Microstructure And Mechanical Properties Of Dissimilar Metal Lap Joints Of Al/Mg By Friction Stir Welding

Mg and Al alloys were widely used in aerospace, rail transportation and automobile industry because of their low density, high weight-to-strength ratio and excellent weight-to-stiffness ratio, so it was particularly significant to connect the Mg alloys and Al alloys into composite structure for reducing their weights.In this thesis, the lap joining experiment of the 3mm thick 5083 Al alloy and AZ31 Mg alloy were carried out. Firstly, the effect of different welding parameters on the microstructure, mechanical properties and microhardness of the joints were investigated, respectively. And then, by using SEM and XRD, the tension-shear fracture was canalized, meanwhile, the precipitation material in transition region of joint was investigated by using EDS, the main conclusions were as follows.By choosed elected proper welding parameters, good surface lap joints were got, otherwise, the flash, groove, weak links, holes and tunnel defects, etc. would appear in the joints; Through analyzing the transition region of lap joint, a transition layer which was different from both sides of the parent materials was formed in the interface, the transition layer contained massive dendritic intermetallic compound Al12Mg17 and columnar grain compound A13Mg2 attributed to the higher spread amount of magnesium than that of aluminum. When a welding speed was given, the strength of joints increased first and then decreased with increasing the rotating speed. When a rotating speed was given, the shear strength increased first and then decreased as welding speed increased. At the welding speed of 50 mm/min, the rotating speed of 1350r/min, the shear strength achieved the maximum, namely,81.6 MPa. "Small plastic pit" and cleavage fracture presented in tension-shear fracture of Aluminum side consisted of Mg-based solid solution and Al-based solid solution, a small amount of intermetallic compounds Al12Mg17 and A13Mg2. Brittle cleavage fracture presented in magnesium side, mainly consisted of Mg and Al, small amount of intermetallic compounds Al12Mg17. The microhardness in lap joints of magnesium alloy and aluminum alloy was distributed as "W" type, the highest hardness value for aluminum was 0.98 HV, and 94.65 HV in weld center, the lowest hardness value appeared in advancing side of heat affected zone, which was about 76.2 HV. For the magnesium alloy, the hardness peak appeared in the weld center, about 81.8 HV, slightly higher than the hardness value of magnesium parent materials, lowest hardness value appeared below the shaft shoulder, about 61.9 HV.