Research On Ultrasonic-arc Behaviors And Ultrasonic Assisted Tig Welding Method

In pursuit of increasing of low welding efficiency caused by not so concentrated arc energy during the conventional tungsten inert gas (TIG) welding process, in this paper the ultrasonic TIG hybrid welding method, in which arc plasma is used as the carrier of the ultrasonic, is proposed. Conventional tungsten inert gas arc (TIG) welding is fit for high-quality welding, with the strong points such as arc stability, easiness to control the heat source, forming an excellent pool and high internal quality of weld beam, but its shortcomings, shallow weld penetration and low welding productivity have hampered the development of the method. The power ultrasound are more and more widely used in welding, up with new developments of ultrasonic vibration-assisted brazing technology, ultrasound-assisted welding methods, ultrasonic frequency pulse welding technology etc. The introduction of ultrasonic has improved the welding process, and refined the after welding- grain size significantly. In view of the relevant merits of both the ultrasound and TIG welding, ultrasonic TIG hybrid welding method was proposed. This method, with the ultrasonic vibration applied to the TIG welding arc space, has an outstanding feature that the introduction of the ultrasonic vibration can produce compression effects of TIG welding arc and improve the arc characteristics, and thus increase substantially in weld penetration depth and improve the mechanical properties of joints. The U-TIG welding arc characteristics, weld forming characteristics, ultrasonic influence on weld microstructure and mechanical properties were systematically studied to clarify the mechanism of ultrasound on the arc.First, in a particular environment, the overall design plan of ultrasonic power supply was made, with completion of design and calculations the rectifier filter circuit, inverter circuit, driver circuit and matching circuit to achieve the ultrasonic frequency AC signal output. Composite welding torch has been designed and optimized using finite element software. The radiating ends of the amplitude transformer, the cooling bodies and the best option of the tungsten fixture were determined. Secondly, the relative intensity of radiation-sound pressure level was researched. The influence of temperature and medium on the distribution of the sound field was investigated to determine the sound field resonant mode. The measured magnification factor of ultrasonic amplitude is closed to the calculated values that the optimization process of the amplitude transformer was verified that met the need of resonance of ultrasonic vibration system and welding requirements. On this basis, the ultrasonic TIG hybrid welding systems has been set up and the welding test proved that the system can meet the testing and designing requirements.Finally the arc characteristic of ultrasonic and TIG arc hybrid welding and the matching process conditions were found. Through the collection of shape of the welding arc and an analysis of the length-diameter ratio of used welding arc, we found that there were relational matches between arc contraction and ultrasound irradiation height and the arc length. The results of the composite welding arc pressure measurements showed that the composite arc peak pressure was significantly higher than conventional TIG welding arc under the same process parameters, and the peak pressure value of the composite arc of different length was related to the height of radiation ends of welding arc. As the welding current increases, the increment of arc pressure of ultrasound-TIG arc hybrid welding reduces. Through the analysis of static characteristic of welding arc, ultrasound TIG hybrid welding arc static characteristic curve integrally shifted up compared with the conventional TIG welding. By calculation, we got the main reason for the overall upward shift of the static characteristic curve of the arc was the voltage drop in the arc pole area.The study of influence of process parameters of ultrasonic TIG hybrid welding on weld seam forming for SUS304 austenitic stainless steel was made, laying a foundation for the practical application of such a method. We found that the welding process was significantly improved only when the ultrasonic input power is greater than the critical value. Meanwhile, the depth-width ratio of weld and its melting area were the combined effect of the height of radiation-side and the matching of the arc length. The thermal cycle during ultrasonic TIG hybrid welding process shows that hybrid welding increases the welding heat input, heat transfer characteristics shows a directional transferring character and high-temperature residence time became shorter. With the mechanical analysis of plasma flow force on the weld pool surface, the introduction of ultrasound changes in the original stress state. The microstructure of weld joint were compared and analyzed, showing that more equiaxed grains appeared in the ultrasound TIG weld central region, and weld micro photo also verified the conclusions of the hardness test. The test results of the mechanical properties of well formed weld beam show that the tensile strength of ultrasonic TIG welded joints are better than conventional TIG welding and hardness test of joints showed that heat-affected zone of ultrasonic TIG welded joints was much narrower. For ultrasonic TIG welding, microstructure homogenization in weld seam is significantly better than conventional TIG welding and ultrasonic TIG welding fatigue tests showed a more obvious advantage.Finally experiment and theoretical calculation were done to verify the mechanism of action between welding arc and ultrasonic vibration. Starting from The Ideal-Gas Equation, we obtained the vibrating form of particles inside ultrasonic field. The enhancement of heat dissipation of arc surrounding medium and the fiercer movement of internal particle make the welding arc contract. And we calculated the plane sound field distribution under different conditions. Under the action of the high-strength ultrasonic vibration, the overall downward ultrasonic radiating force was attached. Force of conventional TIG welding arc changed and the direction of motion of the particles shift to the axis direction.The micro-changes of arc were analyzed by local spectra of the arc and the arc energy distribution was studied by high-speed camera, and the results further confirmed the action between ultrasound and welding arc.