激光抛光增材成形金属结构件熔池运动对组织性能的调控及机理研究

Poor surface quality has been considered as a key challenge for the development of metallic additive manufacturing (AM). Laser polishing is becoming hot topic to achieve high level of automation rapid manufacturing through the integration of design-structure-properties by combining with AM techniques. Previous study mainly focuses on the initial stage about how to optimize parameters in order to achieve low surface roughness of non-AM metal. However, systematic analyses of mechanism of laser polishing AM metallic surface as well as laser-polished surface performance and quality has been rarely studied. In this project, we are planning to explore fundamental mechanism of laser technology for additive-manufactured titanium (Ti) alloy surface enhancement, investigate the interaction between laser polishing and AM rough metallic surface, examine scientific problem of the effect of surface tension and gravity as well as non-equilibrium phase change on surface topography and microstructure, and analyze the principle for AM surface enhancement including microstructure evolution and mechanical behavior through laser polishing technique. The important meaning of expected results lies in investigating mechanism of laser polishing for AM rough metallic surface, examining laws of surface topography and physical-mechanical behavior change caused by laser polishing processing, exploring the method of how to control laser polishing process to achieve high quality surface, and providing theoretical foundation as well as technical support for rapid development and industry application of high performance metallic components made by AM technique.

增材成形金属结构件表面质量问题是目前制约该领域发展的主要技术瓶颈。激光抛光技术有望与增材制造技术相结合，实现设计-结构-功能一体化的高自动化先进制造，因此成为亟需探索的研究热点。激光抛光金属材料的前期研究主要集中在如何优化工艺参数以降低非增材成形表面粗糙度等探索阶段，缺乏对增材成形金属构件表面抛光机理较为系统的研究与分析。本课题拟开展增材选区成形钛合金结构件表面激光抛光技术的基础研究，分析激光辐射引起表层材料快速熔化和凝固等现象，阐明熔池内部表面张力和重力等多向作用力对熔池运动和凝固过程的影响，探索非平衡态固液汽相变优化方法，揭示表层材料显微组织结构成分及其物理-机械行为等演变规律。项目预期成果的重要意义在于：揭示增材成形金属结构件表面激光抛光机理，阐明抛光前后材料表面物理-机械行为等演变规律，为推动高性能金属结构件增材制造技术快速发展和工程应用提供理论指导基础。

金属构件激光增材制造采用高功率激光对金属粉末/丝材进行逐层熔化，通过凝固堆积方式，实现从数模到构件的近净成形，广泛应用于航空、航天、医疗、汽车、轨道交通等领域。随着关键金属构件需求日益复杂化和精密化，如何克服由粉末粘结、阶梯效应、熔池飞溅等造成的表面缺陷，提高尺寸精度和表面质量，开发激光增减材一体化解决方案成为学术界和工业界共同关注的科学问题。近年来，激光抛光技术得到国内外研究人员高度关注，通过调控熔池运动和凝固过程使得初始粗糙表面峰谷高度差减小，进而提高表面光洁度。本项目开展了激光选区成形钛合金构件表面激光抛光技术的基础研究，分析了激光辐射引起表层材料快速熔化和凝固等现象，阐明了熔池内部表面张力和重力等多向作用力对熔池运动和凝固过程的影响，探索了非平衡态固液汽相变优化方法，揭示了表层材料显微组织结构成分及其物理-机械行为等演变规律。项目提出了“熔融金属平整化凝固”控制新方法，发明了沉积增材/层内光整激光快速抛光新方法，解决了曲面形性可控内外抛光技术难题，进一步提出了精确控制激光熔化金属粗糙表面和优化非平衡态固液汽相变新工艺，获得了熔池内部表面张力和重力等多向作用力对熔池运动和凝固平整化过程的影响及其对材料显微组织、热影响区、表面结构和性能影响的演变规律。研究结果实现了选区钛合金构件晶粒形态再凝固和表层金属缺陷调控，使得构件表面粗糙度由Ra 15.0 μm以上降低至Ra 0.05-0.2 μm，并将表面孔隙率降低了30%以上，疲劳强度提高了20%以上。研究成果获得了制造领域顶尖国际会议CIRP2018“最佳论文奖”，西班牙维戈大学Pou 教授和葡萄牙阿威罗大学Davim教授将其编入Elsevier出版的英文专著《Additive Manufacturing and Surface Treatment》，培养了博士生王海鹏获“校级一等优秀毕业生”称号，硕士生马程鹏等荣获“国家奖学金”，获“北航研究生十佳”提名和“北航研究生优秀毕业生”等荣誉，并得到易加三维等业内公司高度关注。