Relating grain size to the Zener-Hollomon parameter for twin-roll-cast AZ31B alloy refined by friction stir processing

An experimentally verified finite element model was used to estimate the strain rate and the temperature values which were, consequently, used in calculating the Zener-Hollomon parameter, Z-parameter, of twin-roll-cast (TRC) AZ31B after being refined by FSP (using range of spindle speeds of 600-2000 rpm and tool feed rates ranging from 75 to 900 mm/min). In the finite element model, an HCP specific Zerilli-Armstrong constitutive relation was used to describe the mechanical behavior of AZ31B. The resulting grain size values were experimentally measured for the observed microstructure of all processed samples. Dynamic recrystallization was identified to be the main mechanism involved in the grain refinement. A linear relation of the form ln d = a × ln Z + b was determined relating the average grain size (d) to the Z-parameter with a and b being equal to −0.23 and 8.79, respectively. These coefficients differed from values reported by others for AZ31 magnesium alloy with this difference being attributed to different material processing techniques used in the as-received condition. The resulting equation can be used in controlling the grain size during friction stir processing by varying the process parameters (spindle speed and tool feed) that would in turn affect the instantaneous value of the Zener-Hollomon parameter.