Abstract: The arc-ultrasonic welding technology is an effective method to refine the weld grain and improve the mechanical properties of joints. To study the excitation and function mechanism of ultrasonic in the arc-ultrasonic MIG welding process and better control the arc-ultrasonic, the MIG welding arc is modulated by the high frequency signal to excite the ultrasonic wave. The electrical and acoustic excitation characteristics of the welding arc are obtained through signal acquisition and analysis during the welding process. The approximate relationship between the input impedance of the welding arc, the sound pressure of the excited ultrasonic and the excitation parameters is preliminarily identified. The effect of arc-ultrasonic on weld appearance and weld metal microstructure is also clarified. It is concluded that external ultrasonic excitation causes the oscillation of welding arc voltage at the base value, and the input impedance of the MIG welding arc to the ultrasonic excitation is related to the excitation frequency. The greater the excitation frequency is, the lower the excited triangular wave current becomes, and then the larger the input impedance gets. The ultrasonic response frequency of the MIG welding arc shows a same value with the excitation frequency, while the ultrasonic sound pressure amplitude is related to the excitation energy. The smaller the excitation frequency is and the higher the excitation voltage is, the greater the excitation energy is, and the greater the ultrasonic sound pressure amplitude becomes. The introduction of ultrasonic arc can increase weld penetration. Compared to conventional MIG welding, the weld penetration obtained using the ultrasonic arc technique can be increased by 23% when U_e is 55 V and f is 70 kHz. The grain size of the weld metal is refined and the precipitates in the weld metal are more dispersed.

Key words: 6061 aluminum alloy, MIG welding, ultrasonic arc, weld appearance, microstructure