DSI/DIC Based Studies On The Local Mechanical Behaviors Of Austenitic/Ferritic Type Dissimilar Welded Joint

Dissimilar metal welding can fully utilize the performance advantages of each constituent material and were increasingly widespread in the advanced equipment manufacturing.Due to significant differences in chemical composition and physical properties between the weld metal and the base metal on both sides of the joints,the local microstructures and mechanical properties of dissimilar welded joints were more complex,and these welded joints often became the weak links in welded structures.As a typical type of dissimilar metal welded joint,austenitic/ferritic dissimilar welded joints have been widely used in engineering structures.Although there has been considerable research on their macroscopic mechanical properties,there was still a lack of systematic research on their local deformation behaviors,microstructural evolutions and mechanical property variations.There was also a lack of indepth understanding of the formation mechanism of the local weak areas.Therefore,combined with the theoretical analysis and numerical simulation,the austenitic/ferritic dissimilar welded joints — HR3C/T92 welded joints were selected as the research object to study the local deformation behaviors,mechanical property variations and the micro-mechanisms in the aswelded state,tensile loading and fatigue loading process by using various advanced testing techniques.The main research contents and results are as follows:To measure the local mechanical properties of dissimilar welded joints,a prediction model of elastic-plastic properties was established based on the depth-sensing indentation(DSI)technique and multi-fidelity neural networks for power-law and linear hardening materials.The relative error of the model was shown to be within ±5% by comparison with experimental results.Combined with the elasto-plastic finite element simulations of indentation,it was further demonstrated that the indentation characteristic parameters(hf /hmax)can not only reflect the influence of the mechanical properties on the indentation contact morphologies,but also the non-linear effects of the tensile and compressive residual stresses.Accordingly,a calculation model for tensile and compressive residual stresses was constructed in this paper by using the basic parameter hf /hmax,and the accuracy of this expression was verified with experimental datasets.For the HR3C/T92 dissimilar welded joints in the as-welded state,the distribution characteristics of the metallographic structures,grain orientations and dislocation structures were studied by using various microscopic analysis techniques including optical microscopy,electron backscatter diffraction and transmission electron microscopy.Based the established multi-fidelity neural networks,the microhardness and elastic-plastic mechanical properties(elastic modulus,yield strength and hardening parameters)distribution characteristics of the HR3C/T92 welded joint were investigated by the DSI tests.The effects of welding thermal cycles on the elastic and plastic properties were also analyzed for the typical regions of the welded joint.To measure the local residual stress distribution in the welded joints,the DSI tests were carried out on the welded joint in the as-welded and annealed states,respectively.The micro-scale residual stress distribution across the welded joint were obtained by the established residual stress calculation model,and the results were compared with those of X-ray diffraction tests.By combining the digital image correlation(DIC)and DSI techniques,the local deformation behaviors and mechanical property variations of HR3C/T92 dissimilar welded joints were investigated during the tensile loading.The macro-scale DIC measurements showed that the strain localization in the welded joint transferred with the increasing applied strains.The plastic strains were initially localized in the HR3 C base material zone,then transferred to the T92 over-tempered zone,and eventually causing failure of the welded joint.The microscale DIC analysis further revealed that the transfer of strain concentration was closely related to the microstructural evolutions of the characteristic areas in the welded joint.In addition,the variations of the local mechanical properties of the welded joints were determined during the tensile deformation process based on the DSI measurements and multi-fidelity neural networks.The results showed that as the tensile strain increased,the elastic modulus tended to decrease,while the yield strength tended to increase;the hardening modulus almost remained constant in HR3 C and WM,and the strain hardening exponent decreased in the regions of T92.Finally,based on the local mechanical properties of the characteristic areas in the welded joint determined by the DSI test,the tensile curve and local strain distribution of the welded joint were simulated using the elasto-plastic finite element technique and compared with the DIC test results.To investigate the local deformation behaviors and mechanical property variations during the fatigue loading process,the DIC and DSI measurements were performed on the HR3C/T92 dissimilar welded joints under different fatigue loads.The macro-scale DIC tests showed that the cyclic mechanical response and failure location of the welded joint were closely related to the applied stress: at low loads,the fatigue damage occurred mainly in the HR3 C base material zone;while at high loads,the fatigue failure tended to locate in the T92 over-tempered zone.The micro-scale DIC measurements further revealed that the load dependence of the fatigue damage location in the welded joint was mainly related to the cyclic slip deformations in the local areas of the welded joint.Based on the DSI measurements and multi-fidelity neural networks,the local mechanical properties of the characteristic zone in the welded joint were determined during the fatigue loading process.The results showed that the elastic moduli and hardening parameters decreased with the increasing cycles,which indicated the reduction of stiffness and ductility of material during the cyclic loading process;The microhardness and yield strengths increased with the increasing cycles,which was related to the strain hardening of material during the cyclic loading process.In the end,combined with the local mechanical properties measured by the DSI tests and the Cowper-Symonds model,the cyclic plastic deformation responses of welded joint under fatigue loading were simulated by using the elastoplastic finite element technique.The results showed that the simulated plastic strain-cyclic number curves and local strain evolutions were consistent with the experimental results.The experimental and theoretical work of this paper not only pioneered a new approach to investigate the local mechanical behaviors of dissimilar metal welded joints,and also provided an analytical basis for the further development of strength design and reliability assessment methods for welded structures.