Ultrasonic nanocrystalline surface modification of low strength aluminum alloy: trade-off between surface integrity and production rate aiming at desired fatigue life

In processing of metallic material by ultrasonic nanocrystalline surface modification (UNSM), obtaining high quality sample with desired surface property needs a careful control on processing factors. The condition would be more critical when the process is designed for match production. In such condition, both of surface integrity and production rate should be taken into account. To meet both of goals, in the present study, an experimental work was made to analyze how the UNSM process factors influences the surface integrity under desired values of production rate. In order to do so, series of experiments based on small fraction central composite design were carried out including static pressure, vibration amplitude, spindle speed, feed rate, and ball size as input parameters. Also, the surface roughness, compressive residual stress, and machining time were considered performance measures. A hybrid optimization approach including gray relational analysis and desirability function was proposed to obtain desired surface integrity under minimum value of machining time. Different criteria based on importance of production rate and surface integrity were separately defined and controlled by variation of weight factors. It is found that considering the production rate as dominant criterion leads to production of samples with maximum high cycle fatigue life. This is due to selection high spindle speed that enhances both production rate and surface quality. On the other hand, considering overall value of surface integrity as dominant criterion of optimization leads to producing sample with desired values of low cycle fatigue life. This effect can be attributed to selection of low spindle speed that deteriorates surface finish but enhances the compressive residual stress.