| Light metal components,represented by high strength aluminum alloy,have been widely used in aerospace,rail transit,warships and other fields.Aluminum alloys are characterized by high thermal conductivity,large linear expansion coefficient,easy oxidation,and large difference in solid-liquid hydrogen solubility.Defects such as deformation,pore and crack are easy to occur during welding,so advanced automatic welding technology and equipment are urgently needed.Variable polarity plasma arc(VPPA)welding technology has the advantages of fewer welding defects,larger weldable thickness and smaller workpiece deformation.It is called "zero defect" welding method for aluminium alloy.At present,the domestic robot VPPA welding power supply is generally dependent on imports,which is not only expensive,but also faces the problems of limited input of core technology.However,the domestic VPPA welding power supply still lags far behind the foreign advanced level in power level,control performance,reliability and process window range,which seriously limits the advanced technology in aluminum alloy welding in China.Promotion and applicationIn order to improve the stability of VPPA welding process,a fully digital 600A high-power robot VPPA welding power source is developed by combining high-power and high-frequency inversion technology with digital control technology,and the pulse piercing welding process of aluminium alloy is studied.Firstly,the research progress of VPPA welding technology and welding power source is analyzed,and the development trend of VPPA welding power source is expounded.Then,the characteristics of plasma arc are simulated numerically,and the influence of key process parameters on the distribution of temperature field and pressure field is explored.On this basis,a control method to improve the stability of plasma arc process is proposed.Aiming at the problem of poor stability of perforated VPPA welding process,the stress state of perforated molten pool is studied,and the necessary conditions to maintain the stability of molten pool are given,and the polarity variation is proposed.A new waveform control method for pulsed plasma arc(P-VPPA)perforation welding.Aiming at the performance requirement of welding power source for aluminium alloy piercing welding process,the overall design scheme of robot P-VPPA welding power source is put forward.The commutation mechanism of main circuit topology is discussed,the key parameters of main circuit are calculated and the key components are selected.A fully digital control system of welding power source based on DSC ARM microprocessor STM32F405RGT6 is constructed.Digital control software for welding power supply based on FreeRTOS kernel environment.Finally,the developed P-VPPA welding power source is integrated with welding robots,high-speed photography system,digital wire feeder and plasma torch,and a robot P-VPPA piercing welding experimental platform is constructed.The performance of the welding power source is tested.The effects of key process parameters on plasma arc characteristics and aluminum alloy piercing welding formation are explored.The results show that the developed P-VPPA piercing welding power supply is stable and reliable,and can realize the precise control of the required welding waveform;the influence of key parameters such as welding current,ion flow rate,variable polarity frequency and tungsten pole shrinkage on plasma arc characteristics is basically consistent with the numerical simulation results;through the welding current,ion flow rate,welding speed and wire feeding,the influence of the key parameters on plasma arc characteristics is basically consistent with the numerical simulation results.The optimum matching of key technological parameters such as speed in pulsed piercing welding of aluminium alloy can obtain better weld formation in a wide process window,and the performance of welded joint is better than that of conventional VPPA piercing welding. |
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