摘要螺旋选晶法是使用定向凝固技术制备单晶材料时经典的选晶方法。在高温合金的定向凝固过程中，当金属液沿一根螺旋管生长时，在螺旋管的顶部可以获得单晶组织。在螺旋选晶器的几何形状限制以及定向热流作用下，晶粒生长受到竞争机制和器壁阻碍的影响，最终筛选出单一取向的单晶体。虽然螺旋选晶法早已在单晶高温合金材料制备中广泛应用，但对选晶器选晶的机理研究并不多。螺旋选晶器分为引晶段和螺旋段，这两部分分别对晶粒的一级取向和二级取向有不同的选择作用。 如今单晶技术多用于制造大型工件，制备成本较高，因此如果使用数值模拟技术来设计工件制造的工艺参数，将大大降低生产成本，提高生产效率。本文以第一代单晶镍基高温合金DD403为例，使用 ProCAST软件的CAFE模型，通过模拟计算研究了螺旋选晶器的引晶段及螺旋段的几何参数对选晶结果的影响作用和选晶机理。 模拟结果表明，引晶段在选晶初期晶粒竞争生长占主导作用。然而引晶段的选晶后期，几何形状阻碍枝晶生长起到的取向优化作用逐渐占主导。螺旋段可以非常快速高效地筛选出单晶，然而对晶粒取向并没有明显的优化作用。而另一方面，最终能生长成为单晶的晶粒与其在螺旋段内的位置密切相关，靠近螺旋段管壁外侧的晶粒生长会受到螺旋段几何形状的严重阻碍，因而逐渐失去生长空间而被淘汰。靠近螺旋段管壁内侧的晶粒生长较少受到螺旋段几何形状的限制，更容易最终生长成为单晶。   41686
毕业论文关键词  螺旋选晶器  定向凝固  引晶段  螺旋段  几何形状 
Title    Simulation of Grain Selection in Spiral Selector               during Directional Solidification                 
Abstract Spiral selector  is widely used in directional solidification to produce single crystal. During the process of direction solidification, single crystal can be obtained on the top of the spiral selector when the melted alloy grows along the spiral. This is because the growth of grains is affected by competitive growth and block of geometry shape under directional heat flow. Although spiral selector has been widely used in the production of  single crystal superalloy,  the mechanism of grain selection hasn’t been studied sufficiently. Spiral selector is composed of starter block and spiral part. These two parts have different influence in primary  orientation  and secondary orientation of grains separately.   Nowadays  single crystal  technique is mostly use in the production of large workpiece, which requires a huge production cost. Therefore, it will considerably decrease production cost and increase production efficiency to use simulation technique in the design of  such workpieces. This work takes  the first generation of single crystal Ni-base superalloy DD403 as example. It uses the CAFE model of the software ProCAST to study the mechanism of grain selection and simulate the influence of the geometry shape of starter block and spiral part on grain selection. The simulation result shows that in the starter bock, grain selection is dominated by competitive growth in the early stage while that’s dominated by geometry shape block in the later stage. The spiral part can select single crystal very efficiently but can’t optimize the orientation obviously. However, the selected single crystal is related closely to its positon in the spiral part. The grain near the outside of the wall will be blocked and the grain near the inside of the wall is much easier to become the single crystal finally.  
Keywords    spiral selector    directional solidification    starter block    geometry shape 凝固组织数值模拟在螺旋选晶过程中的应用研究:http://www.youerw.com/cailiao/lunwen_41830.html