Date of Completion

11-8-2019

Embargo Period

11-10-2020

Keywords

phase-field model, additive manufacturing, selective laser melting, metallic powder, powder-substrate interaction, thermocapillary flow, thermal elasticity, freezing dynamics, freeze concentration, protein pharmaceuticals

Major Advisor

Dr. Tai-Hsi Fan

Associate Advisor

Dr. Robin Bogner

Associate Advisor

Dr. Amir Faghri

Associate Advisor

Dr. Xinyu Zhao

Associate Advisor

Dr. Ying Li

Field of Study

Mechanical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Laser-based additive manufacturing (AM) has been rapidly growing in applied and scientific research areas due to its advantages for manufacturing geometrically complicated parts and functional materials. However, the surface quality and geometric accuracy are great concerns of AM-produced parts because of the formation of surface defects such as voids, notches, or even cracks caused by incomplete melting of powders, lapping, or gas entrapment during melting and solidification processes. From theoretical view points, temperature prediction and multi-phase dynamics at the powder level are of the greatest challenges, yet necessary for better understanding of relevant laser-material interactions as well as for the optimization and rapid design of AM processes. In this thesis, an integrated phase transition dynamics and thermal-fluid-structural analysis using a thermodynamically-consistent phase-field method is developed to predict selective laser melting of a pure metallic powder and its interactions with the substrate. The integrated model couples interfacial evolution dynamics, solid-liquid phase transition, thermal and fluid transport, and the distribution of thermoelastic stress. The temperature-dependent thermophysical properties including thermal conductivity, viscosity, and surface tension are all incorporated in the phase-field model. The computational results demonstrate the development of metal melt pool and the distribution of velocity field and strain and stress across the solid-liquid interface at high temperature and heat flux conditions. In particular, the powder-substrate fusion dynamics and the transient evolution of liquid-gas interfaces driven by thermal capillarity are fully resolved at the powder level under various laser control parameters. As melting and solidification are continuous in the AM process, both have significant impacts on the product quality, stability, and mechanical performance. To further extend the phase-field method to the solidification process at lower temperature, the thesis work emphasizes an additional application of freezing dynamics on a multi-component protein solution. Understanding the role of freezing is an essential step for manufacturing and long-term storage of protein pharmaceuticals. Specifically, the freeing dynamics including heat transfer, phase transition, freeze concentration of solutes, and their interplay with fluid flow in a cylindrical vessel are computationally resolved and validated by experimental data, which would help to mitigate negative impacts on protein stability during freezing and subsequent manufacturing processes. Overall, the main contribution of this thesis is establishing an integrated theoretical framework of thermal fluid dynamics, thermoelasticity, and phase transition dynamics for the prediction of fusion and freezing in critical thermal manufacturing processes.

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