CHAPTER (Darham et al, 2016). Mg alloys need to

CHAPTER 1  2

INTRODUCTION   2

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1.1      Background of
study  2

1.2      Problem
Statements  3

1.3      Significance of
Study  4

1.4      Objectives  4

CHAPTER 2  6

LITERATURE REVIEW    6

2.1      Magnesium
properties  6

2.2      Magnesium alloys  7

2.3      Importance of
alloys  7

Previous Issues  9

Latest Advancements  9

CHAPTER 3  10

METHODOLOGY   10

3.1      Materials  10

3.2      Sample preparation  10

3.3      Mounting  11

3.4      Grinding/Polishing  12

3.5      Sample Characterization  13

3.5.1      Mechanical Test 13

3.5.2      Microstructural
Analysis  14

3.5.3      Phase Analysis  14

CHAPTER 1

 

INTRODUCTION

 

1.1             
Background of study

 

Mg based
alloys are becoming a major industrial materials for today’s advancement
technology and latest manufacturing application. This alloys are favourable as it possess superior properties. With
outstanding combination characteristics of high elastic modulus, good damping
behaviour, very light in weight and low density, Mg alloys stand as one of the
most crucial light weight material in structural application (You et al,
2017). Besides, Mg alloys are common in automotive and aircraft application
including gearbox housing, tack coves and crank case (Darham et al,
2016). Mg alloys need to be developed and enhanced in order to attain high
performance of alloy with excellent workability.

 

For instances
it is very light weight compared to aluminium (Al) and steels. Engineers and
scientist have showed so much interest of Mg alloys for decades due to this
properties. (Qiao et al, 2016). Study of Mg alloys lead to
reasonable prices and properties enhancement of Mg and its alloys will lead to great
use of magnesium. This is because compared to using alternative materials, Mg
alloys results in a 22% to 70% weight reduction. To add their competitive edge, Mg alloy started to
be used in racing cars. Fuel efficiency, increase performance and
sustainability are top-of-mind issues.

 

Mg possess low melting temperature(Tm = 650 °C
and hexagonal crystal structure (HCP). Inadequate slip system cause it to
soften and weak when being exposed to high surrounding temperature. Alloying
helps the materials having good
room temperature strength. New composition of Mg alloys with addition of element
have been developed. Most frequently used alloying elements are zinc
(Zn) and calcium (Ca). Zn will increase the strength of the material at high
temperature exposure because of precipitation hardening. Addition of Ca in Mg
and Al will act as grain refiner (Wang et al, 2017). By alloying, Mg
properties may be improve not only at high temperature but also in ambient
state. In 2008, Wan et al stated that Ca is one of the important element in Mg
alloy as it could improve the ignition-proof of pure Mg. Moreover, Pan et al (2015) stated that Mg2Sn
alloy superior heat resistance properties can fix the weakness of Mg that tends
to soften easily at elevated temperature. In this study, microstructural
and hardness properties on Mg-10Al-6Ca-1Sn and Mg-17Al-4Zn-3Ca-1Sn are carried
out. The characterization will be carried out by X-ray Diffraction (XRD)
technique, Vickers Hardness and Scanning Electron Microscopy (SEM).

 

1.2             
Problem Statements

 

Mg alloys are light in weight and own superior damping capacity.
However, Liang et al (2008) stated that the low strength and ductility
of Mg alloys to Al alloys have limited their application and the higher demands
in advance application requires better mechanical properties. Besides, Jun
(2016) highlighted that Mg-Al suffers poor mechanical properties due to
instability of ß(Mg17Al12) phase at elevated temperatures thus
limit the application of this alloy. Even though those elements added in are to
improve mechanical properties of the alloy, not all properties are developed
and some bad side effects can also be seen from the combination made. Al has
large affinity towards micro porosity while Ca has low protection towards
oxidation process especially in its molten condition.

 

1.3             
Significance of Study

 

New findings
of Mg alloys are very crucial in order to develop the materials to have close
tolerance towards requirement for commercial purposes. In this study, Mg-10Al-6Ca-1Sn
and Mg-17Al-4Zn-3Ca-1Sn samples are carried out so that Mg alloy mechanical
properties can be improved. It is expected to obtain better microstructure,
good damping behaviour, increment of hardness and elastic modulus in Mg alloys
by the addition of few elements like Sn, Al, Ca and Zn. Good mechanical
properties and high strength will result in high durability and long life span
of the products. Improvement of Mg alloy can bring enhancement in the latest
technology production while at the same time widened the field of Mg studies.

 

1.4             
Objectives

 

In this
experiment, few objectives were set.

                               
i.           
To synthesize
Mg-10Al-6Ca-1Sn, Mg-17Al-4Zn-3Ca-1Sn by using Argon-Arc Melting method.

                             
ii.           
To investigate
microstructural characterization of the samples by the application of Field
Emission Scanning Electron Microscopes (FESEM) and X-ray Powder Diffraction
(XRD) machine.

                           
iii.           
To study hardness test and elastic modulus of these alloys by using
Vickers Hardness machine and Resonance
Frequency Damping Analysis (RFDA) respectively.

CHAPTER 2

 

LITERATURE REVIEW

 

2.1             
Mg properties

Pure Mg
possess of superior properties which includes good fatigue and impact strength,
excellent damping properties, excellent strength to weight ratio and also dimensional
stability. In term of weight, Mezbahul et al (2014) stated that Mg is
the lightest metal which it only owns density of 1.738 g/cm3
compared to Fe (7.86g/cm3) and Al (2.70 g/cm3).  This properties is very important
especially in the making of small portable tools such as laptops and mobile
phones and also to the transportation industry. Mg also possess good damping
capacity. Free path for dislocation such as ordering and defect movement and
precipitation collectively comprises solute atom/defect. This free path values
lower dislocation density and/or larger grain size which resulted in better
damping characteristic (Kaya et al, 2017). Damping is very important for
a quieter operation of equipment as it absorb the vibration energy. In
metallurgy, some metal will undergoes some changes due to the processes and
exposure to high temperature environment. However, Mg alloys have dimensional
stability at which is it has predictable and controllable shrinkage rate. The
consequences is, Mg alloys processes can be not needing finishing steps which
would save time in the production as it has low distortion towards the casting
stress.

 

2.2             
Mg alloys

 

Magnesium is the lightest of all light metal alloys
and therefore is an excellent choice for engineering applications when weight
is a critical design element. However, pure
Mg cannot be used in the structural application as it cannot support load from
the external force given when it exposed over long period as Mg is very ductile
and has low elastic modulus. For an example, adding of
aluminium element in an alloy can help increase strength and hardness while at
the same time manage to minimal the increasing of the density.

 

Shuai et al (2016) highlighted that Mg alloy high-to-weight ratio
properties is suitable to be developed in aerospace and automobile application.
Light weight and strong material are needed in order to protect the core
of the car body and at the same time able to save fuel and economic. This is
because heavy body tends to consume more fuel than the lighter one. Light
weight Mg alloy is in demand as it will save more energy and produce less CO2
(Pan et al, 2015). Mg alloy is very friendly to the earth as it can be
reused. In 211st century, it got the title of Green Engineering Material which
have wide application in the field of automobile and aerospace (Wu et al, 2009).

2.3             
Mg-Ca alloy

 

For
Mg alloy application and studies, Mg-Ca alloys is one of the frequently used
ingredients because of the advantages it owns. Ca is an important element
because of its function as grain refiner (Shuai et al, 2016). In 2014, Olga
Kulyasova et al (2014) mentioned that fine-crystalline state of Mg-Zn-Ca showed
micro hardness value of 990 MPa which was absolutely higher than pure Mg (400
MPa). As the amount of Ca in Mg alloy increase, the strength will increase
while elongation will decrease (Du et al, 2016). This
is because of grain structure refinement at 200nm grain size of the alloys.
Grain refiner is needed in order to regulate microstructure of Mg alloy by
grain size reduction and for a better precipitates distribution in the matrix.
According to Darham et al (2016), Scanning Electron Microscope can be used to
see size reduction in dendritic arm and grain size of Mg-Ca alloy.

 

Furthermore,
addition of Ca into Mg will improve the mechanical properties especially at
high temperature. Figure 2.1 shows the binary phase diagram of Mg-Ca. Pure Mg
has melting temperature (Tm) of 650°C. At the Mg-rich end of the phase
diagram, Ca solubility in Mg is 1.34 wt.% and when Ca addition is higher, it
will form eutectic Mg2Ca along with ?-Mg at 16.2 wt.%. The
equilibrium Mg2Ca metallic phase Mg2Ca has Tm
of 715°C which is higher than pure Mg hence, it is relatively stable at high
temperature (Nie and Muddle, 1997). Mg-Ca system (P63/mmc, a = 0.623
nm, c = 1.012 nm) is alike to matrix phase of Mg (P63/mmc, a = 0.321
nm, c = 0.521 nm) of hexagonal crystal structure. The excellent lattice
matching and comparatively high Tm results in uniformly distributed coherent
precipitation and formation of microstructure in Mg2Ca
with thermal stability.

 

Figure
2.1: Eutectic phase diagram of Mg-Ca alloy

 

According
to Hall-Petch et al (1953), the amount of energy needed to move the dislocation
exist in the materials will increase when the grain boundary increased. Finer
grained materials have more grain boundary compared to coarse grained
materials. From the study, it is found that as the amount of Ca in Mg increase,
the hardness of the alloy increased. However, ductility and elasticity are
reduced.

 

2.4             
Importance of alloying

 

Pure Mg is
generally soft compared to other metal but this flaws can be fixed by adding
elements that can be metal or non-metal with higher strength and toughness to
harden the alloy. The most common reason for alloying is to increase hardness
of the material.

 

2.4.1       
Addition of Zn

 

Son et al (2008) stated that increasing
strength of Mg alloys at high or room temperature are based on Mg and Al with
additional combination of  Zn, Ca, Si and
RE. The ternary element Zn is added to the binary alloy to develop the response
process of precipitation hardening (Nie and Muddle, 1997). This is because Zn
is almost three times stronger than Al as it possess better solid solution
strengthening properties especially after being treated and under quenched
state. In 2006, Somekawa and Mukai highlighted that fracture toughness of pure
Mg was low which reported to be only 17.8 MPa m1/2. However, in
Mg-1.6 at.% Zn solid solution strengthening, the value rise by 5.9MPa m1/2
to 23.7 MPa m1/2.

 

2.4.2       
Addition of Sn

 

Sn is beneficial in alloying when it is
combined with Mg and small amount of Al by substituting into few metallic
phases (Toby, 2004). In this study, 1 wt.% of Sn is added to improve properties
of Mg alloys. In 2017, Poddar et al highlighted that (1-10)% of Sn can
improve the creep resistance in Mg alloy. They added that Sn content helps in
refining secondary dendritic arm spacing of the primary ?-Mg
phase. The precipitation of Mg2Sn results in increment of
compressive and tensile strength by which affected to finer grain refinement of
Mg alloys (Cheng et al, Poddar, 2017). Addition of tin (Sn) into
Mg alloys will increase the strength of the materials and improve the
ductility. By the addition of Sn,
yield strength and hardness will increase due to intermetallic phases. Grain refinement
and solid solution strengthening of Mg-Sn give strengthening effect in single
phase alloy where this will result in better mechanical properties to Mg
alloys.

 

By the addition of Sn, it is expected to
give superior properties to the alloy. However, Sn element is quite expensive
which can cause increasing of the production cost. Hence, over addition of Sn
in the alloy can bring limitation to the production too.

CHAPTER 3

 

METHODOLOGY

 

3.1             
Materials

 

Table 3.1
below shows the list of raw materials, chemicals, apparatus and equipment that
will be used in this study.

 

Table 3.1 List of
materials, chemicals, apparatus and equipment

 

Materials/Equipment

Raw materials

Mg ingot, 70% Mg-30% Ca master
alloy ingot, Sn ingot, Zn
chips, Al shot,

Chemicals

Acetone, diamond slurry

Apparatus

SiC abrasive paper

Equipment

List all equipment eg; Linear precision
cutter (Buehler Isomet 5000)

 

3.2             
Sample preparation

 

Sample of
Mg-10Al-6Ca-1Sn and Mg-17Al-4Zn-3Ca-1Sn will be prepared using 99.5% Mg, 70 %
Mg-30 % Ca master alloy ingot, Sn ingot, Zn chips and Al shots. The samples
will be measured and weight accordingly (Table 3.2).

 

Table 3.2 The nominal
composition of Mg-10Al-6Ca-1Sn and Mg-17Al-4Zn-3Ca-
1Sn in wt.%

 

Alloy

Element
(wt.%)

Al

Ca

Sn

Zn

Mg

Mg-10Al-6Ca-1Sn

10

6

1

 

Balance

Mg-17Al-4Zn-3Ca-1Sn

17

3

1

4

Balance

 

The samples
will be melted using an arc meting furnace in argon atmosphere on a water-cooled
copper hearth. The furnace will be cleaned using laboratory tissue immersed in acetone
prior to melting to ensure there is no impurity in the chamber. Then sample will
be placed inside the furnace. The chamber will be vacuumed for at least five
minutes and argon gas will be flushed into the chamber. This process will be
done twice to ensure the chamber atmosphere is filled with argon gas. Air flow
need to be in control to avoid sample got blown away due to high speed of air
flow and the tightly closed door of the furnace to avoid air from diffusing
into the chamber and causing the sample to oxidize. Once purging completed,
titanium (Ti) getter and the samples will then be melted. Ti getter will be
melted before the samples in order to eliminate impurity gases such as oxygen
and nitrogen. Once completed, melting process will continue by melting of the
samples. Chiller is important to cool down the copper hearth by water flowing
as melting process generates high level of heat.

 

 

3.3             
Mounting

 

Samples will
be mounted using cold mounting epoxy and hardener. A PVC hollow tubes will be
used as mould PVC mould will be cut, flatten and cleaned to ensure no leakage
during mounting. After that, mould release will be applied onto the internal surface
of the hollow tube to ease the removal of the mounted sample once it hardened. The
moulds will be placed onto a tape to avoid spilling which will be placed onto a
flat tile. After that, the mixture of transparent epoxy and hardener will be
prepared by ratio 9:1. The mixture of epoxy and hardener will be transferred
into a polystyrene cup and stirred slowly. This is to prevent bubble forming in
the mixture so that it will not cause crack and decreasing in strength later
due to stress concentration. With the sample in each of the labelled tube, the
solvent will be filled in the tube by third quarter from its volume. The
samples were left for 4-5 hours in order to let it harden. The mounting which
the specimen is embedded in the solution/solvent have purposes to ease in handling
and protect the specimen physically.

 

3.4             
Grinding/Polishing

 

After the
mounting processes, the specimen will be grounded using SiC abrasive paper. Metkon
Gripo 2V Grinder-Polisher will be used in this process. The purposes of this step
is to eliminate the damage from cutting, remove contaminants and also repair
the surface of the specimen.

Figure 3.1 Metkon
Gripo 2V Grinder Polisher

The grinding
processes starts from small value of grit to large value of grit which are 120,
240, 600, 800, 1000 and 1200 grit consequently from very coarse grit 120 for
weighty stock elimination to very fine grit 1200. Every change of grit needs to
be followed by change in direction of the grinding which is perpendicular to
the previous direction. This is to clear the lines produced from the previous
grinding therefore making the surface of the specimen to have better surface
finish, smoother and at utmost importance is to not cause disturbance in the
view of the microstructural testing later. Proper grinding will help minimizing
time spending for polishing process. For polishing process, polishing cloths
and diamond slurry will be used to gain a better surface finish that possess
high reflectivity before any microscopic testing can be done. Diamond slurry
will be used as it offers highest level of accuracy that can either be water-based
or oil-based slurries. In this experiment, water-based slurry which is harmless
to the environment with super cleaning properties will be used to polish the metallic
specimen. Polishing is a very sensitive process where some aspects need to be paid
attention to like polishing time and speed, pressure, cloth abrasive and also
the pressure used which is from 6µ, 3µ and last but not least 1 µ in the
correct order. Excess water needs to be avoided to prevent the diamond
particles from slipping away.

 

3.5             
Etching

 

After grinding and polishing, the samples
will be etched by strong acid to remove the surface of the exposed area in
order to achieve a better microscopic view. Etching is done in order to remove
oxide layer that might form at the surface area or any impurities adhered onto
it. The etchants help to cut the unnecessary parts and reveal the protected
parts. Etching is done by immersing sample into the strong acid and is left for
a moment in range 10-60 seconds. The etchants than can be used for Mg alloys
are methanol, hydrochloric acid, nitric acid and also hydrofluoric acid.

 

3.6             
Sample
Characterization

3.61         
Mechanical Test

 

For
mechanical behaviour of the specimen finding, hardness testing will be
conducted by Vickers method. Hardness is one of the material physical
properties where we measure the resistance of the samples towards indentation.
The depth of the sample indented will be varied depends on the hardness of the
specimen. The harder the specimen, the less depth of the indentation would be.
For thin section or small part sample, Vickers Hardness is commonly used for
hardness testing. By using load of 500g and dwell of 15s, a square based
pyramid shape will be used to lift the load. An indenter having an angle of
136° must sufficiently harder than the specimen tested so that it cannot easily
deformed by the force applied.

Figure 3.2 Vickers
indentation and measurement of impression of diagonals (google.com)

 Once the testing over, the width or depth of
the indentation will be measured to determine the hardness of the sample tested.
The Vickers Hardness is the quotient of the tested load applied (500g) for the area
of the indentation in millimetre, mm unit by considering the upside down square
base pyramid. Vickers hardness is identified by using below formula;

 (kgf)

D = average
value of two diagonals, d1 and d2 (mm)

 

3.5.1       
Microstructural Analysis

 

Microstructural
analysis is the reveal of the sample’s structure under microscopic view at a
low voltage in order to quantify microstructural features and characterization
of the specimen. In this experiment, Field Emission Scanning Electron
Microscopy (FESEM) will be used in the electron gun of electron microscope
scanning that minimize destruction and sample charging together with spatial
resolution improvement as the result of high energy of electron. By different
range of magnification following from bigger range to smaller one (200x –
1000x), the feature geometry and structure of the sample can be observed. The
image produced is from a field emission cathode located in the electron gun
that accelerated in a field gradient under vacuum condition. Through electromagnetic
lenses, the beam will be directed on the specimen surface. By comparing the
intensity of the secondary electron caught by the detector, an image of the
specimen surface can be obtained.

 

3.5.2       
Phase Analysis

 

X-ray
diffraction phase/composition analysis (XRD) is commonly used for phase analysis
of crystalline materials either by quantitatively or qualitatively. The
diffraction is a coherent scattering of x-ray by the crystalline substance
which can detect chemical composition, physical properties and the crystal
structure. Firstly, water Chiller pump and x-ray power button need to be turned
on. Once the x-ray button lighted up, warning signal will light up to show that
x-rays are being produced. Sample will be inserted once the voltage and current
are set. Pass ID card in front of the card reader and wait for light to be on
and later press the button of ‘window open’. X-ray tube will strike the sample
and diffraction process will be performed by the red indication light on the
x-ray tube. After that, sample can be safely removed once the procedure is shut
down by pressing of ‘window shut’ button. Next, water chiller pump has to be
turned off and x-ray key is need to be removed.

 

 

Diagram 3.1: Procedure of Sample Preparation and Testing