Numerical Simulation Of Fatigue Crack Initiation Considering Microstructure Characteristics Of Electron Beam Welded Joints

As an excellent joint processing technology,electron beam welding(EBW)is widely used in aerospace manufacturing,especially in the important bearing components of the new generation of aero-engines.This also puts higher requirements on the evaluation of fatigue properties of such components.In the study of structural fatigue,it is difficult to reveal the physical mechanism and process of fatigue damage,and it takes a lot of time and cost by phenomenological model.Therefore,it is a hot research topic to study fatigue failure from the microscopic point of view by numerical simulation.In this paper,the TC4-DT titanium alloy EBW joint is taken as the research object,considering the influence of microstructure non-uniformity in the weld zone(WZ),based on the dislocation slip theory,crack initiation of EBW joint is studied numerically and experimentally.Firstly,based on the traditional Voronoi diagram method and the modified Voronoi diagram method,the equiaxed crystal structure of the TC4-DT base material(BM)region and the mixed crystal region of the EBW joint(including columnar crystal,fine equiaxed crystal and coarse equiaxed crystal)geometric model of the microstructure.The established geometric model agrees well with the metallographic test results.Then the self-written pre-processing program is used to introduce the above-mentioned geometric model into the finite element software ABAQUS to lay the foundation for crack initiation simulation.In this paper,the second development of ABAQUS is combined with the Tanaka-Mura dislocation slip theory and the finite element method.The grain orientation and slip band are imparted to the established microstructure model,and the finite element model of the mixed crystal zone of TC4-DT BM and EBW joint is established.The connection and combination of cracks at the grain boundary are established,the Tanaka method has been improved.Based on the improved simulation method,the fatigue crack initiation process of equiaxed crystal structure in BM of TC4-DT was numerically simulated.The effects of different load levels and grain size on the fatigue crack initiation life were studied.In addition,The fatigue crack initiation process of the welded joint zone was also numerically simulated in the paper.The effects of different load levels on the crack initiation position of the weld zone were studied.The influence of grain size on the crack initiation life of the heat affected zone was investigated.By contrast,a numerical simulation method of fatigue crack initiation considering the microstructure of EBW joints was established,which can predict the crack initiation life and crack initiation position of EBW joints more accurately.Finally,the TC4-DT EBW joint test piece with semi-circular notch was designed and processed.The fatigue damage process of the test piece was interrupted and observed.The fatigue crack initiation process in the WZ was observed.The sub-model technique is used to simulate the fatigue crack initiation process in the WZ of the notched specimen.The simulation results are compared with the test,and the rationality of the numerical simulation method established in this paper is preliminarily verified.