钒合金在航空航天和核工业领域有着良好的应用前景，采用激光熔化沉积（LMD）技术制备钒合金将大量节约成本。针对LMD钒合金晶粒粗大与微观偏析严重的问题，本项目提出在成形过程中耦合超声振动和变质剂，有望显著细化晶粒，改善和消除偏析。目前还未见超声振动与变质剂同时作用下LMD成形的相关报道。本项目将系统研究该成形方式下流场与温度场分布及其对变质剂重排、以及形核与长大机制的影响；针对LMD成形快速凝固的特点，提出采用与钒同为BCC晶格类型具有潜在高效异质形核能力的高熔点金属单质作为变质剂，研究变质剂与熔体的反应与形核机制；研究超声振动和后续热处理对微观偏析的影响。以期深入理解V-5Cr-5Ti合金在多因素耦合作用下LMD成形过程中的微观组织演变规律，获得细化晶粒、消除偏析的机制及方法，为高性能钒合金的LMD制备提供理论与技术基础，形成的相关知识也可为其他合金制备提供指导。

Vanadium alloys are promising candidates for use in the aerospace and nuclear industries. Applications of laser melting deposition (LMD) technique to the fabrications of vanadium components will save costs of both time and materials. Vanadium alloys fabricated using LMD technique show coarse sized grains, and severe micro segregation, which lead to deteriorated properties. A technique of ultrasonic vibration assisted LMD process with additions of grain refiners is proposed in this study to promote the refinement of grains, and the elimination of micro segregation. This technique has been rarely reported. The temperature field and flow field during the fabrication process will be examined, and their effects on the nucleation and growth behavior of vanadium alloys will be investigated. Due to the rapid solidification feature, pure metals with the same BCC crystal lattice with vanadium (which are supposed to have excellent heterogeneous nucleation abilities) are proposed to work as grain refiners. The reactions between these grain refiners and melts, and the heterogeneous nucleation efficiencies of these refiners will be studied. Investigations on the effects of ultrasonic vibration and heat treatments on the micro segregation will also be carried out. The aims of this project are to understand the microstructure evolutions during the fabrication process, and to obtain methods of grain size refinement, and micro segregation elimination. Researches in this project are important for the promotion of properties of vanadium alloys, and both applicable to other alloys produced using LMD technique.