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. / Thesis / Master of Applied Science (MASc)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/23751 |
| Date | January 2018 |
| Creators | Nagaraj, Kishor |
| Contributors | McDermid, Joseph, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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