Active Control Of Metal Transfer In GMAW By Pulsing Laser And Arc Force

This research dedicates efforts to fully decouple the metal transfer mode fromwelding current in GMAW process; therefore, the process stability and weld qualityof GMAW can be fundamentally improved, and the welding parameter andapplication range of GMAW also can be significantly expanded. To this end, aninnovative active control method of metal transfer combining pulsed laser irradiationand arc force regulating is proposed. The molten droplet is irradiated by a laser pulseand thus be vaporized partially. A laser recoil force due to the vaporization is thusproduced which acts as an additional detaching force to enhance the dropletdetachment. Using a sufficiently high laser power would generate adequate laserrecoil force which may detach the droplet despite of the welding current amperage.On the other hand, the droplet can be excited into oscillation by using a specificcurrent waveform to regulate the arc force. The droplet inertia during its oscillationcan be utilized to enhance the droplet detachment. Hence, by coupling the arc forceregulating method into the laser-based control process, a considerable reduction onthe needed laser peak power is expectable.A GMAW system integrating with a pulsed fiber laser is first established toconduct the experiments. The pulsed laser enhanced metal transfer is first in-depthstudied. The laser induced vaporization on the irradiated liquid droplet surface isobserved and verified to confirm the basic principle of laser-controlled metal transfer.The metal transfer in low current GMAW with pulsed laser irradiation is furtherverified. The results of0.8mm ER70S-6welding indicate that the metal transfer canbe fully decoupled from the welding current by using1200W/5ms laser pulseirradiation. Drop spray or small globular transfer can be achieved at any current level,however, the laser peak power is relatively high. Then the effects of related processparameters, such as the laser incident point and angle, on the metal transfer areanalyzed. Optimal values of these parameters are experimentally determined. Thelaser recoil force and laser thermal effect on the droplet are then theoreticallyanalyzed to clarify the droplet detaching mechanism under laser irradiation. Theeffect of laser irradiation on droplet surface tension is also analyzed.The original active droplet oscillation method is further analyzed and improvedin this paper. The current waveform is first simply modified by separating the dropletgrowing and exciting process, and then further optimized. Much stronger dropletoscillation is thus achieved while the average current is significantly reduced. Theoptimized current waveform consists of a growing pulse, exciting pulse and detaching pulse. The droplet grows during the growing pulse then gets elongated bythe exciting pulse. At the end of the exciting pulse, the arc force decreasessignificantly and the droplet is driven to oscillation by the surface tension. The effectof the waveform parameters are studied end their optimal values are experimentallydetermined. The enhancement on the droplet detachment due to the excitedoscillation is experimentally verified. A theoretical model on the droplet oscillationand detachment is established based on mass-spring system. The mechanism ofdroplet oscillation and detachment are clearly revealed through numericalcomputation of the model. The effects of the waveform parameters on the dropletoscillation are also theoretically explained. Their optimal values can be predictedthrough the modelling effort with adequate accuracy and speed.Finally, metal transfer controlling experiments combining the laser and arc forceregulating are conducted. The results demonstrate that the laser peak power can beeffectively reduced by coupling the oscillation method into laser controlling process.The phase match condition between the laser pulse emission and the dropletoscillation are studied. There is an optimal phase match for maximizing theenhancement on the droplet detaching and the reduction of the laser peak power. Wealso tried to apply such laser-arc hybrid control of metal transfer into CO2shieldedarc welding. The laser is responsible to generate an additional detaching force, andthe arc force regulating through reducing the current to base to minimize the repellingarc force when the droplet needs to be detached. Small axial globular transfer issuccessfully produced in CO2welding process.