Diffusion Bonding Of AZ31B Magnesium Alloy And AZ31B/LY12Aluminum Alloy

In this paper, AZ31B/AZ31B and AZ31B/LY12were bonded together with50μm pureCu foil as interlayer by diffusion bonding teehnology. The microstructure, elementdistribution, phase composition, hardness and shear strength of joints were studied with OM,SEM, EDS and XRD, micro-hardness tester and universal tensile testing machine. Moreover,The distribution of axial stress and shear stress in AZ31B/Cu/LY12diffusion bonding jointwere studied using finite element method (FEM). Analysis of the effects on diffusion bondingprocess parameters of joints stress distribution. The main results can be summarized asfollows:AZ31B/Cu/AZ31B could be combined closely diffusion brazing joint with temperature of500~540℃, holding time of10~40min and pressure of0.1MPa. Interface region of diffusionbrazing joints includes three regions:(1) Grain boundary diffusion layer of magnesium sidecomposed of α-Mg(Cu) matrix and along the grain boundary precipitated networkMg17(Al,Cu)12,(2) The fusion region of α-Mg(Cu) cellular structure,(3) The intermediatediffusion layer consist of α-Mg(Cu), Mg2Cu and the eutectic.With the increase of heating temperature and the holding time, Shear strength ofAZ31B/Cu/AZ31B diffusion brazing joints interfacial ascends and then declines. Shearstrength of joints to maximum of69.74MPa in the condition of460℃and120minutes, about50%of the Mg base metal, the fracture was cleavage or quasi-cleavage fracture characteristics.With the increase of heating temperature or the holding time, diffusion region of interfacialhardness and width are increased.AZ31B/Cu/LY12diffusion bonding under the temperature of450~480℃, holding time60~120min, Pressure2.5~5.0MPa, diffusion brazing joint with combined closely can beobtained. The micro-structure of interface diffusion region included diffusion region ofmagnesium base, diffusion region of the middlelayer, diffusion region of aluminum base. The magnesium alloy side of Mg-based solid solution, a few Mg2Al3, Mg17Al12and Mg2Cu phase,and the aluminum alloy side of Al-based solid solution, Mg2Al3, Mg17Al12, a few AlCu andAl2Cu phase. Cu interlayer can effectively hinder the interdiffusion of Mg and Al atoms in thecondition of470℃,that inhibits MgxAlyintermetallic compounds produced.With the increase of heating temperature and the holding time, Shear strength ofAZ31B/Cu/LY12diffusion brazing joints interfacial ascends and then declines. Shear strengthof joints ascends and then declines with temperature rise and could reach to maximum of42.57MPa in the condition of460℃and120minutes. The fracture surfaces of jointsthroughout the interface diffusion region, close to the Al matrix side of the fracture appearbefore the cleavage fracture characteristics, and the magnesium matrix side of the fractureappear before cleavage and intergranular fracture characteristics. With the temperatureincreases width and microhardness increased trend. Close to the side of the aluminum matrixdistribution has a greater gradient to hardness then the magnesium matrix.Fracture surface analysis shows that AZ31B/Cu/LY12joints fracture process crackpropagation path can be summarized as along the side to expand, aluminum alloy weldingseam center area along the expanded and extended along the side of the three types ofmagnesium alloys. AZ31B matrix side presents the fracture cracks and cleavage of theintergranular crack morphology and LY12base side of the body fracture is mixed with Smalldimple fracture morphology and cleavage fracture morphology.Using the second law of diffusion, the joint interface area Mg, Al elements of thediffusion distance was calculated. The results indicated that the theoretical calculation valuewas basically accordance with the actual value.The results showed that near the edge of joints the interface stress magnitude, close to thecenter stress was well distributed. Joints the maximum tensile stress was in the AZ31B matrixside and the maximum compressive stress was in the LY12matrix side. As the heatingtemperature increased, the maximum axial stress increased; with the interlayer Cu thicknessincreasing, the maximum shear stress value increased; when welding pressure increased, thescope of the high stress region increased gradually.