Date of Award

12-2017

Degree Type

Thesis

Degree Name

Master of Science in Materials Engineering

Department

Materials Engineering

Committee Chair

David R. Johnson

Committee Co-Chair

Matthew J. M. Krane

Committee Member 1

Kevin Trumble

Abstract

Elevated temperature tensile testing beyond the non-equilibrium solidus demonstrates the advantage of a new method called the Reheating-Cooling Method (RCM) for determining mechanical properties of Al-Mg binary alloys and cast alloy 7050 aluminum in the solid-liquid two phase mushy zone. Using the RCM, it is possible to achieve semi-solid tensile behavior with standard testing equipment and to retain a nonequilibrium casting microstructure, which is necessary to gain insight into hot tearing. Hot tearing, a virtually impossible to repair defect, is known to worsen with increasing Direct Chill (DC) cast ingot size. Further data gathered from mechanical testing of alloys within the semi-solid mushy zone is necessary to better understand hot tearing and improve ingot production.

First, RCM 0.9 fs modulus of toughness results show Al-10%Mg absorbed an average of 240 kJ/m3 while Al-1%Mg an average of 58 kJ/m3. The modulus of toughness at 0.9 fs matches the expected trend seen in hot tearing susceptibility between the two Al- Mg binary alloys. Longer dwell at elevated temperature during heat up is found to decrease Al-Mg alloy semi-solid modulus of toughness. Secondly, when above 90% of the non-equilibrium solidus, a basic model of diffusion shows a need for diffusion mitigation. Finally, using 7050 at 0.9 fs, a comparison between the traditional direct ramp method to the RCM demonstrates an average reduction from 389 kJ/m3 to 10.6 kJ/m3 in modulus of toughness. When compared to the direct ramp method, the RCM fracture surface contains more evidence of liquid grain boundaries and no sign of plastic deformation, indicating less back diffusion during testing. Increasing strain rate was found to increase specimen modulus of toughness at and above 0.9 fs but lower the modulus of toughness at 0.8 fs. RCM testing shows similarities between the direct ramp method at 0.9 fs and the mechanical response of the same alloy at 0.95 fs.

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