Effects of pre-aging and minor Sc addition on the microstructure and mechanical properties of friction stir processed 7055 Al alloy

Abstract

The effects of Sc addition and pre-aging on the microstructure and mechanical properties of friction stir processed (FSP) 7055 Al alloy were investigated. The addition of Sc effectively inhibited the coarsening of grains in the FSP 7055 Al alloy due to the grain boundary pinning effect of Al3(Sc,Zr) particles. The high density of Al3(Sc,Zr) particles also provided more preferential nucleation sites for the precipitation of the η phase during FSP, and then inhibited the coarsening and increased the density of the η phase in the FSP 7055 alloy. The η′ phase precipitated in the 7055 and Sc-containing 7055 during artificial aging, and then dissolved into the Al matrix during FSP. Thus, pre-aging effectively inhibited the formation of the η phase during FSP, and increased the quantity of the solute atoms in the FSP samples. The mechanical properties of the FSP 7055 Al alloy were improved by the addition of Sc and pre-aging. The aged 7055–0.25Sc Al alloy with FSP showed the highest ultimate tensile strength of 578 MPa; and the aged 7055 Al alloy with FSP showed the largest elongation of 21%. This work provides an effective strategy to improve the mechanical properties of FSP Al alloys.