Study On Microstructures And Mechanical Properties Of Resistance Spot Welded Magnesium Alloy (AZ31B)

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, resistance spot welding plays an important role in material joining techniques, 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 microstructures 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 microstructures and mechanical properties of resistance spot welded magnesium alloy (AZ31B). Microstructures of spot welded magnesium alloy joints, effects of welding parameters on microstructures and mechanical properties of spot welded magnesium alloy joints have been investigated so as to find out the optimized welding parameters and improve joints properties. Furthermore, inoculation in spot welded magnesium alloy nugget has been investigated, which improves spot welded magnesium alloy joints in quality. By analyzing the experimental data, some conclusions have been drawn as follows:(1) Resistance spot welded magnesium alloy (AZ31B) joints contain the nugget and heat affected zone (HAZ) encircling the nugget. Grain boundaries in HAZ melt and grains of HAZ coarsen. Solidification of the nugget begins from the boundaries and heterogeneously nucleates, having a characteristic of epitaxial growth. The growing orientation of crystals is approximately perpendicular to the fusion line. Cellular-dendritic crystals preferentially grow into the interior nugget.(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. Phase structures of weld nugget consist ofα-Mg and littleβ-Mg17Al12 at the grain boundaries.(3) Effects of welding current, electrode force and welding time on resistance spot welded magnesium alloy joints are obvious. An increase in welding current (15-23kA) results in an increase in nugget diameter with corresponding increase in joint strength because of increasing the nugget diameter. Higher welding current (25-27kA) causes metal expulsion during welding operation owing to excessive heat input and results in reducing joint strength; Effects of electrode forces (1.5-4.5kN) were studied. The nugget size and joint tensile shear load have increased with the decrease of electrode forces, but metal expulsion occurs at too low electrode force(1.5kN ); When welding time ranges from 1 to 16 cycles, the nugget size has increased and microstructures have coarsened obviously. The experimental results show that joint tensile shear load at 3000N can be obtained when choosing welding current at 23 kA, electrode forces at 2.5kN and welding time at 6 cycles.(4) 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.(5) Resistance spot weld nugget inoculation process of AZ31B alloy obviously decreases cellular-dendritic crystal area in solidified nugget microstructures, which reduces the weekness in weld nugget. Meanwhile, the fine grains can be obtained. The experimental results show that inoculating effect of K₂TiF₆ on weld nugget is not comparatively good. Cellular-dendritic crystal area is still broad and the grains are coarse. Inoculating effect of KBF₄ is best, which results in little cellular-dendritic crystal area and refining grains. Inoculating effect of K₂TiF₆+ KBF₄ mixtures respectively with a weight ratio of 2:1 and 1:4 is less effective than that of KBF₄ but better than that of K₂TiF₆.(6) Resistance spot weld nugget inoculation process improves mechanical properties of welded joints. Effect of inoculants on improving mechanical properties in incremental sequence is K₂TiF₆, K₂TiF₆+KBF₄(mass%2:1), K₂TiF₆+KBF₄(mass%1:4), KBF₄. Compared with the joint tensile shear load without inoculating process, tensile shear load of joint with K₂TiF₆ increases by 6.58 percents and tensile shear load of joint with K₂TiF₆+KBF₄ (mass%2:1) improves by 16.3 percents while tensile shear load of joint with K₂TiF₆+KBF₄ (mass%1:4) improves by 17.1 percents and tensile shear load of joint with KBF₄ increases by 20 percents. Meanwhile, resistance spot weld nugget inoculation process of AZ31B alloy is a new process in welding field and mechanics of resistance spot weld nugget inoculation process of AZ31B alloy are still to be further investigated.