Date of Award

Spring 2019

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science

Committee Chair

R. Bruce Madigan

First Advisor

Paul Gannon

Second Advisor

Dario Prieto

Third Advisor

KV Sudhakar

Fourth Advisor

Josh Wold

Abstract

PAW Print 3D (PP3D), a wire and arc additive manufacturing (AM) system was developed. PP3D comprises three plasma arc welding torches arranged radially around a central wire feed. Using three torches was hypothesized to eliminate sensitivity to travel direction. Two deposition modes were developed – continuous, which deposited continuous beads, and dabber, which deposited discreet “dabs” of material. Dabber was hypothesized to provide favorable solidification conditions that would refine the as-deposited grain structure. Three sets of process parameters for each deposition mode were developed. Ineffective workpiece melting was observed and investigated during process development. Using COMSOL Multiphysics software and experimental observations, the ineffective workpiece melting was attributed to inefficient convective heat transfer, high applied power density, and low workpiece thermal conductivity. 308L stainless steel linear wall specimens were deposited using each parameter set. Specimen composition, geometry, and microstructure were characterized. Sensitivity to travel direction was reduced, but not eliminated with continuous deposition mode because layer thickness depended upon deposition direction. No travel directional sensitivity was found with dabber mode. Large columnar grains and vertical grain texture were found in dabber mode specimens, indicating that the solidification conditions with dabber mode did not significantly refine the as-deposited macrostructure. Continuous specimens generally had little vertical grain texture, which may have been caused by an atypical weld pool temperature profile associated with the central wire feed. Solidification rates and temperature gradients at the solid-liquid interface were approximated using COMSOL Multiphysics software. Model results generally agreed with trends in deposited metal subgrain size.

Comments

Dissertation presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science

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