Aluminium Alloy Reinforced with Fly Ash: Techniques and Particle Size

Jurnal Tribologi 23 (2019) 13-37

Aluminium alloy reinforced with fly ash: Techniques and particle size

Shankar, S. *, Harichandran, S.

Department of Mechatronics Engineering, Kongu Engineering College, Erode, 638060, INDIA. *Corresponding author: [email protected]

KEYWORDS ABSTRACT Utilization of metal matrix composite materials has grown very fast rate owing to its high weight ratio, low density and low coefficient of thermal expansion. High- temperature resistance, as well as low maintenance, is also an added advantage of using this composites. Fly ash which has low ash density is mainly used as a reinforcement in molten metal, normally reduces overall weight as well as the density of the fabricated composites. Fly ash particles are mainly preferred because of their good mechanical properties and low cost. Ex-Situ methods are mostly solid state processing and liquid state Aluminium alloy processing. Solid state which includes Powder Metallurgy Fly ash (PM). Liquid state processing includes Stir Casting, MMCs Squeeze Casting, Compo Casting, and liquid metallurgy. In Stir casting technique the recent past, several researchers have utilized the Powder metallurgy above said conventional methods to prepare the composites. Comparatively, powder metallurgy for solid state and stir casting for liquid state processing are highly preferred methods. This paper provides a review of the comprehensive literature on the fabrication techniques, the overall performance of variation in fly ash size and weight percent in the preparation of the fly ash-reinforced aluminium alloy composites. Also its corresponding tribological properties such as wear rate and friction coefficient with respect to variation in the content of fly ash are also reported.

Received 22 January 2019; received in revised form 13 April 2019; accepted 28 May 2019. To cite this article: Shankar and Harichandran (2019). Aluminium alloy reinforced with fly ash: Techniques and particle size. Jurnal Tribologi 23, pp.13-37. © 2019 Malaysian Tribology Society (MYTRIBOS). All rights reserved.

Jurnal Tribologi 23 (2019) 13-37

1.0 INTRODUCTION Metal matrix composites (MMCs) are metals which are reinforced with other metallic, organic or ceramic compounds. They are prepared by mixing the reinforcements in the metal matrix. The reinforcements are generally used to improve the base metal properties such as strength, stiffness, conductivity, etc. Aluminium and its alloys attracted the most attention in recent years as a base metal in metal matrix composites (B.Vijaya Ramnath, 2013). Reinforcing fly ash composites are mainly to eradicate the cost barrier for manufacturing automotive and small engine parts. Increase in aging resistance is required for aluminium alloys as this contributes to the development of acceptable mechanical properties. Fly ash is mixed as reinforcement in molten and cast metal which reduces the total weight as well as density (Shukla et al., 2018). Al6061 is the commonly used matrix material due to its unique combination of several properties such as low density, high strength, high corrosion resistance, better mechanical properties, low electrical resistance, and good machinability characteristics. Particles such as mica, alumina (Al2O3), graphite, zirconia, boron and silicon carbide (SiC) are also used as fillers with aluminium alloys. Recently hard ceramic particles like Si3N4 is also used as reinforcement in aluminium alloys to enhance its tribological properties (Haq & Anand, 2018). Although these composites have high specific strength and high specific modulus, the focus is on their cost effectiveness (Nanjayyanamath et al.). Coal fly ash is one of the by-products of coal combustion process from the thermal power plants, and it is the most complex and abundantly available anthropogenic materials. The environmental impact of aerated coal has now been fully recognized. Most methods of ash removal eventually lead to spillage of fly ash in the open land. Improperly disposing of the fly ash and irregular accumulation causes them to be removed over large areas of land resulting in soil degradation as well as causes major risks to human health and the environment. Fly ash particles are very small which normally escape from the exhaust gas purifiers and ease to float in the atmosphere is a major source for gas pollution. Repeated contact with fly ash can irritate the human eyes, nose, throat, skin and respiratory tract and sometimes results with arsenic poisoning (Yao et al., 2014). The prepared composites are normally evaluated for its mechanical and tribological behaviour to conclude its effectiveness in various applications. Composites with fly ash as reinforcement are tested for several applications including brake friction materials (Ahlawat et al., 2019). Normally considering the preparation methodology, tribological aspects of ecological and environmental balance, Green tribology plays a major role (Anand et al., 2017). Fly ash contains approximately 20-60% of SiO2, 5-35% Al2O3, 1-40% CaO, 4-40% of Fe2O3, and 0- 15% of LOI. Stirring is another technique involved in producing MMCs as it follows the conventional metal formation pathway. Gas layers on the surface of the particles may cause floating migration, so normally mechanical agitation is performed to separate the gas layers and also it reduces the surface tension of the molten aluminium. The paddling technique also provides a relatively homogeneous and fine microstructure that enhances the addition of reinforcing material to the molten metal (Bharat Admile, 2014). Recent development in stir casting process involves double agitation method or termed as two-step mixing method. The matrix material is initially heated above its liquidus temperature. Then the heat is removed by cooling process to achieve a temperature between the liquidus and solidus points or to a semi-solid state. At this point, the preheated strengthening particles are added and thoroughly mixed. Again, the suspension is heated to a completely liquid state and then properly mixed once again. The resulting microstructure from double agitation moulding is more uniform than the conventional agitation. The effectiveness of this two-step mixing process is mainly due to its ability to break the gas layer

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around the surface of the particle, which hinders wetting between the particles and the molten metal(Kala et al., 2014). Electromagnetic stirring (EMS) is one such forming process in which the mould cavity is filled by injecting the semi-solid suspension cylinder after the uniform dendritic microstructure which formed during the solidification process. Spherical primers in the eutectic phase were in the early stages of solidification. EMS should be a better replacement system for mechanical agitation to avoid the contamination of alloy and damage to the agitator. The rheological formation is controlled by the fraction of grains and solids using the electromagnetic stirring system(Dwivedi et al., 2015). The percentage difference by weight of the fly ash particles is improved by the compression moulding path. Liquid casting, mainly die casting is preferable because it is the simplest, least expensive and most attractive for mass production. The resulting samples are tested for hardness, tensile strength, compression, impact resistance and density(Lokesh et al., 2013). Powder metallurgy (PM) is a widely used production technique for MMCs. This is due to a deeper understanding of the mechanisms of particles in recent decades which improves microstructure control and reinforcement distribution in particle-based composites(Vogiatzis et al., 2015). Another advantage of particle processing is the ease of obtaining uniform particle distribution in the metal matrix over other fabrication techniques, particle deposition being a common problem in poured MMCs.(Liu et al., 1994). In fabrication methods, Ex-situ method mainly consists of solid-state treatment and liquid state treatment. Normally solid state includes powder metallurgy (PM) and liquid state processing includes stir casting, squeeze casting, compo casting and liquid metallurgy. Several previous works used PM, stir casting, die casting and some other techniques such as high energy ball milling, mechanical alloying, spark plasma sintering, non-pressure infiltration gas injection, ultrasonic cavitation solidification, laser deposition, vortex gel synthesis, etc. for fabricating composites (Aniruddha V. Muley, 2015). This paper consolidates various production methods such as stir casting, squeeze casting, compo casting, powder metallurgy and liquid metallurgy route with their consequences and advantages due to the variation in particle size for the preparation of composites using aluminium alloy and fly ash particles.

2.0 STIR CASTING TECHNIQUE Agitation is a liquid state process for producing composites in which a dispersed phase is mixed with a molten matrix metal using mechanical stirring. Agitation is the simplest and least expensive method of preparation in the liquid state. This normally involves in stir casting technique. Udaya and Moorthy (2013) investigated the influence of wear parameters on the adhesive wear character of aluminium matrix composites (AMCs) using the method of stir casting. They found that the load has the highest influence on wear rate followed by sliding distance, sliding speed, and the weight percentage of reinforcement. The difference in the wear value of composites is not changed anywhere with increasing the fly ash content over 6% under the test conditions due to the limited debris formation. Kumar et al. (2012) investigated the wear rate, and friction coefficient of aluminium reinforced fly ash composites. In this, a feed forward back propagation artificial neural networks (ANNs) model was developed for stir casting method. By using the developed ANNs model, the wear rate and coefficient of friction were predicted. Senthiil and Shirrushti (2017) investigated the properties of the fabricated composites which was produced using stir casting technique from industrial fly ash waste. When compared with the

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unreinforced alloys, the properties of metal matrix composites like specific strength, specific modulus, damping capacity and wear resistance were increased. Narasaraju and Raju (2015) investigated the properties of the composites prepared by reinforcing aluminium alloy (AlSi10Mg) with fly ash and inexpensive rice husk ash. Compared to unreinforcement alloy, 10% fly ash and 10% rice husk ash reinforced aluminium alloy shows the significant improvement. Kumar et al. (2014) investigated the properties of composites which reinforced 4% Mg and 15% fly ash results with high tensile strength and also 4% Mg and 20% fly ash results with maximum hardness. The addition of fly ash and graphite also reduces the specific wear rate. Surappa (2008a) investigated the dry sliding wear resistance of the Al-fly ash composite which was prepared using stir casting method resulted with similar outcomes of Al2O3 and SiC reinforced Al-alloy. Tiwari et al. (2017) compared the properties of the fabricated composites and alloy. The reinforcement of fly ash particles (60-100 μm) in aluminum alloys tends to reduce the tensile properties because the soft aluminum alloy expands during tensile deformation and the hard fly ash particles do not deform at the same rate as the aluminum alloy. It can be seen that the impact resistance of the fly ash composite ADC12 decreases concerning the aluminum alloy. Gikunoo et al. (2005) investigated the properties of the composites prepared using stir casting method which is produced from the addition of fly ash with A535 aluminium alloy which leads to Mg depletion. Kulkarni et al. (2014) investigated the mechanical properties such as tensile, hardness and compressive strength of AMCs which are prepared using stir casting method. Hybrid composites have lower densities when compared with superior properties. Boopathi et al. (2013) investigated the properties of Al-SiC and Al-fly ash composites. The composites are fabricated using stir casting method, and the studies resulted in the wear and corrosion evaluation. Babu et al. (2014) investigated the improvement of properties using the fly ash up to 20% by weight for the Aluminium composites using stir casting route. Mahendra and Radhakrishna (2007) investigated the increment of mechanical properties like, tensile strength, impact strength, compression strength, and hardness while increasing the content of fly ash. Density was found to decrease, at the same time the dry wear, corrosion and slurry erosive wear was increased. Dinaharan and Akinlabi (2018) investigated the composites prepared from Al alloy AA6061, Mg alloy AZ31, with copper and fly ash as reinforcement. Friction stir processing was used to synthesize the composites. Microhardness of the composite was found to increase while increasing the fly ash particle. Canute and Majumder (2018) utilized the stir casting route to produce the hybrid composites, which contains aluminium A356 alloy as a matrix and boron carbide and fly ash as a reinforcement. Further, the developed composites were used in the production of automobile parts which require high wear and thermal resistance. Senapati et al. (2015) prepared the aluminium matrix composites using the liquid stir casting method which utilizes the waste fly ash from the thermal power plant. Ramesh et al. (2018) investigated the properties of fly ash composites prepared using stir casting route. Wear resistance of this composite was much greater than the commercially available unreinforced Al 2024 alloy. When the content of fly ash increases, composite density was decreased resulted in light weight and good strength. Rao et al. (2012) investigated the properties of composites produced from Al alloy- fly ash using stir casting method. The fly ash particles present in the matrix was uniformly distributed and also ensured the better bonding between the fly ash and matrix. The density of the composites and fly ash was indirectly related, which means that the increment of the fly ash content reduces the density of the composite, but increases the hardness of the composites when compared with the base alloy.

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Senapati et al. (2014) investigated the properties using continuous stir casting method, where the metal matrix composite was produced from Al-Si alloy and fly ash. The resulted impact strength was 63.55 J, 67.94 J, 70.75 J for the base alloy LM-6, fly ash mixed alloys MMC-A and MMC-B. The compressive strength of the MMC-A (138.87kN) was greater than the MMC-B (132.11kN). Also, the micro hardness of the MMC-B (74.9 HV) was greater than the MMC-A (71.925 HV). Raju and Kumar (2017) investigated the aluminium and fly ash composites which were produced from stir casting technique. Tensile strength was reduced, and hardness was increased during the increment of fly ash. From the experimentation, 20% of fly ash content provided the desired hardness which also results with the uniform distribution of the particles in the metal matrix composites. Alaneme and Sanusi (2015) utilized the two-step stir casting method to fabricate the Al-Si-Mg alloy composites from the alumina, rice husk ash, and graphite. Wear rate of the composite was low when compared to the fewer graphite composites. Increasing the graphite content 0.5-1.5 wt % reduces the wear resistance. Dwivedi et al. (2015) utilized the electromagnetic stirring process to fabricate the hybrid metal matrix composites from aluminium alloy A356, SiC and fly ash particles. Increasing the fly ash content up to 5% increases the impact strength, tensile strength, fatigue strength and hardness of the hybrid metal matrix composites. If it increased to 10%, 15% or 20% fly ash content it decreased the above-listed properties. Dinaharan et al. (2016) investigated friction stir processing of AA6061 aluminium metal matrix which was reinforced with fly ash particles. Adding the fly ash particles increases the wear resistance of the composites. Senapati et al (2014) investigated the aluminium alloy based matrix composite (AMC) properties prepared from the waste fly ash which was collected from two different industries (Type A and Type B) using the stir casting method. Comparison concluded that the composites from type B provided better mechanical properties. Bera and Acharya (Bera & Acharya) investigated the tribological behaviour of the fabricated LM6/ cenosphere composites prepared from the waste fly ash cenosphere as a reinforcement using stir casting method. 10% of reinforcement provides superior properties than the unreinforced case. Overall, the stir casting method was mainly used to produce the composites from liquid state processing method. Analyzing the above literature, fly ash weight percentage plays a major role which determines the mechanical and tribological properties. Typically, the average weight of fly ash for accomplishing better mechanical properties was in-between 5% -15%. Also, the size of the fly ash was normally within 1μm - 100μm provided better results when compared with unreinforced alloy. Also if the weight percentage of fly ash was increased above the average percentage level which reduces the mechanical properties. Table 1 shows the consolidated works carried out using fly ash as a reinforcement and the stir casting route as a methodology to fabricate the specimens. Various fly ash sizes, % weight/volume fraction with several base materials were also tabulated.

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Table 1: Alloy and fly ash composition in stir casting technique. # Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) 1 A413 3%,6%,9% 75 μm 0.0018 - - (Udaya & 0.0037 Moorthy, 2013) 2 A380 3.25% Cu 9% 0.001 - 0.15 - (K Ravi Kumar et + 8.35% Si 0.003 0.35 al., 2012) + 0.18% Mg + 0.9% Fe + 0.15% Mn + 1.65% Zn + 0.32% Ni + 0.12% Pb + 0.09% Sn + 0.06% Ti + Al. 3 Al6063 5%,10% - - (Senthiil & Shirrushti) 4 AlSi10 10-13% Si 5%,10%,15 0.1- - - (Narasaraju & Mg + 0.2-0.6% % 100 Raju, 2015) (AA60 Mg + 0.6% μm 61) Fe + 0.1% Cu + 0.3- 0.7% Mn + 0.1% Ni + 0.1% Zn + 0.1% Pb + 0.05% Sn + 0.2% Ti + Al. 5 Al6061 Al6061 + 10%,15%,2 100 0.001- - (V. Kumar et al., Mg + 0% μm 0.016 2014) graphite 7 A356 7.08% Si + 0-12% 0.5– 0.0005 - 0.35 - (Surappa, 2008a) 0.41% Mg 400 0.007 0.65 + 0.06% μm Cu + 0.09% Fe + 0.1% Ti + Al.

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) 8 ADC12 10.93% Si 0%,6% 60- - - (Tiwari et al., + 0.73% Fe 100μ 2017) + 2.17% m Cu + 0.13% Mn + 0.12% Mg + 0.95% Zn + 0.18% Ni + 0.08% Pb + 0.02% Ti + Al. 9 A535 6.5-7.5% 0%,10%,15 1-100 - - (Gikunoo et al., Mg + % μm 2005) 0.10% Cu + 0.25% Mn + 0.20% Si + 0.20% Fe + 0.25% Ti + 0.15% others + Al. 10 Al356 0.25% Cu 4%,8%,12 100 - - (Kulkarni et al., + 0.20- % µm 2014) 0.45% Mg + 0.35% Mn + 6.5- 7.5% Si + 0.6% Fe + 0.35% Zn + 0.25% Ti + 0.05% others + Al 12 AA202 5%,10% - - (Boopathi et al., 4 2013) 13 Alumin 20% - - (Babu et al., ium 2014) alloy

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) 14 Al- 4.52% Cu 0%, 5%, 75– - - (K.V.Mahendra, 4.5%C + 0.663% 10%, 15% 100 2007) u Fe + µm 0.131% Mn + 0.075% Ni + 0.029% Pb + 0.021% Sn + 0.013% Ti + 0.118% Zn + Al. 15 AA606 18% 5 μm - - (Isaac Dinaharan 1 and & Akinlabi, 2018) AZ31 16 A356 0.20% Cu 2%,4%, 20 µm 0.0015 - - (Canute & + 0.25- 6%,8% 0.0045 Majumder, 2018) 0.45% Mg + 0.10% Mn + 6.5- 7.5% Si + 0.20% Fe + 0.10% Zn + 0.20% Ti + Al 17 Al-Si 12.2491% 14.2%,13.3 2 - - - (A. Senapati et al., alloy Si + % 40µm 2015) 0.0174% Co + 0.4353% Fe + 0.0800% Cu + 0.1601% Mn + 0.0672% Ti + 0.0944% Zn + 0.0264% Ni +

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) 0.0632% Sn + 0.0199% Cr + 0.0082% Ca + 0.0146% V + 86.7654% Al. 18 Al2024 8%,8.5%,9 40- - - (Ramesh et al., % 100 2018) µm 19 AA202 4.52% Cu 2%,4%,6%, 60 µm - - (Rao et al., 2012) 4 + 1.938% 8%,10% Mg + 0.066% Si + 0.663% Fe + 0.131% Mn + 0.075% Ni + 0.029% Pb + 0.021% Sn + 0.013% Ti + 0.118% Zn + Al. 20 Al-Si 0.01% Co 240 - - (A. Senapati et al., (LM6) + 12.24% µm 2014) alloy Si + 0.43% Fe + 0.08% Cu + 0.16% Mn + 0.06% Ti + 0.09% Zn + 0.02% Ni + 0.05% Pb + 0.06% Sn

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) + 0.01% Cr + 0.008% Ca + 0.01% V + 86.7% Al. 21 Al7075 0.4% Si + 15%,20%,2 - - (Raju & Kumar, 0.5% Fe + 5% 2017) 1.2-2.0% Cu + 0.3% Mn + 2.1- 2.9% Mg + 0.18- 0.28% Cr + 5.1-6.1% Zn + 0.2% Ti + 0.15% others + Al. 22 Al-Mg- 0.48% Mg 0%,5%, <50 - - (Alaneme & Si alloy + 0.52% Si 50%,75%, µm Sanusi, 2015) + 0.073% 100%,99.5 Mn + %, 0.068% Cu 99%, 98.5% + 0.11% Zn + 0.015% Ti + 0.058% Fe + 0.003% Na + Al. 23 A356 6.5-7.5% 0% - 20% 25 μm - - (Dwivedi et al., Si + 0.2% 2015) Fe + 0.2% Cu + 0.1% Mn + 0.25- 0.45% Mg + 0.1% Zn + 0.1% Ti + Al. 24 AA606 0.95% Mg 0%,6%, 2 µm 0.015 - (I Dinaharan et 1 + 0.54% Si 12%,18% 0.045 al., 2016) + 0.22% Fe

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) + 0.13% Mn + 0.17% Cu + 0.09% Cr + 0.08% Zn + 0.02% Ni + 0.01% Ti + Others Al 25 A5083 2%,5% 53– - - (Santhosh, 106 Kempaiah, Sunil, μm & Reddy, 2017) 26 Al+3 5% 53– - - (Fan & Juang, Mg 106 2016) μm 27 Al-Si Al + 11.8% 1 – 15% 50 to - - (Suresh, (LM6) Si + 250 Venkateswaran, alloy 0.32% Fe µm & Seetharamu, + 0.002% 2011) Cu + 0.62% Mn + 0.065% Mg + 0.021% Zn 28 Al6061 0.4% Fe + 0%,3%,6%, - - (Patel, Rana, & + 0.5% Si + 9% Singh, 2017) Al6063 0.3% Cu + 0.3% Cr + 0.9% Mn + 0.2 % Zn + 0.1% Ti + 95.8- 98.5% Al +0.03% Others 29 LM25 0.1% Cu + 3% 3-100 - - (Patil & Motgi, 0.2-0.6% μm 2013) Mg + 6.5- 7.5% Si + 0.5% Fe + 0.3% Mn + 0.1% Ni

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) + 0.1% Sn + 0.05% Ti + Al 30 AA221 3.87% Cu 5%,10%,15 0.01 - 0.1 - 0.5 (Moorthy, 8 + 1.90% Ni % 0.08 Natarajan, Palani, + 1.47% & Suresh, 2013) Mg + 0.51% Si + 0.16% Fe + 0.02% Ti + Al 31 AA606 0.95% Mg 7.5% - - (J. D. R. Selvam, 1 + 0.54% Si Smart, & + 0.22% Fe Dinaharan, + 0.17% 2013b) Cu + 0.13% Mn + 0.09% Cr + 0.08% Zn + 0.01% Ti + Al 32 Al6061 95.8- 10% - - (Malhotra, 98.6% Al + Narayan, & Gupta, 0.15% Mn 2013) + 0.70% Fe + 0.15- 0.40% Cu + 0.15% Mg + 0.4- 0.8% Si + 0.25% Zn + 0.4- 0.35% Cr + 0.05- 0.15% others. 33 Alumin 10%,15%,2 50- - - (Shanmughasund ium 0% 100 aram et al., 2012) alloy µm 34 A356 Al+ 7.08% 6%,12% Narro - - (Surappa, 2008b) Si w size

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) (53– 106µ m) and wide size (0.5– 400µ m) 35 Al-Si 12.2491% 5µm - - (A. K. Senapati et (LM6) Si + to al., 2014) alloy 0.0174% 30µm Co + 0.4353% Fe + 0.0800% Cu + 0.1601% Mn + 0.0082% Ca + 0.0672% Ti + 0.0944% Zn + 0.0264% Ni + 0.0632% Sn + 0.0199% Cr + 0.0146 V + 86.7654% Al 36 LM6 Al 0.1% Cu + 5%,7.5%, 30– 0.2e-5 - - (Bera & Acharya) 0.1% Mg + 10%,12.5% 200 0.45e-5 10-13% Si µm (N/m) + 0.6% Fe + 0.5% Mn + 0.1% Ni + 0.1% Zn + 0.1% Pb

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# Base Chemical Fly ash % Fly Wear Friction References alloy compositi ash rate coefficie on size (mm3/ nt (μm) m) + 0.2% Ti + Al 37 LM6 Al 5%,7.5%, 30– 0.1 - 1.2 0.4 -0.75 (Bera & Acharya, 10%,12.5% 200 (10-11 2018) µm mm3/N m)

3.0 COMPOCASTING TECHNIQUE Compo-casting is a simple and inexpensive casting method. Compo-casting is a type of agitation process in which the metal is heated to a semi-solid state instead of the liquids. Few researchers used this method to fabricate MMCs reinforced with fly ash. Mohammed Razzaq et al. (2017) fabricated the aluminium matrix composites using AA6063 reinforcing with fly ash particles using the compocasting method. The porosity and micro hardness of the composite were increasing due to the increment of fly ash particles and at the same time, the Charpy impact energy and bulk density of the composite was decreased. Rajan et al. (2007) investigated the Al – fly ash composite properties which were fabricated from the modified compocasting cum squeeze casting method. Compocasting and liquid metal stir casting methods provided better distribution of the fly ash particles than other methods. Selvam et al (2016) fabricated the AA6061 aluminium alloy and fly ash particles using the compocasting method. The comparison of base alloy and fabricated composites were made using a scanning electron microscope. The composites were able to carry a high load of above 4.9N due to the better mixing of fly ash particles. Razzaq et al. (2017) investigated the properties of the composites prepared using compocasting method for AA6063 alloy and fly ash reinforcement. The fly ash content was increased, and the resulting mechanical properties like compression, tensile, density, and microstructure, etc. were compared. The compression strength increases and the density was found to decrease during the increment of fly ash particle. Selvam et al. (2013a) investigated the properties of the composites prepared from AA6061 aluminium alloy and fly ash as a reinforcement using the compocasting method. Micro hardness and tensile strength of the composites was enhanced during the addition of fly ash particles. The composition of AA6061 with 12% fly ash provided 132.21% of higher micro hardness and 56.95% of higher ultimate tensile strength when compared with the base alloy. The compocasting method was similar to the stir casting method. The better results were obtained for the composites at below 12% weight percentage of fly ash. Also, the size of the fly ash particles used to prepare the composites should be as same as stir casting method (Table 2). Overall as similar to stir casting, the increment of fly ash above the average weight percentage reduces the mechanical properties.

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Table 2: Alloy and fly ash composition in compocasting technique. # Base Chemical Fly ash Fly ash Wear Friction References alloy composition % size(µm) rate coefficient of alloy (mm3/m) 1 AA6063 <0.01% Cu + 0-12% 44-µm - - (Mohammed 0.19% Fe + Razzaq et 0.51% Mg + al., 2017) 0.04% Mn + 0.86% Si + 0.008% Ti + 0.01% Zn + 0.01% Cr + Al 2 Al356 5%,15% 13µm- - - (Rajan et al., 320µm 2007)

3 AA6061 4-12% 2-3 µm 10-4 - (J. Selvam et 2.5 X 10-6 al., 2016) 4 AA6063 <0.01% Cu + 2%,4%, 44 μm - - (Razzaq et 0.19% Fe + 6% al., 2017) 0.71% Mg + 0.04% Mn + 0.86% Si + 0.008% Ti + 0.01% Zn + 0.01% Cr + Al 5 AA6061 0.95% Mg + 0%,4%, - - (J. D. R. 0.54% Si + 8%,12% Selvam et 0.22% Fe + al., 2013a) 0.13% Mn + 0.17% Cu + 0.09% Cr + 0.08% Zn + 0.02% Ni + 60.01% Ti + Al

4.0 SQUEEZE CASTING TECHNIQUE Liquid metal forging is a process in which the molten metal solidifies under pressure in closed dies placed in-between the plates of a hydraulic press. The other name for the compression moulding method is the liquid phase method. This section consolidates the papers which focussed on squeeze casting route to fabricate MMCs. Prasad and Ramachandra (Prasad & Ramachandra, 2018) investigated the properties of aluminium - fly ash metal matrix composite which was prepared using the squeeze casting method. The prepared metal matrix composites had higher resistance from abrasive wear and also resulted hardness was higher than the unreinforced alloy which was due to the presence of

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harder fly ash as reinforcement. Zhua and Yan (2017) investigated the fabrication of the mullite interpenetrating composite from the A356 alloy – fly ash material using three-stage squeeze casting method. Due to the presence of fly ash as reinforcement, the fabricated composite was used as a breaking material because it contains good potential in resisting wear. Zahi and Daud (2011) utilized the liquid-phase method for fabricating MMCs with the composition of the different type of Al alloys and fly ash particles. The presence of fly ash particles increases the tribological and surface properties when compared with the base alloy. Bharathi et al. (2017) investigated the performance of composites prepared from LM25 and fly ash particles using the stir-squeeze method. Increasing the fly ash content increases the hardness, stiffness and abrasive wear resistance of MMC. Lokesh et al. (2013) fabricated the aluminium metal matrix composite from the matrix using Al-4.5% Cu alloy and reinforcement of fly ash and SiC by squeeze casting method. The increment of fly ash % increases the mechanical properties like hardness, impact, compression, tensile, and wear resistance. Squeeze casting was also another type of liquid state process, which especially applicable for LM alloys. But, the size of fly ash utilized was high (up to 500μm) when compared with the previous methods. Table 3 provides the list of different base alloy, fly ash % and particle size of fly ash used in the squeeze casting process to fabricate the MMCs.

Table 3: Alloy and fly ash composition in squeeze casting technique # Base Chemical Fly Fly ash Wear Friction References alloy composition ash % size rate coefficient of alloy (µm) (mm3/m) 1 LM6 0.002% Cu + 5%, 38.38 - - (Prasad & 0.065% Mg + 7.5%, 94.28 Ramachandra, 0.021% Zn + 10%, (mg/min) 2018) 0.32% Fe + 15% 0.62% Mn + 12.2% Si + Al 2 A356 6.50%-7.50% 50µm, 0.0057 - 0.45 -0.48 (Zhu & Yan, Si + 0.25- 100µm, 0.0128 2017) 0.45% Mg + 250µm, 0.20% Fe + 500µm 0.20% Ti + 0.20% Cu + 0.10% Mn + 0.10% Zn + 0.15% Others + Al 3 Al-Mg 2%,6%, 2µm - - (Zahi & Daud, alloy 8% 2011) 4 LM25 Si-7% 2.5%,5% - - (Bharathi et al., 2017)

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5.0 LIQUID METALLURGY Liquid metallurgical agitation is an economical liquid treatment process which controls process parameters such as melting temperature, particle feed rate, stirring speed, and the time required to achieve uniform particle distribution and also eliminate porosity problems. This section consolidates the fabrication of MMCs which utilized liquid metallurgy process. Kumar and Sreebalaji (2015) utilized three different sizes of fly ash particles and aluminium alloy to fabricate the composites (Al/3.25Cu/8.5Si) using liquid metallurgy process. Increasing the sliding speed and load reduces the coefficient of friction, but the increment of fly ash article size and wt % increases the coefficient of friction. Kumar et al. (2012) fabricated using four different fly ash % to produce aluminium alloy A380 composite using liquid metallurgy method. Addition of fly ash % increases the ultimate tensile strength as well as the hardness and the wear resistance. But, the coefficient of friction and density was found to decrease due to the addition. Only a few works (Table 4) utilized liquid metallurgy route to fabricate the composites which may be due to the high-cost involvement. The average fly ash percentage utilized in liquid metallurgy route was 3% - 12%. The particle size varies from 50 –150 µm.

Table 4: Alloy and fly ash composition in liquid metallurgy. # Base Chemical Fly ash Fly ash Wear Friction References alloy composition % size rate coefficient of alloy (μm) (mm3/m) 1 AA7075 3.25% Cu + 3%,6%, 53–75 0.14 -0.16 0.49 - 0.72 (K. R. Kumar 8.35% Si + 9% μm, & Sreebalaji, 0.18% Mg + 75–103 2015) 0.9% Fe + μm and 0.15% Mn + 103– 1.65% Zn + 125 μm 0.32% Ni + 0.12% Pb + 0.09% Sn + 0.06% Ti + Al 2 A380 3.25% Cu + 3%,6%, 50–75 0.001 - 0.2 -0.35 (Krishnan 8.35% Si + 9%,12% µm, 75– 0.004 Ravi Kumar 0.9% Fe + 103 µm et al., 2012) 1.65% Zn + and103– 0.92% Others 150 µm + Al

6.0 POWDER METALLURGY Powder metallurgy (PM) normally covers a wide range of possibilities in which the materials or components are prepared using metal powders. Normally PM process greatly reduces the need for using several metal removal processes, thus considerably reducing the yield losses during manufacture often at a low cost. This section consolidates the literature which used a powder metallurgy process to fabricate the MMCs. Reddy et al. (2010) fabricated the composite using powder metallurgy method from aluminium alloy and fly ash, in which Pb was used in single action die compaction. Increasing the

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lead (Pb) content up to 10% increases the green density of the composite, after that % its density was decreased. Itskos et al. (2011) prepared the metal matrix composite (MMC) from aluminium alloy and fly ash particles using powder metallurgy. The fly ash used in this process was highly calcareous and siliceous. Addition of 15% wt fly ash provides better wear resistance compared to base alloy. Kumar et al. (2010) investigated the properties of AA6061 alloy reinforced with the fly ash particle composites which were prepared using press sinter extrusion (PSE) and press extrusion (PE) process. Structure and properties of the composites from PSE and PE was compared using the calculated result of this composites from powder metallurgy. This investigation shows that PE had good mechanical properties than the PSE, because it increases the interfacial bonding. Escarlera-Lozanoet al. (2007) investigated the characteristics of corrosion from the Al/SiCp/Spinel composite prepared using recycled aluminium, SiCp and fly ash particles from the powder metallurgy process. Two types (type A and B) of composites were investigated which were differed by % of Si content. Comparison of 8% and 3% Si resulted in the presence of 8% Si provided better properties. Bhandakkar et al. (2014) fabricated the aluminium alloy and fly ash particle metal matrix composites using a powder metallurgy method. Optical microscope and scanning electron microscope images were used to study the flow instability, mechanism failure and microstructural of metal matrix composites. Siddhi Jailani et al. (2011) utilized aluminium silicon alloy and fly ash particles to fabricate the composites using powder metallurgy route. Al alloy with 12% fly ash composites had low density and high hardness than the base alloy. Green density was higher when compared with the sintered density of the Al alloy – fly ash composite. Resulted with lower compressive strength for Al alloy- fly ash composite when compared with base alloy. Also, the increment of fly ash content reduces the compressive strength. Marin et al. (2011;2012) prepared the metal matrix composites from ASTM class C618 and highly calcareous using powder metallurgy method. The increment of fly ash content increases the hardness and mechanical properties, but it oppositely reacted against corrosion behavior of the composites. Kumar et al (2010) utilized hot extrusion and powder metallurgy process for developing composites from an aluminium alloy (AA6061) and fly ash particles. Wear resistance of AA6061-fly ash composites was increased at T6 condition (300 ◦C) when compared to the room and high temperature. Wang et al. (2014) utilized Al-Mg alloy and fly ash particles to fabricate metal matrix composite using powder metallurgy method. When the fly ash content and the normal load was minimum, the wear mechanism had been characterized as abrasive wear and adhesive wear. But, with high fly ash content and high normal load, the wear was characterized into abrasive wear and delamination wear. Moutsatsou et al. (2010) fabricated the Al/fly ash composites from two class C and lignite fly ash using powder metallurgy technique. The increment of fly ash content decreases the density of the composites and also increment of fly ash enhanced the hardness of metal matrix composites. Korpe et al. (2017) used aluminium alloy, boric acid and fly ash particles for the fabrication of metal matrix composites using powder metallurgy technique. The increment of fly ash particle increases the strength of the composites. Yield stress and plateau stress of the composite was in the range of 10-30 MPa and 7-16 MPa respectively. Reddy et al (2013) utilized the powder mixtures of Aluminium-Pb and fly ash for the production of aluminium metal matrix composite using powder metallurgy method. The increment in compaction pressure increases the density, hardness and compressive strength of the composites. Addition of Pb increases the density but also decreases the hardness and compressive strength. Siddhi Jailani et al. (2017) prepared the composite from the Al-Si alloy and fly ash particles which were produced using powder

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metallurgy method. The prepared composite had less wear rate compared with the base alloy and also increases the wear rate when the load and sliding speed increases. Also, the prepared MMC had a high coefficient of friction compared with the base alloy. Moutsatsou et al. (2009) reported the preparation and compaction methods of aluminium metal matrix composites with aluminium alloy-fly ash and Al-Si alloy – fly ash using powder metallurgy method. The increment in fly ash decreases the density of the composites. Subarmono (2011) utilized the fine powders of aluminium and fly ash for the preparation of aluminium metal matrix composite (AMMC) using powder metallurgy technique. Addition of 7.5% wt of fly ash provided better result when compared with others. Bending strength, porosity, hardness, and wear rate of AMMC were 68.5MPa, 5.4%, 62.6VHN and 0.0571mg/MPa respectively. The mechanical properties of AMMC were much better than the base alloy and pure aluminium. Overall, powder metallurgy (PM) was another important technique used to prepare the composites from solid state processing. Various fly ash % and size used to fabricate the composite from the PM process was listed in Table 5.

Table 5: Alloy and fly ash composition in powder metallurgy. # Base alloy Chemical Fly Fly ash Wear Friction References composition of ash % size rate coefficie alloy (μm) (mm3/m) nt 1 Aluminium 0%,5% - - (S Pichi Reddy alloy , et al., 2010) 10%,1 5% 2 Al/Si alloy 10%,1 56 μm 99 - 308 0.4 -0.8 (Itskos et al., 5%, (10-14 2011) 20% m3/Nm) 3 AA6061 Al + 1.0% Mg + 0%,2% 10μm - - (P. Kumar, 0.6% Si + 0.25% Kumaran, & Cu + 0.1% Sn Rao, 2010) 4 Al/Sic/spine Al + 8% Si + 75 μm - - (Escalera- l composite 15% Mg , Al + Lozano et al., (MgAl2O4) 3% Si + 15%Mg 2007) 5 AA6061 5%,10 80 μm - - (Bhandakkar %, et al., 2014) 15% 6 Al/Sic/spine Al + 8% Si + 75 μm - - (Siddhi Jailani l composite 15% Mg , Al + et al., 2011) 3% Si + 15% Mg 7 Aluminium 86.40% Al + 5%,10 20 μm - - (Elia Marin et (Al) silicon 12.12% Si + %, al., 2012; E alloy 0.54% Fe + 12%,1 Marin et al., 0.42% Mg + 5% 2011) 0.17% Cu + 0.063% Zn + 0.287% others

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# Base alloy Chemical Fly Fly ash Wear Friction References composition of ash % size rate coefficie alloy (μm) (mm3/m) nt 8 AA6061 Al + 1% Mg + 2%,6% 50µm, 0.022 - 0.05 - (P. Kumar, 0.6% Si + 0.25% , 10% 100 µm 0.028 0.25 Kumaran, Rao, Cu + 0.1% Sn et al., 2010) 9 Al-Mg alloy 5%,10 5-50 0.1 - 4.5 0.24 - 0.4 (Wang et al., %, μm (mg/min) 2014) 15%,2 0% 10 Al-Si alloy 5%,10 <56 μm - - (Moutsatsou %, and > et al., 2010) 15% 56μm 11 Aluminium 10% <45 μm - - (Korpe et al., alloy 2017) 12 Al-Pb alloy 10% 5-50 - - (Seelam Pichi μm Reddy et al., 2013) 13 Al-Si alloy 86.40% Al + 5%,10 1 - 15 4 -16 0.1 - 0.6 (Siddhi Jailani 12.12% Si + % µm (10-7 et al., 2017) 0.54% Fe + gm/m) 0.42% Cu + 0.063% Zn + 0.287% Others 14 Al-Si alloy 5%,10 56μm - - (Moutsatsou %, et al., 2009) 15% 15 Aluminium 25%,5 - - (Subarmono, alloy %, 2011) 7.5%,1 0%

7.0 FRICTION AND WEAR BEHAVIOUR This paper reviews the production methods of aluminium- fly ash composites. Before utilizing it to the commercial applications, the prepared composites were subjected to several mechanical, corrosion and tribological testing. Normally the wear rate was computed using a pin-on-disc tribometer, and the mechanisms of wear were identified using the wear debris and worn surfaces (Dinaharan et al., 2016). Increase in weight percentage of fly ash in the composites alters the coefficient of friction, wear rate as well as the mechanical behaviour of the prepared composites. In most cases, the hardness, ultimate tensile strength and wear resistance of the composites increased, whereas the coefficient of friction and density of the composites decreased while increasing the weight percentage of fly ash particles. Table 1 presents the composites prepared using stir casting methodology. Normally the wear rate varies with respect to the content of fly ash and also it depends on the base material in which the fly ash is reinforced. The coefficient of friction varies from 0.1 to 0.6. Table 3 presents the composites prepared using squeeze casting

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which also reflects the same tribological behaviour as prepared from stir casting route. Table 4 & 5 presents the results of the composites prepared using liquid and powder metallurgy route. Friction coefficient values ranges from 0.02 to 0.8 which is quiet high compare to casting processes but the range of wear rate is more or less similar.

8.0 CONCLUSION This paper consolidates ex-situ methods in producing aluminium - fly ash reinforced composites which consists of solidstate processing and liquid state processing. Several researchers had utilized powder metallurgy, stir casting, squeeze casting and many other techniques such as high energy ball milling, non-pressure infiltration gas injection, mechanical alloying, ultrasonic cavitation solidification, spark plasma sintering, vortex gel synthesis, and laser deposition, etc. for preparing the aluminium matrix composites. Al-MMCs were mainly suitable for fabricating brake drums because of their high wear resistance. Fly ash particle size and its weight percentage influencing the properties of composites were discussed concerning various base aluminium alloys. Mostly stir casting was preferred by several researchers owing to the production cost. This fly ash fixes the thrift of the fly ash and reduces the production costs, which is an economical and environmentally friendly solution.

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