Alloy Designing And Study Of Microstructure And Properties Of A Al-Li Alloy For Aerospace And Aviation

A novel Al-Li alloy has been studied systemically which covering alloy designing, microstructure evaluation, mechanical and corrosion properties testing. The purpose of this study was to develop a high performance Al-Li alloy in order to meet the ever-increased long-term requirements of aerospace, aviation and armament industries. This work belongs to the "Tenth Five Year Plan" preliminary study of Chinese General Headquarter of Equipment. We own independent intellectual property right for this newly developed alloy which has satisfied the challenging material properties for both aerospace and aviation industries. The innovative work and results are as follows.1. According to its dual material properties, we have employed alloy designing method to study the effect of principle elements and found that Al-Cu-Li ternary system met the requirements and to be one of the practicable selections. The content of Li and Cu was found to be the key factor responsible for strength, while the plasticity was mainly dependent on solid solution which can be roughly estimated by the Li equivalent （Li +Cu/2.82） . According to micro-alloying study, the optimal effects came from the combined addition of Mn+Zr and Mg+Zn. As the result, a new Al-Li alloy for aerospace and aviation with the trademark of 2A97 and normal compositions of 3.7-4.0% Cu, 1.3-1.6% Li, 0.4-0.6% Zn, 0.35-0.55% Mg, 0.2-0.4% Mn, 0.1-0.2% Zr with the balance of Al was registered.2. High temperature plastic deformation behavior has been tested by tensile and compress tests. True stress-strain curves were obtained. Flow stress was modeled based the relationship between flow stress and deformation temperature and rate.. Utilizing Z constant, a formula was expressed as:According to experimental results, the material constants have been figured out as:α=0.017, n=7.67, A=1.1×10¹⁶S^-1, Q=239kJ/mol, Z=εexp（239.02/RT）.We have studied the relationship between microstructure and high temperature plastic deformation condition. Dynamic recovery and recrystallization were proposed and explained in hot deformation process. At 350 - 470℃and 1 - 30S^-1, the compressibility was more than 80% which indicated good plasticity and wide deformation temperature range. According to measured plasticity chart, optimal temperature and deformation rate for good plasticity have been obtained. These results have provided theoretical basis for hot deformation and microstructure controlling.3. Methodology to measure the CCT curve has been investigated. A comprehensive method including in-situ resistance measurement, hardness testing, microstructure analysis, together with DTA, XRD, and TEM has been developed to study phase transformation and measure the CCT curve. The parameter changes during continuous cooling transformation can be reflected rapidly and exactly using this method. The method is applicable to kinetic study for precipitation of supersaturated solid solution and other resistance sensitive phase transformations. The CCT curve measured is the first of its type for Al-Li alloys and is also a rather complete one for aluminum alloys. It is an effective tool for the establishment of heat treatment technology in Al-Li alloy. The CCT curve showed that the precipitation temperature, incubation time and duration of phase transformation are all inversely proportional to cooling rate.4. In addition toδ’, T₁, andθ’, a cubicσ（Al₅Cu₆Mg₂） phase has been observed in Al-Li alloy for the first time. Theσphase was found to appear in T6 aged, two-step aged and three-step aged alloys and it is coherent with matrix in TEM and HREM observations. Furthermore, the amount of a phase is larger than that of T₁ in T6 aged sample. Large amount of T₁ phase has been observed in T8 aged sample where the cubic phase was not found. Finally, we have found that the ageing time and temperature affecting the precipitation of the cubic phase. Andδandθ’ phases appeared in quenched samples which causing nature aging.5. We have studied the microstructure evolution and mechanical property change with regards to heat treatments. The optimal heat treatment process established was the follows: 520℃/2h and water quenching for solution treatment; 165℃/60h for T6 ageing; 6% prior stretching and 135℃/60h for T8 aging; 165℃/36h+130℃/36h for two-step ageing; 150℃/24h+165℃/24h+130℃/48h for three-step ageing. T8 aged samples provided the highest tensile strength and good plasticity. T6 aged samples gave the lowest tensile strength. The difference of tensile strength between two-step and three-step aged samples is negligible, lying between T6- and T8-treated ones.6. Pit corrosion and exfoliation behavior of T6- and T8-aged samples have been investigated through observation of microstructure change before and after immersion into EXCO solution. Electrochemical impedance spectroscopy （EIS） technique was employed too. T8 aged samples have lower exfoliation corrosion susceptibility and small rate of pit corrosion than those of T6-aged ones. The stress corrosion behavior of T6 aged samples forging under different stress was measured by constant-stress method. Under 0.5 Rpo.2, the ratio of non-stress corrosion fracture was found to be 30%.7. With this work, plates and forged samples of 2A97 have been prepared and a systematic evaluation has been carried out by third parts: Beijing Institute of Aeronautical Materials and 703 Research Institute. The testing results indicated that the overall material properties of 2A97 have surpassed the targeted requirements. Some parameters have exceeded the third-generation of Al-Li alloy developed overseas.For example:1) p equals to 2.677 g/cm³; elastic modules E≥80.7GPa, Mechanical properties of T8 aged plate for aerospace is: Rm>560MPa; Rp_0.2 >491MPa; A >7%; K_IC≥26 MPa·m^1/2; the anisotropy of tensile strength and yield strength<5 %. Mechanical properties of T6 aged forging for aviation is: Rm>495MPa; Rp_0.2 >415MPa; A >10%; K_IC≥28 MPa·m^1/2.2) Compared with the widely used 7050, the 2A97 has higher thermal stability, relatively lower elongation, similar fracture toughness, close stress corrosion resistance and fatigue crack growth rate. The bending forming ability and welding quality of 2A97 all satisfy the standard. Welding parameter （ Ar arc welding） is larger than 0.65. 3) The casting property of 2A97 is similar with that of 2195 and 2197. The alloy shows relatively high toughness and deformation ability when hot treated. The heat treating ability is also excellent, exhibiting as wide quenching temperature range, lower quenching rate sensitivity than 2023 and quenching depth which exceeds 80mm.In summary, this new 2A97 has high strength, good toughness, high damage resistance and good weldability. It possesses good malleability and performance which can be utilized in both aerospace and aviation industries. The success of 2A97 simplifies the military material system and cut the cost of alloy production and maintenance. Because of the great potential for practical use, continue research for industrial application has been confirmed as one of the "Eleventh Five Year Plan" projects. Our work supplies new thought for the exploitation of aluminum alloy.