Optimization of squeeze casting process parameters on mechanical properties of SiCp reinforced LM25 composites through Taguchi technique

The aim of the present work is to examine the influence of processing parameters on fabricated composites of LM25 alloy with SiC particle reinforcement through a squeeze casting technique. The following process parameters, like stirring speed from 550 to 750 rpm, SiCP (4 wt% to 8 wt %), and melting temperatures (from 600 to 700 °C) were employed. Then, the processed composites were subjected to microscopy analysis and mechanical tests to ascertain their metallurgical and mechanical properties. SEM micrographs of an LM 25 composite sample show better bonding of SiC particles with matrix, which is due to homogeneous dispersion of SiC particles in the stir casting process. The maximum tensile strength (211 MPa) and hardness (91 Hv) were achieved on the composite samples with processing parameters of 750 rpm stirring speed, 8% SiC proportions, and 650 °C melting temperature, respectively. From the design of the experiment by the Taguchi method, it is observed that the stirring speed plays a significant role in achieving a better distribution of SiC particles in the composite samples than other parameters like SiC weight ratios and the melting temperature of the alloy.


Introduction
For the last two decades, aluminium and its alloys have been considered highly potential materials for applications used in the automotive and aircraft industries. Owing to the presence of hard strengthenedparticles, metal matrix composites are significant and capable of attaining the essential mechanical properties [1][2][3]. At the same time, due to their efficient performance and environmental freedom, these MMCs have benefited from a broad modification in this research zone over the last two decades [4]. The MMCs have materialized into world-class materials in particular domains like thermal, logistics, structural, and transport applications, which can effectively reinstate ferrous-based components in maximum wear resistant applications [5]. In automobile and aerospace applications, fan blades, brake linings, cylinder blocks, pistons, aircraft engines, brake drums and clutches are made up of aluminium composite materials because of good corrosion resistance and light weight [6,7]. In addition, their tribological characteristics are also significant in some applications where homogeneous attributes are to be accomplished right through the automobile body, especially in mechanical structures, chassis, and cross members [8][9][10][11][12]. The size of reinforcement particles, bond potency, and bonding strength are influencing factors in achieving wear resistance in automobile industries. In the past few years, uninterrupted and continuing studies have been conducted to enhance the properties of Almetal matrix composites by utilising various strengthening particles like Zirconium Oxide (ZrO2), Aluminium Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. oxide (Al2O3), Titanium Carbide (TiC), Chromium Carbide (Cr3C2), chromium oxide (Cr3O2) boron carbide (B4C) and other particles to attain maximum effectiveness [13][14][15][16][17].
There are various fabrication techniques (solid, semi-solid, and liquid states) to produce the aluminiumbased metal matrix composites. In the liquid state process, spray deposition, squeeze, and stir casting fabrication techniques are commonly utilised to produce MMCs.The fabrication methods of metal matrix composites play a very important role in the enhancement of mechanical properties. In conventional fabrication methods, problems such as poor wettability, uneven dispersion of particles, and porosity have been degrading the performance of MMCs [18][19][20][21][22]. Therefore, to avoid such issues and to attain homogeneous dispersion in the fabricated MMCs, squeeze casting technique can be preferred in this research. The abrasive wear rate of LM-6 alloy which was composed by conventional casting is higher while contrasted to squeeze casting process. Senthil et al [23] studied the mechanical properties of friction stir processed LM25 alloy with addition SiC particles. They concluded that the increment of SiC particulates in the LM25 alloy would increase the mechanical properties. Arjun Raj et al [24] demonstrated the preparation methods for production of LM 9 MMCs with different reinforcements of boron nitride and tungsten carbides through squeeze technique. Mourad et al [4] investigated the AMCs of scrap aluminum alloy with reinforcements of graphite particulates, SiC and Alumina through squeeze casting and studied theirweldability behavior in the alloys. They reported that the presence of SiC has enhanced the mechanical properties and wear resistance of the alloy under abrasion applications. Kanu et al [25] synthesized the MMCs of LM25 and SiC proportions at 10% through the ultrasonic treatment process. They explored that the fabricated MMCs mechanical properties have improved, but the ultrasonic casting procedure is very difficult and the cost is also high.From these detailed investigations, the LM based alloys are found to be more appropriated for squeeze casting with fitted reinforcement of SiC to enhance the mechanical properties. The squeeze casting aids to developthe entire refinement of the grain size in MMCs with zero percent porosity. In this research, LM25 alloy with reinforcements of SiC particulates was synthesized by the squeeze technique with different processing parameters like various proportions of SiC, Stirring speed, and melting temperature with the help of Taguchi L9 [21]. Based on the literature, those parameters are finalised, and it is important to consider melting temperature and squeeze pressure for improving the mechanical properties. Therefore, pressure is maintained constant at 125 MPa for all levels while varying the melting temperature between 600 and 700°C. These parameters really support the refinement of grain structure and, thus, superior mechanical attributes. Hence, the present investigation focuses on recognizing the influence of SiC proportions from 4 to 8% strengthening particles in the LM25 alloy and investigating the mechanical properties of tensile strength and hardness on composites. Then processing parameters was analyzed through Taguchi based S/N ratios and the grey method. A limitedstudyon analysing the combination of these parameters and influencing these optimisation techniques to find the optimal parameters was reported [26].

Materials and experimentation
In this research work, LM25 alloy was chosen as the base alloy due to its superior castability, maximum strength, and better corrosion resistance, which are more significant aspects for the requirements of industrial applications. This alloy is generally referred to as an Al-Si-Mg alloy, and its chemical compositions are as follows: 6.6 wt% Si, 0.1 wt% Cu, 0.1 wt% Mn, 0.01 wt% Sn, 0.6 wt% Mg, and remaining Al [27]. The calculated quantity of LM25 alloy ingot was melted in a graphite made crucible and superheated at various temperatures from 650°C, 700°C and 750°C. Then, in the lateral stage of melting, the prepared SiC particles with various proportions from 4 to 8% having a size of 20 nm were preheated at 350°C, and finally, processed reinforced particles were added into the molten LM25 alloy in the squeeze casting furnace equipment, and the liquid metal was stirred at the stirring speeds of 550, 650, and 750 rpm, respectively. In these circumstances, 1% magnesium is added during the casting process to improve the wettability of the particulates. Then, the molten alloy and its reinforcements were allowed to solidify under supports of 380 N m −2 axial pressure, and fabricated MMCs were extracted from the metal die having a diameter and length of 50 and 150 mm, respectively. Table 1 shows the processing parameters of the squeeze casting technique through the Taguchi L9 array.
The fabricated MMCs were prepared as per the Taguchi experiments, and then SiCp reinforced MMCs samples were utilized for mechanical tests like tensile strength and hardness as per ASTM A370 standards. Three average samples were prepared for each test. During the testing of hardness and microstructure, samples were prepared from the centre portion of cast composites. For tensile testing, samples were prepared from the right and left sides of cast composites. The HMV-G-Shimadzu and UTN 20-FIE Model is used for conducting hardness and tensile tests. SEM analysis was carried for the investigated samples by the EVO MA18-Zeissmademicroscope.The morphological characteristics of cast samples were examined by an optical microscope, and then optimization of the process parameters by S/N ratios method was performed. Finally, SEM with EDS was used for those optimized specimens (by S/N ratio method) and minitab software was utilized for analyzing the optimization process. Figure 1 depicts the graphical representation of the present research work.

Microstructure of LM25 and SiC composites
Based on the outcomes suggested by Taguchi design, nine squeeze cast samples were successfully produced using a locally fabricated squeeze casting machine. Then, optical microscopy was carried out on the these nine samples to analyze the microstructures and their micrographs are reported as shown in figure 2, figure 3 and figure 4. Figure 2 shows optical micrographs of the samples with 4%SiC, 6% SiC and 8% SiC at 550 rpm stirring speed, and 600°C, 650°C and 700°C respectively. Similarly, figures 3 and 4 show the micrographs of the samples with same percentages of SiC particles and pouring temperatures at 650 rpm and 750 rpm respectively. From these micrographs, it is clearly shown that the presence of SiCp on LM25 alloy results in a more uniform and homogeneous dispersion.Particularly, the sample made with the following process parameters (750 rpm of stirring speed, 8% SiC and 650°C of melting temperature) showed better uniform refinement of SiC in the matrix of α-Al. It is also observed that samples are free frompores and cracks in the as-cast condition. Figures 2(a) to (c) show the microstructure of sample with 550 rpm of stirring speed, 4, 6 and 8% SiC, and 600, 650, and 700°C of melting temperature. Figures 2(b) and (c) exhibit the partial homogeneous dispersion owing to the lesser stirring speed that led to the generation of the agglomeration at a lower level.These parameters influence the composite to improve the mechanical attributes by even dispersion particles in the LM25 alloy base matrix.
Compared to the previous level of 550 rpm stirring speed, the samples produced at 650 rpm stirring speed have better mechanical properties due to the homogeneous dispersion of SiC particles in the LM25 alloy. Similarly, the tensile strength and hardness values of these samples were found to be marginally improved. From figures 3(b) and (c), it is observed that the presence of SiC is clearly visible due to the amount of SiC particles, and it is also observed that no agglomeration of SiC particles is seen in this sample. As evidenced from the  microstructures, the bonding of SiC particles with α-Al matrix is very good at 650 rpm stirring speed compared to other 550 rpm and 750 rpm stirring speeds. The optimum stirring speed produces more uniform distributions of particulates in the Al matrix as well as very good wettability [28]. The optical micrographs were carried out to examine the dispersion of SiC particles in the form of interfacial bonding and matrix arrangements in between the base matrix and SiC particles. Also the small agglomeration and even distribution of nanoSiC powders were dispersed homogeneously in the structure of matrix. However, some micro pores and cracks were formed in the initial level of processing parameter samples due to severe agglomeration and non homogeneous dispersion in the matrix structure [29][30][31].

Mechanical propeties of squeeze casted LM25 alloy and SiC particles
In this section, the mechanical properties of squeeze cast MMCs were explained in detail with respective contour analyses. The main significant aspect of contour analysis is determining the correlation between the different processing parameters. Table 2 shows the taguch L9 based design array of processing parameters with their mechanical responses like microhardness and tensile strength of LM25 alloy and SiC casted MMCs, as well as their standard deviation and error values of mechanical responses. Figures 5(a) to (c) show the contour interpretation of various squeeze casting processing parameters and the microhardness of composites of LM25 alloy and SiC reinforcing particles [32]. In figure 5(a), the contour between stirring speed and various proportions of SiC strengthening particles is displayed. It is explored that the hardness value is increased by the increment of SiC proportions from 6 to 8% with increasing stirring speed from 550 to 750 rpm. Figure 5(b) shows the contour analysis between the various proportions of SiC and melting temperature. From this figure, it is understood that the harndess is increased by the establishment of a SiC proportion of 6 to 8% and a moderate melting temperature 650°C. Figure 5(c) exhibits the contour relation between the melting temperature and stirring speed on the squeeze-cast composites. From figure 5(c), it is implicit that the hardness is enhanced by increasing the stirring speed from 550 to 750 rpm and improving the melting temperature from 600 to 650°C. The above discussion is based on the contour relation among the various different squueze cating process parameters. The proportion of SiC at 8%, maximum stirring speed, and moderate melting temperature are the major reasons for enhancing the hardness. The addition of strengthening particles to the LM25 alloy contributes to the better hardness due to strong interfacial bonding between the base matrix and SiC particles and also keeps away from localised deformation of the matrix alloy.   figure 6(a), it is understood that the tensile strength is improved by increasing the stirring speed from 550 to 750 rpm and adding SiC from 4 to 8%. Therefore, the maximum stirring speed blends the matrix alloy and reinforcing particles effectively and composes the bonding between the SiC and LM25 alloy in a better manner. Figure 6(b) exhibits the different SiC ratio and melting temperatures on the tensile strength of casted MMCs. From this figure, it is concluded that the tensile strength is superior at 8% SiC and moderate at 650°C of melting. Due to the proper blending that is accomplished at 650°C with 8% SiC, it produces a better homogeneous dispersion. The maximum tensile load was applied to the carbide particle region, and failure took place at non mixed SiC zones in the LM25 base matrix alloy. Figure 6(c) illustrates the effect of melting speed and melting temperature on the tensile strength of casted MMCs. From figure 6(c), the maximum tensile strength is attained at the maximum range of stirring speed and a moderate level of melting temperature. From this discussion, the appropriated temperature and maximum stirring speed compose the better interfacial bonding owing to the improvement in deformation resistance attained by the increment of SiC. Similarly, by enhancing the dislocation density, the crack development hinders the SiC particles. Finally, figures 7 and 8 show the overall graphical representation of hardness and tensile strength with different runs.

Optimization of squeeze casting process parameters with Taguchi S/N ratios
In this research, Mini Tab software wasutilized to establish the signal-to-noise ratio values. These values are derived from the processing parameters to influence mechanical outcomes like tensile strength and hardness [33]. Initially,  these parameters are designed by the Taguchi L9 array and add parameters like stirring speed, weight proportion of SiC, and melting temperature as input factors. Then determined S/N ratio values are obtained, and prior to finding the S/N ratio values, the larger the better attributes were considered for this investigation to maximize the    mechanical properties [34]. The attained mean table of S/N ratio clearly exhibits the maximum delta value from their averages of overall S/N ratio values, which is displayed in table 3. From table 3, the maximum delta value of the S/N ratio is attained at the process parameter stirring speed, which contributes as the first rank and whose delta value is 1.72, Next to stirring speed, SiC wt% (0.47 Delta) and melting temperature (0.45 Delta) are the second and third ranks, respectively. But the percentage contribution is very important for analyzing the process parameters; therefore, ANOVA is used to find the optimal parameters based on the maximum percentage contribution.
In this analysis, the 95% confidence level and 5% significance level are fixed for ANOVA analysis, and the P value is achieved as 0.031, which is the reference value of 0.05 in the stirring. Similarly, the maximum percentage contribution is achieved in stirring speed (92.2%), followed by SiC proportions (6.1%) and melting temperature (1.6%), respectively. Table 4 shows the ANOVA for squeeze casting process parameters with their contributed percentages. The R-Sq and its Adj values is 97.12 and 88.48% in the overall model summary, respectively. Figure  9 shows the main effect plots of S/N ratios for various processing parameters. The main effect plot is the most important aspect for enhancing the parameter level; therefore, in this mean effect plot, it is shown that the stirring speed at 650 rpm, SiC proportions at 8%, and melting temperature at 700°C are the optimum conditions for this investigation. From this review, stirring speed is the most important aspect of this research, due to the significant stirring speed, maximum mechanical properties are achieved by the effective casted composite samples. The optimization outcomes clearly indicate the optimal processing parameters, like stirring speed, SiC proportions, and melting temperature, in this order. Based on the optimization results, the samples were further subjected to SEM with EDS analysis to study their microstructural features, and their corresponding runs have lower, moderate, and higher result parameters, i.e., samples 1, 5, and 9, respectively.

SEM with EDS investigations on squeeze cast samples
Based on the optimization results, SEM analysis was performed on samples 1, 5, and 9. Those samples are related to the lower, medium, and maximum mechanical responses of tensile and hardness values, and these values are behind the processed casting parameters like stirring speed, which is the most influencing factor compared to the other parameters like SiC proportions and melting temperature. Samples 1, 5 and 9 belong to the following parameters: 550, 650, and 750 rpm of stirring speed, 4, 6 and 8% of SiC and 600, 700 and 650°C of melting temperature, respectively. SEM micrographs of samples 1, 5, and 9 are shown in figures 10, 11 and 12. From  figure 10, it is observed that the presence of SiC at 4% requires a minimum stirring speed (550 rpm) and a minimum melting temperature (600°C). This micrograph exhibits the accumulation of SiC at a very low level due to the minimum percentage of SiC added, and the appearance of SiC also occurs rarely. At the same time, the minimum stirring speed is not stirred properly in the bonding regions, resulting in poor bonding, which causes poorer mechanical properties. This sample's values of tensile and hardness are 154 MPa and 69 HV, which are much lower than those of samples 5 and 9.Owing to a lesser proportion of SiC and minimum of stirring speed, composites in heterogeneous dispersion and poor wettability are also attained in these samples. From figure 11, it is explored that the moderate level of stirring speed (650 rpm), SiC proportions (6%) and maximum melting temperature (700°C). This micrograph showed better homogeneous dispersion, it is accomplished between the bonding of SiC and LM 25 base matrix alloy. Similarly, the less agglomeration is observed in the samples with better processing parameters due to better mixing and superior wettability. The increased reinforcement particles would help to improve the mechanical properties,especially tensile and hardness, which are 181 MPa  and 84 HV respectively. Figure 12 shows that the addition of SiC strengthening particles reduces the grain size of base matrix LM 25 and owing to the lesser particle size and pinning action, grain boundaries are restrained, limiting the grain coarsening. Similarly, the maximum stirring speed leads to better interfacial bonding in the LM 25 and SiC composites, and the addition of SiC diminishes the gaps in the interdendritic and reduces the residual stresses. It causes no pores, and no cracks were formed in the processed composites.
3.5. SEM with EDS of optimized squeeze cast samples SEM with EDS analysis was carried out on the samples 1, 5, and 9, because these samples were considered to have lesser, moderate, and high-level properties in the optimization study for examining the grain structure and bonding strength.Based on the optimization results, it is observed that the samples with the maximum fraction of SiC showed lower percentages of agglomeration, which eventually improved the bonding strength of the composite samples. SEM with EDS analysis confirmed the presence of SiC distributions, and other elements were also mapped through the intensity peak level. Figures 13 to 15 show the SEM with EDS analysis of samples 1, 5 and 9. From figures 11(a) and (b), 550 rpm, 4% and 600°C composite samples were employed for SEM and EDS. It is shown that the presence of SiC is in the minimum range, and improper blending occurred with minimum stirring speed and a lower melting temperature, so that the bonding was not satisfied while in SEM and a low percentage was measured by the EDS process. Porosity is presents in minor due to poor wettability and processing parameters. From the figure 13(a), it is showed that the presence of SiC particles were clearly mentioned and micro voids were occurred during the SEM analysis [25]. Figures 14(a) and (b) shows the SEM with EDS of sample 5 at 650 rpm, 6% and 700°C. From these figures, it is understood that the SiC is majorly presented and it is clearly confirmed by the peak level. From figure 14(a), it is implicit that the SiC particles were dispersed homogeneously due to the maximum weight ratio presented. The agglomeration and micropores are present very little, and the medium of stirring speed and maximum melting temperature are also the major reasons for improving the even dispersion [35]. Figures 15(a) and (b) shows the SEM and EDS of sample 9 at 750 rpm, 8% and 650°C. From these figures, SiC is presented more and other elements also measured. Figure 15(a) showed that the SiC particle with dendritic structure, which improved the bonding strength, and pores are minor due to the maximum stirring speed and minimum melting temperature [36]. Both figures 12 and 13 exhibit the yielded strengthening particles of SiC agglomeration, which diminish the porosity and also reduce interdendritic gaping by the addition of SiC. The maximum stirring speed is also a significant factor in improving homogeneous dispersion and reducing the inferior dendritic . Similarly, strong bonding can also be generated properly by controlling the process parameters [37].

Conclusion
In this research, metal matrix composites of LM25 alloy with SiC reinforced particles were successfully produced. The conclusions drawn from the present works are as follows: 1. The influence of squeeze casting process parameters like stirring speed, various SiC proportions, and melting temperature is effectively optimised by the Taguchi S/N ratio technique.
2. Samples with 8% SiC, 750 rpm of stirring speed, and 650°C of melting temperature have better mechanical properties (211 MPa of tensile strength and 91 HV of hardness).
3. The contour analysis really supports the correlation between the process parameters. The ANOVA table confirmed that the stirring speed (92.2%) contributed the maximum percentage compared to the other parameters. From the main effect plots, the mean of the S/N ratios proves the stirring speed (1st rank), SiC proportion (2nd rank), and melting temperature (3rd rank). By adding SiC to the LM25 alloy, it exhibits fine interfacial bonding with even dispersion of SiC at maximum stirring speed. The 8% SiC and maximum stirring speed improve the mechanical properties, which are suited for aerospace applications in aircraft pumps and automobile applications in cylinder blocks.

Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.