激光超声复合增材制造逐层交互控形控性新方法及机理研究

Laser additive manufacturing (LAM) can fabricate complex components with shape and property controllability by melting metallic powder layer by layer. However, the internal stress and metallurgical defects will be generated during laser melting/solidification process. The internal stress can result in the deformation and crack of the component, leading to the difficulty of controlling shape, and the metallurgical defects can reduce fatigue performance, leading to the difficulty of controlling property. These severely restrict the development and application of LAM technology. Based on the “layer by layer” deposition character of LAM, we propose a new idea of eliminating both the internal stress and metallurgical defects by ultrasonic peening each layer. Accordingly, a new method is developped by combining laser melting/solidification and ultrasonic peening together, which will significantly enhance the controllability of shape and property. The distribution of internal stress and character of metallurgical defects after LAM will be investigated first, and then the influence of ultrasonic peening on the internal stress and metallurgical defects will be studied. Based on the investigations above, the coupling process of laser melting/solidification and ultrasonic peening will be developped to obtain metallic components with high peformance and no deformation. Meanwhile, a systematically numerical and experimental studies will be carried out to reveal the evlution law of internal stress, as well as the formation mechanism and closing behavior of metallurgical defects under the complex thermal-mechanical coupling effect induced by alternately LAM and ultrasonic peening layer by layer. This study has great significance for promoting the development and application of laser additive manufacturing.

激光增材制造技术通过激光对金属粉末逐层熔化/凝固堆积，实现复杂构件的“控形控性”一体化制造。然而，激光熔化/凝固过程中不可避免的产生内应力和冶金缺陷。内应力造成构件易于变形开裂的“控形”难题和冶金缺陷造成构件疲劳性能较差的“控性”难题，一直制约着该技术的发展和应用。本项目基于增材制造“逐层”堆积的技术特征，提出了采用超声冲击产生塑性变形“逐层”消除内应力和冶金缺陷的新思路，形成激光和超声冲击逐层交互“控形控性”的新方法。通过研究激光熔化/凝固后内应力分布和冶金缺陷特征，以及超声冲击工艺对内应力和冶金缺陷的影响，实现激光增材制造和超声冲击工艺耦合，获得无变形和高性能的金属构件。同时，发展有效的数值计算方法并借助实验表征，揭示在激光和超声冲击“热/机械”交互作用下的内应力演化规律和机理、冶金缺陷形成机理和闭合行为。本项目研究对推动激光增材制造技术发展和应用等具有重要意义。

选区激光熔化（Selective Laser Melting，SLM）是金属增材制造中最具发展潜力的技术之一。该技术具有成形精度高、成形零件性能好等优点，已成为大型结构件制造领域的研究热点。然而目前存在一些问题严重制约了选区激光熔化的进一步发展和应用，其中，残余应力、冶金缺陷和晶粒结构等问题较为突出，有待于深入研究和解决。.针对上述问题，本项目提出了一种新的加工方法——超声冲击辅助SLM成形。以Ti6Al4V粉末为代表材料，开展了一系列的相关实验研究。论文的主要研究内容如下：（1）研究了SLM单道成形样件的表面和截面形貌，探究了不同扫描速度对表面成形质量的影响，确定了适合SLM大层厚成形的激光扫描速度范围，实验结果表明，当速度大于120mm/s时，不利于SLM成形。（2）基于单道实验结果，进行了SLM块体成形实验，测量了不同扫描速度和扫描间距下样件的致密度，并对成形块内部缺陷进行了具体分析，揭示了不同缺陷的形成机理，并得出了适合高效SLM成形的大层厚工艺参数。（3）根据以上实验结果，选取合适的工艺参数，设计了超声冲击辅助SLM成形和SLM直接成形的对比实验，探究了超声冲击辅助SLM成形方法的可行性以及该方法对应力、缺陷和晶粒结构的影响。通过实验结果的对比分析，得出了超声冲击辅助SLM可有效消除样件的残余应力和缺陷，并能生成等轴晶，改善晶粒结构。.同时，本项目将超声冲击辅助方法用于电弧增材制造，以验证超声冲击的有效性。超声冲击辅助大层厚（2mm）的电弧增材制造技术，同样可显著降低残余应力，将粗大柱状晶粒转变为等轴晶粒，对推动电弧增材制造技术发展同样具有重要意义。