Experimental Study And Numerical Simulation On Cyclic Deformation Beaviour Of Aluminum Alloy Welded Joint

Aluminum alloy welded joints are widely used in the vehicle body of high-speed trains,they are often subjected to a certain cyclic loading in service,which can cause the ratcheting effect and accelerate the fatigue failure of structural components.In addition,since the welding is a rapid process of heating and cooling,the thermal cycle of welding in each area of welded joints is significantly different,and the base material and the filled metal component are often not identical and makes the welded joint heterogeneous in composition,organization and mechanics performance,which has an important influence on mechanical behavior of welding structures.Therefore,there is great practical significance and science value to study the cyclic deformation behavior of the aluminum alloy welded joint and evaluate the safety in service of the vehicle body of high-speed trains by combining the microstructures and mechanical properties of welded joints.The main contents are summarized as follows:(1)Microstructure and hardness distribution of two kinds of aluminum alloy welded joints are observed by the optical microscope,scanning electron microscope(SEM),EDS energy spectrometer and micro-hardness tester,respectively.The tensile mechanical properties and strain field distribution of the welded joints are investigated by the DIC technology,and the tensile fracture morphology of the two welded joints is observed by SEM.The results show that an equiaxed grain structure is found at the center of specimens and the columnar crystal structure is found at the edge of specimens.There are many reinforced particles in the welded zone of 6005A-T6 welded joints,the base metal is a completely recrystallized structure,the fusion zone and the heat-affected zone has a similar distribution with reinforced particles.Some heterogeneous coarse reinforced phases appear in the heat-affected zone near the base metal,where the lowest hardness point exists,and the tensile sample breaks is the weakest area of welded joints.The weld zone of A7N01-T4 weld joints are the typical as-cast microstructure consisting of coarse equiaxed grains and columnar crystals,and there is more segregation in the weld zone.The lowest hardness and the lowest tensile strength are found in weld zone,and the tensile specimen also breaks at the weld zone as the weakest area of the welded joint.The grain in the heat-affected zone is coarse,and a coarse reinforced phase precipitates.As high-strength performance zone for welded joints,the base metal is composed of slender fibrous grains and dispersed and finely reinforced phase are found in base metal.(2)Cyclic soft-hardening characteristics and ratcheting behavior of two kinds of welded joints are experimentally investigated,and results show that the 6005A-T6 and A7N01-T4 welded joints exhibit cyclic stability characteristics under symmetric strain controlled loading.Specifically,the accumulation of plastic deformation of welded joints under different asymmetric stress cyclic loadings is not obvious,which implies that the welded joints have a strong ability to resist ratcheting deformation,and a ratcheting shakedown state occurs.Since the cyclic deformation behavior of welded joints is dominated by the weld zone,the cyclic deformation of welded joints can be considered as a result of Orowan’s reinforced mechanism: a larger size of the precipitated phase is found in the weld zone.Since these precipitated phases can only be bypassed during dislocation movement,strain homogenization occur when dislocations bypassing the precipitated phase,and results in homogeneously distributed dislocation entanglements,which hinders further dislocation movement and results in a deformation hardening.As a result,the entire welded joints show a cyclic stability under cycle loading.(3)Based on the homogeneity assumption of different partition zones,the finite element models of two kinds of welded joints of aluminum alloy are established,and the cyclic deformation behaviors are simulated by a rate-dependent metal plastic constitutive model(viscoplastic constitutive model)including in finite element software ABAQUS,the simulations are a good agreement with the experiments.The proposed analysis method provides a theory guide for optimization of welding technology.