Fully penetrated keyhole welding process can be achieved by employing the free burning arc in Tungsten Inert Gas (TIG). The interaction between the heat source and weld pool determines the dynamic behaviour of the keyhole and the resultant weld quality. To determine the energy transfer behaviour, in this paper, the dynamic keyhole behaviour was imaged with the vision system when the heat source was changed by adjusting the welding current or welding speed in real time. It was found that (1) keyhole exit positional and dimensional parameters were all changed as a response to the welding speed changing; (2) keyhole exit size parameters changed much faster than the positional parameters did when the welding current was suddenly decreased or increased; (3) a sharp pulse emerged in the position curve of the keyhole exit center point after the welding current was suddenly changed. The behaviour mechanism was analyzed in view of dynamic changing of the force field and heat field in the arc-keyhole-weld pool system. (C) 2018 Elsevier Ltd. All rights reserved.