Influence of cryogenic deformation and post-deform aging on microstructure and mechanical properties of Al-Cu alloy

Cryogenic forming has been developed as a new forming process for aluminum alloy components with ultra-thin thickness or complex shapes. But their mechanical properties are severely affected by cryogenic deformation and post-deform aging. The cooperative effects were thus studied under different cryogenic deformation and aging treatment conditions. The corresponding strengthening mechanisms were revealed through microstructure characterizations. The changements in dislocation and grain structure were evaluated through X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Precipitation evolutions were reflected by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The strengthening effects were revealed through quantitative strengthening contribution analysis. It was found that excellent mechanical properties could be obtained under the combined contributions of cryogenic deformation and post-deform aging. Large cryogenic deformation can improve the strengths significantly without artificial aging, benefiting from the more produced dislocations and a few deformation-promoting theta '' phases. The highest yield strength reached 397.5 MPa after 30 % cryogenic deformation, which was 7.4 % higher than that of traditional T87. Cryogenic deformation could accelerate the precipitation obviously in artificial aging, which led to the continuous transformation of theta '' phase into dispersive theta ' phase in the peak aging condition. The peak aging time shortened gradually with the increasing cryogenic deformation. Excellent strength could be even obtained by aging 6 h at 175 degrees C for 2 % cryogenic deformation, which is just one-third of the traditional aging time. Moreover, the deformationproduced dislocations would recover in artificial aging, which is beneficial to the optimal combination of strength and toughness through large cryogenic deformation and short-time artificial aging. The processcontrolling window was established, which can provide huge potential to adjust the performance distribution in cryogenic forming for heat-treatable aluminum alloy.