Metal matrix composite fabricated from electrospun PAN, EGNS/PAN nanofibers and AL 5049 alloy by using friction stir processing

This work is an attempt to fabricate aluminum (AA 5049) matrix composites (AMCs) reinforced with electrospun polyacrylonitrile (PAN) nanofibers and consisting of exfoliated graphite nanosheets (EGNS/PAN) by utilizing friction stir processing (FSP) to improve the mechanical characteristics of AA 5049. The electrospinning method was used for fabricating PAN and EGNS/PAN nanofibers. The average diameter of the electrospun PAN nanofibers is 195 ± 57 nm, and after EGNS incorporation is 180 ± 68 nm. Dynamic recrystallization was the main process in the microstructure evolution of the stir zone during the FSP with PAN and EGNS/PAN nanofibers. According to PAN and EGNS/PAN nanofibers were used in the FSP procedure, the grain size reduced as a result of the pinning effects. PAN and EGNS/PAN nanofiber reinforcement enhanced the hardness to 89 and 98 Hv, respectively. Also, the ultimate tensile strength was raised to 291 MPa and 344 MPa, respectively. Tensile strength and hardness of the stir zone increased during the FSP with PAN and EGNS/PAN nanofibers due to the higher density of the strengthening mechanisms of grain boundaries and dislocations. The mechanical characteristics of AA5049 can be enhanced by the procedure of incorporating nanofibers, making them an ideal choice for applications in the automotive and aerospace industries.


Introduction
Ceramic and carbon nanofibers were utilized as the reinforcing fibers for metal matrices such aluminum and magnesium. Using ceramic nanofibers as a reinforcement to improve the mechanical characteristics of metal matrix composites [1,2]. However, the percentage of reinforcement needs to be optimized to improve the mechanical properties (such strength, hardness, and Young's modulus) of the composites [3].
A promising method called electrospinning enables the creation of micro or nanofibers with the potential to form membranes with linked porosity, superior mechanical characteristics, and a large surface area. The technology of electrospinning has numerous applications, including in the fields of medicine [4], energy transportation [5,6], optoelectronic devices [7,8], and water treatment [9][10][11][12][13]. Micro-and nanoscale electrospinning can be used to create electrospun fibers with the potential to incorporate cutting-edge materials to enhance their mechanical or physicochemical properties [14][15][16]. Continuous and non-woven fibers with micro and nanoscale dimensions can be created using the electrospinning technique [17,18]. The electrospun fibers were therefore excellent for potential applications in a wide range of fields, including the conveyance of energy [19], membrane [20, 21], adsorption processes [22], and air filtration [23,24], Among the topics covered are medication delivery [25,26], plat grafting [27], optoelectronic devices [28,29], electrochemical sensors [30], and others. The electrospinning process is defined as the point at which the polymer solution and electric field interact. The voltage applied causes uncompensated charges to accumulate on the polymeric surface. The forces caused the polymer chains to align as they passed through the needle shear. This phenomenon produces a jet and Taylor's 1 3 cone, a conical shape over the solution. A linear jet that leaves Taylor's cone accelerates towards the collector. The intermolecular stability of the polymer is then maintained as the solvent evaporates while the jet is in motion. The fibers are then gathered and spread throughout the collector surface to create a non-woven mesh [13,31,32].
The manufacturing of aluminum matrix composites (AMCs) has recently become more significant due to applications in sectors including aerospace and automotive. This is because they have a number of prospective advantages over conventional alloys, such as great formability, improved wear resistance, higher specific stiffness, favorable fatigue properties, and outstanding strength to weight ratio at low or high temperatures [33].
On the other hand, there are several surface modification processes, including the laser melt technique, centrifugal casting, and plasma spraying, which are used to modify only the surface of a material while leaving the interior unaltered. When using such methods, the bulk of the material loses a tiny number of other attributes like toughness while surface properties like hardness and wear resistance only slightly improve [34]. While all these techniques have been utilized effectively to make surface MMCs, a major concern is that they need material phase transitions throughout the process (from solid to liquid or from liquid to vapor). As a result of the interfacial interactions between the material and the reinforcement caused by the phase change, the interfacial bonding strength is reduced, and harmful phases are formed [35].
A possible solid-state processing method for creating surface MMCs and customizing the surface's microstructure through extreme plastic deformation is friction stir processing (FSP) [36][37][38]. When compared to alternative methods, FSP is quicker, uses less energy, and ensures that the secondary phase is distributed uniformly due to a strong thermomechanical effect [39,40], which improves the mechanical characteristics [41,42]. In FSP, the material is manipulated at temperatures below its melting point and is only mechanically deformed in a plastic way. To create the composite, the reinforcing particles are mechanically combined with the plasticized metal. The fine dispersion of the reinforcement particles without segregation at the grain boundaries is the result of the base material not melting [43]. The pin geometry, the travel and rotational speeds, the tilt angle, and the number of FSP passes are only a few of the processing parameters that have an impact on the FSP process [44,45].
The most common application of FSP is to modify the microstructure and mechanical properties of metallic components with thin surface layers [ Additionally, it was discovered that FSP composites significantly improved in microstructure changes, obtaining equiaxed and refined grain structures as opposed to the base metal's elongated and coarse grains, most likely as a result of the severe plastic deformation and continuous dynamic recrystallization brought on by FSP [59]. Friction stir processing (FSP) parameters were examined for their effects on the production of an AA6061/tungsten carbide nanocomposite. The results demonstrate that using the proper FSP settings results in an AA6061/WC nanocomposite free of voids and defects by uniformly dispersing the WC particles throughout the matrix [45, 60]. To experimentally explore surface generation and the best surface roughness to be achieved in relation to the process parameters, ultra-precision machining of particle reinforced metal matrix composites (MMCs) with polycrystalline diamond (PCD) end mill is carried out [61]. The dynamic cutting force modelling for precise micro milling of particle reinforced MMCs is the subject of a novel investigation presented [62].
Fiber reinforced metal matrix composites (MMCs) are a good option for applications in the automotive and aerospace industries and for military uses like defensive armors due to their mechanical characteristics. This study is an attempt to fabricate, characterize, and assess a metal matrix composite derived from this fascinating material, electrospun PAN, and EGNS/PAN nanofibers, using the FSP method. the analysis of the interfaces and subsequent mechanical characteristics of the metal matrix composites made by FSP, particularly the two PAN/AA5049 composites made from electrospun PAN nanofibers and exfoliated graphite nanosheets.

Electrospinning of EGNS/PAN/DMF
Our previous research [63] described the exfoliated graphite nanosheets (EGNS) fabrication process and procedures. First, sulfuric and nitric acid mixtures were mixed with graphite particles (ratio 4:1 by volume). The graphite intercalation compound was made by stirring the mixture constantly for 24 h at room temperature (GIC). The GIC was then cleaned with distilled water and dried to remove any moisture at 100 °C to produce expanded graphite, the dry particles were heated and thermally shocked for 30 s at 1050 °C. Finally, a 600-ml alcohol solution was poured over one gramme of expanded graphite (alcohol and distilled water with a ratio of 65:35). Before employing the dispersion in the electrospinning procedure, the dispersion was filtered and dried after 16 rounds of sonication to obtain the EGNS. PAN/DMF polymer solution was prepared a ratio of 10 wt. % under continuous stirring at room temperature for 3 h to produce a homogeneous solution. EGNS were dispersed in DMF using ultrasonication for 16 h. The PAN/DMF polymer solution contained 10 weight percent of the EGNS weight concentration. The PAN was subsequently added to the EGNS/DMF suspension to produce a homogeneous and uniformly dispersed EGNS/PAN/DMF polymer solution. After that, the mixture was stirred for 10 h at 60 °C. The polymer solution was placed into a 10-mL glass syringe that was connected to a stainless-steel needle (inner diameter of 0.9 mm). A high voltage power supply was utilized to create a 25-kV electric field between the needle and a grounded, horizontally centered metal collector. The electrospun PAN and EGNS/PAN fibers were collected on a metal collector that was wrapped in aluminum foil. The collector was modified for all tests and placed 20 cm vertically from the needle tip for nanofiber deposition. Figure 1a shows the electrospinning technique.

Friction stir processing (FSP)
Aluminum (AA5049) sheet, whose chemical composition is shown in Table 1, was used to create surface composites. The base metal (BM) AA5049 sheets were wire-cut with dimensions of 100 × 50 × 6 mm. Applying wire electrical discharge machining, a rectangular groove with a depth and width of 3 mm and 1 mm, respectively, was created. The electrospun PAN nanofibers (or EGNS/PAN nanofibers) weighing 8 g were inserted into the wire-cut grooves.  To prevent the escapement of any fabrics from the groove during processing, the groove entrance is initially sealed off using a tool with a shoulder without a pin. A computer numerical control vertical milling center with an FSP tool was used to produce the surface composite. The FSP tool had screwed pin profile, shoulder diameter of 18 mm, a pin diameter of 6 mm, and a height of 5.7 mm. Hardened H13 steel was used to manufacture the FSP tool. Based on the optimum values acquired from the prior study, the FSP process window was chosen [64]. The FSP tool rotation speed and traverse speed were both held constant at 1000 rpm and 40 mm/min, respectively. Figure 1b illustrates the FSP procedure.

Scanning electron microscopy (SEM)
Scanning electron microscopy Model Quanta 250FEG (Field Emission Gun) with EDX Unit (energy dispersive X-ray analyses) at the Tabbin Institute for Metallurgical Studies (TIMS), Egypt was used to analyze the morphology of electrospun PAN,EGNS/PAN nanofiber and the distribution of the nanofiber reinforcement after FSP. The number of the points (about 100 random nanofibers) was correlated with the average diameter and distribution of the electrospun fibers. In each image, at the appropriate magnification, ImageJ software was used to measure the distance from the SEM. Microstructural samples were extracted from the AA5049 plates in the processed zone. Before displaying the surface metallographic, the surfaces of all samples were ground and polished to a mirror-like finish. Following mechanical polishing, the samples were etched with Keller's reagent (190 mL distilled water, 5 mL nitric acid, 3 mL hydrochloric acid, and 2 mL hydrofluoric acid). Also, the morphology of EGNS/PAN nanofiber was studied using a high-resolution transmission electron microscope (HRTEM) (JEOL model JEM 2100) at the Petroleum Research Institute in Egypt.

Mechanical characterization
A Vickers' microhardness tester was used to measure the microhardness at various points in the stir zone, with a 100-g load and 10-s dwell times. According to ASTM E8M-04 standards [65], the tensile sample is manufactured by wire cut EDM with dimensions of 2.5-mm-thick, 4-mm-wide, 58-mm-long, and a gauge length of 26 mm, as depicted in the Fig. 2. Uniaxial tensile tests are conducted with a crosshead speed of 1 mm/min at room temperature.  Fig. 3a. The morphology of EGNS/PAN electrospun nanofibers changed significantly after EGNS was incorporated. In a related nanocomposite, the diameter of electrospun nanofibers can be reduced by adding conductive nanofiller to the polymer matrix [66,67]. Figure 3b shows EGNS/PAN nanofibers with decreased diameter after EGNS addition (180 ± 68 nm). The reduction in fiber diameter reveals that the electrical conductivity of EGNS can predominate in the decreasing of fiber diameter. Since graphene nanosheets have a large surface area, they exhibit powerful interactions with polymer matrices [68,69]. The micrograph of EGNS/PAN composite nanofibers obtained by transmission electron microscopy (TEM) containing 10 weight percent EGNS is shown in Fig. 4. The EGNS layer on the PAN nanofiber surface is visible in the TEM image. By using Raman spectroscopy, the presence of EGNS in the produced EGNS/PAN has been further established. The D-band line (1350 cm −1 ) and the G-band line (1650 cm −1 ) are two significant peaks in the Raman spectra of PAN/GO displayed in Fig. 5. The existence of these two bands strongly suggested that all composite nanofiber membranes have disordered and graphitic layers. The G bands on the EGNS/PAN nanofiber were noticeably higher than the D bands. The EGNS/PAN nanofiber with the I D /I G ratio of 0.95 suggested the presence of disordered graphite since the intensity ratio of the D band and G band is frequently utilized as an indicator for the degree of disorder [70,71].

Microstructural characteristics
Optical micrographs of the FSP AA5049 substrate, PAN nanofiber, and EGNS/PAN nanofiber reinforced AA5049    Fig. 6a-c. A recrystallized fine-grained microstructure is produced within the stir zone as a result of the intense plastic deformation and frictional heating that occur during FSW and FSP. The term for this occurrence is dynamic recrystallization (DRX) [72,73]. At the base of the stir zone, a concentric arc-shaped banded structure of PAN and EGNS/PAN nanofiber can be observed (Fig. 6b, c). This is an inherent feature of FSP and is termed as onion ring structure. An onion-ring structure is produced by the interaction of material flow between the warmer zone at the top and the colder zone at the bottom [74]. The rings of the tool tip during the FSP show that the distribution of Electrospun PAN Nano Fiber 195 ± 57 nm has been spread neatly inside the AA 5049 matrix (Fig. 6b). The presence of nanofibers caused onion rings to occasionally be rich and unreinforced. Regions with and without nanofibers are represented by the dark and light rings, respectively. Figure 6c shows that when particle-rich and particle-free regions were compared, the former had finer grains than the latter. It implies that in the FSP, strong plastic deformation separates the original grains and creates low angle, incorrectly oriented grain boundaries, leading to the growth of a significant number of nucleating zones for recrystallization. High-angle grain boundaries are created by DRX, which changes low-angle grain boundaries into finer grain patterns. Additionally, the "pinning effect" (Zener effect), which occurs when reinforcements are utilized in FSP, may cause these particles to block grain boundaries and stop grain coarsening by limiting their movement. Reinforcements, however, serve as nucleation zones and promote grain breakdown [72,75,76]. The grain structure refinement of the produced nanocomposite was accelerated by including EGNS/PAN nanofiber (180 ± 68 nm) inside the SZ. As a result of the additional dynamic recrystallization (DRX) in  the stir zone, the matrix's grain size is reduced during FSP. It is anticipated that the inclusion of EGNS/PAN nanofibers will result in a more extensive microstructure refinement process when compared to the process using PAN nanofibers and without reinforcing. Figures 7 and 8 from the EDS analysis indicate the distribution of the different constituents in the PAN nanofiber and EGNS/PAN nanofiber reinforced AA 5049 surface composite. The EDS mapping analysis is applied to investigate the distribution of the reinforcements and the detrimental impacts of agglomeration and nanoclusters on the reinforcing nanofibers. In all samples, the reinforcing nanofibers were uniformly distributed in the SZ, and no significant clustering was observed. A typical EDS examination of an electrospun PAN, EGN/PAN nanofibers, the high aluminum concentration was identified, which was located at the boundary of the carbon nanofibers.

Micro hardness behavior of MMNCs
Vickers micro-hardness curves for the AA5049 pure and SZ specimens as performed, with and without reinforcing nanofibers, are shown in Fig. 9. The average micro-hardness in the pure AA5049 and SZ is shown in Fig. 10. The pure AA 5049 has an average micro-hardness of 70 HV. The surface composite's SZ had increased hardness; the main factors contributing to this improvement were the uniform distribution and refining action of the nanofiber reinforcements. In the case of friction stir processed AA5049, the growth of fine grains and dislocations increased the initial hardness to 81 Hv. The hardness was increased to 89 and 98 Hv, respectively, by the reinforcement of PAN and EGNS/ PAN nanofibers. The EGNS/PAN nanofiber has a harder reinforcement than pure AA5049, which results in a stronger reinforcement that strengthens the AA5049 matrix many folds [77]. The homogeneous distribution of electrospun EGNS/PAN nanofiber in the aluminum matrix enhances the strengthening since it serves as a barrier for the movement of dislocations. The variation in the thermal contraction coefficient of the matrix and the reinforcing phase is what causes the fine dispersion of the reinforcement nanofibers in addition to the quench hardening impact [78]. Additionally, compared to those produced by traditional FSP, the hardness values for weld zones under the inclusion of EGNS/ PAN nanofibers are significantly greater. By enhancing localized stresses and hence the driving force for DRX, the inclusion of EGNS/PAN nanofibers results in better stored energy. When EGNS/PAN nanofibers are added during FSP, this inhibits grain growth by pinning the grain boundaries [79]. Therefore, the hardness of the electrospun nanofibers is transferred to the composite, further enhancing its hardness.

Tensile properties
The stress-strain curves and tensile characteristics of the examined composites are shown in Fig. 11. Figure 12 indicates the mechanical properties of the investigated aluminum composites. The yield strength, ultimate strength, Young's modulus, and elongation to break of the pure AA5049 alloys were found to be 125 MPa, 216 MPa, 70 GPa, and 12%, respectively. During FSP, the processing region's microstructure was improved, which improved its mechanical and plastic properties and raised the elongation to break to 24%. The two main factors that had a substantial impact on the mechanical properties of the stirred zone material were dislocation strengthening and grain refining. The tensile performances were greatly increased by introducing particles because of the effects of particle uniformity, grain size refinement, and dislocation pinning [80,81]. During FSP with nanofibers, the stir zone exhibits more severe plastic deformation and a larger dislocation density. Furthermore, it is well known that significant dislocation densities result from the temperature mismatch between the nanofibers and the matrix [82]. Despite the DRX phenomena taking place in the stir zone, the presence of nanofibers in the base matrix causes a high dislocation density around them. These dislocations may contribute to the tensile testing slip mechanism, resulting in a larger work-hardening capability. The pinning action of nanofibers prevents dislocations from migrating to grain boundaries, lowering the recovery rate [83,84]. Additionally, it has been demonstrated that metal surface pre-treatment has a sizable impact on the mechanical performance of adhesively bonded joints. Metal surface pretreatment can increase the strength of the joints since one of the main bonding processes in FSP joints of the metal-composite is the adhesion forces caused by the presence of solidified molten polymer [85]. The ultimate tensile strength of the AA55049 increased to 291 and 344 MPa after being reinforced with PAN and EGNS/PAN nanofibers, with corresponding decreases in elongation (18 and 14.2%). This demonstrated the significant influence of nanofiber on alloy composites. For all composites, the yield strength and the elastic modulus were both improved. The electrospun nanofibers functioned as barriers for dislocation motions in the SZ, improving the mechanical characteristics of the produced composites. Improved mechanical characteristics were achieved as a result of the EGNS/PAN nanofiber's large specific surface area having good interfacial adhesion with the AA5049 matrix. Also, due to EGNS nanofillers' atomic thickness, excellent mechanical strength, and chemical inertness.

Conclusion
This work intends to describe the effects of the use of electrospun PAN and EGNS/PAN nanofibers during the FSP of aluminum 5049 to deal with a number of significant concerns, such as microstructure evolution, EDS, and mechanical characteristics. The findings indicate that: 1. The electrospun PAN diameter was found to be in the range of 195 ± 57 nm, whereas the electrospun EGNS/ PAN nano fibril composite was reduced to 180 ± 68 nm by the presence of EGNS. 2. Microstructure analysis of composite samples revealed that both PAN and EGNS/PAN reinforcement were distributed uniformly inside the AA5049 metal matrix. 3. The FSP procedure results in the production of onion rings, and no voids or porosities with adequate diffusion are seen at the interface of the FSP with PAN and EGNS/PAN nanofibers. 4. EDS results showed The electrospun carbon nanofibers (C) cohesive with the predominant aluminum (AL) element. 5. The superior mechanical qualities of PAN and EGNS/ PAN nanofibers, when introduced into the metal matrix, enhance the metal's strength, microhardness, and mechanical properties as a result to grain refinement, pinning effects, and dislocation density. 6. Microhardness was reported to be 89 and 98 Hv, respectively, for PAN and EGNS/PAN reinforcement, with a significant rise in microhardness along the SZ. 7. The tensile properties of the composites were apparently improved by 35% and 60%, respectively, by the incorporation of PAN and EGNS/PAN nanofibers. 8. The rising need for high-performance and lightweight materials in the aerospace and automotive industries as well as for military applications like defensive armor, the creation of metal matrix composites with PAN nanofibers and EGNS additives employing FSP has been gaining interest.