| Al-Zn-Mg-Cu high-duty aluminium alloys are important structural materials for aerospace, military, shipbuilding and automobile industry. These alloys usually are firstly prepared by casting ingots and formed by plastic processing after homogenization treatment, and then strengthened by solid solution-aging treatment. Because of their large amount of alloying elements, the cast ingots are inclined to form dendritic segregation and generate coarse eutectic phases on the grain boundary, which have an important influence on the homogenization treatment and thermal deformation process. Therefore, it is necessary to investigate the effect of Al-Zn-Mg-Cu as-cast microstructure on the microstructure and performance of homogenization treatment and thermal deformation. The research results will provide theoretical and experimental basis for process establishment of homogenization treatment and thermal deformation of as-cast Al-Zn-Mg-Cu alloys.A novel grain refiner consisting of nanoparticulate TiN suspended in a Ti metal powder(abbr. nano TiN/Ti) was prepared to refine the Al-Zn-Mg-Cu as-cast microstructure, and the effect of homogenization treatment on the microstructure and performance of Al-Zn-Mg-Cu alloy refined with nano TiN/Ti were studied. Extrusion deformation and thermal simulating experiment were conducted after homogenization treatment, the variation rules of microstructue and flow stress under different deformation conditions were analyzed, and the hot compression behaviors of Al-Zn-Mg-Cu alloy refined with TiN/Ti were investigated. The following results were obtained.After one-step homogenization(470?×24h), the ?(MgZn2) and ?(Al2Cu) phases of as-cast Al-Zn-Mg-Cu alloy are relatively fully dissolved, only less MgZn2 phases are left on the boundaries. After two-step homogenization(400?×10h+470?×24h), the ?(MgZn2) and ?(Al2Cu) phases are fully dissolved, and MgZn phases precipitate in the grain in the cooling process. After one-step and two-step homogenization, the grain sizes of Al-Zn-Mg-Cu refined with TiN/Ti are smaller than that of the alloy without refiner, and the hardness values are higher than the as-cast alloy. The hardness values of Al-Zn-Mg-Cu alloy after two-step homogenization reduce slightly than that of the one-step homogenization alloy.During the thermal simulating experiment process, with increase of temperatures and deformation degrees, and decrease of strain rates, it is favorable to the dynamic recrystallization of the Al-Zn-Mg-Cu alloy. At the temperature of 450? and the strain rate of 0.01s-1, the dynamic recrystallization occurs both in Al-Zn-Mg-Cu with refiner and without refiner, and form a large number of fine equiaxed crystal, but there are still some original as-extruded grain boundaries in the alloys without refiner. The TiN/Ti can promote dynamic recrystallization under the same deformation conditions.The flow stress of Al-Zn-Mg-Cu is gradually reduced with the deformation temperature increasing, and gradually increased with strain rate increasing, and it has positive strain sensitivity. For the Al-Zn-Mg-Cu alloy without refiner, at the temperature of 450?, the softening mechanism is mainly dynamic recrystallzation, and the stress-strain curves present an increasing tendency as a whole, the work hardening plays dominant role, at the temperature below 400?, the dynamic softening can balance the work hardening in steady state. For the alloy with refiner, at the strain rate of 1 s-1 and 0.1s-1, in steady state the work hardening and dynamic softening balance, and at the strain rate of 0.01s-1, at the temperature of 300? and 350?, the dynamic softening plays dominant role.At the temperature between 300? and 400?, the flow stresses of Al-Zn-Mg-Cu with TiN/Ti refiner are higher than that of the alloy without refiner. At the temperature of 450?, the flow stress of alloy with TiN/Ti refiner is higher than that of the alloy without refiner at first, but the flow stress of the alloy without refiner increases with the increase of deformation degree, and is gradually higher than that of the alloy with refiner at last. |
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