Impact of micro graphite particles addition on the mechanical behavior of Al2011 alloy metal composites

Micron‐sized graphite particles additions to Al2011 alloy were investigated to know their impact on mechanical properties. The stir cast method was used to manufacture the Al2011 alloy with 2 and 4 wt% of graphite particles reinforced composites. The microstructural analysis and mechanical properties of the synthesized composites were tested. Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) spectrums were used to characterize the microstructure of the samples that were obtained from the casting. SEM micrographs indicated the homogeneous distribution of particles and the EDS patterns confirmed the presence of graphite particles. Al2011 alloy hardness was decreased with the addition of graphite particles. Further, with the addition of 2 and 4 wt% of graphite particles in the Al2011 alloy, the ultimate tensile and yield strengths of composites were increased with increase in elongation. SEM micrographs of tensile fractured surfaces were used to study the various fractured behaviors in the Al2011 alloy‐graphite composites.


INTRODUCTION
Lightweight, high strength, high specific modulus, low co-efficient of thermal expansion, and good wear resistance are just some of the reasons being considered aluminum (Al) composites as a new class of advanced materials. 1 It is difficult to find all of these qualities in a single conventional alloy.Aluminum, magnesium, titanium, copper, zinc, and their alloys are the most used metallic matrices to prepare the metal matrix composites.To make metal matrix composites, these are the alloys what engineers recommend.To prepare these metal composites reinforcements in the form of fibers, whiskers, and particulates can be used. 2Cylinder liners and other rotating and reciprocating parts in car engines are common applications for aluminum alloys and its composites. 3Metal matrix composites (MMCs) have seen increased use in high performance applications like aircraft engines over the past decade due to their enhanced properties. 4Al2011, which can be heat treated, is frequently used as a matrix material in the preparation of MMCs because of its high formability, low corrosion susceptibility, and adequate strength.
Many different matrix materials are employed in the construction of MMCs, but because to their popularity, aluminum and its alloys have reached the stage of industrial manufacturing.The development of low-cost Al-based MMCs with hard and soft reinforcements such as SiC, Al 2 O 3 , ZrO 2 , Graphite, B 4 C, and Mica has received significant attention in recent years.Graphite's strong strength and low density make it a desirable material both in fiber and particle form.Using solidification processes, aluminum graphite particulate is transformed into low-cost MMCs that find widespread use in engineering, particularly in automobile parts, bushes, and bearings.
The addition of hard ceramic particulates to an aluminum based matrix improves the material's strength, stiffness, wear resistance, corrosion resistance, fatigue resistance, and resistance to high temperatures. 5The composites are extremely difficult to machine due to the presence of hard ceramic particles, which are extremely abrasive in nature. 6As a part of this study, efforts were made to develop Al based MMCs reinforced with graphite particles.Particulate graphite reinforced MMCs are used in a variety of engine components due to their potential to act as a solid lubricant. 7These Al-graphite composites can be used in bearings, pistons, piston rings, and cylinder liners.
Particulate reinforced aluminum alloy composites, for instance, have demonstrated notable enhancements in tribological properties like sliding and abrasion wear resistance, as well as seizure resistance.Aleksander and co-authors 8 investigated the wear behavior of 0.2, 0.3, and 0.5 wt% of alumina particles reinforced ZA-27 alloy composites.The primary objective of the study was to examine the effect of adding a negligible amount of Al 2 O 3 nanoparticles on the wear resistance of the ZA-27 alloy, as well as the effect of the nanoparticles' size.Lubricated sliding conditions were used to investigate wear characteristics.The tested nanocomposites were created using a low-cost method that entailed infiltrating the pre-treated nanoparticles (obtained by mechanically alloying the matrix alloy trash with nanoparticles) into the semi-solid matrix.
The work presented here aims to develop Al2011 alloy with 2 and 4 weight percentages of graphite particles reinforced composites using a two-stage melt stirring method at temperatures lower than 750 • C. Particle agglomeration and separation can be prevented by performing additions in two stages.The second goal of this work is to assess the mechanical properties and tensile fractography of the Al2011-graphite composite made through a two-stage addition process.

MATERIALS AND EXPERIMENTAL DETAILS
The matrix material was an aluminum 2011 alloy with a theoretical density of 2800 kg/m 3 , and the reinforcement was graphite with a particle size of 50 to 60 μm.Results from an atomic absorption spectroscopy analysis of the matrix material were summarized in Table 1 at the Central Manufacturing Technological Institute (CMTI) in Bangalore.Reinforcing graphite particles of 50-60 μm in size are being added to composites.
The vortex technique was used to create graphite reinforced Al2011 metal composites.First, the Al2011 alloy was superheated to a temperature of 750 • C in an electrical resistance furnace, after which a calculated amount of Al2011 alloy was added into a SiC crucible.Using a digital temperature controller, it was able to keep the temperature inside the furnace within the tolerance range of ±10 • C. Once the target temperature has been reached, the absorbed gases were released via degassing with solid hexachloroethane (C 2 Cl 6 ). 9 A zirconia-coated mechanical stirrer was used to create a small vortex in the melt.The recommended spindle speed was 300 rpm, and the recommended stirring time was 5-8 min.To improve wettability, graphite powder was heated to 400 • C in a pre-heater.By eliminating the moisture, gas, and other impurities, the surface energy can be increased.Preheated graphite particles were fed into the melt at a steady rate of 1.2-1.4g/sec in two stages.When adding reinforcement in two stages, instead of adding the entire weight at once, the reinforcement was divided into two equal weights.To prevent the particles from clumping together and separating, and to ensure a uniform dispersion of the graphite in the melt, stirring was performed before and after the introduction of reinforcement at each stage.After waiting 5 min, the molten metal was poured into a cast iron mold that had been preheated to 750 • C and measured 120 mm in length and 15 mm in diameter.It was the same for both the 2 and 4 wt% graphite composites in terms of processing.In Figure 1 shows the representation of the Al2011-graphite composites.
After casting the specimen, a scanning electronic microscope (Tescan Vega 3LMU) was used to examine the distribution of reinforcement particles made of Al2011 alloy.Micrographs of the structure of Al2011 alloy and Al2011 composites containing varying amounts of graphite particulates were obtained.The roughness of the specimen was reduced by using 240, 600, and 800 grit papers.A more polished look can be attained by using some polishing paper.After that, it was polished using a velvet cloth and a polishing machine to achieve a relatively flat finish.Samples were rinsed in distilled water to remove any remaining residue from the polishing process.The specimens' surfaces were etched with Kellers reagent to create a contrast.
As needed, the samples were machined in line with ASTM standard E10 10 for hardness testing (Zwick/RoellIndtech of Germany).Hardness was evaluated with a Brinell hardness tester machine.The specimen was indented with a 5 mm ball under a 250 kg load.The data is averaged over the three notches that were used to make the measurement.Utilizing Instron-5980 model universal testing machines and utilizing specimens machined in accordance with ASTM standard E8, 11 tensile properties of Al2011 alloys with varying percentages of graphite particulate reinforced composites were evaluated.Tensile test specimen was prepared with gauge length 45 mm, 9 mm in diameter and overall length of 104 mm.Tensile testing can reveal the material's ultimate, yield, and elongation strengths.Figure 2 shows the tensile specimen.

Microstructural analysis
Casting processes are suboptimal for fabricating metal-matrix composites with graphite particles because of the particles' very low wettability, 12 as well as agglomeration phenomena that cause non-uniform distribution and poor mechanical properties.Incorporating micro-sized graphite particles into an Al2011 alloy matrix necessitated a novel two-stage mixing process and preheating the reinforcing materials.The composites contained between 2 and 4 weight percent of graphite particles.Micrographs taken with a scanning electron microscope of Al2011 alloy containing 2 and 4 wt% graphite particles are displayed in Figure 3A-C.Micrographs taken with a scanning electron microscope display as cast Al2011 (Figure 3A) and Al2011 alloy reinforced with 2 and 4 wt% of graphite particulate composites (Figure 3B,C).There is little to no agglomeration, segregation, or porosity among the graphite particles (Figure 3B,C, 2 and 4 wt%).At 640 • C, the solid dendrites break due to increased friction between the particles and the Al matrix alloy, and the stirring process further induces a homogeneous distribution of the particles.
At the boundary between the graphite and the Al alloy matrix, EDS analysis was performed to verify the presence of graphite.There are traces of Al, Cu, Si, Mg, and C in the interface reaction layer of Al2011 alloy with 4 wt% graphite composites, as shown by the EDS analysis (Figure 4B).The EDS spectrum of as cast Al2011 alloy is displayed in Figure 4A.F I G U R E 4 EDS spectrum of (A) as cast Al2011 alloy (B) Al2011-4 wt% of graphite composites.

Hardness measurements
Figure 5 is a graph depicting the relationship between the graphite content and the hardness of Al2011-graphite composites.A direct negative correlation exists between graphite percentage and the hardness of the composite material, with the hardness falling off gradually as the graphite percentage increases.Since graphite acts as a dispersoid in composites and does not contribute to their hardness, this softening is to be expected.Graphite, a powerful solid lubricant, 13 reduces the material's hardness, allowing the indenter of a hardness tester to move more freely.An optimal amount of graphite should be added to the composite to increase its ultimate tensile strength without reducing its hardness, as this trade-off can have a significant impact on wear resistance in components like engine bearings, piston rings, and cylinder liners.

Tensile properties
Tensile tests on composites synthesized by melt stirring encompassing two step additions of graphite particulates revealed significant increases in the composites' ultimate tensile strength, as shown in Figure 6.As can be seen in Figure 6, the ultimate tensile strength (UTS) of the Al2011 matrix is improved by the addition of graphite particles.By adding more graphite particles, researchers were able to confirm the material's improved properties.When graphite particulates were added at 2 and 4 wt%, the ultimate tensile strength of Al2011 alloy increased by 6.26% and 17.64%, respectively.The

F I G U R E 6
Ultimate strength of Al2011 alloy with graphite composites.
findings confirm the tendencies reported by the original researchers. 14Possibly, the graphite particles are blocking the dislocations in the microstructure, leading to higher UTS.The results of the tests show that the ultimate strength of the base Al62011 alloy is 203.8MPa and the yield strength is 163 MPa.Al2011's UTS increases from 216.59 to 239.79 MPa when graphite content is increased from 2% to 4% by weight.Al2011's yield strength increases from 177 to 197.5 MPa when supplemented with 2 wt% of graphite.The UTS and YS of composites made from Al2011 and 4% of graphite reinforcement are both higher than those of the base alloy by 17.64% and 21.15%, respectively.Graphite's soft dispersoid nature is the likely cause of this phenomenon, as it does not add to the composite's hardness.Instead, the graphite improves ductility by acting as a solid lubricant, allowing the grains to more freely flow along the slip planes and yielding to the indenter's pressure.
Figure 7A-C are showing the stress-strain graphs of as cast Al2011 alloy, Al2011 alloy with 2 and 4 wt% of graphite particles composites respectively.From the figures, it is observed that as weight percentage of graphite content increased in the Al2011 alloy, the strength and elongation of the base matrix enhanced.As-cast Al2011 alloy has tensile strength F I G U R E 7 Stress-strain plots of (A) as cast Al2011 alloy (B) Al2011-2 wt% of graphite (C) Al2011-4 wt% of graphite composites. of 203.4 MPa with 13.5% elongation, further with the incorporation of 4 wt% of graphite particles in the Al2011 alloy, the tensile strength has increased to 238.96 MPa with 15.25% elongation.Better bonding and more uniform distribution of graphite particles in the base matrix may account for the composites' increased strength.Adding graphite particles to the matrix alloy increases the composites ultimate and yield strengths, making it more resistant to external tensile stresses.Other researchers 6,15 showed that the higher strength of the composites was due to the presence of soft particle reinforcing components, as well as restricting plastic flow and triaxiality in the soft and ductile alloy matrix.The strength properties of the metal depend critically on the resistance of the alloy's microstructure to motion owing to displacement.
As can be seen in Figures 6 and 8, the UTS and YS of the composite material improve as the percentage of graphite used in its construction rises from 2 to 4. Increases in UTS and YS can be traced back to a fortification of the Al2011 alloy, which may have occurred as a result of a strengthening of the grains following a reduction in the composite grain measure and the resulting creation of a high dislocation density in the Al2011 framework.Additionally, the graph reveals that graphite reinforced composites with 4% graphite composite show significant increases in ultimate and yield strength.
Figure 9 is a graph showing how the addition of 2% and 4% of graphite content to Al2011 composites affects its ductility.As graphite content increased from 2% to 4% by weight, ductility increased by 15.25%.With more graphite present, the composite material's ductility visibly increases.Since graphite particles are unreactive with the matrix phase, their effect is anticipated to be mechanical in nature.Graphite's presence, as a solid lubricant, facilitates the grain's motion along the slip planes, leading to a notable improvement in ductility.

F I G U R E 9
Elongation of Al2011 alloy with graphite composites.

Tensile fractography
A fibrous mode of fracture is visible on the surface of the monolithic Al alloy sample (Figure 10A).Some of the surface damage has been observed in a fractured area which is located away from the deep voids, revealing the greater pliability of the matrix material.Surfaces that cracked during testing of Al2011 composites with 2 and 4 wt% of graphite are shown in Figure 10B,C.To make composites more malleable, soft graphite particles are often added.The ductile fracture is clearly visible on the SEM micrograph of the composites.In addition, as shown in Figure 10C, during the tensile test, the grains in graphite reinforced composites grew larger and moved along the slip planes, which helped to increase the percentage of elongation.

F I G U R E 10
Tensile fractured surfaces of (A) as cast Al2011 alloy (B) Al2011-2 wt% of graphite (C) Al2011-4 wt% of graphite composites.

CONCLUSIONS
Below findings are the results of the current investigation on the graphite particles reinforced Al2011 alloy composites synthesized by two-stage addition employing in the melt-stirring technique.
1.The Al2011 alloy composites with 2 and 4 wt% of graphite particles were prepared using a melt stirring technique with two stage additions.2. SEM microphotographs show that the graphite particles were uniformly distributed throughout the melt, without any clustering or agglomeration, due to the two-stage addition method used to introduce the graphite during melt stirring.3.As seen in the EDS spectrum, the Al2011 alloy matrix contains graphite particles.4. As the wt% of graphite particles increases, the composites hardness decreased.5. Adding graphite particles to the Al2011 matrix, its ultimate tensile and yield strengths were increased.After addition of 2 and 4 wt% of graphite particulates to Al2011 alloy, the percentage of improvement obtained was 17.64% and 21.15%, respectively.6.The stress-strain plots were showing the impact of graphite particles on the tensile strength with increased ductility.

TA B L E 1 1 F I G U R E 2
Chemistry of Al2011 alloy by weight %.Al2011-graphite composites after casting.Tensile test specimen.

F I G U R E 5
Hardness of Al2011 alloy with graphite composites.

F I G U R E 8
Yield strength of Al2011 alloy with graphite composites.