Study On The Microstructure And Performance Of In-Situ Synthetized TiB Whisker Reinforced TiAl/Ti₃Al Alloy Brazed Joints

TiAl alloy has the advantages of low density,high specific strength,high specific stiffness,good high temperature mechanical properties and excellent oxidation resistance.It is an ideal structural material for hot end components of aerospace vehicles.However,its plasticity and fracture toughness are very poor;when the service temperature exceeds 800℃,the high temperature creep resistance and high temperature oxidation resistance of the material itself also decrease sharply.The room temperature plasticity,high temperature specific strength and fracture toughness of Ti₃Al alloy are much higher than those of ordinary TiAl alloy.However,due to its high Nb content,the density is large.Therefore,in order to give full play to the advantages of the two alloys,it is necessary to solve the connection problem of dissimilar materials.In order to enhance the heterogeneous brazing joints of Ti-47Al-2Nb-2Cr-0.15B（at.%）alloy and Ti-22Al-15Nb-1Mo（at.%）alloy by in-situ TiB whiskers,four kinds of composite powder fillers were prepared by adding different contents of B to the eutectic composition TiH₂-27Ni（wt.%）.In this paper,the brazing process characteristics of the new filler are studied,and the brazing process parameters are formulated.The effects of brazing process parameters（brazing temperature of 1060～1090℃,brazing time of 0～15 min）and in-situ TiB volume fraction on the microstructure and mechanical properties of brazed joints were systematically studied.The relationship between brazing process parameters-TiB whisker size and distribution-central brazing layer Ⅲ structure-shear strength of brazed joints was established,and then the strengthening and toughening mechanism of TiB whiskers was revealed.The optimal process parameters of brazing connection between TiAl alloy and Ti₃Al alloy were obtained,which provided an effective guarantee for the expansion and application of TiAl alloy and Ti₃Al alloy in aerospace field.Four groups of TiH₂-27Ni（wt.%）+B composite powder fillers with in-situ TiB whiskers of 0,10 vol.%,20 vol.%and 30 vol.%in theory were developed by mechanical ball milling method.The microstructure and melting characteristics of the filler were analyzed by SEM,EDS,XRD and DSC,and the activation energy was calculated based on the Kissinger equation to optimize the composition of the filler.The results show that the mechanical ball milling only plays the role of mixed powder,no obvious chemical metallurgical reaction occurs,and the gradual increase of B content does not affect the melting characteristics of the composite powder filler.The composite powder filler with in-situ TiB whisker of 10 vol.%is the optimal filler,and at the heating rate of 10 K/min,its melting range is narrow,and the wetting and spreading ability of the solder on the surface of the base metal alloy is strong,which is most conducive to the subsequent brazing research.The typical interface microstructure of the brazed joint at brazing parameters of1080℃/10 min from the TiAl alloy side to the Ti₃Al alloy side was diffusion layer I（τ₃-Al₃Ni Ti₂,α₂-Ti₃Al）,continuous reaction layer II（Ti₂Ni,α₂-Ti₃Al）,central brazing layer Ⅲ（Ti₂Ni,TiB andα₂-Ti₃Al）and diffusion layer IV（B2 phase,α₂-Ti₃Al）.With the increase of brazing temperature and brazing holding time,the dissolution amount of TiAl alloy and Ti₃Al alloy into the molten filler and the mutual diffusion and metallurgical bonding reaction with the filler elements were enhanced,and the width of the brazing seam and the central brazing layer Ⅲ was increased.When the brazing temperature is lower（1060℃）or higher（1080℃）,the brazing time is shorter（0 min）or longer（15 min）,the central brazing layer Ⅲ would produce holes,which was not conducive to obtaining brazed joints with excellent performance.The volume fraction of TiB was the main controlling factor affecting the interface microstructure of TiAl/Ti₃Al alloy brazed joints and the content,distribution and size of Al₃Ni Ti₂ and Ti₃Al in the center brazing layer Ⅲ.The grain refinement effect of TiB increased with the increase of its volume fraction,which promoted the gradual refinement of the interface structure of layer Ⅲ.However,when the volume fraction of TiB reaches 20%and 30%,the grain refinement effect could not be further enhanced,and the excessive TiB with high melting point and high elastic modulus affected the fluidity and filling ability of the liquid phase in the brazing molten pool and the embrittlement of the brazed joint,resulting in the generation of holes in the brazed joint.The shear strength of brazed joint increased first and then decreased with the increase of brazing temperature,brazing holding time and TiB whisker.The maximum shear strength of brazed joint was 390.73 MPa when TiB is 10 vol.%at brazing temperature of 1080℃ and holding time of 10 min.The brazed joints were fractured in the central brazing layer Ⅲ with continuous block brittle hard Ti₂Ni as the main phase after shear mechanical properties test,and the fracture mode was quasi-cleavage fracture.When the crack propagates to the TiB whisker,there are two kinds of extension paths.No matter which path will increase the fracture energy of the joint,the strength of the brazed joint can be significantly improved.