| Magnesium alloy has the advantages such as low density, high specific strength and Stiffness, which can make equipments decrease weight, save energy and meet the need of environmental protection in the process of application. As a result, magnesium alloy will be widely applied in automobile, information and aerospace industries. As magnesium alloy belongs to Eutectic alloy, what's more, it has a large linear expansion coefficient, a low melting point, a large heat conduction coefficient, and a large heat capacity, it is easy to cause the problems of hot cracking, gas cavity, alloying elements getting burned and weld region softening when the traditional fusion welding technique is used. Therefore, mechanical joining is frequently used in actual application, which greatly restricts the application of magnesium alloy to the structures of automobiles, ships and planes, etc.FSW is a new kind of solid phase joining technique and there is no metal melting in the joint during the welding, which is a solid welding process. High property joints can be obtained by the welding method of FSW, which is without cracks, gas cavities and oxide inclusions, moreover, the special defects of using other melting welding techniques can be avoided. Using FSW technique can weld almost all the metals that melting welding can do, and various defects similar to those of melting welding rarely occur as well. Apart from the advantages of common friction welding techniques, FSW can also join multi-joint modes and different welding positions. Because FSW has the advantages of simple technology, needless cleaning before welding, keeping the metallurgical property of the original alloy , welding various metals and tending to realize mechanization and automation, which shows it has a better application prospect and it is the focus of worldwide attention.The contents of this topic include: Studying on the microstructures and mechanical properties of friction stir welded magnesium alloy plates of 3mm thick by using FSW equipment transformed by the milling machine and self-made stir ring needle; analyzing the effects of the rotating speed of stirring head and the welding speed on the quality of the welding joint, microstructures and mechanical properties.Major conclusions are reached as follows:(1) The welding of magnesium alloy AZ31B is realized by using FSW equipment transformed by the milling machine. When technological parameters are properly chosen, the welding joint with good formability and mechanical properties is available. Owing to heating function of the stirring head and mechanical stirring function, there are four different regions formed in the FSW joint, respectively, welding nugget zone, thermo-mechanically affected zone, heat affected zone and base metal. The welding nugget zone take on basin-shaped or oval"onion circle"appearance, forming tiny equal axial crystals and the composition of phases is the same as base metal. The grains in the thermo-mechanically affected zone and the heat affected zone are not evenly distributed in size, which are obviously deformed.(2) The welding technological parameters decide the heat input in the process of welding. When the other welding technological parameters are fixed, the heat input of the welding increases as the rotating speed of stirring head increases. Grooves, holes and the tunnel-shaped defects occur when the rotating speed is too slow. On the other hand, as the welding speed increases, heat input decreases .But when the welding speed is too fast, holes and the tunnel-shaped defects occur as well. In the experiment, it is discovered that when welding speed is 23.5mm/min and the rotating speed is 1180r/min, the weld of the compact structure is formed.(3) The micro-hardness distribution of the friction stir welding joint is similar to the shape of letter W and the softening occurs. Among which the minimum occurs in heat affected zone on the advancing side. The micro-hardness value on the advancing side is lower than that of the back side. As the welding speed increases, the micro-hardness of the connection slightly increases. However, as the rotating speed of the stirring head increases, the micro-hardness slightly decreases.(4) The tensile fracture position of the friction stir welding joint mostly occur in the heat affected zone on the advancing side. When the joint has the defects such as false connection, incomplete welded,holes and tunnel-shaped defects, the tensile fracture position of the joint is in the weld. The fracture mode of the joint belongs to the mixed fracture of ductile fracture and fragile fracture.(5) The tensile strength of the friction stir welding joint decreases as the welding speed increases. On the other hand, the tensile strength increases as the rotating speed of the stirring head increases but it does not increase unlimited. When the welding speed is 23.5mm/min and the rotating speed of the stirring head is 1180r/min, the tensile strength reaches 173.67MPa, which is 70.89 percent of the tensile strength of the base metal and at this moment the elongation rate is 12.47 percent.(6) When the welding technological parameters are not properly chosen, some inherent defects of friction stir welding may occur in the joint. The defects occurring in the friction stir welding joint of magnesium alloy are divided into two kinds:The first one is the surface defects, which include fin, groove, incomplete welded, cold lap and sink. In the actual application, the defects of fin and sink will reduce the structures and properties of welded joint and the defects of groove and incomplete welding will seriously influence the joint properties, which must be avoided happening.The second one is the defects in the inner weld, which include hole, tunnel-shaped, false connection, crack, gas hole and Inclusion. The formation is closely related to the welding heat input. The quantity of heat input mainly depends on the welding technological parameters. Therefore, in the process of friction stir welding, flawless joint is available when the welding parameters are properly chosen.
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