| Due to the advantages of low density, high specific strength and elasticmodulus, high thermal conductivity, and good damp property as well as highshock resistance, magnesium alloy has great potential in terms of applicationin industries. Steel has been recognized as a kind of structural material whichis most extensively used till now. The development history of materials hasdemonstrated that a more extensive use for any advanced material dependson not only its properties, but also progress in welding technology of thematerials. However, there still exist great difficulties in joining the dissimilarmaterials of steel and magnesium alloy, attributing to the difference inphysical and chemistry properties between the two kinds of materials. Atpresent, several welding methods have been adopted to weld dissimilar materials of magnesium and steel, involving laser welding, laser-TIGwelding, and friction stir welding, whereas it will broaden the applicationscope of dissimilar material joints of steel and magnesium and be of greatimportance if dissimilar materials of steel and magnesium could be weldedby MIG welding successfully, which is one of the most intensively usedwelding techniques in industry currently.In this paper, AZ31B magnesium alloy and Q235steel with the platethickness of8mm and3mm were welded by MIG welding process withmagnesium alloy filler wires. The effects of MIG welding parameters onweld appearance、the fusion depth and weld width; the weldability of thedissimilar materials of magnesium alloy and steel; microstructure andmechanical properties of the dissimilar material welded joints wereinvestigated. The experimental results indicate that due to the poorweldability of the magnesium alloy and low carbon steel, it was difficult fordissimilar materials of magnesium alloy and steel sheets with thickness of 8mm to be welded by MIG welding. However, the weldability of thedissimilar materials of magnesium alloy and steel was improved through theaddition of copper interlayer. During welding, the magnesium alloy basemetal was melted while low carbon steel base metal remained unmelted;copper interlayer was melt completely at the upper region of the joint andmelt partially at the lower region of the joint, which demonstrated that theMg/steel joint belongs to welding-brazed joint in essence. The results ofSEM、EDS and XRD analyses indicate that there was element interdiffusionat the Mg/Fe interface and the Al element was enriched at the interface; Theweld seam was mainly composed of α-Mg; The microstructures were mainlycomposed of α-Mg'Mg2Cu at the upper region of the Mg/Fe interface inthe cross section of the welded joint; And Mg2Cu、Cu-Al-Mg eutectic andCu-Mg-Al intermetallic compound were formed at the lower region of theMg/Fe interface in the cross section of the welded joint. During the weldingprocess, it was easy to generate welding defects such as cracks, porosity and oxidation. The main role of the Cu interlayer was to promote the interfacialreaction and to achieve metallurgical bonding. The experimental resultsindicate that the temperature at Mg/Fe interface had a significant impact onthe bonding properties, and it is easier to realize the joining at thehigh-temperature zone than the joining at the low-temperature region.The research of Mg alloy/steel MIG welding with thickness of3mmindicates that it would be beneficial to achieve the bonding of Mg alloy/steeldissimilar materials with reduced sheet thickness. The welding parametersand the content of Al in the weld had a significant impact on the weldappearance and the microstructure at the Mg/steel interfae as well as themechanical properties of the joints. The results indicate that tensile strengthof the welded joint increased first when the welding linear energy andwelding velocity increased and then the tensile strength decreased when thewelding parameters proceeded to increase above a certain values. At thewelding linear energy of2230～2430J/cm and welding velocity of0.65～ 0.70m/min, good weld appearance shaping was obtained with the maximumtensile strength of121MPa. The results of SEM、EDS and XRD analysesindicate that a continuous and compact reaction layer was obtained at thezone in which the MIG arc heated directly. With the increase in the contentof Al in the weld, thickness of interfacial reaction layer in the Mg/Fe weldedjoint increased, and interfacial bonded strength also increased, resulting inthe enhancement of tensile strength of the welded joint of magnesium alloyand steel. The sound Mg/steel butt joints with average tensile strength of125MPa were achieved with magnesium alloy wire containing6.6wt.%Alelement. In addition, the mechanism of the Al on the Mg/Fe interfacialbonding was also discussed. |
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