Study On The Mechanism Of Rapid Densification Of Electricity-assisted Sintering Based On Multi-physical Field Coupling Combine With Phase-field Method

With the rapid development of technology and industry,the requirement of material preparation technology has been gradually raised,and electric current-assisted sintering technology has gradually come into people’s view.Electric current-assisted sintering is a process of rapid sintering and densification by using large alternating or direct pulse current through the green compact.Compared with other new technologies such as powder laser sintering,its products have better mechanical properties.It can not only reduce the cost and energy consumption,but also shorten the sintering time to ten minutes,which has a very attractive industrial application prospect.The study of electric current-assisted sintering mechanism is the most fundamental and key problem of industrial popularization.It will provide direct guidance for optimization of production process and control of material properties,which help to develop customized new materials with specific microstructure and properties.This paper mainly completes the following three aspects of work for SiC ceramic materials:(i)Different from the classical one-way coupling phase field method,a set of interactive coupled thermo-mechano-electro-diffusional equations combine with phase field method were established to study the forming mechanism of powder in sintering,the evolution of sintering driving force and the effects of various physical fields on sintering densification.(ii)In the simulation of hot-pressing sintering,the variation trend of concentration gradient,temperature gradient and strain gradient driving force in sintering were given,and it was found that the proportion of three driving forces in different sintering stages had all taken an obvious turn,so the three-stage hypothesis of sintering was proposed.(iii)In the simulation of electric current-assisted sintering,the influence of current density on activation energy of sintering was introduced into the coupled equations through diffusion coefficient and the real-time effects of current activation on diffusion were characterized,in this way,the densification behavior of current assisted sintering was simulated.In this paper,the microstructure evolution of hot-pressing sintering with different heating rates was simulated,and the results were basically consistent with the trend of experimental results.It was found that the concentration gradient driving force and strain gradient driving force in the sintering process decrease rapidly with the sintering process.The driving force of temperature gradient increases gradually during the heating stage,which reaches the peak at the end of the heating stage,and then gradually decreases.With the increase of heating rate,the time required for sintering densification is gradually reduced.In addition,under different heating rates,the curves of concentration gradient driving force and stress gradient driving force versus relative density are almost identical,that is,the concentration gradient driving force is only related to the morphology of sintered neck,and has nothing to do with sintering speed.However,the driving force of temperature gradient increases with the increase of temperature rise rate.Moreover,under the assumption of constant temperature sintering,the dynamic relationship between sintering driving force and sintering neck growth rate was fitted,which is true under different phase field parameters and sintering temperature.The change of parameters with different sintering temperature conforms to the expectation of classical kinetic equation.According to the variation trend of concentration,temperature and strain gradient driving force in the sintering process,the relative density characteristic points corresponding to the three stages of initial sintering,middle sintering and late sintering are given,which are called the three-stage hypothesis.The characteristic points corresponding to each stage are basically consistent with the sintering topology model obtained by experimental observation,and Moreover,the three-stages hypothesis was proved to be valid under different external pressure and heating rates.Then,the microstructure evolution of electric current-assisted sintering process was simulated,and it was found that the densification rate of sintered powder was significantly accelerated under the action of electric field.In addition,there is an obvious current concentration effect in the sintered neck.Under the action of current activation,the diffusion mobility of the neck is much higher than other regions,which greatly promotes sintering densification.Furthermore,the joule thermal effect,electromigration effect and current activation effect were quantitatively evaluated.Simulation results show that the effect of current density on activation energy plays a major role in the rapid densification process,and the temperature gradient caused by joule thermal effect has almost no effect on densification.And,as a semiconductor,the current density in sintering is small and the electromigration effect is not obvious for SiC.The diffusion flux distribution also indicates that the activation effect of current will greatly activate the sintered neck due to the concentration effect of current,making the sintered neck grow rapidly and accelerate sintering densification.