Yuan Li1 Zhili Feng2 Ke An3 Hahn Choo1

1, University of Tennessee, Knoxville, Tennessee, United States
2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States

The friction stir welding/processing (FSW/FSP), that uses the unique principles of the severe plastic deformation method, has a great potential as a novel method for the fabrication of bulk ultrafine-grained Mg alloys. Furthermore, there is a great potential to enhance the formability of Mg alloys significantly by using the FSP technique. For example, recent studies show that the FSP can be used for texture manipulation in Mg alloys resulting in significant improvements in mechanical behavior. However, the physics of the microstructure development process during FSP is still not clearly understood. In particular, very limited effort has been made for the fundamental understanding of specific roles that twinning plays in the dynamic recrystallization (DRX) mechanisms, texture changes, and resulting grain refinement. The focus of the current project is to study the transient phenomenon of DRX and critical role that twinning plays in the grain refinement and crystallographic texture development during FSP using in-situ, real-time neutron diffraction measurements.

In this talk, we will present the experimental results from the neutron diffraction measurements performed during FSP of Mg alloy plates using a portable FSP machine at the VULCAN beamline at Spallation Neutron Source. A commercial hot-rolled AZ31B Mg alloy plate was used in this study. The ranges of the FSP parameters used are: 1-5 mm/sec traveling speed and 200-1,500 rpm clockwise FSP tool rotating speed. These conditions allowed us to vary the thermo-mechanical input (in terms of Zener-Hollomon parameter, Z) by more than two orders of magnitude covering both low Z (slip dominant) and high Z (twin prevalent) regimes identified in previous constitutive studies. The main measurement scheme used was quasi-steady state measurement, where the FSP tool is stationary and the Mg plate travels, while neutron-diffraction measurements were performed at a fixed distance from the FSP tool pin (stir zone) over a period of time necessary for adequate statistics. These measurements were performed as a function of distance from the FSP tool pin for a given processing condition. Then, another set of measurements were performed at a different processing condition (Z) and on. We measured five locations each several millimeters apart starting from the tool pin and for four Zener-Hollomon parameters covering low Z, mid Z, and high Z conditions. The variations in the onset and extent of characteristic grain rotations during FSP as a function of location from the tool pin (translated later as a transient evolution along the FSP line) and the Zener-Hollomon parameter (low Z vs. high Z) will be presented. Finally, the in-situ diffraction data will be correlated to our constitutive studies on DRX mechanisms using a Gleeble system.