中文摘要：

针对熔化焊过程建立了宏微观耦合模型，模拟了熔池内不同区域凝固过程中随机取向枝晶的竞争生长过程。通过宏观三维有限元模型计算熔池中瞬态的传热传质过程，利用双线性插值算法将凝固参数传递给微观组织模型。采用元胞自动机法模拟随机取向的枝晶在熔池凝固条件下的竞争生长过程。模拟结果表明，所建立的微观模型能够精确模拟任意生长取向的枝晶。凝固条件中最大温度梯度方向对枝晶竞争过程有明显选择作用，生长方向与最大温度梯度方向相同或接近的枝晶在竞争中具有更大优势。焊缝中的晶粒组织由枝晶簇发展形成，晶粒组织的形貌演变取决于相邻枝晶簇之间的竞争过程，具有择优取向的枝晶簇会逐渐排挤非择优取向的枝晶簇并最终将其阻挡在凝固组织内部，宏观晶粒的取向与其内部枝晶簇的生长方向并不一定相同。熔池中心线附近区域在焊接过程中具有更小的温度梯度、更大的凝固速率以及更大的局部冷却速率，凝固后可以获得更加细小的焊缝枝晶组织。枝晶间距的模拟结果与相应凝固条件下的试验数据符合较好。

英文摘要：

A macro-micro coupled model is developed to simulate the competitive dendrite growths in different areas of the welding pool in the solidification process. The transient solidification conditions in welding pool are obtained by the three-dimensional （3D） macro-scale FEM model. The thermal conditions used in the micro-scale cellular automata model is obtained from the macro-scale FEM model by using the interpolation algorithm. The simulation results indicate that the micro-scale cellular automata model developed in this paper can simulate the morphologies of dendrites with various growth directions accurately. The solidification conditions in welding pool have obvious effects on the competitive dendrite growth. The dendrites with their preferential orientations parallel to the direction of the highest temperature gradient are more competitive. The morphology of grain structure is determined by the competition among different dendritic arrays. The dendritic arrays with more favorable growth direction can gradually crowd out other dendritic arrays and occupy more space through dendrite branching. The area near the central line of welding pool has a lower temperature gradient, a higher solidification rate, and a higher cooling rate in the solidification process, and such solidification conditions lead to the finer microstructure. The simulation results of the secondary dendrite arm spacing are in agreement with the experimental results under the corresponding solidification conditions.