Research On Arc Behaviors And Joint Microstructure&Properties In Ultrasonic Assisted AC TIG Welding Of Aluminum Alloy

In order to solve the problems in alternating current argon tungsten arc (called AC TIG for short) welding of aluminum alloys, such as low welding efficiency, coarse grains, severe loss of strength and porosity, an ultrasonic-assisted AC TIG welding (U-AC TIG) is proposed in this dissertation. It is expected to improve the arc characteristics and weld metal solidification&crystallization behavior and thus obtain high-quality aluminum alloy welds by coupling ultrasonic vibration into the conventional AC TIG arc; meanwhile, the mechanism of such improvements due to the ultrasonic needs to be studied. Hence, this dissertation needs to: build a U-AC TIG welding system in which frequency, amplitude and shape of the end of horn are both adjustable; analyze the arc and melting characteristics in the proposed U-AC TIG welding process; reveal the impact of the parameters; analyze the weld microstructure features, porosity defects, and joint mechanical properties in the U-AC TIG welding process under different frequency and amplitude; find out the correlation between the microstructure feature/the porosity defect and mechanical properties by analyzing the joint fracture behavior. This paper also tried to reveal the mechanism of grain refinement due to ultrasonic vibration, explain the effect of the frequency and amplitude on the grain refinement and degassing, and thus determine the optimal range of frequency. These works would lead to better understanding of the arc behavior and gain refinement in the U-AC TIG welding and provide a fundamental guideline to control the weld quality.According to the characteristics in AC TIG welding of aluminum alloy, a specialized welding torch for the U-AC TIG welding including circuit, the fixture of tungsten and cooling devices are first designed. Considering the potential effect of vibration frequency, three excitation frequency is choosen:20kHz,28kHz and40kHz. In order to enhance the ultrasonic energy absorbed by the welding pool, an horn with concave focusing face is adopted, and the optimal curvature radius of the concave horn is determined by using COMSOL software. The size of horn is optimized by finite element simulation. The resonance frequency and impedance of the ultrasonic vibration system met the needs of welding system. A U-AC TIG welding system with different frequency and the shape of the end of horn is finally established. The system shows excellent stability and durableness under large current. Numerical simulations are conducted to compute the sound field and effect of process parameters thus provides a theoretical basis for the subsequent analysis.The arc behavior in the U-AC TIG welding process is studied by using a high-speed camera and arc variables and arc pressure data collecting system. It is found that the AC-TIG arc is condensed under ultrasonic vibration, and the arc condensing increases with the ultrasound energy. Ultrasonic vibration changes the arc static characteristic curve. During the positive half cycle, it reduces the width of negative resistance characteristics and thus improves the equivalent arc resistance. During the negative half cycle, the decreasing arc static characteristic gradually changes to rising characteristic under ultrasonic vibration. The arc reignition voltage can be reduced by applying ultrasonic vibration. The stability of the AC-arc at the current polarity switching moment is thus improved. In addition, compared with the conventional AC TIG welding, the arc pressure is getting higher when the ultrasound energy increases. Calculations indicate that the ultrasonic vibration also increases the electric field intensity of the arc column and decreases the total voltage drop of the arc cathode and anode.Through aluminum alloy welding tests, the melting characteristic in the U-AC TIG welding is studied. It is found that ultrasonic vibration increases the weld size，especially the weld depth to width ratio. By studying the weld depth to width ratio under different the height of the radiator, the point in the arc space where resonant vibration occurs under different ultrasound frequency to provide an experimental basis for further research. Impact analysis of ultrasonic vibration on the flow and heat conduction of the molten pool demonstrates that weld melting behavior changes from pure melting to centralized melting. The greater the vibration amplitude, this trend is more obvious. Compared with those under20kHz and40kHz vibrations, the pure melting feature under28kHz ultrasound is more pronounced.The microstructure and porosity of the hybrid-welding joint are studied. The mechanical properties are tested. The fracture behavior of the joint is analyzed. It is found that ultrasonic vibration contributes to reduce the width weld fusion zone, refine the grains, change the weld microstructure, and reduce large pores. When the frequency is20kHz, the microstructure of U-AC TIG welding joint is small and partly equiaxed. Compared with the conventional AC TIG welding joint, the number of large pores in the joint is significantly reduced; the pores demonstrate a dispersed micro-porosity distribution. At this point, the joint mechanical properties are improved. When the frequency is28kHz, the weld microstructures are elongated cellular dendrites, and no porosity defects are detected, the mechanical properties are of directionality. Under40kHz vibration the grain refining effect is weak, porosity defects are increased, the mechanical properties are relatively low and shows little difference between conventional joint.Through theoretical calculations and experimental verification，the mechanism of grain refinement due to the applied ultrasound is preliminarily explored. The calculation based on the bubble pulse energy formula indicates that the acoustic cavitation can significantly increase the vibrating frequency of the molten aluminum atoms, help to increase the nucleation rate of weld pool and refine the microstructure of joint. The calculation on the resonance radius of cavitation bubble resonance and transient cavitation threshold under varying frequency indicates that the probability and intensity of cavitation is reduced when the ultrasonic frequency increases, and the grain refinement effect is getting weaker. However, increasing the ultrasonic frequency is beneficial to reduce the porosity defects in a certain range. According to the escape conditions of the pore inside the weld pool and the cavitation resonance theory, the optimal ultrasonic frequency range for the hybrid welding of aluminum alloy can be determined.