Mechanical and Metallurgical behaviour of Aluminum/graphene nanocomposites in Fuselage applications

Abstract

The load bearing structure of airplane called fuselage has to sustain the continuous explosive and implosive stresses generated during time of flight in addition to load of passengers, fuel, cargo and engines. Use of high specific strength materials for such fuselage structure greatly reduces the emissions generated by air vehicles. In the current study, experimental investigations into the processing of Graphene Nanoplatelets (GNPs) reinforced AA2024 metal matrix composites through the fusion of mechanical alloying and stir casting technique by varying the GNPs content from 0 to 2 wt.% with an interval of 0.5 wt.%. The use of micro sized particles of aluminum (75 µm) as launching vehicle helps to carry and launch the nanoreinforcement into molten metal that assists in a uniform distribution by means of mechanical alloying. This helps us in achieving an ultimate tensile strength of 203 MPa for the sample of 1.5 wt.% GNPs. Microstructural evidences from Field emission scanning electron microscopy (FESEM) confirms the uniform distribution of GNPs in the metal matrix. Moreover, FESEM analysis on the graphene pull-out area over fractured surface elucidates an improvement in wettability between the reinforcement and metal matrix. While the X-ray diffraction analysis on the casted samples confirms that the composites are free from major contaminations and intermetallic phases.

Keywords:

• Aluminum nanocomposites
• graphene nanoplatelets
• stir casting
• fractography
• X-ray diffraction

REVIEWING EDITOR:

• Swadesh Kumar Singh, Gokaraju Rangaraju Institute of Engineering and Technology, India

SUBJECTS:

• Manufacturing Engineering
• Materials Science
• Production Engineering

1. Introductions

Realization of materials with the requisite properties is the final frontier in the development of new age technological solutions. The realms of mechanical engineering, aeronautical and automotive sectors have undergone a tremendous revolution owing to the advent of metal matrix composites (MMCs), especially Aluminum metal matrix composites (AMMCs) on account of their high specific strength and less susceptibility to atmospheric attacks. Graphene nanoplatelets (GNPs) are emerging as a popular choice for reinforcement as their relatively large surface area impart unique properties to the MMCs (Bhadauria et al., 2019; Nieto et al., 2017). Inspite of a large number of techniques available for the fabrication of GNP reinforced AMMCs, stir casting is the most sought after owing to its simplicity and cost effectiveness. Natrayan et al. (Natrayan et al., 2019) fabricated graphene reinforced AA8030 composites with different weight percentages of reinforcement by the method of stir casting and observed that a weight percentage of 10% graphene resulted in an increase in the hardness, flexural strength and tensile strength of the composite by 34% than the pure alloy. Kotteda et al. (Kotteda et al., 2022) fabricated fly ash and silicon carbide reinforced Aluminum alloy 6061 and 7075 composites using stir casting process and examined its ultimate tensile strength (UTS), yield strength, wear rate, impact strength and hardness along with scanning electron microscopy (SEM) analysis of the resultant composite. The experimental outcomes showed that all the mechanical properties significantly improved due to the active participation of dispersion strengthening.

Though, high strength materials like carbon fiber are available their usage has been strictly limited to premium products in view of expensive fabrication routes available. As time passes on, increasing the usage of Graphene Nanoplatelets as a viable replacement for Carbon nanotubes witnessed by the researchers due to its exceptional properties. Graphene agglomeration is a problem commonly encountered in the fabrication of GNP/Al composites which has adverse effects on the mechanical properties of the resulting composite. This hindrance can be circumvented by adopting the concept of launching vehicle, i.e. by the pre-distribution of the reinforcement phase. Zhang et al. (Zhang et al., 2022) employed a combination of deformation and pre-distribution to avoid carbon agglomeration in GNP reinforced AMMCs and achieved a 293.3% increase in the strength. Mina et al. (Bastwros et al., 2014) fabricated graphene reinforced Aluminum 6061 composite through the method of hot compaction following ball milling of graphene and Al6061 powders with milling time varying from 30 to 90 minutes with an interval of 30 minutes. The resulting samples were subjected to SEM analysis and Raman spectroscopy. It was observed that the ductility of the composite has been lowered with an increase in the milling time as it was evident from the decrease in the ductile dimples at the surface of fracture, but the flexural strength has increased by 47%. Han et al. (Han et al., 2020) fabricated GNP reinforced Al composites by employing copper as a launching vehicle to prevent carbon agglomeration and utilised hot extrusion process preceded by low temperature ball milling. It was reported that among all the samples, 2.5 wt.% sample exhibited superior mechanical properties; specifically tensile strength of 402 MPa which in comparison with pure Aluminum increased by 130%.

Sethuram et al. (Sethuram et al., 2018) fabricated Al-Sn composites reinforced with graphene by employing a combination of ball milling followed by hot vacuum pressing. Milling time was varied from 2 to 12 hours with an equal interval of 2 hours and the resulting samples were subjected to Energy Dispersive Spectroscopy (EDS) and SEM analysis. The introduction of graphene lowered the density of the composite slightly after sintering, but an increase in hardness by 27% and 48% were reported for the samples with 1 and 2 wt.% of graphene reinforcement respectively. Palei et al. (Palei et al., 2022) employed powder metallurgy to fabricate graphene reinforced AMMCs with weight percentages varying from 0.1 to 0.3% in a planetary ball mill surrounded by inert gas. Milling time was chosen as 5 hours and sintering time and temperature were 5 hours and 550 °C respectively. The samples were subjected to EDS, high-resolution transmission electron microscope (HRTEM), selected area diffraction pattern (SAED), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The Vickers hardness was increased by 153% to 165 ± 8 VHN, while the electrical conductivity was increased by 55% to 59.2 x 10⁶ S/m.

Mohammad et al. (Khoshghadam-Pireyousefan et al., 2020) employed molecular level mixing along with high energy ball milling followed by spark plasma sintering for fabrication graphene oxide reinforced AMMCs. Material characterization techniques such as FESEM and XRD were employed, and formation of Aluminum carbide was not reported. Experimental findings revealed that 1 wt.% of reduced graphene oxide (RGO) yielded best results, while resulting in a 44, 49 and 88% increase in the Vickers hardness, ultimate tensile strength, and yield strength. Wang et al. (Wang et al., 2022) used a combination of powder metallurgy and high energy ball milling to synthesize graphene reinforced AMMCs. Initially, a homogeneous mixture of aluminum powder and graphene was prepared in a planetary ball milling setup, with a 10:1 ball powder ratio and milling time of 2 hours and 150 rpm. Subsequently, sintering was performed at 600 °C with a 4 °C/min heating rate and a holding time of 4 hours preceded by cold isostatic pressing. Zheng et al. (Zheng et al., 2020) adopted a combination of hot extrusion preceded by spark plasma sintering to fabricate 5 wt.% GNP reinforced AMMCs. The resulting composite exhibited 116 and 166% of improvement in the ultimate tensile strength and yield strength respectively in comparison to pure Aluminum. Xudong et al. (Cui et al., 2021) aimed to address the issue of carbon agglomeration by modifying the surface of graphene with cerium nitrate. Later a combination of high energy ball milling (HEBM) followed by hot vacuum pressing was utilized to fabricate Graphene reinforced AMMC. Through the experimental outcomes, 12.3% increase in the tensile strain rate was reported along with a tensile strength of 321 MPa in comparison to 284 MPa of conventionally produced composite.

Lou et al. (Lou et al., 2020) fabricated AMMCs reinforced with 0.5 wt.% GNP using hot and cold pressing. It was observed that the uniformity of GNP reinforcement was determined by speed and time of ball milling along with ultrasonic time. In the current study, 6-hour ball milling time at speed of 300 rev/min resulted in optimal results with low levels of agglomeration and 56.6% increase in the tensile strength and 39.8% reduction in the elongation in comparison to monolithic aluminum. Yu et al. (Yu et al., 2021) fabricated GNP reinforced AMMC with 0.5 wt.% by the method of powder metallurgy; flattening of GNPs was reported as a fallout of mechanical stirring. An increase of 65% in tensile strength was reported with thermal mismatch, grain boundary and load transfer strengthening mechanisms contributing 2, 42 and 56% to the increment in strength. In a comprehensive review by Sadhu et al. (Sadhu et al., 2023) reported that in the research so far pertaining to graphene reinforced AMMC, enhanced tribological properties were obtained with a 61.74% and 12.50% reduction in the wear rate and coefficient of friction respectively, followed by a 4%, 8.15%, 15.40%, 81.25% and 115% enhancement in the electrical conductivity, elongation, thermal conductivity, UTS and hardness were obtained.

Ghasali et al. (Ghasali et al., 2018) adopted microwave and spark plasma sintering (SPS) independently to fabricate GNP and CNT each with 1 wt.% reinforced aluminum composites. Prior to sintering, the reinforcements and matrix materials in powder form were subjected to milling and ultrasonic mixing to achieve uniform distribution. Only traces of aluminum carbide formation were observed in XRD analysis and homogeneous dispersion was identified in FESEM analysis. Microwave resulted in enhanced micro hardness whereas SPS improved bending strength. equal channel angular pressing (ECAP) with a channel angle of 120° was adopted by Ramesh et al. (Ramesh Kumar et al., 2019) to fabricate graphene reinforced AMMCs through mechanical alloying. It was found that carbon agglomeration was extremely low and uniform distribution of reinforcement along with grain refinement was achieved owing to application of back pressure. Niteesh et al. (Niteesh Kumar et al., 2017) fabricated Gr/AMMCs using hot extrusion preceded by powder metallurgy. Grain size has been lowered slightly and UTS increased by 46%. Table 1 illustrates the salient features and enhancement in several properties of GNP/Al cast composites processed through various techniques.

Table 1. Salient features of various Graphene Nanoplatelets reinforced Aluminum composites.

Author and Year of publication with Reference	Salient features	Ball milling time	Weight fraction of GNPs	Matrix	Enhancement
Li et al. (2018)	Ball milling + Cold pressing of Al and GNP Continuous Casting + Rolling SEM, EDS and Raman spectroscopy analysis	4 hr	0.2%	Pure Al (99.99%)	156 MPa UTS was obtained (36.8% increase)
Xie et al. (2022)	Severe plastic deformation and dynamic recrystallization Deformation driven metallurgy TEM, EBSD and Raman Spectroscopy analysis	–	1.5%	Pure Al	468 ± 7 MPa UTS was obtained (293.3% increase)
Zheng et al. (2020)	Shift-speed ball milling (SSBM) process Powder metallurgy + multi-pass hot drawing SEM, HRTEM and Raman Spectroscopy analysis	8 hr (slow) + 2 hr (fast)	5% (vol)	Pure Al (99.99%)	47% increase in compressive Strength
Huang et al. (2022)	Ultrasonic vibration dispersion XRD and SEM analysis	–	0.8%	Al2024 alloy	236 MPa UTS was obtained
Akçamlı et al. (2016)	Mechanical alloying + Pressure less sintering SEM, EDS, Raman spectroscopy and X-ray diffractometry	4 hr	1%	Pure Al	Micro hardness was improved at an exceptional rate up to 1 wt.% of GNPs
Khan et al. (2020)	Ball milling Spark plasma sintering + Solution sonication SEM, TEM, XRD and EBSD	2 hr	0.5%	Al6061 alloy	373 ± 12 MPa UTS was obtained
Khanna et al. (2022)	Dry ball milling compaction and Sintering XRD and FESEM	4 hr	0.25%	Pure Al	109.33 MPa (59.18% increase in compressive strength)
Pérez-Bustamante et al. (2014)	Mechanical alloying SEM, XRD and Raman spectroscopy analysis	5 hr	0.5%	Pure Al	17% increase in microhardness
Khan et al. (2017)	Powder processing route SEM, EDS, XRD and Optical micrography	2 hr	5%	Pure Al (99.5%)	433% increase in compression strength 283% increase in flexural strength 35% increase in hardness
Rashad et al. (2015)	Powder metallurgy method XRD and SEM	1 hr	1%	Pure Al	203 MPa UTS was obtained

From the literature, it is found that the studies on attainment of uniform dispersion of Graphene nanoplatelets reinforcement phase in Aluminum-Copper alloy matrix for the improvement of mechanical and microstructural behaviour through hybrid route involving liquid and powder metallurgy are limited. Also, at par from the literature most of the researchers discussed about the enhancement of composite strength by increasing the nanoreinforcement within the metal matrix up to optimal graphene rates whereas the strength gets effected negatively after the critical rate. Moreover, other parameters such as agglomerations, particle direction and particulate interface of Graphene nanoplatelets with respect to the metal matrix are also needed to study for better understanding about the mechanical and metallurgical aspects of the cast composites. The present work focuses on a study of effect of evenly distributed GNPs reinforcement in the Aluminum launching material on the tensile strength and hardness of AA2024 metal matrix nanocomposites developed through combined ball milling and stir casting. In addition to the above, various microscopic approaches such as Optical microscopy, Fractography and Field emission scanning electron microscopy are also employed to understand the variations in grain size, failure mechanism and dispersion of reinforcement in the metal matrix in detail. In a short note, the main novelty and contribution of the current study include:

• A novel parameter named Launching vehicle to constant nanoreinforcement ratio (5:1) is considered in the present study to fabricate the Graphene Nanoplatelets reinforced Al-Cu alloy metal matrix composites using micro sized particles of pure aluminum powder as launching medium.

• Investigating the effect of dross on GNPs/AA2024 cast composites fabricated by hybrid route involving liquid and powder metallurgy.

• Implementing FESEM analysis on the graphene pull-out area over fractured surface elucidate an exceptional wettability in between the reinforcement and metal matrix.

• Electron Backscatter Diffraction analysis is carried out on superior cast composite sample to strengthen its mechanical behaviour by observing the crystal orientation and morphology of each grain.

• In addition, X-ray diffraction analysis (i.e. a non-destructive technique) is employed to examine the presence of intermetallic compounds and contamination.

2. Materials

2.1. Aluminum-copper alloy (matrix)

Aluminum is characterized by its low density and high strength, consequently, has galvanized researchers to explore its applications in aerospace and automotive applications. Aluminum-copper alloy graded AA2024 is a high-strength and heat-treatable aluminum alloy. It is composed primarily of aluminum, with copper as the main alloying element. Other elements present in smaller amounts include magnesium and manganese. AA2024 exhibits excellent strength-to-weight ratio, making it a preferred choice in aerospace applications, such as aircraft structures and components. Its high strength and fatigue resistance make it suitable for structural parts, including wing and fuselage sections. AA2024 also offers good machinability and weldability, further enhancing its versatility (Chaudhry et al., 2019; Kotteda et al., 2022). Because of these attributes, 2024 Aluminum alloy is considered as a matrix material in the current study. Table 2 shows the chemical composition of the Al-Cu alloy graded AA2024.

Table 2. Chemical composition of Al-Cu alloy (AA2024).

Element	Cu	Mg	Mn	Fe	Others	Al
Weight %	3.9	1.2	0.3	0.1	Traces	Balance

2.2. Aluminum (launching powder)

A common issue encountered while fabrication nanocomposites using stir casting technique is the poor wettability. This occurs due to the difference between the surface tensions of the reinforcement and the composite. Kang et al. (So et al., 2011) electroplated CNT with Aluminum to increase its wettability, Aluminum is chosen at it has the highest surface tension. It was followed by annealing at elevated temperatures. The wettability of the reinforcement was increased due to formation of covalent bonds. Also, the temperature of the pure Aluminum powder is less than the temperature of molten matrix of Al-Cu alloy considered in our study; thus, pure aluminum particles surely melt in the matrix. Therefore, micro sized particles of pure aluminum powder is used as a launching material in the current study. Figure 1 depicts the SEM image of Aluminum launching powder. Moreover, the average particle size of the launching material is 75 µm.

Figure 1. SEM image of Aluminum launching powder.

2.3. Graphene nanoplatelets (reinforcement)

In our never-ending quest for the discovery of materials with exceptional properties, graphene was discovered in 2004. Graphene is the thinnest material known to mankind; it has a two-dimensional structure consisting of a single layer of carbon atoms resembling a honeycomb. Since its discovery two decades ago, it has found numerous applications in a variety of fields, and is considered as a suitable reinforcement in metal matrix composites (Güler & Bağcı, 2020). Due to the inherent manufacturing difficulties and high costs involved, graphene nano platelets are serving as a viable replacement for graphene. GNP consists of a few layers of graphite, with their thickness ranging from 0.7 to 100 nm (Jiménez-Suárez & Prolongo, 2020). It is considered as a wonder material due to its excellent mechanical properties, low weight, and high electrical and thermal conductivity (Lawal, 2019; Navasingh et al., 2019). By considering the aspects discussed, Graphene Nanoplatelets of surface area 300 m²/g is taken as the reinforcement in the present study. Figure 2 shows the FESEM image of Graphene Nanoplatelets.

Figure 2. FESEM image of Graphene Nanoplatelets.

3. Experimental procedure

3.1. Predistribution

All carbon materials will have poor wettability with aluminium. However, recent findings show that ball milling procedure on these reinforcements improved the wettability by destroying the outermost layer that anchors to the matrix (Raju et al., 2016). From the literature (Raju et al., 2016), it is observed that among 1:1, 1:3 and 1:5 the authors reported a superior dispersion of nanoreinforcement at 1:5 ratio of launching vehicle to nanoreinforcement ratio. Hence, the same ratio is considered in our study. Initially, mechanical alloying (MA) is performed between nano reinforcement (Graphene Nanoplatelets) and the launching vehicle independently with a ratio of 1:5 for the purpose of pre-distribution in a planetary ball milling setup. Therefore, the reinforcement particles impinge to the micro sized particles in a layer-by-layer fashion. Later, the pre-distributed powders are incorporated individually into liquid matrix with the help of controlled atmosphere stir casting apparatus to fabricate the final composites. In the initial stage, a planetary ball mill setup with 20 tungsten carbide balls is utilized to mix the reinforcement and launching vehicle in varying proportions. The milling process lasted for 2 hours at a speed of 300 rpm (Sita Rama Raju et al., 2016).

In detail, ball milling is observed to pre-distribute the nanoreinforcement before incorporation into metal melt. The basic activity that takes place in the MA of ball milling process is, during MA the ductile metal gets flattened, and the brittle material gets fragmented during initial hours of ball milling. Simultaneously, fragmented brittle particles get welded to ductile material, while it piles up as laminates embedding the brittle particles in-between. This embedded brittle particles in ductile material assists in a uniform distribution by means of mechanical alloying. In the present study, the ductile material is termed as Launching vehicle that launches the embedded nanoreinforcement.

3.2. Stir casting

Subsequently, the resulting ball milled mixture is preheated to 200 °C before being introduced into the molten matrix phase, which is maintained at a temperature of 700 °C. This temperature is deliberately chosen above the liquidus temperature of both the launching vehicle and matrix phase to ensure proper fusion (Akbari et al., 2013). To prevent any iron contamination from the Inconel crucible and steel components of the setup, all surfaces in direct contact with the molten metal are coated with boron nitrate (Sajjadi et al., 2011). The experimental setup, illustrated in Figure 3, is designed to isolate it from the surroundings by flowing argon gas independently into the crucible chamber and the die chamber. Some unconventional procedures have been implemented during the experimentation to closely observe and elucidate the particle rejection phenomenon previously noted by the researchers (Sita Rama Raju et al., 2016; Valibeygloo et al., 2013). Throughout the experiment, any materials floating on the molten metal surface is collected twice: first, after melting the matrix aluminum at 700 °C, and finally after adding the secondary material, either aluminum powder or ball-milled graphene nanoplatelets with aluminum powder. To prevent oxidation, the argon gas flow is maximized, increasing the pressure in the crucible chamber, and effectively isolating the atmospheric air during the collection of the floating materials. The nomenclature of the various casted composite samples can be found in Table 3.

Figure 3. Process flow of fabricating the composite.

Table 3. Nomenclature of the composite samples.

S. No	Sample name	Nomenclature
1	A	AA2024 pure
2	B0	AA2024 matrix + Al (5 times of 0.5 wt.% of matrix)
3	B	AA2024 matrix + Al (5 times of 0.5 wt.% of matrix) + GNPs (0.5 wt.% of matrix)
4	C0	AA2024 matrix + Al (5 times of 1 wt.% of matrix)
5	C	AA2024 matrix + Al (5 times of 1 wt.% of matrix) + GNPs (1 wt.% of matrix)
6	D0	AA2024 matrix + Al (5 times of 1.5 wt.% of matrix)
7	D	AA2024 matrix + Al (5 times of 1.5 wt.% of matrix) + GNPs (1.5 wt.% of matrix)
8	E0	AA2024 matrix + Al (5 times of 2 wt.% of matrix)
9	E	AA2024 matrix + Al (5 times of 2 wt.% of matrix) + GNPs (2 wt.% of matrix)

The size of each bulk cast sample is 200 mm x 120 mm x 25 mm. Upon fabrication, the composite samples underwent tensile tests using a UTES 40 HGFL universal testing machine as per ASTM E8 standards whereas hardness tests are conducted using a Brinell hardness setup. In addition to the above Optical microscopy, Fractography, Field emission scanning electron microscopy, Electron Backscatter Diffraction (EBSD), X-ray diffraction are employed for sample analysis.

4. Results and discussion

Space exploration is strongly funded by renowned global entities to meet the future needs of space transportation and relocation. Thus, studies focus on developing space crafts that are economically viable by providing high specific strength structures that reduce the dead weight of the transport vehicle. Fuselage is the most critical assembly in an aircraft that has to sustain the explosive stress through its components such as frames and stringers. Figure 4 shows a portion of aircraft fuselage containing frames attached to stringers that hold the outer skin. During space travel, these components get subjected to numerous loads i.e. buckling loads act over the frames in radial direction of the aircraft, tensile loads act over the stringers in longitudinal direction of the aircraft etc. Therefore, developing high specific strength materials to be used as stringers and frames of an aircraft which are metallic in nature (owing to their reliable metallurgical and mechanical behaviour) has been the only alternative for researchers. Aluminum metal matrix nanocomposites are of good choice having potential to satisfy most of the requirements.

Figure 4. Portion of fuselage of aeroplane.

4.1. Experimental findings

A detailed scrutiny of the experimental procedure provided some key insights into the underlying phenomenon causing distinct (a change in reinforcement rejection observed from different investigators) mechanical behaviour in cast aluminum composites as observed. Dross a solid waste mass in aluminum (similar to slag for iron) is observed to hinder nanoreinforcement incorporation into melt. Dross is noted to be produced during every aluminum casting process over the top surface of the liquid metal. When not removed physically, it is observed to trap the materials that are added in the later stages of casting process. Table 4 provides are the experimental findings where particle rejection is recorded by weighting the floating mass content when launching vehicle and ball milled mixture are introduced individually. The difference is expected as GNPs estimate. Such particle rejection is noted by Raju et al. (2016) and Valibeygloo et al. (2013). However, discussion regarding the dross content during casting procedure is not recorded by many researchers. This Table 4 further depicts that, a maximum of 40% of introduced GNPs get rejected from the melt while it can be as minimum as 25% that is mandatory.

Table 4. Estimated GNPs introduced into matrix.

S. No. [i]	Sample	Al-Cu alloy wt. (gm)	Aluminum launching powder wt. (gm)	GNPs wt. (gm)	Dross wt. (gm)	Particle Rejection		Estimated wt. of GNPs rejection (gm) [Vi = Yi-Xi-1]
						Introduced Aluminum launching vehicle only wt. (gm) [Xi]	Introduced ball milled mixture wt. (gm) [Yi]
1	A	2000	–	–	229	–	–	–
2	B0	2000	50	–	256	8	–	–
3	B	2000	50	10	239	–	11	3 (30%)
4	C0	2000	100	–	254	26	–	–
5	C	2000	100	20	227	–	31	5 (25%)
6	D0	2000	150	–	271	43	–	–
7	D	2000	150	30	259	–	54	11 (36.66%)
8	E0	2000	200	–	218	68	–	–
9	E	2000	200	40	254	–	84	16 (40%)

4.2. Metallurgical understanding

Numerous microscopic techniques are used in this investigation to understand the changes in grain size, failure mechanism, dispersion of nanoreinforcement in the matrix, wettability of reinforcement with the matrix and crystallographic orientation of each grain. Further, presence of intermetallic compounds and contamination is observed through X-ray diffraction analysis.

4.2.1. Optical microscopy

Figure 5 depicts the optical micrographs having various grain shapes produced in as-cast nanocomposite samples. Figure 5(a) related to cast sample containing pure matrix shows columnar grains being present with a few equiaxed grains. All the rest of the optical micrographs illustrated contrary grain structure i.e. majority of grains are equiaxed while a few are columnar. Figure 5(d) is observed to possess equiaxed grains in smaller and larger sizes with absence of columnar grains.

Figure 5. Microstructure of various casted samples.

These changes in grain structure from columnar to equiaxed and further decrease in size of equiaxed grains can be correlated as grain refinement. In comparision equiaxed grain produce superior mechanical behaviour than a columnar while smaller equiaxed grains possess better the mechanical behaviour than the larger. Therefore, the best contribution of grain size and shape does matter in understanding the mechanical behaviour (Subbaiah et al., 2019). According to this investigation, sample ‘D’ is noted to achieve best grain refinement and hence owing to hall patch effect. The sample is expected to achieved superior mechanical properties.

4.2.2. Fractography analysis

Figure 6(a) clearly denotes a ductile failure owing to tangible stretched surfaces noted from fractography images of matrix sample. However, these features of ductile failure are observed to degrade for rest of the samples i.e. a decrease in ductility is noted with an increase in wt.% of nanoreinforcement (depicted from Figure 6(b to e)). This behaviour of degradation in ductile nature of composite is eminent numerous other researchers (Palampalle et al., 2018; Prakash et al., 2018), where graphene is not an exception as observed from this investigation. Moreover, voids are marked with circles in the fractography images.

Figure 6. Fractographic images of various casted samples.

Moreover, a few voids or graphene pull-out regions are observed from fractography images which is further investigated by field emission scanning electron microscopy (FESEM) technique. The graphene pull-out region observed (depicted from Figure 7) over fractured surface exemplify a good wettability between the matrix and nanoreinforcement. This region shows flakes of matrix material developed upon solidification of molten matrix in the uneven surfaces or ruptured nanoreinforcement. These solidified flakes anchor into the nanoreinforcement thus effectively transforming the load from matrix to the reinforcement.

Figure 7. Graphene pull-out region of sample D.

4.2.3. Electron micrographs

The FESEM micrographs of various cast composite samples are used for understanding the dispersion of reinforcement within the metal matrix. The individual nanoparticles are shown by pointers, clusters of nanoparticles by means of circles and voids by dotted circles as depicted in Figure 8. It is evident from the FESEM images that clusters of graphene nanoplatelets are noted to decrease with increase in wt.% of nanoreinforcement up to 1.5 wt.%. At the same time, clusters are appeared on 2 wt.% cast sample. Furthermore, the sample with 1.5 wt.% of GNPs in the Al matrix is noted to attain uniform distribution. This tendency of cluster formation on the far side of 1.5 wt.% of GNPs is in line with several investigations using ultrafine (<100 nm) nanoreinforcement particles such as SiC, Al₂O₃, CNTs etc. However, through the quantitative analysis approach, the sample with 1.5 wt.% of GNPs in the Al-Cu alloy matrix with respect to the Orowan’s strengthening effect should possess the superior tensile behaviour (Prakash et al., 2021; Raju et al., 2015).

Figure 8. FESEM images of various samples illustrating dispersion of GNPs in Al matrix.

4.2.4. Electron backscatter diffraction (EBSD) analysis

Hall patch strengthening is inversely related to square root of grain size thus smaller the grain size higher the strength. This phenomenon is impetus from optical microscopic analysis. However, knowing the crystallographic orientation of each grain is important to understand the strength that could be achieved by the composite sample.

The failure of a component begin with formation of a dislocation is one of its constituent grains which further intensify with increase in load thus generating a dislocation forest leading to a permanent failure (Li et al., 2019). To avoid or prolong the failure of a component (increase load bearing capacity or strength) both dislocation forest and propagation of dislocation needs to be restricted. Smaller grains limit the number of dislocations that get generated within hence avoiding dislocation forest. While propagation is restrained by mismatch in crystallographic orientation of lattice planes. Thus, EBSD quantitative analysis is carried out to analyse the crystallographic orientation in grains (Ghazanlou et al., 2021). Presently, the sample possessing finest grain size is compared with the pure matrix sample.

From Figure 9(a), it is evident that most of the cast matrix sample is covered with a single colour representing a similar orientation plane that is a favourable environment for dislocation to propagate through promoting overall failure. While the EBSD image of the sample (From Figure 9(b)) possessing finest grain size clearly depicts a distinctive colour in each adjacent grains elucidating the presence of crystallographic misorientation. This misorientation in adjacent grains restrict the dislocation movement which as a consequence improves the load bearing capacity of a material.

Figure 9. EBSD images of cast composite samples (a) 0 wt.% and (b) 1.5 wt.%.

4.2.5. X-ray diffraction analysis

Every material has a distinctive signature in terms of crystal structure and its primary lattice planes identified from brag’s equation. Graphene as revealed by Senel et al. (Şenel et al., 2019) in their investigation possess broad peak intensities at a 2θ angle of 27 and 55 degrees. From Figure 10, peak intensities of GNPs are absent as X-ray diffraction is sensitive in producing peaks collectively for materials above 5 wt.% of sample. However, the GNPs introduced into the cast composite are ranging from 0 to 2 wt.%.

Figure 10. XRD plots of various casted samples.

Moreover, Fe intermetallic peaks or any other dominant peaks are not registered as evident from comparision of sample B to D with that of sample A. This XRD analysis elucidates that the cast samples are free from major contaminations and intermetallic phases that degrade the strength of the sample.

4.3. Mechanical behaviour

Table 5 depicts an increase in tensile strength with an increase in nanoreinforcement wt.% capped at 1.5 wt.% The % elongation is noted to drop thus increasing the hardness of samples with an increase in nanoreinforcement. These ductility and hardness are contrary properties that possess opposite trends as know by research community (Sadeghi et al., 2022; Samal et al., 2022). While, many researchers projected that 1.5 wt.% of nanoreinforcement to be the barrier to possess superior tensile strength as achieved in this investigation.

Table 5. Mechanical properties of various casted samples.

Sample name	Ultimate Tensile Strength (MPa)	% of Elongation	Hardness (BHN)	Relative density (%)
A	100.46	6.5	47.6	98.88
B	156.25	1.76	59.4	97.2
C	186.93	2.04	61.3	97.6
D	203.51	1.8	81.5	98.2
E	184.62	2.4	91.76	96.8

5. Conclusion

The prominent features achieved through correlation of metallurgical and mechanical behaviour of AA2024/GNPs cast composite samples are briefed as follows.

5.1. Metallurgical insights

• Experimental insights reveal a 25% rejection of GNPs being mandatory due to unavoidable production of dross during aluminum casting.

• Brittle failure mechanism is witnessed from as-cast nanocomposite samples developed.

• The Graphene pulled out region showcased evidence of good wettability through damaged surface achieved during ball milling.

• Hall patch strengthening followed by misorientation of crystallographic planes is noted as the dominant mechanism enhancing the strength of composite.

• Dispersion strengthening achieved from uniform distribution of Graphene within the aluminum matrix is marked to further escalate the composite strength.

• The Aluminum composite samples fabricated are free from a major contaminant called iron.

5.2. Mechanical insights

• 1.5 wt.% of GNPs possessed superior strength with supportive grain size and dispersive strengthening mechanism.

• Hardness is noted to improve with increase in nanoreinforcement while a contrary behaviour is exhibited from % elongation as usual.

Author contributions

Tarun Kumar Kotteda performed the experiments, collected the data, and wrote the original draft; Manoj Kumar defined the methodology; Pramod Kumar supervised the work; Ajay Gupta reviewed and edited the manuscript; Kalidindi Sita Rama Raju reviewed and edited the manuscript; all authors read and approved the final manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data sharing not applicable to this article as no data-sets were generated or analyzed during the current study.