Numerical Simulation On Temperature Field And Flow Field Of Friction Stir Welding Al-Zn-Mg-Cu Aluminum Alloy Thick Plates

Al-Zn-Mg-Cu aluminum alloy has the characteristics of good electrical and thermal conductivity,high specific strength and specific stiffness,low density and high corrosion cracking resistance,and has been widely used in ships,aviation,aerospace and automobile manufacturing.Al-Zn-Mg-Cu aluminum alloy has been widely used in shipbuilding,aerospace and automobile manufacturing due to its characteristics of good electrical and thermal conductivity,high specific strength and stiffness,low density and high corrosion cracking resistance.Friction stir welding（FSW）technology can solve the defects of conventional fusion welding method,such as welding deformation,porosity,crack and residual stress,and get good joint performance.The numerical simulation technology can verify and explain the experiment,which has great significance to the improvement of welding process,welding efficiency and welding cost saving.In this paper,numerical simulation and experiment were used to conduct FSW tests on 20 mm thick Al-Zn-Mg-Cu aluminum alloy plate.The constitutive model of this alloy was established and the effect of processing parameters on the microstructure of this alloy was studied.The functional relationship between friction coefficient and temperature in the process of high strength aluminum alloy thick plate FSW was established,and the influence of welding parameters on welding temperature,plastic flow field,stress and strain was studied.The CA model was used to predict the grain evolution during FSW.The conclusions are as follows:Arrhenius constitutive equation and double multivariate nonlinear regression equation for Al-Zn-Mg-Cu aluminum alloy were established,which proved that Arrhenius constitutive equation can accurately predict the rheological behavior of Al-Zn-Mg-Cu aluminum alloy.Through orthogonal experimental analysis,it is found that the deformation temperature has the greatest influence on the true stress-strain curve of the alloy,followed by the strain rate,and the deformation amount has less influence.The processing diagram of Al-Zn-Mg-Cu alloy was established.It is found that when the deformation amount is 70%,a rheological instability zone exists in the range of deformation temperature from 300℃to 400℃and strain rate of 0.1 s^-1～2.85 s^-1.The deformation temperature of 427℃～500℃and the strain rate of 0.1 s^-1～1.65 s^-1 are the optimal processing parameters.This alloy has a small amount of dynamic recrystallization during hot deformation,but the main softening mechanism is dynamic recovery.The relative content of dynamic recrystallization structure increases with the increase of deformation temperature and strain rate,while the relative content of deformed structure will decrease.The inverse method was used to solve the relationship between the friction coefficient and the FSW temperature of the Al-Zn-Mg-Cu aluminum alloy thick plate during FSW process,which was brought into the numerical simulation calculation,and the error of the temperature field between simulation and the experimental results was kept within 5%,accuracy is better.Through the analysis of the temperature field,it is found that in the process of thick plate FSW,the closer the stirring area is to the shoulder,the higher the temperature,and the temperature of the plate difference between the upper surface and the lower surface is large.The temperature of the welding area will increase with the increase of the rotational speed and decrease with the increase of the welding speed.The material plastic flow field of the aluminum alloy thick plate during the FSW process was simulated while simulating the temperature field,and the grain size and distribution of the aluminum alloy thick plate after welding were simulated by the CA model.It is found that in the FSW process of aluminum alloy thick plate,the material on the forward side will flow forward due to agitation,and the material on the backward side will fill the temporary cavity formed by the material on the forward side in time,which is also the reason why the forward side is more likely to form defects.The change of FSW parameters has little effect on the size of the material mixing region.In the welding process,the material mixing region is larger than that of the forward side due to the accumulation of materials on the backward side.The shape of the stirring pin and the distribution of welding temperature will not only affect the plastic flow zone,making it appear larger in the upper part and smaller in the lower,but also make the average grain size in the upper part of the stirring zone larger than that in the bottom part.In addition,the farther the grains are from the stirring zone,the larger the size.