Year

2016

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, and Mechatronic Engineering

Abstract

Iron aluminide has continuously attractive since the early 20th century, due to a combination of preferred properties including excellent oxidation and sulfidation resistance, considerable high temperature strength and creep resistance, low density and low cost. These properties make Fe-Al series intermetallics the promising replacement for specialized alloy steel in components used for fossil fuel energy systems which require high thermal and corrosion resistance. To date, researches have been focused on improving the high temperature strength and room temperature ductility of the material. Nevertheless, the industrial application of this series of alloys is still limited by its room temperature brittleness, which also leads to a high manufacturing cost of this alloy.

In the present thesis work, an innovative wire-arc additive manufacturing process is used to fabricate iron aluminide alloy in-situ, through separate feeding of pure of Fe and Al wires into a molten pool that is generated by the gas tungsten arc welding process. This new manufacturing process possesses revolutionary time and cost saving in comparison to traditional methods. Since the brittle nature of Fe-Al intermetallics, the feasibility of this process in fabricating iron aluminide have been firstly investigated in this study. Also, in order to optimize the specific manufacturing process, the influences of the manufacturing parameters, such as deposition current, interpass temperature and torch travel speed, on the geometries, material and mechanical properties have been studied. According to the results, suitable parameters for producing the crack-free and cross section symmetric iron aluminide components, 140A deposition current, 400C interpass temperature and 95mm/min tungsten torch travel speed have been determined when the specific deposition energy is kept around 20kJ/g. And it has been proved that the present manufacturing process is capable of producing fully densified iron aluminide structure with designed chemical composition and phase.

Subsequently, the in-depth study on the preferred epitaxial grown grain structures inside the buildup iron aluminide have been performed in order to further understand the influence of certain microstructure on the mechanical properties, especially tensile properties of the material. The tensile results have shown higher strength in the longitudinal direction tensile specimens than in the normal direction specimens, while elongation is correspondingly reduced, which results have indicated that grain refinement procedure is required after the additive manufacturing process to obtain better and more homogeneous mechanical properties within the buildup material.

Afterwards, in order to quantitatively investigate the phase transformations during the multi thermal cycling of the additive manufacturing process, in-situ neutron scattering has been performed on the additive manufactured Fe3Al based iron aluminide sample in a single heating up process. The neutron diffraction data has explicitly exhibited the phase transformation procedures occurred throughout the heating up process. This result also indicates the significance of the homogenization and ordering heat treatment for the asfabricated Fe3Al based iron aluminide, otherwise the imperfectly ordered Fe3Al phase, rather than perfectly ordered Fe3Al phase, would occupy most of the material.

In addition to the material with consistent chemical composition, a functionally graded material with pre-designed chemical composition gradient was manufactured by the wirearc additive manufacturing process. The experimental characterizations have demonstrated that the designed chemical composition in the buildup wall can be accurately achieved by adjusting the wire feed ratio of iron and aluminum wires. The hardness and tensile properties throughout the deposited wall have shown similar properties as in previous experiments. Considering the application prospects of iron aluminide, the corrosion mechanism of the functionally graded material was characterized in different locations by the method of electrochemical impedance spectroscopy. The increasing Al content in the as-fabricated Fe-FeAl functionally graded material buildup wall produces increased electric potentials in the test specimens, which implies an increase of corrosion resistance.

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