| 2XXX aluminum alloys are widely used in the commercial aircraft structures because of their high strength to weight ratio, superior high temperature properties, good damage tolerance, resistance to fatigue crack propagation and fracture toughness. Trace additions of Ag to Al-Cu-Mg alloys with high Cu/Mg ratios stimulate an enhanced age hardening reaction and exhibit excellent thermal stability and creep resistance, which is attributed to the formation of theΩphase in the alloy.Deformation characteristic, processing map and microstructural evolution of an Al-Cu-Mg alloy during hot compression were characterized in the present work. The effects of pre-strain on the properties and microstructures in an Al-Cu-Mg alloy were also investigated. The precipitation behavior ofΩphase at various temperatures in an Al-Cu-Mg-Ag alloy was systematically analyzed and discussed in detail. A type of duplex aging heat treatment was developed to improve the mechanical properties at room temperature and elevated temperatures in a pre-strained Al-Cu-Mg-Ag alloy. The main conclusions can be summarized as follows:(1) The flow stress of Al-3.78Cu-1.67Mg alloy increased rapidly to a peak value at critical stain. The peak stress decreased with increasing deformation temperature and decreasing strain rate, a behavior which can be described by the constitutive equation in hyperbolic sine function with a hot deformation activation energy of 277.8KJ/mol.(2) The optimum hot-working conditions for hot rolling, extrusion and forging were in the temperature range of 390 to 420℃and strain rate range of 0.1 to 1 s-1. The softening mechanism of the Al-3.78Cu-1.67Mg alloy at high Z value was dynamic recovery; continuous dynamic recrystallization occurred as deformed at low Z value. Small Al3Zr and Al20Cu2Mn3 particles dynamically precipitated during hot deformation restrained dynamic recrystallization and increased the deformation activation energy of the alloy.(3) With increasing the pre-strain percents, both the hardness and yield strength of naturally aged Al-3.78Cu-1.67Mg alloy were significantly improved, whereas the variation of ultimate tensile strength is less, and the elongation decreased. In contrast to the resistance of the naturally aged alloy with a pre-strain of 0% to exfoliation corrosion, that of 10% pre-strained alloy was reduced, which was mainly attributed to the increase of dislocations density and grain aspect ratio.(4) TEM observations confirmed the existence ofΩphase in Al-4.66Cu-0.51Mg-0.52Ag alloy when aged for 1h at temperatures in a range of 145 to 450℃. With increasing the aging temperature, the size ofΩphase increased, whereas the number density ofΩphase increased to a peak value at 200℃.(5) A type of duplex aging was employed to obtain moreΩprecipitates by controlling the competitive precipitation betweenΩandθ' phases, increasing the aging hardness of pre-strained Al-4.94Cu-0.43Mg-1.04Ag alloys. Comparing with the conventional T8 peak-aged temper, the tensile strength and yiled Stength of the duplex aging temper at room temperature and elevated temperatures were enhanced by approximately 3-7%, whereas the elongation kept at a high level. The increase in the mechanical properties of duplex aging temper is due to the fact that the recovery of dislocations occurred and the precipitation of 0'phase was restrained effectively at high aging temperature, and moreΩprecipitates were formed during secondary aging. |
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