Agro waste reinforcement of metal matrix composites, a veritable sustainable engineering achievement, or an effort in futility? A critical review

The utilization of agro waste as reinforcement in metal matrix composites (MMCs) has sparked interest regarding its feasibility and sustainability in engineering practices. Compared to synthetic reinforcements, its efficacy as a potentially cost-effective and environmentally friendly alternative has been explored by diverse studies. This review critically examines existing literature on agro waste-based reinforced MMCs, evaluating experimental findings on mechanical, tribological, density, and corrosion performance from a sustainable engineering perspective. Furthermore, it explores the innovative strategy of employing multi-component metal matrix composites to fabricate composites with improved performance attributes. The utilization of multi-component reinforcements has the capability to tackle issues like the challenge of disproportionate reduction in ductility and toughness peculiar to monolithic and hybrid MMCs. Despite promising results in some studies, numerous unexplored research areas and gaps remain, emphasizing the need for further investigation to provide valuable guidance for future research and development of agro waste in sustainable engineering applications.


Introduction
Agro or agricultural wastes, derived from processing various agricultural products like crop residues and animal byproducts [1,2], can take on solid, semi-solid, liquid, or slurry forms [3].Inadequate disposal of these wastes can lead to significant environmental issues.For instance, improper handling of residues from food crops can result in landfill littering [4], greenhouse gas emissions (through the burning of these wastes) [5,6], and waterway pollution (providing a conducive environment for the growth of pathogens and diseases) [7].Also, chemicals from fertilized lands may leach into water runways, causing contamination and posing adverse health risks to the public.It was estimated that the global production of agro waste generation would be 126 million tons in 2020 [8].However, recent reports suggest that annual production is already at an alarming figure of 998 million tonnes [9].This emphasizes the urgent need for global attention to agro waste (AW) management, especially amid the global food crises exacerbated by the aftermath of the COVID-19 pandemic [10][11][12], the conflicts between Russia and Ukraine [13][14][15], Israel and Hamas [16], and several other socio-economic issues contributing to food shortages worldwide.Undoubtedly, implementing effective and sustainable management practices for these wastes is essential.Such practices will play a pivotal role in minimizing environmental hazards and promoting sustainable applications in the face of these global challenges.
Several research efforts have been documented in the literature, focusing on sustainable strategies and the utilization of AW residues, with a particular emphasis on solid wastes.Sadh et al [2] proposed using AW as an essential raw material for producing biofuels, antioxidants, livestock feeds, vitamins and enzymes through the amounts of agricultural waste [54,58,67] and still relies on imported synthetic reinforcements.Additionally, the study aims to uncover potential agro waste residues, that have yet to be explored.It is expected that the insights gleaned from this study will elicit renewed interest in researchers and engineers about the current state of AWreinforced MMCs.Ultimately, this knowledge will aid in shaping future research directions and facilitating the transition towards sustainable practices in engineering, especially in regions grappling with agricultural waste management challenges.

Agro waste classification and production
AWs can be categorized into two main types: animal wastes and crop residues (figure 1).The animal-based AWs are typically sourced from livestock products such as dairy, beef, swine, and poultry birds [3].Conversely, cropbased AWs are sourced from leftovers of plant materials like husks, peels, stalks, shells, leaves, straws, and stems, discarded after harvesting.AWs can further be classified into raw and processed forms, with raw forms referring to direct waste generated in farm fields, while processed AWs are generated from the processing of farm produce by industries, hence called industrial AWs.According to FAO [71], animal wastes can stem from milkproducing livestock such as buffalo, camel, cattle, goats and sheep, along with various by-products such as edible offal, meat, fat, skins and hides from these animals, including pigs and horses; and eggs and meats of poultry birds.Crop residues, on the other hand, can come from by-products of various plant categories, including fruits (such as coconut, dates, bananas plantain, pineapples, and pumpkins), vegetables (like cabbages, chilli peppers, eggplants, and artichokes), fiber crops (such as abaca, jute, kapok, and kenaf), oil crops (such as palm kernels), roots and tubers (including yams, cassava, taro, and potatoes), sugar crops (such as sugar cane), tree nuts (like almonds, cashew nuts, walnuts, and hazelnuts), pulses (including beans, peas, and soybeans), and cereals (such as rice, wheat, maize, barley, rye, millets, and sorghum).
Tables 1 and 2 show an eleven-year regional overview of global agricultural production (2012 to 2022), encompassing both livestock and farm crops, compiled using data sourced from the Food and Agriculture Organization (FAO) of the UN [71].According to the data, Asia emerges as the leading producer of both livestock and plant crops.Figure 2 graphically depicts the trends in livestock and crop production over the specified period.A noticeable decline in global production is evident between 2019 and 2020, attributed to the onset of the COVID-19 pandemic, which emerged in December 2019 in China [72] and led to global lockdowns in 2020 [73,74].In 2022, global production figures indicated a decrease of 0.38% for cereals, 14.36% for fiber crops, 0.46% for tree nuts, and 2.41% for sugar crops, compared to peak production levels observed in 2021, 2019, 2021, and 2019, respectively.Conversely, fruits, oil palm, pulses, roots and tubers, and vegetables witnessed significant increases of 1.11%, 2.12%, 6.60%, 0.76%, and 1.07%, respectively, in 2022 compared to the previous year (figure 2(a)).However, egg production and meat from small game animals such as rabbits and domestic rodents experienced notable declines of 3.51% and 37.35% in 2022, respectively, compared to peak values recorded in 2020 and 2015.Conversely, other livestock products such as dairy, edible offal, meat from livestock, fats from livestock, meat from poultry birds, and skins and hides witnessed substantial increases in 2022 compared to 2021, with growth rates ranging from 0.5% to 4.13% (figure 2(b)).
It is imperative to remark that the data presented in tables 1 and 2 does not necessarily translate to the total global amount of agro waste generated.This is because it is challenging to ascertain the exact quantity of waste generated from plants or livestock, given that multiple types of waste can arise from a single crop or animal.For instance, animals like buffalo can yield more than one type of waste, including bones, faeces, dairy products, and hides.Similarly, cereal crops like maize can generate waste from stalks, stems, and cobs.However, estimating that approximately twice the total production shown in table 1 represents the global waste output is reasonable.With production quantity varying across regions (as indicated in figure 2) for different waste sources, this highlights the complex nature of agricultural waste management.Inadequate management of these wastes can lead to environmental degradation [75,76], especially considering the slow pace of the remediation process [17,77].However, if these wastes can be repurposed for valuable applications, their large amounts show their sustainability potential, which could enhance the remediation turn-around time.

Agro waste as reinforcing material
A reinforcing material refers to a substance added to another material (known as the matrix) to enhance specific properties, thereby improving overall performance.For instance, reinforcing materials can be incorporated into alloy matrices to enhance mechanical properties like ductility, stiffness, hardness, wear resistance, and strength.Reinforcing materials have also provided additional benefits beyond mechanical properties, such as improved corrosion resistance, electrical conductivity, thermal expansion characteristics, thermal conductivity, and flexibility [78,79].
It is essential to consider several factors when selecting materials for use as reinforcement.The chosen material should be defect-free and have smooth and rough surface characteristics.Particles can come in various shapes, including spherical, flaky, continuous, chopped, whiskers, or irregular shapes.Additionally, Kapranos et al [80] suggested that reinforcements must exhibit good chemical matching with the matrix material and possess inherent strengthening properties such as ductility, modulus, and strength.
Qin [81] categorized reinforcement materials into four basic types: fibers, fillers, flakes, and particulates.Fillers are described as powdered substances added to a material to enhance its physio-mechanical properties, while flakes are two-dimensional flat platelet materials that improve stiffness and strength along two directions.Fibers are characterized as elongated structures with an aspect ratio greater than 100, and particulates are small materials with dimensions less than 0.25 μm.However, upon closer examination, it becomes apparent that these categories can be more accurately grouped into two distinct types: artificial reinforcements and natural reinforcements.

Artificial reinforcements
Artificial or synthetic reinforcements are materials crafted through human interactions and activities.Predominantly, commercial reinforcements are of synthetic origin, tailored to meet specific user requirements.These materials are typically derived from synthetic polymers or metallic alloys [82,83] and engineered to demonstrate superior mechanical, thermal, or electrical properties.Synthetic reinforcements (SRs) can take on filler, flake, or particulate characteristics, with fibers being the most prevalent type.These SRs can be chemically synthesized and classified as inorganic and organic [84].Examples of SRs include glass fibers, carbon fibers, aramid fibers, and metallic reinforcements.These reinforcements can be utilized in reinforcing composite materials, each offering distinct attributes and advantages tailored to performance expectations and intended applications.

Natural reinforcements
Natural reinforcements (NRs), known as traditional reinforcements, occur naturally and have been used since ancient times to enhance other materials.The NRs-based materials are abundant and often synthesized to produce SRs depending on specific application requirements.NRs are primarily organic and derived from natural sources such as plants and their residues and animal products.Inorganic NRs derived from synthetic minerals like fibers and metallic reinforcement have also been reported [91].NRs are also called green reinforcement materials (GRM), AWs, biofibers, biofillers, or natural fibers [6,52,92,93], reflecting their derivative source from agricultural-based biomass.Like SRs, NRs are utilized to enhance the mechanical, corrosion, and thermal performane of composites and to produce lightweight materials for various applications in engineering, building, construction, energy storage, transportation, and more [94][95][96][97][98].
Compared to SRs, NRs are environmentally friendly, non-toxic, have low processing costs and energy (<17%), are biodegradable, and sustainable due to their natural abundance.However, they are also a significant environmental concern, prompting research into their potential applications to mitigate pollution [41,57,99].The use of NRs as reinforcing materials faces challenges due to their composition, which includes protein, hemicellulose, pectin, wax, and lignin, making them susceptible to moisture absorption at room temperature, thus impacting their bonding with the matrix material [100].Microstructural defects such as microcracks, interfacial stressors, and high water absorption have been reported with NRs-based composites [101][102][103].
NRs made from materials such as cassava peel ash (CPA) possess suitable elemental compositions and physio-mechanical properties that make them immune to these drawbacks, making them suitable as single reinforcements in metal matrices.When using NRs as reinforcements in MMCs, the characteristics of the metal matrix chosen are also essential.For example, Alaneme et al [105] reported a decrease in hardness properties when rice husk ash (RHA) and alumina were used to reinforce Al-Mg-Si, while Dinaharan et al [106] reported increased hardness using RHA alone with Copper (Cu).This difference is attributed to the superior electrical and thermal conductivities and wear resistance properties of Cu compared to Al Muni et al [62] subsequently validated this observation, reporting improved hardness, wear resistance, and tensile properties in RHA/Cureinforced AMCs.

Composites
A material with two or more distinctive constituent phases is regarded as a composite.A composite usually comprises matrix and reinforcement phases.Matrix materials are typically considered homogeneous substances with a continuous phase but often exhibit relatively lower strength, limiting their applications.However, when reinforcement materials are introduced into the matrix, cohesive and adhesive forces facilitate the binding of reinforcements to the matrix.This enables an efficient load transfer from the matrix to the reinforcements, which act as support providers.The nature of support offered by the matrix depends on its elemental composition.The reinforcement phase can be monolithic (single material), hybrid (two materials) or multicomponent (more than two materials).Composites can be grouped into PMCs (polymer matrix composites), CMCs (ceramic matrix composites), and MMCs.

Polymer matrix composites
PMCs consist of reinforcement materials bound together by a polymer matrix.They are typically processed at low temperatures due to their lightweight nature.PMCs reinforcing materials, including fiberglass, carbon fiber, aramids, and metals, are chosen for their ability to enhance strength, reduce density, resist corrosion, and offer flexibility and chemical inertness [92,107,108].However, they pose environmental concerns as they are primarily non-biodegradable, contributing significantly to pollution.Studies by Hurley et al [109] and Akindele et al [110] have even identified the presence of microplastics in aquatic organisms like fish.Glass fibers are the most commonly used reinforcement in PMCs, constituting about 90% of the materials.However, other substances such as CNTs, SiC, and graphite (Gr) have also been utilized [79,111,112].Research has turned toward bio-composites using agricultural wastes (AWs) to address the environmental drawbacks associated with conventional PMCs.These bio-composites, reinforced with lignocellulose filler materials, offer biodegradability, non-toxicity, and sustainability.Applications of AWs-based PMCs span diverse fields, including packaging materials, microelectronics, turbine blades, medical industries, and the production of hybrid electric vehicles and batteries [113,114].

Ceramic matrix composites
CMCs comprises of ceramic particles or fibers incorporated within a ceramic matrix, offering a unique blend of properties such as thermal stability, fracture toughness, strength, and resistance to crack deformation.These materials are renowned for their durability and reliability in extreme environmental conditions, with operational temperatures reaching approximately 400 °C.They find extensive applications in various sectors, including automotive, energy, and aviation, where they are utilized in engine components, brake discs, heat shields, and turbine blades [115,116].Recent research has explored the use of bio-ceramics and bio-glasses in the development of biomedical devices, while nanostructured hybrid composites incorporating materials like CNTs, SiC, carbon (C), B 4 C, BN, and graphene (Gr) have also been reported [117][118][119][120].Despite their numerous advantages, CMCs may suffer from high abrasion and friction coefficients due to surface roughness, but advancements such as SiC reinforcement have helped mitigate these issues.Notably, SiC-reinforced CMCs have been successfully employed in the production of brake discs for luxury cars, demonstrating exceptional performance over significant distances of about 300,000 km [121].

Metal matrix composites
Metal matrix composites, the focus of this review, are engineered materials consisting of a continuous metal matrix reinforced with materials such as ceramics, fibers, or carbides.They find applications in the aerospace, automotive, electronics, and energy sectors.MMCs offer superior properties, including strength, stiffness, wear resistance, high electric and thermal conductivities, and are less sensitive to moisture than PMCs.Their mechanical performance can be further enhanced using synthetic or natural reinforcements either monolithically or combined as hybrid composites.Table 3 presents a list of metals commonly used as matrices for MMCs, with aluminum being the most researched [122][123][124][125]. Various grades of aluminum, including AA2014 [126], LM13 [127], AA5083 [128], AA2009 [60], AA7075 [129], AA6061 [62,70], ADC12 [68], A356 [130], and AA6063 [128,131], have been investigated.Other metals such as titanium, copper, magnesium, and zinc are also utilized.Copper is well-known for its outstanding electrical and thermal conductivity.In contrast, titanium is remarkably lightweight and strong, with exceptional corrosion resistance and high-temperature performance in demanding conditions.Its biocompatibility further enhances its appeal, particularly for medical implants within the human body.
MMCs are valued for their lightweight nature and improved mechanical properties like strength, ductility, toughness, wear and corrosion resistance, and hardness.Studies have explored replacing SCRs in MMCs with NCRs derived from agricultural waste due to their renewable and cost-effective processing techniques [54-56, 59, 60, 67, 132].Akshay et al [134] report the development of layered MMCs as an effective means to enhance toughness and damage tolerance.Although MMCs are prone to galvanic corrosion between the reinforcement and matrix [133,134], appropriate material selection can mitigate this issue.Researchers have explored MMCs in various applications, such as diesel engine pistons, cylinder liners, brake drums, and rotors [135].Cost of production has been identified as a limitation [136], but the utilization of agro waste presents a solution to this challenge.Thus, Agro waste reinforcements can be used with metal alloy matrix materials to produce monolithic, hybrid, or multi-component composites.

Monolithic metal matrix composites
Monolithic MMCs, often regarded as singly reinforced composites, utilize a single reinforcement material in combination with a matrix.This approach aims to enhance the matrix's properties by adding either a single NR or Sr Studies focusing on SRs have consistently reported improved performance due to their rapid processing and fabrication routes, which enable precise control over their properties.As mentioned earlier, typical examples of SRs investigated in single MMCs include Al 2 O 3 , SiC, WC, Gr, CNT, BN, B 4 C, and SiO 2 .For instance, alumina powder typically comprises Al 2 O 3 at a mass percent of approximately 99%, with minimal impurities such as Fe 2 O 3 , K 2 O, SiO 2 and others constituting the remaining <1% (table 4).This uniform composition makes alumina a consistent and reliable reinforcement material, contributing to the enhanced performance of MMCs.Properties that inform the choice of SRs for metal alloy reinforcement include their density and hardness, modulus of elasticity, and lubricating effect.These properties, as evidenced in the works of [71,127,[139][140][141][142][143], will determine the overall impact of the synthetic reinforcement of the resulting composite's mechanical, tribological, and corrosion properties.
Conversely, natural reinforcements from agro wastes such as CPA exhibit more varied elemental oxide compositions than synthetic reinforcements.For example, table 4 further shows the oxide composition of CPAbased AWs, wherein all the alumina oxides are available (though at varying mass percent), in addition to other refractory oxides, reflecting the complexity of natural reinforcement materials.Table 4 reveals that CPA contains substantial amounts of SiO 2 (44%), Al 2 O 3 (16%), K 2 O (13%), CaO (12%), and Fe 2 O 3 (8%).This compositional variability can significantly enhance the mechanical behaviour, hardness, and wear performance of CPA/MMCs reinforced with single AWs compared to those of Al 2 O 3 /MMCs, despite Al 2 O 3 being present in substantial quantity in the latter.However, further investigations need to be conducted to validate this observation and determine the extent to which CPA/MMCs perform mechanically compared to Al 2 O 3 /MMCs.
Ben et al [54], in their study on green plantain peel ash (GPPA), reported that singly reinforced GPPA/AMCs had relatively reduced tensile strength, modulus, ductility, and hardness compared to the singly reinforced Al 2 O 3 /AMCs.The reason for this observation is consistent with the earlier statement that the density of reinforcements is crucial in the final characteristics of the fabricated MMCs, especially as GPPA comprises K 2 O (81%) and boasts a bulk density of 2.38 g cm −3 , which is ∼40% lower compared to Al 2 O 3 (99%) with a bulk density of 3.98 g cm −3 .Arora and Sharma [42] conducted a comparative study analysis of SiC (SRs) and RHA (NRs) reinforcements in AA6351 monolithic composites using the stir casting technique.The study reported significant improvements in density (1%), hardness (37%), and tensile strength (39%) for the SiC/AA6351 composites.Conversely, the RHA/AA6351 composites exhibited significant enhancement in hardness (16%) and tensile strength (21%) and a slight decrease in density (3.9%).These results suggest that both SiC and RHA reinforcements can effectively enhance the mechanical properties of a metal matrix material, with RHA offering the additional advantage of lightweight MMCs compared to SiC despite the superior properties recorded by the latter.
Adediran et al [146] investigated the effect of SiO 2 reinforcement derived from RHA-based AWs on the alloy of AA6063.SEM micrograph of the study showed a fair dispersion of the reinforcement's particulates within the AMCs (figure 3).Although the study reported enhancement in the hardness, ductility of fabricated composites A1650, C1650, and A1600, with minimal toughness in composites A1250, B1250, and C1250 compared to the unreinforced matrix.RHA is largely comprised of SiO 2 at about 92%, with a density of 2.65 g cm −3 , and as such, can endure the load transfer from the matrix for samples A1650, C1650, and A1600, while the A1250, B1250, and C1250 samples having a cristobalite phase experienced reduced hardness as the silica polytypes were softer.
Bean pod ash (BPA), an agro waste byproduct of bean crops, was used as a single reinforcement with an AA2009 aluminum alloy to form monolithic AMCs of varying weight proportions by Atuanya and Aigbodion [147].BPA nanoparticles (BPAnp) synthesized using double layer feeding stir casting technique showed the formation of SiO 2 , CaCO 3 , NaAlSi 3 O 8 , and Al 4 O 4 C phases (figure 4(a)).The BPAnp/AA2009 AMCs, when compared with the unreinforced matrix recorded significant enhancement in hardness (46.7 HRB to 67.3 HRB) and strength but decreased in fracture toughness.In another study, Aigbodion et al [148] reported a significant increase in the wear performance with rise in the weight variation of BPAnp (figure 4(b)).BPAnp was also reported to impact the fatigue properties of the fabricated AMCs, with a fatigue limit of 75 MPa, 135 MPa, and 167 MPa, reported, respectively, for the unreinforced AA2009 matrix, reinforced 100 μm BPA particulates, and the 55 nm BPAnp [149].
Table 5 presents a comprehensive overview of various studies investigating the experimental performance of agro waste ash (AWA) as reinforcing particulates in the fabrication of monolithic MMCs.The table provides insights into the microstructural features, mechanical behavior, hardness, wear, corrosion performances, and fabrication routes utilized in these studies.The summary of the findings highlights the vast array of reinforcement materials utilized and their respective impacts on the composite performance.Rice husk ash emerged as the most extensively studied agro waste material.Other notable agro waste ash includes coconut husk ash, bean pod ash, palm kernel ash, bamboo leaf ash, sugar bagasse ash, locust bean waste ash, maize stalk, and breadfruit seed husk ash, which have also been investigated in conjunction with the metal matrix.Some agro waste crop residues were also identified in a related review paper [57].Studies involving animal wastes as reinforcements in MMCs are minimal.However, this review reported the use of snail shells [148] and eggshells [150] as reinforcements for AMCs.
It is worth noting that aluminium alloy remains the most studied metal matrix for exploring the reinforcement potential of agro waste, followed by copper.This suggests that there is still a need for further studies with other metals such as titanium, zinc, and magnesium.Despite the advantages of the friction stir processing (FSP) method, the double stir casting method remains the most widely used fabrication technique.This could be attributed to its low processing cost and the versatility offered by the two-step stir cast technique [129,146,151].In contrast, the FSP method has limited applications due to equipment complexity and the Interestingly, none of the reviewed studies reported the corrosion resistance of the fabricated monolithic MMCs, which aligns with the findings noted by Bodunrin et al [152].However, it is worth mentioning that corrosion studies are available on monolithic synthetic ceramic reinforcements in MMCs [134,[153][154][155]. Understanding the corrosion properties of these composites is crucial, as sudden deterioration can significantly impact industrial processes and applications.Insights into corrosion properties can aid in developing strategies, such as coatings, to extend the lifespan of the composite and mitigate against sudden deterioration.Given the potential of agro waste materials as substitutes for synthetic reinforcements, conducting more corrosion-based studies on these materials is imperative.This will help bridge the literature gap on corrosion studies for agro waste-based monolithic MMCs, which is essential for determining their corrosion resistance in various environments.This knowledge will assist industrialists and engineers in making informed decisions regarding the appropriate utilization of these MMCs in different applications.
In terms of the behaviour and performance of the MMCs fabricated using monolithic agro waste ash as natural reinforcements, the study observed the production of lightweight materials with reduced densities for all experimental investigations presented in table 5.The microstructure of these composites typically exhibited uniform distribution and reinforcement homogeneity within the matrix.However, an exception was noted in the study by [156], which reported the formation of agglomeration, clustering, and uneven scattering of the bagasse ash reinforcement in Al-Cu alloy.Mechanical properties including tensile, yield, compressive, and specific strengths were significantly enhanced for all the studies, indicating the effectiveness of agro waste reinforcement.However, there was a reduction in ductility and fracture toughness.Improved hardness was consistently reported with agro waste reinforcement, leading to superior wear resistance of monolithic AWs/ MMCs.This suggests that agro waste materials can bear the load transfer from the matrix.Hard particulates such as Al 2 O 3 , SiO 2 , Fe 2 O 3 , and other elemental oxides with densities more significant than the matrix contribute to the observed superiority in wear and hardness performance.Peak enhancement was, however, observed for bagasse ash-reinforced AA6061 AMCs [157].
The production of monolithic MMCs using synthesized nanoparticles of bean pod ash has been extensively documented [147,149,158].Agro wastes reduced to nanoparticles and used as reinforcements in MMCs tend to exhibit better dislocations, allowing for improved interaction within the composite matrix and resulting in enhanced mechanical properties compared to microparticles, as evidenced in studies by ref. [147,159].Zhang and Chen [160,161] demonstrated that the Orowan strengthening effect significantly increases as the nanoparticle size decreases in MMCs, reaching a peak at a critical size.However, this strengthening effect diminishes below the critical size.This observation aligns with the understanding that the atomic diameter of the matrix influences the critical size of nanoparticles.Nevertheless, the issue of inadequate wetting between ceramic nanoparticles and the molten metal matrix remains a persistent challenge.Casati and Vedani addressed these wettability challenges in a review paper, outlining strategies such as ex situ and in situ production routes [162].Reproduced from [148], with permission from Springer Nature.
Table 5. Overview of some studies investigating singly reinforced agro waste metal matrix composites, detailing their reported performance metrics.

Hybrid metal matrix composites
Hybrid metal matrix composites (HMMCs) usually incorporate two types of reinforcement materials, with one being either natural reinforcement and synthetic reinforcement or a combination of different synthetic or natural reinforcements.The purpose is rarely to explore the possibility of NRs substituting for SRs in the composite to produce cost-effective composite materials with enhanced mechanical properties.Interestingly, some studies [54,151] have reported that combining NRs and SRs in MMCs can produce notably superior composites compared to those reinforced singly.Among the various synthetic reinforcing materials, Al 2 O 3 is extensively researched for its use in hybrid composites.This is due to its superior density (3.98 g cm −3 ) and hardness compared to other synthetic reinforcements such as SiC, SiO 2 , MgO, K 2 O, and Fe 2 O 3 .Additionally, alumina possesses a hexagonal close-packed structure, exhibits solid lubricating effects, and boasts a high modulus of elasticity ranging from 220 to 370 GPa.HMMCs fabricated using Al 2 O 3 /SiC reinforced AA7075 matrix through stir casting have been reported [172].The Al 2 O 3 /SiC/AA7075 HAMCs exhibit homogenous microstructural features, effective bonding at the interface between reinforcements and the matrix, and excellent wear resistance.Similarly, experimental investigations on the influence of Al 2 O 3 /SiC on the performance of AA6061 have also been documented [173].EDX results showed a surface morphology rich in Al, indicating good sample homogeneity (figure 5).Furthermore, hybrid nanocomposites synthesized using FSP routes have been reported, including AA8026 reinforced with TiB 2 /Al 2 O 3 [174] and AA6082 reinforced with TiB 2 /BN [175] synthetic reinforcement particulates.These composites exhibited homogenous dispersion, good interfacial bonding, and no segregation (figure 6), resulting in enhanced mechanical performance in wear resistance, hardness, tensile and yield strengths.
Industrial wastes, including fly ash (FA) and red mud (RM), have shown promise as potential reinforcements in HMMCs.Fly ash and red mud are byproducts of aluminum power plants and are typically considered toxic due to their alkalinity and salinity contents, respectively.Table 6 shows the elemental composition of fly ash and  red mud, with predominant elements including alumina, silicon, and iron oxides.In contrast, potassium, sodium, titanium, and magnesium oxides are present in trace quantities.Kumar et al [176] conducted a study on the fabrication of AA356 HMMCs reinforced with FA/RM particulates using the FSP technique.The research revealed a homogeneously distributed microstructure in which clustered particles were effectively eliminated.Tensile strength, yield strength, hardness, wear resistance, and ductility were generally enhanced, as demonstrated in figure 7.These improvements can be attributed to grain refinements, interfacial bonding, and hard particulates in the reinforcement.Furthermore, FA has been combined with SRs and NRs to fabricate HMMCs with enhanced mechanical and tribological properties [39,177,178].These studies highlight the potential of utilizing industrial waste materials to develop HMMC materials for improved performance.
The microstructure analysis of the AWs-based reinforced HMMCs fabricated using various Al matrix alloys shows excellent interfacial bonding, uniform dispersion and distribution of fillers, and good sample homogeneity between reinforcements and matrices.However, this contrasts with some studies that reported uneven distribution [181] and agglomeration [186] of reinforcements in the surface morphologies of the fabricated composite samples (figure 8).Interestingly, both of these observations involved the use of AA6061 matrices.However, homogenous distribution of reinforcements and robust interfacial bonding were reported when copper, alumina, and graphene were used separately in conjunction with agro waste ash particulates to reinforce AA6061 alloy [54,62,187].These metallic and synthetic reinforcement materials' self-lubricating properties could account for the observed microstructural improvements.
Similarly, the results of mechanical properties from studies presented in table 7 confirm superior performance in various aspects, including tensile, yield, compressive, and impact strengths, elastic modulus, toughness, hardness, and wear resistance compared to the unreinforced matrix.However, it is imperative to note that there are studies that reported a disproportionate decline in specific mechanical properties such as tensile strength [

Clustering of reinforcement particles and porous surface features
There was an initial decline in corrosion resistance, but with increased immersion time, corrosion resistance significantly improved in 1.0 M H 2 SO 4.
Of all the studies presented in table 7, only three examined the corrosion performance of the fabricated AWs-based HAMCs.Alaneme et al [185] evaluated the performance of BLA/SiC-reinforced Al-Mg-Si alloy in both 3.5% NaCl solution and 0.3 M H 2 SO 4 environments.The BLA/SiC HAMCs exhibited good corrosion resistance in the former (a saline solution), while poor performance was recorded in the latter (an acidic solution), indicating inadequate passivation (figure 9(a)).Palanivendhan and Chandradass [43] reported excellent corrosion behaviour of BN/Lemon grass ash (LGA) reinforced ADC12 alloy in a 3.5% NaCl solution (figure 9(b)).Additionally, Ikubanni et al [188] reported a decline in the corrosion rate of Palm kernel shell ash (PKSA)/SiC reinforced AA6063 hybrid composite for long immersion time in 1.0 M H 2 SO 4 environment.These results suggest that AWs-based reinforced hybrid HAMCs will perform optimally in saline solutions and poorly in acidic environments.However, these three studies alone are insufficient to fully assess the corrosion behavior of agro waste reinforcements in hybrid metal matrix composites.Additional research is needed to ascertain the appropriate environments and applications where these composites can be well-suited and strategies to enhance their performance in environments where they perform poorly.

Multi-component metal matrix composites
Multi-component metal matrix composites (Mc-MMCs) represent an innovative approach in materials science involving the incorporation of multiple (>2) reinforcement materials to fabricate composites with enhanced performance characteristics.This concept draws inspiration from the success of multi-component alloys, which have demonstrated remarkable mechanical properties in various applications [191][192][193][194][195]. Integrating multicomponent reinforcement (MCR) materials, including fibers, agro wastes, or ceramic particulates, into MMCs can provide the potential to develop composites with tailored-specific performance requirements.Engineers can further leverage the synergistic effects of these multiple combinations in a bid to overcome any limitations that individual reinforcement materials may impose on the overall composites.
The adoption of MCRs holds the potential to revolutionize material design and optimization.For example, a composite with a superior strength-to-weight ratio and enhanced stiffness can be developed by combining highstrength fibers with nanoparticles of ceramic materials.Additionally, the disproportionate decline in ductility observed in monolithic and hybrid MMCs (tables 5 and 7) can be addressed using MCRs.By strategically selecting and combining MCRs, it is possible to achieve distinct mechanical, tribological, and corrosion properties in the resulting composites.Advanced processing methodologies such as sintering and FSP offer greater control over performance parameters, ensuring uniform distribution and surface morphology of the Mc-MMCs.
Although the concept of MCRs is first proposed in this study, it presents a new frontier in materials engineering, providing an opportunity to engineer novel materials with superior physio-mechanical properties.However, it is still imperative that researchers explore this concept by broadening their scope to include three or more synthetic and natural reinforcement materials to assess their synergistic or antagonist performance.It is worth mentioning that Osunmakinde et al [67] have already taken steps in this direction by investigating the physio-mechanical properties of three agro waste crop residues used to reinforce an aluminum matrix.Their study used multiple agro wastes, namely CPA, coconut shell ash (CSA), and RHA, to reinforce pure aluminium powder via the two-step stir casting methodology.The choice of these reinforcements was based on their wide coverage and reported enhanced performance.For example, table 8 shows that the dominant oxides in CPA are SiO 2 , Al 2 O 3, K 2 O, CaO, and Fe 2 O 3 , while CSA and RHA are also dominated by SiO 2 [39,105,106,146].Literature has documented enhancement in the mechanical characteristics and resistance to wear of MMCs, with observed uniform spread within the matrix of the singly reinforced RHA-AMCs [42,165,167], monolithic CSA-AMCs [70,163,164], and the hybrid reinforced CPA/SiC-AMCs [58].The superior properties of SiO 2 and Al 2 O 3 found in CPA, CSA, and RHA, including high density, elastic modulus, solid lubricating effect, chemical inertness, high strength, and thermal stability, contribute to the enhanced performance of these MCRs.
The physio-mechanical results presented by Osunmakinde et al [67] further demonstrate the potential benefits of the MCR approach for fabricating Mc-MMCs.Uniform dispersion of the MCRs within the matrix, absence of dendritic structure, clustering, and agglomeration (figure 10(a)), and the presence of intermetallic phases (figure 10(b)), resulting in Orowan strengthening mechanisms were observed.The Orowan strengthening mechanism increased the dislocation density of the hard MCR particulates, facilitating the efficient transfer of load-bearing from the matrix to the MCRs.This mechanism is crucial for enhancing the mechanical properties of MMCs and is vital when exploring the strengthening effects of multiple hard particulates in MMCs fabrication [160,161].These mechanisms contributed to the superior tensile strength (33.6%) and hardness (64.3%) reported for the novel Mc-AMCs (figure 10(c)).Notably, the lightweight nature of the composites, with a density of about 2.5 g cm −3 , highlights the sustainability of the engineering approach.Further research is needed to explore additional mechanical, tribological, and corrosion properties of MCRreinforced composites, including combining synthetic and natural reinforcements in varied weight ratios.
8. Sustainable agro waste material engineering practice 8.1.Criteria for sustainable material engineering For a material to be deemed suitable for sustainable engineering applications, it must exhibit five key characteristics viz; readily available (i.e.abundant), relatively ease of processing, cost-effective, renewable, and biodegradable.Interestingly, agro waste satisfies these criteria.AWs are readily available as they come from essential renewable resources [93].Secondly, they require minimal processing, leading to lower energy consumption and reduced pollution potential during production [196].Thirdly, AWs are cost-effective due to their natural recyclability and abundance, resulting in lower processing costs than synthetic alternatives [197].Furthermore, agro waste materials as renewable resources reduces dependence on finite resources [93,198].Lastly, AWs are biodegradable organic byproducts [18], which can be further processed into ash for sustainable engineering repurposing, making them environmentally biodegradable.
Converting agro wastes into ash can indeed lead to atmospheric pollution, posing a challenge to the sustainability of agro waste engineering.However, it is essential to recognize the diverse methods available to reduce emissions from AWs burning and even leverage sustainable advantages through byproduct capture for energy generation.For instance, Darmawan et al [199] demonstrated the utilization of AWs, such as rice waste and palm oil residue, as a cost-effective alternative fuel to mitigate atmospheric contamination and greenhouse gas emissions.They advocate the use of technologies such as biochemical (fermentation and anaerobic digestion), thermal (incineration), and thermochemical (pyrolysis and gasification) methods, which can yield liquid and gaseous energy sources like biofuels.Similarly, Kassim et al [200] introduced the concept of integrated conversion technologies (ICTs) to maximize energy yield.ICTs integrate standalone methods like biochemical, thermal, and thermochemical processes to efficiently process AWs byproducts, aiming to minimize waste and reduce atmospheric pollution resulting from AWs conversion into ash during burning.Moreover, adopting sustainable practices during processing, including efficient conversion technologies equipped with emission control systems, is paramount.By considering these diverse approaches and technologies, it is possible to address concerns regarding atmospheric pollution while harnessing the potential of AWs and integrating energy recovery at the source for enhanced sustainable engineering applications.
Festus et al [201] delineate six resource recovery methods for sustainable engineering namely (1) collecting, (2) sorting, (3) processing, (4) molding, (5) solidifying, and (6) machining and refinements.These methods effectively extract valuable components from waste materials like agro wastes, thus mitigating their environmental impact.Figure 11 provides a graphical overview of these stages, aligning with a circular economy perspective.Reviewing literature across monolithic, hybrid, and multi-component agro waste composite production, it becomes evident that agro waste holds significant engineering potential.Studies demonstrate that transforming these wastes into ash form enables their use as reinforcements in metal matrix composites, yielding enhanced performance results (tables 5 and 7).
However, mere study and experimental characterization of these agro waste materials are insufficient; converting them into engineering materials suitable for industrial applications is necessary.This involves processing them into marketable raw products or integrating them into finished products like their synthetic counterparts.Such an approach fosters an industrial circular economy model, promoting job creation and product development and substantially reducing environmental impact.Notably, this circular economy model is underpinned by five fundamental principles: (1) waste minimization, (2) resource recovery, (3) extended product lifespan, (4) closed-loop systems, and (5) commercialization/industrialization.To ensure that efforts toward leveraging agro waste materials for sustainable engineering are not in vain, commercialization and repurposing within the framework of the circular economy model is paramount.This will also contribute to meeting the UN's SDGs, focusing on climate change resilience (SDGs 13), reduction of fossil fuel consumption (SDGs 14 and 15), and sustainable industrialization (SDGs 12 and 17) by the proposed 2030 deadline [47,48,202].

Progress report on agro waste material engineering
Having identified the inherent potential in agro waste material engineering from existing studies (tables 5 and 7), this section reviews the effort of some researchers who have reported engineering material products fabricated using AWs-based reinforcement.Typical examples include the production of automotive connecting rods, outer panel of the B-pillar, grease for roll-bearing applications, bicycle frame and brake pads utilizing industrial and AW materials.Table 9 presents an overview of documented reports on engineering studies fabricated from agro waste and their performance.
Aigbodion et al [59] conducted a performance analysis of an automobile connecting rod for the Toyota Carina Model I vehicle, fabricated using AA2009 alloy (3.7 wt% Al, 1.4 wt% Cu, and 4 wt% Mg) reinforced with  Microstructural investigation revealed a even distribution of reinforcements within the matrix.Lightweight rods with minimized fuel consumption and brake force compared to the standard rod. [59]

S-glass hybrid composite Abirbara plant
Outer panel of the B-pillar in a sedan car Measured parameters enhanced by 153 MPa, 213 MPa, and 370 N at peak load, respectively for tensile, compressive, and bending strengths, with a water absorption rate of 1.6% within 48 h. [203]

Carbon nanotubes Banana peel waste Grease for roll-bearing
The grease showed good corrosion resistance, elevated dropping point, enhanced viscosity, and increased load-carrying capacity.It exhibits reduced friction coefficient, wear scar diameter, and power consumption, suggesting its potential as an effective lubricant in rolling bearing applications. [204]

AA6063 TiC/Neem leaf Bicycle frame
The optimal combination of TiC and neem leaf ash particles in the aluminum matrix enhanced the mechanical properties. [205] Aluminum dross powder Coconut shells, oil bean stalk, and carbon black
The hardness of the fabricated brake pad exhibited values comparable to those of the commercial variant. [206] BPAnp from AWs (figure 12) via the stir casting and double layer feeding method.A comparative performance analysis of the fabricated connecting rod with the standard rod using SEM analysis of the fabricated connecting rod and a standard rod revealed a higher Al elemental composition compared to the latter's higher Fe elemental composition.This outcome is consistent with the aluminum-based construction of the fabricated rod, which is significantly lighter than its iron counterpart.The lightweight nature of the fabricated AMC-based connecting rod offers strategic advantages, including reduced fuel consumption, as studies have shown a correlation between lightweight materials and decreased fuel consumption [113,207,208].Surface morphology analysis demonstrated homogeneous reinforcement distribution in the fabricated AMC connecting rod (figure 13(a)), with fuel consumption ranging from 0.35 to 0.65 kW h −1 (figure 13(b)), compared to 0.43 to 0.76 kW h −1 for the standard rod.This reduction in fuel consumption underscores one of the many benefits of lightweight composites facilitated by agro waste reinforcements.Additionally, brake force analysis indicated that the AWs-based connecting rod exhibited lower brake force than the standard rod in a Toyota Carina I engine vehicle.This result could contribute to the observed 0.36% reduction in fuel consumption, with the possibility of further reduction through enhanced engineering refinements.Future research could explore the utilization of other agro waste materials listed in tables 1 and 2 to investigate potential synergistic or antagonistic effects.

Challenges of agro waste material engineering
As illustrated in tables 5, 7, and 9, extensive research efforts have explored the potential of agro waste materials as viable engineering reinforcements, aiming to replace costly synthetic alternatives with sustainable natural options.However, findings regarding their impact on composite performance vary, with positive and negative outcomes observed across studies.For instance, Bodunrin et al [152] argued that MMCs reinforced with AWs-  based NRs tend to exhibit inferior properties compared to SRs, citing studies by Alaneme et al [105] and Prasad and Krishna [165,209], which reported decreased hardness and wear performance when RHA was used to reinforce Al-alloys of AA6001 and AA356.2,respectively.However, contradictory evidence exists, with studies by Dinaharan et al [106], Muralimohan et al [68], and Muni et al [62] reporting superior mechanical properties and enhanced hardness and wear behaviour with RHA reinforcement.
The superior properties reported in studies by [62,68,106] are attributed to the choice of matrix used alongside RHA in the composite fabrication.The studies by [62,106] employed a Cu matrix known for its superior electrical conductivity, thermal conductivity, and wear resistance compared to the Al matrix utilized by [105,209].The addition of RHA to the Cu matrix significantly altered the wear rate, addressing the high wear rates typically associated with pure Cu due to micro-cutting wear modes [106].The presence of RHA particles changes the sliding wear conditions, leading to improved wear resistance in the composite.The morphology of the worn surface differs with Cu, exhibiting exposed sub-surfaces with larger craters and significant plastic deformation influenced by frictional heat softening the Cu during sliding wear.The strong interfacial bonding between RHA particles and the Cu matrix facilitates effective load transfer, resulting in homogeneous distribution of RHA particles with good interfacial bonding.Muni et al [62] further validated these findings, reporting improved hardness, wear resistance, and tensile properties in RHA/Cu-reinforced AMCs.They attributed the enhanced hardness and tensile strength to the inclusion of Cu, resulting in approximately 7.4% increase in hardness and 4.1% increase in tensile strength.This aligns with the observations reported by Dinaharan et al Additionally, Muralimohan et al [68] attributed the enhancement in properties to the presence of dispersed hard ceramic (RHA/B 4 C) particulates within the ADC-12 composites.
Significant challenges hindering the industrial adoption of agro waste materials include high processing costs, the need for advanced sorting equipment, and the choice of processing routes.Akbar et al [57] highlights the significant processing costs associated with metals and reinforcement refining, as a major challenge affecting the industrial processing of agro waste for reinforcement material.The sorting of waste requires advanced digital equipment with special sensors and conveyor belt systems, as outlined in the processing stages by Festus et al [201].Additionally, while stir casting is commonly used and yields satisfactory results for some applications, alternative methods like friction stir processing present potential alternatives.Mazaheri et al [210] argued that while other processing methods are susceptible to agglomeration, friction stir processing is not.This observation, however, appears inconsistent considering results from [183,184]; thus, additional investigation is needed under carefully controlled conditions.Furthermore, non-uniform particle size and equipment limitations further hinder SMEs seeking to venture into agro waste engineering for composite fabrication.
Coordination between sustainability research and industrial application is crucial to overcome these challenges and drive progress in the green engineering revolution.High-tech equipment and resource-intensive testing requirements for peer-reviewed experimental efforts present additional obstacles.Engineers must focus on developing cost-effective processing methods like SC and DSC, ensuring proper characterization and rigorous testing for improved performance and safety.Indigenous development of automobile parts using the readily available, renewable, and cost-effective agro waste ash materials by SMEs in sub-Saharan Africa can reduce reliance on imports, while coating technology offers a cost-effective solution to material corrosion concerns.

Further studies in agro waste reinforced MMCs
The critical literature review conducted in this study underscores the significant potential of agro waste reinforcement in metal matrix composites as a sustainable engineering achievement.However, several areas warrant further investigation to advance our understanding and optimize the use of these materials, particularly in addressing the drawbacks identified in the existing literature.
1. Exploration of diverse agro waste sources is paramount, this is because, while research efforts have predominantly focused on food crops such as RHA and CSA, there is a need to broaden the scope to include other agro waste sources such as animal byproducts.Limited studies exist in this area, and exploring a wider range of agro waste materials could unveil new opportunities for MMCs, with superior mechanical performance or improved sustainability compared to currently used options.Future studies should not only aim to investigate the mechanical properties of MMCs reinforced with diverse AWs sources but also address the drawbacks associated with their processing and compatibility with different matrices.
2. Despite adopting similar processing routes, microstructural properties and mechanical performance variations are observed.Further experimental studies are needed to elucidate these variations and identify the underlying factors influencing them.Future studies should focus on systematically analyzing the influence of processing parameters such as pressure, temperature, and duration on the microstructural evolution and mechanical properties of AWs-based MMCs.Advanced characterization techniques could be further employed for deeper insights into the interface morphology and bonding mechanisms.
3. There is a need to pay more attention to how particle size affects the microstructure and mechanical characteristics of AWs-based MMCs.Nanoparticles have been reported for only bean pod ash; there is a need to investigate the nanoparticle performance of the other agro wastes.Future research efforts should focus on determining the optimal particle size range for various AWs reinforcements to offer valuable guidance for composite fabrication.Additionally, these studies should evaluate the potential of different particle sizes to enhance the mechanical properties and microstructural features of MMCs.
4. Assessing the influence of particle size on the microstructural and mechanical properties of agro waste-based MMCs remains underexplored.While nanoparticle performance has been investigated for agro wastes like bean pod ash, comprehensive studies on nanoparticle effects across different agro waste materials are still missing.Future research should thoroughly elucidate the role of particle size in influencing the performance of these composites.By investigating how variations in particle size affect the microstructure, mechanical properties, and processing characteristics of MMCs reinforced with different AWs, future studies can address the drawbacks associated with optimizing fabrication processes and improving the mechanical reliability of AWs-based composites for sustainable engineering applications.
5. Despite the importance of tribo-corrosion in real-world applications, research in this area is limited.Future studies should focus on assessing the corrosion resistance of agro waste reinforced MMCs to provide valuable insights for practical applications.Additional studies into the synergistic effects of mechanical loading and corrosive media on the degradation behavior of these composites are warranted to address observed drawbacks.By thoroughly studying the tribo-corrosion behavior of agro waste-based MMCs, future studies can contribute to enhancing their durability and reliability in demanding environmental and operational conditions.
6.It is worth noting that, while most research has centered on aluminum alloys, there is a need to explore the use of and characterize other metal alloys such as titanium and zinc.Understanding their interaction with agro waste materials can expand the applicability of MMCs in various industries, such as biomedical applications.Future research efforts can address these drawbacks by investigating the compatibility of different metal alloys with AWs and evaluating the mechanical, thermal, and biological properties of resulting MMCs.By exploring a wider range of metal matrices, future studies can identify novel AWs-based MMCs, with properties tailored for specific applications.
7. The concept of multicomponent MMCs is innovative and requires further investigation to evaluate their performance and characterization compared to monolithic and hybrid MMCs.Exploring this area can provide valuable insights into optimizing the design and performance of multicomponent MMCs.Further investigations into the processing techniques and engineering methodologies for achieving uniform dispersion and strong interfacial bonding in multicomponent MMCs are needed.. Advancing the understanding of multicomponent MMCs, future research can address associated drawbacks by facilitating the development of high-performance materials for diverse sustainable engineering applications.

Conclusion
This review extensively examined the engineering application of agro waste-based reinforcements in enhancing the microstructural, tribological, mechanical, and corrosion performance of MMCs.Extensive literature supports the effectiveness of agro waste as a sustainable reinforcement material, particularly when processed into ash form, devoid of impurities that could adversely affect the composite's composition and performance.
Additional key insights garnered from the review include: 1. Asia is the largest global producer of agricultural products, including crop residues and animal byproducts, followed by Africa.
2. Agro waste-based MMCs offer lightweight and cost-effective alternatives to synthetic reinforcements, with processing methods like stir casting and friction stir processing showing promising results in terms of homogeneity and performance.
3. Agro waste has been utilized to reinforce MMCs in various forms, including monolithic, hybrid, and multicomponent MMCs, enhancing mechanical performance, albeit at the expense of ductility and fracture toughness, except for FSP-processed agro waste.
4. Multicomponent agro waste reinforcement MMCs perform better than hybrid or monolithic MMCs but require further investigation with diverse agro waste materials for proper characterization.
5. Further studies are required to assess the corrosion performance of agro waste reinforced MMCs across different types.
6. Industrial product designs utilizing agro waste have yielded results consistent with experimental literature studies, producing lightweight materials that reduce fuel consumption, indicating a valuable avenue for sustainable engineering practices, particularly in the fabrication of automobile parts.
7. Contrary to being deemed futile, agro waste studies represent a valuable effort in sustainable engineering, providing a promising reinforcing material, with further research recommended to explore their full potential and expand their practical application scopes.

Figure 4 .
Figure 4. (a) Formation of crystal nanostructure in BPA-based agro waste reinforcement material.Reprinted from [147], Copyright (2014), with permission from Elsevier, and Wear rate performance of a BPA reinforced AMCs at (b) 100 °C and (c) 200 °C.Reproduced from[148], with permission from Springer Nature.

Figure 8 .
Figure 8.Effect of AWs-based reinforcements on the microstructural features of AA6061 matrix alloy.Reprinted from [181], Copyright (2023), with permission from Elsevier.

Figure 11 .
Figure 11.Circular economy model for sustainable agro waste material engineering.

Table 1 .
Regional livestock production statistics from 2012 to 2022 on a global scale.

Table 2 .
Regional crop production statistics from 2012 to 2022 on a global scale.

Table 3 .
Typical metals used as matrices in MMCs and their area of applications.

Table 4 .
Elemental oxide composition of alumina and CPA-based agro waste.

Table 7 .
Overview of some studies investigating hybrid reinforced agro waste metal matrix composites, detailing their reported performance metrics.

Table 9 .
Review of some agro waste material engineering.