Effects of hot extrusion and heat treatment on microstructure and properties of industrial large-scale spray-deposited 7055 aluminum alloy

Abstract

The industrial large-scale 7055 aluminum alloy fabricated by spray forming technology was subjected to hot extrusion and heat treatment to achieve high strength and ductility. Microstructure of the as-deposited alloy indicates that higher density billets with equiaxed grains (20–40 μm) were fabricated rather than a typical dendritic microstructure of the as-cast alloy. The grains of the as-extruded alloy exhibit fibrous morphology, the original boundaries disappear and fined second phases with size about 0.5–5 μm distribute along with extrusion direction. Meanwhile, the defects could be eliminated by hot extrusion, which resulted in good strength as well as ductility. The ultimate tensile strength, yield strength and elongation of the as-extruded alloy are 345 MPa, 236 MPa and 18.5%, respectively. After heat treatment, the partial recrystallization is observed around the un-recrystallized grains and sub-grains. And the platelet/rod-shaped precipitates (MgZn2) show a uniform distribution in the matrix alloy. The alloy reaches the maximum tensile strength of 730 MPa after T6 temper treatment, associated with a fine precipitation (MgZn2). However, with further deepen aging degree (from T6 to T73 temper), the size of dominant precipitated phases (MgZn2) grows obviously, the grain boundary precipitates transform from continuous to individual ones and the width of precipitate free zone increases. The result shows that the alloy after T7X temper treatment exhibits higher electrical conductivity (>35 %IACS) and facture toughness (>25.6 MPa m1/2) although a 8%–17% reduction in strength compared with that at T6 temper.