Study On Hot Deformation Behavior And Microstructure, Property Of 2297 Al-Li Alloy

Al-Li alloy has been a research focus in the field of aerospace applications, because of its high specific strength, specific stiffness, and excellent comprehensive performance. Hot deformation technology has always been one of the problems that restrict the development of Al-Li alloy plate. In this paper, the third generation of Al-Li alloy 2297 alloy is taken as the research object, systematically studied its microstructure evolution during hot deformation, and analyzed the correlation between the heat treatment and the microstructure, as well as mechanical properties, which provided the experimental basis and theoretical support for controlling finally microstructure and properties, and it is of great significance to achieve practical application of 2297 Al-Li alloy.Constitutive model for predicting flow behavior of the alloy was constructed by isothermal single pass hot deformation experiments. Dynamic softening mechanism of the alloy during hot deformation is closely related to the Z parameter. When the value of ln Z is higher(lnZ>51.70, T<420℃), the cross slip of screw dislocations and the climb of edge dislocations are the main rate controlling mechanisms. When the value of lnZ is moderate(47.10<lnZ≤51.70,T≥380℃), the main rate control mechanism are the screw dislocation cross slip, detachment of three-dimensional dislocation network, partial dislocations depinning, and some dynamic recrystallization(DRX). When the value of lnZ is low(lnZ≤47.10,T≥420℃), the dynamic recovery(DRV) and DRX is the main control mechanism. Based on the flow behavior of the alloy, the critical conditions of DRX was identified: εc/εp=0.294~0.657,ζc/ζp=0.885~0.990, and then DRX kinetics model was constructed.Processing map was constructed to optimize the hot deformation parameter range of the alloy. The suitable heat treatment temperature and strain rate range are 400~440℃ and about 0.1s-1, respectively. In stable domain, the microstructure evolution is domainated by DRV and DRX. The main mechanism of DRX is continuous dynamic recrystallization(C-DRX), and with some partical simulated nucleation(PSN) mechanism. DRX microstructure evolution model of the alloy was constructed. With the increase of strain, initial grain structure→form a large number of subgrains in the deformed grains→the deformed grains are transformed into lamellar structure → collapse of the lamellar boundarie in the trigeminal grain boundaries→spheroidisation and growth begins by Y-junction migration→form new recrystallization grains.Dynamic precipitation and refinement of T1 phase(Al2CuLi) of the alloy during hot deformation(340~500℃) were studied systematically. Coarse T1 phase precipitated at 340~460℃ holding stage. T1 phase dynamic precipitation, at the same time, T1 was obviously refined. Refinement of T1 phase derived from two aspects. One is the coarse T1 phase precipated in holding stage was broken and dissolved during the hot deformation processing. On the other hand, original grain boundaries, a large number of dislocations and subgrain boundaries introduced during the hot deformantion provided a large amount of heterogeneous nucleation site for the T1 phase, which leads to the fine precipitation of T1. When the deformation temperature is higher than 460℃, without precipitation of T1. The β′ phase(Al3Zr) and Mn-containing phase is always stable existence during the hot deformation. The δ′phase(Al3Li) can be precipitated in the quenching process.Double-pass and multi-pass hot compression experiment were conducted to simulate the actual production process of the alloy. The results show that the degree of recrystallization and DRX grain size of the alloy can be controlled by adjusting strain rate in different pass. In the early stage of multi-pass hot deformation, DRV dominated microstructure evolution. When the deformation is more than 40%, recrystallization occurs in the alloy. With the increase of deformation reduction, the degree of recrystallization increased. DRX first occurred in the vicinity of the original grain boundary and second phase particles, and then produced within the deformed grains. Static softening between the deformation pass is beneficial to the DRX, but it is not conducive to the dynamic precipitation of T1 phase.Effect of heat treatment on the microstructure and properties of the alloy plate was studied combined with performance test and microstructure analysis. The suitable solid solution temper and T6 temper were determined at 490℃/1.5h and 175℃/48 h. At T6 temper, tensile strength, yield strength and elongation of the alloy reached 447 MPa, 369 MPa and 8.9%, respectively. Main strengthening phase are δ′phase, θ′ phase and T1 phase. The suitable T8 temper was determined at 6%+160℃/36 h. Meanwhile, tensile strength, yield strength and elongation of the alloy reached 500 MPa, 454 MPa and 10.5%, respectively. The varieties of strengthening phase are still the δ′ phase, θ′ phase and T1 phase. But T1 phase is the main strengthening phase. The predeformation amount is larger(less than 6%), more conducive to the fine precipation of T1 phase, the higher the strength of the alloy, while the elongation decreased. When the predeformation is more than 6%, the strength of the alloy tends to be stable.Effect of hot deformation process and heat treatment on the anisotropy of the alloy was evaluated by the yield strength anisotropy index. The results shows that the cross rolling and the increase of the degree of deformation reduction reduce the anisotropy of the yield strength of the alloy. The precipitatation of δ′ phase during solid solution quenching process leads to the increase of plate yield strength anisotropy index. However, the precipitatation of θ′ phase and T1 phase in T6 treatment are beneficial to reduce the anisotropy index of the yield strength of the plate. T8 treatment process, with the increase of predeformation, the degree of T1 phase inhomogeneous distribution increased in the habit plane, resulting in yield strength anisotropy increases of the plate.Effect of quenching parameters on residual stress of the alloy plate was studied. The results show that the residual stress gradually decreased with the increase of the quenching temperature and the concentration of PAG solution. The stress evolution processing of the characteristic point in the plate with time is analyzed using finite element simulation. The results show that the compressive stress in the core of the plate transformed into tensile stress at 4.9s, after 15 s tended to a stable value 49 MPa. The surface tensile stress is changed into compressive stress at 17 s, after 17 s tended to a stable value-79 MPa.