垂直向上凝固Al-Cu铸件中微观孔洞形成的数值模拟

MODELING OF MICROPOROSITY FORMATION IN UPWARD VERTICALLY SOLIDIFIED Al-Cu CASTING

基于枝晶间流动的Darcy定律,考虑了凝固过程氢的宏观扩散与传输,建立了耦合氢宏观偏析的铝合金铸件微观孔洞形成的数学模型,并进行了实时熔炼的Al-4.5%Cu(质量分数)铸件的垂直向上凝固实验,铸件微观组织和孔洞的分析结果表明:铸件沿着高度方向包括了柱状晶、柱状晶向等轴晶转变(CET)和等轴晶3个区域;柱状晶区域内微观孔洞体积分数存在递减的分布规律,且邻近底部冷模的试样微观孔洞体积分数最大.采用所建立的模型对实验铸件微观孔洞形成进行了模拟,模拟结果与实验结果吻合较好;而当忽略氢宏观偏析时,模拟结果与实验结果存在较大误差.研究表明,氢的宏观偏析对微观孔洞的形核与分布具有重要影响.

Based on Darcy＇s law for interdendritic flow,a mathematical model for hydrogen porosity formation in Al castings was developed,in which the hydrogen macrosegregation including hydrogen diffusion and transport in macroscopic scale was considered.An upward unidirectional solidification experiment of Al-4.5%Cu（mass fraction）casting in situ melted was carried out.The microstructure analysis showed that the casting consists of columnar crystal,CET（columnar-toequiaxed transition）and equiaxed crystal regions along upward direction.It was found that there exists a gradually decreasing distribution of porosity in the columnar region,and the porosity in casting part near the bottom chill is higher.The proposed model was applied to the experimental casting,and the simulation results are in well agreement with the experiment results only when the hydrogen macrosegregation was considered.The hydrogen macrosegregation has significant influence on the predicted porosities,especially their nucleation and distribution.