Study On Microstructure Of Welding Zone For Magnesium And Aluminum Dissimilar Metals By The GTAW

Magnesium alloys are of great potential in light-weight transportation applications for their advantages as lower density, higher intensity, better thermal and electrical conductivity, etc. Aluminum alloys are widely used in transportation vehicles as the main structural material. Mg/Al hybrid structures have advantages for the combination of advantages of both alloys. Joining of Mg/Al dissimilar alloys has great significance.Gas tungsten arc welding (GTAW) of Mg/Al dissimilar metals was conducted using different welding wires. Influence of wire compositions on the microstructure of weld metal in different joints was investigated. Effects of elements Al, Mg and Si on distribution of intermetallic compounds (IMCs) were analyzed. Attempts were conducted to improve the microstructure of welded joints.Microstructure of the Mg/Al dissimilar welded joints was analyzed by optical microscope (OM) and scanning electron microscope (SEM). Fusion zone near Mg alloy consists of melting region and partial melting region. IMCs mainly distribute in the dendrite zone of melting region. According to the analysis of microstructure and crystallization characteristics of the joint, the melting region is composed of a(Mg)+y(Al12Mg17) eutectic structures and some IMCs. The weld metal is composed of coarse or fine a(Al) eutectic structures. Some primary Si, Mg2Si and SiAl4 partials distribute in the fusion zone near Al alloy and weld metal using SA14043 or SA14047 welding wires. While Mg2Si, Al3Mg2 exist in the fusion zone near Al alloy weld metal using SA15356 welding wires.Phase constitution of fusion zone on both Al and Mg alloys were analyzed using EDS and XRD. Mg and Al are the main elements that constitute the fusion zone near Mg alloy. The cellular crystal zone is composed of a(Mg)-γy(Al12Mg17) eutectic structures, the cellular and dendrite zone is a mixture structure of eutectic structures and IMCs. Most IMCs distribute in the dendrite zone of the fusion zone. The fusion zone near Al alloy is composed of a(Al) and eutectic structure of a(Al). IMCs Mg2Si、Al12Mg、AlMg、Al3Mg2 exist in all joints based on XRD analysis. IMCs with low-melting point distribute discretely because of Mg2Si. A large number of IMCs Al12Mg17、AlMg、Al3Mg2 exist in the weld zone when using SA15183 or SA15356 wires.No obvious lamellar IMCs layers were observed in the fusion zone near Mg alloy using SA14043 or SA14047 welding wires and the IMCs mainly distribute discretely in the weld zone. The average micro-hardness of the weld zone is comparatively higher using the SA14047 welding wire and the softening of the fusion zone near Al alloy is mitigated. Thick continuous flake IMCs layer was formed in the joint using SA15183 or SA15356 welding wires. The micro-hardness of the IMCs layer is as high as 250HV50g. The softening of the fusion zone near Al alloy is suppressed using SA15356 welding wire.Thermal joining of AZ31 with 7005 was conducted using GTAW. Influence of elements as Si, Mg on microstructure of the joints was revealed. The formation of IMCs was suppressed to a low level and distribution of the IMCs was improved by the transition of proper alloying element. Thus the microstructure of the Mg/Al joint was improved. The results provide experimental and theoretical basement for the further researches on Mg/Al fusion welding.