Mechanism Of Interfacial Bonding And Soldering Process Assisted By Ultrasound Of2024Al Joint

2024aluminum alloy has been widely used in the field of electronic industryand manufacture of airplane. Cracking and softening can hardly be ignored duringthe welding of2024aluminum alloy as the joining temperature is more than502°C.It is because the strengthening phases of2024aluminum alloy will re-dissolve at502°C. Therefore, the application of2024aluminum alloy has been limited. Theaim of this paper is to solder of2024aluminum alloy at a low temperature. Wesoldered2024aluminum alloy with Sn-based solders and Zn-based solder byultrasonic-associated soldering.The spreading and wetting behavior of solders on the oxide film was firstlystudied. The bonding between solders and oxide film under ultrasound was observed.Secondly, the dissolved behavior of Al in Sn under ultrasound was investigated, andthe bond with aluminum dendrites reinforced was also obtained. The bond ofinterface between Sn/Al has been strengthened by ultrasonic soldering with Sn-xZnsolders. The main problem of the soldering with Zn-based filler metal is tominimize the volume fraction of eutectic. The eutectic in the bond decreased sharplyand even disappeared by increasing the ultrasonic time and holding time, which isalso improve the joints serviceability temperature.The spreading, wetting and bonding between solders and oxide film underultrasound were studied by using micro-arc oxide to simulate the soldering oxidefilm. The results suggest that Sn-based and Zn-based solders can spread and wet theoxide film under ultrasound by TEM and HREM observation of the interface. Thewetting angle were obtuse, but the there are two amorphous layer joining the soldersand the oxide film. Pure Sn can wetting the soldering oxide film of2024aluminumalloy under ultrasound, and bonding of Sn and oxide film was observed. Thewetting conditions of Sn on oxide film under ultrasound were deduced.It is found that the main factors affect the content of Al in the bond areultrasound parameters by study the holding time, soldering temperature. As theamplitude of the ultrasound increased from4μm to8μm, the content of Al in thebond is from0.53%to2.5%. As the ultrasonic time increase to60s, the content ofAl in the bond is up to3%. The migration model of formation of Al dendrites in thebond has been proposed based on the study of dissolution behavior of Al in Sn byincreasing the ultrasonic time and tracing the strengthen phase of the base metal. Aldendrites reinforced bonds were obtained. The process of ultrasonic associatedsoldering with the vacuum evaporating filler metal was presented, and the content of Al in the bond is up to85.4%.High tensile strength of joints with Sn-xZn (x=4,9,20) solders were obtained,which is about4times high than the joint soldered with pure Sn solder. The tensilestrength of joints increases from34-41MPa to158-189MPa. The interface bondingwas strengthened. An amorphous layer was observed at the interface by TEM andHREM observation and analysis, which is Sn61Al34Zn5layer. And the strengthenmechanism of interface was presented.The amorphous layer formed in the ultrasonic process. Small areas of Al aremelted by the local high temperature generatred by ultrasonic cavitaons, and Sn, Znand Al are fully mixed. The cooling rates during cavitation bubbles collapse can becalculated, which is greater than6.6×10~8K/s. It is much higher than the coolingrate for formation of the amorphous alloys. The atoms cluster of Sn61Al35Zn4willsolidify immediately at the temperature of300°C soldering temperature. Theamorphous layer of Sn61Al35Zn4formed at300°C before the solder solidified. Thethickness of the amorphous layer is about15-25nm with pure Al substrate.15-25nm thick amorphous lays are obtained with Sn-4Zn and Sn-9Zn solders. Thethickness of the amorphous layer increased to30-40nm with30s ultrasonic time.And the chemical compositions of the amorphous layers changed lightly..The volume fraction of eutectic is minimized by applying the long ultrasoundand long holding. As the ultrasonic vibration time increases to30s, the striatedeutectic phases change into devoiced eutectic phase of α Al phase and η Zn phase.And the content of the fine eutectic phase declines from12.9%to0.9%.The contentof Al element increases from13%to25%as the soldering time increases from3s to30s. The eutectic disappeared as the holding time is30min. The shear strengthincreases with ultrasonic vibration time increasing. The joints strength withultrasonic vibration of30s is149-153MPa, which is20%higher than that of3s. Thejoints strength soldered with30min holding time is up to177-184MPa, which is40%improved, as the eutectic is disappeared. The fracture model was presented byinvestigating of shear strength, fracture analysis, nano-indentation test.