高压淹没水射流强化材料表面机理及尺度效应研究

Jet flow is considered as the key point of the investigation of surface strengthening by water jet. Planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) techniques are applied to measure quantitatively the flow velocity, jet stream fluctuation, flow structures and their evolution and shear layer cavitation in the jet flow under the jet pressure level of 200 MPa. With TC4 titanium alloy being used as the impinged material, surface strengthening experiment is conducted under different jet pressures, stand off distances and submerged depth respectively. Microstructures and properties are analyzed and surface profiles are measured and analyzed statistically with three-dimensional optical profiling system. numerical simulation of the transient variation of stress and strain is performed with explicit dynamics method. correlation among shear layer cavitation, flow structures and surface profiles is expected to be established through the study. Mathematical model describing the surface strengthening due to shear layer and flow structures of different scales is expected to be built too. The method of differentiate the strengthening between jet impingment and cavitation strengthening, and the micro mechanism of the influence on properties of TC4 titanium　alloy by high pressure submerged water jet are also expected to be established. The study will provide helpful conclusions for cavitation peening and machining techniques for alloys and enhance the understanding of the mechanism of submerged water jet.

以对流场的研究作为水射流强化材料表面机理研究的切入点，运用平面激光诱导荧光法和粒子图像测速技术对200MPa量级高压淹没水射流流场中的流动速度、流束脉动、流动结构及演化特征、剪切层空化形态进行定量测量。进而以TC4钛合金作为研究对象，改变射流压力、靶距与淹没深度，对试样进行射流强化实验。对强化过的试样表面进行显微组织分析与性能测试，采用三维轮廓扫描技术对材料表面的形貌尺度进行描述与统计，用显式动力学方法对材料表面的应力和应变的瞬态变化过程进行数值模拟。本研究拟建立与材料表面形貌相关联的射流流动结构与剪切层空化形态描述方法，获得不同尺度流动结构和剪切层空化对TC4钛合金进行强化的数理模型，建立从材料表面特征识别射流流束冲击强化和空化强化的方法，获得高压淹没水射流影响TC4钛合金性能变化的微观机制。本研究为合金材料的空化强化和射流加工工艺提供有力的支撑，并提升对淹没射流作用机理的认识。

项目研究过程中，对小尺度淹没水射流进行了实验研究与数值模拟研究。通过光学无接触式流动测量，获得了1.5 mm流束直径高压水射流流束中的速度分布与沿程衰减规律，进而利用水射流对Ti-6Al-4V试样进行了冲击实验，获得了试样受水射流冲击后的表面属性变化，试样的测量结果间接反映了淹没水射流的流体动力学行为。通过X射线测定了被水射流冲击的试样表面的残余应力，通过三维光学轮廓扫描技术观测了被冲击试样的表面形貌。借助ANSYS软件数值研究得到了水射流流场中的空化区形态，证实了空化区的产生是由于水射流流场中射流束与环境介质之间的剪切作用所致。试样表面的定量形貌特征也证明了水射流对材料的破坏作用是由于空化泡的溃灭与反复冲击作用所致。结合研究结论对已有的淹没水射流理论进行了完善。项目研究过程中，发表研究论文7篇，其中SCI检索4篇，出版专著1部，培养研究生3名，其中2人已毕业。