细钨钨合金激光增材制造高固相分数两相快速凝固行为研究

An effective way for improving the penetrating performance of tungsten heavy alloys (WHAs) is to refine their W particles (reinforced phases), but it is challenging for the conventional sintering technologies. Laser additive manufacturing (LAM) with the advantages of the layer-by-layer deposition and rapid solidification possesses the potential for producing the WHAs with fine W particles. However, the content (up to 90 wt.%) and melting point of W are high in WHAs, leading to high-fraction solid W and melting matrix co-existing in the melt pool and hence the complex effect of the solidification kinetics conditions, convection and matrix/W wetting on microstructure and properties. Therefore, the successful LAM of high-performance fine-W WHAs requires for understanding the unique high-solid-fraction two-phase rapid solidification behavior. Considering the technical feature of the layer-by-layer deposition of the laser melting deposition additive manufacturing process, this project focuses on the single-track deposition/cladding, and selects the systems with different wetting characteristics and melt-pool fluid fields. By controlling the key processing parameters to obtain a wide range of solidification conditions, and combining the experimental results with solidification theory, the effects of the kinetics conditions coupling with convection and matrix/W wetting on the solidification microstructure (e.g., the size, morphology and distribution of W particles and the texture of matrix) were investigated. The results will be extended to the multi-track, multi-layer process, and can offer the experimental reference and the theoretical instruction for the additive manufacturing with a high solid fraction, and establish the theoretical foundation for the applications of the LAM high-performance WHAs to the military industry.

细化钨颗粒（增强相）是提升穿甲战斗部用钨合金侵彻效能的有效途径，但这对于传统烧结技术仍存在诸多挑战。激光增材制造具有逐层沉积和快速凝固的优势，能解决细钨钨合金制备的问题。但钨合金中钨相占比高达90wt.%且熔点很高，熔池内液态基体与高比例固态钨相始终共存，凝固动力学条件、对流及基体/钨润湿行为对组织性能都有着重要影响，揭示其特有的高固相分数两相快速凝固行为是实现高性能细钨钨合金增材制造的基本前提。本项目拟利用激光熔化沉积增材制造逐道/逐层往复的技术特点，由单道沉积/熔覆为主，选择流场和润湿行为不同的体系，调整工艺参数获取宽范围凝固条件，结合凝固理论，探究动力学条件、熔池对流及体系润湿行为对凝固组织形成（钨相形态、尺寸、分布及基体织构等）的多重耦合作用规律，并推广至多道多层增材制造过程进行验证完善，为高固相分数增材制造提供实验参考和理论依据，奠定增材制造高性能钨合金在兵器工业应用的理论基础。

细化钨颗粒（增强相）是提高钨合金性能的有效途径，但这对于传统烧结技术仍存在诸多挑战。本项目基于增材制造技术“离散+堆积”的原理，将大块钨合金的成形离散为小熔池内的瞬时液相烧结，提出激光瞬时液相烧结钨合金方案，以解决细钨颗粒钨合金制备难题，这其中核心的科学问题就是要探明熔池内高固相分数两相快速凝固行为及组织形成规律。为此，本项目系统开展了过程凝固动力学条件、润湿行为对钨相形态、基体相凝固组织的作用机制，以及熔池对流对钨相分布的作用规律等方面研究，结果表明：.（1）钨合金凝固组织与成形工艺密切相关。随着熔池温度/热输入的增加，钨相逐渐从未熔化的颗粒状转变为发达树枝晶；而随着激光功率或扫描速度的升高，钨合金中基体相则从柱状晶逐渐转变为等轴晶。.（2）当基体相与钨相间润湿良好且熔融基体相在固态钨表面的润湿特征表现为溶解驱动型润湿时，可以出现晶界润湿转变现象且有助于快速致密化。项目利用晶界润湿转变原理，提出多晶钨颗粒分散形成单晶细钨颗粒思路，并探明有效提高多晶钨粉的分散程度的关键工艺参数，解决“粉体流动性”和“细颗粒”间的矛盾。.（3）熔池中强烈的Marangoni对流一方面可避免高固相钨导致高粘度引起的熔池“球化”现象，改善熔池形态及成形质量；另一方面还能充分搅拌熔池，增强固液两相流动，避免钨相团聚或沉降，加上熔池瞬时高温带来的更好的润湿铺展能力，可促进快速致密化和钨相分布均匀化。据此制得了大尺寸、致密、高钨含量（90wt.%）的细钨颗粒钨合金，其平均钨颗粒尺寸仅3.6μm（约为液相烧结传统钨合金的1/9），屈服强度和抗拉强度分别高达1063MPa和1329MPa，显著高于传统液相烧结的钨合金。.相关研究揭示了高固相分数两相快速凝固过程致密化机制，突破了致密、高性能细钨颗粒钨合金的激光瞬时液相烧结技术，可为最终制备出钨颗粒细小、分布均匀的高性能钨合金提供关键的理论依据和成形工艺参考。