Property Evaluation of AA2014 Reinforced with Synthesized Novel Mixture Processed through Squeeze Casting Technique

Abstract

Aluminum alloy–graphene metal matrix composite is largely used for structural applications in the aerospace and space exploration sector. In this work, the preprocessed powder particles (AA 2014 and graphene) were used as a reinforcement material in a squeeze casting process. The powder mixture contained aluminum alloy powder 2014 with an average particle size of 25 μm and 0.5 wt% graphene nano powder (Grnp) with 10 nm (average) particle size. The powder mixture was mixed using the high-energy planetary ball milling (HEPBM) technique. The experimental results indicated that the novel mixture (AA 2014 and graphene powder) acted as a transporting agent of graphene particles, allowing them to disperse homogeneously in the stir pool in the final cast, resulting in the production of an isotropic composite material that could be considered for launch vehicle structural applications. Homogeneous dispersion of the graphene nanoparticles enhanced the interfacial bonding of 2014 matrix material, which resulted in particulate strengthening and the formation of a fine-grained microstructure in the casted composite plate. The mechanical properties of 0.5 wt% graphene-reinforced, hot-rolled composite plate was strengthened by the T6 condition. When compared to the values of unreinforced parent alloy, the ultimate tensile strength and the hardness value of the composite plate were found to be 420 MPa and 123 HRB, respectively.

1. Introduction

The aluminum alloy 2xxx series and composites based on them are considered to be potential alternative lightweight materials for aerospace applications. And they have attracted much attention from materials researchers and scientists because of their high strength, heat treatability, workability, low density, retention of fracture toughness even at cryogenic conditions, easy availability, and low cost [1,2]. Graphene is a 2-D, single atomic layer of sp2-connected carbons with outstanding properties such as excellent heat conductivity, high strength and mechanical modulus. Graphene has a significant load transfer capacity compared to conventional carbon-based nano-reinforcements due to its structural properties [3,4]. Graphene is an efficient reinforcement material for aluminum-based MMCs when used for aerospace applications. Two-dimensional graphene layers always have greater strengthening qualities than carbon nanotubes for the fabrication of MMCs. Reinforcing graphene with a higher specific surface area increases the load transmission of the matrix material by increasing the particle interactions [5]. The reinforced graphene particles create strong interfacial bonding in the finished composite plate and enhance its property requirements due to exceptional mechanical properties. One of the powder processing methods commonly used is the high-energy planetary ball milling (HEPBM) technique. The shear forces in the HEPBM technique cause homogenous dispersion of graphene particles in the matrix metal [6,7]. The HEPBM process also results in cold welding of graphene to the aluminum powder particles [8]. Overall, homogenously dispersed graphene in matrix metal can be obtained by high-intensity ball milling with optimum ball milling parameters. Recent research investigations have highlighted and stressed the use of solid-state processing methods like hot pressing while considering the composite processing and manufacturing approaches [9]. Researchers and manufacturers have noted that the liquid fabrication route is extremely difficult due to the significant density mismatch of graphene, which causes it to float on the molten alloy pool, and poor wettability between the aluminum matrix and graphene during the stirring process. To overcome the issue of achieving homogeneous dispersion of graphene nanoparticles into the Al matrix, various researchers have tried to integrate powder processing into the liquid in situ routes to fabricate MMCs [10,11]. The low surface tension, wettability and interfacial bonding are further impacted by the stacked graphene, which results in low strength and reduced ductility. As a result, it can be difficult to introduce graphene directly into the maelstrom of molten metal by stirring. Other studies address the dispersion issue by ball milling of graphene or pre-dispersion graphene into a liquid medium by ultrasonication, which is then transferred to the molten pool (stir casted) to build the graphene-reinforced composite using the stir casting approach [12,13]. Kumar et.al. reported that to create graphene-reinforced aluminum metal matrix composites, powder metallurgy/processing routes were used. After being hot-extruded, the composites (0.5 wt% graphene) showed excellent tensile properties of 495 and 615 MPa when heat-treated in the T6 condition [14]. To meet fuel tank structural application demand and property requirements and to utilize the excellent mechanical properties with full potential offered by reinforced graphene, not more than 0.5 wt% is required to disperse the reinforced graphene homogeneously and achieve strong interfacial bonding in the final MMC plate [15,16]. Hot rolling is a thermomechanical process that involves plastically deforming the wrought metal to obtain a particular dimension of mechanical properties. To acquire the best physical qualities, the process factors such as rate of heating/cooling and strain rate must be chosen carefully during the hot-rolling process [17]. For larger-scale manufacturing of structural applications like hydrogen fuel storage tanks used in aerospace and space exploration, liquid metallurgy methods like stir casting or squeeze casting are preferred, offering improved yield, lower costs, and simpler operation for the raw material ingot preparation. However, challenges persist due to poor wettability of graphene nanoparticulates during stirring, hindering their dispersion in the molten alloy [18]. In this paper, a novel mixture of aluminum alloy powder 2014 blended with graphene nano powder (reinforcement material) was added into molten aluminum alloy 2014 (matrix material) to fabricate the composite, which was further hot-rolled to produce composite plates.

2. Materials and Methods

2.1. Synthesis of Novel Powder Mixture AA2014/Graphene Nanoparticles

Aluminum alloy (AA) powder 2014 (~25 μm average particle size) procured from Ampal Inc., Palmerton, PA, USA. was considered as the core material to prepare the novel mixture of graphene and AA powder needed for the fabrication of the composites. Two-dimensional graphene nanoparticles of 10 μm length and 5 nm thickness were obtained from Angstrom Inc., Haggerty Rd, Belleville, IL, USA. Five grams of 0.5 wt% graphene nano powder and twenty grams of AA2014 powder were processed using the HEPBM technique. The ball milling process was carried out with tungsten carbide balls, with a ball-to-powder ratio of 5:1, at a speed of 250 rpm for 1 h. The morphology of the novel mixture used to fabricate the graphene-reinforced composite is shown in Figure 1.

2.2. Fabrication of Graphene/AA 2014 Composite

In this work, AA2014 was used as the matrix material for producing an aluminum matrix composite. Figure 2 shows the workflow to prepare the aerospace-grade material of aluminum alloy 2014 reinforced with graphene composite material. In order to prevent interaction with and diffusion of cylinder liner particles with the surface of the shaft into the cast composite, a layer of nano graphite was applied over them. The AA2014 was cut into small pieces and kept in the furnace at a constant temperature of 750 °C to ensure complete melting of the ingot. Subsequently, the graphene particles wrapped with aluminum foil were dropped into the molten material. Before adding the novel mixture into the mold, it was pre-heated at 250 °C to remove moisture, avoid temperature mismatch, and improve dielectric performance. The electric mixer was allowed to spin at a speed of 900 rpm for 5 min to homogenize the molten mixture before it was placed in the pre-heated mold [19]. The novel powder mixture was used to create a graphene nanoparticle-reinforced AA 2014 composite. Following the addition of mixing (shaking and ultrasonication), the unique mixture of molten material was poured into a split mold made from D2 die steel as per the ASTM B686/B686M standard [20]. The mold was heated to 400 °C to prevent the die from cooling too fast. The steel mold had an overall height of 200 mm, a width of 100 mm, and a thickness of 6.5 mm. Following the injection process, hydraulic pressure was applied to the composite melt at a compaction pressure of 110 MPa. In order to prevent the formation of induced residual stress during sample preparation, a wire electric discharge machine (WCEDM) was used to cut the cast samples into the required size [21].

2.3. Hot Rolling Process for Novel Mixture-Reinforced AA2014 Matrix Composite

AA2014 was cut into 50 × 50 × 6.5 mm square plate with the help of wire electrical discharge machining (WEDM) for the hot rolling process. Before the rolling process, the samples were homogenized by keeping them at 490 °C for 12 h to achieve uniform distribution of the graphene reinforcement within the aluminum matrix [22]. The Nabertherm L24/11 muffle furnace was used for the hot rolling process, where a nitrogen atmosphere is maintained to protect the surface from metallurgical changes. During the hot rolling process, the sample temperature was maintained at 480 °C for every pass, and the ramp heating rate was 5 °C per min. The rolling process was carried out using a new field rolling machine, where the roller diameter of top and bottom rollers was 105 mm, rolling force was 1 N/mm, and speed of the roller was 10 rev/min. The casted composite plate used for the hot rolling process had an initial thickness of 6.5 mm. The reduction of plate thickness per pass was 0.1 mm, and finally (after 5 passes), the rolled composite plate thickness was 6 mm. The percentage elongations in the rolling direction and transverse direction were calculated as 6% and 4%, respectively. The percentage reduction in thickness was obtained as 8.3%. The Rockwell hardness values were measured using an applied load of 100 kgf and dwell time of 10 sec as per the ASTM E 18 Standard. The tensile samples were prepared by ASTM E8/E8M substandard specimen size, which is shown in Figure 3 [20].

3. Results and Discussion

3.1. Strengthening Mechanism for AA2014 Graphene-Reinforced Casted Plate

Figure 4 represents the strengthening mechanism of as-casted AA2014 graphene composite plate as per the T6 tempering heat treatment technique. The strengthening mechanism as proposed by Orowan is the homogenous nucleation of the stable h- Al₂Cu inter-metallic precipitate, as shown in Equations (1) and (2) [23].

$${\sigma }_{orowan=MGb}\sqrt{{\displaystyle \frac{f}{r}}}={\displaystyle \frac{MGb}{L}}$$

(1)

$$L=\sqrt{{\displaystyle \frac{2}{3}}Dp\left(\sqrt{{\displaystyle \frac{\pi }{4f}}-1}\right)}$$

(2)

where:

• M—is the Taylor factor
• G—is the shear modulus
• b—is the Burgers vector
• ƒ—is a factor related to obstacles
• r—is a characteristic length related to obstacles
• L—is the obstacle spacing
• Dp—is likely the diameter or some dimension related to the precipitates or obstacles.

Thus, the primary strengthening mechanism is the widespread grain refinement enabled by the evenly dispersed reinforcement (novel mixture) that resulted in the consistent nucleation of the steady θ-Al₂Cu at the grain boundary and the AA 2014 grain [24]. The Hall–Petch theory is another strengthening mechanism to enhance the properties of casted aluminum MMC [25]. The inclusion of graphene nanoparticles alone cannot improve the strength of AA 2014 MMC. Only the T6 treatment process significantly enhances the mechanical strength of MMCs. From the EDS mapping analysis shows that the graphene layers are embedded and interlocked on the grain boundaries. It is understood that the reinforcement was homogenously dispersed during the squeeze casting process. The nucleation mechanics of the steady θ-Al₂Cu intermetallic precipitate were formed on the AA2014 grain boundaries [26]. The graphene nanoparticles were interfaced with AA2014 matrix material after the rolling process, which was evident from the XRD pattern of the T6-treated samples.

3.2. XRD Analysis

In order to identify the phases present, XRD analysis was performed on the AA2014 ingot and the reinforcement (mixture of AA 2014/graphene) and also the casted MMC. The graphene-reinforced casted plate of AA2014 was hot-rolled at 480 °C for each pass. From the XRD peaks, the presence of α-Al in the casted composite and the nature of AA the 2014/graphene powder mixture with embedded/interlocked graphene particles were observed in the as-casted and hot-rolled condition of the composite plate. The absence of aluminum carbide (Al₄C₃) in the hot-rolled composite plate is evident from Figure 5. XRD peak patterns were similarly matched with the novel mixture, in the as-casted and hot-rolled condition of the composite plate.

3.3. Microstructural Analysis

AA 2014 and graphene powders mixed as a casted composite of AA2014 ingots with 99.5% purity were fabricated by mixing of 0.5 wt% (5 g) graphene and 20 g of AA2014 powder. The addition of graphene particles to molten melt is challenging, as it results in floating, non-homogeneous mixing, and agglomeration of graphene particles in the composite cast. The addition of AA 2014/graphene powder mixture (reinforcement) into the molten liquid facilitated their consistent mixing in the molten matrix. Furthermore, the AA 2014/graphene powder mixture acted as an active carrier agent of graphene particles and helped to even the dispersion of graphene particles into the molten melt pool [20,27]. The graphene particles were interlocked and embedded by the casted composite plate, as shown in Figure 6a. The AA 2014 powder protects the graphene particles from direct contact and reaction with the molten melt of AA 2014. After adding the AA 2014/graphene powder mixture to the melt pool, stirring at 500 rpm was carried out for 10 min to avoid the aggregation of graphene and sintering of the novel mixture. Additionally, utilizing the same density of AA2014 powder particles as the molten liquid facilitated the uniform movement and dispersion of graphene-encrusted particles into the molten melt during high-speed stirring at roughly 750 °C [20].

Figure 6b shows that the hot-rolled composite plate was reduced in plate thickness from 6.5 mm to 6 mm. The microstructures observed from optical microscopy show that the solid solution of aluminum grains were well-refined during the hot rolling process, and slightly elongated fine grains were observed in the rolled composite plate. It is noted that the addition of 0.5 wt% graphene particles facilitated the effective grain refinement throughout the hot-rolled composite plate. Figure 7a presents FESEM micrographs of the AA 2014 ingot obtained through the squeeze casting process. From the FESEM micrographs, the dendritic grain microstructure of the as-cast novel mixture-reinforced composite plate was observed. Figure 6a, b shows the graphene particles embedded and interlocked by the solid solution of aluminum grains and surrounded by the intermetallic secondary phases of aluminum grains, as well as the synthesized novel powder mixture with considerable changes in the grain morphology of the composite plate [28]. The existence of the equilibrium phase -Al₂Cu intermetallic compound is clear from the EDS mapping of Figure 7b. The copper and carbon levels match with those of the parent alloy and are greater than the average wt% of the chemical composition in the AA2014 ingot. This equilibrium phase -Al₂Cu intermetallic is formed by dissolving two aluminum atoms, one copper atom, and two more by vacancies, and it is most often seen at grain boundaries [29]. Precipitate formations along the grain boundary are seen, and the graphene flakes resisting the grain growth result in finer grain, as can be clearly seen in Figure 7a. This action caused the grains to be refined and form finer grain structure. The finer grains have higher hardness as per the hall-pitch effect, Figure 8 evidently shows the presence of variation in hardness for the novel mixture at T6 condition as a result of the finer grains.

3.4. Rockwell Hardness Test

Figure 8 illustrates the Rockwell hardness values measured in various phases of the squeeze casted composite plate, which was reinforced by the synthesized novel powder mixture of graphene and AA 2014. The obtained values are compared with the unreinforced parent alloy. The addition of AA 2014/graphene powder mixture increased the hardness values to 58 HRB for the as-cast condition and 123 HRB for the hot-rolled T6 condition of the composite plate. which is 50% more than that of the unreinforced AA2014. The increase in hardness of the novel mixture was caused due to the pinning effect of Al₂Cu, Al precipitates on the graphene flake surfaces resisting further grain growth. This mechanism results in finer grain formation. As per the Hall pitch effect, the finer the grains, the higher the hardness.

3.5. Tensile Behavior

Table 1. Tensile properties of graphene-reinforced AA2014 composite plate along the rolling and transverse directions (RD, TD).

Sample Direction	Procured AA2014	As-Casted Condition	Hot-Rolled and T6
Rolling Direction	185 MPa	220 MPa	420 MPa
Transverse Direction	185 MPa	220 MPa	338 MPa

presents the tensile values of the novel powder mixture-reinforced AA2014 composite plate rolled by multiple passes. The UTS values are noted for the procured condition and as-casted and hot-rolled conditions of the composite plate, The samples of the AA2014 composite plate were prepared and measured in both the rolling and transverse directions (RD, TD), and the values were compared with those of the rolling and transverse directions by the average of three samples. The addition of 0.5 wt% graphene particles to the mixture yielded a UTS value of 338 MPa in TD and 420 MPa in RD, proving the novel mixture improved the mechanical strength of the composite plate, and the UTS values were significantly improved in the rolling direction, also proving that the aluminum grains were well-refined by the rolling direction of the composite plate. The UTS values were increased by 185 MPa to 220 MPa for the as-casted condition, for an increase of 18% compared to the parent AA2014, and the reduction of plate thickness by 0.5 mm in the rolling process resulted in a UTS value of 420 MPa for the plate, which was artificially aged by the T6 condition. The UTS values of the hot-rolled samples exhibited an increment of two-fold compared to those of the as-casted samples, as shown in Figure 9 [30].

3.6. EBSD Analysis

Figure 10 presents the EBSD data for the hot-rolled composite plate. Figure 10a shows the EBSD maps of the AA 2014 with addition of 0.5 wt% novel mixture-reinforced composite plate after hot-rolled condition. After achieving the ultra-fine grain boundaries, it contributes to a significant improvement in the strength of the composite plate. It was analyzed the greater grain orientation along the mixed (101), (111), and (001) composite microstructure despite the fact that the majority of the produced composite employed an average grain size of 50–70 μm. The addition of the novel mixture to the cast was confirmed by the EDS mapping. This behavior confirmed that the aluminum and graphene particles had excellent interface bonding [31]. The processing of the hot-rolled AA2014 composite plates resulted in elongated grain boundaries, closing of micropores and other external defects. The presence of graphene in the composite, on the other hand, resulted in a refined grain structure, which is clearly evident from the pole figure of Figure 10b [32]. When the reinforcement was added into the AA2014, it acted as an inhibitor (pinning effect of the composite, leading to grain growth of the material). The significantly higher specific surface area (SSA) of graphene increased the number of atoms in contact with the nucleation agent and resulted in finer grain formation [33,34]. However, the refined grains depended on the temperature and time profile, as well as the crucial particle size of the composite plate. The greater weight percentage of the graphene particles indicated the agglomeration of particles in the EBSD maps. The rolled samples had graphene particles that were elongated along the rolling direction. These grain boundaries were weaker due to the agglomeration of graphene particles. However, the higher magnification of FESEM analysis ensured that the dispersed graphene particles were very clearly present in the rolled plate. It was noted that the graphene acted as a heterogeneous material and did not interact with the alloying elements and phases present in the microstructure, as observed earlier from the XRD analysis.

4. Conclusions

In this study, a novel powder mixture was synthesized, and the addition of this mixture resulted in the homogeneous dispersion of graphene particles in the AA2014 matrix. The findings from the study are listed below.

• During the stirring process, AA2014 powder in the novel mixture acted as a transporting agent for graphene particles. This resulted in the uniform dispersion of graphene into the final cast.
• By analyzing the FESEM images, it was observed that graphene nanoparticles were homogenously dispersed in the AA2014 MMCs. The analysis also confirmed that the graphene particles were embedded and interlocked throughout the grain boundary of the AA2014 matrix.
• During the hot rolling process, the aluminum grains were well-refined in the rolled plate, which enhanced the mechanical properties of the developed composite.
• The mechanical properties of 0.5 wt% graphene-reinforced AA2014 were exhibited. It was found that the Rockwell hardness increased from 82 to 120 HRB and UTS from 185 to 420 MPa after rolling and the T6 heat treatment process.
• EBSD data confirmed the dispersion of graphene particles and elongation of grains due to the rolling process. The addition of AA 2014/graphene mixture (reinforcement) to the matrix significantly inhibited crystallization during the post-deformation annealing process.
• The addition of graphene nanoplates resulted in a significant grain refinement, with the average grain size decreasing from 120 μm in the monolithic alloy to around 70 μm in the graphene-reinforced composite due to homogeneous dispersion of graphene particles.

The objective of this study was to identify an alternative material and an efficient fabrication technique for a super-lightweight MMC for a Super Lightweight TANK (SLWT) structural application. Overall, this research has helped to understand the fabrication of MMC plates with homogenous dispersion of graphene particles, suitable for large-scale manufacturing processes. It was found that reinforcing metal plate using a combination of graphene mixed with matrix material powder (same as the ingot material used for casting), along with optimal stirring and squeeze casting parameters, facilitates the homogeneous dispersion of graphene particles throughout the composite. Hence, this will serve as an alternate method for the fabrication of lightweight MMCs for SLWT applications.