Study On Microstructure And Mechanical Properties During Extrusion And Heat Treatment Process Of 7050 High Strength Aluminum Alloy With Large Cross-section

Research on the evolution of low angle grain boundary (LAGB) and grain boundary precipitates during the extrusion, solid solution treatment and aging, together with its effect on the properties has been investigated in large-section 7050 high-strength aluminum alloy profiles for the main structure of large aircraft. The research contents include:a study of constitution relation and dynamic recrystallization model of 7050 aluminum cast ingots with homogenization during hot deformation which are calculated fromthe strain and stress data afterthermal simulation experiment and OM observation; a field quantity and microstructure simulation study of strain rate, strain, temperature, grain size distribution and the degree of dynamic recrystallizationin the cross section of this profile after putting the two key models and actual producting parameter into the commercial Deform-3D software; a study of dislocation structure evolution on the formation of low-angle grain boundary by quenching elastic strain energyand grain refinementduring aging by using OM, TEM, andEBSD.The effect of precipitation at sub-grain boundaries on electrical conductivity and impact toughness during aging in 7050 aluminum alloy by OM and TEM and the best fitting model for the mechanism of grain boundary strengtheningaccording to impact energyand tensile test experiment are also preformed based on former studies.The thermal simulation study of 7050highstrength aluminum alloy casting ingots after homogenizing treatment with 490℃/24h shows that different dynamic recrystallization mechanism will occur when the alloy is deformed in different conditions, such as GDRX, CDRX, or DDRX. GDRX can be triggered when deformed at low temperature and high strain rate, i.e.350℃/10s-1, and the parameter a2, the ratio of the peak strain to the critical strain in the dynamic recrystallization, is about 0.2. While at high temperature and low strain rate the grain size is mainly affected by DDRX; at this time, a2 is about 0.4. Between these two conditions the value of a2 is larger due to the completion of these dynamic softening, and CDRX is an obvious phenomenon. Through stress-strain data, the two key models are as follows:Flow stress model: a=0.22-0.104ε+0.885ε2-3.431ε3+6.256ε4-4.328ε5 n=4.501+5.972ε-105.017ε2+550.554ε3-1189.990ε4+923.668ε5 Q-191.916-162.861ε-1059.400ε2 +9463.37ε3-23620.700ε4+19202.600ε5 In A= 30.090-12.645ε-312.021ε2 +2127.186ε3-4964.912ε4+3919.297ε5 Dynamic recrystallization model: εc=0.491-εpSimulationresultsof the extrusion process by Deform-3D software show that the maximum values of the equivalent strain rate, temperature and equivalent strain are grouped close to the concave corners of the thickest section in the 7050 aluminum alloy profile. Maximum temperature, up to 447.7℃, is located at the junction of U-shaped section and the thickest section of the profile at 56.3s of the extrusion process.The trend of the strain rate, temperature and strain distribution of the cross-section of the 7050 aluminum profile is that these will decrease from the edge to the core of the profile. The smallest grainsare distributed aroundthese two concave corners because of the highest degree of dynamic recrystallization. Thegrain distribution of the Deform-3D simulation is explained by the fact that the recrystallization has happened in these two regions and simulation results are consistent with actual 7050 aluminum profiles. Since there are a large number of poorly soluble phases gathered on the new grain boundary of the concave corners with highly recrystallization during extrusion, the strength, elongation and impact energy are low; on the contrary, the center of the thickest portion of profiles has the largest average grain size because of lower dynamic recrystallization, so the strength, elongation and impact energy are higher than that of the corner.Research shows that the dislocation arrays,scattered dislocations during earlier quenchingare rearranged and formed, transform into low-angle grain boundarieswhich separate the coarse spindle-shaped grains of dimension 200 μm ×80μm into fine equiaxed grains of size 20 μm during aging.This caused that the main decrease in the density of GND from 1.83×1013 m2 to 4.40×1011 m-2 occurred in grains with<111> fiber texture.At the rising stage between two-steps aging the scattered dislocations are absorbed by newborn LAGB, which increases their misorientation beyond 3°.So MgZni precipitates on these boundaries and transforms from η’to η. The microstructure evolution makes electrical conductivity rise by 13.04% and impact energy decrease by 53.91% of the decreasment from the end of the first aging to the secondary aging lasting 60 min. At lastthe impact energy is down to 13.2 J and electrical conductivity reaches up to 19.3 mS/min secondary aging processes. The transformation has been accomplished after 121℃/360min+177℃/60min. And consequently, the low-angle grain boundary could be readily viewed in optical microscopy due to the increase of electric potential difference between the matrix and grain boundary in the alloy.Aging of T76, T74 and T73 fail to eliminate the grain size difference of the cross section of 7050 aluminum extrusion profiles. With two-step aging, there are a large number of precipitates on grain boundaries and sub-grain boundary, resulting in a great effect on the mechanical properties of the alloy, especially the impact energy. Mainly finer the grain is in the alloy, worse mechanical properties are. The largest average grain size is located in the center of the profile after T76, up to 13μm, while T74 treatment results in the smallest one at the edge, about 3μm. After different aging treatments, grain size is linear with impact energy within the cross section of profiles. The impact energy after T73 and T74 treatment is greater and better uniformity than that of T76. With the same aging treatment, the impact energy of the center is greater than that of the edge, and the biggest difference is about 20J. In addition, with the aging processing, electrical conductivity increases, and that of the central portion is higher than the edge.The study of the yield stress at the edge, side section and the center of the thickest section in 7050 aluminum extrusion profile under over-aging treatments shows that the Hall-Petch model, compared with Nes model has more accuracy for the grain boundary strengthening of over-aging 7050 aluminum alloy. However, the contribution of precipitation strengthening is more than that of grain boundary strengthening in the yield stress. Also the Taylor factor has a great influence in yield stress by increasing the contribution of precipitation strengthening and the solution strengthening.Consequently, the texture in the center of the thick section portion is mainly Copper which is hard, and the Taylor factor is about 3.925, while at the edge there are more soft Cube texture and Goss texture which lead to the Taylor factor of 2.257.