Master of Science, University of Toledo, 2019, Mechanical Engineering

High pressure die casting (HPDC) aluminum has been widely employed in many industries for its superb cast ability, productivity, flexibility and especially the high strength to weight ratio. The use of aluminum as a light metal reduces weight and the employ of HPDC aluminum enables mass production of thin-walled, complex, intricate components with near net shape. However, one disadvantage of this technology is the inevitable presence of various defects, particularly porosity. The objective of this study was to evaluate and characterize the fatigue behavior of HPDC aluminum with effects of defects. Porosity was the main defect studied. Tensile tests and load-controlled fatigue tests were conducted to generate data for this investigation. The material, A380 HPDC aluminum, was provided by Eaton Corporation, as the sponsor of this study. Tensile tests were performed to obtain basic mechanical properties of the material. Fatigue tests were performed for different conditions to characterize fatigue behavior and evaluate the effects of porosity level, specimen size, mean stress and stress gradient. Two different size specimens were tested under the same condition to evaluate the influence of specimen size on fatigue behavior, and specimens were divided into four levels according to the amount of porosity to study the effect of porosity level on fatigue behavior. As for mean stress, R = 0 tests were conducted to compare to the results of R = -1 tests and rotating bending tests were added for stress gradient evaluation. Stress-life curves were employed for generating relation between stress amplitude and fatigue life, while the significant scatter in the experimental data necessitated a statistical analysis. The fracture surfaces of failed specimens were also examined for crack initiation location and size of defects present.

Committee: Ali Fatemi (Committee Chair); Lesley Berhan (Committee Member); Mohamed Samir Hefzy (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2018, Materials Science and Engineering

The development of new magnesium alloys with improved mechanical properties is important for lightweighting applications, since the current high pressure die cast (HPDC) magnesium alloys, i.e. AM50/60 (Mg-5/6wt.%Al-0.2wt.%Mn) and AZ91 (Mg-9wt.%Al-1wt.%Zn), have limited mechanical properties. Two magnesium alloy systems, Mg-Al-Sn (AT) and Mg-Al-Sn-Si (ATS), were investigated for potential automotive applications. A CALPHAD (CALculation of PHAse Diagrams) approach was used in the development of AT and ATS alloys and to aid in the design of heat treatment schedules. Scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and transmission electron microscopy (TEM) techniques were used to characterize the microstructure of the alloys in the as-cast and heat treated conditions. Mechanical testing was performed on cast specimens, as well as samples cut from thin-wall HPDC components to compare the strength and ductility of these alloys to currently used magnesium alloys. To expand the applications of HPDC components in the transportation industries, further development and optimization of the process is needed. The development of super vacuum die casting (SVDC) process for aluminum and magnesium thin-wall castings were explored using process simulation and experimental validation. Two experimental dies, i.e., a test specimen die and a fluidity die, were designed to evaluate the castability of several new aluminum alloys and optimize process parameters for these alloys. The process conditions were successfully validated in industrial castings such as an automotive door inner and a side impact beam.

Committee: Alan Luo (Advisor); Michael Mills (Committee Member); Glenn Daehn (Committee Member); Gary Kennedy (Committee Member) Subjects: Materials Science