Design of a Co, Cr, Ni and Ti reinforced AA7050 alloyed composite via solid-state thermomechanical processing

In this work, the fabrication of a novel Co, Cr, Ni particle reinforced and Ti alloyed AA7050-T7451 composite is achieved via multi-pass friction stir processing (FSP). Considerable strengthening of the AA7050 alloy was obtained, sufficient to counter the microhardness decrease of ∼90 HV due to the dissolution of precipitates. Furthermore, the solubility of the originally dissolved elements such as Mg was maintained in the Al matrix. It was found that Ti exhibits the highest alloying capability due to its most negative mixing enthalpy, Δ H mix , with the Al matrix, followed by Co and Ni which tend to dissolve upon subsequent FSP passes. A comparative microstructural analysis amongst the progressive FSP passes is presented, which correlates the degree of intermixing with the increasing plastic deformation. Electron back scatter diffraction (EBSD) was used to reveal that the randomization of crystallographic orientation increases with the subsequent FSP passes. Th peak intensity of pole figures decreases from 11.2 MUD in base metal to 2.53 MUD for the SZ after the third FSP pass. An increase in the fraction of high angle grain boundaries occurred from 24.3 % in the base material to 47.1 % in the first pass which further rise to 67.6 % in the third pass. The current study successfully postulates novel alloy designs using FSP, paving the way towards unprecedented compositionally diverse alloys.