CHAPTER 1 2INTRODUCTION 21.1 Background ofstudy 21.
2 ProblemStatements 31.3 Significance ofStudy 41.4 Objectives 4CHAPTER 2 6LITERATURE REVIEW 62.1 Magnesiumproperties 62.2 Magnesium alloys 72.3 Importance ofalloys 7Previous Issues 9Latest Advancements 9CHAPTER 3 10METHODOLOGY 103.1 Materials 103.
2 Sample preparation 103.3 Mounting 113.4 Grinding/Polishing 123.5 Sample Characterization 133.5.
1 Mechanical Test 133.5.2 MicrostructuralAnalysis 143.5.3 Phase Analysis 14CHAPTER 1 INTRODUCTION 1.1 Background of study Mg basedalloys are becoming a major industrial materials for today’s advancementtechnology and latest manufacturing application. This alloys are favourable as it possess superior properties.
Withoutstanding combination characteristics of high elastic modulus, good dampingbehaviour, very light in weight and low density, Mg alloys stand as one of themost crucial light weight material in structural application (You et al,2017). Besides, Mg alloys are common in automotive and aircraft applicationincluding gearbox housing, tack coves and crank case (Darham et al,2016). Mg alloys need to be developed and enhanced in order to attain highperformance of alloy with excellent workability. For instancesit is very light weight compared to aluminium (Al) and steels. Engineers andscientist have showed so much interest of Mg alloys for decades due to thisproperties. (Qiao et al, 2016). Study of Mg alloys lead toreasonable prices and properties enhancement of Mg and its alloys will lead to greatuse of magnesium. This is because compared to using alternative materials, Mgalloys results in a 22% to 70% weight reduction.
To add their competitive edge, Mg alloy started tobe used in racing cars. Fuel efficiency, increase performance andsustainability are top-of-mind issues. Mg possess low melting temperature(Tm = 650 °Cand hexagonal crystal structure (HCP). Inadequate slip system cause it tosoften and weak when being exposed to high surrounding temperature. Alloyinghelps the materials having goodroom temperature strength. New composition of Mg alloys with addition of elementhave been developed. Most frequently used alloying elements are zinc(Zn) and calcium (Ca). Zn will increase the strength of the material at hightemperature exposure because of precipitation hardening.
Addition of Ca in Mgand Al will act as grain refiner (Wang et al, 2017). By alloying, Mgproperties may be improve not only at high temperature but also in ambientstate. In 2008, Wan et al stated that Ca is one of the important element in Mgalloy as it could improve the ignition-proof of pure Mg. Moreover, Pan et al (2015) stated that Mg2Snalloy superior heat resistance properties can fix the weakness of Mg that tendsto soften easily at elevated temperature. In this study, microstructuraland hardness properties on Mg-10Al-6Ca-1Sn and Mg-17Al-4Zn-3Ca-1Sn are carriedout. 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 ductilityof Mg alloys to Al alloys have limited their application and the higher demandsin advance application requires better mechanical properties. Besides, Jun(2016) highlighted that Mg-Al suffers poor mechanical properties due toinstability of ß(Mg17Al12) phase at elevated temperatures thuslimit the application of this alloy. Even though those elements added in are toimprove mechanical properties of the alloy, not all properties are developedand some bad side effects can also be seen from the combination made. Al haslarge affinity towards micro porosity while Ca has low protection towardsoxidation process especially in its molten condition. 1.3 Significance of Study New findingsof Mg alloys are very crucial in order to develop the materials to have closetolerance towards requirement for commercial purposes.
In this study, Mg-10Al-6Ca-1Snand Mg-17Al-4Zn-3Ca-1Sn samples are carried out so that Mg alloy mechanicalproperties can be improved. It is expected to obtain better microstructure,good damping behaviour, increment of hardness and elastic modulus in Mg alloysby the addition of few elements like Sn, Al, Ca and Zn. Good mechanicalproperties and high strength will result in high durability and long life spanof the products.
Improvement of Mg alloy can bring enhancement in the latesttechnology production while at the same time widened the field of Mg studies. 1.4 Objectives In thisexperiment, few objectives were set. i.
To synthesizeMg-10Al-6Ca-1Sn, Mg-17Al-4Zn-3Ca-1Sn by using Argon-Arc Melting method. ii. To investigatemicrostructural characterization of the samples by the application of FieldEmission Scanning Electron Microscopes (FESEM) and X-ray Powder Diffraction(XRD) machine.
iii. To study hardness test and elastic modulus of these alloys by usingVickers Hardness machine and ResonanceFrequency Damping Analysis (RFDA) respectively.CHAPTER 2 LITERATURE REVIEW 2.1 Mg propertiesPure Mgpossess of superior properties which includes good fatigue and impact strength,excellent damping properties, excellent strength to weight ratio and also dimensionalstability. In term of weight, Mezbahul et al (2014) stated that Mg isthe lightest metal which it only owns density of 1.738 g/cm3compared to Fe (7.86g/cm3) and Al (2.
70 g/cm3). This properties is very importantespecially in the making of small portable tools such as laptops and mobilephones and also to the transportation industry. Mg also possess good dampingcapacity. Free path for dislocation such as ordering and defect movement andprecipitation collectively comprises solute atom/defect. This free path valueslower dislocation density and/or larger grain size which resulted in betterdamping characteristic (Kaya et al, 2017). Damping is very important fora quieter operation of equipment as it absorb the vibration energy. Inmetallurgy, some metal will undergoes some changes due to the processes andexposure to high temperature environment. However, Mg alloys have dimensionalstability at which is it has predictable and controllable shrinkage rate.
Theconsequences is, Mg alloys processes can be not needing finishing steps whichwould save time in the production as it has low distortion towards the castingstress. 2.2 Mg alloys Magnesium is the lightest of all light metal alloysand therefore is an excellent choice for engineering applications when weightis a critical design element. However, pureMg cannot be used in the structural application as it cannot support load fromthe external force given when it exposed over long period as Mg is very ductileand has low elastic modulus. For an example, adding ofaluminium element in an alloy can help increase strength and hardness while atthe same time manage to minimal the increasing of the density. Shuai et al (2016) highlighted that Mg alloy high-to-weight ratioproperties is suitable to be developed in aerospace and automobile application.Light weight and strong material are needed in order to protect the coreof the car body and at the same time able to save fuel and economic.
This isbecause heavy body tends to consume more fuel than the lighter one. Lightweight 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 bereused. In 211st century, it got the title of Green Engineering Material whichhave wide application in the field of automobile and aerospace (Wu et al, 2009).2.3 Mg-Ca alloy ForMg alloy application and studies, Mg-Ca alloys is one of the frequently usedingredients because of the advantages it owns. Ca is an important elementbecause of its function as grain refiner (Shuai et al, 2016). In 2014, OlgaKulyasova et al (2014) mentioned that fine-crystalline state of Mg-Zn-Ca showedmicro hardness value of 990 MPa which was absolutely higher than pure Mg (400MPa).
As the amount of Ca in Mg alloy increase, the strength will increasewhile elongation will decrease (Du et al, 2016). Thisis because of grain structure refinement at 200nm grain size of the alloys.Grain refiner is needed in order to regulate microstructure of Mg alloy bygrain size reduction and for a better precipitates distribution in the matrix.According to Darham et al (2016), Scanning Electron Microscope can be used tosee size reduction in dendritic arm and grain size of Mg-Ca alloy. Furthermore,addition of Ca into Mg will improve the mechanical properties especially athigh temperature. Figure 2.1 shows the binary phase diagram of Mg-Ca. Pure Mghas melting temperature (Tm) of 650°C.
At the Mg-rich end of the phasediagram, Ca solubility in Mg is 1.34 wt.% and when Ca addition is higher, itwill form eutectic Mg2Ca along with ?-Mg at 16.
2 wt.%. Theequilibrium Mg2Ca metallic phase Mg2Ca has Tmof 715°C which is higher than pure Mg hence, it is relatively stable at hightemperature (Nie and Muddle, 1997).
Mg-Ca system (P63/mmc, a = 0.623nm, c = 1.012 nm) is alike to matrix phase of Mg (P63/mmc, a = 0.321nm, c = 0.521 nm) of hexagonal crystal structure. The excellent latticematching and comparatively high Tm results in uniformly distributed coherentprecipitation and formation of microstructure in Mg2Cawith thermal stability. Figure2.1: Eutectic phase diagram of Mg-Ca alloy Accordingto Hall-Petch et al (1953), the amount of energy needed to move the dislocationexist in the materials will increase when the grain boundary increased.
Finergrained materials have more grain boundary compared to coarse grainedmaterials. 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 arereduced. 2.4 Importance of alloying Pure Mg isgenerally soft compared to other metal but this flaws can be fixed by addingelements that can be metal or non-metal with higher strength and toughness toharden the alloy. The most common reason for alloying is to increase hardnessof the material. 2.
4.1 Addition of Zn Son et al (2008) stated that increasingstrength of Mg alloys at high or room temperature are based on Mg and Al withadditional combination of Zn, Ca, Si andRE. The ternary element Zn is added to the binary alloy to develop the responseprocess of precipitation hardening (Nie and Muddle, 1997).
This is because Znis almost three times stronger than Al as it possess better solid solutionstrengthening properties especially after being treated and under quenchedstate. In 2006, Somekawa and Mukai highlighted that fracture toughness of pureMg was low which reported to be only 17.8 MPa m1/2.
However, inMg-1.6 at.% Zn solid solution strengthening, the value rise by 5.9MPa m1/2to 23.7 MPa m1/2. 2.4.
2 Addition of Sn Sn is beneficial in alloying when it iscombined with Mg and small amount of Al by substituting into few metallicphases (Toby, 2004). In this study, 1 wt.% of Sn is added to improve propertiesof Mg alloys. In 2017, Poddar et al highlighted that (1-10)% of Sn canimprove the creep resistance in Mg alloy. They added that Sn content helps inrefining secondary dendritic arm spacing of the primary ?-Mgphase. The precipitation of Mg2Sn results in increment ofcompressive and tensile strength by which affected to finer grain refinement ofMg alloys (Cheng et al, Poddar, 2017).
Addition of tin (Sn) intoMg alloys will increase the strength of the materials and improve theductility. By the addition of Sn,yield strength and hardness will increase due to intermetallic phases. Grain refinementand solid solution strengthening of Mg-Sn give strengthening effect in singlephase alloy where this will result in better mechanical properties to Mgalloys. By the addition of Sn, it is expected togive superior properties to the alloy. However, Sn element is quite expensivewhich can cause increasing of the production cost. Hence, over addition of Snin the alloy can bring limitation to the production too. CHAPTER 3 METHODOLOGY 3.1 Materials Table 3.
1below shows the list of raw materials, chemicals, apparatus and equipment thatwill be used in this study. Table 3.1 List ofmaterials, 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 ofMg-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 sampleswill be measured and weight accordingly (Table 3.2).
Table 3.2 The nominalcomposition 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 sampleswill be melted using an arc meting furnace in argon atmosphere on a water-cooledcopper hearth. The furnace will be cleaned using laboratory tissue immersed in acetoneprior to melting to ensure there is no impurity in the chamber.
Then sample willbe placed inside the furnace. The chamber will be vacuumed for at least fiveminutes and argon gas will be flushed into the chamber. This process will bedone twice to ensure the chamber atmosphere is filled with argon gas. Air flowneed to be in control to avoid sample got blown away due to high speed of airflow and the tightly closed door of the furnace to avoid air from diffusinginto the chamber and causing the sample to oxidize.
Once purging completed,titanium (Ti) getter and the samples will then be melted. Ti getter will bemelted before the samples in order to eliminate impurity gases such as oxygenand nitrogen. Once completed, melting process will continue by melting of thesamples. Chiller is important to cool down the copper hearth by water flowingas melting process generates high level of heat. 3.3 Mounting Samples willbe mounted using cold mounting epoxy and hardener. A PVC hollow tubes will beused as mould PVC mould will be cut, flatten and cleaned to ensure no leakageduring mounting.
After that, mould release will be applied onto the internal surfaceof the hollow tube to ease the removal of the mounted sample once it hardened. Themoulds will be placed onto a tape to avoid spilling which will be placed onto aflat tile. After that, the mixture of transparent epoxy and hardener will beprepared by ratio 9:1. The mixture of epoxy and hardener will be transferredinto a polystyrene cup and stirred slowly. This is to prevent bubble forming inthe mixture so that it will not cause crack and decreasing in strength laterdue to stress concentration. With the sample in each of the labelled tube, thesolvent will be filled in the tube by third quarter from its volume. Thesamples were left for 4-5 hours in order to let it harden. The mounting whichthe specimen is embedded in the solution/solvent have purposes to ease in handlingand protect the specimen physically.
3.4 Grinding/Polishing After themounting processes, the specimen will be grounded using SiC abrasive paper. MetkonGripo 2V Grinder-Polisher will be used in this process. The purposes of this stepis to eliminate the damage from cutting, remove contaminants and also repairthe surface of the specimen. Figure 3.
1 MetkonGripo 2V Grinder PolisherThe grindingprocesses 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 forweighty stock elimination to very fine grit 1200. Every change of grit needs tobe followed by change in direction of the grinding which is perpendicular tothe previous direction. This is to clear the lines produced from the previousgrinding therefore making the surface of the specimen to have better surfacefinish, smoother and at utmost importance is to not cause disturbance in theview of the microstructural testing later. Proper grinding will help minimizingtime spending for polishing process.
For polishing process, polishing clothsand diamond slurry will be used to gain a better surface finish that possesshigh reflectivity before any microscopic testing can be done. Diamond slurrywill be used as it offers highest level of accuracy that can either be water-basedor oil-based slurries. In this experiment, water-based slurry which is harmlessto the environment with super cleaning properties will be used to polish the metallicspecimen. Polishing is a very sensitive process where some aspects need to be paidattention to like polishing time and speed, pressure, cloth abrasive and alsothe pressure used which is from 6µ, 3µ and last but not least 1 µ in thecorrect order.
Excess water needs to be avoided to prevent the diamondparticles from slipping away. 3.5 Etching After grinding and polishing, the sampleswill be etched by strong acid to remove the surface of the exposed area inorder to achieve a better microscopic view. Etching is done in order to removeoxide layer that might form at the surface area or any impurities adhered ontoit. The etchants help to cut the unnecessary parts and reveal the protectedparts. Etching is done by immersing sample into the strong acid and is left fora moment in range 10-60 seconds. The etchants than can be used for Mg alloysare methanol, hydrochloric acid, nitric acid and also hydrofluoric acid. 3.
6 SampleCharacterization3.61 Mechanical Test Formechanical behaviour of the specimen finding, hardness testing will beconducted by Vickers method. Hardness is one of the material physicalproperties where we measure the resistance of the samples towards indentation.The depth of the sample indented will be varied depends on the hardness of thespecimen. The harder the specimen, the less depth of the indentation would be.For thin section or small part sample, Vickers Hardness is commonly used forhardness testing. By using load of 500g and dwell of 15s, a square basedpyramid shape will be used to lift the load.
An indenter having an angle of136° must sufficiently harder than the specimen tested so that it cannot easilydeformed by the force applied.Figure 3.2 Vickersindentation and measurement of impression of diagonals (google.com) Once the testing over, the width or depth ofthe 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 areaof the indentation in millimetre, mm unit by considering the upside down squarebase pyramid. Vickers hardness is identified by using below formula; (kgf)D = averagevalue of two diagonals, d1 and d2 (mm) 3.
5.1 Microstructural Analysis Microstructuralanalysis is the reveal of the sample’s structure under microscopic view at alow voltage in order to quantify microstructural features and characterizationof the specimen. In this experiment, Field Emission Scanning ElectronMicroscopy (FESEM) will be used in the electron gun of electron microscopescanning that minimize destruction and sample charging together with spatialresolution improvement as the result of high energy of electron. By differentrange of magnification following from bigger range to smaller one (200x –1000x), the feature geometry and structure of the sample can be observed. Theimage produced is from a field emission cathode located in the electron gunthat accelerated in a field gradient under vacuum condition. Through electromagneticlenses, the beam will be directed on the specimen surface.
By comparing theintensity of the secondary electron caught by the detector, an image of thespecimen surface can be obtained. 3.5.2 Phase Analysis X-raydiffraction phase/composition analysis (XRD) is commonly used for phase analysisof crystalline materials either by quantitatively or qualitatively. Thediffraction is a coherent scattering of x-ray by the crystalline substancewhich can detect chemical composition, physical properties and the crystalstructure. Firstly, water Chiller pump and x-ray power button need to be turnedon.
Once the x-ray button lighted up, warning signal will light up to show thatx-rays are being produced. Sample will be inserted once the voltage and currentare set. Pass ID card in front of the card reader and wait for light to be onand later press the button of ‘window open’. X-ray tube will strike the sampleand diffraction process will be performed by the red indication light on thex-ray tube.
After that, sample can be safely removed once the procedure is shutdown by pressing of ‘window shut’ button. Next, water chiller pump has to beturned off and x-ray key is need to be removed. Diagram 3.1: Procedure of Sample Preparation and Testing