An integrated experimental and analytical approach on mechanical characterization of advanced powder metallurgy aluminium metal matrix composites reinforced with different particulates

Over the last few decades, ‘Discontinuously Reinforced Particulate Composites (DRPCs)’ are a popular class of composite materials with considerable challenge in processing, characterization and machinability because of their increased strength-weight ratio, stiffness, specific strength and oxidization when compared to various metals and their alloys. This paper discusses experimental and numerical investigation on mechanical characteristics of aluminum metal matrix reinforced with various reinforcement particulates such as silicon carbide, aluminium oxide, and zirconium oxide, compaction pressure (kN) and hold time (s) based on Design of Experiments (DOE) and Finite Element Analysis. Initially this paper discusses the process optimization of Aluminum Matrix reinforced with different particulates experimentally to identify the favourable processing conditions by varying reinforcement materials, compaction pressure (kN) and hold time (s) based on TDOE (Taguchi’s Design of Experiments). Further, this paper concentrates to determine ‘maximum principal stress, equivalent elastic strain and equivalent (von-mises) stress’ based on Finite Element Analysis (ANSYS Workbench-2023R1). The results of the experimentation showed that the highest hardness values were achieved with ZrO2 reinforcement material. Increasing the compaction pressure from 8 to 12 kN resulted in a slight decrease in surface roughness and porosity. Higher compaction pressures have assumed to facilitate better particle distribution and improved interfacial bonding, leading to smoother surfaces and lower void content. The simulation results showed that the maximum principal stress achieved were (2235.8 MPa) SiC, (3444.4 MPa) Al2O3, and (3582.5 MPa) ZrO2. The equivalent elastic strain achieved was (0.2488) SiC, (0.2421) Al2O3 and (0.262) ZrO2. The equivalent (Von Mises) stress achieved was (28751 MPa) for SiC, (24880 MPa) for ZrO2 and (26972 MPa) for Al2O3. This experimentation and simulation demonstrated that the PM process can be used to fabricate DRAMMC with different reinforcement particulates. The understanding gained experimentally and analytically from this research can be applied for future processing of Aluminum Matrix Reinforced with different particulates.


Introduction
Aluminium Metal Matrix Composites are the focus of many research studies because of their much desired combination of increased strength, thermal stability and stiffness.In particular Al 2 O 3 , SiC, and ZrO 2 ceramic particles have been used as reinforcements to improve the desirable properties of DRAMMC processed using Powder Metallurgy (PM) process.The PM process involves the formation of composite materials by mixing the metal matrix and ceramic particles, followed by compaction and sintering.The resulting composites have shown remarkable improvements in mechanical properties, such as strength and toughness, and wear resistance compared to conventional aluminium alloys.In recent years, the use of DRAMMC has increased significantly in a range of applications, including aerospace, automotive, and biomedical engineering.However, the lack of understanding of the hardness of DRAMMC remains a critical challenge in their effective application.Hardness is a critical material property that determines the resistance of to plastic deformation in a material, rate of wear, and abrasion.In DRAMMC, the hardness is influenced by various factors, including the standard and volume fraction of ceramic particles, the processing conditions, and the microstructure of the composite.In recent times, the powder metallurgy technique has become superior compared to traditional casting processes while manufacturing metal matrix composites (MMCs) [1][2][3].Currently, metal matrix composites processed through powder metallurgy are being increasingly selected and utilized in diverse fields such as aerospace, automotive, and electronics for component development [4].Furthermore, these techniques have also found application in the field of 3D printing [5].In comparison to stir casting-based composites, composites produced through powder metallurgy exhibit considerably higher porosity and higher hardness.However, this method ensures uniform distribution of reinforcements within the metal matrix, unlike conventional stir casting processes [6].In casting process of Al/SiC composites, silicon carbide particles agglomerate resulting in a weak bond with the matrix [7].Silicon carbide and alumina (Al 2 O 3 ) are commonly used reinforcement materials due to their favourable properties and due to high interfacial bonding with the aluminium metal matrix leading to high wear resistance [8][9][10][11].Numerous researchers have conducted tests to evaluate the physical and mechanical properties of Aluminium Metal Matrix Composite Specimen, processed through powder compaction process [12][13][14][15].The porosity of such composites can be controlled by varying sintering temperature and sintering duration [16][17][18].Further, the size of the reinforcing particle and its volume fraction play a crucial role in physical and mechanical characterization of Aluminium Metal Matrix Composite specimen [19][20][21][22].The effects of SiC and Al 2 O 3 on the mechanical properties of 2xxx Al alloy series through powder metallurgy and conventional methods is studied [23].The incorporation of multiple reinforcing particles, such as SiC, Al 2 O 3 , ZrO 2 etc. into aluminium matrix leads to variations in porosity and consequently alters the structural properties of Aluminium Metal Matrix Composite specimen [24,25].However, the 'Hybrid Metal Matrix Composites' have emerged as pioneer materials with novel properties and have application in Aeronautical and Robotics sector [26][27][28][29][30][31][32].Researchers [27] investigated the microstructural evolution of two types of hybrid composites, namely 'Al/SiC/graphite and Al/SiC/graphene', produced through compaction of powders where, graphene was formed within the aluminium matrix as an in situ nano-structured composite.Further, researchers [28] have examined the sintering temperature effect and compaction pressure effect on 'Al/Al 2 O 3 /WS 2 hybrid composites' produced through powder pressing process.The hybrid composite has demonstrated high density with a high hardness value of 6364 Hv when subjected to a 560 MPa compaction pressure and 600 °C sintering temperature.In hybrid composites, the volume percentage of the reinforcement also plays a significant role in determining their overall properties.FEA simulation and analysis of DRAMMCs using various reinforcement materials and processes have been carried out by various researchers [29][30][31][32][33][34][35].Powder metallurgy process has become standard process for processing discontinuously reinforced aluminium matrix composites from the literature.Though there are studies comparing the powder metallurgy process with deep rolling and hot extraction [36][37][38], powder metallurgy has emerged as a prominent method to manufacture Al matrix based composites.The existing literature has extensively explored their advantageous properties, processing methods, and applications, particularly within aerospace, automotive, and biomedical engineering.However, despite the significant advancements, a notable gap persists in our comprehensive understanding of the hardness characteristics of these DRAMMC.While powder metallurgy (PM) has emerged as a cost-effective and efficient method for producing such composites, there is a need to systematically investigate the interplay between the reinforcement materials (SiC, Al 2 O 3 , ZrO 2 ) and their impact on hardness, surface roughness, and porosity.Moreover, this research also intends to bridge a gap by considering Hybrid Metal Matrix Composites, a relatively novel class of materials with unique properties, for potential applications in aeronautics and robotics.Existing studies have delved into microstructural evolution, sintering conditions, and compaction pressures, but there remains a research niche in exploring the optimal volume percentages of multiple reinforcements in these hybrid composites.Design of experiment plays a crucial role in systematically arranging the parameters to study their individual and combined effects on the fabricated composite [39].This study aims to address these gaps by conducting experimental and numerical investigations into discontinuously reinforced aluminium composites processed through the powder metallurgy route, providing valuable insights into their hardness, surface characteristics, and porosity, thereby contributing to a deeper understanding of their mechanical properties and potential engineering applications.The main focus of this research is to study the hardness, surface roughness and porosity of aluminium metal matrix discontinuously reinforced with silicon carbide (SiC) particles, aluminium oxide (Al 2 O 3 ) particles and zirconium di-oxide (ZrO 2 ) particles.This study integrates experimental and numerical investigations to optimize the mechanical characteristics of aluminum metal matrix composites reinforced with various particulates.Unlike traditional approaches, which often focus solely on experimentation or simulation, this research combines both methodologies to provide a comprehensive analysis.The paper emphasizes process optimization using Taguchi's Design of Experiments (TDOE) and Finite Element Analysis (FEA), offering insights into the influence of compaction pressure, hold time, and reinforcement materials on the material properties.This holistic approach contributes to a deeper understanding of the fabrication process and its impact on composite performance.

Materials and methods
The Aluminum Metal Matrix Composite specimen discontinuously reinforced with Al 2 O 3 , SiC, and ZrO 2 , having 15 Wt% particulates, irregular in geometry with an average particle size of 80-120 microns as specified by the provider Paraswamani Metals Mumbai, were fabricated through a powder metallurgy process using a custom-made AISI 316 L Stainless Steel die.The cylindrical die cavity is 20 mm in diameter and the die is a single action die with 40 mm effective length.Further, Ball Milling is carried on to reduce the particle size to 50 microns.Milling time was for 2 h with 350 rpm rotational speed.Tungsten Carbide balls of size 5 mm is used in a Ball-Powder ratio of 5:1. 5 Wt% Stearic acid is used as process control agent under argon atmosphere.Similarly for the simulation, die with external diameter of 70 mm, internal diameter of 20 mm, with overall length of 60 mm and a cavity length of 40 mm.12 kN compressive force and a temperature of 550 °C is applied.The methodology depicting the process of powder metallurgy to produce DRAMMC with desired reinforcement material with 15 Wt% is presented in figure 1.The process involved varying the compaction pressure at levels of 8, 10, and 12 kN and the hold time at 10, 30, and 50 s, and sintering in a muffle furnace at a temperature of 550 °C in the presence of Argon to avoid oxidation and to achieve optimal density and mechanical properties.The levels of compaction pressure are selected to systematically investigate their direct impact on composite density and structural integrity.Higher compaction pressures and hold times exceeding 12 kN and 50 s resulted in extensive localized densifications in the composite.Effects of the process parameters on the hardness, wear and frictional force were analyzed using a Taguchi L 9 Orthogonal Array which has proved to be a useful tool in analysis conducted by many researchers.Table 1 presents the factors and levels employed in the experimentation.The Brinell hardness value is calculated based on the equation (1).
Where; BHN = Brinell hardness number; P = applied load; π = 3.14; D = diameter of the indentation.Surface roughness values (Ra) have been identified using Talysurf Surtonic Surface Roughness analyzer.Porosity percentage is measured using Gas Porosimeter.
The ANSYS Workbench software for the simulation of the Single-action die compaction and uniaxial cold pressing process begins with Modeling of the geometry of the die and the metal powder compaction process using the 3D CAD software.The 3D model has been imported into ANSYS Workbench for further analysis.This is followed by meshing where, the imported model has been meshed using a hexa-dominant type, all quadrilateral meshing model.The mesh size is selected based on the size of the metal powder particles and the accuracy desired from the simulation results.Further, the material properties such as the 'Young's modulus, Poisson's ratio, and yield strength', as well as the density of the aluminium matrix and the reinforcing particulates (SiC, Al 2 O 3 , and ZrO 2 ) has been input into ANSYS Workbench.This is followed by Boundary Condition Setting where, the boundary condition has been set to represent the conditions in the die compaction process.The normal compaction pressure (10kN) and hold time (30 s) is applied as loads to the Aluminium matrix with reinforcement particles (Al 2 O 3 , SiC, and ZrO 2 ).However, the interaction between the metal powder and the die walls has been considered by means of contact elements to account for the frictional interaction between the powder particles and the die walls during the process of powder compaction.Further, the loading mode has been defined (uniaxial cold pressing) and the finite element analysis has been conducted using the ANSYS Workbench software.The software uses the Mohr-Coulomb theory and an elliptical surface plasticity model to simulate the compaction process.The result of the simulation provides information on the density distribution and pressure distribution of the metal powder during the compaction process.Figure 2 presents the flowchart of the Powder Metallurgy process.Table 2 presents the properties of Aluminum, Al 2 O 3 , SiC and ZrO 2 used in this experimentation as provided by the vendor.

Hardness analysis
The hardness of Discontinuously Reinforced Aluminium Metal Matrix Composites (DRAMMC) is influenced by several factors, including the type of reinforcement material, compaction pressure, and hold time.The type of reinforcement material, such as SiC, Al 2 O 3 , or ZrO 2 , has a significant impact on the hardness of the DRAMMCs.Compaction pressure applied during the compaction stage of the powder metallurgy process, is also an important factor affecting the hardness of DRAMMCs.Higher compaction pressures result in denser, stronger composites with improved hardness.However, excessive compaction pressure can cause damage to the reinforcement particles and negatively impact the hardness of the final composite.Finally, the highest hardness of DRAMMCs is obtained with ZrO 2 reinforcement, under compaction pressure of 10 kN, and a hold time of 30 s.The hardness of DRAMMCs can be influenced by the type of reinforcing material used.ZrO 2 has a high hardness due to its strong chemical bonds and dense crystal structure, making it more resistant to deformation and indentation.However, SiC has lower hardness due to its weaker chemical bonds and less dense crystal structure.At the micro level, the hardness of DRAMMCs is influenced by the arrangement and distribution of the reinforcing particles within the metal matrix.The size and shape of the particles, as well as the orientation and distribution of the particles, can all affect the hardness of the composite.The type of reinforcing material used can influence the type of chemical bonds formed, and thus the hardness of the composite.Microstructure  of DRAMMCs with different reinforcement particles has been presented in figure 3. The experimental and L 9 orthogonal array results are presented in figure 4 and table 3.
The study revealed that the hardness of the composite can be affected by various parameters such as reinforcement material, compaction pressure, and hold time.The experiment was conducted using the L 9 Taguchi's Orthogonal Array and the Main Effect plot (figure 5) was obtained from Minitab software.The results showed that reinforcement material had the highest impact on hardness with a contribution of 71.477%, followed by compaction pressure (26.59%) and hold time (1.933%).Further, highest hardness is achieved with Reinforcement material (ZrO 2 ), Compaction pressure (10 kN) and Hold time (30 s).Table 4 presents the Analysis of Variance of SN ratios for hardness of various Discontinuously Reinforced Aluminium Matrix Composites.

Surface roughness
Surface roughness is a prominent parameter that directly affects the functional and aesthetic material property.In this study, the surface roughness of the specimen was investigated based on varying combinations of reinforcement materials, compaction pressures, and hold times.The surface roughness values (Ra) were measured using the Talysurf Surtonic Surface Roughness Measuring instrument.The results (table 5 and figure 6) have revealed that the choice of reinforcement material has been significant in determining the surface roughness of the AMMCs.Among the three reinforcement materials studied, ZrO 2 reinforced composites exhibited the lowest surface roughness values, indicating a smoother surface texture.This could be attributed to the inherent properties of ZrO 2 , such as its hardness and ability to resist wear.However, SiC reinforced composites demonstrated the highest surface roughness values, indicating a rougher surface texture.In addition to the reinforcement material, the compaction pressure was found to have a noticeable influence on surface roughness.Increasing the compaction pressure from 8 to 12 kN resulted in a slight decrease in surface roughness values.This suggests that higher compaction pressures facilitate better particle distribution and increased interfacial bonding of matrix and reinforcement, leading to a smoother surface.Finally, the hold time during the powder metallurgy process did not exhibit a clear trend in terms of its effect on surface roughness.The variations in hold time (10, 30, and 50 s) did not produce significant differences in the surface roughness values.This  implies that within the studied range of hold times, the duration of the hold time did not have a substantial impact on the surface roughness of the AMMCs.Atomic Force Microscope (AFM) analysis has been conducted over a sweep of 10 μm to understand the effect of the reinforcement material, compaction pressure and hold time on the surface topography and roughness of the composites.
The AFM results (figure 7) revealed that the addition of reinforcement materials to the composites has led to an increase in the surface roughness of the matrix material and peak height.Highest Peaks are found with SiC reinforcement under low compaction pressure and less hold time while relatively less peaks are found with Al 2 O 3 reinforcement.However ZrO 2 reinforcement has resulted in least peaks and corresponding least roughness of the surface under highest compaction pressure (12kN) and hold time (50 s).From the main effects plot (figure 8) it can be concluded that, lowest surface roughness is obtained with ZrO 2 reinforcement material, produced through powder metallurgy with 12kN compaction pressure and 50 s hold time.

Porosity
In this study, the porosity of the Aluminum Metal Matrix Composite (AMMC) was investigated based on different combinations of reinforcement materials, compaction pressures, and hold times.Green Density and Sintered Density are provided in table 6.The porosity was measured by comparing the experimental porosity of   the samples with their theoretical porosity values.The results (table 7 and figure 9) indicated that the choice of reinforcement material has a considerable influence on the porosity of the AMMCs.ZrO 2 -reinforced composites exhibited the lowest percentage porosity values, indicating a higher density and lower void content.Al 2 O 3 -reinforced composites followed closely with slightly higher porosity values, while SiC-reinforced composites showed the highest porosity values among the three materials.Furthermore, increasing the compaction pressure and hold time during the powder metallurgy process led to a reduction in porosity.Higher compaction pressures contributed to better particle packing and reduced inter-particle gaps, resulting in a denser material with lower porosity.Similarly, longer hold times provided more time for particle rearrangement and improved particle bonding, further decreasing the void content within the composites.The findings suggest that the combination of reinforcement material, compaction pressure, and hold time plays a critical role in achieving the desired density and minimizing porosity in AMMC.By controlling these parameters, it is possible to optimize the fabrication process and produce composites with improved mechanical properties and structural integrity.Main effects plot (figure 10) confirm that, lowest porosity is obtained with ZrO 2   11).The SiC reinforcement particles are also known to have a better bonding with the aluminum matrix, resulting in higher load transfer efficiency, leading to lower maximum principal stress values.However, ZrO 2 reinforced composites exhibit higher maximum principal stress due to the lower stiffness and weaker bonding between the reinforcement and matrix phases, resulting in lower load transfer efficiency.

Equivalent elastic strain
The 'Equivalent Elastic Strain' is an important parameter that affects the elastic deformation during the powder compression process.During the FEA simulation of Uniaxial Powder Compression Process to fabricate discontinuously reinforced aluminum metal matrix composites, the Equivalent Elastic Strain was found to vary depending on the type of reinforcement material used.From the simulation it was found that, the Equivalent Elastic Strain for SiC was 0.2488, for Al 2 O 3 it was 0.2421, and for ZrO 2 it was 0.262 (figure 12).The amount of Equivalent Elastic Strain can vary depending on the type of reinforcement material used due to differences in their properties.Further, SiC has a relatively high elastic modulus compared to Al 2 O 3 and ZrO 2 , which results in a lower Equivalent Elastic Strain value.Comparatively, ZrO 2 has a relatively low elastic modulus and high plasticity, which results in a higher Equivalent Elastic Strain value.

Equivalent stress (Von Mises)
Equivalent (Von Mises) Stress is a measure of the total amount of stress in a material under loading.The Equivalent (Von Mises) Stress varies for different reinforcement materials due to differences in their molecular,

Conclusion
From the experimentation, the following conclusions can be drawn.• Increasing the compaction pressure from 8 to 12 KN resulted in a slight decrease in surface roughness and porosity.Higher compaction pressures have believed to facilitate better particle distribution and improved interfacial bonding, leading to smoother surfaces and lower void content.Lowest porosity has been achieved with 12 kN compaction pressure and 30 s hold time.• The selected range of hold time during the powder metallurgy process did not exhibit a clear trend in terms of its effect on surface roughness and porosity.Within the studied range of hold times (10, 30, and 50 s), no significant differences were observed.
• ZrO 2 reinforced DRAMMC showed the potential for achieving both lower surface roughness and porosity simultaneously, indicating improved surface quality and higher density in the composites.
• The simulation results showed that the least principal stress was achieved with SiC and the highest equivalent elastic strain achieved was with ZrO 2 .The highest equivalent (Von Mises) stress achieved was for ZrO 2 .The choice of reinforcement material, its distribution in the matrix, and the application of the right compaction pressure and hold time leads to a higher hardness value.
• This experimentation and simulation demonstrated that the PM process can be used to fabricate DRAMMC with different reinforcement particulates.The choice of input parameters significantly affects the hardness, surface roughness and porosity of the material.The results have provided a better understanding of the stress and strain behavior of the material, which can further be used to optimize the process parameters.

Figure 2 .
Figure 2. Flowchart of finite element analysis process.

Table 1 .
Levels and factors of input parameters.

Table 4 .
'Analysis of variance for signal to noise (SN) ratios'.

Table 5 .
L 9 orthogonal array results for surface roughness.The Maximum Principal Stress on DRAMMC (Discontinuously Reinforced Aluminum Metal Matrix Composites) material during the powder compression process is an important parameter to determine the mechanical properties of the material.The compressive force, die geometry, and temperature play a crucial role in determining the maximum principal stress.After conducting the simulation, the maximum principal stress was found to be 2235.8MPa for SiC, 3444.4MPa for Al 2 O 3 , and 3582.5 MPa for ZrO 2 (figure

Table 6 .
Experimental values of Green Density and Sintered Density (g/cm 3 ).
atomic, and crystal structures, as well as the nature of their chemical bonds.From the results (figure13) it can be seen that, SiC has the lowest Equivalent (Von Mises) Stress of 24880 MPa due to its strong covalent bonds, while Al 2 O 3 has a higher Equivalent (Von Mises) Stress of 26972 MPa due to its ionic and covalent bonding.ZrO 2 has the highest Equivalent (Von Mises) Stress of 28751 MPa, which may be attributed to the fact that it has a combination of ionic and covalent bonding and a lowest elastic modulus compared to the other reinforcement materials.

•
The results of the experimentation confirmed that the highest hardness values achieved were 106.3688BHN, 76.118 BHN, and 103.22 BHN for ZrO 2 , Al 2 O 3 , and SiC reinforcement materials, respectively.Highest hardness is achieved with ZrO 2 reinforcement material, 10 kN compaction pressure, and 30 s hold time.Similarly lowest surface roughness has been achieved with 12 kN compaction pressure and 50 s hold time.