Calculation And Experimental Study On Brazing Residual Stress Of Lattice Sandwich Structure

With the establishment of energy-saving,emission-reduction and scientific outlook on development,the lattice materials have wider application prospects in the field of aircraft engine,high temperature gas cooled reactor and steam turbine.The manufacture of lattice structure is based on the element diffusion method,which leads a large residual stress existing in the brazed joint and seriously affects the safety and reliability of the equipment in the harsh environment.Because of the complexity of the brazed joint in the lattice structure,it is challenging to test directly the residual stress.This thesis studied the brazing residual stresses of butt joints by the method of numerical simulation and its results were verified by a variety of tests.Then the finite element method(FEM)was employed to study the lattice structure.The microstructure and phase transition of brazed joint were obtained by Energy-dispersive X-ray spectroscopy(EDS)and Electron backscatter diffraction(EBSD).At last,the effects of brazing process and the size of lattice structure on residual stress and deformation were investigated by FEM.The main work and conclusions of the thesis are summarized as follow:(1)Based on the creep constiutive equation of Norton,the creep effect was considered during high temperature and then the residual stress of X type lattice structure was simulated.The residual stress in brazing filler metal was larger than base metal.Due to the stress relaxation,the creep factor could not ignored.According to the simulation results of butt joint,the base metal was in 2D stress state,while the brazing filler metal and the transition region were in 3D stress state.The residual stresses of base metal were measured by X ray diffraction and nanoindentation,while the residual stresses of brazing filler metal were obtained only by nanoindentation.The residual stresses along thickness direction were obtained by neutron diffraction.The results obtained by experiments were consistent with FEM results.(2)The simulated and experimental values measured by nanoindentation are 30% and 77%higher than in base metal,respectively.There were two abnormal test points where the residual stresses value were beyond 500 MPa and larger than braing filler metal.The residual stresses distribution in other test points measured by nanoindentation were consistent with FEM.The hardnesses and elastic moduli were different in different test points.These values in brazing filler metal were slightly higher than the base metal and the hardnesses of diffusion region were the largest.(3)There existing a transition region at the junction between base metal and barzing filler metal.Because the B and Si element diffused form brazing filler metal to base metal and some elements in base metal dissolved to brazing filler metal.Some Cr compounds were formed at the same time.The melting point of brazing filler metal decreased with element diffusion and the isothermal transformation occurred here.The solid solution near base metal first began to have the nucleation and growth towards the center of brazing filler metal.The columnar and equiaxed grains were formed in turn.The lattice type of diffusion region was BCC,while the lattice type of base metal and brazing filler metal were FCC.A large amount of B-Cr compounds in grain boundaries lead to chromium depletion.According to the Schaeffler diagram,the martensitic transformation took place with grain refinement.There were a lot of CSL grain boundaries in transition region and base metal,but little in brazing filler metal.Thus it is easy to appear failure in brazing filler metal induced by environment.The stress/strain state of brazing filler metal and transition region were obtained by EBSD.The residual stresses in grain boundaries of transition region were larger than other position where the stresses were consistent with the nanoindentation results.Due to the element diffusion,the residual stresses of transition region were beyond the brazing filler metal.(4)Based on X type lattice structure,the related factors including size effect and brazing process which affect on the brazing residual stress and deformation were studied.The best parameters were obtained: the panel thickness was 2 mm,the core body thickness was 1 mm,the core width was 2 mm,the platform width was 3 mm.The formation of residual stress was closely related to the inhomogeneity of the temperature inside the material.The residual stress could be reduced effectively by staged heating,while the direct heating should be forbidden.The brazing temperature and holding time had little effect on the residual stress.The cooling rate was the main factor affecting the brazing residual stress.With the decrease of cooling rate,the brazing residual stresses were obviously reduced.Thus the cooling rate should be considered seriously during the brazing process.