Mechanism Of Metallurgical Strengthening Of Al Alloy Joints Soldered By Ni-Cu Foam/Sn-Based Composite Solder At Low Temperature

Al alloys,exhibiting high strength,low density,and good electrical conductivity,are commonly used in radio-frequency and microwave high-frequency devices.However,Al alloys will be softened and deformed by using the traditional methods such as fusion welding and friction stir welding（FSW）,which are not suitable for the connection of precise electronic devices.At present,the most common method is to electroplate some metal layers on the surface of the Al alloy to improve its wettability,and then solder with Sn-based solders at low temperature.But,the problems of low strength and poor welding accuracy of Sn-based solders limit the application of low-temperature soldering of Al alloys.Therefore,in this paper,Ni metal foam and Ni-Cu alloy foam were introduced to strengthen Sn-based solders and realized the connection of Ag-plated Al alloys by soldering at low temperatures.The Ni-SAC composite solders with different porosity were used to solder the Ag-plated Al alloys,and the effects of Ni metal foam on the metallurgical reaction and mechanical properties of the Al alloy joints were studied.The results show that the Ni metal foam with 60%porosity had a better strengthening effect because the dense Ni skeleton can effectively support the welding seam and reduce the overflow of Sn solder,then controlled the welding seam accuracy.The shear strength of the Al joints was improved by the double interpenetrating network structure composed of the uniformly distributed Ni skeleton and the intermetallic compounds（IMCs）formed by the reaction.However,with the increase of brazing time,the（Ni,Cu）₃Sn₄ grains coarsened and grew,resulting in a decrease in the mechanical properties of the Al alloy joints.The max shear strength of the joints was 50.25MPa,which was about 50%higher than the traditional low-temperature brazing Ag joints.The high-strength Al alloy joints were rapidly fabricated by using 60%Ni-Cu-SAC composite solders with different Cu contents,and the mechanism of formation and evolution of IMCs in Al alloy joints and their effects on mechanical properties were investigated.The addition of the Cu atom increased the rate of metallurgical reaction,and the Sn solder was completely exhausted in 5min.The formation of the initial phase in the joints was affected by the content of Cu atoms in the Ni-Cu alloy skeleton.In the joints soldered with 60%Ni18Cu-SAC and 60%Ni30Cu-SAC composite solders,the solubility and diffusion rate of Cu atoms are much higher than that of Ni atoms,resulting in a large number of Cu atoms dissolved into the Sn solder and reacting to form（Cu,Ni）₆Sn₅ phase;but in the joints soldered with 60%Ni38Cu-SAC composite solder,a large number of Ni atoms dissolve into Sn solder due to the lower melting point of Ni38Cu alloy skeleton,and the dissolution of Ni atoms will reduce the solubility of Cu atoms in Sn solder,and then a large number of Ni atoms reacted with Sn solder to form（Ni,Cu）₃Sn₄ phase.With the increase of brazing time,the solid-phase diffusion of Ni and Cu atoms was driven by the concentration gradient between the IMCs and the Ni-Cu alloy skeleton,resulting in the phase transition of IMCs.In the 60%Ni18Cu-SAC and60%Ni30Cu-SAC composite solder brazing joints,the Ni atoms diffused from the Ni18Cu and Ni30Cu alloy skeleton into the（Cu,Ni）₆Sn₅ grains and substituted for Cu atoms,when the content of Ni atoms in（Cu,Ni）₆Sn₅ exceeded 24 at.%,the（Cu,Ni）₆Sn₅phase transformed into（Ni,Cu）₃Sn₄ phase;but in the 60%Ni38Cu-SAC composite solder brazing joints,the Cu atoms diffused from the Ni38Cu alloy skeleton into the（Ni,Cu）₃Sn₄ grains and substituted for Ni atoms,when the content of Cu atoms in（Ni,Cu）₃Sn₄ exceeded 8 at.%,the（Ni,Cu）₃Sn₄ phase transformed into（Cu,Ni）₆Sn₅phase.The IMCs formed after the phase transition had a finer grain structure,and formed the double interpenetrating continuous network structure with Ni-Cu alloy skeleton,which could hinder the extension of cracks and significantly improve the shear strength of the soldered joints.The highest shear strength of the joints was 58.3MPa,which was about 16.02%higher than that of the 60%Ni-SAC composite solder brazing joints.