The flow within the melt pool during welding is a major factor that dictates the formation of the final fusion zone shape, solidification microstructure and defects. In this paper, we report the evolution sequence of the arc weld pool flow that observed via fast in-situ synchrotron X-ray imaging. Varying flow regimes attribute to the dominance of characteristic forces within the weld pool during rapid solid-liquid-solid phase transformation. Our analysis indicates the general sequence in the arc, surface tension and gravity driven force domination. Welding process parameters appear to influence significantly in determining the domination interval and the intensity of individual force. In some instances, arc and surface tension driven forces can prevent pores, which causes porosity in final welded structures, escaping from the melt pool. Preliminary relations between power input levels, diffract force domination regimes, flow patterns and pool surface changes in fusion welds are suggested by considering the behavior of multiple weld pools.