Study On Low Temperature Ultrasonic Assisted Brazing Process And Mechanism Of Fine Grained Aluminum Alloy

Aluminum alloy is widely used in automobile manufacturing,rail transit,aerospace,biomedicine,clean energy and electronic devices due to its light weight,high specific strength and good comprehensive mechanical properties.However,with the development of science and technology,the social demand for higher performance aluminum alloy materials is also increasing.Fine grained aluminum alloy has the advantages of ultra-high strength,excellent ductility and good wear resistance,so it becomes one of the candidates for high performance aluminum alloy materials.However,due to the poor thermal sensitivity and weldability of this material,the joining becomes a major difficulty in the application of fine-grained aluminum alloy.The effect of different thickness of filler metal layer and ultrasonic time on the interface metallurgical mechanism of ultrasonic assisted brazing of fine-grained aluminum alloy was studied.Firstly,the temperature field of ultrasonic brazing with different thickness of solder layer was simulated by finite element method,which can provide theoretical guidance for subsequent process test and interface metallurgical reaction.The finite element model was established by using ABAQUS software,and the temperature field of the interface during ultrasonic welding was simulated and analyzed.The results show that with the decrease of the thickness of the solder layer,the temperature rise rate of the Zn interlayer increases significantly,and the time required for the solder to liquefy also decreases.The ultrasonic time needed for 50?m ? 30?m ? 10?m thickness solder liquefying is 1.005 s,0.615 s and 0.29 s respectively.Ultrasonic assisted brazing of fine grain aluminum alloy at 400? was realized,and the microstructure of brazed joint consists of ?-Al??-Zn,diffusion layer and eutectic structure.It is found that with the increase of ultrasonic time,the dissolution and diffusion of Al and Zn elements will be greatly promoted,and the Al content in the brazing seam will gradually increasethe content of ?-Zn and eutectic in the brazing seam decreased,and the ?-Al of content and the thickness of diffusion layer increase gradually,and the full ?-Al solid solution joint will be formed eventually.When the thickness of solder layer is 30?m,the ultrasonic time of 0.5s can not make the Zn interlayer melt completely.The solder joint formed is composed of a large amount of Zn and a thin layer of ?-Al at the interface,and the joint strength is only 45.4MPa;When the ultrasonic time is 1.5s,the solder melts completely and the solder joint is composed of a small amount of dispersed ?-Al,?-Zn solid solution and a large amount of eutectic structure.At this time,the shear strength increases to 123.5 MPa;When the ultrasonic time reaches 7s,the ?-Al solid solution joint can be formed,the ?-Zn and eutectic structure disappear,and the shear strength of the joint can be increased to 213.8MPa.In addition,by controlling the thickness of the solder layer,the metallurgical reaction rate of the interface can be controlled.When the thickness of solder layer is 50?m and the ultrasonic time is 9s,the full ?-Al solid solution joint can be formed.When the thickness of solder layer is 30?m and 10?m,formation time of the full ?-Al solid solution joint is7 s and 3s respectively.The grain size of the full ?-Al solid solution joints with different thickness of solder layer is also discriminative and this leads to a difference in performance.The grain size of the full ?-Al solid solution joints with 50?m thickness of solder layer is the smallest.The average grain size is about 3.47?m and the shear strength can reach 238.5MPa.The full ?-Al solid solution joints with 30?m thickness of solder layer was treated by post weld heat treatment.With the increase of holding time,the interface gradually changes from the full ?-Al solid solution joints to fully diffused layer,and the strength of the joint first increases and then decreases.When the holding time is 10 min,the strength of brazed joint can reach 235.6 MPa.When the holding time is 30 min,the mechanical properties of the base metal deteriorate seriously.The fracture occurs on the base metal side,and the strength of the brazed joint is only 176.3 MPa.