制动盘熔覆层与结合区的激光冲击强化机理及热疲劳特性

In view of the common security risks in high-speed rail brake disc during service caused by the thermal fatigue crack in the combine area (bond area+ heat affected zone) after strengthen by the laser cladding, propose a kind of strengthening method by laser shock processing. The laser shock wave penetrating the cladding layer and effect on the combine area to get the refining columnar crystal structure meanwhile prefabricated residual compressive stress field to meet the demands of improving the thermal fatigue property. Aiming at thermal fatigue working conditions, combining with the finite element numerical simulation and experimental research method, explore the laser shock processing parameters, such as the thickness of cladding layer, energy absorption layer and constraint layer, shock wave energy, spot size, etc. to get the energy utilization mechanism which influence on the grain structure and residual stress field in the cladding layer and combine area; establish the interactive influence of structure strengthening (grain size and distribution) and the stress strengthening (distribution and release of residual stress field) under different temperature amplitude and stress intensity factors on thermal fatigue life and the influence of fatigue crack initiation and propagation behavior; discover the strengthening mechanisms of the cladding layer and combine area by laser shock and the mechanism of reducing the crack sensitivity. The research of this project has both the important theoretical value and practical significant on the development of laser shock processing and the improvement of the thermal fatigue performance of the brake disc.

针对高铁制动盘表面激光熔覆强化后，在服役过程中结合区（熔合区+热影响区）容易产生热疲劳裂纹存在安全隐患的问题，项目提出一种穿过熔覆层作用至结合区并使其柱状晶组织细化的同时预制残余压应力场的激光冲击强化方法，实现一步强韧化熔覆层与结合区，提高激光熔覆强化制动盘热疲劳性能的目的。针对热疲劳工况，采用有限元数值模拟与试验研究相结合的方法，通过对熔覆层厚度、吸收层、约束层、冲击功率、光斑尺寸等激光冲击工艺参数的调控，获得冲击波能量利用机制对熔覆层与结合区晶粒结构和残余应力场的影响规律。研究不同温度幅和应力强度因子下，组织强化（晶粒大小与分布）和应力强化（残余应力场分布与释放）在热疲劳过程中对疲劳寿命的交互作用及其疲劳裂纹萌生与扩展行为的影响规律，揭示激光冲击熔覆层与结合区的强化机理及其降低裂纹敏感性的作用机制。本项目研究对激光冲击工艺的发展和制动盘热疲劳性能的提升具有重要的理论价值和实践意义。

针对高速列车制动盘激光熔覆涂层及其与基材结合区域极易发生热疲劳失效的问题，本项目提出采取激光冲击的方法以实现对熔覆层与结合区的组织细化+预制残余压应力场的双重强化。为此，首先通过调整优化激光功率、送粉量、扫描速度和搭接率等激光熔覆工艺参数，获得了熔覆层厚度、熔覆层表面粗糙度和结合区形貌特征等一系列不同特性熔覆涂层的制备规律，以达到充分发挥激光冲击作用效果的目的。其次，研究了能量为6J～24J激光冲击熔覆涂层后熔覆层与结合区的微观组织结构变化。激光冲击能够显著细化组织，且细化作用随冲击能量的增加而增加。当达到一定细化效果后，继续增加冲击能量作用效果变化不大。关于预制残余压应力场方面，预制压应力的大小、分布与熔覆层组织结构、激光冲击能量密切相关。激光冲击能量越大，涂层表面粗糙度越大。要严格控制熔覆层的厚度，才能确保激光冲击的有效作用至结合区附近。第三，研究了基体试样、熔覆涂层试样和激光冲击涂层试样在室温至600℃的摩擦磨损性能，揭示了磨损失重、摩擦系数、磨损形貌的变化规律，阐明了激光冲击熔覆涂层及结合区的强化机理。最后，研究了基体试样、熔覆涂层试样和激光冲击涂层试样在20℃至600℃的热疲劳性能，揭示了基体试样热疲劳裂纹生长规律，发现了高强高硬涂层会引发脆性裂纹导致涂层的热疲劳性能显著下降，而激光冲击对于延长高强高硬涂层的热疲劳寿命作用非常有限。针对此问题，本项目提出热处理强化和熔覆记忆合金涂层的解决方案并进行了初步试验验证，均取得较好地实验结果。后续将进行深入研究，以获得提高熔覆层与结合区热疲劳性能的更优的解决途径，为激光冲击工艺的应用发展和熔覆涂层热疲劳性能的提升奠定理论基础。