Microstructural effects of multiple passes during friction stir processing of nickel aluminum bronze

Abstract

Friction Stir Processing (FSP) is an emerging technology based on Friction Stir Welding that allows the near surface regions of a material to be selectively processed to improve the material properties. Single and multiple pass samples of nickel aluminum bronze were examined and their microstructures studied and compared using optical microscopy and scanning electron microscopy. FSP conditions were 800 revolutions per minute (RPM), with a tool traversing speed of 101.6 mm per minute (4.0 inches per minute, or IPM), or 1200 RPM with 50.8 mm per minute (2.0 IPM). Quantitative microscopy methods were then employed to determine the approximate volume fraction of beta transformation products and the estimated peak temperatures at various points in the stir zones of the as-processed samples. The results showed that, in material that had undergone multiple, successive FSP passes, microstructural effects of the FSP include a refined, homogeneous grain size and a reduction in the volume fraction of beta transformation products when comparison is made to single pass material. The reduction in beta transformation products indicated that a reduction in the estimated peak temperature in the stir zone had occurred in the samples having undergone multiple FSP passes.