Research On Constitutive Relationship And Fatigue Crack Initiation Of Ti₂AlNb Alloy Welded Joints At Elevated Temperature

The Ti₂AlNb-based alloy is an promising hot section material which is often used to manufacture high temperature structural components like casing of aero-engine.Due to that the high temperature casing is with complex structure,the electron beam welding?EBW?technology is usually applied in the processing of this component.The EBW technology is a vital welding method which has lots of advantages,such as with high power density,narrow fusion zone and small welding deformation.However,the local heat input leads to complicated phase changes and re-crystallizations within the welded joint and results in mechanics inhomogeneity.The fatigue and damage behavior of the Ti₂AlNb EBW joint is closely related to the mechanical properties of different zones of joint,and studies on the mechanical properties of Ti₂AlNb EBW joints at elevated temperature is necessary amd will offer a reasonable way in failure assessments of the joint or even relavant structures.The main contents of the paper are as follows:In order to obtain the mechanical properties of distinct microzones of Ti₂AlNb EBW joints at evelated temperature,a reverse method based on the finite element method and the Ludwick hardening model was introduced to establish the relationship between the load-depth curves of high-temperature nanoindentation test and the Ludwick hardening model.This reverse analysis process aims at sharp tips intenders,which is most commonly used in current high-temperature nanoindentation test.Based on the reverse analysis method,the nanoindentation tests for the various microzones?WM,HAZ,BM?of the Ti₂AlNb EBW joint were conducted from 25?to 500?and the distribution of the mechanical properties within the joint at corresponding tmeperatures are measured.Furthermore,a new unified Ludwick hardening model which can be applied to different microzones of the Ti₂AlNb EBW joints is established.In order to validate the new elastoplastic constitutive model,the specimens from different microzones?WM,HAZ,BM?were prepared for the high temperature micro-tensile tests at 500?.The test results are in good agreement with the ones from the reverse analysis.For the sake of further investigating the fatigue and meso damage behavior of the Ti₂AlNb EBW joint,the fatigue crack initiation and propagation in the microzones of the joint were studied by the the in-situ fatigue testing,and a critical initiation crack length is determined.Based on the Tanaka-Mura dislocation model,the numerical simulations of fatigue crack initiation in different microzones were conducted,in which submodel method from finite element method was applied.