Doctoral Dissertations

Date of Award

5-2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Energy Science and Engineering

Major Professor

Tony L. Schmitz

Committee Members

Brett G. Compton, Sudarsanam Suresh Babu, Katharine Page

Abstract

This study investigates the effects of manufacturing pauses on the mechanical properties and residual stress evolution in additive friction stir deposition (AFSD) of aluminum alloy 7075 (AA7075). Single-walled builds were produced using a closed-loop temperature control system, with intentional 15- and 100-hour pauses introduced midway. Some builds were reheated to 120 C before resumption; others restarted without preheating. Structured light scanning evaluated base plate deflection, and neutron diffraction characterized residual stresses. Samples were sectioned via wire EDM for surface displacement measurements using a coordinate measurement machine (CMM), followed by tensile testing, hardness mapping, and metallographic analysis.

In continuously deposited material, microhardness in the central region increased over 15 weeks post-deposition due to natural aging, with hardness rising from 113 HV to 147 HV. The base plate beneath the deposit consistently showed lower hardness. In paused builds, the material beneath the pause interface aged prior to resumption, leading to elevated hardness in those layers. For example, the 100-hour pause without reheat yielded initial layer hardness of 100–110 HV, versus 90–100 HV in the no-pause sample.

Tensile testing showed that all samples exhibited yield and ultimate strengths between O-temper and T73 conditions, consistent with W-temper natural aging. Elongation at failure ranged from 1.2\%–9.1\%, with the 100-hour pause without reheat producing the lowest ductility, greatest residual stress, and largest EDM deflection. Neutron diffraction revealed tensile residual stress concentration near the base plate in all builds. Paused builds with warm restarts showed increased tensile stress in pre-pause layers, while cold restarts led to sharp stress variations near the pause. Variations in measured $d_0$ spacing reflected changes in thermal history and microstructural evolution.

This is the first comprehensive evaluation of AFSD AA7075 with manufacturing pauses. Motivated by delamination observed in large-scale builds, the study shows that each new AFSD layer contributes to in-situ heat treatment. A pause interrupts thermal continuity, allowing premature aging and introducing mechanical and microstructural heterogeneity. These insights are critical for ensuring consistent performance in aerospace-grade aluminum components and refining AFSD process control strategies.

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