| For enhancing the deep-processing technology and improving the high additionalvalue of magnesium alloy product, which have bring about the unprecedenteddevelopment to the magnesium alloy industry of china. The magnesium alloy diecasting processing develop to maximization and complication, The process of castingand welding composite forming get prior consideration. The conventional fusionwelding joint’s heat affect zone are prone to bubble phenomenon, which are similar tothe porosity grow up at heat treatment. Furthermore, due to the porosity in fusion zoneand weld zone will be widely release, which can be forming porosity defects. So, thisproject using friction stir welding to joint die casting magnesium, in order to solve thewelding difficult of die casting magnesium alloy.Die casting plates of AZ91D magnesium alloy, with thickness of4mm, werewelded by friction stir welding. Then, the microstructure characteristics、tunnel defectsformation mechanism、 mechanical properties of the die casting AZ91D weresystematically characterized with light microscopy, scanning electron microscopy, statictensile test. Results obtained are as follows:①The grains of weld nugget zone are fine and recrystallized. The microstructureof weld centre longitudinal section present a bigger difference: The grains of crownzone at the top of weld nugget are uniform, coarse and highly dense recrystallized, thegrains of the central ring area and bottom zone are relatively fine and recrystallized butless uniform than that of the crown zone. Thermo-mechanically affected zone showpartially recrystallized microstructure, the microstructure close to the nugget zone arerecrystallized, and the microstructure close to the heat affect zone are non-uniform andcourse and the second phase Mg17Al12discontinuous distribute at grain boundary. Themicrostructure of heat affected zone are similar to base metal while the grains are a littlecoarser. Moreover, the transition between the weld nugget zone and thethermo-mechanically affected zone in the advancing side are discontinuous, while thatin the retreating side is gradient.②The experiment results show that: we can judge the tunnel defects of FSW jointof die casting AZ91D magnesium Alloy depend on the key hole shape at the end ofweld. The reason in causing this type defect are dis-proper welding parameters, and thistype defects are usually locate in the boundary between crown zone and ring zone of advancing side, then spread to the bottom of weld followed by parameter changes. Inthis study, when the parameter ratio vr/vw≥10, and keep the welding speed within40~80mm/min and the rotating speed within700~1500r/min, then the joint are not easierto product tunnel defects. This type of defect is very sensitive for the welding speed,however, the rotating speed can be changed in a widely range.③The fracture is located in thermal-mechanical affected zone of advancing side.Tensile rupture is mixed pattern: tough rupture and brittle rupture. On the top of weldpresents intergranular fracture mode. At the centre of weld presents river fracture mode,and fracture surface are perpendicular to the direction of principal stress. On the bottomof the weld presents obvious tearing dimple fracture. The hardness values of weld centreis higher than other zone and the lowest hardness values in heat affected zone ofadvancing side. The hardness value of the weld joint distribution presents the shape of“W”. With a rotating speed of900r/min and welding speed of60mm/min, the jointtensile strength is198MPa, which can be reach to86%of base metal. But the elongationrate is just3%. |
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