Reasearch On Amorphous Ti-based Alloy Foil Filler For Brazing TiAl-based Alloy And The Brazing Technology

TiAl-based alloys is considered as an ideal light-weight high temperature structural material for its excellent properties,including low density,high yield ratio,high specific stiffness,good oxidation resistance and high temperature creep resistance,which make it widely applied in aerospace industry.As a precise and convenient welding method,brazing is a perfect way to gain high quality TiAl alloy joint,for its low bonding temperature and marginal effect on the parent metal.In order to bring the superiority of brazing into full play,a novel Ti-based filler Ti-25Zr-12.5Cu-12.5Ni-3Co-2Mo（wt%）with low melting temperature was optimized from Ti-35Zr-15Cu-15Ni（wt.%）alloy by adding appropriate amount of Co、Mo.In addition,this noval filler was fabricated into amorphous foil filler by melt spinning,and its microstructure and the brazing process characteristics were investigated.Afterwards,the parent alloy Ti-47Al-2Nb-2Cr-0.15B alloy was brazed with this foil filler,and the influence of brazing temperature and holding time on the microstucture and mechanical property of the joints were discussedThe Ti-25Zr-12.5Cu-12.5Ni-3Co-2Mo（wt%）filler alloy in crystalline phase mainly consists ofα-（Ti,Zr）and（Ti,Zr）₂（Cu,Ni）.But after melt spinning,filler achieved better compositional and microstructural homogeneity,and transited into amorphous state mainly consisting of unsteady-state singleβ-（Ti,Zr）,which has lower solid-liquid line and narrower melting temperature interval.The spreading areas of the filler increase with the increase of brazing temperature and sharply uprise when the temperature exceeds 1000℃,Meanwhile,the spreading areas increase smoothly and tends to slow down while the holding time increases.The TiAl joint brazed mainly consists of central brazed zone（Layer II）and two interfacial reaction zones adjacent to the TiAl substrate（Layer I）.When the brazing temperature exceeds 1000℃,the regular continuous lamellar reaction layer I becomes messy bulk and the central brazed layer II transforms from three-phase（α-Ti+γ-（Ti,Zr）₂（Cu,Ni）、γ-（Ti,Zr）₂（Cu,Ni）+α-Ti andγ-（Ti,Zr）₂（Cu,Ni））mixture into two-phase（α₂-Ti₃Al and high Al%（Ti,Zr）₂（Cu,Ni））mixture.The microstructure and width of the reaction layer I and central brazed layer II had no significant change with the increase of holding time,which means that the microstructure of the joint is dominated by brazing temperature.The room temperature shear strength of the TiAl joint increases firstly and then decreases with the increase of brazing temperature or brazing time.A maximum shear strength of 152MPa of joint was achieved with brazing temperature of 1000℃and brazing time of 5 min.The cracks of brazing joints primarily initiate in the central brazed layer II and propagate in（Ti,Zr）₂（Cu,Ni）and Ti₃Al during the shearing process,showing as typical brittle fracture.