Plastic loading induced high strength-ductility for creep aged Al–Cu–Li alloys

Large-curved or high-ribbed large-size integral panels are often subjected to plastic deformation during the loading stage of creep age forming (CAF). Although this plastic loading helps to improve the creep deformation, whether both good strength and ductility of the creep-aged alloy under plastic loading can be guaranteed remains unclear. Thus, under the applied stress ranging from elastic (230/250/270 MPa) to plastic regions (320/ 340 MPa) at 180 degrees C, mechanical properties and microstructural evolution of creep-aged 2195-T4 Al-Cu-Li alloys were systematically investigated. Surprisingly, high yield strength up to 620 MPa and ductility above 9.4% are obtained for peak-aged alloys under plastic loading, increasing by 9.0%-21.5% and 6.7%-30.1% over elastic loading and stress-free aging, respectively. High strength is attributed to the combined effect of high density of dislocations, densely distributed fine T1 precipitates in the matrix, and narrowed precipitate free zones (PFZ) at grain boundaries. Meanwhile, it was observed that shearing of T1 plates happens only as single shearing step, and numerous single-shearing events occur at different locations, accompanied by elastic and/or plastic distortion and progressively segmented rotation (accumulated angle up to 20 degrees) of T1 precipitate, which induce high ductility during plastic deformation. The above good mechanical properties were much less sensitive to aging time for plastic-loading creep-aged alloys relative to stress-free or elastic-loading counterparts, which helps to widen the CAF process window. This paper offers a new strategy of tailoring shape and properties during CAF, which would break the limitation of CAF currently only for the small-curved components.