Investigation of friction stir processing as a tool to improve microstructure and mechanical properties of aluminium alloys

The focus of this project is the improvement of the mechanical properties of light alloys, in particular, aluminium alloys, by microstructural modification using friction stir processing (FSP). Aluminium alloys are useful in many industries, e.g. automobile and ships manufacturing. FSP has emerged as a reliable, effective and efficient thermomechanical process due to its low running cost and inherent simplicity. FSP creates very fine and equiaxed grains in the stirred zone as a result of very complex material flow behaviour and dynamic recrystallization processes. This study aims to first gain an understanding this very complex material flow behaviour during FSP. This study was conducted on aluminium alloys 5005-H34 and 7075-T651 specimens using three different FSP tools with different pin designs, and various combinations tool rotational and traverse speeds. The application of FSP resulted in fine, fully recrystallised microstructures and the processed zone was defect-free for some of the pin designs. Significant grain refinement from an initial pancake-like microstructure with a grain size of about 192 μm in the base material to 10-20 μm in the processed regions was achieved for FSP 5005-H34 aluminium alloy. For FSP 7075-T651 aluminium alloy, grain size decreased from about 50 μm in the base material to 4-10 μm in the processed regions.