Abstract
The conventional heat-treatment standard for the industrial post hot-forging cycle of AA7050 is regulated by the AMS4333 and AMS2770N standards. An innovative method that aimed to improve toughness behavior in Al alloys has been developed and reported. The unconventional method introduces an intermediate warm working step between the solution treating and the final ageing treatment for the high resistance aluminum alloy AA7050. The results showed several benefits starting from the grain refinement to a more stable fracture toughness KIC behavior (with an appreciable higher value) without tensile property loss. A microstructural and precipitation state characterization provided elements for the initial understanding of these improvements in the macro-properties.
Highlights
For AA7050, the AMS4333 and AMS2770N standards require cold working after solution heat treatment and prior to aging [1,2]
The tensile properties of the heat-treatable aluminum alloys are improved by this process because of the pinning of the dislocation structures developed during the deformation, nucleation and refinement of the S’ precipitates in Al-Cu-Mg alloys, θ’ precipitates in Al-Cu alloys, and T1 precipitates in lithium-containing Al-Cu-Mg alloys [3]
A warm deformation process of an AA7050 aluminum alloy showed the possibility to increase the fracture toughness behavior with a KIC data dispersion reduction, without significantly affecting the tensile properties. This treatment may be suitable for all the other 7XXX and 8XXX alloy families where a room-temperature deformation step is mandatory for the fulfilment of the international standards
Summary
For AA7050, the AMS4333 and AMS2770N standards require cold working after solution heat treatment and prior to aging [1,2]. It has to be noted that the AMS4333 standard introduces, after the solution treatment, an intermediate cold deformation step (5% max) before final aging to reach the optimum microstructure and precipitates distribution, improving tensile properties [6,7]. AA7050 represents one of the best performing alloys in the 7XXX aluminum family, for its good balance of high strength, corrosion resistance and toughness [18,19] achieved by controlling the recrystallisation phenomena during and after hot forming processes [20,21,22,23,24,25]. The aim of this paper was to investigate the mechanical properties resulting from the innovative heat and thermo-mechanical treatment (up-setting carried out at temperatures of 423 K and 473 K instead of room temperature as in the standard cycle) and to describe the introduced microstructural and precipitation state modification
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