Microstructural investigation and mechanical properties of Al₂O₃-MWCNTs reinforced aluminium composite

1.   Introduction

Metal matrix composites (MMCs) are extensively utilized in high-performance applications across various industries, including aerospace, electronics, and automotive sectors, due to their superior properties, such as enhanced hardness, increased tensile strength, and improved wear resistance ^[1]. These characteristics, along with the potential for improved strength-to-weight ratios compared to unreinforced alloys, make MMCs highly advantageous. Among the various metals used in MMCs, aluminum is especially preferred due to its lightweight nature, high thermal conductivity, and excellent corrosion resistance ^[2]. MMCs have been widely used in the automotive industry because of their outstanding mechanical and tribological properties. Additional possibilities for increased demand for aluminum come from the electric vehicle revolution, which is entering the market sooner and faster than anyone could expect. Aluminum metal matrix composites (AMMCs) hold immense significance in aerospace applications due to their unique combination of low weight, high strength, and superior mechanical and thermal properties ^[3]. In the aerospace industry, reducing weight while maintaining structural integrity is a top priority to improve fuel efficiency, payload capacity, and overall performance ^[4]. By utilizing hybrid reinforcements such as carbon and ceramic particles, these composites achieve a high strength-to-weight ratio, excellent wear and corrosion resistance, and good thermal stability. This makes them ideal for critical aerospace components such as propellers, rotors, undercarriage structures, fuselage sections, engine blades, tail surfaces, and engine mounts ^[5]. Moreover, hybrid metal matrix composites (HMMCs) also play an important role in improving the mechanical properties of automotive and aerospace products. HMMCs are created by integrating two or more reinforcements with different morphologies that can develop synergistic effects between them, leading to an excellent combination of strength and ductility.

Multi-wall carbon nanotubes (MWCNTs) are particularly effective reinforcements due to their exceptional mechanical strength, low density, high thermal and electrical conductivity, and ability to enhance composite performance through efficient load transfer mechanisms ^[6]. Similarly, Al₂O₃ serves as an effective reinforcement in MMCs, significantly improving hardness, compressive strength, thermal stability, corrosion resistance, and wear resistance, making it valuable for a wide range of engineering applications ^[7]. Furthermore, a study conducted by Murthy et al. demonstrated the effect of the particle size of Al₂O₃ on hardness ^[8]. In this work, Al₂O₃ with different sizes (20, 53, and 88 μm) was used, and the smallest one showed the highest hardness. Finer Al₂O₃ particles provide a larger surface area for bonding with the matrix, improving load transfer and mechanical performance. Additionally, smaller particles contribute to a more uniform dispersion, reducing the risk of weak points and enhancing the composite's overall strength and durability. Finer particles increase the interface area between the matrix and reinforcement, leading to better load transfer and strengthening mechanisms such as Orowan looping and grain refinement ^[9]. Sharma et al. (2019) suggested that Al₂O₃ at 40 μm is sufficient to provide an optimal balance between strength and processability in AMCs ^[10]. On the other hand, Malaki et al. (2021) found that the inclusion of magnesium powder serves as an effective strategy to enhance the wettability of CNTs by reducing the contact angle between the liquid and solid phases ^[11]. This effect is particularly crucial in MMCs, where good wettability ensures uniform dispersion of CNTs, improved interfacial bonding between the matrix, and enhanced mechanical properties.

In a separate study, Shan et al. ^[12] obtained unforeseen results about the effects of Al-Al₂O₃-CNTs using the powder metallurgy approach. The mechanical properties of 0.5% CNTs and 10 wt% Al₂O₃ offered ideal results in terms of yield strength (357 MPa), ultimate tensile strength (404 MPa), and elongation to fracture (9.1%). Chen et al. ^[13] investigated the mechanical properties of ex situ and in situ CNTs formed by γ-Al₂O₃-reinforced aluminum powder. The results showed an increase in the following mechanical properties: yield strength (515 ± 17 MPa), tensile strength (542 ± 4 MPa), elastic modulus (95.6 ± 1.7 GPa), and excellent elongation (10.4 ± 0.8). These approaches have successfully demonstrated the positive impact of hybrid reinforcement with CNTs and Al₂O₃ on aluminum matrix composites. However, the fabrication of these composites requires multiple methods or processes, resulting in additional time and cost investments.

Effectively addressing the challenges of uniformly distributing MWCNTs within the metal matrix is vital for optimizing composite performance. Achieving homogeneous dispersion and strong interfacial bonding is crucial to promoting grain refinement, enhancing load transfer, enabling Orowan strengthening, and mitigating thermal expansion mismatches between the matrix and reinforcements ^[14]. However, attaining an even distribution of reinforcing particles is challenging due to the high viscosity of molten metal and poor wettability between the matrix and reinforcement materials. Mechanical stirring is typically used to produce these composites, but this method has limitations such as air entrapment, oxidation, stirrer blade erosion, composite contamination, and vortex formation, which can trap air bubbles ^[15]. To address these issues, an alternative approach known as electromagnetic stirring (EMS) has been developed. EMS offers several advantages over conventional MMC fabrication methods because of its ability to achieve a more uniform distribution of reinforcement particles within the matrix, reduce porosity, and enhance mechanical properties ^[16]. The use of magnetic fields to generate rotational flow within the molten metal can promote a homogeneous mixture. These results could improve interfacial bonding between the matrix and the reinforcement particles, which is critical for enhancing the overall strength and wear resistance of the composite. Additionally, EMS minimizes the formation of gas inclusions and porosity, which are common defects in other methods, thereby increasing the composite's density and mechanical properties ^[17].

Several authors have successfully utilized the advantages of EMS in enhancing the properties of metal matrix composites. Chandrasheker (2021) investigated Al7050 alloy reinforced with B₄C particles at varying levels (3%, 6%, 9%) using electromagnetic stir casting ^[18]. EMS effectively supported initial particle dispersion, with reinforcements up to 6 wt% resulting in improved hardness of 84.27 HV. Soni (2022) employed EMS to fabricate Al6063-SiC/Al₂O₃ hybrid composites, achieving a 28% increase in tensile strength (134.63 MPa) and a 15.4% increase in double shear strength (94.91 MPa) due to uniform dispersion and enhanced interfacial bonding ^[19]. Similarly, Kantheti et al. (2024) used EMS to disperse 6 wt% graphite and 4 wt% B₄C in an AA7075 matrix, achieving uniform reinforcement distribution and reduced clustering ^[20]. This led to notable improvements, including approximately 18% higher hardness, a 110% increase in impact resistance, an 85.31% increase in yield strength (YS), and an 88.24% increase in ultimate tensile strength (UTS).

Previous research on hybrid reinforcement involving MWCNTs and Al₂O₃ is limited, with only a few studies exploring their combined effects on composite properties. However, very few studies to date have investigated the use of EMS for processing MWCNTs-Al₂O₃ hybrid-reinforced composites. This presents a significant research gap, as EMS has the potential to enhance the uniform dispersion of reinforcements, improve particle-matrix bonding, and minimize defects, thereby addressing key challenges associated with hybrid reinforcement. This study focuses on finding the optimal parameters for incorporating MWCNTs and Al₂O₃ using EMS to achieve a more uniform microstructure distribution and maximize the composite's mechanical properties.

2.   Materials and methods

2.1.   Design of experiments

Orthogonal arrays were used in this study to carefully examine each design component while minimizing experiments through Minitab Statistical Software (Minitab 19). Tables 1 and 2 summarize the condition parameters and design of the orthogonal array L8, with three factors at two levels. In this work, MMCs were produced using varying MWCNT and Al₂O₃ contents (wt%) and EMS stirring durations. Meanwhile, magnesium powder was fixed at 0.3 wt% for all experimental runs. The experiment was conducted using the Taguchi Method; the parameters of this experimental work are presented in Table 1. The selected parameter levels for MWCNTs, Al₂O₃, and EMS duration in the L8 orthogonal array were based on recent literature. The choice of 0.5 and 1.0 wt% MWCNT was based on findings by Samal et al. (2022), who reported that these levels significantly enhanced tensile strength and hardness in AA5052 composites ^[21]. Higher contents (1.5 wt%) caused CNT agglomeration and reduced mechanical performance, making 1.0 wt% optimal. Therefore, the choice to limit CNT content to 0.5 and 1.0 wt% ensures optimal reinforcement while avoiding the negative effects associated with excessive CNT loading.

Table 1.  Condition parameters.

Parameter	Level 1	Level 2
MWCNT content (wt%)	0.5	1.0
Al2O3 content (wt%)	4	6
EMS stirring time (min)	5	10

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Table 2.  L8 orthogonal array.

No of run	MWCNT content (wt%)	Al2O3 content (wt%)	EMS stirring time (min)
1	0.5	4	5
2	0.5	4	10
3	0.5	6	5
4	0.5	6	10
5	1.0	4	5
6	1.0	4	10
7	1.0	6	5
8	1.0	6	10

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In previous studies, Chandas et al. (2020) employed 3 wt% Al₂O₃ combined with 7 wt% SiCp in AA6061 using the stir casting method ^[22]. Similarly, Radha et al. (2022) investigated composites reinforced with 5 wt% Al₂O₃ and 8 wt% TiC using the same technique ^[23]. Building on these findings, the current research explores the mechanical properties of composites reinforced with 4 and 6 wt% Al₂O₃ to evaluate performance within this intermediate range and to identify the optimal alumina content for enhanced material characteristics. The selection of 5 and 10 min for EMS stirring duration was made to evaluate the effect of stirring time on the dispersion quality of reinforcements within the aluminum matrix. A 5-min duration is generally sufficient to achieve initial homogenization, particularly at lower reinforcement levels, while 10 min allows for more thorough mixing, helping to reduce particle agglomeration and improve interfacial bonding. These durations strike a balance between adequate dispersion and avoiding potential drawbacks of prolonged stirring, such as oxidation or increased porosity.

2.2.   Hybrid composite preparation

The alloy utilized in this procedure was a commercially available A356 aluminum alloy from Malaysian Aluminium & Alloys Sdn Bhd in the form of an ingot with a size of 600 × 220 × 70 mm. Its chemical composition, expressed as weight percentages, was determined through spectrometry. The composition comprised 7.38% Si, 0.26% Mg, 0.0463% Cu, 0.25% Fe, 0.190% Mn, 0.0068% Zn, and 0.0411% Ti. For this research, Al₂O₃ was obtained from MERCK with an average size of 63 µm, and MWCNTs were obtained from Sigma-Aldrich, USA, and featured > 95% purity, outside diameter of 20–40 nm, inside diameter of 5–10 nm, and length of 10–30 μm. Magnesium powder with ≥99% purity was obtained from Sigma-Aldrich (ϕ 1 mm) (must be stored in airtight, non-reactive containers in a cool, dry, and well-ventilated area; moisture and heat sources must be strictly avoided, as magnesium reacts with water to produce hydrogen gas, which can lead to explosions).

Figure 1 shows the EMS machine and the fabrication process of the matrix composite. The planetary ball milling process was conducted using Planetary Ball Mill PM 100, maximum speed 650 rpm on the Al₂O₃ for 12 h at 200 rpm, using an alumina jar and alumina balls with a ball-to-powder ratio of 5:1 to reduce particle size. To prevent overheating, the milling cycle included 5-min intervals after every 15 min of operation. The particle size of Al₂O₃ was successfully reduced from 63 µm to 30.2 µm, as confirmed by a particle size analyzer (Malvern Mastersizer Hydro 2000MU & Scirocco 2000 Particle Analyser). The EMS used was a 10 kg aluminum electromagnetic stirring (GW-10) with a power of 15 kW. The melting point of aluminum ingots is 660 ℃; however, the ingots were heated to 700 ℃ to ensure complete liquefaction and facilitate reinforcement incorporation. At the same time, magnesium powder, Al₂O₃, and MWCNTs were thoroughly mixed, wrapped in aluminum foil, and preheated to approximately 200 ℃. Simultaneously, the D2 Tool Steel Mould (60 × 60 × 146 mm, ϕ 25) was also preheated to 80 ℃ to minimize thermal shock during casting; this was controlled using a thermocouple. The EMS current was set to 70 A with a frequency of 50 Hz based on the work by Khanouki (2023), which achieved the best microstructural and mechanical properties of A356 ^[24]. The ingot was then heated to 720 ℃, and a degassing tablet was added to the melt to remove entrapped hydrogen gas and reduce porosity. Then, the preheated reinforcement mixture was introduced into the melt using a steel plunger. The composite was subsequently stirred using EMS to ensure uniform distribution of the reinforcements, enhancing dispersion and minimizing agglomeration. Prior to pouring, flux was added to remove impurities in the molten composite. Subsequently, the molten material was poured at 670 ℃ into a billet mold to produce composite feedstock.

Figure 1.  Electromagnetic stirring process. (a) EMS control panel. (b) EMS machine. (c) Illustration of feedstock production.

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2.3.   Microstructural evaluation and mechanical testing

After casting, the billet underwent material characterization. The Al-MMCs were characterized using several testing and analysis techniques, including optical microscopy, X-ray diffraction (XRD) by PANalytical X'Pert PRO, and mechanical testing. The XRD specifications are shown in Table 3. The microstructural distribution of Al₂O₃-MWCNTs in the composites was examined using a Leica DVM6 optical microscopy and Schottky field emission scanning electron microscope (FE-SEM) (SU5000). Microstructure analysis was performed after grinding the samples with different grit sizes (400,600,800, and 1200), polishing with diamond solutions of varying particle sizes (3 and 1 µm), and etching with Keller's reagent solution. Tensile tests were conducted using a 100 kN Shimadzu universal testing machine at a speed of 5 mm/s. The samples were fabricated into dog-bone shapes according to the ASTM E8M standard for each group, as shown in Figure 2. Three measurements of tensile testing were taken for each sample. Then, the results' average was calculated to ensure reliable data analysis. Vickers hardness tests were conducted using Falcon 400G2, according to ASTM E384-05A, to investigate the microhardness of the composites. A diamond indenter was pressed onto the material with a 1 kg force for a dwell time of 15 s. Subsequently, its diagonal lengths were measured using an optical microscope, and the Vickers hardness number was calculated. Average measurements from five different spots were taken for each composite to obtain reliable results.

Table 3.  XRD specifications.

X-ray source	Cu-Κα radiation (Κα = 1.54187 Å)
Goniometer radius	240 mm
Geometry	Bragg-Brentano (θ-θ system, vertical goniometer)
Scan range of 2θ	3–150°
Minimum-maximum step size	0.001–1.27°
Incident PreFIX module	Fixed divergence slit holder focusing X-ray mirror for Cu-Κα
Diffracted PreFIX module	X'Celerator linear detector

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Figure 2.  Tensile testing ASTM E8M.

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3.   Results and discussion

3.1.   Microstructural evaluation

The development of metal matrix composites (MMCs) relies significantly on the efficient distribution of reinforcements within the matrix, which is frequently influenced by processing methods and reinforcement material quantities. The initial α-Al exhibited a rosette-like and nearly spheroidal morphology, with a plate-like silicon structure formed from a typical dendritic microstructure, as shown in Figure 3. The microstructure becomes densely packed with a small grain size. Here, Al₂O₃ and MWCNTs serve as reinforcements in an aluminum matrix. Figures 4–7 demonstrate the effects of various parameters on matrix composites through optical microscopy. The optical microscopy results reveal an uneven distribution of the microstructure in all samples. Multiple factors may contribute to this issue, including inhomogeneous mixing of the reinforcement, resulting in clustering or irregular dispersion of particles. Subsequently, solidification front instability in the presence of reinforcements can facilitate heterogeneous nucleation, leading to irregular grain development and distribution. Optical microscopy images indicate that increasing the stirring duration to 10 min (samples 2, 4, 6, and 8) typically enhances reinforcement dispersion. This produces a more refined microstructure with reduced macrosegregation, improved uniformity, and smaller grain size. Moreover, a higher concentration of Al₂O₃ (6 wt%) appears to yield a finer grain structure. This grain size reduction can be attributed to the enhanced nucleation effect from the increased presence of Al₂O₃ particles, which act as potent heterogeneous nucleation sites during solidification.

Figure 3.  As-cast sample 500×.

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Figure 4.  Sample 1 with (a) 1500× magnification and (b) 2500× magnification. Sample 2 with (c) 1500× magnification and (d) 2500× magnification.

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Figure 5.  Sample 3 with (a) 1500× magnification and (b) 2500× magnification. Sample 4 with (c) 1500× magnification and (d) 2500× magnification.

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Figure 6.  Sample 5 with (a) 1500× magnification and (b) 2500× magnification. Sample 6 with (c) 1500× magnification and (d) 2500× magnification.

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Figure 7.  Sample 5 with (a) 1500× magnification and (b) 2500× magnification. Sample 6 with (c) 1500× magnification and (d) 2500× magnification.

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According to Li et al., Al₂O₃ plays a crucial role in the uniform distribution of CNTs within the matrix through its vehicle-carrying effect in facilitating CNT dispersion ^[25]. Due to their fine particle size and surface interactions, Al₂O₃ nanoparticles act as dispersing agents, preventing CNT agglomeration, which is a common issue in CNT-reinforced composites. The strong van der Waals forces between CNTs often cause clustering, but when Al₂O₃ particles are introduced, the CNTs adhere to their surfaces, enabling more uniform distribution. These particles serve as carriers, transporting CNTs throughout the matrix and enhancing their mobility, ensuring a well-dispersed network. Reducing Al₂O₃ size is crucial for maximizing its uniform distribution within the matrix. Smaller Al₂O₃ particles possess a higher surface-area-to-volume ratio, significantly enhancing their matrix material bonding. Conversely, a higher concentration of MWCNTs (1 wt%) tends to promote clustering, particularly with shorter stirring durations. This occurs primarily due to strong van der Waals forces between MWCNTs, which promote their entanglement and aggregation, leading to poor matrix dispersion. Although sufficient stirring combined with increased Al₂O₃ can reduce this issue by enhancing mechanical shearing forces and improving particle wetting, MWCNTs still tend to agglomerate within the aluminum matrix composite. This persistent agglomeration can adversely affect the composite's mechanical properties, such as tensile strength and hardness, by forming stress concentration sites that act as potential crack initiation points. Based on Figure 5c, d, a smaller grain size can be achieved using a combination of Al₂O₃ up to 6 wt%, 0.5 wt% of MWCNTs, and 10 min of EMS stirring duration, as represented by sample 4. This enhances the uniform distribution of reinforcements throughout the aluminum matrix.

3.2.   SEI-FESEM

The intermetallic phases are shown in Figure 8 for samples 1–8. Various morphologies are observed in four different spots, including needle-like, rod-shaped, and plate-like structures. These morphologies characterize specific intermetallic compounds formed during solidification. For instance, needle-like features are often associated with brittle phases such as aluminum carbide (Al₄C₃), which typically form at the interface between aluminum and carbon nanotubes during electromagnetic stir casting. These phases develop directionally, resulting in elongated, needle-like structures. Other intermetallics, such as Al-Fe-Si compounds, commonly occur in aluminum alloys and contain trace amounts of iron and silicon, often introduced as impurities or alloying elements. These compounds exhibit similar morphologies and can be identified through XRD analysis. The formation of these intermetallic phases is closely linked to the interaction between the aluminum matrix and reinforcements, including MWCNTs and Al₂O₃, under high-temperature conditions during casting.

Figure 8.  SEI-FESEM for (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, (e) sample 5, (f) sample 6, (g) sample 7, and (h) sample 8.

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The needle-like shape observed in FESEM images is most likely Al₄C₃, characterized by an elongated, fragile morphology. Meanwhile, Al-Fe-Si intermetallics contribute to the hardness and brittleness of industrial aluminum alloys. A Chinese script-like morphology observed in some regions is likely attributable to Mg₂Si, which enhances both toughness and ductility. Similarly, the blocky or chunky morphology is likely associated with Al₂Cu phases, which strengthen the composite by acting as hard, brittle phases that hinder dislocation movement within the aluminum matrix. The formation of spherical microstructures enhances dense packing between barriers, effectively eliminating porosity ^[26]. However, excessive Al₂Cu content can increase brittleness, thus necessitating careful optimization of its volume fraction to achieve a balance between strength and ductility. The presence of the reinforcements was subsequently confirmed by EDS analysis, as illustrated in Figure 9.

Figure 9.  EDS FESEM results.

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3.3.   X-ray diffraction analysis (XRD)

Figure 10 shows the XRD results of sample 4. The XRD analysis of the aluminum matrix composite indicates the existence of several phases, including primary aluminum (Al) and various intermetallic compounds. The pronounced, distinct peaks associated with the (111), (200), (220), (311), and (222) planes signify the predominance of the face-centered cubic (FCC) structure of aluminum, confirming its status as the principal matrix phase. Alongside the aluminum peaks, several intermetallic phases were recognized, such as Al₂Cu, Mg₂Si, Al₈FeSi, and Al₄C₃. The Al₂Cu (θ-phase) was detected between 78 and 82°, indicating its development during the solidification process, which enhances mechanical properties like hardness and strength via precipitation strengthening. The detection of Mg₂Si at approximately 45 and 65° indicates an interaction between magnesium and silicon, which improves wear resistance and mechanical durability.

Figure 10.  XRD results of the composite.

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Additionally, peaks associated with Al₈FeSi, generally observed between 38 and 45°, indicate the development of iron-rich intermetallic compounds frequently present in aluminum alloys. Although Al₈FeSi enhances wear resistance, excessive quantities may induce brittleness, adversely affecting the composite's ductility. The identification of Al₄C₃ at approximately 40 and 50° indicates an interaction between aluminum and carbon, potentially originating from carbon-based reinforcements such as MWCNTs. While Al₄C₃ enhances interfacial bonding, it is brittle and susceptible to moisture, potentially compromising the composite's integrity over time. Furthermore, the presence of Al₂O₃, identified at approximately 78 and 82°, suggests either intentional reinforcement or oxide formation during processing, contributing to improved hardness, wear resistance, and thermal stability. Peaks associated with carbon, detected at approximately 38 and 45°, are presumably attributable to carbonaceous reinforcements, influencing the composite's electrical and thermal conductivity.

The XRD pattern confirms the predominant aluminum phase alongside intermetallic compounds, including Al₂Cu, Mg₂Si, Al₈FeSi, and Al₄C₃, in addition to reinforcing or oxide phases such as Al₂O₃ and carbon. The intermetallic phases significantly enhance the mechanical and thermal properties of the composite through mechanisms like precipitation hardening and dispersion strengthening. However, the excessive presence of brittle phases such as Al₄C₃ and Al₈FeSi may negatively affect ductility. The integration of these phases enhances the composite's mechanical performance, resulting in increased hardness, wear resistance, and thermal stability.

3.4.   Mechanical testing

Based on the design of experiments conducted using the Taguchi method, the mechanical properties of aluminum matrix composites reinforced with Al₂O₃ and MWCNT, presented in Table 4 and Figure 11, provided valuable insights into how variations in composition and stirring time affect key properties such as yield strength, ultimate tensile strength, elongation to fracture, and hardness. Sample 4 stands out as the most successful matrix composite, demonstrating optimal yield strength (94 MPa), ultimate tensile strength (221 MPa), elongation at fracture (11.37%), and hardness (89 HV). This suggests that a mixture of 0.5 wt% MWCNT and 6 wt% Al₂O₃ and a stirring duration of 10 min produces optimal mechanical performance. Conversely, sample 1, with the lowest strength and ultimate tensile strength (UTS) values, illustrates the negative impact of insufficient stirring (5 min) on both strength and reinforcement distribution.

Table 4.  Mechanical testing of the matrix composite.

Experiment	MWCNT content (wt%)	Al2O3 content (wt%)	Stirring duration (min)	Yield strength (MPa)	Ultimate tensile strength (MPa)	Elongation to fracture (%)	Hardness (HV)
As-Cast	-	-	-	69	114	4.82	78
1	0.5	4	5	75	170	9.33	81
2	0.5	4	10	84	206	10.32	85
3	0.5	6	5	82	205	9.23	85
4	0.5	6	10	94	221	11.37	89
5	1.0	4	5	76	201	8.96	87
6	1.0	4	10	93	210	9.34	87
7	1.0	6	5	87	211	9.47	80
8	1.0	6	10	89	212	10.31	81

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Figure 11.  Mechanical properties of the composite.

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Samples containing 1 wt% MWCNT (samples 5 and 7) show increased clustering and reduced uniformity in reinforcement distribution, resulting in lower strength and ductility compared with those with 0.5 wt% MWCNT. According to percolation theory, an optimal concentration of CNTs exists that ensures proper dispersion within the matrix. Exceeding this concentration can cause excessive agglomeration, which diminishes the composite's strengthening potential. This occurs because CNTs have high surface energy and are strongly attracted to each other through van der Waals forces, which causes them to clump together rather than disperse uniformly throughout the matrix. When CNTs agglomerate, they create weak spots in the composite structure, resulting in poor load transfer, increased stress concentration, and a higher likelihood of crack initiation. As a result, instead of improving mechanical performance, excessive CNT content can actually reduce properties like tensile strength, ductility, and hardness. In this case, the optimal concentration of CNTs is 0.5 wt%, and concentrations higher than that lead to matrix composite degradation. The EMS duration is crucial in preventing reinforcement clumping throughout the process. Furthermore, composites with higher Al₂O₃ content demonstrate enhanced mechanical properties. The results confirm the effectiveness of incorporating Al₂O₃ alongside MWCNTs as reinforcement in the matrix composite. This highlights the importance of balancing reinforcement concentrations to avoid excessive clustering, which could impair mechanical performance.

4.   Conclusions

The study aimed to investigate the effects of different parameters of hybrid reinforcement of Al₂O₃-MWCNTs in the A356 alloy matrix on mechanical properties and microstructures. The stirring process, utilizing electromagnetic stirring, successfully integrated Al₂O₃-MWCNTs into the composite, as confirmed by EDX-FESEM and XRD. The microstructure observed by OM was nearly globular or rosette-like. These reinforcements act as nucleating agents, leading to finer and more equiaxed grains that increase the grain boundary area, impeding dislocation motion and thus enhancing the yield strength and hardness through the Hall-Petch effect. Additionally, the thermally stable Al₂O₃ particles pin the grain boundaries, further suppressing grain growth and contributing to the composite's microstructural stability. MWCNTs, known for their exceptional tensile strength and modulus, facilitate efficient load transfer when uniformly dispersed within the matrix. At an optimal concentration of 0.5 wt%, stress is effectively transferred from the ductile aluminum matrix to the stiffer nanotubes, thereby improving both the ultimate tensile strength and ductility. However, at higher concentrations (e.g., 1.0 wt%), MWCNTs tend to agglomerate, leading to stress concentration sites that reduce the composite's mechanical performance. The Orowan strengthening mechanism plays a crucial role, primarily due to the fine size and uniform distribution of Al₂O₃ and MWCNT reinforcements. These non-deformable particles impede dislocation motion by inducing dislocation bowing and looping around them, thereby enhancing resistance to plastic deformation. The quality of interfacial bonding between the reinforcements and the A356 matrix further enhances mechanical performance. Strong interfacial adhesion ensures effective load transfer and minimizes the likelihood of interfacial debonding under applied stress. Al₂O₃ also plays a critical role in promoting the uniform distribution of MWCNTs by acting as a carrier, which reduces agglomeration and supports the formation of a well-connected reinforcement network. Collectively, these mechanisms explain the significant improvements observed in the composite's mechanical properties, with sample 4 (0.5 wt% MWCNT, 6 wt% Al₂O₃, and 10 min of stirring) exhibiting optimal performance: a yield strength of 94 MPa, an ultimate tensile strength of 221 MPa, elongation of 11.37%, and hardness of 89 HV.

Use of AI tools declaration

The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.

Acknowledgments

The study is funded by the Ministry of Higher Education (MOHE) of Malaysia through the Fundamental Research Grant Scheme (FRGS), No. FRGS/1/2022/TK10/UTEM/02/18. The authors would like to thank Universiti Teknikal Malaysia Melaka (UTeM) for supporting this study.

Author contributions

Conceptualization, methodology and writing: Mohammad Na'aim Abd Rahim, Mohd Shukor Salleh, Saifudin Hafiz Yahaya; methodology and material processing: Sivarao Subramonian, Azrin Hani Abdul Rashid, Syarifah Nur Aqida Syed Ahmad, Salah Salman Al-Zubaidi. All authors have read and agreed to the published version of the manuscript.

Conflict of interest

The authors declare no conflict of interest.