| TiAl based alloys have broad application prospects in the aerospace field becausethey posses high Specific stiffness, high specific strength, high resistance tooxidation and creep resistance. However, because of its inherent structural features,poor room temperature ductility and strong resistance of high temperaturedeformation make it extremely difficult to plastic process, their engineeringapplication is limitied. In order to fabricate complex shape parts of TiAl basedAlloys with high-density, low-cost and high-performance, a new technology, whichconsists of Ti/Al composite powder forging and Subsequent reaction sintering, isproposed to overcome plastic processing difficulties of TiAl based Alloys In thispaper. The characteristic of this process is forging Ti/Al composite powder by theuse of characteristics of easy to plastic processing for Ti/Al composite powder, andthen to obtain a high density sintered component by reaction sintering. Based on thisprocess, the problem of hard to processing can be effectively avoided. For thisprocess, the following aspects are studyed. For this process, the following aspectswere studied in the paper.The process of preparation for Ti/Al composite powder billet has been studied. Byobservation of microstructure of Ti/Al composite powder billets with differentsintering processes, the evolution on microstructure of Ti/Al composite powderbillet was obtained at different sintering temperature and sintering time. Formabilityof Ti/Al composite powder billet with different microstructure has been investigatedby hot compression experiments. It was found that deformation of Ti/Al compositepowder billet was effected by plastic deformation, slip slipping and rotation ofparticles, and the best formability of Ti/Al composite powder billet was obtainedafter sintering at450℃for2h.Based on hot compression test of Ti/Al powder at temperature of300~600℃,strain rate0.1~0.001s-1, the effect rules of strain rate, deformation temperature,relative density on the flow stress have been studied. Considering the situation ofcracks and particle segregation in compressed sample, Ti/Al composite powderforging process should be selected at a temperature above400℃and strain rate ofless than0.01s-1, and distortion should not be too large during Ti/Al compositepowder forging.Based on compression test results, the constitutive equations ofTi/Al powder material was established, and produced its thermal processing map,the optimal forging processing parameters is obtained to provide a reference forsubsequent powder forging by the processing map.The deformation densification of the Ti/Al composite powders has been studied under different deformation conditions, the results indicate that Ti/Al compositepowders in the deformation process is easy to dense, its Poisson’s ratio was effectedby relative density, strain rate and temperature. Effects of initial relative density,friction, void shape, deformation degree, deformation temperature and strain rate onthe deformation densification have been investigated, it shows that increase offriction and deformation temperature, deformation degree and decrease of the strainrate can increase the density of Ti/Al composite powders during powder forging.According to fracture limit diagram of uniaxial compression for Ti/Al compositepowder material, the optimal process parameter is determined as500℃,0.001s-1forpowder forging. Based on the plasticity theory of powder material, considering theeffects of temperature and strain rate, the yield criterion of Ti/Al composite powdermaterial was constructed to provide material parameters for the numericalsimulation of powder forging.Compaction of preform of Ti/Al composite powder pressing process wassimulated to get the distribution of relative density inside preform, as well as therelationship of pressing force and the average relative density by the finite elementmethod. According to the simulation results, compaction experiments were carriedout to get the preform with desired density and size for powder forging. The finiteelement model of powder forging process for Ti/Al composite powder has beenestablished by Using Gurson-Tvergaard yield model and ABAQUS software, and ithas been verified to guide the experiments. By simulating the forging process ofconical parts of Ti/Al composite powder, effects of friction coefficient, temperatureand strain rate on the distribution of the relative density, equivalent stress andequivalent strain was studied during forging process, the results show that the bestforging parameters of conical parts for Ti/Al composite powder is500℃,0.001s-1and the friction coefficient of0.1. According to the results, qualified conical parts ofTi/Al composite powder was obtained by the powder forging experiment.Based on reaction tests of Ti, Al foil under different sintering conditions, thereaction mechanism of the Ti/Al at low and high sintering temperature has beeninvestigated systematically. It was found that when Al is not completely consumedduring Ti/Al reaction at the range of520℃~630℃, a unique product TiAl3isgenerated. During Ti/TiAl3diffusion reaction process, TiAl2was generated asintermediate product at the range of630℃~900℃, and TiAl2and Ti2Al5weresimultaneously generated at range of1050℃~1150℃. According to the relationshipbetween the thickness of the reaction products, reaction time and temperature, thekinetic equation of TiAl3, TiAl and the total reaction layer were established, and thecorresponding activation energy and kinetic exponent were determined. The aboveresults can provide a theoretical support to prepare the TiAl based alloy by reaction sintering.The evolution on particle morphology, microstructure of the mixture of Ti and Alelemental powders during mechanical milling has been investigated. reactionsintering experiment of powder forging billets with a certain lamellar distance anddensity, which were prepared by mechanical milling and powder forging, was use toverify reaction kinetic equations and research the evolution law of sinteredmicrostructure. The mechanical properties of sintered TiAl based alloys has beenobtained by high temperature compression tests. The influence of milling time，sintering parameters and compression temperature on the compressive yield strengthof the as-sintered alloy has been revealed. The mechanical property at elevatedtemperature of as-sintered alloy, which was prepared by milling for5h from mixedpowders, subsequent powder forging, final low-temperature presintering at630℃for0.8h and high-temperature sintering at1250℃for5h, was the best, and its yieldstrength reached560MPa at800℃.Through above studies, feasibility of preparing by Ti/Al composite powderforging and reaction sintering process has been proved. The present studies providea strong theoretical and experimental foundation not only for proposed preparationtechnology of TiAl based alloy parts, but also for powder forging and reactionsintering technology of other binary compound... |
PDF Full Text Request