Papers by Author: Qing Yan Xu

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Authors: Qing Yan Xu, Rui Chen, Yu Feng Shi, Bai Cheng Liu
Abstract: In the present investigation, a physically based numerical model was developed to predict the yield stress of Al-7Si-Mg cast alloy during processing. It covered the integrated unit step models of the physical metallurgy of solidification, solid-state of homogenization, and structural hardening of precipitation. The as-cast microstructure of Al-7Si-Mg alloy was calculated based on the cellular automaton method and the evolution of the precipitated phase during aging process was achieved by a precipitation kinetic model involved nucleation, growth and coarsening. The yield stress prediction was achieved by a strengthening model including the effects of as-cast microstructure, solution strengthening and precipitate hardening. The predictions of this model were verified by comparing with experimental measured yield stress which shows that this model is successfully applied to predict the yield stress evolution of Al-7Si-Mg cast alloy.
Authors: Qing Yan Xu, Bin Li, Bai Cheng Liu
Abstract: Aluminum casting is widely used in aeronautical, automobile and other industries nowadays. The Cellular Automaton (CA) method was modified to simulate the microstructure evolution of Al alloy casting. Simulated program code was developed and applied into Al casting production. A nucleation model was investigated based upon the experimental data. The solute diffusion in the liquid and solid phases was also considered in developing a grain growth model. With the developed models, not only grain structure but also dendritic microstructure can be predicted during the solidification process. The microstructure simulation of the Al alloy turbine wheel was studied in detail.
Authors: Z.Y. Liu, Qing Yan Xu, Bai Cheng Liu
Abstract: Physical and mathematical models of microstructure evolution during the solidification process of die casting AZ91D alloy were investigated in this paper. Coupled with solute concentration, a modified three-dimensional cellular automaton (CA) model was proposed. Considering the solute enrichment and the formation of eutectic microstructure, these models can reproduce the whole microstructure evolution process of Mg alloy, from the formation of primary phase to the eutectic transformation. The microstructure of the AZ91D alloy cylinder head cover die casting was simulated with the proposed models. The simulated results are in agreement with the experimental.
Authors: Zhen Nan Fu, Qing Yan Xu, Shou Mei Xiong
Abstract: A cellular automaton (CA)-based model for two-dimensional simulation of the dendritic morphology of magnesium alloys was developed. The model considers solving the solute and heat conservation equations in the modeling domain, including calculation of the solid fraction, the tip velocity, and the solute diffusion process, all of which have significant influence on the dendrite evolution. The microstructure of a step-shape die cast part of AZ91D magnesium alloys was investigated utilizing SEM-EBSD analysis. The microstructure simulation results were compared with the experimental results and they were in good agreement on grain size.
Authors: Qing Yan Xu, Bai Cheng Liu, Zuo Jian Liang
Abstract: Dendritic is the most observed microstructure in metallic materials. Traditional Cellular Automaton model can only predict the grain structure and grain size. It is necessary for us to modify the original model to reflect the real shape of dendrite. In the paper, a mathematical model was established to describe the evolution of the dendritic shape, in which the influence of microsegregation and curvature on undercooling was taking into account. In addition, the growth model was proposed based on the minimum free energy principle. Modeling results indicated the proposed models can predict not only grain structure, but also dendritic morphology. Free growth of equiaxed grains and the competitive growth of columnar grains were simulated. The modeling results were also validated with experiments.
Authors: Dong Pan, Qing Yan Xu, Bai Cheng Liu
Abstract: Directional solidified turbine blades of Ni-based superalloy are widely used as key parts of the gas turbine engines. The mechanical properties of the blade are greatly influenced by the final microstructure. In this paper, a mathematic model was proposed for the three dimensional simulation of microstructure evolution in directional solidification. Based on the thermo model of heat transfer, the grain growth within the blade and the microstructure morphology were simulated via a Cellular Automaton method. Validation experiments were carried out. The simulated cooling curves and microstructures corresponded well with the experimental results.
Authors: Qing Yan Xu, Bai Cheng Liu, Zuo Jian Liang, Jia Rong Li, Shi Zhong Liu, Ha Llong Yuan
Abstract: Single crystal superalloy turbine blade are widely used in aero-engineering. However, there are often grain defects occurring during the fabrication of blade by casting. It is important to study the formation of microstructure related defects in turbine blades. Single crystal blade sample castings of a nickel-base superalloy were produced at different withdrawal rates by the directional solidification process and investment casting. There was a difference between the microstructure morphology at the top part of the turbine blade sample castings and the one at the bottom. Higher withdrawal rates led to more differences in the microstructure and a higher probability of crystallographic defect formation such as high angle boundaries at locations with an abrupt change of the transversal section area. To further investigate the formation of grain defects, a numerical simulation technique was used to predict the crystallographic defects occurring during directional solidification. The simulation results agreed with the experimental ones.
Authors: Bin Li, Qing Yan Xu, Bai Cheng Liu
Abstract: A modified Cellular Automaton model was presented to simulate the evolution of dendritic microstructure in low pressure die casting of Al-Si Alloy, which accounted for the heterogeneous nucleation, the solute redistribution both in liquid and solid, the interface curvature and the growth anisotropy during solidification. The free growth of single equiaxed dendrite and the evolution of multi dendrites with various crystallographic orientations were predicted. The variation of the dendrite tip velocity and local solute concentration at the solid/liquid interface were analyzed. The grain morphology of aircraft turbine wheel casting at different specimen points were predicted and compared with experimental results.
Authors: Heng Shao, Yan Li, Peng Zhao, Hai Nan, Qing Yan Xu
Abstract: Centrifugal pouring is often used in investment casting of large thin-wall Ti castings to promote filling. Shrinkage defects often appear in a Ti casting produced by centrifugal casting. Numerical simulation indicate that shrinkage is caused by these reasons: improper pouring system and thin-wall structure limited feeding of liquid metal from pouring system, and centrifugal force enlarged the shrinkage defects by strengthen feeding of liquid within the casting. Thus centrifugal casting is replaced by gravity casting and a new pouring system is adopted. Obvious shrinkage defects disappear in the new casting process.
Authors: Dong Pan, Qing Yan Xu, Bai Cheng Liu
Abstract: Ni3Al based superalloy has recently been used for the single crystal gas turbine blade. The grain selection behavior in grain selector directly determines the casting’s final microstructure and properties. A mathematical model based on the modified CA-FD method was developed for the three-dimensional simulation of directional solidification process of Ni3Al based single crystal superalloy castings. The microstructure evolution was simulated with the modified Cellular Automaton method. The grain selection process in the grain selector and final microstructure of casting were simulated. The results indicate that the stray grain is easy to nucleate at the middle part of the pigtail because of the discontinuous mushy zones formation. This agrees with previous published experimental results. Based on simulated results, a newly designed grain selector with optimized geometry was proposed to avoid stray grains.
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