In the production of parts for direct industrial application and for developing research purposes, it is of utmost importance to understand the defects associated with the material system. In this work, the microstructural and mechanical properties of 316L Stainless Steel deposits and cast Aluminum A356 and Magnesium AZ91 alloys are investigated. The study first examines the design of efficient gating systems utilizing the Electromagnetic Pump Green Sand system to produce vertically cast A356 plates. A series of numerical simulations were developed for each of the four gating designs in order to compare modeling results with actual castings. The method of four-point bend (FPB) testing was used to obtain information about the effect of oxide entrainment on the mechanical properties of the parts, and from this data, a two-parameter Weibull statistical analysis was performed in order to quantify specimen failure rate for each of the configurations. Metallographic analysis was carried out using optical microscopy, and fractography using Scanning Electron Microscopy (SEM). In keeping with light alloys focus, the determination of superior casting processes for AZ91 alloys is also studied. Passenger car control arms were cast by Indirect Squeeze Cast, Low Pressure Permanent Mold, T-Mag, and Ablation processes. The microstructure, grain size, porosity distribution, and defect analyses were obtained with optical microscopy and the ImageAnalyzer program. The mechanical behavior was characterized from the FPB and tensile tests. The four casting processes were evaluated in terms of reliability again using a Weibull analysis of the ultimate bending strength determined from the FPB test samples. Metallographic analysis was performed on these samples, revealing noticeable microstructural differences between them, with some even showing possible evidence of oxide films. Lastly, the study of process parameters such as beam and laser current, translation speed, and wire feed and deposition rate associated with 316L stainless steel deposits produced by both Laser Engineered Net Shaping and Electron Beam Freeform Fabrication becomes the research objective. Tensile tests, optical microscopy, and SEM were used to determine mechanical properties, characterize solidification grain structure, porosity, secondary dendrite arm spacing, and possible modes of failure.

In this dissertation¡Atwo topics in microbubble systems are investigated¡G1) Pore Formation from Bubble Entrapment by a Solidification Front¡F2) Pore formation in Solid¡C In the first study¡Amechanism of the pore shape in solid resulted from a tiny bubble captured by a solidification front is geometrically and generally investigated¡CPore formation and its shape in solid are one of the most critical factors affecting properties¡Amicrostructure¡Aand stresses in materials¡CFor simplicity without loss of generality, the tiny bubble beyond the solidification front is considered to have a spherical cap in this work¡CIntroducing a geometrical analysis it is found that the contact angle of the bubble cap can be governed by the Abel¡¦s equation of the first kind in terms of displacement of the solidification front¡CThe pore can be elongated, expanded¡Ashrunk and closed¡Adepending on relative variation of the bubble growth rate and solidification rate¡CThe pore can be closed by imposing infinitesimal bubble growth rate-to-solidification rate ratio¡Aand a finite bubble growth-to-solidification rate ratio in order to produce a minimal bubble radius at the contact angle of ¡CA criterion intuitively accepted in the literature¡Astating that closure of a pore is attributed to a greater solidification rate than bubble growth rate¡Ais incorrect¡CThe predicted pore shape and contact angle agree with experimental observations¡CManipulating either bubble growth rate or solidification rate can control pore formation in solid¡C In second study¡Athe shapes of a growing or decaying bubble entrapped by a solidification front are predicted in this work¡CThe bubble results from supersaturation of a dissolved gas in the liquid ahead of the solidification front¡CPore formation and its shape in solid are one of the most critical factors affecting properties¡Amicrostructure, and stresses in materials¡CIn this study¡Athe bubble and pore shapes entrapped in solid can be described by a three-dimensional phase diagram¡Aobtained from perturbation solutions of Young-Laplace equation governing the tiny bubble shape in the literature¡CThe predicted growth and entrapment of a microbubble as a pore in solid are found to agree with experimental data¡CThis work thus provides a realistic prediction of the general growth of the pore shape as a function of different working parameters¡C