Mechanisms For Wetting And Bonding Of Ultrasonic Assisted Brazed SiC Ceramic And Ti-6Al-4V Alloy And Their Brazing Process

SiC ceramic and Ti-6Al-4V alloy have become the important candidates of thestructural materials. However, the poor machining ability for preparing large-size orcomplex ceramic components and the application of the combination of dissimilarmaterials for meeting some special requirements or exerting the good performanceof materials, these problems can be solved by the technologys of joining ceramic tothemselves or alloys. In this study, the joints of SiC/SiC, SiC/Ti-6Al-4V wereachieved by use of ultrasonic-assisted brazing technology in air at a lowertemperature, and aluminium based alloys were used as the main brazing alloys. Thespreading and wetting behaviors, breakage behavior of oxide film, interfacialerosion behaviors, interfcial microstructure and ultrasonic-assisted brazingtechnology were investigated.The spreading behaviors of liquid brazing alloy on the surface of solid basedmaterials under the excitation of ultrasonic vibration were studied. The brazingalloys and based materials showed little effects on the characteristics ofultrasonic-induced spreading, and this spreading behavior had some commonfeatures. In front of the spreading edge, a layer consisted of high densitynanoparticles formed on the surface of based materials, and this spreading behavioroccurred on the solid surface covered by the nanoparticles. The dimension ofnanoparticle was smaller that150nm, and the width of their distribution area wasabout150μm. The ultrasonic-induced apreading behavior also showed some otherspecial characteristics on the microscopic scale: dyssynchrony of spreadingforefront, partial bonding of the interface at the spreading frontier area,nanoparticles engulfed by the liquid brazing alloy gradually, et al. The nanoparticleswere from the liquid brazing alloy. The cavtition bubbles in the inner of liquidbrazing alloy adjacent the spreading edge collapse suddenly, which induced theliquid brazing alloy atomized and appeard many nanoscale droplets. Thesenanoscale droplets were bonded with the solid surface, and their outer surface wereoxidated when contacting with air. At the same time, the collapse of the cavitationbubbles could broken the surface oxide film that enwrapping the liquid brazing alloyat this corresponding areas, so the liquid brazing alloy would spread forward at themicroscale area. An interface structure of liquid brazing alloy/nanoparticles/solidbased materials was formed at the spreading frontier regions, and the surface oxidefilm of nanoparticle delayed the engulfed process of nanoparticles by the liquidbrazing alloy. Under the function of ultrasonic, erosion behaviors were found at the interfacebetween liquid Al-12Si and Ti-6Al-4V alloy. The erosion behaviors consisted ofmany isolated erosion pits, and the erosion pits presented a hemispherical shapewith a more gradual slope at the edge. These small erosion pits had non-uniformdimensions, and the maximum diameter was²5μm. The erosion pits was coveredby an incomplete oxide film and there was a tiny notch present in the middle of thisoxide film. Two kinds of IMC form at the pit interface, and they show differentcharacteristics depending on their positions. Ti₉Al₂₃compounds formed at the walland bottom of the pit and Ti₇Al₅Si₁₂compounds formed at the pit edge. The totalnumber and density of the pits increases with prolonging ultrasonic time orincreasing ultrasonic amplitude, but there is no significant change in the range of pitdiameters. However, when the amplitude reached to6.5μm, some large erosion areawith irregular shape appeared on the Ti-6Al-4V surface, which consisted ofhigh-density erosion pits. The formation of erosion pit could be ascribed to thecavitation phenomonen occurred at or near the liquid/solid interface, and manycomplex effects were generated at the small zones during the bubble implosion,including micro-jets, hot spots, high pressure, and acoustic streaming. The impact ofmicro-jets resulted in breakage of the oxide film on the Ti-6Al-4V surface. Theresultant circular notches served as micro-channels for direct interaction betweenliquid Al-Si and solid Ti-6Al-4V. After the micro-channels opened up, localized hotspots and acoustic streaming caused the solid Ti-6Al-4V substrate to excessivelydissolve into the liquid Al-Si alloy at the notch zones, and granular Ti₉Al₂₃particlesformed at the wall and bottom of pits during this extended dissolution process.When the bubble implosion energy dissipated, a slow further dissolution developedalong the interface between the oxide film and Ti-6Al-4V substrate, resulting in agently-sloped region at the pit edges and laminar Ti₇Al₅Si₁₂phases formed at the pitedges. The large irregular erosion areas were induced by the implosion of cavitationbubble clusters.For Al-12Si/Ti-6Al-4V systerm, a thermostatic treatment process wasperformed after the function of ultrasonic. Ti-6Al-4V alloy dissolved into liquidAl-12Si brazing alloy along the interface between oxide film and Ti-6Al-4V matrix,and the oxide film detached with Ti-6Al-4V and floated in the liquid alloysimultaneously. Holding time for5min after a ultrasonic time of4s could make theoxide film detached the Ti-6Al-4V matrix completely, and Ti₇Al₅Si₁₂compoundsformed at the staight interface. The structure of compounds at the erosion pitsshowed no significant variation, just a layer of Ti₇Al₅Si₁₂compounds formed at theoriginal Ti₉Al₂₃/Ti-6Al-4V interface. Ti₉Al₂₃phases began to form at the inteface and adgacent into the liquid Al-12Si brazing alloy when prolonging the holding timeto8min. For Al-12Si/Ti-6Al-4V interface at620℃, the final steady interfacestructrue was Al-12Si/Ti₉Al₂₃/Ti₇Al₅Si₁₂/Ti-6Al-4V. Second ultrasonic afterthermostatic treatment could break the floating oxide film, but had no effects on thesrtucture of the interfacial compounds.When ultrasonic-assisted brazing SiC ceramics by filling with pure Al, thestrength of joint could reach to⁶5MPa and the fracture pathes were in the jointalloy. However, the intermetallic compounds Al4C3at the interface were potentialthreaten. When using the Al-12Si alloy, the interface was straight and no compoundswere found. Ultrasonic time had little effects on the joint strength, which couldreach to95MPa. The fracture paths of joint were in the inner of the joint alloy orthe SiC ceramic adjacent into the interface. The interface of joints showed threekinds of structural characteristics in TEM: Al-12Si/SiC, Al-12Si/SiO₂amorphouslayer/SiC, and Al-12Si/nanoparticle/SiC. The amorphous layer SiO₂was just theoxide film formed on the SiC surface during heating in air, which was decomposedunder the effect of ultrasonic cavitation erosion. Abnormal Al₂SiO₅compoundscould form at the erosion site when the SiO₂layer was much thicker. Thenanoparticles formed in front of the liquid brazing alloy during theultrasonic-assisted filling clearance process, which were consistent with thenanopaticles found during ultrasonic-assisted spreading process.Ultrasonic-assisted brazed joining SiC ceramic was performed by filling withZn-8.5Al-1Mg brazing alloy. The joint strength increased when prolonging theultrasonic time, and the maximum could reach to¹49MPa. When the ultrasonictime was shorter, the fracture paths mainly occurred at the interface. Afterprolonging the ultrasonic time, the fracture paths existed at the interface and in theinner of SiC and joint alloy both adjacent the interface. The collapse of cavitationbubble could also make the SiO₂amorphous layer of SiC surface eroded by theliquid brazing alloy slowly. When prolonging the ultrasonic time, the erosionamount of SiO₂increased, and the joint strength was promoted. A sandwichstructural filler was used joining SiC ceramic and a SiC ceramic joint with lowcoefficient of thermal expansion was obtained.When SiC ceramic and Ti-6Al-4V alloy were ultrasonic-assisted brazed byfilling with Al-12Si alloy, cracks induced by residual stress could be observed in theinner of SiC ceramic. Newly developed Al-15.5Sn-9.5Si-4.5Zn-0.5Mg brazingalloys were prepared by the addition of element Sn, Zn, Mg, which had a lowsolidus temperature of¹83℃, but its liquidus temperatures was also higher to⁵61℃. An integrated joint was obtained when using the novel filler metals, compared with the inner fractured joint obtained by filling with the common AlSialloy. The novel filler metal was in a semi-solid state at the temperature of183-561℃and showed low deformation resistance, so the shrinkages of the SiCceramic and Ti-6Al-4V alloy had more freedom and a small effect on each otherbefore the solidification of the liquid Sn-rich phases. The results of finite elementcalculation showed the thermal residual stress in the joint was significantlydecreased by use of the novel filler metals. The average shear strength of the jointswas⁷7.8MPa, the fracture paths occurred in the inner of joint alloy adjacent to theSiC side and at the interfaces between Sn phases and SiC ceramic.Ultrasonic-assisted brazing of Ti-6Al-4V alloy and Al1060alloy was carriedout by filling with Al-12Si alloy. The oxide film on the Ti-6Al-4V surface beforedetachment with the Ti-6Al-4V showed significantly effects on the joint strenth, andthe joint strength was only³4.7MPa. The oxide film floating in the liquid alloyand the interfacial compounds showed no effects no the joint strength, and the jointstrength could reach to⁶8MPa.