# Investigation of Mechanical Properties and Microstructure of AZ31-SiC-Graphite Hybrid Nanocomposites Fabricated by Bottom Pouring-Type Stir Casting Machines

Authors: Itha Veeranjaneyulu; Vemulapalli Chittaranjan Das; Srikanth Karumuri
Journal: Advances in Materials Science and Engineering
 (2023)

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## Abstract

In this study, a bottom pouring-type stir casting machine was used to create AZ31 magnesium alloy hybrid nanocomposites with varying weight percentages (0, 3, 5, and 7) of silicon carbide (SiC) and graphite (Gr) particles. Investigations have been made into the mechanical characteristics and microstructural distribution of manufactured hybrid nanocomposites. The outcomes demonstrate that the mechanical characteristics and uniform distribution of SiC and Gr particles are enhanced compared to those of the base alloy. In comparison to monolithic AZ31 alloy, microhardness, ultimate tensile strength (UTS), yield strength (YS), and compressive strength (CS) were raised by about 54%, 68%, 82%, and 107%, respectively. The presence of reinforced particles, the uniform distribution of particles, and the strong interfacial connection between the matrix and reinforcement all contribute to the improvement of mechanical properties. However, the addition of 7 wt. % SiC/Gr showed good mechanical properties compared to the base alloy. The microstructure of nanocomposites was analyzed using a scanning electron microscope (SEM), and particles were described using energy-dispersive spectroscopy (EDS).


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## Body

## 1. Introduction

Magnesium hybrid nanocomposites are manufactured materials with desirable qualities such as strong tensile strength, being compressive, hardness, and stiffness. When compared to aluminium-based materials, magnesium-based alloys and composites have the potential to reduce component weight by about 30%. AZ alloys are magnesium alloys that also contain zinc and aluminium. These nanocomposites are widely employed in engineering applications and are freely accessible. In vehicles and trucks, locomotives, and general aviation, AZ31 magnesium alloys have the potential to replace several standard structural materials as lightweight alloys [1–3]. In recent years, efforts have been undertaken to further enhance these alloys’ characteristics by using reinforcements such as Si3N4 [4], SiC [5], BN-WC [6], Gr-WC [7], TiC-TiB2 [8], and titanium [9]. Due to modifications in the mechanism of solidification for nanosized reinforcements, composites exhibit phenomenal results in terms of strength when nanosized particles are added to the matrix of magnesium. HMMNCs can synthesize via various techniques, but among all, stir casting is a widely accepted and cost-effective technology that has been commercially adopted because of its benefits, which include mass manufacturing, low processing costs, and ease of customization.Nourbakhsh et al. [10] observed good mechanical characteristics of AZ31/SiC nanocomposites by contrasting them to a base alloy produced by squeeze stir casting. Khandelwal et al. [11] studied AZ31/Al2O3 magnesium metal matrix nanocomposites and noticed the improvement in YS and UTS of magnesium MMNCs due to mixing of nano-Al2O3 and in situ reaction of particles. The results were analyzed by OM, SEM, and XRD. Ramanujam et al. [12] prepared biocompatible composite materials processed by a stir casting method. The developed AZ31/eggshell nanocomposites are utilised to produce human bone materials for implants. Cao et al. [13] studied AZ91D nanocomposites. The results indicated that the mechanical properties were raised by incorporation of 1.0 wt. % AINnp into AZ91D. The use of these structural materials is permitted at temperatures of at least 200°C. Torabi Parizi et al. [14] performed a comparative analysis on AZ80/0.5Ca-1.5Al2O3 hybrid nanocomposites prepared via two different casting processes. The results concluded that rheocast nanocomposites had superior properties compared to stir cast. By using two-stage stir casting to create a hybrid composite of Al7075 and (B4C + CDA), Manikandan et al. [15] discovered improved tensile and tribological properties but observed reduced impact strength when compared to Al7075 alloys. Torabi Parizi et al. [16] studied the effect of GNPs on AZ80 composites and concluded that the mechanical properties of AZ80/0.1GNPs composites were improved by 40% and 15%, respectively, compared to those of the base alloy.The innovative aspect of the work is an investigation into the use of bottom pouring-type stir casting machines to incorporate SiC and Gr nanoparticles into magnesium alloy and achieve uniformly dispersed nanoparticles in a hybrid nanocomposite. To the best of our knowledge, stir casting has seldom been used in the manufacturing of nanocomposite materials. Here, a stir casting fabrication technique for the AZ31-SiC nanocomposite is demonstrated. Evaluations of the microstructure and mechanical characteristics, density, and hardness were explored, and the results are described.


## 2. Materials and Methods

### 2.1. Materials and Composite Fabrication

In the fabrication of nanocomposites, AZ31 alloy was the base material, and SiC and graphite particles were used as reinforcements with an average size of 53 nm and 75 nm, respectively. The manufacturing process of hybrid nanocomposites carried out through a bottom pouring-type stir casting machine is shown in Figure1. The setup consists of the graphite crucible, furnace, mechanical stirring unit, argon gas cylinder, temperature detector, and die with a bottom pouring unit. AZ31 ingots were loaded in the graphite crucible, heated up to 650°C, and held till the metal melted into liquid. The raw materials were preheated at 400°C with a soaking time of 1 h. To prevent atmospheric oxidation, argon gas was used to create a vacuum environment in the furnace [17, 18]. The prewarmed reinforcements were released into the graphite crucible, and a stirrer was rotated at 700 rpm. The melted material was poured into a heated mould via the bottom pouring method. The fabricated samples with compositions were designated M1, M2, M3, and M4, as shown in Table 1.Figure 1
Photograph of the bottom pouring-type stir casting machine.Table 1
Designation of specimens.
S. noSpecimensCompositions1M1AZ312M2AZ31 + 3% SiC + 3% graphite3M3AZ31 + 5% SiC + 5% graphite4M4AZ31 + 7% SiC + 7% graphite


### 2.2. Microstructure

The fabricated specimens were prepared by using different grades of polishing paper and a disc-polishing machine. An etchant was used for removing undesirable materials at the surface of the specimen [19, 20]. The microstructure of the monolithic alloy and hybrid nanocomposites was investigated using an SEM (manufacturer: TESCAN and model: VEGA3 SBH). The EDS analysis was carried out over a selected region to confirm the presence of SiC and Gr particles. It revealed the peaks for aluminium (Al), zinc (Zn), silicon (Si), carbon (C), and magnesium (Mg). The formation of magnesium oxide during the solidification process resulted in the peak of oxygen [21]. Hence, these SEM results with EDS analysis were indications of effective incorporation of SiC and Gr particles into Mg matrix composites.


### 2.3. Mechanical Properties

The density of specimens was calculated theoretically as well as experimentally. The theoretical measurement was performed by the rule of the mixture method. The experimental measurement was performed by the Archimedes principle.(1)%porosity=1−measureddensitytheoreticaldensity×100.A Vickers microhardness tester was used to measure the microhardness of all samples while applying a 100 gram load to three different places for ten seconds. The average microhardness values were noted. The photographs of the SEM with EDS, hardness, and density instruments are shown in Figure2.Figure 2
Photographs of SEM with EDS, density, and hardness instruments.The tensile and compressive test was assessed with UTM (INSTRON-E1025) at a ram of 3 mm/min. The specimen was made as per ASTM E8-3 standards for the tensile test.


## 2.1. Materials and Composite Fabrication

In the fabrication of nanocomposites, AZ31 alloy was the base material, and SiC and graphite particles were used as reinforcements with an average size of 53 nm and 75 nm, respectively. The manufacturing process of hybrid nanocomposites carried out through a bottom pouring-type stir casting machine is shown in Figure1. The setup consists of the graphite crucible, furnace, mechanical stirring unit, argon gas cylinder, temperature detector, and die with a bottom pouring unit. AZ31 ingots were loaded in the graphite crucible, heated up to 650°C, and held till the metal melted into liquid. The raw materials were preheated at 400°C with a soaking time of 1 h. To prevent atmospheric oxidation, argon gas was used to create a vacuum environment in the furnace [17, 18]. The prewarmed reinforcements were released into the graphite crucible, and a stirrer was rotated at 700 rpm. The melted material was poured into a heated mould via the bottom pouring method. The fabricated samples with compositions were designated M1, M2, M3, and M4, as shown in Table 1.Figure 1
Photograph of the bottom pouring-type stir casting machine.Table 1
Designation of specimens.
S. noSpecimensCompositions1M1AZ312M2AZ31 + 3% SiC + 3% graphite3M3AZ31 + 5% SiC + 5% graphite4M4AZ31 + 7% SiC + 7% graphite


## 2.2. Microstructure

The fabricated specimens were prepared by using different grades of polishing paper and a disc-polishing machine. An etchant was used for removing undesirable materials at the surface of the specimen [19, 20]. The microstructure of the monolithic alloy and hybrid nanocomposites was investigated using an SEM (manufacturer: TESCAN and model: VEGA3 SBH). The EDS analysis was carried out over a selected region to confirm the presence of SiC and Gr particles. It revealed the peaks for aluminium (Al), zinc (Zn), silicon (Si), carbon (C), and magnesium (Mg). The formation of magnesium oxide during the solidification process resulted in the peak of oxygen [21]. Hence, these SEM results with EDS analysis were indications of effective incorporation of SiC and Gr particles into Mg matrix composites.


## 2.3. Mechanical Properties

The density of specimens was calculated theoretically as well as experimentally. The theoretical measurement was performed by the rule of the mixture method. The experimental measurement was performed by the Archimedes principle.(1)%porosity=1−measureddensitytheoreticaldensity×100.A Vickers microhardness tester was used to measure the microhardness of all samples while applying a 100 gram load to three different places for ten seconds. The average microhardness values were noted. The photographs of the SEM with EDS, hardness, and density instruments are shown in Figure2.Figure 2
Photographs of SEM with EDS, density, and hardness instruments.The tensile and compressive test was assessed with UTM (INSTRON-E1025) at a ram of 3 mm/min. The specimen was made as per ASTM E8-3 standards for the tensile test.


## 3. Results and Discussion

### 3.1. Microstructure Analysis

The monolithic and distribution of reinforcement particles in composites were investigated using the SEM analysis. Figure3(a) shows the SEM image of AZ31 alloy, and Figures 3(b)–3(d) show the composites of 3 wt. %, 5 wt. %, and 7 wt. % of AZ31-SiC/Gr MMCs, respectively. The microstructure of 5 wt. % of nanocomposites has formed the SiC cluster due to the pulling of particles at one place during polishing. The microstructure of these nanocomposites reveals that the reinforcement particles were distributed uniformly in the matrix. This is a result of the ideal mechanical alloying of the particles. Figure 3(c) reveals a small crack in the nanocomposite due to the strength of the composite being reduced slightly.Figure 3
SEM images of (a) AZ31, (b) A31-3 wt. % SiC/Gr, (c) AZ31-5 wt. % SiC/Gr, and (d) AZ31-7 wt. % SiC/Gr.
(a)(b)(c)(d)


### 3.2. Energy-Dispersive Spectroscopy (EDS) Analysis

The elemental analysis of the base alloy and nanocomposites is determined using the EDS analysis, as shown in Figures4 and 5, respectively. It is noted that the structure mostly consists of magnesium, aluminium, and zinc, as shown in Figure 4, whereas in Figure 5, magnesium, aluminium, zinc, and carbon are found in the nanocomposite. However, because of the strong affinity of Mg at the surrounding temperature, oxygen can be seen to be present. It can be assumed that there was no discernible reaction between the matrix and reinforcement material during casting in any of these composites.Figure 4
EDS analysis of AZ31 alloy.Figure 5
EDS analysis of AZ31/SiC/Gr nanocomposites.


### 3.3. Mechanical Properties

#### 3.3.1. Density and Microhardness

An increase in density of hybrid nanocomposites is credited to the inclusion of reinforcements in the semisolid condition as compared with the monolithic alloy which is shown in Figure6. The measured density had lower values than values of the theoretical density. The addition of SiC and Gr particles leads to enhanced porosity and increased density, which contribute to raising the hardness of the nanocomposite. The microhardness of nanocomposites is shown in Figure 7. The AZ31-7 wt. % SiC/Gr nanocomposite was increased by 54% compared with the monolithic alloy [22].Figure 6
Variation of density of nanocomposites.Figure 7
Variation of microhardness of nanocomposites.


#### 3.3.2. Tensile and Compression Properties

A modification in the volume ratio of SiC and Gr particles increases the UTS, YS, and CS of nanocomposites. Figure8 shows that the strength of nanocomposites was increased from monolithic alloys to 7 wt. % of SiC/Gr composites. This may be affected due to grain size of the reinforcement bonding between the reinforcements and matrix [23, 24]. Overall, 7 wt. % of AZ31/SiC/Gr nanocomposites has enhanced the strength compared with monolithic alloys by 68% and 82% of UTS and YS, respectively. The stress-strain curve of the nanocomposites is enhanced with an increased reinforcement. Figures 9(a) and 9(d) show the stress-strain curve for the four specimens. The compression results of the nanocomposites are enhanced due to the reinforcement of SiC and Gr particulates in composites. The strength of the composite depends on the grain size of the reinforcements and the bonding between reinforcements and the matrix. The superior strength is attained due to grain refinement [25, 26]. The interfacial holding between the reinforcement and matrix is satisfactory and the applied pressure might change the magnesium combination lattice on the SiC and Gr particulates. The compressive strength of the monolithic alloy is 173.42 MPa, whereas the improved compressive property was observed for the M4 composite because of the grain refinement. The addition of ceramic particles might diminish the flexibility and enhance compressive properties, as shown in Figure 9. The fracture surface of tensile and compressive specimens is depicted in Figures 10(a) and 10(b) for nano composites [27].Figure 8
Variation of strength of nanocomposites.Figure 9
Stress vs. strain of nanocomposites for (a) specimen M1, (b) specimen M1, (c) specimen M1, and (d) specimen M1.
(a)(b)(c)(d)Figure 10
(a) Tensile and (b) compressive-fractured specimens.


## 3.1. Microstructure Analysis

The monolithic and distribution of reinforcement particles in composites were investigated using the SEM analysis. Figure3(a) shows the SEM image of AZ31 alloy, and Figures 3(b)–3(d) show the composites of 3 wt. %, 5 wt. %, and 7 wt. % of AZ31-SiC/Gr MMCs, respectively. The microstructure of 5 wt. % of nanocomposites has formed the SiC cluster due to the pulling of particles at one place during polishing. The microstructure of these nanocomposites reveals that the reinforcement particles were distributed uniformly in the matrix. This is a result of the ideal mechanical alloying of the particles. Figure 3(c) reveals a small crack in the nanocomposite due to the strength of the composite being reduced slightly.Figure 3
SEM images of (a) AZ31, (b) A31-3 wt. % SiC/Gr, (c) AZ31-5 wt. % SiC/Gr, and (d) AZ31-7 wt. % SiC/Gr.
(a)(b)(c)(d)


## 3.2. Energy-Dispersive Spectroscopy (EDS) Analysis

The elemental analysis of the base alloy and nanocomposites is determined using the EDS analysis, as shown in Figures4 and 5, respectively. It is noted that the structure mostly consists of magnesium, aluminium, and zinc, as shown in Figure 4, whereas in Figure 5, magnesium, aluminium, zinc, and carbon are found in the nanocomposite. However, because of the strong affinity of Mg at the surrounding temperature, oxygen can be seen to be present. It can be assumed that there was no discernible reaction between the matrix and reinforcement material during casting in any of these composites.Figure 4
EDS analysis of AZ31 alloy.Figure 5
EDS analysis of AZ31/SiC/Gr nanocomposites.


## 3.3. Mechanical Properties

### 3.3.1. Density and Microhardness

An increase in density of hybrid nanocomposites is credited to the inclusion of reinforcements in the semisolid condition as compared with the monolithic alloy which is shown in Figure6. The measured density had lower values than values of the theoretical density. The addition of SiC and Gr particles leads to enhanced porosity and increased density, which contribute to raising the hardness of the nanocomposite. The microhardness of nanocomposites is shown in Figure 7. The AZ31-7 wt. % SiC/Gr nanocomposite was increased by 54% compared with the monolithic alloy [22].Figure 6
Variation of density of nanocomposites.Figure 7
Variation of microhardness of nanocomposites.


### 3.3.2. Tensile and Compression Properties

A modification in the volume ratio of SiC and Gr particles increases the UTS, YS, and CS of nanocomposites. Figure8 shows that the strength of nanocomposites was increased from monolithic alloys to 7 wt. % of SiC/Gr composites. This may be affected due to grain size of the reinforcement bonding between the reinforcements and matrix [23, 24]. Overall, 7 wt. % of AZ31/SiC/Gr nanocomposites has enhanced the strength compared with monolithic alloys by 68% and 82% of UTS and YS, respectively. The stress-strain curve of the nanocomposites is enhanced with an increased reinforcement. Figures 9(a) and 9(d) show the stress-strain curve for the four specimens. The compression results of the nanocomposites are enhanced due to the reinforcement of SiC and Gr particulates in composites. The strength of the composite depends on the grain size of the reinforcements and the bonding between reinforcements and the matrix. The superior strength is attained due to grain refinement [25, 26]. The interfacial holding between the reinforcement and matrix is satisfactory and the applied pressure might change the magnesium combination lattice on the SiC and Gr particulates. The compressive strength of the monolithic alloy is 173.42 MPa, whereas the improved compressive property was observed for the M4 composite because of the grain refinement. The addition of ceramic particles might diminish the flexibility and enhance compressive properties, as shown in Figure 9. The fracture surface of tensile and compressive specimens is depicted in Figures 10(a) and 10(b) for nano composites [27].Figure 8
Variation of strength of nanocomposites.Figure 9
Stress vs. strain of nanocomposites for (a) specimen M1, (b) specimen M1, (c) specimen M1, and (d) specimen M1.
(a)(b)(c)(d)Figure 10
(a) Tensile and (b) compressive-fractured specimens.


## 3.3.1. Density and Microhardness

An increase in density of hybrid nanocomposites is credited to the inclusion of reinforcements in the semisolid condition as compared with the monolithic alloy which is shown in Figure6. The measured density had lower values than values of the theoretical density. The addition of SiC and Gr particles leads to enhanced porosity and increased density, which contribute to raising the hardness of the nanocomposite. The microhardness of nanocomposites is shown in Figure 7. The AZ31-7 wt. % SiC/Gr nanocomposite was increased by 54% compared with the monolithic alloy [22].Figure 6
Variation of density of nanocomposites.Figure 7
Variation of microhardness of nanocomposites.


## 3.3.2. Tensile and Compression Properties

A modification in the volume ratio of SiC and Gr particles increases the UTS, YS, and CS of nanocomposites. Figure8 shows that the strength of nanocomposites was increased from monolithic alloys to 7 wt. % of SiC/Gr composites. This may be affected due to grain size of the reinforcement bonding between the reinforcements and matrix [23, 24]. Overall, 7 wt. % of AZ31/SiC/Gr nanocomposites has enhanced the strength compared with monolithic alloys by 68% and 82% of UTS and YS, respectively. The stress-strain curve of the nanocomposites is enhanced with an increased reinforcement. Figures 9(a) and 9(d) show the stress-strain curve for the four specimens. The compression results of the nanocomposites are enhanced due to the reinforcement of SiC and Gr particulates in composites. The strength of the composite depends on the grain size of the reinforcements and the bonding between reinforcements and the matrix. The superior strength is attained due to grain refinement [25, 26]. The interfacial holding between the reinforcement and matrix is satisfactory and the applied pressure might change the magnesium combination lattice on the SiC and Gr particulates. The compressive strength of the monolithic alloy is 173.42 MPa, whereas the improved compressive property was observed for the M4 composite because of the grain refinement. The addition of ceramic particles might diminish the flexibility and enhance compressive properties, as shown in Figure 9. The fracture surface of tensile and compressive specimens is depicted in Figures 10(a) and 10(b) for nano composites [27].Figure 8
Variation of strength of nanocomposites.Figure 9
Stress vs. strain of nanocomposites for (a) specimen M1, (b) specimen M1, (c) specimen M1, and (d) specimen M1.
(a)(b)(c)(d)Figure 10
(a) Tensile and (b) compressive-fractured specimens.


## 4. Conclusions

AZ31-SiC-Gr hybrid nanocomposites have been successfully fabricated using the bottom pouring-type stir casting machine. The hardness, tensile strength, compressive properties, and microstructure analysis of hybrid nanocomposites were evaluated. The results of the present investigation can be summarised as follows:(i)
The hybrid metal matrix nanocomposite with 7 wt. % of Sic/Gr reinforcement particles exhibits extremely low agglomeration and little particle clustering, as revealed by the conclusions of SEM and EDS examination(ii)
A maximum density of 1.88 g/cc and a hardness of 110.83 Vickers hardness number were obtained for M4 (7 wt. % of SiC/Gr) nanocomposites. This was due to the presence of hard ceramic particles, and it shows the enhancement of density and microhardness as compared with the monolithic alloy.(iii)
A maximum UTS of 189.52 MPa and a compressive strength of 320.23 MPa were observed in the combination of 7 wt. % of the nanocomposite. This was due to the perfect interfacial bonding with less clustering of reinforcement particles.(iv)
The mechanical characterization results showed that when an ideal ratio of SiC and Gr was added up to 7 wt. %, attributes including tensile strength, compression strength, and hardness were enhanced by 68%, 107%, and 54%, respectively, in comparison to the base alloy without reinforcements. Finally, the proposed magnesium (AZ31) hybrid metal matrix nanocomposite of 7 wt. % SiC/Gr showed better results.



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*Source: 3402348-2023-01-24.xml*