Numerical Simulation Of Microstructural Evolution And Mechanical Property Calculation Of Aluminum Alloy Friction Stir Welding

Friction Stir Welding(FSW)is a relatively new solid state welding technology.Due to its advantages,e.g.low welding temperature,low welding distortion,no pollution,etc,FSW has replaced many conventional welding techniques in a variety of industrial applications.FSW shows its advantages in light alloys such as A1 alloys and Mg alloys.The effective simulation of FSW process with validation from experiments is very important to the optimization of welding parameters.The combination of finite element model of FSW with material science and thermodynamics provides a new method for the integrated researches on the processing—parameter—mechanical properties.A large number of repetitive experiments can be avoided by using the effective numerical methods,which can be used to extract the regularity and diversity from the welding process under different conditions.Based on the above,it is necessary to establish numerical model to study the friction stir welding,which provides a meaningful reference and guidance for the use of the practical welding technique.In this research,based on the fully coupled thermal-mechanical model,an integrated modeling framework on FSW involving the microstructure,strength and strain hardening evolution has been established and applied to the widely used 6xxx series Al alloys.The microstructure evolution changes including the grain size and precipitation in different areas of the weld joint during the FSW process have been studied,followed by the further study on the post-weld mechanical property and the effect of the post-weld heat treatment.Firstly,by using the FEM software DEFORM-3D,a fully coupled thermal-mechanical model for friction stir welding is established based on the adaptive remeshing technique.During the simulation of the welding process,the point tracking technique is used for analyzing the material flow within the section area around the pin.predicting the shape of the stirring area within the section of the joint as well as the grain size within the nugget zone after reciystallization under different welding conditions,also comparing the grain sizes along the direction of the thickness of the plate sheet.Secondly,the micro structure evolution model of 6xxx series hardening Al alloys is established based on the theory of phase transformation and the precipitation kinetics of the second phase occurring within the super-saturated solid solution due to the temperature variation.By calculating the evolution of the super-saturated solid solution during the isothermal and non-isothermal ageing treatment.the parameters of the microstructure evolution obtained at any time during the both processes can be used to match well with the observed experimental results,which validates the effectiveness of the algorithm.Based on the previous coupled thermal-mechanical model for FSW simulation via different welding conditions,temperature histories can be extracted to calculate the evolution of AA6005A-T6 during the welding process and the evolution morphology of the particles within different areas of joint can be obtained.Thirdly,based on the strength theory of moving dislocations interacting with the second phase particles,the yield strength model and strain hardening model of 6xxx series hardening Al alloys is established under the condition of the microstructure evolution model.With the calculated yield strength and strain hardening from the model algorithm correlates well with the experimental results on the super-saturated solid solution during the isothermal and non-isothermal treatment,the effectiveness of both strength models are 'validated.The evolution of both strength within the AA6005A-T6 weld joint can be calculated under different welding conditions,showing the distribution of yield strength and strain hardening parameters along the.section of weld,accounting for the weakness area appearing within the joint from the views of both static yield strength and dynamic strain hardening.Finally,an integrated numerical model based on the microstructure and initial yield strength model is established for calculating a sequence of process from pre-weld ageing of super-saturated solid solution,to friction stir welding to post-weld artificial ageing.The evolution mechanism of the AA6005A-T6 weld joint applied by combinations of the different artificial time and temperature is analyzed and the evolution morphology of the particles under different combinations can be obtained.In addition,the difference of random distribution within the main solute composition of several commonly used 6xxx series hardening Al alloy is studied,and the distribution range of the characteristics along the sections of different weld joints is obtained.