| In order to exert the excellent performance of magnesium alloys and steels,expand the application range of magnesium alloys,it was important significance to connect magnesium alloys with steels.TIG welding method was widely used in the welding of non-ferrous metals and stainless steel because of its advantages of simple equipment,convenient operation,high production efficiency and good welding quality.Aiming at the outstanding problems of poor metallurgical compatibility,poor interface wettability and low properties of magnesium alloy/steel TIG brazing joints.In this paper,the lap brazing test of AZ31B magnesium alloy and Q235 steel was carried out with Zn and H62 brass alloy interlayers under the condition of welding current of 70 A,welding speed of 25?30 cm/min,argon flow of 12 L/min,welding torch and welding direction of 120°?130°angle.Based on the comparative analysis of weld metal spreading properties of welded joints,the effects of sandwich material,interlayer thickness and post-weld annealing temperature on microstructure and mechanical properties of welded joints were studied.The main conclusions are as follows:(1)The weld of magnesium alloy/steel TIG brazing joints with Zn and H62 brass interlayer is well formed.The microstructure of the interface between magnesium and steel is very dense.A thin AIFe compound layer was formed at the magnesium/steel interface.The wettability of magnesium alloy/steel interface increases with the increase of interlayer thickness.Compared with the joints with Zn interlayer,the interfacial binding length of joints with H62 interlayer is slightly smaller.(2)The weld zone with Zn interlayer is mainly(a-Mg+MgZn)eutectic and a smal amount of Mg17Al12 intermetallic compound.The content of eutectic structure in weld zone increases with the thickness of Zn interlayer.In the magnesium alloy/steel joint with H62 brass sandwich,the Mg-Cu compound region is formed in the heat affected zone of magnesium alloy due to the diffusion of Cu element along the grain boundary of a-Mg matrix,that is,the permeable zone of grain boundary.The irregular embedded structure of the interface near the steel side is formed due to local corrosion,which is beneficial to the improvement of the interface adhesion.The weld zone is composed of a-Mg and radiate eutectic.Compared with Zn sandwich joints,the content of eutectic in weld zone of H62 sandwich joints is less,the grain size of a-Mg grains in heat affected zone increases with the thickness of H62 interlayer,and the width of grain boundary permeation zone of magnesium alloy base metal increases.(3)The micro-hardness of TIG brazed joint of magnesium alloy/steel with Zn and H62 brass sandwich is higher than that of base metal.The hardness increases with the increase of interlayer thickness.The micro-hardness of the heat-affected zone with Zn interlayer is lower than that of the base metal of magnesium alloy by 5%?10%.the hardness of the heat-affected zone with H62 interlayer is increased by 10%?20%.(4)With the increase of sandwich thickness,the tensile strength of the joint increases first and then decreases.The tensile strength of the joints with a thickness of 100 ?m Zn sandwich is the highest(348N/mm),which is 122%higher than that of the non-sandwich joints.For the joints with H62 interlayer,the a-Mg grain growth was inhibited by the compound in the permeable zone of grain boundary,and the grain boundary strength was strengthened to a certain extent.The tensile strength of the joints was up to 409 N/mm,which was 161%higher than that of the joints without interlayer.(5)Compared with the H62 brass sandwich joints without heat treatment,the amount of eutectic in the weld zone of the welded joints annealed after welding is reduced.With the increase of annealing temperature,the grain size of a-Mg in the weld zone and heat affected zone of the joint decreases first and then increases.(6)The micro-hardness of weld zone and heat affected zone after annealing is slightly lower than that before heat treatment.The tensile line strength of the joint increases first and then decreases with the increase of annealing temperature.The tensile strength of the joints annealed at 350?×4 h is the highest,reaching 428N/mm,which is 4%higher than that before heat treatment. |
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