| Magnesium alloys are widely used in aerospace,rail transit,automobile manufacturing and shipbuilding because of their excellent processing properties and good physical properties.Magnesium alloys have become the preferred structural materials for lightweight industrial manufacturing.Friction stir welding(FSW),as a new solid-state welding technology,has many incomparable advantages in the joining of magnesium alloys because of its small welding deformation,low energy consumption and low production cost.However,when magnesium alloys are welded by the conventional friction stir welding,there are usually problems,such as low mechanical properties,large temperature gradient along the thickness of the plates,single form of heat source was insufficient to obtain good plastic softening layer.Therefore,in this study,electrically assisted friction stir welding was used to weld AZ31 magnesium alloy,AZ31 & AZ61 dissimilar magnesium alloy and AZ31 & AZ61 dissimilar magnesium alloy with copper interlayer,respectively.The electrothermal and electro-plastic effect of pulse current helped improve heat input and material flow plasticity to improve the joint strength.At present,there are relatively few studies on friction stir welding of different magnesium alloys as well as the application of pulse current to friction stir welding.Therefore,in the present study,the pulse current of 0A-400 A assisted friction stir welding were carried out on 3mm-thick similar AZ31 B magnesium alloy plates and AZ31 and AZ61 dissimilar magnesium alloy plates and copper interlayer was added between dissimilar magnesium alloys to investigate the effect of pulse current on the microstructure and mechanical properties of welded joints.For the similar magnesium alloy after the interaction of pulse current,pulse current changed the grain size of stir zone,which was consistent with grain size of heat affected zone,and the transition from the stir zone to heat affected zone was smoother.With increasing pulse current,the difference in grain size in the stir zone along the thickness direction decreased.The tensile strength and ductility of welded joints were significantly improved with increasing pulse current,and the fracture position was transferred from the stir zone to heat affected zone.The hardness of welded joint reached the peak at 300A;A large number of precipitates containing aluminum and zinc elements appeared in the joint and were evenly distributed at the grain boundary and promoted precipitation strengthening,which is the main strengthening mechanism for improving the overall mechanical properties.For the dissimilar magnesium alloy joint after the interaction of pulse current,the maximum tensile strength and elongation were 314.28 Mpa and 13.66%.The micro interlocking structure were formed by pulse current and the difference between microstructure of the SZ and TMAZ was reduced;the average grain size decreased and the texture was weakened with increasing pulse current.Precipitated phases were transformed from β-Al12Mg17 and Fe Si phase to β-Al12Mg17 and η-Mg Zn2 phase.By pulse current,the basal slip of long dislocation lines changed into prismatic slip of short dislocation lines and the number of slip systems increased.Furthermore,the formation of nano-scale precipitates at the grain boundary generated the pinning effect,which not only hindered the movement of dislocations,but also promoted the sharp increase of the number of dislocations to simultaneously improve the strength and plasticity.At a pulse current of 300 A,the hardness of the joint was higher.With increasing pulse current,the tearing ridges and dimples were noticeable and the fracture mode changed from brittle fracture(typical river pattern)to ductile fractures.For the dissimilar magnesium alloy joint after the interaction of pulse current and interlayer,the grains were refined,the grain orientation was more random and the texture was weakened.With increasing pulse current,the distribution range of Cu element increased and became more uniform.Strengthening phases such as Mg Cu and Al Cu were dispersed and the overall hardness was increased by 33%.Meanwhile,the highest tensile strength reached 102% and 90.53% of AZ31 base metal and AZ61 base metal,respectively.The pulse current provided energy for dynamic recrystallization,which promoted grain refinement and provided temperature conditions for precipitation of strengthening phases,inhibiting the production of brittle IMCs and promoting the uniform distribution of strengthening phase.Therefore,fine grain strengthening,precipitation strengthening and uniform distribution of copper interlayer are the main mechanisms for the improvement of joint strength. |
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