Compressive Characteristics of Aluminum-Fly Ash Syntactic Foams Processed by Microwave Sintering
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INTRODUCTION
ALUMINUM (Al)-based syntactic foams have received considerable research prominence due to their lightweight and energy-absorption capability for applications including energy absorbers in automobile bumpers, ballistic armor materials, and vibration dampers.[1–3] Their porosities are derived from hollow sphere (HS) microspheres, such as fly ash (FA),[4,5] which are embedded in the matrix to form a closed-cell morphology. Al syntactic foams may be produced via melt-based (e.g., pressure infiltration and stir casting) and powder metallurgy (PM) processes. Melt-based processes are relatively simple, but poor/weak interfaces arise from the poor wettability between the molten metal and FA microspheres.[6] Applied/stirring force may fracture FA microspheres and jeopardize the objective of density reduction.[7] Consequently, the PM processing route is an alternative method with its main advantages being near-net shape ability and lower processing temperature.[8–10] However, a few studies reported PM processing of Al-based syntactic foams. Guo et al. reported the significance of compaction pressure in preserving the structure of FA microspheres in Al/FA syntactic foams. AKEEM DAMILOLA AKINWEKOMI is with Department of Metallurgical and Materials Engineering, Federal University of Technology, PMB 704, Akure, Ondo State, Nigeria. Contact e-mail: [email protected] JELEEL ADEKUNLE ADEBISI is with Department of Materials and Metallurgical Engineering, University of Ilorin, Kwara State, Nigeria. ADEOLU ADESOJI ADEDIRAN is with Mechanical Engineering Department, Landmark University, Omu Aran, PMB 1001, Kwara State, Nigeria. Manuscript submitted October 5, 2018. Article published online July 2, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
Sintering time was between 30 and 360 minutes at about 600 C.[10] Sintering at 710 C for 180 minutes was also reported by Vogiatzis et al.[2] The same authors developed a mathematical model to predict the compressive strength of the foams with respect to compaction pressure.[2] In another study, sintering temperature varied between 610 and 710 C for up to 180 minutes.[7] These few studies[2,7,10] showed that Al syntactic foams were amenable to PM processing, although an appropriate compaction pressure has to be selected to minimize damage to FA and sintering time runs into several minutes. Extended sintering times increase the tendency for the formation of brittle interfacial products between the matrix and FA microspheres.[7,11] These adversely affect strength and fracture of the resulting foam.[12] Thus, microwave (MW) sintering has been investigated as an alternative method. MW sintering is characterized by rapid and high heating rates, which significantly reduced energy consumption, processing time[13,14] and minimized interfacial reactions. These can preserve the structural integrity of the FA microspheres and harness the strengthening contributed by their shells.[11] Despite the obvious advantages offered by MW sintering technique, it is yet to be adequately applied to t
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