THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Thursday, October 5, 2023

Time: 12:00 PM

In-person

MRDC Room 4211

or

Virtual via Teams

Teams Link

will be held the

DISSERTATION DEFENSE
for

Rupesh Rajendran

"Corrosion Mechanisms and Mechanical Behavior of Additively Manufactured 7050-Based High Strength Aluminum Alloy"

Committee Members:

Prof. Preet Singh, Advisor, MSE

Prof. Hamid Garmestani, MSE

Prof. Joshua Kacher, MSE

Prof. Matthew McDowell, MSE/ME

Prof. Christopher Saldana, ME

Jeffrey Eisenhaure, Ph.D., Northrop Grumman Corporation

Abstract:

Metal additive manufacturing (AM), particularly the Laser Powder Bed Fusion (L-PBF) process enables the creation of complex parts with high design freedom, reduced material wastage, and improved performance; challenges that are difficult to overcome using traditional manufacturing processes. High strength aluminum alloys such as AA 7000 series offer superior strength to weight ratio, excellent corrosion resistance, and fatigue properties, making them attractive for applications within the aerospace, defense, and automotive sectors. Nonetheless, the adoption of high-strength aluminum alloys in the realm of additive manufacturing has faced notable constraints. This is attributed to the solidification and hot tearing defects formed during the AM process, stemming from their columnar microstructure. Recent advancements in the inoculation process by addition of nanoparticles/in-situ reactive constituents have demonstrated their ability to promote equiaxed grain growth in the high-strength aluminum alloys, effectively mitigating the defects and enhancing their printability. The rapid solidification rates of L-PBF process along with the presence of inoculating particles and alloy constituents leads to complex and unique microstructures, much different from the traditional wrought alloys. Consequently, there is a lack of in-depth understanding regarding the material behavior of high-strength aluminum alloys fabricated via AM process with the incorporation of inoculants. Moreover, the post-processing treatments required to attain the desired microstructure for improved corrosion resistance and mechanical properties require optimization.

The objective of this research work is two-fold. First, to understand the post-processing effects of an inoculated L-PBF 7050-based high strength aluminum alloy on the microstructure and its evolution. Second, to understand the microstructural effects on corrosion, stress corrosion cracking (SCC) resistance, and mechanical behavior of the alloy. The as printed material is subjected to stress relieving, hot isostatic pressing, and a combination of solutionizing and aging heat treatments. Multi-scale microstructural characterization using SEM, EBSD, and TEM/STEM is utilized to understand the grain size distribution, identify the constituent particles, including their sizes and distribution. Corrosion and SCC behavior of the AM 7050-based alloy subjected to post-processing is investigated using scanning vibrating electrode technique, cyclic polarization, electrochemical impedance spectroscopy, and slow strain rate tests. Mechanical behavior is studied using uniaxial tensile tests and hardness measurements. The results are compared to a commercial grade wrought AA 7050 alloy to understand the key differences. The results show that the size, nature, and distribution of the constituent particles is unique for the AM 7050-based alloy, and it highly depends on the post-processing route. The inoculant addition results in formation of composite like microstructure with intermetallic particles which acts as sites for elemental segregation. In summary, AM 7050-based alloy shows enhanced corrosion resistance and mechanical properties that are on par with those of an equivalent wrought alloy. This research work provides input for post-processing optimization and demonstrates the potential of AM 7050-based high strength alloys as promising candidate for future aerospace applications.