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.

搅拌摩擦加工是一种先进的技术，可以改变金属的微观结构以增强机械性能。先前的工作表明，镁合金的搅拌摩擦加工是可行的。这些作品仅限于小区域或样本量。这项工作的目的是采用螺旋刀具路径策略来加工完整的镁合金毛坯，并研究工艺参数对组织和硬度的影响。搅拌摩擦加工的样品是用刀肩重叠和刀具旋转方向的不同组合制备的。实验结果表明，在工具旋转方向上可以实现完全固结的已加工镁毛坯，这会迫使前进的一面朝工具的“待加工”一面。进一步，旋转方向对镁合金毛坯进行搅拌摩擦加工时，刀具旋转方向对缺陷形成和保持至关重要。对完全固结的加工坯料进行了显微组织研究和硬度测试。加工区域的微观结构由细晶粒组成，带状区域甚至包含细晶粒。铸态镁合金的搅拌摩擦加工还导致第二相的溶解和合金元素在镁基体中的更均匀分布。晶粒细化和第二相溶解导致摩擦搅拌过程中硬度的提高。更高的工具肩部重叠会促进更高的温度和塑性变形，从而导致更多的晶粒细化和第二相溶解，并相应增加硬度。加工样品中平均硬度的局部变化与平均晶粒尺寸的局部变化很好地对应。这项工作表明，通过适当地结合刀具旋转方向和刀肩重叠，可以对大型的Mg合金样品进行摩擦搅拌处理，以实现微观组织改性和提高机械性能。