1. Introduction
With the rapid development of metal matrix composites (MMCs) and especially aluminum matrix composites (AMCs), traditional casting, infiltration, plastic forming, and other methods showed a limited ability to meet the requirements of preparation and processing of new aluminum matrix composites. New powder metallurgy technology, large plastic deformation, and material addition manufacturing have a high controllability and unique role in the preparation and processing of AMCs, which has been widely noted by advanced researchers.
In recent decades, many efforts such as powder metallurgy (PM)
[1][2][3], severe plastic deformation (SPD)
[4][5][6][7][8], nanoscale dispersion (NSD)
[9][10], in situ chemical vapor deposition (CVD)
[11][12], and ball milling followed by post-sintering processes such as forging, extrusion, or rolling
[13][14][15][16][17][18][19] have been undertaken to fabricate MMCs, especially AMCs. Among all the proposed techniques, the PM route has been identified as very promising in order to obtain optimal second phase distributions
[20][21][22]. The basics of this route comprise the deformation, cold-welding and balanced stages
[23]. At the deformation stage, ductile Al powders flatten as a consequence of intense collisions of steel balls with powder particles, consequently providing a larger specific surface compared to spherical particles. Reinforcement clusters are fractured and then dispersed onto the surface of the flattened powder
[24][25][26]. During the cold-welding stage, flattened powders are cold-welded into coarser reinforcement-metal powder particles decorated by reinforcement at the welded seams. It should be noted that once reinforcements are inserted, their further dispersion becomes more and more difficult. Therefore, the minimum necessary time for cold-welding should be precisely set
[27][28].
Powder metallurgy (PM) technology can control the interface of the composite at temperatures well below the melting point of the aluminum matrix, thereby allowing for avoidance of the natural agglomeration of micro-reinforcements into the melt and the floating or settling of reinforcements due to differences between the density values of the melt and the second phase. At the same time, the AMCs have a smaller matrix structure than the liquid phase method. As the basic unit of PM technology, control of powder is key to various new PM technologies.
Some technologies use the reaction activity resulting from the high specific surface area of powder and the external input energy. The dispersion of nano-reinforcements is accomplished through an in situ reaction, such as the reaction of spray deposition of atomized droplets and reactant
[29] or the mechano-chemical reaction of powder and reactant through high-energy ball milling
[30]. Some PM technologies can disperse the reinforcement, control the shape of the powder and refine the powder grains through high deformation during ball milling, such as mechanical alloying of the reinforcement by long-term high-energy ball milling (HEBM) and low-temperature ball milling such as cryo-milling
[31][32][33]. The HEBM route has demonstrated its effectiveness in the destruction of the oxides layers, providing optimal interface bonding between the second reinforcing phase and the matrixes. However, the severe deformation due to ball milling can lead to remarkable damage in very brittle reinforcements such as CNTs.
In general, most of the new PM technologies have been developed to realize the new component design or structural design of AMCs. For example, flake PM (FPM) is a typical method for fine control of powder shape LEBM of mostly ≤ 200 rpm
[24][27][34][35][36], and then for the preparation of the composite structure. The method not only is characterized by a very high specific surface area and flat surface, but also provides sufficient space for the surface in situ reaction
[37][38] and surface dispersion
[26][39][40] to the reinforcement, and can induce flake nano-grain (NG) or ultrafine grain (UFG) in the powder. This fine microstructure can then be retained in the bulk material. Depending on whether or not the flake shape is destroyed during the powder consolidation process, a nano-reinforced body dispersion structure
[25][40][41][42][43] or a layered configuration
[44][45][46][47] can be formed, respectively.
The obtaining of sound mechanical properties in nano-reinforced AMCs can be achieved only if second-phase optimal dispersion is realized during processing
[3][24][48][49]. The second requirement effectively maintains the structural integrity of the nano-dispersions, especially for CNT and graphene
[27][50][51]. The third requirement is interfacial bonding between the nano-dispersal surfaces and the matrix material
[19][25][52]. It is interesting to note that the alignment of the nano-dispersoids has a remarkable influence on the mechanical properties of metal matrix composites
[53]. The new PM technologies, namely FPM, as a powder-smart severe plastic deformation (SPD) method will enable us to overcome the obstinate roadblocks in UFG alloys and MMCs caused by poor dislocation storage and weak strain hardening ability. FPM, as a bottom-up approach, could fulfill a flaky powder geometry known as the building blocks of bulk advanced materials, which was hitherto believed impossible.
The PM route for MMCs production comprises the following main steps: powder processing and consequent powder consolidation. Metal powders are blended with reinforcements by means of techniques such as conventional FPM (C-FPM)
[35][47][54], slurry blending (SB), vapor-based synthesis, shift-speed ball milling (SSBM), and high-shear pre-dispersion and SSBM (HSPD/SSBM), which are then followed by various consolidation routes of the mixture such as hot extrusion
[27][55], spark plasma sintering
[47][56][57], friction stir processing
[58][59], hot rolling
[19][46][60], and others
[5][36]. Significant advances can be underlined in the field of solid-phase powder metallurgy (PM) techniques
[47]. The PM based methods include HEBM
[19][46][55], FPM
[24][34], liquid phase ball milling
[24][61], nanoscale dispersion methods
[9][10], molecular level mixing
[62][63] and in-situ CNTs production combined with low-energy ball milling
[11][37][64], and others
[65]. These methods sometimes require large time consumption when used on an industrial scale. Therefore, it is fundamental to review recent advances in powder-based processing techniques such as FPM. This review specifically focuses on the different FPM approaches, namely C-FPM, SB, SSBM, and HSPD-SSBM. After reviewing the recent advances in FPM approaches, the potential for further optimization and development is indicated.
The main developed powder metallurgy routes are based on the employment of starting spherical powders in order to allow dense compaction of the formed blocks. By contrast, FPM is based on the employment of flake-shaped reinforcements that can be aligned by formation through hot compaction and rolling or extrusion. During flake powder metallurgy, the main processing steps are the precise preparation of flake reinforcements, their proper mixing, consolidation, and alignment.
From the crystal plasticity perspective, the BM process is known as a micro-plastic deformation process, affected by rotational speed and rotational time
[2][27][66][67]. Generally, the collision forces between the milling balls and reinforcement and matrix powders with all their complexity, including compressing and shearing forces, are the origin of the BM process evolution
[27][68]. Specifically, compression leads to the powder’s deformation, flattening and fragmentation, as well as final cold welding. In contrast, shearing effectively promotes the breaking of contamination skins such as alumina skin in Al powders and reinforcement clusters, resulting in uniform dispersion of reinforcements.
2. Conventional Flake Powder Metallurgy (C-FPM)
The FPM via typical dry milling is introduced as powder BM in a dry jar. Specifically, the ingredients blended in the jar are reinforcement, matrix powder, steel balls, and usually a processing agent such as stearic acid, ethanol or similar. The process of flaking for spherical metal powders under the C-FPM route is performed through micro-rolling (ball milling) the starting powders (a).
In current practice, for the first time, the LSBM process is considered a kind of rolling process, but on a micro scale—namely, micro-rolling—because: (1) the LSBM process causes plastic deformation of initial powders, thus leading to significant effects on the geometry of the powders; specifically, particles flatten via a process which is similar to the one which is performed in the conventional rolling process, (2) milling balls can be considered similar to conventional rollers but with a much smaller size, and (3) despite the difference in the source of the generation of imposed stresses, deformation occurred originating from shear stresses upon the powders.
2. Flake Powder Metallurgy via Slurry Blending (FPM-SB)
The strategy of using a slurry-based dispersion process of the reinforcement has emerged as a mighty new route in overcoming the prerequisites needed to produce advanced AMMs
[39][69]. In this technique, the particles’ morphology is modified from three-dimensional spheres to two-dimensional flakes. Subsequently, the surface is modified through organic solutions such as polyvinyl alcohol (PVA) or ethanol. In this way, the flakes can easily attract the second phases onto their surfaces. The produced composite is then compacted and sintered, as illustrated in a.