| 2E12 aluminum alloy is developed a new high-performance aluminum alloy by China. 2E12 aluminum alloy based on 2524 Al-alloy is the most outstanding one, which has good fracture toughness and fatigue resistance, in air high-strength Al-Cu-Mg alloys. It is well known that the properties of alloys are closely related to its microstructure. The deformation behavior and microstructure evolution in a hot compressed 2E12 alloy was investigated by constructing power dissipation map in which work efficiency can be related with microstructural evolution. The effects of the final thermo-mechanical treatment on the mechanical properties, precipitation kinetics and microstructures of 2E12 alloy were investigated with various experimental and testing methods, such as optical microscopy, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction (EBSD), microhardness test, mechanical properties tests and differential scanning calorimetry (DSC). The major conclusions were drawn as follows:(1) During hot deformation, the true stress-strain curves of 2E12 alloy show a steady state flow behaviour including transient and steady state deformation. Flow stress decreases with the temperature increase and strain rate decrease. In the high strain rate and large strain condition, the decrement of flow stress is due to increment of test temperature as a result of deformation heat. The influence of adiabatic temperature rise increases as the test temperature decreases.(2) The material constants of 2E12 alloy are figured out as:a=9.879×10-3 MPa-1, n=6.006, A=2.151×1011 s-1, Q(ΔH)= 154.8kJ/mol. The following relationship between the flow stressσ, strain rate, deformation temperature T and Z parameters was derived:(3) The maps for 2E12 alloy exhibited three domains with higher value of power dissipation:Domain #1 occurs in the temperature range of 325-375℃and strain rate range of 0.01-0.03 s-1, with a peak efficiency of about 22% at about 350℃and 0.01s-1. Domain #2 occurs in the temperature range of 350-450℃and strain rate range of 1.78-10s-1, with a peak efficiency of about 25% at 400℃and 10s-1. Domain #3 occurs in the temperature range of 450-500℃and strain rate range of 0.01-10s-1, with a peak efficiency of about 33% at 500℃and 0.01s-1. It was also found that there would be a wide region where recovery process dominates during deformation in the present alloy for an industrially applicable processing because the efficiency level was quite uniform in the temperature range of 300℃-450℃and the examined strain rate range.(4) The results from processing maps correlated well with the flow curves and microstructural examination. At the deformation temperature of 200℃, it is shown that the microstructure of 2E12 alloy is fibrous and dynamic recovery is oblivious. At the temperature range of 300-400℃, fibrous microstructure gradually transforms to equiaxed subgrain by dynamic recovery and the degree of dynamic recovery also significantly increases with increasing temperature. At the deformation temperature of 400℃, the equiaxed subgrain is apparent. In the low strain rate 3# region of processing map, at the deformation temperature above 450℃, the main softening mechanism of 2E12 alloy is continuous dynamic recrystallization, and the nucleation mechanism for continuous dynamic recrystallization is the reversing or mergering of subgrain. The fine recrystallized grains preferentially nucleate along the original grain boundary and are "chain"-like distribution. The degree of dynamic recrystallization significantly increases as the temperature arises. However, the rate of dynamic recrystallization increases with the decrement of strain rate. At the deformation temperature of 500℃, the strain rate range of 1-10s-1 (3# region of processing map), there also are evidence of redissolution of the particles and intercrystalline cracks.(5) The effect of age hardening and strengthening for 2E12 alloy is significantly enhanced by 40% cold roll after solution treatment, which also significantly accelerates the aging hardening rate and shortens the arrival time of peak aging. The temperature of S1 phase exothermic peak for 40% cold roll after solution treatment is much lower than that for solution treatment. The precipitation volume fraction and precipitation rate of S'phase in 40% cold roll after solution treatment are significantly higher than that of solid solution samples. It is showed that the activation energy of S1 phase in 2E12 alloy of 40% cold roll after solution and solid solution is 86.6kJ/mol and127.6 kJ/mol, respectively.(6) The strength of 2E12 alloy has been significantly improved by 40% cold roll after solution treatment, but its Elongation falls to 4%. The strength and ductility of 40% cold rolled 2E12 after solution were significantly increased by post-aging treatments. A good strength-ductility combination for 2E12 alloy was obatained by 40% cold roll after solution treatment+aging at 175℃/7h (peak aging). The yield strength, tensile strength and elongation of 2E12 alloy treated by this processing reach to～540MPa,～610MPa and～8%, respectively. When 40% cold rolled 2E12 after solution was aged at a relatively high temperature, there were also some improvement in ductility, but the strengthening effect is significantly reduced. The increment of yield strength decreases with increase of aging temperature. In this paper, the optimum final thermo-mechanical treatment for 2E12 alloy is 40% cold roll after solution treatment+aging at 175℃/lh (underageing). The strength and ductility were simultaneously enhanced by this final thermo-mechanical treatment. The yield strength, tensile strength and elongation of 2E12 alloy processed by this final thermo-mechanical treatment are～520MPa which is higher by～40MPa than that of T8,-600MPa and 12.3% which is higher by a factor of～2.6 than that of T8 treated alloys, respectively.(7) The main strengthening phase of the traditional heat treatment 2E12 alloy (T6, T8) is coarse S'phase precipitations, while the strengthening effect of 2E12 alloy treated by final thermo-mechanical processing is mainly from the nanometer scale GPB2/S" phase precipitations and the dislocation strengthening. However, the strengthening effect of over-aging and peak-aging in final thermo-mechanical processing is derived from the small S' phase precipitations and weak dislocation strengthening. SEM fracture surface of cold rolled 2E12 after solution show tensile fracture surface contains a small amount of dimples, torn edges and some smooth regions, which indicate a mixed fracture characteristics. The tensile fracture surface of 2E12 alloy treated by 40% cold roll after solution+aging at 175℃/1h includes a large number of dimples and tearing edges, which shows a ductile fracture. The tensile fracture surface of 2E12 alloy treated by 40% cold roll after solution+aging at 175℃/24h is compose of a large number of edges and a few dimples, which displays a brittle fracture. The brittle tendency increases as the post-aging time increases. |
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