The thermal condition and fluid flow in weld pool determine the formation of humping bead in high speed gas metal arc welding. An external magnetic field is utilized to control the fluid flow and heat transfer in weld pool to suppress the humping bead formation. The measurements of temperature field and molten metal flow on the weld pool surface are conducted under the conditions without and with the external magnetic field. When the external magnetic field is applied; the temperature distribution in the weld pool is more uniform. The backfilling of molten metal and fluid flow are improved, and the longitudinal velocity of backward molten metal flow is reduced. The external magnetic field not only adds a resistance but also decreases the driving force for the flow of the backward molten metal in weld pool so that the momentum of the backward molten metal flow is reduced and the humping bead is suppressed. (C) 2017 Elsevier Ltd. All rights reserved.