Experimental investigation in enhancing the mechanical, wear and corrosion resistance properties of Al6061-SiC-NSA composites fabricated using stir casting process

The study investigates the mechanical, and tribological properties of Al6061-SiC-NSA hybrid composites. The characterization via FE-SEM reveals NSA particle’s crystal structure and agglomeration, with an average particle size of approximately 400 nm. The EDS analysis confirms the presence of oxides (TiO2, SiO2, Fe2O3, Al2O3) along with calcium and potassium. The XRD spectra corroborate these findings, additionally identifying Calcite and intermetallic compounds. The Al6061–5SiC-10NSA composites has improved compressive strength of 254 MPa compared to 220 MPa for base Al6061 alloy. The tensile strength of base Al6061 (100 MPa) decreased to 70 MPa for Al6061–5SiC-20NSA composites. The tensile strength decreases with increasing NSA content. Fracture analysis indicates ductile fracture mechanisms, supported by FE-SEM images displaying honeycomb-like structures and dimples. Impact testing reveals reduced impact strength in composites compared to Al6061 (26J3), with Al6061–10SiC-5NSA exhibiting the best toughness of 22.5 J3. Density decreases in composites, with density of Al6061–5SiC-20NSA composites has reduced to 2.40 g cm−3. The microhardness of Al6061–10SiC-5NSA has better value of 109 HV, whereas further addition of NSA has resulted in decrease in microhardness of composites. Pin-on-disc wear tests demonstrate improved wear resistance with increased NSA content, with Al6061–5SiC-20NSA outperforming other compositions. COF decreases with increased NSA content and sliding speed. FE-SEM analysis of worn surfaces reveals that the major wear mechanism is adhesive wear followed by delamination, further the oxide formation along the surface aiding wear resistance of the composites. Tafel testing indicates decreased corrosion potential with increased NSA content, with Al6061–5SiC-20NSA exhibiting improved corrosion resistance.


Introduction
Now a days the need for light weight and less denser materials for applications such as defence, automobile, aerospace and construction applications.The reason for this surge in research based on light weight materials is owing to their efficiency and low cost.Moreover these materials have the ability to overcome harsh environments such as corrosive and high temperature environments.Hence the light weight and less denser metal matrix composites with hard particle reinforcements such as B 4 C, TiC and SiC particles are considered for the synthesis of these low density composites.This hard particle reinforcement will improve the stability of this less denser composites [1][2][3][4].
Among Various metal matrix composites, the aluminium matrix composites have decent wear resistance, corrosion resistance and mechanical strength based on the type of reinforcement used.However the nano reinforcements have better properties compared to that of micro reinforcements.There are many studies carried out to improve the properties of Al 6061 by adding reinforcements such as Si 3 N 4 , SiC, Gr etc [5][6][7][8][9][10].
A study involving Al 2024 and Si 3 N 4 was conducted by Aayush Bhat et al and reported that the increase in silicon nitride reinforcement has improved the elastic modulus whereas drastically reduced the bending strength due to the brittle nature of silicon nitride particles [11].An another study conducted by Sharma et al involving Al 6082-T6 reinforced with Si 3 N 4 particles fabricated using stir casting process exposes that there is increase in mechanical properties of the composites [12].Anbuchezhiyan et al studied the influenze of Si 3 N 4 in Al6061 matrix synthesized using powder metallurgy process the results indicated that the mechanical properties decreases with increase in reinforcement [13].
Mahmut Can Şenel et al studied the influenze of Si 3 N 4 /Gr addition in Al matrix fabricated using powder metallurgy process.The results revealed that there is increase in compressive strength and wear resistance, whereas the hardness values has reduced with increase in Gr content [14].Bardeswaran et al explored the mechanical and wear resistant properties of Al 7075-Al 2 O 3 -Gr hybrid composites and reported that the coefficient of friction of the composites decreases with addition of 5% Gr and also for combinations containing Al 2 O 3 at 2, 4, 6 and 8 Wt% [15].Sathish et al explored the tribological performance of Al-7Si-Zr-SiO 4 composites under different temperatures.They reported that the composites exhibits much higher wear resistance compared to base Al alloy [16].
Raghav et al investigated the Mechanical and tribological performance of Al-Fe-SiC-Zr hybrid composites fabricated through powder metallurgy process.The results show that the density of Al-10Fe-10SiC-10Zr hybrid composites exhibited the high density of 3.44 g cm −3 .The increase in Zr content has influenced the micro hardness of the hybrid composites.The compressive strength of the Al-10Fe-10SiC-10Zr hybrid composites has increased with increase in SiC and Zr content and found to be around 205 MPa.The wear resistant properties also increased with increase in Zr Content [17].
Lokesh et al conducted studies on Gr reinforced Al6061 composites and found out the tensile strength of Al6061-4 Gr composites is more compared to other proportions and also found out that further increase in Gr content has reduced the ductility further than that of base Al6061 alloy [18].Gopinath et al reported the wear, corrosion and mechanical performance such as microhardness, tensile and compressive strength of Al6061-BN-Al 2 O 3 -C hybrid composites.The results shows that the microhardness and compressive strength of Al6061-30BN-10 Al 2 O 3 -5 C hybrid composites are 63HV and 187 MPa respectively which is improved compared to base Al6061 alloy.The electrochemical corrosion analysis also shows that the Al6061-30BN-10Al 2 O 3 -5 C hybrid composites have improved corrosion resistance compared to that of base Al6061 alloy [9].
Reddy et al investigated the Mechanical, tribological and electrochemical properties of Al-6061-T6 composites reinforced with rice husk ash and bakelite powder.The results shows that wear resistance of the composites increases by 15% and the hardness of the composites increases by 8.5% compared to that of Al-6061-T6 alloy.The electrochemical corrosion behaviour of the composites has improved compared to that of base alloy [19].
Satheesh et al investigated the physical and mechanical properties of Al6061-SiC-Coconut shell ash hybrid composites through double stir casting process.The results show enhanced mechanical properties in Al6061-SiC-8CSA Al6061-SiC-8CSA, with a 46% increase in hardness and 47.31% in tensile strength.However, 10% CSA addition led to a 5% decrease in hardness and 14.80% in tensile strength compared to the 8% CSA hybrid.Despite this, wear loss was lower overall, attributed to CSA's cushioning effect [20].
Borisade et al investigated the integration of waste materials, notably valoxy, as reinforcement in Al-6063based composites, aiming to enhance sustainability and cost-efficiency.Composite samples with varying Al 2 O 3 :Valoxy ratios were fabricated using a double stir casting approach, with results showcasing valoxy's potential sustainability benefits.Notably, samples with 7.5:2.5 and 5:5 ratios exhibited improved properties compared to both the base alloy and Al-6063/Al 2 O 3 samples.These composite configurations demonstrated enhanced hardness (46.45 and 46.21 HB), tensile strength (209.2 and 184.8 MPa), and fracture toughness (6.54 and 4.67 MPa m 1/2 ), respectively.Additionally, the density of the composites decreased with increasing valoxy content, attributed to the lower density of valoxy (1.4 g cm −3 ) compared to alumina (3.89 g/cm 3 ).This research highlights valoxy's promise as a sustainable reinforcement material, offering potential performance improvements while contributing to eco-friendly practices in metal matrix composite fabrication [21].
Sahu et al investigated the mechanical properties of Al 7075/B 4 C/Fly-ash composite synthesized using stir casting method Analysis via SEM and optical micrographs confirms even particle distribution.Micro-hardness tests demonstrate uniformity, supporting potential applications in automotive and aerospace industries requiring lightweight, high-temperature resistant, and wear-resistant composites [22].
Further the researches were carried out using bio waste by-products such as Rice Husk Ash, bagasse ash, Bamboo Leaf Ash and Ground nut shell ash has been documented in various research articles as one of the efficient reinforcements owing to their better properties such as low density, availability , low cost and eco friendliness [23][24][25].
This study introduces a novel method for synthesizing hybrid nanocomposites, combining Al6061 with silicon carbide (SiC) and nutmeg shell ash (NSA) reinforcement via the stir casting process.SiC was deliberately selected as the primary reinforcement to bolster the structural integrity of the Al6061 matrix, while simultaneously enhancing critical properties such as melting point, hardness, corrosion resistance, and wear resistance.Additionally, the incorporation of NSA as a secondary reinforcement offers a sustainable solution, utilizing a byproduct obtained from burning nutmeg shells, a form of biological waste.NSA particles inherently contain various oxide constituents, which play a pivotal role in augmenting the mechanical properties of the composite material.This innovative approach not only advances the development of high-performance materials but also addresses environmental concerns by repurposing waste materials.By utilizing NSA as a secondary reinforcement, the study aligns with the principles of green chemistry, promoting sustainability and minimizing ecological footprint.The resulting Al6061-SiC-NSA hybrid nanocomposites exhibit enhanced mechanical properties, thereby showcasing their potential for applications in industries demanding robust and eco-friendly materials, such as automotive, aerospace, and construction.
Hence in this work an attempt is made to fabricate a Al6061-SiC-NSA hybrid nano composites and explore their tensile strength, impact strength, wear, microhardness, compressive strength, and corrosion resistant properties and a potential proportion of Al6061 based composite material is proposed for various applications such as automotive engines and aeroplane parts.

Materials deployed
In this research work, Al-6061 grade alloy is utilized as base material as shown in table 1.The SiC particles purchased from Merck, India is used as primary reinforcement.The Nutmeg Shell Ash (NSA) is synthesized using muffle furnace by heat treating the Nutmeg shell at 800 degree Celsius for 3 h and used as secondary natural reinforcement.The fabrication of the Al6061-SiC-NSA hybrid composite was carried out employing stir casting process.The stir casting setup depicted in figure 1 comprises a furnace, a reinforcement feeder, and a mechanical stirrer.The furnace serves the dual purpose of heating and melting the materials.Employing a bottom pouring furnace is particularly advantageous for stir casting as it allows for immediate pouring of the mixed slurry post-stirring, preventing the settling of solid particles in the crucible bottom.The mechanical stirrer facilitates the formation of a vortex, essential for mixing the reinforcement materials introduced into the  melt.It consists of a stirring rod and an impeller blade, the latter available in various geometries and blade numbers.Preference is given to flat blades with three blades, as they induce an axial flow pattern in the crucible with minimal power consumption.This stirrer is connected to variable-speed motors, with the rotation speed regulated by the attached motor regulator.Additionally, the feeder, affixed to the furnace, is utilized for introducing the reinforcement powder into the melt.Various molds, such as permanent molds, sand molds, or lost-wax molds, can be employed for pouring the mixed slurry.The maximum working temperature of the furnace is 1200 °C and the maximum capacity is 700 g.The stirrer speed and impeller angle is 390 rpm and 45°r espectively.

Synthesis of nutmeg shell ash (NSA)
The severed pieces of the nutmeg shell were oven dried at 100 °C for 3 h to remove the water molecules.Finally, the oven dried nutmeg shell were heated at 800 °C in the muffle furnace for 3 h.Following the heat treatment the ash is allowed to cool inside the muffle furnace.The cooled Ash is then separated as per the particle size using 500 micron sieve as shown in figure 2.

Synthesis of Al6061-SiC-NSA hybrid composites
The Al6061-SiC-NSA hybrid composites were developed using the stir casting process with different proportions of reinforcements, which is shown in table 2. Initially, the Al 6061 ingots was heated to 850 °C so as to melt, and then the reinforcements were preheated to 200 °C and then added to the molten Al6061 matrix.After adding the reinforcements, in order to maintain uniform solubility of the reinforcements, the graphite stirrer is used to continuously stir the molten composite solution at 500 rpm for the duration of 10 min.Then the Al6061-SiC-NSA hybrid composites were transferred into the rectangular mould and consolidated as a solid composite.

Density analysis and porosity
The experimental and theoretical densities for the Al6061-SiC-NSA hybrid composites were calculated using original weight of the sample and based on rule of mixtures respectively.The porosity of the hybrid composites was calculated based on the experimental densities.

Tensile and compressive test
The casted Al6061-SiC-NSA hybrid composites is machined using Electrical Discharge Machine (E.D.M.) so as to prepare specimen as per the ASTM E8 M-11 standard.The universal testing Machine was utilized to study the tensile strength of the given specimen.The compression test of Al6061-SiC-NSA hybrid composites was also carried out using universal testing Machine as per the ASTM E9-89 standard.

Micro hardness test
The ability of the Al6061-SiC-NSA hybrid composites to withstand indentation was explored using Vickers micro hardness test.The specimens were prepared and tests were carried out as per the ASTM E384-17 standards.The maximum load applied was 1 Kg and dwell period is maintained as 15 s.The test was carried for five different trials and the average value is determined and studied.

Impact test
The impact strength of the hybrid composites was tested using Charpy test following ASTM E23 standard.The energy absorbed by the hybrid composites before fracture is calculated and reported.

Wear analysis
The pin-on-disc wear behaviour for the casted Al hybrid composites was explored using the pin-on-disc method through the standards of ASTM G99-05.The sliding position between the EN 32 steel discs with respect to the sample was placed perpendicularly.The digital weighing balance with 0.001 mg resolution was used to measure the weight of the specimens before and after the test.The computer-assisted data logging system was deployed for analysing the friction mechanism during the trial.The wear rate was calculated for the fabricated composites under different loading conditions like 5, 10, 15, and 20 N. The sliding velocity was kept varied from 0.5 m s −1 , 1 m s −1 , 1.5 m s −1 , and 2 m s −1 with the varying sliding distance from 500 m, 1000 m, 1500 m and 2000 m respectively.

Electrochemical corrosion test
As far as the electrochemical corrosion test is concerned, the corrosion potential (E corr ) and current density (i corr ) were calculated through the potentiodynamic polarization technique.The electrochemical work station setup (Biologic Sp-150) was used with the platinum wire as the counter electrode, Ag/AgCl as the reference electrode.The casted sample was kept as the working electrode.5 wt% NaCl was used as the electrolytic medium for the experiment with −0.5 V to 0.5 V as the initial and final voltage.The potentiodynamic polarization graph was plotted with the scan rate of 10 mV s −1 for the casted composites.The corrosion potential (E corr ) plots and corrosion current density (I corr ) were obtained by interpreting the Tafel extrapolation method and using the EC-lab software.

Characterization of the composites
In this research study, the characterization of the composites was well executed through the effect of Scanning Electron Microscopy (SEM.),Energy Dispersive spectrum (EDS.), and x-ray Diffraction (XRD) application and the corresponding microstructure, lamina morphology, and elemental proportion of the casted composites were clearly examined.The microstructure, surface morphology, and elemental compositions of the casted samples were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive spectrum (E.DS), and x-ray Diffraction (XRD) application.

Characterization of the hybrid composites
The figure 3 is the FE-SEM image of Nutmeg Shell Ash (NSA) particles which is taken using Secondary electron mode at 30X magnification.The structure of the NSA particles are crystal structure in nature and found to be agglomerated.The particle size of NSA particles is approximately 400 nm.The figure 4 shows the EDS image of NSA particles.From the EDS analysis it is confirmed that there is presence of oxides such as TiO 2 , SiO 2 , Fe 2 O 3 and Al 2 O 3 and also we can confirm the presence of Calcium and potassium in the ash.

Tensile and compressive test
A tensile and compressive tests (n = 5) were performed in all samples of different proportions and the results were shown as graph in figure 6.The results confirm that there is improvement in compressive strength of the composites with increase in NSA content, but if the NSA is added above 15% there is no uniform dispersion NSA particles thereby resulting in the reduction of compressive strength of the composites.Moreover the agglomeration of reinforcements also a reason for reduction in compressive strength of the composites.The compressive strength of Al6061 is 220 MPa and it increases up to 254 MPa for Al6061-5SiC-10NSA and it reduces to 240 MPa for Al6061-5SiC-15NSA.The tensile strength of the composites decreases with increase in NSA content.The tensile strength of Al6061 is 100 MPa where as it reduces to 70 MPa for Al6061-5SiC-20NSA composites.The Al6061-10SiC composites has a tensile strength of 88 MPa.This reduction in tensile strength     was due to the increase in brittleness of the composites due to the addition of SiC and NSA particles.Moreover the improper distribution of reinforcement and improper interfacial adhesion of NSA and SiC particles were the reason for this reduction in tensile strength.In order to study the tensile fracture mechanisms of the composite material, the fractured samples were subjected to FE-SEM analysis and the morphology of the fractured samples were explored.The figure 7 represents the FE-SEM image of tensile fractured composites.It can be noted that the morphology of fractured surface resembles honey comb like structure and also possesses holes at some locations, which confirms the fracture mechanism is ductile fracture.The presence of dimples also confirms that the fracture is due to ductile fracture.It can be also noted that there is river like tearing ridge also known as cleavage planes with the evidence of plastic deformation, in addition to dimples at the fractured surface [1,24].

Impact test
The impact test also known as Charpy test was carried out as per the ASTM E23 standard so as to understand the standard deviation of the results.The results are plotted as graph as shown in figure 8. From the graph it is evident that the impact strength of the Composite materials has reduced compared to Al6061 (26 J 3 ).Also Al6061-10SiC composites exhibits impact strength of 24 J 3 compared to other samples.Among the reinforced samples, the Al6061-10SiC-5NSA has better toughness of 22.5 J 3 .The reduction in impact strength observed in the composite materials, compared to the base Al6061 alloy, can be attributed to several factors.Firstly, the incorporation of reinforcement materials, such as SiC and NSA, may introduce discontinuities or weak interfaces within the composite structure, leading to stress concentration points and reduced overall toughness.Additionally, variations in particle distribution and bonding between the matrix and reinforcements can affect the composite's ability to absorb and dissipate energy during impact loading.Furthermore, the specific combination and concentration of reinforcement materials can influence the composite's mechanical properties, including impact strength.In the case of Al6061-10SiC-5NSA composite, the improved toughness compared to other reinforced samples may be attributed to a more favourable balance of reinforcement materials and their interaction with the aluminium matrix, resulting in enhanced energy absorption capabilities [9].

Density and porosity test
The density of the Al6061 alloy is 2.7 g/cm 3 .The composite materials have recorded decrement in the density compared to Al 6061 base alloy.The Al6061-10SiC composite has lesser density value 2.65 g cm −3 compared to that of base Al6061, whereas the Al6061-10SiC-5NSA composites have density of 2.55 g cm −3 which is best among all propositions of the composites having NSA reinforcement.Also the porosity of the Al6061 is found to be around 3% and it is reduced to 2. % for Al6061-10SiC composites and further increases with increase in NSA reinforcement up to 4% for Al6061-5SiC-20NSA composites as shown in figure 9.The decrease in density observed in the composite materials compared to the base Al6061 alloy can be attributed to the lower density of the reinforcement materials, such as SiC and NSA, relative to aluminium.Specifically, the incorporation of SiC and NSA particles into the aluminium matrix reduces the overall density of the composite due to their lighter weight [27].

Microhardness test
The microhardness test were carried out for all propositions of composites and represented as graph as shown in the figure 10.From the experimental results it is clear that the microhardness of the Al6061 alloy is 95 HV and Al6061-10SiC composite has the microhardness value of 100 HV, which is due to the hard nature of SiC secondary reinforcement.The microhardness of the Al6061-10SiC-5NSA composites increased up to 109 HV and then the microhardness started decreasing with further addition of NSA particles, which is due the improper adhesion of NSA particles with base Al6061 alloy [5].

Wear analysis
The wear loss of the Al6061-SiC-NSA composites were assessed using a pin-on-disc apparatus.Wear loss measurements were conducted at various sliding speeds (0.5 m s −1 , 1 m s −1 , 1.5 m s −1 , and 2 m s −1 ) to evaluate the composites' resistance to different velocities.Additionally, wear tests were performed under varying loads to analyze the composites load-bearing capacity.The evaluation also included testing the composites at different sliding distances.Figures 11(A)-(C) represents the wear loss of Al6061-SiC-NSA composites at different loading    enhanced the friction resistance of the composites as shown in figure 12(B).From the figure 12(C) it is understandable that the COF values decreases with increase in sliding speed.This decrease in COF value at higher sliding speed is due to the reduction in contact between the specimen surface and disc over which the specimen is sliding.Overall the composites with high NSA reinforcement has better wear and friction resistant properties [15,16].Figures 13(A)-(C) exhibits the FE-SEM images of Al6061-5SiC-20NSA composites before and after wear test.The figure 13(A) shows the surface of the Al6061-5SiC-20NSA composites before wear test.The figures 13(B) and (C) represents the worn our surface of Al6061-5SiC-20NSA composites after wear test.From the FE-SEM analysis of worn out surface, it can be established that the mode of wear mechanism was adhesive and there is no evidence of adhesive wear.The adhesive mechanism is then followed by delamination.This is due to formation of oxide layers at the surface of the composite specimen.Figure 14.Shows the EDS spectra of  Al6061-5SiC-20NSA composites before wear test.From the image it is clear that there is oxide formation at the surface of the specimen.It is evident there is presence of MgO Al 2 O 3 and SiO 2 which helps in improving the wear resistance of the composites [17].

Electrochemical corrosion test
The potentiodynamic polarization also known as tafel test was used to study the corrosion behaviour of the Al6061-SiC-NSA hybrid composites as shown in figure15.The 3.5% NaCl solution was used as electrolyte to study the corrosion behaviour of Al6061-SiC-NSA hybrid composites.The E corr value, which is also known as corrosion potential is found to be reduced with the increase in NSA reinforcement.The Al6061-10SiC has better E corr value compared to Al6061 base alloy.The addition of NSA particles has increased the corrosion rate and hence the E corr value of Al6061-5SiC-15NSA composites is −0.915 ± 0.018 V which is increased from −0.214 ± 0.002 V for Base Al6061 alloy because of improper dispersion and formation of more galvanic corrosion sites.But interestingly Al6061-5SiC-20NSA composites exhibits − 0.409 ± 0.012 V, which has improved compared to other compositions of the composites resulting in fewer galvanic corrosion sites.On the other hand the I Corr values of the Al6061-5SiC-20NSA composites was also found to be better compared to other samples, which is 0.98 μA cm −2 compared to 1.25 μA cm −2 of base Al6061 alloy.Hence from the results Al6061-5SiC-20NSA composites has better corrosion resistance among hybrid composites [28,29].

Conclusion
In this study, the Al6061-SiC-NSA hybrid composites were produced by Stir casting process.The microhardness, Tensile strength , compressive strength , wear and corrosion properties were premeditated as per the ASTM standards.
• The Al6061-5SiC-10NSA composites has improved compressive strength, which is 254 MPa compared to 220 MPa of base Al6061.The Al6061 base alloy has higher tensile strength of 100 MPa and it reduces to 70 MPa for Al6061-5SiC-20NSA composites with increase in NSA content.
• The impact strength of the Al6061-SiC-NSA composites decreases with increase in NSA reinforcement.
• The density of the Al6061 based composites are found to be lesser compared to that of Al6061 base alloy.The density of Al6061-5SiC-20NSA composites has reduced nearly by 11% to that of base alloy.The porosity of base Al6061 is 3% it further increases up to 4% with increase in NSA content.
• The microhardness of the Al6061 base alloy is 95 HV.The Al6061-10SIC-5NSA composites have better microhardness of 109 HV.
• The Al6061-5SiC-20NSA hybrid composites exhibits better wear and friction resistance at all sliding conditions.The wear mechanism was identified as adhesive wear.• The corrosion study revealed that the Al6061-5SiC-20NSA composites have improved corrosion resistance among hybrid composites.
• From the experimental exploration of Al6061-SiC-NSA composites, it can be established that the Al6061-5SiC-20NSA composites has the better corrosive strength and wear resistance.Even though Al6061-5SiC-20NSA composites has somewhat lesser tensile strength and microhardness, because of its light weight along with good wear and decent corrosion properties the Al6061-5SiC-20NSA composites can be considered for various aerospace and automobile applications.

Figure 5
Shows the XRD spectra of nutmeg shell ash particles.The XRD analysis also confirms the presence of Calcium,TiO 2 , SiO 2 , Fe 2 O 3 and Al 2 O 3 particles in the ash.The JCPDS database confirms the formation of Calcite (CaCO 3 ).It is also noted that there is a formation of calcium chloride Aluminium Silicate intermetallic compounds.Another important finding is the existence of brinrobertsite clay which is the clay having two alternate layers smectite and pyrophyllite.The hydrothermal activity is the reason for the formation Brinrobertsite clay[26].

Figure 12 .
Figure 12.Coefficient of friction of Al6061-SiC-NSA composites at different [A] load, [B] sliding distance and [C] sliding speed.

Table 2 .
Allocated proportions of matrix and reinforcement for the Al6061-SIC-NSA composites.