The heat transferred to the workpiece plays a main role in the control of the full penetration weld bead during AC plasma arc welding of aluminum alloys in the keyhole mode. In order to better understand the effect of the heat generated by the AC arc on the weld formation, a low frequency-changed AC welding power source was developed. When the direct current electrode positive (DCEP) arc initiates, the heat generated in the cathodic region by the high cathodic potential is greater than that in the anodic region when the direct current electrode negative (DCEN) arc initiates. The role of the DCEP arc is not only to remove the oxide film on the workpiece, but also to provide the coming DCEN arc with more heat to melt and penetrate the workpiece. To guarantee the weld formation, the rule of maximum-width cleaning area is not acceptable during AC PAW of aluminum alloys in the keyhole mode.