| Magnesium alloys are the lightest engineering structures.With the development of various high performance magnesium alloys,more and more magnesium alloy structure is applied in every field.Brazing has the advantages of low welding temperature,small distortion,high accuracy of welding parts,and has a unique advantage in the connection of complex structure parts and thin-walled structure parts.Because the variety of magnesium based filler metal is less than that of magnesium alloy,complicated preparation technology,and the cost is high,which limits the application of magnesium alloy brazing structure.In this experiment,Mg-Cu binary eutectic and Mg-Ni-Cu ternary eutectic solder alloy were prepared by diffusion couple electron microscopy(SEM),scanning electron microscopy(SEM),energy dispersive spec-troscopy(EDS),X-ray(XRD),thermal analyzer and so on,and the microstructure,element composition and distribution,phase composition,melting and wetting and spreading characteristics of the filler metals were measured and analyzed.Using filler metals brazing magnesium alloy AZ31Bb,The microstructure and mechanical properties of AZ31B brazed joints under certain conditions were analyzed and tested.The main research contents and conclusions are as follows:(1)The Mg-Cu solder alloy was mainly composed of(?-Mg+CuMg2+Cu2Mg),CuMg2 and Cu2Mg were obtained by the diffusion of Mg-Cu binary diffusion couple at 500?-550? for 4h and 9h.The microstructure of the solder alloy is unevenly distributed,close to the magnesium substrate side have more eutectic and close to the copper substrate side have more compounds.The width of the eutectic layer increases with the raise of the heating temperature and the holding time.At the same heating temperature,with the prolonging of the holding time,the amount of CuMg2 and Cu2Mg compounds increases and becomes coarse.(2)The solder prepared at 500? for 4h,520? for 4h,550 ? for 4h and 550 ? for 9h has an obvious endothermic peak similar to the eutectic reaction.,The melting range of the brazing filler metal at 550? for 4h is the smallest(485.8?488.9?)and the melting point is the lowest,indicating that the composition of the solder is closer to the eutectic composition.The broadening coefficient of that solder is about 89.8%and the wettability is good on the pure copper surface.(3)The Mg-Ni-Cu ternary solder was mainly composed of(a-Mg + CuMg2 + Mg2Ni),Cu2Mg and Cu11Mg10Ni9 were obtained by the diffusion of Mg-Ni-Cu ternary diffusion couple at 500 ?-550 ? for 4h and 9h.With the increase of heating temperature,the intermetallic compound in the solder gradually grows,and the shape evolves from slender granules to slab or even coarse granules.With the prolonging of the holding time,the amount of eutectie decreases but the amount of compounds increases.With the increase of the thickness of the Ni layer,the Mg-Ni-Cu magnesium alloy solder is reduced.(4)The thermal diffusion of Mg/Ni/Cu ternary diffusion couple with different thickness of Ni at 500?-550? for 4h and 9h,the prepared solder has endothermal peaks similar to reaction eutectic.The melting range of the brazing filler metal at 0.05mm,500 ? for 4h is the smallest(479.2?483.7?)and the melting point is the lowest,which indicates that the composition of the solder is closer to the eutectic composition.The solder alloy has good wettability and spreadability on the surface of the copper alloy,and its wetting and spreading performance is inferior to that of the Mg-Cu binary solder alloy.(5)The AZ31B brazing was carried out at 500? for 10min using the Mg-Cu binary,Mg-Ni-Cu ternary solder with the best melting properties.The microstructure of weld joint was compact,The interface between the solder alloy and base metal were well combined.Weld joint shear fracture occurs in the brazed seam,the maximum shear strength were 68 MPa and 61 MPa,the fractures both attrubutes to the mixed-mode of ductile and brittle fractures. |
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