| Due to the super mechanical properties of high-strength wrought aluminum alloy, they have a wide range of applications in many fields. Especially, the high strength aluminum alloy with certain heat resistance will have better application forground in the aerospace industry and deep continental drilling field. In this paper, the widely used and typical alloys(such as Al-Cu-Mg-(Ag) and Al-Zn-Mg-Cu-(Ag) series alloys) were chosen to be investigated. Based on the previous studies, heat resistant Al-Cu-Mg-Ag alloys with various compositions were designed, moreover, the composition, the heat-treatment and deformation parameters were optimized. While the effect of Ag element on the aging process was also studied in the Al-Zn-Mg-Cu alloy, meanwhile, the influence of the order between deformation and heat treatment on the microstructure and mechanical properties was investigated, and the technological processes were optimized. Besides, the electropulsing treatment(EPT) technology was applied in the conventional 2024(Al-Cu-Mg) and 7075(Al-Zn-Mg-Cu) alloys, and the microstructure and mechanical properties of the alloys were analyzed. On the one hand, it expands the applied field of EPT; on the other hand, it provides a new method to improve the microstructure and mechanical properties of Al alloys.In order to study the effects of surplus-phase on the microstructure and mechanical properties of the extruded and T6-temper Al-Cu-Mg-Ag alloys, the high Cu/Mg Al-Cu-Mg-Ag alloys with various compositions were designed in this investigation. The results show that Ag addition changes the precipitates and accelerates the age-hardening rate in the Al-Cu-Mg alloy. When the Cu content is below the solubility limit, the strength of the extruded alloys increases with improving the Cu content, moreover, both the increased Cu and Mg contents can accelerate the precipitation process. However, the excessive Cu and Mg will play a negative role in improving the mechanical properties(especially ductility) of the alloy. As a result, to obtain the superior mechanical properties, the Cu and Mg contents should be controlled in a certain range. When the Cu content excesses the largest solubility, the alloy will retain large amount of surplus phases, which will be harmful for the precipitation and mechanical properties of the alloy, but the appropriate surplus phases will play an effective role in strengthening the alloy at elevated temperature. Based on the results, the optimal composition of the alloy is confirmed as Al-6.3Cu-4.8Mg-0.4Ag, and the microstructure and mechanical behavior of the optimized alloy with various extrusion ratios(extrusion ratios of 17, 30 and 67, respectively) were analyzed. It is found that the sample with extrusion ratio of 30 has the superior comprehensive performance, which was attributed to the fine grains and dispersed precipitates. Meanwhile, it is indicated that there exists a critical extrusion ratio, when the extrusion ratio is below the critical value, the mechanical properties will increase with improving the extrusion ratio. However, while the extrusion ratios excess the value, the excessive deformation will hinder the diffusion of atoms and accelerate the recrystallization and grain growth at elevated temperature, which will be harmful for the mechanical behavior on the contrary.Unlike the Al-Cu-Mg-Ag alloy, the addition of Ag will not change the precipitates in the 7075 Al alloy, but will remarkably accelerate the age hardening rate, moreover, the addition of Ag changes the distribution of the residual phases, which will improve the ductility of the alloy. The superior mechanical properties of the 7075 Al alloy with 0.4 wt% Ag were obtained after the aging time reduced by 50% compared with 7075 Al alloy. While in order to examine the combined effect of plastic deformation and the aging process, the 7075 Al alloy subjected to cold-rolling(CR) deformation and T6 heat-treatment(solid-solution treatment + artificial aging) by different routes: CR+T6 and T6+CR was investigated in this study. The results show that the CR before T6 heat treatment has few influences on the mechanical properties, which is attributed to the consumption of large amount of deformation energy by T6 heat treatment. While the cold-rolling after T6 treatment can improve both the yield strength(YS) and ultimate tensile strength(UTS) of the alloy, and the precipitation strengthening and dislocation strengthening are recognized as the reasons for the improved mechanical properties. In the experiment, the T6+40%(the optimized T6+CR) sample has superior strength with adequate plasticity, and the continued increasing deformation has few effect on improving the strength, moreover, it will induce the great loss of ductility.Besides, the hot rolled 2024 Al alloy subjected to electropulsing was investigated, and the microstructure and mechanical properties were analyzed in this study. The results show that the electropulsing can remarkably refine the microstructure of the hot rolled 2024 Al alloy. After EPT, the orientated and coarse grains turn into the fine equiaxed grains, which are attributed to: on the one hand, EPT accelerates the atomic diffusion and the nucleation of recrystallization; on the other hand, the short-time EPT and rapid cooling followed EPT restrain the growth of the new grains. The fine grains and the dissolution of parts of secondary phases are the primary reasons for the improved mechanical properties. Hence, EPT is a promising method to optimize the microstructure and mechanical behavior of the Al alloy. In addition, increasing the deformation and the cyclic index of EPT has few effects on the microstructure. But we believe that the accurate control of energy input and increasing the cooling rate after EPT may make better results.On the other hand, the optimized cyclic EPT can remarkably accelerate the precipitation of T-phase(Al20Cu2Mn3) in the solid solution treated 2024 Al alloy. Moreover, with increasing the current density and the cyclic index, the uniformly distributed T-phase was obtained, and the particles were surrounded by dislocations after EPT, which was the reason for the improved conductivity and mechanical properties compared with a non-electropulsed sample. On the one hand, EPT promotes the dissolution of the unstable phases; on the other hand, EPT can accelerate the precipitation of stable phase(T-phase). However, when the current parameter excesses a certain range, the value of strength and plastisity droped, and the specimen may even bend severely. While the EPT with various parameters was applied on the solid-solution treated 7075 Al alloy in this study, the mechanical properties of the samples did not approach to the conventional artificial condition. It is indicated that EPT have not accelerated the precipitation of the precipitates with low precipitation temperature. Under the low EPT parameters, EPT can accelerate the natural aging process, while under the high EPT parameters, the dissolution of secondary phases is the primary behavior.When EPT was applied on the as-cast 2024 Al alloy at room temperature, the rod-like second-phases rapidly became the spherical phases. The rapid spheroidizing of secondary phases was attributed to the decreased thermodynamic barrier and the accelerated atom motion induced by EPT. Furthermore, the dissolving process of the randomly distributed second-phase was recognized as the combined effect of the two basic dissolving ways. Besides, the mechanism for rapid spheroidizing of the second-phase was proposed based on not only the accelerated dissolution but also the growth of particles. As a result, the morphology and size of secondary phase could be controlled via changing the cooling speed of the specimen after EPT.The rapid dissolution of secondary phases induced by EPT at low temperature was applied in the solid-solution treatment of 7075 Al alloy. Although the degree of second-phase dissolution was slightly smaller compared with the conventional heat treatment, the rapid process hinder the growth of the grains and decrease the size of recrystallized grains, moreover, the fine grains and rapid diffusion of solute could make the fine precipitates during artificial aging, as a result, the strength of the alloy with EPT is slightly higher than the conventional T6 treatment. Besides, EPT could play an effective role in reducing the distortion and oxidation of the specimens, which provides a new way to improve the surface quality in the heat treated products.After EPT, the T6-temper 7075 Al alloy shows that parts of the intragranular precipitates dissolve in the matrix, especially, the sample exhibits discontinuous dissolution of the grain-boundary precipitates(GBP) under the EPT with certain parameter. After re-aging, the mechanical properties of the re-aged(RA) sample are nearly same to that of T6 temper alloy, meanwhile, the coarse and discontinuous GBP were obtained. The distribution of GBP is recognized as an effective microstructure to improve the stress corrosion crack(SSC) resistance. Hence EPT and the corresponding heat treatment is a new method which can change the morphology and distribution of second phases without loss of strength, which provides a new way to design materials and improve the mechanical properties of the alloy. |
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