| The 7000 series, Al-Zn-Mg and Al-Zn-Mg-Cu aluminium alloys have been known to be strengthened by precipitation hardening with aging treatment. But, among of defects in 7000 Al alloy, casts defects such as gas porosity and shrinkage defect are formed during solidification because of wide mushy zone. This study was undertaken to investigate the effects of applied pressures on aging precipitating behavior, micro and macro-structures and mechanical properties in 7000 series Al alloys. The microstructure of 25, 50, 75 MPa specimens was more fine and dense than that of non-pressurized cast, because of the greater cooling rate of billet by the application of pressure. GP zone solvus was estimated to be 130℃ by DSC results. Growth of precipitates at 180℃ aging treatment was faster than 120℃ aging treatment. Hardness values of as cast specimens was increased by fine grain size and increase in solute amount within matrix with increase in applied pressure. In case of 180℃ aging treatment, aging time obtained highest hardness valuers was shortened than in case of 120℃ aging treatment. In case of 120℃ aging treatment, tensile strength and hardness was represented a similar increasing tendency. Due to fine grain size, tensile strength values of 7075 Al alloy was higher than that of 7050 Al alloy. |
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