Sensing of Metal-Transfer Mode for Process Control of GMAW

Abstract

One of the requirements of a sensing system for feedback control of gas metal arc welding (GMAW) is the capability to determine the metal-transfer mode. Because the operating boundary for the desired transfer mode, expressed as a function of mass input and heat input, may vary due to conditions beyond the control of the system, a means of detecting the transfer mode during welding is necessary. The metal-transfer mode has a large effect on weld pool behavior in GMAW, which, in turn, influences penetration, solidification, heat flow and the available range of welding positions and parameters. A series of sensing experiments was performed during which the audio emissions, welding current fluctuations and welding voltage fluctuations were recorded as a function of the transfer mode. Using laser backlighting and spectral filtering to suppress arc light, high-speed movies of the droplet formation and detachment were taken synchronously with the sensing data. The high-speed film was analyzed to investigate the physics of drop transfer. The film was synchronized with the digitized signal using an LED mounted in the camera pulsed at a set rate and recording the pulses on film and with the sensor data. A second LED was used to mark the film at the beginning of the data-acquisition period. Thus, transfer events recorded on the film can be analyzed and correlated with the digitized sensor data. Data acquired during globular transfer, spray transfer and stiff spray or streaming transfer were observed to correlate with droplet detachment and arc shorting. Instabilities in the transfer mode and mechanisms of drop detachment affecting drop velocity are discussed. Although the current and voltage data are also dependent on the characteristics of the welding power supply, the audio, current and voltage data could be used to discriminate among these different transfer modes.