Modelling And Analysis Of Dynamic Recrystallization In Friction Stir Welding Of Al Plates

AS a kind of solid-state welding method,friction stir welding(FSW)is quite appropriate for Aluminum welding.It has been widely accepted that the microstructure in welded joint has great influence on service performance of the joint.Dynamic recrystallization(DRX),as a major metallurgical mechanism in FSW,could strongly affect the microstructure in welded joint.Consequently,the research of DRX during the welding process could be valuable for control the microstructure in welded joint and improve its performance.However,the properties of aluminum and complexities of deformation in welding process make the DRX behavior in FSW very complex.Up to now,there have been some researches on DRX behavior in FSW based on the after-welding microstructure.Limited by experiment methods,it is difficult to investigate microstructure evolution in welding process directly.Computer modelling,without limitations in experiment method,could let researchers to investigate DRX behavior and microstructure evolution in welding process.In order to achieve the modeling DRX process in FSW,A model based on the Monte Carlo method has been established for DRX modelling.According to the exited Monte Carlo DRX modeling researches,an nuclei model appropriate for Monte Carlo method has been built based on Bailey-Hirsch theory.Then an inhomogeneous distribution model of store energy in grains has been achieved via the relation among stress,store energy and grain size.At last,to make the Monte Carlo model can be applied in real welding process,the Monte Carlo step has been calibrated with real time.Then,Finite element method has been used to stimulate FSW process in different welding parameters.The result getting from FEM modelling agreed well with exited researches.According from modelling result,when the heat input become higher,the zone with maximum strain will move to the bottom of welded joint In addition,the gradient of strain rate could be extremely high and strain rate gradient in advancing side is higher than retreating side.After gotten the necessary deformation data in FSW for DRX modelling,the Monte Carlo modelling has been launched to model dynamic recrystallization process in diferent welding parameters.Welding experiment has also been carried out and the microstructure around the pin and in the joint have been characterized via EBSD technique.Comparing modelling results with EBSD results,it has been approved that the Monte Carlo modelling results agreed well with experiment and can reflect the regularity of microstructure changes and distribution in welding process and welded joint.Finally,the dynamic recrystallization behavior in aluminum FSW has been compressively analyzed by combination of FEM,DRX modelling and EBSD testing results.It has been found that the dynamic mechanism could be variational in different stages of plastic flowing in FSW process.When the material flows through the leading side of pin,the influence of geometric dynamic recrystallization(GDRX)and continuous dynamic recrystallization(CDRX)could be obvious.In the retreating side of pin,continuous and discontinuous dynamic recrystallization(DDRX)could be obvious.And in the tail side of pin,the effect of geometric dynamic recrystallization could be obvious.When heat input getting higher,GDRX and growth of DRX grain can easily happen and DDRX can also occur in some regions.When heat input getting lower,CDRX is easy to occur.What’s more,as the consequence of the difference of strain rate gradient and temperature in the advancing and the retreating side,Thermal-Mechanically affect zone will be wider and the microstructure difference will be less obvious in retreating side.