Effects Of Inoculation On Microstructures And Mechanical Properties Of Resistance Spot Welded Magnesium Alloy

It has become a higher request for weight saving and recycle of materials to reduce the energy consumption and pollution with the rapid development of aerospace, automobile and electron industries. Magnesium alloys, the lightest structural material, have many advantages such as high strength ratio, good castability, high damping capacity and recyclable characteristic, which are praised as the most promising materials to achieve weight saving and recycle. They have been attracting more and more interests. Society's demands promote basic theoretical research and the applications of magnesium alloys in industry fields. The application of any advanced material not only depends on its own properties, but also relies on the technology of material manufacturing. Besides, the resistance spot welding plays an important role in material joining techniques, which is widely applied in aerospace and automobile. However, magnesium alloys are extremely easy to form the oxide film. Furthermore, during welding there are also shringkage cavity and crack. Cellular-dendritic crystals in weld nugget tend to form the weakness in the joint. These factors all reduce the mechanical properties of welded joints, so that weldability of magnesium alloys is weakened. It has become one of main problems for resistance spot welding of magnesium alloys.The present study aims at investigating effects of inoculation on microstructures and mechanical properties of resistance spot welded magnesium alloy (AZ31B). Based on the experimental results, some conclusions have been drawn as follows:(1) Resistance spot welded magnesium alloy (AZ31B) joints contain the nugget and heat affected zone (HAZ). Solidification of the nugget begins from the boundaries, having a characteristic of epitaxial growth. Cellular-dendritic crystals preferentially grow into the interior nugget and the growing orientation of crystals is approximately perpendicular to the fusion line. The width of HAZ is about 0.1-0.2mm, a little narrow, and grain boundaries in HAZ melt and grains of HAZ become coarser than those of base metal.(2) Due to the constitutional supercooling, weld nugget of AZ31B alloy generally contains two different microstructures, the cellular-dendritic crystals at the edge of the nugget and the equiaxed dendritic crystals in the center of the nugget. The phase composition of weld nugget consist ofα-Mg and littleβ-Mg17Al12 at the grain boundaries. (3) The inoculation decreases the cellular-dendritic crystal zone and enlarges the equiaxed-dendritic crystal zone accordingly. Moreover primary dendritic and secondary dendritic become finer. In additon, equiaxed -dendritic crystal tends to form equiaxed crystal. In this experiments, the most appropriate addition for each inoculant is as follows: 0.5mg B , 0.5mg B2O3, 0.8mg KBF4, 1.7mg C2Cl6, 0.1mg MgCO3, 0.8mg CaCO3, 0.2mg K2TiF6, 1.4mg Al-10Sr, 1.6mg Al-5Ti-B.(4) By preliminarily analyzing the refining mechanism of each inoculant, we come to the conclusions as follows. The first, heterogeneous nucleation: TiB2 contained in the Al-5Ti-B alloy can directly act as crystal nucleus forα-Mg, B-contained inoculants such as KBF4, B, B2O3 can react with Mg to form MgB2 which can serve as nucleation site forα-Mg, similarily, C-contained inoculants such as C2Cl6, MgCO3, CaCO3 can react with Al to form Al4C3 which serves as nucleation site while Ti-contained inoculants such as Al-5Ti-B, K2TiF6 refineα-Mg through peritectic reaction: L +(α?Ti)→(α?Mg); the second, solute segregation and adsorption: On the one hand, during the solidification, segregation of K, B, Sr at the liquid-solid interface will lower the liquid line temperature and lower the degree of supercooling accordingly under the same condition, which will result in the bottling , remelting and liberating of secondary dendritics, and make equiaxed crystal further propagate. On the other hand, the solute adsorption will restrict the crystal growth and get more time for nucleation. This kind of inoculants include KBF4, B, B2O3, K2TiF6 and Al-10Sr. It can be seen from the experimental results that the refining mechanism of each kind of inoculant is not the same, and for a given inoculant, it maybe also refineα-Mg under one or two refining mechanism. Generally speaking, the inoculation is a complex phisical and chemical process whose true mechanism has not been cognized. Above is preliminary research and it deserves furth study.(5) The spot welded joints have two failure modes (interfacial failure and button pullout failure) under tensile shear loading conditions. For the button pullout failure, the crack initiates at cellular-dendritic crystals of the nugget and propagates along the cellular-dendritic crystals, heat affected zone (HAZ) and base metal in sequence. After inoculation, the crack propagates along the cellular-dendritic crystals, equiaxed-dendritic crystals, cellular-dendritic crystals, heat affected zone and base metal. The path-length of crack propagation increases after inoculation.(6) The inoculation improves mechanical properties of resistance spot welded joints. Effect of inoculants on improving mechanical properties in incremental sequence is CaCO3, Al-10Sr, C2Cl6, Al-5Ti-1B, K2TiF6, B2O3, KBF4, B and MgCO3. Compared with the joint tensile shear load without inoculating process, tensile shear load of joint with 0.8mg CaCO3 increases by 26.4%, tensile shear load of joint with 0.9mg Al-10Sr increases by 24.3%, tensile shear load of joint with 1.1mg C2Cl6 increases by 21.5%, tensile shear load of joint with 0.9mg Al-5Ti-B increases by 20%, tensile shear load of joint with 0.2mg K2TiF6 increases by 18.9%, tensile shear load of joint with 0.5mg B2O3 increases by 16.2%, tensile shear load of joint with 0.8mg KBF4 increases by 8.7%, tensile shear load of joint with 0.5mg B increases by 7.7%, and tensile shear load of joint with 0.1mg MgCO3 increases by 6.5%.