Numerical study of optimized processing condition in rolling strike ultrasonic nanocrystalline surface modification of copper

Ultrasonic nanocrystalline surface modification (UNSM) is state-of-art surface modification technique in which both the static and dynamic loads are contributed in work hardening of surface layer and amount of residual stress. The former performance causes microstructure refining while the later results to enhanced fatigue life. However, contribution of too many parameters in the process makes finding optimal parameter setting in which the process reaches to desired performance complex. In the present study, an attempt is made to find optimal combination of static force, tool tip diameter, tool tip material, linear velocity and vibration amplitude regarding the maximum plastic equivalent strain and the maximum affecting depth. The process is firstly modeled through finite element simulation in ABAQUS/EXPILICT software, and the results are further verified considering the residual stress distribution. Hereafter, a series of simulation runs are derived based on design of experiment to generate required data for optimization. In this stage, the run data are associated with response surface methodology to find optimal parameter setting regarding maximum PEEQ and maximum effective depth. The results showed that selection of 300 N static force, 9 mm tungsten carbide ball, 10 μm vibration amplitude and 1000 mm/min feed rate is optimum solution causes achieving maximum PEEQ of 3.44 at the depth of 200 μm. By finding optimal results, a series of USAT experiments have been carried out with optimum parameter setting and its hardness distribution, residual tress and microstructure has been compared with as received sample. It is found that the surface residual stress of the sample reaches from +40 MPa and reaches to –60 MPa after UNSM. On the other hand, the hardness of the sample enhances up to 50% after performing process under optimal setting. © 2019 The Authors