| The development of modern industrial raises the requirement of reducing the weight and thickness of metal structure. As a light alloy material, magnesium alloys widely used in automobile manufacturing, transportation, aerospace and other fields, so the connection problems of magnesium alloys got attention. As a solid connection technology, friction stir welding (FSW) can avoid defects generated by conventional welding methods, and obtain good properties joints.Similar FSW joints between twin roll casting magnesium alloy AZ91D and extruded magnesium alloy ZG61 were investigated firstly in this paper. Comparing the difference between forming, microstructure and mechanical properties of two kinds of joints at same parameters, the relationships of the magnesium alloy plastic formation capability with the optional parameter range and formation of joints were analyzed. In addition, dissimilar FSW joints between hot rolled magnesium alloy AZ91D and extruded magnesium alloy ZG61 alloy were researched, the effect of parameters and the position of the base metals on the joints forming, mechanical properties were analyzed, and metal flow pattern and evolution of microstructure were explored in-depth.Similar FSW joints between cast rolled magnesium alloy AZ91D and extruded magnesium alloy ZG61 are obtained with rotation rate 1000r/min and 1200r/min, traverse speed 40mm/min,80mm/min and 120mm/min all those six parameters. Defect-free AZ91D joints are obtained with three parameters when heat input range are from 10r/mm to 15r/mm, while the joints with poor quality on the surface and grooves and tunnel defects inside are obtained when the heat input are improper. Compared to AZ91D joints, there are four joints of ZG61 with good formation on surface and high-quality inside. Plastic deformation capacity of extruded ZG61 is better than cast rolled AZ91D, this conclusion is verified by the results of strain hardening index test.The results of mechanical properties test shows that as for the weld joints of AZ91D, the cracks of defect-free joints locate in base metal, and the tensile strength is 195MPa, and the cracks of defective weld joints locate in weld, and the tensile strength is lower than 195MPa. And as for the weld joints of ZG61, the fracture appears in the thermo-mechanical affected zone of advancing side and extend to the bottom of weld nugget zone, the angle between fracture and stretching direction is 37°, maximum tensile strength of the joints is 251MPa, reaching 85% of the base metal, and the defective weld joints show the brittle fracture along the defects.Clear onion rings structure could be observed in the AZ91D joint, but could not be seen in the ZG61 joints.The time of complete recrystallization of AZ91D joints decrease with the increasing of temperature. The solution treatment of ZG61 joints taken at 430℃, the tensile strength increase first and then decrease with the increasing of time, the highest tensile strength is 256MPa at 2h, reaching 86.77% of base metal, the elongation keep increasing with increasing of time, the maxium is 5.8%.Dissimilar FSW welds between hot rolled AZ91D alloy and extruded ZG61 alloy are obtained with rotation rate 1000r/min, traverse speed 80mm/min,120mm/min and 160mm/min and 240mm/min all those four parameters. Defect-free ZG61/AZ91D joints are obtained with three low traverse speed, severe deformation and metal flow occur in the stir zone under high strain stress and high temperatures, forming lamellar structure and onion ring, defects occur when the traverse speed is 240mm/min. The quality of AZ91D/ZG61 joints are very poor with all parameters.. The microhardness of stir zone of ZG61/AZ91D joints are rang from 76HV to 84 HV and the microhardness of stir zone of AZ91D/ZG61 joints are rang from 72HV to 78HV. Tensile strength of ZG61/AZ91D joints welded at low traverse speed are high, the highest tensile strength reach 86% of ZG61,and tensile strength of joint welded at 240mm/min is only 140MPa. And the tensile strength of AZ91D/ZG61 joints are only 30MPa-50MPa. |
PDF Full Text Request