Study On Mechanism Of Low-power Pulsed Laser Induced And Enhanced Arc Discharge Based On Magnesium Alloy Welding

The low power pulsed laser-arc hybrid heat source possesses many merits, such as low welding energy consumption, high welding quality and efficiency. Therefore, it is of wide application prospect and huge application potential, which makes the laser-arc heat source become one of the research hot spots in welding area. The excellent welding characteristics of this hybrid heat source come from the synergetic effect of laser and arc plasma. Researches on the interactions between laser and arc will be significant for the further development of low power pulsed laser-arc hybrid welding technology. Therefore, aiming at the process of welding magnesium alloy using the low-power pulsed laser-arc hybrid heat source, the particle quantity migration and energy transfer during the interactions between arc plasma and laser keyhole plasma were carefully studied in this thesis, based on behaviors and states of the keyhole and keyhole plasma formed by laser radiation in the workpiece. By synthetically investigating heat source melting ability, plasma behavior and discharge state, the migration of Magnesium atom in the plasma and the behaviors of laser keyhole, the coupling effect between keyhole plasma and arc plasma was revealed. Finally, the research results were used to develop a new high-speed welding technology on magnesium alloy plate using the hybrid heat source to ensure the adaptability of the mechanisms on other welding conditions. The main research contents and results are as follows:(1) Study on enhancing phenomenon during interaction between low-power pulsed laser and arc plasma. In this part, by investigating the effects of hybrid heat source on materials, behaviors and discharge states of arc plasma, and the spectral information of plasma, enhancing phenomenon between laser and arc plasma is studied. Results show that the enhancing effect of laser on arc plasma mainly appears in the process of laser-material-arc plasma interaction; there is great influence of Dla on heat source welding penetration depth when the heat input is confirmed; there is a highly linked relationship among the welding parameters, the state of heat source and welding phenomenon. When the largest welding penetration depth is achieved at one set of welding parameters, the plasma is always in composite shape, with the increased spectral intensity of magnesium atom, decreased electron temperature and increased electron density of the arc plasma.(2) Study on delay phenomenon during interaction between low-power pulsed laser and arc plasma. The delay phenomenon in the hybrid welding process is investigated by the study on the migration of magnesium atom and laser keyhole closing behavior during the laser pulse action. Results indicate that after laser pulse action, the concentrations of magnesium atoms in the arc plasma can not recover to the initial state (the state before laser pulse coming), and there is recovery delay in milliseconds. The delay comes from the lasting interaction between keyhole and arc plasma. The interactions between arc plasma and keyhole plasma greatly influence the shape and behavior of laser keyhole. The keyhole outlet can not close immediately after laser pulse action. Small arc current will lead to a regular keyhole outlet shape and a lasting open state.(3) Study on physical mechanism and process of laser inducing and enhancing arc discharge.â‘ According to the experimental phenomenon and theoretical analysis, the physical mechanism of laser inducing and enhancing arc discharge is proposed:the physical nature of low power pulsed laser enhancing arc discharge lies in the coupling discharge between keyhole plasma and arc plasma. During the coupling discharge, the keyhole plasma connects the arc" plasma, and a composite plasma forms. There are quantity exchanges and energy transfers among the particles in the two plasmas. After laser pulse action, the coupling discharge lasts several milliseconds in the present experimental condition.â‘¡Based on the experimental results, the quantitative relationships between the welding parameters and the inducing effect are confirmed, and a mathematic model is conducted. Consequently, a criterion and a method for optimizing the parameters are proposed to achieve the coupling effect between keyhole plasma and arc plasma.â‘¢The physical process of laser inducing and enhancing arc discharge is discussed as follows:When coupling discharge happens, the keyhole plasma possesses a higher electric conductivity, which improves the energy transfer density. The composite plasma in keyhole part is much closer to local thermodynamic equilibrium state, so the energy transmission efficiency during collisions between electrons and heavy particles is also much higher. Extra energy can be derived by the particles in the keyhole from the arc electric field during coupling discharge, and the amount of energy depends on the density and the length of plasma in the keyhole. After laser pulse action, the coupling discharge effect lasts for a while which prolongs the enhancing effect on the arc plasma. Therefore, the coupling effect changes discharge state of arc plasma, and increases the energy density of arc plasma, as well as the heat penetration depth and welding efficiency of the low power pulsed laser-arc hybrid heat source.(4) Application researches on laser induced and enhanced arc discharge mechanism in high speed welding of magnesium alloy welding process. Parameters of laser power, arc current and heat source distance are carefully chosen according to the obtained optimization method for laser inducing arc discharge, and high-speed welding of magnesium alloy thin plate using low-power pulsed laser+TIG arc hybrid heat source. At the heat input of the laser average power about700W and AC arc current230A, butt welding of2mm thick magnesium alloy can be achieved at the welding speed of6000mm/min. Results show that the welding process is stable and the plates are fully and uniformly penetrated without obvious distortion. The tensile strengthes of the welded joints are almost equal to that of base material. Therefore, it is thought that the mechanism of laser inducing and enhancing arc discharge is available in high speed welding using the low power pulsed laser-arc hybrid heat source.