Study On Microstructure And Mechanical Properties Of Tungsten Inert Gas Arc Joints Of Mg Alloys For Motorcycle Framework

Due to its low density, high specific strength and stiffness, good thermal conductivity and outstanding damping capacity, magnesium alloy becomes an ideal material for energy saving and weight reduction for road vehichles, such as cars, motorbikes and other transportion means. Therefore, using magnesium alloy as motorcycle framework material is technically and economically advantageous to improving riding comfort and fuel economy. As the composite structure of magnesium alloy profiles and castings, the quality of the weld joints is critical for the application.As a part of technical research scheme for magnesium alloy framework production, the present study is devoted to weld joining of hot-extruded AZ71 and AZ71E alloys and die-cast AZ81 alloy plates with tungsten inert gas (TIG) arc welding technology. In the study, the microstructure and mechanical properties of three types of weld joints and the influence of welding current (main processing parameter) were studied systematically in order to provide a guide for the welding in the production of the framework.The study reveals that the cross section of the TIG weld joint consists of three zones, namingly, weld zone (WZ), heat affected zone (HAZ) and substrate material (BM). The microstructure of the heat affected zone is a typically overheated structure and the size of grains is coarser than that of substrate material, while the microstructure of the weld zone appears a typical quenched cast structure.The results show that, with the increase of the current, the weld width increases. When the current is 90A, the weld seam was fine. The increase of the current leads to an increase of the width of heat affected zone (HAZ) and to a grain coarsening in both HAZ and the WZ. Theβ-Mg17Al12 phase in the FZ distributes on the grain boundary and its percentage increases with the increase of the current as well. In tensile test, failure occurres in the weld zone, and the tensile strength and elongation of AZ71 are not as good as the substrate material (313.9MPa,10.5%). With the current increases from 80A to 110A, the tensile strength and elongation increases first and then decreases. When the current is 90A, the tensile strength, elongation of the weld joint reach their maximum values of 281.2MPa and 8.8%, respectively. When the current reaches 110A, the tensile strength and elongation reach their minimum values of 180.2 MPa and 0.9%, respectively. The new phase Al11Ce3 appeares in the joint of AZ71E alloy, mainly distributing inα-Mg matrix with a white needle-shape under scanning electron microscopy (SEM). With the increase of the current, grains coarsen in the WZ and the valume fraction ofβ-Mg17Al12 phase decreases. Thick cellular dendrites occur in the WZ when the current is 100A. In tensile test, failure occurres in the WZ of the AZ71E and the failure strength and elongation are not as good as those of substrate alloy (281.3MP and 9.7%). With the current increasing from 80A to 110A, the tensile strength and elongation increase first and then decrease. When the current was 80A, the tensile strength and elongation are minimum of 112.6MPa and 0.4%, while reach their maximum of 241.9MPa and 3.6% respectively, when the current is 100A. The fracture morphology of the joints are featured bu its quasi-cleavage morphology decorated with thick rod-shaped Al11Ce3. Within the current arrange investaged in this study, the microhardness of WZ is higher than that of substrate alloy but decreases with the increase of current.In the WZ of AZ71 and AZ81 alloys, the tendency of forming pores in the joint becomes obvious due to more gas dissolved in the die-cast AZ81. In tensile test, all joints weld with different current fail in the HAZ in the AZ81 side with more pores. With the increase of current from 80A to 100A, the tensile strength increased from 118.8MPa to 139.3MPa, and the elongation increased from 1.1% to 1.8%, while the tensile strength and elongation of AZ81 magnesium alloy were 121.0MPa and 1.5% respectively. When the current was 100A, the fine weld joint could be obtained. The microhardness of WZ was higher than that of substrate alloy, the hardness variation range of AZ71 HAZ was small with the current change in comparison with that of AZ81.