Study On Microstructure Of Weld Joint Of Aluminium Alloy And Research Of Mechanism Of Ultrasonic Impact

Ultrasonic impact during welding is an improvement of traditional Ultrasonic-TIG welding. This paper emphatically find the regularity of grain refining and behavior of pores in the welded seam and study the mechanism of ultrasonic treatment on the bases of study of other scholars. A new amplitude transformer that could roll on the plate took palce of the old one to improve the performance of the system of ultrasonic impact during welding.To study the theory of how ultrasound promote the process of heterogeneous nucleation, a series of traveling and motionless welding experiments were taken on 2195 aluminum alloy. Ultrasonic treatment on the plate is taken when the arc burn. Lots of equiax grain appeared in metallograph of 2195 aluminum alloy instead of columnar grain while there is no change in the metallograph of pure aluminum. And equiax grain appeared when a few Al2O3 powder were added into the welding pool. Above results show that ultrasound can promote the process of heterogeneous nucleation by changing the degree of supercooling of the melt around the cavitation bubbles.Further study on how ultrasonic treatment broke grain were carried on the basis of above results. Ultrasonic treatment were taken on pure aluminum plate after blowing-out of arc during the process of motionless welding in order to avoid the influence of secondphase particle. Distorted and broken grain were found in that experiment. Columnar grain near the fusion line were impacted by bending force caused by acoustic streaming. And cavitation effect increased the deformation. The grain size decreased from 75μm to 40μm in the center of welding spot under the influence of cavitation effect. Shape factor of grain increases. The ratio of grain which have bigger shape factor increases by 30% to 40%.High power ultrasound promotes the process of heterogeneous nucleation better and broke grain in the center of the welding pool easily. Ultrasound broke the grain in the center where equiax grain grow up hardly to make process of heterogeneous nucleation come into being. After heterogeneous nucleation worked, it’s hard for ultrasound to brake the grain.How ultrasonic treatment influence the stomatal behavior were studied last. The porosity per unit length decreases by 30.4% to 60.8% under ultrasonic treatment which is more remarkable than the change caused by reducing welding speed. The porosity decreases at first then increases when the power of ultrasound increase all the time. The size of the pores in the welded seam has same change with the porosity. The vibration in welding pool were measured to get information of cavitation effect. The degree of cavitation is low when the power of ultrasound is small, which produce nucleus for pore to grow up and escape from the welding pool. On the other hand, high power cause violent cavitation effect which produce too much pore to escape.