Date of Award

12-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Civil Engineering

First Advisor

Ilya Mingareev

Second Advisor

Chiradeep Sen

Third Advisor

Daniel R. Kirk

Fourth Advisor

James R. Brenner

Abstract

Recent advances in additive manufacturing technologies have already led to wide-scale adoption of 3D-printed parts in various industries. The expansion in choice of materials that can be processed, particularly using Fused Deposition Modeling (FDM), and the steady advancements in dimensional accuracy control have extended the range of applications far beyond rapid prototyping. However, additive manufacturing still has considerable limitations compared to traditional and subtractive manufacturing processes. This work addresses limitations associated with the as-deposited surface roughness of 3D-printed parts. The effects of roughness-induced stress concentrations were studied on ultimate tensile strength and fatigue life. The samples were manufactured using a commercial desktop FDM system with a modified version of flat dog bone geometries required in the ASTM D638 standard [1]. The samples were then post-processed with tightly focused laser light produced by a high-repetition-rate ultrafast Yb-fiber Amplitude Satsuma laser using a multi-layer scan approach. This novel post-processing method enables high-efficiency material removal without inducing excessive thermal residual stresses into the material and therefore is suitable for post-processing thermally sensitive materials such as PLA. Taguchi’s Orthogonal Arrays were utilized to create a Design of Experiments (DOE) setup to study effects of four laser parameters; average power, scan speed, repetition rate and number of laser-processed layers to achieve various surface roughness values. A 90% reduction in surface roughness, 20% increase in ultimate tensile strength and 165% increase in high cycle fatigue life were achieved using ultrafast laser processing of 3D printed PLA parts.

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