Date of Award

2026

Document Type

Open 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 Sanders

Committee Member 1

David Labyak

Committee Member 2

Timothy Langan

Committee Member 3

Stephen Kampe

Committee Member 4

Walter Milligan

Abstract

6xxx alloys are widely used in automotive extrusion structures for their high specific strength and formability. Advanced designs require greater formability and strength, creating an opportunity for high-strength, formable alloys. The 6xxx series forms β”-Mg5Si6 precipitates during age hardening and develops a fibrous microstructure during extrusion, both strengthening the alloy. Increasing Mg and Si to form more β” decreases formability. Thus, modifying texture can increase strength without reducing formability. Current alloys like 6005A add Mn and Cr to form dispersoids that inhibit recrystallization and promote strengthening textures; however, high Mn and Cr concentrations reduce formability. Replacing Mn and Cr with elements that form smaller dispersoids could promote strengthening textures by inhibiting recrystallization while maintaining formability. Sc and Zr can form thermally stable, coherent Al3Sc-Zr dispersoids with an L12 crystal structure. However, forming these dispersoids in 6xxx alloys is challenging because V-phase, “AlSc2Si2,” forms during homogenization.

This research proposes a new 6005A-type alloy, “6005+,” where Zr and Sc replace Mn and Cr, and a new multi-step homogenization at 190°C (12 hr), 300°C (10 hr), 400°C (10 hr), and 520°C (2 hr) to form stable Al3Sc-Zr precipitates and avoid V-phase formation. Compression testing of 6005+ showed similar flow stress to a baseline at 500°C and higher from strain rates of 0.1s-1 to 10s-1. Once extruded, the 6005+ had increased strength and similar ductility compared to the baseline. The extruded microstructure had a reduced perioral coarse-grain layer, fewer recrystallized textures, and more strengthening textures. The improvement in recrystallization resistance was caused by the decreased size of Al3Sc-Zr dispersoids compared to Mn-, Fe-, and Cr- containing dispersoids in the baseline alloy. The reduced dispersoid size increased Zener pinning pressure, leading to increased strength at room temperature due to the retention of strengthening textures while having no effect on the flow stress at high temperature.

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