Microstructure evolution and mechanical properties of 7A09 high strength aluminium alloy processed by backward extrusion at room temperature

The present work deals with the microstructural evolution and mechanical properties of 7A09 aluminium alloy in four different conditions, including as-received, annealed, cold-extruded and heat treated conditions. The results indicate that 7A09 alloy treated by annealing can be backward extruded successfully at room temperature with an extrusion ratio of 4. After annealing, although the degree of recrystallisation increases, unrecrystallised grains still exist. Associated with the decrease in tensile strength is an increase in ductility in the 7A09 alloy during annealing. Backward extrusion at room temperature indicates that the strengths of the alloy are moderately improved at the expense of decrease in ductility. After heat treatment (T6), fine precipitates uniformly distribute in the alloy and are very effective in restricting dislocation movement during deformation. Good ultimate tensile strength (590 MPa) is obtained for the alloy treated by heat treatment (T6), with a yield strength of 522 MPa and an elongation to fracture of 10%. The tensile properties of 7A09 alloy in the heat treated condition are a little better than those of 7A09 alloy in the as-received condition. Moreover, dimensional accuracy and surface roughness of 7A09 alloy treated by backward extrusion at room temperature plus heat treatment are also satisfied with the requirement.