| Lightweight of automobiles and railway vehicles, energy saving, environmental protection and security have become the inevitable development trend. The high strength steels are the major automotive lightweight materials, followed by aluminum alloy, magnesium alloy, composite materials, plastics, and so on. The Mg alloy has low density, high specific strength, specific stiffness, and good shock absorption and increasing the amount of magnesium alloy used in automobile design and manufacturing process will be the most effective and directive way to realize lightweight. As a result, welding dissimilar materials of magnesium and steel become an urgent problem to be solved. According to the significant differences in physical, mechanical and metallurgical properties of magnesium and steel, the weldability of dissimilar materials of magnesium/steel is extremely poor, which deteriorats the performance of joints. This has already become one of the key problems of science and technology that limit further development of lightweight of automobiles and railway vehicles. Therefore, it is of great scientific meaning and practical value to conduct the researches of dissimilar materials Mg-steel MIG welding.In the paper, the microstructure characteristics and mechanical behaviors of dissimilar materials MIG welded joints were studied systematically. And the influences of the welding parameters and alloy elements on the microstructures and mechanical properties of the welded joints were investigated. The results showed that the MIG welded joints of dissimilar materials of AZ31 B magnesium alloy and Q235 steel had welding-brazing features, and the welded joints consisted mainly of weld zone(WZ), Mg/steel interfacial zone(IZ), magnesium alloy fusion zone(FZ), magnesium alloy heat-affected zone(HAZ1) and steel heat-affected zone(HAZ2). The microstructures of the magnesium alloy WZ were consisted mainly of α-Mg solid solution and β-Al12Mg17 intermetallic compounds, and the β-Al12Mg17 mainly distributed at grain boundaries, just a little β-Al12Mg17 precipitated from α-Mg crystal. And the porosity defects were easily produced in the magnesium alloy WZ. The Mg/steel IZ consisted of the transition layer I on the magnesium alloy weld side and transition layer II on the steel side. The transition layer I consisted mainly of FeAl intermetallic compounds and α-Mg solid solution, and the transition layer II consisted mainly of α-Fe(Al,Mg) solid solution. Incomplete joining, cracks and other defects were formed easily in the Mg/steel IZ. The magnesium alloy FZ mainly contained a mixture of α-Mg coarse grains and fine grains. The prominent feature of HAZ1 was the α-Mg grain coarsening, and the microstructure of HAZ2 was mainly composed of bainite, pearlite and ferrite. The micro-hardness distribution of the welded joint was uneven. Due to the formation of FeAl intermetallic compounds at the interface, the micro-hardness of Mg/steel IZ increased significantly. Under the action of tensile stress, cracks initiated, propagated and fractured in the end at the Mg/steel interface. And the fracture exhibited a mixed fracture feature of brittle and ductile fracture, and the tensile strength of the welded joint was low(120.2MPa). The brittle FeAl intermetallic compound in the interfacial transition layer and incompletely joining defects were the main factors influencing the mechanical properties of the welded joints.Welding parameters had significant effects on the formation, microstructures and mechanical properties of joints. The bevel angles of magnesium alloy and steel were of 30° and 45°, respectively; the filler metal wire was directed toward the middle and lower surface of the groove surface; the dimension of forming groove was 10 mm ×1.2mm, which was beneficial for the improvement of weld formation and promoting Mg/steel interface reaction. With increasing of the welding current, the size of α-Mg grain in weld, the amount of Al12Mg17 phase in grain boundary and the thickness of the interfacial transition zone all increased while incompleted joining defects decreased, and the tensile strength of MIG welded Mg-steel joints tended to increase. However, under the too large welding current(150A), the weld formation was deteriorated and interface layer oxide inclusions appeared, which resulted in the reduction of MIG welded Mg-steel joint strength. With the increasing of welding speed, the α-Mg grain was refined, the thickness of interface transition layer decreased and amounts of non- welded defects increased, leading to the reduced tensile strength of MIG welded Mg-steel joints. The improvement of weld formation, microstructures and mechanical properties of joints would be helpful under the following welding parameters: welding current of 130 A ~ 135 A, welding speed of 600mm/min ~ 650mm/min, welding line energy of 2100J/cm ~ 2200J/cm.The results of metal(alloy) interlayer showed that alloying elements(Cu, Ni, Al) had obviously effects on the microstructures and mechanical properties of Mg-steel joints. Adding appropriate amounts of Cu(the interlayer thickness of 100μm) would be beneficial to improve the wettability and spreadability of liquid magnesium alloy on solid steel surface. The weld was mainly composed of α-Mg and Mg2 Cu phase. The effects of Cu were to reduce the liquid-solid phase transition temperature, prolong interfacial reaction time, promote the growth of the interface layer and reduce the amounts of incompleted joining defects, resulting in the increasing significantly the tensile strength of Mg-steel joints(180.1MPa). Furthermore, the formation of(Fe, Cu)Al intermetallic compound by adding Cu element reduced brittleness of interface layer and was also beneficial for the improvement of joint strength. However, when excess Cu(the interlayer of 200μm, 300μm) was added, a large amount of eutectic(α-Mg + Mg2Cu) occurred near the interface which greatly increased brittleness, and a large amount of micro-cracks formed, resulting in decreasing joint strength. Adding reasonable amounts of Ni(the interlayer thickness of 100μm) was beneficial to improve the wettability and spreadability of liquid magnesium alloy on solid steel surface. The weld was consisted mainly of α-Mg and NiAl phases, and Mg/steel interface transition layer mainly included NiAl, α-Mg and Fe-Ni solid solution. The interface transition layer was continuous and even with increased width and had reduced welding defects, which significantly increased the tensile strength of MIG welded Mg-steel joints(184.9MPa). The performance of NiAl was better than that of FeAl phase and the formation of Fe-Ni solid solution limited the incompleted joining defects, which led to the reduced brittleness in interface layer and was beneficial to improve the joint strength. However, when added in excess Ni(the interlayer thickness of 200μm), the combination of un-melted Ni interlayer and the transition layer was not tightly with the existence of some cracks, thus significantly reduced the tensile strength of MIG welded Mg-steel joints. The research results of using AZ61 magnesium alloy wire showed that increasing the Al content would be helpful to improve microstructures and mechanical properties of MIG welded Mg-steel joints. The weld consisted mainly of α-Mg solid solution and β-Al12Mg17 intermetallic compound. Compared with AZ31 magnesium alloy wire, the thickness of Mg/steel interface layer and the joint tensile strength increased(136.8MPa).Based on the influences of alloy elements(Cu、Ni、Al) on the microstructures and properties of MIG welded Mg-steel joints, in this paper, the idea of adding Al-Cu alloy interlayer was presented, the influences of the thickness of Al-Cu interlayer on microstructures and mechanical properties of the welded joints were studied, and the thickness of the interlayer was optimized. As a result, the mechanical properties of MIG welded Mg-steel joints were improved significantly. And the tensile strength of the welded joint reached 193.5MPa, which was 84.1% of that of the AZ31 B Mg alloy base metal(230MPa). Therefore, in order to improve the mechanical properties of MIG welded Mg-steel joint, it is more favorable to use the Al-Cu interlayer with the thickness of 100μm. Based on the above research results, the thermal cycle characteristics and the influences of welding parameters on thermal cycle were further studied. Through thermodynamic and kinetic analysis, the growth mechanism of Mg/steel interface transition layer was revealed, and the growth model of interface transition layer was established. The theoretical basis and experimental data could be provided for the improvement of the weldability and the mechanical properties of the welded joints, and development of the Mg-steel welding technology. |
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