超声振动与变质剂耦合激光熔化沉积V-5Cr-5Ti合金的微观组织演变规律

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.

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。

针对航空航天和核工业等领域对高性能钒合金构件增材制造的需求，为解决增材制造钒合金晶粒粗大与微观偏析严重导致性能不高的问题，本项目提出在成形过程中耦合超声振动和变质剂，以达到细化晶粒，改善和消除偏析的效果。搭建了压板式和螺栓式耦合超声场的装置，系统研究了不同超声功率加载对模拟材料316L不锈钢微观组织的影响，发现耦合超声场会改变晶粒的生长方向，在一定程度上促进了柱状晶向等轴晶转变，起到了细化晶粒及枝晶、提升强度的作用，但超声的作用并不显著。未来需进一步改进超声场耦合装置，例如引进变频超声，以促进超声耦合效果，达到组织调控的目的。探索了包括B和W在内的高熔点单质作为变质剂对钒合金以及模拟材料纯铬的微观组织演变的影响，发现B通过包晶反应可在一定程度上细化钒合金晶粒，但效果并不明显，而激光熔化沉积的快速凝固效果不足以维持W在液态熔池中的稳定性，未达到异质形核的效果。系统研究了包括Y2O3、TiC和TiN等在类陶瓷颗粒作为变质剂对钒合金以及模拟材料铁素体、奥氏体钢合金缺陷形成、晶粒形核与生长、第二相颗粒重排的影响，发现陶瓷颗粒的添加可通过增加激光吸收率的方式降低熔体粘度，促进气体逸出熔池，从而降低孔隙率；合适的陶瓷颗粒添加可作为有效的变质剂促进合金晶粒组织从柱状晶向等轴晶转变，且可显著细化晶粒尺寸；合金中陶瓷第二相颗粒同时存在于晶界与晶内，呈三维网络状分布；在细晶强化和第二相强化的作用下，合金的力学性能可得到显著提升。本项目形成的相关方法、技术与理论可为高性能合金的增材制造提供了基础，应用于军工、核能、航空航天等领域中增材制造合金部件的研制。