Tribological behavior of a newly developed AA2014/waste eggshell/SiC hybrid green metal matrix composite at optimum parameters

2.1 Matrix material

In the present investigation, AA2014 aluminum alloy was selected as a matrix material. AA2014 aluminum alloy is typically used in the aviation industry [5]. The hardness of this alloy is very high compared with other existing aluminum alloys [6], but its resistance against rust is poor. The chemical composition of AA2014 is shown in Table 1. Meanwhile, the wear rate of AA2014 alloy is 15×10⁻⁵ mm³/m at optimum parameters of 1.75 m/s (sliding velocity), 34.24 N (normal load), and 1219.63 m (sliding distance). The hardness of AA2014 alloy is 60 BHN.

2.2 Primary reinforcement material

In the present investigation, carbonized eggshell powder, which contained ceramic materials, was selected as a primary reinforcement material, as shown in Figure 1A. The compounds of the eggshell (by weight) are given below.

Figure 1:

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

• Calcium carbonates (94%)

• Magnesium carbonates (1%)

• Calcium phosphate (1%)

• Organic matter (4%)

To remove the upper cover of the eggshells, these were cleaned and dried in sunlight [6]. The dried eggshells were ball-milled to obtain eggshell powder, as shown in Figure 1B. The powder was then carbonized to 500°C for 3 h to remove the carbonaceous materials, as shown in Figure 1C.

2.3 Secondary reinforcement material

SiC was selected as a secondary reinforcement material to further improve the mechanical and wear properties of the composites. A previous study reported that the mechanical properties of the MMCs increased when SiC is used as a reinforcement in an aluminum matrix [9]. SiC also increased the modulus elasticity and resistance against dust, and can work at higher temperatures [10]. The density of the SiC is 3.2 g/cm³, which are very close to the density of aluminum alloy AA2014. SiC also works against acids, alkalis, or molten salts of up to 800°C [11]. For these reasons, SiC is considered one of the best reinforced materials in aluminum-based MMCs [12]. Furthermore, SiC is easily available and its wettability with aluminum alloys is good [13].

2.4 Fabrication of hybrid MMCs

The AA2014/carbonized eggshells/SiC hybrid green MMC used in this study was fabricated by electromagnetic stir casting technique at parameters of 12 A (stirring current), 200 RPM (stirring speed), 180s (stirring time), and 700°C (matrix pouring temperature), and then immediately extruded on a UTM machine at 60 MPa, using a cylindrical H13 tool steel die coated with graphite [5].

The matrix material was heated above its liquidus temperature in a muffle furnace. Carbonized eggshell and SiC particles were also preheated to about 300°C and 500°C, respectively, to avoid the wettability problem. The liquid AA2014 aluminum alloy with a temperature of 700°C was poured into a graphite crucible. After pouring the matrix material, preheated reinforcement particles were mixed with melted aluminum alloy, as shown in Figure 2. A thermocouple was inserted into the graphite crucible for the temperature measurement of the composite during stirring.

Figure 2:

Schematic diagram of the electromagnetic stir casting setup [5].

The size of each reinforcement particle was set at 25 μm. Although it was not easy to obtain the exact particle size (25 μm) of all reinforcement particles (SiC, carbonized eggshell powder), the particle size with deviation 2 μm (25 μm±2 μm) was selected. SiC particles were directly purchased from the market and had an average particle size of 25±2 μm. Carbonized eggshell particles were ball-milled to obtain a uniform particle size (25 μm±2 μm).

2.5 Pin-on-disc apparatus and its parameters

The wear test of the AA2014/carbonized eggshell/SiC hybrid green MMC was carried out on a pin-on-disc apparatus. A hardened mild steel (MS) circular plate was used as a bottom disc, whereas the AA2014/carbonized eggshell/SiC hybrid green MMC was selected as a specimen pin, as shown in Figure 3. Specimen pins used in the wear test are shown in Figure 4. Different process parameters of pin-on-disc machine affect the wear rate and coefficient of friction. A pilot experiment was carried out using a single factor (carbonized eggshell and SiC wt.%, normal load, and sliding velocity and distance) to determine the optimum level of factors in the pin-on-disc machine. Their ranges are given in Table 2.

Figure 3:

Experimental set up of the pin-on-disc machine with specimen pin (AA2014/eggshell/SiC composite).

Figure 4:

Specimen pins (AA2014/eggshell/SiC hybrid green metal matrix composite) used in the wear test.

2.6 Planning of the experiment

Response surface methodology (RSM) refers to the set of statistical and mathematical techniques that are used to develop, improve, or optimize a product or process. RSM is helpful for the modeling and analysis of programs [5]. The experimental work is carried out as per the Box-Behnken Design, using response surface methodology. Based on the experimental design given in Table 3, 46 experiments were performed to test the wear rate of the AA2014/eggshell/SiC hybrid green MMCs. The measured wear rates are shown in Table 3.

3.1 Mathematical modelling

In the present study, ANOVA Table (Table 4) was used to analyze the results for the regression model test, the test for the significance of individual models (sliding velocity and distance, carbonized eggshell and SiC wt.%, and normal load), and test for lack-of-fit.

The Model F-value shown in 195.17 indicates that the model is very important. It is diffcult to say that the large “Model F-Value” is obtained because of noise. Values of “Prob>F” but not more than 0.0500 shows the importance of model terms. Here A, B, C, D, E, A², C², D², E², AB, AC, AD, AE, BC, BD, BE, and DE are all significant model terms. When the value is more than 0.100, this indicates that the model terms are not significant.

When the “Lack of Fit F-value” approaches 2.54, this means this value is not very important. When “Lack of Fit F-value” is large, this may be attributed to noise. Non-important lack of fit is fine. “Pred R-Squared” is 0.9760, which is in good agreement with the “Adj R-Squared.” The “Adj R-Squared” is 0.9885. Meanwhile, the “Adeq Precision” determines the signal to noise ratio, which should be more than four.

Figure 5 shows the relationships between the wear rate of predicted value and that of actual value. The model adequacy is obtained, keeping in mind that the basic statement of regression concept has not been violated. The graph of the residual and normal percentage probabilities is shown in Figure 6, which indicates a straight line. Actual value, predicted value, and residuals (difference between actual and predicted) are shown in Table 5. Residual vs. run and residual vs. predicted graph are shown in Figures 7 and 8 , respectively. Both graphs show that the experiment was conducted randomly and that no similar patterns were observed.

Figure 5:

Correlation between the predicted and actual values.

Figure 6:

Normal probabilities of residuals.

Figure 7:

Residual vs. run.

Figure 8:

Residual vs. predicted.

The regression coefficients of the second order equation (Equation 1) for wear rate were developed by using the experimental data (Table 3) given below.

Wear rate=+7.35+0.19×A–8.0×10–3×B+6.72×10–3 ×C+1.77×10–3×D+0.01×E–0.02×A2–3.83×10–4 ×B2–3.02×10–4×C2–5.30×10–7×D2–6.62×10–3 ×E2–6.67×10–3×A×B+1.67×10–3×A×C–1.44 ×10–5×A×D+8.33×10–3×A×E+5.0×10–4×B×C +8.0×10–6×B×D+1.70×10–3×B×E–1.11×10–6 ×C×D+1.33×10–4×C×E+2.27×10–5×D×E

3.2 Effects of parameters on wear rate

In this study, the effects of sliding velocity and distance, carbonized eggshell and SiC wt.%, and normal load on the wear rate of the AA2014/eggshells particulate/SiC hybrid green MMC were studied. Figure 9 shows the individual parameters’ effects on wear rate. Figures 10 (a–d) show the interactions of parameter A with parameters B, C, D, and E, respectively, to reduce the wear rate of composite. Figures 11 (a and b) display the interaction of parameter C with parameters D and E, respectively, to reduce wear rate. Figures 12 (a–c) present the interactions of parameter B with parameters C, D, and E, respectively, to reduce wear rate. Figure 13 shows the interaction of parameter D with parameter E to reduce wear rate.

Figure 9:

Single factor effects on wear of hybrid composite.

(A) sliding velocity, (B) carbonized eggshell wt.%, (C) normal load, (D) sliding distance, (E) SiC wt.%.

Figure 10:

Interactions of sliding velocity with carbonized eggshell wt.%, normal load, sliding distance, and SiC wt.% to reduce wear rate.

Figure 11:

Interactions of normal load with sliding distance and SiC wt.% to reduce wear rate.

Figure 12:

Interactions of carbonized eggshell wt.% with normal load, sliding distance, and SiC wt.% to reduce wear rate.

Figure 13:

Interaction of sliding distance with SiC wt.% to reduce wear rate.

3.3 Optimum parameters with desirability one

From the ramp function graph (Figure 14), it can be observed that when sliding velocity, carbonized eggshell wt.%, normal load, sliding distance and SiC wt.% are 1.75 m/s, 6.5%, 34.24 N, 1219.63 m, and 11 wt.% respectively, then the optimum value of the wear rate of the AA2014/carbonized eggshell/SiC hybrid green MMC is 8.89×10⁻⁵ mm³/m. ANOVA results in Table 4 show that all the selected parameters (sliding velocity, carbonized eggshell wt.%, normal load, sliding distance, and SiC wt.%) are significant with respect to wear rate. The importance of the process parameters can be ranked based on their F ratios, as shown in Table 4. It can be concluded that sliding velocity contributes most, followed by sliding distance, SiC wt.%, normal load, and carbonized eggshell wt.% in reducing the wear rate of the AA2014/6.5% carbonized eggshell/11% SiC hybrid green MMC.

Figure 14:

Ramp function graph for the minimum wear rate with desirability one.

3.4 Confirmation experiment

Three samples of the AA2014/6.5% carbonized eggshell/11% SiC hybrid green MMC were fabricated by electromagnetic stir casting process, followed by hot extrusion for the confirmation test. Figure 15A and B show the typical microstructures of AA2014/6.5% carbonized eggshell/11% SiC hybrid green MMC. The microstructure images show the uniform distribution of 6.5 wt.% eggshell and 11 wt.% SiC in the AA2014 matrix alloy. From the fabricated hybrid composite samples, three specimen pins were prepared for the wear test. The experimental wear rate (average for three test samples) corresponding to these parameter values (sliding velocity of 1.75 m/s, carbonized eggshell wt.% of 6.5%, normal load of 34.24 N, sliding distance of 1219.63 m and SiC wt.% of 11%) was found to be 9.5×10⁻⁵ mm³/m. The average achievable wear rate with desirability one was 8.89×10⁻⁵ mm³/m. There was only a 6.42% error rate in the experimental and modeled results. Hence, the developed model can be effectively used in the stated process parameter range.

Figure 15:

Microstructure of AA2014/6.5% carbonized eggshell/11% SiC hybrid green metal matrix composite.

Meanwhile, the experimental wear rate of the AA2014/6.5% carbonized eggshell/11% SiC hybrid green MMC at optimum parameters was found to be 9.5×10⁻⁵ mm³/m. The wear rate of the AA2014 matrix alloy was 15×10⁻⁵. Results showed that the wear rate of the hybrid green MMC was reduced by about 36.66% compared with the matrix alloy (Table 6). Figure 16A shows the SEM photographs of the AA2014 matrix alloy, and Figure 16B shows the SEM photographs of the AA2014/6.5% carbonized eggshell/11% SiC hybrid green MMC. The experimental coefficient of friction (average for three test samples) corresponding to these parameter values (sliding velocity of 1.75 m/s, carbonized eggshell wt.% of 6.5%, normal load of 34.24 N, sliding distance of 1219.63 m, and SiC wt.% of 11%) was found to be 0.20. This value is considered acceptable.

Figure 16:

SEM photographs of: (A) AA2014 matrix alloy, (B) AA2014/6.5% carbonized eggshell/11% SiC hybrid composite composites.

3.5 Density analysis

To determine the density of the alloy AA 2014, 6.5% by weight carbonized eggshells and 11% by weight SiC hybrid green MMC, three samples were prepared in the laboratory. It was observed that 6.5% by weight carbonized eggshells was lighter than the AA 2014 matrix and SiC hybrid green MMC. The densities of the above three materials were 2.80, 2.0, and 3.21 g/cm³, respectively. These indicate that when the percentage of the carbonized eggshells increases in AA2014, the density of the composite material decreases. Initially, the eggshell is added as a primary reinforcement material, to which SiC is then added to increase the resistance to wear.