Research On Microstructure Evolution And Deformation Behavior Of 7075 Aluminum Alloy In High Temperature Solid And Semi-solid State

7075 high strength aluminum alloy is widely used in national defense and aerospace for its excellent performance, its processing methods mainly include forging and thixoforming, the former involves high production costs with low efficiency, and requires much machining; the latter can form complex parts with near net shaping, but mechanical properties of the parts is not as good as the forged pieces, moreover, these parts show preference for cracking.This study proposed a compound forming technology by integrating the semi-solid thixoforming and plastic forming, namely the region of billet corresponding to forming the complex shape was deformed in the semi-solid state, while other simple regions were deformed plastically in order to couple the advantages of this two processing methods. In this paper, based on the said compound forming technology, with 7075 aluminum alloy extrusion bar as the raw materials, some first crucial issues were researched, which included the microstructure transformation in semi-solid remelting process, achievement of temperature gradient and deformation behavior in high temperature solid and semi-solid state.The semi-solid isothermal remelting process of extruded bar was studied by the induction heating, then microstructure evolution together with growing rate and mechanism of the grains was analysed. The results showed that, when the holding temperature was at 610~620℃, holding time was at 5~10min, then the semi-solid microstructure with small and round grains can be obtained. The faster heating rate suppressed recovery, which can retain more deformation storage and heighten recrystallization nucleation rate, then recrystallized grains were refined, which caused the thing that grains coarsening rate was slowed down. However, the faster heating rate made less liquid in a short period, which gave grains priority to coarsen quickly by merging.The temperature gradient of extruded bar was achieved through a variable pitch coil, then the microstructure and hardness in different parts were analysed.The results showed that, the highest and lowest temperature of the bar was 620℃ and458℃ respectively. From the high temperature zone to low temperature zone, the microstructure firstly was tiny spherulites, then changed into irregular equiaxed grains in which several insoluble particles, lastly evolved as the microstructure with the characteristic of extruded trace, then the hardness was reduced from 155.6HV to94.3HV.The thermal simulation experiment of extruded bar was conducted in high temperature solid and semi-solid state, then the characteristic of the true stress-strain curves was analysed, the constitutive equation of peak stress was also established, eventually the stress-strain curves were fitted by multiple linear regression. The results showed that, the stress-strain curves at high temperature solid state can be divided into work hardening and steady flowing successively, the stress-strain curves at semi-solid state can be divided into work hardening,rheological softening and steady flowing successively. The average relative error of calculated peak stress and experimental peak stress at high temperature solid and semi-solid state was 3.88% and 4.19% respectively, the average relative error of fitted discrete points and measured curves was 2.19% and 3.12% respectively.The hot deformation behavior was analysed by the microstructure of compressed samples. The results showed that, when the deformation temperature was at 350~490℃, the compression process embodied thermoplastic behavior, and the lower the strain rate was or the higher the deformation temperature was, the higher degree the recrystallization was; when the deformation temperature was at490~540℃, in spite of the sample was in semi-solid state, little liquid phase was generated, so the compression process also showed thermoplastic behavior; when the deformation temperature was at 540~580℃, the liquid phase started to enhance,more fine grains appeared, the proportion of thixotropic behavior began to rise rapidly; when the deformation temperature was above 580℃, the microstructure was semi-solid fine grains, then thixotropic behavior was quite complete, and the liquid phase played an important role in coordinating deformation during hot compression.In this paper, some key preliminary problems about the compound forming technology by integrating the semi-solid thixoforming and plastic forming were solved through a series of researches aforementioned, which can provide powerful help for drafting heating process and selecting forming parameters of specific parts.