Microstructure and mechanical properties of an Al-Zn-Cu-Mg alloy processed by hot forming processes followed by heat treatments

An investigation was conducted to determine the microstructure and tensile properties of an Al-Zn-Cu-Mg alloy processed by spray formed then hot forming processes (forging and extrusion at 703 K). Hot formed samples were solution treatment (743 K/1.5 h) and followed by different heat treatments; T6 aging treatment (393 K/24 h), T74 tempering treatment (393 K for 6 h followed by 433 K for 12 h) and T73 tempering treatment (393 K for 8 h followed by 433 K for 24 h) conditions. The results show that during the T6 aging treatment the number of precipitates is increased which are mainly of η′ and η. The high number of fine percipitates is leading to an enhancement of the tensile strengths and the ductility. The tensile yield strength of T6 and T74 heat treatments are decreasing gradually to different degrees with increasing the testing temperatures up to 473 K and drops dramatically after 473 K. The tensile yield strength (YS) of the T73 tempering treatment sample decreases dramatically after temperature 373 K. The coarse precipitates after T73 tempering treatment were unable to pin grain boundary and dislocation movement during the tensile testing at 373 K. The Precipitates start to dissolve during T73 tempering treatment forming new fine η′ and G.P. zones phases inside the η phase.