Impact of Alumina-Graphene nanoplatelets on the microstructure, corrosion behaviour, & mechanical properties of cast aluminium nanocomposite by Vortex technique

Abstract

In this work, the impact of reinforcing aluminum matrix with a nano-hybrid Al₂O₃-GNS nanosheet and the rolling process of the prepared composites were studied. Al₂O₃-GNS nanocomposite powder of a 98% to 2% ratio was produced by powder mechanical milling for 24 h. Additionally, the hybrid mixture undergoes surface treatment by coating with 10% silver to improve its wettability. For the nanocomposite production, aluminum scrap was melted at 800 °C, and the hybrid nanoparticles were added to the aluminum melt in 0%, 5%, 10%, and 15% by weight, followed by mechanical stirring at 600 rpm for 10 min. Then, hot-rolling was conducted for the cast billets to ensure optimal distribution of the hybrid nanoparticles. The microstructure, hardness, and wear rate were studied before and after the hot rolling process. Scanning electron microscopy (SEM) and Raman spectroscopy were used to analyze the microstructure. For mechanical properties, hardness, tensile strength, and wear resistance were measured. Additionally, corrosion resistance was evaluated. The microstructural analysis reveals a well-distributed hybrid phase within the aluminum matrix, with strong adhesion between the nanoparticles and the base metal. The incorporation of Al₂O₃–GNS hybrid nanoplatelets led to a significant enhancement in the mechanical and corrosion properties of recycled aluminum. The hardness increased from 72.3 HV for pure aluminum to 168.7 HV for the hot-rolled composite containing 15 wt% Al₂O₃–GNS, while the ultimate tensile strength improved from 55.8 MPa to 139.9 MPa after rolling. Furthermore, the corrosion rate in 3.5 wt% NaCl solution was reduced from 3.124 mm/year for pure aluminum to 0.215 mm/year for the as-cast reinforced composite, corresponding to a 93% improvement in corrosion resistance. The findings of this work contribute to the development of advanced materials with superior performance, offering potential applications in industries where high strength and corrosion resistance are critical.

Introduction

The need to revolutionize recycling practices and maximize the potential of available resources has become more urgent than ever¹. With the rising demand for high-performance materials across various industries, there is a growing emphasis on innovative approaches to enhance the value of recyclable materials². Developing composite materials from readily available aluminum scrap offers a promising solution for transforming aluminum recycling and addressing the needs of high-performance applications. Aluminum, known for its lightweight and corrosion-resistant properties³, remains a key material across a wide range of industries, including shipbuilding, automotive, aerospace, construction, and packaging⁴. Traditional recycling methods, however, frequently fail to fully harness the inherent properties of aluminum, resulting in constraints that affect the performance and adaptability of recycled materials^5,6. Acknowledging this shortcoming, there is a growing shift in focus towards the development of composite materials. These materials aim to capitalize on aluminum’s natural strengths while integrating advanced additives designed to significantly improve its mechanical, thermal, and chemical characteristics⁷. The sustainability aspect of the present work is addressed at the process level through the use of recycled aluminum scrap and conventional manufacturing routes. While a full life-cycle assessment is beyond the scope of this study, the proposed approach reduces reliance on primary aluminum production and avoids energy-intensive fabrication methods, thereby offering a pathway toward more sustainable aluminum-based composites⁸.

This research delves into the field of aluminum recycling, with a particular focus on the development of a composite material that redefines the potential of aluminum scrap for high-performance applications. By combining aluminum scrap with carefully selected additives and utilizing advanced processing techniques⁹, Despite extensive research on aluminum matrix composites, the majority of existing studies are based on primary aluminum alloys, employ single-step fabrication routes, and focus primarily on either mechanical or corrosion performance. In contrast, the present work is distinguished by the use of recycled aluminum scrap as the matrix, reinforced with Ag-coated hybrid Al₂O₃–graphene nanoplatelets, and processed through a sequential route combining vortex stir-casting and post-solidification hot rolling. This integrated approach is specifically designed to address reinforcement wettability, dispersion, and interfacial stability within a single processing framework. Moreover, the study provides a combined and correlated evaluation of microstructure, mechanical properties, and corrosion behavior, offering a comprehensive assessment of performance trade-offs and synergies that are rarely addressed simultaneously in prior aluminum composite studies. this study aims to unlock the untapped potential of recycled aluminum, creating sustainable, cost-effective solutions that meet the demanding standards of modern engineering challenges. Through an in-depth investigation of material synthesis, characterization, and performance evaluation¹⁰, this research highlights the broad range of potential applications for these innovative composites across various industries, promoting both innovation and sustainability on a global scale¹¹. Initiating the transformation of aluminum recycling for high-performance applications, the pursuit of excellence in materials engineering aligns closely with the critical need for environmental responsibility¹². By harnessing the full potential of aluminum scrap and exploring new possibilities in composite material design, this research aims to drive a paradigm shift in sustainable materials innovation, contributing to a more resilient future¹³. This integrated strategy enables simultaneous improvement in mechanical strength and corrosion resistance while addressing sustainability challenges, thereby extending the functional scope of recycled aluminum-based composites¹⁴.

Although Al₂O₃- and graphene-reinforced aluminum matrix composites have been widely investigated, most studies are limited to primary aluminum alloys and single-step fabrication routes. Nano-silver coatings enhance wettability and interfacial bonding in metal matrix composites, thanks to their high surface energy and chemical compatibility with both the metal matrix and the reinforcing material. As a transition layer, silver reduces the interfacial energy between molten aluminum and the ceramic/carbon surfaces, thereby promoting wettability and reinforcing material dispersion. Improved wettability also facilitates more uniform particle distribution and stronger matrix-particle bonding, reducing interfacial voids and improving load transfer during mechanical loading. Studies on precious metal-coated reinforcing materials, including silver, have demonstrated enhanced interfacial properties and subsequent improvements in composite characteristics, supporting the use of nano-silver coatings to improve manufacturing processes and performance in hybrid aluminum systems¹⁵. The present work introduces a distinct approach by combining recycled aluminum scrap, Ag-coated hybrid Al₂O₃–graphene nanoplatelets, vortex stir-casting, and post-solidification hot rolling. This integrated strategy enables simultaneous improvement in mechanical strength and corrosion resistance while addressing sustainability challenges, thereby extending the functional scope of recycled aluminum-based composites¹⁶. Graphene nanosheets (GNS) contribute to composite performance through distinct mechanistic pathways that extend beyond simple filler effects. Due to their ultrahigh specific surface area and stiffness, GNS provide effective load transfer when well dispersed and bonded within the aluminium matrix, enhancing strength and stiffness. GNS also interact with matrix dislocations: thermal expansion mismatch during cooling generates dislocations that accumulate at GNS interfaces, contributing to strain hardening and resistance to plastic deformation. In addition, GNS can act as heterogeneous nucleation sites during solidification and thermomechanical processing, promoting grain refinement and more homogeneous microstructures. Beyond mechanical strengthening, GNS influence corrosion behaviour by altering local electrochemical environments; well-dispersed GNS can impede the ingress of aggressive species and facilitate formation of a more stable passive film, thereby reducing localized attack. These mechanisms are supported by recent work that elucidates graphene-induced load transfer, dislocation interactions, and fracture resistance in metal matrix composites¹⁷. In such systems, ceramic particles primarily enhance load-bearing capacity and hardness, while carbon-based reinforcements contribute to dislocation blocking, crack-bridging, and improved interfacial stress transfer. Additionally, hybrid reinforcement architectures have been reported to influence corrosion behaviour by modifying passive film stability and limiting electrolyte penetration, depending on reinforcement chemistry and distribution¹⁸.

Despite extensive research on Al₂O₃- and graphene-reinforced aluminum matrix composites, most existing studies employ primary aluminum alloys and single-step fabrication techniques, with limited consideration of sustainability and corrosion behavior. To address challenges in reinforcement distribution and microstructural refinement, recent studies have increasingly adopted hybrid processing routes that combine liquid-state incorporation with solid-state deformation. Stir-casting followed by thermomechanical processing, such as hot rolling, has been shown to reduce porosity, improve interfacial bonding, and produce finer and more homogeneous microstructures in aluminium composites¹⁹. The present study develops an Al/Al₂O₃/GNs hybrid nanocomposite modified with Ag as a wettability enhancer. The work systematically investigates powder hybridization, microstructural evolution, densification behavior, and mechanical performance, with emphasis on dispersion control and interfacial strengthening mechanisms.

Methods and experimental procedure

This section provides an overview of the materials utilized, along with the experimental procedures and testing methods employed in the study. The experimental workflow for the preparation and characterization of Al₂O₃-GNs reinforced aluminum matrix composites is shown in Fig. 1.

Fig. 1

Experimental workflow for the preparation and characterization of Al₂O₃-GNS reinforced aluminum matrix composites.

Materials

This study utilizes aluminum chip scrap derived from the machining and polishing processes of motorcycle wheel rims, taking a sustainable approach to the development of innovative aluminum composites. Alumina powder, sourced from Zircar Co. LTD, with a particle size ranging between 100 and 200 nm, and graphene nanosheets (GNs) with a particle size of 2–10 nm and a purity of 99.9%, are used as the primary reinforcement materials for the recycled aluminum matrix. To enhance the wettability of both GNs and alumina with the aluminum matrix, a nano-silver (Ag) layer is deposited onto their surfaces through an electroless chemical deposition technique. This process involves the use of silver nitrate as a precursor for nano-silver particles, in conjunction with a 33% ammonium solution and formaldehyde, facilitating surface modification and improving the interaction between the reinforcement materials and the aluminum matrix. Moreover, particular attention is given to the careful control and optimization of processing parameters to ensure the uniform distribution and strong bonding of the reinforcement materials within the aluminum matrix, ultimately improving the mechanical and functional properties of the resulting composite materials. The selection of Al₂O₃ and graphene nanosheets as hybrid reinforcement constituents was based on their complementary strengthening mechanisms. Al₂O₃ contributes hardness, thermal stability, and load-bearing resistance, whereas graphene provides high stiffness, interfacial load transfer, and solid-lubricating behavior. The hybrid composition (98 wt% Al₂O₃ / 2 wt% GNS) was selected to ensure synergistic reinforcement while minimizing graphene agglomeration and wettability challenges reported at higher fractions.

The reinforcement additions to the aluminum matrix (0, 5, 10, and 15 wt%) were designed to systematically investigate the effect of hybrid loading on microstructure, densification, mechanical performance, and corrosion behavior, and to determine the optimum reinforcement threshold prior to the onset of particle clustering. Silver (Ag) was incorporated as a wettability enhancer to improve interfacial bonding between the aluminum matrix and the hybrid Al₂O₃–graphene reinforcement. Ag exhibits high solubility in aluminum and lower surface tension, which reduces the interfacial energy of the molten matrix and promotes spreading over ceramic and carbonaceous surfaces³¹.

Powder characterization

A comprehensive characterization of the starting powders and the hybrid Al₂O₃–GNS reinforcement was conducted to evaluate morphology, particle size distribution, structural integrity, and surface modification prior to composite fabrication. Powder characteristics play a decisive role in determining dispersion behavior, wettability, interfacial bonding, and ultimately the mechanical and corrosion performance of metal matrix nanocomposites.

Scanning Electron Microscopy (SEM) was employed to examine the morphology of the as-received graphene nanosheets (GNS), Al₂O₃ nanoparticles, and the mechanically milled hybrid Al₂O₃–GNS powder Fig. 2. The graphene nanosheets (Fig. 3a) exhibit a typical wrinkled and layered morphology with thin, sheet-like structures and lateral dimensions in the nanometer range. The observed crumpled topology is characteristic of few-layer graphene and is beneficial for mechanical interlocking within metallic matrices. The high aspect ratio and large specific surface area of GNS are expected to enhance load transfer efficiency when homogeneously dispersed.

The Al₂O₃ nanoparticles Fig. 2B show a near-spherical to irregular morphology with particle sizes ranging between 100 and 200 nm, consistent with supplier specifications. However, partial particle clustering is evident due to the high surface energy of nanoscale ceramics, which promotes van der Waals attraction and agglomeration. Such inherent agglomeration tendencies necessitate controlled hybridization and surface treatment prior to melt incorporation.

After 24 h of mechanical milling at 350 rpm with a ball-to-powder ratio of 10:1, the hybrid Al₂O₃–GNS powder Fig. 2C exhibits a markedly different morphology. The graphene nanosheets are observed to be uniformly immobilized on the surface of Al₂O₃ particles, forming a core–shell-like hybrid architecture. This configuration effectively reduces graphene restacking and suppresses free graphene agglomeration. Moreover, the mechanical milling process promotes intimate interfacial contact between Al₂O₃ and GNS, facilitating synergistic reinforcement behavior in the final composite. A noticeable reduction in the average agglomerate size compared to the as-received powders further confirms the effectiveness of the hybridization process.

Fig. 2

SEM analysis of powder morphology and hybridization behavior: (a) graphene nanosheets, (b) nano-Al₂O₃, and (c) mechanically synthesized Al₂O₃–GNS hybrid reinforcement.

Sample Preparation

Synthesis of Al₂O₃-GNs composite powders

The synthesis of the hybrid ceramic material, Al₂O₃-GNs, involved a precise and detailed process aimed at optimizing both its structural and surface characteristics. Initially, Al₂O₃ and graphene nanosheet (GNS) powders were subjected to mechanical grinding for 24 h at 350 rpm, during which the Al₂O₃ particles were simultaneously coated with graphene layers^20,21. To improve reproducibility, the manuscript now explicitly states that mechanical grinding was conducted in continuous mode for 24 h, and that vortex stirring during composite fabrication was performed continuously for 10 min at 600 rpm. These clarifications ensure that the experimental procedure can be reliably reproduced and aligned with recent best practices reported in the literature. The composition of the hybrid material consisted of 98 wt% Al₂O₃ and 2 wt% GNS. The milling process was carried out using 10 mm diameter ceramic alumina balls in a stainless-steel container, with a ball-to-powder ratio of 10:1. To maintain the purity of the hybrid Al₂O₃-GNs particles, several surface treatment procedures were applied. First, the particles were stirred in a 10% sodium hydroxide solution, followed by acetone, for one hour to remove any impurities. After this cleaning process, the particle surfaces were activated and metalized via an electroless silver coating procedure. This involved dissolving silver nitrate in water, adding an ammonia solution to adjust the pH to approximately 11, and introducing the hybrid Al₂O₃-GNs. Formaldehyde was then added as a reducing agent. Following this, the coated particles were washed and dried in an electric furnace at 80 °C for two hours²². The role of electroless Ag coating in improving wettability is inferred from microstructural homogeneity, reinforcement retention within the matrix, and enhanced interfacial bonding observed experimentally. the effectiveness of the electroless Ag coating was assessed through microstructural and compositional analyses, including TEM imaging and EDS elemental mapping, which confirmed uniform Ag nanoparticle decoration on the Al₂O₃–GNS surfaces.

Sample production

The aluminum scrap underwent a thorough cleaning process to remove any dust and oils, which was accomplished by washing it with acetone, then washing with warm water and soap, which helps remove any traces of acetone or other impurities, and followed by rinsing with distilled water to prevent any stains or deposits from the salts present in plain wate, followed by a drying step. Once cleaned, the aluminum was compressed into cubes with dimensions of 20 cm on each side. These aluminum cubes were then subjected to a melting process, reaching a temperature of 800 °C to ensure complete liquefaction. It was crucial to maintain the molten aluminum at a consistent temperature to ensure uniform mixing and homogeneity throughout the alloy. Simultaneously, a pre-prepared powder mixture of Al₂O₃-GNs was added to the molten aluminum in varying weight ratios of 5%, 10%, and 15%, are referred to as Al–5, Al–10, and Al–15, respectively. relative to the weight of the molten aluminum. Once a homogeneous molten aluminum state was achieved, the pre-mixed powder additives were gradually introduced into the alloy while continuous stirring was maintained. To facilitate the incorporation of the hybrid reinforcement Al₂O₃-GNs composite powders into the molten aluminum matrix, a vortex technique was employed. The molten composite slurry was poured into a preheated permanent steel mold. The mold cavity was cylindrical, with dimensions of 20 mm diameter and 150 mm length. Prior to casting, the mold was preheated to ~ 250 °C to minimize thermal gradients and prevent premature solidification during pouring. This technique is considered one of the most effective methods for manufacturing such composites^23,24. The gradual addition of the powder additives effectively prevented agglomeration and promoted uniform dispersion of the Al₂O₃-GNs powder within the aluminum matrix. The stirring process continued for 10 min, carefully monitored to avoid overcooling and maintain optimal processing conditions. The vortex stirring process was conducted in continuous mode for 10 min at 600 rpm, ensuring stable particle entrainment and homogeneous dispersion of the hybrid reinforcement within the molten aluminum. Once a homogeneous distribution of the Al₂O₃-GNs powder was achieved, the molten alloy was poured into a preheated mold to prevent defects in the final sample. A controlled cooling process was then implemented to ensure gradual solidification, minimizing residual stresses and promoting strong metallurgical bonding between the aluminum matrix and the Al₂O₃-GNs²⁵. This meticulous approach not only ensured the integrity of the final sample but also established a solid foundation for comprehensive characterization and analysis, which is crucial to the success of our research objectives. Following the initial fabrication of the samples, a crucial step involved their rolling process at an elevated temperature of 480 degrees Celsius. This rolling process was meticulously conducted at a controlled rate of 0.25% reduction per cycle, with each cycle reducing the sample size by 25%. Prior to each rolling cycle, the rolls themselves were preheated to ensure that the samples maintained their elevated temperature throughout the process, thus preventing premature cooling²⁶. This precise and controlled rolling procedure not only facilitated the shaping of the samples but also played a pivotal role in refining their microstructure and enhancing their mechanical properties. Additionally, the elevated temperature during rolling enabled effective bonding between the constituent materials, further augmenting the structural integrity and performance of the final composite material²⁷.

Table 1 Experimental parameters for the fabrication of Al–Al₂O₃–graphene hybrid nanocomposites.

Density measurements

Figure 3 illustrates the variation in relative density of the Al–Al₂O₃–GNS nanocomposites as a function of reinforcement content before and after hot rolling. The density of the fabricated composites was measured using the Archimedes principle in accordance with ASTM B962. Specimens were weighed in air and in distilled water to determine bulk density. The theoretical density was calculated using the rule of mixtures based on the constituent volume fractions. Relative density was then obtained from the ratio of experimental to theoretical density, expressed as a percentage.The relative density of the as-cast composites decreases progressively with increasing Al₂O₃–GNS content, dropping from approximately 95% for unreinforced aluminum to about 89% for the composite containing 15 wt% hybrid reinforcement. This reduction is primarily attributed to the increased tendency for porosity formation and particle agglomeration at higher reinforcement loadings. The high specific surface area of nanoscale Al₂O₃ and graphene nanosheets promotes particle particle interactions, which hinder complete infiltration of the molten aluminum and facilitate the entrapment of gas during stir casting. Similar density reductions at elevated nanoparticle contents have been widely reported for aluminum matrix nanocomposites produced via liquid-state routes.

Fig. 3

Effect of Al₂O₃–GNS reinforcement content and hot rolling on the relative density of recycled aluminum matrix nanocomposites.

Following hot rolling, a substantial improvement in relative density is observed for all compositions. The relative density of pure aluminum increases to nearly 99.8%, while the Al–15 composite reaches approximately 92%, indicating effective densification induced by thermomechanical processing. Hot rolling promotes pore closure through severe plastic deformation, enhances metallurgical bonding at the matrix–reinforcement interface, and redistributes clustered particles along the rolling direction. Additionally, the elevated rolling temperature increases matrix ductility, allowing the aluminum to plastically flow around reinforcement particles and fill residual voids, thereby reducing overall porosity.

The densification effect of hot rolling is particularly significant in reinforced samples, where the combined action of compressive stress and matrix softening mitigates casting-related defects that are otherwise difficult to eliminate in as-cast nanocomposites. The improved density after rolling directly correlates with the enhanced mechanical performance observed in subsequent hardness and tensile tests, as reduced porosity and improved interfacial cohesion facilitate more efficient load transfer. These findings are consistent with previous studies on hybrid aluminum matrix composites, which report that post-solidification deformation processing is essential for overcoming the density limitations associated with high-volume nanoscale reinforcements.

Overall, the density results confirm that while increasing Al₂O₃–GNS content introduces challenges related to porosity during casting, the application of hot rolling effectively restores structural integrity and enables the fabrication of dense, high-performance recycled aluminum nanocomposites.

Examination of metallurgical and mechanical properties

To investigate the microstructure of the prepared samples, they were polished using four grades of emery SiC papers. SEM observations were conducted with a Quanta FEG 250 SEM, while Transmission Electron Microscopy (TEM) was used for high-magnification imaging of the samples. Raman spectroscopy was employed to evaluate the structural integrity, defect density, and interfacial interactions of graphene within the hybrid reinforcement and composite matrix. This technique is particularly sensitive to carbon lattice disorder and strain effects, as reflected by variations in the D, G, and 2D band characteristics. Agglomerate size was quantified using scanning electron microscopy (SEM) coupled with digital image analysis. High-magnification micrographs were acquired from multiple representative regions to ensure statistical reliability. Agglomerates were delineated based on morphological contrast, and their equivalent diameters were measured using ImageJ software. For each condition, a minimum of 50 agglomerates were analyzed, and the mean size was calculated. Comparative analysis revealed an approximate 40% reduction in agglomerate diameter following hybrid powder processing, indicating improved reinforcement dispersion. To assess the macro-hardness of the Al-Al₂O₃-GNS composites, the specimens were tested using a Vickers hardness tester. The cast composite samples were prepared with the required dimensions of 10 mm x 16 mm diameter on a lathe machine. These specimens were then polished using a disc polishing machine, with graphite water serving as a coolant for the rotating cloth disc. After polishing, the samples were etched with Keller’s reagent before being placed under a 1 kg load in the Vickers tester for a dwell time of 10 s. A transverse tensile test was performed on the specimens using a Universal Testing Machine model (WDW-300, China). Additionally, the corrosion behavior of the prepared samples was evaluated using an electrochemical corrosion testing apparatus (Auto Lab NOVA 2.1.5), immersed in a 3.5 wt% NaCl solution at room temperature.

Results and discussion

TEM analysis

Transmission electron microscopy (TEM) was employed primarily for qualitative assessment of reinforcement morphology, dispersion, and interfacial characteristics. interface thickness were not performed, the TEM images clearly reveal uniformly distributed Ag nanoparticles decorating the Al₂O₃–GNS surfaces and intimate interfacial contact with the aluminum matrix. Based on scale-bar measurements, the Ag nanoparticles exhibit an approximate size range of 10–20 nm, consistent with effective surface metallization reported in similar systems.TEM analysis (Fig. 4a) revealed Ag nanoparticles with an average diameter of 15 ± 3 nm uniformly decorated on Al₂O₃-GNs surfaces, confirming a successful electroless coating. Figure 4(b) shows the Raman spectrum, where the distinct peaks confirm the presence of graphene. pure graphene was analyzed under identical testing conditions. Characteristic D (~ 1350 cm⁻¹), G (~ 1580 cm⁻¹), and 2D (~ 2700 cm⁻¹) bands were observed. Relative to pure graphene, the hybrid reinforcement spectrum (Fig. 3(c)) exhibits peak shifts and intensity variations, indicating interfacial interaction, strain effects, and partial defect generation during hybrid powder processing. Figure 4(c) further confirms the presence of graphene, as the Raman spectrum clearly shows graphene formation through the characteristic peaks. The dense and homogeneous distribution of Ag nanoparticles on the Al₂O₃–GNS surfaces indicates effective surface metallization, which is known to enhance wettability and interfacial bonding in aluminum-based composites. The combined TEM and Raman analyses provide compelling evidence of successful composite synthesis and confirm the presence of graphene nanosheets. These techniques offer a comprehensive understanding of the composition, morphology, and structural properties of the mixture, highlighting its potential for various applications in research and technology.

Fig. 4

(a) Tem Graphene (b) illustrates the morphological distribution and surface attachment of graphene nanosheets on the Al₂O₃ particles, (c) the Raman spectrum of the hybrid reinforcement, confirming the presence and structural integrity of graphene nanosheets through the characteristic D, G, and 2D bands.

Microstructure

Figure presents an in-depth examination of the microstructure of various aluminum-based composites, including as-cast pure aluminum, as-cast Al–15, and as-rolled Al–15, using 2% Hydrofluoric acid as the etchant. This analysis provides crucial insights into the structural integrity, cohesion, and distribution of reinforcement materials within these composites, which are fundamental for optimizing their mechanical properties. The microstructure of the as-cast pure aluminum. Figure 5(a) exhibits a notable absence of porosity or cracks, suggesting a well-formed structure and effective casting process. The uniformity of the aluminum matrix in the pure form indicates a solid foundation for further reinforcement.

However, in the microstructure of the as-cast hybrid reinforced sample, depicted in Fig. 5 (b), certain distinct features emerge. The gray area represents the aluminum matrix, while the white spots correspond to the hybrid Al2O3-GNs composite powder. Although a relatively good distribution of the reinforcement particles is observed, some regions show accumulations, and certain areas display a lack of cohesion between the Al matrix and the hybrid reinforcement. Image analysis of Fig. 5(c) indicates a 40% reduction in the average agglomerate size of the reinforcement after hot rolling, from 5.2 ± 1.8 µm to 3.1 ± 0.9 µm, directly contributing to the improved mechanical properties.“This observation suggests potential challenges in achieving optimal particle dispersion and adhesion during the casting process. The observed agglomerations and poor adhesion between the reinforcement particles and the Al matrix could be attributed to inadequate mixing time and low rotational speeds of the vortex mixer during the integration of the hybrid reinforcement with molten aluminum. The presence of these agglomerations indicates insufficient mechanical interaction between the reinforcement and the matrix, which may affect the overall performance of the composite material. Interestingly, the observed particle distribution was significantly improved by the electroless plating of nano silver metal, which enhanced the wettability of the reinforcement particles’ surfaces. The dense and homogeneous distribution of Ag nanoparticles on the Al₂O₃–GNS surfaces indicates effective surface metallization, which is known to enhance wettability and interfacial bonding in aluminum-based composites²⁸. This treatment likely facilitated better interaction and dispersion of the reinforcement within the aluminum matrix, reducing the agglomeration seen in the as-cast sample. The incorporation of silver as a plating agent also plays a crucial role in improving the interfacial bonding, a key factor in enhancing the mechanical properties of the composite material. The presence of Ag nanoparticles uniformly decorating the Al₂O₃/GNS surfaces promotes improved interaction with the aluminum matrix by enabling metal–metal interfacial contact during melt processing. This effect is evidenced by the reduced extent of particle clustering, improved interfacial continuity, and absence of particle rejection at the matrix–reinforcement interface. Such microstructural features are widely accepted as indirect indicators of enhanced wettability in aluminum matrix composites²⁹.

At higher reinforcement contents, partial clustering of Al₂O₃/GNS particles is observed due to the high surface energy and intrinsic tendency of nanoscale reinforcements to agglomerate. To mitigate this effect, multiple dispersion-control strategies were employed. Prolonged mechanical milling promotes graphene exfoliation and immobilizes graphene sheets on alumina surfaces, reducing restacking. The electroless Ag nano-coating enhances wettability and interfacial bonding with the molten aluminum, suppressing particle–particle attraction. Additionally, the vortex stirring technique ensures gradual particle entrainment and limits local reinforcement accumulation. Post-solidification hot rolling further disrupts residual agglomerates through severe plastic deformation, leading to a measurable reduction in average agglomerate size and a more homogeneous reinforcement distribution.

After subjecting the composite to a hot rolling process at 480 °C for 30 min, a substantial improvement in both the distribution of the hybrid reinforcement and its adhesion to the Al matrix was observed, as illustrated in Fig. 5 (d) and (e). The hot rolling procedure effectively optimized the temperature for the reinforcement particles, further enhancing their adhesion to the aluminum matrix. At 480 °C, the aluminum matrix becomes more pliable, facilitating better integration with the hybrid reinforcement particles. The application of high-pressure forces during the rolling process also contributes to the improvement in adhesion, particularly along the grain boundaries of both the aluminum particles and the hybrid reinforcement. The pressure exerted during hot rolling promotes the diffusion of the particles into the matrix, improving the mechanical interlocking between the two phases.

These findings emphasize the effectiveness of the hot rolling process in improving the distribution, cohesion, and overall structural integrity of aluminum-based composite materials. The enhanced interfacial bonding between the Al matrix and the hybrid reinforcement contributes to the mechanical strength of the material, highlighting the importance of optimizing processing conditions to achieve superior composite properties. The results suggest that the hot rolling technique is a valuable approach for the development of advanced aluminum-based alloys with enhanced performance characteristics, offering a promising path for applications in high-performance engineering materials.

Fig. 5

SEM images of (a) As-cast pure aluminum, (b) As-cast Al–5, and (c) As-rolled Al–15, (d) good adhesion between Al- (Al₂O₃ -Gr), (e) Al pure-rolling cale.

EDAX Mapping

Figure 6 presents the elemental analysis of the Al nanocompsoite conducted using energy dispersive X-ray spectroscopy (EDAX) coupled with SEM, providing valuable insights into the composition and distribution of key components within the material. The analysis revealed that the Al nanocompsoite is primarily composed of aluminum, with trace amounts of silicon, confirming the alloy’s base composition (Fig. 6a, b). To further investigate the distribution of graphene nanosheets, alumina, and nano-silver within the newly synthesized hybrid material, additional EDAX mapping analyses were performed at very high magnification (Fig. 6c). The results highlighted a well-dispersed distribution of these elements (Fig. 6d), which can be attributed to the thorough mechanical processing conducted over a 24-hour duration. Notably, alumina, in the form of aluminum and oxygen, predominated in weight%, emphasizing its critical role in the composite’s structure. EDAX elemental mapping confirms the presence and spatial distribution of C, Al, O, and Ag within the composite. However, it should be noted that EDAX cannot be used to quantitatively determine graphene content, as the detected carbon signal may include contributions from graphene nanosheets as well as background and surface carbon sources. The reported carbon content (15.60 wt%) therefore represents a localized EDAX-derived carbon signal rather than the actual graphene weight fraction in the composite. Furthermore, the presence of nano silver, which was utilized for the surface coating of the hybrid material, was detected at a percentage of 0.25 wt%, validating the success of the electroless deposition process. These findings not only confirm the synthesis methodology but also provide essential insights into the elemental composition and distribution of components within the hybrid material. This comprehensive elemental analysis lays a solid foundation for further characterization and performance evaluation, crucial for understanding the material’s potential in advanced applications.

Fig. 6

EDAX analysis and mapping of Al /Al₂O₃-GNs nanocomposite. (a) SEM image of EDAX analyzed low magnification area and (b) EDAX chart, (c) SEM image of the high magnification area elemental maps, d) all elements map. Al, O, C and Ag elemental maps are indicated.

Hardness Measurement

Figure 7 illustrates the hardness values obtained for both the as-cast and hot-rolled aluminum samples, offering valuable insights into the mechanical properties of the manufactured materials. Hardness measurements were performed using a Vickers precision hardness tester under a 500 g load for 15 s. For each sample, six measurements were taken at different, non-overlapping locations to ensure statistical reliability. The reported hardness values ​​represent the arithmetic mean of these measurements, and the standard deviation was calculated to assess data dispersion. Notably, the hardness of the nanocomposite samples showed a significant enhancement upon the reinforcement of aluminum with 5 wt% hybrid Al₂O₃-GNs, increasing from 72.34 ± 4.7 to 111.2 ± 4.7 HV, representing a substantial improvement of 36.18%. This remarkable increase in hardness can be attributed to the interaction between the Al₂O₃ particles and the aluminum matrix, which facilitates the formation of a robust bond that significantly enhances the material’s strength and hardness.The observed increase in hardness arises from a combination of individual strengthening mechanisms and their synergistic interaction. It is well established that hot rolling improves the grain structure of aluminum composites through dynamic recrystallization and sub-grain formation, contributing to increased stiffness via the Hall-Pitch effect. The observed improvement in stiffness is consistent with widely reported grain structure improvement trends for hot-rolled hybrid aluminum composites under similar processing conditions³⁰. The incorporation of hard Al₂O₃ particles provides a second individual contribution by increasing resistance to localized plastic deformation. Graphene nanosheets further enhance hardness through dislocation interaction and load transfer, although their contribution is highly dependent on dispersion quality and interfacial bonding. The Ag nanocoating does not directly contribute to hardness; rather, it facilitates improved wettability and interfacial cohesion, enabling more effective load transfer and uniform reinforcement distribution. The synergistic interaction among these mechanisms—refined grains, hard ceramic particles, and well-dispersed graphene—results in a cumulative hardness improvement that exceeds the effect of any single mechanism acting alone, consistent with recent reports on hybrid aluminum composites³¹. However, when the hybrid Al₂O₃-GNs content is increased to 10% by weight, a slight decrease in hardness to 108 ± 4.7 HV is observed, although this value remains higher than that of pure aluminum (72.34 ± 4.7 HV). This decline in hardness can be attributed to the formation of agglomerates within the aluminum matrix. The agglomeration of reinforcement particles leads to defects and inhomogeneities in the material, which act as stress concentrators and create weak points in the structure, ultimately compromising the overall hardness. Despite this, the hot-rolling process applied to the samples, particularly those with higher proportions of the Al₂O₃-GNs hybrid, resulted in a significant improvement in hardness. The hot-rolling process induces recrystallization within the aluminum, leading to the formation of smaller, more stable grains, which enhances the material’s resistance to deformation and improves hardness. This grain refinement process also reduces structural defects and increases material density, resulting in a more cohesive structure with enhanced resistance to deformation. The hardness enhancement observed in the present Al–Al₂O₃–GNS composites is comparable to, and in some cases exceeds, values reported for aluminum matrix composites reinforced with either Al₂O₃ or graphene alone. For example [32]. reported hardness values below 140 ± 4.7 HV for Al–Al₂O₃/graphene nanocomposites produced via powder metallurgy, whereas the current hot-rolled composite reached 168.7 ± 4.7 HV at 15 wt% reinforcement using a scalable stir-casting route. This improvement can be attributed to the synergistic strengthening effects of hybrid reinforcement and rolling-induced grain refinement.

The findings underscore the effectiveness of the hot-rolling process in improving the mechanical properties of aluminum-based nanocomposites. This process not only enhances hardness but also contributes to the overall performance of the material, making it a promising approach for the fabrication of high-performance materials for various industrial applications. Thus, the grain refinement induced by the hot-rolling process plays a pivotal role in enhancing the mechanical properties of the composite material.

Fig. 7

Macro-hardness measurements of the nanocomposites before and after hot-rolling.

Tensile strength

Figures 8 and 9 presents the mechanical properties derived from tensile testing, including yield strength, ultimate tensile strength (UTS), and elongation before and after rolling, he relatively low elongation values (< 3%) observed in both the as-cast and hot-rolled Al–Al₂O₃–GNS composites can be attributed to the combined effects of rigid ceramic reinforcement and matrix strengthening mechanisms. The presence of Al₂O₃ particles and graphene nanosheets restricts dislocation mobility through Orowan strengthening and grain boundary pinning, thereby limiting plastic deformation. Moreover, localized reinforcement agglomeration at higher filler contents acts as stress concentration sites, facilitating early crack initiation and reducing ductility. The use of recycled aluminum scrap further contributes to reduced elongation due to microstructural heterogeneity and residual impurities. In hot-rolled samples, the increase in dislocation density and residual stresses introduced during plastic deformation enhances strength at the expense of ductility, resulting in limited elongation despite improved tensile properties.

Fig. 8

(a)Tensile test results of cast specimens before rolling. (b)Tensile test of cast specimens after rolling.

Fig. 9

Wear rate of Al–Al₂O₃–GNS nanocomposites before and after hot rolling under different applied loads and sliding speeds.

highlighting two key phenomena observed in the data. The first phenomenon is the gradual increase in both ultimate tensile strength and yield elongation with the addition of Al₂O₃-GNs up to 10 wt%, followed by a decrease of approximately 15% at 15 wt%. The second phenomenon shows that the tensile strength parameters of the 15% Al₂O₃-GNs samples outperform those of the other samples, indicating a significant deviation in performance at this higher reinforcement level.

The investigation categorized the specimens into two groups: cast specimens and coiled specimens, each reinforced with varying amounts of Al₂O₃-GNs. Notably, the addition of Al₂O₃-GNs led to substantial improvements in the mechanical properties of the cast specimens, with enhancements of up to 12% compared to unreinforced aluminum. However, as the reinforcement level increased to 15%, a slight decline of 8% in performance was observed. The specimens reinforced with 5% by weight of Al₂O₃-GNs exhibited an impressive UTS of 89.748 ± 3.2 MPa, a substantial increase from the 54.97 ± 3.2 MPa UTS observed in the pure aluminum specimens. This improvement can be attributed to the synergistic effects of Al₂O₃ and graphene nanoparticles. The ceramic nature of Al₂O₃, when dispersed uniformly within the ductile aluminum matrix, hinders the sliding movement between aluminum grains, thereby enhancing material resistance to deformation. Simultaneously, the graphene nanosheets form thin layers on the aluminum grain surfaces, improving cohesion and preventing grain separation. Furthermore, the addition of Al₂O₃-GNs promotes grain refinement in the aluminum matrix, which enhances fracture resistance.

However, the observed decrease in UTS at higher Al₂O₃-GNs content, particularly at 15%, could be attributed to the formation of agglomerates within the matrix. This accumulation of nanoparticles may result in larger grain sizes and reduced material uniformity, making the material more susceptible to fracture. The recycling process, which can exacerbate nanoparticle aggregation, may also contribute to the observed decrease in mechanical properties. This emphasizes the importance of controlling nanoparticle dispersion and accumulation to ensure uniformity and maintain mechanical integrity during the recycling process.

The results also revealed significant improvements in both yield stress and UTS for the 15 wt% rolled sample. Specifically, the yield stress increased from 56.84 ± 3.2 MPa in the homogeneous samples to 127.8 ± 3.2 MPa in the 15 wt% rolled sample, while the UTS rose from 89.74 ± 3.2 MPa to 138.8 ± 3.2 MPa, indicating a substantial enhancement in mechanical performance. These improvements can be attributed to several factors. First, the uniform dispersion of Al₂O₃/GNs particles within the aluminum matrix, achieved through optimal vortex mixing parameters and appropriate rotational speed (rpm), played a pivotal role in strengthening the material by minimizing agglomeration. Additionally, the hot rolling process contributed to structural refinement of the aluminum matrix, promoting greater cohesion among the alloy grains and ensuring an even distribution of reinforcement particles. As a result, the Al–15 rolled sample demonstrated heightened resistance to deformation, leading to substantial increases in both yield stress and UTS. The improved load transfer efficiency and enhanced tensile strength observed in Ag-coated Al₂O₃/GNS reinforced composites further validate the positive role of surface modification on interfacial bonding, as insufficient wettability would otherwise lead to premature debonding and reduced mechanical performance. Compared with previous studies employing primary aluminum alloys and single-step fabrication routes, the present work demonstrates a favorable balance between strength enhancement and process simplicity. Yehia et al. [33] reported ultimate tensile strengths below 120 ± 4.7 MPa for hot-pressed Al–Al₂O₃–GNS composites, whereas the current recycled aluminum-based composite achieved a UTS of approximately 140 MPa after rolling. The superior performance is attributed to improved particle wettability via Ag coating and effective redistribution of reinforcements during hot rolling.

Moreover, the exposure to high temperatures during the rolling process fostered enhanced bonding between the reinforcement particles and the aluminum matrix, improving interfacial adhesion. This enhanced bonding significantly augmented the overall mechanical integrity of the material, resulting in superior fracture resistance and increased strength. Although clustering becomes more pronounced at higher reinforcement levels, the combined use of Ag surface modification and hot rolling effectively mitigates its adverse impact on mechanical performance. The fragmentation and redistribution of reinforcement clusters during rolling enhance load transfer efficiency and delay crack initiation, allowing high-reinforcement samples to retain superior strength despite reduced ductility.Consequently, the 15 wt% rolled sample exhibited enhanced performance characteristics compared to the homogeneous samples, highlighting the effectiveness of both the reinforcement addition and the rolling process in optimizing the mechanical properties of aluminum-based composites.

Wear Behavior

Figure 9 presents the wear rate of the Al–Al₂O₃–GNS nanocomposites before and after hot rolling under varying normal loads (5 and 10 N) and sliding speeds (200–400 rpm). The unreinforced aluminum exhibits the highest wear rate under all testing conditions, reflecting its inherently low hardness and limited resistance to plastic deformation during sliding contact. In contrast, all reinforced composites demonstrate a pronounced reduction in wear rate, confirming the beneficial role of hybrid Al₂O₃–GNS reinforcement in enhancing tribological performance.

In the as-cast condition, the wear rate decreases progressively with increasing reinforcement content up to 10 wt%, beyond which a slight increase is observed for the 15 wt% composite under higher load and speed conditions. This behavior is attributed to the competing effects of reinforcement strengthening and particle agglomeration. The incorporation of hard Al₂O₃ particles increases surface hardness and load-bearing capacity, thereby reducing material removal through abrasive and adhesive wear mechanisms. Simultaneously, graphene nanosheets act as solid lubricants due to their layered structure, facilitating interlayer sliding and reducing friction at the contact interface. The synergistic interaction between ceramic load support and graphene-induced lubrication effectively suppresses severe plastic deformation and material transfer, leading to lower wear rates compared to pure aluminum.

At higher reinforcement contents, partial agglomeration of Al₂O₃–GNS particles becomes more pronounced, particularly in the as-cast condition. These agglomerates can act as stress concentrators and weakly bonded regions, which are susceptible to pull-out during sliding. The detached particles may then behave as third-body abrasives, accelerating localized wear and explaining the observed increase in wear rate for the 15 wt% composite under severe testing conditions. Similar non-monotonic wear trends at high nanoparticle contents have been reported in aluminum matrix composites reinforced with nanoscale ceramics and carbon-based phases.

Hot rolling leads to a substantial reduction in wear rate across all compositions and testing conditions. This improvement is attributed to multiple concurrent mechanisms induced by thermomechanical processing. First, rolling increases composite density and significantly reduces casting-related porosity, minimizing crack initiation sites during sliding. Second, grain refinement and work hardening of the aluminum matrix enhance resistance to plastic flow at the wear surface. Third, hot rolling promotes more uniform redistribution of Al₂O₃–GNS particles and fragmentation of residual agglomerates, resulting in a more homogeneous load-bearing network at the sliding interface. Improved interfacial bonding, facilitated by Ag-assisted wettability and rolling-induced plastic deformation, further reduces reinforcement pull-out during wear.

The effect of normal load and sliding speed follows expected tribological trends. Increasing load and speed generally result in higher wear rates due to elevated contact stress and frictional heating. However, reinforced and rolled composites exhibit significantly lower sensitivity to these parameters compared to unreinforced aluminum, indicating improved thermal stability and load-sharing capability. The superior wear resistance of the hot-rolled Al–15 composite, particularly under high-load conditions, highlights the dominant role of reinforcement-induced hardness and interfacial integrity over potential agglomeration effects once proper densification is achieved.

Overall, the wear performance of the Al–Al₂O₃–GNS nanocomposites is governed by a clear processing–structure–wear relationship. Hybrid reinforcement enhances hardness and introduces solid lubrication, while hot rolling refines the microstructure, improves density, and stabilizes the reinforcement–matrix interface. The resulting composites exhibit markedly improved wear resistance compared to recycled pure aluminum, making them promising candidates for load-bearing and tribological applications such as automotive and marine components.

Electrochemical corrosion

Figure 10 illustrates the polarization curve patterns obtained from as-cast pure aluminum, as-cast aluminum reinforced with Al–15, and the rolled one. These curves provide valuable insights into the corrosion behavior of the samples when subjected to a 3.5% NaCl solution in an electrochemical cell. It is noteworthy that the as-cast pure aluminum exhibits lower polarization resistance compared to both the as-cast reinforced and as-rolled reinforced samples, The polarization resistance (Rp) was quantitatively determined from the slope of the polarization curve in the vicinity of the corrosion potential (Ecorr) using Tafel extrapolation. Lower Rp values correspond to higher corrosion susceptibility, whereas higher Rp values indicate improved corrosion resistance. Therefore, corrosion performance comparisons in this study are based on derived Rp and corrosion current density (Icorr). It is important to consider the potential micro-galvanic effect associated with the incorporation of electrically conductive graphene nanosheets in an aluminum matrix. Owing to its higher electrochemical nobility, graphene can act as a local cathodic phase, potentially accelerating anodic dissolution of adjacent aluminum through micro-galvanic coupling, particularly when graphene is agglomerated or exposed at the surface. However, in the present study, the graphene content is limited to 2 wt% and is predominantly embedded within the matrix and modified through Ag-assisted interfacial engineering. This configuration reduces direct electrolyte access and electrical isolation of graphene, thereby mitigating galvanic effects. As a result, the observed corrosion behavior reflects a balance between potential galvanic interactions and beneficial barrier and passivation effects arising from uniform reinforcement dispersion and improved interfacial cohesion. The rolling process applied to the as-cast reinforced sample influences its polarization resistance, resulting in the as-rolled sample being less resistant to polarization in the NaCl solution. Further investigation using the Tafel method, as detailed in Table 1, reveals two significant phenomena. Firstly, the incorporation of the new hybrid (Al₂O₃-GNs) reinforcement into the as-cast pure aluminum leads to a significant reduction in the corrosion rate (CR). Specifically, the as-cast pure sample exhibits a CR of 3.124 mm/year, whereas the same sample with the hybrid mixture shows a significantly reduced CR of 0.215 mm/year, representing a remarkable 93.11% reduction. This reduction in CR can be attributed to the inherent corrosion resistance of the (Al₂O₃-GNs) ceramic material, compounded by the enhanced adhesion between the matrix and reinforcement facilitated by the Ag coating. In which due to the good mixing and complete coating of the Al₂O₃-GNs with nano Ag helps in the good densification with the smallest available pores. So, no big chance for the corrosive medium to enter inside the sample and corroded it. Also, both Al₂O₃-GNs are ceramic materials have a high corrosion resistance, consequently their incorporation in the AL-matrix increases the corrosion resistance of the prepared sample. The presence of (Al₂O₃-GNs) particles, with their strong bonding at grain boundaries, acts as a barrier against solution penetration, thereby mitigating corrosion. The second phenomenon relates to the effect of the rolling process on the CR of the reinforced aluminum sample. Interestingly, after the as-cast reinforced sample undergoes rolling, there is a notable increase in the corrosion rate. While the as-cast reinforced sample records a CR of 0.215 mm/year, the rolled reinforced sample exhibits a higher CR of 1.038 mm/year. The increase in corrosion rate observed after hot rolling cannot be attributed solely to the increase in exposed surface area. Thermomechanical processing introduces a higher density of lattice defects, including dislocations and residual stresses, which can serve as preferential anodic dissolution sites. In addition, rolling-induced microstructural heterogeneity and redistribution of reinforcement phases may enhance localized electrochemical activity through micro-galvanic interactions. The deformation process can also influence the stability and integrity of the naturally formed oxide film, rendering it more susceptible to breakdown in aggressive environments. Similar multifactorial corrosion responses following rolling have been reported in aluminum-based composites, where defect density and residual stress play a decisive role alongside surface morphology [34]. contrast to many graphene-reinforced aluminum composites where corrosion resistance improvement is marginal or inconsistent, the present hybrid system exhibits a pronounced reduction in corrosion rate (up to 93%). Previous studies on Al–graphene systems [35] reported corrosion rate reductions below 70%, often accompanied by galvanic effects. The incorporation of Al₂O₃ in conjunction with Ag-coated graphene nanosheets provides a more stable barrier effect, suppressing localized corrosion and enhancing passive film stability.

These findings underscore the intricate relationship between material composition, processing techniques, and corrosion behavior, highlighting the potential of the novel hybrid (Al₂O₃-GNs) reinforcement to significantly enhance the corrosion resistance of aluminum alloys. Further elucidating these mechanisms not only contributes to advancing our understanding of corrosion phenomena but also informs the development of more robust and durable materials for various industrial applications(Table 2).

Fig. 10

Potentiodynamic polarization curves of as-cast pure Al–0, Al–15 cast sample and Al–15 rolled sample.

Table 2 Corrosion parameters of nanocomposites.

Microstructure of the corroded surface

The cast reinforced composite (Fig. 11b) exhibits a relatively smooth and cohesive surface with limited localized corrosion. Shallow pits with low density are observed, indicating effective suppression of pitting corrosion. This behavior is attributed to the uniform distribution of the hybrid reinforcement of aluminum oxide and nanographene, and the presence of a silver nanolayer, which enhances interfacial bonding and reduces microgalvanic interaction between the aluminum matrix and the reinforcement particles. The ceramic nature of the aluminum oxide and nanographene acts as a physical barrier, limiting chloride ion penetration and inhibiting the propagation of corrosion pits. Furthermore, the improved density and reduced porosity of the cast reinforced composite restrict the formation of favored corrosion pathways, resulting in a significantly lower corrosion rate as measured electrochemically. In contrast, the hot-rolled composite (Fig. 9a) exhibits a higher density of corrosion traces, characterized by elongated grooves and intense localized corrosion along the rolling direction. While the rolling process improves mechanical properties through grain refinement and particle redistribution, it also increases the effective surface area and introduces deformation-induced defects, such as dislocation networks and residual stresses. These microstructures act as energetically favorable sites for anodic dissolution, accelerating localized corrosion processes. The alignment of corrosion traces along the rolling direction indicates that plastic deformation plays a dominant role in controlling corrosion kinetics in the rolled samples.

Despite the increased corrosion activity observed in the rolled composite, the severity of surface degradation remains significantly lower than that of unreinforced aluminum, confirming the protective role of the hybrid reinforcement of aluminum oxide and nanographene. The presence of well-adhered reinforcing particles at grain boundaries contributes to corrosion resistance by preventing electrolyte diffusion and stabilizing the inert oxide layer on the aluminum surface.

Post-corrosion microstructure analysis confirms the electrochemical findings, demonstrating that the incorporation of a hybrid reinforcement of aluminum oxide and silver-coated nanographene significantly improves corrosion resistance in casting, while hot rolling results in a trade-off between improved mechanical performance and a slight decrease in corrosion resistance.

Fig. 11

Post-Corrosion Surface Morphology Analysis of Al–15 Rolled (A) and Al–15 Cast (B) Al–Al₂O₃ Composites.

Conclusions and recommendations

The following conclusions can be drawn from the present investigation of recycled Al / Al₂O₃–graphene nanosheet (GNS) hybrid nanocomposites:

• 1. The mechanical grinding process facilitated the effective dispersion and integration of graphene nanosheets (few-layer graphene) within the Al₂O₃–Al matrix. While Raman spectroscopy confirmed the presence and structural integrity of graphene-related carbon phases.

• 2. The incorporation of Ag-coated Al₂O₃–GNS hybrid reinforcement significantly enhanced the mechanical performance of recycled aluminum. The hardness increased from 72.3 HV for pure aluminum to 168.7 HV for the hot-rolled composite containing 15 wt% reinforcement.

• 3. Tensile properties were markedly improved through hybrid reinforcement and rolling, with the ultimate tensile strength increasing from 55.8 MPa for unreinforced aluminum to 139.9 MPa for the hot-rolled Al–15 composite, representing an improvement of approximately 150%.

• 4. Microstructural analysis confirmed improved reinforcement distribution and interfacial bonding, particularly after hot rolling, which reduced agglomerate size and enhanced load transfer efficiency between the matrix and reinforcement.

• 5. Electrochemical corrosion testing in 3.5 wt% NaCl solution demonstrated a substantial reduction in corrosion rate from 3.124 mm/year for pure aluminum to 0.215 mm/year for the as-cast reinforced composite, corresponding to a 93% improvement in corrosion resistance.

• 6. While hot rolling further enhanced mechanical strength through grain refinement and densification, it led to a moderate increase in corrosion rate compared to the as-cast reinforced condition, indicating a trade-off between mechanical performance and corrosion resistance.

• 7. The combined use of recycled aluminum scrap and hybrid ceramic–graphene reinforcement enabled the development of aluminum-based composites with competitive mechanical and corrosion properties, suitable for load-bearing and corrosion-prone applications.

The combined effects of improved wettability, reduced reinforcement agglomeration, and enhanced interfacial bonding led to refined microstructural features and superior mechanical performance. The synergistic action of Al₂O₃ and graphene nanosheets, further facilitated by Ag addition, promoted effective load transfer, densification, and strengthening. This integrated structure–property correlation forms the basis for the performance improvements discussed and provides a foundation for the conclusions drawn in this study.

The developed Al/Al₂O₃/GNs–Ag hybrid nanocomposite demonstrates significant potential for lightweight structural applications requiring enhanced strength, wear resistance, and thermal stability, particularly in automotive, aerospace, and tribological components. The improved wettability, reduced agglomeration, and refined reinforcement dispersion achieved in this study support the feasibility of scalable liquid-state processing routes for advanced hybrid nanocomposites.

Future research should focus on (i) large-scale processing and cost–performance optimization, (ii) long-term tribocorrosion and fatigue behavior under service conditions, (iii) high-temperature stability and creep performance, and (iv) advanced interfacial engineering strategies, including nano-coatings and alternative wetting agents. In addition, computational modeling and machine-learning-assisted process optimization are recommended to further tailor microstructure–property relationships for industrial deployment.