In order to avoid coarse primary-precipitated ε phase which prolong α+ε→β phase transition time in FeSi2 material prepared by conventional casting, a cyclical superheating process of melt for preparing FeSi2 with complete α+ε eutectic structure was proposed. The effects of melt superheating conditions on the microstructure of FeSi2 were investigated. The results showed that, with the increase of the superheating temperature, the superheating time, the recycling times and the cooling velocity, the size and quantity of the ε phase reduced. Meanwhile, the number of the fine eutectic structure increased. FeSi2 sample with uniform and complete α+ε eutectic structure was successfully prepared in the conditions of melt superheating temperature at 1550°C, superheating time for 10mins, recycling 3 times,melt cooling rate of 30°C/s.
Effect of isothermal annealing on the microstructure evolution of the cold-rolled Mg-9Al-1Zn alloy strip was investigated. It is found that the competition between the precipitation of β-Mg17Al12 phase and recrystallization of α-Mg matrix occur under the conditions of various annealing temperature and time. At a low annealing temperature (523K), the β-Mg17Al12 particles precipitate preferentially at locally high deformation area and grow into the lamellar-cluster with an increase in the annealing time, retarding the recrystallization of α-Mg matrix. With raising the annealing temperature (573~623K), both the precipitation of β-Mg17Al12 particles and recrystallization of α-Mg matrix take place. Both recrystallization and grain growth are prone to proceed without precipitation of β-Mg17Al12 particles when the annealing temperature is 673K. A mechanism for the competitive behavior between the precipitation of β-Mg17Al12 phase and recrystallization of α-Mg matrix at various annealing conditions is discussed.
Surface heat transfer coefficient is a key parameter for accurately predicting the extrusion product temperature near the die exit and then achieving isothermal extrusion by speed controlling. Based on the heat transfer characteristics of extrusion product during cooling process, a dynamic loading method of heat transfer boundary conditions was proposed. The surface heat transfer coefficient model of 7075 Al-alloy extrusion product was established using the dynamic loading method and inverse calculation comprehensively. The model indicated the relationship among surface heat transfer coefficient h, surface temperature T and initial temperature T0 as h=1.16T-0.97T0. Its accuracy is high enough for calculating the surface temperature of 7075 Al-alloy extrusion product. According to the model and the experimental data, the relationship between the product and the measured temperatures can be established. It provides an effective way to solve the problem that the extrusion product temperature near the die exit cannot be directly measured.
In this paper, the compression stress-strain curves of 7050 Al-alloy under various deformation conditions were obtained, and the recrystallization structures were analyzed. The main parameters of dynamic recrystallization model were determined. The rationality of the model parameters were verified by hot compression and extrusion. The results show that the accuracy of the model is high enough for predicting the recrystallization of the 7050 Al-alloy during hot deformation. Comparison of the experiment and calculated results in hot compression shows that the maximum relative error of the recrystallization fraction is 11.4%. Comparison of the experiment and calculated results in hot extrusion shows that the maximum relative error of the recrystallization fraction is 13.0%, and that of the recrystallization grain size is 14.9%.
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