Process And Mechanism Research On Brazing Of AZ31B/3A21

As the lightest and most economical structure metallic material,magnesium alloy has the advantages of high strength and specific rigidity,strong shock absorption and absorption performance,its wide use can not be separated from the connection technology support,especially with the aluminum alloy formed by the composite structure.However,magnesium alloys and aluminum alloys will inevitably produce a large number of harmful intermetallic compounds in the welding process,which seriously affect the joint performance.In this paper,the AZ31 B / 3A21 joint was subjected to ultrasonic assisted low temperature brazing with Sn-Zn brazing filler metal.Through the establishment of dynamic mathematical model,the analysis of the asymmetric interfacial reaction occurs during the brazing process of dissimilar materials was carried out,and the optimum process parameters are obtained by calculation and process test.And the effects of brazing filler metal composition,process parameters(brazing temperature and holding time)on the microstructure and properties of brazed joint were analyzed,the main resultss are as follows:The Sn-Zn brazing AZ31 B / 3A21 dissimilar alloy joint can effectively block the reaction of Mg alloy with Al alloy to produce harmful intermetallic compound and can achieve high quality and reliable connection.The fracture sites of AZ31 B / 3A21 dissimilar metals with Sn-x Zn brazing filler are Mg2 Sn enrichment.The brazing joint of the six brazing materials generates a thin layer of Mg2 Sn intermetallic compound with a few microns on the interface between the brazing filler metal and the Mg base material.In the Sn-3Zn,Sn-9Zn,Sn-20 Zn joints,near the side of AZ31 B magnesium alloy base,it was the block Mg2 Sn intermetallic compound accumulation area.In the Sn-30 Zn joint,the Mg2 Sn thin layer on the reaction interface near the side of the magnesium alloy base material is an easy-broken layer with an average shear strength of 67.26 MPa.In the Sn-40 Zn joint,massive Mg2 Sn is formed in a large amount or even thick layer,the intermetallic compound layer is thick and very brittle,it's an easy-broken layer.The average shear strength of the Sn-50 Zn joint is the lowest,only 20.69 MPa.The Sn-30 Zn brazing filler metal is considered to be the best filler for brazing of AZ31 B / 3A21 dissimilar metals.For the AZ31 B / Sn-30 Zn / 3A21 joint,the Mg2 Sn compound layer is a layer of Mg2 Sn compound at the interface between the Mg alloy and the base metal.The central area of the brazing seam is mainly the bulk Mg2 Sn intermetallic compound and the dispersed chrysanthemum Al-Sn-Zn solid solution;and in the solder seam near the 3A21 aluminum alloy base material side is mainly a layer of Al-Sn-Zn solid solution transition layer.The dispersion of Al-Sn-Zn solid solution in the AZ31 B / Sn-30 Zn / 3A21 connector can effectively reduce the brittleness of the joint,so it can get better performance brazing jointThe elemental kinetics diffusion analysis was carried out for the thin layer of the fracture layer and Mg2 Sn intermetallic compound in AZ31 B / Sn-30 Zn / 3A21 joint.The results show that the diffusion coefficient of Sn and Zn atoms in Mg at the same temperature is about 200 times that of Al,that is,the diffusion of Sn and Zn-Mg is much greater than that of Sn and Zn-Al.Under the same holding time,the brazing temperature has a great influence on the diffusion depth of Mg atom.At the same brazing temperature,the holding time has little effect on the diffusion depth of Mg atom.Different brazing temperature and holding time have great influence on the microstructure of the joints.The length of the holding time has a great influence on the width of the welding seam,which has a great influence on the existence of Mg2 Sn intermetallic compound near the Mg alloy.With the increase of brazing temperature,the number and distribution of Al-Sn-Zn solid solution in brazing seam,the morphology and distribution of Mg2 Sn have changed obviously.When the brazing temperature is 330 ? and the holding time is 5s,the Sn-30 Zn brazing filler metal is used,and the best shear strength of AZ31 B / 3A21 dissimilar metal joint under ultrasonic assisted and atmospheric environment is 69.18 MPa.