Sustainable development on production and characterization of metal matrix composites using stir casting

In this study, four different combinations of aluminium metal matrix composites (AMCs) were produced using a computerized stir casting process. The feasibility of using car Scrap Aluminium Engine Head (SAEH) as matrix material, Fresh Alumina Catalyst (FAC) and Spent Alumina Catalyst (SAC) from petrochemical industries as reinforcement material were investigated. The physical and mechanical properties of the cast samples were tested through density, hardness, tensile, compression, and impact test. Microstructural investigations were carried out using an optical microscope, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Differential thermal analysis (DTA) and Thermo gravimetric analysis (TGA) were also conducted to justify the results obtained. The results indicated that SAEH reinforced with 5 wt% SAC exhibited lower porosity (2.6%) and higher Brinell hardness (71.5 BHN), Vickers hardness (307.1 VHN), tensile strength (217 MPa), and compressive strength (426 MPa) than other composites. Additionally, this composite showed the highest impact strength (0.02375 J mm−2) and DTA value (568.5 μV mg−1). The TGA result showed that all composites had high thermal stability, with the SAC-reinforced composites having the highest thermal stability (100.13%).


Introduction
Sustainable development has become an important global issue, with rising concerns over environmental degradation and resource depletion. One way to promote sustainable development is through the use of alternative materials and environmentally friendly production processes. Metal matrix composites (MMCs) are a class of engineered materials consisting of a metal matrix and reinforcement materials such as ceramics, carbides, and carbon fibers, offering several advantages over conventional materials like steel, aluminum, and titanium. MMCs are known for their enhanced properties, including high strength, stiffness, and wear resistance [1,2]. Several production methods are available to manufacture MMCs, including powder metallurgy, stir casting, squeeze casting, and in situ synthesis, each having its advantages and disadvantages.
Stir casting is an attractive and practical method of production, which allows easy incorporation of a wide range of reinforcement materials, including ceramic and carbon-based materials. It also produces MMCs with a range of compositions and properties, making it suitable for a variety of applications. Several studies have compared the properties of MMCs produced using different methods, including stir casting, powder metallurgy, and in situ synthesis. These studies show that stir casting produces composites with higher strength and ductility compared to the powder metallurgy and in situ synthesis methods [3,4].
This study aims to investigate the feasibility of producing and characterizing aluminum-based MMCs using stir casting with a focus on sustainability. Car scrap aluminum engine head (SAEH) was used as the matrix material, while fresh alumina catalyst (FAC) and spent alumina catalyst (SAC) from petrochemical industries were used as reinforcement materials. The utilization of car scrap aluminum engine head and alumina catalysts from petrochemical industries as raw materials contributes to the sustainable development of the manufacturing industry by reducing waste and promoting the circular economy [5]. Scheme 1 depicts the manufacturing process for the metal matrix composites made using scrap aluminium.
Aluminum is one of the most widely used metals in various industries, but it has some limitations, such as low wear resistance and poor mechanical properties. Therefore, MMCs based on aluminum have been developed to enhance the properties of the metal matrix [6]. Aluminium matrix composites (AMCs) reinforced with ceramic particles like SiC, B 4 C, and Al 2 O 3 can enhance the mechanical properties of the composite, making it an ideal choice for various applications [7,8]. Fly ash, coconut shell ash, and rice husk ash are some of the industrial waste materials that can be used as reinforcements in aluminium matrix composites to reduce the cost of composites, improve physical and mechanical properties, and reduce the thermal expansion coefficient [9][10][11]. Fresh alumina catalyst (FAC) and spent alumina catalyst (SAC) are by-products of the petrochemical industry, and their utilization in MMCs is an attractive solution for waste management and promoting sustainability. FAC is a high-purity form of alumina that is used as a catalyst support in petrochemical processes, while SAC is a spent form of FAC that has lost its catalytic activity [12].
The production of MMCs using stir casting involves several process parameters such as stirring speed, stirring time, pouring temperature, and the weight fraction of reinforcement material [13,14]. These parameters can influence the final properties of the composite material, and their optimization is essential for producing MMCs with desirable properties. In this study, four different combinations of aluminum-based MMCs were produced using stir casting, and their physical and mechanical properties were investigated.
The physical and mechanical properties of the MMCs were evaluated through density, hardness, tensile, compression, and impact tests. The microstructure of the composites was examined using an optical microscope, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) were also conducted to investigate the thermal stability of the MMCs. The results of this study can provide valuable insights into the use of aluminum-based MMCs in sustainable manufacturing processes, offering an environmentally friendly solution to waste management and promoting the circular economy.
Furthermore, the utilization of waste materials such as car scrap aluminum engine head and spent alumina catalyst in the production of MMCs can contribute to the circular economy by reducing waste and promoting resource efficiency. By using such waste materials as feedstock, the manufacturing process becomes more sustainable and environmentally friendly, leading to a reduced environmental footprint.
In conclusion, the development and production of sustainable materials such as MMCs are essential for promoting sustainable development and reducing the negative impact of industrial processes on the environment. The stir casting method is a cost-effective and practical approach to producing MMCs with enhanced properties compared to conventional materials. In this study, the use of waste materials such as car scrap aluminum engine head and spent alumina catalyst as raw materials in the production of aluminum-based MMCs using stir casting was investigated. The results showed that MMCs produced using this method and utilizing waste materials exhibited favorable physical and mechanical properties, making them suitable for various industrial applications. The utilization of waste materials in the production of MMCs not only contributes to sustainability but also promotes the circular economy, making it an attractive solution for waste management in the manufacturing industry.
Novelty statement: This study presents a sustainable approach to producing and characterizing aluminum-based metal matrix composites (MMCs) using stir casting, with a focus on utilizing car scrap aluminum engine head and alumina catalysts from petrochemical industries as raw materials to promote the circular economy and reduce waste. The study evaluates the physical and mechanical properties of four different combinations of MMCs reinforced with fresh and spent alumina catalysts. The results of this study provide valuable insights into the use of sustainable MMCs in various industries, offering an environmentally friendly solution to waste management.

Materials and methods
In this work Scrap Aluminum Engine Head (SAEH), with the grade AC4B (Al-Si-Cu) one of the cast alloys that is frequently used in the automotive industry, is selected as matrix for this investigation and reinforced with 5% catalyst particles, such as fresh and spent alumina catalyst in order to overcome mechanical failure in automobile and aircraft industries. The corresponding chemical compositions of matrices and reinforcement particles are given in table 1.
The composites were made using a computer-assisted stir casting process. Table 2 displays the raw material composition of the aluminum metal matrix composites. Acetone was used to clean the aluminum cylinder head that was obtained from the automobile scrap yard. After being cleaned, the cylinder head is cut down into smaller sizes and transferred to the stir casting furnace. The computerized stir casting parameters are given in table 3. The specific values for these process parameters were obtained through an L9 optimization study using Taguchi-Grey approach [16,17]. It was decided to preheat the combinational steel dies to 300°C so that they would retain their permanent hardness. A temperature of 750°C was set at the resistance-controlled casting furnace. After cleaning, the stirrer rod and stirrer were coated with a non-stick boron carbide coating that could resist high temperatures and prevent the edges of the stirrer from being worn down by erosion. To get rid of any moisture and prevent the reinforcing particles from clustering together, they were heated in a preheater chamber at 300°C. Matrix materials were added to the furnace at a crucible temperature of around 350°C, and the mixture was heated to 750°C to fully melt. In order to minimize the presence of temperature gradients and defects in the final material, the preheating temperature of the mold was closely monitored and precisely controlled. At a stirring speed of 450 rpm, the stirrer rod was turned on, and its vertical jog movement was automated to ensure that the whole melt volume was well mixed with reinforcement. The wettability between the matrix and the reinforcement was enhanced by adding 1% by weight of magnesium to the molten matrix. After forming a vortex, the preheated reinforcement particles were added gently and stirred for 10 min. Finally, a bottom tapping valve is activated to transfer the molten liquid to the die via gravitational force. Each of the other three samples was made using the same process.

Microstructural and elemental composition characterization
The microstructure of the produced composite was intended to utilize a Digi-DMI victory optical microscope at a room temperature of 24°C and Keller's Reagent as the etchant. After adequate etching of the produced samples, the scanning electron microscopic investigation was performed. Carl Zeiss sigma with GEM11 column, USA make, was used for the analysis, which has a resolution of 1.5 nm. The SEM-EDS analysis was performed on each specimen using a Bruker Nano X-Flash detector, German on a 3-point spectrum. The XRD analysis was carried out using Panalytical, Netherlands using a slow scan technique with four degrees per second steps and a scan speed of 0.2 s step −1 .

Mechanical characterization
Mechanical testing of the manufactured composites was performed using Brinell hardness equipment (OPAB-3000) and Vickers hardness equipment (Mitutoyo/HM 200) in accordance with ASTM E18-17 standards, with a 1/16' steel ball intender and a dwell period of 10 s. Tensile testing was performed using a dynamic universal testing machine (INSTRON -8801) using the ASTM B557 standard at room temperature with a load range of 500 N, and the results of ultimate tensile strength, yield strength, and % elongation were recorded. Compression testing for the generated composite was performed on the same UTM machine using the ASTM E9-09 standard, at room temperature, with a load of 600 kN applied progressively at a crosshead speed of 3.0 mm min −1 . Impact testing on the manufactured composite was carried out at room temperature utilizing AIT -300N in accordance with the ASTM E23-16 standard. The generated composites were subjected to differential thermal analysis and thermogravimetric analysis utilizing a Netzsch-STA-F3 Juipitar with a temperature setting of (0°C-1000°C), a heating rate of 100°C min −1 , and a decomposition temperature range of (550°C-700°C). Table 4 shows the physical, mechanical, and thermal properties of the composites produced.

Microstructure evaluation of the developed composites
The microscopic analysis of the produced composites is shown in figure 1. The microstructure image has a scale of 100 μm and a magnification of 100x, with a zoomed inset image at a scale of 20 μm and a magnification of 500x. The specimens were mounted in Bakelite according to ASTM E3-01 and Keller's reagent was used for Aluminium Alloy 2.5% HNO 3 , 1.5% Hcl, 1% HF, 95% H 2 O. The aluminum matrix is indicated in grey, while the mixture of the reinforcement and eutectic phase of silicon is denoted as green [12]. The results in figures 1(a)-(d) show that the typical α-Al phase dendrite is present in a matrix of aluminum. Entrapped air can be observed in figures 1(c) and (d), which may lead to defects in porosity and shrinkage resulting from grains that are loosely packed [18].

Scanning electron microscopic (SEM) analysis
The SEM analysis of the produced composites at a 20-micron scale and 500x magnification is shown in figure 2. The matrix and reinforcement particles are indicated in dark grey and light grey, respectively. The microstructure of both matrix materials, SAEH and LM25, indicates the presence of eutectic silicon and primary α phase aluminum dendrites [12]. The dispersion of reinforcement particles can be clearly seen in figures 2(a) and (b), while few clusters were observed in figures 2(c) and (d) due to the improper distribution of reinforcement particles. In figure 2(d), micro pores and micro cracks were also visible, which can be attributed primarily to the smaller size of the alumina reinforcement added to LM25 alloy, which tends to agglomerate due to increased surface energy, as well as the loosely packed grains in LM25 alloys [19]. This can lead to shrinkage flaws or porosity development. SAEH, which is basically an AC4B type alloy, has closely packed grains due to its superior grain structure, making it free of defects [20]. Therefore, it can be concluded that the dispersion of particles in SAEH + FAC and SAEH + SAC is better than that in LM25 + FAC and LM25 + SAC, as shown in figures 2(a) and (b), respectively, which have no cluster formation.

Elemental mapping analysis
The figure 3 presents the EDS analysis results of the produced composites. The EDS analysis was performed to ensure the homogeneity of the synthesized samples and to check for any contaminants resulting from the synthesis process [21]. The figures 3(a)-(d) show the EDS spectra of SAEH + FAC, SAEH + SAC, LM25 + FAC, and LM25 + SAC, respectively. The EDS spectra indicate the presence of various elements such as Al, Si, Ca, Fe, Cu, O, Na, Mg, and C as the main peaks. The major constituent in all the composites is aluminum and carbon [22]. The figure 3(b) shows the highest amount of Si among all the composites, which is 15.85%. In addition, an additional element Mn is observed in figure 3(a). These results suggest that the composites are homogeneous, and the synthesis process did not introduce any significant contaminants.

X-ray diffraction (XRD) analysis
The XRD analysis results for the produced samples are presented in figure 4. The peaks observed in all samples show similar 2θ values. These results are consistent with the presence of various intermetallic phases of Al 2 O 3 and Si, which were formed due to the diffusion of reinforcement with Al [23]. The major peak observed at 2θ values of 38.5°and 44.5°corresponds to Aluminum, while five minor peaks corresponding to Aluminum oxide, silicon, and Aluminum were observed at 2θ values of 28.5°, 47°, 65°, 78.5°, and 83°, respectively, with different intensities in miller plane [24].The results indicate that the Aluminum peak was observed in three different planes, namely (0 2 2), (0 0 4), and (0 4 4), while the silicon peak was observed in (2 2 0) plane. Figure 5 presents the density and porosity analysis of the produced composites. The theoretical density was calculated using the rule of mixture while the experimental density was obtained using the direct volume method [25]. Additionally, the porosity percentage was calculated using equation no. 3.

( )
Where W m -Weight fraction of matrix, W r -Weight fraction of reinforcement, ρ m -Density of matrix, ρ r -Density of reinforcement, ρ th -Theoretical density of sample, r -radius of the sample, h -height of the sample. The porosity percentages for SAEH + FAC, SAEH + SAC, LM25 + FAC, and LM25 + SAC are 2.4%, 2.6%, 16.6%, and 7.9%, respectively. It is observed that the experimental density of the composites is slightly lower than the theoretical density [26]. The high porosity levels in the composites lead to poor mechanical characteristics and low strength [27]. The optical microstructure image shown in figure 1 supports this observation, as it is evident that the porosity of SAEH + FAC and SAEH + SAC is very low compared to LM25 + FAC and LM25 + SAC, thereby improving the mechanical properties of the composites. Figure 6, shows the tensile test results for the produced composites. The produced composites exhibited varying ultimate tensile strengths. SAEH + FAC had the lowest ultimate tensile strength at 208 MPa, while SAEH + SAC had a higher ultimate tensile strength at 217 MPa. On the other hand, LM25 + FAC exhibited the lowest ultimate tensile strength at 92 MPa, while LM25 + SAC had a higher ultimate tensile strength at 184 MPa. The presence of entrapped air and higher porosity percentages in LM25 + FAC (16.6%) and LM25 + SAC (7.9%) could explain the lower ultimate tensile strength in these composites. In contrast, SAEH + FAC (2.4%) and SAEH + SAC (2.6%) had lower porosity percentages, which could contribute to their higher ultimate tensile strengths. The decrease in grain size observed in the LM25 composites could be attributed to the stirring process during fabrication. Stirring can promote nucleation and growth of fine grains, resulting in a decrease in grain size. However, the entrapped air and porosity in LM25 + FAC and LM25 + SAC could have hindered the stirring process and prevented the formation of finer grains [28]. Therefore, the presence of entrapped air and porosity can have a significant effect on both the ultimate tensile strength and grain size of the produced composites.

Tensile strength and compression strength
Based on the compressive strength test results shown in figure 7, it can be observed that the SAEH + SAC composite has the highest compressive strength at 426 MPa, followed by LM25 + SAC at 285 MPa. On the other hand, LM25 + FAC has the lowest compressive strength at 184 MPa, followed by SAEH + FAC at 384 MPa.
One possible reason for the higher compressive strength in SAEH + SAC and LM25 + SAC composites is the strengthening effect of the ceramic particles. The addition of SAC particles could have provided more resistance against deformation and increased the yield strength of the composite [29]. In contrast, the lower compressive strength in LM25 + FAC and SAEH + FAC composites may be due to the presence of entrapped air and porosity, as well as the lower strength of the FAC particles compared to SAC particles. The entrapped air and porosity can act as stress concentrators and reduce the compressive strength of the composite. Additionally, the lower strength of the FAC particles may have contributed to the lower compressive strength of the composites. Overall, the compressive strength results suggest that the addition of SAC particles can improve the compressive strength of the composites, while the presence of entrapped air and porosity can negatively affect the compressive strength.

Impact strength
The impact strength of the produced composites was evaluated using the Impact test, and the results are shown in figure 8. The impact strength values for SAEH + FAC, SAEH + SAC, LM25 + FAC, and LM25 + SAC are 0.01938 j mm −2 , 0.02375 j mm −2 , 0.03 j mm −2 , and 0.0375 j mm −2 , respectively. The uniform distribution of reinforcement particles throughout the matrix and the strong interfacial bonding between them result in good ductility, but the presence of pores and micro-cracks [30] in the composites leads to decreased impact strength. This observation is consistent with the SEM image shown in figure 2(d) where micro-cracks can be seen. Although the addition of reinforcement particles can enhance the tensile, compressive, and hardness properties of the composites, it cannot increase the impact strength due to the increased brittleness of the composite [31][32][33]. Figure 9 presents the results of the Brinell and Vickers hardness tests conducted on the four produced composites. The Brinell hardness values range from 36.3 BHN for LM25 + FAC to 71.5 BHN for SAEH + SAC. Similarly, the Vickers hardness values range from 133.1 HV for LM25 + FAC to 307.1 HV for SAEH + SAC. SAEH + SAC composite exhibited the highest hardness values due to the uniform distribution of SAC particles and their strong interfacial bonding with the matrix. The variation in the hardness values can be attributed to the difference in microstructure and particle distribution of the composites.

Hardness
3.9. Differential thermal analysis and Thermogravimetric analysis (DTA and TGA) Differential thermal analysis (DTA) was conducted to investigate the exothermic and endothermic transformations occurring during the manufacture of the composite, as shown in figure 10(a). The results  indicate that the reaction starts at around 560°C to 642°C, which shows an exothermic peak. The endothermic peak starts from 642°C, and this heat loss may be caused by incomplete reactions, a slowdown in the rate of reactions, the large size of the reactant particles, the wall of the crucible absorbing the heat, the atmosphere in the furnace [23] and other factors. The TGA analysis, shown in figure 10(b)), indicates that the actual temperature range for the TGA analysis was 0°C to 1000°C and the decomposition of all the samples begins at around 550°C and is complete by 700°C. As the temperature rises, the materials lose density up to 650°C due to material expansion at higher temperatures. Beyond that, the materials start to shrink [34].

Conclusions
Four different combinations of aluminium metal matrix composites (AMCs) were produced using a computerized stir casting process. The study aimed to investigate the feasibility of producing aluminium metal matrix composites using car scrap aluminium engine head as matrix material and fresh and spent alumina catalyst as reinforcement material. The following conclusions may be inferred from the present investigation: