Due to its popularity and high crack sensitivity, 6061 aluminum alloy was selected as a test material for the newly developed double-sided arc welding (DSAW) process. The microstructure, crack sensitivity, and porosity of DSAW weldments were studied systematically. The percentage of fine equiaxed grains in the fully penetrated welds is greatly increased. Residual stresses are reduced. Porosity in the welds is reduced and individual pores are smaller. It was also found that the shape and size of porosity is related to solidification substructure. In particular, a weld metal zone with equiaxed grains tends to form small and dispersed porosity, whereas elongated porosity tends to occur in columnar grains.