Study On Interfacial Reaction Of Ti-Al Laminated Composites Prepared By Ultrasonic Consolidation Assisted Hot Pressing Sintering

Ti/Al metal matrix composite has many advantages such as low density,high strength and oxidation resistance,making it one of the most potential structural materials in aerospace and weaponry.Ultrasonic consolidation technology uses high-power ultrasonic as energy source to achieve fast and high quality solid state joining of the same or dissimilar metal materials.The method of vacuum sintering is to stack the metal foils alternately and make the metal diffusion reaction at high temperature.In order to accelerate the Ti/Al interface reaction,reduce the sintering reaction time and improve the preparation efficiency by ultrasonic consolidation.In this paper,Ti/Al metal layered composites were prepared by two-step ultrasonic consolidation assisted hot pressing sintering and vacuum hot pressing sintering.The evolution of interface microstructure and reaction kinetics were studied by different process temperatures and times,and the influence mechanism of ultrasonic consolidation on subsequent vacuum hot pressing sintering interface reaction was revealed.It is found that there is no new phase formed by diffusion at the interface of Ti/Al by the vacuum hot pressing sintering process at 550℃,and interlaminar cracking occurs at the interface.However,with the ultrasonic consolidation assisted hot pressing sintering process,a diffusion reaction layer is formed at the interface of Ti/Al at 530℃,and a new phase is formed.The interface bonding is stable without obvious defects,and there is a rapid diffusion area（RDA）at the interface.This region often appears preferentially on the interface of Ti layer surface which is acted by the indenter during ultrasonic consolidation.With the increase of temperature and holding time,a diffusion reaction layer is formed at the Ti/Al interface and a reactive phase（Al₃Ti）is formed.The thickness of diffusion reaction layer is positively correlated with hot pressing time and temperature.The activation energy of interfacial reaction is 183.98 k J/mol and the pre influence factor is 2.75×10³.The activation energy of interfacial reaction at FA is 149.39k J/mol and the pre influence factor is 29.9,which are lower than those of vacuum hot pressing sintering process.The activation energy of interfacial reaction at RDA is 266.39k J/mol,and the pre influence factor is 2.55×10⁹,which is higher than that of vacuum hot pressing sintering process.By comparing the concentration gradient of the two processes,it is found that the concentration gradient of FA sintered at 630℃for 1.5 h is 5.96～8.31 wt.%/μm,which is higher than that sintered at 630℃for 1.5 h by vacuum hot pressing sintering,which is2.30～3.08 wt.%/μm,reflecting that the element diffusion rate of ultrasonic consolidation assisted hot pressing sintering is higher than that of vacuum hot pressing sintering.The results show that the concentration gradient of FA sintered at 630℃for 3 h is 0.12～0.42wt.%/μm,which is lower than that sintered at 630℃for 3 h by vacuum hot pressing（0.13～0.78 wt.%/μm）.The results show that there is little difference in the concentration gradient of FA sintered at 630℃for 6 h between ultrasonic consolidation assisted hot pressing sintering and vacuum hot pressing sintering,which is in the range of 0.001～0.009wt.%/μm.The results show that the concentration gradient of RDA sintered at 630℃for1.5 h is 0.1～1.1 wt.%/μm,and that of RDA sintered at 630℃for 3 h is 0.001～0.01wt.%/μm.It can be found that the 1.5 h and 3 h concentration gradients at RDA are similar to those of vacuum hot pressing sintering and 3 h and 6 h at FA,which further indicates that ultrasonic consolidation promotes the diffusion reaction at Ti/Al interface during hot pressing sintering.Through the analysis of oxides at the interface front of the two processes,it can be found that there is continuous enrichment of O element in the area near the reaction layer of Al layer sintered by vacuum hot pressing,while the enrichment of O element in ultrasonic consolidation assisted hot pressing sintering has shown a discontinuous state,even at the interface near the reaction layer sintered at 630℃for 3 h at RDA to the enrichment of O element.The results show that the porosity defects caused by the Kirkendall effect can be observed in the process of sintering at 630℃for 6 h.These porosity defects appear at the interface between the reaction layer and the Al layer.The porosity defects caused by the Kirkendall effect of ultrasonic consolidation assisted hot pressing sintering are larger,and even in some places the Al layer has been consumed,forming the gullies caused by the Kirkendall effect.By phase analysis of EBSD,it is found that the diffusion direction of Ti/Al interface in the initial stage（～1.5 h）of the two sintering processes is obviously different.The single vacuum hot pressing sintering process is the diffusion from Ti layer to the lower Al layer,and the formation of fine Al₃Ti grains dispersed on the near surface of Al layer,while ultrasonic consolidation assisted hot pressing sintering It is found that the diffusion occurs from the Al layer to the lower Ti layer.It can also be seen from the grain boundary diagram that the small angle grain boundary of single vacuum hot pressing sintering is mostly at the interface between the lower surface of Ti layer and the upper surface of Al layer.The small angle grain boundary of ultrasonic consolidation assisted hot pressing sintering is less than vacuum hot pressing sintering,most of them are distributed at the interface between the lower surface of Al layer and the upper surface of Ti layer.Because the grain sizes of Al and Ti foils grow much larger than that of Al₃Ti during the sintering for the process with and without ultrasonic consolidation assisting,the influences of the microstructures of Al and Ti on the interfacial reaction kinetics can be excluded.