A two-dimensional phase field (PF) model was developed to simulate the dendritic solidification in magnesium alloy with hcp crystal structure.By applying a parallel-adaptive mesh refinement (Para-AMR) algorithm,the computational efficiency of the numerical model was greatly improved.Based on the PF model,a series of simulation cases were conducted and the results showed that the anisotropy coefficient and coupling coefficient had a great influence on the dendritic morphology of magnesium alloy.The dendritic growth kinetics was determined by the undercooling and equilibrium solute partition coefficient.A significant finding is acquired that with a large undercooling,the maximum solute concentration is located on both sides of the dendrite tip in the liquid,whereas the maximum solute concentration gradient is located right ahead of the dendrite tip in the liquid.The dendrite tip growth velocity decreases with the increase of the equilibrium solute partition coefficient,while the variation trend of the dendrite tip radius is the opposite.Quantitative analysis was carried out relating to the dendritic morphology and growth kinetics,and the simulated results are consistent with the theoretical models proposed in the previously published works. 相似文献
The morphology and content of the divorced eutectic in the microstructure of high pressure die casting(HPDC) magnesium alloy have a great influence on the final performance of castings. Based on the previous work concerning simulation of the nucleation and dendritic growth of primary α-Mg during the solidification of magnesium alloy under HPDC process, an extension was made to the formerly established CA(Cellular Automaton) model with the purpose of modeling the nucleation and growth of Mg-Al eutectic. With a temperature field and solute field obtained during simulation of the primary α-Mg dendrites as the initial condition of the modified CA model, modeling of the Mg-Al eutectic with a divorced morphology was achieved. Moreover, the simulated results were in accordance with the experimental ones regarding the distribution and content of the divorced eutectic. Taking a "cover-plate" die casting with AM60 magnesium alloy as an example, the rapid solidification with a high cooling rate at the surface layer of the casting led to a fine and uniform grain size of primary α-Mg, while the divorced eutectic at the grain boundary revealed a more dispersed and granular morphology. Islands of divorced eutectic were observed at the central region of the casting, due to the existence of ESCs(Externally Solidified Crystals) which contributed to a coarse and non-uniform grain size of primary α-Mg. The volume percentage of the eutectic β-Mg_(17)Al_(12) phase is about 2%-6% in the die casting as a whole. The numerical model established in this study is of great significance to the study of the divorced eutectic in the microstructure of die cast magnesium alloy. 相似文献
Directionally solidified samples of Mg-32.3 wt pct Al eutectic alloy were produced under an argon atmosphere in a vacuum Bridgman-type
furnace to study the eutectic growth with different growth velocities. Typical features such as steady-state lamellar eutectic
growth, lamellar branching at the quenching interface, and the formation of colony structures due to the impurity of the Mg-Al
binary alloy were observed using a JEOL 6301F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The lamellar spacing
of the two eutectic phases was measured on the transverse sections of the samples. It was found that the relationship between
the measured lamellar spacing and growth velocity agreed well with the prediction of the Jackson-Hunt model. Subsequent studies
of Mg-Al eutectic growth were conducted using a numerical model based on the cellular automaton (CA) method. Taking account
of the solute diffusion, constitutional undercooling, and curvature undercooling, modeling of steady-state lamellar eutectic
growth was achieved. A systematic investigation of the eutectic growth morphology and lamellar spacing of the Mg-Al eutectic
was carried out under directional solidification with different undercoolings, initial lamellar spacings, temperature gradients,
and growth velocities. The results showed that under the interaction between solute diffusion and surface energy, the adjustment
of eutectic lamellar spacing was accomplished by nucleation, lamellar branching, lamellar termination, and overgrowth. The
simulated results were consistent with both the experimental results and the Jackson-Hunt eutectic theory. 相似文献
During the cold-chamber high pressure die casting(HPDC) process, samples were produced to investigate the microstructure characteristics of AM60B magnesium alloy. Special attention was paid to the effects of process parameters on the morphology and distribution of externally solidified crystals(ESCs) in the microstructure of magnesium alloy die castings, such as slow shot phase plunger velocity, delay time of pouring and fast shot phase plunger velocity. On the basis of metallographic observation and quantitative statistics, it is concluded that a lower slow shot phase plunger velocity and a longer delay time of pouring both lead to an increment of the size and percentage of the ESCs, due to the fact that a longer holding time of the melt in the shot sleeve will cause a more severe loss of the superheat. The impingement of the melt flow on the ESCs is more intensive with a higher fast shot phase plunger velocity, in such case the ESCs reveal a more granular and roundish morphology and are dispersed throughout the cross section of the castings. Based on analysis of the filling and solidification processes of the melt during the HPDC process, reasonable explanations were proposed in terms of the nucleation, growth, remelting and fragmentation of the ESCs to interpret the effects of process parameters on the morphology and distribution of the ESCs in the microstructure of magnesium alloy die castings. 相似文献
Fluid flow has a significant impact on the microstructure evolution of alloys during solidification. Based on the previous work relating simulation of the dendritic growth of magnesium alloys with hcp (hexagonal close-packed) structure, an extension was made to the formerly established CA (cellular automaton) model with the purpose of studying the effect of fluid flow on the dendritic growth of magnesium alloys. The modified projection method was used to solve the transport equations of flow field. By coupling the flow field with the solute field, simulation results of equiaxed and columnar dendritic growth of magnesium alloys with fluid flow were achieved. The simulated results were quantitatively compared with those without fluid flow. Moreover, a comparison was also made between the present work and previous works conducted by others. It can be concluded that a deep understanding of the dendritic growth of magnesium alloys with fluid flow can be obtained by applying the present numerical model. 相似文献
The characteristics of defect bands in the microstructure of high pressure die casting (HPDC) AE44 magnesium alloy were investigated. Special attention was paid to the effects of process parameters during the HPDC process and casting structure on the distribution of defect bands. Results show that the defect bands are solute segregation bands with the enrichment of Al, Ce and La elements, which are basically in the form of Al11RE3 phase. There is no obvious aggregation of porosities in the defect bands. The width of the inner defect band is 4–8 times larger than that of the outer one. The variation trends of the distribution of the inner and outer defect bands are not consistent under different process parameters and at different locations of castings. This is due to the discrepancy between the formation mechanisms of double defect bands. The filling and solidification behavior of the melt near the chilling layer is very complicated, which finally leads to a fluctuation of the width and location of the outer defect band. By affecting the content and aggregation degree of externally solidified crystals (ESCs) in the cross section of die castings, the process parameters and casting structure have a great influence on the distribution of the inner defect band.