Research On Acousto-electric Properties Of Arc And Microstructure And Performance Of Joint In Arc-ultrasonic MIG Welding Of 6061 Aluminum Alloy

6061 aluminum alloy is widely selected in the manufacturing industries such as automobiles,ships,aerospace,and Metal Inert-Gas(MIG)welding is often used in its welding production because of simple operation,mature process,and easy to realize automatic welding.But in the actual welding process,there are also many problems for the 6061 aluminum alloy MIG welding such as porosity,coarse grain in the weld metal,and large loss of strength and toughness.In order to solve these problems,the ultrasonic was excited in MIG welding arc by designing an excitation device with ultrasonic pulse.6061 aluminum alloy was welded by arc-ultrasonic MIG welding in this research,and the influence of different excitation parameters on the acoustic-electrical characteristics of the excited arc was studied.With the aid of analysis methods such as microscopic observation and mechanical performance test,the effects of arc-ultrasonic excitation frequency and excitation voltage on the appearance,the number of pores and microstructure of weld metal and joints' mechanical performance were investigated.The acoustic-electrical signal characteristics of arc in the arc-ultrasonic MIG welding were studied.While the excitation voltage was a periodic rectangular wave,and the excited current in the MIG welding arc was a triangular wave with the same frequency.The arc had an input complex impedance to the ultrasonic excitation signal.The greater the excitation frequency was,the greater the input impedance of arc was.The ultrasonic appeared in the MIG welding arc with ultrasound excitation.With excitation frequency ranging from 20 kHz to 85 kHz,the response frequency of the excited ultrasonic signal was consistent.The sound pressure amplitude of excited ultrasonic in the welding arc was related to excitation frequency and excitation voltage,which was greatly affected by the excitation frequency.The sound pressure amplitude decreased with the increase of the excitation frequency,and increased with the increase of the excitation voltage.The effect of excitation frequency and excitation voltage on the appearance and the number of pores of weld was studied.Compared with conventional MIG welding,the gray impurities on the weld surface were reduced in the arc-ultrasonic MIG welding,which was most excellent with excitation frequency of 50 kHz and excitation voltage of 40 V.The number of pores in the weld metal decreased as the excitation frequency increased,and had no linear relationship with excitation voltage.With excitation voltage of 40 V and excitation frequency of 85 kHz,the number of pores in the weld metal was smallest.The effect of excitation frequency and excitation voltage on the weld microstructure was studied.Compared with conventional MIG welding,the microstructure in the middle of weld metal was refined in the arc-ultrasonic MIG welding while the microstructure of the fusion zone was basically unchanged.With excitation frequency increasing from 30 kHz to 50 kHz,the size of the coarse cell dendritic grain was significantly reduced,and equiaxed grain zones was significantly enlarged with the second phase more uniformly distributed in the middle of weld metal.But as the excitation frequency continued to increase,columnar sub-grain began to appear in the weld.With excitation voltage of 36 V and 64 V,the microstructure refinement was more obvious,and became bad at high excitation voltage.So the excitation frequency ranging from 30 kHz to 50 kHz and the excitation voltage ranging from 36 V to 64 V were the ultrasonic excitation intervals with excellent grain refinement effect.The effect of excitation frequency and excitation voltage on the joint performance was studied.Compared with conventional MIG welding,the tensile strength of the arc-ultrasonic MIG welded joints of 6061 aluminum alloy was improved.The arc-ultrasonic with an excitation frequency of 50 kHz,and excitation voltage ranging from 36 V to 76 V could effectively increase the joints' strength.With excitation frequency of 50 kHz and excitation voltage of 76 V,the tensile strength of joints was the highest,reaching 236.5 MPa,which was 7.1% higher than that of the conventional MIG welded joint.The plasticity and impact toughness of joints were improved respectively.The elongation after fracture was increased from 8% to 13.3%,and the impact energy in the V-notch impact test was increased from 12.9 J to 18.7 J.