Slip Casting Preparation Of Fe-based Functional Gradient Materials

Fe-Cu-WC composite material was prepared by slip casting/ infiltration method with raw powder of iron, WC and Cu. The carbonyl iron powder of 4μm, the atomized iron powder of less than 325 meshes, the ultrafine iron powder of 0.6μm and mixed powder by two types of iron powder with certain ratio were select as the base raw powder of the slurry of Fe/WC. The effects of ball milling time, pH value, variety and content of surfactant and mixed powder on slurry fluidity, and types of raw powder, particle size distribution and pre-sintering process on porosity of green sample were studied. The optimum ratios of plaster to water for the mould and infiltration technique were determined. The influence of powder loading, raw powder particle size and its distribution, (NaPO₃)₆, and ratio of plaster to water for the mould on gradient formation of WC was studied. Sedimentation characteristic of composing in slurry of Fe/WC and gradient mechanism of WC in slip casting direction was discussed. Change trend of hardness in equidistance testing position at casting direction was studied to qualitatively discuss the distribution of WC, which would be proved by Metallographic.The result showed that the slurry had best fluidity with ball milling time of 24 hours, pH value of 11 and (NaPO₃)₆ content of 0.5wt%, which could be improved evidently by adding atomized iron powder.Green density of skeletons could be improved in slurry with mixed Fe powder, which was greatly improved with the addition of atomized iron powder. The optimum mass ratios of coarse powder to the fine were 50 to 50 and 60 to 40 for atomized/carbnyl and carbnyl/ultrafine iron mixed powders respectively. Powder loadings with maximum green density for skeletons of carbonyl, carbnyl/ultrafine and atomized/carbnyl powder were 75%, 70% and 80% respectively. The optimum infiltration technique was obtained while sintering at 1150℃for 15 minutes.Sedimentation characteristic in slip casting process was associated with powder loading, raw powder particle size and its distribution, and ratio of plaster to water for the mould, which controled by gravity field and clogging effect of fine particles. The essence of clogging effect was different particles' sedimentation velocities in the first settlement stage. Necessary condition of preparing gradient composite was different particles' final sedimentation velocities in the second settlement stage. Concept of integrative sedimentation factor was proposed, which could guide designing the starting powder.The addition of (NaPO₃)₆ was not helpful to WC gradient formation. For iron-based composite prepared by slip casting/infiltration meathod, successive gradient formation of WC content at casting direction was related to the combination of raw powder. While the slurry consisted of carbonyl iron powder and WC of 7μm, the ratio of plaster to water for the moule at bottom should be not more than 1.15 and powder loading must be not less than 75%. While the slurry consisted of ultrafine iron powder and WC of 7μm, the ratio of plaster to water for the mould at bottom should be not more than 1.15 and powder loading must be not less than 80%. While the slurry consisted of atomized iron powder and WC of 7μm, gradient composite was prepared in any normal range of the ratio of plater to water for the mould. While the slurry consisted of mixed Fe powder, the ratio of plaster to water for the mould must be reduced or powder loading must be increased properly.