Microstructure and mechanical properties of cold sprayed 6061 Al in As-sprayed and heat treated condition

Microstructural and mechanical properties of cold sprayed 6061 aluminum deposits on 6061-T6 aluminum alloy substrates are investigated under various heat treatment conditions, i.e. as-deposited, stress relieved and T6. The local mechanical property variation in the as-deposited material are explored using nanoindentation technique, and correlated with microstructural characterization conducted via electron back-scattered diffraction. It is found that the prior particle boundaries have ~ 0.4 GPa higher hardness than particle interiors, which is attributed to grain refinement in these regions promoted by local dynamic recrystallization. Also, the bulk-scale mechanical properties of the deposits are evaluated by microtensile testing in various post-heat treatment conditions and compared to those of conventionally processed 6061-T6 aluminum. The as-deposited material showed markedly higher ultimate strength (~ 460 MPa) and lower ductility (~ 3%) compared to conventionally processed material and this is attributed to significant cold working during the cold spray deposition process and associated grain boundary strengthening and dislocation strengthening mechanisms. Heat treated specimens showed a slight improvement in both ultimate strength and ductility compared to the as-deposited condition. These improvements are attributed to an improvement in metallurgical bonding at prior particle boundaries and a modest increase in the density of strengthening precipitates. Fractography of the specimens revealed that the heat treatment also changes the fracture characteristics of the cold sprayed 6061 aluminum deposit. The residual stress profiles and bond strength of the deposits are also studied using x-ray diffraction, tensile pull-off and three lug shear testing, respectively.