| Variable polarity arc welding method can clean the oxide film timely,reduce the burning of the tungsten electrode and improve the optimal matching of the welding heat efficiency,which is widely used in aluminum alloy welding.At present,the non-consumable electrode variable polarity arc welding method is mainly divided into the variable polarity gas tungsten arc welding(VP-GTAW)and variable polarity plasma welding(VP-PAW).Those methods use free arc and compression arc as welding heat source respectively,which both have a wide range of industrial applications.VP-GTAW is suitable for thin aluminum plate by thermal conductivity welding,while VP-PAW is a high-energy beam welding method,which is suitable for thick plate by keyhole welding method to achieve single-sided welding double-sided formation.The research on variable polarity arc welding has already made initial progress in process and theory.However,some important physical phenomena in the process of variable polarity arc welding and the mechanism of variable polarity welding process need to be further explored.In this paper,arc shape,arc pressure characteristic and arc spectrum characteristic of VP-GTAW were analyzed by using high-speed photographic equipment,arc pressure acquisition equipment,spectral analysis equipment,electric signal sensor and so on.The thermodynamic characteristics of a variable polarity welding arc are obtained by experiment.In this paper,two experimental platform were set up for a 6 mm thick 2219 aluminum alloy welded by VP-PAW and 4 mm thickness aluminum alloy welded by VP-GTAW.SEM and EDS were used to study the microstructure and composition distribution of welded joint.The tensile properties and fracture morphology of the samples were analyzed.It is found that the tensile fracture of 2219 aluminum alloy weldment appears on the side of the weld in the fusion zone,the fracture mechanism of the specimen is complex,showing a variety of fracture mechanism of the micro-morphology.Based on computational fluid dynamics and electromagnetism theory,a fully coupled mathematical model of tungsten-arc-molten pool is established by a secondary development for FLUENT software.The arc physical field characteristics in DCEN and DCEP phase in VP-GTAW and VP-PAW are analyzed,including the arc temperature,fluid flow,potential,current density and arc pressure.In addition,the mechanism of variable polarity arc heat transfer and its influence on molten pool flow are analyzed,the dynamic distribution of heat flux and wall shear force at on the molten pool surface were revealed during keyhole formation in VP-PAW,which is of great theoretical significance for understanding the physical phenomena of variable polarity arc welding process.The user-defined function(UDF),which are incorperated in FLUENT software,is used to load the heat source and source model alternately.The evolution of molten pool morphologies and flow field in VP-GTAW were predicted.The formation mechanism of the weld defects such as hump and undercut was revealed.The dynamic evolution of heat and force on the molten pool surface during keyhole formation in VPPAW were studied.The temperature and flow field during the keyhole formation and collapse were studied.It is found that there exist blind keyhole molten pool shock phenomenon in aluminum alloy welding process.This is due to liquid metal accumulation in the bottom of the keyhole in the middle of the process,the arc needs to drain the liquid metal,and the keyhole appear "closed-appear-closed" cycle phenomenon.The numerical analysis and experimental study in this research,may provide theoretical guidance to optimize the welding process parameters. |
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