| The high specific strength, high specific stiffness and favorable processing performance Al-Zn-Mg-Cu series alloys, mainly rolled(medium) thick plates, extruded sections or forged pieces, have been widely applied to structural parts in aerospace and transport industries. During multistage hot working, such as rolling and forging, the microstructure and properties of final Al alloy products are greatly influenced by parameters and dynamic microstructural evolution during hot working. Moreover, the static microstructural variations during inter passes holding, cooling or reheating will also determine the microstructure and properties of final products. The studies on static softening of Al alloys lag behind that in steels. Particularly, the theories and physically-based modeling works in Al alloys are destitute.In this works, 7150 Al alloy was the research material. By utilizing advanced Gleeble thermomechanical simulator, a series of single, double and multi stages physical simulated experiments were carried out. The kinetics and mechanisms of static softening during thermomechanical processing were also explored by combining advanced microstructure characterization techniques, i. e. SEM, EBSD and TEM. Further, the interactions among precipitation, recovery and recrystallization were also discussed. Based on experimental results, a quantitative physically-based modeling of microstructure and mechanic during static softening was developed. Because of significant effects of precipitation kinetics on static softening in 7150 Al alloy under various temperatures, the precipitation kinetics over a wide range of isothermal and non-isothermal temperatures were studied by in situ electrical resistivity characterization technique.1) On the basis of single and double stages isothermal hot compression experiments and softening fraction calculations on 7150 Al alloy, the dynamic and static softening kinetics were studied. The results showed that flow curves exhibited clear peaks and some work softening during hot deformation. The flow softening fraction increased with decreasing strain rate and increasing temperature. The static softening during double stages hot deformation increased with increasing strain rate, deformation temperature and delay time. The static softening curves appeared as a typical sigmoidal shape of the type at temperature of 400 oC, while plateaus were observed when temperature decreased to 300 oC. An empirical static softening model based on JMAK model was derived based on experimental static softening kinetics.2) During double stages isothermal hot deformation of 7150 Al alloy, static softening during isothermal holding stages mainly occurred via static recovery in the absence of recrystallization. Static recovery could result in high softening fractions. The appearance of static softening at 400 oC exceeding 100% was observed. Two classes of particles were characterized at 300 oC: Coarser constituent particles were little affected by isothermal holding and had minimal effect on static softening; heterogeneously distributed finer precipitates were formed; their radius increased with increasing isothermal holding time. The heterogeneous nucleation of the fine precipitates on dislocations and their subsequent growth and coarsening was associated with the static softening plateau at 300 oC.3) Integrated physically-based modeling was developed, which related to recovery, precipitate coarsening and high temperature strengthening. And the static softening mechanism was further explained by combining quantitative microstructure results. The results showed that the static softening at 400 oC was only attributed to static recovery with little precipitate affecting. While at 300 oC, the initial static softening was mostly attributed to static recovery contributions. Decreased static recovery contributions were gradually observed during later softening. At the same time, the contributions from precipitates increased. Therefore, the static softening plateau was closely related to the coarsening of precipitates.4) The precipitation kinetics of 7150 Al alloy under isothermal and non-isothermal conditions were studied scientifically by utilizing in situ electrical resistivity characterization technique. The results showed that the precipitation kinetics could be reliably inferred from the evolution of electrical resistivity over a wide range of thermal conditions. During non-isothermal treatments, thermal path dependent precipitation kinetics was found. During continuous cooling processes, fast increase of precipitate fraction, sparse particle with coarse size and heterogeneous distribution were observed. During continuous heating of WQ alloy, very fine precipitates with high density and homogeneous distribution were presented at low temperature because of clear quenching effects and reversed temperature path comparing to continuous cooling. The size of precipitates increased with increasing temperature while the density decreased gradually. Above about 450 oC, only spare Al3 Zr phase was residual due to rapid dissolution of precipitates. During isothermal treatments, typical nucleation, growth and coarsening of precipitates were observed in the low temperature range. The incubation time for nucleation decreased with increasing temperature. Then the idea Ostwald coarsening process started. The quantitative precipitate variations could be obtained according to electrical resistivity results during both non-isothermal and isothermal treatments.5) Based on isothermal and non-isothermal multistage physical simulated experiments and analysis of flow stresses, the effects heating routes on the hot deformation behaviors of 7150 Al alloy during single and double stages hot compression was explored. The constitutive characteristics and static softening kinetics during multistage hot deformation were also studied. The resulted showed that the heating routes affected the dynamic and static softening at 300 oC remarkably. The values of n and ln A of hyperbolic-sine law were evidently influenced by processing parameters, i. e. strain, strain rate, temperature and interval time. Limit influences of various processing parameters on Q were observed. Non-isothermal multistage flow stresses were found to be easier than normal continuous single pass method to manipulate for constitutive equations calculation with better linear fitting results. Clear static softening was found during isothermal multistage hot compression when compared with continuous deformation. Increased fraction softening was observed with increasing accumulative strain, temperature, strain rate and interval time. High fraction softening with short interval and fraction softening exceeding 100% were also found and discussed. The microstructure presented part recrystallization at high temperature and typical recovery at low temperature. Precipitation at low temperature and its dissolution at high temperature made a great influence flow behavior and constitutive characteristics during both isothermal and non-isothermal multistage hot deformation. |
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