The welding defects such as porosity and lack-of-fusion is more likely to occur in the narrow gap gas metal arc welding (NG-GMAW) of aluminum alloy. In this study, a three-dimensional transient numerical simulation was carried out to investigate the molten pool behaviors as well as their influences on welding defects. A pulsed arc welding mode was employed, which is accompanied with elliptically symmetric arc models. Three different weld bead surface shapes i.e. the convex, flat and concave surface were achieved both by experiment and simulation with varied welding current. The flow pattern varied significantly due to the different thermal and force conditions. The formation process of lack-of-fusion defect was analyzed and the effect of flow pattern on porosity in all cases were discussed. The results indicate that the upward fluid flow near the sidewall border of molten pool facilitates the formation of lack-of-fusion defect and the downward and backward fluid flow near the sidewalls may induce the porosity. (C) 2018 Elsevier Ltd. All rights reserved.