目的 激光定向能量沉积（LDED）以其沉积速率高和成本低在制造大型钛航空部件方面具有良好的应用前景,然而制造过程中热量的集聚,使得熔池附近产生了巨大的温度梯度,会诱导Ti6Al4V涂层微观组织及宏观性能发生演变,为了减少沉积层的制造缺陷,获得更好的材料性能,提出一种扫描策略。方法 研究扫描策略对Ti6Al4V涂层沉积缺陷、微宏观组织和性能的影响,采用LDED技术制备10层Ti6Al4V熔覆层,以激光功率、扫描速度、沉积方向3个重要工艺参数为基准,设计二水平三因素正交实验,研究沉积策略对涂层缺陷的影响,并对涂层的物相及显微硬度进行分析和测量。结果 通过4种策略制备的Ti6Al4V沉积层的物相成分相近,沉积层主要由针状α/α′马氏体和网篮状魏氏体相组成,显微硬度在373HV10~388HV10内波动,沉积层的硬度显著高于基板和热影响区。沉积策略会对温度历史产生显著影响,基板的温度越高,则α′针状相的尺寸越小,组织越细化,硬度越高。结论 通过实验和后续的材料表征证明沉积策略是温度变化的关键因素,会显著影响沉积质量。变功率激光通过对温度历史进行优化,可以改善沉积层内部的组织结构,从而获得更少的沉积缺陷和更好的材料性能。
Abstract
Laser directed energy deposition (LDED) has a good application prospect in the manufacturing of large titanium alloy aviation parts due to its high deposition rate and low cost. However, the huge temperature change causes a series of complex physical phenomena such as metal melting and solidification, energy absorption and transfer, material phase change and molten pool flow in the manufacturing process, which induces deposition defects and microstructure and macroscopic performance evolution of Ti6Al4V coatings, bringing great challenges to the engineering application of laser directed energy deposition technology. In order to obtain fewer deposition defects and better material properties, the work aims to study the effect of scanning strategy on the deposition defects, micro and macro structures and performance of Ti6Al4V coatings. Ten Ti6Al4V cladding layers were prepared by LDED technology and laser deposition strategies of variable powers were proposed to optimize the temperature gradient during the deposition process. A two-level three-factor orthogonal experiment was designed based on the three important process parameters of laser power, scanning speed, and deposition direction to study the effect of scanning strategies on coating defects, and the temperature history, phase and microhardness of the coating during the manufacturing process were characterized and analyzed. It was found that the phase composition of Ti6Al4V deposits prepared by the four scanning strategies was similar. The deposits were mainly composed of needle-shaped α/α′ martensite and basket- shaped Widmanstatten phase, and the microhardness fluctuated within 373HV10-388HV10. Compared with the forged Ti6Al4V titanium alloy, the microhardness of the Ti6Al4V coating prepared by LDED process was significantly improved. The microhardness gradually increased from the substrate to the cladding layer, among which the substrate had the lowest hardness, the bottom of the cladding layer had significantly improved hardness due to rapid cooling and grain refinement and the middle and top parts had slightly lower hardness than the bottom due to grain coarsening caused by heat accumulation. The deposition strategy had a significant impact on the temperature history. When the substrate temperature was higher, the size of the α′ needle phase was smaller, the microstructure was finer, and the hardness was higher. Among the four deposition strategies, strategies B and D had lower peak temperatures of the substrate due to faster scanning speeds and cooling times, respectively, while strategy C reduced a certain amount of laser power at the initial deposition position of each layer, and the peak temperature was reduced by an average of 11.72% compared with strategy A. Strategy B and strategy D had various deposition defects such as poor bonding, pores and cracks due to insufficient energy density and large temperature fluctuations. Strategy A and strategy C had a smaller average crystal size due to greater supercooling, obtained higher microhardness, and had fewer manufacturing defects in the deposited layer. Experiments and subsequent material characterization prove that the deposition strategy is a key factor in temperature changes and will have a significant impact on the deposition quality. Variable power laser can improve the internal structure of the deposited layer by optimizing the temperature gradient, thereby obtaining fewer deposition defects and better material properties.
关键词
LDED /
Ti6Al4V /
沉积策略 /
微观组织演变 /
力学性能
Key words
LDED /
Ti6Al4V /
deposition strategy /
microstructure evolution /
mechanical properties
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基金
国家科技重大专项（2024ZD0702901）
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