Keywords

1 Introduction

Material researchers have given much attention on composite materials in the recent past years, because of having desirable properties, unordinarily that could lead to various useful engineering applications in different domains. AMCs which is majorly used in aerospace, defence, nuclear energy, automotive, sports recreational, biotechnology and thermal management must be considered as materials for energy conservation and environmental safeguard [120]. Automotive component application areas of AMCs are fins, gear box housing, disc rotor brakes, rotating blade sleeves, suspension strut, crankshafts, rocker arms, etc. In electronic sector, some of the uses are in the integrated heat sinks, microprocessor lids and microwave manufacturing. Aerospace industries use AMCs as aircraft wings, body structure faming and gears for landing [4]. AMCs have drawn attraction for its economic production and desirable properties. Such desirable properties are higher specific stiffness, specific strength and low coefficient of thermal expansion with better wear and corrosion resistance. Strength along with wear susceptibility is of prime importance for the applicability of AMCs in different service areas [5].

Performance level of MMC depends on:

  • Types of reinforcements combined,

  • Wt% of reinforcing materials mixed,

  • Wettability between matrix and reinforcements,

  • Fabrication route selected,

  • Metallurgical characteristics of the matrix material [5].

Primary challenging task when processing MMCs is to achieve homogeneous and uniform distribution of reinforcements in the matrix as this puts a great impact on the final properties and the desirable quality of the material. Uniform distribution and prevention of segregation or agglomeration of reinforcements help to get better microstructure that leads to achieve good aggregate characteristic profile of all MMCs. Available processing routes of MMCs can be classified into three types, such as (a) Liquid phase (b) Solid phase and (c) Infiltration technique. Semisolid phase processing such as rheocasting and compocasting is also widely used fabrication process, apart from these three techniques. Liquid phase process mainly includes stir casting, high-energy laser melt injection, plasma spraying and squeeze casting, etc. Ultrasonic cavitation-assisted stircasting process as shown in Fig. 1 has ease in breaking the agglomeration of reinforcing particles. Solid phase incorporates mainly powder metallurgy technique, where microwave sintering (shown in Fig. 2) is gradually taking the place of conventional sintering in MMC production. Infiltration route has application in industry for its large-scale production advantage. Figure 3 shows basic flow diagram of liquid metal infiltration technique. In the route of infiltration problem arises when the reinforcement concentration becomes high. Additional processes are required to dilute the reinforcing agent to the required levels in that case. Increased processing times and steps at elevated temperatures often produce a secondary phase as a result of chemical reactions between matrix and particle. This secondary phase usually brittle in nature [100]. Stir casting process has proved to be useful for its simplicity, high productivity and economical [113]. Recently modified stir casting or two-step stir casting are gaining attraction of the researchers for very well dispersion and bonding of reinforcements within the aluminium metal matrix [2]. Particle or fibre reinforced MMC are proofing higher strength and modulus, far better wear resisting behaviour with ease in production and lower cost of preparation [17]. However, too much degradation of ductile behaviour of the MMCs due to higher level of inclusion of ceramic particles is a matter to think about. Interest has been grown by incorporating ceramic particles size of nano-level to produce metal matrix nano-composites (MMNC) that maintain good ductility [78]. Stable and non-reactive performance must be thereby the reinforcements in the range of functioning temperature for better susceptibility to wear [87]. Ceramic powders as reinforcements possess exceptional strength compared to other class of reinforcements; hence, these are majorly useful as primary reinforcements of HAMCs. Ceramics have certain advantages like (a) high hardness, (b) heat resistant, (c) low thermal expansion coefficient, (d) medium conductivity and (e) corrosion resistant. Most commonly used synthetic ceramics reinforcements are SiC, Al2O3, SiC, B4C, ZrO2 and TiB2. Secondary reinforcements are used for the purpose of machinability increment, cost reduction and lowering density [112]. The significant input parameters like particle size and types of reinforcement had percentage contribution of 27.30% and 48.72%, respectively, to control the microhardness of AMC [114].

Fig. 1
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Schematic diagram of ultrasonic-assisted stircasting

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Fig. 2
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Schematic diagram of microwave-assisted hot pressing

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Fig. 3
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Schematic diagram of liquid metal infiltration

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Based on the outcomes of several researches, it has been found that silicon carbide (SiC) can be opted as key ceramic element for aluminium matrix or its alloy base composites [66]. Studies have been done in last two decades for AMC characterization using industrial wastes such as fly ash, red mud, granite dust and agricultural wastes like rice husk ash (RHA), sugar cane bagasse ash (SCBA), coconut shell ash, bamboo leaf ash (BLA), ground nut shell ash, palm oil fuel ash and maze stalk ash. Fly ash is potential discontinuous dispersoids that have got more attention of the researchers among all these waste particulates and results also show to be very promising in tribo-mechanical properties of the fabricated products of AMCs [41]. Strength of aluminium matrix composite not only depends on the reinforcement type, it is also a function of reinforcement size when varied from micro to nano.

Hence, present literature study deals with the different experimental findings achieved by different researchers in the field of Al-MMCs reinforced with micro- or nano-SiC and FA. The microstructural, mechanical, tribological, thermal, machining and some other relevant properties of fabricated AMCs or HAMCs have been progressively reassessed and discussed in detail. Similar experimental conditions have been avoided to focus the objective of this chapter. Major target has been set to illustrate or highlight the influencing parameters and test conditions which greatly affect the properties of ultimate developed AMCs reinforced with this two major synthetic ceramic and agro-industrial waste. Effect of fabrication techniques and reinforcement size (micro/nano) on the distribution of reinforcement in aluminium matrix has been precisely described. This chapter has been organized categorically in five major divisions. First section is introductory part about the reinforcement types and processing methodology of Al-based MMCs. The second, third and fourth sections depict the available research outcomes regarding SiC reinforced AMC, fly ash reinforced AMC and combined SiC, FA reinforced hybrid AMCs. Last section concludes about the prospects in this research area, which helps the researchers from industry or academic to get a motivation about the AMC or HAMC reinforced with micro/nano-SiC and FA.

2 Study on Al-MMC: Reinforced with SiC

In the timeline of AMCs development, different reinforcing materials were used, that are generally grouped into three major types, as (i) synthetic ceramic-based, (ii) industry generated wastes and (iii) agriculture-based wastes. Ceramic particulates reinforced metal matrix composites have favourable properties like high specific strength, better resistance to wear and good capability to retain strength at elevated temperatures [30]. SiC is such a ceramic material which holds high hardness (2800 kg/mm2), high rigidity (elasticity modulus as 410 GPa), good thermal conductivity (100 W/m K for single crystal and 4–20 W/m K for poly crystal), moderate toughness, low specific gravity (density as 3.2 g/cm3), low thermal expansion coefficient (4.3 × 10–6/K) and less responsive against thermal shock (Melting point 2730 °C) [96, 114]. Aluminium or its alloy when reinforced with SiC fulfils the need of lightweight, high strength material in various industries from automotive to aircraft [82]. Many investigations have been executed in last three decades to develop mechanical, tribological and thermal behaviour of SiC reinforced AMCs. Figure 4 presents the future trend and developing areas of AMC synthesis.

Fig. 4
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Trends of aluminium MMC development

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It has been reported by the researchers that both the particle size and dispersion have evident effect on mechanical strength of the discontinuously reinforced metal matrix composites (DRMMCs) composites [132]. Outcomes of the use of micro-size and nano-size SiC reinforced Al-MMC are described below.

2.1 Micro-SiC

2.1.1 Physical and Mechanical Behaviour

Souvignier et al. have shown aluminium matrix composites can be shaped by tool-less manufacturing technique such as extrusion freeform fabrication (EFF). This is one type of rapid prototyping method by multiphase jet solidification technique combined with fused deposition modeling. Silicon carbide particles (5–10 μm) upto 20 vol% were most successful in retaining high density and doubling the strength of the parent metal matrix [117]. The density is prime concern to produce lightweight composite material which needs to be lowered by the inclusion of low density reinforcement addition in to metal matrix. SiC (density = 3.2 g/cc) has slightly greater density than Al-alloy (density = 2.6–2.7 g/cc). The density of AMCs is checked both theoretically (applying rule of mixture) and experimentally (applying Archimedes’ principal). 1.30% increment in the density of the Al6061–SiC composite occurred with the rise of SiC wt% from 0 to 6. This is mainly attributed to the more denser solid structure of constituent SiC particles than that of the Al-alloy [128].

Mechanical strength of micro-size silicon carbide particle produced via conventional stir casting or modified stir casting was evaluated on the basis of tensile, impact, compression load sustainability and hardness behaviour, [7, 14, 46, 49, 80, 115, 126, 128]. The UTS, YS, compression strength and resistance to indentation of Al–SiC composites were significantly increased with increase in silicon carbide wt% or vol% in the parent matrix metal and ductility (% of elongation) droped down [14, 49, 116, 128]. This is in line with the other ceramic particulate reinforced composites. Very small amount of magnesium (Mg) (1–1.5 wt%) could be used as wetting agent with SiC where it forms Mg2Si. This is the compound responsible for achieving more strengthening and better dispersed microstructure of AMC [115]. Comparison between vacuum-assisted high-pressure die casting (HPDC) process and gravity die cast (GDC) process with 10 vol% micro-SiC has cleared that the former process helped to acquire improved particle distribution, reduced porosity and good bonding at matrix/particle interface [46]. Different metal (like Cu and Ni)-coated SiC micro-particles were used to enhance micro-structure and phase composition [35, 47, 48]. Physical and mechanical property results of fabricated 5083 Al-alloy composite using 40 μm SiC show that the ultrasonic assisted stir casting process was beneficial compared to the ordinary stir casting process. This has been attributed to ultrasonic blending process properly distributes the silicon carbide particles into the aluminium matrix by eliminating micro-voids [49]. Micro-hardness of 37 μm SiC reinforced AMC was increased upto 15% because of the strong adhesion between matrix alloy and reinforcement. However, it was found to decrease in hardness at 20% SiC due to the formation of fine cracks at the interface [99]. Uniform particle distribution of SiC reinforcements, reduced porosity level and fine homogeneous microstructural dissipation of the composite through liquid metallurgy process lead to achieve better hardness [7]. Hence, the rotational speed of stirrer and stirring time during addition of particle into the molten aluminium vortex also has influence on the hardness of the composite. It was evidenced that increased stirring speed and time during semisolid condition results into better distribution of silicon carbide in the aluminium matrix, thus enhanced hardness was achieved [84]. Effects of three different Al-alloy (2124, 5083 and 6063), two different sizes of SiC particles (157 and 511 μm) and two different extrusion ratios (13.63 and 19.63) on impact behaviour were studied at different temperature. The result showed toughness of Al6063 composites increased a bit with increase in the size of SiC particle and extrusion ratio. Though test temperature did not possess any significant effect on the failure of the composites. Four-point bending test was conventionally carried out by hot isostatic pressing (HIP) of 30 μm SiC powder in Al-alloy for checking strength of AMC under bending load [142]. Bending strength was not significantly influenced by the SiC particle size distribution after HIP [125].

2.1.2 Effect of Secondary Treatments

Secondary treatments of composites as casted or infiltrated such as hot extrusion or hot rolling generally helped in reaching the targeted mechanical behaviour of particle reinforced MMCs [81]. Hardness, mechanical and sliding wear resistance properties of AMC can be extended as the consequence of heat treatment and ageing of composites [24]. T6 heat treatment both solutioning and ageing were applied on micro-SiC/Al AMC. Despite suggesting T4 treatment for the composite [123], conventional T6 thermal treatment is widely accepted in SiC/Al composite due to the flexibility of artificial ageing [24, 29, 130]. When SiCp content in Al-composites is low (8–14 vol%), overageing (OA) plays more important role than peak-ageing (PA). T6 heat treatment of AA2024/40 µm SiCp composites even exhibited considerably low specific wear rate than grey cast iron as well as H13 tool steel [56]. Main factor to control ageing kinetics of the composites’ matrices is the lattice spacing, which reduces as SiCp vol% increased [57]. More amount of inter metallic dislocation formed as a result of thermal discrepancy in between the aluminium metal matrix and SiC reinforcements was attributed to the better ageing kinetics of Al-alloy. With more SiC particles concentration, this ageing phenomenon (through T6 heat treatment) leads to reduce time required to obtain the maximum hardness [95]. Retrogressed re-ageing (RRA) of the AMC samples at 120 °C for 24 h was used to fully restore the peak aged condition of the T6 treatment. It was concluded from comparative study that T6 treatment improves the mechanical properties of the 7075 Al-alloy system reinforced by 40 μm SiC while RRA made the composite better wear resistant [24]. In due course of enhancing coefficient of thermal expansion (CTE) property of SiC/AMC, T6 tempering is better suggested to minimize the change in dimension at elevated temperatures [29]. Ageing treatment was noticed more beneficial for improving fracture toughness, and its result was found better than that of the Al-borax premixed SiC composites. The improvement attributed to the presence and fine coherent distribution of Mg2Si precipitates formed in the Al (6063) matrix at the time of ageing process [3]. It was noticed that during powder metallurgy processing small ratio of particle sizes between Al-matrix and reinforcement distributed the SiC particles homogeneously. It was well understood that much smaller particle size than the matrix powder size can form cluster very easily in the matrix. This will result an inhomogeneous distribution of reinforcements which was already seen by previous researchers [132].

2.1.3 Machining Behaviour

AMC materials become difficult to machine, especially when hard and abrasive reinforcing elements like silicon carbide (SiC) particles are reinforced. Turning operation was conducted on SiC/AMC to figure out the effect of machining criterion, like feed rate, cutting speed, depth of cut on wear rate of tool and surface roughness of specimen [55, 80]. At the condition of turning without coolant, tool wear was increased mainly with increase in the velocity of cutting. Greater content of SiCp reinforcement increased tool wear; higher cutting speed and lower feed rate were recommended to achieve minimal surface roughness.

2.1.4 Thermal Behaviour

Movement or collision of electron inside any material dominates the heat conduction behaviour of any metal which is denoted by thermal conductivity (W/m K). The presence of ceramic-based SiC particles inside the structure forms resists the motion of aluminium electrons providing conductivity. Thermal conductivity of double phase composite relies on the factors like individual thermal conductivity of the constitutional phases, interaction and spacing between phases and present wt% of phases. Thermal conductivity of the Al-composites drops down with rise in wt% of SiC (referred to Fig. 5), and a reverse phenomenon was detected with increasing size of SiC particle from 70 to 40 μm. This was ascribed to increasing particle size of SiC reduces scattering of electrons and thus mobility of electrons increase [34]. Though SiC reinforced Al-composite had improved heat conductance than the unreinforced matrix alloy [140].

Fig. 5
figure 5

Reproduced with permission from Elsevier, Copyright (2014)

The effect on thermal conductivity with respect to size and proportion of SiC particle [34].

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2.1.5 Tribological Behaviour

Al-SiCp composites processed by liquid metallurgy possess significantly higher wear resistance compared with unreinforced alloy during sliding. Bai et al. reported this increased wear resistance comes from the existing hard silicon carbide particles that can restrict the flow ability tendency of material and avoid the development of iron-rich layers on the composite surface [12]. At the circumstance when the fractural strength of particles is more than applied stress, the SiC particles mainly bear the applied load. The abrasive nature of SiC particles on the mating surface leads to the separation of iron oxide (Fe2O3) layers which was placed onto the mating surfaces of composites provides ten times obstruction to the abrasion phenomenon than matrix alloy. SiC particles were fractured under the action of huge localized strain gradients. These strains were then transferred to the aluminium alloy matrix that nucleates subsurface cracks. This cracks were then propagated by decohesion of particle–matrix interfaces. Shifting of wear from mild to severe generally happened in the unreinforced alloy above a critical load. This phenomenon could be suppressed by the addition SiCp [6]. A mathematical formula was modelled to predict dry sliding wear of MMC and compared experimentally with Al-SiC AMC. In that study adhesion, ploughing and particle fracture are taken as dominating wear mechanism. Hence, sliding friction coefficient is calculated using the following relation (Eq. 1):

$$f = \frac{{F_{a} + F_{p} + F_{f} }}{P},$$
(1)

where P is applied load, total frictional force, Ft is considered to be caused by adhesive force (Fa), ploughing force (Fp) and fracture force (Ff). The particle fracture force can be calculated using the following Griffith equation (Eq. 2), i.e.

$$F_{f} = K_{2} A_{p} f_{v} \frac{{K_{1c} }}{{a^{1/2} }}$$
(2)

K1c is the fractural toughness of the resulting composite, K2 being a geometric factor depends on the type of particles reinforced in the composite, a is the particle size, fv is the particle volume fraction, Ap is the cross-sectional area of scratch [141].

The coefficient of friction of AMC was found 25% higher than the normal casted iron when sliding in similar conditions explored the suitability of SiC/Al-MMC as material for brake rotor applications [79]. The formation of mechanically mixed layer (MML) due to transfer of iron particle from counterface disc to the pin was observed more with wt% of SiC in Al-composites which is another reason for decreasing nature of wear [45]. Impact of SiC particle content, sliding speed along with applied pressure is the prime concern for manufacturing better wear-resistant AMC, being studied by R. N. Rao and S. Das. Wear rate was observed to decrease at a constant rate with rise in SiC proportion and increases as the sliding speed increased. Though coefficient of friction goes up with SiC particle concentration in Al-alloy, it decreased on attaining higher sliding speed. This rise in coefficient of friction attributed to higher frictional force for sliding as microchips were formed due to micro-cutting of counterface by deep penetration of hard SiC particles. It was also seen that there exists a maximum pressure to be applied on the composite samples beyond which wear rate gets seized after remaining constant before that applied load. The temperature also gradually increases at first when applied pressure was increased, and in the latter stages it increased abruptly, when a critical level of the applied pressure was reached [89]. The composite reinforced with coarse grain of SiC micro-particles (37 μm) showed more wear resisting behaviour than that of the composite being produced with finer particles (13 μm) when rubbed against abrasive Al2O3 under dry ambient condition [20]. Wear mechanism map for Al7010-alloys and its composite reinforced with 25 wt% SiC was compared separately depending on wear rate and temperature hike. The plot for composite was divided into four regions—(a) ultra-mild wear, (b) mild wear, (c) oxidative wear and (d) delamination wear, while pure alloy exhibited only two zones—(a) mild wear and (b) severe wear or delamination wear. The plot of wear mechanism for Al7010-25 wt% SiC composites in Fig. 6 depicts the seizure load and transition load (change from mild to severe rate of wear) at different sliding speed. It was found that significantly higher seizure load (~30.7% higher at 0.52 m/s of sliding speed) and transition load (~85.7% higher at 0.52 m/s of sliding speed) required to start transition of mild wear to severe wear in composite in comparison with the base alloy [88]. Similar mechanisms were observed when wear behaviours Al2024/20 vol% SiC composites were investigated by preparing through PM route and artificially ageing at elevated temperature in between 20 and 250 °C. In the prepared composite samples when temperature went above a critical value, a transition of wear mechanism from mild zone to sever zone was found. This critical value of temperature was shifted from 150 to 200 °C by the inclusion of SiC particles than matrix alloy without reinforcements [76].

Fig. 6
figure 6

Reproduced with permission from Elsevier, Copyright (2013)

Wear rate map for Al7010–25% SiCp [88].

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The tribological testing parameters of micro-SiC reinforced AMCs were optimized by Taguchi’s orthogonal array design of experiment. It was noticed that reinforcement volume or weight fraction was the most influencing factor reasonable for the wear properties compared to load and sliding speed whether the composite was either produced by stir casting [38, 39] or PM [101]. Both the main effects and interaction effects of the parameters are studied interaction between load and speed (L × S) which is the significant interaction. Maximum extent of reinforcement content and minimum magnitude of sliding speed, load and sliding time was the optimal combination for minimizing rate of wear [39]. In PM route production, contributions of the grain size and hardness were about 81.57 and 11.09% towards abrasive wear behaviour [101].

Corrosion study in 3.5% NaCl solution was very commonly done on Al-SiC composite processed by stir casting [58, 111] or squeeze casting [40]. Potentiodynamic polarization test using a potentiostat is generally used for precise corrosion test. The magnitudes of corrosion potential (Ecorr) and corrosion current density (Icorr) at open circuit potential (OCP) condition are noted to evaluate corrosive nature of samples. Corrosion resistance improved with increase in vol% of SiC reinforcements. Addition of 10% SiCp in the base alloy was appeared to be optimum proportion for the synthesis of a corrosion-resistant Al–Cu alloy-based composite. Reinforcement particles were agglomerated with further increase of SiCp from 15 to 25 wt%. This agglomeration of SiCp opened cathodic sites and boosted galvanic effect, hence corrosion rate again started to increase. Rapid pitting corrosion started due to adsorption/diffusion of chloride ions of NaCl [111].

2.2 Nano-SiC

2.2.1 Microstructure and Mechanical Behaviour

Literature available so far reveals that use of nano-size silicon carbide particles or fibre in AMC fabrication has been accepted during start of second decade of twenty-first century. Y. Yasutomi et al. have shown interest to understand the reaction due to diffusion of aluminium into nano-SiC fibre while producing electric cable by bundling 27 SiC fibre/Al composite elemental wires together. Generation of Al4C3 and Al2SiO5 at the SiC/Al interface can be converted into molten Al through thermal treatment [137]. Use of magnesium enhances the wettability between Al and SiC, also prevents the formation of Al4C3 by forming a protective layer of MgO at the interfacial layer between nano-SiC and liquid aluminium. It was also difficult to avoid the formation of Al4C3 when lower sizes (like 38 μm) of Al particles are used for AMC produced via powder metallurgy (PM) route. Finer Al particulate was found to be highly active to react and can easily be exposed to surface of SiCp by embedding into the gap among the big Mg particulate [133]. It is being already reported that strengthening effect of nano-SiC particles is superior over micro-size particles [139]. This was attributed to strong refinement of α-Al dendrites in as-cast Al-composites from dendritic crystal with sizes about 200 μm to equiaxial crystal with sizes about 75 μm [131]. The yield strength, ultimate tensile strength and the modulus of elasticity were significantly improvised when nano-size particles were incorporated, though ductility was found to be decreased with nano-SiC particle (50 nm) addition [72]. As reported by A. Mazahery and M. O. Shabani that highest strength values were reached with the inclusion of 3.5 wt% nano-SiC particles. On the other hand, G. Bajpai et al. have found that resistance against tensile and compressive load including hardness of Al-Nano-SiC composites primarily raised up to 2 wt% content of nano-SiC and then fell down at 3 wt% [13, 122]. Similar result was observed by C. C. Gallardo et al. while in their experiment saturation level of hardness and compressive stress occurred at 2.5 wt% of nano-SiC [21]. Hardness of the nano-SiC reinforced Al6061 alloy increased significantly by 73% compared with the base alloy [70]. It has been noticed after reviewing available literatures that 2–4 wt% of nano-SiC gives better result for increasing mechanical strength. Studies have been done to understand the influencing parameters of stir casting such as stirring rate and stirring temperature on the micro-mechanical behaviour of the Al-composites. There exist an optimum stirring rate and temperature for every mixing at which best properties and dissipation of nanoparticulates might be achieved. Stirring temperature of 750 °C and stirring rate of 700 rpm proved to be fruitful for manufacturing AMC containing 1 wt% of 25–50 nm size SiC particle in A356 aluminium alloy [27]. Non-destructive ultrasonic testing helps to minimize the sample count during any mechanical test. Ultrasonic pulse received from x-cut and y-cut transducer and processed via pulse echo overlap method (PEO) had been successfully employed to evaluate the mechanical behaviour of nano-aluminium-SiC composites [33]. Poisson’s ratio indicates that the characteristics of bonding forces whether the material is “stiffer” or “softer” were found to be decreasing with increasing volume % of nano-SiC. L16 Taguchi orthogonal design of experiments was employed for optimizing different process parameters of friction stir processing route during the production of AA6061/SiC nano-composites. Four factors were considered that were tool penetration depth rotational speed, transverse speed and profile of the pin. ANOVA results confirmed that rotational speed as the most crucial parameter and threaded pin profile is better than square pin in achieving higher ultimate tensile strength (UTS) [10, 11, 103]. FESEM and atomic force microscopy (AFM) images reveal that hardness of AMC generally varies inverse to the inter particle spacing of reinforcements [102].

While going for fine nanoparticles, a chance of agglomeration increases during processing of aluminium metal matrix composite via solidus and liquidus routes. Solid methods like high-energy planetary milling [60, 61, 110] could increase hardness value of the aluminium and 1 vol% nano-SiC composite upto 163 HV. Experimental density closely matched with theoretical density while Al 5083 alloy with 10 wt% SiCp was prepared via high-energy ball milling along with a process called as spark plasma sintering (SPS) [15]. Various innovative liquid methods such as stir casting with cryorolling [26], cryomilling [124], solvothermal-assisted grapheme encapsulation of SiC nanoparticles [37], ultrasonic cavitation-assisted stir casting [51, 70, 135] were developed to stop this phenomenon. Study on fracture behaviour have shown higher SiC nanoparticle content increases possibility of failure at the weakly bonded particle boundaries causing higher stress concentration during deformation [136]. Finer grain refinement of α-aluminium and eutectic Si phase can be obtained by better dispersion of nano-SiCp. High-intensity discontinuous ultrasonic treatment (HIDUT) was found to be more effective than high-intensity continuous ultrasonic treatment (HICUT) for improvising mechanical properties. An increase in UTS upto 100 and 126% increase in YS were observed for A413–2% SiCnp composite [28]. UV treatment assisted with squeeze casting can efficiently refine α-Al and eutectic Si phase of aluminium matrix nano-composites (AMNC). This refinement along with increased UV time and applied pressure resulted into improvement in tensile properties of the AMNC [138]. The fractographic investigations have also disclosed predominant nano-void coalescence fracture mechanism due to the uniform distribution of nano-SiC particles [37]. Major influencing fracture mechanism was observed as inter-dendritic cracking, and there was no occurrence of debonding at the interface in between parent alloy matrix and the SiC reinforcement particulates [73].

2.2.2 Effect of Secondary Treatment

Strengthening of nano-SiC reinforced (both in wire form and powder form) was done through annealing [134] T4, T6 heat treatment [8, 59]. A considerable improve in the mechanical strength of Al-alloy was observed. Microstructure analysis using SEM disclosed that T6 heat treatment leads to dissociation of dendrite structure of α-Al and pallet like Si particle globule formation in eutectic phase. Using low ageing temperature for the heat treatment process of SiC particles (<500 nm) reinforced 6061 Al-alloy displayed good stress–strain curve (refer to Fig. 7a) and enhanced hardness (refer to Fig. 7b) while maintaining its ductility [59].

Fig. 7
figure 7

Reproduced with permission from Elsevier, Copyright (2014)

a Stress versus strain curves and b Vickers microhardness graph of Al 6061 alloy and 10 wt% SiC and 15 wt% SiC composites [59].

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2.2.3 Thermal Behaviour

15-nm-sized silicon carbide (0, 0.3, 0.5, 1.0 and 1.5 vol%) was reinforced in 99% pure Al-metal matrix through microwave sintering followed by hot extrusion process. Processed composites were then examined for mechanical strength, fracture behaviour and coefficient of thermal expansion (CTE). Mechanical properties are noticed to be following similar trend as seen by other researchers, best was observed at 1.5 vol% SiC. Thermo-mechanical analyzer with a heating rate of 5 °C/min in between a range of 50–350 °C and argon gas was passed at 0.1 lpm to determine CTE of the prepared samples. Values showed a decreasing trend of CTE with increased addition of hard nano-SiC particles, that indicated better dimensional stability of AMC at higher temperature [94].

2.2.4 Tribological Behaviour

The dominant wear mechanisms in dry sliding wear behaviour of nano-AMC were determined using SEM and EDS microstructural characterization technique on worn surface. The abrasive wear was the controlling wear mechanism for nano-composite sample than sever adhesive wear. The coefficient of friction (cof) values of the SiC reinforced Al-nano-composite was not changed considerably by different applied stresses in between 0.3 and 0.9 MPa. This was attributed to the generation of mechanically mixed layer (MML) by the oxidized debris particle acting as lubricant film at the contact surface of nano-composite [74]. In another study, pin-on-disc dry sliding wear test of Al6061/1.2 wt% nano-SiC (50 nm) was conducted at different load of 10, 20, 30 and 40 N with a fixed sliding velocity of 0.5 m/s for 1000 m sliding distance. Specific wear rate and frictional coefficient decreased with increase of load from 10 to 40 N, whereas normally in case of base alloy Al6061 it has increasing plot. Higher applied load increased temperature at the contact surface causing more oxidization of wear debris from the counterpart (hardened steel disc). The MML layer was formed in case of the nano-composite effectively reduced the wear rate of nano-composite compared to matrix alloy. Surface roughness at the wear zone of the base alloy was reduced from 1.69 to 6.20 μm by the incorporation of nano-size SiC particle via liquid metallurgy. Plastic deformation was reduced to a minimum level as most of the applied pressure was taken by the nano-silicon carbide; hence, the surface roughness of the composite was smoothed. Field effective SEM (FESEM) and SEM of worn surface was checked, and adhesive wear and delamination wear mechanism were found to be presiding. This was being attributed for causing wear in matrix alloy, that changed into abrasion and oxidation in case of nano-composite [70, 75]. A combined dry sliding and corrosive wear tests of nano-SiC composite was performed using high-temperature tribometer (pin-on-disc) with an attached container filled with 3% NaCl solution at 25 °C. 1, 2 and 3% volume fraction of nano-size SiC (25–50 nm) were mixed with Al6061 alloy powder (38–63 μm) using mechanical milling cold pressing and hot extrusion for sample pin preparation. An improved corrosion resisting nature was seen with the increased vol% of nano-SiC due to non-reactivity or inertness of nano-SiC particles with the corrosive solution [75]. Wear test was conducted by placing the sample pin on a flat plate and applying reciprocating motion in between them at a constant rotational speed of 38 rpm for developed Al–SiC nano-composite. Weight loss was reduced by increasing the accumulative roll bonding process (ARB) cycle, and also, the size of SiC particle becomes less than 100 nm which leads to form nano-composite. This refinement of SiC particle reduces weight loss [23].

3 Study on Al-MMC: Reinforced with FA

Fly ash is combustional by-product of pulverized form of coal contain primarily alumina-silicates. Table 1 shows the wt% (approximate) of constituting chemical component of fly ash. Fly ash has lower density in contrast with parent metal (2.6 g/cm3) and other ceramic particulates commonly reinforced in AMCs, such as SiC (3.2 g/cm3) and Al2O3 (3.9 g/cm3). Fly ash particles normally mixed with either solid spheres or hollow spheres. Solid one has industrial name of precipitator fly ash (PFA) and hollow lighter one called as cenosphere fly ash (CFA). Density of precipitator fly ash lies in between 2.1 and 2.6 g/cm3 and often contains pores. The fly ash has particle size ranges of about 1–150 mm during the condition when received from the power plants. Size of cenosphere (CFA) particles usually distributes in between 10 and 250 µm, with approximate density in between 0.4 and 0.6 g/cm3 [96].

Table 1 Chemical composition of fly ash, in weight percent [98].
Full size table

3.1 Micro-FA Particle

3.1.1 Microstructure and Mechanical Charecteristics

Precipitator fly ash has higher modulus of elasticity (143–310 GPa) than base Al-metal (67–79 GPa) [96]. Hence, incorporation of FA can strengthen aluminium metal matrix. Metalographical characteristics of Al–7Si–0.35 Mg alloy composite produced by three different stir casting routes(compocasting, modified compocasting and modified compocasting followed by squeeze casting) were evaluated. Fine fly ash particles with 13 μm average particle size are used as reinforcement. Modified compocasting cum squeeze casting process helps to properly disperse the fly ash micro-particles. Better distribution of FA was then followed by only compocasting process and molten metal stir castinging process with metal moulds [85]. Two types of class C lignite fly ash one with high silicon content (MFA) and one with high calcium content (KFA) were compared in enhancing microstructure of AMC through PM processing. Microstructural morphology and electrochemical behaviour are compared with the help of SEM, EDXS, scanning Kelvin probe force microscopy (SKP-FM), open circuit potential measurements and potentiodynamic polarization. It was noticed that incorporation of the FA particles forms of a strongly heterogeneous microstructure inside AMC [71]. Microstructure and XRD observation revealed that mechanical as well tribological upgrade of AMC was normally attained when finer particulates of high-Ca ash particles were reinforced [25, 50].

Mechanical and wear behaviour of any material potentially help to understand its application area and design of components. Hence, tensile strength, yield strength, ductility, compressive strength, toughness and micro-hardness test have been carried out by most of the researchers during the development of micro-fly ash particle-based AMCs. Several fabrication routes are followed in this regard such as impeller mixing [104], PM [77], stir casting [16], infiltration [62], friction stir processing [90] and compocasting [91]. Overall improved mechanical performance was noted with remarkable increase in tensile strength and hardness. Hardness and tensile strength reduced beyond 10 wt% of FA, attributed to the loss of fly ash due to floatation and removal as dross. Addition of ‘Nucleant 2’ (containing titanium and boron in the ratio 6:1) as grain refiner showed decreasing trend of percentage of elongation with increase in % of FA cenosphere reinforced with LM6 alloy [121]. Rising wt% of fly ash particles in the AA6063 alloy rapidly lowers the energy absorption capacity during fractural failure. The presence of silica and mullite abundance in fly ash particles enhances the brittle and hard nature of aluminium composites when reinforcing particles content increases [90]. Elastic-plastic fracture toughness (EPFM) of AA2024 fly ash composites was in the range of 6–15 kJ/m2 where for the base alloy material the value was found to be 25 kJ/m2 [16]. Machinability (turning) properties were examined minutely when Al-composite was hybridized by FA and graphite. Attempt has been made to optimize the machining parameters using artificial neural network (ANN) technique. Cutting speed and reinforcement percentage were found to be the important parameters which can control the surface finish of the machined composites [64]. Damping capacity of Stircast A356 Al-MMC with 6 and 12 vol% fly ash reinforced composites was 1.2 and 1.5 times the base alloy. Dynamic mechanical thermal analyser (DMTA) was employed to generate the data for damping capacity of the prepared samples, tested under a dynamic load of 10 N and static load of 20 N at 10 Hz [119]. The bending strength of hot extruded aluminium matrix composites reinforced with fly ash (53 μm) was raised from 279 MPa till 302.6 MPa or 8.5% with increasing FA weight percentage from 5 to 12.5 [118].

3.1.2 Thermal and Electrical Behaviour

Thermal properties if compared in between FA and aluminium, it can be seen that precipitator fly ash has thermal conductivity in the range of 0.6–0.16 W/m K and melting point is more than 1200 °C, whereas for Al the values are 237 W/m K and 660 °C, respectively [96]. P. K. Rohatgi and his team have checked pressure infiltrated Al-FA cenosphere composites for some mechanical properties, thermal expansion coefficient (CTE) and chemical reaction between FA and molten aluminium. It was remarked that 20 vol% fly ash showed the significant advantages in aluminium fly ash (ALFA) composites. The density and coefficient of thermal expansion of castings decrease with high applied pressure and infiltration time, their tribological properties improved as the fly ash proportion increases [44, 63, 97]. The thermal elongation of the AA6063-FA decreased with increase in the FA reinforcement content in the matrix. Experimental results showed that the CTE of the FA powder (size 44 μm) increased between 298 and 400 K and remained relatively constant in the range of 400–750 K. The CTE of AA6063 alloy was observed to be in the range of 22.44 × 10−6/K to 24.19 × 10− 6/_K at temperature between 298 and 750 K. With 12 wt% FA content, the CTE of composite reduced to 11.5 × 10−6/K. Compared to 24.19 × 10−6/K of pure AA6063 alloy, which is 88% reduction in vlue [91]. Al-FA composites showed initial increase in CTE upto 400 K then remains almost constant. A large difference of CTE between alloy and FA develops internal stress in the composite resulted into reduction of CTE of the composite [63].

Electrical resistance of precipitator fly ash is in the range of 109–1012 Ω cm which is very much higher than aluminium metal (3.15 × 10–6 Ω cm) [96]. This gives researchers an option to utilize FA for increasing the electrical shielding effect of Al-alloy. Electrical properties like electical conductivity, electromagnetic interference shielding effectiveness (EMSE) of aluminium alloy fly ash composites were examined by sevaral reserchers [31, 85]. EMSE charecteristics of the base matrix (aluminium alloy) were enhanced under the frequency ranges 30 kHz–600 MHz by using the fly ash cenosphere particles. The results showed satisfactory outcome of using fly ash as reinforcing matrial in Al-composites.

3.1.3 Tribological Behaviour

In the year 1997, P. Rohatgi et al. tested stir casted A356/5 vol% micro-size fly ash cenosphere reinforced composite for abrasive wear nature by rubbing against hard SiCp abrasive paper [96]. Wear debris analysis along with SEM analysis of the worn out sample surfaces and subsurface revealed that primary wear on the base alloy was caused through micro-cutting, whereas microcutting and delamination caused due to crack propagation were responsible for the composite wear. The cracks propagated through interfaces of FA, abrasive particles and the alloy matrix, those majorly under the rubbing zone. Higher load reduced the specific wear rates and frictional force during abrasive wear. This is because of wear debris those were accumulated in between the gap of the abrading SiC particles and resulted into lowering the abrasivity of the particles.

Micro-size FA particles mixed with cenosphere and assorted size of 10–100 μm were used to produce AMCs through liquid metallurgy route (squeeze cast and stir cast), and the corrosion behaviour was studied by several researchers [18, 63, 67, 86, 108]. All results show similar result which clears that corrosion resistance of FA reinforced AMC becomes poor with increasing fly ash content. The presence of fly ash particles appeared to initiate pitting corrosion around the particles. Stir caste AMCs containing 5%, 10% and 15 wt% fly ash of size 10 μm were examined under slurry erosive environment, and it was observed that resistance to wear increases with increase in weight percentage of fly ash. This is also seen that erosive wear is extended in the case of basic media than acidic and neutral media [67].

In many literatures, tribological wear performance test using pin-on-disc test rig was conducted under dry sliding condition on Al-fly ash composites reinforced with different wt% [1, 109]. All of them show there is a definite increase in the wear resistance of Al-alloy by the addition of fly ash particles. The wear debris analysis of the specimen concludes that both adhesive and abrasive wear mechanisms dominate wear of composites at light load and denomination is more predominant at heavy load [106]. Taguchi and analysis of variance (ANOVA) techniques confirmed that percentage of FA content and applied load as the primary influencing factors on wear behaviour then sliding speed may be considered secondary factor. Optimized values for minimizing rate of wear were fly ash as 20 wt%, applied load to be 5 N and sliding speed as 1 m/s. Mild wear mechanism was observed when speed becomes higher and applied load was less. On the other hand, severe rate of wear was found at greater applied load and speed [83]. It was also understood that applied load is the main affecting factor that changed the mechanism of wear from mild to severe. Thus, Taguchi and ANOVA combinedly showed a better tool to find out the significance of the influencing parameters in controlling the tribological behaviour of the prepared composites.

3.2 Nano-FA Particle

Nano-size fly ash particles can be produced by high-energy ball milling technique, which will be easy and economical to carry out. This particle size reduction process is applicable to any grade of solid materials and may be scaled up to large-scale production. Improved wettability was observed when nano-aluminium-FA composites were fabricated using ultrasonic cavitation-based mixing route. Melting furnace, ultrasonic probe, ultrasonic generator and transducer and inert gas protection nozzles are used to fabricate the nano-composite. Addition of 3 wt% nano-fly ash particles in AA2024 alloy increases the compressive load taking capacity from 289 to 345 MPa, which is 19.3% improvement [78]. Recently, nano-fly ash (NFA) composite is fabricated through ultrasonic-assisted stir casting process where 76 nm size FA particles are added into Al6063 alloy. Tensile and Vickers hardness increase with increase in NFA weight percentage beyond 2%. Study revealed that friction stir welding process (FSW) better joins Al6063/NFA composites by achieving grain refinement at the stirred zone [42]. Dry sliding test was carried to understand the impact of exerted pressure, sliding speed, sliding time and wt% of nano-FA particles in the Al-metal matrix composites by Katrenipadu and Gurugubelli [53]. Statistically generated regression model using Minitab R17.1.0 tool was used to forecast the wear rate of the developed samples considering FA %, normal loads and time periods as variables. It was reported that resistance against wear out was improved with higher content of fly ash, upto 10 wt%. Effect of nano-size FA addition was checked again by varying weight percentage in between 5 and 10%. Considerable improvement of composite was noticed in hardness and reciprocating tribological behaviour at 10% FA content in the aluminium matrix [53].

4 Study on Hybrid Al-MMC: Reinforced with FA and SiC (Micro-particles)

Concept of hybrid aluminium metal matrix composites came into force at the end of twentieth century, where individual properties of two discontinuous materials are utilized to get desirable characteristics of Al-alloy. Hybrid AMCs can be formed by the combination of synthetic ceramic, industrial and agro waste derivatives.

Comparison was made using erosive wear response between SiC reinforced AMCs and fly ash reinforced AMCs. This study has claimed that Al-SiC better wear resistant in mining environment than Al-alloy and Al-FA MMC, while poor than matrix alloy in basic environment but better than Al-FA [54]. M. O. Bodunrin et al. have found that the more and more study needed to improve the hybrid AMCs that contains fly ash. Comparison also needs to be made with the single reinforced aluminium metal matrix composites that is being reinforced with synthetic reinforcement [19].

Combinations available so far can be categorized in three types as follows:

  1. 1.

    hybrid AMCs reinforced by double synthetic ceramic phases.

  2. 2.

    hybrid AMCs reinforced by one ash from agro-waste derivative and other with one synthetic ceramic particulate.

  3. 3.

    hybrid AMCs reinforced with waste from industry combined with a synthetic reinforcement [19].

4.1 Microstructure and Mechanical Characteristics

Silicon carbide along with FA particles of 53 μm sizes are well distributed into aluminium 7075 alloy through vortex method in different weight percentage ranging in between 0 and 10%. Electron backscattered diffraction (EBDS) images in Fig. 8a of hybrid composite samples show that the average grain size inside the aluminium alloy reinforced with FA and SiC particulates is significantly less than the matrix alloy (refer to Fig. 8b). The developed finer grains are attributed to the dynamic re-crystallization owed to well built plastic deformation, and conclusion has been made from the evidence that the FA particles are actually refining grain size [52].

Fig. 8
figure 8

Reproduced with permission from Elsevier, Copyright (2019)

EBSD analysis for a base alloy and b 10% (SiC + FA) composite [52].

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Charles and Arunachalam [22] have fabricated HAMC by mixing different volume fraction (10, 15 and 20%) of SiC particles and fixed 10 vol% of fly ash (both of size 30–100 μm) in to LM10 Al-alloy using both powder metallurgy (PM) and stir casting route. Hardness (refer to Fig. 9a), tensile strength (refer to Fig. 9b) and dry sliding wear test result exhibited better properties of stir casted specimen than produced via PM route. This was attributed to the strong interfacial bonding created inside the casted specimen than PM specimen.

Fig. 9
figure 9

a Microhardness, b tensile strength of SiC/FA HAMC [22]

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An rising trend of mechanical properties was found with enrichment of SiC particles by keeping fly ash wt% as constant in AA6061 aluminium alloy [105]. Reverse case is also observed by Gikunoo et al. [41] where effect of FA addition into A535 alloy and 10 wt% SiC (size 1–100 μm) AMC through proprietary stir casting technique was investigated. Result shows there was degradation in mechanical characteristics and impact load taking capacity of the MMCs. This was attributed to the depletion of strength between magnesium atoms and the alloy matrix in solid solution and also to the presence of porosity. Also, few findings based on reinforcement particle size are listed in Table 2 and presented with change in percentage of density, tensile strength and hardness.

Table 2 Some selected outcome on effect of reinforcement (FA and SiC) content and particle size on density, tensile strength and hardness of HAMC
Full size table

4.2 Thermal Behaviour

Thermo-mechanical analysis (TMA) was conducted to study thermal expansion coefficient (CTE) of A535 aluminium alloy and its composites when reinforced by FA and SiC. Composite specimens were prepared by combining different proportion of SiC and FA particles in between 5 and 15 wt%. Studies revealed that CTE of A535 alloy reduced with the inclusion of fly ash and silicon carbide as reinforcements. It was also noticed that SiC particulates were more effective in controlling thermal expansion behaviour of the composite than fly ash particles [127]. SiC provides more surface area than FA for good interfacial boding hence provides higher restriction to thermal elongation of matrix alloy. Electromagnetic stir casting (EMS) with stiring speed of 210 rpm setup that was used to fabricate hybrid metal matrix composite (A356/SiC/FA) results into homogeneous spreading of FA and SiC particles in A356 alloy. It was also found that smaller size SiC was deposited at the grain boundaries. HAMC reinforced with different weight fraction combination (0–20%) of SiC and FA (both particle size 25 μm) was heated at 450 °C in electric furnace, and change in dimension of the samples is checked to measure thermal expansion behaviour. A356/15%SiC/5%FA HAMC was observed to be appropriate for various applications for its higher strength characteristics and lower change in dimension during thermal expansion test [32].

4.3 Tribological Behaviour

Pressure infiltration technique was adopted by Escalera-Lozano et al. [36] to prepare hybrid Al/SiCp/spinel composites from α-SiCp (size 75 μm), FA (size 90 μm) and recycled aluminium. Two types of specimens are prepared with SiC and FA volumetric ratio of 3:2. Dextrin was added in a weight percentage of eight along with approximate 0.5 ml distilled water. In one type (for type A), FA is used as-received condition, and for another type (type B), FA is calcinated at 900 °C for 30 min. Aluminium composites reinforced with SiC produced by the melt infiltration process under go the following reaction (Eq. 3) which is relevant to the potential attack of Silicon carbide by the melted aluminium and the ensuing degradation.

$${\text{3SiC}} + {\text{4Al}} = {\text{Al}}_{{4}} {\text{C}}_{{3}} + {\text{3Si}}$$
(3)

Corrosion test was performed with both type A (for 1 month) and B (for 11 months) samples exposed to indoor atmosphere with average relative humidity 54%, ambient temperature (min 15 ± 40 °C to max 26 ± 50 °C). Potential attack of SiC by liquid aluminium was fully avoided by the help of SiO2 present in the FA, the reaction of present carbon in the FA with aluminium continues to form Al4C3. Fourier transform infrared spectroscopy (FTIR), XRD, SEM of powdery corrosion product revealed that calcined fly ash due to the absence of carbon avoids the chemical degradation of the composites rather than as-received condition.

A composite material for bearing bush was suggested using Al-4.5% Cu alloy (fly ash + SiC) after investigation of micro-structure, fluidity, hardness, density, impact load resistance, dry sliding wear along with the slurry erosive wear and fog corrosion test [68]. Samples were prepared after quenching stir casted hybrid composites in hot water by varying combined FA and SiC (both size 1–10 μm) wt% in 5, 10 and 15% (equal proportion). Study shows that mechanical properties (hardness, UTS, compression strength, impact strength) increases while properties like fluidity, density, dry sliding wear, cof, frictional force and corrosion resistance decreases with increasing wt% of SiC and FA. Slurry of silica sand distilled water in ratio 1:2 with different pH values (neutral, basic and acidic) was used for erosive wear test, and the wear in case of basic is more than acidic or neutral media. SEM micrograph of eroded specimen shows pitting at some places.

Thermal treatment like carbothermal reduction of fly ash under reducing environment as shown in Fig. 10 was used to prepare one new in-situ ternary ceramic composite [43, 93]. SEM, XRD and EDX results has confirmed in-situ conversion of SiO2 to SiC in region of Al2O3 available in the FA. Mechanical, physical and tribological performance of novel aluminium fly ash metal matrix composite (ALFMMC) are checked for comparison with base alloy. The Accelerated ageing behaviour showed remarkable increase in strength and wear resistance of developed alloy composite. The author concluded that Al6061PSFA composite has great potential for future application considering production cost as well as material characteristics.

Fig. 10
figure 10

Schemetic diagram of carbothermal reduction process of ALFMC production

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Al-hybrid composites reinforced with both silicon carbide and fly ash was developed for conducting wear test under dry sliding condition using Ducom pin-on-disc tribo tester in accordance with the ASTMG-99, [65, 66, 107]. Shaikh et al. study [107] followed solid-state PM route for the fabrication while Kurapati et al. [65] have followed liquid-state stir casting path. Required proportion of aluminium powder of 48 μm (250 mesh), SiC (400 mesh) and FA-53 μm (270 mesh) powder was prepared by planetary ball mill (FRITSCH Pulvrissettle MM-1552) with steel ball of 8 mm diameter, ball-to-powder ratio was 10:1, grinding at 600 rpm for 20 min. Then powders are pressed with uniaxial hydraulic machine run manually with a pressure of 400 MPa in a die lubricated with zinc stearate, compaction dwell 30 s. Then sintered in electrical furnace of tabular shape under argon gas environment at temperature of 450–470 °C. The samples were gradually cooled to normal temperature in controlled environment. AA2024 alloy with 10 wt% SiC and 10 wt% FA showed higher hardness and better wear resisting property; hence, it was suggested to be used for high wear resisting application. Further, increase in FA content sample showed poor wear resistance due to the highest porosity/cluster formation. Statistical analysis of wear done by Kurapati et al. [65] has reported that wear reduces with increasing wt% (5, 10 and 15%) of FA and SiC in equal proportion. Taguchi L27 orthogonal array test design based on Taguchi’s signal-to-noise ratio and analysis of variance (ANOVA) were used to find out the influencing factors on the wear behaviour of hybrid composite. Rank of factors got after analysis are: (i) applied load (causes 43.83% of the wear), (ii) sliding time (causing 28.47%), (iii) reinforcement wt% (causing 20.10% of the wear). It was also observed by the author that response surface methodology (RSM) model matched more accurately with the experimental results than multiple linear regression model (MLR).

Fig. 11
figure 11figure 11

Two-dimensional (orthographic) coloured map of wear in µm (W). a with respect to sliding time (T, min), load (L, kgf), b with respect to reinforcement wt% (R), load (L, kgf), c with respect to sliding time (T, min), reinforcement wt% (R), [65]

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Contour plot of wear in Fig. 11a–c shows different wear loss with reference to any two effecting parameters.

4.4 Machining Behaviour

EDM done on the hybrid composite specimen prepared using copper electrode for 30 min trail run [22] revealed metal removal rate (MRR) and tool wear rate (TWR) decreases on increasing vol% of SiC and pulse duration. Both of these were higher for PM route specimen and increases for increase in current. This was attributed to the higher interfacial bonding achieved by casting than powder metallurgy route. It was also concluded that three-level full factorial technique can be more efficient to develop mathematical model for predicting MRR and TWR in Electric discharge machining of HAMCs.

5 Conclusions

This present review study has focused on effects of SiC and fly ash on Al-alloy for improving physical, chemical, mechanical, tribological and thermal properties. This chapter explained the works done by the investigators towards the development of aluminium metal matrix composites. Various microstructural changes are described to attribute the outcome of strength and wear behaviour. Therefore, it is expected that bunch of opportunities and scopes will be opened with this overview and that will widen the area of product development techniques toward getting reliable material with high quality. Based on the previous studies, conclusions are drawn as below.

  1. 1.

    Hybrid AMCs reinforced with ceramics have improved the mechanical strength more than single reinforcement.

  2. 2.

    Primary reinforcements must be ceramic for better hardness, mechanical strength and wear resistance. Hence, silicon carbide (SiC) can be effective as ceramic with high hardness.

  3. 3.

    Secondary/complementary reinforcement may be agro/industrial waste for low cost, easy availability, improvement in fractural toughness. Fly ash can be effective as it has balanced content of silica (SiO2) and alumina (Al2O3)

  4. 4.

    Reinforcement wt% must be confined within 15% for enhancing the dominating properties.

  5. 5.

    Modified stir casting (two steps) or semisolid stirring along with heat treatment is economical method and quite useful for well dispersion of the reinforcement in the base matrix.

  6. 6.

    Impact strength (fractural toughness), corrosion resistance and ductility (% of elongation) are reducing with increase in wt% (especially FA), which need to be retained or optimized in the timeline of AMC development

  7. 7.

    HAMCs reinforced with SiC and FA have good potential in strengthening mechanical behaviour though literatures on thermal behaviour, machining behaviour and tribological characteristics of AMCs with such hybrid reinforcements for the applicability in different challenging environment are limited.

  8. 8.

    Nano-reinforcement while used in the Al-matrix holds better behavioural characteristics in contrast with micro-size particles though use of nanoscale particles (both SiC and FA) in fabricating HAMC are still at the developing stage.

  9. 9.

    Ultrasonic-assisted stir casting process has more possibilities than the usual stir casting process in case of nanoparticle reinforced Al-composite production. However, still there exist gap of study for opting efficient techniques for homogeneous spreading of nanoscale reinforcements, as agglomeration is a major issue to be dealt with.