Friction-stir welding (FSW) promises joints with low porosity, fine microstructures, minimum phase transformation, and low oxidation compared with conventional welding techniques. It is capable of joining combinations of alloys not amenable to conventional welding. Certain combinations of FSW parameters were used to create FSWs of aluminum alloys 5083-H18 and 6111-T4, and the physical weld defects were measured. The mechanical behavior of FSW welds made under the most favorable choice of parameters was determined using tensile tests and hardness measurements and was correlated to the microstructures of the weld and base material. Stir zones (SZs) in the 5083 specimens were much softer than the strain-hardened base materials. SZs in the 6111 material are approximately as hard as the base material. Natural aging of 6111 FSW specimens occurred in some parts of the heat-affected zone and produced hardening for up to after welding. Annealing of 5083 FSW specimens produced abnormal grain growth (AGG) for welds produced under certain welding conditions and in certain parts of the weld zone. AGG is more severe for low-heat conditions, i.e., higher tool travel speed but lower rotational speed. The conditions for most favorable FSW are presented, as well as the expected microstructures and mechanical properties, along with the weld conditions that promote AGG.