Torch Optimization And Welding Process Study Of Ultrasonic-Assisted TIG Welding

Ultrasonic-assisted arc welding has so many advantages, such as compressing arc shape, refining weld microstructure, increasing penetration that it can improve welding quality and efficiency. Several welding methods combined with ultrasonic energy have been studied, including TIG welding with DC or AC current (U-TIG) and MAG welding. These new hybrid welding methods have lots of superiorities comparing with traditional welding methods. Until now, combination pulsed TIG welding with ultrasonic energy has not been carried out. Therefore, in order to extend the application of ultrasonic-assisted arc welding, ultrasonic-assisted pulsed TIG welding (U-P-TIG) was studied. Furthermore, for revealing the similarities and differences of ultrasonic effect between U-TIG and U-P-TIG, comparative analysis was conducted. Before welding experiments, focused ultrasonic horns with concave spherical irradiation face were designed to enhance the effect of ultrasound while the welding torch was optimized.The focused ultrasonic horns were designed, while water-cooled device and gas unit of original ultrasonic-assisted TIG welding torch were optimized. Results showed that focused ultrasonic horns could significantly improve the sound field intensity along the axis of the horn. Application of the focused ultrasonic horn could further increase welding penetration and refine microstructure, while enlarge refinement region. The designed water-cooled device could reduce temperature of ultrasonic horn in welding and prolong working hours. The improved gas unit could enhance protective effect of inert gas in U-TIG welding.U-P-TIG welding was studied in depth from four aspects: state of arc shape and weld pool, ultrasonic parameters, welding conditions, microstructure and mechanical properties of welding joint. Results indicated that ultrasound made the arc shape compressed significantly in base current period, upper half of the arc shape compressed and another was slight expanded in peak current period, moreover, changed flow state of the weld pool and made it turbulent. With increase of ultrasonic power, peak current, based current or duty cycle, the ultrasonic effect on weld joint was enhanced, but the degree of increase on weld penetration and microstructure refinement was different. The weld structure with ultrasound was more refined than without one at the same pulsed frequency. Above results showed that ultrasonic effect on weld are more significant when heat input was larger, the larger heat inputted, the longer weld pool lasted, the ultrasonic time extended. Comparative analysis between P-TIG welding joint and U-P-TIG welding joint on microstructure and mechanical properties indicated that the ultrasound enlarged equiaxed grain zone in weld center, weakened columnar crystal between two pools, relieved chromium depletion phenomenon, improved joint strength and elongation.At last, characteristics of ultrasonic effect in U-TIG and U-P-TIG welding were studied. Results revealed that in big or small current condition ultrasound could both compress the arc in U-TIG, but the compression of arc in U-P-TIG only started in the end of peak current time and base current duration. U-TIG weld pool's flow state was about the same as U-P-TIG, but the latter vibration was more severe in its center. Ultrasound could break ferrite skeletons and scatter them. The microstructure was more refined than that of the U-TIG, and the refined grain region was larger. U-P-TIG joints had slightly higher tensile strength than that of U-TIG.