Defects, microstructure and mechanical behaviour upon multi-pass friction stir processing of magnesium alloy with spiral tool path

Abstract

Friction stir processing is an advance technique to modify the microstructure of metals for enhanced mechanical properties. Previous works have demonstrated that the friction stir processing of Mg alloys is feasible. These works are restricted to small areas or sample sizes. The objective of this work is to adopt spiral tool path strategy to process a complete blank of Mg alloy and study the effect of process parameters on the microstructure and hardness. The friction stir processed samples are prepared with different combinations of tool shoulder overlaps and tool rotation directions. Experimental results show fully consolidated processed Mg blanks are achieved with the tool rotation direction, which forces the advancing side towards the ‘yet to be processed’ side of the tool. Further, tool rotation direction plays a crucial role towards defect formation and retention upon friction stir processing of Mg alloy blanks with spiral tool path. The microstructural studies and hardness test are performed for the fully consolidated processed blanks. The microstructure of the processed region consists of refined grains, with banded regions comprising even finer grains. The friction stir processing of the as-cast Mg alloy also lead to the dissolution of second phase and more uniform distribution of the alloying elements in the Mg matrix. The grain refinement and second phase dissolution lead to enhanced hardness upon friction stir processing. Higher tool shoulder overlap promotes higher temperatures and plastic deformation, which leads to more grain refinement and second phase dissolution with corresponding increase in hardness levels. The local variation in average hardness within a processed sample corresponds well to the local variation of the average grain size. This work shows that large samples of Mg alloy can be friction stir processed towards microstructure modification and mechanical property enhancement, with appropriate combination of tool rotation direction and tool shoulder overlap.