Study On Welding Arc Behavior And Process In Pulse VPPA Of Aluminum Alloy

VPPA(Variable Polarity Plasma Arc) welding is a new welding technology for medium thickness plate aluminum alloy welding, properties of welding arc such as energy concentration and high penetrating power will be further played while add high and low frequency pulse in VPPA welding. Low frequency pulse modulation VPPA arc scour liquid pool perodically, which can effectively inhibit generation of hydrogen hole and control the crystallization and forming of weliding line. High frequency pulse modulation compressible arccould compress arc andenhance arc penetrationreduce thermal input and increase arc energy density. Lower the heat input can further reduce the phenomenon of mechanical properties declination which resulted by softening of welded splice in welding process of aluminum alloy. So the pulse VPPA welding mechanism and process has great theory meaning and practical application value.Aluminium alloy double pulse modulation VPPA welding system is established. The system consists of pulse modulation VPPA welding power source, vertical welding workbench, welding pulsed wire feeding machine and welding special cooling water tank.The control centre of double inverter welding power source is MSP430F449 MCU,welding pulse waveform which output frequency is 0.1Hz ~ 1kHZ was designed by C language progrem.Arcshape and properties were collected and analysised by high speed photography and Hanover mass spectrometer in the welding process.Compression degree of the arc shape is associated with high frequency pulse frequency and the welding time constant. Arc shape compression degree will increase with the increase of high frequency pulse frequency. However, under the influence of welding dynamic time response time constant,peak value of arc compression degree will appeared in 500 Hz. It was found out by the analysis of Hanover quality analyzerthat rate of change of rising edge and falling edge was decreased, pulse wave form was distored and pulse wave form effect was weakened when high frequency pulse current waveformwas 1kHZ while compared with 500 Hz.In welding process of 8 mm thick 7A52 aluminum alloy, the welding parameters are: DCEN curren was 180 A, DCEP current was225 A, low frequency pulse frequency was1 Hz, high frequency pulse frequency was 500 Hz, gas flow rate was2.4 L/min, the welding speed was 180 mm/min, wire feed speed was1.8 m/min. Welding joint mechanical properties and microstructure of 7A52 aluminum alloy welding double pulse VPPA were observed and it was found that welding joint mechanical performance of doublepulse VPPA is superior to that of typical VPPA welding.