Porosity formation in pulsed laser keyhole welding was found to be affected by two competing factors: (1) the solidification rate of molten metal and (2) the back filling speed of molten metal during the keyhole collapse process. Porosity (pores/voids) was found in welds when the solidification rate of molten metal exceeds the back filling speed of molten metal. In this study, the use of electromagnetic force was proposed to control the back filling speed of molten metal, and a mathematical model was developed to investigate the effects of electromagnetic force on the transient melt flow, keyhole dynamics, and porosity formation. The results demonstrate that porosity in pulsed laser welding can be prevented by an applied electromagnetic force. Parametric studies to determine the desired strength of the electromagnetic force and its duration were also conducted to achieve quality welds. (c) 2006 Elsevier Ltd. All rights reserved.