多轴应力状态下钎焊接头蠕变损伤与裂纹扩展研究

发布时间：2018-06-18 01:05

本文选题：钎焊接头 + 多轴应力　；参考：《华东理工大学》2016年博士论文

【摘要】：近年来,钎焊技术在航空航天、燃气轮机及燃料电池等高温服役条件下得到了广泛的应用。在高温下,蠕变及蠕变损伤引起的裂纹扩展是导致钎焊接头失效的主要原因之一。钎焊接头中存在的钎焊残余应力、热应力与外加载荷叠加使钎焊接头处于复杂的多轴应力状态,对钎焊接头的蠕变失效产生较大的影响。本文以Inconel625/BNi-2钎焊接头为研究对象,对其多轴应力状态下的蠕变损伤及裂纹扩展性能进行研究,探讨钎焊接头的失效规律,为钎焊接头在高温条件下的寿命预测提供参考。论文的主要研究工作和相关结论如下：(1)对钎焊接头中的基体材料Inconel625合金和钎料BNi-2的蠕变性能进行了试验研究。考虑尺寸效应,特别针对钎料提出了一种与钎焊接头中钎料厚度相类比的小试样。通过高温单轴拉伸(常规试样、小试样)试验分别获得了Inconel625合金和钎料BNi-2两种材料的蠕变应变曲线,并基于Norton方程拟合获得相应的蠕变本构参数,为蠕变损伤本构模型参数的拟合和有限元分析提供了基础数据。(2)结合Kachanov-Rabotnov蠕变损伤模型的优点,通过修正Liu-Murakami模型,建立了描述多轴应力状态下蠕变三阶段的本构模型。基于该模型,利用ABAQUS的CREEP模块嵌入Fortran子程序,对Inconel625合金蠕变裂纹扩展行为进行有限元分析,并通过试验进行了验证。结果表明,修正的蠕变损伤模型可准确描述Inconel625合金蠕变裂纹的扩展情况,解决了Kachanov-Rabotnov模型有限元计算收敛困难的难题,同时避免了Liu-Murakami模型因分析过程中σ1/σeq比值过大或比值小于0而出现与实际情况不符的问题。(3)对钎焊接头Inconel625/BNi-2的裂纹扩展行为进行了研究。根据微观金相组织,将钎焊接头分成母材、扩散区和钎料三个区域。考虑残余应力和热应力的影响,基于上述修正的模型,通过赋予各区域不同属性,对钎焊接头的蠕变损伤及裂纹扩展行为进行了有限元分析。进而采用紧凑拉伸(CT)试样对钎焊接头进行试验研究,获得了钎焊接头的失效区域及蠕变裂纹扩展速率,证明了该修正模型可准确描述钎焊接头蠕变裂纹扩展行为。研究发现扩散区的性能对钎焊接头的蠕变裂纹扩展行为具有较大的影响,提高扩散区的蠕变应变率有利于延长钎焊接头的使用寿命。(4)对钎焊接头蠕变裂纹扩展行为的影响因素进行了参数化分析。对不同钎料厚度、试样尺寸、残余应力和热应力下的钎焊接头蠕变裂纹扩展行为进行了有限元分析,结果表明,适当增大钎料厚度和试样尺寸均有利于提高裂纹扩展的孕育期；消除残余应力不但能够极大地提高钎焊接头的裂纹扩展孕育期,同时还大大降低了裂纹扩展速率,有利于提高钎焊接头的使用寿命；而钎焊接头中热应力的存在改变了接头中的应力分布,对于延长其使用寿命是有利的。(5)基于上述本构模型及研究方法,本文以一航空发动机回热器的设计为例,对回热器整体结构在设计工况条件下(650℃、3MPa)的蠕变损伤和使用寿命进行了模拟分析。结果显示,回热器结构失效位置位于钎缝区域,连续运行34,900小时后,裂纹贯穿整个钎料区域,不能达到连续工作40,000小时的设计要求。当换热主管的壁厚增加50%,连续工作40,000小时后钎料区域的裂纹长度为1.1mm,约占管壁壁厚的1/3左右。此时,回热器能够继续正常工作,满足航空发动机安全运行的要求。然而寿命的延长是以重量的增加为代价的,整个回热器的重量增加约10%。所以进一步提高钎料的抗蠕变性能是提高航空发动机回热器强度、降低重量的重要发展方向。
[Abstract]:In recent years, brazing technology has been widely used in high temperature service conditions such as aeronautics and Astronautics, gas turbine and fuel cell. At high temperature, the crack propagation caused by creep and creep damage is one of the main causes of brazing joint failure. The brazing residual stress, thermal stress and external loading charge are superimposed on the brazing joint to make the brazing filler metal. The welding head is in a complex multi axis stress state, which has a great influence on the creep failure of brazed joint. This paper takes the Inconel625/BNi-2 brazing welding head as the research object, studies the creep damage and crack propagation performance under multi axis stress state, and discusses the failure law of brazing joint, which is the life of brazed joint under high temperature. The main research work and the relevant conclusions of the thesis are as follows: (1) the creep properties of the base material Inconel625 alloy and the solder BNi-2 in the brazed joint are studied. Considering the size effect, a small sample of the brazing joint is presented in particular to the brazing joint in the brazing joint. The creep strain curves of two kinds of Inconel625 alloy and BNi-2 are obtained by the tensile test (conventional and small sample), and the corresponding creep constitutive parameters are obtained based on the Norton equation. It provides the basic data for the fitting of the creep damage constitutive model parameters and the finite element analysis. (2) combined with the creep damage model of the creep damage model. By modifying the Liu-Murakami model, a constitutive model describing the three stage of creep in a multiaxial stress state is established. Based on this model, the finite element analysis of the creep crack propagation behavior of the Inconel625 alloy is carried out by using the CREEP module of the ABAQUS to be embedded in the Fortran subprogram. The results show that the modified creep has been carried out. The variable damage model can accurately describe the creep crack growth of Inconel625 alloy and solve the difficult problem of the convergence difficulty of the finite element calculation of the Kachanov-Rabotnov model. At the same time, it avoids the problem that the Liu-Murakami model is too large or the ratio is less than 0 in the analysis process. (3) Inconel625/ for the brazing joint. The crack propagation behavior of BNi-2 was studied. According to micrometallographic microstructure, the brazed joints were divided into three regions: the base material, the diffusion zone and the solder. Considering the influence of the residual stress and thermal stress, the creep damage and the crack propagation behavior of the brazed joint were finite-element based on the modified model. Then the brazed joint was tested by the compact tension (CT) specimen, and the failure region and the creep crack propagation rate of the brazed joint were obtained. It was proved that the modified model can accurately describe the creep crack propagation behavior of the brazed joint. The effect of increasing the creep strain rate of the diffusion zone is beneficial to prolonging the service life of the brazed joint. (4) the factors affecting the creep crack propagation behavior of the brazed joint are parameterized. The finite element analysis of the creep crack propagation behavior of the brazed joints under the thickness of the solder, the size of the sample, the residual stress and the thermal stress is analyzed. The results show that the proper increase of the thickness of the solder and the size of the sample can improve the incubation period of the crack growth, and the elimination of residual stress can not only greatly improve the incubation period of the crack propagation of the brazing joint, but also greatly reduce the rate of crack propagation and improve the service life of the brazing joint. In changing the stress distribution in the joint, it is beneficial to prolong its service life. (5) based on the above constitutive model and research method, this paper takes an aeroengine regenerator design as an example to simulate the creep damage and service life of the regenerator overall structure under the design condition (650 degrees, 3MPa). The results show that The structure failure position of the regenerator is located in the brazing seam area. After 34900 hours of continuous operation, the crack runs through the whole solder area and can not meet the design requirements for continuous work for 40000 hours. When the wall thickness of the heat exchanger is increased by 50%, the length of the crack in the solder area is about 1.1mm after 40000 hours of continuous work, and it is about 1/3 of the wall thickness of the tube. The device can continue to work properly to meet the requirements of the safe operation of the aero engine. However, the prolongation of the life span is at the expense of the increase of the weight. The weight of the whole regenerator increases about 10%., so the further improvement of the creep resistance of the solder is an important direction for improving the strength of the regenerator and reducing the weight of the aero engine.
【学位授予单位】：华东理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG454

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