Study On The Procedure And Mechanism Of Low Temperature Joined Zr-BMG Joints With The Assistance Of Ultrasound

As a new type of material which combines the characteristics of metal and glass,metallic glasses?MG?have received worldwide attention from the day of inventation.Among MG alloy systems,Zr based metallic glasses?Zr-BMG?have been proved to be a great protential structural and functional material due to its excellent glass forming ability and outstanding mechanical properties.However,owing to the limitation of material size,the major issue in its application process was the joining of Zr-BMG alloys.Brazing was a common method for joining Zr-BMG.But conventional Zr-BMG brazing processes were not efficient to obtain the metallurgical joints due to the unique composition constituent and atomic configuration of Zr-BMG.Therefore,ultrasound-assisted liquid brazing of Zr-BMG joints using Zn-Al filler metal was conducted in this study.The microstructure evolution and interfacial wetting mechanisms of joints in ultrsound field were significantly investigated.The inherent relationship between microstructure and joint properties was analyzed.This work is promised to promote the practical application of Zr-BMG alloys and understand the interfacial wetting mechanisms of amorphous materials in ultrasound field.The effect of brazing temperature on the microstructure and properties of ultrasound-assisted brazing Zr-BMG/Zn-3Al/Zr-BMG joints was studied.The results showed that when the brazing temperature was 400°C,there was only a small amount of granular metallurgical products at joint interface.When the brazing temperature was increased to 415°C,a continuous reaction layer with a thickness of 17?m appeared at joint interface.When the brazing temperature was further increased to430°C,the thickness of the upper and lower interface layers increased to 27?m and22?m,respectively.The shear test results showed that the shear strength of joints obtained at 415°C increased fastest with ultrasound time,and the shear strength of joint brazed at 415°C reached 100 MPa at ultrasonic vibration time of 96s.The influence of brazing time on microstructure and properties of Zr-BMG/Zn-3Al/Zr-BMG joints brazed at 415°C was also studied.The results showed that the phase composition and microstructure were?-Zn pre-eutectic phase and Zn-Al eutectic for filler metal layer and a small amount of Zn₅₀Al₂₅Zr₂₅ reactant paticles for Zr-BMG interfacia layer when the ultrasonic vibration time was between16s-64s.However,the phase composition and microstructure evolved into single?-Zn phase for filler metal layer and a 17?m-thick Zn₅₀Al₂₅Zr₂₅/Zn₂₂Zr composite reaction layer for Zr-BMG interfacial layer when the ultrasonic time was increased to96s.These mutations of the microstructure with ultrasonic vibration time were caused by the unique ultrasonic corrosion characteristics of the Zr-BMG alloy,while the alternative formation of the two intermetallic compounds in the Zr-BMG interface layer was acribed to cycle fluctuation of Al element in liquid filler metal.In order to investigate the effect of Al content on the microstructure of Zr-BMG/Zn-Al interface layer,the microstructure of Zr-BMG/Zn-3Al/1060Al joints at 415°C was investigated.The results showed that during the brazing process,the microstructure of the filler metal layer had been maintained the hypereutectic Zn-Al alloy morphology,which was consisted of Zn-Al eutectic and?-Al solid solution,and the Al content in the?-Al solid solution kept increasing with the increase of the ultrasonic vibration time.The morphology of Zr-BMG interfacial layer was first composed of little Zn₅₀Al₂₅Zr₂₅ reactant particles at 48s,and then envolved to a single-phase Zn₅₀Al₂₅Zr₂₅ reaction layer at 64s,and finally transformed to Zn₅₀Al₂₅Zr₂₅/Al-based diffusion layer composite structure at 96s.The appearance of the single-phase Zn₅₀Al₂₅Zr₂₅ reaction layer proved that the alternative formation of the two intermetallic compounds in the Zr-BMG interfacial layer was acribed to the periodic fluctuation of the Al content in the filler metal layer.However,the formation of the Zn₅₀Al₂₅Zr₂₅/Al-based diffusion layer composite structure was due to the weakening of the ultrasonic cavitation effect which was caused by the decrease of the liquid component in the brazing seam layer.Shear test indicating that the maximum shear strength was 67.8MPa at 64s and the joint was fractured within 1060 aluminum matrix,while joints obtained from other joining times were fractured at Zr-BMG interfacial layer.