| Magnesium and its alloys is the lightest structural material in the metallic materials which can be applied currently.Magnesium and its alloys have the properties of low density,high specific strength,high specific modulus,good damping shock resistance,electromagnetic shielding and recyclables and so on. Stainless steel is one of the most widely used metal materials in industrial production.It has the properties of excellent mechanical strength,good oxidation resistance,high toughness,well weldability,especially the superior corrosion resistance.It is necessary to form a composite structure using the magnesium alloys and stainless steel because of the complementarity,overlapping and generalized of the two kinds of metal materials in practical application.While the bigger difference in crystal structure and physical and chemical performance between the two metallic materials makes the welding of magnesium alloys and stainless steel is very poor.Therefore, it has great significance to seek a new idea and technology that can achieved effective connection of magnesium alloys and stainless steel.This topic studied the low temperature diffusion bonding of AZ31B magnesium alloys and304stainless steel with Ni plating layer,Ni foil and Cu foil as the intermediate transition layer.By means of SEM, EDS,XRD testing the microstructure morphology of the welded joint.The mechanical properties of the welded joint were tested by microhardness tester and other analytical testing methods. Study the relationship of the microstructure and mechanical properties and element distribution when using different intermediate transition layer and welding technology parameters. Analysis the formation mechanism of the bond interface.The study show that the metallurgical connection of AZ31B magnesium alloys and304stainless steel were achieved when using either Ni foil,Cu foil and Ni plating layer.When using the Ni plating layer as the intermediate transition layer,the mechanical properties of the welded joint was not very well.Due to the presence of P in the plating layer formed a large number of brittle phase which reduced the mechanical properties of the welded joint. In the low temperature diffusion bonging between AZ31B magnesium alloys and304stainless steel using Ni foil as the interlayer,the reactive and physical wetting produces a better wettability of Mg-Ni eutectic liquid at the interface of the AZ31B magnesium alloys and304stainless steel.The welded joint include four diffusion layers,the grain boundaries layer,Mg-Ni-Al ternary intermetallic compounds layer,the low-melting eutectic layer,and the diffusion layer on304stainless steel side.The microstructure of interface zone of AZ31B/Ni/304stainless steel composite from magnesium alloys to stainless steel includes the grain boundary penetration layer composed of Mg-Ni-Al ternary intermetallic compounds which was precipitated along the grain boundary of a-Mg solid solution,Mg-Ni-Al ternary intermetallic compounds, eutectic of a-Mg+Mg2Ni,Fe-Ni alloy products.Especially the Mg-Ni-Al ternary intermetallic compounds layer,the Mg-Ni-Al ternary intermetallic compounds is surrounded by eutectic organization (a-Mg+Mg2Ni) to form a composite structure,this composite structure formed a ductile matrix form which is very good for the bonding strength of the joint.With extending the holding time,the micro-hardness and shear strength of the welded joint are presented a trend that first increases then decreases.The shear strength have the maximum value of37MPa at510℃for20min,about27%of the AZ13B base metal.The fracture was showing quasi-cleavage fracture characteristics.Building the material mechanics model of Ni atoms diffusion to the grain boundary of AZ31B magnesium alloy based the study results of the joint with Ni,for the penetration depth of Ni atoms to the grain boundary can expressed by the formula asIn the low temperature diffusion bonging between AZ31B magnesium alloys and304stainless steel using Cu foil as the interlayer,its mechanism is similar with Ni foil.But there is no obvious diffusion layer formation,because the wetting spreading performance of Mg-Cu eutectic liquid on the stainless steel side is poorer.The joint include the grain boundaries layer,a-Mg layer,Mg-Cu-Al ternary intermetallic compounds layer and the low-melting eutectic layer.The microstructure of interface zone of AZ31B/Cu/304stainless steel composite from magnesium alloys to stainless steel includes the grain boundary penetration layer composed of Mg-Cu-Al ternary intermetallic compounds and Mg17(Cu,Al)12,which were precipitated along the grain boundary of a-Mg solid solution, a-Mg solid solution,Mg-Cu-Al ternary intermetallic compounds and eutectic of a-Mg+Mg2Cu.The shear strength have the maximum value of52MPa at530℃for30min,about63%of the AZ13B base metal.With extending the holding time,the micro-hardness and shear strength of the welded joint are presented increase at first,and then decrease. The fracture was showing mixed fracture morphology. |
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