液氮冷却条件下激光快速熔凝Ni-28 wt%Sn合金组织演变  

Microstructural evolution of laser surface remelting Ni-28 wt%Sn alloy under liquid nitrogen cooling

作　　者：曹永青[1] 林鑫[1] 汪志太[1] 王理林[1] 黄卫东[1] 

机构地区：[1]西北工业大学凝固技术国家重点实验室,西安710072

出　　处：《物理学报》2015年第10期311-319,共9页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:51323008;51105311;51271213);国家重点基础研究发展计划(批准号:2011CB610402);国家高技术研究发展计划(批准号:2013AA031103);高等学校博士学科点专项科研基金(批准号:20116102110016);中国高等学校学科创新引智计划(批准号:08040)资助的课题~~

摘　　要：研究了在液氮冷却条件下激光快速熔凝Ni-28 wt%Sn亚共晶合金的组织演化过程.结果显示,熔池从上至下可以分为三个区域:表层为平行激光扫描方向的α-Ni转向枝晶区;中部为近乎垂直于熔池底部外延生长的α-Ni柱状晶区;底部为少量的残留α-Ni初生相和大量的枝晶间(α-Ni+Ni3Sn)共晶组织.激光熔凝区组织受原始基材组织的影响很大,熔池中的α-Ni枝晶生长方向受到了热流方向和枝晶择优取向的双重影响.与基材中存在的层片状、棒状和少量离异(α-Ni+Ni3Sn)共晶的混合组织相比,熔池内的共晶组织皆为细小的规则(α-Ni+Ni3Sn)层片状共晶,皆垂直于熔池底部外延生长,并且从熔池顶部至底部,共晶层片间距逐渐增大.分别应用描述快速枝晶生长的Kurz-Giovanola-Trivedi模型和描述快速共晶生长的Trivedi-Magnin-Kurz模型对熔池表层凝固界面前沿的过冷度进行估算,发现熔池表层α-Ni枝晶和(α-Ni+Ni3Sn)层片共晶的生长过冷度在50.4—112.5 K之间,远大于相应深过冷凝固(α-Ni+Ni3Sn)反常共晶生长的临界过冷度20 K,这说明文献报道的临界过冷度并不是反常共晶出现的充分条件.The substrate of as-cast Ni-28 wt% Sn hypoeutectic alloy immersed in liquid nitrogen is rapidly remolten and solidified by laser surface remelting with a scanning velocity of 10 mm/s and the laser power of 1950 W. The microstructure of the substrate and its effect on the microstructure of the molten pool are investigated by scanning electron microscope carefully. It is found that the substrate of the Ni-28 wt%Sn ingot is composed of coarse primary α-Ni dendrites and the interdendritic （α-Ni＋Ni3Sn） eutectic. The growth orientations of α-Ni dendrites and the interdendritic eutectic are distributed nearly randomly in the as-cast substrate. There are three kinds of microstructure characterstic zones from the top to the bottom of melted pool. The growth directions of α-Ni dendrites with the primary dendritic spacings ranging from 4.19 to 6.91 μm are approximately parallel to the laser scanning direction at the top of the molten pool due to the fact that the temperature gradient at the interface between the molten pool and substrate tends to be parallel to the laser scanning direction. In the middle of the molten pool, the epitaxial α-Ni columnar dendrites are found to be inclined to grow in the direction vertical to the bottom of the molten pool due to the fact that the temperature gradients in most zones of the molten pool are perpendicular to the bottom of the molten pool. The formation of new primary dendrites by the growth of the tertiary arm results in the decrease of primary dendritic spacing in comparison with that at the bottom of the molten pool. There are a small quantity of residual ^-Ni primary phase and a large amount of （α-Ni＋Ni3Sn） eutectic at the bottom of the molten pool. The microstructure of laser remolten zone is greatly influenced by the substrate microstructure, and the growth direction of the α-Ni dendrite in the molten pool is also affected remarkably by both the heat flux and the preferred crystal orientations for dendritic growth. Compared with the mixed lamella, rod and d

关 键 词：快速凝固 转向枝晶 Ni—Sn共晶 过冷度 

分 类 号：TG665[金属学及工艺—金属切削加工及机床]