The Mechanisms And Mechanical Properties Of Ultrasound Assisted Transient Liquid Phase Bonding Of Magnesium Alloy

Among all the welding methods for Mg alloy, transient liquid phase(TLP) bonding has the advantage of obtaining the joint microstructure and mechanical properties close to the base material. However, previous literatures indicated that the bonding temperature was high, the time required for the bonding process was long and a vacuum environment was needed, limiting the industrial application of this bonding method. In this paper, a novel ultrasonic-assisted transient liquid phase(U-TLP) bonding method was developed to joining MB8 Mg alloy, which was commonly used in the aerospace industry. The U-TLP bonding technology was explored with three developed interlayers of Zn foil, Pb foil and brass foil. The microstructures and mechanical properties of joints were investigated by microstructure analysis, X-ray diffraction analysis, constituent analysis, hardness analysis, shear strength analysis, fracture analysis. The mechanisms of U-TLP bonding were investigated on the basis of the experimental results.The U-TLP bonding with pure Zn interlayer indicated that: when applying 1s ultrasound at 370 °C, the Mg/Zn/Mg joint consisted of compounds layer with MgZn and Mg51Zn20, and Mg(Zn) solid solution layer. When the ultrasonic time increased, the compounds layer decreased. The fully Mg(Zn) solid solution joint was obtained by applying 120 s ultrasound at 370 °C. The joint shear strength was 106.4MPa, which fractured in the Mg base metal. In order to shorten ultrasound time, a two-step U-TLP bonding technology was developed, which applying first ultrasound at 370°C and applying the second ultrasound at high temperature. When the bonding temperature increasing to 490 ° C, a full solid solution joint was obtained by applying only 2s ultrasound. The shear strength of the joint was 109.27 MPa.Based on the microstructure and mechanical property studies on the U-TLP boding with pure Pb and brass interlayers. It indicated that the full Mg-based solid solution joint can be obtained in air rapidly with proper interlayer.The oxide film removal process in U-TLP process was that: the ultrasonic induced plastic deformation fractured the brittle oxide film locally, which provided a channel for element diffusion and eutectic reaction to produce the contact reactive liquid. The undermining diffusion and eutectic reaction suspending the continuous oxide film into the liquid phase, which was then been broken into small fragments by ultrasonic cavitation effect and been further squeezed out of the joint by ultrasonic squeeze action. It suggested that the isothermal solidification process was accelerated with the help of ultrasonic, which attribute to most liquid were squeezed out of the joint by ultrasonic and the residual liquid solidified by ultrasonic accelerated diffusion.