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.

Friction stir welding (FSW) has enjoyed great success in joining aluminum alloys. As lightweight structures are designed in higher numbers, it is only natural that FSW is being explored to join dissimilar aluminum alloys. The use of different aluminum alloy combinations in applications offers the combined benefit of cost and performance in the same component. This review focuses on the application of FSW in dissimilar aluminum alloy combinations in order to disseminate research this topic. The review details published works on FSWed dissimilar aluminum alloys. The detailed summary of literature lists welding parameters for the different aluminum alloy combinations. Furthermore, auxiliary welding parameters such as positioning of the alloy, tool rotation speed, welding speed and tool geometry are discussed. Microstructural features together with joint mechanical properties, like hardness and tensile strength measurements, are presented. At the end, new directions for the joining of dissimilar aluminum alloy combinations should guide further research to extend as well as to improve the process, which is expected to raise further interest on the topic.

Poor tribological properties restrict structural applications of aluminum alloys, surface composites of aluminum alloys have gained more attention in material processing. Addition of solid lubricant reinforcement particles along with abrasive ceramics contributes to the enhancement of tribological performance of surface composites. In the present study, solid-state technique, friction stir processing (FSP) was used to develop mono (B₄C) and hybrid (B₄C+MoS₂) surface composites in AA6061-T651 aluminum alloy. The hybrid surface composites were produced by varying an amount of MoS₂. Multipass FSP with different direction strategy was adopted for achieving uniform distribution of reinforcement powders in the aluminum matrix. Microstructure analysis showed a uniform dispersal of reinforcement particles without any clustering or agglomeration in the processing zone. Microhardness and wear performance of mono and hybrid composites improved in comparison to the base metal. Mono surface composite exhibited the highest hardness while hybrid surface composite (75%B₄C+25%MoS₂) achieved the highest wear resistance. This was attributed to the solid lubricant nature of MoS₂. Furthermore, dissolution of strengthening precipitate condition during multipass FSP without reinforcement particles resulted in the reduction of hardness and wear resistance.