Effect of T6 Tempering on the Wear and Corrosive Properties of Graphene and B₄C Reinforced Al6061 Matrix Composites

Abstract

This research study aims to evaluate the wear and corrosive behaviour of aluminum 6061 alloy hybrid metal matrix composites after reinforcing them with graphene (0.5, 1 wt.%) and boron carbide (6 wt.%) at varying weight percentages. The hybrid composites were processed through ball milling and powder compaction, followed by a microwave sintering process, and T6 temper heat treatment was carried out to improve the properties. The properties were evaluated and analyzed using FE-SEM, Pin-on-Disc tribometer, surface roughness, salt spray test, and electrochemical tests. The results were evaluated prior to and subsequent to the T6 heat-treatment conditions. The T6 tempered sample S1 (Al6061-0.5% Gr-6% B₄C) exhibits a wear rate of 0.00107 mm³/Nm at 10 N and 0.00127 mm³/Nm at 20 N for 0.5 m/s sliding velocity. When the sliding velocity is 1 m/s, the wear rate is 0.00137 mm³/Nm at 10 N and 0.00187 mm³/Nm at 20 N load conditions. From the Tafel polarization results, the as-fabricated (F) condition demonstrates an Ecorr of −0.789 and an Icorr of 3.592 µA/cm² and a corrosion rate of 0.039 mm/year. Transitioning to the T6 condition further decreases Icorr to 2.514 µA/cm², Ecorr value of −0.814, and the corrosion rate to 0.027 mm/year. The results show that an increase in the addition of graphene wt.% from 0.5 to 1 to the Al 6061 alloy matrix deteriorated the wear and corrosive properties of the hybrid matrix composites.

1. Introduction

Mechanical components operate under harsh environmental circumstances characterized by significant wear, including elevated applied loads and high working temperatures. The intense operating conditions lead to the failure of components such as pistons, gas turbine seals, and bearings, hence diminishing operational efficiency and increasing overall maintenance costs [1,2]. Researchers have utilized an amalgamation of metal alloys as the matrix material and hard ceramic particles as reinforcement to develop metal matrix composites that demonstrate improved tribological properties at elevated operational temperatures, thus enhancing the performance of the engineering components [3,4]. Numerous studies have indicated an increase in the hardness of metal matrix composites (MMCs) achieved through the incorporation of ceramic particles (such as Al₂O₃, TiO₂, SiC, TiC, etc.) into metallic alloys (including Al alloys, Cu alloys, Mg alloys, etc.). The integration of reinforced hard ceramic particles with the elevated hardness levels of metallic alloys establishes a strengthening mechanism that enhances the tribological properties of metal matrix composites (MMCs) [5,6]. AMNCs play a crucial role in automotive, aerospace, and industrial applications due to their superior mechanical properties and wear resistance. In the automotive sector, they enhance components like clutches, brakes, pistons, and cylinder blocks, improving durability and fuel efficiency. Aerospace applications benefit from their lightweight nature, making them ideal for structural components, heat exchangers, and coatings. These materials also find use in advanced manufacturing, electronic devices, and aerospace coatings due to their customizable properties. Future advancements in nanotechnology and recycling will expand their role in space exploration, renewable energy, and sustainable engineering. Their unique properties position them as key materials for high-performance and eco-friendly innovations. Aluminum composites, despite their advantages, have several drawbacks. High production costs arise due to expensive reinforcement materials and advanced processing techniques. Reduced ductility and fracture toughness make them more brittle, limiting their application in impact-resistant structures. Machining challenges, such as increased tool wear and poor surface finish, add to manufacturing complexity. Additionally, recycling is difficult due to the separation of reinforcements, impacting sustainability efforts [7,8]. Aluminum alloy 6061 (AA 6061) is recognized as the most versatile material within the 6XXX series, attributed to its superior resistance to corrosion, heat treatability, and favourable properties in formability, machinability, and weldability compared to alternative materials. Research shows that this alloy demonstrates robust performance in chloride environments, due to its ability to form a protective oxide layer on its surface. The primary shortcomings of Al6061 are its lower hardness and strength, which restrict its application in the aerospace, automotive, and defence sectors [9,10].

Aluminum 6061 alloy exhibits moderate abrasion resistance. Under dry sliding conditions, the wear rate is generally elevated due to its comparatively softer matrix when contrasted with harder materials such as steel. To overcome these issues, this deficiency can be addressed through hard ceramic reinforcement particles such as carbides, oxides, etc. Research indicates that the frictional properties of metal matrix composites can be enhanced through the incorporation of self-lubricating fillers (reinforcements) including graphene, graphite, silver, calcium fluoride, and molybdenum disulfide, among others [11,12]. Boron carbide (B₄C) is the hardest ceramic substance after diamond and cubic boron nitride and has a lower density and a higher hardness than other materials [5]. Thus, B₄C is the best reinforcement option for high-performance hybrid nanocomposites (HNCs). B₄C’s high melting point of 2450 °C, low thermal conductivity, and great compressive strength make it ideal for high-temperature applications. The composite material has an improved wear resistance due to the excellent hardness and abrasion resistance of B₄C particles. These particles support the matrix, distributing wear and tear over a broader surface and reducing quick wear of the metal matrix. The addition of B₄C particles to the metal matrix composite enhances wear resistance by improving hardness and abrasion resistance. Due to its great wear resistance and low coefficient of friction, graphene is suitable for decreasing friction and wear. Furthermore, graphene’s high Young’s modulus and strength enhance composite stiffness, enhancing wear resistance [13,14]. In this study, the powder metallurgy (PM) route was selected for fabricating the graphene and B₄C-reinforced Al6061 composite due to its advantages over casting-based methods. PM ensures a uniform particle distribution, minimizing agglomeration while preserving graphene’s integrity. As a solid-state process, it prevents porosity and phase segregation, enhancing mechanical properties. Additionally, PM allows for precise microstructure control, improving strength and wear resistance. Although the initial costs are higher, PM offers better material utilization, reduced machining, and fewer defects, making it a cost-effective choice [15,16]. Recently, hybrid aluminum matrix composites (HAMCs) containing graphene nanoplatelets (GNPs) and silicon carbide (SiC) have been fabricated using the powder metallurgy (PM) method. The implementation of dual reinforcements has improved the mechanical and wear properties of composites. The study indicates that hybrid aluminum matrix composites incorporating reduced graphene oxide nanosheets and silicon carbide (SiC) show enhanced wear resistance in comparison to single-reinforced aluminum matrix composites (AMCs) [17].

In a study by Müslim Çelebi et al. [18] the ZA40 alloy is reinforced with different weight percentages of B₄C, from 0% to 3%, to study the wear rate. The findings indicate that the wear loss in the ZA40 alloy, when reinforced with 3 wt.% B₄C, decreased by a factor of 120 in comparison to the unreinforced ZA40 alloy. The notable decrease in wear illustrates the effectiveness of the B₄C reinforcement in improving the wear resistance of the ZA40 alloy. With the synergistic reinforcement of core–shell SiC-GNSs nanoparticles, the wear rate of AlSiGr is 98.0% lower than that of AlSi and can reach as low as 0.0015 mm³/Nm. Martin et al. [19] examined the wear rate of Al10Si composites enhanced by few-layered graphene (FLG). Various concentrations of FLG (quarter, half, one, two, and five weight percent) influence the enhancement of wear rate relative to the Al10Si matrix. Nonetheless, the composites supplemented with two weight percent FLG display the maximum wear resistance. Due to the reinforcement phase network’s intervention against wear pressures affecting the samples, FLG-reinforced composites exhibit superior wear resistance compared to the matrix structures in reciprocal wear tests. The incorporation of graphene significantly influences the lubricant production and reduces wear. This study examines the effect of 3-μm Palm Sprout Shell Ash (PSSA) on Al-Cu-Mg alloy using ultrasonic-assisted stir casting. PSSA (SiO₂, Al₂O₃) reinforcement improved hardness (13.89%), tensile strength (24.04%), and compression strength (32.93%), but reduced ductility (42.87%). Wear tests confirmed the enhanced wear resistance [20].

Based on the literature, the motivation and novelty of adding and varying the graphene (0.5, 1 wt.%) and B₄C (6 wt.%) weight percentage into the aluminum 6061 alloy, will improve the tribological and corrosive properties of the aluminum matrix composites for braking applications for automotive and aerospace sectors. The objective of this study is to evaluate the effects on the tribological and corrosive behaviour of graphene and boron carbide-reinforced Al 6061 alloy matrix composites, before and after T6 heat treatment. The AMCs are fabricated using the powder metallurgy (PM) route. The samples will be analyzed using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM), hardness, pin-on-disc tribometer, and corrosion studies using salt spray and electrochemical methods.

2. Materials and Methods

2.1. Materials Used

The aluminum 6061 alloy (AA 6061) is used as the matrix material, and the reinforcement materials are graphene (Gr) and boron carbide (B₄C). Figure 1 shows that the AA 6061 alloy powder particles has an average size of 10–15 µm and the shape was spherical in nature. The average particle size of the graphene and B₄C are 5–10 nm and 5 µm, respectively. The graphene and B₄C are added into the matrix AA 6061 alloy with graphene contents of 0.5 and 1.0 wt.% and B₄C content of 6.0 wt.%.

2.2. Methodology

Figure 2 illustrates the schematic overview of the fabrication process of the graphene and boron carbide reinforced Al6061 alloy hybrid matrix composites. All three raw powders were weighed as per

Table 1. Description of sample and wt.% of reinforcement in Al6061-Gr-B₄C composites [21].

S No.	Sample Description	Graphene (wt.%)	B4C (wt.%)	Sample Conditions
1	S1-(Al6061-0.5%Gr-6% B4C)	0.5	6	1. As-Fabricated (F) 2. T6 temper (T6)
2	S2-(Al6061-1%Gr-6% B4C)	1	6
3	Al6061 alloy	-	-

, at different weight percentages, and mixed in the highspeed homogenizer. The powder mixture underwent homogenization at a speed of 60 rpm, with 1 h dedicated to forward rotation followed by 1 h of reverse rotation to ensure the adequate mixing of the powders. After that, the homogenized powders were transferred to high-speed ball milling (planetary type) for the purpose of mechanical alloying.

The parameters were set at 250 rpm for 2 h and maintained a ratio of 10 g of ball to 1 g of powder. For every 30 min, the mill was stopped for 5 min to prevent the cold welding and agglomeration of the powder mixture. To avoid the deterioration of the composite strength and properties, 1 wt.% of zinc stearate is added to the powder mixture as a lubricating agent to prevent agglomeration and achieve a fine dispersion of reinforcements into the Al matrix material. It also improves the flow characteristics of the powder, which is beneficial in the compaction and sintering stages to achieve uniform packaging of materials. Then, the ball-milled powder was preheated and kept in the industrial oven for 10 h at 100 °C to remove the moisture content in the ball-milled powder mixture.

To produce the green compact, the ball-milled powder was filled into the cylindrical compaction die made of D2 die steel and pressed using the hydraulic press at the pressure of 450 MPa, for 1 min of holding time, to achieve proper compaction. The green compact was subsequently sintered in a microwave furnace at a temperature of 550 °C and soaked for a period of 2 h. After that the samples were furnace-cooled. The argon gas was passed through the chamber to maintain an inert atmosphere to prevent oxide formation on the AMCs; the flow rate was 0.5 L/min. To perform the T6 heat treatment, the samples that were produced underwent heating in the furnace, at a temperature of 527 °C for a duration of 2 h, followed by water quenching. The AMCs were again heated to 180 °C for 24 h to accomplish artificial ageing. The composite samples were cut using a wire-cut EDM machine, as per the required sample dimensions.

2.3. Characterization Techniques

The Pin-on-Disc tribometer (Model- TR 20, Make-DUCOM Instruments) was used to analyze the wear. The ASTM G99 testing standard is followed and EN 31 steel disc is used. The cylindrical pin sample size is 10 mm in diameter and 30 mm in long. The parameters are selected after the literature review [22,23]. The load is 10 N and 20 N, the sliding velocity is 0.5, 1 m/s, the sliding distance is 1000 m, and the sliding rpm is 500 rpm. The test condition is dry sliding, and it is conducted at room temperature. For each test condition, 2 samples will be tested, and the mean values will be considered for the analysis. To analyze the surface toughness, the Marsurf XR20 surface tester equipment was utilized to measure the roughness of the worn surface. The transverse speed is 0.5 mm/s, and the transverse length is 5.6 mm for all the samples. The samples were immersed in a solution of 5% NaCl for 48 h of exposure time, and the pH value was maintained between 4.5 and 6.5. The applied nozzle pressure is 2 bar, and the test environment temperature is maintained at 35 °C ± 2 °C. The surface area is 1 cm² (10 mm × 10 mm) and the ASTM B117 standard was followed. The electrochemical corrosion testing workstation (make: Ivium, model: Vertex One) is utilized to analyze the property. The working medium is a 3.5% NaCl solution, the applied voltage is ±5 V, and the ASTM G59 standard has been followed.

The microhardness of the graphene and B₄C reinforced AA6061 hybrid composites are listed below in

Table 2. Micro hardness of Al6061-Gr-B₄C composites [21].

S. No:	Sample Description	Hardness (HV)
1.	AA 6061-F	54
2.	AA 6061-T6	61
3.	AA6061/0.5% Gr/6%B4C-F	82
4.	AA6061/0.5% Gr/6%B4C-T6	91
5.	AA6061/1% Gr/6%B4C-F	74
6.	AA6061/1% Gr/6%B4C-T6	83

. The hardness test was performed using the Vickers hardness test machine. The applied load is 200 gf for 15 s of dwell time. An average of 10 readings were taken for each sample. The microstructural studies and mechanical properties were studied in the previous work [21].

3. Result and Discussion

3.1. Wear Behaviour

The fabricated composites were subjected to a wear test to analyze the wear rate and coefficient of friction prior to and subsequent to T6 heat-treatment conditions. Also to analyze the effect of graphene particles and B₄C on the wear properties of the developed composites. The wear rate was calculated using the mass loss method and the following Equation (1) [24]:

Wear rate = Δm/(ρ × d)

(1)

where Δm is mass loss (Initial Mass-Final mass) in g, ρ is the density of the pin (g/mm³), and d is the sliding distance (mm).

3.1.1. Effect on the Wear Rate

Figure 3 represents the wear rate at load conditions of 10 N and 20 N and sliding velocities of 0.5 and 1.0 m/s, as well as before and after T6 heat treatment conditions. Figure 3a represents the wear rate at 0.5 m/s sliding velocity, at 10 and 20 N load conditions. Here, the AA 6061 exhibits the maximum wear rate at both the fabricated and T6 conditions, and when the load increases the wear rate also increases due to the pressure applied. On the other hand, the Al6061-0.5%Gr-6% B₄C (S1) shows a lower wear rate at different load conditions and before and after the heat treatment. According to Figure 3, Al6061-1%Gr-6% B₄C (S2) shows an increased wear rate compared to Al6061-0.5%Gr-6% B₄C (S1). That is due to the increment of graphene content and non-uniform distribution of graphene and B₄C in the Al 6061 matrix. Figure 3b represents the wear rate at 1 m/s sliding velocity; in that condition the sample Al6061-0.5%Gr-6% B₄C (S1) also exhibits an improved wear rate, compared to other samples (AA6061 and S2) in both loading and heat treatment conditions. At both load conditions (10 N and 20 N) and both sliding velocities (0.5 m/s and 1 m/s), the uniformly distributed graphene can form a consistent, thin protective film, which minimizes direct metal-to-metal contact. The application of this solid lubricant film effectively reduces the coefficient of friction, thereby minimizing the wear on the surfaces involved. The inclusion of B4C particles enhances hardness and increases load-bearing capacity. As the load escalates from 10 N to 20 N, the hardness and strength conferred by B₄C particles effectively reduce plastic deformation, while the minimal presence of graphene continues offering lubrication. These two mechanisms help to reduce the wear rate. The T6 condition offers a good refined and uniform distribution of graphene and boron carbide in the matrix, which also helps improve the wear rate [10,23]. Meanwhile, a 1 wt.% graphene addition leads to agglomeration and clustering of particles with B₄C. When the load and sliding velocity are low, the wear rate is also low, and the wear is primarily abrasive and followed by mild adhesive wear. When the load and sliding velocity increase, the wear rate is also high due to the frictional heating and the material softening [25]. The sample Al6061-0.5%Gr-6% B₄C (S1) exhibits a wear rate of 0.00107 mm³/Nm at 10 N and 0.00127 mm³/Nm at 20 N for 0.5 m/s sliding velocity. When the sliding velocity is 1 m/s, the wear rate is 0.00137 mm³/Nm at 10 N and 0.00187 mm³/Nm at 20 N load conditions. Both results refer to the period after T6 heat treatment. T6 heat treatment condition improves up to 70% of improvement in the wear rate compared to all the fabricated samples.

3.1.2. Effect on Mass Loss

The effect of mass loss was studied before and after the T6 heat treatment condition. Figure 4 depicts the mass loss in g of graphene and boron carbide reinforced al 6061 matrix composites, at different loading (10 N,20 N) and different sliding velocities (0.5 m/s, 1 m/s). Figure 4a depicts the 0.5 m/s sliding velocity and Figure 4b depicts 1 m/s sliding velocity. In this condition the sample Al6061-0.5%Gr-6% B₄C (S1) exhibits reduced mass loss compared to the other samples (AA6061, S2). As depicted in Figure 4a, after the T6 heat treatment condition, the mass loss of Al6061-0.5%Gr-6% B₄C (S1) is reduced from 0.0233 g to 0.0158 g at 10 N load condition.

For the 20 N load condition, the mass loss is reduced from 0.0254 g to 0.0163 g for the sliding velocity of 0.5 m/s. As shown in Figure 4b, the sample S1 mass loss is reduced from 0.0356 g and 0.0454 g to 0.0258 g and 0.0313 g, at 10 N and 20 N load conditions, respectively. The maximum mass loss is obtained in the un-reinforced AA6061 alloy which is 0.0953 g and 0.102 g at 20 N load and 1 m/s sliding velocity. The sample Al6061-1%Gr-6% B₄C (S2) induces moderate mass loss compared to the S1 and AA6061 alloy before and after the T6 conditions. The irregular behaviour of S2 at a sliding velocity of 1 m/s may be attributed to an inconsistent dispersion of graphene and boron carbide, particle agglomeration, and interfacial bonding issues between the matrix and reinforcement. The observed effect could also be influenced by the increase in graphene content from 0.5 wt.% to 1 wt.%, as graphene exhibits excellent self-lubricating properties. Mass loss is reduced due to the graphene’s self-lubricating property which induces the thin tribo layer on the surface of the composites. The B₄C particles and T6 heat treatment increase the hardness of the composites, these synergetic properties reduce the mass loss [19,26].

3.1.3. Effect on Coefficient of Friction

The effect of COF analysis was performed on the produced composites after the T6 heat treatment. The COF values were calculated using the following Equation (2) [24];

COF = Frictional Force/Applied Load

(2)

where Frictional force in N, and Applied Load in N.

Figure 5a,b depict the COF analysis of the fabricated and T6 condition composite samples at 10 N, 20 N load, at 0.5 m/s, and 1 m/s sliding velocity. The sample Al6061-0.5%Gr-6% B₄C (S1) underwent the lowest COF of 0.388 and 0.457 at 0.5 m/s sliding velocity and the load is 10 N, 20 N, respectively, after T6 heat treatment condition. After the T6 condition, for 1 m/s sliding velocity, the COF is 0.323 and 0.384 at 10 N and 20 N load, respectively. After the T6 heat treatment condition, the uniformly distributed graphene functions as an effective solid lubricant, minimizing direct contact between metal surfaces. As a result, the coefficient of friction (COF) generally exhibits lower values and an increased stability to resist the friction between the pin and disc surface. The B₄C particles contribute to the preservation of surface integrity and reduce the occurrence of large-scale plastic deformation when subjected to load. Their presence enhances the lubricating properties of graphene, maintaining a consistently low and stable level of friction. The un-reinforced AA6061 alloy shows the highest COF of 0.759 and 0.845 in the fabricated condition. After heat treatment, the COF was significantly reduced. The oxide layer formation occurs at a higher load and a high sliding speed, which significantly contributes to an increase in friction between the sample and the contact surface. The sample Al6061-1%Gr-6% B₄C (S2) exhibits moderate friction compared to the S1 and AA6061 samples prior and after the T6 temper heat treatment [27,28].

Figure 6a,b, show that the frictional force in Figure 6b initially exhibits a non-steady state, which sometimes lasts for more than 60 s. After this period, a steady-state frictional force was established. In both the 10 and 20 N load conditions at a sliding velocity of 1 m/s, Al6061 incurred higher frictional forces, whereas samples S1 and S2 exhibited lower frictional forces compared to the Al6061 alloy before and after T6 heat treatment conditions.

3.1.4. Worn Surface Analysis

The worn samples were examined using the FE-SEM to analyze the surface morphology after the wear test. Figure 7 depicts the worn surface microstructure of all the fabricated and T6 condition samples. The unreinforced pure AA 6061 sample underwent severe delamination due to extensive plastic deformation. Due to frictional heating, the oxide layer also formed a patchy pattern with big pits appearing in the microstructure.

The sample Al6061-0.5%Gr-6% B₄C (S1) worn surface has undergone macro grooves, and flaking of surfaces was observed in the high load and sliding velocity. At the same time, the T6 heat-treated sample has shallow grooves due to abrasion and the less frictional force between the contact surface, which is evident from Figure 7c,d. The sample Al6061-1%Gr-6% B₄C (S2) has severe smearing and patchy surface in the as-fabricated condition. After the T6 condition, micro-cracks, and deep grooves were observed in the microstructure due to the clustering of graphene and B₄C. Figure 8 represents EDAX analysis with mapping of as-fabricated and T6 temper heat-treated samples. From the mapping, it is observed that some Fe and oxides were found in both samples. The Al, Mg, Si, B, C, Cr, and Zn atom peaks were observed in the composite samples. The Fe content was deposited from the EN31 steel disc to aluminum pin composite during the friction and abrasion between the surfaces.

3.1.5. Surface Roughness Analysis

The worn surface of the as-fabricated and T6 temper heat-treated composite samples was analyzed to measure the surface roughness after the wear test.

Table 3. Surface roughness values of Al6061-Gr-B₄C at as-fabricated and T6 temper conditions.

S. No	Sample Description	Ra (μm)	Rq (μm)	Rz (μm)
1.	AA 6061-F	10.096	12.577	51.872
2.	AA 6061-T6	8.174	10.329	42.587
3.	Al6061-0.5%Gr-6% B4C-F	5.023	7.058	31.644
4.	Al6061-0.5%Gr-6% B4C-T6	3.389	4.385	20.513
5.	Al6061-1%Gr-6% B4C-F	6.261	8.568	30.534
6.	Al6061-1%Gr-6% B4C-T6	4.183	5.568	26.313

and Figure 9. represent the surface roughness data and the roughness wave graph of Al6061/Graphene/B₄C at as-fabricated and T6 temper conditions. From the table and figure it is observed that the Al6061-0.5%Gr-6% B₄C (S1)-T6 condition sample exhibits lower surface roughness value compared to other fabricated and heat-treated samples.

To analyze the wear behaviour, wear tests were conducted, followed by surface roughness measurements on the worn surfaces. The provided Table 3 presents roughness parameters: Ra (average roughness), Rq (root mean square roughness), and Rz (maximum height roughness). The results indicate that the unreinforced AA 6061-F sample exhibited the highest surface roughness values (Ra = 10.096 µm, Rq = 12.577 µm, Rz = 51.872 µm), signifying severe wear and material removal. With the T6 treatment, the roughness values decreased (Ra = 8.174 µm), suggesting an improved wear resistance due to increased hardness. In reinforced composites, the inclusion of 6% B₄C and varying graphene content significantly reduced surface roughness, indicating lower material loss and improved tribological performance. The Al6061-0.5%Gr-6%B₄C-F composite exhibited a roughness of Ra = 5.023 µm, while its T6-treated counterpart showed a further reduction (Ra = 3.389 µm). Similarly, the Al6061-1%Gr-6%B₄C-F composite had a slightly higher roughness (Ra = 6.261 µm), which was further reduced to 4.183 µm after T6 treatment.

The wear mechanism can be explained by the synergistic effect of the reinforcements. The B₄C particles improve hardness and act as load-bearing elements, reducing direct metal-to-metal contact and minimizing material removal. Graphene enhances tribological properties by forming a protective lubricating layer on the worn surface, reducing friction and wear. The reduction in surface roughness with T6 treatment is attributed to the higher hardness and strength obtained from the precipitation-hardening process, which enhances the composite’s ability to withstand wear [21,28]. Figure 9 depicts the wave profile of the worn surface of all the 3 different composites prior and subsequent to the T6 heat treatment.

3.2. Corrosion Behaviour

3.2.1. Salt Spray Corrosion

The salt spray corrosion test was conducted to analyze the corrosion behaviour of as-fabricated and T6 temper heat-treated Al6061 hybrid matrix composites. The corrosion rate was calculated using Equation (3) below [29].

Corrosion Rate (mm/year) = {K × ΔW}/{ρ × A × t}

(3)

where K is the corrosion constant (87.6 for mm/year), ΔW is the mass loss in g (initial–final weight), ρ is density (2.69 g/cm³), A is surface area in cm², and t is exposed time (48 h).

According to Figure 10 below, the corrosion rate and mass loss were discussed. Compared to all the other samples, sample Al6061-0.5% Gr-6% B₄C (S1) has a low corrosion rate, which is 0.0109 mm/year, which is T6 heat-treated, and the fabricated sample has 0.0176 mm/year. The weight loss is 0.016 g for T6 and 0.026 g for fabricated conditions. The Al6061 as-fabricated and T6 conditions sample exhibits a higher corrosion rate of 0.0516 mm/year and 0.0353 mm/year, respectively, compared to other samples. The sample Al6061-1% Gr-6% B₄C (S2) incurred a corrosion rate of 0.0326 mm/year and 0.0216 mm/year in the as-fabricated and T6 temper conditions, respectively, and the mass loss is 0.032 g and 0.048 g. The sample Al6061-0.5% Gr-6% B₄C (S1)–T6 condition has a lower corrosion rate because of the lower graphene content and T6 heat treatment.

The mechanism that influences this corrosion resistance is grain refinement and precipitation strengthening. After the T6 tempering heat treatment, the grains were fully refined, and the formation of Mg₂Si precipitates were observed in the matrix. Finer grains possess a greater number of grain boundaries, which can serve as impediments to the passage of corrosive species, such as chloride ions, into the material. These precipitates serve as obstacles for the dislocation movement, impeding the advancement of corrosion-induced cracks, while porosity and inclusions decreased following heat treatment. These defects may serve as nucleation sites for corrosion; thus, their mitigation can enhance the overall corrosion resistance. A denser and more homogeneous oxide layer offers enhanced corrosion resistance. A stable passive layer inhibits the chloride ion (Cl⁻) ingress, hence minimizing pitting corrosion [29]. Whereas the Al 6061 alloy and the Al6061/1% Gr/6% B₄C (S2) has a higher corrosion rate compared to the 0.5% graphene reinforced S1 sample; this is due to the agglomeration and excess graphene reinforcement (1% Gr) in the Al matrix.

Figure 11 represents the corroded surfaces of as-fabricated and T6 tempered Al6061-Graphene-B₄C hybrid composites, which were analyzed using FESEM for the surface analysis. From this microstructure, pitting corrosion and visible grain boundaries were observed. In the fabricated composites cracks propagation and oxide films were observed. The AA6061 alloy composites show more pitting corrosion in an acidic environment. From Figure 11d) The 0.5 wt.% graphene-reinforced S1 sample shows less corrosion and refined grain boundary because of T6 heat treatment. Both S2 and AA6061 alloy have higher corroded surfaces and micro-cracks in the micro-cracks surface. The SEM microstructure analysis confirms that T6 heat treatment significantly improves the corrosion resistance of Al6061-Gr-B₄C composites. Before the T6 treatment, corrosion is more severe due to poor interfacial bonding, residual stresses, and microvoids, leading to deeper pits and extensive corrosion product formation. After the T6 treatment, refining the microstructure, could lead to an improved corrosion resistance. A refined microstructure may reduce the number of preferential sites for pitting initiation, potentially resulting in less pitting corrosion and a smoother surface after exposure to corrosive environments [21,29]. These improvements make T6-treated composites more suitable for applications requiring enhanced wear and corrosion resistance.

3.2.2. Electrochemical Corrosion

The electrochemical corrosion test was conducted to analyze the corrosion behaviour of the graphene and boron carbide-reinforced Al 6061 alloy matrix composites before and after the T6 heat-treated conditions. And the Linear Polarization Resistance (LPR) method is employed to assess the polarization resistance of a metallic surface in a 3.5% NaCl corrosive medium. The sample is a working electrode, mercury is the reference electrode, and graphite is the counter electrode. The corrosion results are tabulated in

Table 4. Electrochemical corrosion results of Al6061-Graphene-B₄C at as-fabricated and T6 temper conditions.

S. No	Sample Description	Ecorr (V)	Icorr (μA/cm2)	Corrosion Rate
				(mm/year)	(mpy)
1.	AA 6061-F	−0.472	6.842	0.075	2.952
2.	AA 6061-T6	−0.357	4.864	0.053	2.086
3.	Al6061-0.5%Gr-6% B4C-F	−0.789	3.592	0.039	1.532
4.	Al6061-0.5%Gr-6% B4C-T6	−0.814	2.514	0.027	1.062
5.	Al6061-1%Gr-6% B4C-F	−0.842	8.762	0.096	3.779
6.	Al6061-1%Gr-6% B4C-T6	−0.896	4.213	0.045	1.836

. Table 4 and Figure 12, illustrate the synergistic effect of the T6 heat treatment and the addition of both graphene (Gr) and B₄C reinforcements on the corrosion behaviour of the Al6061 alloy.

The findings indicate that the integration of a T6 heat treatment with a minimal graphene addition (0.5 wt.%) significantly improves corrosion resistance. In the as-fabricated (F) condition, the AA6061/0.5%Gr/6%B₄C composite demonstrates an Ecorr of −0.789 and Icorr of 3.592 µA/cm² and a corrosion rate of 0.039 mm/year. Transitioning to the T6 condition further decreases Icorr to 2.514 µA/cm², Ecorr value of −0.814, and the corrosion rate to 0.027 mm/year, the lowest recorded among all tested samples. In comparison, the base alloy (AA6061-F) exhibits a higher Icorr of 6.842 µA/cm², and Ecorr value of −0.357 and a corrosion rate of 0.075 mm/year, which decreases to 4.864 µA/cm² and Ecorr value of −0.472 and corrosion rate of 0.053 mm/year following T6 treatment, demonstrating the advantageous impact of heat treatment even in the absence of reinforcement. The open-circuit potential (Ecorr) values in these Al6061 composites indicate that the T6 treatment and the incorporation of moderate graphene levels (0.5 wt.%) with boron carbide (B₄C) might elevate the alloy’s corrosion potential towards a more positive (noble) direction. This signifies a diminished thermodynamic propensity for corrosion, mostly due to two interrelated factors, microstructural refinement, and passive film stabilization [30].

The T6 treatment facilitates the production of finely dispersed Mg₂Si precipitates and other stable second-phase particles, thereby homogenizing the microstructure and reducing local galvanic cells. These homogenized reinforcement particles, in turn, stimulate the formation of a more resilient passive coating on the surface of Al6061 alloy matrix composites. This enhanced microstructure facilitates the development of a more uniform, protective passive coating on the alloy surface. Moreover, B₄C particles, because of their chemical stability and inertness, serve as a formidable barrier that restricts the aluminum matrix’s exposure to the corrosive environment. Its robust ceramic composition can also mitigate micro-scale damage or micro-pathways for corrosive chemicals. Simultaneously, low-content graphene (0.5 wt.%) can enhance this barrier effect due to graphene’s high aspect ratio and chemical inertness, hence reducing localized electron transfer channels that initiate galvanic corrosion. As a result, the AA6061/0.5 wt.% Gr/6 wt.% B₄C-T6 composite attains a uniformly distributed reinforcement phase and a more stable passive film, which together lead to a significant decrease in corrosion current and corrosion rate [31].

4. Conclusions

This research involves the incorporation of 0.5% and 1% wt.% graphene with 6% wt.% boron carbide particles, into an Al6061 matrix, resulting in the effective fabrication of matrix composites by the powder metallurgy method. The as-fabricated Al6061matrix composites underwent T6 temper heat treatment. The study examined the tribological and corrosion resistance properties to investigate and confirm the key inferences between the T6 temper condition sample and the produced composites. The specific inferences are as follows:

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The Al6061-0.5% Gr-6% B₄C (S1) exhibits good wear resistance after the T6 heat treatment condition compared to the fabricated samples. This sample shows less wear loss, lower mass loss, and reduced COF value at a load of 10 N, 20 N, and the sliding velocity of 0.5 and 1 m/s.

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The synergistic effect of graphene’s lubricating property and B₄C’s load-bearing capacity helps to resist wear loss by forming the oxide layer on the composite surface. The worn surface SEM microstructure reveals the change in wear from abrasive to adhesive wear at a higher load and sliding velocity.

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The steady state wear condition was observed in the graphene and B₄C reinforced AA6061 alloy. The SEM images of the worn surface show the microcracks, delamination of layers, and deep grooves. The sample AA6061/0.5%Gr/6%B₄C exhibits a lower surface roughness value after the T6 condition. The Ra value is 3.389 µm and Rz is 20.513 µm.

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Al6061/0.5% Gr/6% B₄C (S1) has a low corrosion rate of 0.0109 mm/year when heat-treated T6 and 0.0176 mm/year when fabricated. The weight reduction is 0.016 g for T6 and 0.026 g for fabricated. In the acidic environment, refined grains, secondary phases, and the formation of an oxide layer on the surface decrease corrosion after the T6 condition.

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In the as-fabricated (F) condition, the AA6061/0.5%Gr/6%B₄C composite demonstrates an Ecorr of −0.789 and Icorr of 3.592 µA/cm² and a corrosion rate of 0.039 mm/year. Transitioning to the T6 condition further decreases Icorr to 2.514 µA/cm², Ecorr to −0.814, and the corrosion rate to 0.027 mm/year.

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The results show that an increase in the addition of graphene wt.% from 0.5 to 1 to Al 6061 alloy matrix, deteriorated the wear and corrosive properties of the hybrid matrix composites.