Mechanical, and corrosive properties of AA7075 aluminium reinforced with rice husk ash particulates

The mechanical and corrosive properties of AA7075 alloy reinforced with rice husk ash (RHA) particles were studied. AA7075 matrix composite reinforced with varying percentage weight compositions of rice husk ash particles (5%, 10%, 15% and 20% wt) were prepared using stir casting technique. The mechanical, and corrosive properties of the new material were compared with that of the matrix alloy. The mechanical properties were also improved due to the automated stirring action employed during the casting process. AA7075-RHA composites had improved mechanical properties compared to the base matrix (AA7075). In terms of tensile strength there was an increase in this mechanical property with an increase in weight fraction of RHA particle (51% at 10% RHA), hardness (25.67% at 20% RHA), while for impact resistance, the reverse was the case as there was a 10.98% decrease in the resistance of the composite as the reinforcement fraction increased from 0%–20% RHA. The progressive decreases in the impact strength of the reinforced composite were probably due to agglomeration and non-homogeneity at higher reinforcements values. Corrosion experiments carried out showed improved properties in the reinforced composite compared to the unreinforced alloy, the corrosive properties improved with an increase in weight fraction of RHA in terms of weight loss (20.63% at 10% RHA), potentio-dynamic polarization (30.7% at 10% RHA) and open circuit potential (1.60% at 20%RH). The developed composite may be suitably applied in high-speed rotating shafts and automotive engineered brake parts.


Introduction
The pace of innovation has resulted in an increasing demand for superior materials that have a good combination of various desirable properties.Strength, toughness, surface hardness, high corrosion resistance, ductility, good weldability, and machinability are all desirable properties while maintaining a lower weight than other materials.To meet this demand, various composite materials have been developed on a continuous basis [1,2].
Aluminium matrix composites (AMCs) are a significant class of advanced technical materials in today's world.AMCs have a unique combination of characteristics that are sought after in a variety of industries and can replace traditional aluminium alloys in a variety of applications [3].The cost of manufacturing AMCs is considerably high, which may limit their extensive use.One way to reduce production costs is to use industrial waste and natural minerals as reinforcing particles [4].In comparison to traditional ceramic particles, rice husk ash (RHA) provides a cost-effective reinforcement.RHA is a type of agricultural waste that is abundant over the world.Rice, bran, and husk are the main components of milled paddy.This husk is used as fuel by rice mills to generate steam for the parboiling process.The volatile substance in the husk evaporates during the burning process, and the leftover husk is converted into RHA [5,6].
Common agricultural waste disposal practices involve techniques like depositing waste in undeveloped areas, along roadsides, or into water bodies.There are instances of open burning, and at times, the remains and their ashes are used as fertilizers.These conventional approaches result in the release of CO 2 through burning and disposal, while further contributing to the problem, methane and other gases are emitted as the agricultural waste decomposes [7,8].Rice husks poses a significant environmental risk, causing damage to the soil and the surrounding area where it is discharged.According to current research findings, RHA comprises approximately 85%−90% amorphous silica [9].Thermal treatment turns silica to crystobalite, a crystalline form of silica.Amorphous silica with high reactivity, ultra-fine size, and enormous surface area is created under controlled burning circumstances.This micro silica can be used to make advanced materials such as SiC, Si 3 N 4 , elemental Si, and Mg 2 Si [10].
This study is dedicated to the investigation of an innovative methodology involving the amalgamation of rice husk ash with aluminium to develop matrix composites utilizing the stir casting technique.Multiple weight ratios of rice husk particles are systematically explored.The primary objective of this research is to conduct a comprehensive assessment of the mechanical, electrochemical, and optical attributes of the resulting composites featuring rice husk ash.This effort is driven by the goal of mitigating environmental impact and optimizing costefficiency within the realm of advanced matrix composites (AMCs) through the development of a novel material characterized by superior performance properties.

Materials
Aluminium alloy 7075 (AA7075) was used as the matrix material.The chemical composition of the matrix material and the RHA particles was determined via x-ray flourescence (XRF).The chemical composition of the matrix material is given in table 1. RHA particulate with an average size of 300 μm was used as the reinforcement material.The chemical composition of RHA is given in table 2.

Composite preparation
Stir casting (with schematic shown in figure 1 was used to develop the aluminium metal matrix composite.A mechanical stirrer is used in the stir casting technique of manufacturing metals to uniformly mix the reinforcement that is introduced into the matrix material.This method was chosen for this study due to its distinct properties in terms of accessibility, simplicity, and affordability.A conventional stir casting system includes a mechanical stirrer, reinforcement particulate feeder, and furnace.The fundamental function of the furnace is to heat and melt the matrix materials prior to casting. The samples were developed by altering the percentage by weight of reinforcement particles from 0 wt% to 20 wt% at 5 wt% intervals, with the balance being the base matrix (AA7075) following the specifications of ASTM B179 standard [11].
In total, 5 samples of different compositions were prepared.Table 3 shows the charge calculation for the metal matrix composite.
All the necessary equipment and materials were assembled, and all personal protective equipment was donned.The reinforcements and matrix were divided according to the mix design.Two patterns were prepared, A cylindrical pattern (200 mm long with a diameter of 20 mm) and cuboid (25 mm × 15 mm × 200 mm).Using the patterns prepared, 10 different mold samples were created: 5 cylindrical molds and 5 rectangle molds.The furnace was ignited, and the metal (base matrix) was introduced at 700 degrees into the oil-fueled tilting furnace.The reinforcement was then placed in the steel cup and fired at 700 °C for one minute.Molten metal was poured and mixed into the steel cup containing the fired forming a composite slurry which was stirred continuously for a period of 10 min at a speed of 400 rpm.The slurry was then poured into the hollow molds and allowed to solidify.This process was repeated for the four alternative material compositions and the control sample.

Microstructural analysis
The samples used for microstructural analysis were each machined to 15 × 15 × 5 mm.Sample surfaces for optical microscopy test were prepared particularly by grinding with various sizes of emery (silicon carbide) papers ranging from 150, 320, 400, 600, 800, and 1200 grits.Polishing cloth and alumina as a polishing paste were then used to polish the specimen.The samples were etched with sodium hydroxide after polishing, and the microstructure was viewed with an optical microscope at magnification of ×100.

Mechanical behaviour 2.4.1. Tensile test
At a predetermined grip separation, samples were inserted into the grips of a Universal Test Machine and pulled until failure occurs.The material specification or the time to failure can be used to calculate the ASTM D3039 test speed (1 to 10 min).A typical test speed for standard test specimens is 2 mm min −1 (0.05 in/min).An extensometer or strain gauge was used to quantify elongation and tensile modulus.

Hardness test
The Vickers hardness of the samples was determined using the Future Tech FV 800 Vickers hardness testing machine.Bakelite was used to keep the samples from shifting when the weight was applied.A diamond indenter was used to indent the material for 10 s at a force of 5 kg.To counterbalance the segregation impact of the matrix reinforcements, four measurements were taken for each sample.

Impact test
The impact test was carried out on the samples using the Charpy apparatus.The sample preparations and testing operations were carried out according to the procedures of [13].2.5.Corrosive behaviour 2.5.1.Weight loss experiment Each sample was machined to the required dimension of 20 mm × 20 mm × 10 mm for immersion test.The samples were weighed and immersed in a 0.5 M hydrochloric acid solution.Individual samples were then tagged and stored for the appropriate amount of time.After the total immersion period of 240 h had expired (weight loss readings were taken at 48 h intervals), the samples were removed from the test solution and washed with distilled water to eliminate corrosion deposits.Before weighing and recording the weight, the sample was allowed to dry.The corrosion rate was calculated using the formula as follows [14][15][16][17]: CR, W, D, A and t are the Corrosion Rate (mm/y), weight loss of the sample (mg), Metal density (2.7 g cm −3) , cross-sectional Area (cm 2 ) and Exposure time (hours) of the sample.

Open circuit potential and potentiodynamic polarization test
Wires with an average length of 110 mm were soldered to the cylindrical aluminium matrix samples and mounted with acrylic resin (versoCit-2), which hardened in a matter of seconds.The mounted samples' surfaces were polished with several grades of abrasive paper up to 2000 microns.The potentiostat was connected to a computer system running the relevant software for analysis (DY2300EN).The working electrode was the mounted aluminium matrix samples, the reference electrode used was Ag/AgCl and the counter electrode was platinum wire.The polarization tests commenced with cathodic polarization at −1.50 V.A potential scan rate of 2 mV s −1 was used.

Results and discussion
3.1.Optical microstructure Plate 1 shows the optical images of AA7075 aluminium with and without reinforcement at 100×.Image (a) shows the internal structure of the aluminium with no reinforcement.A bimodal size distribution of plateshaped ή variants can be seen [18].Image (b) shows uniform distribution of the RHA particles across the aluminium (a darker image).Furthermore, this observation indicates a uniform distribution of RHA particles within the aluminium matrix, effectively occupying active sites for corrosion.This aligns with the empirical data presented in sections 3.6 and 3.7.

Tensile strength
From figures 2 and 3, aluminium-RHA of 10% wt.concentration had the highest tensile stress of 124 MPa.As the RHA weight concentration increased to 15% and 20% the tensile stress values (61.1246MPa, 80.8870 MPa) reduced below the value of the control (82.4864MPa).On an average the increase in concentration of RHA inclusion into the aluminium matrix tends to reduce the load the metal can withstand.Although from the results obtained, if rice husk is dispersed in AA7075 in the proportion of 10% wt.concentration, the aluminium matrix has the potential to withstand 51% more load as compared to the base material.A similar trend was observed in studies conducted by [7] and [19].The rise in strength attributed to the reinforced particles is attributed to the diffusion influence of RH particulates within the aluminum matrix.This diffusion effectively obstructs microvoids and gaps within aluminum's structure [20].

Hardness
Table 4 displays the Vickers hardness values of AMCs with varying weight percentages of rice husk reinforcements.The table shows that adding these particles to aluminum matrix composites improves their hardness when compared to unreinforced AMCs.Vickers hardness values range from unreinforced aluminum (99.48 HV) to AMC reinforced with 20% RHA (125.02HV).Hardness values increase as reinforcement concentration increases; similar trends were observed in [21][22][23].RHA particles in the aluminum matrix that are harder and better bonded prevent dislocations and increase the hardness of AMCs.Research indicates that robust intermetallic bonding between rice husk ash and aluminum contributes to material reinforcement [24].However, it's not necessarily guaranteed that a greater addition of particulate reinforcements will lead to increased hardness.This is due to the limited availability of intermetallic ions for the particles to establish bonds with.

Impact resistance
The graphical representation in figure 4 shows the result of the impact resistance test performed on the RHA reinforced composites.The result showed a steady decline in the impact strength as the % wt.decreased except for the sample with 15%wt.RHA.This anomaly might stem from an uneven distribution of the reinforcement particles in the base matrix.The control sample had the highest impact resistance strength (12.02J) while the composite with 20%wt RHA showed a 10.98% decrease and had the least impact resistance strength (10.70 J), this finding is in correlation with that of [18].

Weight loss test
The graphical representation in figure 5 shows the changes in corrosion rate over time of different compositions of aluminium reinforced with rice husk when immersed in 0.5 M HCl acid environment.The control sample (AA7075) was also used in the analysis of the corrosion rate.From the result analysis, the corrosion rate of the samples in the HCl acid environment decreased over time.Although, at the end of the 240 h period, the Aluminium with 10% in composition of rice husk showed the least corrosion rate while the Aluminium with 5% in composition of rice husk showed the highest corrosion rate at the same period [25].The presence of the fibre reinforcements migrating towards the active sites of the base metals acts as an inhibitive agent preventing the attack of chloride ions on the surface of aluminium [26].

Open circuit potential plots
Figure 6 shows the open circuit potential (OCP) curves of Aluminium-PKS and the control sample in 0.5 M HCl.The aluminium with 5 wt% PKS concentration started with a potential value of −0.696 v and gradually decreased to around −0.696 v after 100 s.After 100 s, the potential value began to rise, eventually reaching −0.676 v after 600 s.After 600 s, the aluminium with 10 wt% PKS concentration had a positive potential value rise from −0.704 v to −0.686 v. Aluminium with 15 wt% PKS concentration produced an overall slow increase with starting and ending potential values of −0.692 v and −0.688 v, respectively.The figure above shows a steady electropositive potential increase with the 20 wt% concentration, with the potential value increasing from −0.682 v to −0.662 v between the 100 s and 600 s time frame.The composition with the least potential value has the least tendency to corrode, and as shown in figure 5, the aluminium with 20 wt% PKS concentration has the least potential value, implying that it corroded least amongst all the samples.The plot's placement indicates the  prevailing intensity of the cathodic reaction relative to the anodic reaction within the system.This positioning underscores the cathodic nature of reinforcement exhibited by RHA.This nature of reinforcement can be attributed to the pronounced prominence of cathodic behaviour within the system's electrochemical response [27,28].

Potentio-dynamic polarization test
Table 5 shows the polarization test results in terms of corrosion rate (CR), corrosion current density (Cd), corrosion potential (Cp), polarization resistance (Rp), and anodic and cathodic Tafel slope.The control sample corroded rapidly in the presence of Cl-ions, with a corrosion rate of 2.300 mm yr −1 ., whereas the aluminium with 5 wt% RHA concentration had the lowest corrosion rate of 1.592 mm yr −1 .Because its particles were more evenly distributed within the matrix than the other RHA reinforcements, the 5 wt% RHA concentration had the lowest corrosion rate [29,30].The disparities in corrosion rates within the composite could also be attributed to the presence of physio-chemical heterogeneous factors, including defects, mechanically compromised surfaces, dislocations, and intermetallic or alloy/reinforcement interfaces.Another study also highlights that the elevated corrosion rate linked with higher composition of RHA may arise from the larger volume of RHA particulates with a lower density (0.3 g cm −3 ) compared to AA7075 (2.7 g cm −3 ), resulting in an increased vulnerability of numerous alloy/reinforcement interfaces to corrosion [12], usually caused by trapped Cl − ions within the intermetallic surface.
The current density values increased as RHA concentration increased from 1.98E-04 to 2.53E-04 A cm −2 .The anodic Tafel slope value also increased from 12.080 V dec −1 to 16.300 V dec −1 , and as a result, an increase in anodic current density triggers metal dissolution processes on the aluminium surface.The aluminium with the highest polarization resistance of 134.10 was obtained with a 15% RHA wt.concentration, similar observation were made in the research of [31,32].The polarization plot of AL-RHA reinforcements and the control sample at 0.5 M HCl is shown in figure 7.

Conclusions
The following conclusions can be made from the foregoing analysis of this research:  i.The mechanical properties of the reinforced AA7075 significantly improved compared to the unreinforced AA7075 alloy.
ii.The tensile strength, hardness of the reinforced AA7075-RH increased with an increase in percentage weight charge of rice husk ash.The optimum increase point for the tensile strength and hardness were 10 wt% RHA and 20 wt% RHA, respectively.
iii.The impact strength of the composites reduced with increase in percentage weight fraction of RHA due to agglomeration of the RHA within the primary phase of AA7075 matrix.The optimum value was observed at the control sample (AA7075) iv.The corrosion rate reduced in all the reinforced samples when compared to that of the control samples.
Based on the research, RHA as a reinforcing material has demonstrated promising potential.This could aid in mitigating the adverse environmental effects caused by agro-waste within our community, while also contributing to a reduction in the procurement cost of aluminium matrix composites.

Recommendations
To unlock the full potential of rice husk ash (RHA) as a reinforcing material, it is advisable to embark on further research with the goal of optimizing the appropriate RHA content within aluminium alloys.Achieving an optimal equilibrium between mechanical properties, impact resistance, and corrosion resistance is paramount when tailoring materials for specific applications.Additionally, broadening the research scope to encompass a wider array of agricultural and industrial waste materials for potential reinforcement in diverse alloy systems offers a promising avenue for both sustainable material development and the efficient utilization of waste.It is strongly recommended that RHA-reinforced materials undergo real-world testing in applications such as highspeed rotating shafts and automotive braking components.

Figure 5 .
Figure 5. Corrosion rate of the Al-RHA composites.

Table 1 .
Chemical composition of AA 7075 alloy.

Table 2 .
Chemical composition of RHA.

Table 3 .
Charge calculations for the AMCs.

Table 4 .
Vickers hardness of the composites.

Table 5 .
Potentiodynamic polarization results for Al-RHA reinforcements and the control sample in 0.5 M HCl.