大倾角悬垂结构激光变向成形精准控制基础研究

The forming of overhanging structure with large inclination angle is an acknowledged difficult problem in the field of metal additive manufacturing. Based on laser cladding with hollow-laser beam internal powder feeding technology, this application proposed precise deposition methods with non-parallel slicing, no “staircase effect” between layers and no support structures for the overhanging structure with large inclination angle. The main contents and innovations are as follows: (1) The structures with the feature of curve, twisting and cavity are abstracted as “arch-shaped” and “fan-shaped” basic forms, and the deposition methods for the special-shapes of inside and outside-curving tracks and variable-height tracks are studied. (2) The mechanisms of melting and solidification, flowing deformation and displacement of the molten pool on the substrate with the inclined angle from 0°-150° are studied, and the control strategy for the measurement and feedback compensation of the deformation of the molten pool height and width is designed. (3) The relationship between single track shape and the processing parameters are studied and modeled, and a traverse repetitive control method is proposed to online measure, memorize and feedback compensate the accumulated errors of uneven surface, which are unavoidable during the variable-height and variable-parameters deposition process. (4) The process control experiments, analyzations and optimizations are implemented on typical overhanging structures with large inclination angle in order to realize precise control with stability, error convergence and anti-interference. This study is expected to greatly increase the maximal inclined angle and significantly improve the dimensional and surface accuracy of the deposition. The aim of this project is to break the bottleneck technology of the deposition of overhanging structure without support, and provide new methods of the laser precise deposition control, in order to realize the deposition of many complex-shaped overhanging structures in engineering, and the on-site repairing of the large-size components, which are difficult to be disassembled, moved and overturned.

大倾角悬垂结构成形是金属增材制造领域公认的难题。本申请基于光内送粉激光熔覆技术，提出沿法向非平行分层、层间无台阶、无支撑变向精准控制成形大悬垂结构的方法。主要研究内容及创新：1）将弯、扭、空腔等典型悬垂结构抽象为拱形、扇形基本形体，研究内外弯和变高异形单道成形控制方法；2）研究0°-150°斜基面悬挂熔池的熔凝、流变、位移机制，设计自动检测斜面熔池高、宽形变并反馈补偿的控制方法；3）研究单道形貌与工艺参数的关系并建模，提出遍历重复控制法，对变高、变参数成形过程难以避免的累积凹凸误差进行在线测量、记忆并反馈补偿；4）进行典型大悬垂结构成形过程控制试验、分析与优化，实现稳定、误差收敛、抗干扰精准控制。研究期望大幅提升成形极限悬垂角，显著提高成形尺寸精度与表面精度。项目为解决工程中大量存在的悬垂复杂结构成形难题，以及难以拆卸、搬动的大型零构件现场修复问题，提供一种激光变向成形精准控制新方法。

送粉式激光熔覆成形是一种自由增材制造方法，然而现有激光熔覆技术仅能实现垂直于水平基面或小角度倾斜基面上的沉积，针对各种悬垂、变截面结构的成形和空间立仰面、扭曲面等的强化修复一般还难以实现，极大限制了成形自由度与功能。本项目采用激光内送粉技术，通过空间精准喷射与悬挂熔池流淌的控制实现了大倾角悬垂结构成形。.建立了斜基面熔池顶点位移模型，通过控制工艺参数实现了抑制斜熔池位移流淌。引入0°~150°空间倾角作为影响熔覆层形貌的工艺参数，同步利用层高测量传感器和定距提升闭环控制算法，实时测量层高数据并控制提升量，采用金相显微镜测量层宽数据。结果表明：在相同功率与扫描速度条件下，层高随倾角变化先减后增，90°时层高最小；层宽则随角度先增后减，90°时层宽最大。利用参数全面试验数据建立了BP神经网络模型，完成对熔覆层高宽形貌的预测模型。设计了逐段闭环控制逼近法，完成了不等高单道的成形，各段堆高控制曲线基本达到收敛、稳定。.成形的304不锈钢悬臂样件成形表面光亮平整，无宏观裂纹，表面粗糙度在1.323µm~9.638µm 之间；成形件中部和悬垂面的显微硬度均在264.1HV~277.2HV之间；拉伸试验表明零件力学性能较好，上屈服强度和最大抗拉强度分别为340.52MPa和765.81MPa，其断后伸长率为11.2%；显微组织整体以均匀致密的细枝晶为主，无明显气孔和夹杂缺陷。采用316L不锈钢等材料实现了火箭喷管、电离室封闭球体、弯管等大倾角悬臂特征结构的无支撑成形。本项目为激光熔覆空间全自由成形与修复提供了新理论与新方法。