Abstract: Spark erosion is a method for producing nickel microspheres. To reduce the size of microspheres, a combined machining technology called ultrasound-aided electrical discharge machining (EDM) is proposed. To investigate the effect of non-electrical parameters on the size of microspheres, the experiments are carried out by changing ultrasonic parameters (frequency and intensity) and dielectric mediums (kerosene and pure water). The morphology and composition of microspheres are characterized by using scanning electron microscope (SEM) and X-ray diffraction (XRD). SmileView software is used to measure the size of the microspheres and OriginPro software is used for statistical analysis to determine the size distribution percentages. The results show the percentage of small particles (in the range of 0 to10 μm) increases significantly by introducing ultrasound. Meanwhile, the percentage of small particles decreases with increasing ultrasonic frequency and increases with increasing ultrasonic intensity. The size of microspheres prepared in pure water is generally smaller than kerosene. In the pure water, the percentage of microspheres with a smaller size (less than 1 μm) and a small size (less than 5 μm) are 6%～10% and 50%～80% respectively. The effect of ultrasound in pure water is better than kerosene on reducing the size of microspheres. The microspheres made up of almost pure nickel (99%) are produced in pure water. In addition, a few hollow microspheres are found in the experiments with ultrasound and the formation mechanism of the hollow spheres is analyzed preliminary.

Key words: dielectric mediums, nickel microspheres, ultrasonic parameters, ultrasound-aided electrical discharge machining