Journal of University of Science and Technology BeijingMaterialsVolume I3, Number I, February 2006, Page 29Application of the gel casting process in iron powder metallurgyChengchang Jia, Weihua Liu, and Zhimeng GuoMaterials Science and Engineering School, University of Science and Technology Beijing, Bejing 100083, China(Received 204-11-17)Abstract: The effects of various gel casting process parameters such as the dispersant and solid loading on the rheology of Fe slur-ries, molding, and sintering behaviors were studied. The relationship between solid loading and viscidity in the process of iron baseis obviously reduced with the increase of the dispersant. The suitable parameters are as follows: the solid loading is 61% and sinter-ing temperature is 11809C. Iron parts with relatively high density and better properties were obtained by the gel casting process.Key words: gel casting; Fe; powder metallurgy; trheology1. Introductionpowder metallurgy parts. It is important to developnew molding processes with which the green com-Powder metallurgy is a technology that manufac-pacts have homogeneous density and better size andtures materials with special properties, as also a proc-shape precision. It is also required that the process beess that makes machine parts with lttle machining.simple and of low cost. Therefore it was of great sig-Powder metallurgy has many merits such as a simplenificance to employ the gel casting process in powderprocess, high materials utilizing ratio, and lesser ener-metallurgy. In the gel casting process, the requirementgy consumption; but sintered parts have generous po-of low viscidity and high solid loading is a pair of in-rosity resulting in their dynamic mechanical propertiesconsistent factors. It is known that adding a suitablebeing affected. Since 1950, powder metallurgy hasdispersant can change the surface electronic charac-developed rapidly and become an important process toteristic of solid particles, and then improve the viscid-manufacture metallic materials and parts along withity of the slurry. The relationship between solid load-the development of new technologies such as hoting and viscidity in the process of iron powder metal-forging, cold isostatic pressure, and hot isostatic pres-lurgy was researched to obtain better microstructuresure, and the successful application of new materialsand properties.such as powder metallurgy super alloys, and disper-sion reinforced materials. Simultaneously, new tech-2. Experimentalniques have rapidly developed and supplied more ef-2.1. Raw materialsfective methods for molding materials. This is in ad-Reduced and carbonyl iron powder were used. Thedition to the penetration of materials science andpurity of the powder used was 99.9%. The particlepolymer chemistry, colloid chemistry, biology, en-size of the iron powder used in this experiment iszyme chemistry, computer science, microelectronics,shown in Table 1. The tap density and particle dis-and so on. In the early 1990s, Professor M. A. Janney,tribution of iron powder were measured. The tap den-at Oak Ridge National Laboratory USA, developed asity of carbonyl iron powder was 4.428 g/cm'. Fig. 1gel casting technique, a new molding technique, usingis an SEM image of carbonyl iron powder. The or-a macromolecule monomer to mold ceramics powder,ganic solvent was anhydrous alcohol, organic mono-combining the traditional ceramicsprocess anmer was acrylamide (AM), crosslinker was N-N'polymer chemistry [1]. Then this process was furthermethylenebisacrylamide (MBAM), initiator was ammo-developed in the field of ceramics [2-5].nium (APS), the dispersant was made by our group. .The requirement was for manufacturing metal parts2.2. Experimer中國煤化工with excellent performance capabilities and complexMixed mateTHCNMH Gand solutionshapes in tandem with the ceaseless applications ofcomposed of!, cil wauviless ethanolCorresponding author: Chengchang Jia, E-mail: jcc@mater.ustb. edu.cn.3(J. Univ. Sci. Technol. Bejjing, Vol.13, No.I, Feb 2006were ball-milled for 10-20 h, to get a thick suspendingMany factors may affect the fluidity of the slurry, suchliquid (slurry). Air was removed, initiator added andas the dispersant, surface, shape of the particles, andwell mixed, and the slurry was put into the mold. Thenthe characteristics of the suspending medium [6]. The-drying treatment was done at room temperature, dese factors must be considered in the gel casting proc-binding at high temperature and sintering, at the endess. The viscidity of the suspending liquid usually be-of which metal parts with near net shape were ob-comes lower when the dispersant added is lower intained.range. However, the viscidity will become higherwhen the number of dispersants added increases. ThisTable 1. Particle size of iron powderis because the motion of the particles is limited by theFabrication methodSize of particle /Content / wt%net structure formed from the interlinking of poly-μumelectrolyte molecules. It is important to investigate theReduced34relationship between the viscidity of the slurry and the503.amount of dispersants to increase the volume of pow-Carbonyl2-5_3:der in the slurry and ensure optimum shapes. Fig. 2shows the effects of the dispersant on the viscidity ofthe slurry; the volume percents of the powder are 40%,45%, and 50% respectively. It can be seen that theviscidity becomes lower with the increase of the dis-persant, and can reach the lowest value. And theamount of the dispersant to get the lowest visciditydecreases with the increase in volume percent of pow-der.800: 600Fig. 1. SEM image of iron powder.In the course of the experiment process the follow-” 400ing need to be paid attention to. In the process ofSolid volume percentmaking a thick suspending liquid (slurry), it is impor-200 .tant to adjust to a suitable ratio between AM and>一 459MBAM that can improve the drying characteristicsand prevent the crack of green compacts. The drying8process of the compacts pulled off the mold ought toDispersant content/ mol%be done under the condition of a higher relative hu-Fig. 2. Effect of dispersant on the viscidity of slurry.midity and the speed must be low, otherwise the de-formation and crack may be extensive. After the dry-This can be explained by Woodcock equation [7],ing shrinkage is stopped, the compacts can be driedunder conditions of a higher temperature and a lower5|2relative humidity to increase the drying speed. Mold-d3πφ”6|ing must be completed during the inertia time, namelythe time from adding the initiator to the gelation tak-where h is a space between particles, d is the diametering place. Therefore, the amount of the initiator mustof particles, φ is the volume percent of powder.be strictly controlled to ensure better fluidity of theThe amount of the dispersant needed in the slurryslurry. The organics in the compacts must be effec-decreases with the increase of solid volume percenttively removed to prevent adverse effects to the finalbecause the space between particles becomes smallerproducts.and the repulsive action becomes greater. DLVO the-ory [8] shows that the potential energy of particles in a3. Results and discussionslurry is a summation of attracting and repulsive po-3.1. Effects of dispersant on the viscidity of slurrytential energy.中國煤化工The fluidity of the slurry is very important to obtainV-=V+Vk,products with better performance and complex shape.JCHCNMHGwhere VA is the ..... ...... ...gy, and Vk is.C.C. Jia et al, Applcation of the gel casting process in iron powder metallurgy31the repulsive potential energy. The slurry is a disper-powder, such as the Quemada model,sive stable state when the repulsive potential energy isηr =(1-q/qm)-,greater than the attracting potential energy.There are mainly two types of theories on the for-where ψm is the maximal solid volume percent, beingmation of the repulsive force [9]: space potential ob-0.63+0.02 for rigid globose particles and low shearstructs the stable mechanism of high polymer mo-limitation and 0.71+0.02 for high shear limitation. Ex-lecules and the electro repulsive stable mechanism ofperimental results on the relationship between viscid-double electro layer. The addition of a dispersant canity and solid volume percent show an agreement withincrease the repulsive action, then improve the disper-the Quemada model.sive characteristic of the slurry.6003.2. Effects of the volume percent of iron powderA higher volume percent of solid powder in the500slurry is expected to get a higher sintered density, buta suitable viscidity and flow characteristic for molding400is also necessary. The relationship between the viscid-ity of the slurry and the volume percent of iron pow-300der was investigated under conditions of better decen-200tralization. The result is shown in Fig. 3. The effect ofsolid volume percent on the viscidity of a thin slurry100can be calculated by two formulas as follows. Ein-stein's formula is based on an ideal state in hydrody-01045namics, and Batshelor's formula considers the interac-Solid content / vol%tion between particles [10].Fig.3. Relationship between viscidity and volume percentEinstein's formula: n.=1+2.5φ,of iron powder.Batshelor's formula: η =1+2.5q+6.2ρ,Fig.4 shows the microstructures of sintered sam-where η, is the viscidity of the slurry, and φ is the solidples with different volume percents of iron powder.volume percent.The results show that the microstructure of 61% solidBut for a thick slurry, experiential formulas arevolume percent is compacter than that of 56%. Theusually used to describe the relationship between thepores are less for 61%.viscidity of the slurry and the volume percent of solid(a)(b100 umFig. 4. Microstructures of sintered samples with different solid volume percents: (a) 56%; (b) 61%.3.3. Effects of sintering temperaturemetallurgy process. Besides, iron powder with differ-In general, in the powder metallurgy process forent particle sizes was used to increase the density ofiron base parts, compacted green parts with a higherthe sintered samples.density can be obtained by press [1 1]; so the sinteringFig.5. shows the fracture sections of samples sin-shrinkage is small. However, in the gel casting process,tered at different temperatures. Fig. 6. is their com-sintering shrinkage is necessary because there arepressive deformation curves, showing the relationshipsome organic matters in the molding parts that mustbetween the中國煤化工can be seenbe removed in the sintering process. Sintering tem-that the compr; sintered atperatures are also higher than in the common powder1360°C is lessefMYHcNMHGnbeconsd-.32J. Univ. Sci. Technol. Beijing, Vol.13, No.1, Feb 2006ered that the grain size had a massive growth duringcrease of the strength. This is consistent with the SEMthe sintering process at 1360°C, leading to the de-image in Fig. 5.a):Fig. 5. SEMs of fracture sections sintered at different temperatures: (a) 1180°C; (b) 1360°C.0.303.4. Properties and shape of sintered samplesTable 2 compares the properties of the samplesfrom the common powder metallurgy and gel castingprocess. It can be seen that their density and propertiesare almost equivalent.Fig. 7 shows the iron parts made by the gel castingg 0.12-process in this experiment. This process has a goodmolding performance. Complex shapes such as screw0.06-thread and traverse hole can be obtained by this proc-ess, which can be hardly obtained by the common1.2.0powder metallurgy process.Deformatin / mmFig. 6. Compression deformation curves: A- -1360°C,B--1180°C.Table 2 Properties of the samples from the different processesProcesspρ(g.cm-3)0b/ MPaHardness, HBCommon P/M7.004507(Gel casting.98610plex shapes and better microstructure and propertieswere obtained. The density was 6.98 g/cm', the com-pression strength was 610 MPa, and the HB was 110.The properties are much better than that made bypowder metallurgy. The suitable parameters: powdervolume percent 61%， dispersant 5%，and sinteringtemperature 1180°C.What needs to be pointed out is that gel casting is aversatile technology and has good prospects.Fig. 7. Iron powder metallurgy parts made by the gelReferencescasting process.[1] 0.O. Omatete, M.A. Janny, and R.A. Strelow, Gelcasting-4. Conclusionsa new ceramici Coram. Soc. Bull,中國煤化工701991),p.1The gel casting process was successfully applied in[2] A.J. SanchezYCNMHG.Millan,etal,the field of powder metallurgy. lron parts with com-Colloidal forming of metal/ceramc composites, [in] 7th.C.C. Jia et al, Applcation of the gel casting process in iron powder metallurgy33Conference & Exhibition of the European Ceramic Society,[7] J. Pugh Robert, Surface and colloid chemistry in advancedBelgium, 2001, p.227.ceramics pressing, Surfactant Sci. Ser., 51(1994), p.136. .[3] K. Scharrer, Thermal process management: Dyson TPM8] Z.Q. Chen, Colloid Chemistry, Higher Education Press,offers cost saving services, equipment and solutions,Beijing, 1984.Ceram. Forum Int, 80(2003), No.5, p.27.9] L.X. He, T.L. Wen, and Z.Y. Lu, Research development[4] M.A. Janney and W. Ren, Gelcast tooling: net shape cast-on electrolyte membrane for ceramic fuel cells made bying and green machining， Mater. Manuf. Process,tape casting, Mater. Sci. Eng, 15(1997), No.3, p.15.13(1998), No.3, p.389.[10] W.J. Si, H.Z. Miao, TJ. Graule, et al, Study on highly[5] S. Dhara, R.K. Kamboj, and M. Pradhan, Shape formingconcentrated Si:N4 slury (I )- the rheological behaviourof ceramics via gelcasting of aqucous particulate slurries,of surry, J. Chin. Ceram. Soc, 24(1996), No.5, p.531.Bull. Mater. Sci, 25(2002), No.6, p.565.[11] C.C. Jia, S.M. Fan, Y. Peng, et al, Sintering mechanism[6] J.L. Yang, Z.P. Xie, Q. Tang, et al, Study on rheologicalof stainless steel in powder injectin molding, J. Univ. Sci.behavior and gelcasting of a-Al2O3 suspension, J. Chin.Technol. Beijing, 7(2000), No.1, p.51.Ceram. Soc., 26( 1998), No.1, p.41.中國煤化工MHCNMH G.