Numerical Simulation Of Thermal Stress In Casting Process Based On Finite Difference Method

Thermal stress in casting process is mainly induced by the hindering of castingalloy contraction. The hindering comes from casting itself and mold (core) becausethe uneven cooling rate in different parts of the casting. It is one of the mainreasons leading to cracks and deformation. Recently, numerical simulation ofthermal stress in casting process has become a hot spot of macro simulation alongwith the maturity of temperature filed numerical simulation. At present, the finitedifference method (FDM) is major in numerical simulation of temperature field,flow field in casting process, while the thermal stress numerical analysis has to usefinite element method (FEM). It will greatly reduce the efficiency and accuracy ofcalculation by using two different software packages (FDM and FEM) to do filling,solidification and thermal stress analysis respectively，and employing two differentmesh model. Therefore, this paper is devoted to the development of integratednumerical simulation software which contains temperature filed and thermal stressfiled modules and is based on single finite difference method mesh.A professional, practical thermal stress field simulation software is developedindependently based on finite difference method and Windows platform. It adoptsone-way coupled method to treat the relationship between heat transfer and thermalstress in casting, and makes full use of the prominent advantages of finite differencemethod in mesh generation and temperature field simulation. The software makesthe pre-processing, temperature field and stress filed unify into one finite differencemethod. It can simulate the residual stress and residual deformation of casting, andpredict location and tendency of cracks in casting.Finite difference mesh pre-processing module was developed. The moduleadopts the method of first generation then fault tolerance to cope with faulty STLfile. It can alter mesh spacing automatically according to the STL solid model,which can simplify the program, increase the mesh model accuracy and improvethe ability of application. In theory, the mesh number is not limited. The dynamicdata structures are used in the module which makes the module consume as little aspossible memory. This module can provide high-quality orthogonal hexahedralfinite difference mesh for the simulation of temperature field and thermal stressfield simulation modules. It achieves the seamless connection between the CADsystem and the thermal stress simulation software. The mathematical models of a one-way coupled thermal-mechanicalsimulation are established, and the finite difference method solutions of temperatureand thermal stress are studied, the temperature field and stress field modules aredeveloped. The temperature field simulation employs fully implicit discretizationscheme, alter the time step automatically, and can provide reasonable increments oftemperature load for the accurate and convergent solution of stress simulation. Thethermal elasto-plastic model is discretized by displacement method using thestaggered grid and non-uniform mesh model, according to resistance against build-up of stress. Line-by-line Gauss-Seidel iterative solution method is present to solvethe discretized equations. The “storing them once, using them multiple times”technique and reducing the calculation domain method both improve the efficiencyof simulation of thermal stress significantly.A typical stress frame castings was selected to verify and validate the thermalstress simulation software. The distribution and variation of stress and deformationin the stress frame during solidification process were studied fully. Subsequentlypouring experiments of the stress frame specimen have been carried out, coolingcurves, dynamic deformation curves and residual stress of main points are measured,which offer the reliable data of software verification. The simulated results are ingood accordance with those obtained from experiments, and the accuracy andreliability of the thermal stress numerical simulation software were validated.The template and the waveguide castings were simulated by the thermal stresssimulation software in order to evaluate engineering ability of the software. Thesimulation results are in agreement with the practical results, which demonstratethat the thermal stress numerical simulation software is a useful tool to predictstress and deformation of casting, and can provide scientific guidance on stressconcentration and dimensional accuracy of the casting.