Creep Ageing Anisotropy Mechanism,Modeling And Control Technology Of Largely Pre-Deformed Al-Cu Alloy

Deformation anisotropy is one of the key factors affecting the forming accuracy and performance consistency of components.Understanding and modeling the material anisotropic creep aging behavior is thus essential to achieve accurate creep aging forming of aluminum alloy components.The in-plane creep-aging anisotropic behavior and mechanical properties of 2219 aluminum alloys largely pre-deformed by unidirectional rolling and cross-rolling(thickness reduction of about82.6%)are investigated.It is found that the samples prepared by unidirectional rolling have a very significant creep-aging anisotropy(inplane creep anisotropy index of 49%)and a marginal yield strength anisotropy(the yield strength anisotropy indices before and after creep were about 3.7% and 6.4%,respectively).The creep rate gradually decreases with the increase of the angle between the applied load direction and the rolling direction.Subsequently,the microstructure(grain morphology,texture,dislocation and precipitation,etc.)of the largely predeformed material before and after creep aging was characterized by XRD,EBSD and S/TEM techniques.The results of the multi-scale characterizations show that the texture and precipitation behavior of the unidirectional rolling samples should not have a significant effect on the creep anisotropy.The dislocation cell(width about 500 nm)in the grain that is elongated along the rolling direction is the key factor to induce strong creep anisotropy.Based on the understanding of the microscopic mechanism of creep anisotropy,it is proposed to use non-proportional cross-rolling instead of unidirectional rolling.Under the same creep experimental conditions,the creep anisotropy indices of the specimens prepared by 1:1 and 3:1 cross-rolling schemes are about 23% and 13%,respectively.The yield strength of the material before creep aging is higher than 470 MPa and the elongation is ～5%.After creep aging,the yield strength slightly decreases(higher than 450 MPa)but the uniform elongation is improved to about 6-8%.The yield strength anisotropy index of the material before and after creep aging is less than 6%.The dislocation structures in the rolling plane of the largely pre-deformed alloys prepared by different rolling processes were characterized using bright-field(TEMBF)and low-angle annular dark-field(LAADF-STEM)imaging modes under transmission electron microscopy.The results show that the dislocation cell morphology in the largely pre-deformed samples prepared by 3:1 cross rolling gradually changed from "elliptical" in unidirectional rolling to "rounded"(with a diameter of 800nm-1.2μm).This randomly distributed "rounded" dislocation cell structure effectively mitigates the creep anisotropy of largely pre-deformed alloys while maintains a high creep efficiency.By introducing an orientation-dependent creep resistance caused by back-stress of the elongated dislocation nanostructure,an internal variable creep model based on physical mechanism is established to accurately describe the in-plane creep anisotropy.Based on the crystal plasticity finite element method,the effects of texture and grain shape on creep anisotropy are considered,and the in-plane creep anisotropy behavior of 3:1 cross-rolled samples is accurately simulated and predicted.