Study on Microstructure of Friction Stir Lap-welded Joint of AZ31B Mg Alloy Continuous Casting and Rolling Plate
AZ31B magnesium alloy continuous casting and rolling plate was subjected to double pass friction stir lap welding with counterclockwise and clockwise mixing head.The macrostructure of transverse sections and microstructure of different position in the lap-welded joint were compared to analyze the influence of double pass friction stir lap welding on the plastic microstructure and forming of welded joints.The results show that the nugget zone(NZ)of joint has especially fine and uniform grain.The microstructure size of transition zone(TZ)fluctuates greatly,the grain size is slightly larger than that of NZ,smaller than that of base material(BM).The interface migration line on advancing side in samples is rolled up in a large arc,which is more abrupt and the welding core is cut off,forming a hook defect.However,the interface migration line of the retreating side slowly penetrated into the middle zone of NZ and gradually disappeared into the micro-structure of NZ.The forming characteristic of the migration line on advancing side is the weakest part of the whole welded joint compared with the defect of the hole and tunnel.The forming properties of the lap-welded joint are influenced by the fusion of microstructure of the interface migration line on advancing side sides.When the rotation direction of the stirring head is the same counterclockwise for double pass friction stir lap welding,a joint with good surface quality,tightly fused microstructures and high strength can be obtained.When the upper plate of the lap joint is on the retreating side,compared to the advancing side,the lap joint can withstand greater shear force.
AZ31B Mg alloy continuous casting and rolling platefriction stir lap-weldinginterface migration
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