Microstructure and texture evolution of complete Mg-3Al-0.2Ce alloy blanks upon multi-pass friction stir processing with spiral strategy

Abstract

The objective of this study is to investigate the microstructure and texture evolution upon friction stir processing with multiple passes and spiral strategy, of Mg-3Al-0.2Ce alloy. Friction stir processing is a promising technique for grain refinement. Complete blanks of magnesium alloy are friction stir processed with all the combinations of two tool rotation speeds and two tool overlaps between passes. Optical microscopy, electron back scattered diffraction and texture analysis of the processed cross-sections are performed. Two distinct regions are observed in the processed blanks: banded region and non-banded region. Banded regions consist of much smaller grains as compared to the non-banded regions. The banded regions appear regularly at a distance corresponding to the tool overlap. The banded regions become more prominent at higher tool rotation speed and tool overlap. For the samples processed with higher tool overlap, the difference in grain sizes of banded and non-banded regions is significant, and interfaces between banded and non-banded regions are easily noticeable, which suggests bimodal microstructure. A tilted basal texture is revealed for all the processed samples. This is different from the oppositely tilted basal texture observed from the single pass friction stir processing of magnesium. The difference is attributed to the presence of resulting microstructure from both advancing and retreating side upon single pass friction stir processing. While for multi-pass friction stir processing with spiral strategy, the microstructure resulting from retreating side alone is present in the processed blanks. The close packing of similar microstructure from retreating sides of successive passes also led to higher texture index.