Al-7%Si合金显微气孔形成的模拟研究

MODELING OF MICROPOROSITY FORMATION IN AN Al-7%Si ALLOY

利用元胞自动机(cellullar automaton,CA)方法,建立了模拟枝晶和显微气孔的二维数值模型.该模型基于凝固过程中显微气孔的形成机理,考虑了枝晶和气孔的形核与生长、溶质和H的再分配与扩散、界面曲率效应等因素.模拟结果可描述气孔与枝晶组织的耦合竞争生长、以及溶质和H的微观偏析.应用该模型对Al-7%Si(质量分数)合金在凝固过程中显微气孔的形成进行了模拟研究,分析了初始H含量和冷却速率对显微气孔形成的影响,将模拟结果与实验结果进行了比较.结果表明:随初始H含量增加,气孔体积分数增大,气孔形核和开始生长所需要的时间缩短.随冷却速率降低,气孔体积分数和最大半径均增大,气孔在较高的温度下开始形核和生长.气孔之间以及气孔和枝晶之间存在竞争生长.

The performance of castings is primarily dependent on the solidification microstruc- tures and defects. Gas porosity is one of the major casting defects existing in the castings of aluminium and magnesium alloys. In this work, a two dimensional （2D） cellular automaton （CA） model is pro- posed to simulate dendrite and microporosity formation during solidification of alloys. The model involves three phases of liquid, gas and solid. The effect of liquid-solid phase transformation on the nucleation and growth of porosity, the redistribution and diffusion of solute and hydrogen, and the effects of surface tension and environmental pressure are taken into account. The growth of both den- drite and porosity is simulated using a CA approach. The diffusion of solute and hydrogen is calculated using the finite difference method （FDM）. The simulations can reveal the coupling and competitive growth of dendrites and microporosities, as well as the microsegregation of solute and hydrogen. The model is applied to simulate the microporosity formation during solidification of an A1-7%Si （mass fraction） alloy. The effects of initial hydrogen concentration and cooling rate on microporosity forma- tion are investigated. The results show that the simulated pressure difference between the inside and outside of a porosity as a function of the reciprocal of porosity radius obeys the Laplace law. With the increase of initial hydrogen concentration, porosity volume fraction increases, and the incubation time of microporosity nucleation and growth decreases, while the porosity density does not increase obvi- ously. With cooling rate decreasing, porosity volume fraction and maximum porosity radius increase, as well as porosity nucleates and starts to grow at higher temperatures. However, the porosity density shows a decreasing trend with the decrease of cooling rate. The competitive growth between different microporosity and dendrites is observed. The porosity nuclei with larger size are able to grow preferentially, while the growth of the small porosity nuclei is inhibited. Because of the effect of gas-liquid surface tension, porosity grows spherically when it is enveloped by liquid. After touching with dendrites, the growth space of porosity is restricted by the complex dendrite network, and thus becomes irregular shape. On the other hand, the growth of dendrite might also be influenced by the nearby porosity. With cooling rate decreasing, the competitive growth between porosities and dendrites becomes more evident, leading to non-uniform porosity size, and more irregular morphology of the porosities with larger size. The simulation results are compared reasonably well with the experimental data.