Study On The Microstructure And Performance Of In-situ Synthetized TiB Whisker Reinforced ?-TiAl Alloy Brazed Joints

TiAl-based alloys possessed high specific strength,good corrosion resistance and oxidation resistance as well as excellent lower density and excellent high-temperature properties,which had been considered as the most potential materials to decrease the application of traditional Ni-based superalloys and heat-resistant steels,and were widely used in turbine blades of aerospace engines.Compared with the traditional Al-based or Ag-based filler metals,Ti-based filler had good wettability on the surface of TiAl-based alloys and the higher bonding strength and high-temperature performance of the brazed joint.Brazing,due to its convenience,cost-effectiveness and lower joining pressure requirement,had been widely used to join TiAl-based alloys.This present paper was mainly to develop a new Ti-based composite powder filler for the brazing Ti-47Al-2Nb-2Cr-0.15B(at.%)alloy,which can in-situ formed Ti B whiskers in the brazed joints.The melting characteristics of the Ti-based composite fillers and corresponding the effect of brazing condition parameters and filler compositions on microstructures and mechanical properties of TiAl-based alloys brazed joints were investigated,the formation mechanism was clarified and the effect of in-situ synthesized Ti B whiskers on the interfacial morphology of brazed seam was revealed in the brazed joint.(TiH₂-66Ni)_1-x(TiB₂)_x(x=0?16.86 wt.%)composite fillers were prepared,corresponding to the ideal volume fraction of in-situ Ti B whiskers ranged from 0 vol.%to 40 vol.%.The microstructures and melting characteristics of composite fillers were analyzed by SEM,EDS,XRD and DSC.Results showed that the components of composite fillers were uniformed distribution and no formed new phases in the short milling time,and the melting temperature interval gradually became wider with increasing of the heating rate.Activation energies of each characteristic temperature point for the composite powder fillers were calculated according to the Kissinger equation,which investigated the composite filler with 7.47 wt.%TiB₂(i.e.,the volume fraction of the Ti B whisker generated is 20 vol.%)had a relatively lower energy barrier.For this composite filler,the spreading areas increased with increasing of the brazing temperature and brazing time,but the spreading rate first increased and then decreased.The inflection point appears at 1230?for 10 min.When the brazing temperature and time were 1230?and 10 min,the analysis of the interfacial microstructure of TiAl-based alloy joints brazed with(TiH₂-66Ni)_92.53(TiB₂)_7.47(wt.%)filler showed that the brazed joint was mainly composed of three parts:interfacial reaction layer?formed by dissolution of TiAl-based substrate,interface reaction layer II adjacent to the substrate side and residual filler layer III.And corresponding the phases were lamellar?₂-Ti₃Al with Al₃NiTi₂in the layer?,Al₃NiTi₂with?₂-Ti₃Al in layer II as well as the rich-Ti+?₂-Ti₃Al,Al₃NiTi₂and Ti B whiskers in the layer III,respectively.Brazing temperature and holding time mainly affected the dissolution rate of TiAl substrate into the liquid filler and the diffusion and distribution of Al element as well as Ti,Ni and B elements in brazed seam.For the holding time of 10 min,the width of the interfacial reaction layer?gradually widened and the layer?gradually narrowed with increasing of the brazing temperature from 1190?to 1250?,meanwhile,more Ti B whiskers distributed in the interfacial reaction layer?.In the brazing temperature 1230?,as the holding time prolonged from 0 min to 20 min,the microstructural morphology of joints gradually homogenized and the width of the interfacial reaction layer?gradually widened,and the Al₃NiTi₂intermetallic compound with block phase always distributed in the interface reaction layer?,the crack was easy to origin from layer?and then propagated in the reaction layer?after longer holding time.The interfacial microstructure of TiAl-based alloy joints brazed with the different composite fillers at 1230?for 10 min were analyzed,results showed that the grain size of Al₃NiTi₂phase in the reaction layer?first refined and then coarsened with increasing of the content of TiB₂.The main reason was that in-situ formed Ti B whiskers with high content tended to agglomerate and reduce the effect of grain refinement.Formation mechanism of the phases in the TiAl-based alloy brazed joint were analyzed by the solubility and diffusion between the parent metal alloy elements and filler elements,revealed the strengthen mechanism of in-situ formed Ti B whiskers on the brazed joint.The shear strength of TiAl-based alloy brazed joints first increased and then decreased with increasing of the brazing temperature,holding time and the content of TiB₂in composite fillers.The maximum shear strength of the brazed joint of 332.87±6.2 MPa was obtained at1230?for 10 min when the content of TiB₂was 7.47 wt.%in composite filler.The fracture path of all brazed joints after shear testing primarily initiated in the layer?mainly consisted of massive brittle Al₃NiTi₂intermetallics,and then propagated in the interfacial reaction layer?,the fracture surface of the joint presented typical brittle fracture characteristics.