Mechanism Of Ultrasonic Assisted Reaction Epitaxy Growth Of Sapphire And Study Of Al/Al₂O₃ Composite Seam Brazing

Sapphire generally refers to single-crystal α-Al₂O₃, it is a kind of optical material with high melting point, high coefficient of thermal expansion and high corrosion resistance. It is widely used in both civil and military fields. Nowadays there are two main demands of sapphire welding, which are large size optical window and complex structure components. However, due to its high chemical stability, sapphire is hard to combine with other metals, and residual stress is likely created in welding seam during cooling since the coefficient of thermal expansion of sapphire is much smaller than other metals. This paper studied the mechanism of reacted epitaxy growth of sapphire in aluminum, and the welding of sapphire using pure aluminum solder, to improve the joint structure, using direct oxidation of aluminium was also studied.Under the assistance of ultrasound, a interface reaction of Al₂O₃ will be created on the sapphire surface which is immersed into liquid aluminum. This study shows that oxygen needed in the reation is from the oxygen in the environment around, and it’s transmitted by the liquid/solid/gas triple line. The collapse of cavitation bubble will cause a sudden high temperature and pressure, urging oxygen to dissolve into liquid aluminum, then dissolved oxygen will react with liquid aluminum forming the Al₂O₃ layer. TEM analysis shows that the reation epitaxy layer grow by steps, with the same crystal orientation of the substrate. There are often stacking faults between the epitaxy layer and the substrate Then the steps devour each other, forming a porous layer, and the thickness of the epitaxy layer will also increase with time passing.Welding of sapphire with pure aluminum brazing solder was studied, when the ultrasonic dipping time is between 50 s and 1000 s, a joint with highest strength of 56 MPa is obtained. The crack of the joint occurred inside the solder metal. When the ultrasonic dipping time reduced to 5s, the surface of the epitaxial layer and the brazing filler metal has become the weak link in the joint, the maximum strength decreased to 43 MPa.In the direct oxidation of aluminum with Mg O powder as the initiator, when the amount of Mg O powder was 15 mg/cm2, it can lead to the growth, the average thickness of Al/Al₂O₃ composite phase goes linear up with the increase of the heating time, based on this method, we tried the process of ultrasonic dipping-direct oxidation-ultrasonic brazing, Al/Al₂O₃ composite phase doped of aluminum brazing joints was obtained, while the thermal expansion coefficient of the seam may be improved, but because of the existence of pure aluminum, the joint strength was not improved significantly, which is up to 53 MPa.Also by the process of direct oxidation method, we usd a ultrasonic dipping-ultrasonic soldering-direct oxidation method to obtain a joint based on 2024 solder containing Al/Al₂O₃ composite phase. In the joint we found he case that Al/Al₂O₃ composite phase and sapphire were connected, it offers the possibility of joint by Al/Al₂O₃ composite phase only. Also, due to the direct oxidation reaction is difficult to control, 2024 alloy and Cu Al compound phase presented in joint, the joint strength is up to 65 MPa.