Metallurgical Bonding Mechanism Of Sapphire/Al Dissimilar Joints Ultrasonically Soldered At Low Temperature

As one of the most widely used optical materials,sapphire is often used in high-power laser devices and optical windows served under high pressure.However,due to its low ductility,the application of sapphire in engineering structures has been hindered,so it is necessary to connect sapphire with aluminum alloy of high toughness and high strength in the manufacturing process.To solve the joining problems of poor wettability and serious residual stress during ultrasound-assisted soldering of sapphire/5A06 Al alloy joint,sapphire and 5A06 Al were ultrasonically soldered with Sn9Zn,SAC305 alloy solders and NiCu alloy foam reinforced Sn9Zn and SAC305 composite solders,respectively.The effects of ultrasonic-assisted soldering parameters and solder compositions on the microstructure and mechanical properties of joints were investigated.The microstructure evolution mechanism of joint and the strengthening mechanism of NiCu alloy skeletons were analyzed.The bonding mechanism at the sapphire/Sn-based composite solder interface was discussed.The metallization of sapphire was successfully realized by ultrasonic assisted dipping in liquid Zn-14Al alloy for 600 s.The influence of ultrasonic power,soldering pressure,soldering temperature,soldering time and other brazing parameters on the microstructure and mechanical properties was systematically studied,and the optimal soldering parameters were determined.The effect of different solder compositions on microstructure and reliability of sapphire/Al alloy joint was studied.When alloy solders were used,cracks appeared at sapphire/Sn solder interface because of the CTE mismatch between sapphire and liquid solder.No cracks could be found at the interface when Sn-based composite solders were used.For joints soldered with Ni10Cu-Sn9Zn composite solder,the soldering seam was uneven and was mainly composed of Sn-based solder,rod-shapedη-Zn phase,α-Al particles and foam skeletons with Al₃Niintermetallic compound（IMC）layer.While in NiCu-SAC305 composite solder joint,fine particles of（Ni,Zn）₃Sn₄,α-Al andη-Zn were largely formed and homogeneously distributed in the solder seam.The mechanical properties of different solder joints were studied.The shear strength values of joints using Sn9Zn and SAC305 alloy solders are always lower than that using NiCu-foam reinforced composite solders.The shear strength of joints soldered with composite solders was higher than that soldered with alloy solders.The strength of sapphire/NiCu-SAC305/Al joint soldered at 300℃for 60 s reached74.42 MPa,which was approximately 24.45 MPa higher than the sapphire/NiCu-Sn9Zn/Al joint soldered with the same parameters.The shearing failure mainly occurred in the soldering seam for all the joints,which indicates a good combination between sapphire and Sn-based solder.The joint remained intact after vibration acceleration test and thermal shock test,indicating good heat resistance and fatigue load capacity.The interfacial bonding mechanism of joints was studied by TEM characterization.An amorphous Al₂O₃transition layer was formed at the sapphire/Sn matrix interface,which could realize the high-strength bonding.Moreover,Zn enrichment was observed at Sn matrix/amorphous Al₂O₃interface.The interface strengthening effect of Zn was analyzed by first-principles calculation.