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Spring 2009 Seminar Series
Monday, May 18, at 3:30 p.m.
Room 264 MacQuigg Labs
Ke Chen
PhD Candidate advised by Dr. Robert Wagoner
Materials Science and Engineering
The Ohio State University
The Origin of Coarse Grain Structure in Friction-Stir Welded AA5083 after Heat Treatment
Abstract
Friction stir welding (FSW) is a new welding technique (1991) that takes place in the solid state with advantages in terms of energy efficiency and high strength welds produced with fine grain size and microstructures. The post-FSW microstructure is determined by the complex thermomechanical conditions in the weld zone that produce dynamic recrystallization, recovery, and possible grain growth. However, FSW regions can exhibit rapid grain growth to form coarse grain structure during post-FSW heat treatment, thus introducing the possibility of degraded mechanical performance. In order to assess the mechanisms for coarse grain structure formation in aluminum alloys, AA5083-H18 sheets of about 1.6 mm thick were friction stir butt-welded using three sets of welding parameters representing a wide range of heat input, then heat treated for 5 minutes at 738 K (465 °C). Coarse grain structure starts at the top surface of the weld and proceeds downward to various degrees depending on the weld condition. Electron backscatter diffraction (EBSD), transmission electron microscopy, scanning electron microscopy, optical metallography, and hardness testing were used to characterize the microstructure and properties throughout the welds. The coarse grain structure was found to be driven by the reduction of interfacial energy instead of strain energy, and produced through the mechanism of abnormal grain growth (AGG). The presence or absence of AGG was consistent with HumphreysÕ model of grain growth informed by the observed microstructures. The principal determinants of AGG in these cases are grain size and grain boundary pinning from second phase particles, with consistent but smaller effects of texture and grain boundary misorientation. For the specific heat treatment used, AGG occurs for all regions with a grain size less than 2 mm and for none of the regions with a grain size greater than 2 µm.
Bio
Ke Chen was born in Zhejiang, China. He received his bachelor degree from Department of Materials Science and Engineering at Hefei University of Technology. He came to United States and started to pursue his PhD degree in 2005 in the Department of Materials Science and Engineering at The Ohio State University. His main research interest is the grain structure stability at elevated temperature. During his graduate study, Professor Robert H. Wagoner has been his advisor.
