Tailoring the structure and mechanical properties of graphene nanosheet/aluminum composites by flake powder metallurgy via shift-speed ball milling
Introduction
Graphene has attracted interest as a novel reinforcement for metal matrix composites (MMCs) due to its extraordinary mechanical and physical properties based on the strong sp2 C-C bonds [1]. The uniform dispersion of graphene is considered to be a prerequisite for achieving high-performance graphene reinforced MMCs [2]. Several techniques have been developed to reach this goal and address the strength and ductility trade-off of the composites [3], e.g. in-situ growth approach [4], molecular-level mixing process [5], electrostatic adsorption [6], slurry blending [7], [8], electroless deposition [9], but most of them are complex and costly for industrial application. In comparison, ball milling [10], [11], [12] has become a widely chosen route for the fabrication of graphene reinforced MMCs due to its simplicity, flexibility, and scalability, especially in graphene/aluminum composites [13], [14], [15], [16]. Nevertheless, as the graphene-metal powder mixture would experience severe deformation during ball milling, a variety of structural factors of the composites evolves asynchronously. For example, a prolonged milling process may promote the dispersion homogeneity [17], but will also damage the structural integrity of graphene and lead to grain refinement of the metal matrix, which may result in the loss of ductility [14].
Considering this, the optimization of the milling regimes should be very important for the fabrication of high-performance graphene reinforced MMCs. However, most studies directly adopted high-energy and/or long-time ball milling, and focused on the influence of different graphene contents on the microstructure and mechanical properties of the composites [1], [11], [14], [17], but the powder evolution regularities have not been fully understood. On the other hand, the evolution of composite powders of metals and nano reinforcements such as carbon nanotubes (CNTs) has been deeply studied [18], [19], [20]. Generally, the deformation of metallic powders during ball milling involved three stages: getting flattened into flakes, cold welded into lamellar-structured particles and then fragmented into smaller particles. Meanwhile, the nano reinforcements were firstly dispersed on the surface of metallic flakes, and then occluded by the metal matrix and trapped in the cold welded particle [10]. It was proposed that, the flake powders, which are more geometrically compatible with the nano reinforcements, are superior to be used as building blocks in the structural design of PM metal matrix nanocomposites, which inspired the development of flake powder metallurgy (Flake PM) route [21]. Typically, Xu et al. [19] proposed a Flake PM route based on the idea of task allocation in a shift-speed ball milling (SSBM) process, where a low-speed ball milling (LSBM) was firstly used to achieve the uniform dispersion of CNTs in Al flake powders without serious structural damage, and a high-speed ball milling (HSBM) was then utilized to achieve good bonding between the CNT/Al flake powders. It supplied a potential route for tailoring the microstructure and mechanical properties of graphene reinforced MMCs. Compared with the one-dimensional CNTs, the dispersion of two-dimensional graphene appears to be more challenging [22]. One of the key problems is the strong Van der Waals forces between graphene sheets and, subsequently, their tendency to aggregate in the metal matrix during processing [6], [14]. Therefore, the control of graphene dispersion during LSBM in SSBM process is the key to prepare high-performance graphene reinforced MMCs, which has not been systematically studied yet.
In the present work, SSBM process was applied to fabricate graphene nanosheet (GNS)/Al composites and realize the tailoring of a series of structural factors. The structural evolution and mechanical properties of the composites fabricated with different SSBM regimes were investigated. It was found that, appropriate LSBM time of GNS/Al flakes was curial for the balance of the grain size of Al matrices, GNSs dispersion and integrity, which could further lead to balanced strength and ductility of the GNS/Al composites. The underlying mechanisms for the powder evolution during SSBM process, the formation of the microstructure and the structure-property correlation of the GNS/Al composites were discussed.
Section snippets
Raw materials
Atomized spherical Al powders (Fig. 1a) with an average diameter of ∼14 μm and a purity of ∼99.8%, provided by Bai Nian Yin Industry & Trade Co., Ltd. (Zhejiang, China), were used. The raw GNSs (Fig. 1b and c) with a wrinkle morphology and a thickness of 3–10 nm of each nanosheet were provided by Morsh Technology. Co., Ltd. (Ningbo, China), which exhibited an ID/IG value of ∼0.12.
Ball milling process of GNS/Al powders
According to the previous reports [18], [19] and the results of our previous tests, ball milling with either too low
Evolution of powder morphology and microstructure in SSBM process
Fig. 2a–d showed the evolution of the GNS-Al powders during LSBM process, the first step of the SSBM process. The Al powders were gradually flattened into flakes as a function of LSBM time. The dimension and microstructure evolutions of the Al flakes were summarized in Fig. 3a and b, respectively. It is shown that the average flake diameter increased gradually as the average flake thickness decreased but the rate of powder flattening slowed down after 6 h LSBM. The domain size reduction and the
Deformation and dispersion of GNS-Al powder in SSBM process
Both powder deformation and GNS dispersion are dominated by the collision force between milling balls and GNS-Al powders. The radial component of the collision force, the compressing force, comes from the direct impact of balls, mainly contributing to powder deformation, e.g. flattening, fragmenting, and cold-welding. The tangential component of the collision force, the shearing force, comes from the side impact, rotation and friction of balls, effectively promoting nano reinforcement
Conclusions
A technique route of Flake PM via SSBM was developed to fabricate and tailor the mechanical behaviors of GNS/Al composites. Much importance was attached to the flaking process to achieve nondestructive but uniform GNS dispersion along with the deformation of Al matrices. The morphology and microstructure of GNS/Al flakes were tailored by varying the LSBM times in SSBM regimes. Based on that, the microstructure evolution and structure-property correlation of GNS/Al composites were discussed. The
Acknowledgment
This work was supported by the Natural Science Foundation of China (Nos. 51671130, 51771110, 51771111, 51371115), the Ministry of Science & Technology of China (Nos. 2016YFB1200506, 2016YFE0130200, 2017YFB1201105), the Ministry of Education of China (Nos. 62501036031, B16032), Aeronautical Science Foundation of China (2016ZF57011), and Shanghai Science & Technology Committee (Nos. 15JC1402100, 17ZR1441500, 14DZ2261200, 14520710100).
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