Dynamic Recrystallization Process and Its Influence on the Microstructure and Mechanical Behavior of a Friction Stir Processed Mg Alloy

In an earlier neutron diffraction study of the longitudinal residual stresses in friction stir welded magnesium alloy plate, a significant softening near the stir zone was observed, which was attributed to the reduction of the yield strengths in the stir zone due to the changes in the crystallographic texture across the weldline. 

Recently, the relationship between the texture variations and mechanical behavior in the friction stir processed (FSP) magnesium plate was further investigated through a fundamental studies of the dynamic recrystallization processes during the FSP.  First, a series of high-temperature compression tests were performed as a function of temperature, strain rate, and loading direction to understand the constitutive behavior during the dynamic recrystallization of the Mg alloy in terms of the grain refinement and the texture development. Subsequently, using the constitutive model as a guideline, a series of FSP specimens were prepared with a wide range of thermo-mechanical input by varying key processing parameters systematically. Neutron diffraction results show a dramatic change in texture in the stir zone, clearly manifesting the effect of processing parameters on the deformation and recrystallization processes during the FSP.

In this talk, using the neutron and x-ray diffraction results, in combination with electron backscattering diffraction and three-dimensional transient fluid dynamics modeling, the recrystallization mechanisms responsible for the microstructure development during FSP of Mg alloy will be presented.  In addition, a potential for the controlled manipulation of microstructures (grain size and texture) using FSP for an enhanced formability of a magnesium plate will be discussed.