Committee

Prof. Donggang Yao – School of Materials Science and Engineering (advisor)
Prof. Preet Singh – School of Materials Science and Engineering
Prof. Meisha Shofner – School of Materials Science and Engineering
Prof. Josh Kacher – School of Materials Science and Engineering
Prof. Jack G. Zhou – Department of Mechanical Engineering and Mechanics, Drexel University

Abstract

The viscosity of the material influences the choice of processing technique. Polymeric materials in the fluid state exhibit a range of viscosity under different shear rates. It enables polymers to be processed using both high and low-viscosity techniques. In comparison, molten metals possess a very low viscosity (several magnitudes lower) compared to polymer melts. Besides, the high surface tension of metals also contributes to a low Ohnesorge number, which is crucial for assessing material compatibility with extrusion-based processes. Therefore, casting is the primary shaping operation for metals. It necessitates additional machining and joining steps to form the final product.

Semi-solid metal alloys offer higher viscosity compared to melts. It allows semi-solid materials to be molded and extruded, unlike molten metals. Dendritic solid grains in semi-solid conditions result in nozzle clogging and liquid segregation and are undesired. A thixotropic microstructure consisting of globular grains in a liquid matrix demonstrates yielding and recovery effects beneficial for extrusion. Smaller globules can also improve resolution by enabling finer nozzle/die diameters. However, current rheoforming and thixoforming techniques produce large grains (>50µm) that hinder processability. An insufficient understanding of the competing fast kinetic and slow thermodynamic effects results in an incorrect selection of processing parameters.

In this thesis work, I will explore the kinetic effects that promote rapid globularization in deformed metal alloys. I will determine the necessary microstructural features that unlock instability mechanisms during heat treatment and study the effects of temperature on globularization time and grain size. I will characterize the melt flow behavior of semi-solid materials and ascertain the impact of grain size on rheological behavior. Through this study, I aim to highlight the importance of rapid thermal processing in generating fine globular microstructures (<10µm). This study can significantly improve the understanding of globularization mechanisms and highlight necessary conditions for high-viscosity processing of metals similar to thermoplastic polymers.