Optimization on the electrical discharge machining (EDM) process parameters of aged AA7075/TiC metal matrix composites

2.1. Materials

AA7075 found widespread application in aerospace where the demand for better strength and low weight was critical. Additionally, it was employed in the fabrication of structural components for the automotive industry and the construction of high-performance bicycle frames. AA7075, a representative Al-Zn- Cu Mg alloy, stood as the most extensively researched series among aluminum alloys. Within the 7xxx series, it held pre-eminence due to its exceptional combination of properties, encompassing high strength, heightened toughness, excellent electrical and thermal conductivity, notable wear and abrasion resistance, resilience to damage at elevated temperatures, commendable creep resistance, fatigue strength and the higher failure elongation. This versatility rendered it the preferred choice across diverse sectors, including submarines, aerospace, ships, prosthetic devices, rail vehicles, pressure vessels, trucks, machinery, and aircraft components such as lower drag brace ventral fins, landing gears and helicopter blades. Its utility further extended to electronics, military applications, and the automotive industry, where it found application in brake calipers, pistons, wheels and rocker arms. The AA7075 aluminum alloy was renowned for its impressive strength-to-weight ratio and was part of the 7000 series of aluminum alloys, recognized for their exceptional strength. The designation “AA” in AA7075 represented the Aluminum Association, the entity responsible for categorizing aluminum alloys. AA7075 was distinguished by its formidable strength, comparable to that of many steels, and was often employed in applications requiring robustness. Additionally, being heat-treatable, AA7075 allowed for the enhancement of its mechanical properties through heat treatment, facilitating the customization of the alloy’s characteristics for specific purposes. While aluminum, known for its robust corrosion resistance, generally exhibited this property well, AA7075, unlike some other aluminum alloys, particularly those in the 6000 series, was not as corrosion resistant; furthermore, AA7075 was commonly considered to have poor weldability, requiring careful welding procedures to minimize cracking, and post-welding heat treatment might be necessary. AA7075 demonstrated moderate machinability, allowing for machining through conventional processes. However, its considerable strength might present challenges related to tool wear during machining. AA7075 exhibited robust fatigue resistance, rendering it suitable for applications with cyclic stress. Despite its notable strength, AA7075 might possess less resilience compared to certain other aluminum alloys. Toughness, a measure of a material’s ability to withstand energy and subjects to plastic deformation before fracturing, was a key aspect to consider. The moderate properties of AA7075 were not enough to fulfil the requirements of the fastest-growing industries such as military, automobile and marine. Hence, the aluminum alloy needed enhancement of mechanical properties (Huda A and Vignesh Kumar V). So, the aluminum alloy 7075 was nominated as the matrix material in this investigation. The titanium carbide was nominated as reinforcements because of its excellent mechanical properties. Titanium carbide (TiC), a refractory ceramic known for its elevated hardness, exceptional wear resistance, and high thermal conductivity, was well-suited for applications requiring efficient heat transmission. Its sought-after properties, including wear resistance, made it ideal for cutting tools, wear-resistant coatings, and scenarios demanding resistance to abrasion. Moreover, titanium carbide typically displayed corrosion resistance, rendering it appealing for use in corrosive environments, further complemented by its inherent hardness. The AA7075 was acquired from Trichy metals, Trichy, Tamil Nadu, India, and it was acquired in cylindrical rod form. The titanium carbide reinforcement was acquired from Sigma Ulrich. The TiC was acquired in the form of powder with a particle size of 10 µm. The particle size of titanium carbide was measured by employing Particle Size Analyzer (PSA). The SEM image of reinforcement TiC is displayed in Fig. 1. The shape of the titanium carbide was elliptical, and it was easily bonded with the matrix material, and the load-carrying capacity was also high. Elliptical shapes, in contrast to spherical particles, could provide enhanced reinforcement along specific orientations. The improved fracture arrest capabilities of elliptical forms, compared to spherical particles, contributed to increased toughness. The crack propagation was easily prevented by this elliptical shape of TiC. The elements presented in the AA7075 are displayed in Table 1, (HassabAlla et al., 2022HassabAlla, M.A.M., Satishkumar, P., Srinivasa Rao, Y., Chebolu, R., Capangpangan, R.Y., Alguno, A.C., Gopal, M., Firos, A., Saravana, M.C. (2022). Investigation of Mechanical Behavior and Microstructure Analysis of AA7075/SiC/B4C-Based Aluminium Hybrid Composites. Adv. Mater. Sci. Eng. 2022, ID 2411848 https://doi.org/10.1155/2022/2411848.). The details of AA7075 and TiC are displayed in Table 2.

2.2. Fabrication and ageing of composites

The AA7075/xTiC (x=0, 4, 8, 12, and 16 wt.%) MMCs were manufactured by utilizing the stir casting setup. The flow chart of the stir casting process of AA7075/TiC MMCs was displayed in Fig. 2. The stir casting setup was displayed in Fig. 3. The acquired AA7075 was sized into a small cylindrical rod by a hacksaw. The reinforcement TiC was preheated at 250 °C for a duration of 30 min. The small-sized matrix AA7075 was placed into the crucible furnace of the stir casting setup. The furnace was heated to 800 °C, the melting temperature of the AA7075 matrix material (Agrawal, et al., 2023Agrawal, A.P., Srivastava, S.K. (2023). Investigating the Effects of Adding Si3N4 on Microstructural and Mechanical Characteristics of AA7075-based TiB2-Reinforced Hybrid MMCs. Int. J. Metal. Cast.https://doi.org/10.1007/s40962-023-01193-5.). The preheated titanium carbide was fed into the crucible furnace of the stir casting setup to mix with the melted AA7075 matrix material. 2 wt.% of magnesium was included in the crucible furnace to improve the bonding between the AA7075 and TiC (Sethi et al., 2020Sethi, D., Kumar, S., Choudhury, S., Shekhar, S., Saha Roy, B. (2020). Synthesis and characterization of AA7075/TiB2 aluminum matrix composite formed through stir casting method. Mater. Today Proc. 26 (Part 2), 1908-1913, https://doi.org/10.1016/j.matpr.2020.02.418.). The mechanical stirrer was utilized to mix the melted matrix AA7075 material and TiC reinforcement. The stirrer was rotated at a speed of 300 rpm for a rotating duration of 10 min. The mixed AA7075 and TiC were poured into the die to solidify the mixed AA7075/TiC composite material. The square-sized die with dimensions of 152x152x22 mm³ was used for stir casting of AA7075/TiC MMCs, and also, a cylindrical-shaped die with a diameter of 32 and a length of 120 mm was used. The casted sample sizes were 100x100x10 mm and 100 mm length x 20 mm diameter. The same procedure was repeated for manufacturing the AA7075/xTiC (x=0, 4, 8, 12, and 16 wt.%) MMCs. The casted AA7075/TiC composites were further aged at 520 °C for a duration of 3 h in the electrical muffle furnace, and the heated AA7075/TiC composite was quenched in water. The quenched AA7075/TiC composites were further heat-treated at 250 °C for a duration of 4 h, and they were cooled in the muffle furnace itself. The aging procedure was followed for aging the AA7075/xTiC (x=0, 4, 8, 12, and 16 wt.%) MMCs.

2.3. Testing of composites

The micro hardness, tensile, compressive, and density tests were conducted on aged AA7075/xTiC (x=0, 4, 8, 12, and 16 wt.%) MMCs to measure the Vickers microhardness, the compressive strength, tensile strength and density. The aged AA7075-based composites were cut into the required ASTM standard shape by utilizing wire cut EDM. The Archimedes principle was utilized to find the density of the aged AA7075/TiC composites. The microhardness of aged AA7075/TiC composites was determined by conducting a hardness test as per ASTM E384 with the help of a Vickers hardness tester (M V - 5, R 1 - 2970 HV). The Vickers hardness test was conducted at the loading condition of 300 g for a loading duration of 10 s. Three trials were done, and the average value had been taken. A composite material’s performance under axial stretching force was assessed through a tensile test, conducted employing a 10 kN Instron computerized universal testing machine (UTM). The test samples were manufactured in adherence to the ASTM E08 standard. The tensile fractured surface was analyzed by utilizing a scanning electron microscope (SEM) to determine the fracture mechanism. The compression test utilized a computerized UTM (UNIVERSAL 2001 - UTE 40) with a 40 KN load capacity. Test samples were manufactured following the ASTM E9 standard. The compressive strength was measured at a crosshead speed of 2 mm×min^-1. The best combination of aged AA7075/TiC composites was determined based on its density, microhardness, tensile strength, and compressive strength. The X-ray diffraction analysis and EDAX microanalysis were done on the best combination of AA7075/TiC composites. The microstructure of the best combination of AA7075/TiC composites was characterized using scanning electron microscopy. This aged AA7075/TiC composite was polished by employing 600, 800, 1000, and 1200 grit sheets. The surface roughness of polished aged AA7075/TiC composite was about 1 µm. To reveal the microstructure AA7075/TiC composite was etched using Keller’s reagent. The constituents of Keller’s reagent were 2.5 ml HNO₃, 1.5 ml HCl, 1 ml HF and 95 ml Distilled Water (Kumar et al., 2023Kumar, A., Kumar, V. (2023). Analysis of Heat Input and its Effects on Microstructure Development and Mechanical Properties of Friction Stir-Processed AA7075 Alloy. JOM. https://doi.org/10.1007/s11837-023-06250-2.). The enhancement of micro hardness causes it to be very hard to machine, and hence the determination of the optimized parameters of machining is needed (Bharat and Bose, 2023______, N., Bose, P.S.C., (2023). Wear performance analysis and optimization of process parameters of novel AA7178/nTiO2 using ANN-GRA method. Proceedings of the Institution of Mechanical Engineers, Part E. Journal of Process Mechanical Engineering 0(0). https://doi.org/10.1177/09544089231156074.). The EDM parameters were optimized for best aged AA7075/TiC composite. The EDM model used is SPARKONIX India Pvt. The dielectric fluid was mixed with chromium for obtaining a better machining surface. The chromium concentration (0, 4, 8 g/litre), pulse on time (120, 240, and 360 µs), and current (5, 10, and 15 amps) were chosen as input parameters (Sivasamy et al., 2021Sivasamy, A., Ravichandran, M, (2021). Variation of electrode materials and parameters in the EDM of an AA7075-TiO2 composite. Materials Testing 63 (2), 182-189. https://doi.org/10.1515/mt-2020-0025.).

The pulse-on time, also known as the “on-time” or “pulse duration”, denoted the duration of an electrical discharge or spark between the electrode and the work piece. This interval encompassed the generation of a high-frequency electrical discharge that produced intense heat concentrated at a specific location on the work piece. As a result of this concentrated heat, material removal occurred through the processes of melting and vaporization. Consequently, the pulse-on time emerged as a critical parameter influencing not only the depth of material removal but also the surface quality and the amount of energy delivered to the work piece. In EDM, the primary role of the current was to initiate electrical discharges or sparks between the electrode and the work piece. This phenomenon occurred when a voltage potential was applied across the two components in the presence of a dielectric fluid. The electrical discharges generated during this process produced significant heat at the contact point between the electrode and the work piece. Notably, the current level had a direct impact on the wear of the electrode, with higher currents leading to more substantial wear, thereby reducing tool life and necessitating frequent replacement or treatment of the electrode. In the context of EDM, the control of the current level was a crucial factor in achieving the required accuracy and precision. By regulating the current, it became possible to manage the depth and shape of the machined features with precision. The pulse on and pulse-off parameters have the same effect in opposite poles. Hence, pulse-on is selected as one of the parameters with current. The response parameters were chosen as the TWR and SR. The pulse-on and current directly influenced TWR and SR more than any other input parameters. Besides, the chromium concentration also directly influenced the TWR and SR of AA7075/TiC MMCs. So the pulse-on, current, and chromium concentration were selected as input parameters. The TWR and SR were selected as response parameters due to the enhanced hardness of aged AA7075/TiC MMC. The enhanced hardness of aged AA7075-based composites was difficult to machine in EDM, and it consumed large current, extended pulse on. Besides, high pulse-on and large current caused a decrease in the life of the electrode and heavily affected the surface finish of the machining surface, requiring secondary operations to obtain a greater surface finish. Besides, more research was done by taking material removal rate (MRR) and TWR, and no research has been done on the optimization of EDM process by taking TWR and SR together. The chromium concentration was taken as one of the input parameters, which caused more heat dissipation from the machining area and hence enhanced the life of the electrode and improved surface finish. An additive helped to improve cooling, reduce tool wear, enhance the flushing of debris, improve surface finish, stabilize the spark, and prevent corrosion. The EDM setup was displayed in Fig. 4. The experimental setup was displayed in Table 3. The L9 orthogonal array was chosen for optimization, and it was done by using GRA. The EDM input parameters with L9 OA were displayed in Table 4 (Bharat et al., 2022b______, N., Bose, P.S.C. (2022b). A Study on Machining Behaviour of Metal Matrix Composite: A Review. Advances in Mechanical and Materials Technology. EMSME 2020, pp. 367-374. Springer, Singapore. https://doi.org/10.1007/978-981-16-2794-1_33.).

The GRA was utilized for the optimization of EDM process parameters of the best combination of aged AA7075/TiC MMC. The first step in GRA involved data pre-processing, wherein the TWR and SR experimental data underwent standardization to fit within the zero to one range. This standardization was necessary when there were variations in the range and unit of one dataset compared to another. The purpose of data pre-processing was to transform a sequence into one that is comparable to the original. Multiple methods for data pre-processing in GRA were available, and the selection depended on the specific parameters of the data series. If the inherent property of the original sequence was characterized by “lower-the-better,” it should be normalized as outlined in the provided equation (Viswanathan et al., 2021bViswanathan, R., Saravanan, K.G., Balaji, J., Prabu, R., Balasubramani, K. (2021b). Optimization of wear and friction parameters in Aluminum7075 hybrid composite. Mater. Today Proc. 47 (Part 14), 4449-4453. https://doi.org/10.1016/j.matpr.2021.05.308.).

Here, i = 1, 2,..., m, and k = 1, 2,..., n represent the original reference sequence and pre-processed data. The notation ${{\gamma }_i}^{*}$ (k) represents the normalized value, ${{\gamma }_i}^{\left(0\right)}$ denotes the intended sequence, $min{{\gamma }_i}^{\left(0\right)}$ (k) signifies the minimum value in the sequence, and $max{{\gamma }_i}^{\left(0\right)}$ (K) represents the sequence’s maximum value. The total number of observations is n, while the number of experiments is m.

The grey relational coefficient (GRC) is a GRA measure used to assess the applicability of two systems or sequences. The GRC is determined by using the following equation.

Where ${∆}_{0i}$ (k) is also known as the deviation sequence and duplicates the difference between ${\mathrm{x}}_0$ (k) and ${{\gamma }_i}^{*}$ (k). In most circumstances, the value is lower and the distinct ability is greater, hence = 0.5 is used.

The grey relational grade (GRG) is typically derived by taking the average value of the GRC. The GRG is used to assess multi-response characteristics and it is determined by using the equation.

The higher the GRG, the closer the linked experimental result is to the ideal normalised value.

3.1. Vickers micro hardness test

The AA7075/xTiC (x=0, 4, 8, 12, and 16 wt.%) composites, both aged and non-aged, underwent Vickers microhardness testing. Three trials were performed, and the average Vickers microhardness value was utilized to analyze the impact of reinforcement and aging. The results of the Vickers microhardness test are presented in Table 5. Figure 5 illustrates the comparison of Vickers microhardness between aged and non-aged AA7075/TiC MMCs. From the obtained results of microhardness, the aged AA7075/TiC MMCs had higher microhardness than its corresponding non-aged AA7075/TiC MMCs due to the obtained fine grain structure and growth of precipitates. Titanium carbide is a resilient and durable ceramic material. Incorporating TiC reinforces the matrix through solid solution strengthening, a process in which improved hardness results from interactions between alloying elements and the reinforcing particles. Fine particles, including TiC, precipitate and act as efficient hindrances to the movement of dislocations. Dislocations, which are defects in the crystal structure of a material and can contribute to plastic deformation, encounter these obstacles. The presence of these hindrances restricts the mobility of dislocations, making their movement through the material more challenging. This enhanced dislocation pinning contributes to an overall enhancement in microhardness (Alam Mohammad et al., 2023Alam Mohammad A., Ya, Hamdan B., Azeem, M., Mustapha, M., Yusuf, M., Masood, F., Marode Roshan,V., Sapuan Salit, M., Ansari, Akhter H. (2023). Advancements in aluminum matrix composites reinforced with carbides and graphene: A comprehensive review. Nanotechnol. Rev. 12 (1), 0111. https://doi.org/10.1515/ntrev-2023-0111.). The enhancement of microhardness in AA7075 resulting from the introduction of titanium carbide (TiC) is primarily attributed to the reinforcing effect of TiC particles within the composite material. As TiC is incorporated into the matrix of AA7075, it acts as a strengthening agent, leading to an augmentation of microhardness. When distributed within the less robust matrix of AA7075, these TiC particles exhibit resistance to deformation, ultimately leading to a general enhancement in the microhardness of the composite.

The presence of TiC particles had the potential to hinder the movement of dislocations within the crystal lattice of the material. Dislocations represented flaws in the crystal structure that could move and play a role in deformation. By restraining dislocation mobility, TiC functioned to fortify the material, leading to a rise in microhardness. Furthermore, the reinforcement from TiC improved the load-bearing capacity of the composite, indicating that these particles effectively carried and distributed applied stresses. This, in turn, reduced the likelihood of localized deformation and contributed to an overall increase in the material’s hardness. The addition of titanium carbide into matrix AA7075 improved microhardness value up to 12 wt.% and decreased by 16 wt.% TiC addition in both the aged and non-aged cases. The higher microhardness of 156 HV was obtained at 12 wt.% TiC addition due to the better bonding of TiC with AA7075 and obtained uniform distribution of TiC in matrix AA7075. The concentration of 12 wt.% TiC was probably an optimal equilibrium between reinforcement and the amount of TiC added. This particular concentration provided a sufficient quantity of TiC particles to enhance microhardness without inducing issues such as agglomeration or excessive reinforcing, leading to an overall improvement in hardness. The synergistic effect arising from the optimal bonding and uniform dispersion probably maximized the reinforcing properties of TiC (Fan et al., 2020Fan, K., Zhang, F., Shang, C., Saba, F., Yu, J. (2020). Mechanical properties and strengthening mechanisms of titanium matrix nanocomposites reinforced with onion-like carbons. Compos. Part A Appl. Sci. Manuf. 132, 105834. https://doi.org/10.1016/j.compositesa.2020.105834.). In comparison to other concentrations, this synergy contributed to an overall enhancement in microhardness. The correct concentration of TiC diminished the likelihood of structural flaws or disruptions within the matrix. A uniform distribution and strong bonding decreased the presence of voids, discontinuities, or other structural imperfections that could compromise microhardness. The decrease in microhardness with the addition of 16 wt.% of TiC was due to the over-accumulation of TiC into matrix AA7075. The decline in microhardness resulting from incorporating 16 wt.% TiC could stem from factors such as agglomeration, over-reinforcement, interstitial alloying, structural changes, or suboptimal processing conditions (Golla et al., 2023Golla, C.B., Babar Pasha, M., Rao, R.N., Ismail, S., Gupta, M. (2023). Influence of TiC Particles on Mechanical and Tribological Characteristics of Advanced Aluminium Matrix Composites Fabricated through Ultrasonic-Assisted Stir Casting. Crystals 13 (9), 1360. https://doi.org/10.3390/cryst13091360.). The specific reasons for this effect would be contingent on the properties of the composite material, the interactions between TiC and the matrix, and the entirety of the manufacturing process.

3.2. Density test

The density of both non-aged and aged AA7075 composites was determined using the Archimedes principle, and the results are presented in Table 6. A comparison of the density of non-aged and aged composites is illustrated in Fig. 6. The incorporation of high-density titanium carbide enhanced the density of AA7075/TiC composites up to a 12 wt.% inclusion and decreased the density with a 16 wt.% inclusion in both aged and non-aged conditions. The addition of TiC, characterized by its high density, contributed to an overall elevation in the composite’s density. When these denser TiC particles were uniformly dispersed throughout the matrix, they introduced additional mass to the material, leading to an increased overall density. At concentrations below 12 wt.%, TiC particles had the ability to effectively occupy voids within the matrix, decreasing porosity and fostering a more condensed structure. This filling process resulted in an augmented overall density of the composite (Manohar et al., 2023Manohar, G., Pandey, K.M., Maity, S.R. (2020). Aluminium (AA7075) Metal Matrix Composite Reinforced with B4C Nano Particles and Effect of Individual Alloying Elements in Al Fabricated by Powder Metallurgy Techniques. J. Phys. Conf. Ser. 1642, https://doi.org/10.1088/1742-6596/1451/1/012024.). Incorporating TiC could lead to heightened porosity and the clustering of particles beyond a certain concentration (16 wt.% in this instance). An excess of TiC might generate spaces or voids between particles, elevating porosity and reducing the material density (Wang et al., 2023Wang, Y., Monetta, T. (2023). Systematic study of preparation technology, microstructure characteristics and mechanical behaviors for SiC particle-reinforced metal matrix composites J. Mater. Res. Technol. 25, 7470-7497, https://doi.org/10.1016/j.jmrt.2023.07.145.). Due to alterations in the composite’s microstructure, the density might be influenced by the ageing process. The precipitation and structural changes induced by ageing could affect the distribution and interaction of TiC particles within the matrix, thereby modifying the overall density. The elevated concentration of titanium carbide in the AA7075 matrix increased porosity, leading to a reduction in density. Notably, the highest density was observed in the AA7075/12wt.%TiC composite for both aged and non-aged composites. The higher density of 2.889 g×cm^-3 was obtained in aged AA7075/12wt.%TiC MMC.

3.3. Tensile strength

The tensile strength of both non-aged and aged AA7075/TiC MMC was determined by conducting a tensile test. From Fig. 7 and Table 7, it can be concluded that the tensile strength of aged AA7075/TiC MMC was higher than that of its non-aged counterpart. Additionally, the addition of TiC to the AA7075 matrix material enhanced tensile strength up to 12 wt.% but decreased it with the addition of 16 wt.%. The higher tensile strength of 312.8 MPa was obtained from aged AA7075/12wt.%TiC MMC. The introduction of TiC particles into the AA7075 matrix had the potential to enhance the tensile strength through mechanisms such as load transfer, Orowan strengthening, and dislocation pinning (Khan et al., 2022Khan, A.H., Shah, S.A.A., Umar, F., Noor, U., Gul, R.M., Giasin, K., Aamir, M. (2022). Investigating the Microstructural and Mechanical Properties of Novel Ternary Reinforced AA7075 Hybrid Metal Matrix Composite. Materials 15 (15), 5303. https://doi.org/10.3390/ma15155303.). The aging process resulted in the formation and growth of precipitates, such as the η’ (eta prime) phase, contributing to the increased tensile strength of the alloy. The effective reinforcement and strengthening provided by the TiC particles could be responsible for the increase in tensile strength of up to 12 wt.% upon the addition of TiC. The bonding strength of TiC with matrix AA7075 is high, which causes the enhancement of tensile strength. Beyond a 16 wt.% concentration, an excess of TiC particles could lead to issues like clustering, agglomeration, or other adverse effects, diminishing the overall effectiveness of reinforcement and potentially inducing stress concentrations. The fractured surfaces of aged AA7075/xTiC (x=0, 4, 8, 12, and 16 wt.%) MMC were analyzed with the help of an SEM. The SEM image of the fractured surfaces of AA7075/TiC MMCs is displayed in Fig. 8. The SEM image of the fractured surface of AA7075 revealed that the fractured surface was due to ductile failure. The addition of TiC into matrix AA7075 converted the fracture mechanism from ductile to brittle. The SEM image of AA7075/12wt.%TiC MMC confirmed that the obtained fracture mechanism was partially ductile and partially brittle. The SEM image of the fractured surface of AA7075/16wt.%TIC confirmed that the obtained fracture mechanism was purely brittle. Images from the SEM were useful in analyzing the fracture surface morphology. The presence of a partially ductile and partially brittle fracture in the AA7075/12wt.%TiC MMC implies a more complicated fracture behavior, perhaps involving matrix and reinforcement interactions (Bedolla-Becerril et al., 2023Bedolla-Becerril, E., Garcia-Guerra, J., Lopez-Morelos, V.H., Garcia-Renteria, M.A., Falcon-Franco, L.A., Martinez-Landeros, V.H., García-Villarreal, S., Flores-Villaseñor, S.E. (2023). Tribological Behaviour of Al-2024/TiC Metal Matrix Composites. Coatings 13 (1), 77. https://doi.org/10.3390/coatings13010077.). The purely brittle fracture in the AA7075/16wt.%TiC MMC, on the other hand, indicates that the material is more prone to brittle failure, presumably due to a greater TiC concentration and clustering.

3.4. Compressive strength

The compressive strength of non-aged and aged AA7075/xTiC (x=0, 4, 8, 12, and 16 wt.%) is displayed in Table 8 and Fig. 9. From Fig. 8, the inclusion of TiC into matrix AA7075 improved compressive strength up to 12 wt.% and decreased by 16 wt.% in both non-aged and aged AA7075/TiC MMCs. The higher compressive strength of 326 MPa was obtained from aged AA7075/12wt.%TiC MMC. In the presence of lower TiC concentrations (up to 12 wt.%), the reinforcement had the potential to markedly enhance compressive strength through mechanisms like load-bearing, dislocation prevention, and plastic deformation. The introduction of TiC particles at an optimal concentration could decrease resistance to compression, leading to an overall increase in compressive strength. Upon surpassing the optimal concentration (16 wt.%), the additional TiC particles might initiate adverse effects, including agglomeration or clustering (Sambathkumar et al., 2017Sambathkumar, M., Navaneethakrishnan, P., Ponappa, K., Sasikumar, K.S.K. (2017). Mechanical and Corrosion Behavior of Al7075 (Hybrid) Metal Matrix Composites by Two Step Stir Casting Process. Lat. Am. J. Solids Struct. 14 (2), 243-255. https://doi.org/10.1590/1679-78253132.). The surplus TiC could induce stress concentrations and act as sites for fracture initiation, leading to a reduction in compressive strength. The clustering of reinforcement might hinder the material’s ability to undergo plastic deformation, consequently contributing to earlier failure. Higher concentrations might also lead to uneven distribution or clustering, resulting in localized weaknesses and diminished compressive strength.

3.5. Metallurgical Examination

The aged AA7075/12wt.%TiC composite underwent metallurgical examination such as EDAX and XRD to determine the presence of elements and precipitates. The EDAX microanalysis is displayed in Fig. 10. The elements Al, Ti, C, Zn, Cu, Mg, and Si were present in the aged AA7075/12wt.%TiC MMC. The XRD result of the aged AA7075/12wt.%TiC composite confirmed the presence of precipitation. The XRD pattern is displayed in Fig. 11. The precipitates Mg₂Zn, Mg₂Si, and reinforcement TiC enhanced the micro hardness, density, tensile, and compressive strength (Prakash et al., 2023Prakash, J.U., Jebarose Juliyana, S., Salunkhe, S., Gawade, S.R., Nasr, E.S.A., Kamrani, A.K. (2023). Mechanical Characterization and Microstructural Analysis of Stir-Cast Aluminum Matrix Composites (LM5/ZrO2). Crystals 13 (8), 1220. https://doi.org/10.3390/cryst13081220.). The high hardness characteristics of Mg₂Zn, Mg₂Si, and TiC enhanced the mechanical properties of the aged AA7075/12wt.%TiC composite.

The SEM image of the microstructure of the aged AA7075/12wt.%TiC composite is displayed in Fig. 12. The titanium carbide is spread uniformly over the entire matrix material. The SEM image of the aged AA7075/12wt.%TiC MMC reveals that there is no oxide formation. The Mg₂Zn, Mg₂Si, and Al₂Cu precipitates are formed during aging, and they are displayed in the SEM image of AA7075/12wt.%TiC composite. The hard nature of Mg₂Zn and Mg₂Si enhances the micro hardness, tensile, and compressive strength of the aged AA7075/12wt.%TiC MMC. The high ductile property of Al₂Cu enhances the ductile property of the aged AA7075/12wt.%TiC MMC (Rajaram et al., 2022Rajaram, S., Subbiah, T., Mahali, P.K., Thangaraj, M. (2022). Effect of Age-Hardening Temperature on Mechanical and Wear Behavior of Furnace-Cooled Al7075-Tungsten Carbide Composite. Materials 15 (15), 5344. https://doi.org/10.3390/ma15155344.).

3.6. Optimization of EDM parameters

The optimization of EDM process parameters for the best combination of aged AA7075/12wt.%TiC MMC was performed using GRA optimization. The EDM input parameters for optimization were wt.% of chromium concentration, pulse-on time, and current. The response parameters for optimization were TWR and SR. The L9 orthogonal array was employed for optimization, and Table 9 shows TWR and SR for the L9 input parameters combination. Each combination of EDM input parameters of L9 was set in the EDM machine separately, and responses (TWR and SR) were noted. The TWR and SR for L9 OA were optimized using GRA optimization, which involved normalization, deviation sequence, GRC, and GRG steps. Table 10 displays the results of GRA optimization for EDM process parameters of aged AA7075/12wt.%TiC MMC. A higher value of the normalization value of L8 represents lower TWR and SR, while a lower value of normalization of L3 represents higher TWR and SR. Similarly, higher values of deviation sequence, GRC, and GRG of L8 represent lower TWR and SR, while lower values of these parameters for L3 represent higher TWR and SR. Therefore, the GRA optimization of EDM process parameters for aged AA7075/12wt.%TiC MMC indicates that lower TWR and SR were obtained from the L8 combination of input parameters, while higher TWR and SR were obtained from the L3 combination. The obtained GRG values of TWR and SR were subjected to Taguchi optimization using Minitab 19 software, as shown in Table 11. From the response table, a higher delta value of wt.% of chromium concentration represents the most influencing parameter on GRG, followed by current and pulse-on EDM input parameters. The optimized combination of EDM input parameters is a high level of chromium concentration (8 g×l^-1), a low level of current (5 amps), and a medium level of pulse-on time (240 µs). The influenced sequence of EDM input parameters is chromium concentration, current, and pulse-on time. The Main effect plot for means of GRG is displayed in Fig. 13, showing that an increase in chromium concentration reduces TWR and SR by improving GRG value, and an increase in pulse-on time reduces TWR and SR up to 240 µs and increases by 360 µs. The increase in current increases TWR and SR due to an increase in heat generation beyond the machining area. The increase in chromium concentration improves heat dissipation capability, resulting in a smooth surface area on the workpiece, and TWR is also reduced. Table 12 provides factor information for ANOVA, displaying the lower, middle, and higher levels of chromium concentration (0, 4, and 8 g×l^-1), pulse-on (120, 240, and 360 µs), and current (5, 10, and 15 amps). Table 13 presents the ANOVA for GRG of optimized EDM input parameters of aged AA7075/12wt.%TiC MMC. From the ANOVA table, it is confirmed that all input parameters are significant, as the P-value of all input parameters is less than 0.05. The contribution percentage of chromium concentration, pulse-on time, and current input parameters on GRG is 64.5%, 7.28%, and 28.2%, respectively, as depicted in Fig. 14. The figure expresses the contribution of chromium concentration, pulse-on time, and current on GRG. Chromium concentration highly influences GRG, followed by current and pulse-on time. Table 14 displays the model summary for ANOVA of GRG. The obtained adj R square value is 99.96, indicating high reliability of the experimental data and accuracy of the experiment. The confirmation test was conducted for the best combination of 8 g×l^-1 chromium concentration, 240 µs, and 5 amps current EDM input parameters, and the obtained TWR and SR are 0.198 and 1.56, respectively (Bharat N et al., 2023______, N., Bose, P.S.C., (2023). Wear performance analysis and optimization of process parameters of novel AA7178/nTiO2 using ANN-GRA method. Proceedings of the Institution of Mechanical Engineers, Part E. Journal of Process Mechanical Engineering 0(0). https://doi.org/10.1177/09544089231156074.).