t=(fi-D)/2) (b) The top view of Gr/Bi damage region

produced by HSS-Co drill at 0.08 mm/rev, 2720 RPM (microscope) , (c) SEM

Figs. 9(a) and (b) show that the damage radial distance decreases as the feed

increases when HSS drills are used. In case of HSS-Co drills, the trend with

various feeds seems to be unclear, but the damage radial distance is large at slow

feed, under 400 rpm, and decreases at higher feed. This damage was caused by

localized heat generation in the Ti alloy around the cutting zone. The lower feeds

resulted in longer tool engagement times between the tool and the workpiece.

Longer machining times led to more heat generation in the Ti alloy. The radial

distance increased with increasing speed for both HSS and HSS-Co drills. Damage

by carbide tools was negligibly small, thus it was not measured.


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Figure 35-Damage radial distance at the Gr/Bi interface

Burrs

Ti alloy burrs were found to depend upon spindle speed and feed. The exit burrs

of Ti were induced on hole quality parameters in Gr/Bi-Ti stacks. Generally,

carbide drills produced smaller exit burr heights, which are under 1 mm in all

conditions. li is dear that speed has a major influence on the exit burr heights.

The exit burr heights increased as spindle speed increased and slower feeds

produced higher exit burr height. This could be due to the low thermal

conductivity of Ti, which heat up rapidly during drilling but does not dissipate

the heat quickly. Heat generation was the primary concern when drilling Gr/Bi-

Ti stacks, because high temperatures in the drilling region negatively effect too!

life and exit burrs. The formation of a Ti burr was affected by several factors

including thrust force and friction heat generated when drilling at high speed,

long tool engagement time, and slow feeds. Prior study proved that thrust force

and feed rate are major factors in exit burr heights. At the same thrust force,

lower feed rate produces higher burr height. Burr height is proportional to thrust

force at the constant feed rate. As the drill pierced through the bottom surface of

the Ti alloy material, the bottom surface layer of the Ti alloy became quite hot

and the ductility increased, allowing the material to flow easier. Also, the last


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thin layer of Ti alloy did not get sheared properly because the cutting edges of

the drill became worn out. When the worn drill exited the Ti alloy, the cutting

edges pushed the Ti alloy outward, creating the burr around the perimeter of the

hole. Rapid tool wear in HSS-Co drills had induced higher thrust force and

frictional heat which in turn produced the largest burr heights at high speed and

low feed. However, carbide drills experienced the 1east wear and resu1ted in the smallest burr heights.

Figure 36-Entrance/exit burr height at the first hole of Ti alloys with various feed and speed: (a)

constant speed, 660 RPM; (b) constant feed (0.08 mm/rev); (c) constant speed (660 RPM); (d)

constant feed (0.08 mm/rev)

Surface Quallty

The surface roughness parameters in Gr/Bi are much higher than those in Ti.The average surface roughness, Ra, in Gr/Bi ranged from1.23 to 5.78 r.tm, while

those in Ti ranged from 0.48 to 2 r.tm. Also, surface roughness of Ti is very close

in both HSS-Co and carbide drilled holes. Surface roughness parameters of Gr/Bi


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are critical in drilling of Gr/Bi-Ti stacks, so Rv (maximum peak-to valley) of

Gr/Bi was chosen to quantify the surface characteristics of drilled holes. Fig. 6

show the effect of feed, speed, and drill material on the surface roughness

parameter, Ry. Overall, HSS-Co drilled holes have higher surface roughness

parameters values than carbide drilled ones, which do not exceed 15 ~lm. Drill

speed is a major factor in Ry of Gr/Bi in HSS-Co drilling, while feed effects

mostly in carbide drilling. It was observed that HSS-Co drill wear was drastically

increased over 1115 rpm, and much heat was generated cutting. In the case of

holes made by carbide drills, drill wear and heat generation were not severe

enough for speed to affect the surface roughness parameter. For both drills,

rougher surface were produced at high feed. In fibre reinforced composites, deeper

fibre pullouts occur while the depth of cut is getting larger.

Fig. 7 show the optical micrographs of a typical surface of the sectioned holes

made by HSS-Co and carbide drills. The surface damage generated in drilling

Gr/Bi is dependent on the relative angle between fibre orientation and the

direction of cutting motion. Fibre pullout, which caused a pitting phenomenon,

general Jy occurred with the fibre at a negative angle to the cutting direction. In

case of positive angle between fibre orientation and cutting direction, the surface

produced tends to be smooth and usually covered with matrix smearing and

crushed fibres. As the depth of pits is dependent on the manner in which the

cutting load was applied and the relative angle between fibre orientation and

cutting direction, it is expected that the holes made by HSS-Co drills have deeper

fibre pullout than those by carbide drills. Torque by HSS-Co drills is at least 40%

higher than that by carbide drills. It can be seen that deeper fibre pullout regions

occurred in -45 plies and 90 plies on a hole made by HSS-CO drill. The

surface made by carbide drills has swallow fibre pullouts only at -45 plies.


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Figure 37-Surface roughness profiles

machining of composite materials - state of the art

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