Papers by Author: Jian Xin Zhou

Abstract: A two dimensional model was employed to simulate the densification process of thick-walled hollow cylinder preform of C/C composite. Using the hydrogen inhibition model, carbon deposition processes under different partial pressures were compared and the efficiency of densification was analyzed. Then the density distribution of some definite time was analyzed to explain the mechanism of hydrogen inhibition. The results showed that with the adjunction of hydrogen, density distribution in preform was improved while the deposition process took a longer time.

Abstract: Thermal stress simulation is an important part in the numerical simulation of casting process. It provides engineers with insights into the evolution of displacement, strain and stress of castings in the solidification process. With thermal stress simulation, some defects of casting, i.e. hot tearing, cold cracking and large deformation can be predicted and the engineers are instructed to optimize and improve the casting process. Based on the finite difference method (FDM), this paper presents an integrated numerical method to simulate the thermal stress and deformation of casting in the solidification process. Practical examples show that the method is capable to predict stress distribution and deformation as well as the defects in the experiment.

Abstract: Computational simulation relies on valid and accurate representations of the geometry of objects being simulated. However, the Stereo Lithography (STL), one of the most commonly used formats for computational simulation, may be invalid with several kinds of defects when the geometry is too geometrically and topologically complex. In many cases, castings have the most complicated geometries in the world, and its geometries in STL format always contain some defects. The implicit surface is a much more robust geometry representation than STL. With the representation in the implicit surface, most of the defects in STL can be cleared and ignored. This paper presents a method to fast construct the implicit surface for arbitrary complex casting geometries in the format of STL.

Abstract: In order to predict the mechanical property during the actual production of gray cast iron, to qualify the casting for the requirement of mechanical property and to control the cost, in this paper a series of anatomy and mechanical property experiments are conducted based on the automobile engine cylinder head processed through the actual production. The actual Brinell Hardness (HB) and the pouring parameters of the cast iron are acquired through the experiment. According to the pouring parameters, the cooling curve is obtained through the Inte CAST software simulation and the cooling rate is also calculated. Based on the existing property prediction model, the Brinell Hardness property prediction model of the engine cylinder head is deduced. The prediction results conform well to the actual data results.

Abstract: A reduced model was used to simulate the CVI process from methane. The scale of the preform was 120mm in diameter and 90mm in height. A random pore model was used to describe the evolution of pores in preform. Parameters such as temperatures and pressures were studied to research the deposition process. Effects of these two parameters are similar but have different impact mechanisms according to the analysis of results. Temperature impacts the reaction rate to improve the efficiency of deposition, while pressure impacts the concentrations of all hydrocarbon and then improves the total amount of carbon which also results in an increase of density.

Abstract: According to the calculation model of the keyhole porfile[1], the 3D point cloud data is calculated. Then, the paper establishes a physical model of the keyhole in the laser welding by using the 3D reverse technology. The adaptive mesh generation of the model is processed by the independent development software. Finally this paper establishes a mathematical model of the temperature field in the laser welding, which considers the factor of the welding velocity. A more accurate temperature field is obtained by using the laser welding solver which is a secondary development of OpenFOAM, adding the Guass surface source and the keyhole volume source, and setting the boundary condition of convection and radiation. The result provides more accurate bases for the welding stress field.

Abstract: A simplified mathematical model for describing the solidification processing of fluid flow, heat and solute transfer in binary alloys is developed. The conservation equations are numerically solved on a staggered grid by using the control volume-based finite difference method. As the interdendritic flow in the mushy zone is governed by Darcy’s law, the Modified Projection Method is presented for solving the momentum and continuity equations. The solidification processing of Fe-C alloy in rectangular domains is simulated and discussed. The numerical result shows that, by using the Modified Projection Method technique, the presented model is able to predict the macrosegregation formation effectively.

Abstract: Many defects relative to stress occur during the complicate casting process, such as hot tearing, residual stress concentration and distortion. Modeling of casting thermal stress during casting solidification process is of great significance to predict and analyze casting stress defects. Involving too many complex influencing factors, the stress simulation is very difficult and retains a hot spot of macro simulation in foundry engineering. Currently most researchers adopt integrated FDM/FEM method, i.e. using finite difference method (FDM) to calculate solidification and heat transferring, while finite element method (FEM) to simulate stress. Some universal commercial FEA packages are usually adopted. This study has tried two kinds of approaches to simulate casting thermal stress. One is based on ANSYS, a well-known powerful FEA analysis software. Another is to develop an independent own copyrighted casting stress simulation system based on FDM. The routes of these two methods were given respectively. To calibrate the simulation system, a stress frame sample and a real practical casting were simulated and pouring experiment was also carried on. The results of simulation were in agreement with the experiment results and practical cases. It indicates that these two approaches can all meet demands. When adopting FDM method, thermal analysis and stress analysis can use the same FD model, which can avoid the nodes matching between different models and reduce the errors of thermal load transferring. It makes the simulation of fluid-flow field, temperature field and stress field unify into one model. This system takes full advantages of mature FDM technology and can be used to simulate the forming of residual stress and predict the occurrence of hot tearing.

Abstract: The nonferrous alloy castings will be much demanded with the development of aerospace, light-weight weapons and automotive industry. Casting CAD/CAE technology has played an increasingly important role in foundry. It can help technician to design casting process and simulate heat transferring and molten metal flowing before actual production. Firstly, the main contents and basic principles, mathematical models of casting CAD and CAE were introduced. Secondly, the casting CAD/CAE technology route was interpreted, and then several CAD/CAE applications of casting process design and simulating were carried out on nonferrous alloys, such as aluminum, magnesium and copper. The results indicate that CAD/CAE technology can be applied to casting process design and predict casting defects which usually occur during the casting mold filling and solidification process, it can provide references to optimize casting process, so as to improve casting quality, reduce the rejection rate and shorten the development cycle of new products.

Abstract: Numerical simulation provides a way to improve our understandings of the heat transfer and fluid flow behaviors of the weld pool during laser keyhole welding. However, current numerical studies are only limited to serial simulations which running on a single CPU. In this study, a parallel numerical study of the heat transfer and fluid flow of the weld pool is presented. A mathematical model considering the effect of Marangoni force, buoyancy force, friction force of the mushy zone region and the effect of keyhole is presented. A combined keyhole volume and surface heat source model is also developed. The coupled transient heat transfer and Navier-stokes equations are solved with a high order accuracy parallel projection method. The simulation code is parallelized with the OpenMP language. It is shown that 200% speedup can be achieved on a shared memory quad-core CPU using the presented parallel simulation system. The simulation results agree well with the in-situ high speed CCD video imaging experiments and the literature results.