42
CHAPTER -5
EXPERIMENTAL INVESTIGATION
CONTENTS
5.1 Matrix Materials: Aluminium Alloys
5.2 Production of the Reinforcement Particles
5.3 Composite Preparation
5.3.1 Composites furnace details
5.3.2 Stirrer Detail
5.4 Preparation of the composites
5.4.1 Stirring procedure
5.5 Heat treatment
5.5.1 Apparatus
5.5.2 Heat treatment Procedure
5.6 Microstructure (ASTM F2450-04)
5.6.1 Specimen preparation
a. Sectioning
b. Grinding
c. Etching
5.6.2 Optical Microscopy Test (ASTM F2450-04)
5.6.3 Transmission Electron Microscopy Test (ASTM D5756-02)
5.7 Density Measurement
5.8 Tensile Test
43
5.9 Compression Test
5.10 Micro-Hardness Test
Test specifications
Theory of operation
Experimental procedure
Precautions
5.11 Electrical conductivity / resistivity
Measurement Technique
Test Specifications
Theory Of Operation
Instrumentation And Experimental Procedure
Precautions
5.12 Damping studies
5.12.1 PUCOT Method
5.12.2 Resonant Bar Method
General theory
Internal friction (Q-1)
Mechanical Loss ()
Damping measurement
5.13 Coefficient Thermal expansion
Test Specifications
Theory of Operation
Experimental Procedure
44
Precautions
5.14 Wear test
5.15 Corrosive- Errosive Tests
5.16 Machinability Test
BIBLIOGRAPHY
45
5.1 Matrix Materials: Aluminium Alloys
Al 6061 is a precipitation hardening aluminum alloy, containing magnesium and
silicon as its major alloying elements. Al 6061 alloys possess good mechanical properties
with ductility and also they are easily weldable alloys. Due to their good properties they
find many applications in different fields and areas. Pretempered Al 6061 alloys means
solutionized, solutionized and artificially aged are purchased (solutionized, stress-
relieved stretched and artificially aged).
The alloy composition of 6061 is
Silicon minimum 0.4%, maximum 0.8% by weight
Iron no minimum, maximum 0.7%
Copper minimum 0.15%, maximum 0.40%
Manganese no minimum, maximum 0.15%
Magnesium minimum 0.8%, maximum 1.2%
Chromium minimum 0.04%, maximum 0.35%
Zinc no minimum, maximum 0.25%
Titanium no minimum, maximum 0.15%
Other elements no more than 0.05% each, 0.15% total
Remainder Aluminum
5.2 Production of the Reinforcement Particles
To minimize oxide content, the as-received -325 mesh nickel and -325 mesh
nanoclay powders were reduced in a hydrogen atmosphere at 500ºC for one hour in a
furnace. For the same reason, the powders were stored and milled in an argon
atmosphere. De-agglomeration was carried out using ball mill with nanoclay and
46
Fig.5.1: XRD pattern of Nano NANOCLAY particulates
7/16” diameter steel balls). In each cycle, 11.90 grams of equii-atomic nanoclay
powder was loaded into the vial along with eleven stainless steel balls for a charge
ratio of 6:1. (Charge ratio is defined as the ratio of grinding media mass to powder
mass.) The mill was run for sixty hours, but interrupted after one, three, five and ten
hours for the purpose of flipping and rotating the vial to avoid powder accumulation .
X-ray diffraction analysis of the final, as well as starting and intermediate,
powders was performed using a PANalyitcal X’Pert PRO X-ray diffractometer system.
Scans were performed using fixed slits (0.04 rads) and an incident scatter slit of 1 degree.
The XRD sample stage was rotated at a specified speed (1rev/min).
Three stages of XRD was carried-out on the nanoclay
1) Un-milled,
2) Ten hours milling and
3) Sixty hours milling.
47
Figure.5.1 (XRD) showed that Si and oxygen peaks in the first case sample
prominently. For 10 hours milling nanoclay shows Si and oxygen peaks but lesser
intensity.
But width of peak reduced, this shows the crystalinite size of the nanoclay decreased,
which indicates that the agglomeration drastically reduced during milling. The angle of
the peak indicates there has been significant diffusion of larger oxygen atoms into areas
of silicon. Due to limitations on the amount of oxygen that can dissolve into the
amorphous Sio2 phase during mechanical alloying, some regions of high O2 concentration
will remain in the powder particles. These regions are manifested in the X-ray diffraction
plot as smaller peaks are shifted to slightly greater angles as compared with the unmilled
sample. These O2 peaks are wider, indicating that the size of crystallites are small. SEM
micrographs of the material at various stages along the path to the final composite are
shown in Fig. 5.2 show the Nano clay after milling. Fig.5.3 displays TEM micrographs of
milled nanoclay.The TEM analysis was limited to the nano particles since the larger ones
were not electron transparent.
Fig. 5.2 SEM Photograph shows NANOCLAY particle
48
However, this analysis should be generally applicable. After XRD and TEM realized that
the purity of NANOCLAY is 95 % and particulate size is 100-200nm
5.3. Composite Preparation
5.3.1 Composites furnace details
The furnace used to prepare the Al MMCs was of the side tilting type. It has an
aluminite crucible fitted in the middle portion, into which the metal is placed for melting.
On the top portion, a motor attached stirrer is placed and there is a provision made for
lowering down of the stirrer into the furnace through the lid.
The opening and closing plug is attached to furnace. The railings are fixed to rails
stand of the furnace and the casting dies/ moulds can be moved on the rails. Also, there
is provision made for the passing of inert gases into the furnace. Central control panel is
provided with all the necessary electrical connections, indicators, controllers, etc.
The furnace is electrically heated 3-phase resistance type with a 12 KW capacity
fitted with three pairs of 14-gauge kanthal Al grade heating coils. The maximum
Fig. 5.3 TEM images for NANOCLAY particulates
49
temperature of the furnace is 1100 C and fitted with integrated differential digital
temperature controller.
5.3.2 Stirrer Detail
The stirrers of centrifugal type with three blades were welded at 45 inclination
and 120 apart. It is coated with alumina and this is necessary to prevent contamination
of the non-ferrous melt into which the stirrer will be dipped during the process.
5.4 Preparation of the composites
Al 6061 alloy and nonclay (30-70nm) were used as matrix alloy and
reinforcements for preparation of composites. Nanoclay of 5-20 wt. % (interval size 5%)
is reinforced with the Al nano clay composites. The alumina crucible containing with the
stainless steel impeller was coated with alumina. The charge of about 5kg was melted
under a flow of argon (5 lts / min) of high purity. The argon gas was also used to create
inert atmosphere to avoid oxidation. The impeller speed was maintained at range
between 400-600 rpm. During stirring, the preheated nanoclay was poured in to the
molten metal for mixing uniformly. Once stirring was completed, the furnace was tilted
and melt was poured into the cast-iron die.
5.4.1 Stirring procedure
Figure.5.5 shows the temperature-time curve indicating the trajectory and the
temperature excursions used in these experiments. As pointed out in Fig .5.4, there are
various phases of the processing scheme.
Phase 1: Maintained molten melt for 60 min in inert atmosphere.
Phase 2: The melt is brought to the stirring temperature (700 C). The stirring temp. was
above the Al molten temperature and complete procedure are given in Fig. 5.4
50
Fig 5.4 Schematic representation of the temperature-time sequence for
Composite preparation
Phase 3: The reinforcement phase is introduced into the melt (slurry) in the semi
solid range such as, shown in Fig 5.5 as Tss (5 min)
Phase 4: From the temperature (Tss, the composites were re-melted to a temperature
above the liquidus temperature (Tmax = 700-720 C). (10 min)
Phase 5: Poured into the mould and suddenly cooled (15 min)
In order to establish the processing macro and microstructure relationship the processing
conditions are as shown in Table 5.1.
Table 5.1 Stirring parameters
Stirring Temperature Stirring Speed Stirring time (seconds)
675C 500 rpm 600 seconds
0
500
1000
50 60 70 80 90
Stirring Addition of Particulate
Phase 1
Phase 3 Phase 4
Tmax
T-liquid
Tss
P
h
a
s
e
2
Time in min
Tem
per
atu
re in
oC
51
5.5 Heat treatment
5.5.1 Apparatus
Electrically heated Furnace with an air chamber as shown in figure 5.5 was used
for the heat treatment of Al matrix alloy and Al-Nanoclay (5%, 10%,15% & 20% wt.)
composites.
Fig.5.5 Ageing Furnace
The furnace is designed such that there is no direct radiation between the heating coil and
the Al alloy. The furnace is provided with temperature controls to maintain the specified
temperatures within 5C accuracy.
The furnace has a separate manual reset safety-cutout, which will turn off the heat
source in the event of any malfunctioning or failure of the regular control equipment.
This safety cutout is set as closely as practicable above the maximum temperature for the
Al being heat-treated. Protective device was installed to turn off the heat source in case
of stoppage of circulation of air, and they were interconnected with a manual reset
control.
52
Generally, after ageing, the Al alloy and composites, are cooled in air media.
Uniform cooling all over the charge is maintained using a fan inside the furnace.
5.5.2 Heat treatment Procedure
Cylindrical tensile specimens with length and diameter of 10 mm and 20 mm
respectively were prepared and aged according to standard as given below (as shown in
the Fig. 5.6), according to ASTM standard .
Fig. 5.6 Drawing of the experimental apparatus used for heat treatment
All the specimens were solution treated at 525C for 24 hours (A-B) in a protective
atmosphere (Nitrogen)
Water quenched (B-C) (temper).
Natural ageing for 24 h at room temperature (C-D)
The specimens were aged at 175 C for different time (E-F)
Finally cooled at room temperature (F-G)
Tem
per
atu
re
in o
C
Time in hrs
0
100
200
300
400
500
600
0 10 20 30 40 50 60
Solution treated
Wat
er Q
uen
ch
Natural ageing
Ageing
A B
C D
E F
G
53
5.6 Microstructure (ASTM F2450-04)
5.6.1 Specimen preparation
a. Sectioning
Specimens were removed from the metal mass by specimen cutter, care was
taken to prevent cold working of the metal, which can alter the microstructural and
complicate interpretation of constituents.
b. Grinding
The rough polishing is done by series of abrasive belts made up of SiC sand
belts. The polishing specimen was done in two stages, rough polishing and finish
polishing. For rough polishing, emery belts of 100, 200, 400, 600, and 1200 (0-emery
paper) were rotated on 500–600 rpm.
In dry grinding, care was taken so that the specimen was not overheated, which
otherwise will affect the microstructure. Progressing from one grit size to the next, the
specimen was turned through 90 and was cleaned with cloth saturated with a water-
soluble ethanol. Polishing machine wheels are used for both polishing stages consists of a
medium-nap cloth (washable cotton), a suspension of MgO size of 5 m particles mixed
in distilled water (50 g per 500 ml of H2O) was used on the wheel for smooth polishing.
Specimens were made to rotate opposite direction of smooth surfaces. Finally, for finish
polishing, a diamond paste (1 m) was used on the wheel.The specimen also rotated
about its own axis across the face of the polishing wheel. And cleaned with alochol then
dried and finaly etched.
c. Etching
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Etchants and etching times used for micro-examination depend on the
composition, phynanoclayal condition and temper of the specimen. Some important
etchants used for microscopic examination of Al alloy and Al composites are given in the
Table 5.2.
Table 5.2 Selected etchants for microscopic examination of Al alloy
5.6.2 Optical Microscopy Test (ASTM F2450-04)
The optical microscope (Fig.5.7) remains the most important tool for the study of
microstructure.
Fig. 5.7 Optical Microscope
Etchant Composition Etching procedure Characteristics and use
Nital
1 to 5 ml HNO3
(con, 100 ml
ethanol (95%)
Immerse specimen for a few
seconds to 1 min. Wash in
water, then alcohol and dry.
Shows general structure
Acetic
Glycol
20 ml acetic
acid, 1 ml HNO3
(con), 60 ml
ethylene glycol,
20 ml water
Immerse specimen face up
with gentle agitation for 1-3
s for as-cast or aged metal
and up to 10 s for solution-
heat-treated metal. Wash in
water, then alcohol and dry
Shows general structure and
grain boundaries in heat-
treated casting.
HF
10 ml HF
(48%), 90 ml
H2O
Immerse specimen face up
for 1 to 2 s. Wash in water,
then alcohol and dry.
Darkens Mg17 Al12 Phase
and leaves Mg32(Al Zn)49
phase white.
55
It was used to study the microstructure of both matrix and composite surfaces. All
examinations of microstructure began with use of the optical microscope starting at low
magnification, such as, 100 X followed by progressively higher magnifications to assess
the bananoclay characteristics of the microstructure efficiently.
5.6.3 Transmission Electron Microscopy Test (ASTMD5756-
02)
Transmission electron microscope (TEM) could be used in applied and
fundamental research in materials science and phynanoclayal metallurgy as shown in Fig.
5.8. TEM was studied on 1 m or 10 . This is particularly applicable to imaging of
crystal in homogeneities, such as, lattice defects, and precipitates. The microstructures of
the Al alloy matrices in the composites were examined by TEM. Thin foils of the
composites were prepared after mechanical grinding to 100 m followed by twinjet
polishing using methanol 25 % Nitric acid mixture was maintained at -40 C and then
chemically polished in a cold HNO3 solution (<250 K) until a hole appeared.
Fig. 5.8 Transmission Electron
Microscope
56
5.7 Density Measurement
Machined and polished composite specimens (10mm diameter and 5mm length,
sample size were considered for density measurement using the Archimedean method at
room temperature (27C and relative humidity of 48%). The beaker with water is
initially kept on the electronic balance (accuracy 0.1 mg) set to read zero. The specimen
initially weighed (W1) and suspended freely into the water filled beaker. The weight of
specimen in the water (W2 mg) shown by the balance represents the volume of the
displaced water (specific gravity of water =1) is equivalent to the volume of the
specimen. The ratio of W1 to W2 represents the density of the specimen.
5.8 Tensile Test (ASTM E8-82)
The tensile properties of the materials viz, ultimate strength and ductility were
evaluated using a standard 40 kN capacity servo hydraulic universal testing machine as
shown in Fig. 5.9(a).
The tensile load was applied in a parallel direction to specimen. In a stress-strain
graph, it can be classified into two: first one strait line (elastic limit) and second region
plastic region (curved shape) as per Hook’s law.
The Young’s modulus were computed. As the load is increased beyond elastic
region then stress is in plastic strain i.e. where Fp = load at proportional limit and Ao is
the area. The maximum stress developed inside the specimen without changing
dimension of the specimen significantly.
Yield point is the maximum point at that point the tensile specimen starts to
deform without increasing the load. Ultimate tensile strength is the maximum stress that a
test specimen as shown in Fig. 5.9(b) can bear before fracture and is based on original
57
area.
The procedures for tensile test are described below.
Fig. 5.9 (a) Universal Testing Machine
64 20 20 7 7
8 12
All Dimensions are in mm
58
Fig. 5.9 (b) Tensile specimen with dimension
Tensile test was conducted as per the ASTM E8-82 standards.
The average area of the specimen was determined using the micrometer.
Then a line was scribed along the bar and the gauge length was marked 180mm
symmetrical with length of bar.
This was divided into twelve equal parts and punch marks were made on these points.
Upper end of the specimen was firmly grip with fixing shackles.
The specimen was placed such that punch marks faced the front of machine.
The lower end of the specimen was gripped taking care not to disturb the fixing of the
extensometer.
Suitable loads were selected in steps of 100 kg and strains are noted.
The load was applied at low speed taking simultaneously observations of load and
strain without stopping the machine.
The failure characteristics after removing the broken specimen from the testing
machine was studied.
The dimension of smallest section was measured, the parts were held together and
gauge length and length between the shoulders and diameters were measured.
All calculations are made and a graph of stress v/s strain was plotted.
5.9 Compression Test (ASTM E9)
The compression tests were conducted on specimens of 12mm diameter and 20
mm length machined from the cast composites (as shown in Fig.5.10). The compression
59
test was conducted as per ASTM E9 standard using universal testing machine
5.10 Micro-Hardness Test (ASTM E 384)
Microhardness test was conducted using a Leitz Wetzlar microhardness tester
equipped (Fig.5.11) with a Vickers diamond pyramid indentor.
Fig. 5.11 Microhardness tester
The illumination and magnification of the microscope attached to the hardness-
testing unit were adjusted prior to taking any measurements. The sample with its polished
surface was placed on the stage and a proper area for indentation was selected. Then, the
12 mm
20 mm
Fig. 5.10 Compression test specimen
60
indentor was brought down to touch the specimen and indentation was made.
The microscope was brought back to focus on the sample surface and the two
diagonals of the pyramid indentation made on the specimen were measured using a
calibrated scale (Least count is 1 m) from which the mean indentation was obtained.
The sample size of six for each condition were taken. Knowing the indentation
length and applied load, the Vickers hardness values (Hv) were obtained from the lookup
tables.
Test specifications
1. ASTM Standard : ASTM E 384
2. Equipment used : Leitz Wetzlar (Germany) micro hardness
tester provided with a Vickers diamond
pyramid indentor.
3. Specimen dimension : 20mm diameter x10mm height
4. Load applied : 1 N
Theory of operation
The Vickers hardness number (HV) is the ratio of the load applied to the indentor
to the surface area of indentation
5.1
Where P - Applied load (kgf)
D - Mean diagonal of the indentation (mm)
- Angle between opposite faces of the diamond indenter
(136).
HV =
2P Sin (/2)
D2
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For a particular load (1kgf), Vickers hardness number is given for indentation of 1 to 200
m, in conversion tables. By measuring indentation produced on the sample, its micro
hardness can be obtained knowing the load applied.
Experimental procedure
The hardness testing unit has a microscope attached to it. Since the indentation
dimension is less than 76m, a fixed load of 100gf is applied on the sample. The sample
surface is subjected to grinding and polishing to attain a smooth metallographic polished
surface. The specimen is placed on the stage of the microhardness testing equipment.
The microscope is focused on to the sample surface to select a particular area on
which indentation has to be made. Then, by a lever movement, the indenter is brought on
the top of the sample. The indenter is then brought down to touch the specimen and
indentation is made.
The microscope is again brought back to focus on the sample surface and the two
diagonals of the pyramid indentation made on the specimen are measured using a
calibrated scale in the interfacing computer and the mean indentation is noted for each
specimen.
A total of six readings were taken and the average value of the indentation length
(discarding highly deviated values of 5%) is obtained. Knowing the indentation length
and applied load, the Vickers hardness values (HV) are obtained from the look up tables
[1].
The microhardness measurements were undertaken for the unreinforced Al 6061
alloy and composites with 5, 10, 15 and 20 weight % nanoclay reinforcement.
Precautions
62
While making an indentation care must be taken to avoid a reinforcing
Particle in the composite specimen, otherwise a great scatter in the hardness
value is obtained.
Load to be applied must be selected suitably for a particular material so that
an optimum indentation size is obtained.
The indentations made on a particular sample should be sufficiently spaced
away from each other. When the load is 1N or less, a metallographic finish is
mandatory so that the indentation is clearly defined.
5.11 Electrical conductivity / resistivity
The electrical conductivity/resistivity are widely investigated properties in
advanced material research. There are several applications like high electrical/ thermal
conductivity of metals and alloys where exploited. The study of electrical behaviour of
metal matrix composites becomes important when these properties have to be combined
with good mechanical properties.
Metal matrix composites possess high electrical conductivity due to the presence
of electrons as charge carriers. In the analysis of MMC resistivity, the particulate
resistivity is generally considered to be several orders of magnitude greater than that of
the matrix alloy since reinforcement is usually a ceramic material.
Hence, the electrical resistance of the interface has no significance in the
conduction process. Further, grain growth, precipitation and structural defects greatly
influence the electrical properties of MMCs.
Measurement technique
The apparatus designed to measure electrical resistivity of the composite
63
specimens in the present work is based on standard four point probe technique, the
schematic diagram of which is shown in Fig 5.12.
Fig. 5.12 Schematic diagram of four point probe instrument
The experiment details are
Test specifications
1. Method : Four-point probe
2. Specimen dimension : 70mmx 5mm x 1mm
3. Equipment used
: Keithley 228A voltage/ current source
Keithley 196 system DMM
Hewlett packard 34401A
Electrical multimeter
Furnace
64
4. Parameters studied
: Resistance measured as a function of
temperature in the temperature range 30C-
300C and measurement of resistance at room
temperature for the aged samples.
C1 & C2 – Current flow
P1 & P2 – Potential
Theory of operation
The resistivity of metals/metal matrix composites is usually very small of the
order of few micro ohms. Therefore, while measuring the electrical resistance of these
materials, the contact resistance becomes much more than the actual resistance to be
measured. Therefore, in the four-point probe method, current and voltage connections
are separated. The sample resistance is calculated from the measured values of voltage
and current using Ohm’s Law. Knowing the dimensions of the samples, electrical
resistivity is calculated.
Instrumentation and experimental procedure
A hot plate with a heating coil embedded inside is mounted horizontally on a
wooden enclosure. The plate can be heated to any desired temperature up to 350C by
passing a current in it through a variable transformer. A chromel - alumel thermocouple
junction is attached to the hot plate and the leads are connected to a Hewlett Packard
multi meter (Digital). Four conducting springs are drilled through a bakelite strip in a
row, the inner two being separated by about 25mm while the outer two equidistant
(10mm) from them.
The top heads of these springs are connected to four terminals fixed outside the
enclosure using tungsten wires. Among these, the two inner terminals are connected to
65
digital micro voltmeter (keitheley 196 systems DMM) and outer two terminals are
connected to a Keithley 228A voltage/current source. The specimen is mounted
horizontally on the hot plate, adjacent to the thermocouple.
The bakelite strip is brought down, so that the free ends of the springs make
contact with the sample and the strip is held firmly in the same position. A constant
current of 2A is passed through the specimen and the corresponding voltage developed is
measured using the micro voltmeter. The average of the six values is taken to avoid any
thermo emf effects. The dimension of the sample is measured using a digital slide
calipers. Hence, the electrical resistivity is calculated in each case using the formula,
= (Rwd) /L m 5.2
Where R= Resistance in ohms,
w= breadth of the sample, mm
d= Thickness of the sample, mm
L= separation between the voltage probes on the sample, mm
The experiment is performed for both as cast and heat-treated samples. In each case, the
temperature is varied from 30C to 300C and the electrical resistance is measured as a
function of temperature. The emf recorded by the multi meter is converted to
corresponding temperature reading using a chromel- alumel conversion chart.
Further, the electrical resistivity measurements were performed at room
temperature for composite samples aged at various temperatures for different intervals of
time.
Precautions
The thickness of the sample should be as small as possible so that sufficient
voltage drop is obtained across the voltage terminals.
66
Fig. 5.13 Schematic drawing of
PUCOT
Fig. 5.14 Damping setup
Drive Crystal (D)
Gage Crystal (G)
Specimen
Proper insulation is provided inside the enclosure so to minimize heat losses.
5.12 Damping studies
Damping Properties has been studied by two methods
1. PUCOT Method
2. Resonant Bar Method
5.12.1 PUCOT Method
Preliminary dynamic shear modulus values were calculated using an assumed
value of Poisson’s ratio of 0.3 and values of dynamic Young’s modulus (E) were
obtained. Specimens are trimmed by using low-speed saw. Measurements of dynamic
shear modulus, damping and the strain amplitude for each specimen were made with the
Piezoelectric Ultrasonic Composite Oscillator Technique (PUCOT).
The PUCOT uses the piezoelectric properties of -quartz crystals. A drive
67
voltage (Vd) from a closed loop crystal driver excites the drive crystal (D) shown in the
Fig 5.13, and Fig. 5.14 to a resonant frequency in the longitudinal mode. The gage
crystal (G), which is attached to the drive crystal with cryanocrylate glue, detects the
vibration as a voltage (Vg) measured by the closed loop crystal driver. A Hewlett
Packard 5302A frequency counter measures the period of the gage crystal.
The crystals are supported by a jig as shown in Fig 5.13 and Fig.5.14. For
measurements of dynamic E and Q-1
, the drive and gage crystal system (DG) was set to
resonate. Values of Vd, Vg and DG (resonant period of DG System) were noted. Then
the specimen (S) of the specified length was attached to the bottom of the gage crystal
with cyanoacrylate glue. The resonant length at room temperature was determined using
the density found by the Archimedes technique and by using the flexural modulus as an
approximate elastic modulus. Possible selected values of the period () were determined
by the availability of the Quartz crystal. The frequencies of crystal used in this study were
60 kHz for the borosilicate glass and 80 kHz for the composites.
The DGS system was adjusted to resonate, and values of Vd, Vg, and DGS
(resonant period of DGS system) were noted. Form this measurement, values of E,
Q-1
and strain amplitude were found. It should be noted that the damping, Q-1
, is defined
as W/2W, where W is that energy dissipated in a full cycle of vibration and W is the
maximum stored energy per cycle (on unit volume basis).
5.12.2 Resonant Bar Method
High damping capacity and lightweight metals have potential uses in weight-
critical structures such as, aerospace, automobile applications. The quantity (E/)1/2, E-
elastic modulus and -density, implies for higher specific stiffness, results in a higher
68
natural frequency and high damping capacity for the component.
The particle reinforced MMCs are one of the important way of getting lightweight high
damping structures.
General theory
The damping capacity can be measured based on four methods:
1) Mechanical loss angle (),
2) Loss tangent (tan),
3) Internal friction ( Q
-1), and
4) Logarithmic decrement (), they are usually used to describe the damping
capacity. In present study the damping capacity described internal friction as below
Internal friction (Q-1
)
Measurement of successive strain amplitudes from the oscilloscope will then yield
the logarithmic decrement as follows.
= ln (An/An+1) 5.3
where An and An+1 are the amplitudes of successive cycles in free decay. The
relationship between , internal friction factor Q-1
, and damping ration is given by
= /2 5.4
Q-1
= 2 = / 5.5
Mechanical Loss ()
The phase log of the strain response behind the stress excitation known as the
mechanical loss angle, the tangent of which is a measure of the fractional loss of
mechanical energy per oscillation cycle.
69
Tan ∅ =δ
π= Q−1 =
lnA i
A i+1
π 5.6
By using resonant-bar techniques, damping capacity (internal frication) loss as a function
of temperature, can be found.
Damping measurement
The flexural resonant bar system was used for determining damping properties of
Al/nano clay MMCs as shown in Fig.5.15. A cantilever plate is excited into fundamental
mode of flexural vibration by an exciting steel ball. After the exciting steel ball hits the
specimen, the amplitude of vibration gradually decreases with time as the vibration
energy is dissipated.
Fig.5.15 Schematic Diagram of the resonant bar damping system
These decayed amplitudes and frequency of vibration are transferred to oscilloscope and
computer by an accelerometer attached on specimen’s end. The temperature of specimen
was measured a thermometer and controlled by a temperature controlled furnace
chamber. The tests were conducted at the temperature between room and 300 C of
Hitting ball
Specimen
Accelerometer
Temperature-
controller
Chamber Temperature
indicator
Computer
through interface
Printer
Oscilloscope
70
interval of 50C.
5.13 Coefficient Thermal expansion (ASTM E831)
The coefficient of thermal expansion of the composites as well as the
unreinforced matrix alloy is determined using thermal mechanical analyzer (TMA)
equipment shown in Fig. 5.16.
Fig. 5.16 Thermal Mechanical Analyzer
The details of the experiment are as follows.
Test Specifications
1 ASTM standard ASTM E831
2 Equipment used Thermal Mechanical Analyzer TMA,
model-943( DuPont )
3 Specimen dimension 10 mm x 5mm x 5mm
4 Temperature range 30C to 400C
5 Rate of heating 5C/ min
6 Method Dimension change or percent linear
change(PLC) or CTE versus temp
Theory of Operation
In the TMA module DuPont 943 equipment, the changes in the linear dimension
71
of a sample as a function of temperature is recorded as percent linear change (PLC),
simply as dimension change or directly as coefficient of thermal expansion (CTE).
Experimental Procedure
The TMA instrument consists of a furnace for heating the specimen and can
operate in the range -70C to 1200C. The sample is mounted at the bottom a sample
holder tube, which is inserted into the furnace. A thermocouple junction is placed in close
contact with the sample to record its temperature. A temperature insensitive quartz probe
is held on the sample at one end and its other end is connected to a Linear Variable
Differential Transformer (LVDT) core.
This probe senses and transmits any small change in the movement of the sample.
A movable core LVDT senses positive and negative deviations of the probe’s position on
the specimen. As the specimen expands, or contracts or otherwise deforms the core on the
probe moves in the annular space of the LVDT. This relative movement produces a
voltage change that is proportional to the linear displacement of the core. The signal is
amplified and processed by a computer data recording system.
The two end faces of the samples were polished with different grits of silicon
carbide papers followed by fine polishing using 1m diamond paste. About six NMCs
specimens were tested. The data were obtained in the form of dimensional change as a
function of temperature in the range 30C-300C, both in the heating and cooling cycles.
The coefficient of thermal expansion (CTE) values were determined on the basis
of calculated slope fit between two selected temperatures on the dimension change versus
temperature curves.
Precautions
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Care must be taken to polish the end faces of the specimens properly so
that better contact is established between the sample and the probe.
Correction to the dimension change of the specimen is applied, if necessary
to account for the dimension change of the probe connecting the sample to
the LVDT.
5.14 Wear test (ASTM G99)
Figure. 5.17 shows the photograph of pin-on-disc type of wear testing machine
used for conduction of wear investigation. The wear specimens are tested under dry
condition. Common methods used to estimate the wear of a specimen are loss of
dimension method, displacement method, loss of weight method and wet wear test.
The specimen is weighed initially and the weight is noted down. Keeping the sliding
distance constant; the sliding pressure is varied by increasing the loads and conducting
the test. The final weight is recorded.
Fig. 5.17 Pin-on-disc test rig
The apparatus consists of a rotating disc (made of EN24 steel of hardness BHN
229 ) of diameter 200 mm which forms the counterface on which the test specimens or
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the pins slide over. Arrangements were made to hold the specimens and for application
of the load on the specimens.
The samples were clamped tightly in the specimen holder and held against the
rotating steel disc. The specimens were cleaned thoroughly and weighed accurately
using a highly reliable and a sensitive balance to an accuracy of three decimals. The
surfaces of the work specimens were observed using a scanning electron microscope.
Disc wear volume was very small, the wear properties of the steel disc are not considered
for analysis.
Cylindrical wear test specimens of diameter 6 mm and length 15 mm were cut,
ground and polished to the required size before testing. The wear tests were carried out
pin-on-disc wear testing machine in accordance with ASTM G99 standards.
The test samples were clamped in the holder and held against the rotating wheel
(made of EN24 steel of hardness BHN 229 ) at a distance of 60 mm from the centre. In
the present investigation, normal loads of 20N, 30N, 40N, 50N and 60N respectively
were applied on the specimen and the speed of the rotating wheel was varied from 200 to
500 rpm in steps of 100 rpm. A standard test procedure was employed for each specimen
as follows:
A standard test procedure was employed for the present wear test.
The specimen of size 8mm diameter and 15mm length was first weighed in an
electronic balance to an accuracy level of 0.01 mg to determine the initial weight.
The specimens were then mounted in the wear testing machine and tested for
different loads and speeds for a duration of 15 minutes.
The specimens were reweighed after the tests to determine the respective weight
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loss through wear.
Each result was obtained from an average of at least three relations.
5.15 Corrosive- errosive Tests (ASTM G67)
The corrosive-erosive wear tests were carried out by corrosive-erosive wear tester
(Fig. 5.18). Three specimens are tested at a time for constant conditions.
Thee specimens of 20mm diameter and 20mm length with smooth finish are
placed at 120 apart circumferentially at 0.1 m radius, to maintain dynamic balance. The
specimen holder was rotated at 1440 rpm using electric motor for maintaining sample
speed of 5 m/s. The experiment was repeated three times for each set of identical test
conditions. The average values of the material removal were computed and travel
distance was varied by varying the duration of the test to obtain sliding distances in the
range of 0.1-100 km. The composition alumina particle was (90-150 m) from 0 to 30-
wt% (interval of 10-wt.%) in the H2SO4 liquid concentrations of 0.01, 0.1 and 1.0 N.
Weight loss was determined by weighing the specimen before and after the tests. Though
the corrosion-erosion wear was measured as the weight loss, the wear rate was calculated
by the following formula.
Vce = W
A t
Fig. 5.18 Corrosive-erosive test rig
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5.7
Where Vce is corrosive-erosive wear rate (cm3m
-2h
-1),
W is weight loss (g),
is specimen density (g cm-3
),
A is the eroded area (m2) and
t is the testing time (hr).
The uncertainty level of the experiment is 2.36 mg.
5.16 Machinability Test
Standard machining tests were carried out by turning the specimens in a CNC
lathe.
Fig. 5.19 CNC machined used for machinability tests
The cutting speeds selected were 200, 315, 400 and 500 rpm. The depth of cut
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was 0.2, 0.5, 0.8 and 1 mm and the feed-rates were 0.1, 0.2, 0.32 and 0.4 mm/rev.
Meanwhile the cutting forces (namely, the tangential, axial and radial forces) were
measured by lathe tool dynamometer.
The tool signature was follows
Table.5.3 Tool Signature
The number of chips produced per gram of the material removed was counted.
Back rake angle 8
Side rake angle 20.5
End clearance angle 12
Side cutting angle 10
Slide cutting angle 75
End cutting angle 80
Nose radius 1 mm