Wetting Spreading Behavior And Interfacial Reaction Of Active Filler Metals On The Surface Of Graphite Under Ultrasonic Vibration

Porous graphite has drawn widely scientists'attention because of its excellent thermal/electrical conductivity,wave/shock absorption and other properties.The wetting and spreading properties of liquid metal on graphite is of crucial importance to composite material preparation and dissimilar material joining.Because of the incompatibility in metallurgical and physical properties of dissimilar materials,molten filler metal cannot spread well on graphite surface when trying to join graphite with metal.Previous researches indicate that,reliable connections can be obtained since graphite can be wetted by active brazing filler metal in high temperature vacuum or protective atmosphere.However,the application of this advanced carbon material is limited because vacuum equipment is expensive and complex.In atmosphere,sound brazing joints cannot be obtained because the oxide film generated on active brazing filler metal surface will affect wetting.Therefore,the wettability of active brazing filler metal on graphite under non-vacuum and low temperature is the key to the application of advanced carbon material.Brazing aided by ultra-sonic vibration was applied in this research to get sound connection of metal/graphite joints under atmosphere and low brazing temperature.The wettability and spreading process of liquid active brazing filler metal on graphite was studied and the factors that affect wetting and spreading were analyzed.The forming mechanism and its composition of oxide film on Sn base active brazing filler metal surface was analyzed,and the result shows that,the oxide film on Sn5Ag8Ti surface was made up with three oxides,Ti,Ag and Sn.And the color was yellow,white,blue,purple and black brown varied with different content of the oxide.The color of oxide film on Sn5Ag2Ti1Al surface was silver since it was made up with Al₂O₃,and the oxide film can be crushed by applying ultra-sonic vibration.The wettability of brazing filler metal on graphite in vacuum and atmosphere was studied,and the result shows that,brazing filler metal Sn5Ag8Ti and Sn5Ag2Ti1Al can wet on graphite surface with acute angle when brazing temperature is 723K in vacuum,40°and 39°respectively.By assisting of untrasonic vibration,the wetting angle of Sn5Ag8Ti and Sn5Ag2Ti1Al on graphite surface was 39°and 14.5°respectively when brazing temperature is 723K in atmosphere.Ultra-sonic vibration can crush the oxide film on active brazing filler metal.Without this restriction,the active brazing filler metal can spread quickly in the direction of sound propagation with the help of sound pressure,and the spreading process was accompanied with atomization phenomenon.The wetting distance and area of Sn5Ag2Ti1Al was longer and larger than that of Sn5Ag8Ti.The interfacial reaction mechanism of brazing joints was revealed.The result shows that,the reaction layer Ti C formed in interfacial area is the key to the wetting of active brazing filler metal on graphite surface,no matter the brazing filler metal is Sn5Ag8Ti or Sn5Ag2Ti1Al and no matter in vacuum or atmosphere.Ti C reaction layer decreases the contact angle.When ultra-sonic vibration is applied,the welding temperature is low and there is oxide film generating on brazing filler metal surface,which will block the diffusion of Ti atoms in active brazing filler metal into Ti C layer.However,with the striping and cavitating effect of ultra-sonic vibration,Ti element can get into graphite and generate Ti C layer with²0nm thickness,even at low temperature.Although the Ti C layer is thin,it still can improve the graphite of wettability.