Effects of waste eggshells and SiC addition in the synthesis of aluminum hybrid green metal matrix composite

2.1 Matrix material

In the present investigation, AA2014 aluminum alloy was selected as a matrix material. AA2014 aluminum alloy is an aluminum-based alloy often used in the aerospace industry. It is easily machined in certain tempers and among the strongest available aluminum alloys. It also has high hardness. The corrosion resistance of this alloy is particularly poor. Chemical composition and mechanical properties are shown in Tables 2 and 3, respectively [20].

2.2 Primary reinforcement material

In the present investigation, carbonized eggshell powder was selected for primary reinforcement material. Figure 1A shows the photograph of hen eggshells. Hen eggshells naturally consist of ceramic materials. The chemical composition (by weight) of a by-product eggshell has been reported as follows: calcium carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%), and organic matter (4%). The hen eggshell was cleaned and sun dried to eliminate the covering layer of eggshell [20]. The dried eggshell was ball milled to obtain eggshell powder as shown in Figure 1B. It was then carbonized to 500°C for 3 h to remove the carbonaceous materials, as shown in Figure 1C. The densities of uncarbonized and carbonized eggshell particles are 2.47 and 2.0 g/cm³, respectively.

Figure 1:

Photographs of (A) eggshells, (B) eggshell powder, and (C) carbonized eggshell powder.

2.3 Secondary reinforcement material

SiC is selected as a secondary reinforcement to further improve the mechanical properties of composites. The introduction of SiC to the aluminum matrix significantly enhances strength, modulus, abrasive wear resistance, and thermal stability. The density of SiC (3.2 g/cm³) is nearer to that of aluminum alloy AA2014 (2.8 g/cm³). The resistance of SiC to acids, alkalis, or molten salts up to 800°C makes it a good reinforcement candidate as a secondary reinforcement for aluminum based Metal matrix composites (MMCs). In addition, SiC is simply available and has good wettability with aluminum alloys [1].

The size of each reinforcement particles is selected at 25 μm. However, it is not easy to obtain the exact particle size (25 μm) of all reinforcement particles (SiC and carbonized eggshell powder). Hence, the particle size with a standard deviation of 2 μm (mean, 25 μm) was selected. SiC particles were directly purchase from the market, with a mean particle size of 25 μm (SD, 2 μm). Carbonized eggshell powder was ball milled to obtain uniform particle size (mean, 25 μm; SD, 2 μm).

2.4 Composition selection

On the basis of the results based on pilot investigations, the compositions of reinforcements (SiC and carbonized eggshell) were selected and are shown in Table 4. The total percentage of both reinforcements varies from 2.5 to 12.5 wt.% fraction in AA2014 matrix material. If the weight percentage of reinforcement increases more than 12.5%, then no more effects on the physical and mechanical properties of hybrid MMC were observed.

2.5 Processing of hybrid MMCs

AA2014/carbonized eggshells/SiC hybrid green MMC used in this study was fabricated by electromagnetic stir casting technique at parameters of 12 A (current), 180 s (time), and 700°C (matrix pouring temperature) and immediately extruded on UTM machine at 60 MPa using cylindrical H13 tool steel die coated with graphite [20]. It has been reported by various researchers that with sufficient time, any type of ash has the ability to absorb moisture from the surrounding air. Basically, when any type of reinforcement was mixed with matrix alloy, then with reinforcement some atmospheric air was also entered in matrix alloy. Carbonized eggshell particles have the ability to absorb the moisture, resulting minimum porosity obtained. Because carbonized eggshell particles have very good reactivity (ability to absorb the air moisture) to air at elevated temperature, the air trapped with carbonized eggshell particles reacts with carbonized eggshell inside the AA2014 melt, resulting in very low porosity formed inside the hybrid MMC. In the present investigation, a precipitation hardening process (heat treatment technique) was used to improve further mechanical properties. The heat treatment process was conducted at a solution time of 4.5 h, an aging temperature of 250°C, and an aging time of 13.5 h [3].

2.6 Corrosion test

The corrosion test of all fabricated composites was conducted in a bath tub with high alkalinity. All samples of composites were immersed in a bath tub with high alkalinity at room temperature 120 h in 3.5 wt.% NaCl aqueous solution. Corrosion rate was calculated at assuming uniform corrosion over the entire surface of the composites condition. The corrosion rate in millimeters per year (mmy) was calculated from the weight loss using the following equation [21], [22], where the constant (K) can be varied:

Corrosion rate (CR)=Weight loss (g)  × KAlloy density (g/cm3)  × Exposed area (A)  × Exposure time (h).

In the given equation, K=8.75×10⁴, exposed area=9 cm², and exposure time=120 h.

2.7 Porosity analysis

The experimental densities of the composites were determined by means of the Archimedes principle. The theoretical densities of composites were calculated using a rule of mixtures, given as follows:

Porosity (P) is the percentage of the pore volume to the total volume with the volume of a substance [3]. It is defined by

3.1 Mechanical behavior

The mechanical properties of AA2014/carbonized eggshell/SiC hybrid green MMCs produced are presented in Figures 2–5. It can be observed from Figure 2 that after the addition of carbonized eggshell particles up to the weight fraction of 7.5% in AA2014/2.5% SiC composites, the tensile strength of hybrid green metal matrix composite is maximum (274 MPa). Further, by increasing the weight fraction of carbonized eggshell particles in AA2014/2.5% SiC composite, tensile strength continues to decrease, and it is lowest at the 12.5% weight fraction of eggshell and 10% weight fraction of SiC. The presence of reinforcements (carbonized eggshell/SiC) produces a significant increase in the work hardening of the material during tensile testing. This increase in work hardening is more significant at a higher weight fraction of carbonized eggshell particles (presence of CaCO₃, Cu, Al₄C₃, SiC, CaSiO₃, Al, MnO₂, Al₂O₃, and Mg₂SiO₄, as shown in Figure 6). However, it decreases at a higher weight fraction of carbonized eggshell and SiC (more than 7.5% of carbonized eggshell and more than 2.5% of SiC). This may be due to the clustering of carbonized eggshell particles in AA2014/SiC composite. It can be observed that after the addition of SiC particles in AA2014/eggshell composite, the heat-treatable property of green MMC significantly improved. After the heat treatment process, approximately 7.66% tensile strength improved as compared with cast AA2014/7.5% eggshell/2.5% SiC green metal composite. However, after the heat treatment of AA2014/7.5% eggshell/2.5% SiC green metal composite, approximately 62.22% tensile strength improved as compared with matrix alloy AA2014. Maximum fatigue strength was also observed for sample designation G3 (AA2014/7.5% eggshell/2.5% SiC green metal composite), as shown in Figure 3. After the heat treatment of AA2014/7.5% eggshell/2.5% SiC green metal composite, approximately 67.77% fatigue strength improved as compared with matrix alloy (90 MPa).

Figure 2:

Tensile strength of hybrid green MMCs.

Figure 3:

Fatigue strength of hybrid green MMCs.

Figure 4:

Hardness of hybrid green MMCs.

Figure 5:

Toughness of hybrid green MMCs.

Figure 6:

XRD results of (A) sample designation G1, (B) sample designation G3, (C) sample designation G5, and (D) sample designation and G21.

From Figure 4, it is observed that there is a general increase in hardness with the increase in the weight fraction of SiC particles in AA2014/2.5% carbonized eggshell composite. Maximum hardness was observed for sample designation G21 (AA2014/2.5% eggshell/12.5% SiC green metal composite). The X-ray powder diffraction (XRD) of AA2014/2.5% eggshell/12.5% SiC hybrid green MMC was conducted, as shown in Figure 6. XRD results showed the presence of SiC, Al₄C₃, Al, CaCO₃, Cu, Mg₂SiO₄, and MnO₂ phases in hybrid green metal matrix composites. It was reported by various previous researcher that the presence of SiC, Al₄C₃, Al, CaCO₃, Cu, Mg₂SiO₄, and MnO₂ phases significantly increases the hardness of composite. Approximately 125% hardness improved after the heat treatment as compared with matrix alloy. It is observed that the toughness (Figure 5) decreases with the addition of carbonized eggshell particles and SiC particles in AA2014 aluminum alloy. The reduction in toughness may be due to the presence of hard phases (Figure 6) in the hybrid MMC.

3.2 Microstructure analysis

Maximum tensile strength and fatigue strength were observed for AA2014/7.5% eggshell/2.5% SiC composite, whereas maximum hardness was observed for AA2014/2.5% eggshell/12.5% SiC composite. AA2014/12.5% eggshell/2.5% SiC composite shows minimum corrosion rate, whereas AA2014/2.5% eggshell/2.5% SiC composite shows maximum toughness. Hence, in view of the above facts, the microstructures of sample designations G1, G3, G5, and G21 were conducted. Figures 7 and 8A–D show a typical microstructure of AA2014 reinforced with SiC and carbonized eggshell (a, 2.5% SiC + 2.5% eggshell; b, 2.5% SiC + 7.5% eggshell; c, 2.5% SiC + 12.5% eggshell; d, 12.5% SiC + 2.5% eggshell) hybrid green metal matrix composites at lower and higher magnification. It shows a uniform distribution of carbonized eggshell and SiC particles in AA2014 matrix. This is an indication that the electromagnetic stir casting process adopted for the AA2014/SiC/carbonized eggshell hybrid green metal matrix composite production is reliable. Figure 9 shows a TEM image of AA2014/2.5% SiC/7.5% carbonized eggshell hybrid green metal matrix composite, which represents dendrites of aluminum and precipitates along the dendrite boundaries of SiC and carbonized eggshell particles. TEM image clearly shows the interface between the AA2014 matrix and the reinforcement materials (SiC particles and carbonized eggshell particles). It shows proper wettability and good interface bonding between AA 2014 matrix, SiC particles, and carbonized eggshell particles.

Figure 7:

Microstructure of AA2014 reinforced (×100) with (A) 2.5% SiC + 2.5% eggshell, (B) 2.5% SiC + 7.5% eggshell, (C) 2.5% SiC + 12.5% eggshell, and (D) 12.5% SiC + 2.5% eggshell.

Figure 8:

Microstructure of AA2014 reinforced (×500) with (A) 2.5% SiC + 2.5% eggshell, (B) 2.5% SiC + 7.5% eggshell, (C) 2.5% SiC + 12.5% eggshell, and (D) 12.5% SiC + 2.5% eggshell.

Figure 9:

TEM image of sample designation G3-AA2014/2.5% SiC/7.5% eggshell composite.

3.3 Density and porosity analysis

Figure 10 shows that both the theoretical and the experimental densities decreased with increasing percentage additions of carbonized eggshell particles in AA2014/SiC composites. Theoretical densities of SiC particles and carbonized eggshell particles were 3.21 and 2 g/cm³, respectively. Although minimum theoretical and experimental densities were observed for sample designation G5, sample designation G3 shows better mechanical properties, as discussed earlier. The overall theoretical density of AA2014/eggshell/SiC hybrid green metal matrix composite decreased with wt.% additions of carbonized eggshell particles. The theoretical density of the composites (AA2014) decreased from 2.8 g/cm³ at 0 wt.% to 2.75 g/cm³ at 7.5 wt.% for carbonized eggshell particles and 2.5 wt.% SiC particles. The experimental density of the composites decreased from 2.78 g/cm³ at 0 wt.% to 2.73 g/cm³ at 7.5 wt.% for carbonized eggshell particles and 2.5 wt.% SiC particles. From the theoretical density analysis of AA2014/7.5% eggshell/2.5% SiC hybrid green metal matrix composite, it can be observed that there was an approximately 1.8% decrease in theoretical density.

Figure 10:

Density and porosity analysis of hybrid green MMCs.

In the present investigation, black deviation bars between theoretical density and experimental density are showing percent porosity, as shown in Figure 10. Shorter black deviation bar between theoretical and experimental densities shows low porosity, whereas longer black deviation bar shows higher porosity. A higher percentage of porosity leads to specimen failure. Minimum porosity was found to be 0.72% for AA2014/7.5% carbonized eggshell/2.5% SiC hybrid green metal matrix composites (sample designation G3).

3.4 Corrosion behavior

From Figure 11, it can be observed that corrosion rate continuously decreases after the addition of carbonized eggshell particles up to 12.5 wt.% in AA2014/SiC composite. The corrosion rate decreased from 5.79 mm/year at 0 wt.% to 3.74 mm/year at 12.5 wt.% for carbonized eggshell particles and 2.5 wt.% for SiC particle. Approximately 35.40% corrosion rate decreases after the addition of 12.5 wt.% carbonized eggshell and 2.5 wt.% SiC particles in AA2014 aluminum alloy. However, after the heat treatment process, the corrosion rate of hybrid green metal matrix composites further increased. A minimum corrosion rate was observed for sample designation G5, but sample designation G3 shows better mechanical properties, as discussed earlier. The corrosion rate of sample designation G3 was found to be 3.83 mm/year. Sample designation G3 (AA2014/7.5% eggshell/2.5% SiC composite) shows approximately 33.85% lower corrosion rate as compared with matrix alloy AA2014, which is acceptable.

Figure 11:

Corrosion behavior of hybrid green MMCs.

3.5 Cost estimation

The cost of AA2014 is approximately 300 Indian rupees (INR) per kilogram, whereas the cost of SiC powder is 700 INR per kilogram [3], [20]. Eggshell particles are available as waste, so these are available free of cost. From Figure 12, it can be observed that the cost of hybrid green metal matrix composites continuously decreases after the addition of carbonized eggshell particles in AA2014/SiC composite. Sample designation G3 (7.5% eggshell and 2.5% SiC) shows better tensile and fatigue strength, whereas sample designations G5 (12.5% eggshell and 2.5% SiC) and G21 (2.5% eggshell and 12.5% SiC) show low corrosion rate and maximum hardness, respectively, as discussed earlier. The costs of sample designations G3, G5, and G21 are estimated at 287.5, 272.5, and 342.5 INR. The costs of sample designations G3 and G5 were found to be approximately 5.83% and 9.16% lower than AA2014 alloy. The cost of sample designation G21 was found to be 14.16% higher than AA2014 alloy, which is not acceptable from the cost point of view of green hybrid metal matrix composite. Minimum cost (272.5 INR) and corrosion rate were observed for sample designation G5. The hardness of sample designation G5 after heat treatment was found to be 110 BHN, which is acceptable from the mechanical property point of view as well as the cost of composite point of view.

Figure 12:

Cost estimation of hybrid green MMCs.