Study On Contact Reaction Brazing Process Between Magnesium Alloy And Steel

Nowadays,vehicle lightweight has become the trend of automobile development,due to the needs of environmental protection,energy saving and safety.With the increase of magnesium alloy and steel consumption,the join between magnesium alloy and steel has attracted the attention of scholars at home and abroad.The solid solubility between magnesium and steel is almost zero,so the solid solution and compound can not be generated under the liquid state and almost no metallurgical reaction happens.The melting point,thermal conductivity and linear expansion coefficient are quite different.It is very difficult to realize the connection between magnesium alloy and steel.But the mechanical properties are not satisfactory,limited to heat input and magnesium alloy properties.Contact reaction brazing is a method to realize the whole reliable connection by producing micro liquid phase on the surface of the material at a certain temperature and pressure,and after a certain time diffusing atomically.During the connection process,Mg and Fe are almost no metallurgical reaction happens.So add Cu intermediate layer to magnesium alloy and steel.Firstly,it can alleviate the difference of thermal expansion coefficient between magnesium alloy and steel,on the other hand,it also can obstruct the formation of brittle phases.AZ31B magnesium alloy sheets and Q235 steel sheets were brazed with 30?m pure copper foil and 30?m Cu-Zn foil,which adjusted the heating temperature and holding time to find the optimal parameters and studied the effects of them on microstructure and mechanical properties of the joints.The results showed that when using the pure copper foil,the main phases of the interface reaction zone are Mg solid solution,Mg₂Cu and AlMg under the conditions of effective process parameters.The maximum tensile strength of brazed joint is 122.6 MPa,when the heating temperature is 570?and the holding time is 30 min,too high or too low,which leads to a decrease in the tensile strength.Meanwhile,the fracture mode is typical ductile fracture.The results showed that when using the Cu-Zn foil,the optimal parameters are that the heating temperature is 570?and the holding time is60 min.Under the optimal parameters,the maximum tensile strength of brazed joint is 79.4 MPa.The main phases of the interface reaction zone are Mg solid solution,MgCu₂?CuMgZn?Cu₃Zn?Al_0.9Mg_3.1.Meanwhile,the fracture of brazed joint is due to the brittle intermetallic compounds.