基于原位合金化的激光-间接电弧复合增材制造技术研究

A new type of laser hybrid indirect arc additive manufacturing based on theories of in-situ alloying is developed to meet high efficiency and quality of additive manufacturing of 7000 series aluminum alloys. Indirect arc between two wires of different materials (Al-Cu and Al-Mg wires) is established to melt wires, leading to formation of droplets. Laser beam is used to hybridize indirect arc at the intersection of the double wire axis, and push droplets into the molten pool of the deposition layer. The third wire (Zn wire) and mixed powder (Ti and B4C) are fed into the molten pool by a wire-powder coaxial device. Laser replenishes heat to molten pool to control solidification of molten pool. A non-equilibrium model of melting three-wires and one-powder with different chemical compositions is also proposed in the time-space domains including formation of droplets. Composition proportion of 7000 series aluminum alloy and the in-situ reaction of ceramic particles (TiB2 and TiC) are achieved. The evolution law of molten pool temperature field and the effect of temperature on equiaxed grains growth and grain refining are further studied to improve the crystallographic features of microstructure texture, which contributes to eliminate hot tearing cracks and cavities. Finally, based on the mathematical characterization of the molten pool shape, the parameters governing thermal outputs of laser and indirect arc are numerically inverted after thermodynamic and microstructural results are predicted. The desired precision control on net shape and properties of additive manufacturing of 7000 series aluminum alloys can thus be achieved by optimizing process parameters.

针对7000系铝合金增材制造易产生裂纹、成形效率低和丝材成型难的问题，本项目提出一种基于原位合金化的激光-间接电弧复合增材制造技术，即在异质双丝(Al-Cu和Al-Mg)之间建立间接电弧使双丝熔化形成熔滴，激光束辅助熔滴过渡至熔池，丝粉同轴装置将第三丝(Zn丝)和混合粉末(Ti和B4C)送入熔池，同时利用激光调节对熔池的热输入，控制熔池凝固和熔积成形。项目构建异质三丝一粉非均衡熔化模型，实现Al-Zn-Mg-Cu合金的成分配比和陶瓷颗粒(TiB2和TiC)的原位生成；研究熔池温度场演变规律，揭示强温度梯度条件下陶瓷颗粒对等轴晶形成和晶粒细化的影响机理，结合对熔池热输入的调控，改善微观组织结构的晶体学特征，抑制热撕裂行为和空腔缺陷产生；基于熔池形貌的数学表征和熔池端热力分布，反求激光-间接电弧工艺参数，得到预期的成形形貌。通过工艺参数优化实现对7000系铝合金增材制造组织和成形的调控。

针对7000系铝合金增材制造易产生裂纹、成形效率低和丝材成型难的问题，本项目提出一种基于原位合金化的激光-间接电弧复合增材制造技术，项目从间接电弧专用电源研发、电弧及熔滴过渡行为、原位合成及晶粒细化机理、基于单摄像机立体视觉的电弧增材工艺过程分析和增材制造Al-Zn-Mg-Cu合金工艺、组织及性能等方面展开研究。自主设计研发了双丝间接电弧增材制造专用电源，可实现交/直流变换、输出电流波形可控。构建了异质双丝非均衡熔化数学模型，分析了不同电流参数下异质双丝交流间接电弧行为及熔滴过渡规律，确定了较适合的双峰值-一基值电流波形，实现了电流参数对异质双丝间接电弧稳定性和熔滴过渡的控制。更进一步地，通过模拟和实验研究了不同激光功率、脉冲宽度、入射位置对熔滴定向过渡的影响规律，建立了电流波形-激光波形匹配控制熔滴过渡机制。研究了不同Ti基晶粒细化剂（粉末状Ti、TiH、Ti+B4C和带状Al-5Ti-1B）对电弧增材制造成形形貌、晶粒形核及晶粒分布等晶体学特征的影响规律，揭示了异质形核颗粒Al3Ti的原位生成和对铝合金性能的强韧化机理，研究结果表明，添加Al-5Ti-B对微观组织和力学性能的改善最优，将试样小角度晶界的比例提高了8.2%，抗拉强度和延伸率在平行增材方向上分别增加了23MPa和2.2%，在垂直增材方向上分别增加了31MPa和8.6%，试样的平均显微硬度提高了5HV，尤其是消除了层间的低硬度区，显著改善了增材制造试样的各向异性。搭建了单摄像机双目立体视觉系统，基于OpenCV开发了适用于熔池宽度提取的图像处理算法，得到了增材制造过程中的熔敷层宽度信息，实现了对增材制造成形的实时监测与调控。进行了间接电弧增材制造Al-Zn-Mg-Cu合金原位合金化机理及组织性能的研究，以5.1kg/h的熔敷效率原位合成了Al-5.7Zn-3.4Mg-1.6Cu(wt.%)合金，其成分配比和主要构成相S(Al2CuMg)、η(Mg(Al，Zn，Cu)2)、T(Al2Mg3Zn3)和η′相与7000系列铝合金一致，显微组织以柱状和等轴晶为主，平均硬度、极限抗拉强度和伸长率分别为98.6HV、243.9MPa和5.9%，略高于铸态7050铝合金。通过本项目的实施，丰富了高效复合增材制造工艺，革新了高性能材料研发与制备方式，实现了异质多丝间接电弧增材制造原位合成7000系铝合金。