Localized Corrosion of Friction Stir Welded Magnesium AZ Alloys

Magnesium alloys, being the lightest engineering alloys, have the potential to replace ferrous and aluminum alloys in structural applications in the transportation sector. Thus far, their use has been limited largely due to challenges associated with their inherently poor corrosion resistance properties, a problem that is accentuated when the alloys are welded. The work in this dissertation elucidated the mechanisms governing the corrosion behaviour of friction stir welded magnesium AZ alloys. The investigation utilized an integrated approach consisting of several corrosion measurement techniques (both bulk and localized measurements) to examine the effect of individual microstructural features on the overall corrosion behaviour of the joint. Dissolution of second phase precipitates was determined to govern the corrosion behaviour of the joint by enhancing the corrosion resistance of the weld nugget. Based on the findings, a mixed potential theory model correlating changes in second phase particle size and distribution to the corrosion behaviour of friction stir spot welded AZ31 was constructed. The mechanism governing the corrosion resistance of similar friction stir welded AZ31B joints did not apply to dissimilar AZ31/AZ80 welds. Instead, the corrosion of dissimilar joints was governed by the galvanic coupling between the dissimilar magnesium alloys. Based on the corrosion studies, plasma electrolytic oxidation (PEO) coatings were identified as a promising corrosion mitigating strategy for welded magnesium structures. The application of this coating to friction stir welded specimens improved the corrosion resistance of both similar and dissimilar types of joints. The corrosion mechanism was also altered, as the corrosion of coated specimens was principally governed by the imperfections of the PEO coating, and not by the microstructural changes caused by the welding operation. The findings of this study provide theoretical knowledge that will enable the optimization FSW processing to permit the use of magnesium alloys in the automotive industry.

Department dc:contributor.department

Materials Science and Engineering

Year dc:date.issued

2019

Author dc:creator

• Savguira, Yury

Advisors dc:contributor.advisor

• Thorpe, Steven J.
• North, Tom H.

• Corrosion
• Friction Stir Welding
• Magnesium
• Microcapillary Polarization
• Microstructural Characterization
• Plasma Electrolytic Oxidation Coatings

Handle dc:identifier.uri

http://hdl.handle.net/1807/97036

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/97036