Numerical and experimental investigation of the mechanical properties of MWCNT/RHA reinforced AlP0507-based hybrid aluminum metal matrix composites

Abstract

The present study investigates the mechanical properties of AlP0507-based metal matrix composites reinforced with multi-walled carbon nanotubes (MWCNTs) and rice husk (RHA) using experiments and numerical simulation. To achieve this goal, an AlP0507-based MWCNT/RHA hybrid metal matrix composite was fabricated by stir casting method. Subsequently, microstructural analysis using field emission scanning electron microscopy (FESEM) and EDAX was carried out for different weight ratios of MWCNTs/RHA. In addition, the tensile, impact, and hardness characteristics of MWCNT/RHA-reinforced aluminum metal matrix composite (AMMC) and hybrid MWCNT/RHA-reinforced aluminum metal matrix composite (HAMMC) were experimentally investigated for different reinforcement weight concentrations. In addition, the tensile, von Mises stress distribution, strain, and deformation behavior of AMMC and HAMMC were numerically investigated using the commercial software Digimat-FE supporting the RVE approach considering particle inclusions. It was also noticed that the optimal addition of RHA/MWCNTs to the AlP0507 melt led to an improvement in the tensile strength, hardness, and impact properties of the composites when compared to the AlP0507 material without any reinforcement. It can also be noticed that after a certain optimal percentage of RHA reinforcement, the tensile strength of AMMC and HAMMC decreases. Further, the examination of the numerical stress distribution facilitates the prediction of the regions that exhibit high levels of stress concentration, potential locations of fracture, or areas where the material may undergo excessive deformation.

Keywords:

• Multi-wall carbon nanotube
• rice husk ash
• aluminum metal matrix composite
• Digimat-FE
• Tensile Properties
• Impact Properties
• Hardness properties

REVIEWING EDITOR:

• Ian Philip Jones, The University of Birmingham, United Kingdom

SUBJECTS:

• Materials Science
• Composites
• Corrosion-Materials Science

1. Introduction

Nowadays, aluminum metal matrix composites (AMMCs) and hybrid AMMC (HAMMC) have attracted significant scientific interest and have become a rapidly expanding research field. In AMMC, one of the components is aluminum alloy/aluminum, which forms a percolation network and is called the matrix phase. Another constituent is embedded in this aluminum and aluminum alloy matrix and serves as reinforcement. Moreover, AMMC materials possess significant potential for utilization in diverse fields including automotive manufacturing, aircraft production, space infrastructure, defense mechanisms, and sporting equipment due to their promising mechanical properties (Ashebir et al., 2022; Srivastava et al., 2014). However, AMMCs have several shortcomings. In particular, the phenomenon of singularity and the presence of many chemically unsaturated atoms on the surface of the dispersed phase (Reddy et al., 2018; Bhandare & Sonawane, 2013). To overcome these shortcomings, the properties of AMMCs can be tailored by varying the type of constituents and their volume fraction to further improve their mechanical properties and provide versatility. In recent years, various nano or micro-sized fillers have been used to further improve their mechanical properties and provide versatility. In recent years, conventional nanosized fillers such as graphene nanoplatelets (GNPs), silicon dioxide (SiO₂), B₄C, TiC, SiC, TiB_2, and carbon nanotubes (CNTs) have been widely used as reinforcing agents. Moreover, regarding AMMC fabrication methods, several methods are used in the production of aluminum composites such as stir casting, mechanical alloying, friction stir processing, and selected laser sintering (Esmaily et al., 2016; Prasad Reddy et al., 2019). However, stir casting is the most popular method among the techniques mentioned above due to its simplicity, flexibility, cost-effectiveness, and minimal ill effects on the reinforcement particles in composite production. In the following few kinds of literature focused on AMMCs and HAMMCs have been presented. Rahman et al. investigated the hardness, tensile, and microstructural properties of AMMCs with various reinforcements. It was reported that the inclusion of 20 wt.% of SiC results in the most notable decrease in wear for the AMMC. The authors also demonstrated that there is non-uniformity in AMMC due to the presence of porosities using microstructural investigation (Rahman & Al Rashed, 2014). Sivananth et al. fabricated TiC-reinforced AMMC using the stir casting method and investigated the tensile strength and brittleness of the composite with various concentrations of TiC particles (10, 12, and 15 wt.%). It was observed that the composites exhibited higher tensile strength in comparison to unreinforced aluminum and the maximum value was attained at 15 wt.% TiC. It was also reported that as the wt.% of reinforcement in the AMMC increased, the brittleness of the stir-casted samples increased due to higher hardness inherited by the TiC and the poor interface bonding between the Al metal matrix and the TiC (Sivananth et al., 2014). Yolshina et al. experimentally studied the tensile strength of AMMCs with graphene and graphite reinforcement. It was demonstrated that the addition of 2 wt.% graphene increased tensile strength by approximately 16%, reducing the density to around 2.4 g-cm⁻³ (Yolshina et al., 2016). Shankar et al. investigated the tensile strength and hardness of AA6061-based AMMC with various glass reinforcement (3 wt.% to 12 wt.%). It was noticed that the introduction of glass reinforcement initially increased tensile strength and hardness but subsequently decreased, reaching a minimum at 9 wt.% due to the uneven distribution of glass reinforcement in the AMMC (Kumar & Shankar, 2012). Sharifi et al. studied the compressive strength of mechanically alloyed aluminum powder with 5 wt.%, 10 wt.%, and 15 wt.% of B₄C nanoparticles. It was reported that pure aluminum exhibited a compressive strength of 130 MPa and hardness of 33 HV, while specimens with 15% reinforcement showed a remarkable 3.5-fold increase in compressive strength and a 5-fold increase in hardness (Sharifi et al., 2011). Moreover, Devaraju and Pazhanivel evaluated the compressive strength of AA1100 metal matrix composites with 2.5 wt.%, 5 wt.%, 7.5 wt.%, and 10 wt.% B₄C reinforcement and also investigated the hardness of the composite (Devaraju & Pazhanivel, 2016). Homogeneous stirring of B₄C within the AMMC was reported to significantly increase hardness, with the lowest wear rate observed at 7.5 wt.% reinforcement concentration. It was also reported that the highest compressive strength was achieved at 7.5 wt.% B₄C concentration. Padmavathi and Ramakrishnan presented the wear and hardness characteristics of the Al6061 matrix reinforced with 0.5 wt.% of MWCNTs and 1 wt.%, 1.5 wt.% of SiC. A positive correlation was observed between MWCNT concentration and hardness improvement. It was also observed that MWCNT inclusion reduced the coefficient of friction, resulting in decreased wear compared to the pure Al6061 matrix (Padmavathi & Ramakrishnan, 2014). James et al. studied the wear, tensile, and hardness characteristics of SiC and TiB₂-reinforced AMMC. It was observed that TiB₂ particles significantly improved wear resistance, but a shift occurred when it exceeded 10% by weight, whereas the SiC particles notably improved the tensile strength and stiffness of the AMMC (James et al., 2014). Imran et al. investigated the tensile strength and hardness of graphite/bagasse ash-reinforced AMMC with graphite concentrations of 1 wt.%, 3 wt.%, and 5 wt.% by weight and bagasse ash concentrations of 2 wt.%, 4 wt.%, and 6 wt.% by weight. Increasing the graphite concentration has been reported to improve both the tensile strength and hardness of AMMC while keeping the bagasse ash content at 2 wt.%. It was also noted that the change in ash content while maintaining the graphite content at 1 wt.% shows the same trend (Imran et al., 2016). Xavier and Suresh investigated the hardness and wear characteristics of AMMC at different concentrations of rock dust particles. It was revealed that AMMC reinforced with stone dust particle specimens exhibited improved wear resistance compared to unreinforced ones (Xavier & Suresh, 2016). In addition to the experimental methods presented in the above-mentioned literature, a comprehensive microstructural numerical model has been proposed to simulate strengthened grain structures in the matrix phase, accurately predicting AMMC performance by considering particle properties such as size, dispersion, aggregation, and necking (Roters et al., 2010; Mingard et al., 2014). Further, the numerical models can include diverse particle shapes, pore volumes, and contact areas (Raether & Iuga, 2006). The representative volume element (RVE) based numerical models have proven effective in depicting macroscopic characteristics of a material’s microstructure (Schmidt & Becker, 2013), focusing on small volumes with distinct morphological features to replicate the overall behavior of AMMC (Ogierman & Kokot, 2014; Amirmaleki et al., 2016). The RVE modeling approach is a cost-effective method for simulating engineering material micromechanical behavior. Furthermore, numerical modes are best suited to evaluate the properties of multiphase materials with different microstructures, so experimental evaluation of properties is a major challenge, whereas for single-phase materials, both experimental and numerical methods are sufficient to evaluate properties (Winkler et al., 2016). To highlight the numerical studies performed related to AMMC are presented below. The 3D microstructure model used to simulate the deformation behavior was reported to be in good agreement with the experimental results. It was also reported that the addition of 1 wt.% graphene improved mechanical properties such as tensile and compressive strength, impact energy, hardness, and wear resistance (Hadad et al., 2020). Nessa et al. proposed 3D microstructures of discontinuous tow-based composites with complex characteristics such as waviness and tow non-uniformity. It has been reported that tow waviness reduces the stiffness of the material by more than 20% (Fereshteh-Saniee et al., 2022). In conclusion, despite extensive research devoted to the characterization of the mechanical properties of micron-sized reinforced AMMCs, there is a limited number of studies devoted to the experimental and numerical characterization of the mechanical properties of RHA/MWCNT-reinforced AMMCs. Therefore, in the current study, the mechanical properties of AlP0507-based metal matrix composites reinforced with multi-walled carbon nanotubes (MWCNTs) and rice husk (RHA) are investigated using experiment and Digimat-FE numerical simulation.

2. Fabrication methods, experimental set-ups, and numerical simulation

2.1. Materials and specimen fabrication

The materials tested in this study were AlP0507 as a parent metal, MWCNT, and RHA as reinforcement with different proportions, which were used to produce AMMC and HAMMC samples using the stir casting method. The specimen geometries followed specifications outlined in the ASTM-E8 (Mohanavel et al., 2018) standard for the tensile specimens, ASTM-E23 (Ozden et al., 2007) for the impact specimens, and ASTM-E18 for the hardness specimens. These specimens’ fabrication begins by putting small pieces of AlP0507 into the crucible having a degasser as well as the cover-all powder. The degasser controls the temperature in the crucible, while the cover-all powder eliminates impurities from the liquid metal. The AlP0507 was subjected to a melting process using an electric furnace, with the temperature being adjusted to 850 °C, as shown in Figure 1(a). At the same time, another container was used to add RHA. Then, the sample was heated for two hours to eliminate any moisture present in the fabricated. The experimental setup involved the use of a mechanical stirrer as shown in Figure 1(b), to mix the heated RHA with the molten AlP0507. The rotation speed of the stirrer was set at 150 rpm. The mixture was subjected to heat and subsequently allowed to undergo solidification for several hours. Then, the solidified stir-casted material, referred to as AMMC specimens, was removed from the crucible shown in Figure 1(c).

Figure 1. Experimental procedure for mechanical properties tests (a) Melting of AlP0507, (b) Stirring of preheated MWCNT and RHA in molten AlP0507, and (c) Sample of fabricated composite.

2.2. Testing machine and experimental setup

A test setup was developed for testing tensile, impact, and hardness testing to examine the mechanical properties of the fabricated AMMC and HAMMCs. The tensile testing was performed using an Instron universal testing machine equipped with a 100 kN load cell at room temperature. To check the hardness of the sample, a Vickers hardness test was carried out using a ball indenter with a force of 100 kg. Hardness was calculated at three different locations and the average of the three measured hardness values was reported. Impact tests were carried out using a Charpy impact test apparatus. It is important to note that throughout the study, each test followed the same general experimental procedure. A complete list of material properties or default values used in this study is shown in Table 1. The standard test specimen dimensions are presented in Figure 2 (a)–(c).

Figure 2. Standard tensile test specimen dimensions (in mm) (a) Tensile test specimen, (b) Impact test specimen, and (c) Hardness test specimen.

Table 1. Mechanical properties of AlP0507, MWCNT, and RHA.

Materials	Young’s modulus (GPa)	Poisson ratio	Density (g/cm^3)	Tensile strength
MWCNT	270	0.16	2.1	62 GPa
RHA	215	0.15	1.8	3.5 MPa
Al P0507	70	0.33	2.7	90 MPa

2.3. Microstructural characterization

It is very important to characterize the microstructure of the fabricated specimens since the mechanical properties of AMMC and HAMMC are highly dependent on their morphology. In addition, the homogeneous dispersion of RHA and MWCNTs in the Al matrix is a key factor for the overall characteristics of RHA/MWCNTs reinforced AMMCs and HAMMCs. Therefore, in the current study, the microstructure characterization of AMMC and HAMMCs with various weight fractions of RHA/MWCNT reinforcements was investigated using a field scanning electron microscope-(FESEM). The investigation focused on examining the distribution of MWCNT and RHA within the AlP0507 matrix.

2.4. Numerical setup

2.4.1. Description of representative volume element (RVE) model

To characterize the tensile behavior of AMMC/HAMMC by incorporating particle inclusions into the RVE approach, Digimat-FE software is used to generate a 3D RVE model to accurately represent the microstructure of AMMC/HAMMC. The RVE model consists of flake inclusions such as MWCNTs, RHA, and aluminum matrix. It is assumed that there is a completely interconnected interface between the considered elements. Moreover, it is assumed that a uniform stress-strain correlation will exist throughout the RVE due to its small size and representative properties. To build the RVE model in Digimat-FE software, specific input parameters are allocated, such as the volume percentage, shape, and phase distribution of inclusions, in particular RHA and MWCNTs, as well as the minimum relative distance between inclusions. The mesh was created using tetrahedral elements. The mesh element size was taken to be 0.05 with a chordal deviation coefficient of 0.10. The number of elements was 51039, in which the number of nodes was 12086. Mixed boundary conditions (MBC) were used in the simulations. MBC involves the imposition of displacements in a designated region of the RVE with the simultaneous application of tractions in another region. In addition, the MBC method uses equations that include constraints to establish a relationship between the degree of freedom (DOF) of nodes located on the outer surfaces of the RVE and the degree of freedom of the reference point. The RVE model in Digimat-FE is built using the input parameters for aluminum, RHA, and MWCNTs, which are listed in Table 1.

After all the necessary pre-processing steps were completed, the model was exported to Digimat Solver. The obtained modeling results were used and the RVE model was validated. An RVE model illustrating the arrangement of inclusions in an AMMC generated by Digimat-FE using tetrahedral elements is depicted in Figure 3. It is important to mention that the RVE model facilitates the visualization and analysis of the distribution of stress within the AMMC material, providing valuable insights into its mechanical response under applied loading conditions. Also, the precise characterization of the dimensions and distribution of particle inclusions within the composite material can be attained by suitably modifying the parameters.

Figure 3. A photographic image of RVE (a) RVE generation of RHA/MWCNTs reinforced AMMCs and (b) Mesh view of RVE model.

3. Results and discussion

3.1. Microstructural analysis

To better visualization of the individual constituents RHA and MWCNTs in the Al matrix, the microscopic image of RHA (in the form of flakes) and MWCNTs (in the form of fibers) is shown in Figure 4 with the help of field emission scanning electron microscope.

Figure 4. FESEM image of raw materials (a) RHA and (b) as-received MWCNTs.

Figure 5(a) and (b) shows the FESEM images of AlP0507 matrix specimens reinforced with 2 wt.% of MWCNT and 6 wt.% RHA, respectively. It is evident from (Figure 5(a) (b)) that the MWCNTs and RHA represented by bright spots are distributed extensively on the surfaces of the Al alloy matrix but found to be negligible across the grain boundary.

Figure 5. Microstructural view of (a) MWCNT reinforced AlP0507 composite specimens; (b) MWCNT/RHA reinforced AlP0507 hybrid composite specimens.

However, in the case of AMMC specimen reinforced with 6 wt.% RHA (Figure 5 (c)), the flake-shaped RHA were observed well dispersed on the surfaces and at the grain boundaries of the matrix. Areas of high intensity in the images indicate the presence of RHA, and the fabric-type lines formed in bundles indicate the presence of MWCNTs. It was noticed in Figure 3 that the distribution of both RHA and MWCNT is widespread on the surfaces of the AlP0507 matrix, mainly along the grain boundaries. It can also be noticed that fabricated AMMC specimens with various concentrations of nanoparticles demonstrate a uniform distribution of RHA and MWCNT particles in the metal matrix. The wetting mechanism observed in the microstructural analysis is explained by the coincidence of the face-centered cubic (FCC) crystal structures of AlP0507, RHA, and MWCNTs.

Figure 6 shows the EDAX diagram of RHA/MWCNT/AlP0507 and MWCNT/AlP0507 composites. It can be seen from the figure that additional elements were found in the RHA/MWCNT/AlP0507 hybrid composite, namely Mg, Si, and Al, which indicates their combination in the structure of the composite. In addition, the identification of aluminum oxides was determined. From the analysis of FESEM images, it can be concluded that MWCNTs and RHA are well dispersed in the metal matrix regardless of the weight fractions of nanoparticles. Moreover, it can be concluded that strong interfacial bonding can be established by incorporating RHA and MWCNT particles at the grain boundaries of the matrix, which increases the strength of the fabricated composite samples. Furthermore, the EDAX analysis revealed the presence of additional minor impurities that cannot be completely excluded.

Figure 6. EDX pattern of specimens (a) MWCNT reinforced AlP0507 composite specimens; (b) MWCNT/RHA reinforced AlP0507 hybrid composite specimens.

3.2. Mechanical properties of AlP0507 hybrid composites reinforced with MWCNT/RHA

In this section, the mechanical properties such as tensile strength, toughness/impact strength, and hardness of the AlP0507 hybrid composites reinforced with MWCNT/RHA sample were investigated using the experimental setup discussed in section 2 of this study to evaluate and compare the reinforcing efficiency of MWCNT and RHA. Table 2 shows the tensile strength, hardness, and toughness of MWCNT-reinforced AlP0507, RHA-reinforced AlP0507, and MWCNT/RHA-reinforced AlP0507 hybrid composite specimens. It is important to note that the mechanical properties characterization was conducted using four different weight concentrations of RHA (2, 4, 6, and 8 wt.%) and weight fraction of MWCNTs (1, 2, and 3 wt.%). Further, it is also important to note that the experimentally obtained Young’s modulus of the composite samples designated as G1, G2, G3, G4, G5, and G6 are presented in Table 3. Moreover, HAMMC was fabricated by selecting the composite with the highest achieved tensile strength among the composites consisting of AlP0507 and RHA at a certain weight percentage. The results obtained are discussed in the following subsections.

Table 2. Tensile strength, toughness/impact strength, and hardness of MWCNT/RHA reinforced AlP0507 composite specimens.

Sample designation	Test specimens	Tensile Strength (MPa)	Vicker Hardness (HV)	Impact Test (J/Cm^2)
G1	Al with 2% RHA	93	37	165
G2	Al with 4% RHA	96	43	175
G3	Al with 6% RHA	98	41	195
G4	Al with 8% RHA	92	39	160
G5	Al with 1% MWCNT	139	51	230
G6	Al with 2% MWCNT	147	53	245
G7	Al with 3% MWCNT	155	56	240
G8	Al with 6% RHA/1% MWCNT	119	47	190
G9	Al with 6% RHA/2% MWCNT	131	50	200
G10	Al with 6% RHA/3% MWCNT	127	46	180

Table 3. Young’s modulus of MWCNT/RHA reinforced AlP0507 composite specimens.

	Sample designation
	G1	G2	G3	G4	G5	G6	G7	G8	G9	G10
Young’s modulus (MPa)	191.73	199.35	206.98	214.61	193.04	201.98	210.93	215.92	224.87	233.81

3.2.1. Tensile properties

Figure 6 illustrates the tensile strength of AlP0507-based composites with various types of reinforcements and concentrations. The corresponding test specimens are shown in Figure 7. From Figure 8 it can be seen that the inclusion of MWCNTs and RHA into AlP0507 led to an improvement in the tensile strength of the hybrid composites compared to unaltered AlP0507 irrespective of types of reinforcements and concentrations. The observed enhancement may be due to the presence of densely dispersed MWCNT/RHA particles along the grain boundaries of the matrix, where these particles effectively retard the movement of dislocations. Moreover, it can be observed from Figure 6 and Table 2 that the tensile strength of AlP0507-based composites reinforced with 3 wt.% of MWCNT, designated as G7 was found to be the maximum (155 MPa) compared to the other composite specimens. The observed improvement in tensile strength properties may be due to the high tensile strength properties of MWCNTs and their interaction with AlP0507. In addition, the strong bond at the reinforcement-matrix interface and the reinforcing effect of grain reinforcement can lead to increased tensile strength. It can also be observed from Table 2 that the tensile strength of AlP0507 reinforced with 6 wt. % RHA/3 wt. % MWCNT composite, sample designated as G9 was found to be the maximum (131 MPa) compared to the other HAMMC specimens. This can be explained by the fact that increasing the concentration of MWCNTs inhibits plastic deformation, thereby improving the tensile strength. However, the introduction of particles led to a decrease in the percentage of elongation, which indicates a decrease in the ability of the material to deform plastically and an increase in its tendency to fracture, which increases its brittleness. From Table 2, it can also be noticed that AlP0507 reinforced with 6 wt. % RHA composite yields the highest tensile strength (98 MPa) among other RHA reinforced AlP0507 composites. The improved mechanical properties observed in RHA may be due to the increased concentrations of carbon and silicon within the material, as well as their interaction with the aluminum matrix. It can be seen from Table 2 that after a certain percentage of RHA reinforcement, the tensile strength of AMMC and HAMMC decreases. This is because the lesser density of RHA where agglomeration of RHA will occur, may reduce the tensile strength. Lastly, it can be concluded that the tensile strength test suggests that the optimal addition of RHA/MWCNTs to the AlP0507 melt led to an improvement in the tensile strength of the composites when compared to the AlP0507 material without any reinforcement. It can also be concluded that after a certain optimal percentage of RHA reinforcement, the tensile strength of AMMC and HAMMC decreases.

Figure 7. Tensile strength test specimens with corresponding sample designation.

Figure 8. Tensile strength of MWCNT/RHA reinforced AlP0507 hybrid composite and composite for various types of reinforcements and concentrations.

3.2.2. Impact properties

Figure 9 shows the impact energy of the MWCNT/RHA reinforced AlP0507 hybrid composite specimens for various types of nanoparticle reinforcements, demonstrating the effect of MWCNT reinforcement at different concentrations. The corresponding test specimens are shown in Figure 10. It can be observed from the Figure 9 that, in line with the tensile properties, the addition of nano-size reinforcements such as RHA and MWCNTs with various concentrations increased impact energy as well. However, it was also observed that in the absence of reinforcement (RHA/MWCNTs), fractures may initiate within the matrix region of AlP0507 and propagate rapidly in multiple directions, leading to a decrease in impact energy. Moreover, from Figure 8 and Table 2, it can be seen that the AlP0507-based composite reinforced with 3 wt.% MWCNTs exhibited a significant ability to absorb a considerable amount of energy during impact tests. The high-impact energy is believed to be due to the presence of densely packed MWCNTs within the metal matrix as shown in the FESEM images presented earlier. Also, the robust semi-metallic nature of MWCNT plays an important role in enhancing the adhesion between the reinforcement and matrix of the composite materials. Furthermore, the strengthened bond between the reinforcing material and the surrounding matrix can be attributed to the strong semi-metallic properties exhibited by MWCNTs, in the case of RHA-reinforced AlP0507-based composite, test specimens reinforced with 6 wt.% RHA sample designated as G7 showed a significant enhancement in impact energy. This phenomenon can be caused due to the existence of densely concentrated RHA particles within the matrix. In addition, the strong semi-metallic properties exhibited by RHA are important for improving reinforcement-matrix bonding in AMMC and HAMMC materials. It can be seen from Table 2 that after a certain percentage of RHA reinforcement, the impact energy of AMMC and HAMMC decreases. This is because the fractures in the unreinforced AlP0507 primarily originate within the matrix region of the material and propagate rapidly in various directions, especially in the absence of MWCNT particles. This phenomenon led to deterioration of the impact strength of the casted material. Nevertheless, the inclusion of densely distributed MWCNT/RHA particles within the matrix could significantly improve the impact strength of the composites. From the results, it can be concluded that the impact properties of AMMC and HAMMC materials were improved by incorporating the optimal concentration of MWCNT and RHA particles into the stir-cast material AlP0507. In addition, it can be concluded that the incorporation of MWCNT and RHA particles into stir-cast AlP0507 matrix hybrid composites contributes to the ability to absorb a significant amount of energy. Moreover, the incorporation of densely distributed RHA/MWCNT particles into the AlP0507 matrix shows a remarkable increase in the impact energy of the composite material.

Figure 9. Impact energy of MWCNT/RHA reinforced AlP0507 hybrid composite and composite for various types of reinforcements and concentrations.

Figure 10. Impact properties test specimens with corresponding sample designation.

3.2.3. Hardness properties

Figure 11 presents Vickers hardness measurements of various AMMC and HAMMC specimens for various types and concentrations of MWCNT/RHA reinforcements. In the figure, the Vickers hardness of AlP0507 was 26 HV. Further, the corresponding test specimens are shown in Figure 12. It can be seen from the Figure 11 and Table 2 that the hardness of AlP0507-based AMMC and HAMMC increases with the increase of MWCNT and RHA content compared to the unreinforced AlP0507. This is because the introduction of MWCNT/RHA particles into the AlP0507 matrix leads to an increase in the hardness of the material, thereby improving its mechanical properties. Further, the significant increase in hardness exhibited by the AMMC and HAMMC samples could be due to various factors. First, there is a noticeable presence of strong and durable MWCNT/RHA particles. These particles are mainly located at the boundaries between grains, although some particles are also found inside the grains of the AlP0507 matrix. The presence of these reinforcements prevents the mobility of dislocations, which leads to an increase in hardness. In addition, it is worth noting that MWCNTs/RHA have a significant surface area-to-volume ratio. The increased surface area provides better interaction between the reinforcing elements and the matrix, resulting in improved mechanical properties. Moreover, the strong and efficient interaction between the MWCNTs/RHA and AlP0507 matrix at the interface promotes successful load transfer from the matrix to the reinforcement materials. The improvement in mechanical properties, in particular hardness, in composites is largely due to the strong interfacial bond between their constituent components. Further, it can also be seen that the AMMC sample with 3 wt.% MWCNTs showed a higher measured hardness value of 56 HV. It was also noticed that the AMMC sample reinforced with 4 wt. % RHA shows the highest hardness value of 43 HV. From the results, it can be concluded that the hardness of AMMC and HAMMC materials was improved by incorporating the optimal concentration of MWCNT and RHA particles into the stir-cast material AlP0507.

Figure 11. Hardness of MWCNT/RHA reinforced AlP0507 hybrid composite and composite for various types of reinforcements and concentrations.

Figure 12. Hardness properties test specimens with corresponding sample designation.

3.2.4. Numerical results

This subsection reports the Digimat-FE analysis results from the unit MWCNT/RHA reinforced AlP0507 composite model described in Sec. 2.4. The composite material parameters used in this study are the modulus of elasticity (E), and Poisson’s ratio (v) is presented in Table 1. To verify the effectiveness of the RVE model in predicting the mechanical response of composite and hybrid composite materials under uniaxial loading conditions, the accuracy and reliability of the RVE model were verified by comparing the tensile strength simulation results with the experimental results for various types of reinforcements and concentration, as shown in Figure 13. It can be observed from Figure 13 that the simulation and the experimental results are in a good agreement with the average error of 8.87%. Therefore, it can be concluded that the 3D microstructure model of RVE can able to predict the interactions between MWCNT/RHA particle inclusions and the aluminum matrix, and further parametric numerical studies can be performed. Therefore, the stress-strain correlation in composites can be accurately predicted using Digimat-FE.

Figure 13. Comparison between simulation and experimental results: composite and hybrid composite at different concentrations.

Figure 14 shows the contour plots of von Mises stress distribution, strain, and deformation at the final moment for each group of specimens under uniaxial loading conditions. Nine composite specimens, designated as G1, G2, G3, G4, G5, G6, G7, G8, and G9 were considered. The stress distribution within the RVE model reveals that the phases consisting of MWCNT/RHA reinforced AlP0507, RHA-reinforced AlP0507, and MWCNT-reinforced AlP0507 show complete interconnectivity. Further, the von Mises stress shows a non-uniform distribution in the AlP0507 matrix. Upon loading, the MWCNT/RHA nanoparticles are subjected to a higher stress than the surrounding matrix, demonstrating the MWCNT/RHA reinforcement mechanism, which is then transferred to the inclusion phase. Maximum stress develops at the MWCNT/RHA-AlP0507 matrix interfaces, especially where the particles bulge and move closer to each other. This means that as the AMMC and HAMMC are further loaded, the MWCNT/RHA nanoparticles will tend to shift, making them more rounded, and the matrix will fracture due to particle slippage. Failures in composite specimens may occur during both the manufacturing and application stages, often due to the presence of small initial flaws that undergo expansion under stress. Moreover, the strain distribution observed in Figure 14 clearly indicates a noticeable strain discrepancy in the x-direction between the inclusion phase and the matrix phase. Specifically, the inclusion phase experiences significantly less strain compared to the matrix phase. The observed phenomenon may be due to the higher Young’s modulus exhibited by the dispersed MWCNT and RHA in comparison to the AlP0507 matrix phase. The difference in strain behavior between the matrix and inclusion phases may directly affect the mechanical properties demonstrated by the respective materials. The MWCNT and RHA reinforcements, which possess increased stiffness and strength, demonstrate a weaker resistance to deformation when compared to the matrix phase. Consequently, the strain values within the inclusion phase are reduced. From the above results, it can be concluded that the examination of the stress situation facilitates the prediction of the regions that exhibit high levels of stress concentration, potential locations of fracture, or areas where the material may undergo excessive deformation. Further, the examination of stress conditions within the RVE model holds significance in acquiring a comprehensive of the mechanical properties of composites and detecting potential failure mechanisms. Furthermore, the above-mentioned result indicates that the inclusion of reinforcements, specifically MWCNT and RHA, plays a substantial role in enhancing both the load-bearing capacity and the overall mechanical characteristics of the composite material.

Figure 14. Strain, Stress, and deformation distributions of RVE model concerning MWCNT/RHA distribution (a) Equivalent von Mises stress distribution, (b) Total strain distribution, and (c) Deformation distribution.

4. Conclusions

In the present study, the mechanical properties of MWCNT and RHA-reinforced AMMC/HAMMC for various weight fractions were investigated through experiment and numerical simulation and the results are presented. Further, some important key conclusions are drawn and presented below:

1. AMMCs/HAMMCs of AlP0507 with varying proportions of RHA and MWCNT were successfully fabricated using the stir-casting process in controlled experimental conditions.

2. The microstructural analysis indicated that the hybrid composite, which comprised 2 wt.% of MWCNT and 6 wt.% of RHA, exhibited a uniform dispersion in the AlP0507 metal matrix. This observation was made specifically in the composite sample containing 6 wt.% of MWCNT. However, it was noted that there was limited dispersion of MWCNT and RHA at the grain boundary in specimens with different concentrations.

3. It was noticed that the incorporation of MWCNT and RHA into the composites and hybrid composites, respectively, results in enhanced tensile strength, impact energy, and hardness compared to the unreinforced AlP0507 material.

4. The simulation results demonstrate a notable concurrence between the anticipated tensile properties of the hybrid composites and aluminum composites, as determined by the RVE model, and the corresponding experimental data.

5. It was observed that after a certain optimal percentage of RHA reinforcement, the tensile strength of AMMC and HAMMC decreases.

6. It was also observed that accurate prediction of the stress-strain relationship in composites and hybrid composites can be achieved by employing an RVE model constructed using Digimat-FE. Moreover, this modeling methodology enables the accurate replication of the observed deformation in an aluminum matrix that incorporates MWCNT and RHA particle inclusions.

Authors’ contributions

The experiments were conducted by NS, who also conceptualized the article and authored the initial draft of the paper. LKS provided technical guidance and contributed to the experimental work during the writing process of the article. MKY was responsible for performing editing tasks, composing the review, and engaging in correspondence about the article. All authors have read and approved the manuscript.

List of Abbreviations
AMMC	=	Aluminium Metal Matrix
HAMMC	=	Hybrid AMMC
SiC	=	Silicon Carbide
wt.%	=	Weight Percentage
TiC	=	Titanium Carbide
AA	=	Aluminium Alloy
MWCNT	=	Multi Wall Carbon Nano Tube
RHA	=	Rice Husk Ash
UTS	=	Ultimate Tensile Strength
HVN	=	Vickers’ Hardness Number
FESEM	=	Field emission scanning electron microscopy
RVE	=	Representative Volume Element
MBC	=	Mixed Boundary Conditions
DOF	=	Degree Of Freedom

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Data availability statement

All the required data is already included in the manuscript.

Additional information

Funding

No funding was received from any agency or organization.

Notes on contributors

Nitin Srivastava

Nitin Srivastava is a Ph.D. scholar in the Mechanical Engineering department of Sharda University, located in Greater Noida, India. He obtained a B.tech degree in Mechanical Engineering from KCC Institute of Technology and Management, Abdul Kalam Technical University, Greater Noida, India in 2014, and an M.tech degree in Mechanical Engineering from Noida Institute of Engineering and Technology, Abdul Kalam Technical University, Greater Noida, India in 2019. He is pursuing his Ph.D. in Mechanical Engineering at Sharda University, India in 2020. He has 8 years of experience in academia. Research interests include Finite Element Analysis, Polymer-based composites, Metal matrix composites, Sandwich composites with diverse core topologies, and Nanocomposites. Material properties characterization of metal matrix composites, such as mechanical, thermal, strength, and flammability. Failure, vibration, stress, and bending, and micromechanical characteristization of nanomaterials based on polymers.