| Aiming at the problems that microstructure evolution law and fracture mechanism of Al/Mg dissimilar metals friction stir welded(FSW)joint are not clear.In this paper,the effects of welding process parameters on the microstructure and properties of 1060 aluminum alloy/AZ31 B magnesium alloy dissimilar metals FSW joint were studied,and the process parameters were optimized by the response surface method.On this basis,the microstructure evolution of each region of the optimized joint was studied by electron backscatter diffraction,and the tensile fracture behavior of the joint was studied.When the stirring pin is biased to the Al-side,the plastic fluidity of magnesium alloy decreases and holes are easy to appear in the weld.When the rotation speed is too slow,the heat input is insufficient,and there are tunnel defects in the weld;when the rotation speed is too fast,the excessive softened metal overflows from the weld due to high heat input,forming flash,and the interior of the weld is prone to incomplete filling due to material loss.When the stirring pin is not biased,the rotation speed is 1200 r/min and the welding speed is between 30～190 mm/min,there are no obvious defects in the weld,but too fast or too slow welding speed is not conducive to the formation of an excellent Al/Mg mechanical interlocking effect.With the acceleration of welding speed,the recrystallization degree of the nugget zone decreases,the average grain size first decreases and then increases slightly,and the tensile strength of the joint first increases and then decreases.The process parameters optimized by the response surface method are as follows: the stirring pin is not biased,the rotation speed is 1200 r/min and the welding speed is 150 mm/min.The microstructure of different regions of the joint varies greatly.From the heat affected zone to the nugget zone,the percentage of low angle grain boundaries and grain size gradually decreases.The heat affected zone is only affected by the thermal cycle,without plastic deformation,and its microstructure is similar to that of the base metal.The thermo-mechanically affected zone and nugget zone are affected by thermal cycle and mechanical stirring.Dynamic recovery mainly occurs in the thermo-mechanically affected zone on the Al-side;while some dynamic recrystallization occurs in the thermomechanically affected zone on the Mg-side,forming a microstructure with the coexistence of coarse grains and fine recrystallized grains.The dynamic recrystallization degree of the nugget zone is high,and the grains are uniform and fine,but the main mechanism of grain refinement of 1060 aluminum alloy and AZ31 B magnesium alloy is different,the continuous dynamic recrystallization and discontinuous dynamic recrystallization,respectively.The aluminum alloy base metal has a strong {112}<111>copper texture,and the magnesium alloy base metal has a basal texture whose <0001>crystal direction is approximately parallel to the normal direction of the plate.After FSW,the {112}<110> shear texture is formed in the thermo-mechanically affected zone on the Al-side,the preferred orientation of grains in the nugget zone on the Al-side is not obvious,and the texture strength is very low.In the Mg-side thermo-mechanically affected zone and Mg-side nugget zone,the grain orientation is <0001> crystal direction is approximately parallel to the transverse direction of the plate,while in the center of the nugget zone,the <0001> crystal direction of the magnesium alloy grain deflects from approximately parallel to the normal direction of the plate to the welding direction.During transverse tension,the material softening caused by the decrease of dislocation density in the heat affected zone and the difference in grain size between the heat affected zone and the thermo-mechanically affected zone on the Al-side lead to the final fracture of the joint near the interface of the thermo-mechanically affected zone/heat affected zone on the Al-side. |
