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多脉冲毫秒激光打孔熔池动态演化机理研究
Alternative TitleDynamic evolution mechanism of molten pool during multi-pulse drilling with a millisecond laser
张越
Thesis Advisor虞钢
2020-08-23
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline一般力学与力学基础
Keyword激光打孔 质量迁移 匙孔演化 熔池驱动力 时空传输特性
Abstract

随着航空航天及汽车工业的迅猛发展,发动机及涡轮叶片等零部件对精微孔的制造提出了大深径比、微小孔径、任意孔型等要求。与传统的机械打孔、电火花打孔工艺相比,多脉冲激光打孔技术具有能量密度大、辐照区域小、能量时空传输可控、易于自动化等优点,广泛应用于上述零件的喷油孔、冷却孔等精微孔的制造。当采用毫秒激光进行多脉冲激光打孔时,基体会产生熔化、蒸发、溅射等物理现象。溅射可以使得基体在短时间内产生大量的质量迁移,因此加工效率较高;此外,脉冲激光能量的时空传输特性高度可控,因此可以在一定程度上对成孔孔型调控,加工质量相对较高。由于毫秒激光打孔技术具有高效率、成孔质量可控的特点,在航空、航天、汽车、医药等领域的精微孔制造有着广泛应用。毫秒激光打孔过程产生了熔融物质,是产生溅射这一高效质量迁移方式的必要条件,但也是匙孔演化过程中产生堵塞等缺陷的主要原因之一。不论是质量迁移方式,还是匙孔演化过程,均与熔池动力学行为息息相关。因此,多脉冲激光打孔过程中的熔池动力学行为及其影响机制,是调控成孔质量的关键科学问题。针对这一科学问题,本文搭建了原位观测实验平台,提出了多相流/多物理场耦合模型。结合实验观测以及数值模拟方法,探讨了多脉冲激光打孔过程的质量迁移机制、匙孔动态演化机制以及激光能量传输影响机制,主要研究内容如下:

首先,通过搭建原位观测系统并结合理论分析,系统的讨论了多脉冲激光打孔过程中的质量迁移机制及其影响因素。原位观测结果表明质量迁移方式及质量迁移效率发生了变化。质量迁移方式发生了熔融喷溅向蒸发去除的转变,并依据质量迁移效率的大小,将多脉冲激光打孔过程分为快速打孔、线性打孔以及缓慢打孔三个阶段。通过理论分析结合解析模型得出,质量迁移效率的转变受到焓变数的影响。若焓变数大于12,则质量迁移效率较高,此时处于快速打孔阶段;若焓变数大于8且小于12,则质量迁移效率降低,此时处于线性打孔阶段;若焓变数小于8,则质量迁移效率低,此时处于缓慢打孔阶段。

随后,提出了自主开发的多相流/多物理场耦合数值模型,系统的讨论了匙孔动态演化规律及其影响机制。通过原位观测手段及数值模拟方法,证明了在多脉冲激光打孔过程中,匙孔形貌的动态演化受到熔融物质流动的影响。此外还指出了在多脉冲激光打孔的不同阶段,熔融物质流动模式发生了转捩,其转捩过程受到熔池驱动力的影响。系统讨论并解释了不同阶段激光加载以及停止时,熔池动力学行为对熔融物质流动状态的影响机制。

进一步通过数值模拟方法,讨论了激光能量的时、空传输特性对熔池动力学行为的影响。并且发现堵塞形貌的产生及消失,受到熔融物质流动状态的影响。在不同的时空传输特性条件下,其影响机制也不同。数值结果表明,在正离焦情况下产生了锥型形貌,匙孔深度的演化速度较慢,熔融层较薄,几乎很少在匙孔内部产生堵塞;在负离焦情况下产生了双曲型形貌,匙孔深度的演化速度较快,熔融层较厚,很大概率会在匙孔内部产生匙孔堵塞形貌;在脉冲间歇时间较长时,匙孔内的熔融物质在脉冲周期结束时基本完全冷却,没有匙孔堵塞产生或者产生后随即消失;若脉冲间歇时间较短,则正离焦情况下堵塞产生于匙孔入口处,负离焦情况下匙孔堵塞往往产生于匙孔中部或底部。

本文的研究结果阐述了质量迁移、匙孔形貌的演化规律,讨论了激光能量时空传输特性对熔池动力学行为的影响机制。首次系统的研究了多脉冲激光打孔过程中的熔池动态演化机理,并阐述了脉冲时序、熔池驱动力、熔融物质流动以及匙孔成孔质量之间的相关关系。对实现高效率、高质量的精微打孔在工程实际应用有着重要指导意义。

Other Abstract

With the rapid development of the aerospace and automobile industry, the demand of high aspect ratio, micro-hole and arbitrary shape hole were proposed for the manufacturing of microhole with high precision applied in engine and turbine blade. Compared with the traditional mechanical drilling and electrical discharge drilling, multi-pulse laser drilling offers several advantages, such as high power density, high spatial and temporal controllability of laser pulse, amenability to automation. With these advantages, this technology is widely used in the manufacturing of spray holes and cooling holes. Substrate material melts, evaporates and ejects once irradiated by millisecond pulsed laser. The machining efficiency is high due to melt ejection which causes intense mass transfer, and the machining quality is relatively high due to high spatial and temporal controllability of laser pulse. By virtue of high machining efficiency and high machining quality, the multi-pulse drilling with millisecond laser are widely used in industries like aviation, aerospace, automobile, and medicine industries. The existence of molten material is the necessary condition for producing the melt ejection, and is also the main reason for causing defects such as keyhole blockage. However, mass transfer and keyhole evolution both are affected by the molten pool behavior. Hence, the key scientific problem for controlling the machinging quality, is the molten pool behavior and its influence mechanism during the laser drilling process. An in situ observation system and a multi-phases/multi-physics coupling model are developed. Combined with experimental observation and numerical simulation method, the mechanism for mass transfer, keyhole evolution and energy transmission are investigated. The main conclusions are as follows:

First of all, by developing an in situ observation system and a theoretical analysis model, the mechanism for quality transfer and its influence factor are systematically discussed. The observation results indicate that the mechanism and efficiency of mass transfer have changed. For the way of mass transfer, there is a transition from melt ejection to evaporation. According to the efficiency of mass transfer, the process of multi-pulse drilling with millisecond laser are divided into three stage, which are rapid drilling stage, linear drilling stage, and moderate drilling stage. As indicated from the theoretical analysis results, the efficiency of mass transfer is affected by the enthalpy change number. If the enthalpy change number is greater than 12, which means the efficiency of quality transfer is high and the drilling is in the rapid stage. If the enthalpy change number is greater than 8 and smaller than 8, which means the efficiency of quality transfer is reduced and the drilling is in the linear stage. If the enthalpy change number is smaller than 8, which means the efficiency of quality transfer is low and the drilling is in the moderate stage.

After that, by developing a multi-phases/multi-physics coupling model, the mechanism for keyhole evolution and its influence factor are systematically discussed. Combined with experimental observation and numerical simulation method, it is proved that the dynamic evolution of keyhole is affected by melt flow. It is indicated that the melt flow pattern changed due to the driving forces of molten pool at different drilling stage. The effect of driving force on the melt flow at different drilling stage has been systematically discussed and explained.

Furthermore, the effect of spatial and temporal transmission characteristic of laser energy on dynamic behavior of molten pool are disscused through the numerical simulation. Research shows that the formation and disappearance of keyhole blockage are affected by the melt flow pattern. The effect of melt flow pattern is different under different conditions. As indicated from the numerical results, in the case of positive defocus, a tapered keyhole is drilled with slow material removal rate, thin melt layer and rarely formed blockage. In the case of negative defocus, a hyperbolic keyhole is drilled with high material removal rate, thick melt layer and probably formed blockage. In the case of long interpulse time, there is almost no molten material remaining at the end of each pulse cycle, and there is no blockage inside keyhole or the blockage disappeared immediately. In the case of short interpulse time, the keyhole blockage is generated at keyhole entrance when the defocus amount is positive, and the keyhole blockage is generated at the middle or bottom of keyhole when the defocus amount is negative.

The mechanism for mas transfer and keyhole evolution, and the effect of laser energy transmission on dynamic behavior of molten pool are investigated. Dynamic evolution mechanism of molten pool during multi-pulse drilling processing is systematically studied firstly, and the correlation between multi-pulse sequences, driving force of molten pool, melt flow and drilling quality is investigated. These provide the necessary theorecial basis to the engineering application of micro-drilling with high efficiency and quality.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/84818
Collection先进制造工艺力学实验室
Recommended Citation
GB/T 7714
张越. 多脉冲毫秒激光打孔熔池动态演化机理研究[D]. 北京. 中国科学院大学,2020.
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