A surface composite layer enhances the mechanical characteristics of a surface while retaining the properties of the base material.Friction stir processing (FSP) is a method for forming surface metal matrix composites (SMMCs) that reinforce a surface with particles.In the current study,a new method entitled friction stir vibration processing (FSVP) was applied to form SMMCs on the surface of AZ91 magnesium alloy with SiC particles as the reinforcing particles.Contrary to FSP,in FSVP,the workpiece was vibrated normal to the processing line while the tool rotated and traversed.The microstmcture and mechanical properties of friction stir (FS) and friction stir vibration (FSV) processed specimens were evaluated.Additionally,the effects of vibration frequency and process parameters on different characteristics of FS and FSV processed specimens were studied.The results showed that the stir zone grains for FSV processed specimens were finer than those for FS processed specimens,and the second phase particles (SiC particles) had a more homogenous distribution in the former specimens than in the latter specimens.This was related to the effect of workpiece vibration during FSVP,which increased the material deformation and led to enhanced dynamic recrystallization and the breakdown of agglomerated SiC particles.The results indicated that the stir zone grain size decreased,and the distribution homogeneity of the SiC particles increased as vibration frequency increased.It was also observed that the stir zone grain size increased,and the mechanical properties of the processed specimens decreased as tool rotation speed increased.