Establishment And Application Of Criterion For Metal Flow Behavior In Friction Stir Welding

Friction stir welding(FSW)is a plastic deformation process with high stress and high strain.The welding process is the result of the coupling of temperature field and stress field.At present,the independent research on the thermal coupling process has been highly summarized,and scholars have begun to pay attention to the research of thermal coupling in welding process.Therefore,in this paper,aluminum alloy 1100plate is taken as the research object.By means of experimental research and mathematical model analysis at the same time,the mathematical model of torque and viscosity based on the law of metal plastic flow is studied.Based on the metal plastic flow,a dynamic detection method for weld defects is proposed.The main contents and results are as followsBy replacing yield stress with flow stress,and combining the mathematical model of strain rate and torque with metal flow,the mathematical model of torque related to dynamic viscosity was established by using flow stress instead of yield stress.The torque calculation model was checked by dimensionless analysis method and 1100friction stir welding experiment of aluminum alloy Within the allowable range of error,the accuracy and universality of the mathematical model of torque based on metal flow behavior are proved.In the analysis of metal flow behavior,it is found that there are two plastic deformation mechanisms in friction stir welding:PLC(portevin--Le Chatelier)effect and dynamic recrystallization.Through the study of PLC effect and dynamic recrystallization,the relationship between strain and strain rate and the relationship between strain and stress in plastic deformation are found respectively.Based on the two laws and truton viscosity model,a thermodynamic coupling criterion of metal flow behavior in friction stir welding(FSW)is established.The new torque mathematical model is checked by dimensional analysis method,and the results are consistent with the basic theorem of dimensional analysis method.The results show that the dynamic viscosity range of friction stir welding is in the range of?=10~4-10~7pa·s.By analyzing the torque data of friction stir welding(FSW)with high frequency sampling,it is found that the torque of friction stir welding shows periodic variation characteristics.Based on the above research on metal flow behavior,a dynamic detection method of weld hole defects based on the periodic variation characteristics of torque is proposed.This method is one of the methods for welding stability stage through high-frequency sampling of torque A cycle is divided into two stages of torque decreasing from large to small a and increasing from small to small.By judging whether the time ratio A/B of two stages A and B is between 0.8 and 1.3,it indicates that there is no cavity defect in this interval.The reliability and feasibility of this method are verified by experiments on aluminum alloy 1100 under different process parameters,but there are also limitations.