PhD Dissertation Defense 

Materials Science and Engineering Graduate Program
Advisor: Professor Jörg M.K. Wiezorek

Title:

Correlating Rapid Solidification Microstructure Morphology and Mechanical Properties in Hypo-eutectic Al- Cu Alloy

Date: 

Wednesday, May 25, 2022

2:00 PM

Join Zoom Meeting:

Link: https://pitt.zoom.us/j/91010447332

Passcode: 089675

Meeting ID: 910 1044 7332

Committee: 

Committee Chair: 

Professor Jörg M.K. Wiezorek,

Department of Mechanical Engineering 

and Materials Science  

Committee Members: 

Professor Guofeng Wang,

Department of Mechanical Engineering 

and Materials Science

Professor Tevis DB Jacobs,

Department of Mechanical Engineering 

and Materials Science

Professor M. Ravi Shankar,

 Department of Industrial Engineering

ABSTRACT:

Scanned laser melting (SLM) and subsequent re-solidification results in the formation of solidification microstructures that form under continuously changing conditions.

This study aims to advance our understanding of the mechanical properties and plastic deformation behavior for the non-equilibrium multiphase microstructures that can be accessed by Al-10at.%Cu hypo-eutectic alloy during rapid solidification (RS) after SLM. We combined nanoindentation with microstructural analyses by SEM and TEM to study processing-microstructure-property relationships. At low scan laser velocity (V_L=3mm/s), a microstructure gradient develops as the solidification rate (V_SL) increases from initially V_SL=0mm/s at the bottom to a maximum of V_SL≈V_L=3mm/s at the top of the melt pool. The cellular α-Al(Cu), located centrally in the melt pool, consistently showed Cu-solute supersaturation by 20% relative to the equilibrium solid solubility limit of ~2.5at% and exhibited 36% hardness increase relative to the α-Al(Cu) in the as-cast state. The experimentally observed solute trapping in the α-Al(Cu) cells of the RS microstructure was found to be consistent with predictions from solidification theory using a cellular/dendritic growth model.

By increasing V_L to higher scan velocities, i.e., 1m/s≤V_L≤4ms, the maximum solidification rates achieved during RS of the alloy melt pools increase accordingly to the m/s range. At these large solidification rates different microstructure morphologies are revealed. They included columnar grain morphologies with continuous θ-Al₂Cu, columnar grains with discontinuous θ/θ’-Al₂Cu and banded morphology region. The nanoindentation of these characteristic RS microstructure morphologies revealed hardness increases relative to α-Al in the as-cast state (1.1GPa) and the supersaturated cellular primary α-Al(Cu) (1.5GPa) observed for RS microstructures after scanned laser melting at low scan speed (V_L=3mm/s). Hardness values of up to 3.32GPa and 2.98GPa have been determined for the columnar grains with continuous θ-Al₂Cu and discontinuous θ’-Al₂Cu, respectively. The banded region showed hardness values between 2.95 to 3.35GPa.

Isothermal annealing of the RS microstructures obtained after scanned laser surface melting at scan speeds of 2m/s and 4m/s at temperatures of 180˚C, 230˚C and 280˚C showed transformations with rapid kinetics establishing unusual populations of second phase Al₂Cu related particles with fine scale and high density. The mechanical property evolution for morphologically distinct regions in the alloy microstructure during annealing has been determined.