Optimization on hardening process parameters of alumnium alloy 7075 based composites using gra approach

Aluminum Metal Matrix Composites (AMMCs) play a vital role in various industries, including aerospace, aviation, maritime, transportation, and automotive, owing to their remarkable mechanical strength, resistance to corrosion, and tribological performance. The age-hardening process for AA7075-based Metal Matrix Composites is essential to meet specific performance requirements, improve material properties (enhancing mechanical properties, tailoring material performance, improving wear resistance, reducing weight, enhancing corrosion resistance, extending service life), and enhancing the overall suitability of these materials for a wide range of demanding applications. The objective of this study is to improve the mechanical and tribological characteristics of AA7075-based composites by utilizing the age-hardening process, rather than introducing additional reinforcements to the base material. Aluminium Alloy 7075 (AA7075) is selected as the matrix material due to its significant need for improvement in mechanical, corrosion, and tribological properties. Nano zirconium dioxide (ZrO2) is chosen as the reinforcement to meet the property requirements of AA7075 for various applications The composites comprising AA7075 alloy and varying weight percentages of ZrO2 (x = 0, 5, 10, 15, and 20) are fabricated using the stir casting method. The fabricated composites are subjected to tensile strength and density tests to determine the optimal combination of AA7075-based composites by employing the Archimedes principle and a Universal Testing Machine, respectively. The manufactured AA7075-based composites were subjected to EDAX and SEM tests to determine the wt% presence of elements of reinforcement and matrix material by employing the S 3000 - HITACHI model. The stir-cast AA7075-based composites were further subjected to hardening under different conditions to enhance their required properties. Age hardening was performed with the help of a Muffle furnace. The input parameters for hardening were age-hardening temperature (200, 300, and 300 °C), hardening duration (90, 180, and 270 min), and cooling environments (Furnace, water, and atmosphere). The chosen response parameters were micro hardness and wear loss. The microhardness and wear resistance of these composites were assessed using a Vickers hardness tester and a Pin-on-Disc testing apparatus. The optimization was carried out using Grey Relational Analysis (GRA). The confirmation test was conducted to determine density, Ultimate Tensile Strength (UTS), micro hardness, and wear loss. The AA7075/15 wt% ZrO2 composite exhibited a uniform distribution of zirconium dioxide in the AA7075 matrix material among the other manufactured composites. The AA7075/15 wt% ZrO2 exhibited higher density of 2.889 g cm−13 and UTS of 316 MPa than other combinations of AA7075 composite materials. Increasing the heating temperature and duration enhanced the micro hardness and wear resistance. Higher micro hardness and wear resistance were obtained with furnace cooling compared to other cooling environments. The optimized age-hardening process parameters are hardening temperature (400 °C), hardening duration (270 min), and cooling environment (Furnace cooling). The enhanced mechanical and tribological properties result from precipitate formation and density enhancement. The confirmation test for the combination specimen yielded higher density, UTS, micro hardness, and lower wear loss values, which are 2.896 g/cm3, 325 MPa, 138 HV, and 27 mg, respectively.


Introduction
Nowadays, growing industries require materials that fulfill industry requirements without negatively affecting the environment, degradability, disposal, and recycling.Composite materials are lightweight and have no impact on the environment.Additionally, aluminum alloy-based composites offer significant advantages, including ease of fabrication, cost-effectiveness, and higher machinability [1,2].Aluminium alloys have been widely employed for automotive components, solar and wind energy applications, and electronic module packaging due to their predominantly anti-corrosion properties, high processability, eco-friendly nature, increased conductivity, and recoverability [3].Due to their exceptional mechanical properties, high strength-to-density ratio, and low overall weight, aluminum-7075 series alloys have been widely utilized in various transportation sectors, including aviation, aerospace, automotive, and maritime industries [4].In the realm of materials science and technology, a fundamental imperative is the advancement of Aluminium Metal Matrix Composites (AMMCs) that exhibit improved mechanical and tribological characteristics [5].The aluminum alloy 7075 serves as the matrix material (continuous phase) and is fortified with diverse properties by incorporating selected individual and multiple reinforcement particles (discrete non-metallic ceramics) like SiC, Al 2 O 3 , Gr, TiO 2 , B 4 C, AlN, fly ash, and others.These resulting composites demonstrate greater strength compared to the unalloyed base material.This improvement in alloy material is essential for the rapid development of technology in various domains of application.Current research pursuits involve exploring how MMCs can modify their physical attributes (such as thermal expansion and density), mechanical characteristics (including tensile and compressive behavior), tribological properties, and other pertinent traits through alterations in the constituent or filler material phase [6][7][8].The test results show that the incorporation of reinforcement particles leads to minor fluctuations in the composite densities across different weight percentages of ZrO 2 particles.Furthermore, the introduction of ZrO 2 particles enhances both the hardness and tensile strength of the composite while also improving its wear resistance [9].By adjusting the treatment parameters (temperature and aging time), heat treatment also has the ability to impart aluminum composites with a desired combination of characteristics.These variables can be adjusted to modify the tribological and mechanical characteristics of composite materials [10].Using the right processing techniques and heat treatment conditions can enhance the characteristics of aluminum alloys.Furthermore, adding ceramic reinforcements to the base matrix improves the characteristics and modifies the way composites age.In contrast to alloys without reinforcement, the precipitation behavior of composites undergoes alterations as a result of the inclusion of ceramic particles into the Al base matrix [11][12][13].Based on the review of existing literature, it appears that there is no prior research conducted on the process of hardening AA7075-based composites.The aim of optimizing the hardening process parameters for aluminum alloy 7075-based composites using the GRA approach is to encompass a range of objectives related to improving the material's properties, reducing costs, enhancing process efficiency, ensuring quality, and considering environmental and application-specific factors.The GRA approach is a mathematical technique used to analyze and optimize complex systems with multiple interacting parameters, making it a valuable tool for achieving these aims.For this study, nano ZrO 2 have incorporated into AA7075 at various weight percentages (5%, 10%, 15%, and 20%) with the aim of identifying the optimal combination of AA7075 composites in terms of both ultimate tensile strength (UTS) and density.The best combination of AA7075-based composites is subjected to different hardening conditions to obtain optimized mechanical and wear resistance properties using the GRA method.The confirmation test will be conducted on the optimized hardening process parameters for the hardened AA7075-based composite to obtain higher density, UTS, micro hardness, and wear resistance without increasing the wt% of reinforcements.

Materials
AA7075 possesses good machinability, weldability, and formability properties with moderate corrosion resistance and mechanical properties.Therefore, AA7075 needs reinforcement to achieve the required mechanical properties for applications such as marine, aircraft, military, structural, transport, and automobile applications.The matrix material chosen for this study is AA7075 alloy.The chemical elements present in AA7075 are Fe, Si, Ti, Cr, Mg, Mn, Cu, Zn, and Al, with contents of 0.5%, 0.4%, 0.2%, 0.15%, 2.5%, 0.3%, 1.6%, 5.5%, and the remaining wt%, respectively.Round rod-shaped AA7075 alloy was used in this investigation.Zirconium dioxide has high tensile strength, high hardness, and high corrosion resistance properties [14].Zirconium dioxide (ZrO 2 ) was chosen as the reinforcement material.ZrO 2 was selected in powder form with a particle size of 50 nm.Zirconium dioxide contains the chemical elements ZrO 2 , Al, Fe, and Pb, with respective concentrations of 99.9 wt%, 0.06 wt%, 0.02 wt%, and 0.2 wt%.The presence of elements in both AA7075 and ZrO 2 was obtained from the supplier's data sheet.Table 1 displays the details of the AA7075 and ZrO 2 materials.
The SEM picture of AA7075 round rod and powder ZrO 2 is shown in figure 1.The AA7075 is shown at 200 μm and ZrO 2 is shown at 10 μm.

Fabrication of composites
The stir casting method was employed for manufacturing composites due to its simplicity, ease of production, and mass production characteristics.Aluminum alloy 7075 was reinforced with nano zirconium dioxide (ZrO 2 ) at weight percentages of 0%, 5%, 10%, 15%, and 20%.The choice of reinforcing from 5% to 20% was based on a review of various literature sources and aimed to analyze the impact of different levels of reinforcement.Overall, the selection of ZrO 2 reinforcement compositions within the range of 0%, 5%, 10%, 15%, and 20% intended to provide a comprehensive understanding of how the presence and amount of ZrO 2 affect the properties of aluminum alloy 7075-based composites.This approach facilitated a systematic investigation and aided in drawing meaningful conclusions regarding the impact of varying reinforcement content on material performance.The aluminum alloy 7075 was sourced from Metals Mart, Trichy, Tamil Nadu, India, in the form of cylindrical rods.These rods were cut into the required smaller sizes for melting in the furnace.The small slices of Al7075 alloy were cleaned using a 100 °C NaOH solution for 20 min.Methanol was used for the subsequent cleaning of the AA7075 alloy.The washed AA7075 was dried before being placed in the graphite melting furnace.Zirconium dioxide (ZrO 2 ) reinforcement was purchased from Sigma Aldrich in powder form, with a particle size of 50 nm.A muffle furnace (Carbolite STF 10/75) with an operating temperature of 1600 °C was employed to preheat the ZrO 2 at 400 °C for 60 min.The degassing method was used to minimize blow holes and porosity [15].
The degassing tablet (hexachloroethane) was employed to achieve better flow pressure of molten metal and flow rate.Cleaned Al7075 was placed into the graphite-based crucible melting furnace for melting.Argon was  used for degassing during the melting of aluminum alloy 7075.The AA7075 was subjected to a temperature of 750 °C to liquefy the matrix, after which preheated zirconium dioxide was consistently introduced into the molten aluminum alloy 7075.Magnesium was added at a concentration of 1 wt% to enhance wettability between the AA7075 matrix and ZrO 2 .The process of mechanical stirring was executed at a rotational velocity of 500 revolutions per minute (rpm) over a duration of 20 min.Stirring served the purpose of mixing the reinforcement with molten aluminum alloy 7075 and achieving a uniform distribution of ZrO 2 within the melted Al7075.The mixed AA7075/ZrO 2 was poured into the mold to cast samples of the required size.Samples measuring 300 mm in length with a 25 mm diameter were cast from the mixed AA7075/ZrO 2 composite.The same procedure was followed to produce specimens with different wt% of ZrO 2 (0%, 5%, 10%, 15%, and 20%).The Al7075/ZrO 2 composites produced were subjected to an aging treatment, which was carried out in a muffle furnace.The manufactured composites were heated at 510 °C for 3 h and subsequently quenched in water at room temperature.The heated AA7075 composites were further treated according to the L9 orthogonal array combination of parameters.The selection of reinforcement wt%, aging temperature, and aging duration was based on literature [16] and factors related to the specific material and desired properties.These factors included material composition, desired properties, phase diagrams, overaging sensitivity, thermodynamics, energy consumption, cost, heat treatment equipment, microstructure analysis, and tolerance for dimensional changes.The input parameters for the L9 array aging process are presented in table 2.

Testing of composites
The produced Al7075/x wt% ZrO 2 (x = 5, 10, 15, and 20) composites underwent EDAX and SEM testing to confirm the presence of Al7075 matrix material and ZrO 2 reinforcement in the produced composites.To confirm the even dispersion of the reinforcement within the Al7075 matrix material, SEM analysis was conducted utilizing the HITACHI model S-3000.The Al7075-based composites were polished using different grit sheet sizes (600, 900, and 1200) to enhance contrast in the SEM images.Keller's reagent was employed to achieve this contrast increase.Subsequently, the stir-cast Al7075/x wt% ZrO 2 (x = 5, 10, 15, and 20) composites underwent tensile testing and density testing.Tensile tests on the Al7075-based composites were conducted according to ASTM E-08 [17], using the Fine TUE-C model from Fine make.Density testing was carried out using the Archimedes principle, with three trials conducted for each manufactured composite.The ultimate tensile strength (UTS) and density of the AA7075/ZrO 2 composites were compared to determine the most suitable combination.The selected best combination of Al7075-based composites then underwent a heat treatment process.The L9 orthogonal array-based heat-treated Al7075/ZrO 2 composites were subjected to microhardness and wear testing to evaluate their microhardness and wear resistance.Microhardness testing was performed according to ASTM E384 [18] using a Mitutoyo-810 series Vickers hardness tester, with a 300 g load applied for 15 s.Three trials were conducted, and the average values were recorded.The sliding wear test was conducted using the pin-on-disc method, which involves sliding a pin against a rotating disc to assess wear and friction characteristics.This test is crucial for materials and tribological research.To assess wear loss, a pin-ondisc wear test was utilized as one of the response parameters.The wear tests were conducted using a pin-on-disc apparatus (DUCOM, Bangalore), with the composites produced being machined to the ASTM G99 dimensions to serve as the test pins [19].
EN31 steel was selected as the disc material due to its inherent properties.EN31 steel typically possesses a high level of hardness, a critical attribute for effective wear resistance.This material can be subjected to heat treatment processes to further enhance its hardness.Thanks to its inherent hardness and chemical composition, EN31 steel exhibits exceptional resistance to wear and abrasion.As a result, it is particularly suitable for applications in which components are subjected to sliding, rolling, or abrasive wear conditions.EN31 steel also demonstrates good tensile strength and yield strength, which ensure the necessary mechanical integrity of components subjected to wear and loading.This unique combination of hardness and toughness is vital for resisting wear and preventing premature failure.Furthermore, EN31 steel tends to exhibit excellent dimensional stability, a crucial factor in maintaining precise tolerances in wear-resistant components.EN31 steel typically exhibits hardness levels ranging from 50 to 60 HRC and tensile strength values ranging from 600 to 850 MPa.For the pin material, an AA7075/15 wt% ZrO 2 composite was selected.The surface of the composite pin used in the wear test was polished to attain a surface roughness of 1 μm.To quantify the wear loss of the composites following the wear test, we utilized a precision electronic weighing machine with an accuracy of 0.0001 mg.The wear loss was assessed by measuring the weight variation of the composites subjected to wear testing.This measurement consisted of weighing the specimen both before and after the wear test.Furthermore, the study utilized the Grey Relational Analysis (GRA) method to establish the sequence of influential parameters in the heat treatment process affecting microhardness and wear resistance.To further characterize the materials, density tests, microhardness tests, and XRD (x-ray diffraction) tests were conducted on confirmation test specimen to ascertain density, microhardness, and the presence of precipitates.

Micro structural examination
The presence of ZrO 2 reinforcement and AA7075 in the manufactured composites was confirmed by the EDAX test.Figure 2 displays the EDAX test results of AA7075/wt% ZrO 2 (0, 5, 10, 15, and 20) composites.The microstructure examination results of the manufactured AA7075-based composites are presented in figure 3. The Al7075/15 wt% ZrO 2 composite exhibits a uniform and dense distribution of zirconium dioxide (ZrO 2 ) reinforcement within the AA7075 matrix material compared to other combinations, as confirmed by SEM images [20,21].
In contrast, the SEM image of the AA7075/20 wt% ZrO 2 composite shows an accumulation of ZrO 2 within the AA7075 matrix material [22].Some areas of the composite surface exhibit porosity, which is attributed to gas escaping during the cooling process [23].Additionally, the formation of oxide on the composite's surface is observed, likely due to atmospheric cooling, as confirmed by the SEM image [24].

Density and tensile testing
The density of the composites consisting of aluminum alloy 7075 reinforced with zirconium dioxide (ZrO 2 ) was ascertained using the Archimedes principle.The Universal Testing Machine (UTM) was employed to measure the ultimate tensile strength (UTS) of these composite materials.The comparison of the density and UTS of the aluminum alloy 7075 reinforced with ZrO 2 composites is presented in figures 4(a)-(b).The inclusion of zirconium dioxide (ZrO 2 ) into the matrix of aluminum alloy 7075 enhances the density of the manufactured composites.The presence of ZrO 2 particles in the aluminum matrix acts as a strengthening agent because it creates barriers that hinder the movement of dislocations within the crystal lattice of the aluminum.Dislocations are defects or imperfections in the crystal structure that can lead to material deformation and failure.By impeding dislocation movement, the composite becomes stronger and more resistant to deformation [25].The AA7075/15 wt% ZrO 2 composite exhibits a higher density compared to the other composites.The density of the AA7075/15 wt% ZrO 2 composite increases with the inclusion of up to 15 wt% zirconium dioxide and decreases when 20 wt% zirconium dioxide is included [14].The high-density zirconium dioxide enhances the density of the manufactured composites when added to the AA7075 matrix material [19].The ultimate tensile strength (UTS) of the AA7075/15 wt% ZrO 2 composites is enhanced due to the inclusion of zirconium dioxide [26].
The inclusion of zirconium dioxide (ZrO 2 ) into the matrix of aluminum alloy 7075 enhances the density of the manufactured composites.The presence of ZrO 2 particles in the aluminum matrix acts as a strengthening agent because it creates barriers that hinder the movement of dislocations within the crystal lattice of the aluminum.Dislocations are defects or imperfections in the crystal structure that can lead to material deformation and failure.By impeding dislocation movement, the composite becomes stronger and more resistant to deformation [25].The AA7075/15 wt% ZrO 2 composite exhibits a higher density compared to the other composites.The density of the AA7075/15 wt% ZrO 2 composite rises with the incorporation of up to 15 wt% zirconium dioxide, but it decreases when 20 wt% zirconium dioxide is introduced [14].The highdensity zirconium dioxide enhances the density of the manufactured composites when added to the AA7075 matrix material [19].The ultimate tensile strength (UTS) of the AA7075/15 wt% ZrO 2 composites is enhanced due to the inclusion of zirconium dioxide [26].The inclusion of zirconium dioxide (ZrO 2 ) into the matrix of aluminum alloy 7075 enhances the density of the manufactured composites.The presence of ZrO 2 particles in the aluminum matrix acts as a strengthening agent because it creates barriers that hinder the movement of dislocations within the crystal lattice of the aluminum.Dislocations are defects or imperfections in the crystal structure that can lead to material deformation and failure.By impeding dislocation movement, the composite becomes stronger and more resistant to deformation [25].The AA7075/15 wt% ZrO 2 composite exhibits a higher density compared to the other composites.The density of the AA7075/15 wt% ZrO 2 composite increases with the inclusion of up to 15 wt% zirconium dioxide and decreases when 20 wt% zirconium dioxide is included [14].The high-density zirconium dioxide enhances the density of the manufactured composites when added to the AA7075 matrix material [19].The ultimate tensile strength (UTS) of the AA7075/15 wt% ZrO 2 composites is enhanced due to the inclusion of zirconium dioxide [26].
As the ZrO 2 content is raised to 20 wt%, attaining a uniform dispersion of the reinforcement particles within the aluminum matrix becomes increasingly difficult.Agglomeration of particles can lead to stress concentration points, reducing the overall strength of the material and possibly increasing porosity, which would lower density [27].The effectiveness of load transfer relies significantly on the quality of the bond formed between the reinforcement (ZrO 2 ) and the matrix (AA7075).In the case of the AA7075/20 wt% ZrO 2 composite, the higher ZrO 2 content may result in weaker interfacial bonding due to reduced contact area and possible formation of weak interfaces.This weaker bonding can lead to a lower UTS.A higher ZrO 2 content can also contribute to increased porosity in the composite.When the ZrO2 particles are not well-distributed or there is inadequate bonding, voids or gaps can form, compromising the material's density and mechanical properties.
The higher ZrO 2 content in the AA7075/20 wt% ZrO 2 composite may lead to issues such as agglomeration, poor dispersion, weaker interfacial bonding, and increased porosity, all of which can contribute to lower density and UTS compared to the AA7075/15 wt% ZrO 2 composite.Achieving an optimal balance of reinforcement content is crucial in composite materials to maximize their mechanical performance [28].

Optimization of heating process parameters
The best combination of zirconium dioxide (ZrO 2 ) reinforced AA7075 composite underwent an age-hardening process.The age-hardening input process parameters included heating temperature, heating duration, and cooling environment, while the response parameters were micro hardness and wear loss.The optimization was carried out using the GRA approach.The best combination, AA7075/15 wt% ZrO 2 composite, was selected based on density and UTS.This optimal combination was then subjected to an L9 orthogonal array experiment, considering input parameters for the age-hardening process.The chosen input age-hardening process parameters consisted of heating temperature (200, 300, and 400 °C), heating duration (90, 180, and 270 min), and cooling environment (furnace cooling, water cooling, and atmospheric cooling).Furnace cooling allowed the heated composite to cool within the furnace, while water cooling involved quenching the composite in water.Atmospheric environment cooling allowed the heated composite to cool in the ambient air [29].
The response parameters chosen for measurement were microhardness and wear loss, and these values were documented using the input parameters from the L9 array.Table 3 displays the L9 orthogonal array with input and response parameters.Notably, trials 3 and 4 yielded the same responses in terms of micro hardness and wear resistance.The GRA was employed to optimize the input parameters for age hardening and their corresponding response parameters.The optimized values from the L9 orthogonal array using GRA are presented in table 4. The GRA table displays the values for the normalized value, deviation sequence value, Grey Relational Coefficient value, Grey Relational Grade, and Rank.It is noteworthy that trials 3 and 4 yielded identical responses.This outcome can be attributed to the variation in heating temperature from 200 °C to 300 °C, the change in heating duration from 270 to 90 min, and the shift from atmospheric cooling to water cooling environment.Consequently, both trials produced similar responses.
The optimal combination of age-hardening input parameters is characterized by a high-level heating temperature (400 °C), an extended heating duration (270 min), and a water cooling environment.This conclusion is based on the rankings obtained from the Grey Relational Grade (GRG) values.
The Grey Relational Grade (GRG) was subjected to Taguchi analysis and ANOVA.The Minitab 19 software was utilized to optimize the GRG using the Taguchi technique and ANOVA.Table 5 displays the response table for Grey Relational Grade (GRG).The sequence of influencing input parameters for micro hardness and wear loss was found to be heating temperature, followed by heating duration and cooling environment.The optimal combination of age-hardening input parameters is characterized by a high heating temperature (400 °C), an extended heating duration (270 min), and a low cooling environment (furnace cooling).This conclusion is based on the observations from response table 5.
Figure 5 displays the main effect plot for the means of GRG.The increase in heating temperature enhances the GRG value by improving micro hardness and wear resistance.Higher temperatures can promote recrystallization and the formation of finer-grained structures.Finer grains often result in increased micro hardness, making the material more resistant to deformation or wear.Changes in the microstructure can also affect the material's wear resistance.For example, a finer-grained structure may reduce the likelihood of abrasive wear [30].
Enhancing the heating duration improves the GRG value by enhancing micro hardness and wear resistance.Prolonged heating provides more time for the recrystallization process, resulting in finer grains and increased micro hardness.Longer heating durations also facilitate the development of a more favorable microstructure for wear resistance [31].In contrast, the furnace cooling environment significantly enhances the GRG value when compared to water and atmospheric cooling.Cooling in a controlled furnace environment often leads to a slower and more uniform cooling rate.This controlled cooling enables the formation of a specific microstructure that enhances properties such as micro hardness and wear resistance, contributing to a higher GRG value [32].
In summary, the mechanisms described in the statements suggest that higher heating temperatures and longer durations promote the formation of a finer-grained microstructure, leading to increased micro hardness and wear resistance.Additionally, controlled furnace cooling is advantageous for achieving the desired microstructure, and thus, a higher GRG value when compared to rapid cooling methods like water cooling or atmospheric cooling.The GRG value serves as a measure of material performance, with higher values indicating superior microstructural properties and, consequently, improved hardness and wear resistance.Table 6 presents the ANOVA results for the Grey Relational Grade (GRG) related to the age-hardening process.The influencing sequence of age-hardening input parameters for GRG is heating temperature, heating duration, and cooling environment.The contribution percentages of heating temperature, heating duration, and cooling environment are 42.08%,37.05%, and 20.21%, respectively.The experiment's reliability is high, as indicated by its high adjusted R 2 value, as shown in table 7 [33].The presence of these precipitates within the crystal lattice significantly increases the material's micro hardness and wear resistance.This occurs because the precipitates act as obstacles to dislocation movement, making it more challenging for dislocations to move and deform the material.Consequently, the material exhibits enhanced strength and greater resistance to plastic deformation, resulting in an elevation in microhardness.Additionally, the presence of hard precipitates also enhances the material's resistance to wear and abrasion.
In summary, increasing the heating temperature and heating duration during the aging process promotes the formation and growth of precipitates within the material's crystal lattice.These precipitates act as strengthening agents, hindering dislocation movement and thereby increasing micro hardness and wear resistance.This process is commonly employed to tailor the mechanical properties of materials for specific  applications, such as in aerospace, automotive, and structural engineering, where high strength and wear resistance are crucial.
The confirmation test was conducted on the combination of high-level heating temperature (400 °C), highlevel heating duration (270 min), and low-level cooling environment (Furnace cooling) to determine density, UTS, microhardness, and wear loss.The results obtained from the confirmation experimental test were as follows: a density of 2.896 g cm −13 , UTS of 325 MPa, microhardness of 138 HV, and wear loss of 27 mg, respectively.These values represented an improvement of 12.19% in microhardness and 34.2% in wear loss compared to the initial settings of the L9 orthogonal array.Table 8 presents the details of the confirmation test.
Figure 6 illustrates the XRD results for the experimental test (A3B3C1) of the confirmation test, revealing prominent peaks corresponding to Al, ZrO 2 , MgSi, MgZn 2 , and Al 2 Cu [34].The formation of secondary phases on the surface was found to enhance the composite's wear resistance and microhardness, with temperature playing a key role in promoting the formation of these secondary phases [35].
For the confirmation test, specimens were prepared using AA7075/15 wt% ZrO2 and subjected to heat treatment at a heating temperature of 400 °C, a high-level heating duration of 270 min, and a lower-level cooling environment of Furnace cooling.Figure 6 displays the XRD results obtained from the confirmation test specimen [36].The worn surface and wear debris of the experimental test specimen from the confirmation test are depicted in figures 7(a)-(b).
The worn surface was analysed by SEM, revealing that the observed wear mechanism was abrasion.The presence of secondary phases prevented the easy breakage of the bonding structure, leading to abrasion during the application of load in the wear test.Examination of the collected wear debris further confirmed that the observed wear mechanism was abrasion.The formation of oxide occurred during the wear test due to the heat generated between the pin and counter disc surfaces.

Conclusions
The AA7075/x wt% ZrO 2 (5, 10, 15, and 20) composites were successfully manufactured using a stir casting setup.The Al7075/15 wt% ZrO 2 composite exhibited a uniform and dense distribution of zirconium dioxide reinforcement within the matrix of AA7075, as confirmed by SEM imaging.The presence and wt% of elements were verified through EDAX testing.The AA7075/15 wt% ZrO 2 composite demonstrated a density of 2.889 g cm −3 and a tensile strength of 316 MPa, both of which were higher than those of other AA7075 composite combinations.This improvement can be attributed to the ability of the zirconium dioxide particles to create barriers that impede the movement of dislocations within the crystal lattice of aluminum, leading to enhancements in density and UTS.The selected input parameters for the age-hardening process included heating temperature (200, 300, and 400 °C), heating duration (90, 180, and 270 min), and cooling environment (furnace cooling, water cooling, and atmospheric cooling).The optimization of age-hardening process parameters for AA7075/15 wt% ZrO 2 composites was accomplished using the GRA technique.The combination of high-level heating temperature (400 °C), high-level heating duration (270 min), and low-level cooling environment (furnace cooling) emerged as the superior set of age-hardening input parameters based on micro hardness and wear loss.Increasing the heating temperature was found to enhance the GRG value, thereby improving micro hardness and wear resistance.Similarly, extended heating durations contributed to higher GRG values by enhancing micro hardness and wear resistance.
Cooling the material in a controlled furnace environment led to a slower and more uniform cooling rate.This controlled cooling allowed for the formation of a specific microstructure that enhanced properties such as micro hardness and wear resistance, ultimately contributing to a higher GRG value.Additionally, longer heating durations provided more time for the recrystallization process to occur, resulting in finer grains and increased micro hardness.
In summary, elevating both the heating temperature and duration during the aging process promoted the formation and growth of precipitates within the material's crystal lattice.These precipitates acted as strengthening agents, hindering dislocation movement and thereby increasing micro hardness and wear resistance.The contribution percentages of heating temperature, heating duration, and cooling environment were determined to be 42.08%,37.05%, and 20.21%, respectively.
The results from the confirmation experimental test showed a micro hardness of 138 HV and a wear loss of 27 mg.These values represented improvements of 12.19% and 34.2%, respectively, compared to the initial settings of the L9 orthogonal array.

Figure 4 .
Figure 4. a.Comparison of density.b.Comparison of UTS.

Figure 5 .
Figure 5. Main effect plot for Means of GRG.

Figure 6 .
Figure 6.XRD image of experimental test specimen for confirmation test.

Table 3 .
L9 with input and its corresponding response parameters.

Table 5 .
Response table for means.

Table 6 .
Analysis of Variance.