ORCID Identifier(s)

0000-0001-7776-6299

Graduation Semester and Year

2020

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Ankur Jain

Abstract

Additive manufacturing (AM) processes involve layer-by-layer addition of material to fabricate a 3-dimensional part. AM offers significant design and manufacturing flexibility compared to traditional manufacturing approaches. The major challenge in polymer-based additive manufacturing (AM) is that printed parts often have poor thermal/structural properties. These properties depend on weld strength of filaments and which intern depends on the degree of healing/neck growth between deposited polymers. The major contribution of this dissertation is understanding the importance of heat transfer during the printing process on the polymer neck growth and presented with techniques to improve the thermal and structural strength of finished part. At first dependence of thermal conductivity of polymer AM parts on the print process parameters is studied. This research developed the understanding through a combination of in-situ high speed imaging and thermal conductivity measurements. Subsequently, measurement of temperature distribution of deposited layer onto pre-deposited layers through in situ infrared thermography is carried out. Degree of healing (weld formation/neck growth) between two polymer filaments deposited is directly governed by heat transfer. This dissertation work measured the temperature profile of the single filament deposited through infrared thermography and showed that hot nozzle tip causes significant temperature to rise in the pre-deposited layer before filament deposition. A novel concept of in situ, nozzle-integrated pre- and post-heating of previously deposited filaments is implemented, and improved weld formation compared to the baseline case is demonstrated. This research work addresses the key broader challenge in polymer AM by developing a novel approach for thermal enhancement of filament-to-filament neck growth process. This is expected to result in parts with improved properties that can be used in applications involving thermal and mechanical loads. Optimization of pre-post heater temperature is carried out to achieve required degree of healing between the filaments. It is expected that with pre-post heating of pre-deposited layers, weld strength improves resulting in higher mechanical strength which intern reduces the void percentage. The experimental study present in this dissertation enables optimization of thermal conductivity of polymer AM parts by developing relationships between print process parameters and thermal properties of interest in a variety of applications. Estimation of heat transfer from newly deposited layer into pre-deposited layers is studied through in-situ IR measurements and demonstrated that classical welding moving heat source analytical solution can be used to predict the temperature profiles of weld zone between two filaments. Further demonstration of novel pre-post heating of polymer layers overcomes the drawback of poor structural strength (due to voids) in polymer AM.

Keywords

Polymer additive manufacturing, Heat transfer, Weld strength

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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