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

54

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

61

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

72

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