Research On The Friction Stir Welding Of Al/Mg Dissmillar Alloys

In this paper, aim to solve the welding problem of aluminum of face-centered cubic structure and magnesium of close-packed hexagonal structure, the friction stir welding (FSW) of aluminum and magnesium is carred on. The dissimilar alloys of 6061 and AZ31B are welded together by FSW after the welding of the same alloy. In the dissimilar alloys welding process, kinds of types of butt and overlap joints are redesigned, and stir heads are as well as recreated based on the characteristics of dissimilar alloys welded joint. Besides, weldability of joints with different parameters is researched, mechanical properties, microstructures and fractures of those joints are also observed.The experimental results show that the weldability of aluminum is better than the magnesium’s. Friction stir welding of aluminum alloy usually owns a good appearance of the joint, a wide range of technical parameters, and in the range of weld speed 60mn/min-22Omm/min, revolving speed 650r/min-1200r/min, joints with good quality shall be got. Magnesium has a bad plasticity and narrow range of parameters, in the FSW process, the Mg adheres to the stir head heavily. In addition, in the range of welding speed 55mm/min-70mm/min, revolving speed 900r/min-1200r/min, the good joint could be barely got.The experimental results of overlap joints of Al/Mg show that a better shape of weld could be got when the Mg plate putter above the Al plate, and the tensile strength of Z-shape joints redesigned first rises then falls along with the increasing of welding speed, when the speed is 60mm/min, the strength comes to the maximum,56.3MPa, the fracture is plastic. XRD results find there exist intermetallic compounds Al12Mg17, Al3Mg2 in the welded joints. Both of the grains of the weld metal and the heat affected zone are fined, own to the decrease of Mg2Si, the hardness of these zones is definitely lower than the base metal’s.The experimental results of the FSW of Al/Mg show that the shape of joint could be improved when welding and revolving speed decrease at the same time,When Al/Mg FSW adopts a butt joint, the shape of joint could be improved when welding and revolving speed decrease at the same time, the maximum strength can approach to 47MPa. The strength shall be further increased to 63MPa,77MPa after putting the Cu or Zn slice in the joint gap, the tensile strength increased 63.8% to the joint without a slice. Fracture morphologies by SEM proved that all joints are fracture brittle. According to XRD and EDS analysis of weld metal, there are much brittle phases Al12Mg17, Al3Mg2 in the joint without slice; in the joint with Cu slice there are compounds such as Al71Zn29, Mg2Zn11, MgZn2, AlMg4Zn11, Al12Mg17; in the joint with Zn slice, there exist such compounds as A12Cu3、Al4Cu9、Al5Cu6Mg2、Al2CuMg, Al7Cu3Mg6. The reason caused the decrease of joints’ strength are brittle phases, i.e., Al-Mg intermetallic compounds. Both of the grains of the weld metal and the heat affected zone are fined, own to the decrease of Mg2Si, the hardness of these zones is definitely lower than the base metal’s.The breakage features of stir-heads are analyzed carefully, the reason bring out fatigue breakage is asymmetric forces to the shoulders of stir-heads, fracture features and mechanism of common screw-thread stir heads are researched then, at the end, a new type of stir heads with a stronger root and no screw is designed. The experimental results show that new type of stir heads can improve the welding process, and has a long service.