Chiang Mai Journal of Science

Fabrication of SiCp/6092Al Graded Composite Materials Via ECAP Process

1. INTRODUCTION

     SiC particle-reinforced aluminum matrix (SiCp/Al) composite gradient materials, as a critical branch of Functionally Graded Materials (FGMs), have garnered significant attention in aerospace, defense, and high-end equipment manufacturing in recent years [1-3]. While traditional homogeneous SiCp/Al composites exhibit advantages such as high specific strength, low thermal expansion coefficient, and excellent wear resistance, their isotropic characteristics struggle to meet the gradient performance requirements under complex working conditions. For instance, spacecraft thermal protection systems demand multifunctional integration including surface ablation resistance, intermediate-layer high thermal conductivity, and substrate toughening [4-6]. Additionally, challenges such as inadequate interfacial bonding strength between SiC particles and the aluminum matrix, along with thermal stress concentration in conventional composites, often lead to material failure. Against this backdrop, gradient design strategies that regulate the spatial distribution and volume fraction of SiC particles enable continuous transitions in mechanical and thermal properties, effectively mitigating interlayer stress discontinuities and enhancing structural reliability. Nevertheless, current research still faces challenges in gradient structure optimization theories, multi-scale interfacial regulation mechanisms, and efficient fabrication processes (e.g., powder lamination sintering, centrifugal melt infiltration, 3D printing). Particularly, balancing interfacial compatibility between gradient layers with manufacturing costs, and establishing quantitative correlation models between gradient parameters and macroscopic properties remain unresolved [7-10]. Therefore, advancing cross-scale design and performance synergy optimization studies of SiCp/Al composite gradient materials will not only propel fundamental theoretical innovations in high-performance metal matrix composites but also provide technical support for engineering applications in next-generation ultra-lightweight impact-resistant structures and high-power electronic packaging devices [11-14].
     Severe Plastic Deformation (SPD) refers to inducing intense plastic deformation in materials under quasi-static pressure, enabling the production of ultrafine-grained materials with submicron or nanometer-scale grain structures. This process significantly enhances the overall performance of alloys. Common SPD techniques include: Equal Channel Angular Pressing (ECAP), High Pressure Torsion (HPT), Cyclic Extrusion Compression (CEC), Accumulative Roll Bonding (ARB) [15-18]. Compared to other SPD methods, ECAP offers the following advantages: ①Simple die structure and low equipment requirements; ②Retention of the specimen’s cross-sectional dimensions post-processing, allowing repeated cumulative deformation after minor surface polishing; ③Uniform microstructural properties after multiple deformations and the ability to produce large-sized specimens.
     In this paper, the gradient composites of SiCp/6092Al were prepared using the severe plastic deformation of ECAP method based on the powder metallurgy method, and the interface bonding characteristics of gradient materials was revealed through compression tests, tension tests, hardness tests, and microstructure observation. The performance characteristics were compared and analyzed, laying the foundation for the later formation of gradient material parts.

2. MATERIALS AND METHODS

     SiCp/6092Al composite material was prepared by powder metallurgy process, which mainly includes powder mixing, cold pressing, hot pressing sintering, extrusion and heat treatment [19-22]. The process is shown in the Figure 1. The heat treatment system adopts the solid solution and aging method, with a solid solution system of 520°C+1.5h, followed by quenching treatment immediately after solid solution, with an interval of less than 5s, the aging system is 170°C+10h. The SEM diagram of initial organization were showed in the Figure 2. From the SEM image, it can be seen that with the increase of silicon carbide content, the agglomeration phenomenon tends to be severe, accompanied by the generation of defects such as cracks.

     The SiCp/6092Al composite material prepared by powder metallurgy process will be used to prepare gradient materials by ECAP method. The matrix process is shown in the Figure 3. Firstly, 15%, 20%, and 25% content SiCp/6092Al composite materials will be prepared separately by powder metallurgy process. Then, the prepared composite materials will be stacked in a sandwich form and gradient composite materials will be prepared by ECAP method. Finally, performance testing and microstructure evolution analysis will be carried out.

     The cross-sectional schematic diagram of ECAP and extrusion route is illustrated in Figure 4 [23–28].
Numerous experiments have shown that the BC route is more likely to form large angle grain boundaries, which can achieve optimal microstructural uniformity.
     The shear strain accumulated during ECAP is influenced by multiple parameters, and its relationship with the number of passes N, the die channel angle φ (inner angle), and the outer corner angle Ψ can be described by the following formula (1-1) [30–32].

$$ \varepsilon = N \left[ \frac{2 \cot(\varphi/2 + \Psi/2) + \Psi \csc(\varphi/2 + \Psi/2)}{\sqrt{3}} \right] $$ (1-1)

     In the formula: ε – strain resulting from multiple-pass extrusion; N – number of extrusion passes; φ – included angle of the die channel; Ψ – the angle subtended by the connecting arc between the outer boundaries of the die channel.
     Based on the finite element simulation software, construct an ECAP process model to simulate and study the deformation laws under different deformation parameters.

2.2 Experiment Introduction
2.2.1 Hot compression experiment
     The experimental material was SiCp/6092Al composite, with its matrix material main chemical compositions (mass fraction) listed in Table 1. The specimens were cylindrical with dimensions of Ф8mm×10mm, as showed in Figure 5. The hot compression tests were conducted using a high temperature electronic universal material testing machine (AG-100KNXplus) under the following conditions: strain rate (0.001mm/s^-1~0.1mm/s^-1), deformation temperature (25°C~450°C), maximum compression reduction is 20%, compression experiment conditions were listed in Table 2. The specimens were heated to the target deformation temperature at a heating rate of 10°C/s, held at the target temperature for 2 minutes to ensure uniform thermal distribution. After compression, the specimens were subjected to grinding, polishing, and etching to prepare metallographic samples. Microstructural observation using a 4XB-TV inverted optical microscope.

2.2.2 Vickers hardness testing
     Considering the unevenness of the material, this article adopts the Vickers hardness testing method, as it can more comprehensively reflect the overall hardness of the material. The hardness measurement was evaluated using a Vickers hardness tester (MH-3L) as shown in Figure 6(a). To ensure the accuracy of the test results, five measurements were taken for each deformation area, and the average value was finally calculated. The macroscopic test load is 5kgf, and the microscopic analysis is 0.5kgf, ensuring that the diagonal length of the indentation is within the range of 20-50 microns to improve measurement accuracy.

3. RESULTS AND DISCUSSION

3.1 Hot Compression Deformation Behavior Analysis Under Different SiCp Content
     The stress-strain curves and microstructure after hot compression deformation under different conditions were showed in Figure 7. It can be seen that under the same SiCp (15%) volume fraction, stress increases with the increase of strain rate. This is mainly because at low strain rates, the matrix has sufficient time to transfer load to the reinforcing particles, resulting in more complete deformation, this also indicates that this aluminum based composite material has strain rate sensitivity; At the same temperature (450 °C) and same strain rate (0.001mm/s^-1), the addition of silicon carbide reinforced particles improves deformation resistance, resulting in an increase in stress with the increase of SiCp content, but high silicon carbide content may cause an increase in porosity; Under the same SiCp (20%) volume fraction condition, stress decreases with increasing temperature. As the temperature increases, the brittle fracture of composite materials gradually shifts from room temperature to a plastic dominated mechanism at high temperatures. The proportion of particle breakage significantly decreases, the softening effect of the matrix increases, and the plastic flowability improves.

3.2 Simulation Results Analysis of ECAP under Different Parameters
     The stress-strain diagrams under different passes at 450°C are showed in Figure 8. The simulation results show that under the BC path condition, there is a significant improvement in the tensile strength of the sample after a single pass of ECAP extrusion, which is related to the increase in strain accumulation degree. After a second pass of extrusion, the tensile strength of the sample is more significantly improved, but the strength improvement in subsequent passes is not significant, indicating that grain refinement in large plastic deformation is not infinite.

     The stress- strain diagrams under different extrusion speed at 450°C and one pass are showed in Figure 9. The simulation results indicate that as the extrusion speed increases, the equivalent strain first decreases and then increases. After deformation, the large strain zone is mainly concentrated at the contact between the specimen and the punch, and the closer it is to this point, the greater the strain. The high stress zone is concentrated around the shear deformation zone, and increasing the extrusion speed can alleviate the stress concentration, but it will weaken the grain refinement effect.

3.3 Experiment Results Analysis of ECAP
     The ECAP experiment conditions is showed in Table 3 and the deformation equipment and ECAP mold tooling is showed in Figure 10. The equipment is a 200 ton press machine (Model: RZU200HF) and the specimens both before and after ECAP processing is showed in Figure 11.

     The metallographic structure after ECAP test under the conditions of SiCp volume fractions of 15%, 20%, and 25% is shown in Figure 12. It can be seen that with the occurrence of large plastic deformation, SiC reinforced particles undergo crushing, and with the increase of silicon carbide content, the crushing effect becomes more pronounced as the load is transmitted to more reinforced particles.

     The distribution of silicon carbide content is 25%-15%-25%. The metallographic structure of the gradient composite material at different positions is shown in Figure 13. It can be seen from the figure that there are monomer component composite material characteristics at positions with silicon carbide content of 15% and 25%, respectively. The interface bonding is good between the silicon carbide content of 15%-25%, and the microstructure has a clear gradient structure distribution. The main reason for interface bonding is that materials with different volume fractions undergo plastic deformation, mechanical interlocking, and atomic diffusion under the combined action of high temperature, high pressure, and high shear stress, thereby achieving direct and firm bonding with different volume fractions.

3.4 Hardness Property
     The HV hardness under different conditions were showed in Figure 14. It can be seen that the hardness increases with the increase of silicon carbide content, but decreases when the silicon carbide content is too high. This is mainly due to the addition of silicon carbide particles hindering dislocation movement (Orowan strengthening) and refining the matrix grains at low content. As the content of silicon carbide increases, the hardness decreases due to an increase in particle agglomeration or interface defects such as pores and cracks. The hardness of gradient composite are higher than those of 15% composite materials, but lower than those of 25% composite materials.

3.5 Tension Property
     The comparison of tensile strength between gradient composite and single volume fraction composite prepared was showed in Figure 15. The tensile strength of single volume fraction composite increases with the increase of SiCp content at low silicon carbide content, mainly due to load transfer strengthening and increased dislocation density. As the content of silicon carbide further increases, the increase in strength decreases or even decreases, mainly due to interface brittleness or residual stress leading to a decrease in fracture toughness. The tensile strength of gradient composite are higher than those of 15% composite materials, but lower than those of 25% composite materials.

4. CONCLUSIONS

     In this work, Gradient composite materials were prepared using ECAP method and the differences in properties and microstructure, that based on the preparation of SiCp/6092Al composite materials with different silicon carbide contents (15%,20%,25%) using powder metallurgy method. The following are the main conclusions from the work:
     1. Based on powder metallurgy process and large plastic deformation ECAP process, aluminum based composites with SiCp volume fractions of 15%, 20%, and 25% were prepared. The hot compression deformation behavior and the changes in hardness and tensile properties after heat treatment were analyzed. The results indicate that hardness and tensile strength increase with the increase of SiCp content, but excessive SiCp content can actually decrease.
     2. Based on the method of powder metallurgy and large plastic deformation ECAP technology, SiCp/6092Al gradient composite materials with good interfacial bonding have been prepared. By comparing the hardness and tensile strength of gradient composite materials and single volume fraction composite materials, it was found that, the hardness and tensile strength are higher than those of 15% composite materials, but lower than those of 25% composite materials. Achieving the preparation of multifunctional integrated materials with surface ablation resistance, intermediate layer high thermal conductivity, and matrix toughening.

ACKNOWLEDGEMENTS

     The authors are grateful for the financial support provided by the Anhui Provincial Key Research and Development Project (JZ2022AKKG0100).

AUTHOR CONTRIBUTIONS

Guo-qiang Gan: Writing-Review & Editing, Resources, Supervision, Project administration, Writing-Original Draft, Funding acquisition. Yong-tai Wang: Investigation, Data Curation. Yu-feng Zhang: Visualization, Investigation, Formal analysis. Ping Li: Conceptualization, Methodology. Rui-zhi Chai: Software, Validation.

CONFLICT OF INTEREST STATEMENT

     The authors declare that we do not hold any conflicting interests.

FUNDING

     This research was financially supported by the Anhui Provincial Key Research and Development Project (JZ2022AKKG0100).

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