Steel 7225 surface ultrafine structure and improvement of its mechanical properties using surface nanocrystallization technology by ultrasonic impact

Abstract

In ultrasonic nanocrystalline surface modification (UNSM) technology, ultrasonic energy is used as a method to apply severe plastic deformations on metal surfaces. Therefore, the surface structure of the metal, which is micro in its normal state, is transformed into a nanostructure up to a certain depth. This method results in mechanical improvements such as hardness, fatigue, yield stress, and surface smoothness. In this research, by designing and manufacturing a vibrating tool and a required fixture for the operation, the effect of UNSM process on steel 7225 was studied. SEM analysis showed that the nanostructure was created at the surface of steel 7225. It showed that the fatigue life and yield stress of processed steel 7225 workpiece increased greatly when compared to those in its similar crude specimen, revealing the efficacy of these operations in increasing fatigue life. Ultimately, the relationship between the number of passes for the application of a number of equal impacts and surface smoothness, as well as the effect of the number of impacts on the hardness of steel at different depths were studied.