Research On Pulsed Ultrasonic Assisted Arc Welding Characteristics

Continuous ultrasonic has been successfully used to control arc welding,which is named continuous ultrasonic-assisted arc welding(CU-AW).To explore the influence of ultrasonic output mode on arc welding,a new method of pu lsed ultrasonic-assisted arc welding(PU-AW)is proposed.In this paper,a PU-AW system was established.The pulsed ultrasonic was obtained by controlling the current output of ultrasonic power.The effect of pulsed ultrasonic on arc shape and droplet transfer was studied.Under the pulsed ultrasonic,the change mechanisms of arc shape and droplet transfer were discussed by numerical calculation and theoretical analysis.The process of pulsed ultrasonic assisted MIG welding(PU-MIG)of 2A14 aluminum alloy was studied.A numerical model of the pulsed ultrasonic field was prepared according to the standing wave equation.The distribution of pulsed ultrasonic field in arc space was calculated.The pulsed peak value and pulsed base value had the same sound fiel d distribution pattern.The intensity of the sound field was increased with the increase of ultrasonic amplitude.The shorter wavelength,the more complex sound field distribution in arc space.The maximum sound pressure at the first resonance height was obviously greater than that of the other resonance height.With the increase of resonance height,the maximum sound pressure decreases gradually.As for arc shape,a kind of pulsed arc shape was obtained by introducing pulsed ultrasonic,when the pulsed ultrasonic frequency was less than 20 Hz.The compression degree of arc shape in the pulsed peak period was greater than that in the pulsed basic period.When the pulsed ultrasonic frequency increased to 20 Hz,the pulsed arc shape was replaced by the stable compression arc shape.With the same average ultrasonic power,the influence of high-pulsed ultrasonic-assisted arc welding on arc shape was more significant than that of the CU-AW.PU-AW has a higher utilization efficiency of ultrasound.The peak power in high-frequency pulsed ultrasound played the leading role in the change of arc shape.As for droplet transfer,the arc length was obviously decreased under the ultrasonic resonance height.The droplet transfer mode of the projected transfer changed to the mixed transfer mode which included projected transfer and short-circuiting transfer.When the frequency of pulsed ultrasonic was lower than 20 Hz,the change of MIG welding arc length was pulsed.The MIG welding arc length was stably compressed when the pulsed frequency was increased to above 20 Hz.The cycle of droplet transfer in the PU-MIG welding was reduced obviously compared with the conventional MIG welding.With the increase of ultrasonic pulse frequency,the droplet transfer period had not changed.As for short-circuiting transfer,the droplet transfer frequency in the PU-MIG welding reached the maximum under the ultrasonic resonance height,which was increased obviously compared with the conventional MIG welding.When the pulsed frequency was 20 Hz and 50 Hz,the effect of pulsed ultrasonic on short-circuiting transfer cycles was better.With the introduction of pulsed ultrasonic,the stability of the welding process was improved according to the shrink of U-I cycle diagrams.The models of conventional arc and ultrasonic arc were established.Based on the numerical simulation of the arc,it was found that the voltage,temperature,and pressure of the ultrasonic arc were increased compared with that of the conventional arc.Combined with theoretical derivation,the increase of kinetic energy of particles in the arc was the primary cause of the increase in the ultrasonic arc temperature.Under the lower pulsed frequency,arc thermal inertia was the direct cause of pulsation change of arc shape.According to the balance of static forces,the acoustic radiation force was the fundamental reason for promoting the droplet transfer.The welding process of 2A14 aluminum alloy in the PU-MIG welding was studied.When the pulse frequency was 1 Hz,the weld surface had obvious fish-scale patterns.The fish-scale patterns of the weld surface disappeared when the pulse frequency increased to 5 Hz.Compared with conventional MIG welding,the weld penetration in the PU-MIG welding was increased with the increase of pulsed ultrasonic frequency.The columnar grains near the fusion line in the MIG w elded joints changed in equiaxed grains.The grain size and the second phase size were decreased by applying pulsed ultrasonic.The microhardness and tensile strength of welded joints in the PU-MIG welding had been improved than that of the MIG.When the ultrasonic power of the CU-MIG welding equaled the peak pulsed ultrasonic power of PU-MIG welding,they had the same improvement of weld appearance and weld microstructure.Therefore,compared with the CU-MIG welding,the effect of pulsed ultrasound on the welding process had better utili zation efficiency of ultrasound.