Study On Technology And Mechanical Properties Of Joint Of Dissimilar AZ31BMg/6061Al Alloys By CMT Welding

In this paper, AZ31 B magnesium alloy and 6061 aluminum alloy were used for the base materials with thickness of 4mm, ER5183, ER4043 and ER4047 were employed for the filler metal. The CMT welding process was employed to achieve sound joint between AZ31 B magnesium alloy and 6061 aluminum alloy thin sheet. The microstructures and properties of the joints were observed by Keyence VHX-900 Digital Microscope and JSM-6480 SEM which is coupled with energy dispersive spectrometer(EDS).The optimal welding parameters which were suitable for welding magnesium and aluminum alloy with CMT welding technology were gained by a lot of experiment. For ER5183 Al-Mg wire, the welding parameters of the joint with good formation were welding speed of 500mm/min, wire feed rate of 5.5m/min, welding current of 93 A, welding voltage of 17.3V. By the same method, for ER4043 Al-Si filler metal, the optimal welding parameters of the joint with good formation were welding current of 118 A, wire feed rate of 5.7m/min, welding voltage of 14.3V, welding speed of 450mm/min.Mechanical property test show that the biggest joint tensile strength of ER5183 Al-Mg and ER4043 Al-Si wires were 17 MPa and 31 MPa. The crack occured in the Mg side fusion zone, hardness test showed that hardness were high here. The feature of the fracture which was analyzed by macrograpy, SEM and EDS was brittle fracture. There were many intermetallic compounds in the fracture. These compounds were rigid, distributing continuously in the fusion zone, was the main influence factor of joint strength.Mixed dendrite grid distribution microstructure of alpha Al, Mg2Al3 and Mg17Al12 are found in weld area on magnesium side by ER5183 Al-Mg alloy wire, and micro-hardness value is more than 175HV; columnar crystal, primary Si and Mg2 Si eutectic are found on the magnesium side weld zone by ER4043 and ER4047 Al-Si alloy wires, and the hardness is about 80~100HV. The weld center and the aluminum side weld zone are mainly columnar crystal, aluminum solid solution, and the micro-hardness is 80~120HV. The weld zone shows uniformly distributed tissue and excellent mechanical properties using ER4043 and ER4047 Al-Si alloy wires.The hardness of Mg/Al welded joints using 3 kind of wires were measured with micro-hardness tester and the results show that magnesium side's fusion zone is the weak part of the joint: hardness increased increased significantly, the tendency of hardness and brittleness is obvious, and the area is prone to some weld defects such as cracks and pores; the fusion zone on the aluminum side is in good conditions. The magnesium lateral fusion zone's hardness becomes bigger with the increasement of Mg and Si elements' content, and more attention should be paid to the Mg and Si elements' content in the wire selection process.Line scanning was done on magnesium side fusion zone with different fillers and the results demonstrated a similar regulation of the element distribution. Magnesium side heat affect zone is mainly Mg element, and Al element is rarely diffused to magnesium base material. On the interface of this area, Mg element reduces sharply with Al element significantly increases; and both elements remain stable in the fusion zone: means that a stable Mg-Al intermetallic compound is generated. The element contents gradually transfered to the weld seam's level as the test area passed over the fusion zone.Stoma defects are easy to appear in aluminum and magnesium alloys welding process. The reason of producing porosity is that the aluminum and magnesium alloys had the high thermal conductivity, molten pool had a higher cooling speed, hydrogen which dissolved in the weld metal at high temperature had no enough time to escape and remained in the weld, in the course of molten pool rapid solidification process.