Optimization of ultrasonic nanocrystalline surface modification of copper and its influence on fatigue life

Abstract

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. In the present investigation, an attempt was made to find optimal setting of UNSM factors viz static pressure, vibration amplitude, spindle speed, feed rate and ball size on surface integrity factors such as surface roughness and hardness. Number of 32 experiments were conducted using small fraction central composite design to correlate input-output relationship. Grey relational analysis and desirability function was also applied to experimental data to identify a setting corresponds to maximum hardness and minimum surface roughness. The optimum samples were also subjected to fatigue tests as well the microscopic observations to find how much improvement occurs as a result of optimization with respect to as turned sample. It is found from both approaches that the setting of 0.03 MPa static pressure, 10 μm vibration amplitude, 100 rpm spindle speed, 0.08mm/rev feed rate and 8 mm strike size is an optimal parameter setting that results in surface roughness of 0.356 μm and hardness of 155 HV. The sample machined by optimum condition result in fatigue life cycles improvement more than 2.5 times in different levels of applied stress. The improvement can be attributed to combined effect of smoothed surface topography, fine microstructure, hardened surface layers and exerting compressive residual stress.