WE Colloquium: Measurement of Flow Stresses at High Strain Rates and Temperatures for Improved Simulation of Friction Stir Welding

Abstract

The modeling of friction stir welding has long been challenged by a lack of accurate flow stress data over the large range of temperatures and strain rates that are typical of the process. Most often, hot compression or hot torsion tests are used to measure flow stresses, but the strain rates and deformation modes associated with these tests do not emulate the high strain rate shearing that occurs near the friction stir welding tool.  An alternate method of measuring flow stresses for the modeling of FSW is proposed, where a flat tool configuration is employed to shear an AA 6061-T6 specimen at different rotational velocities and vertical loads. Temperatures in the specimen and tool are measured by thermocouples and the thickness of the shear layer created by the tool is measured from cross-sections after the experiments are completed.  An analytical model of the shear layer is employed to estimate material viscosity and flow stresses for each test condition. The resulting flow stresses were significantly lower than those measured by hot compression or hot torsion under similar temperature and strain rate conditions.

Bio

Michael Miles is a professor in the manufacturing engineering department at Brigham Young University. He has a B.S. degree in metallurgical engineering from University of Idaho, an M.S. in metallurgical engineering from Ohio State University, and a Ph.D. in Materials Science and Engineering from Ecole des Mines de Paris (Centre de Mise en Forme des Materiaux – Sophia-Antipolis). His research interests include solid state welding processes, microstructure/formability relationships in sheet metal, and modeling of joining and forming processes.