The influence of friction stir processing parameters on microstructure of as-cast NiAl bronze
Abstract
The influence of friction stir processing (FSP) parameters on the evolution of microstructure in an
equilibrium-cooled, as-cast NiAl bronze (NAB) material was evaluated by optical microscopy (OM) and
transmission electron microscopy (TEM) methods. A threaded pin tool was employed and tool rotation
and traversing rates were varied in order to examine the spatial variation of stir zone microstructures in relation to FSP parameters. For processing at low rotation and traversing rates, the
microstructure throughout the stir zone consists of elongated and banded grains of the primary a
and transformation products of the 1 phase. Such microstructures reflect severe deformation at
temperatures up to �900 °C in the a + 1 two-phase region for this NAB material. Increasing rotation
and traversing rates, coarse Widmanstätten a near the surface in contact with the tool became
apparent. The appearance of this constituent reflects nearly complete transformation to 1 during
FSP with peak temperatures of �1000 °C. Also, complex stir zone flow patterns, often referred to as
onion ring structures, become distinct in the mid regions of the stir zones as rotation and
traversing rates increase. Schematic representations illustrating the effect of FSP parameters on
thermal cycles at various locations in stir zones were prepared based on microstructure
observations. Thus, processing at higher rotation and traversing rates results in higher peak
temperatures near the surface in contact with the tool but also in steeper temperature gradients
when compared to lower rotation and traversing rates.
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