|Alternative Title||Dynamic evolution mechanism of molten pool during multi-pulse drilling with a millisecond laser|
|Place of Conferral||北京|
|Keyword||激光打孔 质量迁移 匙孔演化 熔池驱动力 时空传输特性|
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.
|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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