| Nickel-based single crystal(NBSX)turbine blade is typically connected with the powder metallurgy disk using fir-tree joint structure in aero-engine.According to the statistic,up to 20% of structural failures of aero-engine were caused by failure of the tenon joint structure.NBSX superalloys are widely used in aero-engine turbine blades due to its superior fatigue resistance and creep resistance at high temperature,but the fretting fatigue failure is particular serious under the extreme service environment of high temperature,high pressure and high speed.Besides,NBSX superalloys have typical two-phase microstructure,and their mechanical behaviors are related to the crystal orientation,so fretting fatigue damage evolution of which are particularly complicated.So far,most of the researches focus on the fretting fatigue of conventional polycrystalline alloys,and there is a lack of study on the fretting fatigue damage and failure of NBSX superalloys at high temperature.In consideration of the contact form and loading characteristic of tenon joint structure in aero-engine and the microstructure characteristics of the second-generation NBSX superalloy DD6,the present work investigates the fretting fatigue damage mechanism of DD6 and arc-shaped tenon joint structure at high temperature by means of experimental methods and numerical techniques.The main contributions are summarized as follows:1.Experimental investigation on high temperature fretting fatigue of NBSX superalloy.A novel high temperature fretting fatigue test method and fretting fatigue test apparatus are developed.Fretting fatigue tests of NBSX superalloy DD6 in contact with powder metallurgy FGH96 are conducted at four different test temperatures(20 ℃,600 ℃,650 ℃,700 ℃).Fretting fatigue tests under three normal loads(the average stresses of the contact surface are 300 MPa,400 MPa,500 MPa,respectively)and three axial loads(the axial stress are 400 MPa,450 MPa,500 MPa,respectively,and stress ratio is 0.1)at 600 ℃ are also carried out.The results show that fretting fatigue cracks initiate and propagate at the contact leading edge area,where surface delamination,peeling and micro cracks occur.At room temperature,fretting fatigue cracks will initiate and propagate along the direction of 45 ° with respect to the axial loading direction.When the test temperature is above 600 ℃,fretting fatigue cracks initiate along the direction perpendicular to the axial loading direction and propagate for a certain distance,then grow along the direction of 45 ° with respect to the axial loading direction.2.High temperature fretting fatigue damage failure mechanism of an NBSX superalloy.The characteristics and evolution of fretting fatigue damage of an NBSX superalloy are analyzed using scanning electron microscope.In addition,the quasi three-dimensional finite element method is proposed based on the crystal plasticity theory,and the numerical simulations of fretting fatigue behavior of an NBSX superalloy at 600 ℃ are carried out.The numerical results show that the fretting conditions of all tests are partial slip,and the slip area increases with an increase of axial load,while decreases with an increase of normal load.Severe oxidation occurs on the specimen surface due to high temperature,and many micro cracks initiate at the contact leading edge area because of the severe stress concentration.(111)[1(?)0] slip system and((?)1(?))[110] slip system tend to activate alternately,therefore,the fracture shows the characteristics of Mode-I crack macroscopically.When the crack propagates for a certain distance,it will rapidly propagate along {111} planes and eventually result in the final rupture of the NBSX superalloy specimen.3.Fretting fatigue behavior and damage mechanism of arc-shaped dovetail attachment.The high temperature fretting fatigue test method of dovetail attachment has been proposed,and the fretting fatigue tests are conducted at 600 ℃ under three fatigue loads(the maximum fatigue loads are 13805 N,15975 N,18105 N,respectively,and stress ratio is 0.1).It has been found that surface delamination and peeling occur on the dovetail specimen surface,which is accompanied with initiation of micro crack.The numerical results shows that(111)[1(?)0] slip system and((?)1(?))[110] slip system are most easily to activate,which lead to the fretting fatigue cracks initiate and grow along a direction of about 45 ° with respect to the loading direction.The fretting fatigue behavior of dovetail attachment with different contact forms have also been investigated,and it has been found that the contact stress and resolved shear stress are small for the arc-shaped dovetail attachment.In addition,the fretting fatigue life of the arc-shaped dovetail tenon is 1.5 times to that of the plane dovetail tenon when the fatigue load is 13805 N.And the morphology of the fracture surface shows the characteristics of activation of octahedral slip systems.4.Fretting fatigue life prediction method of NBSX superalloy.Resolved shear stress and accumulated dissipated energy are used to describe the degree of activation of slip systems and fretting wear,respectively.It is found that the fretting fatigue life of NBSX superalloy is small under the large resolved shear stress.Besides,with the increase of accumulated dissipated energy,the fretting fatigue life will first decrease and then increase.The effects of both crystallographic slip and wear on the fretting fatigue damage have been comprehensively analyzed,and a damage parameter,RA,is proposed to describe fretting fatigue damage of an NBSX superalloy.The RAbased fretting fatigue life prediction method is developed,the accuracy of which has been proved to be best compared with the previous methods.In addition,the predicted fretting fatigue lives of the DD6 flat specimens and DD6 dovetail specimens agree well with the test results,which basically lie within the ±2N_i scatter band.5.Optimization design method of arc-shaped tenon joint structure(ATJS)based on the free-form meth deformation.A fir-tree tenon joint structure with arc-arc contact form is designed.Based on the theory of free mesh deformation,the mesh parametric design of ATJS is achieved.The design method of ATJS is proposed by considering the crystal orientation and tooth gap.Ten design parameters,that have great influence on the fretting fatigue life,are determined as the design variables according to the results of design of experiments.Subsequently,the Kriging surrogate models are created for fretting fatigue life of tenon and maximum von Mises stress of mortise,respectively,and the optimization design method of ATJS is proposed.The optimal result shows that the stress state between the tenon and mortise has been improved,and the fretting fatigue life of the arc-shaped tenon is increased by 2 times compared with that of the traditional flat tenon. |
PDF Full Text Request