The Origin of Coarse Grain Structure in Friction-Stir Welded AA5083 after Heat Treatment

Chen, Ke
Doctor of Philosophy, Ohio State University, Materials Science and Engineering, 2009.
Committee / Advisors
Robert H. Wagoner PhD (Advisor)
Glenn S. Daehn PhD (Committee Member)
Suliman A. Dregia PhD (Committee Member)

Abstract

Friction stir welding (FSW) takes place in the solid state, thus providing potential advantages of welds of high strength and ductility with fine microstructures. However, post-FSW heat treatment can produce very coarse grains, potentially reducing mechanical properties. AA5083-H18 sheets were friction-stir butt-welded using three sets of welding parameters representing a wide range of heat input. They were then heat treated for 5 minutes at 738 K (465 °C), producing grain sizes exceeding 100 microns near the top weld surfaces, and proceeding toward the bottom surface to various degrees depending on the welding parameters. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical metallography, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Vickers hardness testing were used to characterize the regions within welds. The mechanism responsible for forming the large grains was identified as abnormal grain growth (AGG). Based on a simple model of grain growth, the heat treatment is found to be near the critical condition for AGG to occur, with the principal determinants being the post-FSW grain size and fraction of low angle boundaries. Other possible factors (second-phase particle volume fractions and sizes, textures, dislocation content, and Mg solute distribution) were not significantly different in AGG and non-AGG regions. Based on these results, recommendations for changes in alloy, FSW conditions, and post-FSW heat treatment were identified to eliminate AGG. Here's the link to the .pdf:

http://search.ohiolink.edu/etd/send-pdf.cgi/Chen%20Ke.pdf?osu1244047069

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Henry