Mechanical Properties and Tribological Study of Bottom Pouring Stir-Cast A356 Alloy Reinforced with Graphite Solid Lubricant Extracted from Corn Stover

Abstract

The present work epitomises extracting the graphite (Gr) solid lubricant from the corn stover. The extracted Gr was incorporated as reinforcement in the A356 alloy (Al-7Si), and the effect of the Gr particles on the mechanical and tribological properties was investigated. In spite of this, the input process parameters for the dry sliding wear test at room temperature against the EN31 steel disc were optimised through ANOVA analysis. The fabricated A359—X wt% (X = 0, 2.5, 5, 7.5) composite through bottom pouring stir casting techniques was analysed microstructurally by using XRD and FESEM analysis. The micro Brinell hardness and tensile strength were investigated per ASTME10 and ASTME8M standards. A wear test was performed for the composite pins against the EN31 steel disc according to ASTM G99 specifications. The XRD analysis results depict the presence of carbon (C), aluminium (Al), and silicon (Si) in all the wt% of the Gr reinforcement. However, along with the elements, the Al₂Mg peak was confirmed for the A356—7.5 wt% Gr composite and the corresponding cluster element was confirmed in FESEM analysis. The maximum micro Brinell hardness of 92 BHN and U.T.S of 123 MPa and % elongation of 7.11 was attained at 5 wt% Gr reinforcement due to uniform Gr dispersion in the A356 alloy. Based on the ANOVA analysis, the optimal process parameters were obtained at 20 N applied load, 1 m/s sliding velocity, and 1000 m sliding distance for the optimal wear rate of 0.0052386 g/km and 0.364 COF.

1. Introduction

Wear is commonly observed in all the components moving relative to one another such as automobile connecting rods, pistons, bearings, and cylinder bores. Hence, the consideration of wear is crucial while designing with these elements to ensure superior and accurate performance in all tribological applications [1]. Metal matrix composites (MMCs) have been shown to have improved wear properties, as well as higher specific strength and stiffness than monolithic alloys. As a result, it is important in tribology to investigate MMC wear characteristics from several different perspectives [2].

Aluminium alloys are widely used in the automobile sector given their inherent properties like being lightweight and having high specific strength, better corrosion resistance, and malleable characteristics [3]. The hypereutectic Al-Si alloy with a silicon content of 17% is known to exhibit remarkable wear resistance and load-bearing properties [4]. The mechanical and tribological properties of the Al-Si alloy are modified by reinforcing the primary and secondary reinforcements such as B₄C, Al₂O₃, MgO, SiC, Zirconium Silicate (ZrSiO₄), and Gr [5,6,7] in the matrix. Among these reinforcements, graphite reinforcement was abundantly used to synthesize the aluminium composite to fabricate the brake discs and bearing surface due to the possessiveness of the excellent self-lubricating properties, enhanced durability, and corrosion resistance [5,8,9]. Jaswinder [10] incorporated 6 wt% graphite in the A359-10%SiC composite, and results reveal that the added graphite lowered the wear and frictional properties of the A356-Gr-SiC composite. Chou Shang-Nan [11] analysed the squeeze-casted Al-Si-Al₂O₃ hybrid composite and concluded that a maximum bending strength and fracture toughness of 443 MPa and 11.35 MPa m^1/2 were observed at 5 wt% Al₂O₃ reinforcement. Haji Zamanai [12] studied the stir-cast Al-Si-Al₂O₃-10%ZrO₂ composite. The experimentation confirms that the incorporation of alumina oxide and zirconium oxide improves the compressive strength, yield strength, and tensile strength of the hybrid composite.

Several researchers studied the mechanical and wear properties of the composites reinforced with self-lubricated solid lubricants such as graphite, magnesium, molybdenum disulphide, boron nitride, and other particles. However, nanoparticles are characterised by high cost and difficulty in the fabrication process, limiting their usage in composite synthesis. Generally, solid lubricants extracted from the coal mines cause drastic pollution, soil erosion, and environmental degradation. In addition to this, the extraction process releases large quantities of methane and CO₂ gasses, exacerbating climate change. Hence, in this study, graphite particles were extracted from corn stovers which were abundantly available in the Sambalpur area in the western region of Odisha. The pyrolysis process was used for graphite extraction and the extracted graphite was treated with hydrofluoric acid (HF) to enhance purity. The high-purity graphite was used as reinforcement in the A356 alloy to study its effect on the mechanical and tribological properties. The input process parameters in the tribology test such as applied load, sliding speed, and sliding distances were optimised against the output responses such as wear rate and COF.

2. Materials and Composite Synthesis

The A356 matrix powder (≤20 µm) was procured from Nano Shell Enterprises, Chennai. The elemental constituents present in the A356 powder are shown in

Table 1. Elemental constituents in A356 alloy.

Element	Si	Ti	Mg	C	Fe	Remaining
Wt%	6.56	0.06	0.36	1.65	0.08	Aluminium

. The solid lubricant was extracted from the corn stover, and the extracted graphite was added as reinforcement in the A356 (Al-7Si) alloy. The elemental morphology through FESEM and the EDX spectra for the extracted Gr and A356 alloy are displayed in Figure 1. The SEM micrograph of Gr confirms the hexagonal planar structure (refer to Figure 1a) and no elements other than carbon were identified in the EDX spectra for Gr, as shown in Figure 1b.

Table 2. Synthesized bottom pouring stir-cast A356-Gr composites.

Composite Code	wt% of A356	wt% of Graphite Reinforcement
A356	100	0
C1	97.5	2.5
C2	95	5
C3	92.5	7.5

shows the composites’ compositions. The composites were manufactured using the bottom pouring stir casting technique. The 250 g Al-7Si alloy rods were initially heated and melted in the muffle furnace at 700 °C. The molten Al-7Si slurry was vigorously stirred for a duration of 5 min using a graphite stirrer rotating at a speed of 500 rpm [2,13]. The graphite particles were preheated at 130 °C and then added to the molten metal and stirred at 500 rpm for 5 min for the uniform dispersion of the Gr particles. Thereafter, the composite slurry was added to the mild steel die which was placed below the melting furnace. The schematic diagram for the bottom pouring stir casting and the fabricated composites for the tensile test are depicted in Figure 2a,b, respectively. For microstructural investigation, the casted specimens were polished with different grade emery papers ranging from 200 mesh size to 2300 mesh size, and then polished with the polishing cloth by applying diamond paste size varying from 6 µm to 12 µm, 3 µm to 4 µm, and 0.5 µm to 1 µm. The polished specimens were subjected to etching in Keller’s for 30 s for the proper visualization of the microstructures.

The compression strength and ultimate tensile strength (U.T.S) were measured on the M30 model universal testing machine as per ASTME9 [10] and ASTME8M standards, respectively [7,14]. The Brinell hardness was measured on an MD300 hardness tester according to ASTME10 specifications with the application of a 50 kg load by using a 2.5 mm dia ball indenter. The wear test at room temperature was performed on a DUCOM tribometer with an EN-31 hardened steel disc. The composite pins were prepared according to ASTM G99 standards [15]. The tribology tests were performed at three process parameter levels as tabulated in

Table 3. Input tribological process parameters for ANOVA analysis.

Applied Load (AL in Newtons)	Sliding Distance (SL in Meters)	Sliding Velocity (SL in m/s)
20	1500	1
30	2000	2
30	2500	3

. The process parameters were optimised by using the ANOVA (Analysis of Variance) approach against the minimum wear and COF.

3. Results and Discussion

3.1. XRD Analysis and Microstructural Study

The optical microscope images for the cast A359—2.5 wt% Gr, A359—5 wt% Gr, and A359—7.5 wt% Gr composites are shown in Figure 3a–c. The images were captured under the same magnification for comparison. The graphite particles are identified as black-coloured spots in all composites, and the percentage of these particles is found to improve with Gr reinforcement percentage. The XRD patterns for the composite reveal graphite (Gr), silicon (Si), and aluminium particles at the Gr wt% of 2.5% and 5%. Despite this, oxygen (O), magnesium (Mg), and Al₂Mg elements were detected in the XRD pattern at 7.5% Gr reinforcement, as shown in Figure 4. These added magnesium elements for hydrogen absorption eliminate the blow holes during casting, react with the aluminium matrix, and form the Al₂Mg agglomeration at 7.5 wt% Gr addition. In addition to this, the added Gr reacts with the entrapped oxygen during the solidification process and forms graphene oxide (Gro).

The SEM images of the composites are depicted in Figure 5 and Figure 6. The Gr reinforcements were dispersed uniformly in the A356 alloy up to the addition of 5 wt% Gr due to the uniform stirring and the proper interface bonds in the Gr and A356 alloy particles. However, Al₂Mg clusters were attained in the 7.5 wt% graphite composite, and the corresponding cluster was confirmed in the Al356—7.5 wt% Gr XRD pattern (refer to Figure 4). The disparity in the thermal properties of A356 and the generated Al₂Mg agglomeration causes weakening in the composite bonding strength and load-bearing capability. The EDX spectra for the composites reinforced with 2.5 wt% and 5 wt% Gr reveal the presence of C, Si, and aluminium, as shown in Figure 5a–d. The presence of magnesium (Mg) and oxygen (O) was confirmed at 7.5 wt% Gr addition, which shows the clusters of Al₂Mg and GrO in the A356—7.5%Gr composite, as depicted in Figure 6.

3.2. Hardness of the A356-Gr Composite

The Brinell microhardness of the casted samples was investigated by averaging the five readings taken at different locations on the surface, and the results are shown in Figure 7. It was found that the microhardness improved with the Gr reinforcements up to 5 wt%, and the maximum Brinell hardness of 92 BHN was noticed for A356—5 wt% Gr composite. The hardness value gradually decreased after the addition of 5 wt% Gr reinforcement. There was an improvement in the 6.1%, 13.5%, and 9.87% BHN for the addition of 2.5 wt%, 5 wt%, and 7.5 wt% reinforced Gr compared to the A356 alloy. The maximum hardness of the A356—5 wt% composite was attributed to the even distribution of the graphite particles and the strong cohesive interface bonding among the graphite and A356 particles [16]. The minimal porosity levels lead to strengthened load transfer along the grain boundaries of neighbouring particles, which makes hardness higher at 5 wt% graphite reinforcement. The reduction in the microhardness at 7.5 wt% Gr reinforcement can be attributed to two factors. The presence of brittle Al₂Mg clusters promotes different expansions than the A356 and Gr particles, which promotes the weak interfaces and micropores, and causes the composite to have a hardness lower than the 5 wt% Gr reinforcement.

3.3. Tensile Strength and % Elongation of A356—Gr Composite

The U.T.S and percentage of elongation of the casted specimens are shown in Figure 8. The U.T.S and % elongation were considerably enhanced by 18.72% and 48.12% for the A356—5 wt% Gr composite compared to the A356 alloy. The highest values of 123 MPa compression strength and 7.11% percentage of elongation were attained at 5 wt% Gr reinforcement, and U.T.S and % elongation values were found to be decreased to 109 MPa and 6.8% for the A356—7.5wt% Gr composite. Experimentation on the incorporation of graphite particles strengthens the aluminium alloy. The decrement in the U.T.S and % elongation over the addition of 5 wt% Gr is due to the addition of the increased Gr weight percentage, accelerating the generation of a large number of slip planes at the grain boundaries [17,18]. During the application of tensile loads, the atoms find the easiest path to displace along the slip planes which leads to plastic deformation at the lower intensity of applied tensile loads. This slip phenomenon in the A356—7.5 wt% Gr composite tends to lower U.T.S and % elongation more than the A356 composite reinforced with 5 wt% Gr particles. The presence of various elastic characteristics of Al₂Mg, A356, and graphite generates a tri-axial state of stress, which accelerates crack initiation and propagation at the grain boundaries and leads to the early-stage failure of the composite with 7.5 wt% Gr [19,20].

3.4. Tribological Behaviour of the A356-Gr Composite

The wear track used and the synthesized A356—Gr composite pins for the tribology test are depicted in Figure 9a,b. The wear test was conducted at 10 N, 20 N, 30 N, and 40 N applied loads, 1000 m sliding distance, and 1 m/s sliding velocity. The variation in the COF and wear rate at the constant sliding distance and loads is depicted in Figure 10. It was found that the COF and wear rate were increasing with an increase in the applied loads for all wt% of Gr reinforcements. The composite with 7.5 wt% Gr content exhibits minimal COF and wear loss compared to the composite reinforced with 2.5 wt% and 5 wt% Gr particles. The decreased wear rate and COF were attributed to the self-lubricating properties of the extracted graphite particles [9,21].

The maximum wear rate of 0.040 g/km was observed for the unreinforced A356 alloy which was higher than all wt% Gr-reinforced A356 alloys. By increasing the Gr wt% from 2.5 wt% to 7.5 wt%, the wear rate was found to be decreased due to the existence of self-lubricated Gr particles. A wear rate of 0.041 g/km, 0.026 g/km, and 0.0056 g/km was attained for the A356—2.5 wt% Gr, A356—5 wt% Gr, and A356—7.5 wt% Gr composite, which was 16.3%, 19.8%, and 21.6% lower than the unreinforced A356 alloy. The decrease in the wear rate was attributed to the continuous rubbing of the protruding Gr particles against the disc surface, leading to the improved contact between the composite pin and disc which was prone to the generation of the lubricated tribo-layer, resulting in the minimal wear rate of the composite. The minimum wear rate for the 7.5 wt% Gr composite was due to the higher hardness of the Gr particles compared to the alloy at 5 wt% Gr particles. In addition to this, the generated frictional heat, due to the interaction between the pin’s surface and disc, led to the oxidation of the pin surface. As a consequence, the reduced metal-to-metal interface contact led to a minimal wear rate at 7.5 wt% Gr reinforcement

It was inferred that the COF for all the samples was found to increase gradually with an increase in the applied loads. The minimal COF for the A356—7.5 wt% Gr resulted from the developed mechanically mixed layer (MML) at higher applied loads. The MML lowered the metal-to-metal contact and accelerates the shearing action of the composite pin surface. The generated heat during the shearing led to the detachment of the particles from the surfaces and generated MML, which contains elements from both the pin’s and disc’s surface [22,23]. The MML diminishes the COF during the rubbing of the composite pin against the disc.

The worn surface morphology for the pin surface at a higher applied load of 40 N, sliding velocity of 1 m/s, and sliding distance of 1000 m for the different Gr wt% is shown in Figure 11. The presence of deeper and longer grooves parallel to the sliding direction was observed in all the composites, which confirms the abrasive wear phenomenon. However, there were notable distinctions in the wear morphology of the base alloy and composite surface. In the A356 alloy, deeper grooves containing shallow craters were present, as depicted in Figure 11a. This was due to the continuous contact between the harder disc surface and the softer aluminium alloy [24]. The harder particles on the disc surface penetrated into the base alloy, leading to rigorous surface damage and generating deeper and longer grooves on the pin surface. Moreover, delamination wear and fine grooves were observed on the composites reinforced with 2.5 wt%, 5 wt%, and 7.5 wt% Gr particles. The presence of the Gr particles and the Si content in the A356 alloy led to the generation of microcracks at the higher magnitude of applied loads [25,26,27]. The developed subsurface microcracks propagated and met each other, eventually causing shearing action on the pin surface and resulting in the detachment of material from the pin surface and leading to delamination wear, as shown in Figure 11b–d.

3.5. ANOVA Design of Experiments for Tribological Analysis

The maximum mechanical properties such as U.T.S and % of elongation were attained for the A356—5 wt% Gr composite. Hence, the composite specimen containing 5 wt% Gr was selected for the tribological analysis with a Taguchi L9 orthogonal array. The input parameters such as applied load, sliding velocity, and sliding distance were optimized against the COF and wear rate. The obtained output responses for the experimentation are shown in

Table 4. Input process parameters and output responses.

Run Number	Applied Load (N)	Sliding Distance (m)	Sliding Velocity (m/s)	COF	Wear Rate (g/km)
1	20	1500	1	0.008	0.26
2	20	2000	2	0010	0.28
3	20	2500	3	0.012	0.29
4	30	1500	2	0.036	0.3
5	30	2000	3	0.039	0.31
6	30	2500	1	0.04	0.318
7	40	1500	3	0.042	0.322
8	40	2000	1	0.046	0.342
9	40	2500	2	0.052	0.36

.

The ANOVA (at 95% confidence level) for the wear rate response is shown in

Table 5. ANOVA for wear rate.

Source	DOF	Adj SS	Adj MS	F-Value	p-Value
Applied Load (AL)	2	0.006273	0.003136	227.65	0.004
Sliding Distance (SD)	2	0.001244	0.000622	45.13	0.022
Sliding Velocity (SV)	2	0.000081	0.000040	2.94	0.254
Error	2	0.000028	0.000014
Total	8	0.007625

Model Summary: S R-sq R-sq(adj) R-sq(pred); 0.0037118, 99.64%, 98.55%, 92.68%.

. It was concluded that applied load has attained the maximum sum of squares, and its effects are most influential, followed by sliding distance and sliding velocity. The p-value (allowable range 0 to 0.005) is used to determine the essential input factors. The Analysis of Variance table indicates that load and sliding distance are the two most influential variables that affect wear rate. Sliding velocity, with a p-value greater than 0.05, is considered a non-influential parameter in predicting wear rate. For the given selected output response (wear rate), the R² value of 0.9964 was closely aligned with the adjusted R² value of 0.9855. The R² predicted (0.9268) and R² adjusted (0.9855) are aligned closely and in favourable limits. Therefore, this model is capable of predicting the wear rate within the specified parameter ranges.

The ANOVA for the COF (@ 95% confidence level) is illustrated in

Table 6. ANOVA for COF.

Source	DOF	Adj SS	Adj MS	F-Value	p-Value
Applied Load (AL)	2	0.002217	0.001108	255.77	0.004
Sliding Distance (SD)	2	0.000054	0.000027	6.23	0.038
Sliding Velocity (SV)	2	0.000005	0.000002	0.54	0.650
Error	2	0.000009	0.000004
Total	8	0.002284

Model Summary: S R-sq R-sq(adj) R-sq(pred); 0.0020817, 99.62%, 98.48%, 92.32%.

. It was clear that the applied load has the highest sum of square values and the most significant impact on the COF, surpassing sliding distance and velocity. Similar to the wear rate, the p-value (0 to 0.05) is used to determine the selection of the influential input parameters. According to the p-value in the Analysis of Variance, the most significant factors influencing the COF are applied load and sliding distance. The sliding velocity, which has a p-value larger than 0.05, is deemed a non-significant parameter in predicting the COF. The predicted R² value for COF (0.9332) is close to the R² adjusted (0.9848). Hence, this selected model is within the range of the parameters that can predict the COF.

The obtained quadratic equations for the COF (1) and wear rate (2) are provided below.

COF = −0.0350 + 0.001833 × AL + 0.000006 × SD − 0.00017 × SV

(1)

WR = 0.1541 + 0.003233 × AL + 0.000029 × SD + 0.00033 × SV

(2)

The actual and predicted value graphs for the wear rate and COF are plotted in Figure 12a,b. The obtained points are very close to the straight line and the value falls under the acceptable range. In addition to this, the data points are well distributed, indicating an expected correlation among the predicted and actual responses.

The residual plots for the COF and wear rate are shown in Figure 13a,b. The residue plots demonstrated that all the residues ran between −0.0075 and +0.0075 for COF and between −0.0050 and +0.0050 for the wear rate, and there was no discernible pattern observed for both COF and wear rate. The three-dimensional graph for the effect of input process parameters on COF and wear rate under dry sliding conditions is shown in Figure 14 and Figure 15. The selected process parameters in this experimentation were applied loads of 20 N, 30 N, and 40 N, sliding distances of 1500 m, 2000 m, and 2500 m, and sliding velocities of 1 m/s, 2 m/s, and 3 m/s.

The effect of input parameters on the COF for the dry sliding wear test is shown in Figure 14. Increased COF was observed with an increase in the applied load and sliding velocity. The minimum COF was attained at a sliding distance of 1 m/s and an applied load of 20 N. This was attributed to the lower sliding velocities. The composite pin surface tends to spend a prolonged period of time rubbing against the disc surface, which generates the self-lubricated tribofilm at the tribo pair. There was a 30% enhancement in the COF at 40 N applied load compared to the 20 N applied load. This phenomenon was due to the unstable MML, which was developed at the interfaces at higher applied loads. However, at a lower magnitude of applied loads, the MML was stable, which supports the generation of higher COF values.

When compared to the wear rate for A356-Gr composites, the wear rate for 40 N was significantly higher than that of 30 N and 20 N applied loads, as shown in Figure 15. This might be due to adhesive wear on the pin material, which is caused by a rise in temperature between the contact surfaces at the higher magnitude of applied load on the composite pin. The developed protective mechanically mixed layer on the tribo interfaces due to the detached Gr particles at higher loads (40 N) and sliding distances (2000 m) minimizes the wear rate at higher sliding distances. An increase in the sliding velocity from 1 m/s to 3 m/s leads to a decline in wear rate due to the presence of the protective layer consisting of self-lubricated Gr particles. It was concluded that with a minimum wear rate of 0.00563 g/km and a coefficient of friction (COF) of 0.3763, the best combination of input parameters were 20 N applied load, 1 m/s sliding speed, and 1000 m sliding distance for the A356—5 wt% Gr composite running against the EN-31 hardened steel disc.

4. Conclusions

The graphite lubricant was extracted from corn stover, and the extracted Gr was studied mechanically and tribologically by reinforcing the A356 alloy. The following results were obtained after the experiments.

1.

The U.T.S and % elongation for the A356-Gr were found to be increased up to the 5 wt% Gr addition, and the maximum U.T.S of 123 MPa and 7.11% elongation were attained due to strong interface bonding between Gr and the A356 alloy.

1.

The presence of an Al₂Mg cluster at 7.5 wt% Gr reinforcement led to a decrement in mechanical properties due to the uneven thermal expansions of the cluster and adjacent particles.

2.

The obtained micro Brinell hardness at 5 wt% Gr reinforcement was 13.5%, 6.97%, and 3.37% higher than the A356 alloy, A356—2.5 wt% Gr, and A356—7.5 wt% composite.

2.

The uniform dispersion of reinforcements was identified in the FESEM analysis of the A356—2.5 wt% Gr and A356—5 wt% Gr composite. However, Al₂Mg clusters were confirmed in the A356—7.5 wt% Gr composite due to the non-uniform sintering, and the corresponding Al₂Mg agglomeration was shown in the XRD pattern.

3.

The wear rate and COF values were found to be decreased with an increase in % Gr addition due to the formation of the self-lubricated MML at the tribo interfaces. The minimum wear rate of 0.00563 g/km and COF of 0.3763 were found at 7.5 wt% Gr reinforcement with an applied load of 40 N.

4.

The ANOVA results confirmed that the optimal process parameters for the minimum wear rate of 0.0052386 g/Km and 0.364 COF were 1 m/s sliding velocity, 1000 m sliding distance, and 20 N applied load conditions.