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Researh on Mechanism of Interfacial Formation and Process of Ultrasonic Assisted Brazing of Ti-6Al-4V and SiC

Author: GaoFei
Tutor: ZuoJiuChun
School: Harbin Institute of Technology
Course: Materials Processing Engineering
Keywords: Ti-6Al-4V Alloy SiC Ceramic Cavitaition Erosion Interfacial Reaction Ultrasonic-Assisted Brazing
CLC: TG454
Type: Master's thesis
Year: 2012
Downloads: 21
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Abstract

Silicon carbide （SiC） ceramics have broad application prospects in industry, suchas machine, aerospace, petroleum, nuclear energy, optics and other fields, due to theirgood performances of strong oxidation resistance, good abrasion resistance, highhardness, good thermal stability, low coefficient of thermal expansion, high thermalconductivity, excellent thermal shock resistance, et al. Ti-6Al-4V titanium alloys withhigh strength, corrosion resistance and other outstanding advantages are widely used inaerospace, petrochemical and other fields, and become important structural materials.Research on producing SiC ceramic/Ti-6Al-4V alloy composite components is veryimportant for their application. So the paper made a series of studies on the SiC ceramicand Ti-6Al-4V alloy. The mechanism on the wetting and bonding of Al-12Si brazingalloy to the two kinds of base materials under the effect of ultrasonic vibration weresystemic studied, and the ultrasonic brazing technology to obtained low thermal stressjoints was proposed.There are two main affects on the mechanical properties of the brazed joints: thepoor wettability and bad bonding between the brazing alloy and the base materials in air;the larger residual thermal stress in the brazed joint due to the mismatch of thermalexpansion coefficient between SiC ceramic and Ti-6Al-4V alloy.The ultrasonic assisted interaction behaviors between Al-12Si brazing alloy andTi-6Al-4V alloy were studied. Experimental results showed that numerous2–20μmdiameter-sized and hemispherical pits formed on the Ti-6Al-4V surface. The number ofpits increased with prolonging the ultrasonic vibration duration time, but there is nosignificant change in the range of pit diameters with increasing the ultrasonic vibrationduration time. The formation of pits was result from the implosion of the cavitationbubbles occurred at or near the interface （cavitaion erosion）. The impact of micro-jetsinduced by bubble implosion resulted in breakage of the oxide film on the Ti-6Al-4Vsurface, and the solid Ti-6Al-4V is excessively dissolved into liquid Al-12Si at thenotches under the effects of hot spots and ultrasonic stir, then, abnormal conditionsestablished by bubble implosion made different intermetallic compounds formed at theinterface (granular TiAl₃and laminar Ti₇Al₅Si₁₂). Moreover, phase transformation fromTiAl₃to Ti₇Al₅Si₁₂occurred during the subsequent ultrasonic. Plus a holding timeprocess at620℃after short-time ultrasonic treatment, liquid Al-12Si alloy continuallyinteracted with Ti-6Al-4V along the oxide film/Ti-6Al-4V interface, which made thatthe oxide film floated in the liquid Al-12Si alloy and Ti₇Al₅Si₁₂phase formed at theinterface. When the holding time reached to some extent, TiAl₃phases began to grow atthe interface of Al-12Si side, and accompanied with the Ti₇Al₅Si₁₂phase transformed toTiAl₃phases. But the Ti₇Al₅Si₁₂phase could not wholly disappear. The secondultrasound treatment after holding time could cause the floating oxide film completely broken.The spreading and bonding behaviors in Al-12Si based brazing alloy and SiCceramic system under the effect of ultrasonic vibration were studied. A nano-particlelayer formed on the SiC ceramic and at the forefront of the spreading zone. Theformation of nano-particles was induced by the bubble implosion. The nano-particlelayer favored the spreading behavior of liquid brazing alloy on the SiC ceramic. But thewell bonding could not form at the interface of last spreading zone, and the longerultrasonic vibration duration time was necessary for promoting the bonding. Theholding time treatment after ultrasonic can not changed the situations of spreading andbonding. To analyse the effect of SiO₂layer grow on the SiC ceramic surface on thebonding of Al-12Si alloy to SiC ceramic, the SiC ceramics were suffered pre-heatingtreatment to formed a thicker SiO₂layer. Under the effect ultrasonic, the SiO₂layer waspartially eroded by liquid Al-12Si brazing alloy, and a interfacial chemical reactionAl+SiO₂→Al₂O₃+Si occured at the erosion zones.Ultrasonic-assisted brazing SiC ceramic to Ti-6Al-4V alloy using the Al-12Sibrazing alloy were performed. Due to the mismatch of thermal expansion coefficient ofthe two base materials, a larger residual thermal stress could form in joint during thecooling process after the brazing alloy solidification. A failure occurred in the inner ofSiC ceramic but adjacent to the joint. The addition of elements Sn and Zn to Al-12Sibrazing alloy made the solidus temperature of the brazing alloy largely decreased. Usingthis brazing alloy could also decrease the temperature of generating residual thermalstress in the joints, and SiC/Ti-6Al-4V joints with low-level residual thermal stress wereobtained.

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