铸造充型过程中液固转变影响流动行为的数值计算

Numerical Simulation of Liquid-Solid Conversion Affecting Flow Behavior During Casting Filling Process

为准确预测浇不足及冷隔,在已有的处理液固转变方法基础上,提出基于固相率变化的糊状区流动行为计算模型,该模型可以有效地处理液固转变过程中糊状区不同阶段的流动行为,即高固相率糊状区采用临界固相率方法,低固相率糊状区采用变黏度方法,中等固相率糊状区采用多孔介质拖拽模型。模拟了S型铸型水模拟实验,模拟结果与实验结果吻合很好,验证了不考虑液固转变时所采用模型的准确性。针对简单形状的底注式铸造工艺,对比分析了处理液固转变过程中采用不同控制参数的计算效果,证明了糊状区流动行为计算模型的合理性。

Misrun and cold shut are common defects in casting productions, which could make sur- face accuracy of castings poorer, even leading to cracking and casting scraps in them. The formation pro- cess of misrun and cold shut is hard to be observed directly only by experiment measures, since casting filling process is in a state of high temperature flow inside mold. The key to predict the defects accurately is the way to handle the effect of liquid-solid conversion on flow behavior. On the basis of existing meth- ods for treating liquid-solid conversion, a calculation model of mushy region flow behavior through mea- surement of solid-fraction is developed, which can effectively investigate the flow behavior of mushy re- gion in different stages. Generally, the critical solid-fraction method is adopted for mushy region with high solid-fraction, in consideration of that only the speed of high solid-fraction region is supposed to be zero during casting filling process. The variable viscosity method is applied for mushy region with low solid- fraction, due to casting filling process being unlikely to form toothpaste-like flow. However, the porous me- dium drag-based model is used for mushy region with middle solid-fraction, because only the middle solid- fraction region can be equivalent to porous medium. Combining the above three methods, a flow-field cal-culation program considering the effect of liquid-solid conversion on flow behavior during casting filling process is developed, in which finite volume method （FVM） is included for discretization equations; the pressure implicit with splitting of operator （PISO） algorithm is added for coupling pressure and velocity; the volume of fluid （VOF） algorithm is also combined for interface tracking. An numerical simulation of water-filled S-shaped channel is performed in the case of taking no account of liquid-solid conversion, and the simulated results coincide better with the experimental results, which certifies for its accuracy as an adopted model. Since the bottom filling casting craft is commonly used in single-shape casting, a com- parison between the calculated results obtained using other single models and those using this model at different control parameters, is needed. The better agreement between them indicates that this new model is appropriate for calculating the flow behavior in mushy region.