超声辅助激光/锻造同步复合金属成形技术基础研究

Laser metal forming is an important means to realize metal additive manufacturing and remanufacturing. However, there are usually defects in the laser forming parts, such as porosity, cracks, residual tensile stress and poor reliability. Against the shape-and-performance-control problem, in this project, a new method on ultrasonic assisted laser/forging synchronous hybrid metal forming is proposed. The fundamental scientific problem, the relevant theoretical basis and the technological method are studied. The main innovations include: 1) to establish a new ultrasonic assisted laser/forging synchronous hybrid metal forming process, and then to obtain a high-performance additive manufacturing method; 2) to explore the multiphase non-Newtonian rheological properties for the metal during ultrasonic forging after laser heating, and then to develop a new constitutive model of metallic materials in hybrid forming process; 3) to explore the effects of the laser parameters, the forging pressure, the ultrasonic parameters, the laser/forging distance and the laser/ultrasonic coupling type on the macroscopic shape accuracy and the microstructures of the forming parts, and then to establish a new shape-and-performance-control method for laser metal forming by ultrasonic forging. The project will establish the process flow for ultrasonic assisted laser/forging synchronous hybrid metal forming and related theoretical models, and the research findings are expected to apply to the direct forming of complex metal parts.

激光金属成形是实现金属增材制造和再制造的重要手段，然而，目前激光金属成形件经常存在着气孔、裂纹、残余拉应力和性能稳定可靠性较差等缺陷。为此，本项目针对激光金属成形的控形控性难题，提出了一种超声辅助激光/锻造同步复合金属成形新方法，对其基本科学问题、相关理论基础和工艺方法开展研究。主要创新点包括：1）建立一种超声辅助激光/锻造同步复合金属成形新工艺，进而获得一种高性能增材制造方法；2）探索金属在经过激光加热熔化后再进行超声锻造时的多相非牛顿流体流变特性，建立复合成形过程中的金属材料本构新模型；3）探索成形件宏观形状精度与微观组织结构受激光参数、锻压力、超声参数、激光/锻造作用距离和激光/超声耦合方式的影响规律，建立采用超声锻造对激光金属成形进行控形控性的新方法。本项目将建立超声辅助激光/锻造同步复合金属成形的工艺流程和相关理论模型，研究成果有望应用于复杂金属件的直接成形制造。

高端装备关键部件对于高质量表面改性、增材制造与再制造具有广泛需求，然而，目前激光金属成形件常存在气孔、裂纹、残余拉应力和性能稳定可靠性较差等缺陷。为此，本项目针对激光金属成形的控形控性难题，引入超声微锻，围绕激光金属成形与超声/振动的耦合机理及其数值模型、激光/超声微锻耦合作用下的金属熔池温度场及流场、超声空化对激光凝固成形过程半固态区的作用机理、超声/振动对激光金属成形微观组织与成形缺陷的影响机制、超声/振动辅助激光金属成形中的控形控性方法等方面开展研究。主要结果包括：将声辐射压力梯度以源项引入到Navier-Stokes方程，建立了超声/振动辅助激光金属成形数值模型，数值模拟表明同步超声微锻促进了熔池流动，显著影响熔池传热传质特性；超声空化显著影响激光凝固成形过程半固态区，细化激光金属成形微观组织；超声/振动显著影响激光金属成形宏观形貌，抑制气孔、熔合不良等缺陷，降低残余应力，抑制裂纹萌发，提升成形件力学性能与耐腐蚀性能；试验结果表明，在2.25kW超声功率作用下，316L不锈钢激光金属成形宏观形貌高度降低66.57μm，宽度升高154.24μm，熔覆层平均显微硬度提高23.26%，腐蚀电流密度较无超声条件降低了一个数量级。项目科学意义在于：构建了激光/超声多物理场耦合数值模型，揭示了激光/超声复合能场作用下的熔池流动特性；综合了半固态区空化泡破裂引起微射流的“微观”机制计算，以及引入机械效应和声流效应的“宏观”数值模拟，揭示了超声对于激光金属成形宏微观形貌与缺陷抑制的影响内在机制。本项目研究促进了金属构件高性能激光金属成形的相关理论发展，成果有望应用于高端装备关键部件的激光表面改性与再制造。