**1. Introduction**

Among the advanced particulate reinforced metal matrix composites (PRMMCs), Al/SiC composite is the most commonly studied one due to its potentially high tensile strength and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

elastic modulus at room or elevated temperatures, low thermal expansion coefficient, high thermal and electrical conductivity, excellent corrosion resistance, good wear resistance, mechanical properties, ductility, low cost and wide range of applications [1–3]. Normally, two routes (i.e. powder metallurgy (PM) and melt based approach) are used to fabricate PRMMCs with 'net shape' or 'near-net shape' forming. In the fabrication of Al/SiC composites via a melt process, SiC often reacts with molten Al to degrade the reinforcement strength and the interfacial strength [4], and the uniform distribution of SiC particles especially those with nanosizes are difficult to be realized. This deficiency can be largely avoided by PM process which can offer more control over reinforcement distribution and require less energy input than the conventional foundry route. A common PM process of Al/SiC composite consists of cold compaction in a closed-die or in an isostatic pressing followed by sintering. To fabricate Al/SiC composites with superior performances, large amount of physical and numerical work was carried out using PM method in the past few decades.

distribution of nano SiC particles. Jamaati et al. [8] investigated the effects of SiC particle size (2 and 40 μm, respectively) on microstructure and mechanical properties (tensile strength and elongation) of Al/SiC (with 10 vol.% SiC) composite fabricated by accumulative roll bonding. It was found that the composite strip with 40 μm particle size became uniform with high bonding quality and without any porosity sooner than the strip of 2 μm particle size. For both

MPFEM Modeling on the Compaction of Al/SiC Composite Powders with Core/Shell Structure

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To determine the compressibility behavior of Al-Cu/SiC composite powder mixtures which include 4 wt.% (weight fraction) Cu and 5–20 wt.% SiC, a double action die compaction with the pressure ranging from 50 to 450 MPa was performed by Ghiţă and Popescu [9]. And empirical equations were proposed to describe densification mechanism of the composite powders and predict the optimal pressure applied. Li et al. [10] carried out physical experiments to study the distributions of SiC particles in different positions of the Al/SiC composite samples (with 35 vol.% SiC) formed by equal channel angular pressing and torsion (ECAPT) and found that the shear strain could create significant influences. And from the compaction stage to the angular pressing stage during ECAPT, the distribution homogeneity of SiC particles increases greatly, implying that this forming method can realize relatively homogeneous SiC distribution. With the addition of nanosized SiC particles (0–7 wt.%), Moazami-Goudariz and Akhlaghi [11] conducted physical experiments on the compaction of Al/SiC composites. In their work, the effects of morphology, microstructure, size, apparent density, flowability, and hardness of the produced powder mixtures on their compaction behavior were investigated. The results showed that the chemical composition and the nano SiC content created effects on the compaction behavior as well as properties of the compact. Al/SiC composites with different contents (up to 15 vol.%) and sizes (3, 6, and 11 μm) of SiC particles were fabricated using conventional PM route [12], where the effects of the size and content of SiC particulates on the microstructural and corrosion behavior of the composite were studied. Meanwhile, El-Kady and Fathy [13] also studied the effects of SiC particle size (70 nm, 10, and 40 μm) and content (5 and 10 wt.%) on both physical and mechanical properties of Al/SiC nanocomposites produced with PM followed by hot extrusion. Majzoobi et al. [14] investigated the tribological properties of Al/SiC nanocomposite prepared by hot dynamic compaction (with the strain rate of 10<sup>3</sup> s−1), where the content of SiC nanoparticles was respectively 0, 5, and 10 vol.%. After

sizes, different cycles could lead to different tensile strength and tensile elongation.

compaction, the relative density of the composite compact can be up to 98%.

Recently, the mechanical properties and corrosion behavior of Al/SiC (comprising 20 vol.% SiC) composites fabricated by vacuum hot pressing sintering at 700°C under a pressure of 20 MPa were studied by Zhang et al. [15], in their work the high relative density of 99.65 ± 0.08% for the Al/SiC composite can be obtained. Cold isostatic compaction of Al/SiC composite powders with different content of nano SiC particulates as the reinforcement was experimentally performed by Bajpai et al. [16], where various properties such as hardness, density, porosity, compressive strength, indirect tensile strength and the microstructure of the samples were measured and characterized. The micrograph shows the uniform distribution of nano SiC particles in the aluminum matrix. With micro wave sintering and hot extrusion, Penchal Reddy et al. [17] fabricated nano-sized SiC (0, 0.3, 0.5, 1.0 and 1.5 vol.%) reinforced Al metal matrix composites and studied the structural, mechanical and thermal properties of the developed Al/SiC nanocomposites. Results indicated that hot extruded

Physically, many researchers studied the effects of SiC content and particulate sizes using various forming methods. Ling et al. carried out experiments to study the PM fabrication of Al/ SiC composites with SiC content ranging from 0 to 30 vol.% (volume fraction), where four PM methods (such as sintered, cold isostatic pressed (CIPed) and sintered, hot isostatic pressed (HIPed), and sintered plus HIPed in the same HIP cycle) were considered and the results of relative density, mechanical properties, and fractography were characterized and compared [1]. They found that the sintered plus HIPed technique can yield the best bulk composites. When the SiC content is within 10 vol.%, the matrix is more likely suffered to ductile failure. With a higher SiC content, the factors such as the interfacial bond strength, pore structure evolution, and the cracking within particles can all determine the mechanical properties of the composite products. For the Al/SiC composite powder comprising 40 vol.% SiC, Sridhar and Fleck respectively performed isostatic and closed-die compaction [5]. They found that for a given SiC powder content, the compaction pressure to achieve a given relative density increases with the decrease of the SiC particle size. The measured yield surfaces after each compaction indicated that the shape depends on the deformation path, with greatest hardening along the loading direction. Tavakoli et al. [2] studied the consolidation behavior of Al/SiC composite powders (with reinforcement SiC up to 50 vol.%) during pressure cycling (ranging from 90 to 360 MPa with 1 Hz) at room temperature in uniaxial compaction experiments to address the effects of compaction mode and SiC content on the densification, microstructure and mechanical properties of Al/SiC composites. And corresponding comparisons were made with monotonic compaction. The results showed that the pressure cycling can enhance the densification of Al/SiC composite powder, and the densification rate increases with the SiC content. Using HIP, Tang et al. [6, 7] studied the consolidation of Al/SiC composite powder with 6.5 vol.% nano-sized SiC particles (25 nm in size) synthesized via cryomilling and followed by hot rolling. Microstructural investigation indicated that the nano-sized SiC particles had been dispersed homogeneously in the reinforced regions in the composites and the tensile strength of the composite was improved greatly. However, coarse-grained SiCp-free regions were observed to be formed during HIPing, which improved the ductility but to a certain degree decreased the strength. In short, SiC nanoparticles located at grain boundaries can contribute to limiting grain growth, but it is difficult to achieve the complete uniform distribution of nano SiC particles. Jamaati et al. [8] investigated the effects of SiC particle size (2 and 40 μm, respectively) on microstructure and mechanical properties (tensile strength and elongation) of Al/SiC (with 10 vol.% SiC) composite fabricated by accumulative roll bonding. It was found that the composite strip with 40 μm particle size became uniform with high bonding quality and without any porosity sooner than the strip of 2 μm particle size. For both sizes, different cycles could lead to different tensile strength and tensile elongation.

elastic modulus at room or elevated temperatures, low thermal expansion coefficient, high thermal and electrical conductivity, excellent corrosion resistance, good wear resistance, mechanical properties, ductility, low cost and wide range of applications [1–3]. Normally, two routes (i.e. powder metallurgy (PM) and melt based approach) are used to fabricate PRMMCs with 'net shape' or 'near-net shape' forming. In the fabrication of Al/SiC composites via a melt process, SiC often reacts with molten Al to degrade the reinforcement strength and the interfacial strength [4], and the uniform distribution of SiC particles especially those with nanosizes are difficult to be realized. This deficiency can be largely avoided by PM process which can offer more control over reinforcement distribution and require less energy input than the conventional foundry route. A common PM process of Al/SiC composite consists of cold compaction in a closed-die or in an isostatic pressing followed by sintering. To fabricate Al/SiC composites with superior performances, large amount of physical and numerical work

Physically, many researchers studied the effects of SiC content and particulate sizes using various forming methods. Ling et al. carried out experiments to study the PM fabrication of Al/ SiC composites with SiC content ranging from 0 to 30 vol.% (volume fraction), where four PM methods (such as sintered, cold isostatic pressed (CIPed) and sintered, hot isostatic pressed (HIPed), and sintered plus HIPed in the same HIP cycle) were considered and the results of relative density, mechanical properties, and fractography were characterized and compared [1]. They found that the sintered plus HIPed technique can yield the best bulk composites. When the SiC content is within 10 vol.%, the matrix is more likely suffered to ductile failure. With a higher SiC content, the factors such as the interfacial bond strength, pore structure evolution, and the cracking within particles can all determine the mechanical properties of the composite products. For the Al/SiC composite powder comprising 40 vol.% SiC, Sridhar and Fleck respectively performed isostatic and closed-die compaction [5]. They found that for a given SiC powder content, the compaction pressure to achieve a given relative density increases with the decrease of the SiC particle size. The measured yield surfaces after each compaction indicated that the shape depends on the deformation path, with greatest hardening along the loading direction. Tavakoli et al. [2] studied the consolidation behavior of Al/SiC composite powders (with reinforcement SiC up to 50 vol.%) during pressure cycling (ranging from 90 to 360 MPa with 1 Hz) at room temperature in uniaxial compaction experiments to address the effects of compaction mode and SiC content on the densification, microstructure and mechanical properties of Al/SiC composites. And corresponding comparisons were made with monotonic compaction. The results showed that the pressure cycling can enhance the densification of Al/SiC composite powder, and the densification rate increases with the SiC content. Using HIP, Tang et al. [6, 7] studied the consolidation of Al/SiC composite powder with 6.5 vol.% nano-sized SiC particles (25 nm in size) synthesized via cryomilling and followed by hot rolling. Microstructural investigation indicated that the nano-sized SiC particles had been dispersed homogeneously in the reinforced regions in the composites and the tensile strength of the composite was improved greatly. However, coarse-grained SiCp-free regions were observed to be formed during HIPing, which improved the ductility but to a certain degree decreased the strength. In short, SiC nanoparticles located at grain boundaries can contribute to limiting grain growth, but it is difficult to achieve the complete uniform

was carried out using PM method in the past few decades.

22 Powder Technology

To determine the compressibility behavior of Al-Cu/SiC composite powder mixtures which include 4 wt.% (weight fraction) Cu and 5–20 wt.% SiC, a double action die compaction with the pressure ranging from 50 to 450 MPa was performed by Ghiţă and Popescu [9]. And empirical equations were proposed to describe densification mechanism of the composite powders and predict the optimal pressure applied. Li et al. [10] carried out physical experiments to study the distributions of SiC particles in different positions of the Al/SiC composite samples (with 35 vol.% SiC) formed by equal channel angular pressing and torsion (ECAPT) and found that the shear strain could create significant influences. And from the compaction stage to the angular pressing stage during ECAPT, the distribution homogeneity of SiC particles increases greatly, implying that this forming method can realize relatively homogeneous SiC distribution. With the addition of nanosized SiC particles (0–7 wt.%), Moazami-Goudariz and Akhlaghi [11] conducted physical experiments on the compaction of Al/SiC composites. In their work, the effects of morphology, microstructure, size, apparent density, flowability, and hardness of the produced powder mixtures on their compaction behavior were investigated. The results showed that the chemical composition and the nano SiC content created effects on the compaction behavior as well as properties of the compact. Al/SiC composites with different contents (up to 15 vol.%) and sizes (3, 6, and 11 μm) of SiC particles were fabricated using conventional PM route [12], where the effects of the size and content of SiC particulates on the microstructural and corrosion behavior of the composite were studied. Meanwhile, El-Kady and Fathy [13] also studied the effects of SiC particle size (70 nm, 10, and 40 μm) and content (5 and 10 wt.%) on both physical and mechanical properties of Al/SiC nanocomposites produced with PM followed by hot extrusion. Majzoobi et al. [14] investigated the tribological properties of Al/SiC nanocomposite prepared by hot dynamic compaction (with the strain rate of 10<sup>3</sup> s−1), where the content of SiC nanoparticles was respectively 0, 5, and 10 vol.%. After compaction, the relative density of the composite compact can be up to 98%.

Recently, the mechanical properties and corrosion behavior of Al/SiC (comprising 20 vol.% SiC) composites fabricated by vacuum hot pressing sintering at 700°C under a pressure of 20 MPa were studied by Zhang et al. [15], in their work the high relative density of 99.65 ± 0.08% for the Al/SiC composite can be obtained. Cold isostatic compaction of Al/SiC composite powders with different content of nano SiC particulates as the reinforcement was experimentally performed by Bajpai et al. [16], where various properties such as hardness, density, porosity, compressive strength, indirect tensile strength and the microstructure of the samples were measured and characterized. The micrograph shows the uniform distribution of nano SiC particles in the aluminum matrix. With micro wave sintering and hot extrusion, Penchal Reddy et al. [17] fabricated nano-sized SiC (0, 0.3, 0.5, 1.0 and 1.5 vol.%) reinforced Al metal matrix composites and studied the structural, mechanical and thermal properties of the developed Al/SiC nanocomposites. Results indicated that hot extruded Al/SiC nanocomposites (with 1.5 vol.% SiC) exhibited the best mechanical and thermal performance as compared to the other developed Al/SiC nanocomposites.

Al/SiC binary powder mixtures, there are still some problems left unsolved, which include: (1) it's really difficult to precisely control the distribution of SiC reinforced particles; (2) it cannot realize full densification especially when SiC particles formed agglomeration or clusters; (3) the local density and stress distributions in the compact are non-uniform. With these problems, a new binary Al/SiC composite constructed by the compaction of powders with core (SiC)/shell (Al) structures were designed and the whole process was simulated by MPFEM method from

MPFEM Modeling on the Compaction of Al/SiC Composite Powders with Core/Shell Structure

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In this chapter, uniaxial die compaction of Al/SiC composite core/shell powders with different initial packing structures was numerically reproduced using DEM-FEM coupled MPFEM modeling from particulate scale. The effects of external pressure, initial packing structure, and SiC content (composition) on the packing densification were systematically presented. Various macro and micro properties such as relative density and distribution, stress and distribution, particle rearrangement (e.g. sliding and rolling), deformation and mass transfer, and interface behavior between particles were characterized and analyzed. Some interesting results have been obtained, which can provide the materials scientists and engineers with valuable references to the realiza-

tion of fully dense and high performance Al/SiC composite compacts in PM production.

The simulation method used in current work is MPFEM. In this method, the initial random powder packing is firstly generated by DEM and then imported into FEM model, where each particle is fully discretized into finite element meshes. **Figure 1** respectively gives the schematic diagram of an individual composite particle with core/shell structure and corresponding mesh division as well as the numerically generated initial packing structure in the closed die before compaction when the SiC content is 25 vol.%, where each core (SiC)/shell (Al) composite particle includes respectively 200/1700 nodes and 173/1552 elements. After all the parameters and conditions are determined, the program will be complied and run in the commercialized MSC.Marc software. For simplicity, the details of DEM model are not given here, interested readers can refer to [26–29, 45] for more information. In comparison, each initial ordered binary packing is generated by the intrinsic function in MSC.Marc software based on geometry. The initial random or ordered packing structure was then imported into MPFEM model as the input. **Figure 2** shows the packing morphologies of Al/SiC composite powders before compaction. Here three initial ordered packings, i.e. simple cubic (SC), hexagonal close packed (HCP) and honeycomb structures are considered. The composition of the composite powder can be adjusted by the thickness of Al shell, which is represented by *R* – *r*, where *R* and *r* are respectively the radius of the composite Al/SiC particle and the radius of SiC particle therein. It needs to clarify that the SiC content used in this chapter all refers to volume fraction. For each initial packing structure before compaction, corresponding parameters such as the size ratio between the radius of SiC particle and the radius of the whole composite particle (*r*/*R*), SiC content (vol.%), and packing density (or relative density) are listed in **Table 1**.

particulate scale. Currently, no relative literature has been found.

**2. Simulation method and conditions**

**2.1. Simulation method**

Aforementioned researches in physical experiments indicate that most of the previous work was mainly focusing on the sintering stage or the forming stage, comprehensive studies on the compaction of Al/SiC composite powders are less conducted. Actually, most of the densification takes place in the compaction stage by rate-independent plasticity [1]. And the relative density (defined as the volume of the powder divided by the volume occupied by the die) and corresponding packing structure of the compact can determine the subsequent sintering process as well as the final properties of the sintered component. Therefore, the researches on the compaction of Al/SiC composite powders when subjected to external energy have increasingly attracted the materials scientists and engineers' interests in the past few years. Nevertheless, even though physical experiments can reproduce the relationship between relative density and compaction pressure and/or temperature, they are unable to quantitatively characterize the local density distribution, stress distribution, and particle motion behavior for pore (or void) filling in situ, especially the nonlinearity features in geometry, materials, and contact during compaction all increase the difficulties of physical experiments [18–22]. Most importantly, it's really hard for researchers to accurately control the uniform distribution (ordered or disordered) of reinforcement (SiC) in the metal (Al) matrix, these disadvantages in physical experiments can be conquered by the so called numerical simulations.

Numerically, various models or methods were proposed or used to simulate powder compaction densification in PM process. For example, a traditional macro continuous FEM (finite element method) simulation model, in which the powder mass is regarded as a continuum with uniform void distribution, was proposed to solve the problems arising from physical experiments. In addition to the relationship between overall relative density and compaction pressure, this method can also be used to analyze local relative density and distribution, stress distribution, and powder displacement in the compact upon compaction from macro continuous scale. Therefore, as reported in the authors' previous researches, the single-action die compaction of pure metal powders [23, 24] and composite powders [25] has been systematically investigated by this method. Even though the traditional FEM can to some extent solve the problems in physical experiments, it is really hard to deal with the important issues like dynamics and contact mechanics from particulate scale based on the aforementioned continuum assumptions. However, this will be overcome by molecular dynamics based DEM (discrete element method) simulation. DEM has been widely applied to generate various packing structures of spherical and non-spherical particles [26–29], but its effectiveness in modeling the compaction of powders is restricted to limited relative density (e.g. *ρ* < 0.85) [30]. For higher relative density and extra large plastic deformation in PM compaction, a new method MPFEM (multi-particle FEM) has been developed and successfully applied in modeling this process [31–43], but less work was conducted on the compaction of Al/SiC composite powders from particulate scale. Recently, using MPFEM the authors successfully modeled the single action die compaction of Al/SiC composite mixtures [44], and the results show that the packing state, size and number of SiC particles in the initial packing structure can significantly influence not only the densification of the Al/SiC composite powders upon compaction but also the properties of the compacts. Even though our previous numerical simulation can reproduce the compaction of Al/SiC binary powder mixtures, there are still some problems left unsolved, which include: (1) it's really difficult to precisely control the distribution of SiC reinforced particles; (2) it cannot realize full densification especially when SiC particles formed agglomeration or clusters; (3) the local density and stress distributions in the compact are non-uniform. With these problems, a new binary Al/SiC composite constructed by the compaction of powders with core (SiC)/shell (Al) structures were designed and the whole process was simulated by MPFEM method from particulate scale. Currently, no relative literature has been found.

In this chapter, uniaxial die compaction of Al/SiC composite core/shell powders with different initial packing structures was numerically reproduced using DEM-FEM coupled MPFEM modeling from particulate scale. The effects of external pressure, initial packing structure, and SiC content (composition) on the packing densification were systematically presented. Various macro and micro properties such as relative density and distribution, stress and distribution, particle rearrangement (e.g. sliding and rolling), deformation and mass transfer, and interface behavior between particles were characterized and analyzed. Some interesting results have been obtained, which can provide the materials scientists and engineers with valuable references to the realization of fully dense and high performance Al/SiC composite compacts in PM production.
