In situ Al3Ti/Al-5.5Cu composites fabricated by powder metallurgy and subsequent forging were subjected to multiple pass friction stir processing (FSP) with and without active cooling. The forged sample exhibited lower strength and ductility due to the presence of coarse Al3Ti clusters with a size range of 50-100 mu m and coarse matrix grains. Four-pass FSP in air resulted in the refinement and redistribution of the Al3Ti clusters, and the generation of micron matrix grains, thereby increasing the strength and ductility of the composites. Furthermore, coarse Al2Cu particles dissolved and re-precipitated due to a relatively long duration of thermal exposure. Additional two pass FSP with rapid water cooling (FSP-water) dissolved most of the Al2Cu into the matrix and retained the solutes in solution due to the short duration of thermal exposure. Meanwhile, ultrafine matrix grains with a high density of dislocations were obtained. These microstructural changes led to significant increase in strength and a decrease in ductility in the FSP-water sample. After aging, the FSP-water sample exhibited further increased yield strength and ultimate tensile strength due to the precipitation of metastable Al2Cu phases. However, the ductility did not decrease due to the decrease of dislocation density after aging. (C) 2012 Elsevier B.V. All rights reserved.