UNIVERSITY OF NAIROBI DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING

Transcription

1 UNIVERSITY OF NAIROBI DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING OPTIMIZATION OF HEAT TREATMENT OF A MULTICOMPONENT SECONDARY CAST ALUMINIUM ALLOY PROJECT CODE: TMO O2/2015 PREPARED BY: WAMBURU PETER GITEI MUADH KHAMIS OMAR ZAKII FAIZ KHAMIS F18/36047/2010 F18/36222/2010 F18/23326/2008 SUPERVISED BY: DR. T. M. OCHUKU Project report submitted in partial fulfilment of the requirement of the award of the Degree of Bachelor of Science in Mechanical Engineering of the University of Nairobi APRIL 20 th, 2015 i

2 DECLARATION We hereby declare that this research is our original work and has not been presented for a degree award in this or any other institution. WAMBURU PETER GITEI SIGNATURE: DATE: MUADH KHAMIS OMAR SIGNATURE: DATE: ZAKII FAIZ KHAMIS SIGNATURE: DATE: This project report has been submitted to the Department of Mechanical and Manufacturing Engineering, University of Nairobi, with my approval as the supervisor; DR. T. O. MBUYA SIGNATURE: DATE: ii

3 DEDICATION This report is dedicated to our loving parents for their relentless support and encouragement throughout our degree programme. iii

4 ACKNOWLEDGEMENTS We would like to acknowledge the department of Mechanical and Manufacturing Engineering for providing us with the necessary facilities to carry out this project. We thank our supervisor Dr. Ochuku for giving us his valuable time, advice and guidance without which the completion of this project would not have been possible. We would also like to thank Mr. Njue for his assistance in the workshop and metallurgy lab. Last but not least, we thank God for the gift of life, our health and all the blessings that have enabled us to get this far and complete this project. iv

5 TABLE OF CONTENTS DECLARATION i DEDICATION iii ACKNOWLEDGEMENTS iv LIST OF TABLES vii LIST OF FIGURES vii LIST OF ABBREVIATIONS viii ABSTRACT ix Chapter 1 1 INTRODUCTION Background Problem Statement Justification Objectives Project report 4 Chapter 2 5 LITERATURE REVIEW Cast Aluminium-Silicon Alloys Common Cast Aluminium Alloys & their Applications Alloys and A Alloys 319.0, A319.0, B319.0 and Alloy Alloy A Alloys A380.0 and B Alloys A390.0 and B Alloy Alloy Literature Survey on Aluminium Alloys for Automotive Piston Applications Pistons for Gasoline and Diesel Engines Automotive Piston Operating Conditions Piston Materials Microstructure of Al-Si Alloys & the Influence of Alloying Elements Silicon (Si) Magnesium (Mg) 19 v

6 2.4.3 Copper (Cu) Strontium (Sr) Iron (Fe) and Manganese (Mn): Titanium (Ti): Zirconium (Zr) Heat Treatment of Cast Aluminium Alloys Solution Heat Treatment Quenching Aging/Precipitation Hardening 26 Chapter 3 28 METHODOLOGY Introduction Experimental Work Heat Treatment Microstructural Examination Hardness Testing 29 Chapter 4 30 RESULTS AND DISCUSSION Effect of Heat Treatment on Alloy Micro-hardness : Discussion of the Variations in Micro-hardness with Increasing Solution Heat Treatment Time Effect of Heat Treatment on the Alloy Microstructure : Discussion of the Visible Changes in the Microstructure with Increasing Solution Heat Treatment Time 38 Chapter 5 39 CONCLUSIONS & RECOMMENDATIONS Conclusions Recommendations 39 REFERENCES 40 APPENDIX i vi

7 LIST OF TABLES Table 1.1: Typical Properties of Cast Aluminium Alloys [1] 1 Table 2. 1: Composition of Common Commercial Alloys [5] 10 Table 2. 2: Mechanical Properties of Piston Alloys at Various Temperatures [6] 16 Table 2. 3: Physical Properties of Aluminium Piston Alloys [6] 17 Table 4. 1: Average Micro-hardness Values of Heat Treated Samples 30 Table 4. 2: Standard Deviation of Micro-Hardness Values 30 LIST OF FIGURES Figure 2. 1: Important Piston Terminology [7] 13 Figure 2. 2: Operating Temperatures in Automotive Engines [7] 14 Figure 2. 3: Piston Types for Different Combustion Engines [6] 15 Figure 2. 4: Illustration of Aluminium-Silicon Phase Diagram 19 Figure 3. 1: Vicker's Hardness Tester LV-800 AT 29 Figure 4.1: Micro-hardness Variation with Solution Heat Treatment 31 Figure 4.2: Micro-hardness Variation with Ageing Time for 1hr Solution Heat Treated Specimen 31 Figure 4.3: Micro-hardness Variation with Ageing Time for a 2hr Solution Heat Treated Specimen 32 Figure 4.4: Micro-hardness Variation with Ageing Time for a 4hr Solution Heat Treated Specimen 32 Figure 4.5: Micro-hardness Variation with Ageing Time for a 6hr Solution Heat Treated Specimen 33 Figure 4.6: Average Micro-hardness Variation with Ageing Time 33 Figure 4.7: As-Cast Alloy Microstructure 35 Figure 4.8: 1hr Solution Heat Treated Specimen without Artificial Ageing 36 Figure 4.9: 2hr Solution Heat Treated Specimen without Artificial Ageing 36 Figure 4.10: 4hr Solution Heat Treated Specimen without Artificial Ageing 37 Figure 4.11: 6hr Solution Heat Treated Specimen without Artificial Ageing 37 vii

8 LIST OF ABBREVIATIONS β α-al Al Si Be Sr Ti Zr Cr Cu Fe K Mg Mn Na Ni HV SiC Beta phase Aluminium matrix Aluminium Silicon Beryllium Strontium Titanium Zirconium Chromium Copper Iron Potassium Magnesium Manganese Sodium Nickel Vickers Hardness number Silicon Carbide viii

9 ABSTRACT Aluminium alloy castings have increasingly become of greater importance in engineering applications due to their comparatively favourable properties as compared to ferrous alloys. In the automotive industry, stringent global regulations on carbon emissions have led to a shift towards aluminium as the main manufacturing material for most vehicle components due to its high strength to weight ratio. Aluminium alloy pistons for vehicles were of particular interest in this research work. The aim of this project was to optimize the microstructural and mechanical properties of aluminium alloy pistons through T6 heat treatment i.e. solution heat treatment, quenching and artificial aging. Due to the importance of aluminium castings in automotive piston applications, a study on the optimization of heat treatment parameters is imperative. Heat treatment offers an economical way of improving mechanical performance. The heat treatment process was conducted in an air circulated furnace at the University of Nairobi. The Solution heat treatment was carried out at times of 1hr, 2hrs, 4hrs and 6hrs at a temperature of 490 o C. This was followed by quenching with water at room temperature for each specimen. The specimens were left to naturally age for 24hrs before the commencement of artificial aging. The specimens were artificially aged at a temperature of 170 o C at times of 2hrs, 4hrs, 6hrs, 8hrs, 10hrs, 12hrs and 24hrs. Microstructural analysis and Micro-hardness testing was then carried out. The hardness values obtained were found to increase significantly with solution heat treatment. The peak hardness was found at a solution heat treatment time of 1hr with artificial ageing of 4hrs. The peak hardness value was 166.9HV. This was a particularly interesting observation as it is theoretically expected that with increasing solution heat treatment time more Al2Cu particles will dissolve in the α-al matrix resulting in increased hardness of the alloy. However, this is not the case because with prolonged solution heat treatment time the size of the individual precipitates increases which in turn diminishes the micro-hardness. For artificial ageing, it was found that the maximum hardness was attained at times of 4hrs for solution heat treatment of 1hr, 2hrs and 4hrs. However, for a solution heat treatment time of 6hrs an ageing time of 4hrs produced the peak hardness value. A solution heat treatment time of 1hr and 4hrs of artificial aging, thereby present significant energy savings as prolonged heat treatment times are redundant and unnecessary. ix

10 The as-cast microstructure was found to contain unevenly distributed dendritic silicon particles. The intermetallic phases observed were Al2Cu and AlCuNi. With increasing solution heat treatment time, the Al2Cu dissolves in the aluminium matrix and this is evident by their decrease in size in the microstructure. Changes in the size of the AlCuNi are also evident as they decrease both in size and number. Microstructural changes are not visible with an increase in ageing time. A sub-micron scale (i.e. nano-scale) is necessary for observation of these changes. From the data obtained, the recommended solution heat treatment time for a multicomponent piston alloy is 1hr with an artificial ageing time of 4hrs. x

11 Chapter 1 INTRODUCTION 1.1 Background The importance of cast aluminium alloys in today s industrial world cannot be understated. The demand for these alloys has increased due to their numerous cumulative advantages over ferrous alloys, more so, in automotive applications where stringent global regulations on carbon-dioxide emissions have caused a shift to materials with a high strength to weight ratio with aluminium being at the forefront. Properties displayed by these alloys, without considering the expanded capabilities of metal-matrix and other composite structures, are shown in Table 1.1: Table 1.1: Typical Properties of Cast Aluminium Alloys [1] Tensile Strength (MPa) Yield Strength (Mpa) Elongation, % 1-30 Hardness, HB Electrical Conductivity, %IACS Thermal Conductivity (W/m.K at 25 0 C) Fatigue Limit ((MPa) Coefficient of Linear Thermal Expansion ( ) x 10-6 / 0 C at C Shear Strength (MPa) Modulus of Elasticity (GPa) Specific Gravity

12 The physical and mechanical properties of aluminium castings may be altered through: Alloying composition: The composition of alloys determines the potential for achieving specific physical and mechanical properties. Alloy content is designed to produce characteristics that include castability as well as desired performance capabilities. The interaction of alloying elements is recognized in promoting desired microstructural phases and solid-solution effects for the development of these properties. Cooling rate during and after solidification: The conditions under which solidification takes place determine the structural features that affect the physical and mechanical properties of an alloy. Casting process: There are a large number of casting processes, and each imposes different rates of heat extraction, solidification rates, and means of compensating for solidification-related microstructural and macrostructural tendencies. Solidification: Engineered castings are susceptible to internal and superficial defects. The complex geometries of shaped castings, fluid dynamics, and solidification mechanics combine to present unique and difficult challenges to the objective of dense, discontinuity-free parts. Internal porosity can result from shrinkage and hydrogen porosity, as well as from visually detectable defects such as misruns, cracks, moisture reactions, folds, and tears. Non-metallic inclusions affect mechanical properties and nucleate hydrogen pore formation. Pore volume fraction and the geometry and distribution of internal voids reduce tensile properties, fatigue strength, toughness, and ductility, while surface defects strongly influence mechanical and fatigue performance. Heat treatment: Mechanical properties can be altered by post-solidification thermal treatment, including annealing, solution heat treatment, and precipitation aging. Post-solidification densification: Hot isostatic processing (HIP) of castings can result in improved levels of internal soundness, higher tensile properties, ductility, and fatigue performance. This report s main concern is with heat treatment of cast Al-Si based alloys for automotive piston applications and thus that shall be the main focus. 2

13 Solution heat treating, quenching and ageing are the basic heat treatment operations for aluminium alloys. A proper selection of these operations can achieve optimum combination of strength and ductility of the material. The purpose of solution heat treatment is to put the maximum practical amount of hardening solutes such as copper, magnesium, silicon into solid solution in the aluminium matrix and the purpose of quenching is to preserve the solid solution formed at the solution heat treating temperature by rapidly cooling to some lower temperature, usually near room temperature. Quenching not only retains solute atoms in solid solution, but also maintains a certain minimum number of vacancies that assist in promoting the low temperature diffusion required for precipitation. The rapid quenching rates improve the strength. Furthermore, the purpose of ageing is to increase strength and resistance to corrosion by forming Guinier-Preston (GP) zones and precipitating second-phase particles from solid solution obtained from quenching. There are two types of ageing for aluminium alloys: natural ageing and artificial ageing. Most of the heat treatable alloys exhibit age hardening at room temperature after quenching, called natural ageing. By reheating the quenched material to an elevated temperature, the solute content will be precipitated from solid solution gathering necessary energy for diffusion from heat, called artificial ageing which greatly affects the mechanical properties of aluminium alloys. Artificial ageing increases the tensile properties of the alloy system with a decrease in ductility and toughness. 1.2 Problem Statement This project is an attempt to investigate the influence of heat treatment on the microstructure and micro-hardness of aluminium alloys for automotive piston applications. 1.3 Justification Aluminium has rapidly become one of the most important engineering materials due to its favourable mechanical properties. It is thus imperative to continuously investigate and research on how these mechanical properties can be further optimized in an efficient and economical way. 3

14 Heat treatment provides one such way of drastically improving the mechanical properties of aluminium alloys. This strengthening mechanism is particularly important for components such as pistons which are subjected to adverse, high temperature environments. 1.4 Objectives The objectives of this project are as follows: To carry out T6 Heat Treatment of aluminium alloys used for automotive pistons. To conduct microstructural analysis using optical microscopy. To evaluate micro-hardness properties of the heat treated aluminium alloys. 1.5 Project report This project report is divided into five chapters. Chapter 1 highlights the background of this project, the problem statement, justification and specific objectives. Chapter 2 presents the literature review on the fundamentals of aluminium alloys, a literature survey on aluminium piston alloys, microstructure and mechanical properties of the alloys and the heat treatment process. Chapter 3 presents the methodology used in conducting this project. Chapter 4 reports the experimental results obtained and a detailed discussion of these results. Finally, Chapter 5 gives a summary of all findings and conclusions drawn from this work with recommendations. 4

15 Chapter 2 LITERATURE REVIEW 2.1 Cast Aluminium-Silicon Alloys Aluminium-Silicon alloys are rapidly becoming of greater importance to engineering industries as they exhibit high strength to weight ratio, high wear resistance, low density, low coefficient of thermal expansion, etc. Silicon imparts high fluidity and low shrinkage, which results in good castability and weldability. Effects of silicon in the Al-Si alloys are as follows: (i) Thermal expansion is reduced substantially by silicon (ii) Magnetic susceptibility is only slightly decreased by silicon (iii)the lattice parameter is decreased slightly by silicon (iv) Machinability is poor because of the hardness of the silicon [2] Common examples of aluminium applications in vehicles cover: power trains, automobile chassis, body structure (frame) and air conditioning. Aluminium castings have been applied to various automobile parts for a long period. As a key trend, the material for engine blocks, which is one of the heavier parts, is being switched from cast iron to aluminium resulting in significant weight reduction and increased efficiency. Aluminium castings find the most widespread use in automobile and aerospace industries where weight is an integral factor in overall performance. In automotive power trains, aluminium castings have been used for almost 100% of pistons (as either forged or cast aluminium pistons), about 75% of cylinder heads, 85% of intake manifolds and transmission (other examples rear axle, differential housings and drive shafts etc.) For chassis applications, aluminium castings are used for about 40% of wheels, and for brackets, brake components, suspension (control arms, supports), steering components (air bag supports, steering shafts, knuckles, housings, wheels) and instrument panels. Aluminium alloys have also found extensive application in heat exchangers. Modern high performance automobiles have many individual heat exchangers, (e.g. engine and 5

16 transmission cooling, charge air coolers (CACs), climate control) made up of aluminium alloys. Commercial uses for hypereutectic alloys are comparatively limited because these are the most difficult Al alloys to cast and machine due to the high Si contents. Once high Si content is alloyed into Al, it adds a large amount of heat capacity that must be removed from the alloy to solidify it during the casting operation. Major variation in the sizes of the primary Si particles can be found between different areas of the cast structure, causing significant deviation in the mechanical properties for the specimen. The primary crystals of Si must be refined so as to accomplish better hardness and wear resistance. Due to these reasons, hypereutectic alloys are not very cost-effective to fabricate because they have a broad range of solidification that results in poor cast ability and requires extra foundry processes to control the microstructure and the high heat of fusion. On the other hand, the usage of hypoeutectic and eutectic alloys is very widespread in the industries, because they are: a) More efficient to produce by casting b) Simpler to control the cast parameters c) Easier to machine than hypereutectic alloys. 2.2 Common Cast Aluminium Alloys & their Applications Some of the common commercial alloys are outlined below, together with their various applications in industry Alloys and A242.0 Alloys and A242.0 are used extensively for applications where strength and hardness at high temperatures are required. Typical applications include: heavy-duty pistons, motorcycle, diesel and aircraft pistons, aircraft generator housings; and air-cooled cylinder heads. These alloys have good fluidity, are fair for pressure tightness, and show fair resistance to hot cracking and solidification shrinkage. Arc and resistance methods are good for welding these alloys. Gas welding is satisfactory, but brazing is not recommended. The resistance of these alloys to most forms of common corrosion is fair but some additional protection can be gained by using chemical conversion coatings. 6

17 2.2.2 Alloys 319.0, A319.0, B319.0 and Alloys and A319.0 exhibit good castability, weldability, pressure tightness and moderate strength and are stable in that their casting such that mechanical properties are not affected by fluctuations in impurity content. They are also resistant to hot cracking and solidification shrinkage. Alloys B319.0 and show higher strength and hardness than and A319.0 and are generally used with the permanent mould casting process. Characteristics other than strength and hardness are similar to those of and A Typical applications for sand castings of these alloys include: internal combustion and diesel engine crankcases, gasoline and oil tanks, and oil pans [3]. Permanent mould cast components include: water-cooled cylinder heads, rear axle housings and engine parts Alloy Applications for are similar to those for Alloy has excellent casting characteristics and has largely replaced alloy This alloy displays good fluidity, resistance to hot cracking and solidification shrinkage. It also provides good finishing characteristics. Permanent mould castings of this alloy are used for machine tool parts, aircraft wheels and hand wheels, pump parts, tank car fittings, marine hardware, valve bodies and bridge railing parts, as well as for aileron control sectors, rudder control supports, fuselage fittings and fuel tank elbows for airplanes and missiles. Automotive applications include miscellaneous castings for trucks and trailers, spring brackets, cylinder heads, engine blocks, passenger car wheels and transmission cases. Uses for sand castings of include flywheel housings, automotive transmission cases, oil pans, rear axle housings, brackets, water-cooled cylinder blocks, various fittings and pump bodies[3]. This alloy is used in various marine applications in the T6 condition where pressure tightness and/or corrosion resistance are major requirements. 7

18 2.2.4 Alloy A356.0 Alloy A356.0 has higher strength and considerably higher ductility than It has these improved mechanical properties because impurities are lower in A356.0 than in Due to its strict impurity level tolerance, this alloy is difficult to recycle. It displays very good castability and is also corrosion resistant. Typical applications are airframe castings, machine parts, truck chassis parts, aircraft and missile components, and structural parts requiring high strength [4] Alloys A380.0 and B380.0 These alloys are used for casting general-purpose die castings. Fluidity, pressure tightness and resistance to hot cracking are all good. These alloys has a tendency to be abrasive during machining [3]. They have good mechanical properties and are used to make housings for lawn mowers and radio transmitters, air brake castings, gear cases and air-cooled cylinder heads. These alloys have only fair resistance to most corrosive atmospheres, and the protective value of chemical conversion coatings is poor Alloys A390.0 and B390.0 These companion alloys are hypereutectic aluminium-silicon alloys. The optimum structure of either alloy must consist of fine, uniformly distributed primary Si crystals in a eutectic matrix. This alloy does not require heat treatment, which may eliminate internal stresses that may cause fatigue failure. For alloy B390.0, the die-castability rating is good, and relatively thin and intricate sections can be produced. Pressure tightness and resistance to hot cracking are good [3], [4]. Resistance to die soldering is excellent. For A390.0, the permanent mould castability for this alloy preferred. Sand castability is not used because the slower cooling rates adversely affect casting microstructure. Pressure tightness and resistance to hot cracking are good. Gating designs for proper directional solidification and feeding are essential for sound castings. Pressure tightness is rated good. The low coefficient of thermal expansion, high hardness and good wear resistance of these alloys make them suitable for internal combustion engine pistons and blocks and cylinder bodies for compressors, pumps and brakes. 8

19 2.2.7 Alloy Alloy is an aluminium-magnesium alloy possessing a high, stable combination of strength, shock resistance and ductility. It is suited for parts in instruments and computing devices where dimensional stability is of major importance. In addition to the high ductility and tensile strength of 535.0, the Charpy impact is kg, which makes it suitable for shock-resistant applications. In addition, this alloy doesn t require heat treatment. Brackets, C-clamps and machined parts that need strength as well as impellers [4], optical equipment and similar applications requiring a high polish or anodized finish are its typical uses. In many cases, this alloy has replaced grey iron and malleable iron because its use reduces weight without sacrificing strength. The alloy has fair casting characteristics and attains its high physical and mechanical properties immediately upon casting. This fact is important to remember because most highstrength aluminium alloys change their properties as a result of age hardening. These properties remain constant for within the entire range of temperatures from -60 to 107 o C. The alloy shows fair fluidity with little tendency toward hot tearing. Because of the almost complete absence of heavy metals, the corrosion resistance of is extremely high. In addition, its relatively high magnesium content gives it further protection against corrosion from mild alkalis or salt spray Alloy Alloy is employed when a combination of good mechanical properties without heat treatment is needed. It also shows good shock resistance, corrosion resistance, machinability and dimensional stability. No distortion is exhibited when is heated. After brazing, the alloy will regain its original strength by natural aging. Alloy has fair to good castability. Although its pressure tightness and resistance to hot cracking are only fair, the alloy s fluidity and solidification shrinkage tendency are rated as good. The alloy has good natural resistance to corrosion, and good additional protection is received from chemical conversion coatings The alloy is used for marine castings, farm machinery, machine tool parts and other applications in which the part must have good strength or impact resistance [3]. 9

20 Table 2.1: Composition of Common Commercial Alloys [5] Alloy Si Cu Mg Fe Mn Ni Zn Ti A A A A A B A B A *Compositions are in wt. % and single values are maximum limits. The balance is aluminium and other trace elements such as Cr, Pb and Sn. 10

21 2.3 Literature Survey on Aluminium Alloys for Automotive Piston Applications Pistons for Gasoline and Diesel Engines In an internal combustion engine, pistons convert the thermal energy into mechanical energy. The functions of the pistons are: to transmit the gas forces via the connecting rod to the crank shaft, to seal - in conjunction with the piston rings - the combustion chamber against gas leakage to the crankcase and to prevent the infiltration of oil from the crankcase into the combustion chamber, To dissipate the absorbed combustion heat to the cylinder liner and the cooling oil. Aluminium alloys are the preferred material for pistons both in gasoline and diesel engines due to their specific characteristics: low density, high thermal conductivity, simple net-shape fabrication techniques (casting and forging), easy machinability, high reliability and very good recycling characteristics. Proper control of the chemical composition, the processing conditions and the final heat treatment results in a microstructure which ensures the required mechanical and thermal performance, in particular the high thermal fatigue resistance[6]. The continuing development of modern gasoline and diesel engines leads to specific objectives for further piston development: reduction of piston weight, increase of mechanical and thermal load capacity, lower friction and thus improved scuffing resistance, etc. In addition, the basic requirements for durability, low noise level and minimum oil consumption have to be taken into account. These goals are achieved by a targeted combination of high performance aluminium piston materials, novel piston designs and the application of innovative coating technologies. For future development, new aluminium materials using e.g. powder-metallurgical production methods or aluminium-based metal matrix composites produced by various methods as well as other lightweight materials such as magnesium alloys, carbon, etc., are being investigated. However, the on-going improvements achieved with cast and forged aluminium alloys reveal that aluminium piston materials still offer great optimization potential and will continue to play a dominant role as piston material in the future. 11

22 2.3.2 Automotive Piston Operating Conditions Pistons are subjected to high mechanical and thermal loads. The mechanical loads on the piston result from extreme pressure cycles with peak pressures up to 200 bar in the combustion chamber and Huge forces of inertia caused the by extremely high acceleration during the reciprocating motion of pistons. These mechanical loads are superimposed by thermal stresses which are primarily generated by the high temperature gradients prevalent on the piston top. Ever rising demands regarding power density as well as the need for reduced emissions, low noise and more efficient fuel and oil consumption are the main engineering challenges for engines. For the pistons, these challenges translate into maximum strength requirements in the relevant temperature range combined with minimum weight. In gasoline engines, the thermal loads have risen significantly during the last few years as a result of higher power demands. Also the stresses at average ignition pressure have increased as a consequence of the introduction of knock control, direct fuel injection and turbocharging. Moreover, high speed concepts have led to an increase in inertia load. The requirements for pistons for diesel engines are even more demanding. Modern diesel engines for passenger cars (equipped either with direct injection or super-charging with charge cooling) operate with injection pressures up to 2,000 bar, mean effective pressures over 20 bar, peak pressures of 170 to 200 bar, and achieve specific powers of up to 80 kw per litre. But also the demand for ever lower exhaust gas emissions asks for significantly improved piston material characteristics. The different elements of the piston system are indicated in figure 2.1: 12

23 Figure 2. 1: Important Piston Terminology [7] The thermal loads on the piston result from the combustion process with peak gas temperatures in the combustion chamber between 1800 and 2600 C depending on type of engine, fuel, gas exchange, compression, and fuel/gas ratio. Exhaust gases have temperatures between 500 and 800 C. Combustion heat is transferred to the chamber walls and piston top primarily by convection. The heat is then dissipated by the water cooling of the chamber walls and by the oil cooling of the piston. A large share of the heat absorbed by the piston top is transferred by the piston ring belt area. The remainder is essentially removed by the oil lubricant impinging on the underside of the piston. The resulting temperature profile within the piston is schematically outlined in figure 2.2: 13

24 Figure 2. 2: Operating Temperatures in Automotive Engines [7] Piston Materials Pistons are produced from cast or forged, high-temperature resistant aluminium silicon alloys. There are three basic types of aluminium piston alloys. The standard piston alloy is a eutectic Al-12%Si alloy containing in addition approx. 1% each of Cu, Ni and Mg. Special eutectic alloys have been developed for improved strength at high temperatures. Hypereutectic alloys with 18 and 24% Si provide lower thermal expansion and wear, but have lower strength (see tabled property data on Table 2.2 and Table 2.3 below). In practice, the supplier of aluminium pistons use a wide range of further optimized alloy compositions, but generally based on these basic alloy types. The majority of pistons are produced by gravity die casting. Optimized alloy compositions and a properly controlled solidification conditions allow the production of pistons with low weight and high structural strength. Forged pistons from eutectic and hypereutectic alloys exhibit higher strength and are used in high performance engines where the pistons are subject to even higher stresses. Forged pistons have a finer microstructure than cast pistons with the same alloy composition. The production process results in greater strength in the lower temperature range. A further advantage is the possibility to produce lower wall thicknesses - and hence reducing the piston weight. 14

25 Also aluminium metal matrix composite materials are used in special cases. Pistons with Al2O3 fibre reinforced bottoms are produced by squeeze casting and used mainly in direct injection diesel engines. The main advantage, apart from a general improvement of the mechanical properties, is an improvement of the thermal fatigue behaviour. Figure 2. 3: Piston Types for Different Combustion Engines [6] 15

26 Table 2. 2: Mechanical Properties of Piston Alloys at Various Temperatures [6] 16

27 Table 2. 3: Physical Properties of Aluminium Piston Alloys [6] The two micrograph images below further illustrate the typical microstructures to be expected: (a.) Eutectic Piston Alloy (b) Hypereutectic Piston Alloy The microstructure of piston alloys typically contain a modified eutectic aluminium matrix with dendritic silicon particles. The intermetallic phases present are Al2Cu and AlCuNi. The intermetallic phases are hardener precipitates and are responsible for impeding dislocation motion and thereby improving the mechanical properties of the alloy. 17

28 2.4 Microstructure of Al-Si Alloys & the Influence of Alloying Elements Microstructural characteristics depend not only on factors such as the solidification condition but are also closely related to the initial chemical composition of the alloy. These microstructural parameters have a significant influence on mechanical properties, these parameters include: Secondary Dendrite Arm Spacing (SDAS), the grain size, the shape and the distribution of the Al-Si eutectic and the volume fraction of the intermetallic phases [8]. Usually the practice of silicon eutectic modification is performed for Al Si casting alloys, mainly to improve their mechanical properties through the refinement of the silicon particles' natural brittle acicular/lamellar structure. Gruzleski and Closset [9] established a modification rating system of the silicon particles for hypoeutectic Al Si alloys to quantify the degree of refinement. This classification is based on the change of the structure and on the size of silicon particles and it is divided into 6 levels, where level 1 is unmodified and level 6 is a completely modified alloy with a fibrous silicon eutectic structure. This rating system is the basis for an American Foundry Society (AFS) standard wall chart used to qualitatively evaluate the silicon modification level Silicon (Si) Aluminium with silicon as a major alloying element is the most common of the aluminium casting alloys due to the impact of fluidity. Aluminium-silicon alloys supply a good combination of mechanical properties and castability and for this reason; they are widely used in the automotive and aerospace industry. Silicon increases the fluidity in aluminium casting alloys and reduces the solidification interval and hot tear tendencies. Adding more than 13 % makes the alloy extremely difficult to machine but the volume shrinkage is reduced. Mechanical properties depend more on the way silicon particles are distributed than the amount of it in the alloy. Alloys where silicon particles are small, round and evenly distributed usually display high ductility. Silicon is inexpensive and one of the few elements that can be added without increasing weight. Porosity slightly decreases with increasing silicon content. 18

29 Aluminium-silicon alloys are divided into three groups: 1. Hypoeutectic containing 5-10% silicon 2. Eutectic containing 10-13% silicon 3. Hypereutectic containing 13-25% silicon Figure 2. 4: Illustration of Aluminium-Silicon Phase Diagram Magnesium (Mg) Mg is added to Al Si alloys mainly to improve their mechanical properties through the precipitation of the Mg2Si phase, which changes into a solid solution during the solution treatment and precipitates as small particles during the aging treatment.[10] Some researchers have reported that, beside the effect of mechanical property improvement, Mg additions of up to 1 wt. % produce an increase in the level of modification of the silicon eutectic for Al Si Mg and Al Si Cu Mg alloys but without achieving a fibrous structure level. This modifying effect disappears for Mg additions higher than 1 wt. %.[11], [12]. However, most of the information on the effect of Mg as a modifier agent has been reported in combination with Sr 19

30 in a qualitative manner, without any quantitative data to support the modification level due to the Mg content by itself [8] Copper (Cu) Copper as an alloying element increases the strength, hardness, fatigue, creep resistance and machinability in an aluminium-silicon alloy. Strength and ductility are dependent on how copper is distributed in the alloy. Copper is found dissolved in the dendrite matrix or as aluminium-copper rich phases. Alloys with dissolved copper in the matrix show the most increase in strength and retention of ductility. Continuous networks of copper at the grain boundaries increase the strength to appreciable levels but the ductility decreases. Increasing the content of copper in the alloy gives a higher hardness but porosity formation increases. Aluminium-silicon alloys that contain 1.5 % copper have the optimal mechanical properties comparing to alloys having lower or higher content of copper. Al-Si-Cu-Mg alloys contain several inter-dendritic phases and the more Cu is added, the larger the area fractions of Cu-rich compounds. The size of the particles is largely influenced by the solidification rate, SDAS. The solidification rate impacts on the segregation profiles of Si and Cu but seem to be stronger for Mg. Pore fractions and sizes of the alloys are not dependent on Cu content but are influenced by the mode of solidification.[13] The tensile strength seems to be favourably influenced by the addition of Cu at the expense of ductility which is lowered due to the increased levels of intermetallics. The tensile strength decreases simultaneously by decreasing the cooling rate and SDAS of the alloys. Depending on the component design, castings that are directionally solidified and well fed, are beneficially alloyed with higher levels of Cu.[13] Strontium (Sr) Modification is one of the important melt treatments because it improves the mechanical properties by changing the microstructure of the material. Unmodified alloys contain silicon in the form of brittle flakes which leads to poor ductility of the material. The commonly used modifier is of Sr in Al-Si alloy industry because it is easy to handle, has good modification rate, a long incubation time and a low fading effect. One important feature of strontium is in consistently producing castings of good quality. 20

31 Sr addition in A357 alloys depress the growth of the α -Al dendrite and Al-Si eutectic and influence their growth mechanism. This effect was more significant with higher cooling rate as in the case of permanent mould casting. Due to addition of this modifier Primary α -Al dendrites become more equi-axed and Al-Si eutectic much finer. The microstructure shows the primary α-al phase and the eutectic distributed among the globular grains of primary phase. Sr addition to hypoeutectic Al-Si alloys results in a finer lamellar or fibrous eutectic network [14] Iron (Fe) and Manganese (Mn): The solid solubility of iron in aluminium is very low with the result that most iron forms intermetallic compounds, the nature of which strongly depends on other present impurities or alloying elements [15]. It is expected that with increasing the amount of iron from a critical percentage, the ß-FeSi5Al intermetallic inevitably would be formed. This type of intermetallic has the most significant effect on the mechanical properties of Al-Si alloys. It especially decreases ductility because this compound tends to form thin platelets which are very brittle and have substantially low bond strength with the matrix. In addition, platelets and needles of ß intermetallic have the main negative effects on fluidity, castability, and dendrites channel feeding of the alloy causing unsoundness of casting [16]. A wide range of AlFeSi particle types has been reported. These can generally be divided into three different morphologies: polyhedral crystals, Chinese script, or thin platelets. These phases are dominant in slowly cooled castings, whereas the metastable phases Al6Fe (orthorhombic) and α/-al20fe5si2 (cubic) only occur in rapidly quenched material. Since many commercial aluminium alloys contain manganese, it is to be expected that the cubic α- AlFeSi phase will be found in these alloys rather than the hexagonal α -AlFeSi phase.[16] Manganese is also able to change the morphology of the iron-rich phases from platelets to a more cubic form or to globules. These morphologies improve tensile strength, elongation, and ductility. If the iron content exceeds 0.45 wt. %, it is reported that the manganese content should not be less than half of the iron. A manganese concentration over 0.6 wt. % causes segregation whether the iron content is 0.8 or 2.3 wt. %. Thus, manganese is more powerful than iron in causing gravity segregation.[16] 21

32 2.4.6 Titanium (Ti): Titanium is added to aluminium primarily as a grain refiner. The grain refining effect of titanium is enhanced if boron is present in the melt or if it is added as a master alloy containing boron largely combined as TiB2. Some improved corrosion properties can be obtained from increasing the Ti contents in aluminium alloys to a level above the normal practice for grain refinement. However, increasing the Ti content above the peritectic point, 0.15%, can influence the grain refinement and cause casting difficulties. It was found that with normal Ti contents in the range of 0.015%, the grain refinement is effective. However, upon larger Ti additions to levels around 0.15% the grain structure becomes coarser[17]. Among the Ti-containing alloys, the increase in Ti content to 0.15% improved their wear resistance as a result of increase in the micro hardness due to the presence of relatively hardphase Al3Ti. However, these alloys showed higher wear rates (thus lower wear resistance) compared with the binary alloy due to the tendency for embrittlement and micro cracking brought about by Al3Ti particles. Heat treatment of the Ti-containing alloys improves further their wear resistance.[18] Zirconium (Zr) Additions of Zr in the range of 0.1 to 0.3% are used to form a fine intermetallic precipitate that inhibits recovery and recrystallization. An increasing number of alloys, particularly in the aluminium-zinc-magnesium family, use zirconium additions to increase the recrystallization temperature and to control the grain structure in wrought products [19]. Only a few studies are available on the effects of Zr on cast aluminium alloys. Zr is used as a grain refiner to reduce the Al cast grain size and consequently improve strength and ductility. It was reported that a minor addition of 0.15% Zr can significantly improve the hardness of A319 Aluminium alloys in both as solutionized and age-hardened conditions because of the precipitation of coherent coarsening-resistant Al3Zr dispersoids during solution heat treatment. The authors attributed increments in strengthening to Al3(Zr Sc) dispersive strengthening and substructure strengthening [20]. 2.5 Heat Treatment of Cast Aluminium Alloys The metallurgy of aluminium and its alloys offers a range of opportunities for employing thermal treatment practices to obtain desirable combinations of mechanical and physical properties. Through temper selection, it is possible to achieve properties that are largely responsible for the current use of aluminium alloy castings in virtually every field of application. The term heat treatment is used to describe all thermal practices intended to 22

33 modify the metallurgical structure of products in such a way that physical and mechanical characteristics are controllably altered to meet specific engineering criteria.[1] One or more of the following objectives form the basis for temper selection: Increase hardness Improve machinability Improve wear resistance Increase strength and/or produce the mechanical properties specified for a particular material condition Stabilize mechanical and physical properties Ensure dimensional stability Alter electrical characteristics Alter corrosion resistance Relieve residual stresses The versatility of aluminium is reflected by the number of alloys that have been developed and commercially used. A wide range in desirable combinations of mechanical and physical properties can be achieved through the heat treatment of many of these alloys. To achieve any of these objectives, parts may be annealed, solution heat treated, quenched, precipitation hardened, over-aged, or treated in combinations of these practices. The Aluminium Association has standardized the definitions and nomenclature applicable to thermal practice types and maintains a registry of standard heat treatment practices and designations for industry use: F, as-cast O, annealed T2, annealed (obsolete designation; use O instead) T4, solution heat treated and quenched T5, artificially aged from the as-cast condition T6, solution heat treated, quenched, and artificially aged T7, solution heat treated, quenched, and overaged 23

34 The heat treatment of aluminium alloys is based on the varying solubilities of metallurgical phases in a crystallographically monotropic system. Since solubility of the eutectic phase increases with increasing temperature to the solidus, the formation and distribution of precipitated phases can be manipulated to influence material properties. In addition to phase and morphology changes associated with soluble elements and compounds, other (sometimes desirable) effects accompany elevated-temperature treatment. Microsegregation in all solidified structures is minimized or eliminated. Residual stresses caused by solidification or by prior quenching are reduced, insoluble phases may be physically altered, and susceptibility to corrosion may be affected Solution Heat Treatment Solution heat treatment must be applied for a sufficient length of time to obtain a homogeneous supersaturated structure, followed by the application of quenching with the aim of maintaining the supersaturated structure at ambient temperature. In Al-Si-Cu-Mg alloys, the solution treatment fulfils three roles: [21], [22] (i) Homogenization of as-cast structure. (ii) Dissolution of certain intermetallic phases such as Al2Cu and Mg2Si. (iii) Change of the morphology of eutectic silicon. The segregation of solute elements resulting from dendritic solidification may have an adverse effect on mechanical properties. The time required for homogenization is determined by the solution temperature and by the dendrite arm spacing. Hardening alloying elements such as Cu and Mg display significant solid solubility in heat-treatable aluminium alloys at the solidus temperature; this solubility decreases noticeably as the temperature decreases. The changes in the size and morphology of the silicon phase have a significant influence on the mechanical properties of the alloy. It has been proposed that the granulation or spheroidization process of silicon particles through heat treatment takes place in two stages: (i) fragmentation or dissolution of the eutectic silicon branches and (ii) spheroidization of the separated branches [23]. During solution treatment, the particles undergo changes in size and in shape. In the initial stages, the unmodified silicon particles undergo necking and separate into segments, which retain their original morphology. As a result of the separation, the average particle size decreases and the fragmented segments are eventually spheroidized. The 24

35 spheroidization and the coarsening of eutectic Si can occur concurrently during the second stage. The solution treatment process needs to be optimized because too short a solution treatment time means that not all alloying elements added will be dissolved and made available for precipitation hardening, while too long a solution treatment means using more energy than is necessary. The solution heat treatment may be carried out in either a single step or in multiple steps. Single-step solution treatment is normally limited to about 495 C, in view of the fact that higher temperatures lead to higher thermal stresses induced during quenching and the risk of the incipient melting of the Cu-rich phases [24], [25]. This incipient melting tends to lower the mechanical properties of the casting. Solution treatment at temperatures of 495 C or less, however, is not capable of maximizing the dissolution of the copper-rich phases, nor is it able to modify the silicon particle morphology sufficiently. In Al-Si-Cu-Mg alloys having a low magnesium content (0.5 wt.%), Ouellet et al. [24] used a solution temperature of 500 o C because, at 505 o C, fusion of low melting point phases can occur; Wang et al. [26], on the other hand, reported that, for a similar alloy with a solution temperature of 520ºC, mechanical properties increase without any observable localized melting. The time at the nominal solution treatment temperature must be long enough to homogenize the alloy and to ensure a satisfactory degree of precipitate solution. In alloys containing high levels of copper, complete dissolution of the Al2Cu phase is not usually possible. The solution time must then be chosen carefully to allow for the maximum dissolution of this intermetallic phase, bearing in mind nevertheless, that solutions treating the alloy for long times are expensive and may not be necessary to obtain the required alloy strength. Moreover, the coarsening of the microstructural constituents and the possible formation of secondary porosity which result after prolonged annealing at such temperatures can have a deleterious effect on the mechanical properties [27] Quenching The objective of quenching is retention of the highest possible degree of solution with the lowest level of induced residual stresses and the least warpage or distortion consistent with commercial or specified requirements. The quench rate is especially critical in the temperature range between 450 C and 200 C for most Al-Si casting alloys where precipitates form rapidly due to a high level of supersaturation and a high diffusion rate. At higher temperatures the supersaturation is too low and at lower temperatures the diffusion 25

36 rate is too low for precipitation to be critical. 4 C/s is a limiting quench rate above which the yield strength increases slowly with further increase in quench rate. Faster rates of quenching retain a higher vacancy concentration enabling higher mobility of the elements in the primary Al phase during ageing. An optimum rate of quenching is necessary to maximize retained vacancy concentration and minimize part distortion after quenching. A slow rate of quenching would reduce residual stresses and distortion in the components, however, it causes detrimental effects such as precipitation during quenching, localized over-ageing, reduction in grain boundaries, increase tendencies for corrosion and result in a reduced response to ageing treatment [28], [29]. The best combination of strength and ductility is achieved from a rapid quenching. Cooling rates should be selected to obtain the desired microstructure and to reduce the duration time over certain critical temperature ranges during quenching in the regions where diffusion of smaller atoms can lead to the precipitation of potential defects [30]. The effectiveness of the quench is dependent upon the quench media (which controls the quench rate) and the quench interval. The media used for quenching aluminium alloys include water, brine solution and polymer solution. Water used to be the dominant quenchant for aluminium alloys, but water quenching most often causes distortion, cracking, and residual stress problems [31], [32]. With a slow quenching in air, very different precipitation features are normally evidenced. By air quenching, the material remains at high temperatures for a longer period, which enhances the diffusion of silicon and magnesium Aging/Precipitation Hardening Age-hardening has been recognized as one of the most important methods for strengthening aluminium alloys, which involves strengthening the alloys by coherent precipitates which are capable of being sheared by dislocations [33]. By controlling the aging time and temperature, a wide variety of mechanical properties may be obtained; tensile strengths can be increased, residual stresses can be reduced, and the microstructure can be stabilized. The precipitation process can occur at room temperature or may be accelerated by artificial aging at temperatures ranging from 90 to 260 o C. After solution treatment and quench the matrix has a high supersaturation of solute atoms and vacancies. Clusters of atoms form rapidly from the supersaturated matrix and evolve into GP 26

37 zones. Metastable coherent or semi-coherent precipitates form either from the GP zones or from the supersaturated matrix when the GP zones have dissolved. The precipitates grow by diffusion of atoms from the supersaturated solid solution to the precipitates. The precipitates continue to grow in accordance with Ostwald ripening when the supersaturation is lost. The length of each step in the sequence depends on the thermal history, the alloy composition and the artificial ageing temperature. 27

38 Chapter 3 METHODOLOGY 3.1 Introduction The aim of this research project is the optimization of heat treatment parameters of a cast aluminium piston alloy. The samples used for this paper are as-cast bars from scrap aluminium alloy pistons. Specimens were machined from these cast bars and heat treatment and microstructural analysis was subsequently carried out. The heat treatment of the samples was carried out in an air circulated furnace, the micrograph images and the hardness test were obtained using equipment in the University of Nairobi. 3.2 Experimental Work Heat Treatment The heat treatment was carried out in an air circulated furnace. The specimens were machined from two different cast bars for a T6 heat treatment operation i.e. solution heat treating, quenching and artificial ageing. Solution heat treatment was carried out at a temperature of 490 o C at time intervals of 1hr, 2hrs, 4hrs and 6hrs. Each sample was then quenched in water at room temperature - approximately 26 o C. This was subsequently followed by artificial aging at a temperature of 170 o C for a period of 2hrs, 4hrs, 6hrs, 8hrs, 10hrs, 12 hrs and 24hrs Microstructural Examination Before microstructural examination could be carried out, sample preparation was done. Each specimen was ground using SiC (Silicon Carbide) paper of grades 240, 320, 400 and 600 under a flow of water to remove any deep scratches or visible surface imperfections. Each specimen was washed between each paper finish to remove any remaining particles and rotated through 90 o before moving on to the next grade of paper. After the last grade the specimen was washed well in water, methanol and subsequently dried. The samples were then successively polished on rotating wheels impregnated with diamond paste of 6μm, 1μm and 1/4μm respectively till all surface scratches were removed. Test specimens were then washed with water and dried. Micrograph images were thereby taken for microstructural examination and analysis. 28

39 3.2.3 Hardness Testing The Hardness test used for the analysis was Vickers Hardness test which was conducted according to ASTM E384 (Standard Test Method for Micro hardness of Materials). Hardness measurements were taken for each sample using Vickers Hardness Tester LV-800 (shown in figure 3.1). The micro hardness tester is equipped with a diamond indenter of square shaped base and an angle of 136 o between face. The available loads of the testing machine in SI Units ranged from to 294.2N. The load used for this experiment was 29.42N for a dwell time of 15sec. The samples for micro-hardness were prepared in a similar way as the metallographic samples. Twenty indentations were taken for each sample to get a representative (accurate) hardness of the material since taking few readings would give an inaccurate representation of the material hardness by all the micro-indentations falling on either the hard intermetallic phases or on the soft aluminium matrix. Figure 3. 1: Vicker's Hardness Tester LV-800 AT 29

40 Chapter 4 RESULTS AND DISCUSSION 4.1 Effect of Heat Treatment on Alloy Micro-hardness Table 4.1 shows the average micro-hardness test values for the specimens when aged and solution heat treated at various time intervals. Prior to heat treatment the average microhardness value obtained for the as-cast alloy was 88.09HV. The attached appendix gives the hardness values obtained for 20 indentations for each specimen. Table 4. 1: Average Micro-hardness Values of Heat Treated Samples Solution treatment time 1hr 2hrs 4hrs 6hrs Ageing time (h) Hardness (HV) Hardness (HV) Hardness (HV) Hardness (HV) Table 4. 2: Standard Deviation of Micro-Hardness Values Solution treatment time 1hr 2hrs 4hrs 6hrs Ageing time (h) Standard Deviation Standard Deviation Standard Deviation Standard Deviation

41 HARDNESS (HV) Figure 4. 1: Micro-hardness Variation with Solution Heat Treatment Solution Heat Treatment - 1hr Ageing time (h) Figure 4. 2: Micro-hardness Variation with Ageing Time for 1hr Solution Heat Treated Specimen 31

42 Figure 4. 3: Micro-hardness Variation with Ageing Time for a 2hr Solution Heat Treated Specimen Figure 4. 4: Micro-hardness Variation with Ageing Time for a 4hr Solution Heat Treated Specimen 32

43 Figure 4. 5: Micro-hardness Variation with Ageing Time for a 6hr Solution Heat Treated Specimen Figure 4. 6: Average Micro-hardness Variation with Ageing Time 33

44 4.1.1: Discussion of the Variations in Micro-hardness with Increasing Solution Heat Treatment Time The hardness values increased from the as-cast value of 88.09HV with increasing solution time. The values increased to 138.7, 134, 128.5, and for solution times of 1hr, 2hrs, 4hrs and 6hrs respectively without artificial ageing. When artificial ageing was carried out, peak hardness was observed at an ageing time of 4hrs for solution heat treatment times of 1hr, 2hrs and 4hrs. However, for a solution heat treatment of 6hrs, a peak hardness value was obtained after 6hrs of ageing. The overall peak hardness value was found to be 166.9HV for a solution heat treatment time of 1hr at 490 o C with 4hrs of artificial aging at 170 o C. It is of particular interest that peak hardness is obtained after 1hr solution heat treatment and an ageing time of 4hrs as it presents significant energy savings as longer solution heat treatment times become redundant and unnecessary. It is evident that increased heat solution time causes the dissolution of more copper atoms in the α-al matrix. However, prolonging the solution heat treatment durations may cause grain growth. The increase in the grain size causes a decrease in the mechanical properties (i.e. hardness) of the alloy. The amount of undissolved Al2Cu phase is decreased in the specimens solution treated for longer periods and there are still some undissolved Al2Cu at the grain boundaries. The decrease in hardness with increasing aging time can be explained by the increase rate of growth of the precipitates at higher temperature. As the precipitates grow, the aluminium crystal lattice is strained, creating a hardening effect, once the precipitates exceed a critical size, the lattice will shear and the crystal lattice strain will be reduced. 34

45 4.2 Effect of Heat Treatment on the Alloy Microstructure The microstructure of the samples were examined with the light optical microscope before and after the heat treatment. Figure 4.21 shows the microstructure of the Aluminium alloy before heat treatment. Al2Cu (AlCuNi) Si α-al 50μm Figure 4. 7: As-Cast Alloy Microstructure The as-cast microstructure shows an unmodified aluminium-silicon eutectic structure. The α- Al matrix contains dendritic silicon particles. Cu particles are not visible on the as-cast microstructure. The phases present in the as-cast are Si, Al2Cu and AlCuNi precipitates. The shape of the second phases are flake like with rough/sharp edges. The micrographs of the solution heat treatment specimens have been displayed on Figures 4.8 to 4.11 below: 35

46 Spherodization of Si particles α-al Necking of Si particles 50μm Figure 4. 8: 1hr Solution Heat Treated Specimen without Artificial Ageing Si AlCuNi Fragmentation of Si particles 50μm Figure 4. 9: 2hr Solution Heat Treated Specimen without Artificial Ageing 36

47 Al 2 Cu Si α-al AlCuNi 50μm Figure 4. 10: 4hr Solution Heat Treated Specimen without Artificial Ageing Necking of AlCuNi Particles α-al Spherodization of Si particles 50μm Figure 4. 11: 6hr Solution Heat Treated Specimen without Artificial Ageing 37