Research On Ti-Based Filler Metal And The Brazing Technology For Brazing TiAl Alloy And Ni-Based Superalloy

Possessing high specific stiffness,high specific strength,excellent oxidation resistance and excellent high-temperature mechanical properties,TiAl alloys have great potential to replace Ni-based superalloys and heat-resistant steels as a lightweight high temperature structural materials.Compared with Ni-based superalloys,the density of TiAl alloys is only half of Ni-based superalloys and the application of TiAl alloys in aerospace vehicles can save the weight of aircrafts significantly,thereby increasing work efficiency and reducing fuel consumption.Brazing is considered to be an effective method for joining TiAl alloy to Ni-based superalloy.The present work attempts to develop a novel Ti-based filler with low melting temperature for brazing TiAl alloy and GH536 nickel-based superalloy and the focus is placed on the brazing technology of these two alloys.The brazing process characteristics of the Ti-based filler and effects of brazing parameters on the microstructures and mechanical properties of TiAl alloy and Ni-based superalloy joints were investigated,and the formation mechanism was also clarified.A novel Ti-30Zr-10Fe-5Cu-4Ni-3Co-2Mo?wt.%?filler alloy was developed,whose microstructure,melting characteristic and spreadability were analyzed by SEM,EDS,XRD and DSC.The filler foil prepared by rapid solidification technology shows excellent compositional and microstructural homogeneity,low melting temperature and narrow melting temperature interval.The spreading areas of the filler increase with the increase of brazing temperature and brazing time.The typical interfacial microstructure of the joint is TiAl/layer?/layer?/layer?/layer?/GH536.Layer?is composed of Ti₃Al and a small quantity of TiAl.Layer?consists of Al₃NiTi₂.Layer?mainly contains AlNi₂Ti in which a small quantity of Cr-rich?Cr,Ni,Fe?_ss,Ni-rich?Cr,Ni,Fe?_ss and?Ni?_ss+TiNi₃ disperse.Layer?consists of four kinds of phases:Cr-rich?Cr,Ni,Fe?_ss is the matrix in which Ni-rich?Cr,Ni,Fe?_ss,AlNi₂Ti and?Ni?_ss+TiNi₃disperse.Brazing temperature affects the dissolution of TiAl and GH536 alloy into the molten filler alloy as well as the content and distribution of Al,Cr,Fe and other elements in the brazing seam,which decide the microstructure of the joint.During the brazing process,layer?is formed at the TiAl/filler interface firstly,meanwhile the Ni-rich?Cr,Ni,Fe?_ss reaction layer is formed at the filler/GH536 interface.With a further diffusion of alloying elements,layer?and layer?are formed successively on the side of the layer?which is far from TiAl alloy,and layer?is formed at the interface of GH536 alloy.The room temperature shear strength of the brazed joint firstly increases and then decreases with the increase of brazing temperature and brazing time.The maximum shear strength of the brazed joint reaches 183 MPa at 1150?for 10 min.It can be found that the cracks of all joints primarily initiate and propagate in layer II and the Al₃NiTi₂ intermetallic compounds occupy most of the fracture surfaces,showing the characteristics of brittle fracture.Meanwhile,some cracks extend to layer I and layer III.The hardness distribution curve of the joint is"M"shape and it tends to be more flat with the increase of brazing temperature.