强制流动对Mg–9%Al合金定向凝固组织演化的模拟研究

Simulation of forced flow on the evolution of directional solidification microstructure of Mg-9%Al alloy

基于欧拉多相流技术与元胞自动机方法构建了镁合金凝固组织预测模型,研究了无流动、x方向流动和y方向流动三种条件下Mg–9%Al合金定向凝固过程中的成分和组织演化过程.研究结果表明,无流动作用时镁合金枝晶呈现互为60°夹角生长,并在凝固后期出现与一次枝晶呈60°夹角二次枝晶形貌,凝固组织具有密排六方(HCP)晶体结构特征,验证了该模型的可靠性.由于x方向的流动作用下,与无流动结果相比较,迎流方向枝晶生长较快并出现发达的二次枝晶形貌,研究表明由于枝晶前端排出的溶质受流动影响被运输到枝晶后端区域,从而使前端区域具有更大的过冷度,提升了凝固驱动力.由于y方向流动的存在,枝晶呈现不对称生长,其中部分枝晶生长方向偏转约3°,研究表明迎流枝晶前沿的溶质被运输并富集在其他枝晶生长区域,最终使其发生了生长偏转;进一步分析枝晶生长前沿的流场和成分分布信息发现,枝晶生长过程中流动u1∇c1引起的值在固液界面处的不对称分布是导致枝晶生长发生偏转的主要原因.

In this paper,a solidification model of magnesium alloy based on the Eulerian multiphase flow technique and cellular automata method is proposed to investigate the evolutions of aluminum concentration and solidification microstructure during the directional solidification of magnesium alloy with 9%(mass fraction)aluminum concentration under three types of boundary conditions,i.e.,no flow,forced flow in the x-direction,and forced flow in the y-direction.The numerical simulations reveal that the dendrites of magnesium alloy grow at an angle of 60°to each other for the no-flow condition.In addition,secondary dendrites are also found in the late period of solidification,which grow at an angle of 60°to the primary dendrites.Both characteristics of primary dendrites and secondary dendrites demonstrate that the simulated solidification microstructure has a characteristic of crystal solidification with the hexagonal closed-packed structure,which confirms the reliability of the model.For the condition of forced flow in the x-direction,the main difference between this case and the no-flow condition is that the dendrites grow faster along the direction of the melt flow.Moreover,the characteristics of well-developed secondary dendrites are also found in the late solidification period for this condition.The main reason for current solidification phenomena is that the rejected solute in the vicinity of liquid–solid interface is transported along the melt flow during the solidification and accumulates in the rear of the dendrites.The decrease in aluminum concentration due to the melt flow in the area of dendritic tips will increase the supercooling,which finally advances the dendrite growth.Conversely,for the condition of forced flow in the y-direction,asymmetric growths of dendrites appear,and the preferred orientation for some dendrites has deflected about 3°compared with the no-flow condition.The numerical simulations indicate that the rejected solute is transported along the melt flow from one side of the dendrite to another side due to the forced flow in the y-direction,which results in the asymmetric u1∇c1 distribution of aluminum concentration and the forming asymmetric characteristics of dendrites.Meanwhile,represents the u1∇c1 formation mechanism of the asymmetric morphology of dendrites.The results show that it promotes solidification when>0 and u1∇c1 suppresses the dendrite growth when it is negative.The in the vicinity of the liquid–solid interface is seen as asymmetric distribution due to the forced flow in the y-direction during the solidification,which finally results in the deflection of the dendrite growth.