Study On Microstructure And Properties Of Al Alloy Ultrasonic Brazing Joints With Porous Ni-Sn Composite Solder

7075Al alloy acceptes extensive attention due to its excellent mechanical properties.However,strengthening phase may re-dissolve when suffering high temperature,which seriously deteriorates the mechanical properties of Al alloy.Therefore,Low temperature soldering 7075 Al alloy was proposed.Because of the poor mechanical properties of Sn-based and Zn-based lead-free solder,as well as the difficulty to break the oxide film located on Al substrate surface,a continuous reticulated foam Ni reinforced Sn composite solder was employed to solder 7075Al alloys with ultrasonic assistance at low temperature.Al/x%Ni-Sn/Al joints were prepared by using different porosity Ni-Sn composite solder.The effect of brazing process on joint microstructure and mechanical properties was investigated.The evolution mechanism of the joint structure and the strengthening mechanism of Ni-foam on the joint were analyzed.The bonding mechanism of Al/Ni-Sn interface and the interface reaction mechanism of Ni/Sn were discussed.The effect and mechanism of ultrasonic brazing process on the microstructure and properties of hig porosity porous Ni-Sn solder brazed joints were investigated.The tensile strength of 98%Ni-Sn and 80%Ni-Sn composite solder at room temperature was 49.77MP and 54.79MP,respectively,which was 34.72MP and39.74MP higher than that of pure tin solder.When pure Sn was employed as the interlayer,the structure was composed of?-Al and?-Sn,whereas the large-size?-Al particles were mainly distributed on the upper surface of brazing seam,making the structure extremely uneven.While in the joints of Al/98%Ni-Sn/Al the microstructure maily consisted of?-Al,?-Sn,Ni₃Sn₄ and Ni skeletons.With the ultrasonic vibration time and brazing temperature increase,the number of Ni3Sn4 particles increased gradually,with the refinement of?-Sn phase.While the ultrasonic vibration time was too long,the Ni skeleton would be broken.The TEM characterization of the Al/Ni-Sn interface showed that the Al base metal and the Ni-Sn composite solder were connected by a transition layer of Al₂O₃ about 100nm,which is amorphous near the Al/Al₂O₃ interface,and most of the transition layer was crystallized to form?-Al₂O₃.When soldered at 280?for 120s,the maximum strength of Al/Sn/Al and Al/98%Ni-Sn/Al joints was 60.93MPa and 69.08 MPa,respectively.The effect and mechanism of ultrasonic brazing process on the microstructure and properties of low porosity porous Ni-Sn solder brazed joints were investigated.When 80%Ni-Sn was employed as the interlayer,the microstructure maily consisted of?-Al,?-Sn,Ni₃Sn₄ and Ni skeletons.When the ultrasonic power was 115W,the bonding ratio was low,while the brazing microstructure becom uniformly when it raised to 120W.The growth rate of Ni3Sn4 drastic increased when the joint was holding for 30s before and after ultrasonic brazing.When the low-porosity porous Ni-Sn composite solder was used as the filler alloy,the Ni₃Sn₄ layer on the surface of the solder will greatly inhibit the diffusion rate of Sn and Ni elements.During the heat preservation,Ni3Sn4 layer could fully grow and peeling off under the function of grain boundary grooving and the Gibbs-Thomson effect.With the assistance of ultrasonic cavitation effect the growth of Ni₃Sn₄ would be greatly improved,as well as the spalling rate.While the ultrasonic vibration time and brazing temperature had a little effect on its growth.The maximum shear strength of the joint was 77.44MPa,which was 16.51MPa higher than that of Al/Sn/Al joints and 8.36MPa of Al/98%Ni-Sn/Al joints.When 60%Ni-Sn composite solder was used as the intermediate layer,the lower interface of Al substrate between solder alloy tends to have unconnected area,what's more there were many larger size pores in the middle layer,and the joint had a poor strength.