Microwave processing of porous carbon nanotube-reinforced magnesium alloy composites

Abstract

Lightweight structural materials with enhanced mechanical properties are required for minimising weight and energy consumption in aerospace and automotive applications. Porous magnesium (Mg)-based composites are useful for achieving these goals because they have low density, high damping properties, and high specific strength. However, the low strength of Mg and processing difficulty inhibit its use for structural applications. The aim of this research work is to develop a rapid microwave sintering scheme for fabricating porous carbon nanotube-reinforced magnesium alloy composites with enhanced compressive and energy absorption properties. The new sintering scheme synergises powder metallurgy (PM) with energy-efficient microwave (MW) sintering to fabricate porous magnesium alloy composites. With this proposed method, rapid sintering could be achieved in 8 minutes, a significant reduction of about 80% processing time when compared with conventional sintering methods. The melt processing technique is commonly used for processing porous Mg and can produce a closed-cell structure. Nonetheless, the process requires the use of expensive foaming agents and suffers from difficulty in controlling the pore structure and the formation of brittle secondary phases, which degrades the strength of the porous material. PM can be used to alleviate some of these difficulties because it offers a means of fabricating near-net shape parts, minimising interfacial reactions, and controlling pore size and shape. When combined with MW sintering, processing is rapid, significant cost and energy savings are achieved, and materials with improved properties could be produced. Nevertheless, the MW processing of Mg is still very challenging. The inhomogeneous field distribution in the MW cavity can lead to the appearance of hot spots and damage the processed parts. In addition, Mg shows poor coupling with MW at room temperature, thus only limited heating can be realised. These challenges underscore the difficulties in utilising MW for sintering Mg-based powders. With a view to overcoming these challenges, an energy-based sintering criterion was developed and incorporated into a finite element model (FEM) to establish the critical processing conditions required for developing a rapid MW sintering scheme. Thus, the whole sintering process could be simulated. The FEM model predicted the electric field distribution in both the MW cavity and the compact, as well as the heating behaviour of the compact in response to the applied field. Analyses of the FEM results revealed that the heating time, carbon nanotube (CNT) filler, the size of the compact, and the thickness of the external susceptor were the critical processing conditions that strongly influenced the MW sintering scheme. A densification model included in the FEM analysis also facilitated the prediction of the evolution of density in the compact and the estimation of its apparent sintering activation energy. The proposed MW sintering scheme was implemented experimentally by fabricating porous CNT-reinforced Mg-alloy composites. Green samples were fabricated by the PM technique using fugitive and permanent space holder (SH) techniques and MW-sintered in 8 minutes. Accordingly, about 68% and 81% reductions in energy consumption and processing time were accomplished when compared with other sintering methods. The gains recorded could be attributed to the synergy between the CNTs, which acted both as reinforcement particles and internal sources for volumetric heating, and silicon carbide/graphite hybrid susceptor, which heated the samples externally until they coupled with and directly absorbed MWs. This rapid sintering scheme significantly saved time and energy, minimised the oxidation and formation of brittle secondary phases in the samples, and produced quality porous Mg alloy composites. Through design and calculations, SH agents were utilised to control the pore structure, density, and design floatability property into the porous samples. Enhancement in the compressive strength and energy absorption properties were achieved by nano-reinforcement of the pore wall of the porous samples using CNTs. For porous samples with an open-cell structure and 49% porosity, a density, a compressive strength and an energy absorption capacity of 0.92 g/cm³, 47 MPa and 16 MJ/m³, respectively, was achieved at 3% CNT volume fraction. Furthermore, porous samples with a low density of 0.79 g/cm³ were fabricated by hybridising open and closed-pore structures into the AZ61 matrix. These samples exhibited remarkable buoyancy property and floated on water without recourse to any surface or chemical modifications. A static water contact angle of ~45° indicated that the surface of the sample was hydrophilic. Therefore, the buoyancy property exhibited by this group of porous samples was confirmed to be due to its hybrid pore structure and low density. The self-propulsion of a microboat fabricated from the porous sample was found to be dependent on the concentration of ethanol fuel used and a velocity as high as 93 mm/s was recorded at an ethanol concentration of 100% (v/v). These properties make these porous composites potentially useful for energy absorption and buoyancy applications. A rapid MW sintering scheme has been successfully developed in this research work for fabricating porous Mg alloy composites with enhanced compressive and energy absorption properties. The proposed scheme not only significantly reduced the sintering time, but also overcame the problem of the formation of brittle secondary phases in porous Mg alloy composites. CNT served as both MW susceptor (for achieving rapid sintering) and reinforcement particles (for strengthening the cell wall) for the porous composite samples. Pore hybridisation was successfully utilised for controlling the density and achieving floatability property in the composite samples. The modelling work and experimental investigations can contribute to providing useful knowledge in choosing the critical parameters relevant to the MW sintering process and developing MW technology for sintering porous Mg-based composites with enhanced mechanical properties. Extension of the FEM model to investigate the effect of the sintering atmosphere and to account for compact shape change during densification, as well as investigating the crashworthiness of the porous composites under dynamic and static conditions, are recommended for future works.