【作者】 王强；

【导师】 占小红； 王磊磊；

【作者基本信息】 南京航空航天大学 ， 能源动力， 2023， 硕士

【摘要】 航空发动机被誉为“现代工业皇冠上的明珠”,而叶片则是航空发动机的心脏。TC4钛合金由于其优异的综合性能,被广泛用于航空发动机叶片制造,但由于恶劣的服役环境,钛合金叶片极易发生损伤而失效。目前对于受损发动机叶片的处理方式主要为整体替换,该方法维修成本高且会造成大量材料的浪费。激光熔覆技术作为修复与再制造领域极具发展前景的技术之一,能够在高质量修复的基础上有效降低航空发动机的使用成本。因此,本文基于实验与仿真相结合的方法,开展TC4钛合金激光熔覆修复过程的有限元模拟及组织形貌研究。重点分析了曲面凹槽结构与叶片损伤件熔覆过程中的热、力分布及其变化过程,以期为钛合金叶片损伤修复的工艺参数优化及应力控制提供一定的理论指导。首先,开展了不同工艺参数下的单道激光熔覆实验,获得了熔覆层高度及宽度的分布情况。在此基础上构建了TC4钛合金激光熔覆过程单道及曲面凹槽结构的有限元热-机耦合模型,分析了激光功率、扫描速度及送粉率对单道试样温度场与应力场分布的影响。结果表明,熔覆过程中的温度越高,试样的应力峰值越小,但冷却后的应力值反而明显升高。其次,基于上述结果构建钛合金局部曲面凹槽结构模型,开展了不同扫描方式及搭接率下的多层多道激光熔覆修复仿真研究,提取了不同区域位置热循环曲线与应力历程曲线,分析了温度场及应力场的分布与变化规律。结果表明,修复过程应力主要集中在侧壁、熔覆起始点以及凹槽底部。采用每层反向的摆动扫描方式能够获得更小且对称的应力场。此外,试样应力峰值随搭接率的升高先减小后增大,在50%搭接率时获得最小值724 MPa。然后,采用上述模拟后获得的最佳工艺参数,开展了TC4钛合金局部曲面凹槽构件的激光熔覆修复实验,研究其宏观形貌及熔覆层内部不同区域的组织形态变化。同时,通过无损检测方法获取试样表面及侧面应力分布。结果表明,熔覆层内的组织主要为柱状晶,而在层间区域与顶部则出现了等轴晶。另外,实验与模拟的应力果吻合良好,验证了模拟结果的准确性。最后,对两种不同损伤形式下的钛合金叶片开展激光熔覆修复过程温度场与应力场分析。构建了实际钛合金叶片有限元模型,分析了热循环过程对应力场分布的影响机理,对比了两种损伤形式下的应力分布情况,并开展了实际钛合金叶片损伤件的修复实验。结果表明,熔覆层周围应力随着热循环次数增加而逐渐上升。上表面损伤件修复再冷却后应力达到580 MPa。连接处损伤件叶根处由于榫头的约束,冷却后达到了737 MPa。熔覆过程中拉应力主要分布在修复区上半部分,压应力则分布在下半区域和底部过渡区。更多还原

【Abstract】 The aeroengine is known as the "pearl on the crown of modern industry",and the blade is the heart of the aeroengine.TC4 titanium alloy is widely used to manufacture aeroengine blades due to its excellent comprehensive performance.However,titanium alloy blades are vulnerable to damage and failure because of the harsh service environment.At present,the main treatment method for damaged engine blades is to replace them as a whole,which has high maintenance costs and can cause a large amount of material waste.As one of the most promising technologies in the field of repairing and remanufacturing,laser cladding can reduce the cost of aeroengine effectively on the basis of high-quality repair.The finite element simulation and microstructure of TC4 titanium alloy during laser cladding repair are studied in this paper based on the combination of experiment and simulation.The thermal and stress distribution and its change process in the cladding process of curved groove structure and damaged blade parts are analyzed.It is expected to provide some theoretical guidance for the optimization of process parameters and stress control of repaired titanium alloy blades.First of all,the single pass laser cladding experiments were carried out under different process parameters,and the distribution of the height and width of the cladding layer was obtained.On this basis,the thermos-mechanical coupling model of TC4 titanium alloy laser cladding process with single pass and curved groove structure was established.The temperature field and stress field of single pass sample were obtained and analyzed under different laser power,scanning speed and powder feeding rate.The results show that higher temperature in the cladding process can reduce the stress peak value.But the stress value increases significantly after cooling.Secondly,the simulation of multi-layer and multi-pass laser cladding repair process is carried out under various scanning method and overlap ratio based on the above established model of titanium alloy local curved surface groove structure.The thermal cycle curves and stress curves under different regions were extracted,and the distribution and variation of temperature field and stress field were analyzed.The results show that the stress in the cladding process tends to be concentrated on the side wall,the starting point of cladding and the bottom of the groove.A smaller and symmetrical stress field can be obtained by using the reverse swing scanning method for each layer.Furthermore,the stress first decreases and then increases with the increase of the overlap ratio,and the minimum value of 724 MPa is obtained at 50 % of the overlap ratio.Then,the laser cladding repair experiment of TC4 titanium alloy local curved groove components was carried out using the optimized process parameters and scanning methods obtained from the above simulation.The macro morphology and microstructure of the cladding layer under different regions were studied.At the same time,the stress distribution on the surface and side of the sample was obtained by nondestructive testing.The results show that the microstructure of the cladding layer is largely composed of columnar crystal,and equiaxed crystal is distributed in the interlayer area and the top layer.In addition,the simulated stress results are in good agreement with experimental results,which verifies the accuracy of the established model.Finally,the temperature field and stress field of titanium alloy blades were analyzed during the repair process under the two different damaged forms.The finite element model of the actual titanium alloy blade was constructed,and the mechanism of the influence of thermal cycle on the stress field distribution was analyzed.The stress distribution is compared under the two different damaged forms.What’s more,the repair experiment of actual titanium alloy damaged blade was carried out.The results show that the stress around the cladding layer gradually increases with the increase of the number of thermal cycles.The stress reaches 580 MPa after repairing and cooling the damaged part on the upper surface.Due to the constraint of the tenon,the damaged blade root at the connection reached 737 MPa after cooling.The tensile stress is observed to be distributed in the upper half of the cladding layer,while the compressive stress is distributed in the lower half and the bottom transition zone.更多还原