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Title: | Microstructural Characterization of Al-Fe Intermetallics in Aluminum Alloy 7xxx during Controlled Solidification |
Authors: | Nagaraj, Kishor |
Advisor: | McDermid, Joseph |
Department: | Mechanical Engineering |
Keywords: | Microstructural Characterization |
Publication Date: | 2018 |
Abstract: | Aluminum 7xxx series alloys have been a major focus for lightweight automotive structural components to achieve the targeted weight reductions by auto industries and, in turn, to increase the vehicle fuel efficiency. AA 7xxx series wrought alloy components have been widely used by the aerospace and automotive industries for many decades due to their low density and high strength. However, although near net shape casting of these alloys has many benefits versus wrought alloys; this processing route has been a challenge for the researchers and the auto industry because of limitations due to castability issues such as like hot tearing and die soldering. One possible mitigation strategy involves the addition of iron (Fe) as one of the major alloying element and then subsequently optimizing the alloy chemistry and solidification parameters in terms of die soldering. The objective of this thesis is to determine the microstructural evolution of any Al-Fe intermetallic phases with respect to cooling rate during solidification for a range of AA 7xxx series alloy compositions. Fe was added at three different levels in a total of nine alloy composition developed from a Taguchi experimental matrix based on the interaction of three composition levels for four alloying elements. The alloys were cast using a custom built casting rig while the cooling rates were measured along the length of a directionally solidifying sample. The thermocouple measurements were analyzed to determine the velocity of the solid/liquid interface, overall cooling rate and thermal arrest points for later correlation to variations in the microstructural development of any Al-Fe intermetallic phase particles present in the experimental alloys. Metallographic samples were taken at locations with iv known cooling rates to determine the resultant microstructure. Scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS) were performed to obtain elemental analyses of the Al-Fe intermetallic phases for present in the samples. The Fe maps obtained by EDS were processed and analyzed using Image-J software to determine the size distribution and area fraction of the Al-Fe intermetallic phases as a function of alloy composition and solidification rates. Also, a regression analysis was used to develop a statistical model to predict the variation of intermetallic particle size and area fraction of the Al-Fe intermetallic phases as a function of alloy composition and cooling rate. Based on the experimental investigation and analysis of the nine Al 7xxx-Fe alloys the results can be summarized as follows: (1) Cooling rate has a strong influence on the chemistry and morphology of the Fe intermetallic particles: It was determined that the dominant intermetallic species changes from the equilibrium Al3Fe to the metastable Al6Fe alloys for cooling rates in excess of approximately10 °C/s. (2) Alloy cooling rate does significantly affect the area fraction of the Fe intermetallic particles. It was determined that the morphology of the Al-Fe particles transitions from a relatively low aspect ratio particles to a high aspect, needle-like particles for cooling rates less than approximately 10 °C/s. (3) Alloying elements such as Zn, Cu, and Mg does not influence the Fe intermetallic chemistry and the area fraction of the intermetallics. |
URI: | http://hdl.handle.net/11375/23751 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Kishor Nagaraj June 2018-MASc.pdf | Microstructural Characterization of Al-Fe Intermetallics in Aluminum Alloy 7xxx during Controlled Solidification | 8.84 MB | Adobe PDF | View/Open |
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