Friction stir extrusion is a derivative process of friction stir welding for joining dissimilar materials. The process forms a mechanical joint through extrusion to form interlocking features between two metals. When joining AA6061-T6 to mild steel through extrusion of a dovetail, much of the T6 heat treatment of the aluminum in and around the weld is lost during the process. This paper details the examination of a post-processing aging process to return AA6061 to the T6 condition. Through a 10-h precipitation hardening process at 170 °C. The welds experienced a 23.9% increase in the tensile strength from 2193 N as-welded to 2718 N after 10 h. Hardness measurements showed a return close to T6 conditions in both the weld nugget and extruded material, 67.5–83.5 HRF and 54.7–81.8 HRF, respectively, during the heat treatment period. The steel showed no substantial changes in hardness during the heat treatment. Scanning electron microscopic (SEM) analysis revealed significant changes in the sizes of two species of inclusions within the aluminum during both the welding and post-processing.