Creep Damage And Crack Growth Analysis Of The Brazed Joint Under Multi-axial Stress State

In recent years, the brazing technology has been widely used in the aerospace, gas turbine and fuel cell and so on. The crack growth caused by the creep and creep damage is one of the main failure modes for the brazed joint at high temperatures. The superimpositions of the brazing residual stress, thermal stress and the stress from the external loads produce a multi-axial stress state in the brazed joint, and it has a remarkable effect on the failure of the brazed joint. Therefore, the present thesis, focused mainly on the brazed joint of Inconel625/BNi-2. The creep damage and crack growth under the high temperature and multi-axial stress state were stuided in order to explore the failure behavior of the brazed joint caused by the creep damage and crack growth. The result can be used to predict the life of the brazed joint working at high temperature. The main work and conclusions of the thesis are summarized as below:(1) The creep performance of the substrate metal and the brazing filler metal were tested. Considering the size effect of the brazing filler metal, a small specimen, which thickness can be compared with the thickness of brazing filler metal in the brazed joint was proposed in the testing procedure. The creep strain curves of the two materials were obtained by the high temperature uniaxial tensile test (conventional specimen and small specimen), respectively. And Norton creep constitutive equations of these two materials were also acquired. The obtained data is used for the fitting of the parameters in the creep damage constitutive model and for finite element analysis in following sections.(2) Combined with the merits of the Kachanov-Rabotnov creep damage model, a modified Liu-Murakami creep damage model which can describe the three stage of the creep process under multi-axial stress state was proposed. Based on the modified creep damage model, a subroutine was compiled using the Fortran language and incorporated into the ABAQUS by the module of CREEP. The creep crack growth of Incone1625 was analyzed by the finite element method and then verified by the experimental data. The results show that creep crack growth behavior corresponds well with the experimental data. Meanwhile, the modified model avoids the convergence difficulties of the Kachanov-Rabotnov creep damage model and the abnormal phenomenon caused by the ratio of σ1/σeq too large or less than 0 in the Liu-Murakami constitutive model.(3) The creep crack growth (CCG) behavior of the brazed joint Inconel625/BNi-2 was studied. According to the microstructure analysis, the brazed joint is divided into three regions:substrate, diffusion zone and brazing filler metal. Based on the modified model, considering the residual stress and thermal stress, the creep damage and crack growth of the brazed joint was analyzed by the finite element method and the material properties of the three regions are included. And then, using the CT specimen, the failure region and CCG of the brazed joint were obtained by the experimental test. Compared with the experimental data, the modified model can accurately describe the creep crack growth behaviors of the brazed joint. The results also show that the mechanical properties of the brazing diffusion zone have a large effect on the creep crack growth of the brazed joint, and increasing the creep strain rate can prolong the life of the brazed joint(4) Based on the proposed model, the related factors which affect on the creep damage and crack growth behavior of the brazed joint were studied. The effect of the different brazing filler metal thicknesses, specimen sizes, residual stresses and thermal stresses on the creep crack growth behaviors of the brazed joint were analyzed by the finite element method. For the brazed joint of the Inconel625/BNi-2, reasonably increasing the thickness of the brazing filler metal and specimen size can improve the incubation time of the crack growth; Releasing the residual stress generated in the brazing process can greatly extend the incubation time of the crack growth and decrease the creep crack growth rate; while the thermal stress produced at the start-up period of the working conditions is beneficial to the strength of the brazed joint.(5) Based on the above constitutive model and theoretical methods, the creep damage and the life of an aero-engine recuperator were simulated at the design operation conditions (650 ℃,3 MPa). The results show that the failure region of the recuperator is located in the brazing filler metal of the brazed joint. When the creep time is 34,900 h, the crack runs throughout the whole region of the brazing filler metal and the leaking phenomenon occurs. So the recuperator does not meet the requirements of continuous operation for 40,000 hours. When the mainfold wall thickness of the recuperator increases by 50%, the crack length in the brazing filler metal is about 1.1 mm with the creep time of 40,000 h, taking up to about 1/3 of the wall thickness. The recuperator can still work normally in this circumstance, and meeting the design requirements of the aero engine. However, the prolonged life is achieved at the cost of the increasing weight of the recuperator, which is about 10% of the total weight of the recuperator. Therefore, improve the creep performance of the brazing filler metal is the key issues to increase the strength and reduce the weight of the recuperator.