Date on Master's Thesis/Doctoral Dissertation

5-2018

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Industrial Engineering

Degree Program

Industrial Engineering, PhD

Committee Chair

Li, Yang

Committee Co-Chair (if applicable)

Nagy, Peter

Committee Member

Starr, Thomas

Committee Member

Chou, Kevin

Committee Member

DePuy, Gail

Author's Keywords

additive manufacturing; nondestructive evaluation; ultrasonics; in-situ monitoring; interfacial stiffness

Abstract

Metal Additive Manufacturing (AM) is increasingly being used to make functional components. One of the barriers for AM components to become mainstream is the difficulty to certify them. AM components can have widely different properties based on process parameters. Improving an AM processes requires an understanding of process-structure-property correlations, which can be gathered in-situ and post-process through nondestructive and destructive methods. In this study, two metal AM processes were studied, the first is Ultrasonic Additive Manufacturing (UAM) and the second is Laser Powder Bed Fusion (L-PBF). The typical problems with UAM components are inter-layer and inter-track defects. To improve the UAM process, an in-situ quality evaluation technique was desired. Several NDE techniques were tested in a lab environment before ultrasonic NDE was chosen as a practical, robust, and cost-effective NDE tool. An in-situ monitoring setup was designed and built on an UAM system. NDE results showed interesting features that were simulated through analytic and finite element wave-propagation models. AM layers with defects were characterized as an intact layer and a finite interfacial stiffness spring. The spring stiffness coefficient is a quality parameter that was used to characterize AM layers through a model-based inversion method. In-situ and post-process NDE provided an understanding of defect generation and propagation in UAM. A novel solid-state repair mechanism based on Friction Stir Processing (FSP) was proposed and demonstrated. The quality of L-PBF components depends on several factors including laser power, scan speed, hatch spacing, layer thickness, particle shape/size distribution and other build conditions. Developing process parameters for a new material is an expensive and complex optimization problem. Post-process ultrasonic NDE tests revealed that the model-based in-situ quality monitoring developed for UAM is also applicable to L-PBF Additive Manufacturing. A similar NDE set-up was designed and installed on an open-architecture L-PBF system. A layer-by-layer bond quality evaluation demonstrates the ability to detect good-quality bonds hidden behind poor-quality regions for Inconel 625 alloy. A cost-effective, process parameter development methodology has been proposed and demonstrated.

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