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, window frames, etc., owing to its smooth surface finishing and good corrosion resistance. Despite these good properties, the AA6063 metal matrix is a medium-strength alloy with a Brinell hardness of 25 HB [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 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 nano-tubes (CNTs), graphite, and tungsten carbide (WC), that impact positively on the hardness and wear characteristics of AMCs [5–7]. Bodunrin et al. [8] further classified Al-reinforcements into agro waste, synthetic ceramic, and industrial waste particulates. Like the SCRs, the use of agro waste as reinforcements has 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 have been found to be relatively cost-effective owing to their abundance in nature [9] compared to SCRs [10, 11].
Furthermore, literature reports indicate favourable engineering properties, encompassing mechanical strength, wear resistance, corrosion resistance, and porosity, for NCRs [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 production of AMCs include rice-husk ash (RHA) [12–14], coconut-shell ash (CSA) [15–18], palm-kernel shell ash (PKSA) [6, 19, 20], corn cob ash (CCA) [21], date palm seeds ash (DPSA) [22], bean pod ash (BPA) [23–26], groundnut shell ash (GSA) [27], plantain peel ash (PPA) [28, 29], bamboo leaf ash (BLA) [30, 31]. For example, when combined with alumina as reinforcements in Al-Mg-Si matrix, RHA was reported to produce lightweight composites [9]. AMCs reinforced with agro waste particulates have equally demonstrated enhanced hardness with increasing percent weight variation as reported for RHA/SiC [32], RHA/B4C [33], RHA/Cu [13], CSA/Al2O3 [18], CSA [34], BPA [26], and DPSA with an optimum increase at 7.5 wt.% [22]. 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 [9], CCA/SiC [21], BLA/Al2O3 with about 9% reduction for a 40% Al2O3 reduction [35], and GSA/SiC [27].
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 [36]. Poor wettability, agglomeration, and uneven distribution of reinforcement particulates contribute significantly to mechanical properties such as hardness in AMCs [28]. The use of Mg has been recommended to improve wettability [9], while production methods such as two-step stir casting have been found to ameliorate issues relating to agglomeration and non-uniform distribution of particulates [37]. Increasing the stirring time and speed in the double-step stir cast method has also been reported to significantly influence the hardness property of the AMC [7]. Commonly investigated Al alloys reinforced singly with either SCRs or NCRs or a hybrid of SCR-NCRs include AA2014 [38], A356 [39], LM13 [40], AA5083 [41], AA7075 [42], Al6061 [14, 18], AA2009 [26], ADC12 [33], and AA6063 [6, 41]. Studies involving the use of AA6063 have employed fabrication routes like ballistic impact [41] and the compo-casting method [6], with limited information on the use of a double stir-cast route, particularly for SCR-NCR hybrid reinforcement. While some studies have reported the use of Plantago major peel ash (PMPA) as reinforcement in AMCs [28, 29], there is no report, to the best of the authors knowledge, on incorporating Manihot esculenta peel ash (MEPA) as a reinforcing element in fabrication of hybrid AMCs (HAMCs).
Manihot esculenta (ME) and Plantago major (PM) are agricultural products largely consumed across the globe in raw or processed forms. ME, also known as Cassava, is widely cultivated in Nigeria at ~ 60 million tonnes as at 2022 [43], surpassing global production by over 18% [44]. Conversely, PM, also known as Plantain, is predominantly cultivated in Uganda at ~ 10 million tonnes [43]. Globally, ME and PM are majorly produced in sub-Saharan Africa at ~ 208 million and ~ 30 million tonnes, respectively [43]. This regional prominence serves as motivation for selecting these crops in this study. Furthermore, these food crops have been associated with the indiscriminate disposal of solid wastes, contributing significantly to environmental pollution [45–47]. Repurposing these solid wastes for engineering applications offers an environmentally sustainable solution with socio-economic benefits for sub-Saharan Africa and helps reduce the considerable costs incurred in importing SCRs. This research effort aims to comparatively evaluate the impact of utilizing MEPA and PMPA as reinforcements in In-situ AA6063/Al2O3, focusing on hardness and wear performance. Additionally, the research will explore the potential of using NCRs from MEPA and PMPA as single reinforcements with AA6063 in the absence of Al2O3 to produce environmentally sustainable AMCs for strength-based and wear-resistant engineering applications. The two-step stir-casting procedure will be utilized in this study to guarantee the even dispersion and distribution of particulate reinforcements within the matrix.