1 Introduction

The AA6063 alloy is renowned for its good mechanical properties and formability, mainly suited for aluminium (Al) extrusion [1]. Comprising elements such as Al, magnesium (Mg), and silicon (Si), this alloy forms an intermetallic compound (Mg2Si) with good heat-treatment properties, facilitating easy weldability. The alloy is commonly used for applications such as door frames, roofs, and window frames, owing to its smooth surface finish and good corrosion resistance. Despite these promising properties, the Brinell hardness number (BHN) of the medium-strength AA6063 metal matrix is relatively low at 25 BHN [2]. This relatively low hardness value and wear resistance of the AA6063 alloy further limits its use in the automobile and aerospace industries, making it unsuitable for high-strength applications despite its lightweight and unique thermal properties. In a bid to improve its hardness and wear properties, researchers have considered the use of different reinforcements with varying results. For instance, the use of precipitation hardening by artificial ageing to improve wear resistance [3] and in-situ Mg2Si reinforcement of Al–Si12–Mg(5,10,20) to enhance hardness properties and wear resistance [4] of the AA6063 matrix have been documented in the literature.

The quest for enhanced mechanical and tribological properties has continued to spur research interest in innovative methods for reinforcing aluminium matrix composites (AMCs). Among these, the utilization of synthetic ceramic reinforcements (SCRs) like alumina (Al2O3) with a density (3.9 g/cm3) greater than Al (2.7 g/cm3) holds the capability to enhance the hardness and wear properties of AMCs significantly. There are also documented reports of other SCRs like titanium carbide (TiC), silica (SiO2), silicon carbide (SiC), carbon nanotubes (CNTs), graphite, and tungsten carbide (WC), that impact positively on the hardness and wear characteristics of AMCs [5,6,7,8,9]. Bodunrin et al. [10] further classified Al-reinforcements into agro waste, synthetic ceramic, and industrial waste particulates. Like the SCRs, the use of agro waste as reinforcements have also garnered research interests, aiming to repurpose engineering practices to ameliorate environmental pollution caused by the indiscriminate disposal of agricultural solid wastes. These solid wastes, known as natural ceramic reinforcements (NCRs), are employed in ash form and are relatively cost-effective owing to their abundance in nature [11] compared to SCRs [12, 13].

Furthermore, literature reports indicate favorable engineering properties for NCRs, including mechanical strength, wear resistance, corrosion resistance, and porosity [6]. NCRs exhibit high strength, improved hardness, and enhanced wear resistance as the load on the matrix is efficiently transferred onto these reinforcements. Examples of NCRs that have been utilized in the production of AMCs include rice-husk ash (RHA) [14,15,16], coconut-shell ash (CSA) [17,18,19,20], palm-kernel shell ash (PKSA) [6, 21, 22], corn cob ash (CCA) [23], date palm seeds ash (DPSA) [24], bean pod ash (BPA) [25,26,27,28], groundnut shell ash (GSA) [29], plantain peel ash (PPA) [30, 31], bamboo leaf ash (BLA) [32, 33]. For example, when combined with alumina as reinforcements in the Al–Mg–Si matrix, RHA was reported to produce lightweight composites [11]. AMCs reinforced with agro waste particulates have equally demonstrated enhanced hardness with increasing percent weight variation as reported for RHA/SiC [34], RHA/B4C [35], RHA/Cu [15], CSA/Al2O3 [20], CSA [36], BPA [28], and DPSA with an optimum increase at 7.5 wt.% [24]. In contrast, a decrease in hardness has also been reported for AMCs with increasing weight ratio variations in NCRs/SCRs such as RHA/Al2O3 with about 10% decrease reported [11], CCA/SiC [23], BLA/Al2O3 with about 9% reduction for a 40% Al2O3 reduction [37], and GSA/SiC [29].

In-situ composites have multiple phases, with the reinforcing phase generated within the matrix during production. The in-situ processes can be used to fabricate reinforced composites with varying characteristics, such as ceramic or ductile phases and continuous or discontinuous morphologies [4]. In-situ AMC fabrication benefits from weight reduction, enhanced mechanical properties, and relatively low cost [38]. Poor wettability, agglomeration, and uneven distribution of reinforcement particulates contribute significantly to mechanical properties such as hardness in AMCs [30]. The use of Mg has been recommended to improve wettability [11], while production methods such as two-step stir casting have been found to ameliorate issues relating to agglomeration and non-uniform distribution of particulates [39]. Increasing the stirring time and speed in the double-step stir casting method has also been reported to significantly influence the hardness properties of AMCs [7]. Commonly investigated Al alloys reinforced singly with either SCRs or NCRs or a hybrid of SCR-NCRs include AA2014 [40], A356 [41], LM13 [42], AA5083 [43], AA7075 [44], Al6061 [16, 20], AA2009 [28], ADC12 [35], and AA6063 [6, 43]. Studies involving the use of AA6063 have employed fabrication routes like ballistic impact [43] and the compo-casting method [6], with limited information on the use of a double stir-cast route, particularly for SCR–NCR hybrid reinforcement. Furthermore, several studies have reported the pivotal role of tribological properties in manufacturing sectors such as aerospace, automotive, and machining of hard-to-cut materials [9, 16, 17, 27, 45,46,47,48,49,50,51,52,53,54,55,56,57,58].

Prasad and Krishna [58] studied the tribological performance of monolithic RHA/A356.2 AMCs fabricated using a mechanical stirrer. They utilized the pin-on-disc dry sliding wear technique for assessment and reported significant improvement in the hardness and wear resistance of the composites. Muni et al. [16] observed enhanced hardness properties in hybrid RHA/Cu/AA6061 AMCs manufactured via the two-step stir casting method. The investigation involved different RHA/Cu concentrations, specifically 6:0, 6:3, 8:3, and 10:3 ratios. Apasi et al. [17] assessed the wear characteristics of a monolithic CSA-reinforced Al–Si–Fe alloy produced via stir casting, while Lakshmikanthan and Prabu [56] studied the mechanical and wear properties of CSA/AA6061 AMCs. The studies reported enhancement in the wear resistance of the monolithic composites during dry sliding wear testing conducted with a pin-on-disc apparatus. Subramaniam et al. [57] utilized the stir casting method to reinforce AA7075 with CSA/B4C, resulting in hybrid AMCs. Their tribological investigations, employing varying wt.% of 0:3, 3:3, 6:3, 9:3, and 12:3 for the CSA/B4C hybrid reinforcement, revealed significant improvements in the composite's tribological properties, obtained via the pin-on-disc dry sliding wear tests conducted. Aigbodion et al. [27] utilized the double stir casting and double layer feeding method to fabricate BPAnp-reinforced AA2009 AMCs, followed by wear testing using a pin-on-disc heated at 373.15 and 473.15 K. Their study revealed a significant increase in the wear behavior of the monolithic AMCs. Bhuvaneswari et al. [9] explored the tribological performance of AA219 alloy reinforced with sea-shell powder (SSP) biocomposite fabricated via stir casting. Their experimental findings, analyzed using surface methodology for tribological analysis, demonstrated a notable enhancement in the hardness and wear resistance of the monolithic SSP/AA219 AMCs. Palanivendhan and Chandradass [46] examined the mechanical and wear properties of GSA and hemp fiber ash (HFA) as reinforcements in AA6063 hybrid AMCs manufactured via the stir casting method. They investigated various GSA/HFA ratios (2.5:0, 0:2.5, 5:0, 0:5, 7.5:0, 2.5:2.5, 2.5:5, and 5:2.5), noting a decrease in hardness and a relative enhancement in wear performance. Furthermore, they conducted additional research on the tribological behavior of ADC12 alloy reinforced with boron nitride (BN) and lemon grass ash (LGA) prepared through the stir-squeeze casting method, with varying wt.% compositions of 0:6, 1.5:6, 3:6, 4.5:6, and 6:6 for BN/LGA [47]. Their findings, obtained using a linear reciprocating tribometer, indicated improvements in the hardness and wear resistance of the BN/LGA/ADC12 hybrid AMCs.

The review of existing studies underscores the potential of monolithic and hybrid composites (MHCs) to address the mechanical and chemical limitations associated with unreinforced Al alloy matrices in sustainable engineering applications. Green manufacturing, increasingly popular among researchers, explores agro waste-based NCRs as alternatives to the costly SCRs for developing innovative MHCs [51]. Despite the extensive studies conducted on agro waste reinforcements, many potential agro waste materials remains unexplored or lack investigations into their tribological properties. Among these mterials are the Manihot esculenta peels (MEP) and the Plantagor major peels (PMP), which are the focus of this study. Research on the tribological performance of MEP and PMP ashes in hybrid composites have been limited, with existing studies concentrating solely on the physio-mechanical properties of these composites, leaving their tribological performance unexplored. [30, 59]. However, there have been extensive reports of the Manihot esculenta peels ash (MEPA) and Plantagor major peels ash (PMPA) as reinforcement and replacement materials in polymer composites, and admixture concretes [60,61,62,63,64,65,66,67]. This knowledge gap motivated the investigation of the mechanical and tribological performance of MEPA and PMPA as reinforcements using the two-step stir casting method to fabricate in-situ Al2O3/AA6063 MHCs, presenting a novel approach in this study. Comparative analysis between MEPA/Al2O3/AA6063 and PMPA/Al2O3/AA6063 reinforced MHCs further enhances the novelty of this research.

The choice of MEP and PMP agro waste byproducts for this study is premised on their widespread global consumption and extensive cultivation in sub-Saharan African countries. According to FAO statistics [68], the global production of Manihot esculenta (ME), commonly known as Cassava, and Plantagor major (PM), also referred to as Plantain, in sub-Saharan Africa was approximately 208 million and 30 million tonnes, respectively in 2022. ME is largely cultivated in Nigeria at ~ 60 million tonnes [68], surpassing global production by over 18% [69], while PM is predominantly cultivated in Uganda at ~ 10 million tonnes [68]. Considering these statistics, it can be inferred that a significant portion of these food crops, especially the peels, will become environmental waste and significant contributors to environmental pollution [70,71,72]. To mitigate against the problem arising from the indiscriminate disposal and environmental pollution caused by these solid wastes, repurposing them as sustainable materials for engineering applications is crucial. This approach will further reduce the substantial costs associated with processing and importing of SCRs.

This research aims to contribute to the discourse on agro waste reinforcement of MMCs by exploring the potential of using NCRs (MEPA and PMPA) as single reinforcements with AA6063 in the absence of SCR (Al2O3) to produce environmentally sustainable lightweight AMCs for strength-based and wear-resistant engineering applications. Novel weight percentage variations will be employed for fabricating MEPA and PMPA-reinforced Al2O3/AA6063 MHCs. In addition, a detailed analysis of the oxide compositions present in the MEPA and PMPA particulates and their influence on the density and tribological properties of the fabricated AMCs will be conducted, providing further novelty to this study.

2 Experimental methodology

2.1 Processing

Material selection and preparation Materials used for this study are novel and include ingots of AA6063, alumina powder of 26 μm particle size, MEPA powder, and PMPA powder. Spectral analysis was carried out on the AA6063 matrix, and the findings are presented in Table 1. The ME and PM peels were acquired from a regional commodity marketplace in Ede, Nigeria. The peels were incinerated separately inside a crucible and placed in an oven for 24 h at 353.15 K. The ashes were removed and conditioned inside an electric muffle furnace maintained at 923.15 K for 180 min to eliminate carbonaceous constituents. Figure 1 shows the powdered ash samples prepared for MEPA and PMPA utilized in this study.

Table 1 Result of Spectra analysis for AA6063 alloy and its chemical composition
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Fig. 1
figure 1

Powdered ash samples of a MEPA and b PMPA, used as reinforcements

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Composite fabrication The experiment was formulated to yield a 10% weight ratio reinforcement of SCR (alumina) and NCRs (MEPA and PMPA). Table 2 presents the novel weight percentage (wt.%) variations of 2, 4, 5, 6, 8, and 10% employed in fabricating the MEPA/Al2O3/AA6063 and PMPA/Al2O3/AA6063 HMCs in this study. The matrix was heated to approximately 1023.15 K ± 20 K using a gas-fired charged crucible furnace to obtain a molten liquid, which was subsequently left to cool. Conversely, the SCR and NCRs were preheated simultaneously at 523.15 K to allow for easy distribution of reinforcements within the alloy of the molten matrix. Adopting the double-stir casting methodology, the preheated reinforcement particulates and 0.001 weight fractions of Mg (for improving wettability) were added to the molten matrix and agitated for 10–15 min. The slurry of the molten composite was heated to an elevated temperature of 1143.15 K ± 20 K and agitated using a mechanical agitator (300 ± 50 rpm) for about 15 min for sample homogeneity. Sand moulds prepared using the method described by Festus et al. [48, 49] were used to receive the molten mix for the casting process, forming composite ingots. The ingots were subsequently machined and cut into test pieces, as detailed in Table 2, to facilitate further experimental investigations. It is imperative to highlight that the hybrid composites of MEPA/Al2O3/AA6063 and PMPA/Al2O3/AA6063 were prepared independently, utilizing the same methodology. The as-cast HAMCs were hot-mounted and metallographically prepared by grinding and polishing the surfaces of the specimens.

Table 2 The weight percentages of (a) MEPA/Al2O3 and (b) PMPA/Al2O3 reinforcements
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2.2 Experimentation

Microstructure The surface morphologies of the MEPA and PMPA reinforcements were characterized using a JSM-7900F JEOL field emission scanning electron microscope (SEM) equipped with an OXFORD elemental dispersive X-ray (EDX) device for elemental composition identification.. The chemical constituents of the elemental oxides were identified through X-ray fluorescence (XRF) spectroscopy of the as-prepared powders. Elemental phases present in the MEPA and PMPA powder samples were analyzed at 0.017 step size, 30 s per step screening rate, 40 kV (voltage), and 45 mA (current) using a Rigaku X-ray diffractometer.

Density For each as-cast HAMC fabricated in this study, the theoretical \({(\rho }_{th})\) and experimental (\({\rho }_{ex})\) densities were determined using Eqs. (1) and (2), respectively, per the ASTM D2734 standard [73]. A pycnometer was used to estimate the theoretical densities of Al2O3, MEPA, and PMPA powders. The experimental density was estimated by measuring the sample mass (m) in air using a high-precision digital weighing scale, whereas the volume (V) of the HAMCs was measured using the principle of flotation.

$$\rho _{{th}} = Wt._{{AA6063}} \times \rho _{{AA6063}} + Wt._{{SCF}} \times \rho _{{SCF}} + Wt._{{NCF}} \times \rho _{{NCF}}$$
(1)
$${\rho }_{ex}=m/V$$
(2)

Hardness The hardness investigations of the fabricated HAMCs were conducted per the ASTM E10-23 standard [74] using the Innovatest Falcon 500 hardness tester. The indenter was placed in contact with the composite specimen and a test force (FN) of 980.67 N was applied perpendicular to the surface for a dwell time of 10 s. The size of the indenter’s ball diameter (D) used was 10 mm, while the indentation diameter (d) was measured in perpendicular directions with a 20 × microscope affixed to the Innovatest Falcon 500 hardness tester. The Brinell hardness of the as-cast HAMCs was estimated using Eq. (3). The average of the three indentations was used to obtain the BHN of each as-cast HAMC sample.

$$BHN = (2 \times F_{N} )/\pi D(D - \sqrt {D^{2} - d^{2} } )$$
(3)

Wear Using a pin-on-disc wear tester device, an arid wear test was conducted on the as-cast HAMCs per the ASTM G99 standard [52]. Each HAMC specimen's initial weight (WINITIAL) was determined with a high-precision digital weighing scale. The test specimens were positioned onto the sample holder, and a 10 N load (FLOAD) applied. The test specimens were rotated under this load conditions for 20 cycles. The test specimens final weight (WFINAL) and the sliding distance (DSLIDING) were measured. Using the experimental density of the test specimens, the volume of wear loss (\(\Delta {V}_{LOSS}\)) and specific wear rate (KS) were computed from Eqs. (4) and (5) [75]. The wear coefficients (K) are estimated by taking the average hardness (HC) of the corresponding composite into consideration and using Archard’s wear equation given by Eq. (6) [76].

$${\Delta V}_{LOSS} ({mm}^{3})=({W}_{INITIAL}-{W}_{FINAL})/\rho$$
(4)
$${K}_{S} ({mm}^{3}/Nmm)={\Delta V}_{LOSS}/( {F}_{LOAD}\times {D}_{SLIDING})$$
(5)
$$K ({mm}^{3}BHN/Nmm)=({\Delta V}_{LOSS}\times {H}_{C})/( {F}_{LOAD}\times {D}_{SLIDING})$$
(6)

3 Result and discussion

3.1 Microstructural results

The structural features and elemental compositions of the as-prepared ash samples of MEPA and PMPA are shown in Figs. 2 and 3, respectively. SEM results for MEPA particles revealed a roundish-shaped surface with some longitudinal portions (Fig. 2a), consistent with [63, 77]. For PMPA particles, the surfaces were angular in shape, some portions were roundish, with fibrous and spongy distributions, (Fig. 3a), which aligns with the observation by [78,79,80].No defects or surface contours were observed on the surface morphologies of the MEPA and PMPA particulates. This observation is significant as wettability is influenced to a large extent by the surface layer [81, 82]. The EDX spectra further revealed that MEPA particles contained significant wt.% of elements like oxygen (46%), silicon (21%), potassium (9%), aluminium (9%), calcium (5%), and iron (5%). Trace amounts of Mg, Ti, phosphorus (P), and Sulphur (S) were also observed in the MEPA particles. This observation suggests that silica (SiO2) is most likely to dominate within the MEPA particles, with the particles also containing substantial portions of Al2O3. For the PMPA particulates, EDX spectra show substantial wt.% portions of potassium (56%) and oxygen (38%) elements, with trace quantities of P, Mg, S, Si, and chlorine (Cl), which is consistent with the elemental compositions reported by [83,84,85]. Oxides of potassium (K2O) are likely dominant in the PMPA surface. The dominance of potassium in PMPA particles have also been reported by [80, 83, 86, 87], with wettability and mechanical properties found to significantly impact the nature of oxides on the surface layers [88,89,90].

Fig. 2
figure 2

SEM/EDX investigations of MEPA (a) and PMPA (b) reinforcements

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Fig. 3
figure 3

SEM results for MEPA (i) and PMPA (ii) HAMCs

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The surface morphologies of the fabricated MEPA and PMPA Al2O3/AA6065 hybrid composites are shown in Fig. 3. A distinct interfacial appearance is observed amongst the MEPA and alumina reinforcement particles, which are homogenously distributed along the visible grain boundaries within the matrix (Fig. 3i[a-e]). This showed one of the benefits offered by the two-step stir casting methodology in relieving the surface tension between the reinforcements and the matrix. This observation aligns with the study by [59], who also reported uniform distribution of the reinforcing phase within the aluminium-rich matrix. The PMPA HAMCs, on the other hand, showed uniform dispersion of reinforcement within the matrix, with visible grain boundaries. Clustering of reinforcements was observed in PMPA-2 HAMCs (Fig. 3ii[a]). The PMPA composites also showed high porosity levels with mesopores and micropores evident in Figs. 3ii(c, d), which agrees with the observation by [80, 83, 87]. As a polymeric material, agro waste-based ceramic reinforcement contains lignin, which inhibits efficient bonding between the matrix and the reinforcements, owing to its smooth surface,. The effect of the cellulose and hemicellulose bonding becomes pronounced with an increase in the weight percent of PMPA particulates [91], as evidenced by Fig. 3iic, d.

XRF results showing the oxide compositions of the MEPA and PMPA ash samples and the as-received alumina powder are presented in Table 3. As suggested by the EDX results, the MEPA particulates were dominated by oxides of SiO2 at 44%, Al2O3 at 16%, K2O at 13%, CaO at 12%, and Fe2O at 8%. The oxide composition observed in the MEPA particles is consistent with previous reports, indicating the presence of various oxides [60, 62, 92, 93]. The silica content measured in this study (44%) falls within the range reported in other studies (36.79% to 83.00%). The XRF results for PMPA particulates correlate with the EDX elemental peaks observed in Fig. 2b, as oxides of K2O (81%) and CaO (5%) dominate. Other studies have reported similar observations regarding the oxide composition of PMPA. However, the potassium (K) content in this study (81%) exceeded the range of 45.16% to 68.37% found in previous studies [83, 84, 94, 95]. For the Al2O3 particulate, 99 mass% of alumina particles were observed, which was consistent with [11]. A closer look at Table 3 reveals that the first eight oxides were present in varying quantities in the SCR and NCR particulates, suggesting that these oxides would largely impact the mechanical performance of the resulting HAMCs.

Table 3 XRF oxide results for (a) MEPA, (b) PMPA, and (c) Al2O3 particulates
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The XRD diffractogram in Fig. 4 exhibited matched peak patterns for MEPA and PMPA particulates. In Fig. 4a, the XRD peaks confirm the presence of abundant silica (SiO2) in an amorphous phase structure, followed by the monoclinic hatrurite (Ca3SiO5) and hexagonal wüstite (Fe2O) phase lattice structures. This observation aligns with the EDX result depicted in Fig. 2a. Additionally, previous studies [79, 96] have also confirmed the presence of amorphous silica peaks in MEP particles. For PMPA particles, the XRD diffraction pattern in Fig. 4b confirms the dominant presence of potassium compounds. The observed peaks correspond to the face-centered cubic potassium chloride (KCl) and monoclinic potassium carbonate hydrate (K2CO3·1.5H2O) phase structures, consistent with findings reported by previous researchers [80, 83, 86, 87] and the EDX results shown in Fig. 2b for the PMPA sample. Additionally, peaks indicative of silica (SiO2) and calcium metaphosphate (Ca3P2O8) phases were observed in the XRD diffractogram for PMPA particulates.

Fig. 4
figure 4

XRD diffraction pattern results for MEPA (a) and PMPA (b) reinforcements

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3.2 Density and void fraction results

The densities of Al2O3, MEPA, and PMPA measured using the pycnometer were 3.9796 g/cm3, 2.6792 g/cm3, and 2.3821 g/cm3, respectively. Table 3 revealed that the denser oxides with the highest mass% exerted a dominant influence on the overall density of the reinforcement particulates. The measured powder densities were consistent with their literature values within an uncertainty of 0.19%, 1.18%, and 1.37%, respectively, for Al2O3, MEPA, and PMPA particles.

A graphical plot showing the variations in theoretical densities, experimental densities, and void fractions of the MEPA and PMPA particulates was presented in Fig. 5. Theoretical densities of the as-cast HAMCs were estimated through the rule of mixtures and the measured densities of the powder samples per Eq. (1). Comparatively, theoretical densities for MEPA/Al2O3/AA6063 HAMCs ranged from 2.69 to 2.78 g/cm3, while PMPA/Al2O3/AA6063 HAMCs varied from 2.66 to 2.78 g/cm3. Likewise, experimental densities of MEPA-HAMCs varied from 2.64 to 2.74 g/cm3, while that of PMPA-HAMCs ranged from 2.59 to 2.74 g/cm3. With increasing wt.% of NCRs (MEPA or PMPA) and decreasing SCR (Al2O3) particulates in the matrix (AA6063), a consistent decline in density values below that of the matrix was observed, confirming the production of lightweight HAMCs. This supports Islam et al.’s [51] observation that NCRs are more suitable for producing low-density reinforced metal matrix composites (MMCs) than SCRs. Other studies on NCRs-reinforced MMCs also reported the production of lightweight composites [15, 30, 93, 97].

Fig. 5
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Effect of wt.% variations on densities and percent porosity of the MEPA/PMPA HAMCs

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Void fractions for each of the wt.% variations for MEPA-HAMCs and PMPA-HAMCs were estimated as percent porosity and found to be within the acceptable 4% for cast aluminium composites [11]. However, the void fractions were comparatively lower in MEPA/Al2O3/AA6063 HAMCs (< 2%) than in PMPA/Al2O3/AsA6063 HAMCs (> 2%). Void fractions in AMCs are associated with hetero-phases like reinforcements or precipitates, resulting in local deformation that affects the mechanical properties of the resulting AMCs [98,99,100]. The sudden rise in the void fraction observed in the MEPA-10 reinforced AA6063 composite could be attributed to the inherent porosity associated with the MEPA particulates. MEPA has been documented to undergo the formation of mesopores and micropores during the carbonization process [101,102,103,104,105,106]. The activation process, influenced by impregnation ratios and activation temperatures, plays a crucial role in creating the porous nature of MEPA [104, 106]. For instance, Andiani et al. [104] observed the formation of hollow structures in MEPA, resulting in porosity at an activation temperature of 973.15 K. SEM micrograph studies, such as those depicted in Fig. 3i(e) and reported by other studies [104, 105], have confirmed the formation of a rough and porous surface in biochar-derived MEPA. This observation is consistent with the study by Hartini et al. [107], which reported the formation of pores in MEPA-activated carbon derivatives within the structure of γ-alumina composites.

The porous properties of the MEPA particulates explained why the void fraction was significantly and consistently lower in the hybrid MEPA/Al2O3/AA6063 composite than in the single MEPA-reinforced AA6063 composite (MEPA-10). This is because the alumina particles, with their uniform shape and relatively dense properties, acted as fillers in the hybrid AMCs, effectively occupying the pores and spaces between the less dense MEPA particles. This improved packing efficiency resulted in a more uniform distribution of particles throughout the matrix, significantly reducing the formation of voids during compaction and consolidation processes and enhancing the structural integrity of the composite, even with a decrease in alumina particles and an increase in MEPA particles. The interfacial solid bond between alumina and the matrix material, which previously reduced void formation, became evident upon the complete removal of alumina from the composite, resulting in a sharp rise in void fraction of the MEPA-10 monolithic AMC (Fig. 5a). In the absence of a synthetic filler agent, such as alumina, the less dense MEPA particles, with their non-uniform dispersion or clustering properties, result in a high degree of void formation.

Interestingly, Hartini et al. [107] reported a decrease in total pore volume with increased activated carbon weight percentages. However, further investigations are needed to understand the interaction and the contributions of hard particulates, such as Al2O3, Fe2O3, and CaO, present within the MEPA particles (Table 3a) in maintaining the structural integrity of singly-reinforced MEPA/AA6063 AMCs at increasing weight percentages in the absence of SCRs like alumina. This is crucial for determining the potential application areas for the monolithic MEPA/AA6063 composite, where the naturally abundant MEPA waste could potentially substitute for the relatively expensive alumina particles. For the PMPA particles, the sharp rise observed with PMPA-6 in Fig. 5b is attributed to the processing conditions during the composite fabrication, including mixing and compaction. Inadequate compaction and mixing issues, particularly for aluminium, have been reported to lead to void formation or porosity within composite structures [108,109,110,111,112]. This explained why the acceptable porosity level in aluminium-cast composites was benchmarked at 4% [11, 113].

3.3 Hardness performance

Figure 6 depicted the graphical comparative performance of hardness tests conducted on the MEPA and PMPA-reinforced HAMCs. For the MEPA HAMCs, the hardness of the reinforced composites increased with increasing weight variation of MEPA and decreasing wt.% of Al2O3 particulates. Other studies have reported similar improvements in hardness for MMCs reinforced with MEPA [59, 93]. Compared to the singly reinforced ALMA-0 (74.70 BHN), the singly reinforced MEPA-10 (107.47) showed better hardness at 22.62%. This could be linked to the combined density contributions of the oxides present in MEPA particles (Table 3a) and the reduced porosity (< 2%) observed in Fig. 5a. Even dispersion of reinforcements within the matrix reduces the interparticle distance [28] resulting in increased density of crystal defects, due to the accumulation of dislocations within the composite. The elevated density of dislocations augments the surface area [114] of the MEPA particles, enhancing resistance to further material deformation while suppressing grain boundary effects [115]. This factor significantly contributed to the overall hardness observed with increasing MEPA wt.%.

Fig. 6
figure 6

Wt.% variation effect on hardness performance of MEPA/PMPA HAMCs

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Conversely, for PMPA-reinforced HAMCs fabricated in this study, hardness declined with a rise in wt.% of PMPA and a decrease in Al2O3 wt.%. Notably, the sharp increase observed in the PMPA-2 (8 wt.% Al2O3—2 wt.% PMPA) compared to ALMA-0 (10 wt.% Al2O3—0 wt.% PMPA) was due to the combined densities of Al2O3 (3.98 g/cm3) and K2O (2.35 g/cm3) of the reinforcements in the matrix. However, a reduction in hardness was observed as the wt.% of Al2O3 decreased and the wt.% of PMPA increased. This outcome was expected for two primary reasons: firstly, the K2O density was significantly lesser than that of Al2O3, and secondly, Fig. 5b confirms a significant increase in porosity levels (> 2%) for PMPA particles compared to the MEPA particles. Porosity weakens the composite’s structure by creating pores, preventing the formation of localized stress fields [116, 117]. This allows the material to undergo plastic deformation, resulting in a decreased hardness. A closer examination of the hardness values revealed that the singly reinforced PMPA-10 (86.03 BHN) experienced a 1.84% decrease compared to the singly reinforced ALMA-0 (87.64 BHN). This further confirms that the crystal planes glide more smoothly when the dislocation densities were significantly reduced, adversely affecting the hardness of the fabricated HAMCs. However, Okafor et al. [67] reported an increase in hardness when using PM-based fibers in polyester matrix composites.

3.4 Wear assessment

The mass loss of MEPA and PMPA-reinforced AA6063/Al2O3 HAMCs before and after the conducted wear test, under a 10 N applied load and 15 mm sliding distance, is presented in Table 4. For the MEPA-HAMC specimens, the mass loss ranged from 0.031 to 0.090 g. The singly reinforced ALMA-0 exhibited the highest wear loss at 0.090 g, whereas the singly reinforced MEPA-10 showed the lowest wear loss at 0.031 g. It was also observed that the mass loss decreased with an increase in MEPA wt.% at 27.7% (MEPA-2), 1.1% (MEPA-4), 7.9% (MEPA-5), 25.8% (MEPA-6), 27.5% (MEPA-8), and 3.1% (MEPA-10), compared with ALMA-0. Volumetric loss ranged from 11.72 mm3 (MEPA-10) to 32.91 mm3 (ALMA-0). The MEPA-HAMCs experienced a gradual decline in volumetric loss with an increase in the MEPA wt.%. The reductions in mass and volumetric losses are due to the impact of MEPA particle reinforcements on the improved hardness of the fabricated HAMCs. Complex composites contribute to grain refinement due to increased density dislocations within the material [40]. Archard’s equation postulates that the wear rate is inversely related to hardness [76, 118], thus validating the enhanced wear resistance observed. The singly reinforced MEPA-10 particle had the maximum hardness result (Fig. 6), thus recording the least wear rate (Table 4). Wear resistance for MEPA was observed to increase with rising wt.% of MEPA to a maximum value of 1.28 mm/mm3, while specific wear rate and wear coefficient declined with reducing wt.% of MEPA particles (Fig. 7a). Olaniran et al. [59] reported similar results, showing enhanced wear resistance with MEPA-based reinforcement in Al–Mg-Si/SiC matrix composites.

Table 4 Effect of (a) MEPA and (b) PMPA weight variations on mass loss before and after wear
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Fig. 7
figure 7

Effect of wt.% variation on the wear assessment of MEPA a PMPA b HAMCs

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The PMPA-based HAMC specimens exhibited mass losses ranging from 0.029 to 0.090 g. PMPA-2 exhibited the lowest mass loss, which increased with ascending PMPA wt.%. The volumetric loss for the PMPA HAMC also varied from 10.808 mm3 to 32.906 mm3, with PMPA-2 recording the most negligible loss. The volumetric loss also increased with an increase in the PMPA wt.%. The consistent rise in volumetric and mass losses accounted for the elevation in the wear rate of the fabricated PMPA-reinforced HAMCs. The steady rise in the wear rate observed in Table 4 for the PMPA HAMCs is consistent with the decrease in hardness (Fig. 6). This finding was validated by Archard’s wear-hardness equation [76]. Wear resistance was lowest at the singly reinforced PMPA-10 HAMCs (5.39 mm/mm3) and highest at PMPA-2 (13.88 mm/mm3). However, the wear resistance of PMPA-10 was 18.35% superior to that of ALMA-0 while decreasing with an increase in the PMPA wt.%. However, it is essential to note that the wear resistance, as indicated by the wear coefficient, decreased with increasing weight percentage of PMPA particulates (Fig. 7b). This implies that more surface materials were removed from the HAMCs during sliding, owing to a consistent decline in the hardness of the PMPA HAMCs. The void fraction (> 2%) observed in the PMPA HAMCs (Fig. 5b) may potentially contribute to reduced mechanical properties, which could impact the wear resistance at the contact surface during the rubbing action between the pin and rotating disc.

Hybrid MMCs have been recognized for their superior wear resistance compared to pure matrix alloys [119]. Palanivendhan and Chandradass [46] attributed this to plastic deformation, abrasion, strain rate, and tribo-oxidation during tribological investigation. The wear performance obtained with MEPA particulates in this study aligns with the findings from other studies on NCRs-reinforced MMCs. These studies have consistently shown a significant enhancement in wear resistance with an increase in wt.% of reinforcements, including RHA [15, 44], fly ash [55], BPA nanoparticles [27], CSA [57], locust beans waste [120], and sugarcane bagasse ash [48]. The worn surfaces of the MEPA and PMPA HAMCs are depicted in Fig. 8, revealing transverse sliding tracks indicative of structural changes due to wear mechanisms. Examination of the composite alloy revealed cavities and large grooved regions on the worn surface, with broken ceramic particles found in cavities and loose fragments between the surfaces, indicating an abrasive wear mechanism. The wear mode appeared to be abrasive, with crack propagation observed along the transverse and longitudinal directions. Crushed and fragmented particles were also present, along with patches, indicating material removal from the surface.

Fig. 8
figure 8

Surface microstructure morphology showing wear characteristics of MEPA (a) and PMPA (b) HAMCs

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Interestingly, the wear resistance performance results obtained with the PMPA hybrid AMCs contradict the existing literature on agro waste reinforcement in MMCs. This discrepancy could be attributed to several factors. Firstly, the softening effect of K2O is known for its potential to act as a flux, thus lowering the melting point of the material [121, 122]. This reduces the hardness of the material, making it more malleable and prone to deformation. Secondly, K2O, due to its high stiffness, forms brittle phases when it reacts with other elements [123, 124], weakening the structural composition of the composites and making them more susceptible to abrasion and wear. For instance, Gao et al. [125] observed the formation of brittle (K.Na)Al2O3 phases during the sintering of K2O and Al2O3 resulting in a deliquescence effect. Thirdly, K2O can influence microstructural changes in materials, leading to defects such as voids or porosity, as evidenced in previous studies [126,127,128,129]. The instability of potassium with varying oxygen content, coupled with the formation of the KAlO2 phase in the composite structure, can act as a potential stress concentration point, promoting crack initiation and propagation. This accounts for the significant decline in the hardness and wear performance observed in this study for the PMPA/ Al2O3/AA6063 hybrid composites.

Comparatively, the wear properties of the MEPA-based HAMCs outperformed those of the PMPA-based HAMCs (Table 4 and Fig. 7). This superiority stemmed from the greater hardness and enhanced wear resistance exhibited by the composites fabricated using MEPA, attributable to the oxide compositions present in the MEPA particulates. In contrast, the PMPA particles are predominantly composed of K2O, which has a lower density than Al2O3. The rationale behind the correlation between oxide compositions, particularly hard particulates such as silica, alumina, and hematite, is rooted in the intrinsic properties of these oxides. One major contributing factor is the density of the oxide, which plays a crucial role in enhancing the hardness and wear resistance of the hybrid MMCs. Several studies have provided evidence that supports this relationship. For instance, Alaneme et al. [130] and Apasi et al. [17] underscored the significant hardness and wear resistance enhancement attributed to oxides such as SiC, SiO2, Fe2O3, and Al2O3, in composite materials. Similarly, studies by other researchers [10, 11, 131,132,133,134,135,136] further corroborated these findings, highlighting the pivotal role of specific oxide compositions in improving the mechanical properties of MMCs. This suggests that the oxide density and intrinsic properties, particularly in hard particulates, are key factors driving the enhanced hardness and wear resistance observed in hybrid MMCs.

Consequently, the composites fabricated with PMPA exhibit reduced hardness and wear resistance. Additionally, while the specific wear rate and wear coefficient decreased with increasing weight ratio for the MEPA particulates, these wear properties showed an opposite trend, increasing with the rising weight fraction for PMPA particles. In addition, considering the density, hardness, and wear performance of the hybrid MEPA- and PMPA-reinforced Al2O3/AA6063 composites reported in this study, future research could aim to optimize the compositions and processing parameters to further enhance their mechanical properties and expand their potential applications in various industries. Given their lightweight nature, the composites fabricated in this study hold promise for application in automotive parts, aerospace components, sports gears, and portable devices. However, additional investigation into the corrosion resistance performance of these composites in diverse environments is necessary, along with an assessment of their mechanical properties such as ductility, toughness, and strength. The superior hardness and wear properties observed in the MEPA-reinforced composites suggest their potential application in abrasive wear scenarios, such as cutting tools and wear-resistant coating materials.

4 Conclusion

This study presents valuable comparative insights into the use of two solid agro wastes (MEPA and PMPA) for the in-situ reinforcement of AA6063/Al2O3, focusing on their mechanical and tribological properties. The findings of the experimental investigation are summarized as follows.

  1. 1.

    The SEM investigations confirmed the uniform dispersion of the reinforcement materials within the AA6063 matrix. The XRD patterns confirmed the presence of SiO2 and K2O peaks in the MEPA and PMPA particulates, respectively, indicating their elemental compositions.

  2. 2.

    Lightweight composites were fabricated with MEPA and PMPA reinforcements. The sharp increase in void fraction observed in the MEPA/AA6063 composite was attributed to the inherent porosity of the MEPA particles, stemming from the micropores and mesopores within them.

  3. 3.

    MEPA reinforced AA6063/Al2O3 HAMCs demonstrated an increase in hardness with rising MEPA wt.%, surpassing the hardness of the singly reinforced AA6063/Al2O3 AMC. Conversely, the hardness of PMPA/AA6063/Al2O3 reinforced HAMCs significantly reduced with increasing PMPA wt.%, with the singly reinforced PMPA/AA6063 recording the lowest hardness value.

  4. 4.

    MEPA and PMPA particulates, when used singly to reinforce the matrix, enhanced the hardness of AA6063 from 25 to 107 BHN and 86 BHN, respectively, showing their potential as lightweight alternatives to SCR-based materials for Al-reinforcements.

  5. 5.

    Volumetric loss and wear rate in the MEPA-based HACMs decreased with increasing MEPA wt.%. Conversely, the wear rate increased with increasing PMPA particles, aligning with the reduction in hardness reported for PMPA/AA6063/Al2O3 HAMCs.

  6. 6.

    The decline in the wear resistance of the PMPA/Al2O3/AA6063 HMCs was attributed to microstructural changes in the material due to the softening effect of K2O and the formation of brittle phases. The MEPA/AA6063/Al2O3 HAMCs demonstrated superior wear resistance compared with the PMPA/AA6063/Al2O3 HAMCs.

  7. 7.

    The singly reinforced MEPA/AA6063 AMC showed enhanced hardness and wear behaviour compared to the singly reinforced AA6063/Al2O3 and PMPA/AA6063 AMCs. MEPA particles show promise as a cost-effective alternative to the relatively expensive Al2O3, suggesting sustainable engineering applications for Manihot esculenta solid wastes.

The comparative analysis indicated that the fabricated MEPA/AA6063/Al2O3 HAMCs had better mechanical properties than the PMPA/AA6063/Al2O3 HAMCs, with enhanced hardness and wear performance. This superiority is primarily attributed to the elemental composition and oxides found in MEPA. Both solid agro wastes, MEPA and PMPA, emerged as promising materials of engineering interest for enhancing the hardness and wear performance of AA6063, suggesting their potential applications in the automotive and automobile industries.