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Title:	Microstructural refinement and corrosion resistance enhancement of A356 alloy via semisolid processing and severe plastic deformation
Authors:	Mohamed A. Mansour Gebril (P72668 )
Supervisor:	Mohd Zaidi Omar, Prof. Dr.
Keywords:	Universiti Kebangsaan Malaysia -- Dissertations Dissertations, Academic -- Malaysia Alloy Corrosion Plastic deformation
Issue Date:	19-Jan-2020
Description:	Improving the engineering properties of A356 alloy provides an appealing option for the automotive and aircraft engine industries. However, the solubility limitation of silicon (Si) in aluminum contributes to the precipitation of flake-shaped Si particles with sharp edges. Presence of Si particles in flaky or acicular shape acts as a stress riser and facilitate the crack propagation through the eutectic phase as well as weaken the stability of the protective layer. Thus, this study aimed at refining and redistributing the Si particles and the eutectic phase through applied T6 heat treatment to refine the Si particles of semisolid A356 alloy followed by severe plastic deformation (SPD) to refine the microstructure. The as-cast, rheocasting using cooling slope technique and thixoformed samples subjected to heat treatment prior to processed by equal channel angular pressing (ECAP) and high pressure torsion (HPT) at room temperature for microstructure refinement. After processing, the microstructures were evaluated, hardness measurements were recorded and corrosion resistance was examined. The ECAP process was initially applied following route A (where the sample is not rotated between each pass) and Bc (90° rotation of samples around the linear axis between each pass) through a constrained channel of a die that was bent abruptly close to 120o. The HPT process was applied for 0.75, 1, and 5 turns accordingly with the pressure of 6 GPa with a rotation speed of 1 rpm. The change in microstructure was evaluated using optical microscope, field emission scanning electron microscope (FESEM) and field emission transition electron microscope (FETEM). The study also includes quantitative metallography analysis to measure the grain size according to the ASTM E112. The corrosion test was successively performed at room temperature using Gamry 3.2 potentiostat, which was attached with three-electrode, forming an electrochemical cell setup in a 3.5% NaCl solution. The results of surface analysis revealed the potential of two SPD techniques in producing an ultrafine-grain structure with equiaxed grains. The brittle Si particles were effectively fragmented and redistributed in homogenous microstructure. The grain size reduction from 170 μm of as-cast to 23 μm after combing heat treatment and cooling slope followed by ECAP (ECAPed heat-treated cooling slope) sample after six passes route A, while reduced to 160 and 180 nm after 5 turns of HPT process of heat-treated cooling slope and thixoformed samples. The hardness of heat-treated cooling slope casting samples increased with ECAP process, from 61 HV, for the as-cast alloy, to 134 HV, after six passes route A. The hardness of heat treated cooling slope and thixoformed samples improved with HPT process to, 211 and 199 HV, respectively, after 5 turns. The corrosion resistance of heat-treated cooling slope casting sample improved with corrosion rates from 0.0424 to 0.00120 mmy−1 after six passes of route A, while the corrosion rate of heat-treated cooling slope and thixoforming samples reduced to 0.00062 and 0.00134mmy−1 after 5 turns of HPT process respectively. The results showed that the grain refinement of thixoforming A356 alloy using HPT process was more effective than the ECAP process. In this work the microstructure refinement of A356 alloy by combination of semisolid process and heat treatment prior to processing by ECAP and HPT led to significant improvement of hardness and corrosion resistance due to refining and redistribution of Si particles and intermetallic compounds in Al matrix.,Ph.D.
Pages:	172
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

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