Btp Final Report 123

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EFFECT OF PROCESS PARAMETERS ON

MICROSTRUCTURE, MICRO HARDNESS AND SURFACE

FINISH, FRACTOGRAPHY OF ALUMINIUM ALLOY THIN

STRIP OBTAINED BY TWIN ROLL CASTING PROCESS

A Project submitted for the partial fulfilment of the requirements for

the award of the degree of

BACHELOR OF TECHNOLOGY (HONS.)

IN

MECHANICAL ENGINEERING

BY

B.Venkata Sainath

(10MF3IM05)

Under the guidance of

PROF. S.K. PANDA

DEPARTMENT OF MECHANICAL ENGINEEERING

INDIAN INSTITUTE OF TECHNOLOGY

KHARAGPUR -721302, INDIA

November, 2013



DECLARATION BY STUDENT

I certify that

a) The work contained in this report has been done by me under the

guidance of my supervisor.

b) The work has not been submitted to any other Institute for any degree or

diploma.

c) I have confirmed to the norms and guidelines given in the Ethical Code of

conduct the Institute.

d) Whenever I have used materials (data, theoretical analysis, figures and

text) from other sources, I have given due credit to them by citing them inthe text of report and giving their details in the references. Further, I have

taken permission from the copyright owners of the sources, whenever

necessary.

DATE: Signature of the Student



CERTICATE

This is to certify that the project entitled “Effect of process parameters

on microstructure, porosity, micro hardness and surface finish of

aluminium alloy thin strip obtained by twin roll casting process” submitted

by Mr B. Venkata Sainath (10MF3IM05) to the Department of Mechanical

Engineering, Indian Institute of Technology, Kharagpur, in partial fulfilment for

the award of degree of Bachelor of Technology (Hons.) is a bonafied recordwork carried out by him under my supervision and guidance. The project report

has fulfilled all the requirements as per the regulations of this Institute and, in

my opinion, has reached the standard needed for submission.

DATE: _______________________

Professor S. K PandaDepartment of Mechanical Engineering

IIT Kharagpur.



ACKNOWLEDGEMENTS

I would like to acknowledge and extend my heartfelt gratitude to

my guide, Prof. S. K Panda, Mechanical Engineering Department,

Indian Institute of Technology Kharagpur for guiding me through the

entire course of the project. Without his guidance, I would not have

been able to complete my work. I would like to express my sincereregards and thanks to the entire faculty members of the department.I

would like to extend my gratitude to Prof.Sudipto Ghosh from

Metallurgical & Materials Engineering Department for allowing us to

use twin-roll casting apparatus present in the casting lab of

Metallurgical & Materials Engineering Department.

Date:

B.Venkata Sainath

10MF3IM05

Department of Mechanical Engineering

IIT Kharagpur



CONTENTS

1. Introduction

2. Literature review

3. Preliminary work

4. Objective

5. Conclusions

6. References



Introduction:-

Aluminium alloy sheet metals (strips) are used for car bodies in automobile industry

and aircrafts in aviation industry. Conventional method of producing theses thin strips

involves supplying alloy materials as cast slabs. These slabs are first sawed, scalped and

homogenized or several hours. They are then hot-rolled in single-stand or tandem hot rolling

mills, at a strip thickness of 6-2.5mm (0.24 in - 0.1in) and coiled at a temperature of

approximately C as shown below.

Fig1: Conventional hot strip production

This process is highly capital, energy and labour intensive, especially for producing

thin metal strips. The problem associated with this process is that it involves high production

cost but low productivity. An alternative and more economical named “Twin Roll Casting”has been developed which can overcome the drawbacks in the conventional method of strip

production. This technology has been used for producing stainless steel strips in plants in

Japan and the USA.

A number of new variants of this technology have been developed and some are still

under development. We will be limiting our discussion to the vertical twin roll casting. In this

process, molten metal is directly poured from tundish through a nozzle into the gap between

two rotating cylinders of equal diameter. The melt gets solidified and rolled by the time it

comes out of the gap between rollers. The strip undergoes rapid solidification in this case than

what it does in thin slab casting. If we use roll made of steel, aluminium strip may stick to the

roll. In order to prevent that lubricant needs to be sprayed on the roll. Thermal conductivity of

the roll plays significant role in cooling of the metal strip. If thermal conductivity is high,

cooling rate of the strip is high and so it gets solidified rapidly. It means the casting speed can

be increased.

Steel is not the best material for the roll from the point of the cooling of the strip.

There are few researches about roll materials. The heat transfer between the melt and the rollaffects the cooling condition of the strip. When the heat transfer is large, the strip can be



cooled enough to increase the roll speed. Lubricant is essential in order to prevent the sticking

of the strip to the roll if the roll is made up of steel. However, the use of the lubricant gives

bad effect on the heat transfer between the melt and the roll. Therefore, no use of the lubricant

is better in order to increase the roll speed. So in our experiments, copper material (thermal

conductivity is very high) is used for rollers and no lubricant is used.

Fig2: Schematic of vertical twin roll casting process

Though twin roll casting process is energy-saving, space saving and requires low

equipment cost and running cost, it has some disadvantages. It is difficult to cast aluminium

alloys having wider freezing range using this process. The strip is broken when it comes outof the gap between rollers and cannot be cast continuously. The strip is broken because it is

not cooled enough. In order to cast aluminium alloys having wider freezing range, the cooling

ability of the roll should be large enough. The microstructure of aluminium alloy strip

obtained through this process usually columnar in structure but equiaxed structure is more

suitable to have better mechanical properties. Besides this, the microstructure has inclination

of the grain, centre segregation etc.



Literature Review:-

It has been well known that the twin roll strip casting was first conceived by Henry

Bessemer in 1856. Due to its many advantages, continuous strip casting has become more and

more important during the last 50 years. The modelling of twin roll strip casting process of

aluminium alloys and the effect of process parameters such as rpm of rollers, roll-gap etc, on

characteristics of the strip has been investigated by many researchers. A brief overview of the

research done by various researchers on various defects in the twin roll strip casting of

aluminium alloys is discussed below.

These defects include surface bleeding, hot tearing, buckling, sticking , internal

defects such as channel segregation , deformation segregation and banded structures etc.

Surface bleeds are pockets of solute-rich material which form on the strip surface andcontain a higher concentration of intermetallic. The size of the surface bleed varies from :

0.05 mm long and 0.01 mm deep to 1.5 mm long and 0.1 mm deep. An example is shown in

fig3.

Fig3: Surface bleed in twin roll cast Al-0.3 wt% Fe alloy

Surface bleeds are more frequent and more severe at low specific loads and fine

gauges. Eventually, at high enough loads, surface bleeds become much smaller and less

frequent. The amount of surface bleeds was found to depend critically on alloy. It is proposed

that surface bleeds are the result of a gap opening up between the roll and the semi-solid

sheet. The space is then partially filled with solute-rich liquid. It is suggested that the

susceptibility to bleeding for different alloys is determined by the freezing range of the alloyafter the liquid fraction has reached a few percentage. When the strip is rolled in the caster



any remaining gap between the roll and strip is closed up and the surface bleed appears to

remain relatively undeformed.

Hot tears, also known as hot cracking, are failures in the casting that occur as the casting

cools. This happens because the metal is weak when it is hot and the residual stresses in the

material can cause the casting to fail as it cools. Hot tearing occurs in the twin roll casting of

thin strip, especially in alloys with long freezing ranges, such as AA 5182.Often the tearsfilled with solute rich material, in which case they are visible on metallographic sections of

the sheet. Sometimes, however the tears are only revealed during subsequent deformation,

when they open as cracks in the surface of the sheet.

Hot tearing can be explained as the result of differential rolling in adjacent parts of the

strip. Consider a situation in which the solidification front in the caster advances locally as a

result of a local variation in the heat transfer between the strip and the roll. Material in the

advanced region ahead of the main solidification front will, at any stage of the freezing

process, be less solid and have a higher fraction liquid than the material on either side of it.

When the harder material begins to support a rolling load, it will be extendedhorizontally along the rolling direction as it is reduced in thickness vertically. The softer

material, which forms a continuum with the hard material, will also be extended horizontally

along the rolling direction, but, because it has a high fraction liquid, hot tears will form as a

result of the tensile stresses that are imposed on it by the harder material on either side.

Sticking is also an important problem for the twin roll casting. When the sticking

happens, it becomes difficult to continue the roll casting. The temperature of the roll surface

especially affects the sticking. The strip sticks to the roll as the temperature of the roll surface

becomes higher. The temperature becomes lower as the thermal conductivity becomes larger.The copper roll is suitable for the roll caster than the steel roll in order to prevent the sticking.

If copper roll is adopted in the HPTRC (hydrostatic pressure twin roll casing), the strip does

not stick to the roll without the lubricant. The load affects the sticking. The strip tends to stick

when the load is large. The load of the HPTRC is 1/10 to 1/100 of the load of the CTRCA

(Conventional twin roll casting). The strip is not liable to stick to the roll in the HPTRC.

Some of the major internal defects found in the Al-strip obtained by twin roll casting

are channel segregation, deformation segregation and banded structures. Channel segregates

are cylindrical low melting point regions oriented in the casting direction (see Fig5).

Deformation segregates are equiaxed regions of low-melting-point material distributed in a

central band. (See Fig4). When a banded structure forms, the outer surfaces of the sheet havevery different grain structure and secondary arm spacing when compared to that within a

central band .

All the different morphologies are thought to be the result of the combined

solidification and rolling process. The different structures can be discussed in terms of

decreasing the specific load at fixed sheet thickness. This is equivalent to increasing the

casting rate.

For thick sheet under high loads, defect-free structures are formed. As the casting velocity is

increased the depth of the sump increases. Deformation of the semi-solid leads to liquid being

squeezed back towards the sump. When liquid flows from a cold to a hot region in a casting,



the liquid must change its composition and this melts solid. Flow in one region leads to

melting, to further flow and, thus, to channel formation.

Channels are formed in a number of casting situations where liquid metal flows

between the dendrites from a cold to a hot region. Usually in twin roll casting the channels

are formed in the central plane of the sheet and have an almost constant spacing. As thecasting rate is increased the channels are shorter and occur over a central band. As the casting

rate increases further deformation segregates are formed.

Fig4. Deformation segregates in a twin roll-cast Fig5.Channel segregates in AA6111. (a) lengthwise

AA6111 alloy: (a) longitudinal and (b) transverse section; (b) transverse section.section.



Preliminary work :-

An attempt was made to design the twin roll casting apparatus using a 2×2 gear box

and an idler gear as shown below. The rollers can be rotated with four different speeds using

the gear box for a given rpm of the input shaft attached to motor. By using variable frequency

drive the motor can be made to rotate with various rpm and we can obtain various rpm of the

roller. But using variable frequency drive may increase the cost of the setup. The gap between

the rollers is fixed in this case and so the thickness of the strip obtained will also be constant.

Two different motors can be used for rotating two rollers .But the rpm of the rollers may not

be exactly same and this may lead to relative motion between the melt on either side of the

roll-gap when it is entering and exiting through gap between rollers. It also unnecessarilyincreases the number of gears and shafts required .

Fig 6: Schematic of the gear mechanism in twin roll casting apparatus



Later it was thought that designing the whole apparatus and manufacturing the

individual components or assembling the individual components is a time consuming process

and it was decided to purchase the apparatus from a vendor. The specifications of the

apparatus are obtained by considering the desired dimensions of the strip which is suitable

for various tests such as tensile test in both longitudinal and transverse section, deep drawing

test etc., and from the design specifications by various researchers. The range of temperaturefor the sensors for measuring the temperature at inlet and exit gap is made considering the

fact that the melting point of pure aluminium is 660 .

Specifications of the twin roll casting apparatus :-

1. Material of the roll – Cu (Electrolyte )

Size : diameter :300 mm, width :150mm - 2 pieces

2. R.P.M of roller : 20 – 500 rpm

3. Stand height : 24’’

4. Roll gap : 0 – 3 mm

5. Sensors :

a) for measuring roll gap – LVDT sensor

b for measuring rpm of the roller (20 - 500 rpm)

c) for measuring the temperature of molten metal at the inlet (450 -900 and

exit of gap (100-500 ) between rollers .

d) for measuring the roller force / load (0-2.14 metric tonne)

Calculation for max value of the roller force to measured by sensor :-

Maximum value of separating force = 0.14 KN/mm

Width of the roller = 150 mm

Maximum value of the force to be measured by the sensor = 0.14*150

= 21 KN= 2.14 metric tonne



OBJECTIVE

The primary objective of the project is to obtain the aluminium alloy strip using the

vertical twin roll casting and compare its properties such as uniaxial tensile strength

,ductility,microstructure etc, with the strip obtained through conventional casting and rolling .

Objective of the project:-

To Study the uniaxial tensile strength, microstructure, micro-hardness of the strip.

To Study the surface finish of the strip.

To Study the fractography of the strip and model how process parameters(roll-

gap,rpm of the roller etc,) are affecting them .

Uniaxial tensile strength test :-

Uniaxial tensile testing is the most commonly used for obtaining the mechanicalcharacteristics of isotropic materials and the most common testing machine used in tensile

testing is the universal testing machine. A tensile specimen is a standardized sample cross-

section. It has two shoulders and a gauge in between. The shoulders are large so they can be

readily gripped, whereas the gauge section has a smaller cross-section so that the deformation

and failure can occur in this area. A standard specimen is prepared in a round or a square

section along the gauge length, depending on the standard used

The initial gauge length Lo is standardized (in several countries) and varies with the

cross-sectional area (Ao) of the specimen. According to United States ASTM standards,

Lo√Ao ratio for a sheet type specimen is 4.5. The test process involves placing the test

specimen in the testing machine and applying tension to it until it fractures. During the

application of tension, the elongation of the gauge section is recorded against the applied

force. The data is manipulated so that it is not specific to the geometry of the test sample. The

elongation measurement is used to calculate the engineering strain, ε, using the equation

where Δ L is the change in gauge length, L0 is the initial gauge length, and L is the

final length. The force measurement is used to calculate the engineering stress, σ, using thefollowing equation:

Where F is the force and A is the cross-section of the gauge section. The machine

does these calculations as the force increases, so that the data points can be graphed into

a stress-strain curve. In our case, the test specimen is nothing but the aluminium alloy strip

obtained through twin roll casting. Properties that are directly measured via this tensile test

are ultimate tensile strength, maximum elongation and reduction in area.



Microstructure :

Many times, the physical properties and, in particular, the mechanical behaviour of a

material depends on the microstructure. Microstructure is subject to direct microscopic

observation, using optical or electron microscopes. The microstructure of an alloy depends on

such variables as the alloying elements present, their concentrate ions, and the heat treatment

of the alloy (i.e., the temperature, the heating time at temperature, and the rate of cooling to

room temperature). The surface of a specimen(strip) to be examined is scanned with an

electron beam in SEM, and the reflected (or back-scattered) beam of electrons is collected,

then displayed at the same scanning rate on a cathode ray tube (similar to a CRT television

screen).The image on the screen, which may be photographed, represents the surface features

of the specimen. The surface may or may not be polished and etched. Magnifications ranging

from 10 to in excess of 50,000 times are possible, as are also very great depths of field. A

photograph of the microstructure of A356 sample is as shown below.

Fig : Microstructure of A356 cast by a HPTRC (hydrostatic pressure

twin roll casting ).



Micro-hardness:-

The term "microhardness" has been widely employed in the literature to describe the

hardness testing of materials with low applied loads. A more precise term is "micro

indentation hardness testing." In micro indentation hardness testing, a diamond indenter of

specific geometry is impressed into the surface of the test specimen using a known applied

force (commonly called a "load" or "test load") of 1 to 1000 gf. Microindentation tests

typically have forces of 2 N (roughly 200 gf) and produce indentations of about 50 μm. The

two most commonly used microhardness tests are tests that also can be applied with heavier

loads as macroindentation tests are:

1. Vickers hardness test (HV)

2. Knoop hardness test (HK)

In microindentation testing, the hardness number is based on measurements made of

the indent formed in the surface of the test specimen. The hardness number is based on thesurface area of the indent itself divided by the applied force, giving hardness units in

kgf/mm². Microindentation hardness testing can be done using Vickers as well as Knoop

indenters. For the Vickers test, both the diagonals are measured and the average value is used

to compute the Vickers pyramid number. In the Knoop test, only the longer diagonal is

measured, and the Knoop hardness is calculated based on the projected area of the indent

divided by the applied force, also giving test units in kgf/mm².

There is some disagreement in the literature regarding the load range applicable to

micro-hardness testing. ASTM Specification E384, for example, states that the load range for

micro-hardness testing is 1 to 1000 gf. For loads of 1 kgf and below, the Vickers hardness

(HV) is calculated with an equation, wherein load ( L) is in grams force and the mean of two

diagonals (d ) is in millimeters:

Both the Knoop and Vickers indenters require prepolishing of the strip surface to achieve

accurate results.

Surface finish:-

Surface finish, also known as surface texture, is the characteristics of a surface. It has

three components: lay, surface roughness, and waviness. Lay is the direction of the

predominant surface pattern ordinarily determined by the production method used. Surface

roughness commonly shortened to roughness, is a measure of the finely spaced surface

irregularities. In engineering, this is what is usually meant by "surface finish".Waviness is the

measure of surface irregularities with a spacing greater than that of surface roughness. These

usually occur due to warping, vibrations, or deflection during machining.



The most common method is to use a diamond stylus profilometer. The stylus is run

perpendicular to the lay of the strip surface. The length of the path that it traces is called

the measurement length. The wavelength of the lowest frequency filter that will be used to

analyze the data is usually defined as the sampling length. Most standards recommend that

the measurement length should be at least seven times longer than the sampling length, and

according to the Nyquist – Shannon sampling theorem it should be at least ten times longerthan the wavelength of interesting features. The assessment length or evaluation length is the

length of data that will be used for analysis. Commonly one sampling length is discarded from

each end of the measurement length. 3D measurements can be made with a profilometer by

scanning over a 2D area on the surface of the strip.

Fractography:-

Fractography is the study of fracture surfaces of materials. Fractographic methods are

routinely used to determine the cause of failure in engineering structures, especially in

product failure and the practice of forensic engineering or failure analysis. In material

science research, fractography is used to develop and evaluate theoretical models of crack

growth behavior. An important aim of fractography is to establish and examine the origin of

cracking, as examination at the origin may reveal the cause of crack initiation. Initial

fractographic examination is commonly carried out on a macro scale utilising low

power optical microscopy and oblique lighting techniques to identify the extent of cracking,

possible modes and likely origins.

Optical microscopy or macro photography are often enough to pinpoint the nature ofthe failure and the causes of crack initiation and growth if the loading pattern is known.

Common features that may cause crack initiation are inclusions, voids or empty holes in the

material, contamination, and stress concentrations. "Hachures", are the lines on fracture

surfaces which show crack direction.

In many cases, fractography requires examination at a finer scale, which is usually

carried out in a scanning electron microscope or SEM. The resolution is much higher than the

optical microscope, although samples are examined in a partial vacuum and colour is absent.

Improved SEM's now allow examination at near atmospheric pressures, so allowing

examination of sensitive materials such as those of biological origin. The SEM is especiallyuseful when combined with Energy dispersive X-ray spectroscopy or EDX, which can be

performed in the microscope, so very small areas of the sample can be analysed for their

elemental composition.

Using this technique, the cracks in strip obtained through twin roll caster can be

examined, if present and their causes and so the process parameters affecting the crack

initiation, propagation and growth can be studied. This analysis will be useful in reducing the

the cracks and hence adding more value to the strip.



Conclusion:-

1. An attempt was made to design the twin roll casting apparatus for obtaining the thin

aluminium strip using a 22 gear box and an idler gear.

2. Specifications of the twin roll casting apparatus were found by considering the tests to

be performed on the strip and using work done by various researchers.

References:-

1. Toshio Haga, Kenta Takahashi, Masaaki Ikawa, Hisaki Watari: A

vertical-type twin roll caster for aluminium alloy strips.

2. Toshio Haga, Kenta Takahashi, Masaaki Ikawa, Hisaki Watari: Twin roll

casting of aluminium alloy strips.

3. D. J. Monaghan, M. B. Henderson, J. D. Hunt and D. V. Edmonds:

Microstructural defects in high productivity twin roll casting of aluminium

alloys.

4. M. Yun, S. Lokyer, J.D. Hunt: Twin roll casting of aluminium alloys

Btp Final Report 123

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