Nanoparticle promoted solution treatment by reducing segregation in AA7034

Abstract

The zinc-rich aluminum alloy AA7034 offers the highest tensile strength among all commercial aluminum alloys. However, the commercial AA7034 alloy was produced by Rapid Solidification. At a slow cooling rate, the high-alloying of AA7034 produces coarse secondary phases, causing severe segregation at interdendritic regions, and difficulties in later solution treatment. Nano-treating, by introducing a small volume fraction of nanoparticles, was applied to AA7034 alloy to successfully modify the microstructure and promote solution treatment at a slow cooling rate of 0.25 K/s. Primary phases were refined and the volume fraction of eutectic phases was modified. A modified local solute redistribution equation (LSRE) was applied to show how nanoparticles change the volume fraction of the eutectic phase by altering the viscosity of the liquid and the permeability of the coherent solid network in the terminal stage of solidification. In particular, by adding 1.0 to 2.0 vol percent of nanoparticles, the eutectic phases were not only refined and much more homogeneously distributed, but also their volume fractions were reduced by 24.5% and 30.1% respectively after down-hill casting. The modified microstructures enabled by nanoparticles allowed an accelerated solution treatment as well. The dissolution of the refined and distributed secondary phases was more complete during solution treatment. The nano-treated AA7034 also exhibited high strength and reasonable ductility after extrusion and heat treatment. The nano-treating approach has great potential to make conventional casting applicable to the difficult-to-cast/heat-treatable alloys through refined microstructure and promoted solution treatment.