Rheo-Extrusion of an Alloy Specially Developed for Rheo Process Based on Al–Zn–Mg–Cu System

Abstract

This study has been directed towards developing 7xxx aluminum alloy for rheo-extrusion. Rheo extrusion was done by a co-rotating twin-screw extruder as a new semi-solid process for production of 7xxx aluminum alloys with high integrity. A super high-strength aluminum alloy was thermodynamically developed with nominal composition of Al-14Zn-9Mg-5.2Cu. The rheo-formability of alloy had been assessed by thermodynamic criteria and mechanical properties. The newly developed alloy showed good rheo-formability in terms of low temperature sensitivity of solid phase and reasonable mechanical properties. The results showed that optimized mechanical properties and microstructure was obtained at 0.6 solid fraction and 450 rpm for screws. The average grain size changed from 300 by conventional casting to 16 lm by rheo-extrusion process, also shape factor changed from 0.3 to 0.9. The mechanical properties of the rheo-extruded samples at these conditions were: UTS of 682, 621 MPa 0.2 % proof stress and 10 % elongation. Furthermore grain coalescence and columnar growth of solid/liquid interface were the main mechanisms that could deteriorate the rheo-formability. Also results showed that increasing of rotation speed could refine grain size and eliminate the grain coalescence but could not overcome instability of interface.