The diffusion bonding of AZ31B Mg alloy and Q235 steel was investigated with a Zn-5Al alloy as interlayer and under different holding time ranging from 3 to 1 200 s. The microstructure and phase compositions of bonded joints were characterized by scanning electron microscopy( SEM),energy dispersive spectrometer( EDS) and X-ray diffraction( XRD)methods. The shear strength of Mg alloy/steel joints was measured by tensile tester. It was found that the microstructure of bonded joints evolved dramatically along with the prolongation of holding time. Under the holding time of 3 s,the main part of joint was composed of MgZn_2 phase and dispersed Al-rich solid solution particles. When increased the holding time more than 60 s,the excessive solution of AZ31B into the interfacial reaction area led to the formation of coarse phase and eutectic microstructure,and also the complex Fe-Al and Mg-Al-Zn IMCs at transition layer closed to Q235 steel side. According to the tensile testing characterizations,the joints obtained under holding time of 3 s exhibited the best shear strength of 84 MPa,and the fracture occurred at the intermediary part of joint where the flexible Al-rich solid solution particles could help to impede the microcrack propagations. With prolonging the holding time to 600 s,the shear strength of joints was deteriorated enormously and the fracture position was shifted to the transition layer part closed to Q235 steel.
