Wear and Frictional Behaviour of Al 7075/FA/SiC Hybrid MMC’s Using Response Surface Methodology

In the present research work, an effort was made to study the wear and frictional behaviour of Aluminium metal matrix composite (Al 7075as a base alloy and fly ash (FA) and silicon carbide (SiC) as reinforcements) by using the stir casting method. To carry out this work, the wt.% of reinforcements FA (2.5%, 5%, 7.5% and 10%) and SiC (2.5%, 5%, 7.5% and 10%)5%, 10%, 15% and 20% were used. Initially, the mechanical studies were conducted, and to arrive the mechanical properties obtained at 20 wt.% of FA and SiC. Later on, the composite was fabricated by 20 wt.% of FA and SiC reinforcements were used to check the wear and frictional behaviour on a pin-on-disc machine in dry condition. The dry sliding wear behaviour was carried out at various input parameters such as applied force (10 N, 20 N, and 30 N), sliding velocity (1.5 m/s, 3 m/s, and 4.5 m/s), and sliding distance (1000 m, 2000 m, and 3000 m). Further, a scanning electron microscope (SEM) was used to study the microstructural behaviour of the composites and the worn surfaces during wear test. A response surface methodology (RSM) is a reasonable and accurate method for conducting the experiments and identifying the optimal wear parameters. Moreover, the RSM helped to identify the most significant factor, which was the influence on the wear rate. Finally, it was found that the applying load is the utmost significant factor that affects wear rate. The sliding velocity and sliding distance are found to have less influence on the performance indicator.


Introduction
In recent years, the usage of aluminum metal matrix composites (AMMC's) are increasing in aerospace, naval, and automobile industries due to their superior mechanical, tribological, microstructural properties and lightweight. In general, AMMC's are modified based on the said application and numerous benefits, namely low coefficient of friction, adequate wear resistance, superior strength-to-weight ratio, and high corrosion resistance. Therefore, wear is a significant parameter duly considered while designing and manufacturing any machine elements and ensuring the performance is improved [7,21]. Through the particle dispersion, solid lubricant in the aluminum alloy can display better potential for wear resistance. Moreover, graphite is added in aluminum alloy because of better properties like low friction, chemical inertness, film-forming ability, and inherent absence. The addition of graphite particulates with aluminum alloys can diminish the wear rate and reduce flexural strength and hardness [12]. Most researchers add both ceramics and lubricants to develop hybrid composites to overcome the above problem, leading to better mechanical and tribological properties [17,18,26,27,30]. Al 7075 has owing the high strength and greater potential in dry sliding wear applications [2,9]. Many researches [19,24,29] were done to study the wear behaviour of Al 7075 alloy reinforced with various ceramic reinforcements like silicon carbide (SiC), titanium carbide (TiC), alumina (Al 2 O 3 ), graphite (Gr), titanium boride (TiB 2 ), etc. An investigation was conducted on the effect of SiC and its particle size of Al 7075-SiC composites and observed a significant enhancement in the composite mechanical and tribological properties of the composite when compared with base material [16]. Added to this a study [5] was carried out on the performance of wear on Al 7075 reinforced with Al 2 O 3 composites and it was found that the adding of ceramics improves the wear resistance.
Further, an investigation [3] has been carried out on Al 7075 reinforced with B 4 C composites and observed that the enhancement in hardness and wear resistance, but there is a reduction in COF. Baskaran et al. [6] discussed the wear behaviour of Al 7075 reinforced with TiC and it was observed that the behaviour of wear reductions under dry conditions. Ranjan et al. [14] fabricated a composite using Al 7075 reinforced with TiB 2 particles and identified the tribological behaviour on developed composites, and they mentioned higher wear resistance with increasing the TiB 2 particles. Recently, Idrisi et al. [11] fabricated a composite by Al6061 reinforced with SiC particles in various fractions using an ultrasonic vibration-based stir casting process. After adding SiC particles, the author's observed the density, hardness, tensile, and compressive strength increased compared to the traditional stir casting process. In [10], the authors reported the hardness decreased with an increase in the graphite percentages within aluminum matrix. Moreover, Akhlaghi et al. [1] fabricated a composite by varying the Gr from 5% to 20% and Al2024 as a matrix. After the experiments conducted, the authors reported that the composite's hardness and toughness reduced after adding the Gr as reinforcement. Further, Baradeswaranet al. [4] discussed after adding the Gr with Al 7075, the hardness decreased, whereas the tensile strength was found increasing. In [15], Mohankumar et al. fabricated a composite with Al 359 as a matrix and B 4 C and fly ash as the reinforcements using the stir casting technique. They discussed the hardness, increase in tensile strength, and increase in wear resistance after adding fly ash and B 4 C content. Dipankar et al. [8] fabricated a composite with the base metal Al 2024 and different weight percentages of SiC particulates and conducted the mechanical and tribological tests. The authors concluded that because of SiC particulates, the mechanical and tribological properties improved than the matrix alloy. In [22], the authors discussed about the dry sliding wear behaviour of Al 6082 reinforced with red mud particles. The authors observed that with higher content of red mud particles, the hardness, yield, and tensile strength increased, where as a reduction in the impact strength was observed. Further, due to the involvement of high red mud particles, the wear rate of the specimens decreased with an increase in the sliding distance. Later on, Samal et al. [20] studied the mechanical and sliding wear properties of TiC reinforced AA5052 composites. The mechanical properties enhanced with excellent interfacial bonding and uniform dispersion, while the composites exhibited higher wear resistance than the base alloy.   Therefore, the current research work is to develop the commercially based Al 7075 reinforced with SiC and fly ash powder-based hybrid composites using the stir casting technique. Further, mechanical properties such as hardness, tensile strength, and tribological characteristics such as wear rate and coefficient friction were studied with respect to the SiC and FA content. RSM has been used for modeling the process parameters (i.e. applying a load, sliding speed, and sliding distance) and planning the number of experiments. The literature study observed that most of the alloys chosen as matrices had been the aluminium alloys; some studies have been reported on the 7xxx series alloys reinforced with SiC, TiC, Al 2 O 3 , Gr and TiB 2 particles. Significantly less attention has been given to the AA 7075 alloy matrix composites, which have the highest hardness among all Al alloys. Therefore, in the present investigation, SiC and FA particles were used as reinforcements for an aluminium (AA 7075) alloy hybrid metal matrix composite under heat-treated conditions using Response Surface Methodology. The complete flow chart of the present research work is shown in Fig. 1.

Materials and Methods
In the present section, the chemical composition of the base alloymaterials, fabrication of the composite, instruments required to conduct the mechanical and tribological tests, and the response surface methodology used for modeling and analysis of the problem are discussed.

Materials and Instruments
The matrix Al 7075 the reinforcement's fly ash and silicon carbide were procured from Venuka Engineering Private Limited, Hyderabad. The composites were prepared using a stir casting process. The mechanical properties such as hardness and tensile strength of the fabricated composites were measured on a Vickers hardness tester and Universal Testing Machine (UTM). The tribological properties such as wear and coefficient of friction were measured on pin-on-disc apparatus and the scanning electron micrographs (SEM's) were taken  from a scanning electron microscope. Further, the chemical composition of the matrix Al 7075 alloy is given in Table 1.

Fabrication of the Composite
Al 7075 as a matrix and the reinforcements of 2.5%, 5%, 7.5%, and 10% of fly ash and silicon carbide prepared a composite using stir casting (ref. Figure 2 (a)). Nearly 1000 g of Al 7075 alloy small blocks are placed in a graphite crucible and melted up to the temperature of 850°C. After the complete melting of aluminum, a stirrer was introduced into the crucible for mixing the reinforcements uniformly. The weight percentages such as 2.5%, 5%, 7.5%, and 10% of fly ash and silicon carbide powder were introduced into the crucible to prepare composite. The stirrer was rotated at 500 ± 10 rpm for 5 min, and it was made by stainless steel sustained at 850°C temperature. The stirring was continued until it was to ensure proper mixing of reinforcements in a matrix. After completion of adequate mixing, the liquid metal was poured into a preheated mild steel mould. Further, the obtained composite specimens are shown in Fig. 2 (b) were kept under T 6 heat-treated conditions for conducting the hardness, tensile strength, and wear tests.

Density Test
Measurement of density is one of the important properties to identify the porosity levels of the Al 7075 alloy and fabricated composite specimens. The experimental and theoretical densities were calculated to determine the porosity level in the base alloy and fabricated composites. The Archimedes principle (Eq. (1)) is used to determine the experimental density values, and the theoretical density values were determined by rules of mixtures (Eq. (2)).

Hardness and Tensile Test
Once the composites were prepared, the specimens are prepared for the hardness tests as per ASTM E10 standards. Moreover, determining the hardness value is an essential parameter for all fabricated composites. The conventional Vickers microhardness tester with load 1 kg was used to measure the hardness values for both base and fabricated composite materials at room temperature. The test was conducted at room temperature (28°C) and the measurement of hardness was taken at five different places on each sample to obtain an average value of hardness. Further, the tensile specimens were machined into a cylindrical shape as per ASTM E8 standards. The prepared specimens were tested on a universal tensile testing machine at room temperature. For obtaining the accurate result, three samples were tested in each category, and the average value is reported in this study.

Wear Test
To measure the wear properties on a pin-on-disk testing machine ( Fig. 3) initially, the developed composite specimens were prepared with a diameter and height of 8 mm and 50 mm, respectively. The counter face of the disc was made up of EN31 hardened steel with a thickness and diameter of 5 mm and 90 mm, respectively. Before experimenting, both sides of the specimens were polished by using different sizes of emery papers. Further, the wear test was conducted by varying the sliding speed of 1.5-4.5 m/s, the sliding distance of 1000-3000 m, and the applied load of 10-30 N under ambient conditions. While experimenting before and after each test the weight of the specimens was measured using a weighing machine with a precision of 0.001 mg. The wear rate was calculated using the difference between the volumetric loss per unit sliding distance earlier and later of each test. Moreover, normal force, friction force, and sliding surfaces of contact were continuously recorded by a load cell to measure the COF. The morphologies of the worn surfaces of the tested samples were examined using SEM to study the wear mechanism for the composites.

Response Surface Methodology (RSM)
RSM adopts both mathematical and statistical techniques, which are helpful to determine both the modeling and analysis of the problem. Moreover, this method will also help estimate the coefficients, study the responses based on the combinations, fit with the experimental data, predict the response, and check the fitted model's adequacy [28]. The sliding speed, sliding distance, and applying load are selected as independent variables and the wear rate and coefficient friction were treated as response variables. The initial input variable sliding speed was varied between the two levels 1.5 and 4.5 m/s relatives to the center point 3 m/s. The second input variable, the sliding distance, varied between the two levels 1000 and 3000 m relative centre point 2000 m. Further, the third input variable, applying load, varied between the 10 and 30 N with the centre point 20 N. Alongside, the wear rate and COF are the dependent variables that are to be examined with respect to the input variables.

Density
The variation of the experimental and theoretical densities of the composite materials are shown in Fig. 4. It was observed that the density of the fabricated composite (after adding the reinforcements such as, fly ash and SiC) was low compared to the base material. Moreover, it was observed that the density decreases when the percentage of reinforcements increases. Similar trends were also observed the previous researchers [8,20,22]. Further, the density of the experimental was low compared to the theoretical density. It may be due to the porosity increases while fabricating the composite.

Microhardness
The surface of the fabricated hybrid composite samples and base material were prepared to determine the hardness value. Figure 5 shows the Vickers microhardness values for both base and composites. It was observed that the hardness of the fabricated composite specimens was increased compared to the base aluminum 7075 alloys. Moreover, it is clearly understood that the hardness of the fabricated composite increased while increasing the weight percentage of SiC and fly ash reinforcements. The hardness of the base material to composites increased from 175 VHN to 190 VHN. It may be  because fly ash particulates consist of high in silica and alumina, which are hard. The combining effect of SiC and fly ash resists the indention energy, thus causing an enhancement in the hardness. Due to the better strain energy, the hardness of the developed composites is increased at the peripheral of the particles dispersed in the matrix. Therefore, the uniform dispersion of reinforcements causes the enhancement of the hardness and rigidity of the composite. Figure 6 shows the variation of tensile strength, yield strength, and young's modulus with varying SiC and fly ash. The tensile strength, yield strength, and young's modulus increased with an increase in the SiC and fly ash content and it is significantly greater than the strength of the base alloy. It may be because

Tensile Strength
SiC and fly ash reinforcements exhibit a perfect bonding with Al 7075 alloy, which helps in enduring more loads compared to Al 7075 alloy. The structure and properties of SiC and fly ash particles build a strong interface between matrix to reinforcements and showing better tensile strength.

Dry Sliding Wear Behaviour
After conducting the mechanical tests the hardness and tensile of the developed hybrid composites were achieved the best results at 10% reinforcements of SiC and fly ash. Further, the dry sliding wear behaviour was conducted at 10% of SiC and fly ash hybrid composites. The central composite design is basically used to design the experiment, develop the models, and analyze the results, which was successfully used previously in many machining and tribological studies [23,25]. Table 2 shows the input  parameters with their levels used for the current study. Based on the number of input variables and levels (shown in Table 2) the number of experiments were planned for the present study is given in Table 3. The significance values of the coefficients were tested at a 95% confidence level using the MINITAB 16. Later on, the developed mathematical models were established to estimate the wear rate and COF. The mathematical expressions for the hybrid composites are given below.  The Fig. 7 illustrates the wear rate and coefficient of friction of the developed composite specimens with respect to sliding distance under various load conditions from 10 to 30 N. From Fig. 7 (a), the wear rate is high when the sliding distance is small and slow; the wear rate decreased with an increase in the sliding distance. It was obvious that the wear rate is high when the applied load is 30 N compared to 10 N. It may happen the wear rate is more when a high load is applied on the specimen due to the contact area between the specimen and disc is more than the small load. With the increase in the applied load, the surface of the composites undergoing higher temperature condition, which loses its thermal stability and bonding. Therefore, the plastic deformation of the sliding surfaces was more which caused a higher wear rate [13]. Furthermore, an increase in the applied load led to the formation of larger grooves on the specimen surface. These large groves resulted in high deformation with severe wear loss. Overall, the decrease in the wear rate was credited to the composites' higher hardness, which reduced the contact area during the sliding action. Similarly, from Fig. 7 (b), the coefficient of friction is small initially and slowly increases when the sliding distance increases. The continuous sliding action due to the higher sliding distances ensured higher roughness above the surface, increased the contact duration among sliding mating parts, resulting in a higher coefficient of friction. Further, the coefficient of friction is high at 10 N than 30 N, and it may be the reason for less load.

Testing the Data and Competence of the Model
The normal probability graph of the residuals for wear rate and COF of the fabricated hybrid composites are shown in Fig. 8. It was observed that the residuals were falling on the straight line and the means of errors were distributed normally. Moreover, it can also be confirmed that there was no predictable pattern observed because all the run residues lay on or between the levels. The ANOVA technique was used to test the significance of the established model and the coefficient specifies the goodness of fits of the model. The ANOVA results related to the wear rate and COF for their levels, factors, and their interactions are given in Tables 4 and 5.In the present problem, the value of the coefficient for wear rate (R 2 = 0.9666) and coefficient of friction (R 2 = 0.9610) indicates that only less than 5% of the total variance. Further, the adjusted coefficient for wear rate (adjusted R 2 = 0.9365) and coefficient of friction (adjusted R 2 = 0.9259) are high, which means the significance of the model is high. Based on  the above results, the predicted R 2 value is also better than the adjusted R 2 . The ANOVA result of wear rate and coefficient of friction for the developed hybrid composites is shown in Tables 4 and 5 which indicates that the predictability of the developed model for wear rate and coefficient of friction is at a 95% confidence level because the value of p is less than the 0.05 in most of the cases even though the value of p is more than the 0.05 such parameters are not that much influence on the significant level. It was observed that the individual effect of all the three parameters on wear rate is significant, whereas square terms of sliding speed and applied load are also significant. With a p value of less than 0.05, the interaction terms of sliding speed, applied load, sliding distance, and speed are significant influences on the wear rate of the composites. Similarly, the individual and square terms of sliding speed and applied load are found to significantly affect the coefficient of friction, whereas all the interaction terms between the input variables influenced the friction coefficient value between the mating sliding parts of composite specimen and rotating disc. Figures 9 (a), (b), and (c) show the 3D surface and 2D contour plots of the wear rate were plotted from Table 3, to identify the responses of various combinations of independent variables. Figure 9(a) shows initially, the wear rate is higher with an increase in the applying load and is lower with the sliding distance. Later on, the wear rate was decreased with the increase in the sliding distance and applying load. It may be due to the reason at initial due to more applying load the specimen more contact with the disc and resulting in more wear. Moreover, Fig. 9(b) shows the wear rate was low at initial, and slowly wear rate increased when the sliding speed and applying load increases, but the wear rate was low at the end of the sliding speed. Further, Fig. 9(c) shows the wear rate increases when the sliding distance and sliding speed were small and slowly decrease when the sliding speed increases. It was observed that the Fig. 9 3D surface and contour plots for predicting the wear rate (a) sliding distance vs applying a load, (b) sliding speed vs applying load, and (c) sliding speed vs sliding distance applying load had the most dominant effect on wear rate concerning sliding distance and sliding speed. Figures 10(a), (b), and (c) show the 3D surface and 2D contour plots of the COFconcerningthree input variables such as sliding distance, applying load, and sliding speed. From Fig. 10(a) it was observed that the COF is high when the sliding distance is small and the applying load is small. Similarly, from Fig. 10(b) it was obvious that the COF is low when the sliding speed is high concerning applying load. It can be observed that the presence of blue regions shows in the contour plot has the minimum COF. Finally, Fig. 10 (c) shows that it has been observed that the COF is high when sliding speed low but, COF decreases slowly when increasing the sliding speed and distance. Figure 11 shows the SEM worn surface morphology of the selected specimen with the reinforcement of 10% FA and 10% after SiC. It was noted that the analysis of worn surfaces indicates that the delamination, abrasion, and adhesion mechanisms are leading in this alloy. Further, the delamination, abrasion, and oxidation on wear mechanisms were recognized and examined. Severe surface wear, delamination, peeling of the matrix, grooves, ploughing, scratches were recognized in the composite. Moreover, the aluminum alloy contacts with the EN31 steel disk to form an adhesive layer at the contacting asperities. It may happen due to the high frictional temperature produced during sliding contact at the interface. Also, in the case of the base alloy, the appearance of shear-like crack characteristics on worn surfaces as represented by arrows reveals that the dominance wear mechanism is delaminated.
The SEM micrographs of the worn surface of Al7075 hybrid composite (10% FA and 10% SiC) under the sliding distance of 1000 m, the sliding velocity of 1.5 m/s, and normal loads of 20 and 30 N at room temperature were shown in Fig.  12 (a) and (b). The composite material at 30 N shows wide and heavy delamination grooves on the surfaces, whereas distinct grooves and ridges running parallel to one another in the sliding direction, as shown with a yellow mark ( Fig. 12 (b)), are noticed composites.

Confirmation Tests
The wear behaviour of Al 7075 + 10%FA + 10%SiC hybrid composite was conducted with various input parameters using a central composite design (Table 3). The confirmatory experimentations were accompaniedto check the precision of the established model and results are displayed in Table 6. Further, the acquired investigational results were well correlated with the predicted results by the developed model. Moreover, it was observed that all the obtained confirmatory results produced an error value which is less than 5% specifies more reliability. Based on the above results, it can be ensured that the developed model was generating good and accurate results. Hence, the developed hybrid composite has been generated superior wear-resistant properties in the long run, which can be suitable for automobile, aerospace, and marine applications.

Conclusions
Based on the above experimental work, the subsequent conclusions are made as follows: & The matrix Al 7075 and reinforcements FA and SiC with different weight percentages vary from 0%, 2.5%, 5%, 7.5%, and 10% were effectively fabricated using the stir casting technique. & The composite materials exhibit improved mechanical properties with an increase in reinforcement content of 10% of SiC and 10% FA, whereas hardness and tensile strength were found to be 175 to 190 VHN and 498 to 526 MPa, respectively It can be concluded that this behaviour is due to the difference in the elastic constants and load transfer from the matrix to reinforcement. When compared with the matrix material, an increase in UTS was recorded as 5.3% and 11.2%, respectively. & The wear rate was found to be lower with a higher sliding distance, whereas, with higher applied load, it was observed that the wear rate was increased. & Central Composite Design was used to estimate the wear rate at various input parameters such as applying load, sliding distance,and sliding speed. The optimal wear rate was attained. The developed model has shown an error estimation of less than 5% which validates the design of wear experiments. & Moreover, the developed statistical ANOVA effects were revealed that the applying load was contributed toa high wear rate concerning sliding distance and sliding speed. & Further, the morphology study has been studied for optimal wear rate specimen and identifying the ploughing, asperities, adhesion, and sliding direction. & Al 7075/SiC/fly ash hybrid composites could be potential material in aerospace, automobile, and marine applications with higher mechanical and wear characteristics.