Research On Mechanism Of Droplet Resonance And Active Control Of Metal Transfer In GMAW

In GMAW,the transfer of droplets from the molten welding wire to the molten pool due to various forces is called metal transfer.Heat input and mass transfer in this process are directly related to the process stability,spatter,bead formation,and ultimately the welding quality.Most of the previous studies aimed to change the transfer mode and improve the transfer stability,little has been done for the control of the transfer parameters.In this paper,the forced oscillation of droplets under alternating mechanical force is deeply studied by experiments and analytical models.Further,the decoupling control and open-loop control of metal transfer based on droplet resonance are studied.In addition,the arc variation,molten pool behavior,and fluctuations of welding electrical signal during the free oscillation of droplets are systematically studied.A scheme of feedback control of metal transfer is proposed based on a low-cost method to measure the droplet size online.An experiment system is first established centering on an electrode excitation device.The natural frequency and forced oscillation of the droplet are studied.The relationship between the natural frequency and droplet size is determined by observing the selfattenuating oscillations of droplets of different sizes.A continuously vibrating welding wire is used to stimulate the forced oscillation of the droplet,then the deformation,amplitude,and phase variation of the droplet are studied.The results show that when the excitation amplitude is small,the droplet experiences three regions successively as the droplet size increases: sub-resonant region,main resonant region,and super resonant region.In the main resonant region,the amplitude of the droplet increases greatly,reaching a maximum value when the natural frequency equals the excitation frequency.When the excitation amplitude is large,the droplet resonance can promote droplet detachment,inducing reduced transfer size and increased transfer frequency.It is verified that the metal transfer based on the droplet resonance is independent of the welding current.The double-mass-spring model is constructed to simulate the forced oscillation of the droplet.The amplitude,oscillation phase,and force condition of the droplet during the forced oscillation are studied,and how the droplet resonance promotes metal transfer is explained based on the simulation results.Secondly,the online detection of droplet size based on the free oscillation of the droplet is studied.Oscillation stimulated by current pulses with different parameters is studied,and the excitation intensity is explained to be related to droplet inertia and arc climb.It is found that once the excitation pulse ended,the change of the arc length and arc structure is mainly due to the droplet oscillation,while the fluctuations of the molten pool surface have a neglectable impact on the arc.The relationship between the welding voltage fluctuation and the droplet oscillation is studied experimentally.The welding voltage increases when the droplet moves toward the molten pool,and vice versa.Acquiring the droplet oscillation frequency online by analyzing the welding voltage data is feasible because the fluctuation frequency of the welding voltage signal is the same as that of the droplet oscillation.The droplet size can be further obtained according to the pre-calibrated relationship between the natural frequency and the droplet size.To improve the measurement accuracy,the welding voltage data is removed DC components,filtered,added windows,and appended zeros in sequence before processing FFT analysis.The accuracy of this measurement method is high,and the error is mainly due to the slightly increased droplet size in the analyzed period,and then the limitation of the resolution of the FFT.The online detection of droplet size provides a basis for the online feedback control of metal transfer.Finally,schemes of open-loop control and feedback control of metal transfer parameters are proposed.The open-loop control is based on droplet resonance,which occurs only when the excitation frequency equals the natural frequency of the droplet.By changing the excitation frequency,the detached droplet size and transfer frequency can be accurately adjusted.The metal transfer and weld bead formation are both significantly improved with the increase in excitation frequency.Current pulses are used to stimulate droplet oscillation or detach the droplet in feedback control,and the appropriate pulse parameters for the detaching pulse are selected according to droplet size measured before,i.e.,providing personalized detaching parameters for every single droplet.After the detaching pulse,the droplet detachment is detected and verified by checking the voltage spike along with the break of the liquid neck of the droplet.The feasibility of feedback control is experimentally verified,and the results show that the droplet is detached reliably in every feedback cycle,and the parameters of the detaching pulse are consistent with the expected values within the error range.