Research On Defect Prediction And Relationship Between Microstructure And Properties Of Friction Stir Welding

Friction Stir Welding(FSW)technique is widely used in aluminum alloy joint.The FSW joint has some excellent characteristics such as high strength,low residual stress and small distortion.However,there are also many problem of FSW technique,such as internal defects are unpredictable,process window establishment is costly and joint performance is inhomogeneous.In this study,Alloy 7N01 which was used in the high-speed train industry was chosen to be the object,based on the combination of thermo-flow coupling simulation and experimental test,the prediction method of FSW void defect and the microstructure-property relationship were studied.The main research work of this paper is listed below.Properties and constitutive model of 7N01 aluminum alloy.The tensile test between 298 K-773 K showed that the material softening with increasing temperature.The isothermal compression experiments of 7N01 aluminum alloy revealed that alloy has steady rheological characteristics and dynamic softening trend.A flow stress constitutive model which is proportional to the value of the Z parameter was established.The specific heat and thermal conductivity both demonstrated a sharp peak at 573 K which is indicated by thermophysical properties tests.The properties test result and constitutive equation of material are the basis for the FSW thermo-flow coupling simulation.Weld defect prediction and process parameter window estimation.The thermo-flow coupling model of the FSW process was established.The welding temperature was verified by thermocouple temperature measurement and material migration was verified by micro-arc oxidation layer marker insert technique,respectively.The influence of welding process parameters on temperature field,velocity field and material migration behavior were analyzed.Based on the area of the high temperature region and the number of rotating particles through the thickness direction,the formation conditions of the void defect are clarified and the defect prediction method is established.Furthermore,a defect-free FSW process window of 7N01 aluminum alloy was established.The relationship between microstructure and mechanical properties.The microstructure analysis shows that the average grain size,the low-angle grain boundary ratio and the second phase particles dimension at the top of the weld nugget are higher than the middle and bottom.The delamination properties test results show that the microhardness in the top of the weld nugget is significantly lower than that at the middle and bottom,and the tensile properties in the top layer of the joint are also significantly lower than those at the middle and lower layers.Based on the temperature distribution result from top to bottom of weld joint,the top layer has higher welding temperature and larger strain,which leads to the abnormal growth in the recrystallized grains,seriously precipitated and grown in the second phase,significantly reduced in microhardness and tensile properties,the top layer is a weakest part of the welded joint.In summary,the research on material constitutive model,void defect prediction and microstructure-property relationship enriches the basic theory of FSW technology,it has important engineering significance of improving the quality of FSW joints and extending the application of FSW technology in aluminum alloy welding.