In this study, extruded 2195-T8 plugs that were friction welded into the friction stir weld of 2195-T8 base metal plates were examined. This study characterizes the resulting microstructure and microhardness of the plug weld interfaces with the friction stir welded material and base metal in an effort to identify the extent of the thermal, thermomechanical, and mechanical effects introduced by the friction plug welding process. A zone of recrystallized material was observed around the plug weld circumference. The thickness of the recrystallized layer was measured to be 30–122 μm. The hardness measured near the plug weld interface was found to be 110–130 HK100g , or approximately 35% less than the base metal hardness. By characterizing the hardness of these zones, insight can be gathered into the transformation of material from the friction plug welding process around the fusion zone, and areas that may control the fatigue behavior of the joint.