AZ61 Magnesium Alloy Friction Stir Joining Process Research

Magnesium alloy has the advantages of low density,high strength,good damping and shock absorption,strong electromagnetic shielding,etc.It has a very wide range of applications in rail transit,aerospace,medical equipment,3C and other fields.Traditional welding is difficult to guarantee the quality of the connection area.Friction Stir joining（FSJ）is a new solid-phase joining technology,which can obtain a better performance connection area by joining magnesium alloys.In this thesis,the FSJ test was carried out on the AZ61magnesium alloy plate,and the influence of process parameters on the microstructure,tensile properties,microhardness,wear resistance and corrosion resistance of the connection zone was analyzed.The temperature field in the connection process was analyzed by using DEFORM-3D simulation software,and the simulation was consistent with the test temperature results.The main research content of this thesis is as follows:（1）The stirring tool of AZ61 magnesium alloy FSJ with a thickness of 5 mm,the surface of the shoulder is in the shape of a spiral groove ring,the stirring needle is a smooth circular platform,the inclination angle of the stirring tool is 0°,and the pressing amount of the shoulder is 0.1mm.FSJ process parameters The rotation speed of the stirring tool is 1000～1200rpm,and the forward speed is150～250mm/min.The surface of the connection area obtained under these process parameters is well formed.Internally free of defects.（2）The microstructure of the FSJ junction zone of AZ61 magnesium alloy was studied.Within the range of process parameters,the central grain of the connection zone is significantly refined.The size of the central grain in the connection zone increases with the increase of the rotation speed of the stirring tool,and decreases with the increase of the forward speed of the stirring tool;The dual effects of thermal cycle and stirring and extrusion of the stirring needle and the shoulder lead to dynamic recrystallization of some grains;the heat-affected zone is only affected by the friction stir thermal cycle during the whole process,which promotes the growth of some grains.The existence of the second phase was not found through the metallographic and XRD patterns.（3）The properties of the FSJ junction zone of AZ61 magnesium alloy were studied.The fracture of all FSJ samples occurred on the forward side,and the tensile strength of the connection zone was the highest,which was 89.9%of the base metal,when the rotation speed of the stirring tool was 1000rpm and the forward speed was 150mm/min.The distribution of microhardness in the joint area has no obvious rules,and the hardness of the entire joint area fluctuates between 65 and 78HV_0.3,and there is no obvious difference in the hardness of each area.The average friction coefficient of the joint area obtained under each process parameter is partially smaller than that of the base metal,indicating that the wear reduction is stronger than that of the base metal,and the wear amount is greater than that of the base metal,indicating that its wear resistance is lower than that of the base metal.The wear mechanism is mainly abrasive wear and adhesive wear.Through 12h and 24h salt spray corrosion,it was found that the corrosion time of the connection zone obtained under each process parameter increased,and the corrosion rate of the connection zone increased,but compared with the base metal,the corrosion growth rate was smaller;the electrochemical corrosion performance of the connection zone was better than that of the base metal Poor,when the rotation speed of the stirring tool is 1200rpm,and the forward speed is150mm/min,the corrosion performance is better than other process parameters.This is mainly because the heat input is the highest under this process parameter,the dislocation density in the connection area is reduced,and the electrochemical corrosion resistance is improved;The stress corrosion test shows that the fracture strength and elongation after fracture of the samples obtained under the corrosion environment of 3.5wt.%NaCl solution in the base metal and various process parameters are lower than those in the air environment,and there is no obvious necking phenomenon,showing the characteristics of brittle fracture.Under some process parameters,the performance is better than that of the base metal.（4）Based on the hyperbolic sine function constitutive model of fluid mechanics,a numerical model of the FSJ temperature field of AZ61 magnesium alloy was established,and the simulation analysis of different positions in the connection process（connection front section,middle section,and tail section）was carried out using DEFORM-3D simulation software.Obtain the temperature change curve of the feature point at this location,and verify the accuracy of the model through experiments.