Abstract Increasing demand of lightweight structures with exceptional properties elicits materials processing and manufacturing technologies to tailor blanks in order to achieve or enhance those prerequisite properties. Friction stir processing (FSP) is a solid-state material processing technique, which was derived from friction stir welding (FSW). Initially, FSP was invented to refine the microstructure in way that superplasticity in a material can be achieved. Afterward, FSP has gained much more attraction as a solid-state grain refinement technique to improve the mechanical, tribological, and corrosion properties in a wide range of low strength non-ferrous and high strength steels. FSP is well capable to produce material with microstructure in range of few micron to nanoscale, depending on the processing conditions. Researchers have investigated FSP at different process parameters such as tool rotation and travel speeds, number of passes, and additional cooling in order to evaluate the impact on the resulting properties for different alloys. Recently, FSP has begun to modify the microstructure and properties in hard alloys and superalloys with some modifications in FSP tooling system. Furthermore, FSP has shown great potential to repair or modify the weld or coating structure by microstructure refinement. Therefore, the present review will discuss the state-of-the-art of FSP under the main categories of microstructure evolution, and effect of process parameters. This review also provides a comprehensive summary of research progress on FSP in different materials i.e. aluminum, magnesium, copper, and steels with the contents much emphasized on the microstructure refinement in terms of average grain size and resulting properties like hardness, tensile, wear, and corrosion. Finally, FSP as a new post-processing approach in weld or coating structure has been discussed.

中文翻译：

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摩擦搅拌加工作为固态晶粒细化技术的最新发展：微观结构演变和性能增强

摘要 对具有特殊性能的轻质结构的需求不断增长，需要材料加工和制造技术来定制坯料，以实现或增强这些先决条件。摩擦搅拌加工 (FSP) 是一种固态材料加工技术，它起源于搅拌摩擦焊 (FSW)。最初，FSP 的发明是为了细化微观结构，从而实现材料的超塑性。后来，FSP 作为一种固态晶粒细化技术，在广泛的低强度有色金属和高强度钢中改善机械、摩擦和腐蚀性能，获得了更多的吸引力。FSP 能够很好地生产具有从几微米到纳米级微观结构的材料，具体取决于加工条件。研究人员研究了不同工艺参数下的 FSP，例如工具旋转和行进速度、道次和额外冷却，以评估对不同合金所得性能的影响。最近，FSP 已开始修改硬质合金和高温合金的微观结构和性能，并对 FSP 工具系统进行一些修改。此外，FSP 已显示出通过微观结构细化修复或修改焊缝或涂层结构的巨大潜力。因此，本综述将在微观结构演化的主要类别和工艺参数的影响下讨论 FSP 的最新技术。本综述还全面总结了 FSP 在不同材料（即铝、镁、铜、和钢的含量非常强调在平均晶粒尺寸和由此产生的性能（如硬度、拉伸、磨损和腐蚀）方面的微观结构细化。最后，讨论了 FSP 作为焊接或涂层结构中一种新的后处理方法。