The influence of friction stir processing parameters on microstructure of as-cast NiAl bronze

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 alpha and transformation products of the beta phase. Such microstructures reflect severe deformation at temperatures up to similar to 900 degrees C in the alpha + beta two-phase region for this NAB material. Increasing rotation and traversing rates, coarse Widmanstatten alpha near the surface in contact with the tool became apparent. The appearance of this constituent reflects nearly complete transformation to beta during FSP with peak temperatures of similar to 1000 degrees 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.