Functionally graded materials (FGMs) have been actively researched owing to their potential applications in aerospace, biomedical engineering, sensing, and energy. FGMs exhibit locally different properties because they are manufactured from multiple materials. Recently, mechanical processing has been utilized to produce FGMs, during which the plastic strain induced by burnishing causes hardening of the surface and increases the compressive residual stress. In our previous study, we reported that the component of shot peening media diffused to the shot-peened surface during hot-shot peening. Hot-shot peening can be used to control the local material composition, while shot peening, which induces plastic deformation, can be used to improve the mechanical properties of the material. The two effects of hot-shot peening can be combined to produce FGMs. The results of this study showed grain refinement and work hardening at 0.25 mm in the hot-shot-peened surface. A tensile residual stress was observed on the bare surface, whereas a high compressive residual stress was observed on the hot-shot-peened surface. A media component diffused layer was not observed on the bare surface, although it was observed on the high-injection air pressure hot-shot-peened surface. Furthermore, a diffusion layer with a larger media component was observed on the hot-shot-peened surface in which the grain boundaries had increased. This was due to the dynamic recrystallization induced by the hot-shot peening process. Thus, the media component diffused layer observed on the hot-shot-peened surface can be grow thicker by increasing the number of grain boundaries.
