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 α and transformation products of the β phase. Such microstructures reflect severe deformation at temperatures up to ∼900 °C in the α+β two-phase region for this NAB material. Increasing rotation and traversing rates, coarse Widmanstätten α near the surface in contact with the tool became apparent. The appearance of this constituent reflects nearly complete transformation to β 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.