Investigation on Thermal Effect Induced by Ultrasonic Vibration on Surface Deformation Behavior During Micro-forging

In order to investigate the relationship between ultrasonic energy and thermal energy in micro-forming processes, as well as the effects of the resulting heat on the surface deformation. A novel ultrasonic-assisted micro-forging test system is designed, which can offer 60 kHz frequency to the pure copper specimens, face-centered cubic, and proper melting point under amplitude from 0–3.5 μm. A piezoelectric transducer was also used to provide vertical vibration at a lower frequency of about 21 kHz and 1 kHz under the same amplitude for comparison. Furthermore, a technique for directly measuring the surface temperature of the specimen was proposed. A thermoelectric couple with a diameter of 0.1 mm was put into the machined dimple in the specimen surface with a depth of 0.2 mm and width of 1 mm. The entire process was monitored by a high-sensitivity dynamic force sensing system to ensure correct frequency and to distinguish between acoustic softening and impact effects. The experimental results indicate that the increase in surface temperature caused by acoustic softening is very limited, even at high frequencies. The significant increase in temperature is mainly due to the impact effect, which results in high deformation rates. However, as the amplitude continues to increase, the thermal expansion effect caused by the continuous increase in surface temperature will enhance the surface deformation resistance, but this indirectly enhances the impact effect, resulting in more reduction of surface roughness.