Study On The Hot Deformation Behavior And Microstructural Evolution Of 7075 Aluminum Alloy

Aluminum alloy is considered to have many advantages, such as low density, good corrosion resistance, good electrical and thermal conductivitiy, excellent machinability and easy recycling. The strength of aluminum ally can be enhanced by solid solution strengthening, precipitation strengthening and strain hardening. As structural material, aluminum alloy has become one of the hot topics in the field of automobile, aerospace and electronics. With the increasing pressure of population, non-renewable resources and environment protection, there has been recently a growing demand for producing light weight vehicles in order to reduce the energy consumption and CO2 emission. Researches to increase the strength and elongation, improve the plastic deformation capability, simplify fabrication technics and reduce manufacturing cost are now attracting worldwide attention in aluminum alloy fields.It is difficult to fabricate aluminum alloy sheet by traditional technology which has complicated process, energy inefficient, complex phase composition, low elongation and poor plastic deformation capability. Twin roll casting (TRC) technology is applied to fabricated aluminum alloy by its short flow and near net shape process. It combines casting and hot rolling into a single step with high cooling rate, which can refine grain and second-phase size, reduce segregation, improve plastic deformation capability, and also shorten the fabrication process of aluminum alloy. The forming mechanism of TRC microstructure during strip casting process, the microstructure evolution of 7075 alloy sheet during rolling process, and factors which influence the microstructure were studied systematically. In addition, the constitutive model was established by regression model. The rheological behavior and microstructural evolution during hot deformation were studied with the processing maps. And, deformation parameters were determined to improve the integrated properties of aluminum alloy. Furthermore, the effect of second-phase particles on the microstructure evolution and deformation behavior was also investigated, which revealed the particle stimulated nucleation mechanism in TRC alloy.High-strength 7075 aluminum alloy strip with 4.5 in thickness and 150mm in width was sucessfully fabricated by TRC technology in this paper. The fabrication parameters of TRC were also determined that pouring temperature was 680℃, rolling speed was 5mpm, and roller gap was 4mm. The forming mechanism of TRC aluminum alloy microstructure during strip casting process was studied. The results showed that dendrite structure with short second dendrite arm spacing developed from surface to the center of TRC strip along the thermal gradient. The grain size of TRC alloy was much smaller than that of traditional casting alloy due to its higher cooling rate during TRC process. Furthermore, the intermetallic compounds were also refined and distributed uniformly, which could improve the deformation capability of aluminum alloy.Based on the hyperbolic sine equation, the constitutive equation of 7075 aluminum alloy was established by regression model. The rheological behavior and microstructural evolution during hot deformation were studied. And the predicted results and experimental data showed good agreement. In the safe deormation domain, the peak value of power dissipation reached 32%, and the deformed microstructure was fine-grained and uniform. In the instablity domain, the deformed microstructure was inhomogeneous where recrystallized grains were presented in center area and large grains in surface area. The optimal rolling process parameter was determined as 663K-723K,0.01s-1-0.1s-1. During this domain, recrystallized grain were observed and distributed homogeneously, which could improve the deformation capability of aluminum alloy.The hot deformation behavior of 7075 aluminum alloy sheets with the thickness of lmm fabricated by twin roll casting and sequent rolling process were also investigated. The constitutive model was established and also was modified by considering the deformation conditions. The largest elongation of 235% was obtained at 773K,0.001s-1 and 200% was obtained at relatively high strain rate of 0.005s-1. Fine grained microstructure with high angle grain boundary was considered as main reason for large elongation.In this paper, the effect of partcle distribution on the microstructure and deformation behavior was also studied. High solidification rate in TRC alloy induced small particles (-1 μm) in homogeneous distribution. However, inhomogeneous particle distribution was observed in PMC alloy, wherein large fraction of fine particles with the size around 0.1 μm and some remarkably large particles over 5 μm were concurrent. The relatively high fraction of particles about 1 μm in TRC alloy attributed the homogeneous recrystallized microstructure with fine grains induced by particle simulated nucleation, which was characterized by the rotated cube recrystallization texture at high temperature. This was beneficial to high ductility and formability of TRC alloy sheets during hot tensile deformation. However, fine particles about 0.1 μm in PMC alloy inhibited recrystallization and resulted in partial recrystallized microstructure which was dominated by mixture of rolling and recrystallization texture. Meanwhile, remarkably large particles over 5 μm initiated the cracks at the particle-matrix interface and also the cracks inside particles, leading to low fracture elongation.