Study On Contact Reactive Brazing Of Magnesium/Steel Dissimilar Metals

Magnesium alloys as the lightest metal engineering materials, has been widely used in automotive, aerospace and other important areas with low density, high specific strength and specific stiffness, advantages of recycling and so on. And steel is still the material basis of modern society, and one of the most used materials in the world with low price, reliable performance. If realizing the connection of magnesium and steel, can make the two play their respective advantages, and is of great significance in reducing the weight of structure, saving resources and other fields. But due to the huge physical and chemical differences between magnesium and steel, to realize the connection between them is very difficult, which also attract many scholars’attention and research.Contact reactive brazing is an advanced connection technology, which is reliable for dissimilar metals’effectively connection, and can choose appropriate intermediary materials according to different base composition, realizing the connection of dissimilar metals with good performance. In this thesis, using AZ31B magnesium alloy and Q235low strength steel as base materials, adopting the method of contact reactive brazing, adding different compositions of interlayers connected magnesium/steel dissimilar alloys. Through analyzing the variation of microstructure and mechanical properties of the joints under different process conditions, and the effects of different interlayer composition on mechanical properties of the joints, discussed the formation mechanism of joints.By hot dipping pure zinc interlayer on Q235surface, had realized the connection of AZ31/Zn/Q235by contact reactive brazing. The maximum shear strength was57MPa under the process of355℃connecting temperature, average rate of110℃/min, and insulation30s. Analyzing the fracture compositions of joints which had the best shear strength by XRD, can reach that the Fe11Zn40phase was a key to joint fragile, and the Fe-Zn intermetallic which was hard and brittle in pure zinc interlay was inevitable, which directly influenced the performance of the AZ31/Zn/Q235joints.Adding Al element into pure zinc plating bath, had formed the Zn-3A1, Zn-5A1, Zn-8A1three kinds of coating composition. And the maximum shear strength of three kinds of joints were higher than joints of AZ31/Zn/Q235. This was because that the transition layers in Zn-xAl coating were Fe2Al5compound layers, the performance of which were superior to Fe-Zn transition layers; and the fracture location of AZ31/Zn-xAl/Q235joints were MgZn2area which contained soft Al solid solution. The joints of AZ31/Zn-5Al/Q235because of the reasonable structures of hard sandwiched soft area, namely the Al solid solution particles which had uniform sizes distributed dispersively in MgZn2matrix, so the performance of joints can be the highest, which was96MPa.Through analyzing process that interface element diffusion, liquid generation and extension, solidification and so on, of the Zn/5A1/Q235joints which were under the optimum technological parameters, discussed the connection mechanism of joints. In the stage of liquid generation and extension, Al solid solution particles in interlays began to dissolve with the diffusion of Al element, and the size became little. So if we want to get good performance, we must strictly control the size of Al solid solution particles in this stage.