Effect of Waveform and Shielding Gas on Melting Rate and Bead Geometry for GMAW-VP and Correlation to Kinematics of Metal Transfer

Due to the importance of variable polarity in GMAW welding (GMAW-VP) with great potential for overlaying and root pass applications, it is fundamental to assess the effect of process parameters on the bead formation and wire-burn rate, specially the latter, since higher melting rate is achieved by this process during negative polarity. Moreover, although the well-known importance of metal transfer, there is lack of information about this phenomenon for the variable polarity version. Therefore, this work aims to assess the effect of two different waveforms previously investigated, in which current pauses exist before and after the main pulse during positive polarity, since this condition leads to better stability. The shielding gas effect was also investigated over the bead geometry, melting rate, deposition rate and deposition efficiency. Bead-on-plate weldments over SAE1020 carbon steel were performed with AWS ER 70S-6 wire with 1,2 mm of diameter. Contact tip working distance was kept constant at 20 mm. The GMAW-VP was set at 30 and 50% of time in negative polarity. The employed shielding gases were Ar+2%O2, Ar+5%O2, Ar+8%CO2 and Ar+15%CO2 at 14 l/min. The arc length was kept approximately the same during the runs by varying the wire-feed speed. Also, the relationship between wire-feed speed and travel speed was kept constant in order to keep the same amount of deposited material. Concerning the metal transfer studies, high-speed filming was carried out at 2000 fps with shadowgraphy synchronized technique when using Ar-2%O₂ as shielding gas. It is possible to conclude that the better understanding on the relationship among bead geometry, parameters and consumables leads to assure a desired bead profile. Moreover, the melting rate analysis allows selecting more coherent parameters and consumables to assure lower material losses and high productivity.