Service Failure Mechanism And Life Evaluation Of Ni-based Superalloy Electron Beam Welding Joints

Electron beam welded structures of superalloys in aero-engines endure harsh conditions such as high temperature,high pressure,and high speed and are susceptible to service damage resulting from high-temperature fatigue,creep,and fatigue-creep interaction.As people’s demands for the performance and lightweight of aero-engines continue to grow,the performance and quality requirements for superalloy electron beam welded structures become more stringent.However,the welding process’ s inevitable introduction of welding defects and microstructure inhomogeneity has complicated the service failure mechanism of superalloy electron beam welded structures,posing significant challenges to the safe and reliable operation of aero-engines.As a result,there is an urgent need to develop and enhance service life prediction and evaluation methods for related welded joints.Therefore,this paper aims to conduct systematic experimental and theoretical research on the electron beam welding joints of high-temperature alloys in a certain type of domestic aero-engine combustor case(GH4169)and flame tube(GH3536),with the goal of uncovering the failure mechanism and developing a life prediction model for service.The research involves material failure experiments under actual service conditions,as well as the use of multi-scale characterization methods to clarify the service failure mechanism of related welded structures under fatigue,creep,and fatigue-creep interaction.An accurate service model is then constructed based on these findings to serve engineering applications.This study is driven by the country’s major needs and is expected to provide guidance and reference for the life extension and life determination evaluation of superalloy welded structures,ultimately contributing to the development of China’s aeroengine industry.The following sdudies are carried out in this disseration:(1)Study on fatigue crack initiation mechanism and crack propagation mechanism of GH4169 electron beam welded joint in casing.Through the in-situ fatigue test system,the initiation and propagation of fatigue cracks were observed in real-time.Results show that the stress concentration at defects initiates fatigue cracks,and the propagation path is significantly influenced by the weld texture.In addition,due to the strength mismatch and microstructure difference of welded joints,the crack growth rate curves of base metal and welded joints cross.Specifically,it was found that the fatigue crack growth rate of the welded joint is low at low stress intensity factors due to the shielding effect caused by weld strength mismatch and the crack closure effect caused by the weld texture.However,under high stress intensity factors,the shielding effect gradually disappears.The base metal with finer grains and higher ductility can coordinate larger plastic deformation and passivate cracks,resulting in better fatigue crack growth resistance than welded joints.(2)Study on fatigue life evaluation method of GH4169 electron beam welding joint under service condition.Firstly,the fatigue experiments of GH4169 electron beam welding joints at 400 ℃,500 ℃ and 550 ℃ were carried out,and the bilinear characteristics of S-N curve of GH4169 fatigue at high temperature were first found.Then,three different fatigue life evaluation methods were established for different application scenarios.In the first method,a fast low-cycle fatigue life prediction model was established considering the influence of temperature and initial defects,which was mainly aimed at the specific working conditions with the maximum stress of 650 MPa and the fatigue life class of100,000 cycles,and the life prediction within ±3 times the dispersion zone could be achieved.In the second method,a fatigue life prediction model considering defects and crack growth is established to realize the fatigue life prediction under each cycle.Finally,considering that the initial defects of large welded structures could not be accurately characterized and confirmed due to the limitation of detection means,probabilistic fatigue evaluation analysis of GH4169 electron beam welded joints was carried out to meet the requirements of engineering applications.(3)Study on high temperature fatigue failure mechanism and creep characteristics of GH3536 electron beam welding joint for flame tube.The study found that after rapid cooling,the weld pool of GH3536 electron beam welding head forms a supersaturated solid solution with developed dendrite and subgrain structure,which eliminates the macro segregation of strengthening elements in the base metal and makes the weld stronger than the base metal.Due to the excellent plasticity of GH3536,it is hardly affected by welding defects,and the welded joints fail in the base metal area under high temperature,normal temperature fatigue or creep.The study also found that the high temperature fatigue damage of GH3536 is mainly grain boundary damage,supplemented by slip deformation,and the room temperature fatigue damage is mainly torsional deformation and slip damage between grains.The Kachanov-Robotnov continuous creep damage model can effectively predict the creep performance of GH3536 electron beam welded joint without considering the influence of welding process.(4)Evaluation of fatigue creep interaction properties and life of GH3536 electron beam welded joint.First,the fatigue creep interaction experiments of GH3536 electron beam welded joints at 650 ℃,750 ℃,850 ℃ and 900 ℃ were carried out to obtain the basic data required for life prediction.Then,the mechanism of fatigue creep interaction failure under different temperatures and loads is analyzed.At 650 ℃,fatigue damage takes the dominant role,at 900 ℃,creep hole damage takes the dominant role,and at intermediate temperature,there is obvious fatigue creep interaction.In addition,at the same temperature,the proportion of fatigue damage increases gradually with the increase of load retaining stress.Finally,taking the average strain rate at half life as the damage parameter,a fatigue creep life evaluation method based on the ductile exhaustion theory was developed,and an effective evaluation of the fatigue creep service life of GH3536 electron beam welded joint was realized.