激光动态凝固组织的凝固特征及其形核机理

THE SOLIDIFICATION CHARACTERISTIC AND THE NUCLEATION MECHANISM OF THE LASER DYNAMIC SOLIDIFICATION STRUCTURE

本文研究了激光快速动态凝固下的凝固组织特征,讨论了在激光与金属交互作用时,其熔池内的不同区域结晶组织特征和形核机理。在激光与金属的交互作用的能量密度为2.85×10~3～1.70×10~4J/cm^2,计算冷却速度达8.0×10~4℃/s 条件下,发现激光动态凝固组织主要为不规则胞状,胞状树枝状和等轴树枝状等。在此基础上,探讨了激光动态凝固组织的形核的二类机理。一类是以未完全熔化的晶粒残骸作为形核质点;另一类是以固—液界面上的微熔晶粒作为形核的基质。提出了相应的形核模型。本实验结果表明,激光动态凝固之后,在其自由表面上存在两种混晶情况,其一是尺寸相差甚大的不同尺寸晶粒的混合共存;另一是柱状晶与等轴晶的混合共存。

In this paper,the characteristic of solidified structure under dynamic soli- dification by laser irradiation was studied,the characteristic of the solidified structures of the various zones in the laser interacted molten pool and nuclea- tion mechanism were discussed.The experimental condition was that the ener- gy density of the interaction of laser with metal was 2.85×10~3—1.70×10~4J/ cm^2.Correspondingly,it is found that the characteristic of the solidified structure under dynamic solidification is mainly irregular cellular structure, cellular dendritic structure,equiaxed dendrite structure and so on. The nucleation mechanism could be classified into two types:the one is that the remained grains which were not melted away serve as the nucleus of crystallization of new grains,and the other is that the partly melted grains which existed on the solid-liquid interface in the molten pool serve as /he sub- strafe of nucleus.The corresponding nucleation models are suggested.In this experiment,it is also found that two types of mixed grains are existed on the free surface which is processed by laser dynamic solidification.The one is that the grains of various sizes co-existed,and the other is that the columnar grains and the equiaxed dendrite grains co-existed.