Study On The Rapid Sintering Densification Of Iron-base Material Based On An Electric Field

In this paper, pure iron powders compact is rapidly sintered successfully by the method based on an electric field. The thermal simulation instrument Gleeble-1500D is mainly used in this experiment. Microstructure of the compact is researched with the help of metallographic microscope and scanning electronic microscope (SEM). The process of sintering densification and the influence of sintering parameter on it are studied deeply. At the same time, the densification mechanism of electric- field and traditional sintering is comparatively studied by using the theory of sintering kinetics. Finally, the NdFeB magnetic material is rapidly sintered as a test example by electric-field sintering method.The results of this study show that: Pure iron powders compact can be perfectly sintered by the method based on an electric field. The sintering densification process is began at 350℃ or so, and completes at about 1000℃. The whole process needs only several minutes. The relative density of the sintered compact is over 95%.Both the sintering temperature and sintering time have great influence on the sintering densification. There is an optimum sintering temperature and an optimum sintering time, under them, the best density can be obtained. In this study, the optimum sintering temperature is 1100℃, and the optimum sintering time is 4 minutes. The statistical results of the porosity show that the higher the density of the green compact is, the higher the porosity is. But this change is not very obvious from the data of porosity.The electric field will affect the porous compact in two sides: one, the electric field can produce Joule caloric at the contact surface and inside of the grains; the other, the electric field can cause field emission, which result in diffusion at low temperature. Just because of such contributions, the system can obtain huge energy at a low temperature easily. That accelerates the diffusion and completes the sintering in a rather short time at a low temperature.In the process of sintering, the amount and size of the pores is reducing with the rising of the temperature. In fact, the big pore turns into a small one and the small pore disappears. Finally, there are only small pores in the sintered compact. Field emission can not happen because of the gas in the compact below 200 °C, at this temperature, there are lots of big pores in the compact which is a green one yet. Field emission happens at 350°C because the gas is out. In the course from 350*C to 700 °C, net-shaped pores begin to close, globular pores begin to form, and the amount and size of the pores begin to reduce. In the course from 700 °C to 1000*C, about half of the pores disappear. The porosity of the compact is reducing with the rising of the temperature. The sintering densification of the compact almost completes at 1000 *C.The sintering of pure iron powders compact under the traditional and electric-field method is analyzed and compared by kinetics theory, and get that: the sintering mechanism of traditional method is volume diffusion. The sintering process under electric field can be divided into two stages. The sintering mechanism of the first stage is mostly volume diffusion, at the same time, a little evaporation-condensation and viscose flow included. Evaporation-condensation and viscose flow are the key mechanism in the second stage, and a little volume diffusion included. According to the research results, in the same stage, the sintering characteristic exponent is reducing with the rising of the temperature, that is, the proportion of the evaporation-condensation and viscose flow are rising.The preliminary experiment results show: the NdFeB magnetic material with a rather density and magnetic performance can be obtained by the sintering methodbased on an electric field in a short time. Compared with the traditional sintering method, the distribution of neodymium phase by electric field sintering method presents discontinuous thin spotted state. Such distribution can improve the corrosion resistance of NdFeB magnetic material. The experiment results not only prove the special action of the electric-field sintering to iron-base material, but show that preparing NdFeB magnetic material by electric-field sintering method is quite feasible.