| High-strength aluminum alloys have low density,high specific strength,and good mechanical processing performance.They are widely used in the fields of aviation,aerospace and military.However,due to its unique physical properties,its weldability is not good.Modern industry puts forward stringent requirements for high efficiency and high quality welding methods for aluminum alloys.Compared to TIG welding or MIG welding,the VPPA(Variable Polarity Plasma Arc)welding method for aluminum alloys has many advantages such as energy concentration,large arc stiffness,large penetration depth,and small post-weld deformation.Adding high-low-frequency pulses in a typical VPPA will further fully reflect its advantages of energy concentration and strong penetration ability,and achieve good welding result.This article used C programming language,established the MSP430F449 as the core high-strength aluminum alloy pulse modulation VPPA welding system and welding process detection and analysis system.The VPPA welding system includes a pulse-modulated VPPA welding power source and a vertical welding working system;its inspection and analysis system includes a high-speed camera,a HANNOVER mass analyzer,and an arc pressure test sensing device.The secondary development of welding procedures will eventually output multiple target VPPA welding voltage and current waveform.The mechanical characteristics of the pulsed VPPA are significantly different from those of the free arc.In this paper,the mechanical properties of the arc are studied quantitatively based on the analysis of the pulsed VPPA arc morphology,it is combined with the characteristics of the pulsed VPPA voltage and current waveform.Used the arc pressure sensing device,the center pressure of the pulsed VPPA arc was tested and analyzed.When the plasma gas flow rate is changing,the center pressure of pulsed VPPA arc is increasing in proportion.Plasma gas flow rate and welding current play an important role in the center pressure of pulsed VPPA arc,and current has more influence on pulsed pressure.With the same other conditions,when the variable cycle is keep same and the negative polarity time is increasing,the decreased rate of the center pressure of pulsed VPPA arc is slower by 0.8% than that of the typical VPPA arc.The impulse effect is weaken due to the voltage and current waveform distortion at the high frequency of 1kHz,the average central pressure of the pulsed VPPA at the high frequency of 500 Hz is about 0.32 kPa higher than the high frequency of 1kHz,which is increased by 0.81%.Because of the difference between high and low energy increases,the arc is unstabitily,and several peaks appear in the voltage and current probability density curve.This paper studies the welding characteristics of high-strength aluminum alloy pulsed VPPA under different welding specifications and obtains a good weld.Adopting reasonable welding specification parameters and controlling measures to ensure the “force” and “hot” balance of the perforated molten pool in the vertical welding process is the key to obtaining a good weld.After a large number of tests,by changing welding current,ion gas flow rate,low frequency frequency,and positive and negative polarity time ratio,and the variation of weld forming was analyzed.Finally,the 6mm thick 7075 aluminum alloy test plate welding process was realized,and the better welding process parameters of the 6mm thick 7075 aluminum alloy test plate were obtained,which provided a practical reference for the research of the VPPA welding process of medium and thick plate high-strength aluminum alloy pulse.In this paper,mechanical properties test,fracture scanning and micro-structure observation of 6mm thick 7075 aluminum alloy pulse VPPA welded joints were conducted.Due to the mixing action of high and low frequency pulse modulation,the mechanical properties of pulsed VPPA welded joints are superior to those of typical VPPA welded joints,no porosity and other defects. |
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