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

CORROSION MECHANISMS AND MECHANICAL BEHAVIOR OF ADDITIVELY MANUFACTURED 7050-BASED HIGH STRENGTH ALUMINUM ALLOY

ABSTRACT

The inherent benefits of metal additive manufacturing (AM) such as its ability to design complex parts, high efficiency, and lower lead times, when combined with the material benefits associated with the high strength aluminum alloys – high strength to weight ratio, high corrosion resistance and fatigue properties makes them attractive candidates for the aerospace, defense, and automotive industries. However, adoption of high strength aluminum alloys in the AM space has been particularly limited due to hot tearing and solidification defects associated with their columnar microstructure. Recent developments in inoculation process by addition of nanoparticles/in-situ reactive constituents have proven to promote equiaxed grain growth in high strength aluminum alloys mitigating the defects and improving their printability. The AM process with extremely high solidification rates, like for laser powder bed fusion (LPBF) along with the presence of inoculating constituents and alloying elements in high strength aluminum alloys makes the resulting microstructure complex and unique, compared to the traditional wrought alloys. As such, the material behavior of AM high strength aluminum alloys is not well understood, and the post processing treatment done to impart a balance of corrosion resistance and mechanical properties needs to be optimized for the desired microstructure.

Aluminum 7050 alloys are precipitation strengthened alloys used for aerospace applications where high corrosion and stress corrosion cracking (SCC) resistance is required in addition to the mechanical property requirements. The objective of this work is to understand the influence of microstructure on corrosion and mechanical behavior of AM 7050-based aluminum alloy in response to various post-processing conditions and thermal treatments. AM 7050-based alloy fabricated via LPBF process with an equiaxed grain structure is subjected to stress relieving, hot isostatic pressing, and a combination of solutionizing and aging treatments. Multi-scale microstructural characterization using SEM, EBSD, and TEM is utilized to understand the grain size distribution, identify the constituent particles, their size and distribution. Corrosion and SCC behavior of the AM 7050-based alloy subjected to post-processing and thermal treatment combinations is investigated using scanning vibrating electrode technique (SVET), cyclic polarization, electrochemical impedance spectroscopy, and slow strain rate tests under different environments. Mechanical behavior is studied using uniaxial tensile tests and hardness measurements. The goal of this research is to correlate microstructure with the corrosion, SCC, and mechanical behavior of AM 7050-based aluminum alloy and to understand the underlying corrosion mechanisms. The results of this work will be beneficial for post processing optimization, microstructure-sensitive design, and adoption of LPBF 7050-based aluminum alloy for future aerospace application.