采用Ni--28\%Sn, Ni--30\%Sn, Ni--33\%Sn和Ni--35\%Sn(质量分数)共4种成分的亚共晶和过共晶合金, 考察了Ni--Sn共晶合金在激光快速熔凝过程中的组织演化过程. 当激光扫描速率较低时, Ni--28\%Sn亚共晶合金和Ni--35\%Sn过共晶合金的熔凝组织主要由细化的初生枝晶(亚共晶合金为α-Ni相, 过共晶合金为Ni3Sn相)和枝晶间共晶(α-Ni+Ni3Sn)组成; 近共晶合金Ni-30%Sn和Ni-33%Sn的熔凝组织基本相似, 熔池从底部到顶部存在柱状共晶团向等轴状共晶团的转变. Ni-Sn亚共晶和过共晶合金熔池底部皆存在少量粗大的残留初生相. 激光快速熔凝后, 相比基材中所存在的层片和棒状共晶的混合组织, 熔池内的共晶组织皆为层片状共晶, 层片间距相比基材明显减小, 并呈现垂直于熔池底部外延生长的特征. 通过对比4种成分合金在不同激光扫描速率下的熔凝组织, 获得了激光快速熔凝条件下 Ni-Sn合金共生生长区的成分范围以及临界激光扫描速率. 应用描述快速枝晶生长的 KGT模型和快速共晶生长的TMK模型对熔凝组织进行了分析, 模型计算结果与实验结果符合良好.
The microstructure evolution of Ni–Sn alloys (Ni–28%Sn, Ni–30%Sn, Ni–33%Sn and Ni–35%Sn) near eutectic during laser rapid solidification has been investigated. In low velocity laser scanning, the microstructures of Ni–28%Sn and Ni–35%Sn hypereutetic alloys consist of refined primary dendritic phase, which is α–Ni phase for the former and Ni3Sn phase for the latter, and (α–Ni+Ni3Sn) eutectic phase. However, the microstructures of Ni–30%Sn and Ni-33%Sn near–eutectic alloys consist completely of (α–Ni+Ni3Sn) eutectic, and they undergo a morphological transition from the columnar to equiaxed eutectic from bottom to the top of molten pool. There is a small amount of residual coarse primary dendritic phase of substrate in the bottom of molten pool for these four Ni–Sn alloys. With increasing laser scanning velocity, compared with the mixed lamella and rod eutectic microstructures in the substrates, for these alloys eutectic in molten pool is completely composed of lamella eutectic, and lamella eutectic spacing is reduced significantly after laser rapid solidification. Besides, lamella eutectic in molten pool grows epitaxially along the normal to the molten pool interface with the substrate. Moreover, the composition range and the critical laser scanning velocity are also obtained for coupled eutectic growth during laser rapid solidification. In the present work, further analysis of microstructure evolution is given by using KGT and TMK models, showing a good agreement between the simulated and the experimental results.
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