High strain rate superplasticity via friction stir processing (FSP): A review

Abstract

High strain rate superplasticity by the friction stir processing (FSP), an adaptation of the friction stir welding (FSW), is summarized in this overview article. As a common severe plastic deformation (SPD) processing technique, the microstructures prepared by FSP are characterized by fine grain sizes, being homogeneous with fragmented and dispersed particles, and having a high proportion of high-angle grain boundaries. These attributes are beneficial to the superplastic forming operations at high strain rates and low temperatures. In this monograph, the principles of superplasticity are reviewed, where the importance of grain boundary sliding (GBS), strain rate sensitivity index, grain refinement, and deformation temperature on the remarkable enhancement of ductility is emphasized. Afterwards, FSP is introduced and the effects of the main processing parameters on the heat input and grain size are critically discussed. Finally, the recent progress in the application of FSP for processing of superplastic materials is thoroughly overviewed and the influence of thermal stability against grain growth, addition of alloying elements to form pinning particles, external cooling for obtaining ultrafine grained (UFG) microstructure, FSP process variables such as tool rotation rate and traverse speed, and multi-pass FSP is summarized. Accordingly, this overview presents the opportunities that FSP can offer for controlling the superplastic behavior of materials.