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Date of Award


Document Type

Campus Access Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering (PhD)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Paul G. Sanders

Committee Member 1

Douglas J. Swenson

Committee Member 2

Stephen A. Hackney

Committee Member 3

Walter W. Milligan

Committee Member 4

Mei Li


Further lightweighting of aluminum-intensive vehicle (AIV) body structures is of high priority to increase vehicle range. 7000-series alloys can offer in-service strength levels of 500 MPa which could provide a 30% weight reduction over existing 6000 series structural parts. However, these alloys do not have a stable T4 temper and are not able to be conventionally stamped due to low formability. Hot stamping has been demonstrated as a viable method to forming 7000-series part where a blank is solution heat treated, formed and simultaneously quenched, followed by a two-step artificial age treatment raising the part to its in-service strength.

A key challenge to implementing a hot stamped and two-step aged 7000-series part is that they are not readily joinable via self-pierce riveting (SPR), because they lack room-temperature ductility. Precipitation-hardened alloys like 7000-series alloys are susceptible to localized planar slip, which degrades the local ductility needed to form a successful SPR joint. In this work a strategy to reduce planar slip was proposed, in which a dispersion of sub-micron sized, non-shearable, Mn-based dispersoids was added to a dense distribution of nano-sized strengthening precipitates. Model Al-Zn-Mg alloys with various additions of Mn were cast, homogenized, and fabricated into wrought sheet material that was solution heat treated and two-step artificially aged.

In the two-step aged condition, the addition of Mn increased ultimate tensile strength, but did not affect tensile yield strength. Mn additions improved ductility due to the formation of Mn-dispersoids that facilitated dislocation cross-slip onto adjacent slip planes, resulting in a more finely-spaced arrangement of slip bands that reduced grain boundary stress localization. A more homogenous slip distribution was observed in TEM and slip-line spacing was quantified using EBSD data collected from strained specimens that were thermo-mechanically processed to have similar grain sizes.