Micromanufacturing of composite materials: a review

The concept and/or definition of micromanufacturing has been addressed by a number of researchers and industrial personnel, as reported in earlier studies [2, 11–15]. The simplest definition of micromanufacturing is a system for producing small-dimensional parts occupying less space and consuming less resources and energy by downsizing the complete production process. Since the equipment size is reduced, the mass of the system can be reduced dramatically. This results in reduced energy consumption, overhead cost, materials requirement, noise, and pollution and eventually facilitates a more environmentally friendly and viable production process. Due to a reduced manufacturing cycle and higher tool speed, micromanufacturing also leads to higher production rates. A study conducted in [21] demonstrated the influence of miniaturization and pointed out that a 1/10-scale reduction of the production facility may result in a 1/100-scale reduction of energy consumption when compared to that of a conventional production system/factory. The most noteworthy improvement of micromanufacturing is its ability to produce components having a feature size of less than 100 μm [22, 23], close to the size of a human hair. Figure 1 shows micromachined parts made of aluminum oxide (Al₂O₃)-reinforced 316 L stainless steel composite materials that combine the hardness of ceramic and the strength of steel. A novel method, called the 'soft molding technique,' was used. The authors [20] claim that the hardness of 316 L stainless steel was improved by 1.8 times, enabling the parts to be stronger, harder, and more wear resistant.

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There are a number of techniques and processes that are used to manufacture miniaturized components. Based on the materials used, micromanufacturing can be broadly classified into two major categories: silicon and nonsilicon. In nonsilicon-based micromanufacturing, the materials usually dealt with are metal, ceramic, polymeric, and composite. Consequently, a number of different manufacturing techniques are involved and are being practiced in the industry. A useful, simple, and all-embracing classification is presented in table 1, including typical applications and common materials used. There is also a common trend of combining multiple processes together, termed hybrid micromanufacturing [24]. The type of energy involved in these processes includes mechanical, electrical, chemical, electrochemical, laser, and electron beam [25]. The working principles entailed include mechanical forces, thermal effects (melting/vaporization), dissolution, ablation, recomposition, plastic deformation, consolidation, lamination, and sintering [24, 26]. Similar to macroproducts, microcomponents, based on the way they are produced, can also be classified as subtractive, additive, joining, forming, and hybrid processes [3–5, 27–30].

With the rapid development of the modern manufacturing industry, composite materials are being extensively used as advanced multifunctional materials in various fields, such as electronics, aeronautics, medicine, automobiles, and machining tools, due to their unique properties that eliminate traditional limitations due to the physical and mechanical performance of monolithic materials [34, 35]. For example, tungsten carbide (WC) has good hardness and wear resistance but poor strength and toughness [36, 37], while high strength steel has excellent strength and toughness but low hardness and wear resistance [38–40]. A layered composite of WC and high strength steel can combine their advantages and be used in many engineering applications.

A composite material can be defined as a material fabricated from two or more integral materials with considerably different physical and chemical properties that, when combined, produce a material with characteristics different from the individual constituents. The individual constituents stay separate and distinct within the completed structure, differentiating composites from mixtures and solid solutions [41, 42]. Thus, every composite material, by definition, has essentially two components, a matrix, i.e. a continuous phase, which is armored by a reinforcement, i.e. a discontinuous phase. In cases where there are three or more constituents, the composite is termed to be a hybrid composite [43, 44]. Composites are available all around us in nature, e.g. wood, bone, tissue, etc. In industry, most of the composites are based on metals, ceramics, and polymers [17]. Composites can be classified in various ways. A useful and all-embracing classification can be made based on matrix and reinforcement, as shown in table 2. The MMCs containing reinforcement, e.g. ceramic particles, whiskers, and fibers, are gaining much importance these days. The CMCs are considered to be the newest entrants in the field [45]. In this study, particular focus is given to micromanufacturing of metallic- and ceramic-based composite components.

The need for new materials is constantly important to manufacturing industries. Better mechanical properties, reduced weight, and lower cost are the key factors for developing new materials [46]. Since current bulk materials eventually reach their limits, engineers are looking to composites to obtain extra strength, stiffness, and durability [47]. Metals and their alloys are largely manufactured and shaped in bulk form; however, they can also be intimately combined with another material in order to improve their performance. The resulting materials are MMCs [48]. Significant advancement in the development of MMCs has been achieved over the past few decades, with their incorporation into important industrial applications. These pioneering materials have opened up infinite possibilities for present material science and development [49]. The characteristics of MMCs can be designed into the material, custom-made, and dependent on the application [49]. When compared to polymer matrix composites, MMCs offer improved material properties. For example, compared to resin, metal matrices provide higher tensile and shear molduli, higher melting temperature, a lower thermal coefficient of expansion, better dimensional stability, better joinability, high ductility and toughness, and the ability to be fully dense [50, 51]. MMCs essentially consist of a metal or alloy as matrix material and a reinforcement of different kinds and shapes. Table 3 presents a detailed classification of MMCs with typical examples of materials used, their applications, and fabrication techniques.

Metal and metallic composites have been extensively used in industry; however, in certain applications, particularly involving high temperatures, they have reached a limit in their potential for further development. Ceramics, on the other hand, offer the advantage of operating at substantially high service temperatures. In addition, low density, high hardness, and chemical inertness extend potential ceramic performance limits beyond those achievable by metallic materials. Nevertheless, ceramic materials have intrinsic limitations of brittleness and poor strength reliability. In an effort to overcome these drawbacks, significant progress has been reported in the last two decades. The most important development appeared in the form of composites, i.e. the combination of multiple constituent phases with suitable microstructures in order to obtain the desired properties. Generally, ceramic composites are composed of two or more distinct ceramic phases combined on a microstructural scale to provide the properties that cannot be achieved by monolithic materials [63, 64].

In the arena of composite materials, CMCs are considered to be the latest entrants with a set of impressive properties and potential applications [45]. Due to advancements in the manufacturing process and technology, a wide variety of types of CMCs have come into use to meet the continuously growing industrial demand. These can be categorized into four major groups: (1) reinforced CMCs, (2) graded and layered composites, (3) refractory composites, and (4) nanostructured composites. Similarly, there are a number of different techniques for manufacturing CMCs, which are classified based on the materials used, kind of composites, typical applications, and fabrication principle. Table 4 summarizes different matrices and fiber materials used in CMCs and different fabrication techniques with examples.

Composite materials are popular because of their heavy industrial use, providing cheaper, lighter, and stronger alternatives to monolithic materials [76]. For example, the use of layered composite materials has recently received significant attention due to their lower cost and attractive mechanical, electrical, and magnetic properties. Laminated composite materials provide customizable properties for specific applications requiring high impact and fracture resistance [77, 78]. Such composite materials have a uniform distance between composite layers and are typically fabricated by physical vapor deposition (PVD) techniques, e.g. magnetron sputtering [79–81] and electron beam deposition [82–84]. Among mechanical means, rolling, hammering, and swaging are used to bond alternating sheets to fabricate a composite. However, rolling or roll bonding is reported to be one of the popular methods, due to good potential for commercialization due to its comparatively simple processing and low cost. Rolling can also be accompanied by a cold or hot system to customize the material functionalities. Usually, bottom-up PVD methods provide more refined and even microstructures while top-down mechanical techniques produce coarser, nonuniform microstructures. Nevertheless, uniformity of microstructures can be controlled to some extent during rolling by choosing materials with similar hardness and strain hardening rate, providing uniform layer deformation and reducing layer pinch-off.

Stover et al [85] fabricated a layered, microcomposite material of nickel (Ni) and Al using a repeated cold rolling method with initial billet thicknesses of 25 and 18 μm, respectively. They investigated the effects of thickness reduction and observed that at higher thickness reduction, less uniform layer deformation and more pinch-off occurs, compared to more gradual and smaller reduction at lower thickness reduction (figure 3). Eizadjou et al [86] implemented accumulative roll bonding (ARB) to fabricate an Al/Cu layered composite at room temperature and demonstrated the generation of nanostructured layered composite with superior properties. An increase in the number of passes may result in equiaxed grains with enhanced hardness [86]. As reported in [87], the flow stress ratio of two materials should be similar and chemically stable to achieve good metallurgical bonding between layers. Adding heat during microrolling can improve further grain refinement [88]. Another emerging approach is to add nanoparticles to improve the properties of composite materials. Yousefian et al [89] used titanium dioxide (TiO₂) nanoparticles and observed significant improvement in tensile strength of aluminum MMC during microrolling. Thus, there are numerous techniques that could be implemented to enhance the properties of composite materials. However, the choice of materials used for microrolling is still limited, leading to a considerable scope of future research to attempt different combinations of materials with a number of potential future applications. In addition, as the trend is to obtain finer grain structure by improving material properties, severe plastic deformation (SPD) could be used to fabricate composite materials. Though there has been remarkable progress in SPD of bulk materials, their use in composite materials is limited [90–92]. The application of SPD in microrolling of composite materials may significantly improve the material properties and, therefore, deserves further research.

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MDD is a fundamental microforming process and regarded as one of the most applicable sheet metal forming processes. It is widely used in many industries to produce microcomponents, such as microcylinder cups, rectangular cups, conical cups, spherical cups, hollows, and box-like parts [76, 93]. However, most of these microcomponents are made from bulk materials. Limited attention is paid to MDD of composite materials, though composite materials exhibit a number of attractive properties, as stated earlier. Laminated composite materials, for instance, can be used in manufacturing parts with different inner and outer conditions, such as corrosion, wear resistance, and thermal and electrical conductivities [94–96].

MDD of composite materials appears to be more complex than conventional forming methods. There are several associated parameters which cause problems. A number of defects may occur in the drawn parts, which must be accurately addressed and properly controlled to reduce the number of no-go parts as well as production costs. The parameters taken into account during MDD of composite materials include blank holding force (BHF), applied punch force, material properties of the blanks, thickness of the blanks, stacking sequence of the blanks, velocity of the punch, lubrication, and temperature conditions. In addition, the type of reinforcement used and its shape and size are also important. These factors are responsible for affecting the final products and can regulate the wrinkling effect, tearing effect, and fracture defects.

Many of these parameters are thoroughly discussed in conventional deep drawing as well as MDD of monolithic materials; however, they are not as developed in the case of composite materials. Jia et al [76] examined the effect of heating during MDD of 50 μm of an Al–Cu composite material (figure 4) and successfully fabricated a microcup without considerable fractures or wrinkles. Another important parameter is BHF, as reported in [94], which discusses MDD of an Al 1050 and stainless steel (SS) 304 laminate composite. Yin et al [18] examined the MDD of a cobalt (Co)-Al–Cu laminate composite and observed that an increase in holding time during heat treatment can improve the properties of the formed parts. Consequently, the interaction between the die and specimen plays a significant role in MDD of composite materials. The formability can also be greatly influenced by employing appropriate lubrication. Nanoparticle-based oil lubricant can improve drawability and reduce the forming load of composite materials, as observed in [93], which discusses MDD of an Al–Cu composite material. There has been some progress in MDD of composite materials; however, the literature still does not describe the interfacial behavior of different laminates and/or matrix-reinforcement interactions during the course of MDD. The material flow characteristics of different layers (prevention of defects, e.g. wrinkling and fractures) during MDD needs to be acutely analyzed. Investigation of the stress and strain distribution of the drawn composite parts also requires further research to develop this promising field.

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In addition to the aforementioned microforming techniques, microextrusion, microstamping, microbending, microembossing, micropunching, microblanking, and microcoining are also attracting attention in the forming of microproducts that have a variety of applications. However, not all of these techniques have been implemented in the manufacturing of composite-based microproducts. Examination of the potential for using the above techniques to fabricate microparts from composite materials is clearly lacking [97–104]. However, there have been some innovative microforming methods reported in recent years for the fabrication of composite parts. Patel et al [105] reported a novel microblast-driven microforming method and fabrication of a bismuth oxide (Bi₂O₃)-reinforced Al composite micropart. A mathematical model and critical forming processes have also been proposed. Bending and contact strengths of a carbon-reinforced silicon nitride-silicon carbide (Si₃N₄-SiC) composite were examined by Dusza et al [106] and its fracture mechanisms were illustrated. Cui et al [107] fabricated a microlaminated titanium boride-titanium aluminum (TiB-TiAl) composite sheet by employing a multistep heat treatment and pressing process. Zhang et al [108] conducted a simulation of a tin oxide/silver (SnO₂/Ag) particulate-reinforced MMC through microextrusion. The effects of the extrusion angle, the extrusion ratio, and the ram speed on the deformation and redistribution of particles during microextrusion were studied using the finite element (FE) method. When composite materials are microformed, the scenario may generally appear different than that of conventional techniques. The deformation behavior of reinforcement (particle distribution) used in the matrix plays a significant role in microforming. The size and type of reinforcement used and its interaction with the matrix material and tools may be a matter of substantial significance, which needs to be thorougly investigated.

The microstructure and grain distribution of composite materials play a significant role in the machining of composite materials. The grain size of commonly used engineering materials subject to micromachining falls in the range of 100 nm–100 μm [115, 116]. The radius of the tool edge (i.e. roundness) and feed rate value are frequently considered to be in the range of several hundreds of nanometers to several micrometers, which is also comparable to crystalline grain sizes. Therefore, the influence of grain size, grain distribution, and overall crystallographic nature of the composite materials plays a crucial role in micromachining, as detailed in [115, 117, 118]. It has been reported that a homogeneous grain size distribution has a positive effect on better dimensional accuracy and high surface quality. The influences of metallurgical phases on cutting forces were studied by Vogler et al [119]. An FE model was proposed in [120–122] to evaluate the stress, strain, temperature, and damage distribution due to changes in grain size and grain distribution.

Another important parameter is the reinforcement used in a composite. Because of the presence of particulates/fibers, the material removal rate and chip formation mechanism appear differently in composite materials. Teng et al [114] carried out an FE simulation for the cutting mechanism of SiC/Al MMCs reinforced with micro- and nanosized particles using the FE method (ABAQUS). They reported that nanosized particles remained intact without fracture during the machining process and were more likely to produce continuous chips, in contrast to microsized particles that were easy to break and tended to form discontinuous chips (figure 5). Thus, a better machined surface quality with less defects can be obtained from nanosized, reinforced MMCs compared to their microsized counterparts. Based on the literature surveyed, the machining mechanism for composite materials is not yet fully understood, especially for micro- and nanoparticle-reinforced composites. Further investigation (theoretical and experimental) is required to reveal the fundamentals of micromachining of composite materials, in terms of stress and strain distribution, tool wear, failure mode, chip formation, and particle behavior.

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Another important factor is the effect of strengthening. Machining is principally a process where materials are continuously or discontinuously fractured and then driven away under comprehensive fracture criteria [123]. The improved mechanical properties of composite materials, such as yield strength and toughness, considerably influence the material's fracture characteristics. Several authors have attempted to estimate the reinforced yield strength by virtue of different strengthening mechanisms [124–126]. There are three key strengthening mechanisms: (1) Orowan, (2) increased dislocation density, and (3) load-bearing strengthening [124–128]. Zhang et al [124] proposed a model to predict the yield strength of nanoparticle-reinforced MMCs and revealed that the increased yield strength is governed by a number of factors, such as size and volume fraction of nanoparticles used, the difference in CTE values between the fiber and matrix, and the change in temperature after processing. A mathematical equation proposed to predict the increased yield strength is shown in equation (1):

$$\begin{eqnarray}&&{\sigma }_{yc}={\sigma }_{ym}(1+{f}_{Orowan})(1+{f}_{dislocation})(1+{f}_{load-bearing}),\end{eqnarray} \tag{ 1 }$$

where ${\sigma }_{ym}$ is the yield strength of the matrix material, and ${f}_{Orowan},$ ${f}_{dislocation},$ and ${f}_{load-bearing}$ represent the aforementioned three strengthening mechanisms.

However, when dealing with practical conditions, the process of material removal appears much more intricate because of the complicated microstructural effects and cannot be explained by only yield strength. Therefore, further study is necessary to understand the fracture mechanism and chip formation during machining of composite materials [123–132].

Components made from laminate composites often need to be microfeatured through various machining operations, such as microperforation, microsawing, microrouting, and microgrinding. However, microperforation, or making a hole, is perhaps the most important and frequently used machining technique in laminated components [133]. Delamination may appear during machining of laminate composites which results in severe reductions in the load-carrying capacity of the component and, therefore, must be avoided. Delamination not only decreases assembly tolerance and bearing strength but also has the potential for long-term performance deterioration under fatigue loads [134–136]. Delaminations may be initiated by three mechanisms, as shown in figure 6: peeling up of the top layer, punching out of the uncut layer near the exit, and the thermal stress mode [137, 138]. Peel-up delamination occurs around the entry periphery of drilled holes (figure 6(a)). When the drill edge contacts the surface, a peeling force through the slope of the drill bit flutes results in separation of the plies from each other. Push-out delamination occurs at the exit periphery around the drilled holes (figure 6(b)). Thermal stress mode appears due to the heat that is generated during drilling because of the high-speed tool-specimen contact (figure 6(c)). It has been found that the delamination associated with push-out is more severe than with other mechanisms. Henceforth, most of the studies have paid more attention to push-out delamination [139–144].

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In many cases, micromachining of composite materials using conventional techniques or tool materials is difficult due to the presence of the abrasive reinforcing constituents, which may cause several problems, such as delamination, poor surface quality, and severe tool wear [145, 146]. Furthermore, conventional material removal methods often introduce surface flaws, cracks, and residual stresses in composite materials [146, 147]. In some cases, such as difficult-to-machine parts and ceramic-based brittle-like composites, conventional techniques fail to provide the desired machining performance. Many such limitations were overcome by the advent of nonconventional micromachining techniques. At present, there are a number of different nonconventional micromachining techniques being extensively used in many industrial applications, such as laser, electrical discharge machining (EDM), electrochemical machining (ECM), abrasive jet machining, and microgrinding. These processes are used to perform precision machining of composite materials where the material removal process is not affected by hardness, strength, or toughness of the specimen materials [148, 149].

Laser micromachining is an important modern technology for machining difficult-to-machine composite materials. Lie et al [151] produced holes of a few hundred micrometers in diameter in SiC/SiC composite using a picosecond laser. This was also done by Biswas et al [145] to microcut Al₂O₃-Al composite. Wang et al [150] combined laser heating with conventional machining for cutting silicon copernicium (SiCp)/2024 Al composite. Figures 7(a) and (b) present some typical examples of laser machining. Similarly, EDM is also used to machine parts made of composite materials. In this process, no mechanical force is applied; and it is independent of the specimen's hardness. Li et al [152] produced microholes in zirconium diboride (ZrB₂)-SiC-graphite composite using EDM. Paul et al [153] investigated microdrilling of newly developed SiC-20% boron nitride composite. Similarly several authors examined EDM micromachining of composite materials, such as SiCp–Al [149], SiC–titanium carbide nitride (Ti₂CN) [154, 155], and SiC/Al [147]. Consequently, ECM was reported to be one of the most suitable processes for difficult-to-machine materials without a heat-affected zone. This process has been well exploited in various applications, offering a higher machining rate, better precision, and good control over the machined surface [156, 157]. Examples include machining a 400 μm hole in Al-Al₂O₃-boron carbide (B₄C) hybrid MMCs [158], AA6061-titanium diboride (TiB₂) [157], and Al-6% MMCs [159]. For machining glass and glass fiber-reinforced plastic (GFRP) composites, abrasive jet machining could be a suitable technique, as reported in [160]. Likewise, ultrasonic grinding is a preferred method for machining ceramic-based composites, as reported by Zhao et al [161], and grinding Al₂O₃-zirconia (ZrO₂). Although the above results present various techniques for micromachining of composite materials, the fundamental mechanisms are not yet well understood [39, 40]. In addition, studies on the machinability of microreinforcement composite materials are plentiful; and there is inadequate literature on nanoreinforced composite materials. Therefore, further research is necessary to gain a comprehensive understanding of nanoreinforcement of composite materials.

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Size effects can be generally defined as the deviations from the expected results which occur when the geometrical dimension of a process or specimen is changed or typically reduced [230]. There are three main categories of size effects based on density, shape, and microstructure [230], as presented in figure 13. Accordingly, in microforming, there is another type of size effect called the tribological size effect [14]. These size effects generate a number of different problems, including mechanical, tribological, and scatter of material behaviors, which have been extensively studied by many authors [73, 231]. Subsequently, in order to deal with size effects, various strategies have been reported, such as microstructural refinement [103] and applying heat during micromanufacturing [232]. Size effects in composite materials appear to be even more serious; particularly the size and amount of reinforcement used in the composite which are important factors, as detailed in Wisnom et al [233]. It was reported that there is a tendency for the strength to decrease with increasing specimen volume. It was also found that the size effect reduces with increasing scale. However, scaling laws in composite materials are not straightforward, rather they are substantially intricate due to the heterogeneous nature of their microstructures involving variations in fiber diameter and length, fiber/matrix interface, ply layer, free edge effects, and stress gradients. Size effects in composite materials also depend on the length and diameter of fibers, their volume fraction, and the manufacturing technique used [234, 235]. Presently, a major worldwide research effort is underway regarding the possibility of producing very high-strength composites in fiber, whisker, or laminate form. A significant diameter size effect in such composites might well provide a complementary avenue for improved strength properties by obtaining finer filaments.

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The characteristics of a composite material can be attributed to three main factors: (1) the reinforcing element or fiber, (2) the matrix material, and (3) the fiber/matrix interface. The fiber/matrix interface in composite materials is of great importance as the internal surface area occupied by the interfaces is considerably high. For example, for a composite material containing a moderate fiber fraction, it can be as high as 3000 cm² cm⁻³ [51]. The factors that influence the interface area in a composite include surface roughness of the reinforcements (most fibers or reinforcements show some degree of roughness [236]), crystallographic nature of the interface, and interactions at the interface. Figure 14 shows a typical example of an interface of a composite material. The bonding that takes place at the interface can be of the mechanical, physical, or chemical type. It should be noted that maximizing the bond strength is not always the objective. If the interface is too strong, i.e. stronger than the reinforcement, it will cause embrittlement; and the interface will have the lowest strain to failure. Therefore, an interface with an optimum interfacial bond strength is preferred with an enhanced toughness but without much sacrifice on the strength parameters. Optimum interfacial bond strength can be obtained in two ways: treatment of the fiber or reinforcement surface or modification of the matrix composition. Therefore, during micromanufacturing, close attention should be given to make sure the optimum interface strength is maintained in order to obtain the desired quality of the composite materials.

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In composite materials, residual stresses (RSs) can be generated by thermal mismatch between the reinforcement and matrix materials or between alternating sheets during the manufacturing process. It may severely impact the strengthening of microcomposite components. Therefore, unlike conventional methods, micromechanics-based simulation methods are preferred for developing a comprehensive analysis of the effects caused by thermal RSs during the manufacturing of microcomposites. It is reported that thermal RSs can considerably decrease the yield strength and ultimate tensile strength of composite materials. Therefore, consideration of thermal stresses is essential for a realistic prediction of the overall elastoplastic characteristics of composite materials during micromanufacturing [238].

Haghoo et al [239] conducted a simulation on carbon nanotube (CNT)-reinforced Al composites. The variables considered included fiber volume fraction (FVF), aspect ratio and directional behavior of CNTs, degree of CNT agglomeration within the matrix on the elastic modulus, thermal expansion behavior, and the overall elastoplastic response of CNT-reinforced Al composite materials. Aghdam et al [240] simulated the influences of manufacturing parameters and FVF on residual stresses in SiC/Ti composites. The effects of the coating, interaction layer, and stress relaxation were also considered. As can be seen in figure 16, a 3D representative volume element (RVE) consisting of a quarter of fiber, coating, interaction layer, and corresponding matrix material was used to evaluate RSs within the MMC. Stress contours for the axial stress distributions in the fiber and matrix of a SiC/Ti-alloy composite system are plotted in figures 18(c) and (d). Dimensions of the constituent of the RVE were obtained by calculating the fiber volume fraction using the following equations:

$$\begin{eqnarray}&&\displaystyle \frac{\pi {R}_{1}^{2}}{4{a}^{2}}={v}_{f}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{R}_{2}={R}_{1}\sqrt{(1-{\nu }_{c})},\end{eqnarray} \tag{ 3 }$$

where v_f and v_c are the fiber and coating volume fractions, respectively, R₁ and R₂ are the outer and inner radius of the coating, and a denotes the width of the RVE, as shown in figure 18.

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Jia et al [76] developed an FE simulation model to examine the deformation behavior of a two-layer Al–Cu composite (∼50 μm thickness) during MDD by employing a continuum shell element in ABAQUS software (figure 17). The Voronoi tessellation model was introduced to represent the grains of the laminate composite material for addressing the size effects of the blank during micromanufacturing. Each Voronoi tessellation was assigned with different mechanical properties based on experimental data, thereby conserving the grain heterogeneity. Therefore, accurate results can be obtained [241, 242]. Application of nanolubricants during MDD of an Al–Cu composite was investigated in [93], and significant improvement in the formability of the drawn parts was reported. Adding nanolubricant also reduces the drawing force, providing more uniform thickness distribution, and improves the surface quality of the microcomponent drawn. According to the literature reviewed, the existing simulation works are mostly based on microforming techniques, particularly microrolling and MDD. There are also a few research studies on various composite materials, including MMCs and CMCs, to evaluate fiber/matrix interactional behaviors during their manufacturing processes. However, a limited number of studies were observed to simulate other micromanufacturing techniques, such as micromachining and microcasting of composite materials. This leads to a substantial scope of further research, with numerous possibilities for obtaining exciting results to solve many of the practical problems.

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A composite of WC and high strength steel could be developed to manufacture a composite microdrill (CMD) with outstanding overall performance. The outer WC with nanocrystalline grains can offer high hardness, wear-resistance, and rupture strength, while the inner steel material could provide high strength and fracture toughness. A direct powder/solid, consolidation-extrusion forming technology could be implemented to fabricate a CMD. The schematic diagram of the proposed microextrusion system is presented in figure 18. Die 1 will be used for powder solidification and extrusion forming, while dies 2 and 3 will be used to fabricate the CMD. Dies 2 and 3 will be removable and could be disassembled conveniently when the extrusion process is complete. The values l_s, l_f, and D represent the shank length, flue length, and diameter of the CMD produced, respectively. Microdrills several hundreds of micrometers in diameter will be produced. Die 3 will be rotatable and is designed as split construction with cutting grooves because of the undercut on the part during the extrusion process. Die 3 will be designed based on the flute structure of the CMD produced.

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In addition, FE modeling could be employed to simulate the behavior of WC powder and the bonding phenomena with high strength steel. Constitutive relationships that describe the behaviors of both materials during solidification could be included in the FE model. Because the material properties of the steel and WC are very different, nonuniform deformation will occur during the composite extrusion processes. Consequently, in microextrusion, the size of the deformation zone is comparable to the size of constituent grains; and, therefore, grain orientation has an impact on strain distribution. Each single grain has to deform not only in accordance with the shape of the tool but also its favorable orientation. The random orientation and size of each single grain leads to inhomogeneous material behavior, so that scatter increases with decreasing specimen size. Therefore, a careful consideration of these parameters is necessary for successful simulation as well as experimental investigation to fabricate such a microproduct, due to future evolutionary prospects.