Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. Preet Singh, Advisor, MSE

Prof. Josh Kacher, MSE

Prof. Mathew McDowell, ME/MSE

Prof. Richard Neu, ME/MSE

Prof. Naresh Thadhani, MSE

Yudie Yuan, Ph.D., Novelis Corporation

"Understanding the Mechanisms and Parameters Affecting the Structural Corrosion of Sheet Gauge 7xxx Alloys"

Abstract:

The light weighting of the automotive structures is an effective method for increasing the fuel efficiency of the vehicles. Historically, use of aluminum alloys (5xxx and 6xxx) have been restricted to hang on parts and outer skin applications. Due to its high strength to weight ratio 7xxx series alloys offers exciting alternative options for replacing high strength steels in the automotive load bearing or structural applications. This class of alloys derive their strength via precipitation hardening mechanism. The nano sized precipitates are formed after the heat treatment (ageing) of the alloy at the super saturated solid solution state. Factors such as solutionizing temperature, duration, quench rate and subsequent ageing temperature affects the precipitates size and distribution. One of the key roadblocks for 7xxx alloys in penetrating such market is due to the concerns regarding the structural forms of corrosion such as intergranular corrosion (IGC) and stress corrosion cracking (SCC).

The objective of this work is to understand the effect of processing sequence, alloy composition, and joining methods on different forms of corrosion in sheet gauge 7xxx alloys. Grain size will be controlled by two distinct methods, namely recovery anneal of 2mm F-temper and reducing the percentage of the cold work on 10mm hot rolled material. The samples will be then subjected to IGC, SCC and hydrogen permeation tests to determine the underlying mechanism of the stress corrosion cracking. Additionally the grain boundaries will be characterized using scanning transmission electron microscope (STEM) for determining width of precipitate free zone, size and continuity of grain boundary precipitates between the finer and coarser grain materials. In alloy composition work, the effect of controlled addition of major alloying element (Cu) and minor alloying element (Zr) to 7xxx matrix and its resulting microstructure will be evaluated for its corrosion and stress corrosion cracking resistance. Specific focus will be on the effect of testing environment and second phase constituent particles on the localized corrosion resistance of these alloys. Finally, the gradient microstructure of 7xxx resistance spot welded (RSW) joints will be characterized and evaluated for its corrosion resistance. This work would also focus on evaluating the corrosion performance of dissimilar joints (7xxx to 5182) and post weld heat treatment. The concepts of the localized electrochemical technique will be extensively applied. This research initiative influences the 7xxx sheet gauge development in alloy design, manufacturing process selection and its end application.