Microstructural Evolution during Friction Stir Spot Welding of AZ31 and Its Effect on the Corrosion Resistance of the Joint

Abstract

In recent years, the corrosion resistance of friction stir spot welds made in magnesium alloys has been the focus of many research projects. Previous reports documented the cathodic behavior of the stir zone (SZ) with respect to the base material, causing increased localized corrosion in the region adjacent to the weld, which is known as the thermo-mechanically affected zone (TMAZ).1,2 While the electrochemical interaction between the SZ and the base metal have been thoroughly studied, the cause for the ennoblement of the weld nugget remains largely unknown. Various studies suggested that the microstructural evolution occurring during the welding process was responsible for the increased corrosion resistance of the stir zone, but the specific contribution from each microstructural feature has not been examined. The present study examines the effect of several microstructural features that are altered during the welding process, ultimately determining their individual contribution to the overall effect on the corrosion characteristics of the joint. Specifically, this investigation examines the effect of (i) residual stress, (ii) grain size and orientation, and (iii) second phase precipitation on the corrosion behavior of FSSW in magnesium alloys. The effect of microstructural variation on the corrosion resistance properties of the stir zone has been evaluated through characterization and electrochemical testing. The corrosion potential and current of the stir zone were determined using microcapillary polarization in 0.1M NaClO4, a localized electrochemical technique developed in previous work.2 Changes in grain size and orientation were monitored through electron back scattered diffraction (EBSD). The effect of residual stress was examined through a series of stress relieving heat treatments. Finally, the effect of second phase particles was investigated by electron microscopy (EM) and nuclear magnetic resonance (NMR). The effect of friction stir spot welding on the microstructure and electrochemical properties of AZ31 is summarized in table I. Each microstructural change was found to have a unique effect on the electrochemical properties of the stir zone region. The relatively small changes in grain size and residual stress induced by the welding operation have been shown to have negligible effects on the corrosion behaviour of the weld nugget.3 Investigation of the grain orientation revealed weaker basal texture in the stir zone than in the base metal. As a result, changes in grain orientation were found to be responsible for the reduction in the corrosion potential and current of the stir zone. Changes in the size and presence of second phase precipitates (Mg-Al and Al-Mn intermetallics) were determined to be the most dominant factor determining the electrochemical properties of the stir zone. Dissolution of these intermetallics eliminated the harmful microgalvanic cells formed between the particles and the surrounding matrix, as well as increased the overall aluminum content in the α-Mg matrix. Ongoing work is focused on producing a model that correlates the welding parameters to the microstructural features present in the SZ region and its subsequent electrochemical properties.