Mechanism Of Interfacial Formation And Progress Of Ultrasonic Assisted Brazing Of Sapphire Using Zn-Al Filler

Sapphire has outstanding light-transmitting features in a wide wave lengthspectrum from infared to microwave. As a windows material, it has been applied inmissile fairing, satellite, fighter, and other high-speed aircrafts because of itsexcellent optical properties. Sapphire also has high strength, high hardness, goodcorrosion resistance, high temperature resistance and many other advantages. Thus,sapphire is widely used in many high-tech fields, for instance, solid laserinstruments, light emitting diode substrate and semiconductor packaging. Due to thetechnology limitation, it is hard to produce large-size sapphire. Therefore, makinglarge-saize sapphire windows by joining small-size pieces are widely concerned.There are still some problems in the joining of sapphire by metal materials, such asreaction inertia, low wetting tendency, high joining temperature and serious residualstress. In this study, a method to braze sapphire using Zn-5Al filler with theassistance of ultrasound was proposed, interfacial reaction was investigated, andlow-stress sapphire joints were made. The structure, interfacial combiningcharacteristic, mechanical properties and fracture behavior of the joints wereanalyzed.The results indicate that the Al element in Zn-5Al filler can react with sapphireon the surface, and the filler metals and sapphire are joined together by theinter-layer of reaction product, Al2O3. As the ultrasound is applied longer, the Al2O3generated on the sapphire surface tends to be thicker.As a result of the high residual stress arises during the welding coolingprogress, cracks exist in the base sapphire and at the metal/sapphire interfaces of thejoints produced by ultrasonic brazing using Zn-5Al. The joint strength has arelationship with the thickness of the weld. The strength gets higher with thedecrease of the thickness. The average shear strength is as high as87MPa when theweld is20μm thick. The joints always break down at the interface of sapphire andfiller. This means that the interface between sapphire and Zn-5Al is the weakest partof the joint.The cracks and other defects in the sapphire joints can be eliminated by addingan intermediate layer of55%SiCP/A356composites. Composite weld can beobtained when the intermediate layer is200μm thick, and the volume fraction ofSiC particles in the weld can be up to46%. The average and highest shear strengthof the joints with the weld of120μm are133MPa and166MPa, respectively. The strength doubles compared to the sapphire joints without a composite intermediatelayer.